Water Cooled Rooftop Solar Panels | Hacker News Reader

Water Cooled Rooftop Solar Panels

evergreenoffgrid.com

125 pointsgardenfelder161 comments

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.

I feel the failure mode should be pretty good though, degrade to a normal solar setup - probably with less airflow and higher temps but not worse than that.

Heat degrades panel longevity too so I'd actually assume a water-cooled system to outlive a normal one, in the long run I would be surprised if it wouldn't end up being cheaper.

I do agree with the simplicity of a solid state setup though. Whether it makes sense probably depends on where you live. Thinking about it, temporarily running "warm" water might help with keeping snow away, though I am aware that many setups won't generate much in the winter anyway.

Longevity doesn't really matter as most roofs are only good for 25-30 years. As such you are removing the panels anyway to repair the roof, there is not much additional cost to just replace them. Odds are newer solar technology will exist by then and so you will want to upgrade.

My tile roof is 120 years old, on a house that is 220 years old.

Yo my knowledge it has had basically no maintenance in that time.

Most tiles roofs have underlayment (around 30 years) or some other waterproofing that needs to be serviced. You move the tiles over, replace, then move them back. The tiles, alone, don't make a waterproof roof.

What's under your tiles?

All the tiled houses I lived in (in Melbourne, Australia) had nothing under the terracotta tiles. Standing in the loft, you saw the underside of the tiles.

Late in the day when the sun was low, you would see chinks of sunlight through the gaps between tiles.

Romantic, but sadly thermally unsuitable in many places.

Nearly all houses have the thermal insulation flat across the ceiling of the room below, rather than diagonally across a slanted roof.

Having it flat means you need far less insulation for equivalent results.

Downside is the loft space is effectively unheated/uncooled.

This wouldn't work where I am, where it's windy when it rains. The tiles themselves don't make a roof waterproof.

Yeah our (concrete tile) roof is about 80 years old. The only maintenance has been replacing broken tiles after a particularly bad hail storm.

Wait really? Why are we using regular (asphalt?) shingles?

The weight of roofing construction above the truss/rafter may be estimated with the calculator below[1]:

Clay tile (10.8 lbm/ft2, 52.7 kg/m2)

Concrete tile (9.3 lbm/ft2, 45.4 kg/m2)

Slate - 3/8" thick (12.8 lbm/ft2, 62.5 kg/m2)

Slate - 1/2" thick (18.8 lbm/ft2, 91.8 kg/m2)

Slate - 3/4" thick (26.0 lbm/ft2, 127 kg/m2)

Asphalt roll roofing (1.1 lbm/ft2, 5.4 kg/m2)

Asphalt shingle or asphalt-fiberglass composite shingle (2.7 lbm/ft2, 13.2 kg/m2)

Metal roofing - aluminum or steel - panel, sheet or shingle (1.2 lbm/ft2, 5.9 kg/m2)

[1] https://www.engineeringtoolbox.com/roofing-materials-weight-...

Installing tiles requires more skill than asphalt.

In California, because of clumsy rules around workers compensation insurance, low-skilled high-risk labor is much cheaper than high-skilled labor. As a homebuilder explained it to me: if you hire actual roofers you have to pay very high insurance rates (roofing is quite dangerous). But if you hire day laborers and tell them to install the roof, you don't. So they prefer roof designs that day laborers can install.

There are also earthquakes, during which I'd rather have shingles than concrete tiles above me.

But there's wildfires, in which case you want tiles not extremely flammable petroleum products above you.

Asphalt shingles are usually quite resistant to fire. IIRC, wildfires most often enter a structure through a window.

Installing concrete tiles requires pretty much no skill. Anyone could do it after watching a 20 min youtube video. They literally just interlock and hold each other in place. You only need to nail down edge ones. A whole roof can be done in a day by 2 people - and most of the time is spend hauling the tiles to the roof - they're heavy!!

What about the rest of the country. What state predominantly installs tiled roofs?

