THIS POST IS A SUMMARY OF A COMPREHENSIVE PRESENTATION OF GEOTHERMAL HEATING BY PROFESSOR JOHN O’SULLIVAN OF THE UNIVERSITY OF AUCKLAND, NEW ZEALAND.

LINK TO SOURCE DOCUMENT: https://www.irena.org/-/media/Files/IRENA/Agency/Events/2014/Jun/2/1_Sullivan.pdf?la=en&hash=7B8C0F4B0F52944710B21166BFC30352DD7AC288#:~:text=Enthalpy%20is%20the%20measure%20of,discussing%20types%20of%20geothermal%20systems

Everywhere in the world there is an upward flux of heat at the surface of the earth arising from radioactive decay in the interior. The average value of the heat flux is 65 milliwatts per square meter ~65mW/m2. The corresponding average temperature gradient is 30C/km or 0.03C/meter but the gradient will vary according to the thermal conductivity of the rock). In geothermal areas, found at plate boundaries and other geologically active regions, the heat flow is much greater. The type of geothermal system depends on the heat flow and the geological setting. They range from warm water systems where there is no fluid movement and no boiling, up to two-phase systems with convection and lots of boiling underground. Power is the rate at which energy is being produced (or used). Energy is measured in Joules (J) Power is measured in Watts (W). 1 Watt = 1 Joule/second. 1 kW = 1000 W & 1 MW = 1000kW. Enthalpy is the measure of energy contained in water or steam (kJ/kg). Before discussing types of geothermal systems we need to quickly review the thermodynamic properties of water. As liquid water is heated it changes phase by boiling to become steam (a gas phase) The boiling point of water depends on pressure.

Before discussing types of geothermal systems, we need to review the mechanisms for heat transfer that operate in a geothermal system. They are Conduction, Convection, and Counter-flow. In conduction, heat flows from a hot temperature to a cold temperature without fluid movement. The rate of heat flow = conductivity x temperature gradient.

Convection – in hot geothermal systems there is a large scale movement of water (called convection), with hot water rising. Heat moves with the hot water. Heat flow = mass flow x enthalpy Requires pathways for water to move: PERMEABILITY.

Counter-flow: In some geothermal systems there is a boiling zone containing water and steam. Here water trickles down and steam rises. This process is called counter-flow Heat flow = steam rising – water trickling down. Also requires pathways (permeability) Transfers heat (even though it may not transfer much mass) because the enthalpy of steam is higher than the enthalpy of water.

Warm water systems: In warm water systems the heat transfer mechanism is conduction alone. There is no convective circulation of groundwater. Boiling does not occur in the reservoir even during exploitation. They are mainly used for direct use, nonelectrical purposes. Binary plants are now being produced that can use water at 90C.