An Integrated Hybrid Power Supply for

Distributed Generation Applications Fed by

Nonconventional Energy Sources

ABSTRACTA new, hybrid integrated topology, fed by photovoltaic (PV) and fuel cell (FC) sources and

suitable for distributed generation applications, is proposed. It works as an uninterruptible power

source that is able to feed a certainminimum amount of power into the grid under all conditions.

PV is used as the primary source of power operating near maximum power point (MPP), with the

FC section (block), acting as a current source, feeding only the deficit power. The unique

“integrated” approach obviates the need for dedicated communication between the two sources

for coordination and eliminates the use of a separate, conventional dc/dc boost converter stage

required for PV power processing, resulting in a reduction of the number of devices,

components, and sensors. Presence of the FCsource in parallel (with the PVsource) improves the

quality of power fed into the grid by minimizing the voltage dips in the PVoutput.Another

desirable feature is that even a small amount of PV power (e.g., during low insolation), can be

fed into the grid. On the other hand, excess power is diverted for auxiliary functions like

electrolysis, resulting in an optimal use of the energy sources. The other advantages of the

proposed system include low cost, compact structure, and high reliability, which render the

system suitable formodular assemblies and “plug-n-play” type applications. All the analytical,

simulation, and experimental results of this research are presented.

1.INTRODUCTION

Fig.1 Stationary Fuel Cell Power Systems with Direct FuelCell TechnologyTackle Growing Distributed Baseload Power Challenge

Anthony Leo, Vice President of OEM and Application Engineering, FuelCell Energy, Inc.

Global demand for electric power is on the rise, while tolerance for pollution and potentially hazardous forms of power generation is on the decline.Traditional forms of power generation primarily made up of centralized fossil fuel plants — are becoming less favored in lieu of clean,distributed power generation technologies.Emissions from coal and other fossil fuel-powered plants include large amounts of carbon dioxide(CO2) greenhouse gas and pollutants that include nitrogen and sulphur oxides (NOx and SOx).In addition, all forms of centralized power generation use a grid of high-voltage transmission lines to carry energy to consumers.These transmission lines represent a problematic infrastructure for theutilities, due to real or perceived health threats, NIMBY issues, and the loss of energy during transmission as much as 20% depending on the distance from the central power source to the user.Unlike the backup power solutions normally associated with Distributed Generation (DG), new technologies offer baseload power solutions that operate 24/7 and can dramatically reduce dependence on the power grid. Stationary fuel cells, wind farms, and solar arrays are coming of age,while micro-turbines and combustion generators continue to improve.However, only fuel cells andfossil fuel-based generation technologies offer 24/7 distributed power.Of this group, fuel cells offer the cleanest and most efficient form of distributed power generation available.

Why Fuel Cells?

The need is clear and well recognized for clean, safe, and reliable forms of energy that can provide prescribed levels of power consistently, and on demand.Yet, most forms of non-combustion electric generation have limitations that impact widespread use of the technology, especially as a primary source of electric power (i.e., baseload power). Solar energy, for example, depends on the sun.Extended days of cloudy skies can severely limit the generation of electricity, and power availability is generally considered to be between 25 to 35%.Wind turbines are designed to turn kinetic energy into electricity.They too, depend on factors that cannot be controlled. In this case, the presence of wind and a certain minimum wind velocity are required. As a result, power availability is judged tobe in the range of 30 to 35%.Geothermal sources require heat energy from underground geothermal fields, which mean they are restricted to certain geographic locations. Similarly, hydroelectric plants are confined to locations near major rivers and are also somewhat constrained by nature.Thus, without adequate and consistent sun, wind, heat, and water flow, such sources of power generation are limited by the whims of nature and cannot be considered as reliable sources ofbaseload power when and where needed. Fuel cell technology, on the other hand, has advanced to the point where it is now a viable challenger to combustion-based plants for growing numbers of baseload power applications.Today, fuel cells are reaching their potential as the cleanest and most reliable sources of distributed power generation.With 95% power availability and electric power generation efficiency of 47%, they represent a viable means of producing Ultra-Clean power, reliably, consistently, and on demand.

While the need to ensure the availability of hydrogen has been seen as a concern in the operation of fuel cells,Direct FuelCells® (DFCs®) developed by FuelCell Energy, Inc. are unaffected by such limitations because they use natural gas and biofuels (gases from food processing and wastewater treatment) as a source fuel. Furthermore, with system adjustments, these fuel cells can also operate with a wide range of alternate fuels, including ethanol and propane.Direct FuelCells have even been shown to generate clean power from diesel fuel and coal gas, fuels traditionally considered to be high pollution sources. DFCs internally reform hydrogen from the source fuels and emit dramatically reduced CO2 greenhouse gas compared to fossil fuel power plants, and only negligible amounts of pollutants, such as NOx and SOx.

How Fuel Cells Work

In essence, fuel cells are electrochemical devices that combine fuel with oxygen from the ambient air to produce electricity and heat, as well as water.The non-combustion,electrochemical process is a direct form of fuel-to-energy conversion, and is much more efficient than conventional heat engine approaches. CO2 is reduced, due to the high efficiency of the fuel cell, and the absence of combustion avoids the production of NOx and particulate pollutants.Fuel cells incorporate an anode and a cathode, with an electrolyte in between, similar to a battery.The material used for the electrolyte and the design of the supporting structure determine the type and performance of the fuel cell. Figure 1 illustrates the process for FuelCell Energy(FCE) Molten Carbonate Fuel Cells (MCFC). Fuel and air reactions for the molten carbonate Direct FuelCell occur at the anode and cathode, which are porous nickel (Ni) catalysts.The cathode side receives oxygen from the surrounding air. As can be seen in Figure 1, hydrogen is created in the fuel cell stack through a reforming process,which produces hydrogen from the reforming reaction between the hydrocarbon fuel and water. The gas is then consumed electrochemically in a reaction with carbonate electrolyte ions that produces water and electrons.The electrons flow through an external circuit to provide the power to the fuel cell load, and then return to be consumed in the cathode electrochemical reaction. The O2 supplied to the cathode, along with CO2 recycled from the anode side, reacts with the electrons to produce carbonate ions that pass through the electrolyte to support the anode reaction.The electron flow through the external circuit produces the desired power (DC current). An inverter is used to convert the DC output to AC.

Figure 2. Fuel Cell Processes for a Direct FuelCell

Figure 3. Block Diagram of a Molten Carbonate Direct FuelCell

A DFC powerplant consists of the fuel cells described above (arranged in stacks to provide the required system voltage and power) and the equipment needed to provide the proper gas flow and power conversion, which is referred to as Balance of Plant (BOP). The power plant process is illustrated in Figure 2. Fuel and water are heated to the required fuel cell temperature in a heat recovery unit (HRU), which transfers heat from system exhaust gases.The heated humid fuel stream is sent to the fuel cell stacks where, as described above, the fuel is converted to hydrogen, most of which is used in the electrochemical reaction. Residual fuel — i.e., fuel not consumed in the electrochemical reaction — is supplied to a catalytic reactor to heat incoming air.The heated air flows to the cathode to provide the cathode reactants (oxygen from the air and carbon dioxide from the anode reaction).Cathode exhaust gas exits the system through the heat exchanger used to preheat the fuel and water supplied to the HRU.

Figure 3 shows a typical DFC power plant, with a cutaway illustration of one of the fuel cell modules,showing the fuel cell stacks within the module.There are two main pieces that make up the balanceof plant equipment: the Mechanical Balance of Plant (MBOP), and the Electrical Balance of Plant (EBOP).The MBOP consists of such functions as water and fuel treatment, preheating and humidification of the fuel to be supplied to the anodes of the fuel cells, and supply of the air to the system.The EBOP encompasses such subsystems as the DC/AC converter, power metering, switching equipment, and the voltage transformer. As with other types of power plants, fuel cells can offer the benefits of cogeneration, known as Combined Heat and Power (CHP). A bottoming process, in that heat can be extracted in the production of electric power,cogeneration using fuel cells can represent a significant opportunity to increase the efficiency of the power plant. A diagram showing the setup for extracting “waste” heat during the generation of electrical power is shown in Figure 4.

Direct FuelCell power plants have an exhaust temperature ranging from 650°F to 750°F.This heat energy can be captured to provide heat for buildings, swimming pools, and other facility needs. In fact, the already high efficiency of fuel cells can be increased from around 47% to as much as 80% or more, depending on design and installation parameters.* How important is CHP? According to the U. S. Combined Heat and Power Association, more than $5 billion dollars in heating costs are saved annually by building owners in the U. S., and energy consumption is being reduced eachyear by some 1.3 billion BTUs.While these figures cover all types of CHP, including systems incorporated in plants that burn fossil fuels, the benefits of cogeneration in a fuel cell operation are two-fold: 1) the increase in efficiencypreviously mentioned, and 2) the fact that Ultra-Clean,quiet fuel cell plants can be located within or near the facility where the electricity is to be used.This is a distinct advantage over conventional central plants that are usually located too far from heat users to allow for effective utilization of waste heat. There are other CHP considerations regarding the tradeoff between heat and electricity that highlight the benefits of fuel cells over turbines and other combustion generators.Electricity generated during a cogeneration process has a significantly greater value than that of the associated waste heat, in fact, up to 10 times as much.Thus, the generation of electricity is paramount in the economic efficiency equation, since the more electricity that can be produced by the power plant, the less of this relatively high priced electricity must be purchased from the grid.

With traditional sources of power generation — e.g., reciprocating engines, microturbines, etc. — CHP can mask the underlying electrical power generation efficiency of the power source.Whatever CHP adds to the overall efficiency,economics will be driven by the actual electrical powergenerating efficiency of the plant.Thus, in the case of a gas turbine, operating at 25% electric power generation efficiency and a reciprocating engine at 35% electrical power generation efficiency, considerably less of the overall output of the system — percentage wise — is in the form of electricity. In contrast, the Direct FuelCell operates at 47% electrical power generation efficiency.The bottom line: fuel cells offer the distinct advantage of a higher ratio of electricity to heat — electricity that would be relatively expensive if it had to be purchased from the grid — while capturing much of the waste heat generated for the CHP process.

* Alternatively, instead of the waste heat generated by the fuel cell being used for cogeneration, the heat can be transferred to a turbine, which converts the heat to mechanical energy and then to electrical energy. Such a process can increase electrical efficiency by 10 - 15 percentage points. Simpler power generation bottoming approaches, such as powering an Organic Rankine Cycle (ORC) with DFC exhaust heat, can increase electrical efficiency by 2 - 3 percentage points. In a system where DFC waste heat is provided to support gas distribution pressure letdown (DFC-ERG), efficiencies in the mid-60% range can be achieved. These types of heat to electricity approaches are effective in larger grid connected applications where there may not be a local user of thermal energy.

At the Leading Edge of Technologywith Direct FuelCells

Some 60 stationary power plant installations using FuelCell Energy’s DFC fuel cells supply baseload power in five countries.To date, the company’s DFC systems have produced more than 180 million kilowatt hours of electric power for customers. Facilities in which systems have been installed include hospitals, hotels,wastewater treatment plants, prisons, food and beverage processing plants,manufacturing plants, universities, government institutions, and utilities.

For these types of applications, fuel cell technology, and in particular, FuelCell Energy Direct FuelCells,offer important performance and cost-saving benefits over traditional methods of generating power.For example, in food and beverage processing, digester gases are produced by the fermentation of organic matter.Direct FuelCells can use the methane from the digester gas to produce electricity and heat (which is used by the digesters).This avoids the need to flare unused gas, and provides power in a much cleaner manner than if the gas were used in a combustion-based generator.

Hospitality represents another market for which fuel cells are especially suited.The success of a hotel depends largely on its ability to provide an attractive, clean, and quiet environment.Virtually silent and unobtrusive, fuel cells provide Ultra-Clean baseload power to the hotel, while simultaneously generating heat as a usable byproduct for swimming pools, domestic hot water, and building heat.The presence of this “green” technology is also seen by the hotel industry as a unique marketing opportunity: some companies request that their employees only stay at “green” hotels.

Manufacturing plants employ fuel cells for both baseload power and peak load management.Waste heat produced in power generation is often used in the manufacturing process to augment or replace existing heating systems.

The medical industry, including hospitals, nursing homes, and other critical care facilities require reliable baseload power around the clock, and they have a need for cogeneration heat. Ultra-Clean, quiet fuel cells offer a viable alternative to dependence on conventional fossil fuel-based utilities for baseload power.

