Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar thermal electricity
The high concentrating ratio (hundreds of suns) of the tower concept achieves high
temperatures – 565° for molten salts, 550°C for the steam – thereby increasing the
efficiency at which heat is converted into electricity. Improved efficiency also means
a lower cooling load, thus reducing water consumption in plants in arid areas. It would
also reduce the performance penalty of dry cooling. In addition, the concept is highly
flexible; designers can choose from a wide variety of heliostats, receivers, transfer fluids
and power blocks (some plants have several towers that form one power block). The
potential for cost reduction in thermal storage is particularly impressive. Solar towers using
molten salts as HTFs and storage media need three times less storage media than current
trough plants, thanks to the larger temperature difference between the hot and cold molten
salts.
The possibilities of even higher temperatures should be explored using different receiver
technologies. One option is supercritical steam cycles, such as those used in modern coalfired
power plants, which reach overall efficiencies of 42 to 46% with supercritical and ultrasupercritical
designs (thermal-to-electric efficiencies of 45% to 50%). Typically, modern
coal-fired power plants use steam at up to 620 °C and 24 MPa to 30 MPa, but by 2020 could
reach 700 °C and 35 MPa, using nickel-based alloys to achieve overall efficiencies
approaching 50%. The application of this technology to solar towers, however, will require
some adaptation.
Direct steam generation (DSG) will pose particular challenges in synchronising solar fields
with receivers and supercritical steam turbines. A continuous management of solar
collectors will be needed to avoid problems during start-up and variations caused by
clouds and at sunset. Solar towers with high-temperature HTFs and storage may prove
more capable of fulfilling these requirements, as they disconnect solar heat collection and
power generation. Superheating with some fuel, or full hybridisation in solar-gas or solarcoal
plants (see below) could also help address these challenges. Solar towers would need
to be paired to fuel each single supercritical turbine, whose minimum electric capacity
today is 400 MW.
High-temperature tower concepts also include atmospheric air as the HTF (tested in
Germany with the Jülich solar tower project) with solid material storage. Solar-to-electricity
efficiencies of up to about 25% can be delivered by such towers, but it is not yet clear if
the gain in efficiency may compensate for the cost and complication of the cycle.
Molten salts decompose at higher temperatures, while corrosion limits the temperatures
of steam turbines. Higher temperatures and efficiencies could rest on the use of fluorideliquid
salts as HTFs up to temperatures of 700°C to 850°C, with closed-loop multi-reheat
Brayton cycles using helium or nitrogen, which have initially been developed for hightemperature
nuclear reactors (Figure 8.3). On top of higher plant efficiency, such power
systems operate at relatively high pressure and power densities that implies smaller
equipment than for steam cycles with their large low-pressure low-power-density
turbines, so they could cost less. The preferred heat-storage medium in this case would
be graphite.
Solar-based open Brayton cycles offer a completely different way of exploiting the higher
working temperatures that towers can achieve. Pressurised air would be heated in the solar
receivers, and then sent directly to a gas turbine, at a temperature exceeding 800°C. The
pressurised air can be further heated in a gas fired combustion chamber to reach 1300°C,
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© OECD/IEA, 2011