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Forum Index : Solar : Keeping roof PV panels cool(er).
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| neil0mac Senior Member  Joined: 26/12/2009 Location: AustraliaPosts: 210 |
For those with an evaporative cooler, don't literally 'blow your money out the window'. Re-use it! A louvred ceiling vent with a duct to the back of the panels that have the lower edge and both sides sealed to the roofing material can reduce the heat that radiates onto the back of the panels from the ceiling cavity as well as taking away some of the direct heat from the sun. (A 'skylight' in reverse.) The extra power generated can 'pay for' running the cooler? |
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| VK4AYQ Guru  Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Neil My cooler is 300 watt so wouldnt get any gain just a loss of around 200 watts. All the best Bob Foolin Around |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Thats what i thought as most swampys i have looked in have a 1hp motor and thats @ 750watts. I think Neilmac meant if the swampy was running on a hot day to vent the house out through under the panels and not a door or window. This only works to some degree unless you have ducting in the ceiling from all rooms, otherwise one room gets all the air flow and/or all the hot air from the other rooms. It was tv earlier where they had roof top gardens around the panels and commented the plants help cool the panels on hot days. Im not sugesting you move the vegie garden on the roof here, but growing green grass around ground mounted panels in summer would be a big advantage. Over a span of summer you might even benefit more from watering the grass around the panels than you would splashing water over them on the odd hot day as has be sugested before. Pete. Sometimes it just works |
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| neil0mac Senior Member Joined: 26/12/2009 Location: AustraliaPosts: 210 |
Spot on. Surely people understand that no idea fits all circumstances. Fitting cap to head ???? |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Neil I see what you mean about the evaporation cooler for the house discharging over the panels, I am putting together one for the house that runs off two 12 volt panels and a backup battery with total use of 100 watts for next summer so that may be practical. Also I saw a system that draws the air from an underground source in this case a number of clay pipes connected in parallel two foot below the vegie garden as the ground temp was 23 C and it worked by convection. Also made the vegies grow better as it warmed the soil a bit and the water grew the vegies and not just wasted to the atmosphere. More projects got to live to 129. All the best Bob Foolin Around |
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| Jarbar Senior Member Joined: 03/02/2008 Location: AustraliaPosts: 224 |
Hi all, my plan is to duct air from under the house with 150mm pipes into 3x100mm pipes.Each one will have slots at spacings to match panels and be situated at the lower end of panels (3 rows)with the slots angled to create the greatest flow possible over the rear of panels.The barrel fan originally from a ducted heating system consumes 125w and delivers a large volume.Will find a thermostat to switch on and off. What do you think. Anthony. "Creativity is detirmined by the way you hold your tounge".My Father "Your generation will have to correct the problems made by mine".My Grandfather. |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Air is a poor conductor and i would doubt if the return will justafie the running costs. I would think something like some cheap irrigation poly pipe stuck to the back(underside) of the panels themselves with water circulated from a tank through the tubing would do better at maybe a lower running cost. I would tend to look towards a heat exchanger / heat sink method rather than air or flood system. Pete. Sometimes it just works |
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| neil0mac Senior Member Joined: 26/12/2009 Location: AustraliaPosts: 210 |
Ummm? What about the original V-dub? (No radiator in that one.) Ok. A mate one and dropped it into 2nd gear at 90MPH. I asked, "What happened?" He replied , "It just screamed!" |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
There was a story on TV last night about cooling building roof's. The best approach was to cover a flat roof with a couple of inches of dirt and plant a few grasses and creepers. Dropped the roof temperature from over 40 deg C to 20 dec C in full sunlight, ambient air was about 22 from memory. They also had a solar panel on the roof and it performed better because the plants were keeping the air around them cooler. Interesting stuff. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Its just a right bugger getting the mower up there. 
Perhaps a goat could be handy?? 
