Comments on: Why do power plants need water? http://amrelbeleidy.com/2010/02/why-do-power-plants-need-water/ I write when I have something to say, my thoughts are not necessarily connected Tue, 07 Feb 2012 20:51:08 +0000 hourly 1 http://wordpress.org/?v=3.3 By: Erika http://amrelbeleidy.com/2010/02/why-do-power-plants-need-water/comment-page-1/#comment-160 Erika Mon, 22 Feb 2010 23:02:42 +0000 http://www.amrelbeleidy.com/?p=192#comment-160 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. 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.

]]> By: Joe http://amrelbeleidy.com/2010/02/why-do-power-plants-need-water/comment-page-1/#comment-159 Joe Mon, 22 Feb 2010 17:01:40 +0000 http://www.amrelbeleidy.com/?p=192#comment-159 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 :) 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 :)

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