Power generation and its possible environmental effects-3
Discharges to surface waters and into the subsurface
The chemical composition of fluids from geothermal reservoirs varies over wide ranges, from near neutral and relatively dilute values (<200 ppm chloride at pH > 9, Hveragerdi/Iceland) up to acidic and highly saline concentrations (150’000 ppm chloride at pH < 5, Salton Sea/USA). In addition, significant amounts of SiO2 as well as further elements like B, As, Li and Hg can be present. Concentrations in liquid effluents vary not only locally but also with the (condensing or notcondensing) power plant type. Discharge of effluents into surface waters like rivers is applied only at a few places (e.g. Wairakei/New Zealand); in many countries there are legal restrictions with discharge limits. Retention and treatment techniques are available. Groundwater contamination can happen at improperly cased drillings or with leaking retention ponds. Unexpected discharges of power plant operation (Diesel fuel, lubricants, biocides etc.) cannot be excluded but are usually manageable. The environmental effects of fluid reinjection into the subsurface manifest themselves rarely at the surface, except for seismicity (see below). Of course, contamination of aquifers used for irrigation or for drinking water must be avoided by all means.
Water consumption, waste heat
Water is needed in rather large quantities (up to 1’000 m3/day) already for exploration and test drillings, which can cause problems in semiarid or arid climate. Steam type power plants (direct, single or double flash, see Table 1) do not need external cooling water since the steam gets condensed and recirculated. On the other hand, water-cooled binary plants need large quantities whereas air-cooled plants (e.g. Figure 4) do not need any water.
All heat – power conversion systems produce waste heat, which can attain significant portions. This applies to geothermal power generation too; the waste heat fraction depends on the conversion technology/power plant type. Geothermal power plants release considerably larger waste heat quantities, due to the lower conversion efficiency, than other power plant types. A comparison with other technologies is given in Figure 6. For geothermal, the waste heat is released at the plant site. When the heat is considered which is wasted with other technologies during mining and transport (coal, oil, gas, uranium) than the geothermal option shows clear advantages. On the other hand, the geothermal cooling facilities like cooling towers are, related to unit capacity (MWe), are significantly larger than for other technologies.
Figure 6. Waste heat (MWt) per unit electric capacity (MWe) of geothermal power plants, in comparison with other types of power generation. From DiPippo (1991), modified.prohibit the heat transfer over greater distances. The now emerging Hot-Dry-Rock, HDR (also called Enhanced Geothermal Systems, EGS) technology could site the power generation at places where heat consumers exist. Such projects are now under development in Basle and Geneva, Switzerland (Haering and Hopkirk 2001). HDR/EGS system operates in a closed system: water is circulated via injection and production wells through an artificially created heat exchanger at several kilometers depth. A further solution to avoid discharge to the atmosphere or hydrosphere is the so-called cascaded use. This consists of a chain of applications with stepwise decreasing temperatures, e.g. from industrial uses of hot wastewater through balneology down to fish farming. Geothermal direct-heat applications can be attached to geothermal power generation systems efficiently. Applications needing temperatures not higher than 65 ºC might be attached (cascaded) in series to the power plant fluid outlet line (Lund and Boyd, 1999). There is a whole spectrum of possible direct-use applications suitable for stepwise reduced temperature levels, so ensuring the minimum practicable effluent throw-away energy. The direct-use applications depend on the required fluid temperature; with decreasing temperature the following applications can be considered for cascading: industrial use, space heating, diverse balneological and bathing uses, greenhouse and soil heating, de-icing of parking lots and pavements, production of domestic hot water, air-conditioning, heat pump applications, assorted low-temperature washing, low-temperature drying, fish farming etc. Figure 7 shows a schematic cascading sequence, with progressively lower temperatures.
Figure 7. Schematic flow diagram of a cascading sequence.
Examples for cascading use exist already since many years in Italy and Japan; they are described in detail in Lund (1987) and Minohara and Sekioka (1980), respectively. Further cascading examples can be found in Dickson and Fanelli (1990, 1995).
Reference :
Rybach, L. (2003): Geothermal energy: sustainability and the environment. Geothermics
Tags: drinking water, Environmental effects, geothermal power generation, geothermal power plants, groundwater contamination, power generation, surface water