WORLD ENERGY [R]EVOLUTION A SUSTAINABLE WORLD ENERGY OUTLOOK
After an intermediate scaling up to 30 MW capacity, solar tower developers now feel confident that grid-connected tower power plants can be built up to a capacity of 200 MWe solar-only units. Use of heat storage will increase their flexibility. Although solar tower plants are considered to be further from commercialisation than parabolic trough systems, they have good longer-term prospects for high conversion efficiencies. Projects are being developed in Spain, South Africa and Australia. Parabolic dish: A dish-shaped reflector is used to concentrate
9 energy technologies | RENEWABLE ENERGY TECHNOLOGIES
sunlight on to a receiver located at its focal point. The receiver moves with the dish. The concentrated beam radiation is absorbed into the receiver to heat a fluid or gas to approximately 750°C. This is then used to generate electricity in a small piston, Stirling engine or micro turbine attached to the receiver. Dishes have been used to power Stirling engines up to 900°C, and also for steam generation. The largest solar dishes have a 485-m2 aperture and are in research facilities or demonstration plants. Currently the capacity of each Stirling engine is small — in the order of 10 to 25 kWelectric. There is now significant operational experience with dish/Stirling engine systems and the technology has been under development for many years, with advances in dish structures, high-temperature receivers, use of hydrogen as the circulating working fluid, as well as some experiments with liquid metals and improvements in Stirling engines — all bringing the technology closer to commercial deployment. Although the individual unit size may only be of the order of tens of kWe, power stations of up to 800 MWe have been proposed by aggregating many modules. Because each dish represents a stand-alone electricity generator, there is great flexibility in the capacity and rate at which units are installed to the grid. However, the dish technology is less likely to integrate thermal storage. The potential of parabolic dishes lies primarily for decentralised power supply and remote, stand-alone power systems. Projects are currently planned in the United States, Australia and Europe.
Thermal Storage: Thermal energy storage integrated into a system is an important attribute of CSP. Until recently, this has been primarily for operational purposes, providing 30 minutes to one hour of full-load storage. This eases the impact of thermal transients such as clouds on the plant, assists start-up and shutdown, and provides benefits to the grid. Trough plants are now designed for 6 to 7.5 hours of storage, which is enough to allow operation well into the evening when peak demand can occur and tariffs are high.
In thermal storage, the heat from the solar field is stored before reaching the turbine. The solar field needs to be oversized so that enough heat can be supplied to both operate the turbine during the day and, charge the thermal storage. Thermal storage for CSP systems needs to be generally between 400°C and 600°C, higher than the temperature of the working fluid. Temperatures are also dictated by the limits of the media available. Examples of storage media include molten salt (presently comprising separate hot and cold tanks), steam accumulators (for short-term storage only), solid ceramic particles, high-temperature phasechange materials, graphite, and high-temperature concrete. The heat can then be drawn from the storage to generate steam for a turbine, when needed. Another type of storage associated with high-temperature CSP is thermochemical storage, where solar energy is stored chemically. Trough plants in Spain are now operating with molten-salt storage. In the USA, Abengoa Solar’s 280-MW Solana trough project, planned to be operational by 2013, intends to integrate six hours of thermal storage. Towers, with their higher temperatures, can charge and store molten salt more efficiently. Gemasolar, a 19-MWe solar tower project operating in Spain, is designed for 15 hours of storage, giving a 75% annual capacity factor (Arce et al., 2011).
figures 9.3: csp technologies: parabolic trough, central receiver/solar tower and parabolic dish
PARABOLIC TROUGH
CENTRAL RECEIVER
PARABOLIC DISH
CENTRAL RECEIVER
REFLECTOR RECEIVER/ENGINE
REFLECTOR HELIOSTATS ABSORBER TUBE
SOLAR FIELD PIPING
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