Solar Water Heating: Which way would you go?

Discussion in 'Off Grid Living' started by Asia-Off-Grid, Nov 24, 2016.

  1. Asia-Off-Grid

    Asia-Off-Grid RIP 11-8-2018

    Greetings Fellow Primates:

    I want to pipe, or wire, a solar water heating system at the farm. Considering how cheap solar panels are these days, I have been considering using electric panels and heating elements, rather than to pipe a thermosiphon (passive) or pump (active) system.

    First, money isn't really an issue here. I don't think the cost difference would be much, either way. In fact, I would have to build the solar collector for the water pipes to be run through, if I were to use a passive or active system. I would also have to pipe the entire hot side from the tank to the collector, and then into the existing water system. So, it does seem as though it would be a much easier route, to toss a panel or two on the roof, and wire it / them to a heating element, or two, in a tank. Then, just wire that tank into the existing water system.

    So, which way would you folks go, if you were starting fresh? Active / Passive water heating? Electric Panel water heating?
    UncleMorgan likes this.
  2. Tempstar

    Tempstar Old and crochety Site Supporter+

  3. UncleMorgan

    UncleMorgan I like peeling bananas and (occasionally) people.

    Going solar is always a tradeoff in cost vs. efficiency. Flat plate is less expensive and less efficient than tracking panels, etc., for just about every design choice.

    Going solar-electric for heating usually costs more than going solar-thermal for heating.

    With solar-electric you have the cost of an expendable battery bank that will have to be replaced over a period of years--but you can use the stored electricity for anything.

    Solar thermal has no battery bank, and a bulk hot water storage tank is pretty cheap. But you can only use the heat for hot water and area heating.

    Before you can realistically pick a system, you'll need to know exactly how much energy you'll need to harvest--regardless of the way you harvest it.

    Your location and annual sunfall make a big difference, too.

    If price is no object, I'd look into a Tesla Wall and Tesla Solar Roof. If they aren't taking orders yet, they soon will be.
    BlueDuck and Asia-Off-Grid like this.
  4. Asia-Off-Grid

    Asia-Off-Grid RIP 11-8-2018

    Idea of cost? Estimated only, say 40 gallons?

    Sorry. I meant price was not an issue between solar electric, and passive / active solar collector installation.

    Building the collector wouldn't take a lot to do, I'm sure. But, an electric generating panel wouldn't need batteries, either, to heat the water. It could just heat during the light of day. Use all the water we want then, and shower in the evening. Wouldn't really even need hot water until the following day. However, storing it in an insulated tank wouldn't be a bad idea either. The sun would easily heat 302 to 455 liters (80 to 120 gallons), before lunch.

    The type of collector I am considering, would be more like a pool heater. It would be two or more panels arranged in series.


    Last edited: Nov 24, 2016
  5. UncleMorgan

    UncleMorgan I like peeling bananas and (occasionally) people.

    What I did (for max cheap & easy) was snag an old pressure tank from a well--the kind that doesn't have a bladder in it--and put it in a box along the south side of my house. I insulated the box with ordinary fiberglass batting, and slapped a piece of scrap glass across the front. Painted everything black inside and then spliced it into the feed to my standard electric hot water heater.

    So however much pre-heating I get chops the top right off the electric bill for water heating.

    I been saving about $125.00 a month ever since I put it in, and it cost about $65.00 to build.

    One was fine for household hot water. For a pool or space heating, try four hooked up in parallel, with a pump for fast circulation.
  6. Airtime

    Airtime Monkey+++

    For the simple task of collecting solar energy to be used to generate hot water, the efficiency of solar collectors (evacuated tubes or flat panels) are way more effective per square foot compared to converting the energy into electricity and then using it to resistively heat water.

    PV panels are generally in the area of 16-17% efficient converting the solar energy into watts of electricity.

    Flat panel collectors can be in the ball park of 70-75% efficient capturing the solar energy.

    So, assuming the losses in the systems are comparable (losses in the wiring vs loses of heat in the pipe runs) for both systems, you will need about 4 times the area of PV panels to yield the same amount of hot water as flat panel collectors. Evacuated tube collectors work a bit better in cold ambient temperatures and can be more efficient than flat panels, they do cost more. You can make pretty good flat panels yourself rather easily too.

    To just heat water for domestic use (don't need water over 150F) it's a no brainer to me, flat panel collectors and several hundred gallon hot water heat storage tank (also easy to build).

    Asia-Off-Grid likes this.
  7. Asia-Off-Grid

    Asia-Off-Grid RIP 11-8-2018

    Fair enough. Okay. You've convinced to go that route, a collector.
    Thanks, fellers. I truly appreciate your input.
  8. azrancher

    azrancher Monkey +++

    Right, a flat plate 4'X10' collector is the "best" way to go, yes vacuum tubes are more efficient, but cost wise simple is best, the 70's had all kinds of collectors, the water tank in a box, flat plate, and tracking polished aluminum troughs,... the troughs are most efficient, however they move, moving stuff needs maintenance, you don't maintain it, it is worthless.

    I use 8 - 4'X10' Gobi collectors.

    Altoidfishfins likes this.
  9. Cruisin Sloth

    Cruisin Sloth Special & Slow

    I use Collectors (evacuated tube type), times 5 around here . NOW in the winter wetness , were airtight jacketed DHW .

    ONLY use electric PV or turbine for a diversion load or a waste not (Midnite controller )
    Winter time , HOT showers for everyone !!!
    TinyDreams and Ganado like this.
  10. arleigh

    arleigh Goophy monkey

    i did an experiment years ago,
    I made a flat panel using 5/8" copper tubing and a double glassed boxed in 10 gallon tank.
    In the same light at the same time the panel produced 160 degrees and the tank produced 130 degrees.
    Since the tank held more water we choose it to supply the house at the time .