Florida has a lot of houses with tile roofs. I think Arizona, New Mexico, and Texas also have lots of places with tile roofs due to the Southwest aesthetic.

California also has wildfires and at least in SoCal the homes primarily use a fire resistant tile or slate.

Haha, no. Some high end housing uses clay or tile, but the vast vast majority of roofs in California are Asphalt shingle.

[https://kelly-roofing.com/what-roof-styles-are-popular-in-ca...]

As an Australian who has moved to the USA, I don't know why it's done. I don't know a single person in Australia who has ever replaced their roof. My dad lives in the same house I was born in 40 years ago, same roof. I've been in Colorado for 4 years. We had a new roof put on 2 years ago, then another 6 weeks ago due to a bad hail storm.

A quick search suggests that it's because historically clay tile roofs didn't do well in environments that regularly drop below freezing (like Colorado). It sounds like newer materials can handle freezing weather fine, so our usage of asphalt probably is just a cultural habit at this point.

Slate handles cold weather and lasts for hundreds of years. It also looks fucking awesome.

We don't use it anymore because it's expensive.

Also because no one knows how to handle it anymore and ignorant roofers will break your really expensive slate roof.

I did find it super weird the first time I heard of Americans using asphalt shingles. Everything over here is either corrugated steel (often powdercoated) or tiles, apart from some fancy upmarket properties with zinc roofs.

As child poster notes, the weight is a factor, and US houses uses wood frames for construction, where warmer parts may use brick construction. Can carry heavier roofs.

My house is wood frame, brick veneer. It was relatively cheaply built government housing back in the 40s/50s. It seems to have held up just fine with the weight of concrete tiles.

If the roof is covered with solar panels, the roof itself will last a lot longer since it's not directly exposed to the sun.

Except solar panels don't cover the whole roof, and that last 10% means the whole roof must come off

Winter freezing conditions seem like that could suck out any potential ROI. In freezing temperatures, you would need to drain the system and/or add a heating element to ensure the water does not freeze and burst the outdoor pipes. Warming water in freezing conditions is going to be energy intensive. Maybe you could get away with some anti-freeze additive to lower the threshold point.

Just drain them in the fall. It’s not like you need to wait till the last minute.

With irrigation it can be kinda tricky to judge when to blow them out. But with this? Just when you’re averaging temperatures of 10-15, blow the pipes out.

Why wouldn't you get a way with anti-freeze?

I ended up just getting a bigger PV system and a heat pump hot water system. The clincher is - how much hot water do you actually need? Just running for about two hours at 1kW draw, the heat pump generates all the hot water I need for the next 24 hours.

So since I have limited roof space, it’s better for me to use that for PV and just use a few kWh of that per day for water heating, instead of dedicating space to a solar hot water collector.

The idea behind the combined panels addresses the limited space concern - these are full-size PV panels with a back plane that harvests the heat and cools the panel up front.

There is that idea where you use water to pump heat under your house over the summer and use it for heating over the winter, and vice versa for cooling.

Ever since I've learned that cold solar panels are more efficient, I've dreamed about water-cooled PV that uses the heat for warm water supply.

One thing I'm worried about in such an integrated solution is water leakage. It looks like you'll need quite a few of these elements to form your roof, and there's got to be a joint between each pair of elements, and each joint has the potential for leaks, for example due to thermal expansion and contraction.

With traditional roof shingles (at least as they are done in central Europe), the tiles overlap, so that you don't need a perfect fit, and when the tiles expand at higher temperatures, the overlap still keeps water out. Doesn't seem like that simple mechanism could work if water flows through the solar tiles.

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.

Well that sounds like you'd have a higher failure rate in the margins then, because current solar panels can practically be thrown on the roof with nails and they'll work for 25 years.

There's value in simplicity, and something that's going to be ignored on a roof for multiple decades needs to have an idiot proof installation process.

