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What is geothermal energy?
Geothermal means heat from the Earth. The centre of the Earth is hot and largely composed of molten and semi-molten rock. This heat is the result of a few different processes including: left over heat from the formation of the planet; the gravitational pressure of the Earth itself, and the radiogenic decay of various elements in the mantle and crust. Convection of the molten interior (the mantle) is the main driver of tectonic plate movement. Heat flows from the mantle into the base of the crust and then via conduction to the surface where it is lost to space.
Fig 1. Mantle convection may be the main driver behind plate tectonics. Courtesy University of Sydney.
Because heat is constantly flowing outward from the Earth’s centre, temperature increases with depth into the crust everywhere. But in some places it gets hotter shallower because molten rock (magma) is very close to the Earth’s surface, such as along the edges of tectonic plates and Mid Ocean Ridges for example. New crust is constantly being created at the ridges and consumed along the margins, resulting in magma being emplaced close to the Earth’s surface and heating the rocks above and around it.
What can geothermal energy be used for?
Throughout history people have used this heat as a source of energy and put it to work for them. The Ancient Greeks, Romans, Native Americans, Maori, Japanese and Chinese have all used natural hot springs and steaming ground for health, cooking, food preservation and heating buildings.
Currently there are 3 main categories of geothermal energy use: Direct Use, Ground Source Heat Pumps (GSHP) and Electricity Generation.
Fig 2. Geothermal energy uses. Courtesy Geothermal Education Office, Tiburon, California, USA
Direct Use
Globally geothermal energy supplies >100,000 MWth for direct use applications. This is where the heat is used directly for some industrial purpose, and is not converted to electricity first. There is a huge range of industrial applications where low grade heat between about 50oC and 150oC is needed, and these temperatures are usually occur within 2-3 km of the Earth’s surface. Often quite shallow groundwater systems will be within this useable temperature range.
Low temperature geothermal fluids and steam have been used for heating swimming pools, health spas, food processing, greenhouses, aquaculture, desalination, timber and paper processing, refrigeration, equipment sterilisation, milk pasteurisation, and a host of other industrial uses where heat rather than electricity is needed. A future innovation is the use geothermal energy to produce green hydrogen for industrial uses and transport fuel.
The single biggest direct use of geothermal energy is in heating and cooling buildings. It can be used to heat single buildings or entire districts. One of the best known district geothermal heating systems is in Paris, France.
Current world use of geothermal energy for direct use applications is over 107,000 MWth and more than 283,000 GWh per year (Lund and Toth, 2020). It is a widely present but under-used resource.
Ground Source Heat Pumps (GSHP)
Ground Source Heat Pumps take advantage of the relatively stable temperature conditions that occur within the top couple of hundred metres of the ground surface to heat and cool buildings. The system is made up of an underground heat exchanger - which is basically a buried loop of pipes - and a pump. The pipes are buried to a depth where the ambient temperature is between about 5oC and 30oC, but most systems are quite shallow – about 6 metres deep - where the ground is usually around 10oC to 16oC. Heat from the ground warms the fluid in the pipes, which is pumped to surface where it is used in standard HVAC systems to heat the air in the building. In summer, the system is reversed to cool the building, the fluid in the pipes draws heat from the building and dissipates the heat into the ground. A by-product of GSHP is the production of hot water for domestic use.
GSHP are a passive system and very efficient using between 25-50% less electricity than air source heat pumps and conventional air conditioning systems and are also less polluting, with between 44% to 72% less greenhouse gas emissions (US DOE, 2020; Hanova and Dowlatabadi, 2007).
Fig 3. Residential heat pump diagram. Courtesy Encyclopaedia Britannica, Inc (2012)
GSHP are very widely used throughout North America, Europe and east Asia particularly Japan and Korea, where cooling of buildings is a significant energy demand. This is well proven mature technology, which can be applied to single buildings or larger developments and can be retro-fitted to existing buildings.
Electricity Generation
Geothermal energy also has some very good characteristics that make it attractive for generating electricity. Its biggest advantage is it is truly baseload. The Earth never stops generating heat and so geothermal electricity can be generated 24/7. Geothermal plants generally have a very high capacity factor which means they are very reliable. Geothermal energy is a sustainable and renewable resource. If properly managed this type of resource can be reliably used for decades. The first Geothermal electricity plant was built at Larderello, Italy in 1914 and the Larderello field is still in production with a total net capacity of 769 MW. Wairakei, New Zealand (352 MWe) and The Geysers, USA, celebrated their 50th Anniversaries in 2010 and are still producing. The Geysers is also the world’s largest geothermal electricity generator with an installed capacity of over 2000 MW.
Geothermal electricity is low emission and low polluting. The main waste products are hot water, silica and calcite. Hot water extracted from the geothermal reservoir for use in the power plant, is usually reinjected back into the reservoir and recycled through a closed loop system. Geothermal electricity generation has a small footprint particularly when you consider that the fuel source is co-located with the power plant.
