Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar thermal electricity
Most first-generation plants have little or no thermal storage and rely on combustible fuel as
a firm capacity back-up. CSP plants in Spain derive 12% to 15% of their annual electricity
generation from burning natural gas. More than 60% of the Spanish plants already built or under
construction, however, have significant thermal storage capacities, based on two-tank molten-salt
systems, with a difference of temperatures between the hot tank and the cold one of about 100°C.
Photo 8.2 Cleaning of parabolic troughs at Ain Beni Mathar (Morocco)
Key point
Reflective surfaces in STE plants need to be cleaned regularly.
Beyond incremental improvements in size, performance and costs, parabolic troughs would
possibly experience more significant change if other heat transfer or working fluids could
replace the synthetic oil, which limits the working temperatures to less than 390°C. The main
options are:
• water/steam. Direct steam generation (DSG) in the collector fields would allow high working
temperatures and reduce investment costs, as no heat-transfer fluid (HTF) and heat exchangers
would be necessary. Work is needed to ensure the separation of water and steam, and handle
the circulation of high-temperature, high-pressure working fluids, which is a challenge with
mobile receivers. Furthermore, DSG does not lend itself to easy storage (see below);
• molten salts. This solution simplifies storage, as the HTF becomes the storage medium. Salt
mixtures usually solidify below 238°C and are kept above 290°C for better viscosity,
however, so work is needed to reduce the pumping and heating expenses required to
protect the field against solidifying. The 5-MW Archimede plant in Sicily uses this
technology developed by Italian government agency ENEA and Archimede Solar Energy. It
is a solar fuel saver integrated in a larger natural gas plant;
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© OECD/IEA, 2011