What is Geothermal Energy?

Geothermal energy is heat energy that comes from within the earth. Heat is taken from liquid and vapour circulating in pores and fractures in the rock, or directly from the rock itself.

 

Elements of a Geothermal System (https://www.gns.cri.nz)

Groundwater flows down to depths of a few kilometres, gets heated up and rises to the surface by convection.

  • Heat source: Underlying the system at deep down is hot magma.
  • Rising water: Groundwater near the magma becomes heated and more buoyant than the surrounding colder waters and rises through pathways that lead to the surface (hot water is also less viscous than cold water).
  • Hot water plume: The rising water is discharged at the surface through hot springs and steaming ground.
  • Interaction: The rising fluids interact chemically with the surrounding rocks and their temperature is moderated by mixing with cooler water and by local boiling.
  • Counter flow: The upward flow of hot water from deep down creates a downward counter-flow in the surrounding area so the surrounding cold water moves downwards.
  • Convection system: The movement of the two types of water create a circulating convection system that is a very efficient way to transfer heat.

A schematic showing how groundwater is warmed.

Extracting the Heat

How do scientists and engineers turn geothermal heat and steam into something you can use for heating? First, specialist geologists and engineers locate an economic source of geothermal energy. Site selection is based on heat content, fluid content, and permeability of the rock.

Bubbling mud, geysers and hot pools are good indicators of geothermal activity at the surface, but what is going on underground? There are a vaiety of exploration methods used to find out:

  • Areas are explored, analysed and mapped
  • Geological, geophysical and geochemical data are combined
  • Field models are developed
  • Geothermal potential is assessed
  • Drilling sites are identified

 

Geothermal Drilling

Drilling is used for geothermal exploration and for generating electricity and direct heat use applications.

A bore hole is created by rotating a drill bit on a hollow drill string. As the drill penetrates the ground, steel pipe casing is added to keep the hole open until the required depth is reached. Drilling fluid is circulated to clean cuttings from beneath the bit and carry them back to the surface. The well is extracting geothermal heat. The pressure of the system must be monitored as temperatures can be over 100°C.

 

Advantages of Using Geothermal Energy

  • Clean – has a low environmental impact
  • Abundant – present throughout New Zealand
  • Reliable – is a renewable resource independent of weather and climate
  • Low carbon – has lower greenhouse gas emissions compared to fossil fuel alternatives