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Water Cooled Rooftop Solar Panels

125 points by gardenfelder 3y ago

gardenfelder 3y ago
>Some combinations were just meant to be. Peanut butter and jelly, ice cream and spoons, solar panels and canals, and solar photovoltaic (PV) thermal integrated roofing. Who’da thunk.
- pstuart 3y ago
  Not clear on how that works with the plumbing re: tiles, but this idea has long intrigued me (I figured just using standard panels with a thermal block on the back. I hope this is successful, we need all the options we can get for energy independence.
jrmg 3y ago
Before PV became efficient and cheap enough, it was not uncommon to see ‘solar panels’ on roofs in California that actually contained water pipes rather than silicon. Water circulated through them and was used either as regular hot water for the house, or in a ‘heated’ pool.
- jeffbee 3y ago
  You still see new buildings in California with rooftop solar water heaters. I can't make it pencil out but builders seem to disagree. Here's one I happen to know of which is especially baffling because they have no rooftop PV despite an overabundance of space. https://www.google.com/maps/@37.8027685,-122.2898842,118a,35y,348.28h,40.15t/data=!3m1!1e3?entry=ttu https://www.google.com/maps/@37.8027685,-122.2898842,118a,35...
  - Scoundreller 3y ago
    Maybe a LEED credit (scam) thing?
  - bombcar 3y ago
    Part of it is licensing/permits/requirements - you can often add rooftop solar water with no additional permits or a minor one, whereas solar PV is an electrical permit at minimum, and likely plans and licensed electricians, too.
    - hedora 3y ago
      This is doubly true in California, where adding PV means dealing with PG&E. Also, if you're grandfathered in on a NEM 2 electricity bill rate, then they apparently penalize you for upgrading your installation (by forcing you onto NEM 3).
- philjohn 3y ago
  You see them far more than Solar PV in Denmark as well, because IIRC you get taxed per kWh consumed, no matter where it's generated, so it's more economical to use the roof space for domestic hot water instead.
  - Scoundreller 3y ago
    I wonder how many semi/complete guérilla setups there are.
  - jacknews 3y ago
    The output of your own panels that you use yourself is taxed per kWh? That's insane. And unenforceable, surely?
    - SoftTalker 3y ago
      Welcome to Scandinavia. Everything is taxed. It's the price of "free" healthcare and all the other social benefits.
      - jacknews 3y ago
        I'm not against taxes, and the Scandinavian model seems to lead to decent societies, but this seems unnecessarily 'invasive'.
    - Scoundreller 3y ago
      Seems pretty easy to enforce (at a binary level) with sat maps.
    - philjohn 3y ago
      Not sure about Denmark, but in the UK part of the MCS signoff to allow you to export to the grid and receive payments is fitting of an generation meter. It's a holdover from the old FIT days (because you were paid per kWh generated), and has never been deprecated - your actual export (and any payment per unit) is recorded on your smart meter anyway.
    - 3y ago
- Freestyler_3 3y ago
  Australia has had houses with tubes on the roof to heat the pool for a long time already.
  - friend_and_foe 3y ago
    I don't see the benefit here. In the summer you want a cool pool, and in the winter you need that heat to hit your house to reduce heating costs. Unless you live somewhere so warm in the winter that you still have to cool your house, in which case a pool would be pretty temperate and need no heating. If you've lived in a house like this, what's the energy benefit?
    - bluGill 3y ago
      In most cases the average temperature is a bit lower than what your comfortable swimming temperature is. Sure it gets hotter than that some days, but pools are very slow to heat up/cool down and so they generally track the average temperature not the peak temperature. (you are probably swimming when it is peak temperature, and not at night when it is colder outside)
      - friend_and_foe 3y ago
        Yeah I suppose. I enjoy the unpreditctability outside, it's a bit of a philosophical point for me I guess, I don't want the whole world finely tuned to keep me in my comfort zone. A nice cool dip, even a little uncomfortably cool, is exilirating to me.
- Guvante 3y ago
  The ROI on a solar water heater is terrible. Cost us $10k before rebates (half that with) and saved us less than $50 a month in electricity. If I stuck around for a decade it might have paid off but given a family member's unit didn't last that long (the part that held water went bad and a replacement was $4k, not exactly economical on a ⁴ year old system) Ended up it was a pop up company designed to appropriate the green money by "improving your A/C efficiency" which of course probably only lasted a year or two given it was just blasting foam into the vents...
  - smolder 3y ago
    There are a lot of grifters out there wasting peoples money spent with good intentions, which is really frustrating. Sometimes it's not even a grift, just incompetence will lead to poor ROI, because contractors lack the skills to be efficient or do quality work. I'm somewhat convinced I'm going to have to become a home builder myself to end up with something that's long-lasting, comfortable, healthy, and energy efficient, all without being robbed blind.
    - friend_and_foe 3y ago
      It's really starting to get that way isn't it? You basically have to build your own house to get something that isn't made of paper.
- nonfamous 3y ago
  My Dad did something like this for our pool in Australia in the '80s. He ran a bunch of black PVC pipe in a zigzag pattern on the garage roof and ran water through it from the pool filter. It was pretty effective at heating the pool!
  - capitainenemo 3y ago
    We have the same, and it works great. Was (relatively) cheap, durable, and also helps a lot to keep the upper level of the house cool.
- dymk 3y ago
  Those were likely evacuated tube solar collector systems. They can boil water in direct sunlight when there's snow on the ground.
- Scoundreller 3y ago
  While it’s a lot less energy efficient, once saw a video of someone that replaced one of his tank-based residential water heater elements with a combo 240v/24v element and attached a panel to the 24v element with a temperature-driven cutoff relay. Set the tank to 50C or whatever and the element to a higher point to dump extra heat when possible without getting too hot. A very cost efficient approach that probably gets you some double digit reduction without a lot of risky plumbing or freezing issues.
- deleted 3y ago
  [deleted]
- puzzledobserver 3y ago
