Solar Energy Perspectives - IEA

Chapter 8: Solar thermal electricity
Seasonal storage for CSP plants would require much larger capacities still, and as noted in
Chapter 7 it would more likely rest on stone storage, if it ever comes to fruition. The volume
of stone storage for a 100 MW system would be no less than 2 million m 3 , which is the size
of a moderate gravel quarry, or a silo of 250 metre diameter and 67 metre high. This may not
be out of proportion, in regions where available space is abundant, as suggested by the
comparison with the solar collector field required for a CSP plant producing 100 MW on
annual average on Figure 8.7.
Figure 8.7 Comparison of the size of a 100 MW solar field
and its annual 67-m high stone storage
100 MW (annual average) collector
Source: Welle, 2010.
Daily storage facility
Annual storage facility
Key point
Annual thermal storage for CSP plants may emerge as a viable option.
Stones are poor heat conductors, so exchange surfaces should be maximised, for example,
with packed beds loosely filled with small particles. One option is then to use gases as HTFs
from and to the collector fields, and from and to heat exchangers where steam would be
generated. Another option would be to use gas for heat exchanges with the collectors, and
have water circulating in pipes in the storage facility, where steam would be generated. This
second option would simplify the general plan of the plant, but heat transfers between rocks
and pressurised fluids in thick pipes may be problematic.
Investment costs for annual storage would be less rapidly amortised than in the case of daily
storage cycles, as it would in one sense provide service only once a year. On the other hand
it would require very inexpensive storage media, such as stones; and the costs of heat
exchangers, pumps and pipes would probably be much smaller per MW th than for daily
storage, as heat exchanges would not need to take place at the same speed. So annual storage
may emerge as a useful option, as generation of electricity by CSP plant in winter is
significantly less than in other seasons in the range of latitudes – between 15° and 35° –
where suitable areas for CSP generation are found. However, sceptics point out the need for
much thicker insulation walls as a critical cost factor.
There is another way to store the energy of the sun – thermo-chemical storage. In the case of
low-temperature heat, as described in Chapter 7, the chemicals would probably remain
inside some storage tank, and heat exchanges between the solar collectors and the storage
tank will be done using some HTF. With high-temperature heat, however, the chemical
reaction could take place in the receiver itself. This not only allows for storage, but the
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