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Thread: Solar Drainback Water Heating System

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    DIY Junior Member Alphacarina's Avatar
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    Sep 2009
    Location
    Biloxi, MS
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    26

    Default Solar Drainback Water Heating System

    I'd like some input from you guys who are familiar with drainback systems

    Currently, I have an open loop system which I'm going to convert to a drainback system to make things 'automatic' so I won't have to worry about draining my panel on cold nights. There are other benefits too, but freeze protection is the major goal

    As usual, before I make any changes I've been reading up on the subject and I find that I'm giving up some things I don't really want to (like efficiency) by converting away from open loop, so I'd like to minimise those losses. No matter what I do, it's going to use more electicity than my system does now because I'm going to need a whopping 1/8th HP pump for the new drainback feature in addition to my current 1/40th HP circulator. I'd like to increase efficiency and minimise that wasted electricity if I can

    With an open loop system, the collector is always full of pressurized water which gets heated as the panel warms in the sunshine - Very efficient . . . . nothing else comes close. One small 1/40th HP circulator which uses very little electricity. It can even be solar powered if you like

    With a standard drainback system, the panel is always empty when the pump isn't running, so the panel heats up much faster, since it's not heating any water. Once the cooler water from the drainback tank is introduced to the hot panel, it doesn't take long before the pump stops running and the water drains out of the panel again. You get lots of pump cycles on that high HP pump and it wastes lots of electricity to lift the water all the way up to the panel each time the pump cycles - About a 25 foot lift in my case. Lifting a column of water 25 feet every time the pump cycles on makes no good sense . . . . at least not to me

    What I would like is for my panel to stay full of water until there is no more opportunity for solar gain . . . . so that anytime the sun is shining on my panel, it's heating WATER and not just an empty panel. Heating an empty panel and then filling it with water isn't good for the panel - You get lots of rapid expansion and contraction when you fill a hot panel with water suddenly a hundred times per day and that ultimately shortens the life of the panel

    I would like the panel to be full all day and then a single drainback when the sun goes down and then only one refill for the day the next morning once the collector temp says there is energy to be harvested. Fewer pump starts and stops and only one 'big lift' each day . . . . less energy wasted, fewer cycles on my pump, no more filling a hot panel with cooler water a hundred times per day - What's not to like?

    There are some very sophisticated, pretty complicated, quite expensive, patented commercial setups available which do exactly what I want . . . . keep the panel full all day and then drain it every night - But I'd prefer something much simpler and something I can do myself without spending a fortune. I have a complete open loop system which has been working very well for the past 3 years that I designed myself, built with readily available parts which didn't cost me a fortune - I probably have about $2500 in the whole thing. A single 40 sq foot collector, two very well insulated 55 gallon storage tanks, a timed switch to add energy if needed after a couple days of no sun, a single 1/40th HP circulation pump and a pretty sophisticated, fully adjustable differential controller - Actually I have 2 different controllers and I play with them all the time, tweaking things for best efficiency. My system provides nearly 100% of my needs for about 9 months per year and a pretty large percentage of what I need during the winter 3 months

    Back to my proposed drainback system - What I would like to do is put a check valve just above my drainback pump so that the distilled water it pumps out of my drainback heat exchange tank up to the panel cannot drain back everytime the pump stops . . . . the feed pipe and the panel would remain full of distilled water. The rest of the system design would be similar to a standard drainback. Two differential contollers . . . . one to control the drainback pump and run it when the collector temps are hotter than the bottom of the drainback heat exchanger and a second one to circulate the domestic water through the drainback heat exchanger and into the storage tanks whenever the drainback tank temp is hotter than the bottom of the storage tank. A 3/4 inch pipe from the top of the collector would allow the heated water to fall back into the vented drainback tank whenever the pump is running. I have a vent at the top of the panel now which would also remain . . . . a second air source to make sure the panel drains completely . . . . when I want it to at the end of the day

    My 'secret weapon' would be a tee just above the check valve on the output of the pump which would lead down to an electrically operated valve, which when opened would allow the collector to drain through that valve around the pump and back into the drainback tank. I'd like for that valve to be a 12 volt DC valve which needs 12 volts to remain closed - You can probably guess where I'm going next . . . . a PV panel on the roof to generate the voltage to keep the valve closed. When the sun goes down, the valve opens and the collector drains

    I have some 'fail safe' ideas to make sure the panel still drains in the unlikely event the 12 volt valve should fail in the closed position. I also have a plan which would prevent it draining the panel everytime a cloud hits the PV panel but those are just little 'tweaks' I can play with and perfect over time and they don't affect the basic premise of the system operation

    Any discussion pro or con on my idea(s) is welcomed . . . . especially if you have any suggestions to make it better

    Thanks,

    Don
    Last edited by Alphacarina; 12-22-2009 at 09:03 PM.

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