Generating electricity from solar water heaters via steam?

[dan_aka_jack]

Here's the deal:

PhotoVoltaic solar panels are expensive and inefficient.

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

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

edit:

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

[Muddypause]

Well, for what it's worth, here are some of my thoughts:

First off, you are going to have to generate steam, which means more energy input than if you just want hot water. I don't know much about steam turbines, but I would imagine that you are going to need quite a lot of steam to make the turbine run. I'm also wondering if it needs to be steam under pressure (ie superheated steam). I have a feeling that a turbine only gets to be efficient when it is running at high speed. But it may be possible - it is certainly possible to get small, domestic sized water turbines.

But the thing about a solar heat collector, is that it is usually used to heat a tank of stored water, and because of the way we tend to use domestic hot water, it can spend several hours harvesting small amounts of energy from the sun and adding it to the storage tank. But if my physics is right, it can never make the stored water any hotter than the collector gets. This would seem to imply that the only way to get steam is if the water boils in the collector. It may be possible to do this, but I wonder how much and how long for? And I wonder if the collector is able to withstand this.

But if this is no good, maybe you could use the collected heat to boil a fluid with a lower boiling point that the water in the collector. I guess this would need to be a closed loop, where the re-condensed fluid is cycled through the system again.

But I think that the real issue that decides it's viability, is working out just how much energy you can expect to get out of such a system. An evacuated tube collector may (ideally) collect up to 80% of the heat energy that shines on it from the sun, but the process of converting that into electricity may be the killer. At every stage that energy is converted, stored or moved around, there will be losses. There is the amount of energy that can usefully be used to make the steam, the efficiency of the turbine, the efficiency of the generator, the efficiency of any storage medium (batteries), and the efficiency of any inverters that you may use. You may be doing well to end up with only a small fraction of the energy input as useful energy output. I think it may be a good idea to work backwards through the system - figure out how much electricity you want to produce, account for all the possible losses and inefficiencies, and then see how that squares with the amount of energy you can expect to collect from a solar panel during a typical UK day.

I realise that this is me doing an impersonation of a wet blanket - what you suggest may be possible, but I don't think it will be all that straight forward. However, just as a last thought - quite a lot of domestic energy consumption is used to heat water, so maybe you could work out an offset. Install a solar hot water system, and offset the energy savings against the electricity you use.

[Martin]

I think it's a "scale" thing - you have to get over the hump of the fact it takes one heck of a lot of energy to get water to change state - something like 16 times the amount used to heat the same body of water by 1 degree (latent heat of vapourisation of water), so I have a feeling you'd need ACRES of panels
Good thought though! 8)

[Muddypause]

I've just looked up the specifics, if you want to look into it further:

The Specific Heat Capacity of water is about 4.2J/g/K. This means it takes about 4.2 Joules to raise the temperature of 1 gram of water through 1 degree. So, for example to raise the temperature of a litre of water (about 1kg) from 30 C To 100 C will take 1000 x 70 x 4.2 Joules = 294kJ.

The Latent Heat of Vaporisation of water (the amount of energy required to vaporise 1g of water at 100 C) is about 2.26kJ. This means it takes 2,260 kJ to turn one litre of water at 100 C into steam.

So, to turn 1 litre of water, which starts off at 30 degrees C, into steam, will take 2,554kJ (plus insulation losses).

1 joule per second is a Watt, so 1 kW delivers 1kJ in 1 second, or 2,554kJ in 42½ minutes.

It might also be useful to know the expansion rate - at 1 bar (atmospheric pressure), 1 litre of water will turn into around 1,600 litres of steam.

[dan_aka_jack]

Wow, thanks loads for all the replies. I'm always very impressed with the replies I get from this site!

Hmmm... I've got to say that I'm going off the idea of using solar to power a steam turbine... my current obsession is Stirling engines... I'll write more in a little while...

[Martin]

the "fringe" idea I rather like for it's utter simplicity is an old diesel Lister stationary engine, running on filtered chip fat that turns something like a 3kw alternator. There are sites on the net where people have got systems up and running for under £1,000. If you are using the "average" daily amount of about 10kw/h, you can then see that running the genny for 3-4hours a day will suffice to charge your battery banks. You've then got all that lovely hot water from the cooling that can be used too!) 8)
They can also be very effectively silenced - a really good silencer, and a large bore chimney, put the genny in an insulated (strawbale?) enclosure, and you'll have a mellifluous, gentle sound while it's running as it's very slow-revving, totally unlike the yammering modern generators! 8)

[dan_aka_jack]

Here's a link for a discussion on Stirling engines... including a link to a 1kW Stirling engine that will be available in the UK in 2007!

http://navitron.org.uk/forum/index.php?topic=150.0

[kenboak]

Hi,

Technically it's possible and was done in the late 1970s at the White Cliffs Solar Steam Project - however this used a large solar mirror array to concentrate the sun's energy onto a boiler heat exchanger.

Here's the link for the technically inquisitive

http://people.linux-gull.ch/rossen/solar/wcengine.html

The efficiency returned would be in the low 20%, which is 5 or 6% better than photovoltaic conversion - however this has been superceded by using a Stirling engine and a solar dish.

I have boiled my Navitron solar panel a couple of times when the pump stopped, and that produced more scary steam than I would care to deal with.

Steam power is not a friendly DIY sort of topic, it is very dangerous, and I would not recommend it.




Ken