An Overview of Solar Technologies

An Overview of Solar Technologies

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IEEE power & energy magazine

U.S. Government work not protected by U.S. Copyright

may/june 2009

N By Benjamin Kroposki, Robert Margolis, and Dan Ton

NOW IS THE TIME TO PLAN FOR THE integration of significant quantities of solar energy into the electricity grid. Although solar energy constitutes a very small portion of our energy system today, the size of the resource is enormous: The earth receives more energy from the sun in one hour than the global population uses in an entire year. In addition, the solar photovoltaic (PV) industry is growing very rapidly, sustaining an annual growth rate of more than 40% for the last decade. The combination of this rapid growth, falling costs, and a vast technical potential could make solar energy a serious contender for meeting our future energy needs. The steep growth in solar energy markets during the past decade has been driven by a growing concern about climate change, the adoption of state-level renewable portfolio standards and incentives, and the accelerated reduction in system costs. In particular, both the number and scale of distributed PV installations have been growing very rapidly (Figure 1). With the maturation of distributed and utility-scale PV (Figure 2) as well as concentrating solar power (CSP), solar technologies can—and likely will—begin to penetrate a range of energy markets and play an increasingly significant role in meeting our nation’s energy demand. However, as the market share of PV and CSP grows, concerns about the potential impacts on the stability and operation of the electricity grid might create barriers to their future expansion.

Solar Basics PV and CSP technologies both use the sun to generate electricity. But they do it in different ways. PV— or solar electric—systems use semiconductor solar cells to convert sunlight directly into electricity. In contrast, CSP—or solar thermal electric—systems use mirrors to concentrate sunlight and exploit the sun’s thermal energy. This energy heats a fluid that can be used to drive a turbine or piston, thus producing electricity. (The heated fluid can also be used directly for applications such as industrial process heat.) Simply put, PV uses the sun’s light to generate electricity directly, whereas CSP uses the sun’s heat to generate electricity indirectly. Again, PV and CSP both use the sun to generate electricity. But they use different forms of the sun’s radiation. The earth’s surface receives sunlight in either a direct or diffuse form. Direct sunlight is solar radiation whose path is directly from the sun’s disk and shines perpendicular to the plane of a solar device. This is the form used by CSP systems and concentrating PV systems, in which the reflection or focusing of the sun’s light is essential may/june 2009

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PV Shipments (MW)

Although solar energy constitutes a very small portion of our energy system today, the size of the resource is enormous.

3,800 3,600 3,400 3,200 3,000 2,800 2,600 2,400 2,200 2,000 1,800 1,600 1,400 1,200 1,000 800 600 400 200 0

3,733 Rest of the World Europe Japan United States 2,500

Source: PV News, March 2008 Revised: 04-02-08

1,782 1,194 748 201 47 55 58 60 69 78 89 126 155

277

386

547

solar radiation on a latitude-tilt surface in the United States. The maps show that direct sunlight is even more concentrated in the Southwest than global radiation is. Hence, the desert Southwest is often referred to as “the Saudi Arabia of the sun.” Both maps show that the United States has world-class resources, and our worst resources are as good as the typical resources of Germany and Japan, which historically have had much larger solar markets.

