5 Kw Fuel Cell System

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Residential Fuel Cell System

ABSTRACT

This paper reports on the combination of solid oxide fuel cell generators fueledwith biogas as renewable energy source, recoverable from wastes. The solid oxide fuelcells have gained much importance in the recent years for residential fuel systems. SOFCscould improve and promote the exploitation of biogas on manifold generation sites as smallcombined heat and power especially for farm and sewage installations raising the electricalconversion efficiency on such reduced power level. The design of an independentstationary residential fuel cell system with a generation capacity of 5 KW along with theprocess of producing biogas from animal waste is studied. This document compiles andestimates the biogas data that is required for the production of hydrogen to supply the fuelcells and presents the thermodynamics and electrochemical conversion processes. Thispaper also presents the power conditioning system of the fuel cell system to provide therequired voltage and power to the application.

INTRODUCTION

The interest in the distributed generation has increased significantly in the recentyears. It is believed that the distributed generation market will be between US $10 and $30billion by 2010. Due to environmental concerns, more effort is now being put into theclean distributed power like geothermal, solar thermal, photovoltaic, and wind generation,as well as fuel cells that use hydrogen, propane, natural gas or other fuels to generateelectricity without increasing pollution.

There are five major types of the fuel cells in current technology. Among these five,Alkaline Fuel Cells (AFCs) have been used in the NASA space program since 1960s.Polymer Electrolyte Membrane (PEM) fuel cells have very fast slew rates and lowoperating temperatures and are being used in electric vehicles. Phosphoric Acid Fuel Cells(PAFC) are very tolerant to impurities in the fuel steam and by far are the most mature interms of system development and commercialization. Over 200 stationary units withtypical capacity of 200 kW have been installed in the United States. Molten Carbonate FuelCells (MCFC) and Solid Oxide Fuel Cells (SOFC) both operate at high temperature 600-1,0000C, and are targeted at medium- and large-scale stationary power generation. InSOFC, a solid ceramic material is used for the electrolyte, and viable fuels can be used

without a separate reformer. The byproduct: hot water and heat can be used for heating.Much research has been done towards the residential application of SOFC. One of themajor obstacles of its commercialization is the high cost of installation. In recent years, theproduction costs of fuel cells keep decreasing.

Biomass offers worldwide large exploitation potential among renewable energysources. Biogas fuel feeding presents an attractive option among emerging application forfuel cells, especially for the high temperature ceramic type solid oxide fuel cells (SOFCs).


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Compared to natural gas it shows advantages of being indigenous and renewable, free ofnon methane hydrocarbon, with the exception of landfill gas and containing a large fractionof methane reforming agent CO2. Biogas fabrication inherently is a friendly and zestfulway to process waste streams of variable nature (sewage sludge, liquid organic industrialeffluents, farm residues, and animal waste, and landfill, municipal and industrial solid

organic residues).

Rising energy prices, broader regulatory requirements, and increased competition inthe marketplace are causing many in American agriculture's livestock sector to consideranaerobic digestion of animal wastes. They view the technology as a way to cut costs,address environmental concerns, and sometimes generate new revenues. While hundreds ofanaerobic-digestion projects have been installed in Europe and the U.S. since the 1970s, itwas not until the 1990s that better designed, more successful projects started to come online in the U.S. Today, there are an estimated 40 farm-scale projects in operation on swine,dairy, and poultry farms across the country. This paper studies the process of production ofbiogas through anaerobic digestion and the conversion of biogas to hydrogen and the

production of electricity from hydrogen by fuel cells. This paper calculates the amount ofbiogas needed and the amount of manure required for producing 5 KW power from the fuelcells. The total study can be briefly given as below.

Animal Waste (Biomass)Methane (Biogas- along with other gases)

Methane Hydrogen (fuel for fuel cells)

Hydrogen Electricity (through fuel cells)

ANAEROBIC DIGESTION

Anaerobic digestion works in a two-stage process to decompose organic material(i.e., volatile solids) in the absence of oxygen, producing bio-gas as a waste product. In thefirst stage, the volatile solids in manure are converted into fatty acids by anaerobic bacteriaknown as "acid formers." In the second stage, these acids are further converted into bio-gasby more specialized bacteria known as "methane formers." With proper planning anddesign, this anaerobic-digestion process, which has been at work in nature for millions ofyears, can be managed to convert a farmer's often problematic waste-stream into an asset.Figure 1 shows the basic components of the anaerobic digestion process to produce biogas.

The key by-products of anaerobic digestion include digested solids ( useful as a soilamendment) and methane, the primary component of "bio-gas," which can be used to fuel avariety of cooking, heating, cooling, and lighting applications, as well as to generateelectricity. Capturing and using the methane also precludes its release to the atmosphere,where it is 20 times more damaging to the ozone layer than carbon dioxide.


