HYBRID FOSSIL-GEOTHERMAL POWER PLANTS.

In this paper we introduce the so-called hybrid concept for the simultaneous conversion of high- and low-grade thermal energy into work. In particular, we consider hybrid fossil-geothermal systems in which the low-grade geothermal energy is used for preheating the working fluid in a power cycle at lower temperatures, while the high-grade energy derived from a fossil fuel is utilized for heating in the higher temperature range. In this manner the entropy production associated with the poor utilization of geothermal energy in a binary-fluid or a flash system is reduced. The performance of the hybrid system is analyzed through the introduction of a number of figures of merit. It is shown that from a thermodynamic viewpoint the hybrid system is superior to a combination of two individual state-of-the-art systems, one for the fossil energy and the other for the geothermal energy. This is evidenced by the ability of hybrid systems to generate more work than would be possible in individual plants. It is recognized that geothermal fluids at relatively low temperatures ( less than 150 C congruent 302 F) are below the limit of usefulness for the production of electricity in binary-fluid or flash systems and that such fluids are wasteful in space-heating applications. Thus, it is emphasized that the hybrid arrangement possesses the important advantage of being able to utilize such marginal geothermal resources to produce electricity.