Low-enthalpy geothermal resources are typical finite heat capacity sources,
while the environment which acts as heat sink may be regarded infinite.
Therefore the temperature-enthalpy curves of heat source and heat sink have
an almost triangular shape. Applicability, advantages and disadvantages of
different ideal power cycles in such a setting, like the Carnot cycle, the Lorentz
cycle and a cycle with triangular shape, are discussed.
Different optimisation criteria like thermal efficiency, power output and resource
utilization may be applied to improve the design of the plants. Every criterion
would result in a different design.
A couple of power plant concepts were developed to get close to the ideal
cycle. The most prominent like the organic Rankine cycle (ORC) and the Kalina
cycle will be presented, including suitable optimisation approaches.
Cold production competes fairly well and will be addressed also. Since combined
generation of heat and power, or heat, power and cooling, is expected to
significantly improve the economics plant, promising methods of coupling of the
processes will be discussed briefly.