ES2011-54833

1 Copyright © 2011 by ASME

Proceedings of ASME 2011 5th International Conference on Energy Sustainability & 9th Fuel Cell Science, Engineering and Technology Conference

ESFuelCell2011 August 7-10, 2011, Washington, DC, USA

ESFuelCell2011-54833

COMPARING UTILITY CONNECTED TO STAND ALONE MICRO-GRIDS: FROM THE VIEWPOINT OF A UTILITY ENGINEER.

Mike Hoffman Pacific Northwest National Laboratory

Portland, Oregon, USA

Brian Russo Pacific Northwest National Laboratory

Richland, Washington, USA

ABSTRACT

This paper presents project examples of grid and grid-isolated micro-grids. It discusses micro-grid drivers; economic, environmental, and financial issues; and tools to plan and design micro-grids. Tools covered include modeling software resource options, generation types, fuel options, and services to provide smooth transitions (including required equipment and software controls), ranging from those of minimal expense to maximum functionality. The need for, and construction of, real-time operational interfaces is also addressed, with a focus on real world complications and guidance regarding implementation of planning design and of a micro-grid.

This paper attempts to present a broad overview of micro-grids, including project examples, modeling tools, technology options, and practical and business insights to enable interested parties to quickly come up to speed on micro-grid basics. The included information should give interested parties the tools and resources to move forward on their own projects, assuming these parties have sufficient knowledge and experience with electrical distribution systems.

INTRODUCTION

When comparing utility-connected to standalone micro-grids, a range of important technical, operational, planning, design, and installation issues need to be examined. Utility engineering focuses on always keeping the power on (reliability at a reasonable cost) as well as rate recovery, which is based on the concept of a project being both used and useful to the entire rate base (all customers), safety for line workers, and use of proven technology.

This paper defines micro-grids as multiple generation sources with communications and control capability that supply multiple structures (not a backup generator in a single building). Another definition, courtesy of Lawrence Berkeley Lab, Consortium for Electric Reliability Technology Solutions (CERTS), is “an interconnected network of Distributed Energy Resources (DER) connected and separate from the electricity grid” [1].

A key feature of a micro-grid, is its ability, during a utility grid disturbance, to seamlessly separate and isolate itself from the utility grid with little or no disruption to the loads within the micro-grid (e.g., in the CERTS Micro-grid concept, impacts on power quality). Additionally, when the utility grid returns to normal, the micro-grid automatically resynchronizes and reconnects itself to the utility grid in equally seamless fashion.

WHY BUILD A MICRO-GRID? - SITUATIONS AND DRIVERS:

Strong grid situations are usually found on continental landmasses containing widespread transmission networks. The drivers tend to be reliability (industrial plants), economics (reducing peak loads with renewables integration or readily available resources), and limiting greenhouse gases in a changing regulatory environment (with the expectation of future carbon regulation).

Weak grid situations are usually found on islands. The drivers are economics (price of fuel oil), reliability (loss of a single transmission line or generation resource can cause system collapse), and abundance of renewable resources (solar, wind, ocean energy and in some situations geothermal energy).

Proceedings of the ASME 2011 5th International Conference on Energy Sustainability ES2011

August 7-10, 2011, Washington, DC, USA

ES2011-54833


Off grid situations occur where there is no access to a larger electrical grid, e.g., (islands, remote villages, or military forward operating bases [FOBs]). The drivers are a need to sustain comfort and communications in a hostile environment and provide essential services (e.g., water pumping, sewage treatment, and equipment maintenance).

Technological distinctions between grid connected and stand alone micro grids:

When connected to a multiple utility, regional or national grid system, a micro grid acts as a controllable resource/load within the power system. It does not impact the reliability or stability of the power system because of its ability to act as a controlled and grid compliant entity that it operates within normal utility rules, standards and regulations. A grid connected micro grid is synchronized using paralleling switchgear on a medium voltage distribution system (4160 V to 35,000 V) in parallel and frequency matched to the main grid. High-speed digital relaying connected via high-speed communications channels is used for sensing of system instability and disconnection from the main grid when required and has a Supervisory Control And Data Acquisition (SCADA) system to ensure that local generation and loads are properly matched and protected, including coordinated fault protection.

Stand alone micro grids are isolated from a main grid system, use generation that can include internal combustion engines, micro turbines, photovoltaic panels, wind generators, fuel cells to supply power to a load that is generally smaller than 10 MW. Many standalone micro grids are operated at 600 V or lower, allowing the use of low-voltage switchgear and

eliminating the need for distribution transformers. Voltage regulation for the isolated micro grid is usually controlled through the use of a voltage versus reactive current droop controller. This means as reactive current generated by the local source becomes more capacitive the voltage set point is reduced, to reduce circulating currents. This is the control scheme used by many generator manufacturers to load balance multiple internal combustion engine resources via a low-speed communications channel.

