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Main technology options available today

Geothermal energy is energy stored in the form of heat beneath the surface of the earth. Based on the available resource geothermal energy can be used for power and/or heat generation. High and medium temperature resources are used for electricity production while medium to low temperatures are merely used in direct heat utilisation applications, e.g. district heating, agriculture and industrial processes etc. Low to very low shallow ground resources are used for heating and cooling in buildings via ground-source heat pumps (GHP). Fig. 1 shows the most widely spread binary cycle configuration for power production.

Figure 1. Schematics of a binary cycle power generation plant

Binary cycle configuration

• Use of two separate loops for the geothermal fluid and the working fluid
• The working fluid is vaporized and led to the turbine cycle
• Better performance for medium temperature (< 180 °C) reservoirs
• Better environmental management

A more recent option is the Enhanced Geothermal System (EGS) or Dry Hot Rock System that exploits the heat of deep ground formations (hot rocks). The principal of the technology is the injection of water through pipes at suitable hot-rock sites. The water is heated and consequently collected back via another pipe system (Fig.2).

Figure 2. Scematics of the Enhanced Geothermal System

Combined heat and power (CHP) systems is an option for the utilization of rejected heat in cases where the demand is available in close vicinity of the plant. While electricity efficiency is low, typically around 10% due to relatively low operating temperatures, CHP total efficiencies can be as high as 97%. There is a small number of DH systems that utilize geothermal energy mainly supplemented by another heat source (eg fossil fuel or biomass). Other geothermal heat applications include direct use in balneotherapy, greenhouse heating, drying processes etc.

The most wide application of geothermal energy refers to the use of geothermal or ground-source heat pumps (GHP/GSHP) that utilize the earths seasonal temperature in order to provide space or water heating/cooling in buildings. Both open and closed-loop configurations are available depending on the available resource. Figure 3 shows a variety of piping configuration of such systems. The suitability of each loop type is dictated mainly by the land availability, soil type and system capacity.

Figure 3. Several configurations of a GHP closed loop system. (Source:The Encyclopedia of Alternative Energy and Sustainable Living)

Average electricity production cost for geothermal systems in heavily depends on site characteristics and ranges from 40 to 80 €/MWh. For experimental EGS systems production cost may reach 170 – 350 €/MWh. Average cost of heat from GHP is around 60 €/MWhth, but is strongly dependent on site and systems characteristics that may alter overall cost significant.

Global capacity of geothermal electricity production in 2009 systems was 10.7 GWe generating

67.2 TWh of electricity. Worldwide investment exceeded 2 billion € in 2008, increased by almost 40% from 2007, and global employment of the sector reached about 25 000 people.In the EU gross electricity generation from geothermal sources was 5.77 TWh in 2007. Direct heat use was 9.2TWh in 2008 while useful heat from GHP reached 17TWh. The following table provides the targets for electricity and heat generation from geothermal energy in the participating countries.

1. Source: EUROSTAT, 2. Source: NREAPs, 3. Accordign to Croatian Energy Strategy the potential for geothermal energy is 810 MWt and 46 MWe.

Territorial Imprint

Power generation geothermal plants induce several Health and Safety (H&S) issues during all the phases of development and operation. The two major hazards is the exposure to H2S emissions in case of accidental leackage and the pollution of groundwater resources with reinjected geothermal fluid. Well blowouts, although rare, are another technology specific hazard that may result in the realease of sulfate gases and other liquid pollutants.

One major issue is the stimulation of seismic activities due to the drilling in the cases of EGS. This is a rather important issue as several cases in the EU as well as in the US have indicated that EGS may cause frequent low to medium size earthquakes.

Geothermal power generation does not come without environmental considerations. Typical plants require 0.5 to 3.5 hectares of land per MW. Competition with other land uses is rarely the case, except in cases where traditional use of resources (e.g. balneology) imposes restrictions to industrial uses.

The development of geothermal energy is supported by national RES supporting schemes, eg. feed-in-tariffs. In some cases benefits to the local communities are provided. The following table provides information on such schemes.

SWOT Analysis

PG: power generation, DH: direct heat, GHP: Ground heat pumps

• Enermed Project - Composante 2 - Geothermal Energy - Branch Questionnaire