cooling towers

[kev67]

We went on a field trip to Didcot Power Station yesterday, which was pretty interesting and worth doing if you get the chance. I don't know whether it's possible to visit other power stations. There are two stations there: Didcot A which is coal and Didcot B which is gas. Didcot A was built earlier so they built six massive great cooling towers which are a blot on the landscape that you can see from ten miles in each direction. When they came to build Didcot B, the local residents did not want to see any more cooling towers, so they built about 17 much lower condensing units, which aren't visible from far away. The only problem is that each one of those units takes about 1MW to run. From an environmental point of view, it was probably better to have the cooling towers.

[greenacy]

from the info you have presented I would have to agree. Do you happen to know what is required to maintain the larger cooling tanks?

[LMagic007]

This might seem like an ignorant question, but I cant help but wonder about the efficiency design of a power station that requires cooling towers. i.e. surely all that waste heat could be harnessed and fed back into the system somehow ? i.e. to reduce the water heating requirement to produce steam for the turbines.

I suspect there is an explanation, but I also know that much waste has occurred for decades during an era of cheap energy. I wonder perhaps if they might help allow the plant to be more responsive to grid demand fluctuations, by being able to more rapidly adjust and maintain the optimal the operating temperatures. i.e. presuming a more passive approach ( recirculate heat and reduce burn requirement for feedwater to steam conversion ) might not have that degree of flexibility and responsiveness. Just wondering.

[forfismum]

One of the worst manipulations carried out in the bygone days of environmental awareness was to show cooling towers with the vapour coming out and to represent this to a gullible public as toxic smoke.I think Greenpeace were the perpetrators of this one and it may well have been Didcot that was used in the photies.

[kev67]

Originally Posted by greenacy

from the info you have presented I would have to agree. Do you happen to know what is required to maintain the larger cooling tanks?

Do you mean the cooling towers for Didcot A? I don't think they use anything like as much energy. I saw some water being sprayed along the sides of the towers about a few metres up, but I don't suppose that would use all that much energy.

[kev67]

[quote=LMagic007;237900]This might seem like an ignorant question, but I cant help but wonder about the efficiency design of a power station that requires cooling towers. i.e. surely all that waste heat could be harnessed and fed back into the system somehow ? i.e. to reduce the water heating requirement to produce steam for the turbines.

There's a theoretical maximum for the efficiency of heat engines, which means that waste heat is inevitably ejected. I think the formula is

1 - Tc/Th where Tc is the heat of the steam exiting the turbine and Th is the heat of the steam after it has been heated by the furnace before it goes into the turbine. The point of using the river water to cool the steam exiting the turbine is to lower Tc and improve efficiency. CHP stations use the low grade waste heat to heat homes as well as produce electricity. Overall efficiencies can be in the order of 80% provided there's a demand for the heat. CHP stations tend to me more efficient at producing heat than electricity so they can be inefficient if used primarily to produce electricity. CHP also depends on there being buildings nearby to use the heat along with a heat distribution system.

[Besoeker]

Originally Posted by LMagic007

This might seem like an ignorant question, but I cant help but wonder about the efficiency design of a power station that requires cooling towers. i.e. surely all that waste heat could be harnessed and fed back into the system somehow ? i.e. to reduce the water heating requirement to produce steam for the turbines.

It's a good point. I'm not a power station engineer so I have no direct answer but the first project I was given after graduating was in the power station of the company that sponsored me. It was a very, very long time ago but from what I recall....
Passing the output steam from the turbine into a condenser obviously reduces its pressure thus increasing the pressure differential across the turbine and thus increasing its output. The condenser is a water to water heat exchanger. The primary water, the condensed steam, goes back to the boiler. The secondary water goes to the cooling tower and gets recirculated from there.
My recollection is that the water in the cooling tower wasn't much above ambient. Maybe there just isn't enough energy to make recovery cost effective?

[LMagic007]

I wonder is the sheer volume of water used to cool, might explain its lower temperature. It would be interesting to know if it's a turbine efficiency measure, an optimal operating temperature stability measure or whether its just based on a design formulated in an age where energy was relatively cheap or some other reason yet mentioned.

I did a bit of Googling and came up with a few articles that may be of interest though they differ slightly in that they seem to largely focus on using gas turbine fired waste heat to power secondary steam turbines, rather than steam heat recovery.;

Coal company to build western Downs power station - ABC News (Australian Broadcasting Corporation)

CS Energy: Glossary

Cogeneration-Combined Heat and Power (Electricity) Generation (Research Note 21 1998-99)

Australian Coal Association

http://www.futurepundit.com/archives/002148.html

http://www.newscientist.com/article/dn5039-system-converts-smokestack-heat-to-electricity.html

[Wobs]

Originally Posted by LMagic007

This might seem like an ignorant question, but I cant help but wonder about the efficiency design of a power station that requires cooling towers. i.e. surely all that waste heat could be harnessed and fed back into the system somehow ? i.e. to reduce the water heating requirement to produce steam for the turbines.

I suspect there is an explanation, but I also know that much waste has occurred for decades during an era of cheap energy. I wonder perhaps if they might help allow the plant to be more responsive to grid demand fluctuations, by being able to more rapidly adjust and maintain the optimal the operating temperatures. i.e. presuming a more passive approach ( recirculate heat and reduce burn requirement for feedwater to steam conversion ) might not have that degree of flexibility and responsiveness. Just wondering.

When you see a large power station with the cooling towers emitting lots of steam, it would seem they are wasting lots of energy, and indeed they are emitting huge amounts of heat. But the heat is at a low temperature, as all or most of the useable heat has been used up somewhere in the process (pre-heating etc). Many power stations have greenhouses nearby using much of the waste heat, and Drax has in the past asked for more greenhouses to be built nearby to use even more of the heat.

Apparently you could have a greenhouse the size of Yorkshire to use all the heat of drax, although this could be a bit of an exaggeration. Although if you were to use more of the heat, at some point, you would lose efficiency elsewhere.

[Besoeker]

Originally Posted by LMagic007

I wonder is the sheer volume of water used to cool, might explain its lower temperature.

Yes, the mass of water would have a direct bearing.
That said, the cooling water is circulated so there would be a finite mass in the system. The cooling tower has to be able to extract as much heat from the cooling water as it absorbed in the condenser.
As Wobs suggested, the temperature difference from ambient may make further extraction of heat impractical.