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Thread: Photovoltaic to heat water vs solar water heater

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    DIY Junior Member rains808's Avatar
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    Cool Photovoltaic to heat water vs solar water heater

    Hi - my first post. We may be building a house this year in Hawaii. We're on the west side of the Big Island and get tons of sunshine all year round. I'm wondering if considering the cost of installation & maintenance whether it might be better to use a regular electric water heater in conjunction with Photovoltaic roof panels (net metering system) rather than a solar water heater + collector. In other words, heat water electrically with the power generated from the PV panels during the day. Would need a timer ("little grey box") to switch the power to the water heater off during the evening/night. Also would need an oversized water heater to store hot water overnight. My thinking is that this would eliminate one system: the hot water collector panel and circulating pump and the plumbing that goes with it. Would probably mean a few more PV panels to generate some more KWH to cover the use of the water heater. I hope I explained this well enough - any thoughts on this much appreciated. Aloha, Rich

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    In the trades Dana's Avatar
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    In Hawaii a combination of PV + hybrid tank hot heaters actually MORE efficient than a thermal hot water, and lowers the cooling load inside the building too.

    In heating-dominated climates there's always the issue of hybrid tanks adding to the space heating load, but in cooling dominated climates with high-priced electricity (such as in HI) they're a slam-dunk.

    Some independent testing seems to indicate that the larger the tank the more efficiently the hybrid heat pump tank works, but I don't have web-published data on that. The 80 gallon Steibel Eltron Accelera tests better than any of the smaller- tank hybrids out there, and has good first-hour ratings for an electric tank- you can even fill a decent sized soaking tub with it before the resistance elements would kick on.

    A couple grand for a hot water heater may seem like a lot, but at Hawaii's electricity rates it'll pay back well within it's anticipated lifetime, since it uses dramatically less power for hot water while reducing your cooling power use at the same time. The installed cost will still be less than that of a fossil-fired tankless, (and less than even an in-line batch-solar hot water heater of lower net efficiency.)

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    General Engineering Contractor ballvalve's Avatar
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    I would do the net metering and preheat the incoming water for a basic water heater with some roof mount rig. An uninsulated water tank in a glass box used to be made that closed up at night with insulation - sun runner - Or just a basic recirc rig panel- no need for freeze issues, so its easy.

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    In the trades Dana's Avatar
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    Roof top batch heaters are pretty cheap, and a better solution than homebuilt hacks like that, but probably a lower ROI than the heat pump water heater approach.

    With a heat pump water heater you're really getting as much air-conditioning out of it as you are hot water- you couldn't ask for a nicer fit for net-metered PV in that climate, since most homes there rely on air-conditioning anyway. With cooling loads 300 days out of the year means you don't HAVE to up-size the PV array to get hot water. With any kind of batch solar you'd either have to oversize the sucker 3x to be able to coast through the rainy/cloudy days, or have some sort of backup HW heater. With the solar thermal sized at a more appropriate 50-70% of the hot water load the house would be still be using more power, since there's no AC being offset by the HW heater. At an average room temp of 78F room the COP/water-only of the Stiebel Eltron would be closing in on 3, but the net reduction on power use from the "free" air conditioning it provides would be nearly 2x the savings on hot water heating.

    Stated another way: With solar thermal hot water you're bringing heat into a house that already has a cooling load, so any standby or distribution loss from that loop is simply added to the AC load. With a heat pump water heater 2/3 of the heat going into the water comes OUT of the cooling load, and the water coming into the house is never above room temp, so there's no heat being leaked into the house except through the walls of the tank. The only extra heat being added anywhere inside the thermal envelope of the house is the power to run the heat pump, and most of that ends up in the water first, the majority of it going down the shower drain rather than leaking back into the house through the tank walls. It's basically just a little air conditioner constantly sucking heat out of the house, heat that would otherwise be removed with similar net efficiency by the main air conditioning.

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    DIY Junior Member rains808's Avatar
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    Hi Dana,
    We have a solar water heater in our current house with an 80 gal - it saves the hot water stored during the morning hours (tends to cloud up in the pm, so we had the collector situated more to the SE) for showers for the spouse and me the next morning. Only time we turn the breakers on is if we have guests. I'll look into the different tanks available (have to see what's here, otherwise pay thru nose to ship a special item).

