TODAY’S STUDY: WHAT UTILITIES NEED TO KNOW ABOUT SOLAR TECHNOLOGY
Solar Fundamentals; Volume 1: Technology
Becky Campbell and Daisy Chung, February 2015 (Solar Electric Power Association)
This report serves as one component of a multi-part series of publications that SEPA plans to produce throughout 2015. The purpose of this effort is to provide a broad introduction to several facets of the solar industry, including: a discussion of different technologies; an update on the current state of the U.S. market; a summary of project financing options; and, an overview of some of the solar integration challenges that utilities are encountering (or soon will be).
SEPA undertook this effort to assist in educating those seeking to become more familiar with the solar industry. Whether you are reading this publication as a new utility regulator seeking information to better inform your decision-making process or as a student researching potential career paths, the goal of this series is to distill information into short publications that any individual can use to gain practical knowledge of the industry.
This portion of the series introduces solar technologies, explaining each technology’s applications. There is a brief section that describes the ancillary components that make up a photovoltaic system and explains how these components can be used to optimize energy generation. This report also describes solar insolation, explaining how it impacts energy generation and illustrating where solar energy is a viable option. A final section highlights important considerations in siting a solar project including opportunities to maximize system production and avoid unexpected project development challenges.
Types of Technology
Solar generating technologies can be generalized into two groups: photovoltaics and concentrated solar power. Photovoltaics (PV) are semiconducting materials used to convert sunlight into direct current (DC) electricity. Concentrated solar power (CSP) uses a collection of mirrors to concentrate solar thermal energy, which in most cases drives a steam turbine, thus producing alternating current (AC) electricity. This section will describe both technologies in detail, examining the various options available for each and their potential market applications.
Put simply, PV cells are composed of semiconductor materials that exhibit the photoelectric effect – that is, materials that display properties that allow them to absorb the photons in sunlight and, in turn, release electrons which can be captured to generate electricity. As indicated in Figure 1, individual PV cells are combined to form PV modules (or panels), and modules are connected to form PV arrays. 1 PV arrays are connected to accessory components to form a solar system (see the Balance of System section for further discussion).
PV technologies are primarily differentiated based on the nature of the absorber material responsible for converting light into electricity. This section will focus on crystalline silicon and thin film PV technologies, which represent the most commonly used PV technologies as of this date. A brief discussion of emerging PV technologies is also included.
Crystalline silicon (c-Si) is the most commonly used PV technology in the world. This prevalence is due, in part, to a mature process technology that greatly benefited from the knowledge of the semiconductor industry. Typically, a c-Si module consists of a dozen or more individual PV cells electrically wired together. Crystalline silicon PV can further be divided into four broad categories — monocrystalline, multicrystalline (or polycrystalline), ribbon, and ‘super’ monocrystalline. Mono- and multicrystalline technologies accounted for approximately 90 percent of the total global PV manufactured in 2013. 2 Monocrystalline cells are composed of a uniform material grown from a single crystal of silicon, while multicrystalline cells are made up of materials from several, smaller crystals. The process of “growing” a monocrystalline cell is slower and more expensive than the process used to create multicrystalline cells. Because the composition of monocrystalline cells is uniform throughout, they are generally more efficient at converting sunlight into electricity than multicrystalline cells.
Unlike crystalline silicon, where the substrate is nearly always glass, thin-film PV can use a range of both rigid and flexible substrates, such as metal foils (steel or aluminum) or plastics. Also unlike c-Si, which requires a manufacturing process that produces modules batch by batch, thin-film manufacturing processes can, in principle, continually produce modules at higher speeds. Thin-film can be grouped into three categories: amorphous silicon (a-Si), cadmium telluride (CdTe), and copper gallium indium diselenide (CIGS). In 2013, CdTe accounted for approximately 54 percent of global thin-film production, while a-Si and CIGS accounted for 23 percent each. Collectively, thin-film technologies account for approximately 10 percent of the global PV market share.
Efficiency describes the effectiveness of a technology at capturing the energy in sunlight and converting it to usable electricity. The highest recorded efficiency in a laboratory environment for a c-Si PV cell is approximately 25 percent, but commercially available modules have efficiencies closer to 20 percent. 7 The highest observed laboratory thin film efficiencies have surpassed 21 percent, but commercially available technologies range from approximately 10 to 15 percent. Monocrystalline modules maintain a sizeable efficiency advantage to thin-film products, but some thin-film technologies have started to surpass multicrystalline modules in efficiency.
When comparing efficiencies of modules, it is important to take into account the price compared to the estimated lifetime generating capability. Purchasing a nominally more efficient module for a significantly higher price is not necessarily a sound investment. It may be worth exploring whether lower-cost, lower-efficiency modules will produce a more attractive return on investment over a project’s lifetime. The National Renewable Energy Laboratory (NREL) has several tools that help users estimate PV production. PVWatts is a simple tool that quickly and easily estimates production based on location and efficiency assumptions. 8 The System Advisor Model (SAM) is a more advanced tool capable of modeling both the performance and economics of a PV system, based on inputs such as location, specific system components, and system costs.
