Glossary for the topic geothermal energy

Glossary

More information: At this place we can only offer definitions of some of the terms used on our pages. The German Geothermal Association provides a detailed encyclopedia of geothermal energy on its website (only in German): www.geothermie.de

Kinds of geothermy

Definition "Deep geothermy": Deep geothermy consist of systems which exploit geothermal energy with deep boreholes and using the energy directly, without raising its temperature.

Types of use of geothermal energy

Balneology/ thermal spa: Balneology is the doctrine of the therapeutically application of natural medical springs, healing gases and peloids by bathing, drinking and inhaling.; A hot spring resort uses a thermal spring for therapeutic application. Exposable thermal springs are used for bathing and healing since time immemorial. Since the 15th century bc ancient bathing facilities are verifiably known using thermal springs.; Thermal water is groundwater with a surface temperature of 20 °C gushing from natural springs or exploited with production wells.Source: Deutscher Heilbäderverband

CO2 production: Extraction of CO2 from groundwater e.g. for the beverage industry.

District heating: District heating is a system for distributing heat generated in a centralized location for residential and commercial heating requirements such as space heating. The term district heating is used for systems supplying cities or parts of a city with heat. The term local heating is used for systems supplying buildings or parts of building or small housing estates with heat of their own local heat production.

Space heating in terms of local heating: The catch phrase local heating describes the transfer and/or storage of thermal energy between buildings for heating purposes if the heat transfer distance is short compared to district heating.; Local heating systems are small decentralized units in contrast to district heating systems. They are working with relatively low temperature transfers. Accordingly it is possible to use relatively low order lost heat of cogeneration units or the heat production of solar collector systems or of low temperature ground heat systems. Using local heating systems increase the efficiency of the utilization of primary energy. As part of the orientation to renewable energy resources the development of local heating systems is very important to reduce the amount of the total energy expenditure distributed as high-order energy like electricity or hydrogen.; The thermal power of typical local heating systems has the range from 100 kilowatt and some megawatt. Local heating systems supply a residential area or a municipality with heat. For this purpose it is possible to integrate long-term heat storage in the system. However, this makes no sense for only one building.

Greenhouse: Heating of greenhouses with ground heat for a protected and controlled propagation of plants.

Power generation: Electric power generation is defined as production of electric energy. To produce electric power out of hydrothermal energy geothermal water with temperatures from 100 °C upwards is needed.

Potable/ service water: Drinking water is sweet water with a very high degree of purity, so it is suitable for human consumption. It is usable for drinking and for cooking food. Drinking water should contain a minimum concentration of mineral substances but no disease-causing microorganisms. The most common dissolved mineral substances are sodium, calcium, magnesium, potassium, chloride, bicarbonate, and sulfate. The summation of their concentration is called water hardness. The quality requirements for drinking water are defined in Germany in the DIN 2000 and the drinking water regulation (TrinkwV).; Process water (also referred to as service water or industrial water) is water for the technical, industrial, agricultural or domestic application and unfit for human consumption. Anyway, it has to meet all technological requirements of the respective processes. For example, cooling water must be so designed, that cooling units are not clogged by algae or lime. For this reason the water has to be cleaned up depending on the application (e. g. demineralized water for use in steam turbines).

Other: Uncategorized usage.

Unused: No recent usage.

Unknown: No known type of usage.

Production development concepts

Doublet: For hydrothermal utilization water will be hoisted from deep water-bearing formations (aquifers) and a heat exchanger accumulates the geothermal energy. Basically, the cooled water, with low mineralization, could be dumped on the surface (waste water system, receiving stream). Of course, in many cases it is necessary to recharge the withdrawal aquifer with the cooled water or to reinject the water for reasons of disposal engineering. The cooled water will be reinjected into the aquifer by a second borehole at a sufficient distance from the production borehole (doublet operation). In general is a combination of a number of production and reinjection boreholes possible.; The classic system of a doublet consists of two vertical drillings at a sufficient distance. Nowadays production wells and injection wells are drilled at the same well site. That is possible because of the development of producing zones with directional drilling. Besides the hydraulic connection with the aquifer is better than with vertical drillings. Additional, the facility on the surface needs less space, the technical facilities could be installed in one place and this is why there is no need for long above-ground connecting pipes.

Singlet: The geothermal exploitation with a single well system or singlet only needs a production well because the extracted water will not be reinjected. This is the case for example in the balneological utilization.