PV Technologies

Just as PV and CSP are different forms of solar energy technologies, there is also a range of different PV figure 1. The shipment of PV cells and modules continues to show tremendous materials and designs for generating growth (data: Prometheus, 2008). electricity—crystalline silicon, thin to the electricity-generating process. Flat-plate, or non- films, concentrating PV, and future-generation PV, along concentrating, PV systems can also use direct sunlight. with associated balance-of-systems components. Figure 3 shows the annual direct-normal solar radiation Crystalline Silicon map of the United States. Other solar radiation is diffuse, meaning the sunlight Silicon was one of the first materials to be used in early PV reaches the earth’s surface after passing through thin cloud devices, and it continues to dominate the commercial solar cover or reflecting off of particles or surfaces. Global ra- cell market at more than 90% of market share. Cells using diation is the sum of the direct and diffuse components silicon have been labeled as first-generation PV. Pure silicon of sunlight. And this global radiation, as well as direct is “doped” with minute amounts of other elements such as boor diffuse radiation alone, can be used by flat-plate PV ron and phosphorus, which produces positive- and negativesystems to generate electricity. Figure 4 shows the annual type semiconductor materials, respectively. Putting the two materials in contact with one another creates a built-in potential field. And when this semiconductor device is illuminated, the energy of the sunlight frees electrons that then move out of the cell—due to the potential field—into wires that form an electrical circuit. This “photovoltaic” effect requires no moving parts and does not use up any of the material in the process of generating electricity. As shown in Figure 5, a typical solar cell consists of a glass or plastic cover or other encapsulant, an antireflective surface layer, a front contact to allow electrons to enter a circuit, a back contact to allow the electrons to complete the circuit, and the semiconductor layers where the electrons begin and complete their journey. ’90 ’91 ’92 ’93 ’94 ’95 ’96 ’97 ’98 ’99 ’00 ’01 ’02 ’03 ’04 ’05 ’06 ’07

figure 2. Nellis Air Force Base in Nevada boasts a 14-MW PV system, currently the largest PV system in the United States (credit: U.S. Air Force). 24

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Thin-Film PV Second-generation PV devices are a more recent development, and they rely on layers of semiconductor materials that may/june 2009

are much thinner than those in silicon cells. The thickness of a crystalline silicon (c-Si) cell may be 170 to 200 µm (10-6 m), whereas the active region in a thin-film cell is on the order of only 2 to 3 µm thick. (For comparison, a human hair has a thickness of about 80 µm.) If silicon is used, it is typically in the form of amorphous silicon (a-Si), which has no discernible crystal structure; in addition, microcrystalline silicon thin-film devices are under development. But other thin-film materials have also been developed and commercialized, including cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS). These PV devices require much less material than do traditional c-Si devices. So why don’t we just use thin-film cells if they have these desirable qualities? A key factor is that thin films, when compared with c-Si cells, generally have a lower solar conversion efficiency—which is the percentage of the sun’s power shining on the cell that is converted into electrical power by the cell. For example, if 1,000 W of solar power illuminate a cell, and 200 W of electricity are generated, then the cell has a solar conversion efficiency of 20%. A commercial silicon cell may have an efficiency of around 20%, whereas a commercial CdTe cell’s efficiency is in the range of 11%. The thin-film cell uses less material and can be deposited with a method that is much less energyintensive than silicon. Less material also equates to lighter weight. And some thin-film technologies do not rely on rigid wafers; rather, they can be deposited on flexible substrates of stainless steel or plastic (Figure 6). Flexibility may be a desirable aspect depending on the application. So, in theory, thin-film PV should be less expensive to manufacture and easier to integrate into a wide range of applications. However, in practice, the jury is still out with respect to which PV technologies will be dominant in the marketplace.

a lens, but at about 1/400 of the cell cost. Figure 8 shows a typical basic concentrator unit that consists of a lens to focus the light, cell assembly, housing element, a secondary concentrator to reflect off-center light rays onto the cell, a

figure 3. Solar radiation map of the United States, showing the intensity of direct-normal sunlight averaged over a year (source: NREL).

