Capacity Factor and Renewable Energy
When it comes to several renewable energy sources such as solar power, wind power and hydroelectricity, there is a third reason for unused capacity. The plant may be capable of producing electricity, but its fuel (wind, sunlight or water) may not be available. A hydroelectric plant's production may also be affected by requirements to keep the water level from getting too high or low and to provide water for fish downstream. However, solar, wind and hydroelectric plants do have high availability factors, so when they have fuel available, they are almost always able to produce electricity.
When hydroelectric plants have water available, they are also useful for load following, because of their high dispatchability. A typical hydroelectric plant's operators can bring it from a stopped condition to full power in just a few minutes.
Wind farms are variable, due to the natural variability of the wind. For a wind farm, the capacity factor is mostly determined by the availability of wind. Transmission line capacity and electricity demand also affect the capacity factor.
Solar energy is variable because of the daily rotation of the earth and because of cloud cover. However, according to the SolarPACES programme of the International Energy Agency (IEA), solar power plants designed for solar-only generation are well matched to summer noon peak loads in areas with significant cooling demands, such as Spain or the south-western United States., although in some locations solar PV does not reduce the need for generation of network upgrades given that air conditioner peak demand often occurs in the late afternoon or early evening when solar output is zero. SolarPACES states that by using thermal energy storage systems the operating periods of solar thermal power (CSP) stations can be extended to meet baseload needs. The IEA CSP Technology Roadmap (2010) suggests that "in the sunniest countries, CSP can be expected to become a competitive source of bulk power in peak and intermediate loads by 2020, and of base-load power by 2025 to 2030".
Geothermal has a higher capacity factor than many other power sources, and geothermal resources are available 24 hours a day, 7 days a week. While the carrier medium for geothermal electricity (water) must be properly managed, the source of geothermal energy, the Earth's heat, will be available for the foreseeable future. Geothermal power can be looked at as a nuclear battery where the heat is produced via the decay of radioactive elements in the core and mantle of the earth.
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