The Direct Use of Geothermal Energy for Residential Heating and Cooling
Photograph of a geothermal manifestation in Myvatn, Iceland where surface waters interact with heat from a geothermal system at depth to cause boiling. This geothermal manifestation is located on the rift zone between the North American and European tectonic plates. Photo by Ben Smith
by Benjamin D. Smith
Our worldwide demand for energy is constantly growing. We are faced with a big question - How can we satisfy this expanding hunger for energy in a sustainable way? Innovative and renewable solutions are essential as we seek answers. This paper explores one solution with great potential that has hardly been tapped - the direct use of geothermal energy for residential heating.
In some parts of the world, high temperature geothermal resources can be used to generate electricity where residential heating is provided secondarily by either using the electricity or residual heat left over from producing the electricity. This traditional use of high temperature geothermal energy resources is geographically limited to those locations where high temperature resources can be found. In contrast, lower temperature geothermal resources can be found more abundantly worldwide for direct use residential heating. Direct use geothermal heating, has the potential to meet the market demands of a much larger demographic than traditional geothermal energy. The figure below shows the potential heat uses of geothermal energy across the temperature spectrum. Temperatures below 150°C have low efficiencies for electric power generation and a high cost per kWh; however, these temperatures provide a wide range of applications for direct use. While there are many industrial applications, I will primarily explore residential space heating, as it is one of the largest categories of energy consumption. According to the U.S. Energy Information Administration (EIA, 2019), 50% of residential energy use is consumed by space heating and water heating.
Figure 1: Direct uses of geothermal energy (Ragnarsson, 2016)
Geothermal direct heating efficiencies
At a geothermal electric power plant, a typical steam turbine converts less than 50% of thermal energy into electrical energy. Before the electricity can be used by consumers, it must be carried along transmission and distribution lines where further energy losses occur. . At the end point of residential use, the electrical energy must then be converted back into thermal energy for space heating with additional energy loss. This is obviously a very inefficient process. On the other hand, a geothermal direct heating system, uses the thermal heat for space heating without the wasteful energy conversion steps described above. It is far more efficient for an end user to directly use geothermal heat sources instead.
Here in Iceland, nearly 90% of the houses are heated using direct geothermal district heating (Gunnlaugsson, 2008), yet this was not always the case. Coal use to be the dominate heat source but now, with geothermal heat, Iceland boasts the cleanest air in Europe.
Where can low temperature geothermal resources be found and used?
Low temperature heat sources can be found in sedimentary basins in many parts of the world. Some projects are investigating the use of old oil and gas wells for heat production. The areas that prove to have the largest potential for geothermal district heating are places with cold winters and hot fluids near the surface. However, with intelligent utilization, even areas with no geothermal anomalies have shown potential for geothermal heating and cooling using ground source heat pumps (GSHP).
Figure 2: Locations with accessible low temperature heat (Limberger et al. 2018)
Ground Source Heat Pumps
Even in areas with no geothermal anomalies, there is significant potential for residential geothermal heating and cooling. For example, ground source heat pumps (GSHP) can be used to transfer heat from your house into the ground in the summer and pull that heat back into your home in the winter. GSHPs are able to use the ground as a thermal battery because ground temperatures have far less fluctuation than the seasonal and hourly temperature ranges you will see inside your home. If you want to improve the sustainability of heating and cooling your home, you can easily find GSHP resources for your city and state with a quick Internet search. GSHP applications can take on many forms. In some locations, GSHP systems can be designed to provide heat to an entire community. In other areas, individual home applications may be more practical. In any event, there is an exciting future for GSHPs and the technology is worthy of your consideration.
Conclusions
When it comes to energy, we need to think outside the box. There are geothermal energy opportunities all around us, whether they are hot geothermal reservoirs suitable for electric power generation, lower temperature direct use geothermal opportunities, or ground source heat pump opportunities. I am excited to continue exploring these innovative opportunities in my studies at the University of Reykjavik.
Contact: bendsmith917@gmail.com
Work Cited
U.S. Energy Information Administration (2019). Independent Statistics and Analysis. Use of energy in homes. Accessed 2020 Jun 4. https://www.eia.gov/energyexplained/use-of-energy/homes.php
Ragnarsson, Árni (2016). Overview of direct geothermal applications and uses worldwide. IIIGGDP Roundtable. Harpa Conference Center. Reykjavik, Iceland. April 25 – 26, 2016. ÍSOR – Iceland GeoSurvey.
Gunnlaugsson, Einar (2008). DISTRICT HEATING IN REYKJAVIK PAST – PRESENT – FUTURE. United Nations University. Geothermal Training Program.
Limberger J, Boxem T, Pluymaekers M, Bruhn D, Manzella A, Calcagno P, Beekman F, Cloetingh S, Wees J-DV (2018). Geothermal energy in deep aquifers: A global assessment of the resource base for direct heat utilization. Renewable and Sustainable Energy Reviews. (82) 961–975. doi:10.1016.2017.09.084