Concentrating PV Another type of second-generation PV device is the highefficiency multijunction cell that uses compounds from the group III and group V elements of the periodic table of elements. This type of multijunction cell (Figure 7), for example, could be a top layer of gallium arsenide, a middle layer of gallium indium phosphide, and a bottom layer of germanium. Very high efficiencies—close to 40%—can be generated by this scheme. This is because each layer in this multijunction cell is designed to absorb and use a different portion of the solar spectrum. Again, if this type of cell has such high efficiency, why don’t we just use this for our applications? The answer has to do with cost: These III-V materials are expensive to produce. One way to overcome the cost issue is to use these cells in a concentrator system, in which a relatively inexpensive lens or mirror can be used to focus sunlight on just a small area of cells. So, for example, if a 10-by-10-in lens focuses this area of incident sun onto a 0.5-by-0.5-in cell, the concentration factor is 400x (100 in2 / 0.25 in2). This cell with the lens can produce as much power as a 10-by-10-in cell without may/june 2009

figure 4. Annual solar radiation on a latitude-tilt surface in the United States (source: NREL).

Sunlight Antireflection Coating Transparent Adhesive Cover Glass

n-Type Semiconductor p-Type Semiconductor

Current Front Contact

Substrate Back Contact

figure 5. The basic components of a silicon PV cell (source: NREL). IEEE power & energy magazine

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concentrator systems could be very effective in largescale power generation. The STAR Center unit (Figure 9) produces 20 kW of electricity—enough to power about five homes. Systems like these may someday supply power to entire communities. The challenge lies in developing concentrating systems that balance the overall system-level costs so that they can be competitive in the marketplace. Again, the jury is still out with respect to which type of concentrating PV system, if any, will be able to compete widely in the marketplace.

Future Technologies

figure 6. Copper indium gallium diselenide can be deposited on rolls of lightweight, flexible polymer material to produce thin-film solar modules (used with permission from Global Solar Energy).

mechanism to dissipate excess heat produced by concentrated sunlight, and various contacts and adhesives. Notice that the module uses 12 cell units in a 2x6 matrix. These basic units may be combined in any configuration to produce a module with the desired power output. Concentrator systems currently under development range in concentration levels from tens (10x) to hundreds (100x). Although they are not suitable for small projects,

Scientists are exploring approaches for third-generation solar cells. One pathway is that of very high-efficiency cells, with their attendant high costs. The push is toward reaching the theoretical limits of various material systems and device configurations. The highest-efficiency device to date is a GaInP/GaAs/GaInAs multijunction, which has a 40.8% conversion efficiency under 326x concentration. The other pathway is that of very low-cost cells, but with lower efficiencies. An example is the so-called dyesensitized cell, which operates under a totally different physical paradigm that uses dye molecules adsorbed into very small spheres of titanium dioxide. To date, this photoelectrochemical device has been able to generate electricity at efficiencies exceeding 10% on small areas. Again, this efficiency is relatively low, but the simplicity of the materials and structure make the device very inexpensive to manufacture.

Sunlight Fresnel Lenses

Metal Grid

Housing Secondary Optics

Ga0.5In0.5 P Cell

Cell Assemblies Cell Assembly

Ga(In)As Cell

Tunnel Junctions

Secondary Concentrator Electrical Contact Cell and Prismatic Cover Solder

Ge Substrate and Cell

Copper Heat Spreader and Electrical Contact Conductive Adhesive Metal Contact

figure 7. A typical multijunction solar cell design showing three layers, each of which absorbs a different portion of the solar spectrum to use in generating electricity (source: NREL). 26

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Module Housing

figure 8. Concentrating PV system that uses a lens to concentrate sunlight onto a high-efficiency solar cell. may/june 2009

Other third-generation solar cell approaches, some of which are still in the conceptual phase only, include the advanced cells based on quantum dots, organic PV, intermediate-band cells, and multiple-exciton generation. Verified efficiencies are typically in the single digits. However, there is considerable opportunity for technological innovation and improvement, and the potential for very low manufacturing costs has created a serious buzz about these technologies in both the research and development (R&D) and investment communities.