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    In the trades Dana's Avatar
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    I'm pretty sure you can get the GE & Rheem models HI, if not the Stiebel Eltron. You can get the the Rheem Ecosense ($1300/50 gallons) through the Home Depot in Kailua Kona without extra shipping, and you can get the GE Geospring (~$1000/50 gallons) through Sears. You'd have to call to see what's in-stock- it's not clear from their websites, but store-pickup at these outlets usually has no additional cost even when it's not an item usually stocked. (I would think that it's a standard stock item in HI though, since cooling dominated climates are the biggest/best markets for these.) I'm not sure if HI offers rebates on them, but there might be some efficiency kick-back available. When there's a choice, bigger volume==better for both efficiency & function- the 50 gallon GE GeoSpring is probably a better choice overall than the 40, etc.

    I'm not sure if AO Smith has a local distributor, but the 80 gallon Voltex is another one to look into. It's about $2K from online sources, the 60 gallon Voltex runs ~$1500. The increased performance of a larger unit may be worth it, or maybe not, for a 2-person household. Most of them have an "efficiency mode" where the resistance heater elements are inhibited vs. "hybrid" mode where the resistance elements reduce the recovery time, and only the heat pump runs. If you get something big enough for your own needs it would be similar to your current solar setup- you only run it in "hybrid" mode when the guests are around otherwise leave it in heat-pump-only "efficiency" mode. Clearly 80 gallons works for you, but it could well be that 50 gallons does too, unless you have a big tub to fill. Unlike your current solar setup it's not time-linked to when the PV is putting out, you don't have to adjust your habits or the orientation of the PV array to accommodate the water heater.

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    General Engineering Contractor ballvalve's Avatar
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    1000$ to 1600$ for complicated water heaters that have a rather long history of failures seem like a life cycle cost analysis would handily eliminate them over a passive solar and 300$ electric water heater.

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    In the trades Dana's Avatar
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    A 12 year warranty and reduced AC load in the land of 25 cents/kwh is good enough for me. Clearly YMMV.

    The 50 gallon $1000 GE from Home Despot will pay for the $700 difference in price between that and a purty-good 50 gallon standard electric in reduced air conditioning + water heating cost WELL before the warranty period is up- probably in less than 3 years at HI electricity prices even in a 2- person water-sipping household.

    How long can that "long history of failures" be, pray tell? (Given that they're ALL new enough to still be within warranty!) Got any references to point to?

    Batch solar/passive solar is still not super cheap, about half the installed cost of active solar even for those without freeze protection.

    But go ahead, show us the lifecycle analysis of both alternatives, based on 25 cent electricity and a HI location, and DO reference your sources, s.v.p. For simplicity, assume an air conditioning COP of 4 for the whole house AC, and an EF of 2 (rather than the measured 2.3), if that makes the math easier, and assume you replace the hybrid &/or standard tank on a 10 year basis, 20 years for the passive solar.

    The fact that the hybrid removes more energy from the room air than it consumed from the power lines means you'd already need at least a 50% fraction from the passive solar + standard electric system just to match it for the hot-water heating power. If the COP of the AC is 2x that of the water heater, the reduced electricity use by the AC for that 50% of water heat is half, or another 25%. The batch solar end would have to exceed 75% of usage to almost-match the whole-house performance of the hybrid, and at those levels you have to design in heat dumping etc to avoid the steam explosions. You may able to support your assertions on the lifecycle cost analysis without some real systems and real data, but the lipstick on bathroom mirror math sez "No way, Jose!"

  9. #9
    General Engineering Contractor ballvalve's Avatar
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    In Hawaii, my lipstick says all hot water from the roof, or 90%. Now I don't know about you, but I just do not have time in my life to screw with phone calls to customer -non-support and wait for the guy to show when you have a life to live.

    The ge-ecoscrew has a pile of folks itching for a class action suit.

    I know you love heat pumps, but the complexity and cuts in manufacture make them a crapshoot now.

    I have a 1950's fridge that is the best on earth, huge copper coil on the rear. Probably run forever. Its a heat pump that was built by people without bean counters and outsourcing, and CARED about well done. Now its the bottom line, and the CEO got 200 million in stock options. In 1950, he got a living wage a bit higher than his factory floor workers.

    that 200 million +++++ to the CEO's of the heat pump companies make them all into ethically bankrupt prostitutes.