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Figure 1. Basic components of an anaerobic-digestion systemSource: ATTRA - National Sustainable Agriculture Information Service,

http://attra.ncat.org/attra-pub/anaerobic.html

The plug flow type of anaerobic digester is the most commonly used digester. Thisconsists of a cylindrical tank in which the gas and other by-products are pushed out one endby new manure being fed into the other end. This design handles 11-13% solids andtypically employs hot-water piping through the tank to maintain the necessary temperature.Most appropriate for livestock operations that remove manure mechanically rather thanwashing it out.

Procedure

The digestion process is not difficult but requires long period of time. The digestertank is filled with water and then heated to the desired temperature. "Seed" sludge from a

municipal sewage treatment plant is then added to about 15% of the tank's volume,followed by gradually increasing amounts of fresh manure over a three-week period untilthe desired loading rate is reached. Assuming that the temperature within the systemremains relatively constant, steady gas production should occur in the fourth week afterstart-up. The bacteria may require two to three months to multiply to an efficientpopulation.


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Temperature within the digester is critical, with maximum conversion occurring atapproximately 95F in conventional mesophilic digesters. For each 20F decrease intemperature, gas production will fall by approximately 50 percent. Even more significant isthe need to keep the temperature steady. Optimal operation occurs when the methaneformers use all the acids at approximately the same rate that the acid formers produce them.

Variations of as little as 5F can inhibit methane formers enough to tip the balance of theprocess and possibly cause system failure.

Bio-gas produced in an anaerobic digester contains methane (60-70%), carbondioxide (30-40%), and various toxic gases, including hydrogen sulfide, ammonia, andmercaptans. Bio-gas also typically contains 1-2% water vapor. At roughly 60% methane,bio-gas possesses an energy contentof 600 Btu/ft3.

Uses of Bio-gas

Because of the extreme cost and difficulty of liquefying bio-gas, it is not feasible

for use as a tractor fuel. Bio-gas has many other on-farm applications, like cooking, heating(space heating, water heating, and grain drying), cooling, and lighting. Bio-gas can also beused to fuel generators for producing steam and electricity.

While methane is a very promising energy resource, the non-methane componentsof bio-gas tend to inhibit methane production and, with the exception of the water vapor,are harmful to humans and the environment. For these reasons, the bio-gas producedshould be properly "cleaned" using appropriate scrubbing and separation techniques.

Digester Design Factors

Digesters are installed primarily for economic and environmental reasons. Digestersrepresent a way for the farmer to convert a waste product into an economicasset, whilesimultaneously solving an environmental problem. Under ideal conditions, an anaerobic-digestion system can convert a livestock operation's steady accumulation of manure into afuel for heating or cooling a portion of the farm operation or for further conversion intoelectricity. The solids remaining after the digestion process can be used as a soilamendment, applicable on-farm or made available for sale to other markets.

Anaerobic digestion requires careful consideration of many factors. They can bequite costly to install. A straightforward batch-loading design will involve an air-tight tank,means of mixing the contents of the tank and maintaining a constant temperature, and ameans of collecting the gas with appropriate safety precautions. Additional hardware willinclude regulators, flame traps, pressure gauges and relief valves, a hydrogen-sulfidescrubber, and means of removing the carbon dioxide. The size of the system is determinedprimarily by the number and type of animals served by the operation, the amount ofdilution water to be added, and the desired retention time. The most manageable of thesefactors is retention time; longer retention times mean more complete breakdown of themanure contents, but require a larger tank.


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BIOGAS TO HYDROGEN CONVERSION

Methane steam reforming (MSR) is a major route for the industrial production of H2. Thethree main reactions in a MSR reactor are represented by following equations.

CH4 +H2OCO + 3H2; H298 = 206.2 103

kJ/kmol; . (1)

CO+H2O CO2 +H2; H298 = 41.1 103 kJ/kmol; (2)

CH4 + 2H2OCO2 + 4H2; H298 = 164.9 103 kJ/kmol; .. (3)

Fig 2: Steam Methane Reforming Process in a Fuel ProcessorSource: [3]Hydrogen from Methane in a single step process, Chemical Engineering

Science

Reforming reactions (1) and (3) are highly endothermic and thermodynamicallyfavored by high temperature and low pressure. On the other hand, the watergas shift(WGS) reaction given by (2) is favored at low temperature, but it has no pressuredependence. MSR is generally operated at a temperature of 750900oC due to the overallendothermic nature of the reactions. Although high-temperature operation is indispensablefor a substantial conversion of CH4, it facilitates the reverse WGS reaction, giving theproduct gas containing 810% CO on a dry basis. For the purpose of obtaining the productgas with less CO and more H2, it is conventional that the MSR product gas is fed to another

reactor where the temperature is kept as low as 300400oC for the WGS reaction to takeplace prior. To obtain the H2 product stream, the effluent is then cooled and fed to amulticolumn pressure swing adsorption (PSA) process.