Applications for PV technologies vary widely. This technology is easily scaled to suit energy needs of any size. In some of its smallest-scale uses, PV is used to power calculators, street lights, and water pumping stations, but it is also commonly used to meet larger energy needs. PV is widely deployed to generate on-site energy for residential and commercial users. It is also increasingly deployed through utility-scale power projects — projects that can range from five to hundreds of megawatts in capacity and directly supply power into the electric grid (similar to a traditional power plant). PV projects can easily be built and powered up in phases, making it convenient to expand projects over time as demand increases. While most PV projects are interconnected to the electric grid, off-grid PV systems are possible with proper use of storage technologies (albeit often cost prohibitive for significant energy needs).
PV can be mounted for use on rooftops or at ground-level, and is increasingly incorporated into accessory structures, such as parking canopies and pergolas. Some of the emerging technologies directly integrate PV into building materials, such as windows, roofing tiles and shingles.
Concentrated Solar Power
Concentrated or concentrating solar power (CSP) refers to the general technology of redirecting sunlight via mirrors and concentrating it to a focal point, where it is used to form thermal energy. Mirrors used in CSP have specified reflectivity and are set in strategic shapes/placement. They serve as the “collector” of sunlight, and reflect it to a central “receiver.” The receiver absorbs the focused solar thermal energy, becoming a heat source. This heat source may boil water or other fluids to form steam that spins a turbine to generate electricity. Turbine-generated electricity from CSP works in a similar manner as coal, nuclear, oil and natural gas turbine generators with the only significant difference being the heat source.
There are three main established CSP technologies, easily distinguished by their mirror configurations. In increasing complexity, these technologies are linear concentrators, dish/engines and power tower systems.
Linear Concentrator System
The linear concentrator system features a set of linear collectors and receiver tubes. The collectors face the sun to focus its energy on the receiver, which is placed in parallel above the collector. The linear receiver tube contains water or another heat-transfer fluid, which absorbs the heat of the focused sunlight. The heated fluid is used to generate steam, which, in turn, powers a turbine to generate electricity.
There are two types of linear concentrator systems. The most common and proven is the parabolic trough system. It consists of parabolic, or near u-shaped, mirrors placed in rows that run north-south and make use of single axis tracking to maximize sun exposure. A linear receiver tube is placed parallel to each row of mirrors. The curvature of the u-shaped mirrors collects sunlight and reflects it onto the dedicated receiver tube. Figure 5: An illustration of a parabolic trough power plant. (Credit: DOE/NREL 1996)
The second, newer type is called the linear Fresnel reflector system. In general, linear Fresnel systems operate similarly to parabolic trough systems; however, this system uses a shared receiver placed higher above multiple rows of mirrors, which use tracking and are flat or nearly flat. The equipment setup of the linear Fresnel reflector system is simpler than the parabolic trough system.
The dish/engine system describes the combined usage of a dish-shaped collector (or “concentrator”) attached with a centrally mounted engine unit that serves as a receiver and electricity generator. Each dish apparatus tracks and concentrates sunlight onto an engine, similar to the operation of a satellite dish, and can be made of large concave mirrors or many small, flat mirrors mounted into a dish shape (often the cheaper of the configurations). The engine absorbs the thermal energy of the concentrated sunlight through its receiver, where a heat-transfer medium is heated. The heated medium then drives a spinning generator, commonly through moving pistons in an electricity-generating Stirling engine.
Power Tower System
The power tower system also concentrates sunlight like the dish/engine system, but on a massive scale. It collects sunlight via many large, flat, ground-mounted mirrors, called heliostats, placed in a circular pattern around a receiver tower that can be hundreds of feet tall.
Each heliostat tracks and concentrates sunlight onto the receiver tower, where the absorbed heat produces steam to power a conventional turbine generator. Because the scalable configuration demonstrates favor in economies of scale, the power tower system exhibits the largest deployment on a per-system basis, to as large as 200 megawatts.
CSP Efficiencies and Advancements in Technology
The Department of Energy’s SunShot Vision Study compares the annual average efficiencies as well as technology improvement among CSP system designs. 14 Both are important factors because they directly affect project viability and upfront investment ultimately influencing delivered cost of electricity and annual revenue. 15
Average annual efficiencies are used as a comparison by the Department of Energy (DOE) because they are closer to actual, rather than ideal, design-point, solarto-electric conversions of an operating CSP facility. The table below identifies the average annual efficiencies for the CSP technologies discussed in this section.
Balance of System (BOS)…Solar Irradiance…Siting Considerations …