Thermodynamics of Deep Supercritical Geothermal Systems

Abstract

The study of water is very important in science and for many industrial applications, such as steam turbines, high pressure devices and supercritical cycles. In aquifers, hydrocarbon reservoirs, geothermal systems and atmosphere-ocean interactions, water is one of the most important fluids in action. Specifically, geothermal reservoirs contain water in a wide thermodynamic range. The temperature gradient between the core and the surface of the Earth results from a continuous flow of natural heat, embracing all kind of thermal energy resources that can be classified in terms of their profoundness, going from shallow depths (102 m), to medium depths (103 m), and to even deeper depths (104 m), reaching the immense temperatures of molten rocks. Warm aquifers below oil reservoirs, hot dry rock, submarine reservoirs, and engineered geothermal reservoirs are examples of profound geothermal systems. The fluids in very deep reservoirs can be at supercritical thermodynamic conditions, at more than 400°C and pressures larger than 220 bar, having higher fluid density, more volumetric enthalpy and heat available. They could provide twenty times as much power, per unit fluid volume, as the normal geothermal fluids used today. During the span of human life, deep geothermal energy represents practically an infinite potential. The aim of this paper is to provide a computational description of water thermodynamics at supercritical conditions showing the power it represents. The water properties are defined in terms of the Helmholtz potential or free energy, and are calculated for single phase, liquid, vapor and supercritical, as functions of density or pressure, and temperature. These properties are internal energy, enthalpy, entropy, specific isochoric heat, specific isobaric heat, compressibility, bulk modulus, Joule coefficient, speed of sound, thermal expansivity and thermal diffusivity.