If users have stand-alone systems—with no grid connection—then the BOS will include batteries and charge controllers to produce electricity at night or during

PV Balance of Systems Balance of systems (BOS) includes all of the components of a PV system beyond the actual PV module that produces the power. A frame structure might be needed to hold the module; keep it oriented toward the sun; and stabilize it in the outdoor elements, such as wind and snow. PV systems produce direct-current (dc) electricity. Therefore, if alternating current (ac) is required for a particular application, the BOS must include an inverter. This component usually decreases the overall system efficiency by some 5–10% and typically has the greatest reliability problems of any component in the system. System efficiency can be boosted by attaching a tracker to the solar modules. Single-axis trackers (Figure 10) aligned with the axis in a north-south direction allow the module to follow the sun’s progress across the sky from east to west during the day. Dual-axis trackers further refine the module’s orientation, allowing the sun to always illuminate the cells perpendicular to the plane of the module (Figure 11). This geometry facilitates maximum energy output from the system; yet, it adds cost and also typically requires additional spacing to prevent one module from shading an adjacent one. Flat-plate PV systems do not need trackers but will produce about 25% more energy if one is used. However, concentrator PV systems require direct-normal radiation, making a tracking system essential. These concentrator systems also require a lens or mirror to focus the sunlight onto the solar cells. Fresnel lenses extruded from acrylic are a typical option for concentration. But mirrors can also provide concentration. They require a thin metal layer that must be protected from oxidization and adhered to a rigid smooth surface (like glass). This requires a sturdy frame and robust tracking mechanism to maintain optical accuracy and structural integrity against wind. In most cases, residential and commercial PV systems are directly connected to the grid without energy storage. Users draw power from the utility during hours of darkness or on cloudy days. As we move toward a “smart” grid in the future, more systems may incorporate battery energy storage to increase reliability—if battery costs decrease or grid reliability is an issue. One potential way to incorporate energy storage with PV systems would be to use the plug-in electric hybrid vehicles as energy storage for the PV systems if these vehicles are used on a large scale. may/june 2009

figure 9. A high-performance concentrating PV system operated by Arizona Public Service (used with permission from APS).

figure 10. Curved plastic Fresnel lenses focus sunlight onto a small area of high-efficiency PV cells using one-axis tracking (used with permission from PVI).

figure 11. This two-axis tracking system keeps the solar modules facing directly at the sun throughout the day, which boosts the power output (used with permission from NREL). IEEE power & energy magazine

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cloudy conditions. The batteries store excess power that is generated from the PV array to be used later, and the charge controllers regulate the current to the batteries to prevent overcharging.

Steam Condenser Electricity Thermal Storage Tanks Receiver Generator Turbine Parabolic Troughs

figure 12. Schematic showing the basic operation of a parabolic trough CSP system.

CSP Technologies CSP technologies use mirrors to reflect and concentrate sunlight onto receivers that collect the solar energy and convert it into heat. This thermal energy can then be used to produce electricity via a steam turbine or heat engine driving a generator. CSP systems are typically classified by how the various systems collect solar energy. The three main systems are the linear, tower, and dish systems.

Linear CSP Systems

figure 13. Field of parabolic troughs at the Solar Energy Generating Station in Kramer Junction, California (source: NREL).

Steam Condenser Electricity

Generator Turbine

Receiver Linear Fresnel Reflectors

figure 14. Schematic showing the basic operation of a linear Fresnel reflective system. 28

