Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 207

ing through the body. The more complete the alternating heat transmission is per-
formed inside the regenerator, the bigger the mean temperature difference be-
tween working and compression cylinder and thus the efficiency of the Stirling
motor.
If the displacing piston is coupled to the working piston in the appropriate
phase angle via a driving mechanism or a vibratory system, the whole system can
serve as thermal engine.
In terms of mechanical design, single and double-acting machines are distin-
guished. In single-acting machines only one side of the compression or expansion
piston undergoes pressure fluctuations inside the working space, while the pres-
sure of the working gas is effective on both sides of the piston of double-acting
machines; in the latter case, they simultaneously work as compression and expan-
sion piston /5-21/, /5-24/.
Stirling machines can also be distinguished into kinematic and free piston Stir-
ling engines.
- Kinematic Stirling engines perform power transmission via a link mechanism.
A generator can be coupled to this gear by a shaft, leading to the exterior.
- Free piston Stirling engines lack mechanical interlinkage between the working
piston, the displacement device and the environment. Both pistons move freely.
The converted energy can be transferred to the exterior by an axial generator,
for instance. Mechanical interlinkage is replaced by an interior spring damping
system; this is why only two movable parts are required. The machine is her-
metically sealed, so that tightening issues are avoided. Free piston Stirling ma-
chines present the theoretical benefits of a simple structure and a high reliabil-
ity, but are currently far behind in terms of development when compared to ki-
nematic machines.
The machines applied for dish/Stirling systems use helium or hydrogen between
600 and 800 °C as working gas temperatures. Power output of the Stirling motor
is controlled by varying the working gas mean pressure.

5.4.1.2 Plant concepts

Because of their size and space requirements individual dish/Stirling systems are
suitable to supply power to small and medium grids (micro and mini grids). When
combined with batteries and/or additional generators operated by fossil or bioge-
nous combustibles, they are suitable for the energy supply of rural communities.
Since, in this respect, they have to compete with a multitude of other renewable
sources, current developments concentrate on automated operation and cost-
cutting.
Alternatively, dish/Stirling power plants can be interconnected to provide lar-
ger quantities of heat and power. The biggest park was commissioned in 1984 in
California and consisted of 700 individual collectors and a central thermal engine
of an electric overall capacity of almost 5 MW.