Hudson

The thread about nuclear power stations reminded me of a very important aspect of power policy that I haven't seen mentioned elsewhere. This might already be UKIP policy for all I know but here goes anyway.

Combined Heat and Power (CHP).
Most modern power stations, including nuclear, use steam turbines operating on the Rankine reheat cylce to drive generators. The theoretical maximum thermal efficiency of this cylce is 37%, assuming no losses from pumping power, turbines, generator or heat loss. In reality, the best coal fired stations (Drax or Rugeley B) manage about 33% (I know, I calculated it from measurements taken on site).

That means over 65% of that energy is being thrown away.

The problem is that the steam needs to be condensed before it can be pumped back INTO phpbb_the boiler. This means for every 1000 tonnes of coal, 650 are simply making pretty clouds on the tops of cooling towers. There is already a way to reclaim that energy.

CHP uses steam from the condensors to provide process heat for industry and central heatings for homes. By using CHP with exisitng turbine systems, cooling towers become almost redundant and thermal efficiencies of 60 to 80% can easily be achieved. The long term benefits in terms of fuel cost and pollution are self evident.

So why don't we use CHP now? Put simply, cost. To cobble a CHP system onto eg. Rugeley B would cost as much as building a new power station and, as the station will be closed anyway by CO2 emissions legislation in 2012, would never pay back.

If Britain is to build new nuclear power stations, or even further gas turbine + steam ones like Deeside (efficiency 42%), then it would be environmental and commercial insanity to not build them as CHP systems.

Unfortunately, this stuffs up the nuclear option. Hiding reactors in mountains is great for PR and safety, but it rather negates the use of CHP. By the time that steam reaches any centre of population, about half of its energy will have been lost in pipe friction and heat transfer. Pumping power would be enormous and the cost of sufficinetly big bore pipes (to reduce pumping power) with adequate insulation (to minimise heat loss at low flow velocity) would be prohibitive. CHP systems need to be close to the end user, unlike electricity only power stations.

Perhaps the answer is build nearer to the major cities but bury the reactors deep underground, such that the public can be assured no major incident would likely wipe their town off the face of the earth. Keep the turbine hall and control room above ground of course; we don't need power station workers to live like coal miners.

Cost is an issue, but sustainable energy policy is never going to be cheap. Wind farms etc are not the answer as they simply cannot meet demand, even if the whole of Britain were to be covered in them. Only UKIP can 'find' the money for such huge schemes out of the public purse because Britain would no longer be carrying the financial millstone of EU membership.

If we are to build any more steam turbine generators then we must use CHP. Even if you care not for environmentalsim or wasted uranium, throwing away useable energy can only ever be bad practice.

Well done to anyone who read all of that! :wink:

komerad

As a contract fitter having worked on many UK power stations I thought it was a very informative and interesting posting. Cheers :!:

Paul Birch

Steam turbines aren't limited to 37% efficiency, though simple versions may be. There is no upper limit to the efficiency under high pressure. At 220 atmospheres steam condenses to water without any phase change. There are also many other routes for generating power, such as MHD systems. That's not to say that CHP can't be a useful option too.

social observer

I thought that posting was superb, we need a library of well thought out policy suggestions and arguments right across the board so that in meetings, debates and interviews UKIP can consistently come across as knowing what they are talking about and being able to stop lesser informed representatives in their tracks.

BTW, I read it all and wasn't lost one bit, very well put across.

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Alex McKee

Absolutely. Does UKIP have an energy policy committee?

If so, hudson you should apply to join it!

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Hudson

Thanks, chaps. What's the salary for a 'scientific advisor'? :wink:

Steam condensing to water is a phase change!

