Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 447

Figure 14.3.22 Heat pipes heat exchanger with Sodium for a Stirling engine.
solar dish is 4.4 m, the concentration factor 800 and the receiver metal temperature
about 650 ◦ C, resulting in an overall electrical efficiency of 21%.
14.3.18 Power system
The heat-to-electricity conversion in solar dish technology is usually based on a Stirling
reciprocating engine, while a Brayton cycle gas turbine is rarely adopted. In Stir-
ling engines for solar applications (Kongtragool and Wongwises, 2003), the working
fluid is usually hydrogen or helium. The principle of Stirling engines consists of fluid
compressed to about 20 MPa, heated to temperatures generally higher than 700 ◦ C
owing to the high concentration ratio and then expanded to produce power. The cycle
concludes with the cooling of the fluid (further details about the Stirling engine can be
found in Section 14.5). Heat pipes with sodium as the intermediate fluid are usually
adopted for these engines so as to achieve uniform and controlled temperatures of the
fluid (Figure 14.3.22).
Brayton engines, on the other hand, use air as the working fluid, with typical
maximum pressures of 0.25 MPa and turbine inlet temperatures of 850 ◦ C. In this kind
of application conversion efficiencies are limited to 25-27%. In Brayton cycle systems
the receiver is a volumetric absorber where solar concentrated radiation passes through
a quartz window and is absorbed by a honeycomb-like matrix which provides a high
exchange surface (Figure 14.3.23).
An example that represents an attempt to apply a gas turbine cycle to a solar
dish system is the adaptation of the Garrett Turbine Engine Company's automotive
gas turbine (Stine and Diver, 1994). The gas turbine is based on an open cycle with a
centrifugal compressor and a radial turbine operating at 87,000 rpm. Because of the
target operating temperatures of this engine, 1371 ◦ C (2500 ◦ F), a ceramic turbine and
ceramic hot-section components are currently under development. At such operating