Alternative Uses of Geothermal Energy

Abstract

Currently in Poland, geothermal energy is only used for heating purposes, like the classic geothermal heating plants, as well as balneological and recreational objects. However, there are many suggestions on using geothermal energy to produce mechanical energy and electricity. This text refers to the thermodynamic and energetic determinants of the use of low-temperature geothermal energy for electricity production. To model the conversion of thermal energy (geothermal energy) into mechanical energy a Stirling engine was used. This engine was selected as the alternative device to the expensive and technologically difficult Organic Rankin Cycle (ORC). The Stirling engine runs on a gas cycle (without phase transition). As the working medium to the Stirling cycle you can use argon or helium, and also common gases such as carbon dioxide, nitrogen or air. It is also possible to use hydrocarbons too. The first of these are expensive and make it difficult to attain the proper level of airtightness in the device, but provide a higher cycle efficiency. The last have explosive properties, which are of course undesirable. However air and carbon dioxide are widely available and do not carry any hazards to the environment or people, although they do not guarantee as much efficiency as other gases. Considerations have been factored in for the relatively low temperatures of the upper heat source, corresponding to the Polish conditions of extraction, a temperature around 80C. The lower heat source is in the form of a natural heat receiver, for example surface water or air. The concept was presented in the form of an additional component installed on the operating geothermal installation, as a heat exchanger producing electricity in a Stirling engine specifically adapted for this purpose.