Why do power plants need water?

If you put the world’s power plants on a map, you are sure to notice one thing, most of them are next to water. You might think that water and electricity are not a good mix, but in this case it is. Every power plant needs a lot of water, thirsty beings they are. Those which are not next to a sea or river will need to be supplied from the nearest location. So why on earth do they need all this water?

The answer is simple: Cooling. Power plants produce a lot of heat, how much depends on the type and efficiency. Coal power plants have percent efficiencies generally in the high 30, gas 40′s and combined cycle gas turbines max out at 50%. This means that a CCGT (combined cycle ..) produces as much heat as it does power, and this heat needs to go somewhere.

Now once the water is taken in and used for cooling, the heat has been transferred there, and now the water is hot. This is sometimes annoying for the fish if you are going to dump hot water at 90 degrees back on them. So what can we do?

One factor is  the water body you are dumping the water in, if you are dumping some hot water in the Med then you won’t really affect the sea temperature, although you might cause some damage to the local wildlife. But in a smaller water body you are bound to cause severe damage to the wildlife, and so the power plant location becomes vital. Some power plants in countries that care about fish, are limited to how much power they can produce just because they would dump too much hot water in the surrounding areas.

So what can you with a lot of water at 90 degrees other than dump it? In cold countries you can create a district heating system. (these also work with combined heat and power (CHP)). Put the hot water in pipes and pump them around the surrounding houses, they use this water to heat up their homes and save money and energy. In warmer countries the same principles can be used for a cooling system, through absorption chillers for example, which use a heat source for cooling.

This however all depends on the location of the power plant, if it’s in the middle of nowhere, then this might be difficult as there are no houses around.

So power plants produce loads of heat, they need water to cool down, the water becomes hot, we need to watch out where we dump it or we could use it for district heating/cooling. This means we need to think about this, among a million other things, when we are deciding where to locate the power plant.

Sustainable Energy

February 22nd, 2010

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Joe

I am a big fan of district heating (I actually design district heating systems for a living these days). The city I used to live in has just built a fairly large CCGT and because it’s also connected to the local district heating network, they’re approaching 90% efficiency.

Minor clarification for you on the temperature of cooling water: power stations use A LOT of water for cooling, which means that the overall temperature rise is just a few degrees when it is put back into the sea. If cooling towers are used, then the water just goes straight out the chimney.

But it’s much better to use district heating anyway :)

February 22, 2010 5:01 pm

Erika

Yes, power plants are thirsty animals, and as such there are some tricks to make them more efficient regarding their water consumption. The key to this is the employed water-cooling system. The one you mention is the simplest (and therefore cheapest, if the siting allows) once through or open cycle cooling. In this system, water is taken from the reservoir, passed through the power cycle and then rejected back to the heat sink (i.e. the sea, lake or any other large enough water body) a few degrees warmer. Leaving aside more advanced options, the other widely spread water cooling system is a closed cycle one, in which the water used in the cycle is condensed and cooled by means of heat and mass transfer in cooling towers, before being re-circulated again. Fresh water is taken from the reservoir only to make up for evaporation (around 5%). This option should not be dismissed as it is used, for example, in around 50% of power plants in the USA or in the UK.

Why choose a more complicated and expensive cooling tower setting instead of going for a one through one? After all, it evens reduces plant’s efficiency a bit. There are a number of reasons. Lack of water availability (power needs to be produced even in water-scarce –and warm, may I add- locations!), power plants so large the nearby water reservoir could not act alone as a heat dump or sink… and this leads us to the third reason, environmental concerns. Those tree huggers and fish lovers out there have made a point, and regulation is in place to limit the maximum temperature water can be dumped back into a reservoir, which is generally below 30C.

Once through-cooled power plants manage this, as Joe mentions, by having a rather large water intake flow rate. I found out (http://web.mit.edu/12.000/www/m2012/finalwebsite/solution/energy.shtml) a fossil-fuel fired power plant with this cooling system can have a water withdrawal flow rate of 20,000 to 50,000 gal/MWh (sorry about imperial units!) while a recirculating one is only 500 to 600 gal/MWh. The implications for the environment are obvious here, just think about the chemicals that go into this water to avoid pipe fouling or microorganism-siting in the pipes. All this water will end up back in the ocean or lake; and despite pre-discharge water treating some alterations always remain. The impact is minimized in closed-cycle cooled power plants, and the water conditions to avoid equipment damages can better be managed.

Of course one also argue that cooling towers look horrendous and that they have an effect on local humidity… but then again, there are people who wouldn’t want a wind turbine on their backyard at any price! So if you need to have a fossil fuel fired power plant around and the local heat demand doesn’t justify DH, it better have recirculating cooling.

February 22, 2010 11:02 pm

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