Prisons have well defined power and heat loads, and benefit from the reliability gained by paralleling a fuel cell powerplant with the grid. A power loss at one of these facilities would cause concerns about security, even though short-term backup systems exist.Many prisons also house a medical facility, for which reliable power and heat is essential.Wastewater treatment plants produce biogases that are released in the form of atmospheric emissions. Fuel cells take advantage of the biogas by reforming it into usable hydrogen that provides a source of fuel for the fuel cell power plant.The biogas is thus consumed and emissions are negligible. By all standards of measurement, FuelCell Energy is the leading manufacturer of stationary fuel cells in the world.The reasons for this encompass more than company size, capability, and years of experience.They are based on two factors: a) the innovation and advancements in fuel cell technology pioneered by FuelCell Energy, and b) participation by the company with government agencies, such as the U.S.Department of Energy (DOE) National Energy Technology Laboratory (NETL) and the Solid State Energy Conversion Alliance (SECA), in technology research and development. Such co-development efforts have led to global recognition for the company’s achievements in advanced research in such areas assolid oxide fuel cells (SOFCs) and the successful development and commercialization of the Direct FuelCell. At present, FCE manufactures three fuel cell systems:DFC300MA, DFC1500, and DFC3000.The DFC300MA provides 300 kW of continuous power output. Suitable applications for this product include supermarkets,medium-sized (300-bed) hotels, and small commercial businesses.The DFC1500 produces 1.2 MW of continuous baseload power. It is ideal for large hotels,convention centers, and facilities with similar levels of power consumption.The DFC3000 produces 2.4 MW of baseload power.The unit is designed to meet the power needs of hospitals, universities, large manufacturing complexes,and utility/grid support.Multiple DFC systems can be combined to provide larger power outputs for largeutility/grid support applications up to 50 MW. FuelCell Energy also supplies DFCs for a unique grid support application through its distributor, Enbridge, Inc. In this application, the Direct FuelCell is combined with a turbo expander to extract energy at natural gas letdown stations. Known as the DFC-ERG, the product is commercially available.Other high-performance systems, such as the high efficiency Direct FuelCell/Gas Turbine hybrid (DFC/T), are in developmental stages Some of the advancements and enhancements in technology that help to distinguish Direct FuelCells include the following: • DFCs create the hydrogen gas needed by the anodes within the fuel cell module from readily available hydrocarbon fuels, such as natural gas.This process is called internal reforming, and external equipment (which adds cost and requires energy input) is not required.

Internal reforming is possible due to the relatively high operating temperature (650-750°F) of Direct FuelCells.This operating temperature has other advantages. For example, because of the high temperature, non-precious metals can be used for the anode and cathode instead of platinum, resulting in significant cost savings. Also, the exhaust from the system is high-grade heat, capable of supporting a variety of heat recovery options, including steam generation. • DFCs can run on natural gas, or biogas, such as digester gas from wastewater treatment and food processing plants.Some states consider fuel cells running on biogas to be a renewable energy source, which means the fuel cells qualify for additional financial incentives.With adjustments to the system, DFC plants can also run on propane, diesel fuel, ethanol, coal gas, and other hydrocarbons. • FuelCell Energy’s DFC power plants are complete systems, which include all of the “Balance of Plant” equipment needed to convert natural gas or treated digester gas into grid quality electricity. Simply stated, this means all aspects of the plant, including accessories and supporting equipment — heat exchangers, power electronics, control logic, and even valves, fittings, piping,etc. — have been designed and selected to optimize the complete power system package.

FUEL CELL TECHNOLOGY AND APPLICATION

1.INTRODUCTION

In order to move towards a sustainable existence in our critically energy dependent society there is a continuing need to adopt environmentally sustainable methods for energy production, storage, and conversion.

The use of fuel cells in both stationary and mobile power applications can offer significant advantages for the sustainable conversion of energy. Benefits arising from the use of fuel cells include efficiency and reliability, as well as economy, unique operating characteristics, and planning flexibility and future development potential. By integrating the application of fuel cells, in series with renewable energy storage and production methods, sustainable energy requirements may be realised.The need is clear and well recognized for clean, safe, and reliable forms of energy that can provide prescribed levels of power consistently, and on demand.Yet, most forms of non-combustion electric generation have limitations that impact widespread use of the technology, especially as a primary source of electric power (i.e., baseload power). Solar energy, for example, depends on the sun.Extended days of cloudy skies can severely limit the generation of electricity, and power availability is generally considered to be between 25 to 35%.Wind turbines are designed to turn kinetic energy into electricity.They too, depend on factors that cannot be controlled. In this case, the presence of wind and a certain minimum wind velocity are required.

As a result, power availability is judged to be in the range of 30 to 35%.Geothermal sources require heat energy from underground geothermal fields, which mean they are restricted to certain geographic locations. Similarly, hydroelectric plants are confined to locations near major rivers and are also somewhat constrained by nature.Thus, without adequate and consistent sun, wind, heat, and water flow, such sources of power generation are limited by the whims of nature and cannot be considered as reliable sources of baseload power when and where needed. Fuel cell technology, on the other hand, has advanced to the point where it is now a viable challenger to combustion-based plants for growing numbers of baseload power applications.Today, fuel cells are reaching their potential as the cleanest and most reliable sources of distributed power generation.With 95% power availability and electric power generation efficiency of 47%, they represent a viable means of producing Ultra-Clean power, reliably, consistently, and on demand.While the need to ensure the availability of hydrogen has been seen as a concern in the operation of fuel cells,Direct FuelCells® (DFCs®) developed by FuelCell Energy, Inc. are unaffected by such limitations because they use natural gas and biofuels (gases from food processing and wastewater treatment) as a source fuel. Furthermore, with system adjustments, these fuel cells

can also operate with a wide range of alternate fuels, including ethanol and propane.Direct FuelCells have even been shown to generate clean power from diesel fuel and coal gas, fuels traditionally considered to be high pollution sources. DFCs internally reform hydrogen from the source fuels and emit dramatically reduced CO2 greenhouse gas compared to fossil fuel power plants, and only negligible amounts of pollutants, such as NOx and SOx.

2. FUEL CELL FUNDAMENTALS

2.1 Description

A fuel cell is conventionally defined as an “electrochemical cell which can continuously

convert the chemical energy of a fuel and an oxidant to electrical energy by a process involving

an essentially invariant electrode-electrolyte system” [1]. For a hydrogen/oxygen fuel cell the

inputs are hydrogen (fuel) and oxygen (oxidant) and the only outputs are dc power, heat, and

water. When pure hydrogen is used no pollutants are produced, and the hydrogen itself can be

produced from water using renewable energy sources such that the system is environmentally

benign. In practice hydrogen is the best fuel for most applications. In addition to hydrogen some

fuel cells can also use carbon monoxide and natural gas as a fuel. In these reactions, carbon

monoxide reacts with water producing hydrogen and carbon dioxide, and natural gas reacts with

water producing hydrogen and carbon monoxide, the hydrogen that is produced is then used

as the actual fuel.

2.2 Electrochemistry

The basic physical structure of all fuel cells consists of an electrolyte layer in contact

with an anode and cathode electrode on either side of the electrolyte. The electrolyte provides a

physical barrier to prevent the direct mixing of the fuel and the oxidant, allows the conduction of

ionic charge between the electrodes, and transports the dissolved reactants to the electrode. The

electrode structure is porous, and is used to maximise the three-phase interface between the

electrode, electrolyte and the gas/liquid, and also to separate the bulk gas phase and the

electrolyte. The gas/liquid ionisation or de-ionisation reactions take place on the surface of the

electrode, and the reactant ions are conducted away from or into the three-phase interface [2]. A

schematic representation of a fuel cell with the reactant/product gases and the ion conduction

flow directions through the cell is shown in Fig.1.

Fig.1 Basic working concepts [2]

In theory a fuel cell is capable of producing an electric current so long as it supplied with

fuel and an oxidant.In practice the operational life of the fuel cell is finite, and fuel cell

performance will gradually deteriorate over a period of time as the electrode and electrolyte

age. However, because fuel cells operate with no moving parts, highly reliable systems are

achieved [3].

2.3 Efficiency

The thermal efficiency of the fuel cell can be defined as the percentage of useful electrical

energy produced relative to the heat that would have been obtained through the combustion of

the fuel (enthalpy of formation). In the ideal case, the maximum efficiency (or thermodynamic

efficiency) of a fuel cell operating irreversibly can be expressed as the percentage ratio ofGibbs

free energy over the enthalpy of formation, that is,

where DG is change in Gibbs free energy and DH is the enthalpy of formation of the

reaction. For the hydrogen/oxygen fuel cell the thermodynamic efficiency limit at the higher

heating value (HHV) is equal to 83% [3].

In practice the efficiency of the fuel cell can be expressed in terms of the percentage ratio of

operating cell voltage relative to the ideal cell voltage as

where Vcell is the actual voltage of the cell and Videal is the voltage obtained from Gibbs

free energy in the ideal case. The 0.83 is from the thermodynamic limit (HHV). In the non-ideal

case the actual operating voltage is less than the ideal voltage because of the irreversible losses

associated with the fuel cell electrochemistry. There are three primary irreversible losses that

result in the degradation of fuel cell performance and these are activation polarisation,ohmic

polarisation, and concentration polarisation [1-3]. Fig.2 illustrates the effects of the irreversible

losses on cell voltage for a low temperature, hydrogen/oxygen fuel cell.

Activation polarisation is caused by limited reaction rates at the surface of the electrodes, and is

dominant at low current density and increases marginally with an increase in current density.

Ohmic polarisation is caused by the resistance to the flow of ions in the electrolyte and to the

flow of electrons through the electrode materials. This loss is directly proportional to the current

density.

Concentration polarisation is caused by a loss of concentration of the fuel or oxidant at the

surface of the electrodes. These losses are present over the entire

current density range but become prevalent at high limiting currents where it becomes difficult to

provide enough reactant flow to the cell reaction sites.

Fig.2 Ideal and actual voltage/current curves of a low temperature hydrogen/oxygen fuel cell (From [2,3]).

2.4 Advantages

The main advantages of fuel cells are:Efficiency - Fuel cells are generally more efficient

than combustion engines as they are not limited by temperature as is the heat engine.

Simplicity - Fuel cells are essentially simple with few or no moving parts. High reliability may

be attained with operational lifetimes exceeding 40,000 hours (the operational life is formally

over when the rated power of the fuel cell is no longer satisfied) [3 -5].

Low emissions - Fuel cells running on direct hydrogen and air produce only water as the

byproduct.

Silence - The operation of fuel cell systems are very quiet with only a few moving parts if any.

This is in strong contrast with present combustion engines.

Flexibility - Modular installations can be used to match the load and increase reliability of the

system.

2.5 Disadvantages

The principal disadvantages of fuel cells, however, are the relatively high cost of the fuel

cell, and to a lesser extent the source of fuel. For automotive applications a cost of US$10 to $50

per kW and an operation life of 4000 hours is required in order to compete with current internal

combustion engine technology. For stationary combined heat and power systems a cost of

US$1000 per kW and an operation life of 40,000 hours is required [5,6]. The current cost of a

fuel cell system is around US$3000 per kW for large systems with additional costs required for

the heat exchanger in the combined heat and power systems. The cost of fuel cells will be

brought down with mass manufacture and costs of US$100 per kW have been predicted as the

production of fuel cells expand over the following few years [5].

FUEL CELL CLASSES

There are five primary classes of fuel cells, identified by their electrolyte, which have

emerged as viable systems [2]. Although the most common classification of fuel cells is by the

type of electrolyte used, there are always other important differences as well. Each fuel cell class

differs in the materials of construction, the fabrication techniques, and the system requirements.

The potential use for different applications is inherent in the main characteristics of each fuel cell

class [2].

Solid Oxide (SOFC):

The solid oxide fuel cell operates between 500-1000C. The electrolyte in this fuel cell is

a solid, nonporous metal oxide and the charge carriers are oxygen ions. The electrolyte always

remains in a solid state adding to the inherent simplicity of the fuel cell. The solid ceramic

construction of the cell, can minimise hardware corrosion, allows for flexible design shapes, and

is impervious to gas crossover from one electrode to the other. Due to the high temperature

operation, high reaction rates are achieved without the need for expensive catalysts and also

gases such as natural gas can be internally reformed without the need for fuel reforming.

Unfortunately the high operating temperature limits the materials selection and a difficult

fabrication processes results. In addition the ceramic materials used for the electrolyte exhibit a

relatively low conductivity, which lowers the performance of the fuel cell.