Types of geothermal resources
Volcanogenic systems
World-wide about 30 nations currently generate electricity from geothermal energy and most of this production uses relatively shallow, very high temperature and high permeability geothermal resources related to active volcanicity. Although these resources are geographically discrete, only about 6 - 7 % of the total known global potential for this type of geothermal resource is currently used, so there is scope for geothermal generation from these resources to expand (IEA GIA, 2019; Huttrer, 2020).
Hot Sedimentary Aquifer Systems (HSA)
Even away from volcanic areas there is still a huge amount of heat energy beneath our feet waiting to be accessed. Electricity can be generated from geothermal resources with temperatures as low as 95oC but commercial viability of any given project is contingent on the temperature and permeability of the resource. These two factors largely control the resource’s productivity.
Hot Sedimentary Aquifer (HSA) resources are geographically more common than conventional volcanogenic resources. HSA are generally deeply buried, fluid filled sandstone or limestone reservoirs with relatively high natural porosity and permeability. Groundwater within a deeply buried aquifer will be at the same temperature as the surrounding rock. If the local geothermal gradient is moderately high, then at depths between 2 and 3 km the ambient rock temperature will be between 100oC to 150oC and so will any fluid present in the pore spaces of that rock. These temperatures are ideal for direct use applications and for electricity generation using Organic Rankine Cycle binary plants.
Until recently, Australia’s only commercial geothermal electricity was generated in Birdsville, using 98oC hot water from a bore into the Great Artesian Basin. Many of the HSA resources investigated to date in Australia have been identified through historical oil and gas drilling.
Fig 4. The three geothermal energy systems. Courtesy Greenrock Energy.
Engineered Geothermal Systems (EGS)
As a rule of thumb the average geothermal gradient is about 30oC/km, so to reach temperatures above 200oC needed for steam flash power generation, boreholes would need to be drilled to about 6 -7 km deep. At these depths, rocks are generally hard, abrasive, and impermeable metamorphic or igneous rocks. However there are areas where high heat production from crustal rocks locally increases the geothermal gradient. This additional heat is produced from rocks with higher than average content of the radiogenic elements Potassium (K) Uranium (U) and Thorium (Th). These elements are present everywhere and are constantly releasing thermal energy as they undergo radioactive decay over time. Most rock types contain some K, U, Th but granites tend to be the rocks most enriched in these elements. Even in granites, the amounts of these elements are very small, but when the volume of rock is extensive (cubic kilometres) the amount of heat produced is significant. The average continental heat flow is about 60 – 80 mW/m2 but there are local areas throughout Australia for example, with heat flow approaches 120mW/m2 or more, due to the presence of buried high heat producing granite bodies.
The fundamental process of electricity generation from EGS is the same as a volcanogenic or HSA system. Water is pumped from surface down an injection well, flows through a network of fractures throughout the hot granite rock, which heats the water. The heated water is then brought to surface through a second well and used to generate electricity. The main point of difference is the need to artificially enhance permeability in the reservoir.
Fig 5. A detailed view of an Engineered Geothermal System (EGS). Courtesy Greenrock Energy.
The ideal reservoir resembles a large network of small interconnected fractures which permeates the rock mass – similar to the thousands of tiny tubules which make up the radiator of a car. This acts as a huge surface area throughout the rock able to exchange heat with the transporting water. In EGS permeability of the reservoir is increased using a technique called hydraulic stimulation to extend and enhance existing fractures in the rock.
EGS have tremendous potential but commercial and technical challenges exist. The technology is advancing quickly but not yet at a point where these systems can be routinely deployed everywhere with commercial success. Significant leaps in technology are needed to be able drill deep into hard rock while controlling the well, and then engineer the subsurface heat exchanger to optimal permeability. Most of this technology development is adapted from the petroleum industry. There are however many hybrid HSA - EGS projects operational, particularly in Germany, which demonstrate that the technology works.
References
Hanova, J., and Dowlatabadi, H, 2007. Strategic GHG reduction through the use of ground source heat pump technology. Environ. Res. Lett. 2 (2007) 044001 (8pp).
Huttrer, G.W., 2020. Geothermal Power Generation in the World 2015-2020 Update Report. Proceedings World Geothermal Congress, 2020, Reykjavik Iceland April 26 – May 2, 2020.
IEA GIA (International Energy Agency Geothermal Implementing Agreement), 2019. Geothermal Trend Report 2016/17. Available online.
Lund, J.W. and Toth, A.N. (2020). Direct Utilization of Geothermal Energy 2020 Worldwide review. Proceedings World Geothermal Congress, 2020, Reykjavik Iceland April 26 – May 2, 2020.
US DOE (United States Department of Energy), 2020. Geothermal Heat Pumps. Available online: Geothermal heat pumps | Choosing and installing geothermal heat pumps