Pete. Sometimes it just works |
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KarlJ Guru Joined: 19/05/2008 Location: AustraliaPosts: 1178 |
No way in hell id be sticking anything to the back of my panels, the weight could peel the backing off them quite easily and destroy them, at $500 a panel makes for an expensive mistake. I think its a good idea Neil but what happened to the tracker? dont tell me you've folded and gone for my first suggestion? ie Pay Nu-Energy to put up 1.5KW and play from there with trackers cooling etc etc etc etc and in your case etc! Karl Luck favours the well prepared |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
It would only need to be in contact with the panel not attached to the panel itself. As for the VW example given, one needs to note the thermal difference in temperture between engine block and air temp is greatly different, so air cooling will work but when you have a few degrees difference the thermal conductance of the air is next to nothing. A solid or a liquid have much greater thermal transfere properties than air ever will. Think about it?? why do we use heat sinks if air is so good of a conductor. It needs a solid to disperse the heat over a larger area to be able to disapate it. I would think air with a thermal difference of 2-5 degrees would do next to sweet FA if discharged under the panels, unless the underside was boxed in and designed to disperse the flow across the panels under side surface. Then this open another can of worms. Cool air sinks and the expectation of it to cool the panel surface is going against physics. Its not that im against the idea but can not see how the exspence of set up and running costs could deliver a return. At least with a liquid the heat can be carried away and dealt with elsewhere. If a solid was used to increase the surface area to allow for better thermal convection and dispersion into the surrounding air, the thermal transfere becomes more effecient and hence the panels do to. All of which sound good in theory but not so easy to apply. Pete. Sometimes it just works |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi All I think that all us needed is to make sure that ambient air can circulate by convection behind the panels and put in the garden to lower the ambient a few degrees as Gizmo suggested, one of the problems of collecting sunlight energy is the residual heat, and that's just part of the equation. All the panel makers rate their panels for a European temperature and we aren't in Europe we have a higher temp and are always trying to get the rated power out of the panel, just forget it and put on a couple of extra panels to make up the losses. Remember that it is self equalizing to some degree as in the winter the panels work closer to their correct temperature and put out a bit more but the days are shorter, in the summer the output is lower but the days are longer. The worse case is the panels on a roof with the heat buildup to some degree trapped due to poor air circulation. The best case is a free standing tracker subject to ambient air temperature with a slight breeze and natural convection of the air flowing across the panels back and front. from what I have seen in the past this will increase the output by 15 % over the same array laying on a hot roof. All the best Bob Foolin Around |
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| neil0mac Senior Member Joined: 26/12/2009 Location: AustraliaPosts: 210 |
I think that some are trying to evaluate an idea from a different climatic viewpoint. Air from an evaporative cooler is predominantly under 30 Deg C - maybe a little higher, but not much. Air in a roof cavity radiating onto the back of an already heated solar panel can be around the 70C mark. If, as suggested, the edges of the array on the roof are sealed to the roofing material as well as practicable, the cool exhaust air has to move upward behind the panels with any air at a slightly elevated temperature rising first. The summary dismissal of the V dub is only partly relevant. The speed of the air behind the panels is very much lower and has more time for the molecules to become excited to say nothing of the relevant surface areas of the two. |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
I do agree in what you say, "But" as i said it opens a new can of worms. Any actaion you take to make the idea work can have a bigger adverse reaction if the swampy is not running circulating air flow. Sort of damed if you do and damed if you dont. So is the cost of setup and running it going to give a positive return?? (i think not) Me thinks a tracker in open air off the roof is far cheaper and a far better return, if you have the space. As for the VW, my point was there is a big difference in air cooling something that is over 100c compared to a much lower base temperature. The % of thermal loss is far greater when the difference in temperture is much wider. Im not sure on the 70C as i can place my hand on a solar panel on a hot day, but not on a VW motor. Pete. Sometimes it just works |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Neil I read what you are saying and I think the only way to resolve the gain on the panel is to set one up as you suggest, and blow the air in and read before and after temperature and before and after output power, if this is a substantial gain then it is worth doing but it must give an increase of a substantial amount to make worthwhile, and also work it in with the tracking device as the gain from the tracker is a solid 30% a day compared to non tracked panels, if the both ideas can be incorporated and give say another 10 % it could be a worthwhile exercise. All the best Bob Foolin Around |
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GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
I see many new systems that are installed close to the roof surface, not real smart. I had some say, in my own install, and my panels have free air circulation behind the panels, and on a white roof, the temp maxes out at around 55C in the summer. Gordon. become more energy aware |
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| neil0mac Senior Member Joined: 26/12/2009 Location: AustraliaPosts: 210 |
OK, Gordon. It would be interesting to know just how much difference the gap you have between panels and roof makes to the radiation onto the back of the panels. Not that I expect you'll change it just for anyone's curiosity! So 55C is your figure. What would that be in terms of a 40C daytime temperature? What would it be in terms of an overnight minumum temp of 35C? It seems to me that the early morning sun would already have considerable bit of help in heating up the panels to above 55C. (Methinks downwind might almost see snow all year round. (Well, not quite!) P.S. I really would to see 'efficiency ratings' quoted at a realistic 'Aussie temperature! |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Neil I agree with you on getting panels rated to Australian conditions, the problem with that is the standard adopted for panels isn't generated for Australia, and what area of Australia would be the standard temperature zone, as Australia has most climatic zones at any one time, where I am the ambient midday air temperature varies from 20 C in winter to 40 c in summer and this is a temperate zone. Metal objects in the direct sunlight can reach 90 c and the internals of a solar water heater I have I have measured at 120 c and that was with water in the collector at 90 c its a good steam generator in the summer. So I think we are stuck with the performance graph supplied and some common sense construction methods in our racking or tracking devices. All the best Bob Foolin Around |
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| neil0mac Senior Member Joined: 26/12/2009 Location: AustraliaPosts: 210 |
Bob, Climate variability is a problem for everyone. My 'beef' with the standard is that it would just as easy to rate them internationally at a 45C rating and let those in cooler climes, say, "Wooppee!, we can expect better than that.". When it is all said and done, even temperate areas can have panels operating at 50 - 55C. |
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