Standards for micro-grids and with main grid systems:

This section is meant to be only a brief listing of a range of US and international standards related to micro grids and their interaction with main grid systems. In the United States IEEE initiative P2030 (smart grid operability of energy technology with electrical power system and end-use applications and loads) and IEEE Interconnection Standard 1547 (Standard for Interconnecting Distributed Resources with Electric Power Systems) impact energy storage systems, distribution grid management, voltage regulation, management of island its systems along with integration of electric transportation and vehicle systems. International standards of relevance are IEC 61970—301 (application programming interface for energy management systems), IEC 61968 (standards for information exchange between electrical distribution systems) and IEC 61850 (substation automation).

Project examples of grid connected micro-grids:

The business case for micro-grids is just starting to be made, with the bulk of micro-grids currently situated in

Figure 1. CERTS Micro-grid Example1


industrial (refinery) or campus situations. However, as utility rate structures begin to include dynamic pricing and standardized interconnection policies, micro-grids will play a larger role in utility planning and programs [2].

Industrial situations: Petroleum refineries are a key element of national economies and the energy ecosystem. They are also complex, potentially toxic if disrupted, expensive to operate, and difficult to restart if shut down. A well-documented case of micro-grid implementation is in the Motor Oil Hellas petroleum refinery in Greece. This refinery is the largest privately held industrial complex in the country. A micro-grid was installed to increase the reliability of the refinery’s electrical network and to reduce energy costs. The system relies on high-speed digital relays to manage load shedding and system control and averages 22 ms response times. While this facility is tied to Greece’s national grid, there are other examples of refinery situations (e.g., a petroleum refinery in the middle of Saudi Arabia's Empty Quarter), which operate entirely off grid and rely on a micro-grid for complete energy control.

Campus situations: The University of California, San Diego is demonstrating integration of onsite renewable energy production with its combined heat and power (CHP) plant and other local generation resources. The system will include two 13.5 MW gas turbines, a 3 MW steam turbine, and 1.2 MW of solar cells and will provide 80 percent or more of the campus’ annual average power use [3].

Eco-districts – combined heat and power: Denmark is the prime example of centralized and decentralized combined heat and power plants. CHP plants are spread across the country, producing both the heat and power used in district heating systems. The Danish Energy Agency regulates prices for heating and electricity to ensure fair rates for end-users. 670 CHP plants produce electricity and heat under various forms of ownership, including utility owned plants, municipal plants, and local cooperatives. Another use for energy from CHP plants is in balancing energy needs during periods of low wind production.

Utility scale micro-grid: Micro-grids at the distribution feeder level are still non-existent. However, Portland General Electric in Salem, Oregon is in the process of constructing this type of micro-grid using American Recovery & Reinvestment Act (ARRA) funding. The goals of the project include building a self-healing feeder that can automatically isolate itself from the grid, operate as a high reliability island served by several resources;demand response, energy storage, and distributed generation. This grid will create a reliable power zone for customers in a service area.

The key to the islanding scheme is a 1,300 kWh battery and a 5 MW power conditioning system to transition smoothly from grid power to backup generation. Additionally, the energy storage system will reduce peak demand except when charging in off-peak hours.

Residential Micro-grid: California leads the field in creating the first Micro-Grid, Distributed Energy Resource Community. A private, commercial micro-grid in Sacramento, California called 2500 R Street will feature solar generation, energy storage, and 34 Leadership in Energy & Environmental Design (LEED) platinum homes. This community will have a utility advantage, dispatching and controlling solar resources discreetly or in aggregate, which could improve utility transmission and distribution congestion. Additionally this community will have the advantage of potential for capital investment while at the same time improving grid reliability and shaving system peaks [4].

Other cities where U.S. Department of Energy (DOE) ARRA-funded micro-grids projects have been approved include Austin, Texas (Pecan street project); Arlington, Texas (University of Texas test bed project); Stamford, Connecticut (911 emergency center); Detroit, Michigan (Next Energy Center -- DC micro-grid); San Diego, California (Borrego Springs); and Texas (renewable based micro-grid power systems for communities (colonias) around the US/Mexico border) [5].


Project examples of isolated micro-grids:

Off-grid – island/remote village micro-grid: From large islands such as Sri Lanka and Hawaii, extending to the smallest islands (where individual facilities are powered by individual generators), the largest factor driving power cost is the price of fuel. Wind and solar resources play an important role in the effort to keep electricity rates down by reducing fuel costs. Another characteristic of island grid systems is that they are electrically fragile. This fragility is due to maintaining relatively few generators and using usually long transmission or distribution lines. Extreme voltage and frequency fluctuations occur when a line or generator is out of commission. When frequency drops too low, damage can occur to generators causing breakers to open and then system failure.

For many decades, smaller islands and remote villages have relied upon diesel generators to provide power. As fuel costs have risen, DOE has supported research on “free fuels” (e.g., solar and wind) to reduce the use of diesel fuel. The DOE tribal energy program website [6] has a wealth of information applicable to micro-grid implementation, including analysis, economics, business and finance, demand-side options, biomass, geothermal, solar, and wind energy resources.

