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Power stations and Cooling towers

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twozzer
227207.  Mon Nov 05, 2007 6:41 am Reply with quote

This may seem a very naive question (probably because it is!) but why, when a conventional coal/gas/whatever Power station uses vast amounts of energy to turn water into steam to drive turbines, does it then use a whole bunch of Cooling towers, presumably to turn the very hot water back into cold water? Doesn't this rather defeat the object somehow? Out of all the energy used to create electricity, how much is then wasted by heat energy lost to the atmoshere? Or could it be that I really don't understand how power stations work?

 
npower1
227256.  Mon Nov 05, 2007 7:56 am Reply with quote

Interestingly doing a Google search turns up plenty of information on how they work but not why they are needed.


You are essentially correct. More steam requires more water. Cooling towers allow the steam to turn back into water so that there is approximately a closed system.

How much energy is input and how much actually ends up getting to your kettle appears to be a controversial subject. Certainly, only a small percentage of the input (coal, gas, etc) is translated into electricity. How much is lost via transmission lines from the power station to your kettle is a 'green' issue resulting in 'vested interests' type arguments.


*** pats myself on the back for not guessing at any numbers***

 
cnb
227257.  Mon Nov 05, 2007 8:00 am Reply with quote

You've mentioned how the turbines are driven in your post, so you obviously have a fair idea of how it works, you've just missed one point...

It's the fact that the liquid water is turned to steam in the boiler that is essential. Once the steam has passed through the turbine, it HAS to be cooled back to liquid water so that it can go through that cycle again. It only needs to be cooled just enough to condense it.

Normally the condensing is performed using cold river or sea water sprayed over the steam pipes - it is this cooling water that you see rising from the cooling towers, not the steam generated in the boilers.

It is possible to use the heat generated to provide space heating for nearby buildings. You have to dump enough heat to cool the steam though, so you still need the cooling towers for the summer! Heating systems using steam from the local power plant has been in use in some parts of the world for decades, but is only recently starting to become common here in the UK.

 
cnb
227271.  Mon Nov 05, 2007 8:13 am Reply with quote

My last post was intended as a response to the OP, this one is a follow-up to npower1's post...

According to Elexon, the company who manage the 'settlement system' for the UK's electricity markets, transmission losses (in the cabling and transformers) account for approximately 1.5% of the total energy transmitted.

The National Grid website shows current demand standing at 47GW, which means that right now the transmission system is acting like a 700MW heater - and using up the entire output of a small power station (Rye House, for example, has an output of 715MW).

 
twozzer
227363.  Mon Nov 05, 2007 11:49 am Reply with quote

Thanks for the replies so far, they certainly help. However, if you put steam into a turbine and get out electricity and, after that, you still have steam, why not just put that through another turbine? Answer, I suppose, is that the steam is no longer under pressure?
Anyway, I agree it would still seem to make sense to use that heat somehow, even if it meant power stations nearer cities or adjacent to some other industry that could use it. Why, oh why, are we so inefficient with our energy use!!!
(Loved the reference to Rye House, incidentally, I used to watch speedway there as a lad)

 
AlmondFacialBar
227365.  Mon Nov 05, 2007 12:00 pm Reply with quote

twozzer wrote:
Thanks for the replies so far, they certainly help. However, if you put steam into a turbine and get out electricity and, after that, you still have steam, why not just put that through another turbine? Answer, I suppose, is that the steam is no longer under pressure?


that's exactly the reason! the steam thus has to be cooled down, recycled, and pressurised again.

twozzer wrote:
Anyway, I agree it would still seem to make sense to use that heat somehow, even if it meant power stations nearer cities or adjacent to some other industry that could use it. Why, oh why, are we so inefficient with our energy use!!!


in germany and austria you'll find some of the more modern residential developments have that. dunno the english word for "blockheizkraftwerk" (nice german compound, that, innit?), and wiki doesn't even seem to have an article on it in any language except german, but essentially they are tiny power stations with an output between 15 kw and 10 mw, designed to supply heat and electricity to the neighbourhood they're in. because they're right where the power and heat is needed they run at up to 90% efficiency. any electricity they produce that's not needed locally is automatically fed into the public grid. good things, those, i'm surprised they obviously haven't caught on elsewhere.

:-)

AlmondFacialBar

 
cnb
227367.  Mon Nov 05, 2007 12:07 pm Reply with quote

I picked Rye House as I grew up near it too... I remember when they blew up the old cooling towers... the Broxbourne Mercury ran a competition to win the chance to push the plunger!

The original reason why Britain was so inefficient was, I suspect, primarily that we had lots of cheap coal...

Once the pits were closed however, you'd have thought the 'dash for gas' would have meant that in places like Rye House, where fancy new super-efficient gas-powered plants were built, that they could have included a way of using the excess heat to warm the warehouses on nearby industrial estates... Why not? I can't find a source for this, but I'm sure I remember hearing that it was something to do with damaging energy markets by introducing a local monopoly on heating. Can anyone verify that?

 
cnb
227368.  Mon Nov 05, 2007 12:14 pm Reply with quote

Indeed AFB, and since 2000 or so, the British government has been promoting 'micro-CHP' (that's your english translation - the CHP stands for Combined Heat and Power) too. There still aren't many around yet though.