  10. #10
    DIY Junior Member albeas's Avatar
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    Quote Originally Posted by Dana View Post
    A 12 year warranty and reduced AC load in the land of 25 cents/kwh is good enough for me. Clearly YMMV.

    The 50 gallon $1000 GE from Home Despot will pay for the $700 difference in price between that and a purty-good 50 gallon standard electric in reduced air conditioning + water heating cost WELL before the warranty period is up- probably in less than 3 years at HI electricity prices even in a 2- person water-sipping household.

    How long can that "long history of failures" be, pray tell? (Given that they're ALL new enough to still be within warranty!) Got any references to point to?

    Batch solar/passive solar is still not super cheap, about half the installed cost of active solar even for those without freeze protection.

    But go ahead, show us the lifecycle analysis of both alternatives, based on 25 cent electricity and a HI location, and DO reference your sources, s.v.p. For simplicity, assume an air conditioning COP of 4 for the whole house AC, and an EF of 2 (rather than the measured 2.3), if that makes the math easier, and assume you replace the hybrid &/or standard tank on a 10 year basis, 20 years for the passive solar.

    The fact that the hybrid removes more energy from the room air than it consumed from the power lines means you'd already need at least a 50% fraction from the passive solar + standard electric system just to match it for the hot-water heating power. If the COP of the AC is 2x that of the water heater, the reduced electricity use by the AC for that 50% of water heat is half, or another 25%. The batch solar end would have to exceed 75% of usage to almost-match the whole-house performance of the hybrid, and at those levels you have to design in heat dumping etc to avoid the steam explosions. You may able to support your assertions on the lifecycle cost analysis without some real systems and real data, but the lipstick on bathroom mirror math sez "No way, Jose!"
    Thanks for providing a detailed comparison of the best water heating options in Hawaii. Unlike the other poster you actually have facts to back up the info you put out.
    I have a friend that is considering solar for a new house here on Oahu in Kapolei area (lots of sunshine). Conventional wisdom seems to be that solar water is the best bang for buck, followed by PV. I told him I thought he might be better off going all PV on the roof and using a hybrid water heater. My reasoning was primarily based on best utilization of roof space, and installation complexity and cost of all the separate plumbing that goes with solar hot water. I hadn't thought about the hybrid actually being more efficient. He always runs the Air Conditioning, so reducing that load would help. But I don't know whether the hot water heater will be in a garage or inside the Air Conditioning boundary of the house, so I don't know if it will actually contribute to cooling the house.

    On a separate note, I installed the GE hybrid heater over a year and half ago and it's been great. It dropped my average electric bill by about 1/3 from ~$150 to ~$100. After rebates the added cost of the hybrid paid off in less than a year. I did the install myself and it wasn't too bad. I'm also on Oahu and live in an old house built on a slope, with water heater underneath the house exposed to outside air. The old heater was a shorty 30 gal model, so the GE was too tall for plug and play. I poured another concrete slab right next to the old one but just downhill to allow more height. I then went to home depot and bought four 18" flexible hot water connection hoses and matched the gender on each end so that I could put two on hot and two on cold. This allowed me to use the existing plumbing connections that were directly over the old heater. The height difference also provides a natural heat loop.

    IRT the rant about product origin, mine was actually made in China. But GE just opened a new plant in Louisville, KY to manufacture all of their hybrid water heaters.
    http://www.industryweek.com/articles...aspx?ShowAll=1
    http://bluelivingideas.com/2012/02/2...-water-heater/

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    General Engineering Contractor ballvalve's Avatar
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    Well, good on GE! And that explains all the clamoring for the class action suit on the early GE chinese models.

    Many factorys are headed home because of shipping and the impossible management of quality. Not to the mention the cheapest natural gas on most of the earth.

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by ballvalve View Post
    that 200 million +++++ to the CEO's of the heat pump companies make them all into ethically bankrupt prostitutes.
    From what I've seen of ethical standards and financial accounting of U.S. based companies, the execs were "ethically bankrupt prostitutes" before they landed in the CEO chair.