The wind energy resource is particularly useful because of its focus on wind/diesel hybrid systems. A detailed presentation prepared by the National Renewable Energy Laboratory (NREL) documents the implementation of wind, solar, and diesel energy storage systems in Alaska, Chile, and California. The presentation [7] contains technical drawings and equipment currently in use at remote locations.

Over the last decade, village photovoltaic (PV) systems paired with batteries (for energy storage) have given rise to an entirely new business opportunity. These systems have powered lighting, refrigeration, and communications, greatly raising the standard of living in remote locations. The systems have run parallel to the new concept of micro finance at the village level and have used that as a funding source. In fact, one recent winner of the General Electric (GE) Ecomagination competition was Mera Gao Micro-grid Power [8], a provider of a PV/battery system.

Military micro-grid: A February 2008 Defense Science Board (DSB) report entitled “More fight – Less Fuel” [9] included drivers for many changes in the US military. It concluded that fuel demand compromises operational capability and mission success, requires excessive support forces, creates additional risks for logistics, and greatly increases costs and military installations and situations worldwide. Subsequently, security, reliability, and energy efficiency have become top priorities both at home and abroad. From the perspective of micro-grids, utility scale (on grid) and village scale (off grid) are the two types of situations used. The ability to “island” military facilities was the second priority, after more efficient fuel use.

Following recent reductions in military spending, military contractors seem to have followed the advice of the DSB report. Systems spending has been reduced and focus has shifted to include military energy issues, partly because of their potential for revenue generation. This following press headlines and Lockheed micro-grid animation further demonstrate this change of focus [10]:

Figure 2. PGE – Salem Feeder Micro-grid1


Expanding Military Interest in Microgrid Technology Fuels $4 Billion Industry, reveals SBI Energy [11]

Energy, Environment Opportunities Grow Despite DOD Belt-Tightening – NYT [12]

Utility Scale – on-grid examples: Fort Bragg in North Carolina has one of the world's largest micro-grids (Figure 2). It was created by integrating a variety of distributed generation technologies. The bases electrical distribution network, along with centralized energy management, communications, and information technology infrastructures manage the system. The project’s goal has been to enhance reliability and reduce energy costs. The components of the micro-grid include; diesel backup generators, fuel cell, 5 MW gas turbine and an energy management system to control power flows.

Figure 3. Fort Bragg System Overview

Fort Sill, Oklahoma was selected [13] to demonstrate a field scale, renewable-focused intelligent micro-grid. This demonstration will be housed on a part of Fort Sill’s field artillery training center campus. The purpose of the demonstration is to validate the ability to network renewable and advanced distributed generation, quantify the carbon reduction of using these technologies, and show that mission-critical power requirements can be met in a secure and reliable manner. Based on the results of the demonstration, a document will be developed to implement micro-grids at other Department of Defense (DoD) facilities.

FE Warren Air Force Base in Wyoming has been the site of testing [14]wind resources for their ability to extend diesel generator fuel supplies. Half of the base was islanded and carried by backup generation and on-base wind generators with no perceived impact on power quality. This was an important step in proving that renewables can extend the time emergency power is available for a grid-connected base.

Off grid (FOBs) example: Smart and Green Energy (SAGE) is described in a Scientific American website article entitled, “Can a Micro-grid Protect U.S. Troops in Afghanistan?” [15]. The article hypothesizes that using load

balancing generators to reduce Army fuel consumption could reduce fuel use by as much as 17%. The Army is following a precedent set by the Marines, which sent an initial wave of PV panels to reduce generator use last fall. The goal of this effort is to reduce the amount of fuel delivered across the rugged and dangerous terrain of Afghanistan. This effort would create the Army's first field-ready micro-grid.

Figure 4. SAGE – Forward Operating Base Concept

Drivers – economic, environmental, reliability, and security

Economics drive everything related to micro-grids, both utility-connected micro-grids (regulatory standard and customer equality, which makes it harder to find funding) and remote micro-grids (fuel cost). This paper will not include the issue of subsidies and tax credits. The accounting sector can better present work in that area and its inclusion here would compromise the intended brevity of this work.

Currently the best locations to practically demonstrate micro-grids, both grid-connected and remote, are Hawaii and Alaska because they have high power costs and the most economic and current high-value opportunities – based on the cost of fossil fuel (i.e., diesel). A spreadsheet of diesel fuel costs, generation, and maintenance costs for paybacks with commercial generator sets based on the reduction in fuel use made possible by renewables (PV systems, Wind) can easily be generated. Lifecycle costs for localized power generation would be lowered by displacing diesel fuel use with PV, wind, geothermal, small hydro, and energy storage.

