A Thermodynamic Study of Heating with Geothermal Energy

A thermodynamic analysis of various methods of heating while using either purely geothermal energy or a combination of geothermal energy and conventional energy has been completed. Since geothermal energy, unlike conventional fuels, has an available energy that is markedly lower than its energy, a first law of thermodynamics performance parameter cannot accurately portray the thermodynamic performance and the second law based performance parameter (effectiveness) is used as the objective function during optimization. Results of the analysis present the optimum performance of several systems as a function of heating temperature and geothermal resource temperature. These results show that when only heating is required at a temperature substantially below the existing geothermal resource temperature a conventional direct geothermal heating system is thermodyanmically desirable. However, when heating is desired at temperatures near that of the geothermal resource, systems that use heat pumps can provide superior performance. Furthermore, for the usual range of water-dominated geothermal resource temperatures (up to about 500 or 525 K) some geothermally based heating systems have performance superior to that of fossil-fired furnaces and electric resistance heating systems at heating temperatures up to, and somewhat above, that of the resource temperature. At higher heating temperature, however, it is shown that furnaces and electric resistance heating systems improve their relative position and can have superior performance.