GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon
Guest Blogs

Why Aren’t Solar Water Heaters More Popular?

Despite widespread global success, solar water heating is virtually unheard of in the U.S.

Solar thermal collectors can be found on less than 1% of houses in California despite decades of advocacy and widespread use in other parts of the world. A desire for simple solutions to climate change may have played a part in its struggle. Photo courtesy Paul Kuenn.

This post originally appeared at Ensia and is republished here with permission.

For Gershon Grossman and Ed Murray, 1978 was a big year. Grossman, then a solar energy pioneer at the Technion, Israel’s premier technological institute, was launching the first International Conference on the Application of Solar Energy. Murray, an idealist attending college, joined an upstart solar heating company in Sacramento, California’s capital, drawn by a prescient concern about climate change and, as he puts it, an impulse to “save the world.” For both, the excitement was palpable. Solar water heaters were surging into the market, solar thermal energy showed broad potential, and the two were riding the wave.

Four decades later, however, they live in two different worlds. In Israel, 85% of households get hot water from a dud shemesh, or “sun boiler.” But in the U.S., despite decades of advocacy by Murray and others, the number of households that have a solar water heater is less than 1%. In California, many people don’t even know the technology exists.

America’s solar water heating deficit is often portrayed as a historical accident driven by the vagaries of politics and comparatively cheap fossil fuels. However, interviews with academic and commercial players on the front lines of the solar thermal industry, and an in-depth report on the now-expired California Solar Initiative–Thermal (CSI-T) program, suggest that the desire for simple, “magic bullet” solutions to climate change has also played a significant role in relegating this practical technology to the sidelines.

A mandate, an election and two roads diverged

Heating water accounts for 25% of residential energy use worldwide, mostly achieved by burning fossil fuels. Solar water heaters do the job without combustion. Unlike solar photovoltaic (PV) systems, which convert sunlight into electricity, solar thermal systems collect solar energy as heat. Solar water heaters transfer this heat to water in a holding tank. Other energy sources, such as natural gas or electricity from a power grid, serve as a backup for cloudy days.

By tapping the sun, solar water heaters can reduce a household’s water heating fuel consumption 50% to 70%. And Israel is just one of dozens of countries with a variety of climates where this technology has been deployed. Solid performance and wide applicability have made the technology one of Project Drawdown’s top 50 climate change solutions.

So why did solar thermal technology soar in Israel and sputter in California, setting Grossman and Murray on such different life paths? A pair of political decisions in the 1970s and 1980s had dramatic impact.

The Yom Kippur War of 1973 and subsequent oil embargo made energy independence a matter of national security worldwide, but the pinch was particularly painful in countries lacking oil production. For Israel, the threat was existential; as former Israeli prime minister Golda Meir famously quipped, “[Moses] took us 40 years through the desert in order to bring us to the one spot in the Middle East that has no oil.” In 1976, Israel mandated solar water heaters for all new residential buildings up to eight stories tall—a mandate that was extended to all residential buildings in December 2019.

For Grossman, now a professor emeritus at the Technion and head of the Energy Forum at the Neaman Institute for National Policy Research, mandating solar water heaters made sense environmentally, even beyond Israel’s political agenda. “You just can’t argue with the numbers on how much [energy] you can save using solar water heating instead of electrical heating.”

The United States also felt the jolt of the oil embargo and feared running out of domestic oil. Supported by President Jimmy Carter’s 1978 federal tax credits for renewable energy, Americans installed nearly 1 million solar thermal systems by 1990, supplied by more than 200 U.S. manufacturers, including leading corporations such as Grumman Aerospace Corporation and Sears Roebuck.

However, in contrast to Israel, America’s commitment to renewable energy proved ephemeral. Under President Ronald Reagan, the federal incentives lapsed, dealing the solar thermal industry a body-blow. “We went from 650 companies in California that were installing solar [water heaters] to about 37 overnight,” recalls Murray, who is currently the president and CEO of two California companies dedicated to manufacturing, distributing and installing solar thermal systems, as well as president of the California Solar and Storage Association.

Recent attempts to revive the residential solar water heater industry have had limited success. The CSI-T program, begun in 2010 as a larger push to incentivize solar installations statewide, aimed to add 200,000 systems, but received only 6,237 applications for residential retrofits in 10 years, according to the program’s December 2019 report. “I could put a sign over the front door of my office that says ‘free solar water heating,’ and they’d probably still stay away in droves,” Murray says with a wry laugh.

Larger installations for apartment complexes, hotels and universities, and home pool heating have helped keep Murray’s solar thermal businesses afloat despite the lack of other residential demand. Ironically, the commercial sector isn’t as robust in Israel because the country’s original mandate only applied to residential properties—a move Grossman views as a significant oversight. Indeed, Grossman believes that an industrial mandate could increase Israel’s renewables usage up to fivefold.