Polymer Electrolyte Membrane (PEMFC):

The polymer electrolyte membrane fuel cell operates at 50- 100C. The electrolyte in this

fuel cell is a solid ion exchange membrane used to conduct protons. Hardware corrosion and gas

crossover are minimised as a result of the solid electrolyte and very high current densities as well

as fast start times have been realised for this cell. However due to the low temperature operation,

catalysts (mostly platinum) are needed to increase the rate of reaction. In addition heat and water

management issues are not easily over come in a practical system, and tolerance for CO is low.

Alkaline (AFC):

The alkaline fuel cell operates between 50-250°C. The electrolyte in this fuel cell is KOH,

and can be either mobile or retained in a matrix material. Many catalysts can be used in this fuel

cell, an attribute that provides development flexibility. The ACF has excellent performance on

hydrogen and oxygen compared to other candidate fuel cells. The major disadvantage of this fuel

cell is that it is very susceptible to CO2 and CO poisoning and hence its use with reformed fuels

and air is limited.

Phosphoric Acid (PAFC):

The phosphoric acid fuel cell operates at 200C with phosphoric acid (100%) used for the

electrolyte. The matrix universally used to retain the acid is silicon carbide, and the catalyst is

Platinum. The use of concentrated acid (100%) minimises the water vapour pressure so water

management in the cell is not difficult. The cell is tolerant to CO2 and the higher temperature

operation is of benefit for co-generation applications. The main limitation of the PAFC is the

lower efficiency realised in comparison with other fuel cells.

Molten Carbonate (MCFC):

The molten carbonate fuel cell operates at 600C. The electrolyte in this fuel cell is usually

a combination of alkali carbonates retained in a ceramic matrix. At the high temperature of

operation the alkali carbonates form a highly conductive molten salt, with carbonate ions

providing ionic conduction. The high reaction rates remove the need for noble metal catalysts

and gases such as natural gas can be internally reformed without the need for a separate unit. In

addition the cell can be made of commonly available sheet metals for less costly fabrication. One

feature of the MCFC is the requirement of CO2 at the cathode for efficient operation. The main

disadvantage of the MCFC is the very corrosive electrolyte that is formed, which impacts on the

fuel cell life, as does the high temperature operation.

In addition to the five primary fuel classes, there are two more classes of fuel cells that are

not distinguished by their electrolyte. These are the Direct Methanol Fuel Cell (DMFC),

distinguished by the type of fuel used, and the Regenerative Fuel Cell (RGF) distinguished by its

method of operation.

4. FUELS FOR FUEL CELLS

4.1 Fuel Requirements

In theory, any substance that is capable of being chemically oxidised at a sufficient rate at

the anode of the fuel cell may be used as a fuel. In the same sense, any substance that is capable

of being reduced at the cathode of the fuel cell at a sufficient rate may be used

as an oxidant [2]. In practice, hydrogen is the best fuel for most applications. The low-

temperature fuel cells such as the AFC, PEMFC, and PAFC, are electrochemically constrained to

hydrogen fuel use only, while the hightemperature fuel cells such as MCFC and the SOFC, in

addition to hydrogen can also use carbon monoxide and natural gas as a fuel. In these reactions,

carbon monoxide reacts with water producing hydrogen and carbon dioxide, and natural gas

reacts with water producing hydrogen and carbon monoxide, the hydrogen that is produced is

then used as the fuel. Similarly, oxygen is the most common oxidant because it is readily and

economically available from air.

4.2 Advantages of Hydrogen

The wide spread use of hydrogen as the fuel choice for fuel cells has the following benefits [1,5]:

High electrochemical reactivity when suitable catalysts are used, and high energy

content(kJ/kg),

The oxidation of hydrogen is a simple and environmentally benign reaction that makes

zero

emissions power systems possible,

The source of energy production is not constrained to any particular fuel type and hence

provides the basis for a rapid progress towards a sustainable transportation and electricity

system,

An increase in retail price competition as a result of the many fuel sources available

4.3 Sources of Hydrogen

Unfortunately, hydrogen does not occur naturally as a gaseous fuel and must be produced

from another source. Potential sources of hydrogen include, such as fossil fuels (coal, oil, or

natural gas), a variety of chemical intermediates (refinery products, ammonia, methanol), and

alternative resources such as bio-mass, bio-gas, and waste materials. Hydrogen can also be

produced by water electrolysis, which uses electricity to split hydrogen and oxygen elements [6].

The electricity for the water electrolysis can be generated from conventional sources or from

renewable sources. In the longer term, hydrogen generation could be based on photo-biological

or photochemical methods [7].

4.4 Fuel Processing

Some fuel processing will almost always be required in order to produce useful hydrogen

rich gas from another source. Most of the hydrogen currently produced on the industrial scale is

through the steam reforming of natural gas, which produces carbon dioxide as a by product.

While this method of hydrogen production is generally the most economic, it is not sustainable in

the long term and can serve only as an intermediate step, as is the same for all fossil fuels.

However the sustainable production of hydrogen can be achieved with current technology

through bio-fuels or by the electrolysis of water using renewable energy sources such as hydro-

power, solar energy or wind energy [4,8].

4.5 Hydrogen Economy

The emergence of a true hydrogen economy, based upon hydrogen for energy storage,

distribution, and utilisation would be a major advantage for the wide spread application of fuel

cells. Although there is already an existing manufacturing, distribution, and storage infrastructure

of hydrogen, it is limited. The infrastructure costs associated with a large scale hydrogen

distribution, is often cited as the major disadvantage for the wide spread use of hydrogen as

“a major world fuel and energy vector” [3]. In addition there are concerns that because of the

relatively low density of hydrogen it is not viable for energy storage, particularly in mobile

applications, and there is also concern in regard to the safety of hydrogen [5].

It can be argued however, that with good integration practices for both distributed fuel cell

power supplies and mobile power applications, natural gas or off peak electricity can initially be

used for hydrogen production, removing the initial requirement for the large infrastructure costs

associated with a hydrogen distribution system. In this case, the hydrogen can be produced as

needed, in quantities to match the incremental growth of fuel cells applications. The reforming of

natural gas, and the use of electricity from coal fired power plants can be used as an intermediate

step and does not constrain the hydrogen use to a particular fuel type [5].

4.6 Hydrogen storage

In addition the storage of hydrogen, although not limited to, can be achieved in the simple

form of a compressed gas. While the use of compressed hydrogen gas in stationary applications

presents a viable option there is concern that the insufficient density of storing hydrogen as a

compressed gas limits its inclusion in mobile applications. This is however not necessarily the

case, as is illustrated with the design approach of using compressed hydrogen gas storage in the

ultra light fuel cell vehicles termed hypercars [5].

Finally, the safe storing hydrogen as a compressed gas are in many ways less stringent than

the safe storing of alternative fuels such as methanol, petrol, or natural gas. The hydrogen gas

would be stored in extremely strong carbon fibre cylinders. Because of the rapid diffusion of

hydrogen any spill will dissipate quickly. Hydrogen is also non-toxic and requires a four fold

higher concentration than petrol to ignite [5].

Additional methods of hydrogen storage include [3]:

1. Storage as a cryogenic liquid,

2. Storage as a reversible metal hydride,

3. The use of metal hydride reactions with water, and

4.The use of carbon nano-fibers.

The first three methods of hydrogen storage are currently available and are generally

well understood processes. The fourth method, which uses carbon nano-fibres for hydrogen

storage is not yet practical although considerable efforts are being invested into making this a

feasible technology.

FUEL CELL APPLICATIONS

As a result of the inherent size flexibility of fuel cells, the technology may be used in

applications with a broad range of power needs. This is a unique feature of fuel cells and their

potential application ranges from systems of a few watts to megawatts. Table 1 illustrates some

typical fuel cell applications for the different fuel cell types.

Table 1. Typical Applications [3]

Note: CHP - Combined Heat and Power

Fuel cell applications may be classified as being either mobile or stationary applications. The

mobile applications primarily include transportation systems and portable electronic equipment

while stationary applications primarily include combined heat and power systems for both

residential and commercial needs. In the following, fuel cell applications for transportation,

portable electronic equipment, and combined heat and power systems are addressed.

5.1 Transportation Applications

Cars

All the world leading car manufacturers have designed at least one prototype vehicle using

fuel cells. Some of the car manufacturers (Toyota, Ford) have chosen to feed the fuel cell with

methanol, while others have preferred to use pure hydrogen (Opel has used liquid hydrogen,

General Motors has stored hydrogen in hydride form). In the short term there is a general

trend for the car manufacturers to use reformed methanol as the fuel type for the fuel cell.

However, over in the long term hydrogen remains the fuel of choice for the majority of the car

manufacturers. Since 1994, Daimler-Benz working in collaboration with Ballard, built a series of

PEMFC powered cars. The first of such vehicles was fuelled with hydrogen, and in 1997

Daimler-Benz released a methanol fuelled car with a 640 km range. Plans are to offer a

commercial vehicle by 2004 [2].

In 1996, Toyota built a hydrogen-fuelled (metal hydride storage) fuel cell/battery hybrid

passenger car, which was followed, in 1997 by a methanol-fuelled car built on the same RAV4

platform. Renault and PSA-Peugeot Citroën are currently working on an improved design based

on the results obtained from the FEVER prototype. General Motors, Volkswagen, Volvo, Honda,

Chrysler, Nissan, and Ford have also announced plans to build prototype PEMFC cars

operating on hydrogen, methanol, or gasoline. International Fuel Cells, Plug Power, and Ballard

Power Systems are each participating in separate programs to build 50 to 100 kW fuel cell

systems for cars [2].

NECAR Program

The NECAR program, initiated in 1994, was designed in 4 phases leading to 4 prototypes of

electric vehicles. The aim of this program was to show the feasibility of such a vehicle and then

to improve the technology during each of the design phases.

The latest in the series is NECAR 4, which uses the 5-seater Mercedes Class A vehicle as

the platform. Incorporating a PEMFC using hydrogen stored in a cryogenic tank, it offers a

maximum speed of 145 km/h and an operating range of 450 km. A compressor maintains the fuel

cell under pressure. Air and hydrogen pass through a humidifier and a thermal exchanger before

enter to the fuel cell. A condenser recovers the water produced by the fuel cell. An air radiator

evacuates excessive heat. NECAR 4 can accelerate from 0 to 60 km/h in 6 seconds.

Buses

In 1993, Ballard Power Systems demonstrated a 10 m light-duty transit bus with a 120 kW

fuel cell system, followed by a 200 kW, 12 meter heavy-duty transit bus in 1995. These buses

use no traction batteries and operate on compressed hydrogen as the on-board fuel.

small fleet of hydrogen-fuelled, full-size transit buses for demonstrations in Chicago,

Illinois, and Vancouver, British Columbia. The marketing phase is envisaged for 2002 [2].

5.2 Portable Electronic Equipment

In addition to large-scale power production, miniature fuel cells could replace batteries that

power consumer electronic products such as cellular telephones, portable computers, and video

cameras. Small fuel cells could be used to power telecommunications satellites, replacing or

augmenting solar panels.

Micro-machined fuel cells could provide power to computer chips. Finally, minute fuel cells

could safely produce power for biological applications, such as hearing aids and pacemakers [6].

Unlike transportation applications where fuel cells are competing with the internal combustion

engines to indirectly produce a mechanical output, in portable electronic equipment fuel cells are

in competition with devices such as batteries to produce an electrical output. As a result fuel cells

can offer a viable alternative to batteries and several low power fuel cells are currently being

manufactured for this application.

5.3 Combined Heat and Power Systems

The primary stationary application of fuel cell technology is for the combined generation of

electricity and heat, for buildings, industrial facilities or stand-by generators. Because the

efficiency of fuel cell power systems is nearly unaffected by size, the initial stationary plant

development has focused on the smaller, several hundred kW to low MW capacity plants. “The

plants are fuelled primarily with natural gas, and operation of complete, self-contained,

stationary plants has been demonstrated using PEMFC, AFC, PAFC, MCFC, SOFC technology”

[2].

Fuel Cells in India:A survey of current developments

India, with over a billion people, many of whom lack access to reliable power, represents a huge prospective market for fuel cells. It is one of a number of countries including former Soviet states, newly emerging economies and economies in transition that will increasingly shape the 21st century supply and demand for energy. As a nation it has a history of technological innovation, a track record of technological leap-frogging and a highly skilled and adaptable work force. However, this is tempered by a huge gap between the wealthy and the poor, an energy distribution system that is unreliable and non-pervasive and problems with theft of electricity from the grid.