  Roof-mounted solar water heaters were very common in South India too. Our childhood home had two solar panels heating a tank of water. Worked very well in summer, and there was a backup electric heater for the winter months. We also got some rebate from the electricity supplier, if I remember correctly. The entire system was quite simple, and hasn't needed any maintenance in close to 20 years, I think? What are the benefits of solar electric panels compared to water heating panels?
  - pfdietz 3y ago
    No moving parts, nothing to leak, and a joule of electricity is more valuable than a joule of low grade heat. It's my understanding that if you want solar hot water these days you just get PV and a heat pump water heater. Sure, the heat pump has moving parts, but it's not on the roof.
    - Scoundreller 3y ago
      And in a hot climate, the heat pump water heater provides a little cooling (when you dump the hot water outside).
hathawsh 3y ago
Nice idea. I have questions: - Are the pipes and liquid heavy enough to increase strain on the roof and panel mounting hardware? - How does this solution deal with the risk that the pipes might freeze, expand, and burst? - Would it make sense to run an antifreeze and water mix, similar to cars, to prevent freezing?
- Freestyler_3 3y ago
  Yes its heavier. They would have to keep the water moving or for heavy winters have it drained. Also possible to have a liquid in there with much lower freezing point like you said. The problem I think is that when the panels need cooling the most the system to disperse the heat to would need to be huge, because you just have a large surface heating up water fast and you don't want the house floor to heat up and your water tank for showering is already at max temp. Also gotta be careful not to evaporate the pool.
cogman10 3y ago
Winterizing the lines, leaks, and the added weight are the downsides to this. If you are in a climate where you don't need to winterize, you are probably in a climate where you can get away with a heat pump on your water heater/pool/AC. In fact, they make AC adapters that dump excess heat into the pool. I really want this to work, but heatpumps just make a lot more sense with a lot less cost.
- btbuildem 3y ago
  From what I've seen, solar preheaters in the northern climes tend to use antifreeze instead of water, and have an exchanger in the loop. This way they can withstand below-freezing temps overnight, and be functional in the sunshine of the day. The additional benefit here being that you potable water isn't travelling through materials that are potentially degrading in the sun.
  - f-securus 3y ago
    I think you would only do it for the hot water input which isn’t potable after it runs through a traditional hot water heater?
    - idiotsecant 3y ago
      Why isn't hot water potable?
      - e2e8 3y ago
        According to the EPA: "Hot water dissolves lead more quickly than cold water and is therefore more likely to contain greater amounts of lead. Never use water from the hot water tap for drinking, cooking, or making baby formula." The applicability of this advice is probably quite variable. [0] https://www.epa.gov/lead/why-cant-i-use-hot-water-tap-drinking-cooking-or-making-baby-formula https://www.epa.gov/lead/why-cant-i-use-hot-water-tap-drinki...
        3y ago
ortusdux 3y ago
Tech Ingredients on Youtube did an interesting video about actively air-cooling panels. In the video he uses 3 watts of fans to cool a 100 watt panel, which increases its output by 6 watts. The gains are modest (~5%), but his argument is that the lower operating temp will increase the longevity of the panels, which should make the ROI positive. I'm sure the designers of the panels could engineer more efficient solutions. It did get me wondering about the importance of the roof underneath the panels. Most people install black panels over top black asphalt shingles for aesthetic reasons, but I wonder if some other variation would increase efficiency and/or longevity. https://www.youtube.com/watch?v=Mt9qLRN7JvA https://www.youtube.com/watch?v=Mt9qLRN7JvA
- Scoundreller 3y ago
  Looks like you beat me to it by a minute, but worth pointing out those numbers were at around 60w of output. The difference probably increases at full output.
- Freestyler_3 3y ago
  White background with double sided panels.
- pengaru 3y ago
  You could configure the installation to create a natural convection current of air traveling between the panels and the roof deck, so no power wasted at all on the "fan". Then you could take it a step further by running some "earth tubes" up to the low/entry point of this cooling channel, so the incoming air for the convection current is cooler. All passive.
  - friend_and_foe 3y ago
    This is similar to my plan. I plan to cool and warm my house using an earth tube or two, which leaves electricity required for very modest use, lighting, charging phones and what not and a refrigerator. Combined with a white or silver metal roof and a good gap between the solar panels to allow wind to flow through in leu of an attic should do me just fine, and on top of that the inside air will always be fresh from outside. The energy saving will be tremendous, and digging a trench for the earth tubes will actually cost less than installing a central air conditioning unit. Additionally you don't really need insulation, since fresh temperate air is always cycling in and is available pretty much endlessly, the limiting factor on efficiency is air flow, not insulation. Also, no condensation in the walls; they'll always be above the dew point in the summer, and in the winter the air will be well below 100% relative humidity since the ambient air is being warmed, not cooled, and so the walls will never be below the outside ambient temperature in the winter. The only downsides I can see are that I can't set the temperature, it would be whatever it is in the ground, which at the right depth should be around 73F year round, and 100% relative humidity in the summer. That and that the house must be designed for maximum air flow but I don't really see that as a downside, people have designed houses this way forever until the advent of air conditioning, and to be honest I'm not a fan of a house design that doesn't take air flow into consideration.
PaulKeeble 3y ago
