ELECTRICITY GENERATION

Electricity generation mainly takes place in conventional steam turbines and binary plants, depending on the characteristics of the geothermal resource. Conventional steam turbines require fluids at temperatures of at least 150 °C and are available with either atmospheric (back-pressure) or condensing exhausts. Atmospheric exhaust turbines are simpler and cheaper. The steam, direct from dry steam wells or, after separation, from wet wells, is passed through a turbine and exhausted to the atmosphere With this type of unit, steam consumption (from the same inlet pressure) per kilowatt-hour produced is almost double that of a condensing unit. However, the atmospheric exhaust turbines are extremely useful as pilot plants, stand-by plants, in the case of small supplies from isolated wells, and for generating electricity from test wells during field development. They are also used when the steam has a high non-condensable gas content (>12% in weight). The atmospheric exhaust units can be constructed and installed very quickly and put into operation in little more than 13-14 months from their order date. This type of machine is usually available in small sizes (2.5 - 5 MWe).
The condensing units, having more auxiliary equipment, are more complex than the atmospheric exhaust units and the bigger sizes can take twice as long to construct and install. The specific steam consumption of the condensing units is, however, about half that of the atmospheric exhaust units. Condensing plants of 55 - 60 MWe capacity are very common, but recently plants of 110 MWe have also been constructed and installed Generating electricity from low-to-medium temperature geothermal fluids and from the waste hot waters coming from the separators in water - dominated geothermal fields has made considerable progress since improvements were made in binary fluid technology. The binary plants utilize a secondary working fluid, usually an organic fluid (typically n-pentane), that has a low boiling point and high vapour pressure at low temperatures when compared to steam. The secondary fluid is operated through a conventional Rankine cycle (ORC): the geothermal fluid yields heat to the secondary fluid through heat exchangers, in which this fluid is heated and vaporises; the vapour produced drives a normal axial flow turbine, is then cooled and condensed, and the cycle begins again .By selecting suitable secondary fluids, binary systems can be designed to utilise geothermal fluids in the temperature range 85-170 °C. The upper limit depends on the thermal stability of the organic binary fluid, and the lower limit on technical-economic factors: below this temperature the size of the heat exchangers required would render the project uneconomical. Apart from low-to-medium temperature geothermal fluids and waste fluids, binary systems can also be utilised where flashing of the geothermal fluids should preferably be avoided (for example, to prevent well sealing). In this case, down-hole pumps can be used to keep the fluids in a pressurised liquid state, and the energy can be extracted from the circulating fluid by means of binary units.Binary plants are usually constructed in small modular units of a few hundred kWe to a few MWe capacity. These units can then be linked up to create power-plants of a few tens of megawatts. Their cost depends on a number of factors, but particularly on the temperature of the geothermal fluid produced, which influences the size of the turbine, heat exchangers and cooling system. The total size of the plant has little effect on the specific cost, as a series of standard modular units is joined together to obtain larger capacities.
Binary plant technology is a very cost-effective and reliable means of converting into electricity the energy available from water-dominated geothermal fields (below 170 °C).
A new binary system, the Kalina cycle, which utilizes a water-ammonia mixture as working fluid, was developed in the 1990s. The working fluid is expanded, in super-heated conditions, through the high-pressure turbine and then re-heated before entering the low-pressure turbine. After the second expansion the saturated vapour moves through a recuperative boiler before being condensed in a water-cooled condenser. The Kalina cycle is more efficient than existing geothermal ORC binary power plants, but is of more complex design.
Small mobile plants, conventional or not, can not only reduce the risk inherent to drilling new wells but, what is more important, they can help in meeting the energy requirements of isolated areas. The standard of living of many communities could be considerably improved were they able to draw on local sources of energy. Electricity could facilitate many apparently banal, but extremely important operations, such as pumping water for irrigation, freezing fruit and vegetables for longer conservation.
The convenience of the small mobile plants is most evident for areas without ready access to conventional fuels, and for communities that would be too expensive to connect to the national electric grid, despite the presence of high voltage transmission lines in the vicinity. The expense involved in serving these small by-passed communities is prohibitive, since the step-down transformers needed to tap electricity from high voltage lines cost more than US$ 675,000 each, installed, and the simplest form of local distribution of electricity, at 11 kV using wooden poles, costs a minimum of US$ 20,000 per kilometre (US$ 1994). By comparison, the capital cost (US$ 1998) of a binary unit is of the order of 1500-2500 US$/kW installed, excluding drilling costs. The demand for electric capacity per person at off-grid sites will range from 0.2 kW in less-developed areas to 1.0 kW or higher in developed areas. A 100 kWe plant could serve 100 to 500 people. A 1000 kWe plant would serve 1000 to 5000 people