What is Geothermal Energy?
Geothermal Energy depends on the Heat of the Earth.
The Deeper you go into the Earth’s crust and Subsoil, the Hotter the Rock and Groundwater. Geothermal Energy is based on a set of Techniques that allows us to Recover this Heat, at varying depths and Temperatures. This Renewable Energy allows the Heating & Cooling of Houses, Buildings, Factories, and Agricultural Buildings. If used to turn a Turbine, this Heat can be transformed into Electricity.
Water Tables and Aquifers
The Term Geothermal Energy is formed from the Greek words Ge (Earth) and Thermos (Hot). Underground heat is stored in Shallow Groundwater, Deeper Aquifers (Rock Reservoirs Containing Groundwater) or Granitic Rocks. This Heat Energy comes in roughly equal parts from the Residual Heat produced by the formation of the Planet (4.5 Billion Years Ago) and from Natural Radioactivity.
The Geothermal Gradient
The Thermal Gradient of each part of the Globe measures the Average Temperature Increase with Depth. The Gradient has an Average Value of 3 °C per 100 m depth (i.e. 30 °C Increase per 1 000 m). This Average figure depends on the Physical and Geological conditions of the Region. The Subsoil of a Volcanic area, for example, experiences very rapidly rising Temperatures.
The Different Types of Geothermal Energy
Geothermal Techniques differ according to the Temperature, the depth and the use of this heat.
There are Generally Three Categories
Very Low Energy Geothermal
The Temperature is Lower than 30°C and the Depth of Treatment Ranges from 10 to 200 m. It is used to heat (or cool) Individual Houses, High-Rise Buildings, and Commercial Buildings. The Calories are collected by raising up Groundwater or by introducing Vertical or Horizontal “Geothermal Probes” into the Ground. Heat Pumps are used to Raise the Temperature or Lower it to Provide Air Conditioning.
Low Energy Deep Geothermal
The Temperature of the Water is between 30° and 90°C and the Depth of Operation is between 200 and 2 500 meters. Water from Aquifers Rises to the surface and then returns to the Depths, thanks to Two Boreholes that form a “Doublet”. This Technique makes it possible to heat neighbourhoods with Thousands of Inhabitants or Industrial Parks, via Heating Networks installed on the Surface
High Energy Geothermal
Dealing with Temperatures exceeding 150°C, it mainly consists of Producing Electricity from Deep-Water Steam Sources. Another Technique consists of Injecting Water into a Hot, dry Deep Rock and then recovering it at the surface. High-Temperature Geothermal Energy can also be used for Cogeneration to Recover Heat & Produce Electricity at the same time.
| Temperature | Depth | Targets | Methods | |
|---|---|---|---|---|
| Very Low Energy Geothermal Energy | Less than 30 C | From 10 to 200 m | Individual Houses Buildings Shopping Centers. | Horizontal Networks, Vertical Probes, Groundwater collection. With Heat Pump support. |
| Low Energy Deep Geothermal Energy | Between 30 and 90 C | Between 200 and 2 500 m | Neighborhoods Industrial Parks. | Doublets (2 Vertical Drillings in Aquifers). |
| High Energy Geothermal Energy | More than 150 C | Between 1 500 and 5 000 m | Power Generation plants. | Deep Drilling in Aquifers or Water Injection in Deep Rocks. |
Advantages and Limitations of Geothermal Energy
Geothermal Installations have the advantage of occupying little surface space, being Non-Polluting and Soundproof. They do not consume Water since it is returned Underground. After the Investment Phase, Operating Costs are Low.
But Geothermal Reservoirs tend to be depleted after 30 or 50 years as they are Exploited. Deep Boreholes must be monitored for Seismic Shocks. At the surface, long-distance heat transport generates significant Heat Loss. It is therefore preferable to use the heat in the immediate vicinity of the Boreholes or to install a Heat Network to distribute it in the Area.