The primary reason why most panels don't do this is longevity. A panel is designed to be put onto the roof and stay there for 25 years, it is entirely solid state and its weather hardened to survive hail, wind and lots of sunshine. The moment you introduce water to just the panel you make the panel more efficient but you also drastically increase its failure rate, water corrodes and over 25 years its much more likely to cause a failure. On top of that the pump is a moving part and its going to require replacements over the lifetime of the panel. In many countries around the world Solar heaters for water are quite common. Using tubes to heat water you can get near 99% efficient capture of the suns energy to heat which compares very favourably to the 20% of a solar panel to electricity. But their rated lifetime is quite a bit shorter and their pay off period looks quite different as a result. So while there are benefits to combining them it increases the cost considerably and decreases the longevity and it just doesn't make much economic sense most of the time. Besides on a hot sunny day your panels are going to do great anyway, its clouds that really reduce the collection. As an example my system will give me 34KWh on a cool 100% sunny day but on a hot day its more like 31KWh. But on a cloudy rainy day it can get down to 7KWh. The heat isn't as big as an issue as its made out to be.
- hef19898 3y ago
  As long as the utilits scale projects in the UAE don't use active cooling or systems tracking the sun, I don't see a reason why I would do it.
  - idiotsecant 3y ago
    Utility scale projects in the uae don't want hot water. Your house does, though.
- MichaelZuo 3y ago
  As long as it's properly designed, built, and assembled, keyword properly, then a 25 year service life is absolutely achievable for the median customer even with an integrated water cooling system.
  - 3y ago
Scoundreller 3y ago
this fellow added fan cooling to his panels. An immediate net benefit on output (5.5w increased output for 3w of fan) at 60w of output. Then there’s the longevity improvements. Really starts around 15 minutes but a good video overall. https://youtu.be/Mt9qLRN7JvA https://youtu.be/Mt9qLRN7JvA
- friend_and_foe 3y ago
  That guy got me on the solar water heater bandwagon. I plan to build one out quite similar to his, using a thermosiphon rather than active pumping. The downside to this is that the tank must be above the panel, which means it will cast a shadow on the panel at some point during the day, and also added structural complexity to keep a heavy water tank on a roof, but proper placement of the tank will ensure that the shadow gets cast in the summer when heat isn't as much of a hot commodity.
derriz 3y ago
Putting solar panels on roofs doesn't make a lot of sense to me. Per kWh, rooftop domestic PV costs can be unto 10 times more expensive on a levelized cost basis (https://www.lazard.com/media/2ozoovyg/lazards-lcoeplus-april-2023.pdf https://www.lazard.com/media/2ozoovyg/lazards-lcoeplus-april...) compared to a ground based utility/grid scale installation. In fact, adding any complexity to a solar panels hardly makes any sense any more because of the incredible fall in panel prices (by 90% in about 12 years). This price fall has made a lot of solar ideas defunct. For example, when panel prices were much higher, solar plants often had mechanical systems to orient panels to track the sun - but now nobody bothers - it's cheaper and simpler to just install an extra 10 or 20% more panels and have a no-moving-parts/low maintenance system. Same with this idea - the extra complexity and maintenance cost of adding active cooling to a panel to increase efficiency by 10% - I cannot see the economics stack up. For the same reason, the enthusiasm for the idea of installing solar plants in north africa and exporting power over interconnectors to Europe is gone. It's cheaper to install twice as many panels in a field in cloudy northern Europe (where capacity factors are half of what they would be in the Sahara) than deal with the costs and complexity of underwater interconnectors and transmission system upgrades.
- AlanSE 3y ago
  > For the same reason, the enthusiasm for the idea of installing solar plants in north africa and exporting power over interconnectors to Europe is gone. It's cheaper to install twice as many panels in a field in cloudy northern Europe (where capacity factors are half of what they would be in the Sahara) than deal with the costs and complexity of underwater interconnectors and transmission system upgrades. I don't know about that, because long-range transmission significantly reduces the intermittency, even for the same longitude. It's not just about smoothing out the curve of solar production, demand patterns (due to weather, industry, behavior) will also have variations from place-to-place and these allow opportunities for power export. Some places will have more dispatchable and expensive power plants than others, which also creates an export opportunity so that power is exported so that one country can burn less fuel. However, I admit this argument is slightly undercut by the question "can't we just build solar panels in the country with dispatchable dirty power?" True, but there's also a connection to wind power which is super local and random.
pfdietz 3y ago
I wonder if there would be a good return for a selective surface on top of PV. Make it reflective for near IR below the bandgap of the PV cells, but transparent for light of shorter wavelengths and opaque in the far IR for high emissivity for radiative cooling.
KaiserPro 3y ago
When I had my panels installed, I had the option to "embed" them, rather than have them mounted over the top of the roof tiles. I chose over the top for three reasons: 1) ease of replacement, panel dimensions change, so getting a new panel the right size in the future could be hard. 2) faff. Getting the roofer to talk to the solar people and agree on dimensions would have been expensive and time consuming. Assuming that they would actually agree to do it. 3) Thermal bridge. Because there is an airgap between the panels and the roof, I get a radiative barrier between my roof and the sun. combined electric and thermal panels seem like a grand idea, and perhaps I might get some in the future (not inside 10 years though) But, having them as your roof tiles I'm less keen on. It makes the MPPT less efficient and there are more connectors. for over roof type panels though, I think they are a slam dunk.
DrNosferatu 3y ago
Price per KWh?
happytiger 3y ago