I imagine many customers will opt for simplicity, some will opt to pay more for properly built installations with integrated cooling and enjoy the advertised benefits, and some will opt for cut-rate installations that will prematurely fail, thinking they got a deal.

That seems to be the general pattern whenever a new, non-trivial, innovation is introduced.

People will pay for the system with the best ROI, which is a simple system with simple installation and simple failure modes.

If this is really a win (see other posts - I question that), then solar panels can be made with integrated pipes for the cooling, and pumps that will last. We can even make a wire/water pipe all in one unit so you just have one connector to install. The pump likewise can be combined with other parts you have to have. All of this will reduce costs for this.

Again, I question if the ROI can be good enough to be worth it, but if it really is a win there are lots of options to reduce costs.

People often mixup performance of some component of the system, with total system performance, and system cost performance. Even this breakdown is probably oversimplifying.

I would suspect you're right on simpler being better here. That doesn't mean this isn't worth exploring. But one practical tradeoff calculation to make to see how viable this is right now is take the component cost of the cooling system and put that into more "simple" solar panels and see what trade of power generated is.

If that benefit isn't a win of the cooling system, then a lifetime reliability of the cooling system being less than a lifetime reliability of the solar electric components can only make that tradeoff even worse.

But making that calculation might still give someone a target for cost, performance specs to become a viable market option.

Based on the numbers I have provided for the best days a day (of which I might get 50 a year) we can get 9% more output, but over a year that is maybe a few percent. If the pump needs replacing on typical pump periods of 5-10 years that will cost more than I would gain as each is going to be £200 (100 replacement and 100 for the pump) and that alone is more than a few percent of the system. If it drops the life of the panels to 20 years or requires one maintenance on the roof all the savings disappear instantly. Also if the panels cost more than 3% more than the electrical gains aren't worth it alone.

The problem is the gains are just too small and the risks and extra costs far too high.

Its not really fair because what you would do is flow this hot water into a water cylinder and get ~6KWh of water heating out of it a day. It will produce a lot more heat than that, you get nearly 4x as much heating as you do electricity but i can't use it, I don't need 120Kwh of heated water for showers and in the winter its not going to be enough to heat the house. That is fairly substantial amounts of extra power but I don't need that amount of panels of water heating I likely only need 1 solar heater, the rest is going to be wasted and not help in cooling the panels.

Who ever its for its not for people like me.

I wonder if it would be interesting for a hybrid pool heating & filtration system where you are already in for a cost of a pump.

The differential cost would be for an additional panel array loop and some isolation valves etc.

Solar pool heating is very much an established thing, and it does eat up a good chunk of a roof. So yea, if the quantity of hot water is an issue, that would certainly be a good place to dump it.

On the other hand, pool water has chemistry which may make it unsuitable for using in combined panels. Especially if it's a salt water pool, you don't want that water anywhere near electronics. The typical pool solar heating panels are basically just thin black plastic that the water flows through.

I read that the primary issue for similar combined water and PV systems is that it's great for making a large amount of slightly warm water, but it can't make water hot enough for general domestic use while keeping it cold enough to improve the solar panels efficiency significantly. Also when the sun is shining, there isn't much demand for heating, so you can't use the hot water in radiators.

I've heard it works well with swimming pools, as the temperature of the water required in the pool is cold enough to help the panels keep cool, and you generally want a pool to be warm on hot days in summer when you will use it most.

More generally, the trade off is:

buying/deploying more panels.

So adding water cooling adds materials and work and potential repairs etc. Translate that into money, and use that to buy and install more panels, which one wins?

Same applies to more efficient panels, trackers that follow the sun and most other solar innovations. As panel prices drop the calculus shifts more and more towards more panels.

There are some constrained environments that drive innovation in size/weight or whatever but the mass market is driven by electricity output/dollars.

I'm not sure you'd actually need a pump to get water to most rooftops. Your municipal water pressure should be enough to push it through.

I'm not sure corrosion is that big of an issue either. Plumbing in people's houses often lasts far longer than 25 years.

