PV as a heat shield

murphaph

So, we're in our new dormer bungalow since Xmas and have had our first few days of 30°C temps recently. It is certainly significantly more comfortable at night than in our old apartment, probably down to the timber frame superstructure not acting as a thermal store at all. The roof tiles are presumably acting as a thermal store however.

So, as we see PV as something we'll be doing in the foreseeable future and because the rules here are such that slightly over 7kWp makes little sense and over 10kWp makes essentially no sense, we'll almost certainly be installing slightly under 10kWp and almost completely covering our 100m² SSW facing 45° pitch with panels.

Then I started wondering how effectively the panels can block the suns rays from heating up the tiles during the day. I am assuming the underside of the panels gets very hot, but that this heat rises up along the 45° roof due to convection and cooler air (albeit at 30°C or whatever) comes in to replace it and that the tiles become hot, but not as hot as they would do if the suns rays could directly heat them.

Any thoughts on this? Can PV help keep our house 2 or 3 degrees cooler on those hot summer nights? (we're in Berlin by the way)

quentingargan

I'd be surprised if there isn't a good layer of insulation between your roof tiles and your ceiling, especially in Germany. But if your PV panels are slightly above the roof, which is usually the case, there is a cool vented area between the panels and the roof itself. The panels do get hot themselves, but the tiles would be cooler.

Should do. Conservation of energy and all that. The PV is extracting energy from the sun so less heat on the roof.

As to the science of it, no idea how you'd calculate it other than empirically.

Impetus

murphaph wrote: »

So, we're in our new dormer bungalow since Xmas and have had our first few days of 30°C temps recently. It is certainly significantly more comfortable at night than in our old apartment, probably down to the timber frame superstructure not acting as a thermal store at all. The roof tiles are presumably acting as a thermal store however.

So, as we see PV as something we'll be doing in the foreseeable future and because the rules here are such that slightly over 7kWp makes little sense and over 10kWp makes essentially no sense, we'll almost certainly be installing slightly under 10kWp and almost completely covering our 100m² SSW facing 45° pitch with panels.

Then I started wondering how effectively the panels can block the suns rays from heating up the tiles during the day. I am assuming the underside of the panels gets very hot, but that this heat rises up along the 45° roof due to convection and cooler air (albeit at 30°C or whatever) comes in to replace it and that the tiles become hot, but not as hot as they would do if the suns rays could directly heat them.

Any thoughts on this? Can PV help keep our house 2 or 3 degrees cooler on those hot summer nights? (we're in Berlin by the way)

One has intimate knowledge of several properties, including one in Malta in a nine building skyscraper development. The apartment in question is on a high floor – higher than any skyscraper in Ireland. But it is not a penthouse – which would suffer max heat. The daytime temp in the summer outside can reach 40C+ and above. So the guy above probably has a higher A/C bill.

While today, Malta is a neutral independent republic within the EU, in the past it got infected with poor British construction standards (eg Ireland has allowed itself to occupy a similarly dumb position). The MT government places low emphasis on the need for insulation not unlike the situation in GB. Hot countries need heavy insulation just as much as cold countries. Malta has also copied the ejit British wiring system, with three pin flat non-European standard plugs with fuses (for the past 50 years circuit breaker technology has worked faster than fuses). The Maltese run a high amp feed around the place, in the rest of Europe there is a direct (lower amp) connection between each socket and its circuit breaker and this incorporates an RCD to minimise shock / fire risk.

I know other places, eg not quite in France, but close, who use better than French standards, where the windows are quad glazed, and the typical apartment is fed with water at 5C and 80C which goes into heat exchangers in each building to replace A/C. One can leave the system on while away for several weeks, so that it is pleasantly cool on return home, and it will not dent your electricity bill. The electricity generation is 100% green – ie hydro or recycling in the incinerator). Therefore almost zero Co2 gas or other carbon energy cost. The 80C water from the incinerator gives you hot water, and can spew out heat from your heat exchangers in the winter if you need it. Incidentally the electricity is charged at 10c per kW/h at retail. The devices in these apartment only consume 30w/hr of power (to heat or cool the place). The heat is being generated by the trash you and your neighbours send to the local incinerator, as well as wood waste imported from the neighbouring waste-material dumb EU countries on a ‘we will take it off your hands’ basis). (For anyone worried about incinerators, this little country has one of the longest life expectancies in Europe, and has been using incineration since the 1850s – because landfills are illegal and financially non-viable).

So will solar block out excessive heat from your home? Perhaps marginally. But you really need serious thermal insulation. Something that EU countries are poor at, compared with other European countries. Iceland, Switzerland, Liechtenstein and Monaco come to mind as leading in the field of making the most of what you have got in the energy space. At least if you install solar PV it will partially/entirely cover your A/C running cost. The hotter the sun gets, the more power is generated. However this will not solve the warm night-time issue and electricity is grossly over-priced in DE.

murphaph

Yeah electricity is shockingly expensive here. I guess that's why PV is still quite popular.

We have 300mm of rock wool between the rafters but the heat can still to an extent penetrate the rafters themselves.

We don't believe we need AC. I leave the high efficiency UFH circulation pump running and this is according to my logic going to transport energy from the ground and first floor to the cellar, evening out the temps in the whole house. We also have exterior roller shutters to keep the sun's rays out in high summer.

The cellar stays at 20° while the ground floor is around 22°. This is totally comfortable obviously. The first floor (former) is where it gets a touch warmer but not at all unbearable. I'm just wondering would the PV bring the additional benefit of dropping the temps by a couple of degrees and it seems most think it would.

Another crazy scheme I am considering is a trickle pipe along the ridge line, pumping groundwater onto the tiles during the summer months. We have very high (cheap to extract, only need to go down 5m or so) low iron but hard groundwater. I believe the limescale would not be a huge problem as rainwater is acidic and would wash it away. Iron would be a problem as it leaves brown stains.

Calahonda52

Some physics here first
https://www.designingbuildings.co.uk/wiki/Decrement_delay
and
http://www.greenspec.co.uk/building-design/decrement-delay/

You can do the math of the rockwool vs say full wood fibre.

there is no doubt at all that the PV panels will stop the solar rays striking the roof so the delta t in the math for the roof tiles being heated by air even at 30 deg c, and some heat from the panels, will be several multiples less than tiles which are being baked by the sun.
The heat radiated from the panel will be purely heat, and no light, the sun does both at about 1,400 watts/ metre squared per second

http://www2.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html
and
http://environ.andrew.cmu.edu/m3/s2/02sun.shtml

dathi

Impetus wrote: »

. The electricity generation is 100% green ? ie hydro or recycling in the incinerator). Therefore almost zero Co2 gas or other carbon energy cost. DE.

well thats bull anyway when you burn waste the majority of the calorific value comes from the plastics being burnt . and all plastics come from fossil fuels so there is plenty of co2 added to the atmosphere.