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solar electric to power electric water heater

Post in 'The Green Room' started by smabon, Feb 5, 2012.

  1. smabon

    smabon New Member

    Joined:
    Jan 29, 2009
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    Loc:
    Lunenburg, Ma
    So I have been heating my house with wood for the last three years and the only thing I now use my oil boiler for is hot water. I would love to stop paying for oil. I have been wondering if the is a small electric solar system that I could get to power a new electric water heater. I know nothing about solar power. Is my idea a waste of time? How big of a solar system would I need. As usual money is tight. Would I be able to get a solar system for under $1000 not including the water heater? I imaging that there is a way to set it up so that if there isn't enough power stored up from the solar system that the house electricity would take over. I also might use the electric solar system to help run the blowers on my stove. Any info would help. Thanks for your time.

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  2. DBoon

    DBoon Minister of Fire

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    Jan 14, 2009
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    Take it in steps, and do some of the work yourself. Direct electric resistance HW heat is roughly the same cost as oil at $3/gallon, and electricity is probably more competitive if you are only using your boiler for hot water.

    Look at getting a good electric hot water heater - Marathon makes one that will last a lifetime.

    Cut your usage any way you can with low flow faucets and pipe/water heater insulation. Put a heat trap on the output of the hot water heater. Make all future appliances the most efficient ones you can afford.

    Next step (as money permits), add a solar hot water panel (you could build this yourself to save money), or a heat pump water heater (reduce electricity costs by half, but not so efficient in a northern climate in the wintertime).
  3. jimbom

    jimbom Combustion Analyzer

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  4. peakbagger

    peakbagger Minister of Fire

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    Do the math, once you can fill in all the details I expect you will figure out the answer.

    PV panels are about 15% efficient at converting sun light to electricity. When you figure in the rest of the system to convert DC to AC and line losses , figure on 70% of the power going into the panels come out at AC power to the electric heater. Compare that to a Solar hot water system which is in the 60% efficient range at converting sunlight to hot water. If you want to continue on with solar electric, search for the PV watts program and punch in your data. It will figure out how much power you will get from a system in your specific location. Using that you can figure out waht size PV array you need.

    I can save you a lot of time and tell you its not a good idea. Spend your money on solar hot water system. If you get the right tank, you can get one with electric backup and bascailly can shut down the boiler for 6 months. I have a coil of wire and breaker for my backup electric element and have never hooked it up.

    By the way if you are set up with tankless heater set up on you boiler where the boiler has to be kept hot all summer to get hot water, serious consider putting in a "hot water maker" sytle tank where the boiler heats and insulated tank of hot water via a seperate zone and switch you boiler to cold start. Before you do it make sure it can be cold start by manully shutting it off when you dont need it and see if it leaks, some older boilers can be cold start as they leak whne they are cycled but others can.
  5. semipro

    semipro Minister of Fire

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    I agree with Peakbagger. Collecting heat in its native form for use as heat is much more efficient and less costly than going PV.
  6. Slow1

    Slow1 Minister of Fire

    Joined:
    Nov 26, 2008
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    Loc:
    Eastern MA
    Being that you live in MA there is one wrinkle in the whole calculation that you should look at. MA has a rather unique SREC system in place in that it has a 10 year floor mandated. This means that if you put in the 'right' system for PV solar you can get significant long term SREC payments that are worth at least 2X the value of the electricity. This seriously changes the equation. Unlike markets where the floor fell out of the SREC prices, this one is mandated to hold - right now they are trading much higher, but don't count on that long term.

    At any rate, the point here is that you may be able to put up a PV array that pays for itself in 5 years or so. What you end up doing with the power is up to you.

    To the decision on using it for heating your water - you may want to consider an air-source heat pump. Assuming the location is heated by your wood and/or your don't mind cooling the air wherever the hot water tank is then you can reduce the amount of electricity required to heat the water quite a bit. Down side here of course is that the heat is being pulled from the home - this could be an issue in the winter.

    The good side of going PV to collect the energy (despite the inefficiency of collecting) vs direct heat via hot water collection panels, is that the PV is much simpler and thus easier to maintain. Of course I have heard that 'modern' solar DHW systems are much better and easier to maintain than older systems.

    Best of luck, whatever you decide.
  7. GaryGary

    GaryGary Feeling the Heat

    Joined:
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    Loc:
    SW Montana
    Hi,
    Agree with the others that for heating that solar thermal is more efficient and cheaper.

    Just as an example, I have a DIY PV system and a DIY solar water heating system. Just by coincidence they both produce about 3300 KWH of energy a year (about $330 worth where we are).

    The 2.2 KW PV system (2 years ago) cost $10,000 before rebates and $6500 after federal and state rebates.

    The solar thermal system cost $1000 before rebates and $500 after the the state rebate.

    Now days, the PV system would be a bit cheaper as PV has dropped since I did mine, but the basic idea is that DIY solar thermal water heaters are about 10 times more cost effective than DIY PV systems for heating water.

    http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm

    Both systems are very low maintenance.


    Gary
  8. Rhone

    Rhone Minister of Fire

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    Nov 21, 2005
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    824
    x2 on PV. The cost & inefficiency is the reason I'm not a fan of them. However, sunlight converts to heat very well so solar hot water panels make a lot of sense. Since you live in MA, you can DIY but I don't know about doing it for $1000. I got this solar kit http://www.radiantsolar.com/solar_packages_and_pricing.php but you can see the individual pricing from http://www.radiantsolar.com/solar_component_pricing.php. There's 2 types of solar water heating, one is a drainback system the other is closed loop glycol. I prefer the latter even if slightly less efficient. Since MA does not allow you to do your own plumbing for anything tied to potable water or sewer, if you do the closed loop glycol you can do it yourself. A drainback would have to be installed by a licensed plumber since it is tied to potable. That's something to consider (and the glycol in the system has to be special glycol that's safe to drink).