Market overviewIndia has experienced dramatic economic growth over the past decade, with GDP growth of around 6% per year since the early 1990’s, when market liberalisations began, and peak GDP growth of 8% in 2005-6. Some analysts have predicted 10-12% growth per year over the next decade, although many suggest more modest rates of 3-5%. Either way, given the vast economies of scale in India, even relatively modest GDP growth may result in large numbers of people being lifted out of poverty and joining the growing middle-class in India who are fuelling a consumer boom. Whilst affordability of conventional power resources (e.g. diesel generators) is out of reach for a large proportion of India’s population, there are tens to hundreds of millions of people who can afford electrical power: a massive market by any standards. One of the biggest challenges to India’s continued growth is the development ofan effective infrastructure. Two issues key to this are provision of reliable distributed power (particularly for remote locations) and energy for transport. Remote power generation is important in India as many rural locations lack regular access togrid power. A variety of non-conventional energy resources are currently exploited to meet the heating and power needs of rural populations. Some of these have severe drawbacks wood and cow-dung burning stoves, for instance, have been linked to respiratory diseases which are thought to cause 484,000 deaths per year in south east Asia. In 2004, 69% of India’s population relied on biomass for cooking. There is a great need for clean, reliable primary sources of power in rural communities but, above all, that power needs to be affordable. Power distribution is also a problem for urban India. Most businesses and better-off householdshave backup diesel generators, which add to urban air pollution, and are used during frequent power outages. The Indian Power Ministry estimates that around 40% of power is stolen from the grid by informal and illegal hook-ups to the overhead or underground network, putting further strain on a system that is already struggling to meet existing demand. The Indian government plans to build nine 4000 MW coal fi red power plans in the next five years, although there is no concrete strategy to meet growing demand. Aware of the shortages of supply, many Indian companies have their own private distributed power supply either for primary or backup power. The lack of reliable electrical power in India has a very real potential to curb future economic growth. India has a large and growing market for personal transportation, particularly 2- and 3- wheeled vehicles. Around 1.3 million 2-wheelers are sold per month. Out of these, 750,000 are bicycles, 500,000 are motorbikes and 100,000 are scooters. This gives a total of around 16 million 2-wheelers sold per year in India. These fi gures are only expected to rise as India’s economy grows. Indeed, the Indian automobile sector has been growing at a rate of 16% each year since 2001. Many of India’s cities suffer from severe air pollution caused partly by the large number of mopeds and scooters with ineffi cient and polluting engines. There have been moves by some companies to introduce battery-powered bikes (e.g. Ultra Motors, who have

produced over 100,000 units in two years). The power requirements of bikes and scooters is relatively small and could easily be met by Direct Methanol Fuel Cells (DMFCs) or hydrogen- powered Proton Exchange Membrane (PEM) fuel cells. There is currently some discussion as to whether DMFC should be the initial route for India, followed by hydrogen-fuelled PEM units for transport in the longer term. REVA electric car company, manufacturer of India’s fi rst electric car, has a fuel cell version of the REVA-EV at developmental stage. The large potential market for fuel cells in India raises the prospect for signifi cant economies of scale in the commercialisation of fuel cells, although this potential has yet to be realised. There has been a concerted effort in some Indian cities to move to less polluting forms ofpublic transport. Delhi, which ranked as one of the world’s worst cities for urban air pollution, banned diesel buses from its streets following a Supreme Court ruling in 1998 and now runs its 10,000-strong fl eet of buses on compressed natural gas (CNG). This is part of a progressive plan to ‘green’ all municipal vehicles, including taxis. In addition to buses, some 3-wheelers and taxis in Delhi already run on CNG and number in the tens to hundreds of thousands. Eventually, Delhi plans to move away from CNG to using hydrogen in internal combustion engines, and eventually to install fuel cells. A demonstration project for setting up a hydrogen dispensing unit at a fi lling station in New Delhi has been sanctioned between the Ministry of New and Renewable Energy Sources (MNES) and the Indian Oil Corporation. This project would enable dispensng of neat hydrogen and hydrogen-CNG blends as automotive fuel, but is still in the developmental stage. Cities such as Delhi have a history of ‘leapfrogging’ in technological innovations: diesel buses were replaced by CNG buses in just 5 years. In 2004, it was announced that 8 fuel cell buses will be introduced in Delhi as part of a UN DevelopmentProgramme project, but this appears not to have materialised. The Indian military runs a wholly-CNG fl eet of vehicles and there are moves to make all government vehicles run on CNG, again as a preliminary to establishing an all-hydrogen fl eet. A project for the introduction of hydrogen-CNG blends on a trial basis in existing CNG vehicles is currently in the planning stage. Indian railways have plans to build a hydrogen fuel cell powered train, using a 500 kW PEM module. In both public and personal transport, some preliminary activities are ongoing in hydrogen/fuel cell development, but the market is still far from maturation.

Indian fuel cell activityThe majority of organisations involved with fuel cells in India are R&D orientated, with a smaller number involved with distribution and manufacture. This refl ects partly the strength in fuel cell research in some of India’s leading Institutes of Technology and partly the nature of several companies who are working on fuel cells but have yet to commercialise them.

Organisations involved in fuel cells are mainly grouped around the New Delhi/Maharashtra region. Maharashtra is India’s most industrialised state, containing such manufacturing centres as Mumbai, and sees the most activity on fuel cells. New Delhi, India’s centre for IT and telecommunications, is also home to several fuel cell companies and R&D organisations. Karnataka, Uttar Pradesh and Tamil Nadu are home to companies engaged in signifi cant fuelcell development.

Of the companies actively engaged with developing fuel cells in India, most are concentrating on small stationary units. Delivering distributed power is one of the key challenges for the energy sector in India. Industrial users have long used conventional sources to supply their own distributed power and are now looking towards fuel cells to provide either stationary backup power or the main source of power in future. India currently struggles in effective power distribution in domestic and agricultural settings as well as some urban areas. Stationary fuel cells could fi ll this gap and are being promoted by various government programmes. The automotive sector is the next most common application focus of fuel cell manufacturers in India.

The Ministry of New and Renewable Energy Sources (MNES), one of the world’s few dedicated ministries focusing on alternative energy sources, is committed to increasing the use of renewable energy sources both in order to guarantee the country’s energy security and ensure a more environmentally sustainable energy base. The Ministry has strengths in ‘conventional’ renewables such as wind, solar photovoltaics, solar thermal, small hydro, biogas and biomass. From these sources, MNES has an installed capacity of around 7200 MW, around 6% of the installed power generation capacity in the country. MNES estimates that there is a potential of around 80,000 MW of power from renewables. MNES also sponsors a number ofemerging and new technologies, in particular fuel cells and hydrogen energy. In 2003, the Ministry set up a National Hydrogen Energy Board (NHEB). NHEB is chaired by Ratan Tata, CEO of Tata Group, one of India’s biggest companies. The Board published a National Hydrogen Energy Roadmap in 2005 which set out an ambitious transition to a hydrogen economy by 2020. This plan envisaged a move away from petroleum based automobiles and power generation at present to bio-fuel and synthetic fuel based vehicles, electric and hybrid vehicles and power generation to, ultimately, a carbon-free, hydrogen based economy. The Roadmap envisages that one million hydrogen fuelled vehicles will be on Indian roads by 2020 and there will be 1,000 MW of aggregate hydrogen power capacity in the country by 2020. The development path outlined in the Roadmap would fulfi l several of the government’s key objectives, namely energy independence, rural electrifi cation, poverty reduction, and environmental protection. An investment of 2.5 billion rupees (around $6.1 million) is envisaged between 2006 and 2020 in order to realise this. Of this, around 4% would be for research, development and demonstration, with the remaining 96% going towards creating an infrastructure for hydrogen production, storage, transportation and distribution. The Roadmap was approved by NHEB in early 2006 and identifi ed two major initiatives; the Green Initiative for Future Transport and the Green Initiative for Power Generation. The fi rst of these aims at developing hydrogen powered internal combustion engine and fuel cell based vehicles ranging

from small two and three wheeled vehicles, cars, taxis, buses and vans. This will be done by a gradual shift to hydrogen fuel blended into the fuel of existing CNG vehicles starting with 5-10% hydrogen blends and increasing to 30%. This will be followed by 100% hydrogen ICE vehicles in the next 10-20 years. India hopes to ‘leapfrog’ conventional technologies by introducing new hydrogen technology in markets such as 2- and 3-wheeled bikes and auto rickshaws, which make up some 81% of the vehicles sold each year in India. By adopting established technologies early over short development cycles, India hopes to eventually be a regional leader in hydrogen powertrain vehicles. The second strand of the Roadmap aims to develop and demonstrate a hydrogen powered engine/turbine and fuel cell based decentralised power generating system of ~1000 MW aggregate capacity by 2020. Both of these initiatives will be conducted as a public-private partnership and will be an industry- driven planning process, guided by government with support from research organisations, academia and NGOs. Given the ambitious timescale for the introduction of widespread hydrogen technology (less than 13 years), a loose interpretation of hydrogen technology will probably by used. The objectives in the Roadmap are deliberately vague; the commitment to 1 million hydrogen fuelled vehicles will probably include a large proportion of vehicles with hydrogen-CNG blends fuelling Internal Combustion Engine (ICE) powertrains, rather than fuel cell powertrains. The commitment to 1,000 MW of aggregate hydrogen generating capacity by 2020 is likely to include a large proportion of hydrogen used for combustion in conventional thermal power plants. A smaller amount of power will probably come from PEM fuel cells running on direct hydrogen and MCFCs running on natural gas although their widespread introduction will likely be later than 2020. Nonetheless, the Roadmap is an important step in the direction of fuel cells and a hydrogen economy. Expertise in production and infrastructure will have to be developed for hydrogen to be used in CNG blends. The resulting capacity for hydrogen production and distribution may pave the way for more widespread adoption of hydrogen fuel cells.

The Roadmap addresses issues relating to hydrogen production, storage and delivery, seen as key challenges to successful implementation of a hydrogen economy. Most hydrogen in India is currently produced by steam reformation of methane. A smaller amount is produced as a by-product of the chlor-alkali industry, and indeed US-based Hydrogen Engine Centre recently shipped a hydrogen fuelled generator to Grasim Industries’ chlor-alkali plant in India. In order to guarantee energy security and address environmental concerns, the roadmap sets out plans to research hydrogen production from coal gasifi cation, nuclear energy, biomass, biological, photovoltaic electrolysis of water and other renewable methods. Coal gasifi cation is commercially available and there are plans to set up a pilot plant. The biological route for hydrogen production is still in a pre-commercial stage although a demonstration plant has been set up. Production of hydrogen from nuclear-powered electrolysis has not yet been conducted and is partly dependent on India’s nuclear programme and continued negotiations with the United States. Hydrogen production from other renewable sources, such as solar energy (electrolysis, photolytic, photo-electrochemical and thermal splitting) is proposed in the Roadmap but is not apparently being worked on at present. A recent Indian government report concluded that no single production technology is likely to meet the requirement of hydrogen for the new and emerging applications in power generation and the transport sector in the near and medium term. Therefore, all possible production options should be pursued for now. Hydrogen storage is seen as another key challenge, and various goals, including effi ciency of storage, useful life cycle, compactness and cost need to be met if the Roadmap is to achieve its objectives

by 2020. Some of the Indian Institutes of Technology have been very active in developing techniques for hydrogen storage. Metal hydrides with 2.4% storage are claimed by Indian scientists. The Indian Institute of Technology (IIT), Madras aims for 3-5% storage in light metal hydrides and carbon materials. IIT Madras has been experimenting with carbonnanomaterials and has achieved reproducibility of 2% hydrogen storage. Metal hydride storage currently shows most promise for small 2- and 3- wheel vehicles whilst compressed hydrogen storage tanks are more likely to be used for buses and larger vehicles. Regarding hydrogen combustion and fuel cell technologies required to fully implement theRoadmap, a dedicated hydrogen combustion engine is said to be in development. PEM Fuel Cells for stationary applications and automobiles will be bought from commercial manufacturers outside India (refl ected in the proportion of the Roadmap’s budget set for R&D). Solid oxide fuel cells for large stationary applications are in the early stages of being developed by the Central Glass and Ceramics Research Institute (CGCRI). Several major motor manufactures are said to be entering into partnership with the government on developing hydrogen power, including Asok-Leyland, Tata Motors and Mahindra Motors. These manufacturers are actively looking for international partners. In December 2006, the Indian government unveiled a roadmap for the development of the Indian automotive sector, a sector that employs over 13 million people and contributes 5% of India’s total GDP. India currently produces 10 million vehicles per year (including 2- and 3- wheel vehicles).