The limiting psychology of renewables

The woes of the American solar water heater industry go far beyond politics, however. The industry also suffers a more insidious challenge: For the average consumer, “going solar” means just one thing: solar PV.

Solar thermal technologies, including solar water heating, provide a direct, thermodynamically efficient and cost-effective method for decarbonizing heating. And for households in mild climates with low electricity bills, “solar water heating can be one of the simplest ways … to use renewable energy and save on energy bills,” says the CSI-T report.

But it’s solar PV that has exploded into the global electricity sector, thanks to manufacturing innovations and strong government support. Leveraging economies of scale, the price of solar PV panels has dropped by over an order of magnitude in the past decade. In California, additional boosts came from government-instituted solar feed-in tariffs, cheap financing plans and private-sector investments. And, in a major coup for the industry, California mandated solar PV on new residences up to three stories starting 2020.

On the other hand, California’s residential solar water heater industry finds itself in a vicious cycle of low consumer demand and high prices. As the CSI-T report notes, “In contrast to conventional gas and electric water heaters, which are typically installed by plumbers, solar water heaters are installed by a range of firms and public entities.” In other words, consumers must actively seek out solar water heaters by relying on nonstandard sales channels.

This additional friction reduces consumer demand among all but the most motivated consumers, leading to higher marketing costs that drive up the customer’s bottom line. Prices in California are further exacerbated by past industry failures, which have led to strong, self-imposed regulations in the name of consumer satisfaction, says Murray. For example, after many cheaper solar water heating systems froze during the unprecedented 1990 freeze in California, only more expensive systems were allowed through the CSI-T program.

All told, the cost of the average solar water heater sold in California through the CSI-T program was $7,400, compared to less than $1,000 for a fossil fuel alternative. By contrast, a solar water heater in Israel can cost as little as US$700.

Today, drumming up excitement for solar thermal remains difficult. According to CSI-T report interviews with solar water heater adopters, “Some interviewees remarked that it seemed tough to get others interested, theorizing that PV was so dominant in neighbors’ minds that solar water heating hardly registered.”

“It’s just the sizzling, sexy PV [that] really captivates the audience,” says Murray.

Portfolios, not magic bullets

Entrepreneurs routinely caution, “Fall in love with the problem, not the solution.” In this case, the problem is carbon emissions, and, against entrepreneurial advice, individual governments have tended to fall in love with just one solution. For Israel, Cyprus, Hawaii and others, solar water heaters were that solution. For California, it’s solar PV.

By committing to a specific technology, governments fall prey to a conceptual error that science journalist Ed Yong recently referred to as a “monogamy of solutions.” (Interestingly, he argues this fallacy also shapes the government’s response to Covid-19.) Rather than embracing the growing portfolio of technologies available to solve the carbon emissions problem, going all-in on one satisfies the very human need for “magic bullets.”

Europe’s Green Deal may model such a “portfolio” approach for the rest of the world, according to Bärbel Epp, a German physicist-turned-journalist with nearly two decades of experience studying the global solar thermal market. According to Epp, representatives from the European solar thermal market have lobbied the European Commission for over a decade to use solar thermal technologies to decarbonize the heating sector.

“It took [the solar thermal industry] I don’t know how many years, at least 10, of just continuously repeating the sentence that heat is 50% of our final energy consumption in Europe. … It was hard to lobby in Europe, but it’s now obvious that we have to do something for heat.” Whether these efforts will succeed in providing solar thermal a seat at the table remains to be seen.

To Grossman, solar water heaters are the first piece of Israel’s portfolio. As Israel struggles to meet its Paris Agreement goals, Grossman says he believes solar PV panels will take their place alongside solar water heaters on Israel’s rooftops.

Back in Sacramento, Murray is still battling for solar thermal. This year, he’s lobbied the California legislature to extend the state’s recently expired solar thermal subsidy program for one more year, citing Covid-19 as a barrier. The legislature hasn’t budged, but Murray vows he’ll keep going. He may be a lot older than he was in 1978, but the idealism is still alive.


-Dina Berenbaum and Manoshi Datta wrote this story as participants in the Ensia Mentor Program. The mentor for the project was Peter Fairley. Berenbaum is a data scientist and environmental technology enthusiast. Datta is a computational biologist and freelance writer.

36 Comments

  1. User avater
    Jon R | | #1

    “You just can’t argue with the numbers on how much [energy] you can save using solar water heating instead of electrical heating.”

    Show me the macro-scale numbers (eg, $/ton) as compared to utility scale wind and PV solar with heat pump water heaters.

    +1 on not falling in love with solutions (like thermal solar and residential scale PV).

    1. Tyler Keniston | | #5

      I agree. Analyzing the whole get's complicated fast though. Is grid-scale PV lighter on the environmental scale than residential solar-water when all inputs and trade-offs are accurately accounted for? I.e. mining, forestland loss, etc. The answer may very well be yes, but do we have the resolution of analysis to make such a call? Maybe it's different ballparks, I don't know.

