TCASE 6: Cooling water and thermal power plants

Heat engines require cooling, to turn heat energy into mechanical energy (and then, via a turbine-connected generator, to electrical energy). This is an unavoidable physical principle, and is typically exploited via the Carnot cycle. Usually, this cooling requirement uses water.

Why do I raise this point? Because it seems to be a source of much confusion (innocent and deliberate) amongst the energy illiterate, especially when mounted as an argument against nuclear energy generation (and, implicitly, as a reason for adopting renewable energy). For instance, Friends of the Earth have decried:

Nuclear power plants consume large amounts of water –35-65 million litres daily. Indeed nuclear power is the thirstiest of all energy sources. A December 2006 report by the Commonwealth Department of Parliamentary Services states: “Per megawatt existing nuclear power stations use and consume more water than power stations using other fuel sources. Depending on the cooling technology utilised, the water requirements for a nuclear power station can vary between 20 to 83 per cent more than for other power stations.” Global warming and water shortages are likely to exacerbate problems experienced by the nuclear power industry during heatwaves in recent years. Nuclear power plants in several countries, including France and the US, have had to operate at reduced capacity, or to shut down temporarily, because of reduced water supply or to avoid breaching regulations limiting the heat of expelled water.

So what’s the story? Are water limitations and discharge regulations destined to be a major limiting factor for nuclear power, especially for places that are experiences increasing water shortages, such as Australia? The short answer is no — this is classic FUD. For the longer answer, read on.

All thermal power plants, by definition, make use of heat engines with heat exchangers, and so require cooling (although this need can be reduced in various ways, as explained below). This includes coal-fired, nuclear fission, oil-fired, conventional gas-fired, solar thermal and geothermal power stations. The renewable energy sources that don’t have this cooling requirement are hydropower, wind, wave, tidal and solar photovoltaic power.

Water is used in two ways in thermal power plants: (a) Internal steam cycle: to create steam via the energy source (fossil fuel combustion, fission chain reaction, heat exchange with deep rocks [hot dry rock geothermal] or a heat transfer fluid [concentrating solar power]) and convey it to an electricity-generating turbine, and (b) Cooling cycle: to cool and condense the after-turbine steam (this condensation dramatically decreases the volume of the expanded steam,creating a suction vacuum which draws it through the turbine blades), and then to discharge surplus heat to the environment.

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