Av Henrik Holmberg, stipendiat ved Fakultet for ingeniørvitenskap og teknologi, Institutt for energi- og prosessteknikk, NTNU
Geothermal energy has received increasingly international interest during the recent years. One example of this is the yearly geothermal conference at Stanford University in USA where both the number of publications and participating countries has increased rapidly during the last 5 years. Geothermal energy is commonly used both for heating demands and for electricity production and the theoretical potential is enormous. In a MIT-rapport from 2006 it is estimated that the resource can contribute with up to 100.000 MW electricity in USA within the next 50 years [1].
Geothermal energy referrer to the thermal energy that is produces in earth’s crust through breakdown of radioactive isotopes and the heat that is transported outwards from earth’s interior. The concept geothermal energy includes both deep geothermal energy systems where heat is mined from depths of several kilometers and shallow geothermal systems where wells with depths of a few hundred meters are used in ground source heat pump (GSHP) systems. While shallow geothermal energy is indeed an important part of the geothermal sector, deep geothermal energy is the focus for this text.
Deep geothermal energy has long been tightly associated with the geographically constricted and naturally occurring hydrothermal systems in volcanic active regions, see figure 1. In recent years it has been pointed out that engineered geothermal systems (EGS) can provide a way for geothermal energy to grow outside its geographical constraints and thereby to reach a significant share of its huge global potential.
Figure 1. Manifestation of hydrothermal system in Iceland.
The research on EGS or, hot dry rock (HDR) systems as it was first named started in the seventies. Primarily sedimentary basins and the periphery of the active regions were of interest in the subsequent projects aiming to extract heat from 4-5 kilometers depth through artificially created or stimulated fractures. A milestone was reached in 2006 when electricity production could be started from a research project in Soultz, France. Since then, commercial companies have been joining the field, and amongst others, USA and Australia have invested significant amounts into the research and support of geothermal energy. 3rdof may this year, electricity production started at the commercial EGS power plant in Habanero, Australia [2], where heat is extracted from a sedimentary basin. Although these pilot-plants have a relatively low power output in the range of 1 MWe, they are important proofs to the validity of the concept with artificially created geothermal systems.
Geothermal energy is ideal as a base load resource for direct usage of heat. Through history, geothermal energy has been used to cover direct heating purposes such as space heating, bathing and agricultural demands. In the development of EGS it is electricity production that has been in focus and thus areas with the highest geothermal potential have been sought. While low temperature resources that can be exploited for direct heat-purpose have been neglected for some time. This was recently pointed out in the IEA- roadmap for geothermal energy [3], which urges countries to also asses their potential for low temperature applications.
Low temperature resources can as well be used to produce electricity with binary cycles at temperatures lower than 100 º C. This has much in common with heat recovery from low grade waste and the efficiency for such a process is bounded to be low by the laws of thermodynamics. Thus electricity production would only be considered if there were no other way of disposing the heat, or in remote locations outside the electricity grid. Trough direct use of geothermal energy a high efficiency is ensured, while the resource can be used to displace for example electric resistance heating or other high grade fuels that could be used for electricity production.
District heating provides an ideal way to distribute low grade energy. And it accounts for 85 % of the direct use of geothermal energy worldwide [4]. The stable nature of geothermal energy makes it a suitable base load candidate in a district heating grid. However, district nets often operate at excessively high temperatures. In Scandinavia it is common with production temperatures around 80-90 °C and return temperatures around 65 °C. In some systems even higher temperatures can be found. High temperatures are often related to requirements from industrial processes while domestic consumers in general have a significantly lower temperature demand. Future district heating nets are likely to be operated at lower temperatures as the heating demand of buildings decrease; this also reduces transmission losses and promotes renewable energy resources such as solar and geothermal.
The primary focus of my research is an EGS in which the heat transfer is based on primarily thermal conduction [5]; in theory this gives a reliable system with a predictable long term performance. The amount of energy extracted from the system is, however, in direct proportion to the potential between the temperature of the inlet fluid and the targeted reservoir temperature. Thus a shift towards lower temperatures in district heating can have a tremendous impact on the accessible geothermal potential. Even though the geological conditions in Norway are less favorable compared to other places where EGS projects have been initiated, an EGS could be built based on what is considered accessible depths (4- 5 km) to provide hot water in the temperature range of district heating.
The geothermal sector in going through an expansive phase, currently it is the shallow geothermal installations that grow fastest while the knowledge base is being build up on deep geothermal and EGS. The potential for EGS is, however, tremendous, and the development is driven forward with successful projects like the ones mentioned earlier. In a world craving for energy, geothermal energy could be one of the major players and EGS is probably the way to unleash the potential. It remains however, to be proven that the installations can sustain long term heat production and that the concept can be applied to a variety of different site-conditions.
References:
1. http://mitei.mit.edu/publications/reports-studies/future-geothermal-energy, accessed 20-06-2020
2. http://www.geodynamics.com.au/home.aspx, accessed 20-06-2020
3. IEA 2020, http://www.iea.org/publications/freepublications/publication/name,3988,en.html accessed 20-06-2020
4. Lund J.W., D. H. Freeston., T. L. Boyd (2020) Direct Utilization of Geothermal Energy 2020 Worldwide Review, Proceedings World Geothermal Congress 2020
5. Holmberg. H., O. K. Sønju., E. Næss (2020) A novel concept to engineered geothermal systems, Proceedings, Thirty-Seventh Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California.
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