Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 467

Table 14.5.4 Performances at on-design conditions for five selected technologies

Indirect Direct

Molten Salts Superheating Direct

Indirect (Andrea (Andrea saturated Direct

Andasol Giostri Giostri (A. Giostri saturated

(A. Giostri et al., 2012) et al., 2012) et al., 2013) (Colzi et al.,

et al., 2013) Archimedes Supernova Novatech 2010)

HTF working 297.3-391.0 300.0-550.0 - - -

temperature ( ◦ C)
T steam SH ( ◦ C) 371 525 540.0 270.0* 260.0*

Pressure solar field (bar) 25.0 15.0 100.0 66.0 45.0

Pressure at turbine 95.0 115.0 81.0 55.0 40.0

inlet (bar)
Steam mass flow 63.5 47.1 56.8 77.2 18.0

@turbine inlet (kg/s)
RH temperature ( ◦ C) 371 525 - - -

RH pressure (bar) 14.5 14.5 - - -

Condensing pressure (bar) 0.096 0.096 0.096 0.096 NA

N ◦ of regenerators 6 7 7 3 4

Temperature at boiler 234.8 278.9 - - -

inlet ( ◦ C)
Temperature at solar - - 260.4 195.9 NA

field inlet ( ◦ C)
Net power output (MW) 50.0 50.0 50.0 50.0 11.0

Thermal input (MW) 144.5 128.9 130.2 156.7 35.8

Power block efficiency (%) 34.6 38.8 38.4 31.9 30.7

*Saturated steam.

of the main targets for solar power cycle design because of the reduced size of the
collector area, and thus lower costs, for a given power output. Most proposed Stirling
applications (Stine & Diver, 1994) are for small (10 to 100 kW) engines placed at the
focus of a parabolic dish concentrator. In point of fact, for small power output, the net
efficiency of Rankine or Brayton cycle-based engines is seriously degraded, favouring
the high efficiency potential of the Stirling engine. On the other hand, component size
and costs increase significantly for large-scale Stirling engines.
The ideal Stirling cycle results from two constant-temperature and two constant-
volume processes with working fluid in the gas phase. Figure 14.5.5 shows the four
processes in pressure-volume and temperature-entropy diagrams. Energy is exchanged
(either produced or absorbed) by the cycle only during the constant-temperature
processes; however, heat must be transferred during all four processes.
Because the processes at constant volume (2-3 and 4-1) involve an equal amount
of heating and cooling of the working fluid, in the hypothesis of ideal gas behaviour,
a regenerator may be used which transfers the heat internally to the cycle, between
expanded and compressed gas. As has already been pointed out, with an ideal heat
exchanger, heat would only be introduced and discharged from the cycle at constant
temperatures, ideally obtaining the same efficiency as the Carnot cycle. However, the