What is geothermal energy?
The word “geothermal” comes from the Greek words ‘geo’ meaning earth, and ‘therme’ meaning heat. In other words, geothermal energy is the energy derived from the heat of the Earth.
How is the heat available?
This heat is generated by flow of heat from the Earth’s interior caused mainly by the radioactive decay of isotopes in the mantle and the crust. The heat conducts slowly continually to the surface along a temperature gradient which varies normally between 15°C/km to 30 °C/km. There are, however, some areas where the geothermal gradient is higher and thermal energy is accessible at shallow depths.
The Ring of Fire
Areas where large quantities of accessible thermal energy are located are usually along major plate boundaries where earthquakes and volcanoes are concentrated as shown in the figure below. One such location shown is the Eastern Caribbean.
The Ring of Fire. Areas represented in red are expressed as areas of increased seismic activity and volcanism.
Why is the Eastern Caribbean ideal for geothermal energy?
- There are 21 potentially ‘live’ volcanoes spread across 11 islands within the Eastern Caribbean.
- As a result, there are active magmatic heat sources beneath most of the islands which heat the surrounding rocks.
- The intersecting fault systems, formed from the active tectonic activity, creates pores and crevices (also known as fracture networks) that allow seawater or rainwater to seep into the hot rock.
- The hot water can be obtained by drilling wells and can be used to produce electricity (see below for an explanation on this). The above allows for the formation of geothermal systems.
Geothermal Systems
Geothermal systems are described by:
- A heat source: Usually the magmatic body heats the surrounding rocks through conduction.
- Rising water: Rainwater or seawater that seeps through pores and crevices becomes heated. As a result, it becomes more buoyant than the surrounding cold waters and rises through pathways that lead to the surface
- Hot water plume: The rising water is discharged at the surface through thermal manifestations (for example the Boiling Lake in Dominica, fumaroles etc.)
- Interaction: Rising fluids interact chemically with the surrounding rocks
- Counter flow: The upward flow of hot water from depth creates a downward counter-flow in the surrounding area so the surrounding cold water moves downward.
- Convection system: The movement of two types of water creates a circulating convection system.
Figure 3: Elements of a geothermal system as explained above. Source:GNS
How can we find geothermal systems in the Eastern Caribbean?
Possible geothermal locations can be located from geothermal exploration methods. Geothermal exploration gathers and evaluates information about a potential geothermal field. Some of these techniques include:
Geological Surveys
By studying and analysing surface formations or thermal manifestations, possible geothermal sites can be located. Surface mapping of numerous thermal springs, solfataras, fumaroles and steam vents, cold discharges, fossil alteration areas and phreatic craters were some of the initial steps to mapping the geothermal prospects in the Eastern Caribbean. The geological background information is used to determine possible geothermal locations and is used with other geothermal exploration.
Geochemical Surveys
This involves analysing gas, fluid and solid samples collected from surface geothermal features (e.g. hot pools, fumaroles, geysers) as well as subsurface environments. Parameters tested include temperature, isotopic rations, conductivity and rations of common elements and compounds. The data obtained can be used to determine whether geothermal systems are water or vapour dominated, estimate minimum temperature at depth and homogeneity of water supply, infer chemical characteristics of deep geothermal fluid or determine the source of recharge water.
Geophysical surveys
Unlike geochemical and geological surveys, geophysical surveys can help image the possible subsurface of the geothermal reservoir by measuring the physical properties related to the geothermal area. Some types of geophysical methods usually conducted in geothermal areas include: Magnetotellurics, Seismology, Gravity and Magnetics. The objectives of geophysical exploration of geothermal areas include finding geothermal prospects, outlining drilling fields, locating reservoirs, estimating the properties of the system and locating potential well targets.
Reservoir/ Conceptual Modelling
Data from the geophysical exploration methods can be integrated into a conceptual model. The use of more than one data reduces the uncertainties associated with each geothermal exploration technique. Conceptual models are descriptive or qualitative models incorporating the main features of geothermal systems in question. They define the general flow pattern, temperature and/or pressure conditions, define the geology to highlight the existence of the cap rock and explain the nature of the heat source of the geothermal system. Conceptual models help locate favourable drilling geothermal wells.
Drilling Geothermal Exploration wells
Geothermal exploration wells are drilled to confirm the presence of the geothermal reservoir. Several analyses can be performed on these wells to further constrain the geothermal area and identify the possible energy output.
How is geothermal energy extracted and what can it be used for?
Geothermal energy can be extracted by drilling wells. The steam or hot water produced from the wells can be utilized to power turbines for power generation.
Figure 4: The process by which heat is extracted. A well is drilled to a required depth. The hot water/ steam produced from these wells can be used to power turbines for power generation.
Advantages of Geothermal energy (C-L-E-A-R) within the Caribbean
- Costs- Most islands within the Eastern Caribbean are dependent on imported oil causing high electricity prices. The use of geothermal energy can help reduce the cost of electricity in these islands.
- Low Carbon emissions- low greenhouse gas emissions compared to fossil fuels alternatives
- Efficient- Once the geothermal reservoir is managed appropriately the power output can remain steady.
- Abundant- Thermal energy is constantly available in the Earth. The flow of heat from the Earth’s interior to the surface is estimated at 47 terawatts.
- Reliable- unlike solar and wind it is independent of the weather.
Disadvantages
- High initial capital costs- the costs for drilling geothermal wells are very expensive
- May release harmful gases- Underneath the earth’s surface, there is a substantial amount of greenhouse gases. Harnessing of the heat can potentially lead to the migration of these gases to the surface of the earth and pollute the air.
- Some geothermal reservoirs can be depleted if not managed appropriately.
Read more on the advantages and disadvantages of geothermal energy here.
Can geothermal activities trigger earthquakes?
Renewable Energy World reports that geothermal drilling does not cause earthquakes. There are, however, some instances in which micro earthquakes can occur. These include injecting water into geothermal reservoirs or fracturing geothermal reservoirs whose permeability is usually low (more applicable to Enhanced Geothermal systems). For more information on Enhanced Geothermal systems please see the video below:
