Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 459

However, the design of the storage system is more complex because of the aforemen-
tioned mixing issues.
Lastly, an indirect configuration can adopt a solid as storage medium . When using
concrete, for example, the solar energy of the solar field is transferred through the
HTF to the solid storage material system. The storage material comprises a tube heat
exchanger which transfers the thermal energy from the HTF to the storage and vice
versa. This heat exchanger has a significant impact on overall investment costs and,
moreover, the design of geometry parameters such as tube diameter; the number of
pipes is also a key issue, increasing engineering costs. While experimental tests have,
however, shown long-term instability of the media (Gil et al., 2010), this technology
is potentially advantageous for the very low cost of thermal energy storage media.
Another possibility is phase-change materials as storage media. The overall concept
of this type of storage system is the same as in solid systems, but the storing material has
a melting temperature within the range of the charging and discharging temperatures
of the HTF (Bauer, Laing, & Tamme, 2011; Steinmann, Laing, & Tamme, 2010).
For the sake of completeness, there is also the possibility of obtaining an indirect
storage system in which the secondary fluid is steam (Beckam and Gilli, 1984; Slocum
et al., 2011). In principle the advantage of steam storage lies in its direct use in the
power block Rankine cycle. The disadvantage is related to the high storage pressures,
thus limiting its application to small-size storage tanks, exploiting the buffering of
the system. Examples of steam storage are the central-tower commercial plants PS10
(10 MW el ) and PS20 (20 MW el ) built in Spain and which started operating in 2007 and
2009 respectively; both of them feature sliding pressure steam accumulators which are
able to provide approximately 1 hour of operation at 50% load (Gil et al., 2010).
14.5 FROM HEATTO POWER
This section discusses the part of the plant dedicated to the conversion into electricity
of the thermal power collected in the solar field. Operating plants are based on two
different technologies: Rankine cycle and Stirling cycle. In large-scale CSP, which can
be either linear focus or power tower, technologies, the power cycle is based on the
water Rankine cycle, which is used also in other solid fuel-based technologies such
as coal plants and waste-to-energy plants. In solar dish, thermal conversion is based
on the Stirling cycle, which, though it has a fairly high efficiency even at small power
output, is difficult to scale up.
One of the main characteristics required to power cycles in solar plant is a high
conversion efficiency even at partial load; solar-based plant are characterized by daily
start-up and shut-down procedures, as well as by partial load operating conditions as a
consequence of the variation in solar radiation during the day. In reality, the adoption
of large storage systems (i.e. 7.7 hours as in Andasol) reduces the operating time at
partial load, something which cannot be avoided.
An example of thermal input for a power cycle with different storage sizes and
solar multiple, and in different seasons, is shown in Figure 14.5.1. At large storage size,
power production can begin before sunrise because the thermal input stored during
the previous day can be used.