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Linear CSP collectors capture the sun’s energy with large mirrors that reflect and focus the sunlight onto a linear receiver tube. The receiver contains a fluid that is heated by the sunlight and then used to create superheated steam that spins a turbine driving a generator to produce electricity. Alternatively, steam can be generated directly in the solar field. In this method, no heat exchanger is used, but the system uses costly pressure-rated piping throughout the entire solar field and typically has a lower operating temperature. Linear concentrating collector fields consist of a large number of collectors in parallel rows that are typically aligned in a north-south orientation to maximize both annual and summertime energy collection. With a single-axis sun-tracking system, this configuration enables the mirrors to track the sun from east to west during the day, ensuring that the sun reflects continuously onto the receiver tubes. The predominant CSP systems currently in operation in the United States are linear concentrators using parabolic trough collectors (Figure 12 and Figure 13). In typical systems, the receiver tube is positioned along the focal line of each parabola-shaped reflector. The tube is fixed to the mirror structure, and the heated fluid—commonly a hightemperature oil—flows through and out of the field of solar mirrors to where it is used to create steam, and the steam is then sent directly to the turbine. Currently, the largest individual trough systems generate 80 MW of electricity. However, individual systems being developed will generate 250 MW. In addition, individual systems can be collocated with power plants. This capacity would be constrained only by transmission capacity and availability of contiguous land area. Trough designs can incorporate thermal storage. In such systems, the collector field is oversized to heat a storage system during the day that in the evening can be used to generate additional steam to produce electricity. Parabolic trough plants can also be designed as hybrid systems that use fossil fuel to supplement the solar output during periods of low solar radiation. In such a design, a natural-gas-fired heater or gas-steam boiler/reheater is used. Troughs may may/june 2009

PV and CSP technologies both use the sun to generate electricity, but they do it in different ways. also be integrated with combined-cycle natural-gas- and coal-fired plants to improve the plant heat rate or provide a peaking boost to the steam turbine in a combined-cycle plant, much like a duct burner does. A second linear concentrator technology is the linear Fresnel reflective system (Figure 14). Flat or slightly curved mirrors mounted on trackers on the ground are configured to reflect sunlight onto a receiver tube fixed in space above these mirrors. A small parabolic mirror is sometimes added atop the receiver to further focus the sunlight.

Electricity

Steam Condenser Receiver Feedwater Reheater

Generator Turbine Steam Drum

Power Tower CSP Systems In this CSP technology, numerous large, flat, sun-tracking mirrors, known as heliostats, focus sunlight onto a receiver at the top of a tower (Figure 15). A heat-transfer fluid heated in the receiver is used to generate steam, which in turn is used in a conventional turbine generator to produce electricity. Some power towers use water/steam as the heattransfer fluid. Other advanced designs are experimenting with molten nitrate salt because of its superior heat-transfer and energy-storage capabilities. Individual commercial plants can be sized to produce up to 200 MW of electricity. Two large-scale power tower demonstration projects have been deployed in the United States. During its operation from 1982 to 1988, the 10-MW Solar One plant near Barstow, California, demonstrated the viability of power towers, producing more than 38 million kWh of electricity. The Solar Two plant was a retrofit of Solar One to demonstrate the advantages of molten salt for heat transfer and thermal storage. Using its highly efficient molten-salt energy-storage system, Solar Two successfully demonstrated efficient collection of solar energy and dispatch of electricity. It also demonstrated the ability to routinely produce electricity during cloudy weather and at night. In one demonstration, Solar Two delivered power to the grid for 24 h a day for almost seven consecutive days before cloudy weather interrupted operation. Currently, Spain has several power tower systems operating or under construction. Planta Solar 10 (Figure 16) and Planta Solar 20 are water/steam systems with capacities of 11 MW and 20 MW, respectively. Solar Tres will produce some 15 MW of electricity and have the capacity for moltensalt thermal storage. Power towers also offer good longer-term prospects because of the high solar-to-electrical conversion efficiency. Additionally, costs will likely drop as the technology matures. may/june 2009

Heliostats

figure 15. Schematic showing the basic operation of a power tower (or central receiver) CSP system (source: NREL).

figure 16. Heliostats surround part of the central power tower at Planta Solar 10 in Spain (used with permission from Solucar Energia/Abengoa).

Power Conversion Unit

Concentrator Electricity

figure 17. Schematic showing the basic operation of a dish/engine CSP system (source: NREL). IEEE power & energy magazine

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PV—or solar electric—systems use semiconductor solar cells to convert sunlight directly into electricity.

