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Author Topic: Generating electricity from solar water heaters via steam?  (Read 4083 times)
dan_aka_jack
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« on: July 05, 2006, 08:26:25 PM »

Hi folks,

This is my first post on this great forum - it looks like a very useful site!

I'm currently exploring the possibility of using a solar water heater to boil water to power a steam turbing electricity generator.  My thinking goes like this:

1) PhotoVoltaic solar panels are expensive and inefficient.

2) Evacuated tube solar hot water panels are pretty efficient and are capable of reaching very high temperatures.

3) So - would it be possible to generate electricty by using evacuated tube solar hot water panels to boil water (or perhaps another liquid) and then use that steam to drive a steam turbine and generator?

You could re-claim a lot of the water and heat by sending the exhaust steam through a heat-exchanger/condenser.

Would it work?!

Thanks,
Jack

Stats and links:

* Evacuated tube solar heaters have an efficiency of almost 80%
* PV cells have efficiencies of only 7-15% (although I thought they could achieve almost 30%)
* http://en.wikipedia.org/wiki/Steam_turbine
* Wier Services - manufacturers of industrial steam turbines
* Peter Brotherhood - manufacturers of industrial steam turbines
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Ian
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« Reply #1 on: July 06, 2006, 12:36:50 PM »

Theoretically, I think it would work. But I think it is very much like a school science paper question where "everything else not mentioned is unimportant". The trouble is, in real life just about everything has an influence.

Steam turbines are a bit like water turbines - you drive them hard - pretty much at full pelt and you get predictable results. The trouble is, thermal solar energy outputs is a bit erratic and not at all predictable in good old UK weather. You only get maximum energy output very occasionally.

The energy required to raise water a few degrees is miniscule compared to the energy required to generate steam (which is precisely why steam is used - because of its inherent energy). You would need quite a few panels (maybe a few hunderd Huh) - proabably arranged in series. Assuming that everything could take the pressure, soft water were to be used, and you could source a correctly sized turbine and matching generator, plus all the safety systems, you may get some steam that turns a turbine - but most of the time the steam would just be heating the turbine and not rotating it.

Why not try it and see ? (I think I will pass on this one as I have about 0.1% confidence level of any kind of useful result).

I think i would want to use all those panels for a more predictable and useful benefit.

There is nothing wrong with the principles, its just that real life gets in the way.

I hope this helps.

Regards,
Ian
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NickW
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« Reply #2 on: July 06, 2006, 01:49:35 PM »

From what I recall the best steam engines can only convert 15-20% of heat in steam to electricity so in reality they are no more efficient that PV. PV has of course the advantage of no working parts. Furthermore I doubt that for most of the year the panels would be able to convert the water to superheated steam. Another point is that I'm not sure that the panels are designed to routinely operate at these sort of temperatures.

Regards

Nick
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wyleu
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« Reply #3 on: July 06, 2006, 02:04:38 PM »

Striling engines are fascinating bits of kit, but examining the sites of companies that manufacture them commercially tend to have very specific applications and in some cases actively disuade interest of this sort. I have not constructed any kit but have had various explanations as to why they are inappropriate. Heat difference is a key part and actually generating a decent difference in heat in a domestic environment that can generate real power is quite complicated. As Nick has said there needs to be fairly constant different conditions required and the stop start nature means that the difficulty of initiating movement can be quite a problem, i.e. they can be a person to start under gradually changing conditions. Solar furnaces seem to be making moves in the stirling direction but again this is hardly back garden technology.

The whispergen being offered by powergen contains a stirling engine of new zealand design. This uses the heat of a domestic gas boiler to generate localised grid locked electricity. I went a fair way down this path with powergen but in the end they admitted it required about 10 to 15 minutes of continual gas burn to actually start generating which is great if you are a big gas user but matched against my own gas usage statistics it wouldn't be doing much generation, so it is probably another of those 'look how green we are' type projects that Energy companies are want to produce at times of media attention.

This said I would love to be proved wrong in this dismissal as it's a lovely idea and just to wet the appatite I post this link to various home built devices
http://www.rotarystirlingengines.com/rotacola.htm

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dan_aka_jack
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« Reply #4 on: July 07, 2006, 02:10:20 PM »

Hi folks,

Thanks loads for the excellent replies - I'm very impressed with this forum!

I'm going off steam turbines... Wyleu has got me all excited about Stirling engines!  If I may, please can we start a new discussion of Stirling engines.. I'll post a link in a few minutes...

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dan_aka_jack
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« Reply #5 on: July 07, 2006, 02:30:47 PM »

Here's the link for the new "Stirling engines" thread:

http://navitron.org.uk/forum/index.php?topic=150.0
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Ivan
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« Reply #6 on: July 18, 2006, 01:58:31 AM »

Don't run the poor old steam engine down too much - actually they can achieve 50% efficiency - ever wondered how your electricity is produced? Mainly from a coal-powered, gas-powered or nuclear-powered steam engine.

The steam engines used are a little more complex than an old steam loco, and have multiple turbine blades, and most importantly run in Rankine cycle (closed cycle) - which means that the steam is condensed on the downwind side of the turbine - causing a partial vacuum, which increases the efficiency. Also, imagine that the steam is condensed into water at 99C, and then reheated to 100C, you can see that the efficiency would be much higher than in an open-cycle like the old steam loco, that simply puffs away the exhaust steam.

You could lower the temperatures required by using a volatile circulation fluid such as refrigerant gases, thus avoiding the need for high temperature super-heated steam.

Using a steam-engine (or rather an expansion engine or turbine) to generate power from solar is actually quite feasible, and should easily be competitive with PV in terms of efficiency. However the total lack of off-the-shelf equipment, and the lack of research in this area would make it a long project, nevertheless a highly worthwhile one.

Ivan
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renewablejohn
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« Reply #7 on: November 06, 2007, 07:30:13 PM »

Yes it can be done but first a warning most steam engines work at a minimum pressure of 150 PSI which when connected to a boiler can result in a large explosion.  On outputs less than 500kw I would recommend a steam engine rather than steam turbine as the turbine losses are high at these low power levels. The solar tubes could be formed into an array and using thermal oil as the transfer medium a Gekakonus boiler can convert this energy into steam.
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Ivan
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« Reply #8 on: November 09, 2007, 03:35:08 PM »

Renewablejohn,

The Gekakonus system looks interesting. I guess the smallest power output of these is probably quite high. Any suggestions for relatively low power output? I'd love to experiment with a small system (ideally under 10kW, but up to about 30kW would be feasible).


Ivan
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renewablejohn
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« Reply #9 on: November 09, 2007, 04:44:00 PM »

Ivan   Have been told that the system is scaleable so you could try an experiment with an indirect cylinder  and see how much steam it will produce through the coil. Just remember the  steam safety valves when you come to pressurise the system. Personally I would leave the design to the experts 
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