Solar electricity generation using Stirling engines?

[dan_aka_jack]

In this thread, Wyleu enthused about using Stirling engines to generate electricity from solar.  Stirling engines seem very promising: they're efficient, quiet and have a favorable power-to-size ratio.  Apparently solar stirling engines can achieve 35% efficiency.  Which leads to the obvious question: why hasn't every roof got one?

I've heard there are several problems, e.g.:

1) Creating a large enough heat differential in a domestic environment
2) Keeping the Stirling engine lubricated and maintained

Are there any commercial, roof-mountable solar stirling engines out there?

Some links:

http://en.wikipedia.org/wiki/Stirling_Engine
http://auto.howstuffworks.com/stirling-engine1.htm

http://www.stirlingenergy.com/whatisastirlingengine.htm  (Stirling Energy are currently installing the world's biggest solar farm in the USA - based on Stirling engines)
lots of handy links
In 2007 we'll be able to buy a gas boiler that sports a 1kW stirling engine licensed from SES (BIG NEWS!!!)
Lots more links
http://www.bullnet.co.uk/shops/test/stirling_engines.htm - buy toy Stirling engines (including a solar one) in the UK
NASA's stirling engines
http://www.energyinnovations.com/sunflower.html - concept for a solar-powered combine heat & power stirling engine
http://www.bekkoame.ne.jp/~khirata/english/others.htm - lots of links
http://www.bekkoame.ne.jp/~khirata/academic/kiriki/solar/seino.html - experimental 1kW solar powered stirling engine

Videos:

* Mechano stirling engine!
* Solar model stirling engine

[dan_aka_jack]

More info on the SunPower / MicroGen combined-heat-and-power domestic Stirling engine:

http://www.microgen.com/ - the UK distributers
http://www.sunpower.com/index.php?pg=48 - SunPower's pre-production Stirling engines (available for purchase, apparently)
http://video.google.com/videoplay?docid=6529537814937384757 - A video interview with a SunPower representative

[NickW]

I've just read the info on the microgen domestic CHP units. If these are as simply as they say to install (and I heard somewhere cost about £2k?) these really do offer a major advance in energy effciency and an opportunity to produce large amounts of peak rate electric for very little additional fuel. I understand that 5 million units would have the equivalent output of Drax power station.

[wyleu]

The economics of these devices are predated on existing energy prices where electricity is four times more expensive than gas. Now this is not a situation that is liable to remain so for too long. A kilowatt/hr is a kilowatt/hr and it will probably end up where it has a stronger 'value' than money. Ask the people stranded in New Orleans if they would have rather have had money or electricity, the conclusion was generally the later.  I looked quite hard at the Pwergen offering the whispergen and certainly at the start it looked very positive but, and there always seems to be one, in that particular deal you were tied into Powergen for any excess energy sales, the system didn't actually generate for the first 15 minutes of use, and it would shut down , like most grid connected systems, if there was a power cut.
Certainly there is a fondness for 'alternatives' but by and large the mechanism seems to be you relieve the power company of the responsibility of generating power by paying yourself for the equipment which they then define the operating financial parameter for.
It might benefit by spreading the timings of your usage but ultimately if it is popular you will still be hitting it at the same time as all the other users so will probably be charged accordingly whilst you will probably generate at the same time as lots of others so you will receive a very low rate of return. This isn't the only justification but I came away feeling very suspicious of the deal and it often seemed to involve the power company saying trust us, and frankly I don't.

If the situation has changed then please leap in and point out any errors I have made, things do change quickly and I havn't looked much in the last year or so.

[NickW]

Most peoples boilers are set to come on at the same times as peak demand for electricity occurs - in the winter months. My boiler on times are 5.30-8.00 and 16.15 to 21.30. In view of this the electricity generated by micro CHP is just at the right time. As the micro CHP operates at efficeincies of 90%+ whereas the gas fired power station is at best 45% that is one hell of a saving in gas. If you compare this to solar PV - which produces power at the times of lowest demand - sort of puts things in perspective.

What I liked about the microgen boiler was its ability to be used as generator in the event of a power cut - producing upto 1100w - which will keep basic services going.