The current generation, transmission, and distribution infrastructure of the United States is relatively mature and was built for peak generation needs that did not envision the current capability of two-way power flows. Consequently, much generation has been built for urban load centers. The Federal Energy Regulatory Commission (FERC) has encouraged, and legally mandated, this long-distance power delivery system


with open access on the transmission system. Consequently, transmission lines are hard to site and build into load centers, which creates another opportunity for micro-grids to cost-effectively supply local power to urban areas. Then environmental benefit of both types of micro-grids is that they reduce the user’s carbon footprint. This is true in both cases, and particularly obvious with remote micro-grids and diesel fuel, but also very implicit wherever fossil resources (e.g., coal) are used in grid-connected systems. Using natural gas-fired internal combustion engines for backup generation is much cleaner than burning coal and a central plant. Another benefit, both economic and environmental, is that local generation of energy is more efficient relative to the 3 to 7% transmission and distribution losses that occur with central plant generation.

Micro-grids have the potential to increase local reliability because of the detailed planning needed to increase automation, generation resource control, energy storage, communications capabilities, and energy management systems within facilities found in the local grid. This will mean fewer blackouts due to transmission and local incidents on the distribution system because automated feeder switching will be in place – similar to the Portland General Electric example mentioned earlier. From an end-user standpoint, this means business can continue even when others are unable to produce or sell products. Additionally, the benefits flow back upstream when system peaks in local generation are used to relieve larger grid and/or independent system operators (ISOs), such as New York or New England, provide a market for the services.

Security benefits relative to military situations both on grid and off grid are obvious. For commercial or utility micro-grids, the benefits of maintaining water pumping, sewage pumping, and other essential services (police, fire, government) during a larger outage are also obvious. From a business perspective, high-quality power can be a large business advantage, large enough even that a business could choose to site their operations in areas where utilities offer this high level of service.

Financial modeling tools to plan/design

Equipment & Resource Options: The following programs for financial and resource modeling are supported by the Canadian and United States governments. There is no cost to download or use them. They are not specifically configured for micro-grid analysis but are useful and often used to find off-grid technology solutions to energy resource costing and planning. Verbiage below was taken from an energy storage paper by Mike Hoffman.

RETScreen [16] This clean energy project analysis software is a unique decision support tool developed with the contribution of numerous experts from government, industry, and academia. The software, provided free-of-charge, can be used worldwide to evaluate the energy production and savings,

costs, emission reductions, financial viability, and risk for various types of renewable-energy and energy-efficient technologies (RETs). RETScreen allows the user to assess a variety of different, clean energy technologies. These include wind energy, small hydropower, PV (on-grid and off-grid), biomass, and others. RETScreen compares these clean energy technologies to a conventional “base case” specified by the user, to determine the financial viability of the proposed technology and to determine how cost effective the clean technology is relative to conventional technologies. The key outputs of this software are as follows:

Development of a suite of new models to evaluate energy efficiency measures for residential, commercial, and institutional buildings; communities; and industrial facilities and processes.

Expansion of the RETScreen Climate Database to 4,700 ground-station locations around the globe and incorporation of the improved National Aeronautics and Space Administration (NASA) Surface Meteorology and Solar Energy Dataset for populated areas directly into the RETScreen software.

RETScreen allows users to model energy storage, in the form of a battery, for off-grid applications such as PV projects. Version 4 of RETScreen, uses the same model for off-grid wind and other technologies. RETScreen also suggests battery size to the user based on a formula involving the desired number of days of autonomy. The formula calculates the appropriate battery size by month, and the suggested battery size is the maximum of these monthly values over the year.

HOMER [17] helps users optimally determine the characteristics of micro power (kilowatt to 100s of kilowatt) generation units. In contrast to some of the other models considered, HOMER does not model the entire electricity grid, so the location of storage units and other components within the system modeled are not explicitly considered. Its principal value is that it helps users optimally configure localized electricity projects. The developers of HOMER are working on another model called ViPOR, which will model the entire grid.

HOMER provides the user with considerable flexibility. It can specify buying and selling constraints, and allow for the electricity generated to supply a user-specified range of the expected load. HOMER models transmission line capacity. Additionally, the storage unit can be used to model arbitrage. HOMER also has the advantage of highly refined time slices – it has the capability of modeling by minutes within a given year.

Hybrid2 [18] was designed to analyze a wide range of hybrid power systems. The models may include multiple electrical loads, multiple wind turbines of different types, PV systems, multiple diesel generators, battery storage systems, and a range of power conversion devices. Control strategies


may be studied with the models, which include generator and interactions between diesel generator sets and energy storage. An economic analysis tool calculates the project’s economics.

Hybrid2 has a graphical user interface (GUI) and includes a glossary of terms associated with hybrid power systems. It also has a library of equipment to assist in designing hybrid power systems. The library covers a range of commercially available equipment and uses the manufacturers' specifications. It also provides sample power systems and projects that can be used as templates.

