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Talking Point

With an increasing need for low-cost, low-carbon renewable energy, the hidden potential of deep geothermal energy needs to be considered more closely, says Róisín Goodman

Geothermal energy - the energy stored in the form of heat below the earth’s surface - has been used for space heating and bathing since Roman times.

More recently geothermal resources have been used for the supply of hot water for district heating schemes for homes, agriculture and horticulture, industrial applications, as well as to generate electricity.

A couple of recent projects clearly demonstrate the potential.

Working for the Sustainable Energy Authority of Ireland (SEAI), SLR has completed a Play Fairway Analysis of the deep geothermal resources of the Republic of Ireland.

The project is a first step in efforts to reduce the cost of geothermal exploration in Ireland by reducing the geological risk of identifying geothermal resource targets.

The study assessed the geothermal exploration risk by analysing the various attributes of the subsurface of Ireland to a depth of 5km.

The resulting series of geothermal resource risk maps published by SEAI is now helping to advance the exploration for geothermal resources in Ireland for district heating and the generation of low-carbon electricity.

The analysis will also encourage more exploration companies to get involved, increasing investment in exploration and the likelihood of success in the development of geothermal resources in Ireland.

Geothermal development company GT Energy has also recently been granted planning permission for its first geothermal electricity generating station in Ireland.

The plant, to be based at the Greenogue Business Park, Rathcoole, County Dublin, will be capable of generating up to 3.6MW of electricity which will be fed into the National Grid.

“Geothermal resource risk maps are helping to advance the exploration for geothermal resources in Ireland”

Deep geothermal resources are usually found below 1km and are commonly subdivided into hydrothermal and engineered geothermal systems (EGS).

In Europe hydrothermal resources are present in deep aquifers where heat can be easily extracted due to the presence of water as a heat transfer medium.

Doublet hydrothermal systems produce hot water from a production well and reinject to the aquifer using an injection well.

Single borehole hydrothermal systems can use a closed loop heat transfer system contained within a single borehole.

There are three basic technologies for generating electricity from geothermal energy.

Dry steam power plants and flash steam power plants can use water from the geothermal production well at temperatures greater than 182°C.

Binary cycle plants use geothermal water below 100°C to heat a “working fluid” such as isopentane, which is vaporised and used to turn turbine/generator units and is the technology that will be needed to generate electricity from Ireland’s low enthalpy geothermal resource.

Geothermal projects are characterised by high capital costs for exploration, drilling wells and installation of plant, but this is balanced by low operating costs.

Costs for geothermal projects are variable worldwide because exploration and drilling costs vary.

Nonetheless, the benefits could be far-reaching.

  • Róisín Goodman is an associate with SLR Consulting

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