Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar thermal electricity
least costly option available on a large scale, pumped hydro, offers a round-trip efficiency of
about 80%. Using thermal storage one needs to collect 2% to 5% more thermal energy if it
goes to storage; as losses are only thermal, their cost is also lower as they do not imply
running turbines in vain. Using electricity storage one needs to produce 25% more electricity
if it needs to be stored.
Storage is a particular challenge in CSP plants that use DSG. Small amounts of saturated
steam can be stored in accumulators, but this is costly and difficult to scale up. Effective fullscale
storage for DSG plants is likely to require three-stage storage devices that preheat the
water, evaporate the water and superheat the steam. Stages 1 and 3 would be sensible heat
storage, in which the temperature of the storage medium changes. Stage 2 would best be
latent heat storage, in which the state of the storage medium changes, using some PCM.
Sodium nitrate (NaNO 3 ), with a melting temperature of 306°C, is a primary candidate for this
function.
Thermal storage in STE plants can be used for a variety of purposes. The first objective is to
make the capacities “firm” despite possible variations in the solar input. STE plants, especially
linear designs with abundant HTF, already offer some thermal inertia, plus the spinning
inertia of their turbines.
Thermal storage also allows shifting the production in time, usually on a daily scale. The idea
is to produce electricity when it is most valued by utilities. This concept is all the more
important when peak demand does not coincide, or coincides only partially, with the
sunniest hours. Figure 8.5 illustrates the daily resource variations (DNI) and the flows from
the solar field to the turbine and storage and from the field and storage to the turbine, in
a CSP plant generating electricity from noon to 11 pm.
Thermal storage could also increase the capacity factor and achieve base load electricity
generation, at least during most of the year. Alternatively, it could be used to concentrate the
generation of electricity on demand peak hours.
Instead of adjusting the size of the solar field for different uses with the same electrical
capacity, as is often suggested, the designers would more likely consider solar fields of the
greatest possible size for large CSP plants inserted in grids, given the inherent optical
limitations to heliostat field size (see Chapter 8) and the limitation of linear systems due to
pumping losses through large fields. They would then adjust the turbine capacity to the
possibilities of the solar field, serving different purposes, as illustrated on Figure 8.6.
At the top of Figure 8.6 (as on Figure 8.5) the storage is used to shift production from the
sunny hours to the peak and mid-peak hours – say, noon to 11 pm. The solar field is thus
roughly the same size as that of a CSP plant of the same capacity without storage 1 – which
is expressed by a “solar multiple of 1”. Such a design would likely fit the conditions of
California. At the middle of Figure 8.6, the storage is used to produce electricity round the
clock in a smaller turbine. This leads to a solar multiple greater than 2. Electricity is less
costly this way, but has to compete with cheap base load power producing plants, usually
coal or nuclear. At the bottom of Figure 8.6, the storage is used to concentrate the
1. If a solar multiple of 1 is precisely defined as the size of the field that optimally feeds the turbine during the sunniest hour of the
year, then a CSP plant without storage will in practice have a solar multiple of about 1.1 to feed the turbine during more hours, at
the cost of small amounts of dumped energy.
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