Distribution system modeling tools: This is a brief overview of tools normally used by utilities or consulting engineers who model 4kv to 35kv electrical distribution systems. Optimizing a micro-grid and its control system would require familiarity, if not actual experience, with tools of this nature. The purpose of this section is to name the tools and provide references: OpenDSS [19] (open source), SynerGEE [20], CYME [21] and Milsoft [22]. These software packages support reliability calculations, protection and coordination, switching and contingency planning, multiyear analysis and load forecasting, operations and real-time support, as well as economic evaluation of options. They generally do not support analysis of energy storage or distributed generation. Because customer requests drive commercial software development, options for energy storage and distributed generation resources likely will not be included until customers request them.

Generation options

The following sections illustrate resources that power micro-grids. These resources range from conventional diesel backup generator and across the technology spectrum to the most energy-efficient resource available.

Energy Efficiency (including Demand Response): Initially, the most cost efficient power resources are energy efficiency measures, which require less energy generation. Whether on-grid or off-grid, insulation can be used in hot or cold climates to radically reduce the need for generation resources. Once insulation is in place, heating and cooling technologies (i.e., heat pumps) can be the next major contributor to reducing the need for generation. These technologies can be supplemented with energy-efficient lighting (i.e., fluorescent or Light Emitting Diode [LED]). The downside of these efficiency technologies involves costs associated with building to higher standards (usually very small cost) or retrofitting existing construction to include these technologies. Taking these energy-efficiency measures can usually be justified through lifecycle cost analysis.

Internal Combustion Engines (ICE) (Fossil fuels - liquid & gas): Diesel backup generators (BUGS) are the most common form of generation used in grid-connected or remote micro-grids. BUGS are found in commercial buildings, on

military bases, and in remote locations around the world. They have the advantage of being ubiquitous technology with a pervasive distribution and support system. Because of the military’s focus on load balancing between generators in micro-grids, vendors such as Caterpillar and Woodward are developing systems to allow control of legacy and other vendor’s equipment with a single controller. This control requires a communications link, which is usually a data cable and knowledge of each generators sensors and control points. Currently this load balancing capability exists only in larger (500 KW or bigger) generators, but it will soon be available for much smaller systems.

Micro turbine technology: Capstone is the epitome of micro turbine technology. These units range from 60 kW up to 1000 kilowatts. The technology has proven itself in the last decade in a wide range of environments, with over 5,000 units used in a full range of extreme environments (e.g., arctic, desert, marine). The advantage of micro turbine systems in hot climates is that waste heat potentially can be used, along with a direct evaporation cooler, to provide cooling. In temperate climates, micro-turbines can provide both heating and cooling from waste heat streams. Another advantage of micro turbines is that they have much longer maintenance cycles than diesel generators (40,000 hours [five years]) between core change out versus thousands of hours for diesel generators. Combustion turbine technology is a sister technology of micro turbines. they are larger, more efficient, and have a better heat rate. Combustion turbines range from one megawatt through hundreds of megawatts and are used by utilities, industrial plants, and energy companies around the world.

Photovoltaic (PV) systems: PV systems are covered in more detail in this paper because they are the focus of most on-grid and off-grid micro-grid systems.

Systemic pressures to pursue sustainable, net-zero buildings and communities, reduce emissions, and to “go green” have brought renewable energy technologies to the forefront of public policies, government politics, and business practices. Due to the higher capital costs and lower capacities historically associated with solar PV technologies, their installation rate has lagged behind other common renewable energy technologies. However, the synergistic combination of more efficient, low cost, processing methods; production economies of scale; a highly competitive environment; proactive government action; and public interest has allowed PV systems to flourish.

Traits of PV Systems: A number of characteristics that span most module and mounting options can typify PV systems. First, because of the inherent nature of electricity generation via PV effect, PV arrays are highly reliable and often designed with no moving components (the natural exemption being axis tracking systems). Second, PV arrays require careful integration into electric grids because of the


intermittent nature of the solar resource. Consequently, it is not uncommon to limit the amount of PV derived electricity that can be delivered to a local grid. Careful monitoring is required to accommodate the natural fluctuations and variability of solar electricity and to ensure grid reliability.

Related to the natural variability of the solar resource, capacity factors (i.e., the ratio of the output of a power plant over a period relative to its nameplate capacity) for most solar arrays are lower than most other renewable energy technologies. Capacity factors above 20% are generally regarded as good, and rates above 25% typically require more advanced systems employing axis tracking mounts. Consequently, solar arrays frequently struggle to cost-effectively produce electricity unless economic incentives are available or government policies create a premium for solar electricity. However, the cost per watt of PV systems has dropped faster than nearly any other commercial renewable energy technology, allowing PV arrays to become increasingly cost competitive.

Inverter Technologies: The nature of the PV effect results in the production of direct current (DC) energy. To be used in the national grid, solar derived electricity must be converted, via an inverter, into alternating current (AC). As with PV modules, inverters have improved in terms of reliability, efficiency, and price [28]. Consequently, energy output per unit of installed capacity has increased while PV array operations and maintenance (O&M) costs, and the levelized cost of produced electricity have decreased.