      Perhaps we should just dump the money we're spending on solar into agricultural reforms and innovations, education, and health. But in loving the problem, not the solution, we of course should do other things. Maybe some solar.

      1. User avater
        Jon R | | #8

        IMO, it's about economic efficiency. There are roughly X dollars available and looking at ALL the options, how should it be spent for the most environmental effect?

        Every time I see solar on a residential roof, I think "there is a missed opportunity to do much more for the environment". Utility scale PV or wind would have done about 5X more for the same cost. Even with some adjustment for other factors and "complicated", it's clearly the way to go. Residential solar (PV or thermal) is a waste, let's get on with doing renewable energy right.

        1. Roger Smith | | #16

          Sure, but offsetting residential behind the meter is more valuable than dumping wholesale power on the grid to be distributed.

  2. Charlie Sullivan | | #2

    The real question is why it costs >10X as much in the US as in Israel. For $700 it would be cost effective; for $7400, it's not cost effective. Without solving that problem, there's not much else to discuss.

    See also Martin's 2014 article https://www.greenbuildingadvisor.com/article/solar-thermal-is-really-really-dead, and, for a counter point, a solar thermal installer's 2017 rebuttal https://www.greenbuildingadvisor.com/article/solar-thermal-is-really-really-dead.

    1. User avater Expert Member
      Dana Dorsett | | #3

      >"The real question is why it costs >10X as much in the US as in Israel. "

      The real answer is that the freeze risk in Israel is exceptionally low compared to most of the US, which allows them to use much simpler technology with no pumps or other moving parts, and fewer local regulations that would interfere with being able to import cheap batch heaters that are popular in most of India & parts of China. When extra tanks & pumps are involved the installed costs go up by a lot, and when freeze control (other than a simple heat tape on the exterior plumbing) is critical costs go up an order of magnitude. The average January overnight low temp in Haifa or Tel Aviv is about 50F-55F, much more temperate than even Sacramento, with average overnight lows around 40F, and many nights that hit below the freezing mark in the pre-dawn hours.

      If California got serious about adjusting regulations to fit the local climates, this type of solar technology would be applicable in the temperate coastal regions of CA and parts of the Central Valley, not so much at elevation in the Sierra or eastern side thereof.

      Take a look at a few year round temperature profiles- the picture is pretty obvious how simple it is in Israel even to otherwise not crazy-cold parts of CA:

      https://weatherspark.com/y/1157/Average-Weather-in-Sacramento-California-United-States-Year-Round

      https://weatherspark.com/y/98222/Average-Weather-in-Tel-Aviv-Israel-Year-Round

      https://weatherspark.com/y/1925/Average-Weather-in-Barstow-Heights-California-United-States-Year-Round

      1. Roger Smith | | #17

        I think it's also how we build our homes. Many are multistory with the water heater in the basement. You have to integrate the roof into that system at substantial retrofit expense. On top of that it's boutique, there are few manufacturers, you have to freeze-proof, etc.

        FWIW, I tried to push SHW in CT and even while we had substantial incentive for it there were few takers. The manufacturers I worked with just a decade back aren't around locally anymore either (back to Germany I think)

    2. Tyler Keniston | | #4

      Charlie, you linked the same link twice... probably meant: https://www.greenbuildingadvisor.com/article/solar-thermal-is-not-dead

      1. Charlie Sullivan | | #7

        Thank you Tyler, indeed that's what I mean to link.

    3. CarsonB | | #10

      I would be surprised if you could get the roofer and plumber to install that for $700 even if the panels were free, at least in my area.

  3. User avater
    BlueSolar | | #6

    Dana, you really think freeze resistance alone will be the reason why solar water heaters cost ten times as much here as they do in Israel? That doesn't seem plausible. What would make it cost so much beyond using propylene glycol as the working fluid? The heat exchanger?

    Solar thermal is cheaper everywhere, not just Israel. Colombia is another example of massively lower cost systems, where they even had a novel home-grown design a while back, that came from some big thinkers in a pioneering community down there.

    Solar PV costs more here than almost everywhere else too. Our fear-driven, extremely risk-averse regulatory environment is extremely costly. $7,000 for a water heater is just insane, a devastating price for something we would want people to adopt.

    There's a lot of room for improvement in how we build roofs, where if we applied a systems approach we could reduce the cost of both thermal and PV, but the big wins look regulatory. Beyond that it looks like it will take big thinkers to move the needle. Hybrid PV and thermal would be ideal, like DualSun is doing, especially if it could be a whole roof.

    1. Kevin Dickson, MSME | | #9

      I know you find it hard to believe, but Dana is exactly right about the solar thermal DHW cost problem. I worked with the researchers at NREL, and although the market existed from about 1978 until roughly 2014 when PV costs dropped, there was never a cost effective thermal system design identified for North America. And of course now it's too late.

