Water for people.pdf - WHO Thailand Digital Repository

D E V E L O P I N G E N E R G Y T O M E E T D E V E L O P M E N T N E E D S / 2 6 3Thermal power generation requires substantial amounts ofwater at every stage of the energy cycle, from mineral extraction todelivery of power to the transmission/distribution system. But by farthe greatest need for water comes from the cooling of turbines inthe power plant. Power plant condenser coolant makes significantdemands on water resources and water abstraction for this purposeis highest in regions with high installed capacities of thermal powergeneration: withdrawals in the eastern states of the United Statesand central Europe are in the range 10 to 1,500 mm/year, and 1 to10 mm/year for much of eastern China, the Nile basin and northernIndia, compared with less than 1 mm/year for drainage basins in therest of the world. The amount of water required depends on thetype and size of power plant, and especially on the kind of cooling.‘Once-through cooling’, in which condensed water is directlydischarged, is responsible for thermal pollution of naturalwatercourses with attendant risk to downstream aquatic ecosystemsand water users. ‘Tower cooling’ involves the turbines being cooledand the hot water being sent to a cooling tower, reused severaltimes and eventually discharged from the plant.Cooling towers enable the water to be recycled to thecondensers, requiring less than 3 percent (per unit of energygenerated) of the water withdrawals of once-through cooling.However, although tower cooling requires much lower withdrawals,it consumes twice as much water per unit of energy as oncethroughcooling since water is evaporated in the cooling process,thus requiring the addition of ‘make-up water’. One of theadvantages of tower cooling is that water is discharged back towaterways at much cooler temperatures, thereby protecting aquaticecosystems and downstream water uses. Further, the heat stored inthe cooling water can be extracted for utilization in district heatingsystems or for industrial heating purposes. Most water withdrawn bythermal power plants is returned to waterways without majordegradation (except for thermal pollution caused by once-throughcooling).Table 10.5: Advantages and disadvantages of various forms of energySystem Technical advantages/disadvantages Ecological advantages/disadvantagesGeothermal Advantages Advantagesenergy• Geothermal power stations are very reliable compared • Geothermal energy is an abundant, secure and, if properlyto conventional power plants. They have a highutilized, renewable source of energy.availability and capacity factor. Geothermal power plants • Geothermal technologies, using modern emission controls, haveare designed to run 24 hours a day, and operation is minimal environmental impact. Modern geothermal plants emitindependent on the weather or fuel delivery.less than 0.2% of the carbon dioxide of the cleanest fossil fuel• Geothermal resources represent an indigenous supply of plant, less than 1% of the sulphur dioxide and less than 0.1%energy, providing energy supply security, reducing the of the particulates. Geothermal plants are therefore a viableneed for fuel imports and improving the balance of alternative to conventional fossil fuel plants, particularly withpayments. These issues are particularly important in respect to greenhouse gas emissions.developing countries, where geothermal resources can • Geothermal power stations have a very low land areareduce the economic pressures of importing fuels, and requirement.can provide local technical infrastructure and employment.DisadvantagesDisadvantages• Geological uncertainties. • Geothermal energy produces non-condensable gaseous pollutants,• High initial investment. mainly carbon dioxide, hydrogen sulphide, sulphur dioxide and• High-tech technology. methane. The condensed geothermal fluid also contains dissolvedsilica, heavy metals, sodium and potassium chlorides andsometimes carbonates. However, modern emission controls andreinjection techniques have reduced these impacts to a minimum.• Geothermal energy production has been associated with inducedseismic activity. However, this is a debatable issue as mostgeothermal fields are located in regions that are already prone toearthquakes. In production plants where reinjection maintainsreservoir pressures, seismic activity is not found to be greatlyincreased.