Turbines aren't limited to 37%, but the thermodynamic cylce as a whole is. Typical turbine efficiencies are 85% for high pressure down to about 65% for low pressure in a three stage unit, at which point pressure can be slightly below atmospheric. To limit the minimum circuit pressure to 220 bar would be to throw away even more energy! It's all about the difference in enthalpy bewteen the HP inlet and the LP outlet, hence full expansion to saturated vapour state is used. The only way to raise output is to raise maximum pressure (or steam mass flow rate), but that doesn't effect theoretical efficiency as you have to burn more fuel. However high you superheat the steam, full expansion thermal efficiency is limited to 37%.

The proofs are really boring and arcane and involve steam tables, temperature-enthalpy charts and a spreadsheet, but if anyone is really interested I could try typing them up.

Paul Birch

Not when you go around and above the critical point it's not.

There are many thermodynamic cycles that can be used with steam turbines (strictly speaking, an infinite number). Many of those have efficiencies considerably in excess of 37%. Since plasma-core reactors could in principle have thermodynamic operating temperatures up to ~1TK, with the heat sink being anywhere down to the temperature of free space ~3K, theoretical thermodynamic efficiencies of 99.9999999...% are possible - effectively unlimited. Of course, for temperatures above ~10,000K the description "steam" turbine would be somewhat inappropriate. I'm pretty sure I've seen reports of existing nuclear power plants (from twenty years or so ago) that have overall efficiencies in the 40-50% range.

Hudson

Ok, I was referring to the Rankine reheat cylce, my apologies if that was not made clear enough.

There is no such thing as sublimation between liquid and vapour states. The critical point is a state point at which transitions between liquid and vapour are not clear i.e. composition at the phase boundary cannot be defined. Why try to operate at the critical point? You may as well try to operate at the Triple point! Besides, there is much greater enthalpy available in the superheat region without having to reach 220 bar.

Show me a plasma-core reactor, magnetohydrodynamic generator, cold fusion reactor, Schauberger hydro-vortex or phase-plasma rifle in the 40 Watt range that is sufficiently developed to be a direct replacement for steam turbines at a similar cost while reducing CO2 emissions and I'll be delighted to embrace it (not literally, of course).

I do suggest we limit UKIP energy policy to those technologies that are available now rather than the 'Energy of the Future' articles out of New Scientist. Otherwise, we would be as Hitler telling his bunker mates that the war will be won by some secret wonder weapon in April 1945. Nobody believed him, either.

Paul Birch

And my point is that not all power stations use that cycle. CHP is a good way of improving total efficiency, but not the only one.

Did I say there was? Sublimation is a phase change between solid and vapour.

Of course it can be defined, but not in terms of abrupt phase changes. The critical point is the point above which there is no distinction between gases and liquids. That means that by going around the critical point one can go from liquid to gas and gas to liquid by a continuous thermodynamic process with no latent heat of evaporation. This saves the energy that in steam engines (including steam turbines) is usually wasted in boiling the water. It makes it easier to get a more efficient cycle.

I don't entirely agree with this, but even so, higher efficiency technologies are available now. They were twenty years ago, anyhow - perhaps we've gone backwards. That wouldn't surprise me.

Hudson

Maybe we should abandon steam turbines altogether and go for UHF molecular resonnance electrolysis?

Could you give an example of these alternative efficiency improvements? CHP isn't the only one of course, but it is one that is practicable today and could be installed in a new build power station now. The immediate problem is how to honour those preposterous 'greenhouse gas' agreements.

Supercritical cycles are fine as long as you don't want to use steam. According to the 1982 IAPS formulation, the maximum enthalpy available above the critical point, without exceeding 1000 bar, is 2200 kJ/kg. The minimum enthalpy is about 2150 kJ/kg before you reach the mixed phase boundary. That means 50 kJ/kg is available to drive your turbine. In the superheat region there's about 1400 kJ/kg available from existing technology before you reach the saturated vapour line. That's going from 15.3MPa down to 5KPa (151 to 0.05 bar). There is no data for steam above 1000 bar.

As I said, it's all about the change in enthalpy. I'd be interested in a supercritical cycle proof if you have one.