Monday, 28 August 2017

How much water do power stations use?

It is world water week and the power generation sector is often blamed for high water consumption - how high is it really? First the headline figures. Current water consumption by UK power stations comes to around 13 l/person/day [1][2]. That is less than a tenth of the amount we use at home [3].

However, the amount of water used depends on the technology. Wind and solar power generation need no water at all but most nuclear and conventional power stations use water for cooling. Coal power stations use more water than gas, partly because they use water for cleaning sulphur out of the flue gases as well as for cooling. Nuclear power plants also use more cooling water than gas. This is partly because they are less efficient so they have to dump more heat and also because they don't generate any hot flue gases, so all their heat has to be dumped through the water cooling system.

Open cycle cooling systems use less water but consume more than closed cycle systems.
The amount of water used can be a great deal more than this. For example a hydroelectric power stations needs a huge amount of water flowing through it but does not consume any - what goes in one side comes out the other. Most coal and gas power stations use water for cooling but there are two types. In an open cycle cooling system a great deal of water flows through but very little is lost to evaporation. Most comes out the other side a few degrees warmer than it went in. These cooling systems can use as much as 160 litres/kWh generated but they may consume as little as 0.4 l/kWh. In the UK some coal and gas plants are open cycle and all nuclear ones. In a closed cycle cooling system (most coal and gas plants), the cooling water is mostly recycled but some is lost to evaporation. These systems typically lose around 1 l/kWh.

Water consumption by different kinds of power stations with closed or open cooling systems. Data from [1].

All nuclear power stations are open cycle - usually on the coast and vulnerable to jellyfish.

Power stations using open cycle cooling need a great deal of water. If they don't have enough they have to reduce power output or shut down. This is why most of our nuclear powers stations are on the coast, because there is always plenty of sea water and the increased water temperature near the outflow has the whole ocean to dissipate away into. However using sea water brings some problems such as having the filters clogged by blooms of jellyfish. Bristol University is working on an early warning system for this [4].

Closed cooling systems have their problems too. They are more expensive to build and they require more energy to run. Air cooled systems that use no water at all are more expensive again, and use more energy [1].

Irrigating biofuel crops consumes far more water than is used in power stations.
A full life cycle analysis should include plant construction and fuel preparation as well as operations. Plant construction is significant for solar PV panels but per kWh water consumption is still less than half that used by a closed cycle gas power station [5]. For fuel preparation, mining coal and refining methane adds less than 0.5 l/kWh. However for biofuels irrigating the crops can take another 30-150 l/kWh making this by far the most thirsty type of power [1].

Power stations use little mains water - much less than other sectors.
We get our water from the mains which is treated to guarantee it is safe to drink. Power stations do not need such clean water and they mostly get it from the sea or from rivers and lakes. In terms of mains water use, the energy and water sectors are responsible for only 3% of non-domestic use. The highest consumers are manufacturing industry and agriculture [6].

Main water consumption by sector. Data from [5] based on 2006/2007.

Water use can double in future scenarios but the more wind and solar power used the less we need.
As for future scenarios, the more wind and solar we use the less water we need. The Newcastle team looked at six different scenarios and the high renewables one was the only one where water consumption was less in 2050 than now. The highest consuming scenario relies heavily on nuclear power but most of the extra water was seawater. Scenarios with a lot of coal power result in increasing freshwater consumption and cleaning up coal with carbon capture and storage increases water use further. However, even in the worst scenario water consumption was not more than double what it is today [1].

[1] Byers EA, Hall JW, Amezaga JM. Electricity generation and cooling water use: UK pathways to 2050. Global Environmental Change 2014, 25, 16-30.
[2] Overview of the UK population (ONS)
[3] The amount we use (Discover Water)
[4] Power stations to get early warning against jellyfish invasions (Guardian) Oct 2016
[5] J Meldrum, S Nettles-Anderson, G Heath and J Macknick Life cycle water use for electricity generation: a review and harmonization of literature estimates (IOP Publishing Ltd) March 2013 Environmental Research Letters, Volume 8, Number 1
[6] Freshwater availability and use in the United Kingdom (WRAP)

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