      You are right about roofs. Elon Musk's big thought is quite simple: a solar PV shingle that costs the same as regular shingles and lasts much longer and looks better. As usual, volume is what drives costs down, Tesla understands this well, and they will eventually get there.

      Hybrid panels like Dual Sun do not have the volume potential of Tesla, so their idea will never achieve the required cost goals. And of course, that's why Elon ruled out that concept.

      1. Burninate | | #11

        What made it cost effective in Beijing but not warmer US areas like the Gulf Coast or West Coast? Is it just that we're spoiled by cheap-verging-on-free energy prices and we've become accustomed to a perfectly reliable supply?

        https://www.ars.usda.gov/ARSUserFiles/50301000/Graphics/Climate_china.pdf

        https://planthardiness.ars.usda.gov/PHZMWeb/

        "A report from NREL showed that the typical solar water heater installation costs between $5,000 and $10,000 in the United States, while Chinese consumers pay between $300 and $1,000. Annually, China installs 6 million solar water units."

        https://www.eenews.net/stories/1059983772 has a few different ideas as to the difference

      2. User avater
        BlueSolar | | #12

        Yeah, but the question is *why* specifically a system costs $700 in Israel and $7,000 in the US. That's an enormous difference. What explains the difference? Your point doesn't address this.

        Verbal intuitions and generalizations aren't a valid way of knowing things like this. To truly know why a system costs $6,300 more in the US, we'd have to rigorously investigate the question and trace all the costs. In this case, they're not the same systems – apparently the Israeli systems don't have freeze protection, while the American systems do. That implies glycol as the working fluid here, which implies an indirect system as opposed to direct water heating.

        It's also unclear if we're comparing active vs. passive systems (pumps or not). There are all kinds of solar water heaters, and people aren't always clear on what type they're talking about, and don't always use the same terminology.

        I'm skeptical that glycol vs water explains the $6,300 difference. Reporting that an agency reported that the cost of solar water heaters in the US didn't fall over some period doesn't actually intersect with my claim – it's not new information, nor does it explain *why* Israelis are spending $6,300 less on their systems.

        1. User avater
          Jon R | | #14

          When I check prices on a couple of small pumps, a simple controller and a heat exchanger, it's more like +$630, not +$6300.

          1. User avater
            BlueSolar | | #15

            Interesting, so it looks like freeze resistance isn't going to be a big reason for the delta. So I assume this was for an indirect/glycol system? A direct system wouldn't use a heat exchanger, right?

      3. User avater
        BlueSolar | | #13

        I forgot what I said about the whole roof hybrid system. So you're saying Elon Musk ruled out hybrid PV-thermal systems? I wasn't aware that this ever came up with him. Did he talk about this publicly at some point?

        In general, I wouldn't have a reason to care that Musk or someone else had ruled out hybrid solar. He has his own context, business goals and interests, and so forth. His beliefs and conclusions wouldn't normally be relevant to anyone else.

        I'm very interested in factory-made components or even entire houses. It opens up a lot of opportunities for better construction, materials, sealing, etc. When it comes to roofs, I'm most interested in my "uni-roof" concept – a single seamless roof structure that would be made in a factory, transported to the home site, and laid on top. It would be made of a strong, tough, and light material like a good fiberglass (e.g. S-glass fibers instead of E-glass), basalt fiber reinforced polymer (similar fiber strength to S-glass), a carbon fiber composite, Kevlar or other aramid fiber, etc.

        Such a roof would be a heck of a lot stronger and weather resistant than the piecemeal plywood/OSB + felt paper mishmash that is typical today. It would be easy to make such a roof completely self-supporting – it wouldn't need rafters, joists, etc. underneath. You might just have a ridge beam or something, or maybe just a skeletal framework with far fewer members than usual. Overweight workers and firefighters would be able to tromp all over the roof, no problem.

        Such a roof could be optimized for solar PV by implementing a standard snap-in interface for panels and solar tiles/shingles. Solar wouldn't penetrate the roof – it's much smarter to just have a snap-in interface or some other toolless method. Installation would be ridiculously easy, simple, and cheap – and totally DIY-friendly. That alone would chop thousands of dollars from the cost of solar.

        Other savings would come from the simpler structure of the panels. They wouldn't have to be built like tanks like they are now. They're currently designed to be used on random roofs that were not designed for solar at all. Solar panels are not complementary or integrated with the roof. They're at odds with it, racked up above now-redundant layers of tile or shingles, laying on racks that are punched through the roof. They could be much thinner and sleeker if designed to just snap into a roof designed for them.