DESIGN OF COMPLETE FUEL CELL SYSTEM

Numerous calculations are involved in the design of a 5 KW residential fuel cell system. Itis desired to find the number of cows (cattle) required to produce 5KW power, the size of


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the digester tank, the amount of manure and biogas produced, the retention time of themanure in the digester tank, the heat energy involved in the conversion of methane tohydrogen, the volume of methane required to produce sufficient amount of hydrogen toproduce 5 KW electricity.

This paper focuses on the design of a 5 KW solid oxide fuel cell (SOFC) power generationsystem to provide non utility and ultra clean residential electricity. The aimed fuel cellstack outputs 22 V to 41 V dc. For residential applications, the needed output is two splitphase 60 Hz, 120V ac, the 5 KW fuel cell stack is supplemented by a 5 KW battery pack tomeet peak power demand of 10 KW.

The generating capacity of a typical hydrogen fuel cell is 167 watts at a voltage output of 1volt. For higher output and terminal voltage, cascades of series- parallel connections of fuelcells- will meet the required electric load demands.

For a 5 KW residential cell system, the number of fuel cells required in a stack is

No. of fuel cells = 5000/167 30 fuel cells in series.

The output voltage of the fuel cell stack is Vo = 30 * 1 = 30 V .. [1]

The current passing through the fuel cell stack is I = 5000/30 167 A .. [2]

It is known that current is the rate of charge. Hence

I = (n * e-)/t . [3]

where n: no. of electrons passing through the external load connected to the fuel cell.e-: charge of an electron = 1.602*10-19 coult: time of discharge of the electron

*Note: The time of discharge of the electron is the time taken by the electron to travel fromone electrode of the fuel cell to the other. The time of discharge depends on the distancebetween the electrodes. The time of discharge of a typical hydrogen fuel cell ist = 1.0813 * 10-17 sec.

Hence the number of moles of electrons (from [3])

n = I *t / e- = (166.67 * 1.0813*10-17)/ 1.602*10-19

n = 11249.70 moles of electrons

The electrochemical reactions taking place in the fuel cell are

At anode A, 2H2 = 4H+ + 4e- [4]


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At cathode K, 4H+ + 4e- + O2 2H2O . [5]

And the overall reaction is

2H2 + O2 2H2O ... [6]

Hence from [4] it can be taken that the molar ratio of H2 to the electron is 1: 2.

Hence the number of moles of H2 ,

nH2 = 11249.70/2 = 5624.85 moles of H2

The reactions occurring in the conversion of methane to hydrogen are

CH4 +H2OCO + 3H2;

CO+H2O CO2 +H2;

The overall reaction is

CH4 + 2H2OCO2 + 4H2;

From this it can be observed that the molar ratio of methane to hydrogen is 1: 4.

*Note: It is taken that the complete chemical reactions take place in the fuel reformationi.e., conversion of methane to hydrogen. The rates of chemical reactions are consideredwhen the dynamic model of the fuel cell system is designed. The rates of reactions depend

on the partial pressures of the reactants and the products. The dynamic model of the fuelcell system is required to analyze the transient changes in the electrical load. To study howfast the fuel cell can respond to the sudden changes in the load can be determined by thedynamic model. The time of response of the fuel cell system depends on the rate of changein the input to the fuel cell (i.e., hydrogen). This in turn depends on the rate of reactions ofthe fuel processor.

Hence the number of moles of methane

nCH4 = 5624.85/4 = 1406.21 moles of methane

Ideal Gas Equation

PV = nRT [7]

Where P: pressure of the processorV: volume of methanen: no. of moles of methaneR: Universal Gas Constant


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T: temperature of the reaction

P = 1 atm , n = 1406.213, R = 0.0826 litre/atm/mol-K, T = 800+273 = 1073 K

From [7] , volume of methane required is

V = nRT/P = 1406.213*0.0826*1073/1 = 124632.386liters of methane

According to ATTRA -National Sustainable Agriculture Information Service,

One cow yields 12.5 gallons of manure per day. This amount of manure produces 46 cubicfeet of biogas through anaerobic digestion.

Hence 1 cow produces 12.5 gal/day manure 46 cubic feet of biogas 1302.574 litersof methane.