There was a manufacturer doing this a few years back when we did solar on our house and I looked deeply into this problem. The issue they had was that they had two manufacturers with different guarantees — one for the water and one for the panels. Love the idea — works like a charm (I got my hands on their unit and tested it out — glorious), but when you’re deploying capital to prepuchase an offset to alternative expenses it needs the warranty or it’s foolish to do the deal. Now there is another manufacturer (dualsun.com) claiming they own the global patents to the idea (don’t know how, they weren’t the first to commercialize it). The issue is definitely the incompatibility of materials. The engineering required to make a plastic water system last 25-30 years is so much more expensive than what it takes to make a pv unit last that long, so it gets expensive. Further, the warranty is dependent on the small volume manufacturer still being in business in 20 years to have any warranty claim at all, and with the turnover in solar a lot of homeowners aren’t willing to take that chance. So you get a double edged disincentive. It’s expensive to buy, and the warranty uncertainty is high. It’s all the classic problems of something that needs a mass market success to get to scale, but needs that scale to deliver mass market success. If someone could bond the warranty support or similar solution, it could take off. I wanted to use it to heat a hot tub or a pool and that had added complexity as the chlorine degraded the system and it wasn't advised. Another twist. :) Hopefully advances in materials and manufacturing will make it viable because it’s a GREAT idea and it WORKS! It definitely kept the panels cooler and it definitely heated water well. Edit: For those interested this is the product I know of… no affiliation with them though. https://dualsun.com/en/products/dualsun-spring https://dualsun.com/en/products/dualsun-spring
1024core 3y ago
My parents (who live in a sunny, hot part of India) are in the habit of taking their baths early in the summer (before 7AM). That's because the water in the pipes becomes too hot for bathing after the sun has been up for a while. :-D
swader999 3y ago
Too bad with the bulk of panels and especially their components made with coal energy, panels net out to the same carbon footprint as clean natural gas. https://environmentalprogress.org/big-news/2023/7/3/solar-panels-are-more-carbon-intensive-than-experts-will-admit https://environmentalprogress.org/big-news/2023/7/3/solar-pa...
- jupp0r 3y ago
  You fall into the trap of confusing necessary and sufficient. Switching to solar is necessary, but it is by no means sufficient to become carbon neutral. Other things have to happen, but that doesn't mean that we should stop rolling out solar. Following your argument to the extreme, no solar panel would have been produced, ever.
  - swader999 3y ago
    Yeah a little bit but so does everyone else that thinks these things are the solution and we should dive right into it.
- sunshinesnacks 3y ago
  Except that is very, very far from being an established fact. Note that minimum CO2 from gas referenced here is still higher than the range alleged by the lone researcher referenced in that article: https://www.nrel.gov/docs/fy21osti/80580.pdf https://www.nrel.gov/docs/fy21osti/80580.pdf. Claiming that everyone else is off by an order of magnitude starts to raise some questions, like, how is there 10x more energy going in and selling solar panels is still profitable? Not all solar is made in China, either. Edit to add: I now see that the EP article only claims that solar might have higher CO2 impacts than gas with carbon capture. Which is an order of magnitude lower than regular gas CCs. And is not widespread, off the shelf, or cost competitive with grid energy prices most places…
Projectiboga 3y ago
This wasn't as much for solar efficiency as for cooling a structure. From the article, Water Cooled Roof Under the wrong conditions, your attic can reach temperatures as high as 150 degrees. Not only is this detrimental to standard asphalt roofing materials, but that heat is cooking your house! You waste hard earned energy cycling cool air in and throughout your home just to fight back against this suffocating blanket of heat. So in addition to carrying away heat from your panels, the system is carrying heat away from your attic as well. Once again, we’ve discovered an efficiency gain that which seems almost hidden by the primary features. So now, water heating equals panel cooling AND roof cooling, which of course means you require less energy for cooling your home.
MaXtreeM 3y ago
This is something I wish I could find more actual data than people opinions. On the surface it seems like a natural fit to use the black surface to also heat water and also increase longevity and efficiency of solar panels. But many people argue that this doesn't make sense from economic point of view and can decrease longevity of the overall system. Sadly right now I don't think there are enough sources to make confident decision. I am in the process of building a house in central Europe. My source of heat will be a ground-source heat pump from boreholes, which means using it as cooling for solar panels in the summer would be quite cheap (I specifically ordered a model of the heat pump wich has cooling mode build in). Added benefit would be that heating the boreholes in summer should increase their longevity because there is a potential of "sucking out the holes" (don't know if this is the right term in english) after decades of using them as a heat source - again something I could not find data supporting this claim so I don't know if I should be worried about this. My combination seemed like a great fit to install water cooled PV but probably will not do it just because there is not a single contractor who does it and nobody who has it installed to give me advices. I hope that in 25 years when I should suspect to do a PV change it will be decided if it is a good idea or not.
- perlgeek 3y ago
  > probably will not do it just because there is not a single contractor who does it and nobody who has it installed to give me advices. And this is why subsidiaries for pilot projects are super important. As a society, we really should try out such ideas, but of course, the first few times you do it, it's more expensive and error-prone, so there should be some subsidiaries or other incentives (tax reduction?) to kickstart this.
sparrowInHand 3y ago
So, this discourages installing in hurrican prone areas? But why not just add a mechnaism so the install can be folded away before a storm arrives?
zzbn00 3y ago
This type of technology has been around for a while, there are some commercial offerings in the UK -- search for PV-thermal. Should be a useful source of heat into a heat-pump system, either for space or water heating... The whole idea though probably makes more sense though on easy-access flat roofs rather than pitched roofs so why the tile? (And why do we still make pitched roofs if we are doing energy efficiency? Much better to condense out as much of the internal moisture as you can to recover the heat from there)