Even better than Germany's record is Denmark, where every thermal power plant built since about 1980 has been a CHP system - even the big ones.

 
AlmondFacialBar
227369.  Mon Nov 05, 2007 12:15 pm Reply with quote

cnb wrote:
I picked Rye House as I grew up near it too... I remember when they blew up the old cooling towers... the Broxbourne Mercury ran a competition to win the chance to push the plunger!

The original reason why Britain was so inefficient was, I suspect, primarily that we had lots of cheap coal...

Once the pits were closed however, you'd have thought the 'dash for gas' would have meant that in places like Rye House, where fancy new super-efficient gas-powered plants were built, that they could have included a way of using the excess heat to warm the warehouses on nearby industrial estates... Why not? I can't find a source for this, but I'm sure I remember hearing that it was something to do with damaging energy markets by introducing a local monopoly on heating. Can anyone verify that?


GRAH! sounds very thatcherite indeed, would i be correct there? in germany, a couple of cities use their waste incinerators as heat and power stations. the power goes into the grid, and the heat is routed to the council estates for cheap and efficient heating. my hometown does that, actually, it seems to work quite well.

thanks cnb! :-) yes, denmark is way ahead with the micro-chp's, that's where germany got the idea from.

so the incinerators would be waste chp's, right?

:-)

AlmondFacialBar

 
npower1
227370.  Mon Nov 05, 2007 12:19 pm Reply with quote

AFB,

I'm no linquist but I suspect that your German word roughly translates to 'local' or 'locality'.

As to why we (the UK) don't generate power locally it is a mixture of 'market forces', privatisation, a lack of the government having no ability to plan ahead (except for spending huge amounts on future nuclear armaments and failing and unwanted massive IT systems), basically piss poor planning.

 
AlmondFacialBar
227372.  Mon Nov 05, 2007 12:21 pm Reply with quote

actually, "block" means exactly the same in german as in english in this context, so i guess the translation would be block chp. :-) pisspoor planning... yes... you ain't seen it till you've been to ireland, actually...

:-)

AlmondFacialBar

 
Jenny
227380.  Mon Nov 05, 2007 1:12 pm Reply with quote

Blockheizkraftwerk means something like block heating power plant doesn't it?

They do have something similar in Sweden - when I was visiting my nephew in Stockholm this summer he pointed one out to me in their neighbourhood.

 
AlmondFacialBar
227425.  Mon Nov 05, 2007 2:55 pm Reply with quote

Jenny wrote:
Blockheizkraftwerk means something like block heating power plant doesn't it?

They do have something similar in Sweden - when I was visiting my nephew in Stockholm this summer he pointed one out to me in their neighbourhood.


well done! that's exactly what it means! ;-)

:-)

AlmondFacialBar

 
MatC
227618.  Tue Nov 06, 2007 5:44 am Reply with quote

Alan Simpson MP has been campaigning for this for years, and written a great deal about it for the Morning Star; some of his articles are archived here http://www.alansimpsonmp.co.uk/content/climate.htm

 
PDR
255556.  Thu Jan 10, 2008 8:54 am Reply with quote

twozzer wrote:
Thanks for the replies so far, they certainly help. However, if you put steam into a turbine and get out electricity and, after that, you still have steam, why not just put that through another turbine? Answer, I suppose, is that the steam is no longer under pressure?


I think it's a "law of diminishing returns" thing. Consider a steam engine. In its simplest form it is a cylinder into which steam is admitted at high pressure/temperature (the two are essentially the same thing). The steam expands, pushing the piston down, and then it is exhausted from the cylinder. But mechanical compromises mean that the "exhausted" steam is still at higher than atmospheric pressure and so still contains some energy, so we can add a second cylinder to extract some of this, and even a third - creating a "triple-expansion" steam engine which has much higher thermal efficiency than the simple single-cylinder model.

We could continue adding cylinders, but there's a snag. Each cylinder is handling steam in a more expanded form than its predecessor, and so must be much larger. The larger cylinder has greater contact areas (ie greater friction) and higher masses (both physical and thermal), so the mechanical efficiency progressively drops as the pressure of the steam is reduced in each stage. If you were to scheme out a 10-stage expansion engine the final cylinder would probably be the size of a politician's ego whilst contributing fractions of a per cent to the total output (again, like a politician). In general terms a three-stage system offers the optimum compromise between size and efficiency, although some quadruple-expansion engines have been used succesfully they generally aren't worth the effort. If steam engines aren't your thing then the same holds true for extracting the gas energy from the exhaust of a turboprop/turboshaft engine with multi-stage turbines.

So, getting back to the power stations - these are heat engines and the energy is most easily and efficiently extracted where the heat difference is greatest. This requires the smallest physical plant and has the highest efficiency. As the pressure/heat reduces it becomes harder to extract the enrgy, and so you get to a point where it simply isn't worth the effort. If you can use the heat directly (ie in CHP systems) then fair enough, but trying to convert this energy into mechanical work is just too inefficient to be viable.

PDR

 

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