[wyleu]

How is the switchover managed?

You must isolate the DNO supply from your energised kit in the event of a failure in supply.

[NickW]

I've no idea wyleu but in the brochure it said that this option was possible.

Nick

[wyleu]

The pdf crashed my browser ! I think I've requested details

The issue of switchover is probably a question our friends in the turbine ongrid section could answer with ease...

[NickW]

I understand that the manufacturers intend to produce a unit that runs on LPG. This would be a practical addition to an off grid system incorporating PV, wind, and solar thermal. The CHP unit would provide a useful baseload of heat and power at the time it is most needed. OK the purists will say that LPG is not offgrid but unless you have a reliable source of hydro or a submarines worth of batteries complete off grid is fairly impractical.

[wyleu]

I have now downloaded the brochure so I'll have another look...

[Ivan]

Stirling engines tend to be very large if they are to take advantage of low temperature differential. There is a practical solar stirling engine that has been produced, but it is the size of an enormous car trailer, and generates a tiny amount of power - a few hundred watts, i think. The technology just doesn't seem to work with solar. The efficiency of 35% or so, is not really that impressive - you can get 30% out of a good diesel generator. There are quite a few people that have designed CHP units around diesel generators running on waste veg oil.
Ivan

[wyleu]

What do you think of the Combined heat units like the Whispergen offering, Ivan?

[Ivan]

The concept is a great idea. The practice is always a compromise. The most disappointing thing about the whispergen is that it is only 35% electrical output at low heating load. If you turn up the heating, it just switches on additional conventional burners, without producing extra electricity.

I think CHP is a fantastic idea. It is not generating any kind of renewable energy - it is generally all fossil-fuelled, but what it does achieve is decentralising electricity production - I think this is a key concept to maintaining our current lifestyle as we burn up most of the remaining fossil fuels. By generating power near to where it is needed - ie your household requirements and your neighbour's requirements, it is possible to very significantly reduce powerline losses - which can be 40% or so.

The whispergen is expensive, and unlikely to make a huge impression, but I think it demonstrates an interesting and useful concept. In theory, if you can produce electricity at 35% efficiency, and sell it at the same tarrifs that renewables qualify for, then it should be possible to heat your house for free - I guess that makes the concept quite attractive!

I prefer a more conventional approach - take a look at this newsgroup. It is written by people doing the same thing with more conventional diesel engines, on much much smaller budgets but running off waste vegetable oil - a virtually free non-fossil fuel. http://ph.groups.yahoo.com/group/Lister_CSOG/

Ivan

[dinitro]

I'm sold

http://www.powercubes.com/listers_1.html
http://www.powercubes.com/listers.html
http://www.powercubes.com/listers_14.html

Ivan or anyone else thinking of selling this off the shelf?  I like the listeroid engine, the fact its quite small and runs on SVO.
The only thing I'm put off by is coupling to an alternator to get that 50Hz off??

dinitro

[Ivan]

The idea is very simple, and very easy. It does require a fair bit of fiddling around, though.

THere is no reason why you could not do it with a modern 1500rpm diesel generator, but it does take the fun out of it a little. And really you need to have a water-cooled system to get a good heat recovery. Also water-cooled generators are more efficient.

The reason that the Listers are popular is that they are old, reliable and cheap. Also, as they are low-speed generators, they have excellent efficiency, with very low exhaust temperatures (off-load exhaust temperature is around 50C!).

I have one of these on my driveway, which is planned to be incorporated into a CHP system. I will take a slightly different route though. My plan is to couple the Lister to a Navitron 2kW wind turbine PMG alternator, producing three-phase ac at 220v. This will then be rectified, and being three phase, the ripple will be within 10% bounds, meaning that I can feed it directly into a SMA grid-feed inverter, so the power can be fed directly into my existing household mains without isolating from incoming supply. Any surplus would then be sold back to the utility companies. This is probably easier, safer, and more convenient than a 50Hz feed, and would be perfectly regulated, as the AC waveform is built up from DC, so engine-hunting would not be an issue. Also, if it was producing less power or more power than I need, the import/export meters will take care of any deficit/surplus.

Ivan