125 points by gardenfelder 3y ago

>Some combinations were just meant to be. Peanut butter and jelly, ice cream and spoons, solar panels and canals, and solar photovoltaic (PV) thermal integrated roofing. Who’da thunk.

pstuart 3y ago
Not clear on how that works with the plumbing re: tiles, but this idea has long intrigued me (I figured just using standard panels with a thermal block on the back. I hope this is successful, we need all the options we can get for energy independence.

jrmg 3y ago

Before PV became efficient and cheap enough, it was not uncommon to see ‘solar panels’ on roofs in California that actually contained water pipes rather than silicon. Water circulated through them and was used either as regular hot water for the house, or in a ‘heated’ pool.

jeffbee 3y ago
You still see new buildings in California with rooftop solar water heaters. I can't make it pencil out but builders seem to disagree. Here's one I happen to know of which is especially baffling because they have no rooftop PV despite an overabundance of space. https://www.google.com/maps/@37.8027685,-122.2898842,118a,35y,348.28h,40.15t/data=!3m1!1e3?entry=ttu https://www.google.com/maps/@37.8027685,-122.2898842,118a,35...
- Scoundreller 3y ago
  Maybe a LEED credit (scam) thing?
- bombcar 3y ago
  Part of it is licensing/permits/requirements - you can often add rooftop solar water with no additional permits or a minor one, whereas solar PV is an electrical permit at minimum, and likely plans and licensed electricians, too.
  - hedora 3y ago
    This is doubly true in California, where adding PV means dealing with PG&E. Also, if you're grandfathered in on a NEM 2 electricity bill rate, then they apparently penalize you for upgrading your installation (by forcing you onto NEM 3).
philjohn 3y ago
You see them far more than Solar PV in Denmark as well, because IIRC you get taxed per kWh consumed, no matter where it's generated, so it's more economical to use the roof space for domestic hot water instead.
- Scoundreller 3y ago
  I wonder how many semi/complete guérilla setups there are.
- jacknews 3y ago
  The output of your own panels that you use yourself is taxed per kWh? That's insane. And unenforceable, surely?
  - SoftTalker 3y ago
    Welcome to Scandinavia. Everything is taxed. It's the price of "free" healthcare and all the other social benefits.
    - jacknews 3y ago
      I'm not against taxes, and the Scandinavian model seems to lead to decent societies, but this seems unnecessarily 'invasive'.
  - Scoundreller 3y ago
    Seems pretty easy to enforce (at a binary level) with sat maps.
  - philjohn 3y ago
    Not sure about Denmark, but in the UK part of the MCS signoff to allow you to export to the grid and receive payments is fitting of an generation meter. It's a holdover from the old FIT days (because you were paid per kWh generated), and has never been deprecated - your actual export (and any payment per unit) is recorded on your smart meter anyway.
  - 3y ago
Freestyler_3 3y ago
Australia has had houses with tubes on the roof to heat the pool for a long time already.
- friend_and_foe 3y ago
  I don't see the benefit here. In the summer you want a cool pool, and in the winter you need that heat to hit your house to reduce heating costs. Unless you live somewhere so warm in the winter that you still have to cool your house, in which case a pool would be pretty temperate and need no heating. If you've lived in a house like this, what's the energy benefit?
  - bluGill 3y ago
    In most cases the average temperature is a bit lower than what your comfortable swimming temperature is. Sure it gets hotter than that some days, but pools are very slow to heat up/cool down and so they generally track the average temperature not the peak temperature. (you are probably swimming when it is peak temperature, and not at night when it is colder outside)
    - friend_and_foe 3y ago
      Yeah I suppose. I enjoy the unpreditctability outside, it's a bit of a philosophical point for me I guess, I don't want the whole world finely tuned to keep me in my comfort zone. A nice cool dip, even a little uncomfortably cool, is exilirating to me.
Guvante 3y ago
The ROI on a solar water heater is terrible. Cost us $10k before rebates (half that with) and saved us less than $50 a month in electricity. If I stuck around for a decade it might have paid off but given a family member's unit didn't last that long (the part that held water went bad and a replacement was $4k, not exactly economical on a ⁴ year old system) Ended up it was a pop up company designed to appropriate the green money by "improving your A/C efficiency" which of course probably only lasted a year or two given it was just blasting foam into the vents...
- smolder 3y ago
  There are a lot of grifters out there wasting peoples money spent with good intentions, which is really frustrating. Sometimes it's not even a grift, just incompetence will lead to poor ROI, because contractors lack the skills to be efficient or do quality work. I'm somewhat convinced I'm going to have to become a home builder myself to end up with something that's long-lasting, comfortable, healthy, and energy efficient, all without being robbed blind.
  - friend_and_foe 3y ago
    It's really starting to get that way isn't it? You basically have to build your own house to get something that isn't made of paper.
nonfamous 3y ago
My Dad did something like this for our pool in Australia in the '80s. He ran a bunch of black PVC pipe in a zigzag pattern on the garage roof and ran water through it from the pool filter. It was pretty effective at heating the pool!
- capitainenemo 3y ago
  We have the same, and it works great. Was (relatively) cheap, durable, and also helps a lot to keep the upper level of the house cool.
dymk 3y ago
Those were likely evacuated tube solar collector systems. They can boil water in direct sunlight when there's snow on the ground.
Scoundreller 3y ago
While it’s a lot less energy efficient, once saw a video of someone that replaced one of his tank-based residential water heater elements with a combo 240v/24v element and attached a panel to the 24v element with a temperature-driven cutoff relay. Set the tank to 50C or whatever and the element to a higher point to dump extra heat when possible without getting too hot. A very cost efficient approach that probably gets you some double digit reduction without a lot of risky plumbing or freezing issues.
deleted 3y ago
[deleted]
puzzledobserver 3y ago
Roof-mounted solar water heaters were very common in South India too. Our childhood home had two solar panels heating a tank of water. Worked very well in summer, and there was a backup electric heater for the winter months. We also got some rebate from the electricity supplier, if I remember correctly. The entire system was quite simple, and hasn't needed any maintenance in close to 20 years, I think? What are the benefits of solar electric panels compared to water heating panels?
- pfdietz 3y ago
  No moving parts, nothing to leak, and a joule of electricity is more valuable than a joule of low grade heat. It's my understanding that if you want solar hot water these days you just get PV and a heat pump water heater. Sure, the heat pump has moving parts, but it's not on the roof.
  - Scoundreller 3y ago
    And in a hot climate, the heat pump water heater provides a little cooling (when you dump the hot water outside).