  13. #13
    In the trades Dana's Avatar
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    Is shipping from Louisville to Hawaii any closer/cheaper than f.o.b. Guangzhou, Hong Kong, or Shanghai?

    I wouldn't hold my breath on that gas pipeline from the Wind River range or the Catskills out to the Big Island... :-)

    If the hybrid HW heater isn't parked inside the building envelope there's no AC benefit, but if you're a mid-day-showerer it might deliver somewhat higher efficiency if located outside in the heat during it's heating recovery.

    There are hybrid solar panels out there too- PV covered thermal panels, which can max the solar utilization of limited roof area, but the "rest of system" costs for solar HW is still likely to higher than the GE hybrid.

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    General Engineering Contractor ballvalve's Avatar
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    Quote Originally Posted by Dana View Post
    Is shipping from Louisville to Hawaii any closer/cheaper than f.o.b. Guangzhou, Hong Kong, or Shanghai?

    I wouldn't hold my breath on that gas pipeline from the Wind River range or the Catskills out to the Big Island... :-)

    If the hybrid HW heater isn't parked inside the building envelope there's no AC benefit, but if you're a mid-day-showerer it might deliver somewhat higher efficiency if located outside in the heat during it's heating recovery.

    There are hybrid solar panels out there too- PV covered thermal panels, which can max the solar utilization of limited roof area, but the "rest of system" costs for solar HW is still likely to higher than the GE hybrid.
    As to the shipping, yes, it is cheaper, and it makes jobs for YOU because your customers are not selling crack in the back alley, but turning wrenches in America. Are Americans so dumbed down by decades without a real war that they have forgotten that jobs AT HOME keep the dollars at home? Pathetic.

    Thanks for providing a detailed comparison of the best water heating options in Hawaii. Unlike the other poster you actually have facts to back up the info you put out.
    When your biased-"factual" poster's chinese GE piece of crap compressor dies just outside of or even inside of warranty, take a few weeks off work to try and get it fixed. Just search the disaster of the chinese little refrigerators on top the GE junksprings on any forum. dana has a special love for anything with a compressor in it, so your kudos is misplaced.

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    In the trades Dana's Avatar
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    Not so, the love affair with compressors- they're a PITA. But when heat pumps can provide a significant boost to efficiency, they have an appropriate place, and displacing resistance electricity use (especially EXPENSIVE resistance electricity such as in Hawaii) is one of those uses.

    I'm more partial to variable refrigerant volume systems with scroll compressors than the rattle-bang units used in refrigerators & hybrid hot water tanks. Standing next to the compressor of a 2 ton mini-split running at mid speed it can seem quieter than your refrigerator. The compressor is nearly silent, but you hear the fan. But being a DC-drive fan you don't even get much of the 60 hertz buzz you hear with old-school air conditioner units. I they would use scroll compressors rather than cheapo reciprocating units on hybrid hot water heaters they would be more conditioned-space friendly, to be sure.

    I'm pretty sure my customers aren't selling crack in the back alley, but blaming the state of the US labor markets on the "rise of the rest" is a mis-placed gross oversimplification. Manufacturing is still very strong in the US, but it has become highly automated, with much lower labor content than in 1960 or even 1980. The value-added of Chinese assembly is still a small fraction of the sticker price for something like a water heater, with most of the dollars going to the (US-based) parent company and distributors.

    I'm an electrical engineer- my customers are both domestic & international. A few years ago my company designed a chip for a company in India that was designing an avionics product for a UK company as part of a contract they had for a Czech aircraft manufacturer. The silicon was fabbed in Japan, packaged into military-grade ceramic in Taiwan, assembled onto printed circuit boards with other components in Italy, and tested in the UK before shipment to the client in the Czech Republic. My client was the firm in India. Most of the money in the deal went to the UK company. The Japanese & Taiwanese who did the most fundamental piece of the manufacturing got less than 10% of the total deal, but more than the Italians.

    As the Chinese get richer, their services cost more, and some of the manufacturing goes elsewhere, sometimes even back to the US where the communication is easier, deals happen faster, and there are lower shipping delays or costs etc. But I'd be surprised if the labor-hours content of the US manufactured version was anywhere near what it was at the Chinese plant, but the labor COST to the manufacturer might be higher.

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