Simply running some pipes behind the solar panel and having that fill your hot water tank or flush your toilet wouldn't be terribly difficult, although maybe hard to retrofit.

> I'm not sure you'd actually need a pump to get water to most rooftops. Your municipal water pressure should be enough to push it through.

You'd use a closed loop, not fresh water continuously. What a waste of water otherwise...

Isn't the context of this article using the heat from the solar panels to heat water for your home? You wouldn't use a closed loop for that.

Typically the way this works is its a closed loop with antifreeze in it for cold weather and it coils through a water cylinder to do the heating. So the water that flows through the roof panels never goes anywhere.

There are a few reasons for this, mainly the extremes of weather both hot and cold you don't want 90C water in your shower nor to freeze water in the system and break it. All of which adds to the cost any complexity but if you dont do it they don't work.

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.

Utility scale projects in the uae don't want hot water. Your house does, though.

So? drag the heat out via Peltier and have a safe layer of material between solarlayer and heat transport?

34kwh!? That’s a pretty beefy setup.

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

Still not worth it aside from the "neat" factor. 3 watts net increase won't pay for the cost of the fan over the lifetime of the panel, and a moving part like a fan won't make it to 25 years (5 years if it's lucky).

Many people run Noctua fans for more than 10 years.

25 years in all weather, not even a Noctua fan is making it that long. Its not the cost of the panels that is the issue its getting them on the roof, repairing them is enormously expensive as they have no gaps between them and buying replacements a decade later is almost impossible as people are finding out. Longevity to maximise returns is a necessity with an investment that takes 7 years to break even.

Sure, in a PC case. Not outside exposed to the elements.

Presumably the fan isn’t really exposed to much other than atmospheric air at atmospheric. There’s generally Less dust outside than inside. I’m doubting humidity has much effect on fans.

I suspect biggest failure mode for fans are the bearing’s lubrication drying out. Sometimes this can be serviced (with much difficulty) or take the efficiency hit and use a thicker grease that doesn’t evaporate as fast.

Nice thing is that they’re not a critical part here, and it’s diminishing returns so if you have multiples and 1 fails, you’re still ok.

The combined heat and humidity cycles of being on a roof would be quite damaging to any electro-mechanical machine. Especially one that isn't designed for that kind of environment. I'd be surprised if a PC fan made it more than 2 or 3 years in that environment.

Probably depends a lot on the specific climate though.

Exposed to the elements on a roof?

The fans in air conditioners seem to handle the elements fine.

The fans in air conditioners also need a 3-inch x 2-inch capacitor to start.

Compressor fans are expensive, heavy, and energy intensive, so what exactly have we gained?

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.

This is why the panels are raised off he roof with racking. Its allows the air to flow underneath them to cool them. The issue with heating is you have a mostly black object getting ~1400Watts/m^2 and just 20% of it is turned into electricity and the rest is heating the panel. Its a lot of heat.

True Black objects get incredibly hot in the sun I've long thought that Solar Panel should be combined with Thermoelectric generator but I guess the economics don't stack up yet. Thermoelectric is not very efficient and progress is painfully slow.

For thermoelectric generators you need a cool side. Where would you get that?

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.

I don't think earth tubes are ideal for such direct plumbing into living spaces.

My understanding is that they tend to develop mold and stagnant moisture will pool in the underground portion. Imagine a hot and humid day, the humid air will condense on the cool walls while passing through the underground section of the tube. Do you really want that to be your fresh air duct?

But it seems like a useful mechanism for delivering ground-coupled outdoor air to an outdoor application, which could be a heat exchanger participating in regulating indoor temperatures.

Yes, my design right now includes a HEPA filter on the intake, preventing spores from ever even getting in the tube, a slight grade downwards towards the intake and a water catchment system at the end with a float tank activated pump to pump the water out periodically.