The government believes the sector will continue to grow and account for more than 10% of GDP by 2016. Within this context, the market for hydrogen and fuel cell vehicles is also expected to grow, especially if the government sticks to NHEB’s ambitious plans for 1 million hydrogen vehicles by 2020. A report on the energy R&D working group for the Eleventh Five Year Plan (2007-2012) recognises that there is no ‘silver bullet’ for solving India’s energy needs and that clean coal technology, nuclear and solar will have to be pursued in parallel together with other energy resources. Steps recommended for uptake in the Five Year Plan include developing hydrogen production, storage and end use technologies as alternative energy carrier. However, hydrogen technology comes fairly low down the list of priorities for energy, and nuclear and clean coal technology appear to be given more priority. India is heavily dependent on imported fossil fuels for meeting its ever-increasing energy demands. This is particularly true for the transport sector, as India currently imports about two-thirds of its requirement of petroleum products. There is great potential for replacing petrol fuelled vehicles with CNG, and eventually hydrogen. This does not necessarily fulfi l India’s key objective of security of fuel supply since CNG and hydrogen are still largely derived from imported oil and gas and are also sensitive to variations in global prices. Ulimately, India hopes to produce hydrogen domestically and much R&D has focussed on ways to do this.

R&D, manufacturing and commercialisation

The last few years has seen considerable research activity in hydrogen and fuel cells in India, despite the apparent lack of funds available in the sector. In the fi rst 5 months of 2007, three separate workshops on fuel cells took place in Indian Institutes of Technology (IITs). Fuel cell technology is taught as a subject at several Indian universities (see Key Players) although there is perceived to be a skills shortage, particularly for fuel cell manufacture and maintenance. Despite the strong R&D base in Indian universities, there is currently little fuel cell manufacturing

expertise. It is likely that fuel cells would be manufactured outside India then imported and sold with local partners, although effective maintenance may still be an issue. Already several companies in the stationary power and automotive sectors are said to be looking for international collaborations on fuel cells for distributed generation and transport applications. Through the MNES, India has been supporting a broad-based research, development and demonstration programme on hydrogen production, storage and use. Several research, scientifi c and educational institutions, laboratories, universities and industries are involved in implementing various projects on hydrogen energy technology. Some of the R&D work sponsoredby the MNES includes solar hydrogen production from water; synthesising hydrogenabsorbing alloys; development of polymer membrane gas fi lters and demonstration of stationary hydrogen generators and hydrogen fuelled agricultural engines. Much of this research seems to concentrate on the development of hydrogen for combustion, rather than for fuel cells, but this is seen as a necessary ‘path’ to eventual adoption of fuel cells. The Department of Science and Technology (DST) has established a Centre for Fuel Cell Technology (CFCT) located in Chennai with the specifi c objective of demonstrating and validating commercial applications of PEM fuel cells in collaboration with industry. Their work involves designing, developing and testing PEM fuel cell stacks for electrical power and heat production.CFCT has created 1-10 kW stacks for remote power generation which are expected to be available for commercialisation before the project’s end in early 2009. The Council for Scientifi c and Industrial Research (CSIR), an industrial R&D organisation comprising different institutes including the National Chemical Laboratory (NCL) and the Central Glass & Ceramic Research Institute (CGCRI), are actively researching PEM and other FC technology and have conducted several successful demonstrations. To date, no standard test procedures for fuel cells, stacks and systems exist in India, and this is one service that would currently have to be carried out abroad. The Working Group on R&D for the 11th Five Year Plan recently recommended that research should be carried out on system as well as materials development for low temperature fuel cells (alkaline and PEMFC); high temperature fuel cells (MCFC and SOFC); high temperature reversible fuel cells; and DMFCs. In particular membranes, bipolar plates, catalysts and electrodes need to be researched. Other materials that are being proposed for development are low cost hydrogen sensors and heat exchangers. Whether funding will be made available and research in these areas remains to be seen.There are plans at the Indian Institute of Technology (IIT) Madras to develop PEMFC technology for decentralised power generation and automotive applications. At the Central Glass and Ceramics Research Institute (CGCRI), a new generation of high temperature SOFCs are reported to be under development. The Institute is working on SOFCs and is planning activities in collaboration with Bharat Heavy Electrical Ltd., one of India’s leading companies. Indian Institute of Science, Bangalore is working on the development of alkaline and DMFCs. The Council for Scientifi c and Industrial Research have a mission project involving a number oflaboratories including the National Chemical Laboratory (NCL), Pune, Central Electrochemical Research Institute (CECRI), Karaikudi and others. NCL and CECRI have developed a PEM FC stack which they report is set for commercialisation.Cost and durability remain the biggest challenges to successful fuel cell adoption. Economies of scale are diffi cult to achieve from only a small number of fuel cell systems. High cost also remains a barrier to potential customers and the lack of manufacturing expertise in fuel cells means that working fuel cells have largely been restricted to demonstration projects. Cost reductions must be realised in raw materials, manufacturing of fuel cell stacks and components,

though the actual reduction in cost will be largely dependent on fuel cell type and application. Without a widespread tradition of fuel cell manufacturing in India (unlike in parts of Europeand North America) critical economies of scale have not been realised and it is more likely that an Indian company wishing to use a fuel cell in its products would buy an off-the-shelf fuel cell from abroad. This has happened with REVA motors who announced in 2005 it would be buying fuel cells for its FC car from Canada’s Hydrogenics. Despite the large amount of R&D going on in Indian research institutes, it is more economical to buy critical FC components overseas and import them (despite high import tariffs in India, some of which are now being relaxed). This is refl ected in the allocation of budget by NHEB – the majority of funding goes to commercialisation rather than fundamental research, despite what various otherreports may recommend. India has the potential for vast economies of scale if fuel cells fi nd commercial applications in transport and distributed generation - it is this potential demand that could bring the price of fuel cells down, although like elsewhere, price and demand for fuel cells is a chicken-and-egg question. India has a history of ‘leapfrogging’ in technology – mobile phones replaced land lines in parts of India before the land line infrastructure was even built. The growth rate of mobiles in India now far exceeds that of land lines - 50% per annum growth compared with 3% for land lines. India may do the same with FCs, not for the novelty of the technology, but because there is a very real demand for clean, reliable distributed generation and urban transport.

Summary: challenges and opportunities Despite some investment in fuel cell technology and a good amount of expertise among the academic community, India remains a relatively small market for fuel cells at present. Providing distributed and backup power generation are the two big immediate challenges in the stationary power sector, with energy security and environmental sustainability being two equally important longer term challenges. India’s government is committed to renewable energy technology, including fuel cells. Despite high profi le objectives, such as the National Hydrogenenergy roadmap, the biggest challenges to fuel cell commercialisation remains affordability and the shortage of skills in manufacturing and maintaining fuel cells. However, with the potential for vast economies of scale and a history of technological leapfrogging, the outlook for fuel cells is optimistic in the longer term.Providing reliable electrical power to meet growing demand is one of the biggest challenges facing India’s energy sector. India currently struggles to deliver power to the poor and to agriculture – those that need it most. Industry, long familiar with the insecurity of the grid supply, has largely invested in private distributed generation systems. Many companies in the high tech ‘service’ industry, including IT and call centres, also have their own uninterrupted power systems, largely diesel fuelled at present. Given the problems of air pollution in India and its commitment to environmental sustainability, there is a huge potential demand for clean forms of micro-generation, including fuel cells. The problems facing adoption of fuel cells for mirco-generation remain manufacturing fuel cells that are suffi ciently durable andcheap that they are adopted on a widespread basis, and generating suffi cient renewable hydrogen or other fuel. Indian companies are beginning to buy fuel cell stacks from established North American and European manufacturers and tailor them to their requirements, rather than build their own systems from scratch. At the same time, government sponsored R&D is continuing, although little commercial activity has so far resulted from this. However, in the

immediate term the Indian government will attempt to meet the growing demand for electricity by building large coal-fi red (and eventually nuclear) power stations. India estimates it needs to sustain economic growth at nearly 10% per year for at least 25 years to end poverty and meet the demands of a population expected to reach 1.6 billion by2050. Meeting these twin challenges of sustained economic growth and a burgeoning population means that the country will have a huge demand for energy over the next few decades. At the same time, India is committed to ‘energy independence’ and environmental sustainability. India imports 2 million barrels of oil per day and under a ‘business as usual’ scenario envisaged by the International Energy Agency, 5 million barrels of oil would be imported by 2030. There is a huge opportunity for hydrogen to replace oil in transport in the next few decades and help meet the goals of sustainability and energy security. Indeed, the NHEB envisages 1 million hydrogen vehicles by 2020 (largely hydrogen-blend ICE initially, but with increasing numbers of FC vehicles). For this plan to be fully effective and in keeping with the Indian government’s goals of reducing its reliance on foreign oil and cutting pollution, hydrogen would have to come from sources other than petroleum. Plans for coal gasifi cation and hydrogen production from photovoltaic electrolysis of water are already afoot, but in the long term, hydrogen could come from biomass. There could also be potential for bio-alcohols ton be used in fuel cells, although culturally this may not be acceptable in all parts of India. The NHEB and MNES also appear committed to investing in a hydrogen infrastructure; initially for hydrogen-CNG blends to fuel automobiles in some of the larger cities.The strong government support being given to clean, effi cient forms of power generation in the stationary and transport sectors gives some room for cautious optimism for the Indian fuel cell market. Indian government, at city, state and national level, already has a history of supporting new and renewable energy resources, and the fact that India has a national Ministrycommitted to this is encouraging. India has a unique potential demand for remote and transport power and the economies of scale possible in the Indian market makes means that hydrogen and fuel cells may have a promising future in the long term. This will partly depend on the ambitious National Hydrogen Energy Roadmap being implemented fully (although its objectives are unlikely to be met by 2020); production, storage, and distribution issues being addressed and the cost of fuel cell technology being brought to within reach of more people. All of this has to take place against a backdrop of growing demand for reliable power, which will come from other forms of energy if hydrogen and fuel cell power is not available at pointof need.

Key players engaged in FC development

Acme Telepower is a company founded in 2003 which provides power solutions in the telecoms sector and services in the agriculture and environment sectors. There is a large requirement for stationary power in the telecoms sector in India and the company is currently looking for collaborations with fuel cell companies.Bhabha Atomic Research Center, Mumbai (BARC) is carrying out basic research on anode, cathode, electrolyte and interconnecter materials for SOFC technology in conjunction with Indian Institute of Technology, Chennai.Banaras Hindu University (BHU) is carrying out research and development of metal hydride storage methods for hydrogen and the use of hydrogen in internal combustion engines. In 2001, the institute started a fi eld trial with 10 motorcycles and added a three-wheeler to the fl eet in

2004. The demonstration programme is supported by India’s Ministry of New and Renewable Energy Sources (MNES).Bharat Heavy Electrical Ltd (BHEL) is working on developing PAFC and MCFC for distributed power generation and also focuses on preparing catalyst and fuel reformers to be used in fuel cell power plants. They have demonstrated distributed power systems. Bharat Heavy Electricals Ltd is involved in the development of PAFCs and plans to develop a 50 kW stack. One possible application of this technology is in remote stationary power and backup powerfor villages.Birla Hitech focuses on the development of clean energy technology products with a particular focus on PEM fuel cells. It is planning to set up dedicated manufacturing facilities for the construction of PEMFC components (bipolar plates, stacks etc.) and systems in India. It is currently seeking partners for a joint venture, and is said to have initiated correspondence with a number of US fuel cell companies. It has already forged links with leading R&D agencies and institutions in India.Central Electrochemical Research Institute (CECRI), Karaikudi is said to have has developed and tested a MCFC stack.Eden Energy, an Australian producer of hydrogen fuels and equipment agreed to form a joint venture with Larsen & Toubro Ltd (L & T). The three-stage accord with Mumbai-based L & T, India’s biggest engineering company, will by late 2008 lead to a 50:50 joint venture for the manufacture in India of Eden’s entire range of equipment for hydrogen and so-called Hythane, a mix of hydrogen and natural gas. The hydrogen produced will mainly go towards providing materials for glass production, food oil hydrogenation and metal annealing, although in the long term, Hythane could be used as a fuel for vehicles.FuelCell Energy: The US based provider of fuel cell power plants is said to be working with Air Products Inc. to build a hydrogen power station by combining their Direct FuelCell power plants with Air Products’ advanced gas separation technologies. The plans are to set up the first unit in Gujarat. The company is also said to be in talks with Indian companies Reliance, a diverse chemical and energy conglomerate, construction and engineering fi rm Larsen & Toubro Limited (L&T), and Kirloskar, manufacturer of water pumps, for possible tie-ups. FuelCell Energy has received a US$20 million subsidy from the World Bank for promoting fuel cell power generation in developing countries, including India. The company is not planning to set up its own subsidiary in India but is looking for private Indian partners. Fuel Cell Energy will be operating in India in phases beginning by the end of 2007 or early 2008. Initially the company plans to source fuel cells from US units for power generation in India. After that the company will begin R&D for possibilities of fuel cell production in India.Gas Authority of India Ltd (GAIL) is interested in promoting fuel cells for applications including industrial and residential power and auxiliary power. It claims it could provide the fuel infrastructure in India for a wide range of suitable fuels, including hydrogen, natural gas, propane, butane and methanol. It is also interested in taking part in fuel cell fi eld trials. GAIL seeks to be actively involved in establishing a fuelling infrastructure for fuel cell vehicles in India.Haldia Institute of Technology, Chemical Engineering Department based in West Bengal is currently carrying out projects on solar hydrogen production and the development of PEM fuel cells. They have developed one polymer electrolyte fuel cell and are currently developing microbial fuel cells.

