Geothermal resources with the temperatures over 150°C are used for conventional electricity generation. Binary fluid technology has been developed to generate electricity from medium-to-low temperature resources and to increase the utilization of thermal resources by recovering waste heat. The binary system utilities a secondary working fluid, which has a low boiling point and high vapour pressure (Freon, isopenthane, etc.) and can be operated at 85-170°C.
Various conversion systems exist for generating electricity from geothermal fluids.
1. Direct dry steam
Steam dominated fields produce only steam and these fields are the simplest to exploit. Steam from the well is filtered and simply passed through a turbine then a condenser. Cooling water for the condenser is provided by a conventional cooling tower (Fig. 1).
Ref. : http://www.digtheheat.com/geothermal/dry_steam_plant.html
2. Separated steam
Water dominated systems produce both steam and water which have to be separated by a separation vessel located at the wellhead. While steam phase is sent to the turbine, water phase is injected back to the reservoir or discharged to the surface waters.
2.1. Direct non-condensing cycle (back pressure turbine) The simplest and the cheapest cycle used to generate electricity is the direct intake non-condensing cycle. Steam from the separator is simply passed through a turbine and exhausted to the atmosphere. There are no condensers at the outlet of the turbine (Fig. 2)
2.2. Condensing cycle
Condensing cycle is a thermodynamic improvement on the non-condensing cycle. Instead of discharging the steam from the turbine to the atmosphere it is discharged to a condensing chamber that is maintained at a very low absolute pressure-typically about 0.12 bara. Because of the greater pressure drop across a condensing turbine approximately twice as much power is generated from a given steam flow, at typical inlet conditions, compared with an atmospheric exhaust turbine.
2.3. Double flash cycle
The waste water from the separator is sent to a flasher at a lower pressure to produce more steam for a low pressure turbine. Usually a dual pressure or pass-in turbine used. Double flash cycle increases the power generation of the plant.
2.4. Multi-flash cycle
The maximum power would be extracted from the water dominated fields by using an infinite number of flash vessels connected in cascade under economical restrictions.
2.5. Binary cycle
If the geothermal well produces hot water instead of steam, electricity can still be generated, provided the water temperature is above 85°C, by means of binary plants. These plants operate with a secondary, low-boiling point working fluid (freon, isobutane, ammonia, etc.), in a thermodynamic cycle known as the Organic Rankine cycle (Fig. 3). Heat is transferred from the geothermal fluid to the monclerbinary fluid via heat exchangers where the binary fluid is heated and vaporised before being expanded through a turbine to some lower pressure/temperature.
2.6. Hybrid fossil-geothermal systems
Geothermal fluid is used for preheating or superheating at a fossil fuel plant to improve the expansion efficiency.
2.7. Total flow
Two phase geothermal fluid is used to generate electricity by machines such as biphase turbine, gravimetric loop machine, Armstead-Hero turbine, Robertson engine which are not yet economically proven.
1.Armstead, H.C., Geothermal Energy, E&F.N. Spon, NY, 1983, 2nd Ed.
2.Dickson, M.F., Fanelli, M., Geothermal Energy, John Wiley&Sons, 1995.
3.Barbier, E., “Nature and Technology of Geothermal Energy: A review”, Renewable and Sustainable Energy Reviews, V.1, No.1/2, pp.1-69, Pergamon Press, 1997.
DIRECT USE OF GEOTHERMAL RESOURCES
The medium-to-low temperature geothermal resources (T < 150°C) can be used to supply heat energy to residences and industrial applications. These resources can be used for residential heating, commercial greenhouse heating, aquaculture and industrial processes.
Geothermal heat energy is cheaper than fossil fuel energy. It is possible to reduce the cost of using heat energy 80% by using geothermal energy. By the use of geothermal energy, pollutants produced by fossil fuels can be reduced or eliminated.
Direct use systems generally consist of three main components.
1. Production unit: It consists of the well that brings hot fluid to the surface. 2. Mechanical installation: Heat exchangers, control units and pipes that transport heat to the place where it is necessary. 3. Discharge system: Injection well that is used to discharge cold geothermal fluid to the reservoir.
The medium-to-low temperature geothermal resources are mainly used in district heating systems, residential heating, greenhouse heating and aquaculture. In district heating systems, hot fluid is transported to office buildings and residences from geothermal wells by means of mechanical installation. Geothermal district heating systems are approximately 30%-50% more economical than natural gas systems. An example geothermal district heating system is shown schematically in Fig. 1.
Geothermal district heating systems are capital intensive. The main costs are initial investment costs, for production and injection wells, downhole and transmission pumps, pipelines and distribution networks, monitoring and control equipment, peaking stations and storage tanks. Operating expenses, however, are comparatively lower than in conventional systems, and consist of pumping power, system maintenance, control and management. A crucial factor in estimating the initial cost of the system is the thermal load density, or the heat demand divided by the ground area of the district. A high heat density determines the economic feasibility of a district-heating project, since the distribution network is expensive. Some economic benefit can be achieved by combining heating and cooling in areas where the climate permits. The load factor in a system with combined heating and cooling would be higher than the factor for heating alone, and the unit energy price would consequently improve.
Space cooling is a feasible option where absorption machines can be adapted to geothermal use. The technology of these machines is well known, and they are readily available on the market. Geothermal space cooling has expanded considerably since the 1980s, following on the introduction and widespread use of heat pumps. The various systems of heat pumps available permit us to economically extract and utilize the heat content of low-temperature bodies, such as the ground shallow aquifers, ponds, etc. (Fig. 2)
Geothermal energy use in greenhouse heating and aquaculture is widespread. The first application of greenhouse heating in Turkey was in Kızıldere, Denizli in 1985. The total area of geothermal greenhouses in Turkey is 31 hectares and the total heating capacity is 69.61 MWt.
The most common application of geothermal energy in agriculture is in greenhouse heating, which has been developed on a large scale in many countries. The cultivation of vegetables and flowers out-of-season, or in an unnatural climate, can now draw on a widely experimented technology. Various solutions are available for achieving optimum growth conditions, based on the optimum growth temperature of each plant (Figure 3), and on the quantity of light, on the CO2 concentration in the greenhouse environment, on the humidity of the soil and air, and on air movement. The walls of the greenhouse can be made of glass, fibreglass, rigid plastic panels or plastic film. Glass panels are more transparent than plastic and will let in far more light, but will provide less thermal insulation, are less resistant to shocks, and are heavier and more expensive than the plastic panels. The simplest greenhouses are made of single plastic films, but recently some greenhouses have been constructed with a double layer of film separated by an air space. This system reduces the heat loss through the walls by 30 -40%, and thus greatly enhances the overall efficiency of the greenhouse. Greenhouse heating can be accomplished by forced circulation of air in heat exchangers, hot-water circulating pipes or ducts located in or on the floor, finned units located along the walls and under benches, or a combination of these methods. Exploitation of geothermal heat in greenhouse heating can considerably reduce their operating costs, which in some case account for 35% of the product costs (vegetables, flowers, house-plants and tree seedlings).
Some examples of industrial processes in which geothermal energy is used are wood drying, food dehydration, gold mining, milk pasteurization and spas.
What is geothermal energy?
M.H.Dickson and M.Fanelli, Istituto di Geoscienze e Georisorse, Pisa, Italy