At the core of the earth temperatures of more than 5000°C prevail. Commercially viable reservoirs of heat for geothermal applications generally feature temperatures of up to 200°C.

geothermal-plant-nesjavellir-iceland (ucsusa)
© Gretar Ívarsson

Geothermal energy

The further away from the core of the earth towards the earth’s crust, the lower the temperatures. Temperatures in the earth’s crust which has a thickness of up to 40 km several hundred degrees Celsius are common. From the surface of the earth towards the core, the temperature gradient is roughly 30°C per km. Most of this heat originates from radioactive decay of elements in the earth’s core. The heat released in those processes has been absorbed by the rocks. As these rocks have a high heat capacity, but a transfer heat rather slowly, heat from the earth is classified between renewable and fossil energies: Only a sustainable utilization can guarantee a balance between heat generated in the core and heat used for other purposes.

In volcanic regions temperature anomalies are in magma chambers near to the surface are common and lead to higher temperatures in lower depths. Due to the high share of drilling costs to the overall costs when trying to tap the geothermal potential, economic utilization of geothermal heat for electricity generation can be found in such areas. Most prominent examples are Iceland and New Zealand. In Germany those areas are seldom, and geothermal energy is not used for a significant amount of electricity generation.

For power generation (e.g. via a steam power process) temperatures should be as high as possible. In order to reach those temperatures, drillings have to be deep. This increases costs of a geothermal power plant. Commercially viable reservoirs of heat generally feature temperatures of up to 200°C. This implies however low Carnot efficiencies.

When using a reservoir with a temperature of 160°C, the expansion of the steam in the turbine cools the fluid to about 50°C. Carnot factors of 25.4 percent can be obtained. This is of course the highest theoretical efficiency. In real power plant processes the efficiency will therefore be considerably lower.

Possibilities for using heat from the earth include the following heat sources:

  1. Naturally occuring steam and hot water springs
  2. Artifical deep drilling into hot aquifers
  3. “Hot-dry-rock” cooling

Due to the lack of heating reservoirs, option 3 is the main form investigated in Germany. The generating capacities are roughly 10 MW. The Hot-Dry-Rock procedure involves drilling into hot and dry rocks, which is fractured by detonations. Water is then pumped into the drilled holed and is extracted again via a second borehole as steam.

Der Dampf bzw. das heiße Wasser hat Temperaturen bis 200°C und gibt seine Energie über einen Wärmetauscher an einen sekundären Wasser-/Dampfkreislauf ab. Der Sekundärkreislauf ist der Kreislauf eines Dampfkraftprozesses.

Low efficiencies and high costs to deep drilling operations for geothermal energy result in high costs of power generation.

An advantage of geothermal power plants is their ability to run as base-load power plants, as heat supply through the earth is constant and not fluctuating.