Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 205

Tube receiver. The directly radiated tube receiver is the simplest type of a solar
receiver suitable for operation with a Stirling motor (Fig. 5.21, left). The heating
tubes of the Stirling motor, through which the working medium flows, serve as
absorber surface. Thus the receiver tubes are directly heated by the concentrated
solar radiation. The volume of the tubes filled with the working medium should be
as low as possible to keep the dead volume of the engine low. The shape of the
receiver must fit to the geometry of the focal spot produced by the concentrator.
Heat pipe receiver. For heat pipe receivers (Fig. 5.22, right) a phase-change heat
transfer medium (e.g. sodium) is applied. Since this heat transfer medium under-
goes an evaporation and condensation cycle, the latent evaporation heat is trans-
ferred from the radiated absorber surface to the heater and from there to the work-
ing medium of the Stirling motor, while the temperature is almost kept constant.
Subsequently, the condensate is re-transferred to the heating zone via a capillary
structure. Due to the heat pipe principle this structure requires comparatively high
efforts in terms of production engineering. However, this concept offers the ad-
vantage that high or extremely different heat flow densities may be homogenously
transferred onto the Stirling heater thanks to good heat transmission. It is also
beneficial that the heat pipe receiver may comparatively easily be combined with
other types of operation; i.e. in addition to solar radiation it can also be operated
by liquid or gaseous, fossil or biogenous fuels /5-22/.
Such receivers are most commonly designed as cavity receivers. The concen-
trated radiation passes through a small aperture and impinges on a cavity. The
actual absorbing surface, which is subject to temperature rise due to the incident
radiation, is positioned behind the focal spot. Because of this geometric position,
the absorber surface is bigger than the aperture; the radiation intensity which im-
pinges on the receiver is thus reduced. Yet, with regard to cavity receivers heat
losses are relatively low since only a small portion of the diffuse radiation emitted
by the absorber is lost by the aperture and convection losses, caused for instance
by wind.
Stirling motor. Thermal energy provided by concentrated solar radiation can be
converted into electrical energy using a Stirling motor with coupled generator.
Stirling motors belong to the group of hot-gas machines and use a closed system;
i.e. within the working cycle always the same working gas is used /5-23/. Con-
trary to Otto or Diesel engines, energy is provided by external heat supply, so that
Stirling motors are also suitable for solar operation.
The basic principle of a Stirling motor is based on the effect that gas performs a
certain volume change work in case of a temperature change. The process is based
on isothermal compression of the cold and isothermal expansion of the hot me-
dium at a constantly low volume, in case of heat supply, and, at a constantly large
volume (isochorous), in case of heat removal (Fig. 5.5 (c)). Periodic temperature
change - and thus continuous operation - can be ensured by moving the working
gas between two chambers of constantly high and constantly low temperature.