Geothermal system archetypes: classification, applications and technology development opportunities

Figure 1: Geothermal archetypes

Geothermal is a renewable energy source that has the potential to be low or zero GHG emissions, but can be “baseload” in character available 24/7/365 and dispatchable or accessible on demand.  Geothermal resources are also used for Long Duration Energy Storage (LDES).

There are a number of ways that thermal energy held in rocks and/or fluids within the rocks in the subsurface can be harvested or reinjected.  Figure 1 illustrates a classification of these system archetypes.  It is based on an original diagram published by the British Geological Survey (1).  We have added several system types and provided additional detail on the source temperatures, distinguishing the use of heat pumps (Carnot cycle) for the provision of heat versus turbines (Rankine cycle) for the generation of electricity.

The classification assumes an average geothermal gradient of around 25 ºC/km and therefore is representative of most of the world’s continents away from areas of active extension or convergence of tectonic plates and volcanic hot spots, such as Ireland and the UK.  Therefore “shallow”, less than around 500 m depth, also means “low temperature” or in the case of hydrothermal reservoirs, “low enthalpy”, with temperatures up to 25 ºC.

Figure 2: Use of geothermal energy in the world. Data from Lund & Toft 2022 (2) and IRENA (3).

Shallow geothermal systems, depicted on the left-hand side of the diagram, are of two basic types and both most often involve the use of heat pumps to refine and boost the temperature of the thermal energy harvested in the geothermal boreholes.  A little over half of the world’s geothermal energy use is through shallow geothermal heat pumps (Figure 2).

There are two types of shallow system.  The most common is “closed loop” which is where heat is harvested to the borehole by conduction into a U tube or coaxial tubing in the borehole which contains a circulating fluid such as water or glycol-water mix.  There is no exchange of fluid between the rocks and borehole, the inefficiencies of conduction in rocks is allowed for through careful design of the number and length of borehole heat exchangers (BHEs) relative to the know heating (and cooling) loads through each year of the project.

“Open loop” is where there is an aquifer or groundwater resource in the shallow subsurface from which water can be produced, some heat extracted, and then most often re-injected into the rocks.  Although this system is much more efficient than closed-loop, requiring fewer boreholes, it is less common because the risks to the aquifer particularly if it is also a potable water source, need to be either avoided or carefully managed.

In our classification diagram, we have included mine water sources.  This system type is getting a good deal of attention in the UK for example, where old abandoned coal mines often naturally occur underneath or alongside centres of heat demand.  The water filled shafts and tunnels basically act as a big underground heat exchanger and that heat can be extracted either by open loop, or closed loop.

In deep geothermal systems, typically much deeper than 500 m, the heat is can be used for heating as with shallow systems or if above 80 ºC at the surface, for electricity generation.  All the heating systems we know of, except one, in the world are open loop and do not use heat pumps.  Most of the classic geothermal power generation plants sit in geologically hot areas where there are very hot waters or brines that flash as steam at the surface to drive conventional turbines.  But there is also a growing minority of binary turbines, where geothermal heat is used to heat another working fluid, with a lower vapour (or flash) temperature than water, making electricity in places like Nevada in the USA.

It is also worth pointing out clearly, that although the system depicted second from right to generate power is labeled “Enhanced Geothermal System”, in fact all current geothermal power plants use naturally occurring fluids contained withing naturally occurring permeability.  EGS is an exciting topic of research, development and demonstration, to create artificial permeability and even introduced water in otherwise hot, but dry rocks.  However, there are no commercial deployments on this technology as yet.

Similarly, we have depicted Deep Closed Loop Geothermal Heat Pump (GTHP) Array in the middle of the chart, but there is even less field pilot data on such deep closed loop borehole heat exchangers, let alone a commercial deployment.  Nevertheless, the technology is attracting attention and investment (e.g. Eavor, Greenfire) and Causeway is putting research effort into it too, because higher source temperatures in principle improve the efficiency of our heat pump systems.  Deep closed loop offers the same advantages as shallow closed loop, avoiding fluid exchange, contamination and induced seismicity, but is similarly burden the slowness of conduction in rocks.

In the next part of this series we will discuss the economics of these different archetypes and what that means for geothermal market development in example geographies.

(1) ABESSER, C, BUSBY, J P, PHARAOH, T C, BLOODWORTH A J, WARD, R S. 2020. Unlocking the potential of geothermal energy in the UK . British Geological Survey Open Report, OR/20/049. 22pp.

(2) LUND, J.W. & TOTH, A.N. 2021. Direct Utilization of Geothermal Energy 2020 Worldwide Review.  Proceedings World Geothermal Congress 2020+1 Reykjavik, Iceland, April – October 2021.

(3) https://www.irena.org