Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 179

- The Clausius-Rankine cycle (steam power cycle/two phase cycle) makes use of
the phase transformation of matters. Such phase transformations correspond to
isothermal heat addition and large additions of specific volume. Their technical
application is easy (isotropic compression/decompression, isothermal heat ad-
dition and dissipation). This is why such processes were first technically ap-
plied (Fig. 5.5 (e), (f)).
For current industrial applications Joule and Rankine cycles are most commonly
applied.
- For the Joule cycle the working medium "ambient air" is aspirated and com-
pressed prior to adding heat. Heat can either be added by caloric devices or in-
ternal combustion (e.g. by combustion of natural gas). For solar applications
heat is transferred directly from the absorber to the working medium of the en-
ergy conversion process. The volumetric absorber itself has a very large sur-
face to benefit both heat transfer and radiation absorption. Since pressurised air
is used as working medium such an absorber must be of closed design. Indirect
heat addition, for instance by means of a heat transfer medium, is disadvanta-
geous since the working medium air only has a poor thermal conductivity and
thus requires large surfaces for heat transmission.
- The Clausius-Rankine cycle, by contrast, requires a phase change medium to
allow for isothermal heat addition. In most cases water is applied, but there are
also processes using organic working media for low-temperature applications
(so-called Organic Rankine Cycles (ORC)). At the beginning, the liquid work-
ing medium is highly pressurised and undergoes a phase change while heat is
added. The now gaseous material is subsequently expanded, possibly after fur-
ther heat has been added. Afterwards condensation is performed under low
pressure while heat is dissipated.
All above-mentioned cycles have in common that heat is first applied to increase
the volume flow of a gaseous working medium. Subsequently, during its expan-
sion, this volume flow performs mechanical work in pressure engines, which can
either be designed as oscillating machines of varying working volume (i.e. recip-
rocating engines) or as machines with stationary flow (i.e. turbo-machines or tur-
bines). For large-scale power plants dealing with large volume flows almost ex-
clusively turbo-engines are applied.
Turbines are referred to as turbo-engines which first transform the potential en-
ergy of a flowing working medium into kinetic energy and afterwards into me-
chanical energy of the rotating turbine shaft. The medium flows through the tur-
bine either axially or radially, causing it to rotate. A stator whose blades form
nozzles causes the working medium to first expand and at the same time acceler-
ates the rotor. Inside the rotor coupled to the turbine shaft the kinetic energy of the
working medium is subsequently converted into shaft torque. The combination of
rotor and stator is referred to as turbine stage; for instance, in large turbines up to
sixty subsequent stages are implemented. Inevitable friction, inconvertible kinetic
energy at the turbine exit and so-called gap leakages are considered to measure