Indian Institute of Chemical Technology (IICT) and BHEL, have developed catalysts and two fuel reformers for producing hydrogen from methanol. Performance tests are being conducted. A heat recovery system has been attached to the fuel cell system to utilise waste heat.Indian Institute of Science (IISc), Bangalore and Central Glass & Ceramic ResearchInstitute (CGCRI), Kolkata are involved in developing SOFC systems. A methanol reformer was developed and integrated with a fuel cell system by IISc, Bangalore. Work on developing a DMFC is underway at IISc. In addition, research on SOFC is being done at IISc and CGCRI.Indian Institute of Technology, Kharagpur, Department Metallurgical and Materials Engineering have been working on materials for solid oxide fuel cells since 2003. They have developed SOFC anodes and electrolytes and are currently developing a complete SOFC unit of 5-10 kW capacity.

IN THE PAST, centralized power generation was promoted. The power generation units were

generally built away from the populated areas but close to the sites where the fuel (i.e., fossil

fuel) was available. This kept the transportation cost (of the fuel) to a minimum and eliminated

the possibility of pollution in populated areas. Such schemes remained quite popular until

recently despite drawbacks such as Ohmic (i2R) losses (due to transmission of electricity

through cables over long distances), voltage regulation problems, power quality issues, and

expansion limitations.With the power demand increasing consistently, a stage has come when

these centralized power generation units can be stressed no further. As a result, the focus has

shifted to generation (and consumption) of electric power “locally” leading to “distributed power

generation systems” (DGS) [1]–[4]. At the same time, increased awareness about the importance

of a clean environment and the quickly vanishing fossil fuels

Fig. 1. Various HDGS configurations. (a) Conventional, multistage topology

using two H-bridge inverters [4], [6]. (b) Modified topology with only one H-bridge inverter [4].

(c) Proposed topology. λ denotes solar insolation (Suns have given impetus to the idea of local

power generation using nonconventional energy (NCE) sources (e.g., photovoltaic (PV) cells,

fuel cells (FC), wind energy, etc.), which may suit a particular region and provide power at

various load centers along the main power grid. Most of these sources are pollution-free and

abundant. Unfortunately, they are not so reliable. For example, the PV source is not available

during the nights or during cloudy conditions. Wind energy may or may not be available. Other

sources, such as fuel cells may be more reliable, but have monetary issues associated with them.

Because of this, two or more NCE sources are required to ensure a reliable and costeffective

power solution. Such integration of different types of energy sources into a DG system is called a

hybrid distributed generation system (HDGS) [4]–[20]. A combination of PV and FC sources

forms a good pair with promising features [6] for HDGS applications. Of course, the slow

response of the FC needs to be compensated with an ultracapacitor or a battery (ultracapacitor is

preferable due to its high energy density) [4], [11]. A brief survey of the literature dealing with

HDGS systems is presented next. Among the earlier work, Tam and Rahman [5] have proposed

an HDGS configuration shown in Fig. 1(a). It consists of two inverters, operating in parallel,

whose outputs are tied to the grid through a single, multiwinding, step-up transformer. The

drawback with this otherwise elegant scheme is that it does not utilize the available sources

efficiently as maximum power point tracking (MPPT) is not implemented. Later, Ro and

Rahman [6] improved upon this system by introducing a two-loop controller with MPPT. Tao et

al., [12] have proposed a multiinput, bidirectional dc–dc converter configuration involving a

combination of dc-link and magnetic coupling. The configuration offers high boosting capability

and galvanic isolation. However, it consists of multiple power processing stages with an

additional dc/ac inverter stage for feeding ac loads. Thus, it requires a large number of devices.

Monai et al., [13] have proposed another HDGS configuration involving PV, FC, and battery

sources that can meet the fluctuating requirements at the load end. The authors have proposed a

modified Euler-type moving average prediction model for proper sharing of load among the

various sources. The proposed system is a good solution for high power standalone or utility

applications. Agbossou et al. [14] have proposed a hybrid system in which the excess energy

generated by the renewable sources is used for producing hydrogen. Rajashekara [15] proposes

another useful system, with PV and FC sources, for space applications. This system uses the PV

power optimally by diverting the excess power for production of hydrogen through electrolysis.

Another HDGS system with wind, PV, and battery sources has been discussed by Valenciaga

et al., [7], where they propose a control technique that not only maintains the load demand but

also the state-of-charge of the battery. Blaabjerg et al., [16] have given an elaborate review of the

various control and grid synchronization techniques used in HDGS systems. Some other useful

HDGS configurations, based on FC, PV, and wind sources, have also been reported [17]–[20].

Recently, a PV- and FC-fed hybrid topology has been proposed [4], which makes use of a

dedicated boost type dc–dc converter for each of the two sources, with a common inverter stage

[Fig. 1(b)] for grid connection. The dc–dc converter stages aremeant for boosting the low PV and

FC voltages. The PV side converter also takes care of MPPT. One of the problems with this

system is that, at very low insolation levels, the PV side dc–dc converter must be cut off to

prevent its inefficient operation [21]. Thus, at small power levels, generation by the PV source

remains unutilized. This may be acceptable for highpower applications, but could be a matter of

concern for residential or medium-power installations [22]. If the function of the PV side dc–dc

converter is merged with the inversion stage, as shown in Fig. 1(c), even a small fraction of

power generated during low insolation can be utilized. Most HDGS configurations discussed

earlier use an H-bridge inverter topology for interfacing with the grid, which either needs a line

frequency bulky transformer at the output or high dc link voltage at the inverter input. This paper

proposes an integrated solution for PV/FC-based HDGS using a new configuration depicted in

Fig. 1(c). The proposed system uses an inverter with boosting capability, which eliminates the

requirement of high dc voltage at the inverter input, thereby saving the cost of high-voltage

buffer capacitor. Various other features, working principle, control strategy and simulation, and

experimental results for the proposed configuration are described in the subsequent sections of

this paper.

Fig. 2. Proposed Configuration. (a) Block diagram showing the basic concept.

(b) Detailed view of (a) along with the electrolysis application.

II. BASIC CONCEPT, OPERATING MODES, AND SALIENT FEATURES OF THE

PROPOSED SYSTEM

The proposed topology is built around a buck-boost inverter topology capable of inversion (dc–

ac), boosting and bucking the voltage and MPPT. The basic idea behind the proposed integrated

configuration is shown in Fig. 2(a). A detailed view of Fig. 2(a) is shown in Fig. 2(b) along with

an example (electrolysis) application. A combination of PV and FC sources feeds the

configuration. While the PV source directly feeds the inverter through a buffer capacitor, CPV,

the FC source is interfaced through a buck-boost type dc–dc converter, as shown in the figure.

An extra block is added across CPV to divert the excess power generated by the PV source.

The proposed system is designed to meet a certain minimum active power demand (Preq) from

the grid side. PV is the main source, which is continuously made to track the MPP, while feeding

the required amount of power into the grid. The FC source, with buckboost type dc–dc converter,

acts as a current source in parallel with the PV source. It is only used to supplement the PV

source during low or zero insolation. Thus, FC supplies only the deficit power into the grid. On

the other hand, any “excess power” generated by the PV source is conditioned and diverted to an

auxiliary application such as electrolysis, to produce hydrogen, which can be stored for later use

by the FC source. This results in an optimal utilization of the available sources, rendering a

highly economical system [14]. The aforesaid description leads to the following three modes

in which the proposed system operates:

1) Mode-I: Only PV mode (only PV provides power).

2) Mode-II: Hybrid mode (both PV and FC provide power).

3) Mode-III: Only FC mode (only FC provides power).

These operating modes are summarized in Table I.

Fig. 3. Circuit schematic of the proposed integrated configuration for hybrid

distributed generation system. xi denotes the corresponding state variable. Due to its unique

hybrid integrated nature, the proposed configuration offers several desirable features as outlined

next:

1) It obviates the requirement of a boost converter stage for conditioning the PV power, as

shown in Fig. 1(c).

2) Out of the two capacitors Cdc and CPV [Fig. 1(a)], only one is required [Fig. 1(c)].

3) Presence of FC in parallel reduces the fluctuations in the PV voltage due to changing

environmental conditions. This, in turn, reduces the fluctuations in the power fed into the grid.

Consequently, the grid voltage profile improves [23].

4) Elimination of dips and surges in the PV voltage increases the speed of MPPT.

5) In two-stage systems [4], usually, both PV array and dc capacitor (Cdc) voltages are sensed

for MPPT and power control, respectively. In the proposed system, Cdc (or CPV) appears right

across the PV terminals due to elimination of the dedicated boost stage on the PVside [Fig. 1(b)].

Hence, only one sensor is adequate.

6) It eliminates the requirement of extra hardware for communication and coordination between

various sources [24] to generate and use the available energy optimally.

7) The special configuration, in which the PV and FC sources are connected, ensures that the FC

section works as a current source irrespective of the voltage magnitude at its output. This

facilitates an appropriate adjustment of the PV voltage for PV’s operation close to MPP.

8) Proposed configuration is a compact, low-cost, and reliable solution for HDG applications.

III. CIRCUIT OPERATION AND ANALYSIS

Fig. 3. shows the complete circuit schematic of the proposed configuration along with the

electrolysis application.With refer-

Fig. 4. Inductor currentwaveform in discontinuous conductionmode showing the three

constituent intervals

TABLE II

STATE OF THE DEVICES, GOVERNING EQUATIONS AND ACTIVE CURRENT PATHS

DURING THE POSITIVE HALF-CYCLE OF THE GRID VOLTAGE

ence to the general description of the proposed topology [Fig. 2] in Section II, the following

three sections can be identified in Fig. 3:

1) Inverter section for grid interfacing.

2) A buck-boost type dc–dc converter for conditioning the

FC power.