    Since MA incentives are based on the output of the panels and State & Federal credits must be SRCC rated to quality, it makes the most sense to buy pre-made SRCC rated solar panels. If from that company, after federal & state incentives each 4x8 panel (30+ year panels, aluminum frame, special glass, copper, special coated fins, and condensation removing device) would be $24 each panel then you'd need the mounts (which you'd get get 30% federal tax credit on). So, figure about $40-$50 a panel in the end and 30% off everything else because now you're using SRCC rated panels. I recommend the Flat panel style, not evacuated tubes.

    The rest I personally prefer a kit. You have to handle overheating (which is much less a problem with FP). Have a means to fill and drain, design it so you can shut off & isolate the motor for replacement, prevent backflow, prevent thermosiphoning at night, I enjoyed the kit as all I had to do was run the pipe. My friend did a DIY system (evacuated tubes) and hasn't had as much luck as other DIY have posted (probably because he went with evacuated tubes). He's ended up three times where his system overheated and dumped steaming glycol all over his basement (twice because of a power failure), and his tank overheated and dumped steaming water all over his basement once when he left for a few days. I would stick with flat panels.
  9. GaryGary

    GaryGary Feeling the Heat

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    SW Montana
    Hi,
    So, the incentives in MA bring the price for a solar collector from about $800 per collector down to $40 a collector? That is one sweet deal -- or am I reading that wrong? That would certainly be a good argument for going with commercial collectors.

    You made the case for closed loop glycol kit systems -- I'll make the case for A full DIY drain back system:

    This is the drain back design I use: http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm and the newer version that does space and water heating: http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm
    These systems are full DIY (not kits), and lots of people have built them for around $1000 (before rebates) -- some of the documented here: http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Example1KSystems

    - The drain backs of this type are simpler and less expensive and have fewer components than closed loop systems (no expansion tank, PRTs, or fill valves).

    - If the plumbing for the drain back is installed correctly (with all lines sloping toward the storage tank), they are essentially maintenance free and provide bombproof freeze protection. The closed loop systems required yearly checks on the antifreeze and replacement if it has degraded. If you don't do the checks, and the antifreeze goes bad, it can damage the system in a couple ways.

    - Drain backs are less susceptible to overheating in that the controllers allow a maximum tank temperature to be set, and once that temperature is reached, the pump is shut down and the collector drains back to the tank. The collector is subjected to high temperatures, but it should be built to take these temperatures. With glycol systems, there is always antifreeze in the collector, and when that antifreeze heats beyond a point 1) the antifreeze can degrade from the high temperatures, and 2) the pressure/temperature relief valve can vent and make big green puddles.

    - Given that its inexpensive to add some additional collector area and tank capacity, its easy to oversize the systems and get a better yearly solar fraction. It only costs about $6 per sqft to add collector area and a bit over $1 per gallon to add tank capacity, it cheap to add another 30 sqft or more to get better winter performance.

    - This drainback design does not need a heat exchanger between the collector loop and the heat storage tank, and that coupled with the better heat transfer capability of water vs antifreeze makes for an efficiency gain.

    - Since there is no antifreeze in the system, there is no risk of someone using the wrong (toxic) antifreeze which coupled with a heat exchanger failure could put toxic antifreeze into the potable water side.


    On the negative side, there are times when it does not make sense to use a drain back -- mostly when the plumbing can be run with a slope toward the tank, or when the collectors are mounted WAY above the heat storage tank. Drain backs also typically need a somewhat larger pump than closed loop systems.

    I don't understand the bit about a drain back requiring a plumber and closed loop not? Both systems are connected to the potable water, else how would they heat it? Can you clarify that?

    All that said for the full DIY approach, I am in the process of adding a section to Build-It-Solar that has some good kit approaches for people who just don't want to build the whole thing. If anyone has had a good experience with a kit, or has seen one they like, please let me know.

    Gary
  10. Jerry_NJ

    Jerry_NJ Minister of Fire

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    Loc:
    New Jersey USA
    I didn't read all posts in detail, so my point my already be covered. If, just count me as in agreement with that post.

    If you are going to heat water only when the sun is out, cut out the middle man (the electricity) and heat the water in a solar heat collector. I know in the summer my garden hose that is just rolled up on the side of the house, it isn't black and it isn't spread out with a southern exposure, and it will get the water in the hose hot enough to burn the hands in an hour or two of sunshine.

    I believe solar water heaters/collectors are widely available, or one could just use black water pipe and a pump. In cold weather the pipes would have to be protected, under glass and it may be necessary to drain the collector on real cold nights.

    Other than a pump, if one is needed, there are no moving parts, should last forever. I understand in hot sunny climates like Israel this type water heater is widely used.
  11. colebrookman

    colebrookman Minister of Fire

    Joined:
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    Loc:
    Middlefield, Ma
    All that said for the full DIY approach, I am in the process of adding a section to Build-It-Solar that has some good kit approaches for people who just don’t want to build the whole thing. If anyone has had a good experience with a kit, or has seen one they like, please let me know.Gary

    I'm anxious to see that section, I'm trying to heat my 250 gal. hot tub in a way that keeps it simple. Many thanks for your great web site Gary. Be safe.
    Ed
  12. GaryGary

    GaryGary Feeling the Heat

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    Hi Ed,
    Thanks for the reminder -- I've got some material for the section, but still not complete.

    I'll add a note to this thread when its up.

    Gary

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