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 a low-enthalpy geothermal reservoir and its
environment show a 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 in detail, including suitable optimisation approaches. Since combined
generation of heat and power, or heat, power and cooling, is expected to
significantly improve the economics of a plant, promising methods of coupling of the
processes will be discussed briefly.