3) A buck type dc–dc converter for conditioning and diverting excess PV power for electrolysis

application. The inverter consists of two buck-boost converters connected back to back [25], as

shown in Fig. 3. It uses two pairs of controllable devices. Each pair (SWpa and SWpb or SWna

and SWnb) is operated for one half-cycle of the grid voltage. One of the switches, SWpb (or

SWnb) in the pair is kept ON for the entire positive (or negative) half-cycle of the grid voltage,

while the other,SWpa (orSWna) operates at high switching frequency with sine triangular pulse

width modulation (SPWM). The converters are made to operate in discontinuous Current mode

(DCM) to feed high-quality current waveforms into the grid. The converter is designed to

operate in critical conduction mode (boundary of continuous current mode (CCM) and DCM)

around the peak of the grid voltage, where, twice the rated average power is being delivered to

the grid. The critical conduction mode and DCM operation are shown in Fig. 4. When SWpa is

ON, the inductor “L” stores energy. When SWpa is OFF, this stored energy is transferred into

the capacitor “Cf ”. Since the ON/OFF times are modulated in a sinusoidal manner, current fed

into the grid is also sinusoidal. The state equations governing the operation of the inverter during

the positive half-cycle of the grid voltage are shown in Table II along with the corresponding

active current paths. The operation and analysis during the negative half-cycle of the grid voltage

is analogous. As the inverter operates in DCM, there is a complete energy transfer from the

buck-boost inductor to the grid during each high-frequency switching cycle. Assuming the

switching frequency to be an integer multiple of the grid fundamental frequency

(i.e., fs(= 1/Ts) = 2n × fg (1/Tg )),

each cycle of the

grid voltage can be divided into “2n” parts or intervals. During each such interval, a definite

amount of energy is being transferred. Assuming grid voltage to be constant during each

switching cycle, the duty ratio, DON(k) for the kth(k = 1, 2, . . . n) switching cycle is given by

DON(k) = M×sin(π × (k/n)), where, M=Vsin/Vtri (1) where Vsin and Vtri are the amplitudes of

the rectified sine wave and triangular wave, respectively. Energy “Ek(inv)” transferred in the kth

switching interval with modulation index “M” is given by

where VPV is the average voltage across the PV array. Thus, the total energy transferred during

a fundamental cycle of the grid voltage is given by

The FC side buck-boost converter can be operated in DCM or CCM, depending on the control

technique used. Due to the inherent nature of the buck-boost converter’s operation, the input

source never sees the output capacitor CPV. In other words, the input energy is first stored in

inductor Lbb, which is then transferred on to the output capacitor. The energy drawn from

the FC source during one fundamental cycle of the grid voltage is given by

where, VFC and IFC are the average values of FC voltage and FC current, respectively. Since

IFC= DON(FC)×IL(bb) and VFC is a function of IFC, (4) may be rewritten as follows:

where IFC and IL(bb) are the average values of the FC source current and FC side buck-boost

inductor current, respectively, and DON(FC) is the turn “ON” interval [Fig. 4] of the FC side

buck-boost converter. The buck converter is added to the system (as shown in Figs. 2(b) and 3)

to condition the excess power (Pex ) generated by the PV source for electrolysis process. The

buck converter has the advantage of having high current at the output. This helps in increasing

the production of hydrogen by electrolysis [14]. The electrolysis load is represented by a

resistive load (REL) in

Fig. 3. IV. MPPT ALGORITHM AND CONTROL STRATEGY

The modulation index, M of the grid-connected SPWM inverter is adjusted to enable the desired

power flow. The amount

Fig. 5. I–V and P–V characteristics of the PV array with β curve. of power depends on the MPP,

which is being tracked. Thus, the value of M is computed by the MPPT algorithm used.

Various MPPT schemes are available. Unfortunately, popular MPPTtechniques, like hill

climbing or incremental conductance method cannot be applied because they provide slow

tracking as they use a fixed and small incremental change in the modulation index to track MPP.

During mode II, when FC is also feeding power into the system, small incremental changes in

the modulation index will not be able to balance the input and output power across CPV,

resulting in the rise of voltage across CPV. This is equivalent to shifting the PV operating point

toward open circuit condition (OCC) resulting in underutilization of the PV source. Also, as the

change in the modulation index can be implemented only at the start of the fundamental cycle of

the grid voltage, the probability of underutilizing the PV power becomes high using slow MPPT

algorithms. Therefore, it is imperative to use a fast MPPT scheme, which can counter the

drawbacks of the slow schemes. A. MPPT Scheme Used in the Proposed Topology

A fast MPPT scheme, called the “β” method [26], was suitably modified and used for the given

application. The scheme is based on the observation that the value of an intermediate variable

“β”, defined only at MPP condition, varies with in a narrow band (βmax − βmin) as theMPP

varies fromPMPP(max) to PMPP(min) over the full insolation and temperature range (λmax,

Tmax to λmin, Tmin), as shown in Fig. 5. β is a subset of β_, which is applicable to any point on

the P–V curve, including MPP. β is obtained [26] by using the MPP condition, ∂P/dV = 0 and is

given by,

where Io is the reverse saturation current of the diode. Therefore, by tracking β using large

iterative steps, the operating point can be quickly brought into a narrow band of MPP [Fig. 5].

Also, controlling “β” indirectly controls the operating voltage (OV)

Fig. 6. Flow-chart of theMPPT algorithm used in the proposedHDGSscheme.

Mmin and Mmax denote minimum and maximum values of the modulation index. of the PV

array, which helps in balancing the power across capacitor CPV. It is important to note that

problemsmay arise during the lowinsolation phase when the generated PV power is very small

and the value of “M”, as computed by the MPP algorithm, is small. Under such conditions, as

FCcontinues to feed the deficit power, this will result in a voltage increase acrossCPV due to

imbalance between the input and output power. This may lead to shifting of OV of the PV array

toward the OCC. The array will remain at OCC even when high or normal insolation is restored

and the system will continue to draw the required power from FC. To avoid such a situation, the

algorithm has been suitably modified to ensure that the PV array voltage is always less than or

equal to a critical voltage (Vcrit ), which is less than the PV array’s VOC. Whenever the array

voltage goes beyond Vcrit (which occurs only when the power drawn from the FC is not fed into

the grid due to the low value of M) and array power is less than the minimum power Pmin, the

modulation index is set to a fixed value (Mcon) by the algorithm. This ensures that VPV ≤ Vcrit .

The complete flowchart of the proposed MPP tracking algorithm is shown in Fig. 6. In the given

flowchart, the average values of VPV and IPV are processed to obtain the current value of β_.

Then, PPV is calculated and its value is compared with the set minimum value. The value of β_

is also checked for whether its magnitude lies within the MPP range. If the PV power is less and

β_ lies within the range, it corresponds to the minimum insolation condition. This initiatesmode

III operation with a set, fixed value of “M”. An additional advantage of the β method is that β_

and β_ critvalues themselves serve as an indicator of low- or zero-

Fig. 7. Simplified block diagram of the proposed HDGS configuration showing

the control variables. The arrows indicate the power flow direction along with the notations.

insolation conditions, which helps in deciding the intershuffling of operation between modes I–

III.

B. Control of Converters and Intershuffling Between Modes The power transferred to the grid

depends upon the value ofM and VPV during mode-1 and upon M, VPV and DON(FC) during

modes II and III. Equations (3) and (5) show that the amount of power transferred through the

inverter is determined by the values of VPV and M. The control scheme adjusts the value of m

according to the MPPT requirement, which, in turn, adjusts VPV. The power supply from FC

(controlled through DON(FC)) is not affected by this because the FC source is not a stiff dc

source. The Basic power flow scheme and the control variables used in the proposed HDGS

configuration are shown in Fig. 7. Excess power, Pex generated by the PV source is diverted for

electrolysis application through a buck converter. Hydrogen, the end product of electrolysis, is

stored and may be used by the fuel cell to feed Pdef at a later time. The overall control scheme

comprises of three independent control loops,whichwork in coordinationwith each other. These

are:

1) SPWM control of inverter along with MPPT.

2) FC side dc–dc converter control.

3) Excess power control (control of dc–dc buck converter).

Fig. 8 shows the complete control strategy and the truth table for the logic controller used in the

proposed system.When Preq > PPV, the logic controller turns ON the control for extracting

Pdef from the FC and turns OFF the control diverting Pex for electrolysis and vice versa. The

individual controllers corresponding to the excess or deficit power are controlled by

the logic controller and are not turned on simultaneously.Details of the two controllers are

described next.

1) SPWM Control of Inverter Along With MPPT: The inverter is operated using the SPWM

technique to feed the sinusoidal current into the grid during all the three operating modes

[Fig. 8(a)]. This control loop ensures the operation of the PV system at an optimum voltage

corresponding to the MPP of the PV array in modes I and II. As described earlier, the MPPT

algorithm also predicts the condition for entering Mode-III (only FC mode) based on the values

of β_, VPV, and PPV. In this operating mode, the controller shifts to a fixed modulation index,

Mcon, which maintains a critical voltage “Vcrit” across the PV array. This is achieved by

incorporating this feature in the MPPT algorithm itself. The value of Mcon can be calculated as

Fig. 8. Complete control strategy used in the proposed system. (a) Inverter

control. β_ is computed with in the MPPT block; (b) FC side buck-boost converter

control. (c) Buck converter control. (d) Truth table of the logic controllers. follows. Equating the

input and output energy of the converter in steady state for the kth division, assuming unity

power factor operation (i.e., grid voltage and current in phase and both sinusoidal) yields

where, Vg and Ig are the peak values of the grid voltage and current, respectively, while Ipk is

the peak value of idc. Simplifying (7) near the peak of grid voltage (i.e., k = n/2), using (1), and

solving for “M”(= Mcon) gives

During mode-III, the possibility of reverse current flowing into the PV array is prevented due to

the presence of a protection diode, DPr [Fig. 3].

2) FC side dc–dc converter control: Fig. 8(b) shows the control block used for the FC side dc–dc

converter. If PV power falls below Preq, the logic controller activates the control of the buck-

boost converter to feed deficit power from the FC source. It is observed from (5) that the power

drawn from the FC source can be controlled by modulating DON(FC), irrespective of the voltage

across CPV. Effectively, the converter acts as a current source [Appendix A], feeding current,

corresponding t the deficit power, Pdef . The deficit power that should be fed into

the grid can be calculated as

Another way to determine Pdef is by subtracting Preq from the average output power, Po

.However, a PI controller is required in this case to correctly maintain the required power

reference and DON(FC). This is because Po may already contain a component supplied by the

FC. The presence of PI, however, slows down the response. Therefore, in this research, (9) has

been used to determine Pdef . The ratio Pdef /VPV provides the desired value of iFC, which is

“(1 − DON(FC))” times the average value of the inductor (Lbb) current. This is the desired

reference current, Iref L(bb) that should flow through Lbb and is given by

A fast hysteresis current controller makes the buck-boost inductor current track this reference by

controlling DON(FC), which results in the FC source supplying Pdef .

3) Excess Power Control: The logic controller activates the control of the buck converter, when

PV power is more than the required power. Fig. 8(c) shows the control scheme. In the

control block of the buck converter, the average output power, Po is computed using the sensed

grid current (assuming constant Vg ). Po is compared with the reference power (= PPV − Preq)

to calculate the error. The error is then fed into a PI controller, which modulates the duty ratio,

DElec of the buck converter

V. DESIGN PROCEDURE

Design of the proposed HDGS system involves the design of the three power converters

described in the previous section. It includes the determination of the values of capacitors and

inductors, which is presented in this section, and the ratings of the various power devices used in

the system, which is obtained with simulations in Section VI. Table III summarizes the design

values of the various components.

A. Design of Inverter It involves the design of buck-boost inductor L, capacitor Cf and inductor

Lf [25]. Design values of the parameters are

where ΔVg and ΔVPV are the allowed maximum ripple in the grid and PV voltage, respectively,

PPV(rated) is the maximum rated power extracted from the PV array and fc is the lowest cutoff

frequency determined by the circuit parameters of the inverter configuration. l can have any

value satisfying (11), but is optimally chosen such that the converters operate in critical

conduction mode at the peak of the grid voltage.

B. Design of FC Side dc-dc Converter The FC side dc–dc converter is required to supply the

deficit power to the grid. It acts as a voltage-fed controlled current source. Thus, a proper design

of inductor Lbb is critical. As the FC supplies the deficit power under the control of a hysteresis

controller, the design of Lbb should ensure that the converter operates in CCM. Let ΔI be the

allowed ripple in the inductor current. Then, the values of inductor and capacitor are given by

where VFC and ΔVFC are the average value of and the allowed ripple in the fuel cell voltage,

respectivelyC. Buck Converter Design Design value of the inductor used in the buck converter is

given by

where, VEl is the output voltage across the electrolysis load and ΔIbuck is the allowed ripple in

the buck inductor current.

VI. SIMULATION AND HARDWARE RESULTS

The proposed configuration, along with the control scheme [Fig. 8], is simulated using the

MATLAB/SIMULINK. Design values of the components obtained in Section V [Table III] were

used. PV and FC sources are realized in the MATLAB Function with the help of their governing

equations [26]–[28]. FC steady state response is considered with the assumption that the FC

source is coupled with an ultracapacitor to improve its transient response. The inverter and the

boost converter are simulated in the SIMULINK using the state equations given in Table II.

Specifications of the PV and FC sources, used in the simulations, are as follows: PV source:

Open circuit voltage, Voc ≈ 125 V ; short-circuit current, Isc = 7.6 A. FC source: Voc = 69 V ;

Isc = 42 A. The system is required to feed 500 W (=Preq) into the grid. The inverter system is

designed for 1 kW power. The ratings of the various devices used in the system were ascertained

by simulations and are listed in Table III. Fig. 9 shows the simulation results of the basic HDGS

configuration without excess power control. Results corresponding to low, medium, and high

values of λ are shown. It is observed that beginning at t = 0 s, when λ is negligible, FC supplies

the entire Preq (=500 W). Around t = 1.2 s onwards, λ value picks up and the PV power

increases. The power drawn from the FC automatically decreases, such that PPV(t) + PFC(t) =

Preq. As the PV source operates near MPP, an optimal utilization of the two sources is ensured.

