I think it's a perfect idea for concentrators.

For flat panels? Not sure if it'll work out cheaper than PV + ST arrays next to each other, and how effective is the T part of it anyway? I'm sure there's a niche market where you don't have space for both, but I'm not convinced it will become the standard solution.

PVT panels do work and produce great electrical yields as a result of the cooling effect of the Thermal side.

The key consideration is sizing the array and a heat dump of some sort so that when the water in your cylinder has been heated there is still the ability to continue drawing off heat. Bear on mind that with so many panels, typically at least 10, there is going to be an awful lot of heat!



Solar energy systems
2Kwp System Photovoltaic Combined PVT a Combined PVT b
Roof area required Sq M 15.1 18.55 17.13
Typical annual yield (kWh electric) 1667 1999 2171
Typical annual yield (hot water) 0 6339 3650
Savings per annum £927 £1995 £1548

This table shows the difference between expected output of PV and PVT systems

Combined PVT a= Thermal bias system (more heat)

Combined PVT b= Electrical bias system (more electricity)

These figures assume: 41.3p per kWh Feed In Tariff (FIT)

50% of generated power is exported at 3p per kWh

Cost of domestic electricity is 12.9p per kWh
3,800 kWh domestic hot water required, of which PVT supplies 60%

Renewable Heating Incentive rate 18p per kWh (to be agreed)

Hey, go easy with the spam patrol. Neil has made an interesting contribution, here and in the past.

Thanks Neil.

PV-T seems like a really neat idea as the physics of the two things complement each other, but I'm yet to be convinced that the numbers really work out. What would be a very interesting addition to GM's chart would be separate PV and thermal installs, and the relative prices. (and details of exactly what systems are being referred-to).

One big problem is that you have to compromise the thermal design by making it flat panels rather than evacuated tubes, which makes the summer peaking much worse. On the other hand there is an area mismatch in that a typical domestic house needs 4-6m2 of solar thermal and 30m2 of PV. So if you provide 30m2 of both don't you just get a very expensive install and _waaaay_ too much hot water all summer (which you somehow need to get rid of). (I guess this counters the reduced autumn/winter output of flat panels). Maybe it would work well if combined with some kind of interseasonal store arangement, but those remain expensive for their effectiveness so far as I can tell.

And then I don't see how the temps can work particularly well. In summer your water is likely to be circulating at quite high temps 40-60C (because that's how how you are trying to get it to make it useful as DHW), and regularly 60-85C. How much cooling to the PV aspect does that really provide? Do PV panels alone run at 100C inthe summer or 60C? If the latter they aren't going to see much actual cooling for the thermal aspect. Typical PV reduction in power output is 0.5%/degreeC, so if panel is at 60C instead of 100C then that's 20%, which is clearly worth having. But if it's 60C instead of 50C then you are making things worse. I've just failed to find data for real-world PV panel temps during the summer. I guess I should measure my own alongside the already-measured thermal and see what the difference typically is.

I'd _like_ this to work well - it has potential as an integrated-sytem design (PV+solar thermal+long-term store), but it's not obvious how to get the overall engineering right, and someone has to work out how to do it cheaply enough for it to make more sense than just leaving a corner of the PV install free for solar thermal.

As Niall fits them perhaps he can comment further on the data to back up the numbers in the summary he quoted?

(That's a typo above BTW in NiallMac's post: GBP 1548, not 15,485 :-)

Damon
How are you going to make it 'cold', and how cold is cold?

Ah, found a useful graph showing typical solar panel temps for various cases against PV electrical output. Halfway down this page:
http://www.newformenergy.com/photovoltaic-thermal

I take from that, that the issue of temperature mismatch between ideal electrical output and ideal thermal output does exist, but the advantage of this set-up is that you can play them off against each other to some degree and choose which it is you want to try and maximise.

Interestingly that manufacturer provides two different panels which have different PV/thermal ratios. I don't understand why this is necessary, as I expect this ratio to be mostly a feature of the temp of the circulating fuid, not the panel design.

