Concentrating Solar Power
Concentrating solar power (CSP) can be considered either in the context of photovoltaic technology (PV) or in terms of solar thermal energy. There are two main types of concentrating solar power systems: Concentrating Photovoltaics (CPV) and Concentrating Solar Thermal (CST). Both applications require sites that are very sunny and that offer expansive, flat, wide open spaces. These technologies are currently being developed for utility-scale applications, though research is being done in micro-CPV as well for residential applications. CSP’s ability to store energy makes it viable as a source of baseload power.
In the U.S., the Southwest has the most favorable conditions and solar resources to deploy CSP technologies. In fact, a study done in 2005 by DOE NREL for the Western Governors’ Association determined that seven States in the Southwest (AZ, CA, CO, NV, NM, TX and UT) have a the combination of solar resource and available suitable land to generate up to 6,800 GW of solar capacity. Today, the entire electric generating capacity of the entire U.S. is about 1,000 GW. Currently, there are nine CSP generating projects installed or in development in the Southwest, and one in development for Florida. Once all these facilities are operational, they will generate about 1,868+ MW combined. The cost of electricity from CSP technology is currently about 13-17 cents/kWh, but could be reduced to 7-8 cents/kWh in the next twelve years, depending on the solar resource and DOE research grant support.
Concentrating Photovoltaics (CPV)
Concentrating Photovoltaics uses Fresnel concentrating lenses to focus direct sunlight onto high efficiency solar cells that are designed to operate with concentrated sunlight and convert the light directly into electricity. Fresnel concentrating lens can also be used with conventional PV panels. These magnifying lenses focus and concentrate sunlight approximately 500 times onto a relatively small cell area and operate similarly to glass magnifying lenses. Through concentration, the required silicon cell area needed for a given amount of electricity is reduced by an amount approximating its concentration ratio (500 times). In effect, a low cost plastic concentrator lens is being substituted for relatively expensive silicon.
High efficiency solar cells are much smaller than those used in conventional PV modules, they cost more and they are more efficient, with efficiency capacities nearing 30 percent. But because the lenses must be pointed at the sun, the use of concentrating collectors is limited to the sunniest parts of the country. Some concentrating collectors are designed to be mounted on simple tracking devices, but most require sophisticated tracking devices, which further limit their use to electric utilities, industries, and large buildings.
Concentrator systems can potentially reduce the costs of PV electricity because the materials used to concentrate the sunlight are generally less expensive that the cost of the additional PV cells that would be required to produce the same amount of electricity in a nonconcentrating solar thermal energy system. The smaller size of concentrating PV cells further decreases the relative cost of concentrating solar thermal systems.
Concentrating Solar Thermal (CST)
Unlike photovoltaic cells that harness the movement of electrons between its layers when the sun strikes it, solar thermal power works by utilizing the heat of direct sunlight. Solar thermal power plants are attracting renewed interest as governments around the world are making concerted efforts to develop alternative sources of energy in light of rising fuel prices and climate change. Solar thermal power plants can compete with conventional fossil fuel burning power plants to produce electricity, and CST electricity is generated pollution free.
Large-scale solar thermal power plants are under construction or planned in California, Florida, Spain, and Algeria. Algeria, a leading world oil exporter, is planning to develop 6,000 megawatts of solar-thermal electric-generating capacity, which it will feed into the European grid via an undersea cable. The electricity generated from this single project is enough to supply the residential needs of a country the size of Switzerland. ACCIONA Energy has connected its "Nevada Solar One" solar thermal plant in the State of Nevada (USA). Its 64 MW capacity make it the largest solar plant to be built in the world in the last 17 years. Close to $250 million were invested in the facility, which took 16 months to be built and will produce 134 million kilowatt hours a year. This is Acciona’s first venture into solar thermal energy, a renewable technology in which the company is currently developing several significant projects.
Parabolic-trough systems concentrate the sun’s energy through long rectangular, curved (U-shaped) mirrors. The mirrors are tilted toward the sun, focusing sunlight on a pipe that runs down the center of the trough. This heats the oil or other thermal transfer fluid flowing through the pipe to about 400°C which is then pumped through a series of heat exchangers to produce superheated steam to power a conventional steam generator to produce electricity. Since 1985, nine parabolic trough-type solar thermal power plants in California have fed more than 8 billion kWh of solar-based electricity into the Southern Californian grid. These nine distinct solar thermal power plants located in the Mojave Desert total 360 megawatts, by far the largest central solar power station in the world. (That’s enough electricity to power about 360,000 homes.)
A parabolic dish/engine system uses a mirrored dish (similar to a very large satellite dish). The dish-shaped surface collects and concentrates the sun’s heat onto a receiver, (a Stirling engine) which absorbs the heat and transfers it to fluid or hydrogen gas within the engine. The heat causes the fluid/gas to expand against a piston or turbine to produce mechanical power. The mechanical power is then used to run a generator or alternator to produce electricity. The energy conversion process requires no water and the engine is emission free. The Stirling dish is a 30-year-old technology that’s just now becoming cost-effective thanks to big solar-power orders from utilities. In California, the CPUC approved a PG&E solar thermal project, Solel-MSP-1. The Mojave Solar Park, to be constructed in CA’s Mojave Desert will deliver 553 MW of solar power, the equivalent of powering 400,000 homes. When fully operational in 2011, this solar plant will cover up to 6,000 acres, or nine square miles and will use 1.2 million mirrors.
A "power" tower is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, movable mirrors (called heliostats) to focus the sun’s rays upon a central receiver atop a collector tower (the target). The high energy at this point of concentrated sunlight is transferred to a substance that can store the heat for later use. The most recent heat transfer material that has been successfully demonstrated is molten salt. Sodium is a metal with a high heat capacity, allowing that energy to be stored and drawn off throughout the evening. That energy can, in turn, be used to boil water for use in steam turbines. Water had originally been used as a heat transfer medium in earlier power tower versions (where the resultant steam was used to power a turbine). This system did not allow for power generation during the evening. Tower power plants are in the experimental stage; they generate the hottest steam and offer good conditions for high efficiency.
In Spain the 11 MW PS10 solar power tower was recently completed with 624 mirrors and a 100 meter tower; a 20 MW tower is expected to be completed in 2008.