Low temperature geothermal engines and experimental work under the CNR geothermal project (Italy)

Abstract When dealing with accessible energy sources, the production of mechanical energy can be attained in several ways. In a few cases, mechanical energy is directly available from its primary sources such as wind, waves, water fall by gravity, tides; however, mechanical energy is usually obtained by means of conversion from other energy sources and, to this effect, a very important role is played by the conversion of the chemical energy of coal, natural gas and oil either in internal combustion engines or in fluid-cycle thermodynamic systems. These systems can run on any heat source and are based on thermodynamic cyclic transformations, accomplished by a working fluid, capable of turning part of the source heat into mechanical work. The system features, the selection of the working conditions and the choice of the working fluid must be selected each time, according to the available energy source characteristics and the mechanical power required. Steam is the most widely employed fluid in high- and medium-power systems, fed by fossil fuels or a nuclear source. In the case of geothermal energy, two solutions are generally proposed: if dry, medium-temperature steam (above 150°C) can be directly obtained from the geothermal well, the predominant technology employed is the steam thermodynamic cycle, in which the geothermal steam operates as a motive fluid and the energy it is endowed with is partly converted into mechanical power. On the other hand, wherever geothermal wells containing lower-temperature fluids, such as hot water ranging from 80 to 150°C, are available, the use of steam cycles does not appear as a convenient solution. High conversion efficiency must not be expected at such low temperatures: it is well-known that, according to Carnot theorem, the maximum theoretical yield diminishes as the thermal source temperature decreases. The conversion process will be favoured if the thermodynamic cycle is improved and adapted to the working conditions given by the geothermal source: it is thus important to widen our survey to the examination of different working fluids and to investigate the best working conditions for each. In the case of organic fluids, dealt with in this report, their conversion efficiency is acceptable and approaches the maximum theoretical value for different temperature conditions and for different power requirements. It is worth remembering that low-temperature organic fluids may be employed in heat recovery from industrial sources and for the utilization of heat obtained from solar sources. This report is not meant to be a technological investigation but rather a general description of these cycles. Part 1 summarises the basic concepts of fluid cycles. Part 2 describes the work being carried out in Italy by the CNR Geothermal Energy Project.