Turning Solar Heat Into Electricity

Producing electricity from the energy in the sun's rays is a straightforward process: direct solar radiation can be concentrated and collected by a range of Concentrating Solar Power (CSP) technologies to provide medium to high-temperature heat. This heat is then used to operate a conventional power cycle, for example through a steam turbine or a Stirling engine. Solar heat collected during the day can also be stored in liquid or solid media like molten salts, ceramics, concrete or, in the future, phase-changing salt mixtures. At night, it can be extracted from the storage medium and, thus, continues turbine operation.

Solar thermal power plants can be designed for solar-only or hybrid operation, as in California where some fossil fuel is used in case of lower radiation intensity to secure reliable peak-load supply. Thermal energy storage systems are extending the operation time of solar thermal power plants up to a 100% solar share. For example, in Spain the 50 MWe AndaSol plants are designed with six to 12 hours thermal storage, increasing annual availability by about 1,000 to 2,500 hours.

Electricity from solar thermal power is also becoming cheaper to produce. Plants operating in California have already achieved impressive cost reductions, with generation costs ranging today between 14 and 17 US cents/kWh. However, costs are expected to drop closer to 7-8 US cents in the future. Advanced technologies, mass production, economies of scale and improved operation will together enable a reduction in the cost of solar electricity to a level competitive with fossil-fueled peak- and mid-load power stations within the next ten to 15 years.

TECHNOLOGY, COSTS AND BENEFITS

Four main elements are required to produce electricity from solar thermal power: a concentrator, a receiver, some form of transport or storage, and power conversion. The three most promising solar thermal technologies are the parabolic trough, the central receiver or solar tower, and the parabolic dish.

Parabolic trough systems use trough-shaped mirror reflectors to concentrate sunlight on to receiver tubes through which a thermal transfer fluid is heated to roughly 40ooC and then used to produce superheated steam. They represent the most mature solar thermal power technology, with 354 MWe of plants connected to the Southern California grid since the 1980s and more than 2 million square metres of parabolic trough collectors. These plants supply an annual 800 million kWh at a generation cost of about 14-17 US cents/kWh.

Further advances are now being made in the technology, with utility-scale projects planned in Spain, Nevada (USA), Morocco, Algeria, Italy, Greece, Israel, Egypt, India, Iran, South Africa and Mexico. Electricity from trough plants are thus expected to fall to 7-8 € cents/kWh in the medium term. Combined with gas-fired combined cycle plants - so-called ISCC (Integrated Solar Combined Cycle) systems - power generation costs are expected to be in the order of 6-7 € cents/kWh in the medium term and 5 € cents/kWh in the long term.

Central receiver (solar tower) systems use a circular array of large individually tracking plain mirrors (heliostats) to concentrate sunlight on to a central receiver mounted on top of a tower, with heat transferred for power generation through a choice of transfer media. Following completion of the first 10 MWe PS-10 demonstration tower plants, currently under construction in Spain, and with further scaling up to 30-50 MW capacity, solar tower developers feel confident that grid connected tower power plants can be built up to a capacity of 200 MWe solar-only units with power generation costs then comparable to those of parabolic troughs. Use of thermal storage will also increase their flexibility.

Current trends show that two broad pathways have opened up for large-scale delivery of electricity using solar thermal power. One is the ISCC-type hybrid operation of solar collection and heat transfer combined with a conventional power plant. The other is solar-only operation, with increasing use of a storage medium such as molten salt. This enables solar energy collected during the day to be stored then dispatched when demand requires.

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