MUSE – Differences between deep and shallow geothermal energy

The term geothermal energy covers a wide field of applications at different depth and temperature levels or based on different technologies to extract geothermal heat. Although all kind of geothermal energy originates in the Earth’s interior, the technologies clearly differ in their range of application and concepts. In this blog we will have a look on the terms “shallow“ and “deep geothermal” and try to explain how these systems differ and what they have in common.

What do “shallow” and “deep” geothermal energy have in common?

One has to bear in mind that the terrestrial heat flux alone, ranging between <50 mW/m² and 200 mW/m², is not strong enough to supply geothermal applications. Therefore, using geothermal energy in all of its different kinds means harvesting the heat stored in porous, water filled rocks of the subsurface, which is available around the year at the same capacity. Using it sustainable requires appropriate planning and resource management!

Both systems, “shallow” and “deep geothermal” allow for heating, cooling and underground heat storage and, apart from petrothermal energy (e.g. Hot Dry Rock), they refer to the same technological principles for heat recovery from the subsurface.

Ok, and how do “shallow” and “deep” geothermal differ?

Figure 1: Overview of ambient heat use supplied by heat pumps.

First, let’s take a look at the terminology: The terms “shallow” and “deep” refer to the depth of the heat absorber, which can harvest heat either from subsurface water with an open loop system or from the solid ground with a closed loop heat exchanger. There is no uniform definition for “shallow” and “deep”. In most countries, the depth separation is regulated by the Mining Act requiring a permit for drillings (mostly between 100 and 400 meters below surface). The Mining Act influences the drilling market and the depth range. For depth levels up to around 150 meters, a large market exists offering low drilling costs, which makes shallow geothermal energy affordable to private households as well. In contrast, deep geothermal energy requires long term planning and high investments with regard to drilling costs.

Additionally, temperature and capacity ranges separate “shallow” and “deep” geothermal. Shallow geothermal systems operate at temperature levels between 0°C and up to 30°C, which is considered as atmospheric ambient temperature – for this reason it can also be called geothermal ambient heat (see figure 1).  In contrast to the direct use of deep geothermal, shallow geothermal energy requires a heat pump to process the heat for space heating (indirect thermal energy use). However, in contrast to deep geothermal it allows direct (free) cooling, which makes it very attractive in urban areas. Shallow geothermal energy provides capacities up to 5 MWth, for individual buildings or de-centralized 5G low temperature heating and cooling grids. Due to the higher temperature levels between 30°C and up to 200°C deep geothermal is predominately used in industrial processes and conventional, centralized 2G to 4G heating networks. Moreover, deep geothermal energy allows for producing electricity at temperature levels above 90°C, which makes it attractive for combined heat and power applications.

Figure 2: Principle of exergetic prioritization – The relationship between required temperature level and matching renewable heat sources.

Our conclusion – geothermal energy is a real all-rounder!

There is no uniform and strict borderline between “shallow” and deep” geothermal. Both concepts contribute to the transition towards a future carbon free energy landscape. The selection of the right heating or cooling source, considering also other renewables, should match the required exergy level of your thermal application (see figure 2). Therefore, exergetic prioritization will be key for reaching the transition towards green energy supply. Why using a high enthalpy energy source like green gas or biomass for a low enthalpy use, such as space heating?

Summary

CharacteristicShallow GeothermalDeep Geothermal
Depth rangeNo permission for drilling, standard drilling range up to around 150 metersPermission for drilling required, drilling depths >150 meters to 5.000 meters
Temperature level0 °C to <30 °C30 °C to 200 °C
Thermal capacities<10 kW to <5 MW1 MW to >50 MW
Electricity productionNo electricity production possibleBinary circle: 90 °C – 200 °C Direct use: >200 °C
CoolingFree coolingForced cooling (adsorption, absorption)
Heat supplyIndividual buildings 5G heating and cooling networksIndustrial heat 2G to 4G heating networks

For more information about MUSE and geothermal energy in general, please contact our project coordinator:
Gregor Goetzl,  gregor.goetzl@geologie.ac.at

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