Solar Energy Perspectives - IEA

Chapter 8: Solar thermal electricity
• nano-fluids. Dispersing solid particles in fluids enhances thermal conductivity, but
particles rapidly settle in fluids. Nano-particles, possibly enhanced with surfactants/
stabilisers, would remain in suspension almost indefinitely, and have a surface area per
unit volume a million times larger than that of micro particles, offering improved heattransfer
properties; and
• pressurised gas, currently under testing at the Plataforma Solar de Almeria, Spain.
Additional work is needed to improve heat transfer in the receiver tubes, and to ensure
control of the solar field, which is more complex than the standard design.
Linear Fresnel Reflectors could provide a lower-cost option, in particular for direct steam
generation, thanks to fixed receivers and pipes. One weakness might be the greater cosine
losses when the sun is low in the sky, which would tend to restrict electricity generation to
the middle of the day.
Solar towers and dishes
Solar towers represent a less mature technology than trough plants. However, they hold the
promise of greater efficiencies and ultimately lower costs than all other STE technologies,
whether with or without storage. Some commercial tower plants now in operation in Spain
and in construction in the United States generate the steam directly in the receiver. At least one
other, in Spain, uses molten salts as both the HTF and storage medium. Others still may break
ground in the United States (Figure 8.2). One advantage of using molten salts as HTF is that
this can be done at low pressure with effective thin-wall solar receivers. Another is that it can
avoid the investment and temperature differences of heat exchangers between the HTF and the
storage medium.
Figure 8.2 Working scheme of a molten-salt solar tower
Collector field Molten salt system Power block
Receiver
Generator HP turbine
IP/LP turbine
Steam turbine generator
Receiver tower
Hot salt
Hot salt
Molten
salt loop
HP steam
Superheater
Reheater
Steam gen./evaporator
Reheat
steam
Condenser
Condensate
tank
Heliostats
Source: SolarReserve.
Cold salt
Thermal storage system
Feedwater preheaters
Steam generation system
Key point
Molten-salt towers represent the best option today for CSP with large thermal storage.
145
© OECD/IEA, 2011