I built an open sided tin lean-to off the shack to have some sheltered workspace from the rain and blazing sun. The gutter is plumbed to a collection tote for garden water. I dropped a sump pump in and hosed the tin rooftop down on an oppressively hot noon day. Other than evaporative losses the free rain water was being collected and recirculated.
The gravel temp in the sun was in the 90s. The gravel temp under the shade of the enclosure was 70ish and air temp was around 83 and stuffy feeling from the ceiling area. The underside of the tin was 125-130F before wetting. After a few minutes of hosing the top of the roof, underside temps dropped to 88-90 and the feeling of coolness from above was a notable relief, like a welcome breeze.
The sump pump draws .05 amps @115vac when pushing up to the roof through an unthrottled hose.
The collection water out the downspout was easily hot enough for a good shower the entire cycle. 20 minutes later the roof was back to 130F so there is certainly all the hot water you can want on a tin roof during a sunny day. This is only 12x24
If that roof was over an air conditioned space i am certain it would reduce the heat that the air handler is having to work with. Water is ~830x denser than air which means it can carry away and/or store much more BTU than the same cubic volume of air. Hence the 40° temp drop in minutes.
Cooling and hot water for the electrical cost of a window fan. Im going to use it to heat the kids pool and cool my workspace.
Along the same line I have an old desert tan shade cloth that I used to block the morning sun from my sawmill building. I am tempted to stretch it over the roof of my garden shed(12x24) that is in full sun to see what difference a little shade would make.
Quote from: stavebuyer on July 26, 2021, 07:09:52 PM
Along the same line I have an old desert tan shade cloth that I used to block the morning sun from my sawmill building. I am tempted to stretch it over the roof of my garden shed(12x24) that is in full sun to see what difference a little shade would make.
Shade makes all the difference I find, I hate working in the sun actually. Unless it's a warm early spring thawing day lol
Curious as to how the cooling would be reduced once the water in your tote had been heated after a cycle or two across the roof? Would you need a larger tank or long buried loop for the system to remain effective?
The tote was just a numbers run for today to see if it was worth the effort. Im gonna use an 18ft round pool that we just stood up. about 7,000 gallons of water. I think it would take about a week to raise that 5 degrees.
I stuck my hand in the top of the tote and couldnt feel any temp rise. As the delta T shrinks it will get less effective at cooling the roof down. A 20g water heater tank or something however could probably stay at 120f all the time. Be handy for an offgrid gravity fed hot water supply. Solar could easily run the pump. Tank in the rafters.
I always kinda thought about cooling my future cabin roof with a recirculated coi pond. Keep the water turning over and help keep out solar attic gains during the noon sun.
if the water temp starts to rise and decreases cooling, you could add a thermostat to trun on a pump and add well water as needed. keeping the tote in the shade will help. the tank in the rafters is nice to decrease the height to pump to the roof, and therefore decrease power use or increase flow. if water evaporates, it pulls a lot of BTUs via the phase change from liquid to vapor. you could also pump the hot water to a radiator and heat a kiln or something, or to the outside to drop the water temp. I am sure you have a few old radiators around.
IMO the biggest cooling effect you are seeing is evaporative cooling. Same concept as a cooling tower with the same limitations. In dry climates the same effect is used for indoor cooling using something called a swamp cooler. It works as liquid water absorbs energy when it evaporates into vapor. The energy it absorbs cools the surroundings. The best possible cooling would be to cool the surface down to the wet bulb temperature which is related to relative humidity. In reality the process end up several degrees warmer than wet bulb. In dry desert climates where there is lot of heating during the day it works quite well but on the east coast where its green and possibly influenced by the ocean, the relative humidity is quite high during the summer so the wet bulb is getting close to the dry bulb. Therefore evaporative cooling is not as effective. In very hot dry condition, sometimes they skip the shade and just mist fine water droplets into the air, it cools the air a bit and the cooler air is denser and it drops to the ground.
Yeah ive heard people talk about swamp coolers online. Try it "back east" and youll just have a hot mushroom house.
When i first hosed the roof and aluminum top containers, they flashed to steam. It is certainly evaporative cooling in the beginning. The longer you run the water the cooler the tin gets. If evaporative losses were a concern just keep the water going. Im my case its rainwater and we have plenty of it. I put about 50 gallons in the garden every week.
Yep, you just learned about evaporative cooling. If the air is already full of water, humidity, the air can not absorb much more evaporative water. So it has a small benefit, depending on climate.
If you could plumb in a heat sink to take the hot water run off from the roof, transfer it via a heat exchanger, then put it back on the roof to repeat the cycle...you would have a open air swamp cooler. The best heat sink is a tank of water with a copper hose coiled through it...redneck hot tub.
The best example I've seen of utilizing evaporative cooling is utilized by roofers. In the dead of summer with all the heat, most wear long sleeves and jeans. They sweat because of the heat and clothes, but it stays on the body to evaporate. That is how they stay cool. Simple and effective.
im using a pool. I figure 7000 gallons of water with an 18ft surface is gonna dissipate most of the days heat each evening unless i put a cover to retain it.. which might make sense to extend pool season. A big thermal flywheel.
or cover with a silver tarp in the day when not swimming to keep the heat out. May want a filter for the dirt on the roof, gravity fed.
Mike, is the pool water chlorinated? If so it might a little rough on the metal. Just a thought.
Yeah its chlorinated. The roof is used old thick tin off a chicken barn, with many coats of silver slop. Doubt itll harm that asphalt based sealant (brake cleaner does!) But ill keep tabs on it. Thanks.
It does wash down a good amount of bugs and pollen at the first pass. I will rig it up to dump the first 5 gallons into a settling bucket with a screen on top. Chicken treats. I use old boot socks as a filter on 2" pipes like the bottom of a tote for instance. Just drained my full tote into the pool so i can move it. Crystal clear.
Half of the goal is to warm the pool a bit too so no desire to reject heat from it. When its blazing hot and im working under that tin ill cycle the switch now and then until im not working there anymore. Shop cools a bit, pool warms a bit.
you can always monitor the pool. our pool is 85° now with passive solar, i.e. just sitting in the sun. running the water on the roof when the temp is lower outside than the water, will help cool it but cost more electricity. or you can cover during the heat of the day when no one is swimming. that will keep it cool for when you need it.
About 50 years ago there was an article in Mother Earth News by a guy who had made the same discovery as you. He ran a copper pipe down his ridge and drilled holes for the water to run down his roof. He collected the water and ran it into some sort of storage tank to use as hot water. Eventually, he put glass over the top of the whole roof to make it more efficient. He was credited with starting or restarting the whole solar hot water boom.
When I first moved down here to South Florida lots of the old houses still had 4 X 8' panels alongside a black painted water tank for hot water. They worked fine as long as demand was fairly low.
i have 7 sheets of that 4x8 frosty glass from a rooftop solar water system. when the roof goes they never get put back up. probably use it for a greenhouse in the future.