        I'd also have a lot of built-in safety features, like standard clips or rings for fall prevention straps and lines, probably on all the ridges. That's so basic, and it's crazy that it's not standard. And the edges of the roof, at the overhangs, would have drop-down rails with a sliding ladder brace, maybe a simple looping chain that slid horizontally along the rail, with some snap-lock deal to keep your ladder from falling.

        On the solar thermal, at minimum we could leverage our wonderful new panel snap-in standard – the solar water heater panels would snap in just like the PV. And maybe we'd have two or three standardized panel sizes, where all roofs would support maybe two of the XL, a bunch of the main standard size, and a few of the smalls (maybe the XL size would be intended for thermal collectors).

        But beyond that bare minimum, we could make the entire roof a thermal collector, designed to have some synergy with the PV. A roof material like fiberglass or other composites makes it possible to integrate thermal collectors, in the form of piping, tubes, capillaries, whatever. These would probably occupy the top surface of the roof, and the PV panels would be designed to mate to said surface such that the thermal collectors are cooling the PV panels, which has the added bonus of increasing their performance. We'd a collect a ton of thermal energy this way, and would probably cool the roof and attic to an extent equal to some amount of insulation + a good radiant barrier (and I'd probably integrate several inches of insulation and a radiant barrier directly on the the bottom of this roof assembly).

        This would be a mother of a water heater, and could also serve a radiant floor system, with some thermal energy left over that we'd have to figure out how to use productively. We might need a radiator on the side of the house to dump the unused thermal energy sometimes – that would still be a win because we cooled the roof, reduced our house cooling needs, increased PV performance, etc.

        A modern roof built from modern materials like this opens up a lot of possibilities. I doubt that Musk's analysis of hybrid PV considered the option of whole-roof systems like this, or a fiber-reinforced composite uni-roof with a standardized panel snap-in system.

    2. User avater Expert Member
      Dana Dorsett | | #19

      >"Dana, you really think freeze resistance alone will be the reason why solar water heaters cost ten times as much here as they do in Israel?"

      Pretty much.

      But there is the substantially lower cost of labor too.

      >"That doesn't seem plausible."

      Sure it does. Batch heaters are ULTRA simple, and with dramatically fewer labor hours at a lesser skill requirement needed to install them.

      >" What would make it cost so much beyond using propylene glycol as the working fluid?"

      Well, there is the matter of pumps, storage tanks, control systems, etc.

      Typical Israeli solar water heaters are dumber than a box o' rocks, usually with NO controls or wiring, relying on a thermosiphon (or evacuated tubes) tied directly into a storage tank integrated into the system to move water from the collector into storage. Deluxe versions have a single heating element serving as back up for days of excessive water use or rare stretches of zero sun. The only installation plumbing is the cold line in, and the hot line out of the batch tank. Often there usually aren't even expansion tanks or pressure relief valves. ( The local water supply system serves those functions).

      >"Colombia is another example of massively lower cost systems, where they even had a novel home-grown design a while back, that came from some big thinkers in a pioneering community down there."

      Yep- batch heaters of various designs are cheap and available in most of Latin America, just as they are in China and India.

      Plumbing safety regulation & certification is the only hurdle for importing these sorts of water heaters in the US, but if you live anywhere freeze-up is a real hazard you may have to build your own freeze control mitigation on the exposed potable plumbing.

      Installation labor rates are lower in places like Columbia (or even Israel, where the all-in construction worker compensation is still under USD$20/hour, even a licensed plumber only runs ~USD$25/hour), and there are fewer installer licensing requirements and fewer local regulatory inspection agencies to deal with. A rooftop water heater installer in the US costs at least 5x per hour what it costs in Israel, more than 10x what it costs in Columbia.

      Many batch heaters are DIY-able. A real issue for a DIY installation on a roof top would be verifying that the structural capacity of the roof can handle the weight of the water heater full of water. Ground mounts are dead-easy if you have sufficient unshaded yard area.

      1. Josh Durston | | #20

        I agree, typical solar thermal systems here, have heat exchangers, pumps, vents, multiple fluid types, antifreeze sensitivities to breakdown from overheating, and rely on properly setup controls and sensors.
        This is why I think solar PV to hot water is probably simpler and cheaper even if you need a bit more square footage on the roof due to lower PV versus thermal efficiencies.
        Electrical components don't freeze, and wires are easier to run on roofs and attics than pipes.

      2. Charlie Sullivan | | #21

        Dana, I think the one I linked below for $1k is the kind of system you are talking about--that would indicate that they have gotten over whatever hurdles to be able to import and sell these in the US.

        1. User avater Expert Member
          Dana Dorsett | | #29

          >" I think the one I linked below for $1k is the kind of system you are talking about--that would indicate that they have gotten over whatever hurdles to be able to import and sell these in the US."