I'd love to hear how that plays out after a decade of use. It's something I've considered experimenting with for a future ADU/guest-house on my desert property, but it seems too troublesome for my liking.

Yeah it sounds troublesome at first, but then you realize that digging a trench and laying some pipe is cheaper, easier and less maintenance than having a machine with moving parts and duct work and all that. In a desert I'm guessing in north america, the only things you have to consider are that the ground may be very rocky and also the pipe won't be in wet soil which will mean it will cool less quickly and you have to run a longer pipe. The good side is you're not likely to have tree roots crack the pipe and youre very likely to keep the humidity well below 100% even in the summer. I'd say go for it, deserts are very mold free.

You might also consider radiative cooling with some of that super white paint that radiates heat faster than it collects, since you'll have less shadows and clouds it should radiate rather well, downside being colder nights and winters.

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.

White background with double sided panels.

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...) 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.

For some homeowners looking at a residential installation, the added independence itself is a feature (as is the autonomy to be able to slightly accelerate the world's shift to renewables -- the average person does not get to participate in deciding when and where to build a grid-scale solar installation).

Of course, reducing a home's energy consumption would be even more preferable than just offsetting it with localized generation - but reality is complicated.

I think the autonomy is the big thing. Redundancy is nice, but many or possibly most residential rooftop solar installs are grid tie only; adding the ability to power locally when the grid is down adds signifigant expense.

Tarriffs are a thing to. It's common for rooftop generation to offset grid usage, but if you used your construction budget to buy into a larger project, you would likely not be able to offset usage, you'd more likely earn wholesale rates on generation at the facility and pay retail rates on usage at home. Probably you get more generation capacity, and maybe that works enough, but probably not. There's also a lack of these projects to buy into.

As a homeowner, the last thing I want to do is own and maintain the equipment to provide electric power. I will gladly pay a utility to take care of this for me. A home solar system with battery storage and an automatic transfer switch and interconnected to feed power back to the grid is a complicated setup and I want no part of owning or maintaining that.

I think that's why parent post said some home owners. Some people like coconut. As a person i, however, hate coconut.

All comes down to reliability, no?

People probably said the same in the transition from simple and reliable ice boxes with ice block delivery to electric fridges.

They are doing this in the UK, a high voltage DC line is being connected to Northern Africa where power where about 2.5-5GW of power will be produced. Its very viable economically but I also think we have more than enough roof space for solar panels and surround sea for offshore wind to meet our power needs for the foreseeable future.

Even if a commercial installation is half the price per KWh the economics still work out personally. The reason is a combination of taxes, profits and substantial business overheads that make personal Solar substantially cheaper than the current grid power which is all tied to gas prices. I am about on average 9p a KWh over the life of a Solar system which I can export for 15p currently and buying from the grid is 45p through the day. Its a substantial saving yet wholesale prices are hovering on average around the same 9p. The overheads of the grid and businesses are most of the cost and substantial.

You're talking about Xlinks? They are nowhere near doing it. They have been talking about it since 2018 and trying to get investors to give them money but despite favourable press and interest from politicians, a heavyweight board, nobody who has looked at their business plan has stepped up to the plate to back the project. Desertec is the other one which has raised nothing despite touting a similar idea since 2008.

Xlinks is going nowhere - in fact the founder, Simon Morrish, seems to be mostly spending his time on other projects these days.

Even their website claimed last year that the £16B project could be profitable if a CFD at £48/MWh (CDF is a type of contract electricity suppliers bid for in UK auctions to win 15 year contracts to supply electricity). Unfortunately local solar pv, on-shore and off-shore wind auction prices in the last auction round were all lower than this.

Regarding local subsidies and supports for rooftop solar, of-course it can make sense for an individual. My point is that on aggregate, it does not make sense - if the government is going to spend $X to encourage the production of solar PV, then supporting rooftop solar provides a poor return on the subsidy. And the difference is more than half the price - typically the multiple is 3 or 4 times cheaper.

> 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 [...] compared to a ground based utility/grid scale installation.