Arguably, the largest systemic shift in inverters relative to PV arrays is the development and increasing deployment of micro inverters. Whereas traditional inverters convert the current from a large number of panels from DC to AC, micro inverters typical convert the output of a single module to AC. These inverters have several advantages including the minimization of long DC line runs (and their subsequent line loss), redundancy, and panel level employment (as opposed to string or array level) of maximum power point tracking (a technology used to control the current flowing through a PV system to optimize the power output). The principal disadvantage is that these systems are more expensive and require additional O&M.

PV Technologies and Mounting Methods

Since the development of the first solar cell sixty years ago, an explosion of research as yielded dozens of different material combinations that exhibit the PV effect. A number of these material systems are still experimental or commercial only for relatively exotic applications (e.g., space satellites). Of the dozens of options available, a handful have become relatively common (Table 1):

Monocrystalline Silicon. Monocrystalline solar cells feature a single, thinly cut silicon crystal per individual PV cell. The use of a single crystal allows electrons to flow relatively freely when exposed to sunlight. This allows greater operational efficiency, which results in higher electricity production per unit area of material. However, it also requires somewhat expensive processing techniques, which can translate to higher expenses.

Polycrystalline Silicon. Polycrystalline silicon solar cells feature several silicon crystals per cell. The interface between crystals results in barriers to electron migration, which results in lower efficiencies than with monocrystalline cells. However, these cells are typically cheaper to manufacture than monocrystalline cells.

Amorphous Silicon. Often referred to as a-Si, amorphous silicon is classified as a “thin film” PV technology because of the minimal amount of material needed to manufacture the cell. This small amount of material also is beneficial because it allows some a-Si modules to be flexible. However, the amorphous nature of the material poses a hindrance to electron migration, which then results in lower cell efficiency. The principal benefit of these cells is lower fabrication costs, which can result in lower system costs.

Cadmium Telluride (CdTe). CdTe cells are also regarded as thin film PV cells. They also employ lower cost fabrication techniques that result in low dollar per watt module costs. However, the efficiency of these cells tends to be lower than monocrystalline silicon and high quality polycrystalline solar cells. The use of cadmium, a known toxic heavy metal, is also often a concern.

Table 1. Common PV Module Technologies

Cell Type Cost (large scale array),

$/watt (DC) Efficiency

(typical), % Lifespan, years Footprint, sf/kW (DC) State of

Technology Monocrystalline Silicon 2.29* [23] 22-25 25-30 40 – 50 Commercial Polycrystalline Silicon 1.85 [24] 15-20 25-30 55 – 70 Commercial Amorphous Silicon <1.0** [25] 5-10 25-30 100 – 200 Commercial CdTe <1.0*** [26] 15-17 25-30 50 – 60 Commercial *Solar Buzz 2011 ** Renewable Energy World 2010a ***Renewable Energy World 2010b.


Table 1 documents several of the most salient qualities for common solar cell materials. Note that cost data represents the cost per watt on a module basis and does not include typical balance of system (BOS) components including mounts, inverters, and wiring. Currently, module costs typically outweigh BOS costs. However, as module costs drop, BOS costs will represent a larger fraction of the gross array investment. Eventually, BOS costs may outweigh module costs for an array and subsequent drops in module cost will have smaller net impacts on the cost of a solar array. In short, focusing on module cost to the detriment of BOS costs will keep net PV array costs unexpectedly high. Lastly, the cost represents the dollar per watt for a DC system, which means that after inverter losses [27], the cost per watt (AC) will be higher.

Other semi-common solar cells are fabricated from gallium arsenide (GaAs), Copper indium gallium (di)selenide (CIGS), conductive organic compounds, and various other multi-junction combinations of semiconductors. However, these cells are uncommon generally because of system cost, fabrication complexity, and material scarcity.

PV Mounting: In addition to the module material, PV array mounting technology can heavily influence energy production and system economics. The most common array mounting technique is to ground mount the array at a fixed angle – typically the site’s latitude, which generally optimizes energy production. Axis tracking arrays rotate the panels to follow the progression of the sun across the sky. Single axis tracking mounts rotate the panels across one axis (typically the north-south axis). Dual axis tracking mounts rotate the panels along the north-south and east-west axis, which allows for the most optimal level of solar energy harvesting. Axis tracking systems are more complex than fixed axis systems, increase initial system costs (typically by $1 – 2 per watt), and have additional O&M needs.

Lastly, PV arrays can be mounted on buildings. While building mounts can result in suboptimal module orientation, they are less expensive than ground mounts and usually result in lower overall system costs.