Here's a useful post linking to various PV-T suppliers and projects, including a (probably now outdated) IEA-SHC summary report showing types and outputs (but no prices). http://claverton-energy.com/pipermail/generalnewssnippets_claverton-energy.com/2010-March/000228.html

Seems things have moved on somewhat over the weekend. Interesting article.
http://www.sciencedaily.com/releases/2011/01/110125141810.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily+%28ScienceDaily%3A+Latest+Science+News%29&utm_content=Google+UK

Posted By: wookey
And then I don't see how the temps can work particularly well. In summer your water is likely to be circulating at quite high temps 40-60C (because that's how how you are trying to get it to make it useful as DHW), and regularly 60-85C. How much cooling to the PV aspect does that really provide? Do PV panels alone run at 100C inthe summer or 60C? If the latter they aren't going to see much actual cooling for the thermal aspect. Typical PV reduction in power output is 0.5%/degreeC, so if panel is at 60C instead of 100C then that's 20%, which is clearly worth having. But if it's 60C instead of 50C then you are making things worse.

Would this scenario ever occur? (PV panel @ 50c and Water @ 60C)

In standard solar thermal system, the solar controller will turn off the pump if the water in the cylinder approaches the temperature of the panel (i.e. you could configure the controller to turn off the pump cylinder return temp is within 10C of the temp of the panel).

Surely a PV-T system would be no different? If the PV-T panel is 50C and the return from the water cylinder is 60C then the solar controller would turn off the pump. You wouldn't be able to pump 60C water around the panel.

If the PV-T panel temperature increased to 75C and the return from the water cylinder is 60C then the solar controller would turn on the pump until the temperatures equalised to within 10C of each other (or whatever value you have set on the solar controller).

If the sun goes in, then no water would be circulating and the PV-T temperature would drop. If you have a drainback system, then the temperature would drop quicker because there wouldn't be any water in the panel to hold the temperature for longer.

Once you had a full tank of hot water, the PV-T panel would revert back to just being a PV panel, except if the sun spikes the PV temperature 10C above the temperature in your hot water cylinder.

I think to do a reasonable comparison you'd have to look at the cost of buying both PV and ST panels vs buying PV-T.

A lot of people seem to be using immersun style devices to heat their water from PV rather than from ST, though what will happen to their FIT payments when we eventually all get smart meters I'm not sure.

I did see an article about using these PV-T panels in conjunction with a heat pump for heat during winter months. It sounded interesting as the input temperatures from the ST part would normally be too low for water heating, but would provide sufficiently warm water for a heat pump to improve its COP rating. It sounds complex and no use to us as the outbuilding we'd be putting solar onto would be in total shade in winter, in the show of the house.

I think with all of these products, they have their own market and work well within it. We would only be able to site a 2KW PV array on our outbuilding. This would not be sufficient to allow us to heat water (with an immersun type device) and supply our electricity needs. A PV-T would use the same restricted roof space and would provide us with water heating and electricity, reducing our electricity bill and also (hopefully) allowing us to leave our gas boiler off all summer.

For someone with space for a south facing 4KW array, they could probably supply all their energy needs and push the excess into an immersion heater.

--FIT--

I'm sure I've read on a few FIT websites that until smart meters are rolled out, there is no way to measure the export so the FIT gives everyone 50%. Dunno, I don't have solar so I haven't really looked into it that deeply, I just assumed that once smart meters come in they will measure export as well as import of energy. Bit of a missed opportunity if they don't.

--ST HEATDUMP--
This is being discussed on a different thread by someone who is fitting ST panels, on that one I mentioned putting a couple of radiators in our woodshed to dump the heat too - helping to season our woodpile quicker.

However I think I'd be inclined to fit a drainback system, so once the heatbank is hot the pump would stop and the water would empty from the panel, reverting it back to just a PV system. It seems the simplest solution and I'm all for simple solutions. :)

Posted By: Pile-o-StoneI'm sure I've read on a few FIT websites that until smart meters are rolled out, there is no way to measure the export so the FIT gives everyone 50%.

SSE used to fit an export meter free of charge on request. We have one.
I understand they stopped offering that option a while a go (> 1yr?).
Some other suppliers would fit one, but only for a fee (in the examples I saw, the fee was so large as to make the procedure very unattractive).
If you have an export meter then the export element of the system is already based on the meter readings.

I just assumed that once smart meters come in they will measure export as well as import of energy. Bit of a missed opportunity if they don't.

Part of the problem with the smart meter situation is that at present it's just a snappy name. There isn't an agreed definition or specification as yet. Worse that that, I understand (from various topics on the Navitron forum) that some of the meters currently described as "smart" can't even be installed as replacement for a "dumb" meter if the user has microgeneration!