Dish/Engine CSP Systems Dish/engine systems produce relatively small amounts of electricity—typically in the range of 3 to 25 kW— compared with other CSP technologies. A solar concentrator, or dish, gathers the solar energy coming directly from the sun. The resulting beam of concentrated sunlight is reflected onto a thermal receiver that collects the solar heat (Figure 17). The dish is mounted on a structure that tracks the sun continuously throughout the day to reflect the highest percentage of sunlight possible onto the thermal receiver. The power conversion unit includes the thermal receiver and the engine/generator. The thermal receiver is the interface between the dish and the engine/generator. It absorbs the concentrated beams of sola r energ y, conver ts t hem to heat, and transfers the heat to the engine/generator. A thermal receiver can be a bank of tubes with a cooling fluid—usually hydrogen or helium—that typically is the heattransfer medium and also the working fluid for an engine.

figure 18. Silhouette of a Stirling Energy Systems (SES) 25-kW dish/Stirling system moving into its stowed position at the end of the day (used with permission from Stirling Energy Systems). 30

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Alternate thermal receivers are heat pipes, where the boiling and condensing of an intermediate fluid transfers the heat to the engine. The engine/generator system is the subsystem that takes the heat from the thermal receiver and uses it to produce electricity. Currently, the most common type of heat engine used in dish/engine systems is the Stirling engine (Figure 18). A Stirling engine uses the heated fluid to move pistons and create mechanical power. The mechanical work, in the form of the rotation of the engine’s crankshaft, drives a generator and produces electrical power.

Thermal Energy Storage In a CSP system, the solar field includes the mirrors, trackers, and receivers needed to collect the sun’s thermal energy. A second component is the power plant, which may include a turbine and generator or a Stirling heat engine. A third component is thermal energy storage (TES) (Figure 19), which addresses one of the challenges facing the widespread use of solar energy—the problem of reduced or curtailed energy production when the sun sets or is blocked by clouds. TES provides a workable solution to this challenge. In a CSP system, if the receiver contains oil or molten salt as the heat-transfer medium, then the thermal energy can be stored for later use. This allows CSP systems to be a cost-competitive option for providing clean, renewable energy.

figure 19. Thermal-storage tanks for a Planta Solar 10 power tower near Seville, Spain (used with permission from Solucar Energia/Abengoa). may/june 2009

Growing in Energy Market Share Renewable energy technologies, such as PV and CSP, are expected to become a larger part of our energy portfolio during the next couple of decades. As stated earlier, the reasons may/june 2009

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4 Residential and Commercial Rates Utility Generation

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2005 Year

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Cumulative U.S. Installed Capacity (GW)

The Future of Solar

Cost of Energy in Cents/kWh ($2005)

Cumulative Installed Base of PV (GW)

Base +NetMeter +full ITC +Intercon

Several TES technologies have 26 been tested and implemented since 1985. Two-tank systems have one Historical Projected 24 tank containing fluid at high tem22 perature and another tank at lower 20 temperature. In the direct two-tank 18 system, the heat-transfer fluid from the collectors and in the tanks is 16 the same; in an indirect two-tank 14 system, the heat-transfer fluid 12 transfers its heat to the fluid in the 10 tanks through a heat exchanger. 8 A third option is the single-tank 6 thermocline system, which sets up a temperature gradient in a tank 4 that contains a solid material such 2 as sand. 0 Research on advanced heat-trans2001 2002 2003 2004 2005 2006 2007 2008 2009 20102011 2012 2013 2014 2015 fer fluids and novel thermal-storage Year concepts seeks to increase efficiency and reduce costs for TES. Scien- figure 20. Grid-connected distributed PV growth from 2001 through 2006, protists are working to identify and jected to 2015 (source: DOE RSI study report “PV Market Penetration Scenarios”). characterize novel fluids and mate- The low end of the growth range (base case) assumes a partial extension of the rials that possess the physical and federal investment tax credit; the high end assumes the implementation of a full chemical properties needed to im- set of solar-friendly policies. prove thermal storage. For example, phase-change materials (PCMs) allow large amounts of for this expectation include the growing concerns about energy to be stored in relatively small volumes, result- climate change, the adoption of state-level renewable porting in some of the lowest costs for storage media of any folio standards and incentives, and the accelerated reducstorage concept. Initially, PCMs were considered for use tion of system costs. As PV and CSP technologies mature, with parabolic trough plants that used a synthetic heat- they have the potential to supply a significant share of our transfer fluid designed to withstand high temperatures in nation’s electricity demand. the solar field. PCM thermal storage is now being considered for application with direct steam generation in the parabolic trough solar field. 80 18 Incorporating TES into CSP Targets Historical 16 power plants allows utilities to en70 hance dispatchability. As TES tech14 60 nologies improve and allow for lon12 ger storage periods and lower costs, 50 more utilities may consider CSP as 10 Sy ste a viable alternative to or supplement 40 m Pri for power plants that depend solely 8 ce Ra 30 on fossil fuels. ng