          I didn't say that those hurdles were insurmountable, though most manufacturers haven't jumped through those hoops. The one in the link is an evacuated tube model, which is typically slightly more expensive (but more effective in cooler weather, and more freeze robust) than thermosiphoning flat panels. Both types have been available in the US for more than 40 years. But most international manufacturers/vendors have not bothered to make that effort due to the very small size of US market, where energy from other sources is comparatively cheap, and typical per-person hot water use is much higher, insolation levels lower, requiring larger units than typically found in Israel or Columbia.

          Very few US families could manage with the average size of water water heater in these Israeli pictures- most would need something 2x that size (or more):

          https://22dpwr1zij0x2s69j227c6gj-wpengine.netdna-ssl.com/wp-content/uploads/solar-water-heaters-on-roof.jpg

          https://static.timesofisrael.com/www/uploads/2019/09/F170621MA003.jpg

          (Note that those are all flat panel, not the more expensive evacuated tube type.)

          Freeze control is still an issue for exposed plumbing to/from evacuated tube type water heaters- an installed-cost adder for most US locations.

          There really isn't any mystery why the average solar water heater in Israel costs only ~10% of what it takes to deliver solar hot water for the average US household.

          1. User avater
            BlueSolar | | #31

            What are those things in the pictures? Thermosyphon? I've never seen solar water heaters like those, with upright tanks. They look goofy, and I wonder what the engineering implications are of doing thermosyphons with horizontal tanks vs. upright tanks. The hot water moves up into the tank and then what? I wonder what the implications are of the tank orientation.

            I thought about having a whole-roof thermosyphon collector where the ridge of the roof would be a huge, long tank, probably unobtrusive, hard to tell a tank was there because it would drop down into the attic. But it's probably better to just store it somewhere lower, and dump a lot of the heat into the ground and save it for space heating during the winter. I read about a guy who dug a big thermal storage cavity under his driveway – tons of energy for the winter.

            Why would Israelis use much less hot water than Americans? Do they not shower every day? Bummer. I read that they have real showers, higher flow rates than our 2.5 gpm, but maybe they shower less. The Passive House models on water use were apparently bogus because they relied on German assumptions of weekly showers or something – reminds me of my Russian roommate in college. Israelis have had much higher water prices in the past, so maybe that's why, but their huge new desalination plant is supposed to be selling water dirt cheap now.

      3. User avater
        BlueSolar | | #30

        Thanks Dana, good points. By the way, I forgot to ask what's the consequence of freezing in a solar hot water system? Why does it matter that a system might freeze once in a blue moon? Would pipes burst, or is it just the inconvenience of no hot water from that particular source? In many cases there would be a backup gas tankless or something, so it wouldn't matter unless freezing broke stuff. Would it be easy to design a system such that freezing did no harm?

        Where did you get those Israeli wage figures? All-in for under $20 an hour? That's surprising given that Israel is a developed and arguably socialist country where government eats ≈36% of GDP, so their taxes are high. I figured it must be expensive to hire and employ people there, maybe not as bad as France, but UK-ish or California level overhead. $20 is remarkable, and I wonder if it's a recession dip.

  4. Charlie Sullivan | | #18

    One way to get a handle on the costs is to look at kits:

    $3000 for a kit with a glycol loop, a flat plate collector, and a tank with built-in heat exchanger.
    https://www.sunmaxxsolar.com/product/heliomaxx-80g-glycol-solar-hot-water-flat-plate-collector-kit/

    $1000 for a rooftop tank plus evacuated tube thermosiphon system.
    https://www.sunmaxxsolar.com/product/thermopower-vts-15-tube-40g-thermosyphon-solar-hot-water-kit-with-heat-pipes-copy/

    Then there's installation and permitting, both of which are higher than they would be if the systems were more popular.

    You can argue that it's a shame that installation and permitting are expensive and try to get those costs down, partly by getting the volume up, and 25 years ago, that was the right answer. But the PV + heat pump water heater solution is now cheap enough that it's really hard to beat, so it's really unlikely that you'd ever get enough systems being installed to compete well with that approach.

  5. User avater
    Paul Kuenn | | #22

    Just scrolling through the stories and then I see a photo of my house long ago. Hmm? Nice article. I now have 4 Heliodyne collectors up there taking care of DHW & heating. All works great and we figure about 65% of the years energy used in our zero energy home and we're in dark & cloudy NE WI. Most of the solar thermal I install are ground mounts. Easier to trench than dealing with weak roofs and nightmare plumbing. I use mostly Craig's List/roofing contractors found collectors that work just as good as new 40 years later. Saves thousands of dollars for the customer and lots available from roof tear offs. At latitude 45 we need expensive additional equipment but new solar PV run variable DC pumps last forever. The only things I'm called to replace every 10 years is the anti-scald mixing valve needed as storage tanks can get near boiling without a well thought out heat dump.