hathawsh 3y ago

Nice idea. I have questions: - Are the pipes and liquid heavy enough to increase strain on the roof and panel mounting hardware? - How does this solution deal with the risk that the pipes might freeze, expand, and burst? - Would it make sense to run an antifreeze and water mix, similar to cars, to prevent freezing?

Freestyler_3 3y ago
Yes its heavier. They would have to keep the water moving or for heavy winters have it drained. Also possible to have a liquid in there with much lower freezing point like you said. The problem I think is that when the panels need cooling the most the system to disperse the heat to would need to be huge, because you just have a large surface heating up water fast and you don't want the house floor to heat up and your water tank for showering is already at max temp. Also gotta be careful not to evaporate the pool.

cogman10 3y ago

Winterizing the lines, leaks, and the added weight are the downsides to this. If you are in a climate where you don't need to winterize, you are probably in a climate where you can get away with a heat pump on your water heater/pool/AC. In fact, they make AC adapters that dump excess heat into the pool. I really want this to work, but heatpumps just make a lot more sense with a lot less cost.

btbuildem 3y ago
From what I've seen, solar preheaters in the northern climes tend to use antifreeze instead of water, and have an exchanger in the loop. This way they can withstand below-freezing temps overnight, and be functional in the sunshine of the day. The additional benefit here being that you potable water isn't travelling through materials that are potentially degrading in the sun.
- f-securus 3y ago
  I think you would only do it for the hot water input which isn’t potable after it runs through a traditional hot water heater?
  - idiotsecant 3y ago
    Why isn't hot water potable?
    - e2e8 3y ago
      According to the EPA: "Hot water dissolves lead more quickly than cold water and is therefore more likely to contain greater amounts of lead. Never use water from the hot water tap for drinking, cooking, or making baby formula." The applicability of this advice is probably quite variable. [0] https://www.epa.gov/lead/why-cant-i-use-hot-water-tap-drinking-cooking-or-making-baby-formula https://www.epa.gov/lead/why-cant-i-use-hot-water-tap-drinki...
      - 3y ago

ortusdux 3y ago

Tech Ingredients on Youtube did an interesting video about actively air-cooling panels. In the video he uses 3 watts of fans to cool a 100 watt panel, which increases its output by 6 watts. The gains are modest (~5%), but his argument is that the lower operating temp will increase the longevity of the panels, which should make the ROI positive. I'm sure the designers of the panels could engineer more efficient solutions. It did get me wondering about the importance of the roof underneath the panels. Most people install black panels over top black asphalt shingles for aesthetic reasons, but I wonder if some other variation would increase efficiency and/or longevity. https://www.youtube.com/watch?v=Mt9qLRN7JvA https://www.youtube.com/watch?v=Mt9qLRN7JvA

Scoundreller 3y ago
Looks like you beat me to it by a minute, but worth pointing out those numbers were at around 60w of output. The difference probably increases at full output.
Freestyler_3 3y ago
White background with double sided panels.
pengaru 3y ago
You could configure the installation to create a natural convection current of air traveling between the panels and the roof deck, so no power wasted at all on the "fan". Then you could take it a step further by running some "earth tubes" up to the low/entry point of this cooling channel, so the incoming air for the convection current is cooler. All passive.
- friend_and_foe 3y ago
  This is similar to my plan. I plan to cool and warm my house using an earth tube or two, which leaves electricity required for very modest use, lighting, charging phones and what not and a refrigerator. Combined with a white or silver metal roof and a good gap between the solar panels to allow wind to flow through in leu of an attic should do me just fine, and on top of that the inside air will always be fresh from outside. The energy saving will be tremendous, and digging a trench for the earth tubes will actually cost less than installing a central air conditioning unit. Additionally you don't really need insulation, since fresh temperate air is always cycling in and is available pretty much endlessly, the limiting factor on efficiency is air flow, not insulation. Also, no condensation in the walls; they'll always be above the dew point in the summer, and in the winter the air will be well below 100% relative humidity since the ambient air is being warmed, not cooled, and so the walls will never be below the outside ambient temperature in the winter. The only downsides I can see are that I can't set the temperature, it would be whatever it is in the ground, which at the right depth should be around 73F year round, and 100% relative humidity in the summer. That and that the house must be designed for maximum air flow but I don't really see that as a downside, people have designed houses this way forever until the advent of air conditioning, and to be honest I'm not a fan of a house design that doesn't take air flow into consideration.