Deep vertical heat exchanger: Deep vertical heat exchangers are closed systems, installed in wells with more than 400 m depth. The used technique is similar to the technique of a shallow borehole heat exchanger. In the closed system of the deep vertical heat exchanger circulates a heat carrier medium down to 3000 m depth. The transfer of the heat of the rocks to the circulating fluids in the heat exchanger goes through the backfilling material and the casing of the borehole. The cold fluid is piped down in the annular space of the double tube system (coaxial tube) regulated by quantities. It will be heated up convectively because of its slow motion (5 – 66 m/min) and rises to the surface in the isolated internal pipe.; In the heat pump on the surface the warm fluid will be cooled down to 15 °C and then it will be reinjected in the annular space with a gear pump. Ammonia is often used as fluid. The withdrawal of thermal energy cools the rock; the arising horizontal temperature gradient causes the temperature flow from the surrounding rock.; Deep vertical heat exchangers are not relying on good permeable aquifers and for this reason they could be installed everywhere. The use of existing deep wells is favorable because of the heavy investment costs of this technique. Deep vertical heat exchangers have closed circuits so there is no interference with the chemical balance of the rocks. Solution or precipitation reactions are ruled out compared to Hot-Dry-Rock-Systems.

Geothermal utilization of mines: Exhausted disused mines and gas reservoirs are conceivable deep geothermal projects. With reservations, deep tunnels are usable for geothermal projects. The temperature of the formation water is depending on the depth between 60 and 120 °C hot. Existing wells and shafts could be used for the geothermal exploitation.; The technology to exploit geothermal energy have to be integrated into the safekeeping of the used mine in a way that the under public law normalized safekeeping aims, to keep disused mines free from dangers (§ 55 paragraph 2 Federal Mining Act and § 69; 2), could be accomplished with the additional installations.

Mining

Permit area: Anybody, who wants to explore geothermal energy in an area free for mining needs a concession. Anybody, who wants to exploit geothermal energy needs an authorization or the mine's property (§ 6ff BBergG). The differentiations between these mining rights are not trivial for geothermal energy. Since the application of the classical term deposit is not suitable for geothermal energy, for known reasons, in contrast to other mining free natural resources. Therefore, the assessing of mining rights for the exploitation of geothermal energy has to consider special constraints to satisfy the statutory provisions of the mining laws of the federal states. In addition, these criteria have to meet the needs of the potential geothermal energy users and of the applied technology.; The Federal/State Committee of mining commissioned the Ad-hoc-Working group "Assessment of geothermal fields" with the preparation of these constraints. The report is adopted in the meantime by the Federal/State Committee of mining and serves as a recommendation for all mining authorities. Hence, a broad national approach is ensured.

MD: The MD (measured depth; also along hole depth) of a well describes the length of the borehole, which is normally larger than the true vertical depth.

TVD: The TVD (true vertical depth) of a well describes the length of a perpendicular line from the surface to the bottom of the well.

Production data

Calculation of installed capacity

Data of installed geothermal power and used energy are of particular interest to the general public. These data are also used on an international scale. They are published annually in the Trend Report of the International Energy Agency Geothermal Implementing Agreement (IEA-GIA) and every 5 years on the occasion of the World Geothermal Congress organized by the International Geothermal Association (IGA). Unfortunately not every operation company of geothermal installations provides appropriate data. Therefore, the installed geothermal capacity must be calculated.

The basis is initially the installed geothermal capacity:

(1) P = ρF * cF * Q * (Ti - To) with

^-3

^-1

With ρF = 998 kg m^-3 and cF = 4181 J kg^-1 K^-1 follows the calculation formula:

^-3

With Q ( l s^-1) as flow rate during operation (or the permissible production rate according to the water act or the open-flow potential) and Ti the temperature at the wellhead (or the reservoir temperature). If either of them is not available then the calculation of the installed capacity is not possible. The scheduled value for the return temperature is 20 °C; this is by definition the minimum temperature for thermal water. The geothermal utilization of thermal water for a thermal bath starts above this temperature. Otherwise it has to be heated up adequate to the requirements. Thus the application of equation (2) is only feasible for Balneology / thermal spa. Nevertheless, it can be used for calculations for greenhouses, service water and other applications.

For this kind-of use you may assume a perennial production (8750 h) and calculate the annual energy production E:

(3) E = 8,76 * P in GWh/awith

P as the (calculated) installed power [MWt].

It is not possible to make a valid calculated estimation of the installed power of geothermal application in district and space heating with equation (2), because there are no applicable boundary conditions of the output temperature.

Normally the installed capacity of these facilities is known and the annual production data is collected on a regularly basis. Nevertheless, it is also possible to estimate the annual geothermal production based on the installed geothermal capacity provided by the operator. Therefore, the assumption is that a district heating or space heating installation is running 2190 full-load hours per year. This is equal to a load factor of 0.25 and similar to equation (3) follows:

(4) E = 2,19 * P in GWh/awith

P as the geothermal installed power [MWt].

Bear in mind, that normally geothermal central heating systems consist of several energy sources to compensate energy losses and peak loads. Therefore the installed geothermal power must be distinguished from the total installed power; both values are available in the catalog.

Source: PESTER, S., SCHELLSCHMIDT, R. & SCHULZ, R. (2007): Verzeichnis geothermischer Standorte – Geothermische Anlagen in Deutschland auf einen Blick – Geothermische Energie 56/57: 4-8.

Geothermal Information System

Current research data on the potential and use of geothermal energy