Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 446

pedestal support, respectively. Figure 14.3.21a shows the components and working
principle of a 10 kWe prototype, Eurodish, developed in Europe 9 . Figure 14.3.21b,
shows a more diffused configuration based on pedestal support (Stirling Energy
Systems, USA); in this case the solar concentrator dish structure supports an array
of curved glass mirror facets.
With a solar dish collector, concentration can be as high as 3000, with potential
high temperatures on the absorber and, consequently, on the thermal engine. The
optical design of components and the accuracy of their manufacture determine solar
radiation interception and limit the concentration factor. Cost optimization indicates
10-12 m as a maximum diameter for the reflector, therefore limiting the single solar
dish to a net power output of about 25-30 kW with a solar radiation of 1000 W/m 2 .
Nevertheless, solar dish technology can be applied even to multi-MW solar power
plants thanks to its modularity: two examples consist of Maricopa Plant in Arizona
and a Power Purchase Agreement (2010) for the project of a 664 MW plant in Calico
(USA) involving about 26,000 dish Stirling “Sun Catcher'' (25 kWe) made by Stirling
Energy Systems. Recent market evolution seemed to convince the project's owner to
convert the Calico project entirely to PV technology.

14.3.17 Receiver
The receiver of a dish Stirling has two functions: absorbing solar radiation reflected
by the concentrator and transferring this energy to the working fluid of the thermal
engine. Usually, the receivers used in parabolic dishes are of the cavity type to reduce
radiative and convective losses (Mancini et al., 2003). In commercial solar dishes, two
kinds of receiver have been used:

• Tube receiver : the absorber consists of several tubes which transfer the heat directly

to the working fluid of the thermal cycle. The high temperatures of these absorbers
(up to 800 ◦ C) make it difficult to use selective coatings due to the great overlap
between absorbed and emitted radiation.

• Reflux receiver (heat pipes): these receivers use a primary loop with liquid metal

(usually Sodium) that evaporates on the absorbing surface. The liquid metal then
condenses, transferring the heat to the working fluid of the thermal cycle. This
solution implies two heat transfer loops, but takes advantage of the high heat
transfer coefficients (800 W/cm 2 ) and the metal condensation with a more uniform
heating of the working fluid.

The absorbing surface of the receiver is generally positioned behind the concentrator
focus in order to limit the intensity of the incident thermal solar flux to values of
approximately 750 kW/m 2 .
Recently, some models have been proposed in the power range of a few kW in order
to obtain cost reductions through lighter structures and standardized low-maintenance
engines. Infinia is a Stirling manufacturer that proposes a 3.2 kWe solar dish Stirling
(Infinia Company), based on a Beta-free piston configuration (see Section 14.5 for
details on the Stirling cycle) with helium as the working fluid. The diameter of the

9 Project co-financed by the European Community (under Contract N. Jor3-CT98-0242).