    1. Kevin Dickson, MSME | | #24

      Reply to Paul Kuenn,

      It's good to hear that you are reusing older but functional solar thermal collectors. It's a great use of an undervalued resource.
      Unfortunately, when the system needs a new controller, pump or tank, the typical thermal system owner opts for system removal, even though it is probably the wrong decision. That's why there is a good supply of used panels.

      Reply to Blue Solar:
      Elon Musk always reverts to first principles and cost estimates when pursuing his goal of reducing global CO2 emissions. The attractive concept of hybrid panels has been well-known for 30 years. I wrote this article in 2008 when I thought hybrid panels could be the winning technology:

      https://greenbuildingindenver.blogspot.com/2008/12/domestic-hot-water-for-zero-energy.html

      System efficiency is also a big issue:
      Here is a report from NREL that highlights just how a bad real life system efficiency tends to be:

      http://www.nrel.gov/docs/fy08osti/43188.pdf

      So did Elon Musk consider PV/thermal solutions to put on roofs? Yes, and he rejected them.

      1. User avater
        BlueSolar | | #26

        Kevin, I'm not talking about hybrid panels. I'm taking about a whole-roof integrated thermal collector, and some PV on top. Hybrid panel cost estimates wouldn't be relevant here.

        It doesn't matter if Elon Musk rejected some broad concept. It wouldn't be relevant to any specific design or innovation. Since innovation is always possible, and there are so many dimensions and aspects of a system, device, or implementation where innovation can take place, it's not possible for anyone to know that some broad concept doesn't work. Musk would have no way of knowing what other people might come up with in their implementations of something like hybrid solar. Neither would you.

        The whole-roof concept I sketched out, with built-in solar thermal collection, is a good example. If someone's assumptions are that solar has to be implemented as "panels", then an integrated whole-roof system throws their assumptions and math out the window. There are endless other innovations and modifications possible. It's not possible to know everything that's possible when it comes to other people's minds.

  6. CarsonB | | #23

    $600 worth of solar panels would be able to roughly power a 3.75 EF hpwh on a net metering contract according to my back of an envelope calculation. It's not clear if the article included labor. The PV is less than the $700 system in isreal and would be easier to install and likely maintain, and any any excess power could be used for other purposes. The numbers of course depend on climate, but solar thermal efficiency drops too. If that's correct, why would solar thermal even be worth considering?

    days per year * daily usage / efficiency = yearly energy useage of hpwh.
    365*12.03/3.75 = 1,170.92 kWh.
    Assuming an average of 4 sunlight hours per day, a $300 385 watt panel would make 385*4*365~=562kWh. So two of those panels would be needed to produce enough power for the hpwh.

    https://www.energy.gov/energysaver/estimating-costs-and-efficiency-storage-demand-and-heat-pump-water-heaters
    https://news.energysage.com/what-is-the-power-output-of-a-solar-panel/
    https://unboundsolar.com/9434371/heliene/solar-panels/heliene-385-black-mono-solar-panel

    1. User avater
      Jon R | | #25

      Look at macro scale numbers and you will find that any kind of solar on a residential, grid connected house isn't the way to go. But politics and poor laws distort things - it might make sense to your pocketbook.

      1. CarsonB | | #28

        Jon, agreed. But cheaper grid scale solar should make the price comparison even more skewed towards PV. For me personally, the very generous metering contracts and federal/local subsidies made it cheaper to pay for PV through my loan than my electric bill.

  7. Tom Gocze | | #27

    Solar thermal in Israel is just simpler and hence inexpensive. And they have an industry in place manufacturing for many years. Do they ever have to think about freeze protection?
    The US Solar Thermal industry is pretty non existent. As one who makes thermal components and deals with contractors, contractors love that there are minimal call backs with PVs. Solar thermal, at some point, requires pump, controller and antifreeze maintenance. If you ignore an antifreeze system, it becomes acidic and causes corrosion failures.
    An orphaned PV system can be brought back to life without too much trouble. Not necessarily so with old solar thermal systems. And once you get it running, an older system is probably going to need more love to keep it going. (At what cost per hour?)

    This is coming from a guy who has 40 years committed to solar thermal. (Still do it, but make most of my living doing other energy related things.) At this point, I think I missed the bus and should have switched to solar electric. PVs and heat pump water heaters are hard to ignore for reliability.
    Solar thermal needs low cost collectors and simpler system design with lower cost hardware.
    These systems cannot continue to use high cost heating system or European hardware.

    my 2 cents worth,
    Tom Gocze
    American Solartechnics

  8. KurtGranroth | | #32

    I live in Phoenix AZ, the sunniest large city in the world, and so solar water heaters should be a shoe-in here, if anywhere. I've never gotten one, though, due to a reason that I almost wonder if it isn't a reason since it's so rarely talked about.

    The reason I haven't gotten a solar water heater even with pervasive solar energy available is one of "time of use".

