This section refers to geothermal systems for applications that require a large amount of heat energy, e.g., district heating, industrial processes, or electricity generation. Large amounts of geothermal heat can be accessed by drilling deep boreholes into the Earth. In some cases, very large wellfields consisting of many shallow boreholes can also produce large amounts of heat for space heating (and cooling) applications. In the Irish geothermal context, geothermal electricity production will require very deep boreholes in excess of four kilometres deep.
What is a geothermal doublet?
A geothermal doublet is a pair of wells used to extract geothermal energy from the subsurface. A doublet consists of one abstraction and one injection well. The doublet can be shallow or deep, and deeper doublets will be capable of extracting more heat. Naturally heated groundwater is pumped to the surface where heat is harvested, and the cooled water is reinjected into the subsurface to be recirculated and warmed by the hot rocks. The doublet requires careful management of the system to ensure the rate of heat and water extraction is balanced. Deep geothermal doublets are commonly used across Europe for district heating (e.g., Paris basin, France) and horticulture (e.g., the Netherlands).
How much energy can be generated?
It is difficult to estimate energy potential without site-specific geological and engineering studies. Over 40 deep doublets (1.5 - 2 km) in the Paris basin, France, produce just over 240 MWth, heating 187,000 households and saving 240,000 tonnes of CO2 compared to natural gas. Preliminary studies have estimated that a doublet in the Dublin basin could provide anywhere between 3 and 14 MWth. However, more deep subsurface information is needed.
How much does it cost?
In general, geothermal energy requires a high capital investment because drilling is expensive. However, once the system is installed the operational expenses are very low. As geothermal energy is local, secure and always on, this means that the cost of the heat is very stable and not dependent on market fluctuations. Using figures from the Paris basin (2014) as an example, a 1.5 km to 2 km geothermal district heating doublet cost between €10M and €15M to install (including surface plant). For higher temperature geothermal electricity generation, costs can range from €25M to €40M (including surface plant).
Does using geothermal energy cause earthquakes?
There is the potential for deep geothermal systems to generate small earthquakes, but these are generally too small to be felt by humans. This seismic activity is the result of rocks cracking when water is circulated at depth through open pathways in the subsurface. Small earthquakes can also occur when cooled water from the surface is recycled and reinjected deep underground to be heated up again. A small number of geothermal projects in Europe have been stopped due to earthquakes that could be felt at surface – these projects are generally in areas of higher natural seismicity than Ireland, or where injection rates have been too high. In the majority of well-run projects, and with good understanding of the subsurface, very low levels of seismic activity will occur (where felt these will be equivalent to e.g., passing traffic). Many projects with small amounts of seismic activity as described have continued to operate with the support of local communities. As a precaution, sophisticated and very sensitive seismic monitoring systems are installed in all medium and large-scale geothermal energy projects.
Does deep geothermal energy pollute underground water systems?
Deep geothermal systems extract water from rock units hundreds or thousands of metres below the surface of the Earth. This water is naturally saline and non-potable so deep geothermal projects do not interfere with the drinking water supply used by humans and animals. The saline water that is produced from geothermal wells to extract heat is always kept within a closed system of piping and is then re-injected into the deep reservoir that it came from. Because of this, there is little to no possibility for it to interact with shallow groundwater aquifers or surface water. The extraction of water (and heat from that water) does not require the injection of chemicals into the rock system. All geothermal projects are also governed by strict EPA and European regulation and legislation.
Do geothermal systems use all the water and affect wells/aquifers?
As discussed above, deep geothermal systems do not use groundwater from freshwater aquifers but draw it from deeper rock units that are unfit for human consumption, so they do not directly affect water wells or aquifers. Geothermal boreholes will be regulated and must be constructed to ensure adequate protection for sensitive aquifers.
Shallow open-loop geothermal systems can draw water from aquifers and therefore environmental extraction regulations will apply. The extracted water is re-injected to ensure minimal overall loss to the aquifer, and properly designed systems (e.g., proper borehole design and maintenance) should not introduce pollutants to the aquifer.
Is geothermal energy a renewable resource?
Yes, there is a constant supply of heat flowing from the core of the Earth. Groundwater that transports the heat to surface from a geothermal reservoir is recycled by injecting the produced water back into the Earth to be reheated and used again. Geothermal reservoirs are carefully monitored and managed to prevent derogation of the heat (small reductions in temperature over time).
Does geothermal energy produce greenhouse emissions?
At source, low- to medium-temperature geothermal energy is a carbon-free source of heat (this is the type of geothermal setting we have in Ireland). However, geothermal installations do have some degree of a carbon footprint due to their construction methods (e.g., drilling or excavation with diesel engines) and materials (e.g., steel casing, heat pumps). Technological developments such as electric drill rigs can be used to drive down the carbon footprint of these installations. Traditional geothermal power plants in volcanic regions can produce very small amounts of carbon dioxide relative to fossil-fuelled power plants (in the same way that volcanoes naturally emit carbon dioxide and other gases). However, newer 'binary' geothermal power plants release no emissions. The geological setting of Ireland means that the only geothermal plants built here would be binary plants. Geothermal district heating systems would also have no emissions.
Do geothermal projects have negative visual impacts or are they noisy?
Geothermal installations are mostly underground and as such have a negligible visual impact, especially when compared to, e.g., wind or solar farms. A geothermal well will have a temporary surface impact while it is being drilled. For deep wells the drilling rig itself can be up to 60 m tall but this is temporary (usually only on site for a few months). Modern geothermal drilling rigs are designed to operate in urban environments where minimising noise is paramount. Once drilling is complete and the rig is removed, the remaining well head (pipes) will occupy only a very small area of land. Frequently for district heating projects in urban areas the well heads will be housed within or next to a plant building.