Additional Information

Wind: large wind turbines (1 MW plus) are a commercial product generally used in utility-scale projects by independent power developers for sale to utilities (power purchase agreement). Projects range in size from 50 MW to over 300 MW. These large systems are not normally included in micro-grid projects (practically or conceptually) except for rare examples (i.e., FE Warren Air Force Base project). If wind generation is considered, the issue of radar interference needs to be addressed. The following headlines and their links should provide minimal background on wind generation issues and potential technology vendors:

Wind Turbine Projects Run Into Resistance – Barstow CA [28]

Wyden, Merkley, Praise DOD Decision to Approve New Wind Farms in Oregon [29]

A presentation [30] from GovEnergy, entitled Energy Security & National Defense: Shepard’s Flat Wind Farm gives full coverage of the issues and politics of this issue.

Top 10 large wind turbine manufacturers by annual market share in 2009 from Wikipedia [31].


1. Vestas 12.5% 2. GE Wind Energy 12.4%

3. Sinovel 9.2% 4. Enercon 8.5%

5. Goldwind 7.2%

6. Gamesa 6.7%

7. Dongfang Electric 6.5%

8. Suzlon 6.4%

9. / Siemens Wind Power 5.9% 10. REpower 3.4%

Biomass, Combined Heat and Power – CHP (Waste to Energy), Hydro, and GeoThermal:

These generation systems will not be covered in this paper because they are currently infrequently implemented in micro-grid use.

Enabling technologies: services provided, smooth transitions, required equipment, and control

Two technologies, high-speed supervisory control and data acquisition- (SCADA-) enabled utility islanding and energy storage are required to make smooth transitions and provide uninterrupted power supply (UPS) to the distribution system or buildings involved.

SCADA system: High speed relaying is necessary to make fast transitions (ms) to assure the safety of equipment and loads. The utility gold standard is Schweitzer Engineering Lab [32] (SEL) – the company that created digital relaying. SCADA systems provide a real-time operational interface for micro-grids. SCADA systems also enable paralleling of generations sources to allow energy sales back to the grid.

Energy Storage: This topic covers the battery flywheel and power conditioning systems needed to integrate resources and provide UPS-like capabilities for a smooth transition to and from grid power. Two resources for this topic are the Electricity Storage Association [33] (ESA) website and the Sandia National Labs [34] (SNL) website.

Summary of Technology Considerations/Limitations: use commercially available technology, use consultants (don't expect miracles) with technology specific experience and references, talk with your utility, if grid connected, there is a maintenance budget (nothing lasts forever), consider potential escalation of fuel prices if using fossil fuels, simple & dumb systems with reliable operators can outperform a badly programmed complex, simple systems cost less and function all complex systems are expensive, reliable power is priceless in

an emergency, be prepared to learn a lot and enjoy the trip because life is too short not to.

Getting it built – real world considerations and advice [35]

Assess load to be served (size, location, and criticality)

List of energy sources onsite and offsite

Define resources already in place and additional resources needed to serve critical loads

Define thermal requirements and applications

Describe deliveries and storage of all fossil fuels

Identify critical facilities based on mission accomplishment

Analyze potential threats, list unacceptable risks, and estimate the duration of impact

Establish the energy preparedness and operations plan

Define a “Plan B” for continuing operations

Be aware of functional agreements with utilities (electric and gas)

Inventory and manage emergency generators

Identify remedial actions for unacceptable risks

Other Practical Considerations:

funding based on project size & specific financing options

decide whether to use consultants

tailor the product to your site’s mission

assure information security (cyber)


REFERENCES

[1] CERTS – Consortium for Electric Reliability Technology Solutions, 2011, “Distributed Energy Resources Integration,” http://certs.lbl.gov/certs-der.html

[2] U.S. DOE – U.S. Department of Energy, 2009, “Smart Grid System Report,” p. 18, http://www.oe.energy.gov/DocumentsandMedia/SGSRMain_090707_lowres.pdf

[3] R&D Magazine, 2011, “Microgrid to Deploy at UC San Diego Campus,” http://www.rdmag.com/News/2010/04/Industries-Energy-Utilities-Microgrid-to-deploy-at-UC-San-Diego-campus/

[4] PRWeb, 2011, “First Micro-Grid, Distributed Energy Resource Community Coming to California,” SFGate, http://www.sfgate.com/cgi-bin/article.cgi?f=/g/a/2011/02/01/prweb5023374.DTL

[5] http://www.gc.doe.gov/NEPA/documents/CXdbRawData.xls

[6] U.S. DOE – U.S. Department of Energy, 2008, “Tribal Energy Program: Information Resources,” http://apps1.eere.energy.gov/tribalenergy/wind.cfm

[7] Baring-Gould, E. I., n.d., “Wind/Diesel Power Systems Basics and Examples,” http://apps1.eere.energy.gov/tribalenergy/pdfs/wind_akwd04_basics.pdf

[8] Defreitas, S., 2011, “Ge Ecomagination: Mera Gao Micro-grid Power,” Earthtechling, Llc., Http://Www.Earthtechling.Com/2011/02/Ge-Ecomagination-Mera-Gao-Micro-Grid-Power/

[9] DSB – Defense Science Board, 2008, “Report of the Defense Science Board Task Force on DoD Energy Strategy,” Office of the Under Secretary of Defense for Acquisition, Technology, and Logistics, Washington, D.C., http://www.acq.osd.mil/dsb/reports/ADA477619.pdf

[10] http://www.youtube.com/watch?v=m59oyx0cwEw

[11] SBI Energy, 2011, “The World Market for Microgrids,” SBI Energy Publication No. SB2835891.