    Ideally preheating with the panel then to the tank is superior in out put.

    If I were better off I"d get a heat exchanger and fill the solar heaters with antifreeze and the heat exchanger could heat the water inside the house , eliminating the problem of freeze damage .
    TinyDreams likes this.
  11. Tempstar

    Tempstar Old and crochety Site Supporter+

    If I remember correctly, the 10 tube collector was around $600. It came with a spare tube as well. The last system I installed was 2 panels, 120 gallon tank with the solar loop inside, pump and expansion tank. We heated the master bath floor and supplied the input to the Rinnai tankless which hardly ever ran.
    The one on my house is now 10 years old and my water heater has been off for 10 years. We drain the hot water in the mornings with our 2 showers and it's good to go again by 10 a.m. if the sun is out, noon if it's cloudy, and 5 p.m. if it's rainy. It will on occasion cause the TP valve on the water heater to pop in the summer.
    Nicest thing is that the tubes contain no water.
    TinyDreams, Asia-Off-Grid and Ganado like this.
  12. Cruisin Sloth

    Cruisin Sloth Special & Slow

    I have that system , I use Propylene glycol . Still can be poison
    Pressure in the collector system is one 1.5 BAR (one BAR= One Atmosphere of 14.7 psig) House system is 3.7 Bar ,so if the heat exchanger fails , the collector side will be over pressured & the alarm will sound . keeping the Propylene glycol in the system loop & the pressure accumulator and gauge will show a DHW pressure & in the low pressure heat collection side.

    Built mine 10 + years ago on a 3 story roof on my rancher house. PV panels to spin the solar transfer pumps .
    Tempstar likes this.
  13. Cruisin Sloth

    Cruisin Sloth Special & Slow

    UPDATE TO 2020 updating an old thread ( Heart slump thinking about "Paul")
    I pulled all off and went with PV to heaters , much less work / maintenance
    Never would i go with collectors for DHW.
    Meat likes this.
  14. Meat

    Meat Monkey+++

    Dang. RIP.
  15. TnAndy

    TnAndy Senior Member Founding Member

    How about a good description of what you did, as I'm going to do something soon. I have 20 -175w panels I took down (Replaced with 300w) looking for a good excuse to use. What do I need in the way of heating elements, controls, etc ?
  16. Cruisin Sloth

    Cruisin Sloth Special & Slow


    I used two 10 watt solar panels to control the electrics , meters , relays etc.

    This is to ensure the heaters can be turned off when 140 * has been reached .

    All the panels are in parallel. and the pair of 10W 12Vdc solar units put out 17Vdc and one amp max. So the meters and relays amp gauge all have ample power.

    I used 40 Amp DC relays from Volvo 2001-2007 cars to feed the heaters , 2 heaters , 2 relays , each DHW tank has a AC thermostat and will not arc or burn the contacts on the power the relay draw 30-55 Mila amp . the relays are to close the circuit so they are normally open. Once the sun hits the small 10 W panels the system comes alive to show the voltage of the MAIN 4 panels of 350 Watt in parallel , no amps till the relays close and that’s when the 10 watt units reach enough power and the 1400 Watt main panels are producing . I can see it start at 5 amps at 28-32 Vdc then ramp up to 30 Amp max at 36.9 Vdc giving me 1100 watts of heating .

    MC4 feeds 4 strings #10 gauge thru 15 amp glass car fuse rail (pico 941-11 , 30 amp per fuse max ) then the output side go’s to a #2 gauge cable down 2 floors 25 feet (large cable size for less voltage drop) feeds 50 amp breaker #4 gauge soft copper welders wip wire), then thru 2 amp meters , one analog hard wire and the other is Digital to relays Ground is the same but no fuses. The 12Vdc panels is similar but the fuse is down in the grey control box . the grounds of both panels are connect . the 12 volt side feeds the thermostats that control the relays and the meters .

    IMG_2774. IMG_2775. IMG_2777. IMG_2779.

    Attached Files:

    Last edited: Dec 13, 2020
  17. TnAndy

    TnAndy Senior Member Founding Member

    Panels are Solarworld 60 cell, mid 30ish volts by 5-6amps.....they're packed away right now so I can't get to the specs off the back. I used them in pairs on a 24v setup, they did 68-70 volts on the Outback CC input side.
    Looking forward to your post.

    Thanks !
  18. Cruisin Sloth

    Cruisin Sloth Special & Slow

    Close enough Andy , I ran mine in strait parallel due to amps and the heater element is a direct short or 1.2 too 2 Ohm's , the 120 or 220 volt units in a DHW tank are replaced. Only part i use in the original system is the thermostats , top and bottom , Now I have never had my bottom one ever open up the circuit .Day is not long enough here.
    Im replacing blades on a shear today and maybe sunday , hence why im ready to leave the ranch. Never been busier now im retired from what ,Work without pay !
  19. Cruisin Sloth

    Cruisin Sloth Special & Slow

  20. DKR

    DKR Raconteur of the first stripe

    My brother had a place in Tucson (6610 E Speedway Ave if you want to look it up) that was solar heated with flat panels.
    The system also provided domestic hot water. over 4K sq ft interior.

    He sold it and the new owners ripped the system out for being "unsightly" - the house now uses a natgas water heater designed for commercial dishwashers to provide hot water to the heating loop.

    So, even in AZ, not many use solar heating, a shame...

    had fun windows tho..
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