This is an odd either/or attitude.

If you own a house with a roof, there are factors that decide whether putting PV on your roof is economically sustainable for you or not. If you do the analysis, and the answer is that it pays off in a sufficiently short time frame, then install PV, even if utility-scale PV somewhere else would be 100x cheaper.

As long as the hypothesized utility PV installations don't bring down the electricity cost so much that your own installation becomes unsustainable, the two are basically independent.

> 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.

In general, yes, more interconnection and better transmission can only be good.

But in the specific example I gave, the finances just don't work. Underwater HDVC is just too expensive and panels are too cheap - even with the value of the uncorrelated intermittency.

And even if the numbers could be made work financially, I can't see European countries lining up to become dependent on fixed infrastructure in politically unaligned and/or unstable countries like Algeria or Libya - especially given recent experience with Russia. Securitywise, Nord Stream has shown that underwater infrastructure is vulnerable to attack and difficult to protect/guard.

Winter solar in Northern Europe, when it produces 10-15% of summer, is not displacing the winter heating load supplied by fossil fuels. North African solar could do that.

Keep in mind the political instability in NA and geopolitical risks.

> rooftop domestic PV costs can be unto 10 times more expensive on a levelized cost basis ... compared to a ground based utility/grid scale installation

Your "can be" hides a lot of sins. Looking at the data you quote domestic PV also "can be" 1/2 the cost of of ground based utility installation.

The truth is between those extremes and will vary a lot depending on situation. But here is something that doesn't vary so much: the difference between the retail cost and wholesale cost is around a factor or 3. Translation: utility solar has to be around 3 times cheaper than domestic before it makes sense to a household.

I don't think it's terribly surprising there are a lot of places that factor of 3 tilts the balance in favour of domestic PV.

> Putting solar panels on roofs doesn't make a lot of sense to me. Per kWh, rooftop domestic PV costs can be u[p]to 10 times more expensive on a levelized cost basis

Yeah an idea I've had kicking around for a bit: if my roof is a good place for solar, it should make some kind of sense to lease it out to someone else who owns the panels and their output. I've never heard of anyone making that arrangement. There are people near me who own property with no water rights who are able to lease that land out to solar utilities.

I'm not against solar and I do think that we need to have every available square inch of the world making energy if we're to survive the next century. I'm just saying that the current incentives kind of don't support it.

Up until 2019, exactly that type of scheme was very popular with installers in the UK.

Interesting! What happened in 2019? And sorry I'm speaking in kind of broad strokes here; I live in the US and don't own my roof so I haven't gone as deep into this as I'd like.

The economics changed: up until that point the government would pay feed-in tariffs for solar. In 2019 they stopped that scheme, so the value to the actual owners of the panels dropped past the point where they could install them "for free".

The price of panels has dropped since then, but I have to assume that it's not by enough to make resurrecting the idea worthwhile.

A residential roof is too small to make that idea interesting to anyone who is looking to rent space for solar panels.

Leasing a solar system on one’s roof is a thing, either fixed or as a power purchase agreement: https://www.sunrun.com/solar-lease

Leasing the roof itself to the solar company less so. Though the difference isn’t clear.

In theory you get a deal on your power, but it really depends on the terms of the lease compared to your local utility’s prices.

- the grid investment is nil,

- it increases the resiliency of people in emergency situations like storms, hurricanes, tornadoes, earthquakes, mudslides

- there is LOTS of real estate so the "hidden cost" of dedicating land to solar panels isn't needed.

Finally, I feel residential solar is a ripoff currently, because there is some degree of price fixing. 10x more expensive for LCOE? That is a policy problem, not a real economic reality. 2x or even 3x I could see, but 10 fucking times?

Some thing with batteries to consumers. The tool makers like lawnmowers / power tools / etc are simply raking consumers over the coals with vastly overpriced batteries compared to what the BEV companies are getting.

Maybe the magical hand of economics will finally deliver for the end consumers, but currently it is not.