PaulKeeble 3y ago

The primary reason why most panels don't do this is longevity. A panel is designed to be put onto the roof and stay there for 25 years, it is entirely solid state and its weather hardened to survive hail, wind and lots of sunshine. The moment you introduce water to just the panel you make the panel more efficient but you also drastically increase its failure rate, water corrodes and over 25 years its much more likely to cause a failure. On top of that the pump is a moving part and its going to require replacements over the lifetime of the panel. In many countries around the world Solar heaters for water are quite common. Using tubes to heat water you can get near 99% efficient capture of the suns energy to heat which compares very favourably to the 20% of a solar panel to electricity. But their rated lifetime is quite a bit shorter and their pay off period looks quite different as a result. So while there are benefits to combining them it increases the cost considerably and decreases the longevity and it just doesn't make much economic sense most of the time. Besides on a hot sunny day your panels are going to do great anyway, its clouds that really reduce the collection. As an example my system will give me 34KWh on a cool 100% sunny day but on a hot day its more like 31KWh. But on a cloudy rainy day it can get down to 7KWh. The heat isn't as big as an issue as its made out to be.

hef19898 3y ago
As long as the utilits scale projects in the UAE don't use active cooling or systems tracking the sun, I don't see a reason why I would do it.
- idiotsecant 3y ago
  Utility scale projects in the uae don't want hot water. Your house does, though.
MichaelZuo 3y ago
As long as it's properly designed, built, and assembled, keyword properly, then a 25 year service life is absolutely achievable for the median customer even with an integrated water cooling system.
- 3y ago

Scoundreller 3y ago

this fellow added fan cooling to his panels. An immediate net benefit on output (5.5w increased output for 3w of fan) at 60w of output. Then there’s the longevity improvements. Really starts around 15 minutes but a good video overall. https://youtu.be/Mt9qLRN7JvA https://youtu.be/Mt9qLRN7JvA

friend_and_foe 3y ago
That guy got me on the solar water heater bandwagon. I plan to build one out quite similar to his, using a thermosiphon rather than active pumping. The downside to this is that the tank must be above the panel, which means it will cast a shadow on the panel at some point during the day, and also added structural complexity to keep a heavy water tank on a roof, but proper placement of the tank will ensure that the shadow gets cast in the summer when heat isn't as much of a hot commodity.

derriz 3y ago

Putting solar panels on roofs doesn't make a lot of sense to me. Per kWh, rooftop domestic PV costs can be unto 10 times more expensive on a levelized cost basis (https://www.lazard.com/media/2ozoovyg/lazards-lcoeplus-april-2023.pdf https://www.lazard.com/media/2ozoovyg/lazards-lcoeplus-april...) compared to a ground based utility/grid scale installation. In fact, adding any complexity to a solar panels hardly makes any sense any more because of the incredible fall in panel prices (by 90% in about 12 years). This price fall has made a lot of solar ideas defunct. For example, when panel prices were much higher, solar plants often had mechanical systems to orient panels to track the sun - but now nobody bothers - it's cheaper and simpler to just install an extra 10 or 20% more panels and have a no-moving-parts/low maintenance system. Same with this idea - the extra complexity and maintenance cost of adding active cooling to a panel to increase efficiency by 10% - I cannot see the economics stack up. For the same reason, the enthusiasm for the idea of installing solar plants in north africa and exporting power over interconnectors to Europe is gone. It's cheaper to install twice as many panels in a field in cloudy northern Europe (where capacity factors are half of what they would be in the Sahara) than deal with the costs and complexity of underwater interconnectors and transmission system upgrades.

AlanSE 3y ago
> For the same reason, the enthusiasm for the idea of installing solar plants in north africa and exporting power over interconnectors to Europe is gone. It's cheaper to install twice as many panels in a field in cloudy northern Europe (where capacity factors are half of what they would be in the Sahara) than deal with the costs and complexity of underwater interconnectors and transmission system upgrades. I don't know about that, because long-range transmission significantly reduces the intermittency, even for the same longitude. It's not just about smoothing out the curve of solar production, demand patterns (due to weather, industry, behavior) will also have variations from place-to-place and these allow opportunities for power export. Some places will have more dispatchable and expensive power plants than others, which also creates an export opportunity so that power is exported so that one country can burn less fuel. However, I admit this argument is slightly undercut by the question "can't we just build solar panels in the country with dispatchable dirty power?" True, but there's also a connection to wind power which is super local and random.

pfdietz 3y ago

I wonder if there would be a good return for a selective surface on top of PV. Make it reflective for near IR below the bandgap of the PV cells, but transparent for light of shorter wavelengths and opaque in the far IR for high emissivity for radiative cooling.

KaiserPro 3y ago

When I had my panels installed, I had the option to "embed" them, rather than have them mounted over the top of the roof tiles. I chose over the top for three reasons: 1) ease of replacement, panel dimensions change, so getting a new panel the right size in the future could be hard. 2) faff. Getting the roofer to talk to the solar people and agree on dimensions would have been expensive and time consuming. Assuming that they would actually agree to do it. 3) Thermal bridge. Because there is an airgap between the panels and the roof, I get a radiative barrier between my roof and the sun. combined electric and thermal panels seem like a grand idea, and perhaps I might get some in the future (not inside 10 years though) But, having them as your roof tiles I'm less keen on. It makes the MPPT less efficient and there are more connectors. for over roof type panels though, I think they are a slam dunk.