To produce 124632.386 liters of methane the number of cows required is

No. of cows = 124632.386/1302.574 96 cows

Hence with 96 cows the amount of manure produced is 1200 gal/day.

According to ATTRA, the dimensions of the digester tank for 100 cow dairy herd is

18 ft diameter*18 ft height (cylindrical tank). The capacity of the tank is 32,250 gal.

The retention time is 15 days in the tank.

35 % of the biogas produced is used to maintain the temperature in the tank.

POWER CONDITIONING SYSTEM

This paper uses an inverter system that supports the commercialization of a 5 kWsolid-oxide fuel cell (SOFC) power generation system to provide non-utility and ultra-cleanresidential electricity. The aimed fuel cell outputs 22 V to 41 V dc. For residentialapplications, the needed output is two split-phase 60 Hz, 120 V ac, the 5 kW SOFC issupplemented with a 5 kW battery pack to meet peak power-demand of 10 kW. The

general inverter system configuration is shown in Fig. 3. The dc voltage from the fuel cellis first boost up to 350-450 V by a dc-dc converter then a dc-ac inverter with output filter iscascade-connected to produce ac voltage. The battery can be added to either the lowvoltage side or the high voltage side of the converter.


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Fig 3: Components of Power Conditioning System of a Fuel Cell systemSource: [5] A New Low Cost Inverter System for a 5 KW fuel cell

The dc-dc converter uses phase shifting to control power flow through atransformer with a full bridge on the low voltage side and a controlled voltage doubler onthe high voltage side. The transformer provides voltage isolation between the fuel cell andthe ac output voltage improving overall safety of the system. A voltage doubler on the highvoltage side decreases the turns ratio of the transformer, which reduces leakage inductanceand makes the system more efficient and easier to control. And at the same time, thevoltage and current stresses on the low voltage side are also minimized. A high voltagebattery pack is added after the voltage doubler as transient power for load dynamics. Thusthe capacitance of the high voltage side capacitors are minimized, which will significantlyreduce the total cost of the system.

The overall residential fuel cell system is shown in the figure 4.

CONCLUSION

This paper mainly concentrated on the design issues of an independent residentialfuel cell system. The power generation system is not connected to the grid but it suppliespower to the farm house. A typical 5 KW fuel cell system is taken into consideration. It iscalculated that 96 cows are required to yield 1200 gallons of manure per day. This manurewhen subjected to anaerobic digestion for 15 days in a cylindrical 18 feet diameter , 18 feettall tank, it produces biogas which consists of 60 -70 % of methane, the rest consisting ofcarbon dioxide, oxides of nitrogen and other gases. This biogas is sent to the fuelprocessing system which converts methane into hydrogen. It is noted that 99% of methaneis converted into hydrogen. This is a highly endothermic reaction which requires atemperature of 800o C. The hydrogen obtained is passed into a fuel cell stack consisting of30 fuel cells in series. The output voltage of the fuel cell stack is 30 V with a power of 5KW. The obtained power is in DC form. This is converted into 120V ac split phase powerby a power conditioning system consisting of a dc-dc boost converter followed by an

inverter. An auxiliary power supply of 5KW is taken by a battery connected to the lowvoltage side of the dc-dc converter.


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96 Cows are required in the Herd

Cylindrical Anaerobic digestion Tank, 18ft diam * 18 ft tall

Fuel processor, Methane to Hydrogen conversion at 800oC

Fuel cell stack, 30 fuel cells in series

Power conditioning system, DC DC Converter and Inverter

Residential Farm HouseFigure 4: Complete Residential fuel cell system


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REFERENCES

[1] ATTRA - National Sustainable Agriculture Information Service,

http://attra.ncat.org/attra-pub/anaerobic.html

[2] B. Balasubramanian, A. Lopez Ortiz, S. Kaytakoglu, D.P. Harrison, Hydrogen frommethane in a single-step process, Chemical Engineering Science, Journal of PowerSources

[3] K. Denno, Power System Design and Applications for Alternative Energy Sources,Prentice Hall Advanced Reference Series Engineering

[4] Jan Van herle, Yves Membrez, Olivier Bucheli, Biogas as a fuel source for SOFC co-generators, Journal of Power Sources 127 (2004) 300312

[5] Jin Wang, Fang Z. Peng, Joel Anderson, Alan Joseph and Ryan Buffenbarger, A New LowCost Inverter System for 5 kW Fuel Cell, 2003 Fuel Cell Seminar Special Session on fuelCell Power Conditioning and International Future Energy Challenge Eden Roc Hotel,Miami Beach, Florida

[6] Aashish Mehta, The Economics and Feasibility of Electricity Generation usingManure Digesters on Small and Mid-size Dairy Farms


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5 Kw Fuel Cell System

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