A few years ago, solar panels dropped a lot more than 10% in efficiency when they got hot. I can see investing in water cooling them back then, but suspect that technology trends have already made this product obsolete.

If they really can cool the panels at a price + reliability + installation complexity comparable to conventional panels, it might be worth it.

However, our heat pump water heater is under 10% of our electricity bill, so if it's cheaper to just add 10% more panels + batteries, then this product seems like a non-starter to me. I guess it's possible this would make sense for heating swimming pools (they create an extremely large reservoir for the waste heat during the summer).

Rooftop solar makes sense when independence is a key need. Where I live (San Francisco Bay Area, California, USA) the power utility is extremely corrupt. This makes their service the terrible combination of unreliable (sometimes being disconnected for several days at a time) and extremely expensive (they charge about half a dollar per kWh). Being as independent from them for my energy needs as possible means buying solar, and my roof is the only open space I have available.

What you say in your last paragraph also applies to rooftop solar.

You have to weigh up better efficiency far away against local generation and account for the extra cost of transmission. If PV costs are dropping faster than transmission, which they are, then local generation gets more desirable.

> 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.

Are you quite sure, when you factor in the cost and availability of land?

Yes - all the serious proposals are dead (the xlinks guy is a grifter) because underwater HDVC costs have risen while panel prices are a tenth of what they were.

Not only the finances, but (as I described in another comment), the security aspects of depending on fixed infrastructure in unaligned and relatively politically unstable countries are very unpalatable. Such infrastructure is incredibly vulnerable and difficult to protect.

here in south africa where we have load shedding and sometimes don’t have power 8 hours a day, so they are necessary if possible.

What % overbuild do I need to generate power at night?

Clearly a sarcastic question, but as solar panels become vastly cheaper, the battery storage requirements to effectively guarantee power on demand become much less onerous. The kWh of backup required for a solar PV system sized to maximum demand is much more than (e.g.) a PV system sized to 2x maximum demand.

Hydrogen storage has a round trip efficiency of about 40%, so you need less than 2.5 times overbuild.

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.

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.

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?

Why isn't hot water potable?

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-drinki...

In addition to this, tank water heater systems include a sacrificial anode rod.

Water heater tanks include different metals in contact with the water, which creates a galvanic cell. Over time this corrodes the least noble metal. The rod is added in the design to corrode before pipes do. The rod is usually magnesium, aluminum or aluminum-zinc. It slowly but steadily leeches out the rod metal into the hot water supply.

Serious question: do we still install lead pipes anywhere? If no, when would be the threshold year where installation started to drop off for various reasons? I have heard that pex plumbing has taken over, but I have no idea if that is only for residential, but commercial use is something else entirely.

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.

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.

The output of your own panels that you use yourself is taxed per kWh? That's insane. And unenforceable, surely?

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.

Welcome to Scandinavia. Everything is taxed. It's the price of "free" healthcare and all the other social benefits.

I'm not against taxes, and the Scandinavian model seems to lead to decent societies, but this seems unnecessarily 'invasive'.

In Sweden you only get taxed if you exceed half a megawatt.

Seems pretty easy to enforce (at a binary level) with sat maps.

I wonder how many semi/complete guérilla setups there are.

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!

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.

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.

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?

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.

And in a hot climate, the heat pump water heater provides a little cooling (when you dump the hot water outside).

Those were likely evacuated tube solar collector systems. They can boil water in direct sunlight when there's snow on the ground.

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...

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.

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.

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,35...

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.

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).

Maybe a LEED credit (scam) thing?

Australia has had houses with tubes on the roof to heat the pool for a long time already.

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?

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)

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.

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?

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.

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.

>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.

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.

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

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.

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.

> 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.

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.

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.

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

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

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)

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?

Price per KWh?

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-pa...

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.

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…

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.

Yeah a little bit but so does everyone else that thinks these things are the solution and we should dive right into it.

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