Non-conventional thermodynamic converters for low temperature geothermal applications

Abstract

Non-conventional thermodynamic converters suitable for low-temperature geothermal applications have been considered in the present paper. The first type, called a thermogravimetric system, TG, runs with a direct cycle: this is capable of transforming thermodynamic energy into mechanical-electric energy. The second type, called a thermogravimetric heat pump, TGHP, works with an inverse cycle. In this case the mechanical energy is supplied to the system through a feed pump and not by a compressor as in a conventional plant: heat is delivered within a range of 50–80 °C. The values of the cycle extreme temperatures make a TGHP appear more promising than a thermogravimetric system and therefore it has been analysed in more detail. In particular, a regenerative TGHP is characterised by COP values that are larger than those of a TGHP in its basic configuration and, furthermore, such values are larger than those of a conventional heat pump. In the present investigation, both the thermogravimetric system and the TGHP use demineralized water as a carrier fluid. Due to the environmental impact and other basic reasons, PP 50 (C 5F 12) and HFC 134a (C 2H 2F 4) at present appear to be the most promising working fluids.