At t = 3s onwards, λ value improves further and the entire Preq is supplied by the PV. PFC

reduces to zero. Any excess power generated by the PV is also fed into the grid, because there is

no excess power diversion. It is observed that the proposed system is able to supply the required

power (Preq) under all conditions and for all the three modes as shown in the figure. In addition,

there is a smooth transition between the various modes (denoted by T1 and T2 in Fig. 9) while

maintaining Preq. Fig. 9 also shows the variation of other parameters like voltage, current,

modulation index (M) of SPWM, etc., during transition and operation in various modes.

Fig. 9. Simulation results of the integrated hybrid configuration showing transition

from mode III to mode II and then to mode I. T1 and T2 denote the transition between mode III

to mode II and mode II to mode I respectively.

Fig. 10. Simulation results of the integrated hybrid configuration operating in

electrolysis mode (mode I to mode III and then to mode I). T1 and T2 denote the transition

between mode I to mode III and mode III to mode I respectively. Fig. 10 shows simulation

results obtained by incorporating excess power control. The excess PV power (Pex ) is diverted

for electrolysis application through a buck converter. This prevents any grid voltage surge in

mode-I due to pumping. of excess power into the grid. Further, Preq. is always fed into the grid

with PV array operating near MPP. A small-step increment in the environmental conditions near

7.9 s shows corresponding increase in diverted power in steady state with the PV array

operating at MPP.

Fig. 11. Simulation results. Performance comparison of the proposed HDGS

system with and without an FC source in parallel with the PV source. To demonstrate the

usefulness of placing an FC source in parallel with the PV source, simulations were performed

with and without the FC source in the circuit, for a step change in insolation. Fig. 11 shows the

results. It is observed that the presence of FC eliminates the transient dips in the PV array voltage

and current, which, in turn, improves the MPPT speed and efficiency of the system. Further, the

grid current total harmonic distortion (THD) during the transients is lower in the presence of the

FC. An experimental prototype of a 300 W system was built to verify the simulation results and

other theoretical claims. Due to the nonavailability of the fuel cell source, a DC source was used

in the experiments, which can be justified by the fact that the performance of an FC source, with

a reformer and an ultracapacitor, is close to a stiff dc source [4]. Component values used in the

prototype are: CPV = 2000 μF; L = 310 μH; Cf = 2.2 μF; Lf = 3.25 mH; Lbb = 0.7mH and

Lbuck = 2 mH. Controllable power devices used are MOSFETs IRFP460, while the power

diodes used are CSD20060. Other specifications include VPV ≈ 90 V, VFC = 50 V and Vg =

140 V. To implement the control scheme depicted in Fig. 8, TMS320LF2407 digital signal

processor was used. A DSP controller was preferred to enable the computation of the average

values of the sensed PV voltage and current using fast fourier transform and implementation of

the MPPT algorithm.Also, it was used for computation of Po , Iref L(bb), etc. LEM current

sensors are used for sensing the instantaneous values of iPV, iL(bb) and igrid. The PI and

hysteresis controllers were implemented using JFET input-stage operational amplifiers TL084

having high input impedance, bandwidth, and slew rate. Fig. 12 depicts the PV side waveforms

for mode II. Fig. 13 shows the current and voltage waveforms of the dc source usedin place of

the FC source. Fig. 14 shows the voltage and current waveforms of the inverter section across Cf

and inductor “L” respectively. Fig. 15 gives the grid-side experimentalwaveforms of voltage,

current, and power. It is observed that the grid current

Fig. 12. Experimental plots showing the current, voltage, and power waveforms on the PV side

for Mode II.

Fig. 13. Experimental waveforms (Mode II) of the dc source current, voltage, and power. A dc

source was used in place of the FC stack.

Fig. 14. Experimental waveforms (Mode II) of the buck-boost inductor (L) current of the inverter

section and voltage across capacitor Cf .

Fig. 15. Experimental results (Mode II) showing the waveforms of grid voltage,

grid current and power fed into the grid.

THD is on the higher side. This can be attributed to a slightly distorted grid voltage.

VII. CONCLUSION

A compact topology, suitable for grid-connected applications has been proposed. Its working

principle, analysis, and design procedure have been presented. The topology is fed by a hybrid

combination of PV and FC sources. PV is the main source, while FC serves as an auxiliary

source to compensate for the uncertainties of the PV source. The presence of FC source improves

the quality of power (grid current THD, grid voltage profile, etc.) fed into the grid and decreases

the time taken to reach theMPP. Table IV compares the system performance with and without

the FC block in the system. A good feature of the proposed configuration is that the PV source is

directly coupled with the inverter (and not through a dedicated dc–dc converter) and the FC

block acts as a current source. Considering that the FC is not a stiff dc source, this facilitates PV

operation at MPP over a wide range of solar insolation, leading to an optimal utilization of the

energy sources. The efficiency of the proposed system in mode-1 is higher (around 85% to 90%)

than mode 2 and 3 (around 80% to 85%). A laboratory prototype of the proposed system has

shown encouraging results in terms of efficiency, complexity, reliability, EMI concerns, and

other features. Table V compares the proposed system and some of the existing HDGS

configurations with respect to various parameters and features.

APPENDIX

The state-space-averaged [29] equations for the integrated system, consisting of FC side dc–dc

converter and the inverter, corresponding to the positive half-cycle of the grid voltage are

[Fig. 3]

Fig. 16. Equivalent large signal model of the proposed configuration in continuous

time domain during positive half cycle of the grid voltage.

Continuous time-domain large-signal equivalent circuit corresponding to (A1)–(A6) is shown in

Fig. 16. It is observed that the FC source in conjunction with the buck-boost converter acts

as a current source (L1 tends to be large).

REFERENCES

[1] J. Kabouris and G. C. Contaxis, “Optimum expansion planning of an

unconventional generation system operating in parallel with a large scale

network,” IEEE Trans. Energy Convers., vol. 6, no. 3, pp. 394–400, Sep.

1991.

[2] P. Chiradeja and R. Ramakumar, “An approach to quantify the technical

benefits of distributed generation,” IEEE Trans. Energy Convers., vol. 19,

no. 4, pp. 764–773, Dec. 2004.

[3] Y. H. Kim and S. S. Kim, “An electrical modeling and fuzzy logic control

of a fuel cell generation system,” IEEE Trans. Energy Convers., vol. 14,

no. 2, pp. 239–244, Jun. 1999.

[4] K. N. Reddy and V. Agarwal, “Utility interactive hybrid distributed generation

scheme with compensation feature,” IEEE Trans. Energy Convers.,

vol. 22, no. 3, pp. 666–673, Sep. 2007.

[5] K. S. Tam and S. Rahman, “System performance improvement provided

by a power conditioning subsystem for central station photovoltaic fuel

cell power plant,” IEEE Trans. Energy Convers., vol. 3, no. 1, pp. 64–70.

[6] R. Kyoungsoo and S. Rahman, “Two loop controller for maximizing performance

of a grid-connected, photovoltaic fuel cell hybrid power plant,”

IEEE Trans. Energy Convers., vol. 13, no. 3, pp. 276–281, Sep. 1998.

[7] F. Valenciaga and P. F. Puleston, “Supervisor control for a stand-alone

hybrid generation system using wind and photovoltaic energy,” IEEE

Trans. Energy Convers., vol. 20, no. 2, pp. 398–405, Jun. 2005.

[8] N. Kato, K. Kurozumi, N. Susuld, and S. Muroyama, “Hybrid powersupply

system composed of photovoltaic and fuel-cell systems,” in

Telecommun. Energy Conf., Jpn, Oct., 2001, pp. 631–635.

[9] T. Senjyu, T. Nakaji, K. Uezato, and T. Funabashi, “A hybrid power system

using alternative energy facilities in isolated island,” IEEE Trans. Energy

Convers., vol. 20, no. 2, pp. 406–414, Jun. 2005.

[10] Y. M. Chen, C. S. Cheng, and H. C.Wu, “Grid-connected hybrid PV/wind

power generation system with improved dc bus voltage regulation strategy,”

in Proc. IEEE APEC, 2006, pp. 1089–1094.

[11] W. Gao, “Performance comparison of a fuel cell-battery hybrid power

train and a fuel cell-ultra capacitor hybrid power train,” IEEE Trans. Veh.

Technol., vol. 54, no. 3, pp. 846–855, May 2005.

[12] H. Tao, A. Kotsopoulos, J. L. Duarte, andM. A.M. Hendrix, “Multi-input

bidirectional dc–dc converter combining dc-link and magneticcoupling for

fuel cell systems,” in Proc. IEEE Conf. Ind. Appl., 2003, vol. 3, pp. 1136–

1142.

[13] T. Monai, I. Takano, H. Nishikawa, and Y. Sawada, “A collaborative

operation method between new energy-type dispersed power supply and

EDLC,” IEEE Trans. Energy Convers., vol. 19, no. 3, pp. 590–598, Sep.

2004.

[14] K. Agbossou, M. Kolhe, J. Hamelin, and T. K. Bose, “Performance of a

stand-alone renewable energy system based on energy storage as hydrogen,”

IEEE Trans. Energy Convers., vol. 19, no. 3, pp. 633–640, Sep.

2004.

[15] K. Rajashekara, “Hybrid fuel-cell strategies for clean power generation,”

IEEE Trans. Ind. Appl., vol. 41, no. 3, pp. 682–689, May/Jun. 2005.

[16] F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview

of control and grid synchronization for distributed power generation systems,”

IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409, Oct.

2006.

[17] A. Di Napoli, F. Crescimbini, S. Rodo, and L. Solero, “Multiple-input

dc–dc power converter for fuel-cell powered hybrid vehicles,” in Proc.

IEEE IAS, 2002, vol. 3, pp. 1578–1585.

[18] J. Zhenhua and R. A. Dougal, “A hybrid fuel cell power supply with rapid

dynamic response and high peak-power capacity,” in Proc. IEEE APEC,

2006, pp. 1250–1255.

[19] D. Das, R. Esmaili, L. Xu, and D. Nichols, “An optimal design of a

grid connected hybrid wind/photovoltaic/fuel cell system for distributed

energy production,” in Proc. IEEE IECON, 2005, pp. 2499–2504.

[20] S. Nakahara, M. Kobayashi, Y. Sawada, I. Takano, and H. Nishikawa, “A

collaborative operation study of hybrid new energy type dispersed power

supply system,” in Proc. IEEE PESC, 2006, pp. 999–1004.

[21] M. Jantsch and C. W. G. Verhoeve, “AC PV module inverters with full

sine wave burst operation mode for improved efficiency of grid connected

systems at low irradiance,” presented at the 14th EC PVSEC, Barcelona,

Spain, 1997.

[22] F. Blaabjerg, C. Zhe, and S. B. Kjaer, “Power electronics as efficient

interface in dispersed power generation systems,” IEEE Trans. Power

Electron., vol. 19, no. 5, pp. 1184–1194, Sep. 2004.

[23] A. Woyte, V.V. Thong, R. Belmans, and J. Nijs, “Voltage fluctuations

on distribution level introduced by photovoltaic systems,” IEEE Trans.

Energy Convers., vol. 21, no. 1, pp. 202–209, Mar. 2006.

[24] K. J. P. Macken, K. Vanthournout, J. Van Den Keybus, G. Deconinck,

and R. J. M. Belmans, “Distributed control of renewable generation units

with integrated active filter,” IEEE Trans. Power Electron., vol. 19, no. 5,

pp. 1353–1360, Sep. 2004.

[25] S. Jain and V. Agarwal, “A single-stage grid connected inverter topology

for solar PV systems with maximum power point tracking,” IEEE Trans.

Power Electron., vol. 22, no. 5, pp. 1928–1940, Sep. 2007.

[26] S. Jain andV.Agarwal, “A newalgorithm for rapid tracking of approximate

maximum power point in photovoltaic systems,” IEEE Power Electron.

Lett., vol. 2, no. 1, pp. 16–19, Mar. 2004.

[27] J. M. Correa, F. A Farret, J. R. Gomes, and M. G. Simoes, “Simulation

of fuel-cell stacks using a computer-controlled power rectifier with the

purposes of actual high-power injection applications,” IEEE Trans. Ind.

Appl., vol. 39, no. 4, pp. 1136–1142, Jul/Aug. 2003.

[28] G.Walker, “EvaluatingMPPT converter topologies using a MATLAB PV

model,” J. Electr. Electron. Eng., Aust. IE Aust., vol. 21, no. 1, pp. 49–56,

2001.

[29] D. M. Mitchell, dc-dc Switching Regulator Analysis, 3rd ed. New York:

McGraw-Hill, 1998.