[12] Snider, A., 2011, “Energy, Environment Opportunities Grow Despite DOD Belt-Tightening,” The New York Times, http://www.nytimes.com/gwire/2011/01/26/26greenwire-energy-environment-opportunities-grow-despite-21875.html?pagewanted=2

[13] Abdallah, T., 2009, “Charting a Course to Energy Independence,” GovEnergy 2009, Providence, RI, http://www.govenergy.com/2009/pdfs/presentations/Security-Session04/EnergySecurity-Session04-Abdallah_Tarek.pdf

[14] Warwick, W.M., Myers, K.S., and Seifert, G.D., 2010, “Demonstration of Security Benefits of Renewable Generation at FE Warren Air Force Base,” Technical Report PNNL-20045, Pacific Northwest National Laboratory, Richland, WA.

[15] Maron, D.F., 2010, “Can a Microgrid Protect U.S. Troops in Afganistan?,” Scientific American, http://www.scientificamerican.com/article.cfm?id=can-a-microgrid-protect

[16] Natural Resources Canada, 2010, “RETScreen® International,” http://www.retscreen.net/ang/home.php

[17] NREL – National Renewable Energy Laboratory, n.d., “HOMER: The Optimization Model for Distributed Power,” https://analysis.nrel.gov/homer/

[18] Baring-Gould, I., n.d., “The Hybrid Power System Simulation Model,” http://www.ceere.org/rerl/rerl_hybridpower.html

[19] Dugan, R., Henry, R., McDermott, T., and wsunderm1, 2011, “OpenDSS,” Distribution System Simulator, http://sourceforge.net/projects/electricdss/

[20] http://www.gl-group.com/en/8672.php

[21] CYME International, 2011, “CYME: Power Engineering Software Solutions,” http://www.cyme.com/

[22] Milsoft Utility Solutions, Inc., 2010, “Industry News,” http://www.milsoft.com/smart-grid/

[23] solarbuzz®, 2011, “Retail Pricing Environment: Module Pricing,” http://www.solarbuzz.com/facts-and-figures/retail-price-environment/module-prices

[24] solarbuzz®, 2011, “Retail Pricing Environment: Module Pricing,” http://www.solarbuzz.com/facts-and-figures/retail-price-environment/module-prices


[25] RenewableEnergyWorld.com eds., 2011, “First Solar No Longer Number 1,” http://www.renewableenergyworld.com/rea/news/article/2010/11/first-solar-no-longer-number-1

[26] RenewableEnergyWorld.com Ed., 2011, “First Solar No Longer Number 1,” http://www.renewableenergyworld.com/rea/news/article/2010/11/first-solar-no-longer-number-1

[27] NREL – National Renewable Energy Laboratory, 2006, “A Review of PV Inverter Technology Cost and Performance Projections,” Navigant Consulting Inc. Subcontract Report NREL/SR-620-38771, Burlington, MA.

[28] Vestel, L.B., 2010, “Wind Turbine Projects Run Into Resistence, The New York Times, http://www.nytimes.com/2010/08/27/business/energy-environment/27radar.html

[29] Wyden, R., 2010, “Press Release of Senator Wyden: Wyden, Merkley Praise DOD Decision to Approve New Wind Farms in Oregon, “http://wyden.senate.gov/newsroom/press/release/?id=081a8b6a-789a-4e22-a014-77e71502fc25

[30] Cotton, C., 2010, “Energy Security & National Defense: Shepard’s Flat Wind Farm,” GovEnergy 2010, Dallas, TX, http://www.govenergy.com/2010/Files/Presentations/Energy%20Security/Energy%20Security%20S4%20-%20CCotton.pdf

[31] Wikipedia, 2011, “List of Wind Turbine Manufacturers,” http://en.wikipedia.org/wiki/List_of_wind_turbine_manufacturers

[32] SEL – Schweitzer Engineering Laboratories, Inc., n.d., “Home page”, http://www.selinc.com/default.aspx

[33] ESA – Electricity Storage Association, 2009, “Home Page,” http://www.electricitystorage.org/ESA/home/

[34] SNL – Sandia National Laboratories, 2011, “Energy Storage Systems Program (ESS),” http://www.sandia.gov/ess/

[35] ORNL - Oak Ridge National Laboratory, n.d., “Federal Energy Management Program: Performing Energy Security Assessments – A How-To Guide for Federal Facility Managers,” http://www1.eere.energy.gov/femp/pdfs/energy_security_guide.pdf

ES2011-54833

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