Utility-Scale and Distributed Solar Energy Generation
Solar energy technology can be applied at varying scales, including utility-scale generation and distributed generation.
Solar energy technologies can be broadly defined as those activities, applications, or devices designed to harness energy from the sun in order to perform useful work. Solar energy technologies can be applied at various scales ranging from utility-scale generation to distributed generation. While a growing industry, solar energy currently contributes only a small fraction of the U.S. energy mix.
What Is Utility-Scale Solar Energy Generation?
Utility-scale solar energy plants generate a large amount of electricity that is transmitted from one location (the solar energy plant) to many users through the transmission grid. A utility-scale solar development can consist of hundreds to thousands of solar collectors. Continued growth of utility-scale solar energy generation is expected as renewable energy becomes a more important component of the U.S. energy mix. One challenge in generating electricity from solar energy is that sunlight is intermittent (it occurs only during daylight hours and can be obscured by clouds, rain, fog, and other climatic conditions). The intermittency of sunlight creates concerns for the stability and reliability of utility-scale solar energy developments. This issue can be alleviated at solar energy facilities that have heat or electrical storage capacity or that are part of a hybrid system (e.g., collocated with one or more conventional fossil-fueled generation technologies).
What Is a Capacity Factor?
The capacity factor of a solar energy power plant is the ratio of the actual output of the plant over a period of time and its output if it had operated at full capacity the entire time. Capacity factor needs to be considered in deciding whether a solar energy project should be developed. For solar energy power plants, the capacity can be viewed as a combination of: (1) how much of the sunlight received by the collector array is converted to electricity (its electrical conversion efficiency); and (2) how much of the time the solar energy plant can operate. The latter depends on how long the sun shines and whether the plant has a means to store energy that can be used to operate the plant when the sun is not shining. For example, photovoltaic (PV) cells convert only about 10 to 20% of the radiant energy into electrical energy, while dish engines can convert over 30% of the sunlight to electrical energy. Electricity conversion efficiencies for PV systems can be increased to 36% by using high-performance semiconductors, concentrating lenses, and dual-axis tracking. Parabolic trough plants have an annual capacity factor of about 25% without thermal storage and up to 70% or more with it. In comparison, fossil-fuel plants convert 30 to 40% of their fuel's chemical energy into electrical energy.
What Is Distributed Generation?
Distributed generation is the generation of small-scale solar energy that is distributed and used over a local area. For example, individual homes, farms, or businesses may have their own solar units to generate electricity or heat for personal or business use. Distributed solar units are much smaller than utility-scale facilities (e.g., kilowatts vs. megawatts). In many locations, excess electricity not used by the owner of a distributed solar system can be sold to the local utility and distributed for more widespread use. Solar systems can also be installed to generate hot water for use in individual buildings or facilities (e.g., homes, hospitals, dormitories, and swimming pools). Unlike utility-scale solar energy generation, distributed solar energy generation can be more readily utilized in any geographic location.