Applications

The high investment costs limit electrical power generation to only a few specific geological regions with near-surface steam storages. Otherwise, the normal temperature gradient of 3.5K per 100 m of depth requires too deep borings (for example, 5000 m) to get the required temperatures to produce electricity.

Using lower temperature heat is, however, often very plausible for heating buildings - ideally, groups of buildings or large buildings. For a single family home, the costs are usually prohibitive. Groups of buildings can be supplied with geothermal heat via a grid distribution system, as is done in Paris. The required deep drilling and the necessary appliances (primary circle system for aquifer and a secondary system for users) are expensive, so can only be justified if shared by a network of users. High-performance housing is not an ideal target group because the investment/gain ratio is rather poor, given the very small demand.

Near-surface geothermal systems, in which a heat pump can achieve nearly constant heat fluxes throughout the year, are a much more promising approach. Such systems can also be used for preheating/cooling outside air before it enters the air heating unit (AHU), and may thus significantly reduce the heat losses of the ventilation system. Such a configuration also avoids the freeze-up problem of high-efficiency ventilation air-to-air heat exchangers, as shown in Figure 12.10.2.

Source: H. Erhorn and J. Reiss, Fraunhofer Institut für Bauphysik, Stuttgart

Figure 12.10.2 Ground-coupled air heat exchanger for preheating air in high-performance houses

Source: H. Erhorn and J. Reiss, Fraunhofer Institut für Bauphysik, Stuttgart

Figure 12.10.2 Ground-coupled air heat exchanger for preheating air in high-performance houses

12.10.3 Design insights

Heating

Heat from near-surface layers can be taken from groundwater directly by using a fountain technology. Heat can also be extracted from near-surface earth layers by means of horizontally or vertically installed heat exchangers (see Figure 12.10.3). The extracted heat will be transformed by heat pumps to a higher temperature level. The lower the temperature of the heating system, the higher the efficiency of the heat pump. The heat pumps normally run in a monovalent mode, using buffer storage to harmonize the in-stationary demand of the heating during the day and the different seasons. Typical load values for heat pumps are 5 kW to 100 kW.

Investment costs required for adapting the heat source from the earth to the heat pump are in the range of €500 to €800 per kW. Systems with low outputs between 2 kW and 3 kW, for instance, are in the region of approximately €3500. The highest costs are caused by horizontally installed earth heat exchangers because extensive excavation work is required. A garden area measuring approximately 1.5 to 2.0 times the heated area of the house is needed to bury the heat exchanger. The use of

Source: H. Erhorn and J. Reiss, Fraunhofer Institut für Bauphysik, Stuttgart

Figure 12.10.3 Comparison of: (a) a direct-use system with (b) a vertical geothermal heat exchanger coupled to a heat pump system

Source: H. Erhorn and J. Reiss, Fraunhofer Institut für Bauphysik, Stuttgart

Figure 12.10.3 Comparison of: (a) a direct-use system with (b) a vertical geothermal heat exchanger coupled to a heat pump system groundwater via fountains leads to approximately 35 per cent higher annual costs due to the intensive maintenance costs required for these appliances. Investment expenditures for a heat pump and buffer storage with a range of performance between 5 kW and 100 kW amount to €500 and €800 per kW. Costs for a small heat pump with an output of between 2 kW and 5 kW (including buffer storage, piping and installation) are about €8000 to €10000.

The heating energy-related costs for the whole system in conventional houses amount to 5 cents per kWh (assuming electrical power costs for the heat pump of 5.5 cents per kWh). Because of the high standard costs for the installations, costs may easily increase to factor 2 and more in highperformance houses. Normally, however, costs are 25 per cent to 40 per cent lower than for solar-assisted heating systems. Only fossil and biomass-fired systems are usually a little more economical than earth-coupled heat pumps.

Preheating fresh air

Channelling the fresh air through earth channels before entering the AHU leads to a highly efficient use of geothermal heat. The installation is usually very cost efficient because the construction hole can be used without any extra costs for moving the earth. The extra costs for a single family home are usually less than €200, and a saving potential of approximately 1000 kWh can be used. This leads to heating energy-related costs of approximately 5 cents per kWh. Because homeowners may elect to install the heat exchanger themselves to save money, special care should be taken regarding the following points: the tubes should be generously dimensioned to minimize pressure losses and thereby minimize the needed fan power; they should also be sloped to drainage so they can be flushed for cleaning, and protected from dirt and debris by a filter for incoming air.

References

BMU (Federal Minister for the Environment, Nature Conservation and Nuclear Safety) (2004)

Geothermie - Energie für die Zukunft, BMU, Berlin FIZ (Fachinformationszentrum Karlsruhe) (ed) (2004a) Geothermie, CD-ROM Energie, CD-ROM

Datenbanken über erneuerbare und konventionelle Energien, Eggenstein-Leopoldshafen FIZ (Fachinformationszentrum Karlsruhe) (ed) (2004b) Geothermie: Basic Energies 8, Stuttgart IRB (Fraunhofer-Informationszentrum Raum und Bau) (ed) (no date) Erdwärmenutzung, IRB-

Literaturdokumentation, no 7046, Stuttgart, Germany Sanner, B. and Bussmann, W. (eds) (2004) Erdwärme zum Heizen und Kühlen [Geothermal Heat for Heating and Cooling Purposes]: Potentiale, Möglichkeiten und Techniken der Oberflächennahen Geothermie, Geothermische Vereinigung e.V., Kleines Handbuch der Geothermie, vol 1, Geothermische Vereinigung e.V., Geeste

Websites

Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (BMU): www.erneuerbare-energien.de

Geothermische Vereinigung e.V (GtV): www.geothermie.de International Geothermal Association (IGA): www.iga.igg.cnr.it Schweizerische Vereinigung für Geothermie (SVG): www.geothermal-energy.ch

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