0 2020

figure 21. The DOE’s goal is to make solar electricity from PV cost competitive with conventional grid electricity by 2015 (source: DOE). IEEE power & energy magazine

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CSP technologies use mirrors to reflect and concentrate sunlight onto receivers that collect the solar energy and convert it into heat.

However, as their market share grows, concerns about the potential impacts on the operation and stability of the electricity grid might create barriers to further expansion. Wind power is already gaining considerable market penetration at the transmission level, and its variable nature increases the complexity of operating the bulk power grid. Additional challenges are likely to emerge as additional nondispatchable sources, such as PV, are added to the electrical distribution network. In 2008, the U.S. Department of Energy (DOE) published a set of reports—the “Renewable Systems Interconnection

Transmission System

Distribution System

System Operators

Retail Utilities

(RSI) study”—to address technical, regulatory, and business issues that could potentially limit the market uptake of distributed PV and other renewable technologies. One key finding was that grid-integration issues are likely to emerge much more rapidly than many analysts expect. In some U.S. regions, barriers to future growth related to grid integration could emerge within the next five to ten years. In California, for example, a number of new homes are currently being built with PV systems as a standard feature. With these types of developments already occurring in the marketplace, it

Weather Station SEGIS

Peak Generation Demand Management Transmission Deferral

Energy Storage

RECs Distribution Deferral Grid Regulation Inverter/ Controller/ EMS

System Monitoring and Control

Internet Data Uplink Green Power Bill Reduction Backup Power Monitoring Services O and M Services

Metering Data Validation

Electric Power

Loads

Monitoring Gateway

Plug-In Hybrid Vehicle

Residential or Commercial Building

Value Information

Operations Information

figure 22. Integration of SEGIS with home loads and the electric power system (source: NREL). 32

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is clearly time to plan for the integration of significant quantities of distributed renewable energy into the electricity grid. The U.S. grid-connected PV market grew rapidly during the past five years. Annual grid-connected PV installations increased from 10 MW in 2001 to about 180 MW in 2006. This growth resulted in a cumulative installed base of about 480 MW of grid-connected PV in the United States at the end of 2006. This accelerated growth of the PV and CSP industries is simply the tip of the iceberg. Policy developments at both the federal and state levels, coupled with technological improvements, are helping to create a more receptive market for solar technologies in the United States. Indeed, scenarios developed as part of the RSI study indicate that annual installations of grid-connected PV in the United States could reach 7.1 GW by 2015, resulting in a cumulative installed base of up to 24 GW by 2015. As shown in Figure 20, three key regulatory and policy drivers were found to have a significant impact on PV adoption rates: lifting net-metering caps/establishing net metering, extending the federal investment tax credit, and improving interconnection standards.

