In a solar thermal power plant, mirrors are made to concentrate sunlight trapped in a pipeline to heat oil, molten salts or other chemicals which can trap heat for a longer duration of time. This heat is used to generate steam in a boiler that runs a turbine to produce electricity. CSP technologies vary in how they concentrate sunlight.
For example, in parabolic trough systems, parabolic trough-shaped mirror reflectors concentrate sunlight on thermally efficient receiver tubes placed in the trough’s focal line. A heat transfer fluid is circulated through the receiver tubes and heated to temperatures up to 400°C.
The fluid passes its heat to other working fluid, typically water or steam, through a series of heat exchangers, which is further used to drive a conventional turbine generator. Other systems include the power tower system and the parabolic dish system.
CSP has higher operating efficiency and lower cost compared to PV. While even the most efficient solar PV can generate about 20 MW per sq km (km2), for solar thermal generation it is about 35 MW/km2. CSP also has major advantages in energy storage, a critical component of technologies harnessing intermittent energy sources like wind and solar.
The storage allows for higher plant capacity factors, compensation for vagaries in solar radiation, increased ability of the plant to provide firm capacity and consequently greater carbon dioxide (CO2) emissions reductions.
These assume greater importance in India, given its weak inadequate grid and generation capacity, translating to limited grid back-up capability. CSP systems can be easily integrated into conventional power plants as they utilize the same generator as most other fossil fuel based thermal power plants.
The exact cost of CSP systems is currently a grey area, primarily because of a lack of standardization as the technology is still developing. Apart from technology, the plants vary greatly in specifications like storage capacity, efficiency of solar field, and generation capacity of the plant.
However, qualified cost forecasts put down the capital costs of a parabolic trough plant—with 12 hours of storage capacity, no fossil fuel back-up and a nearly 55 per cent capacity factor— at US $4,816 per kW (total plant capacity of 100 MW) in 2004, and US $3,220 per kW (total plant capacity 400 MW) in 2020.
The respective costs of electricity are US 10.37 cents and US 6.21 cents per unit. These costs are far lower than even the most efficient solar PV systems on per watt basis. CSP plants present the possibility of a high local content during construction and operation and maintenance, unlike solar PV where silicon—the major cost component—has to be imported. Additionally, carbon credits can serve as an income stream to reduce the cost.
Over the past two years, rising oil prices have brought CSP systems into sharp relief. Countries such as the US and Spain have taken the lead; Australia, Israel, Morocco and even the oil-rich countries of Iran and Algeria have shown progress. In Spain, construction of the world’s biggest solar thermal power stations—Andasol 1 and 2—are almost complete.
By early 2008, as much as 4,000 MW of solar thermal was in the pipeline in Spain and five more plants are underway. The German company Solar Millennium AG and the Spanish plant builder Cobra is building Andasol. They are working on liquid salt, which can be heated to 500oC—oil can be heated to only 400oC. The plants would use liquid salt to store heat during the day to generate electricity at night.