DrNosferatu 3y ago

Price per KWh?

happytiger 3y ago

There was a manufacturer doing this a few years back when we did solar on our house and I looked deeply into this problem. The issue they had was that they had two manufacturers with different guarantees — one for the water and one for the panels. Love the idea — works like a charm (I got my hands on their unit and tested it out — glorious), but when you’re deploying capital to prepuchase an offset to alternative expenses it needs the warranty or it’s foolish to do the deal. Now there is another manufacturer (dualsun.com) claiming they own the global patents to the idea (don’t know how, they weren’t the first to commercialize it). The issue is definitely the incompatibility of materials. The engineering required to make a plastic water system last 25-30 years is so much more expensive than what it takes to make a pv unit last that long, so it gets expensive. Further, the warranty is dependent on the small volume manufacturer still being in business in 20 years to have any warranty claim at all, and with the turnover in solar a lot of homeowners aren’t willing to take that chance. So you get a double edged disincentive. It’s expensive to buy, and the warranty uncertainty is high. It’s all the classic problems of something that needs a mass market success to get to scale, but needs that scale to deliver mass market success. If someone could bond the warranty support or similar solution, it could take off. I wanted to use it to heat a hot tub or a pool and that had added complexity as the chlorine degraded the system and it wasn't advised. Another twist. :) Hopefully advances in materials and manufacturing will make it viable because it’s a GREAT idea and it WORKS! It definitely kept the panels cooler and it definitely heated water well. Edit: For those interested this is the product I know of… no affiliation with them though. https://dualsun.com/en/products/dualsun-spring https://dualsun.com/en/products/dualsun-spring

1024core 3y ago

My parents (who live in a sunny, hot part of India) are in the habit of taking their baths early in the summer (before 7AM). That's because the water in the pipes becomes too hot for bathing after the sun has been up for a while. :-D

swader999 3y ago

Too bad with the bulk of panels and especially their components made with coal energy, panels net out to the same carbon footprint as clean natural gas. https://environmentalprogress.org/big-news/2023/7/3/solar-panels-are-more-carbon-intensive-than-experts-will-admit https://environmentalprogress.org/big-news/2023/7/3/solar-pa...

jupp0r 3y ago
You fall into the trap of confusing necessary and sufficient. Switching to solar is necessary, but it is by no means sufficient to become carbon neutral. Other things have to happen, but that doesn't mean that we should stop rolling out solar. Following your argument to the extreme, no solar panel would have been produced, ever.
- swader999 3y ago
  Yeah a little bit but so does everyone else that thinks these things are the solution and we should dive right into it.
sunshinesnacks 3y ago
Except that is very, very far from being an established fact. Note that minimum CO2 from gas referenced here is still higher than the range alleged by the lone researcher referenced in that article: https://www.nrel.gov/docs/fy21osti/80580.pdf https://www.nrel.gov/docs/fy21osti/80580.pdf. Claiming that everyone else is off by an order of magnitude starts to raise some questions, like, how is there 10x more energy going in and selling solar panels is still profitable? Not all solar is made in China, either. Edit to add: I now see that the EP article only claims that solar might have higher CO2 impacts than gas with carbon capture. Which is an order of magnitude lower than regular gas CCs. And is not widespread, off the shelf, or cost competitive with grid energy prices most places…

Projectiboga 3y ago

This wasn't as much for solar efficiency as for cooling a structure. From the article, Water Cooled Roof Under the wrong conditions, your attic can reach temperatures as high as 150 degrees. Not only is this detrimental to standard asphalt roofing materials, but that heat is cooking your house! You waste hard earned energy cycling cool air in and throughout your home just to fight back against this suffocating blanket of heat. So in addition to carrying away heat from your panels, the system is carrying heat away from your attic as well. Once again, we’ve discovered an efficiency gain that which seems almost hidden by the primary features. So now, water heating equals panel cooling AND roof cooling, which of course means you require less energy for cooling your home.

MaXtreeM 3y ago

This is something I wish I could find more actual data than people opinions. On the surface it seems like a natural fit to use the black surface to also heat water and also increase longevity and efficiency of solar panels. But many people argue that this doesn't make sense from economic point of view and can decrease longevity of the overall system. Sadly right now I don't think there are enough sources to make confident decision. I am in the process of building a house in central Europe. My source of heat will be a ground-source heat pump from boreholes, which means using it as cooling for solar panels in the summer would be quite cheap (I specifically ordered a model of the heat pump wich has cooling mode build in). Added benefit would be that heating the boreholes in summer should increase their longevity because there is a potential of "sucking out the holes" (don't know if this is the right term in english) after decades of using them as a heat source - again something I could not find data supporting this claim so I don't know if I should be worried about this. My combination seemed like a great fit to install water cooled PV but probably will not do it just because there is not a single contractor who does it and nobody who has it installed to give me advices. I hope that in 25 years when I should suspect to do a PV change it will be decided if it is a good idea or not.

perlgeek 3y ago
> probably will not do it just because there is not a single contractor who does it and nobody who has it installed to give me advices. And this is why subsidiaries for pilot projects are super important. As a society, we really should try out such ideas, but of course, the first few times you do it, it's more expensive and error-prone, so there should be some subsidiaries or other incentives (tax reduction?) to kickstart this.

sparrowInHand 3y ago

So, this discourages installing in hurrican prone areas? But why not just add a mechnaism so the install can be folded away before a storm arrives?

zzbn00 3y ago

This type of technology has been around for a while, there are some commercial offerings in the UK -- search for PV-thermal. Should be a useful source of heat into a heat-pump system, either for space or water heating... The whole idea though probably makes more sense though on easy-access flat roofs rather than pitched roofs so why the tile? (And why do we still make pitched roofs if we are doing energy efficiency? Much better to condense out as much of the internal moisture as you can to recover the heat from there)