Green Building Forum - PV-T Thu, 16 Apr 2026 18:25:17 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=99679#Comment_99679 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=99679#Comment_99679 Wed, 19 Jan 2011 17:24:58 +0000 tony
Seems they dump heat in the summer to increase pv efficiency/output by cooling the panels]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=99684#Comment_99684 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=99684#Comment_99684 Wed, 19 Jan 2011 17:52:31 +0000 evan
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.]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=99685#Comment_99685 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=99685#Comment_99685 Wed, 19 Jan 2011 18:26:45 +0000 SteamyTea PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=99998#Comment_99998 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=99998#Comment_99998 Sun, 23 Jan 2011 06:17:33 +0000 NiallMac
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)]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=99999#Comment_99999 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=99999#Comment_99999 Sun, 23 Jan 2011 07:08:07 +0000 SteamyTea Can I draw your attention to this page:
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=289&page=1#Item_1]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100010#Comment_100010 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100010#Comment_100010 Sun, 23 Jan 2011 10:19:34 +0000 marktime
Thanks Neil.]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100014#Comment_100014 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100014#Comment_100014 Sun, 23 Jan 2011 12:08:06 +0000 DamonHD
Rgds

Damon]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100118#Comment_100118 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100118#Comment_100118 Mon, 24 Jan 2011 13:51:32 +0000 wookey
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 :-)]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100122#Comment_100122 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100122#Comment_100122 Mon, 24 Jan 2011 14:28:55 +0000 DamonHD
But in this case I'd be allowing it to run hot (to DHW temps) in summer to maximise the DHW element, and cold the rest of the time to maximise PV output.

Rgds

Damon]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100138#Comment_100138 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100138#Comment_100138 Mon, 24 Jan 2011 18:04:24 +0000 SteamyTea How are you going to make it 'cold', and how cold is cold?]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100144#Comment_100144 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100144#Comment_100144 Mon, 24 Jan 2011 18:14:21 +0000 DamonHD
Dunno.

Rgds

Damon]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100284#Comment_100284 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100284#Comment_100284 Tue, 25 Jan 2011 18:02:07 +0000 wookey 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]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100292#Comment_100292 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100292#Comment_100292 Tue, 25 Jan 2011 18:48:15 +0000 SteamyTea 100/130=0.769
Its a negative slope
So
-0.769WpK^-1

What is the size of that test panel?

If you heated water from 10C to 50C your performance on the PV side would drop to:

(130/160)*100=81%

Still need to know the size of the test panel to get a true picture.]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100317#Comment_100317 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100317#Comment_100317 Wed, 26 Jan 2011 06:48:41 +0000 SteamyTea 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]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100497#Comment_100497 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=100497#Comment_100497 Thu, 27 Jan 2011 12:38:40 +0000 NiallMac
As I stated earlier, the key is what you do to extract that heat - and believe it or not, I've seen one installation where a beer cooler was used to draw out the excess heat and provide a higher PV yield.

Wookey, the reason for the two panel types is that they market one where the heat is to be used for more than just hot water. This would typically be a new extremely well insulated property where because of the number of panels a useful contribution can be made to space heating. This is a fairly standard thermal panel.

Where there is no useful purpose in collecting the heat, the domestic hot water having been attended to, then a panel with less insulation qualities , i.e. a different glass, is used.

But back to my first point. You've got to draw that heat off, otherwise the whole thing will be counter productive.]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174526#Comment_174526 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174526#Comment_174526 Thu, 25 Apr 2013 11:35:19 +0100 Pile-o-Stone 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.]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174540#Comment_174540 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174540#Comment_174540 Thu, 25 Apr 2013 12:35:10 +0100 Chris P Bacon
From what I remember of some other previous discussions (maybe 12-18 months ago) about PV-T the economic case for installing them was difficult to justify.]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174544#Comment_174544 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174544#Comment_174544 Thu, 25 Apr 2013 12:42:20 +0100 Pile-o-Stone PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174550#Comment_174550 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174550#Comment_174550 Thu, 25 Apr 2013 13:08:15 +0100 SteamyTea Putting up, or changing panels to higher output ones may not be that expensive.

When I think about ST systems that are backed up with other heat sources, it may be that we think that the ST should take priority, this may not be the most cost effective or practical method.
Taking a 200 to 300 lt tank as an example (not every one has room for a 1000 lt one), if the water temperature is set to 60-70°C and then the solar stopped adding, the next day there may not be enough sun and the alternative starts the heating.
If during the day the sun comes out and the tank is already up to temp, then no extra can be added.
Would it not be better to have 2 different temps, say 45°C for the alternative and the 60-70°C for the solar, that way solar can always top up and there should be adequate hot water.]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174560#Comment_174560 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174560#Comment_174560 Thu, 25 Apr 2013 13:29:14 +0100 Pile-o-Stone
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.]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174580#Comment_174580 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174580#Comment_174580 Thu, 25 Apr 2013 14:53:14 +0100 skyewright Posted By: Pile-o-StoneA 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.
FIT applies to the reading on the Total Generation Meter (TGM). ImmerSUN & its ilk do their stuff between the TGM and the DNO meter, so no reason for FIT to be affected at all.

If the smart meter is capable of measuring export and replaces the "deemed 50% export" then unless real export was exactly 50% it's reasonable to suppose it would affect the export payment (for those that don't already have an export meter). Even with "deemed 50% export", that bit of the payment is usually much smaller than the FIT anyway (especially so for the people who had their systems fitted before the tariffs started to change).

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.

Unless PV efficiencies have increased a lot since I last looked a 2kWp PV array might be around 10m2?
10m2 of flat plate ST (in the form of PV-T) is quite a lot of ST so it would be sensible to at least consider some form of heat dump or other means of preventing stagnation (since you mention a gas boiler, is it reasonable to guess there's no GSHP loop to consider for the role?)]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174583#Comment_174583 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174583#Comment_174583 Thu, 25 Apr 2013 16:02:48 +0100 Pile-o-Stone
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. :)]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174585#Comment_174585 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174585#Comment_174585 Thu, 25 Apr 2013 16:21:08 +0100 Ed Davies 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%No. FIT, contrary to its name, is paid for generation, not feed in. You get the FIT payment for electricity you generate whether you use it yourself or export it.

There is an additional small payment (3 to 4.5p/kWh) for export. For those who don't have export meters (as a lot haven't pending the introduction of smart meters) this is “deemed” to be 50% of the amount generated.]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174599#Comment_174599 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174599#Comment_174599 Thu, 25 Apr 2013 17:32:57 +0100 skyewright 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!]]> PV-T https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174600#Comment_174600 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6841&Focus=174600#Comment_174600 Thu, 25 Apr 2013 18:23:33 +0100 SteamyTea Posted By: skyewrightUnless PV efficiencies have increased a lot since I last looked a 2kWp PV array might be around 10m2?
10m2 of flat plate ST (in the form of PV-T) is quite a lot of ST so it would be sensible to at least consider some form of heat dump or other means of preventing stagnation (since you mention a gas boiler, is it reasonable to guess there's no GSHP loop to consider for the role?)Or just fit less of the T part, may change the resistance of 2 or 3 modules though, so may need to be either:
On own string if enough of them
Micro inverters if only 2 or 3.

Think it was sour old mate JS Harris who said about this.]]>