Moving to Cost Parity on the Grid In 2006, the United States announced a national solar initiative—the Solar America Initiative—to be led by the DOE’s Solar Energy Technologies Program. The overall goal of the initiative is to make PV-generated electricity cost competitive with conventional energy sources across the United States by 2015 (Figure 21). For CSP, the Western Governors’ Association concluded that CSP could provide electricity at 10 ¢/kWh or less by 2015 if 4 GW of CSP plants were constructed in the southwestern United States. This lower cost is due, in part, to the economies of scale in production. In addition, the extension of the solar investment tax credit for eight years beginning in 2009 could lead to many gigawatts of CSP projects being developed in this region. Spain is also actively constructing trough and tower CSP plants, with the goal of installing 500 MW of CSP power by 2010, using a 0.21 ;/kWh feed-in tariff. As many as a dozen 50-MW plants may be involved, with thermal storage being an important part of the projects.

Integrating with the Smart Grid of the Future As the electric power system moves toward a smart grid in the future, solar technologies will need to adapt to be more compatible with the grid. One important aspect of the smart grid is that it will seamlessly integrate many types of generation and storage systems with a simplified interconnection process analogous to “plug and play.” This means that solar energy systems will need to change from their current role as passive participants on the grid to a state in which they may/june 2009

communicate back to the utility and actively participate in its operations. Figure 22 shows a future solar energy system integrated with various loads, energy storage, and the electric power system. Termed the “Solar Energy Grid Integration System,” or SEGIS, this solar energy system of the future actively participates in electrical markets, is able to provide improved reliability to local customers, and participates in grid functions. Implementation of the SEGIS will require changes to the existing interconnection standards, and these changes will be necessary as solar energy systems become a larger player in the electric power system.

Acknowledgments The authors would like to acknowledge Don Gwinner at the National Renewable Energy Laboratory (NREL) for providing information in this article, as well as editing all the articles in this issue. We would also like to thank the NREL GIS team for providing new solar resource graphs.

For Further Reading B. Kroposki, R. Margolis, G. Kuswa. J. Torres, W. Bower, T. Key, and D. Ton. (2008). Renewable systems interconnection (RSI): Executive summary. NREL/TP-581-42292. [Online]. Available: http://www.nrel.gov/docs/fy08osti/42292.pdf P. Denholm and R. M. Margolis, “Evaluating the limits of solar photovoltaics (PV) in electric power systems utilizing energy storage and other enabling technologies,” Energy Policy, vol. 35, no. 9, pp. 4424–4433, 2007. P. Denholm and R. M. Margolis, “Evaluating the limits of solar photovoltaics (PV) in traditional electric power systems,” Energy Policy, vol. 35, no. 5, pp. 2852–2861, 2007. International Energy Agency (IEA). (2008). Trends in photovoltaic applications—Survey report of selected IEA countries between 1992 and 2007. Report IEA-PVPS T1–17:2008. [Online]. Available: http://www.iea-pvps.org/ products/download/rep1_17.pdf International Energy Agency (IEA). (2008). Photovoltaic power systems programme (PVPS) annual report 2007. [Online]. Available: http://www.iea-pvps.org/products/download/ IEA%20PVPS%20Annual%20Report%202007-web.pdf K. Zweibel, J. Mason, and V. Fthenakis. (2008, Jan.). Solar grand plan. Sci. Amer. [Online]. pp. 63–73. Available: http://www.sciam.com/article.cfm?id=a-solar-grand-plan S. Grama, E. Wayman, and T. Bradford, “Concentrating solar power—Technology, cost, and markets—2008 industry report,” Cambridge, MA: Prometheus Inst. Sustainable Develop., 2008.

Biographies Benjamin Kroposki manages the Distributed Energy Systems Integration Group at NREL. Robert Margolis is the lead analyst for the Solar Energy Technologies Program at NREL. Dan Ton leads the Systems Integration team in the Solar p&e Energy Technologies Program at the DOE. IEEE power & energy magazine

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