    That is, the water is only going to be heated while the sun is out, during the peak of the day. Having hot water when the sun is down requires super insulated tanks of stored hot water. But every time you tap into the stored hot water tank, the refilling action will introduce (relatively) cold water, thus diluting the overall temperature. Every time a hot water valve is activated, the subsequent water draw will be cooler than the last time.

    The key for efficient solar water heating, then, would be to ensure that all hot water usage scenarios happen during the day, when the sun is out. That way, the refill water is also already hot.

    Let's look at the primary users of hot water, though. The biggest usage by far is going to be showers, followed by clothes washers, then dishwashers, and finally cooking.

    Unless you are an athlete and shower after working out, you are almost surely going to take a shower either in the morning after waking up or at night before going to sleep -- in both cases, the sun will not be out and you'll be getting water from the storage tank.

    Clothes washers, at least in my household, are typically run at night, when electricity is cheaper due to our Time-of-Use plan. Plus, pre-pandemic, we wouldn't even be home to do laundry during the day in the first place.

    Dishwashers are similar to clothes washers in prioritizing electricity use and avoiding the daytime.

    Some of the cooking hot water might coincide with solar water heating time but we're also talking about (by far) the least amount of usage, so hardly worth doing.

    Based on this thinking, I'm assuming that the "backup" resistive heater intended for use when it is cloudy would actually activate nearly every day.

    So am I missing something fundamental or are solar water heating systems really aimed at people who can completely readjust their living schedules to match up with the sun?

    1. User avater
      BlueSolar | | #35

      Hi Kurt, I lived in Phoenix too, and Tucson. I assumed that a solar water heater would be paired with a conventional backup or complementary unit. More specifically, I assumed that a gas tankless water heater would be a perfect pairing. The solar water heater would provide a large tankful of hot or warm water and the tankless, on demand heater would take that water and add any additional heat needed to achieve the specified temps. Since the input temperature would be much warmer than default cold water, much less natural gas would be consumed over the course of a month or whatever.

      I also had the impression that solar water heaters were somewhat more useful during typical showering times than you imagined. I don't have any expertise here, but I thought the indirect types that use glycol as the working fluid, instead of pumping water up to the roof, were a lot more useful than the direct types. I don't know if they store the heat more efficiently, or for longer, but I had the impression it was something along those lines. There would be a heat exchanger involved. I also thought that the tanks stored hot water fairly well over say 24 hour spans.

      1. KurtGranroth | | #36

        Yeah, from my research on the topic, all solar water heaters do have a conventional backup unit... but I would stress the "backup" part of that. Even DIY solar water heaters are notably expensive -- usually markedly more than even a hybrid heat pump water heater! At that cost, I'm not going to view the solar water heater as just assisting on heating the water but rather it darn well better be doing almost 100% of it. Regularly having to use the conventional heater component would make it nigh impossible to break even on the thousands spent on the solar heater installation.

        My understanding, too, is that the water heated by the solar collectors is in a super insulated tank and so can remain hot for 24 hours, as long as it is not touched. My concern is more that the hot water is being used when no hot water is being generated and so the incoming cold water will decrease the temperature, perhaps notably, and perhaps enough to always trigger the backup unit.

        Say, for example, that I have a 100 gallon tank that was heated by solar to 120 degrees and remains so overnight. In the morning, Person 1 takes a 10 minute shower consuming 20 gallons, all easily hot enough. The insulated hot water tank replenishes the used water with cold water, meaning that the tank now has 80 gallons of 120F and 20 gallons at 50F. Person 2 takes a shower, consuming 20 gallons, all hot enough but now we're looking at 60 gallons at 120F and 40 gallons at 50F. Person 3 takes a shower... is the overall water temp still high enough when 40% of it is "cold" or has the "backup" unit already kicked in? 20 more gallons down and now we're looking at 40 gallons at 120F and 60 gallons at 50F. The conventional heater is now on full bore to handle the hot water for Person 4 and beyond, definitely.

        Am I still missing something? With my above example, the conventional/backup heater is going to kick on every single day, regardless of how well the solar heater is working and at thousands of dollars, I can't see how it would be worth it?

        That said, I have thought along your line of thinking a few times. What if I could "pre-heat" my water before it enters my conventional heater? Using the sun to do that would be ideal. Could that somehow be done for $100-$200? It would be worth doing, at that point, even in just a "helper" mode... but cost is everything.

  9. David Bainbridge | | #33

    For those with excess photovoltaic energy it would be helpful if the USA had equipment that would more easily allow that energy to be directed to heating water. There are devices in Europe and Australia for instance that allows people to heat their water directly from PV panels but nothing is available in the USA. This completely avoids all the hassle of maintenance and plumbing necessary with a solar thermal water heater.

    1. User avater GBA Editor
      Martin Holladay | | #34

      David,
      For more information on this issue, see "PV Systems That Divert Surplus Power to a Water Heater."

Log in or create an account to post a comment.

Related

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |