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Using solar thermal tanks that don't pressure test

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  • Using solar thermal tanks that don't pressure test

    I have 3 old but never installed solar thermal tanks, 82 gallon and 2 -120 gallon, that don't pressure test on the panel side. They are electric assist. I'm guessing that some time in their life, 30 years, that they got laid on their side and transported, thus the coil broke, probably at the top of the coil. I'm installing a glycol pressurized system and have a new tank that will replace one of the others. Not sure which one to replace but probably the 82 gallon tank.
    My question is can I keep 2 of the tanks, plumbing them to the new tank as additional storage, using the one good coil as the source of hot water? I have another 80 gallon tank that also doesn't pressure test but I've caped off that coil and just use the electric assist to heat the water. That system has a wood stove insert as a backup and only heat a 525 square foot space. That system works well.
    I'm skeptical that I can heat an additional 200 gallons of water from one good tank connected to my 10 solar panels without relying completely on the electric assist from all three tanks.

    My house is 1800 square feet, adobe, slab on grade with insulation under the slab. Current heat source is a 24KW electric boiler with infloor heat. That boiler sucks a lot of juice on a cold night. I'm hoping that this solar system will replace most of the need for the electric boiler.
    Any advice is greatly appreciated.

  • #2
    When you write "tank", do you mean a tank with a coil internal to the tank that has fluid circulating through that coil and also circulating through the solar collector(s) ?

    Technically, tanks with internal coils that act as heat exchangers are known as batch heaters (or batch coolers). The size of the heat transfer area ( the actual surface area of the coil) is primarily determined by the surface area (size ) of the solar collectors array. The larger the array, the more heat transfer surface (the coil area) you'll need.

    Your skepticism is well placed.Too small a heat transfer surface (or too large a collector surface for the amount of heat exchanger surface - in this case, the coil surface area), less heat will be transferred to the water in that one tank.

    Also, with only one coil in one tank, unless you've got a recirculation pump on the potable water in the tanks, the water in the other two tanks will not get much, if any heat.

    What you have is an exercise in heat exchanger sizing and some heat exchanger design. For a 10 collector system, a single coil will very likely be grossly undersized.

    I'd give serious consideration to an external shell and tube heat exchanger with the glycol circulating on the shell side and the potable water on the tube side. Unfortunately, a decent S & T HX will probably cost a lot more than the energy it will produce over its life.

    Short answer. What you have now won't work anywhere near adequately. Reason: Not enough heat transfer surface.

    Before you go further, get the design and annual heat load for the dwelling. Then determine how much of that load you want to displace with solar. Then determine how much solar energy is available at your site and design a system to meet that need based on available solar resource and climate. That system design includes the heat exchanger design and sizing.

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    • #3
      Thanks J.P.M. I do plan to use a recirculating pump to equalize the temperature in all 3 tanks. And I will be relying on the heating elements in all three thanks as backup. However, I'm not sure I understand your suggestion about an external shell and tube heat exchanger. Some more information would help.

      I live in Colorado at 7300 feet elevation and solar energy is abundant. We have maybe 25 or 30 cloudy days here where I live. It is a high mountain desert.

      The house is my design and build and I have a pretty good idea of the heat load if you are referring to electricity use. If I could offset 1/2 of that with this thermal system, I would be happy, more, the better. But I've done no design and sizing calculations I acquired this system from a guy who needed to dump it because he was leaving the state. I got enough copper pipe and fittings to more than make up for the cost of acquisition. I jumped into the project without doing the research on how to use what I have to best advantage I figured with 300+ gallons of hot water, I would have heat to spare. Then I discovered the problem with the tanks. So I'm now trying to make the best of this situation, understanding that one good, tested tank was not enough to service my needs given the size of the collector system.

      I'm still optimistic that the one good tank can heat the glycol in the other 2 tanks, using a recirc .pump appropriately sized. It adds another dimension to the project, and based on your comments, I think it will work. Maybe not to my expectations but at this point I'm kinda out of options. I'm not in a position to add 2 more new 120 gallon tanks. I gotta go with what I have.

      Thanks for your help. Your comments are valuable. Please clarify the external shell and tube heat exchanger.

      Comment


      • #4
        Originally posted by Stephentk View Post
        Thanks J.P.M. I do plan to use a recirculating pump to equalize the temperature in all 3 tanks. And I will be relying on the heating elements in all three thanks as backup. However, I'm not sure I understand your suggestion about an external shell and tube heat exchanger. Some more information would help.

        I live in Colorado at 7300 feet elevation and solar energy is abundant. We have maybe 25 or 30 cloudy days here where I live. It is a high mountain desert.

        The house is my design and build and I have a pretty good idea of the heat load if you are referring to electricity use. If I could offset 1/2 of that with this thermal system, I would be happy, more, the better. But I've done no design and sizing calculations I acquired this system from a guy who needed to dump it because he was leaving the state. I got enough copper pipe and fittings to more than make up for the cost of acquisition. I jumped into the project without doing the research on how to use what I have to best advantage I figured with 300+ gallons of hot water, I would have heat to spare. Then I discovered the problem with the tanks. So I'm now trying to make the best of this situation, understanding that one good, tested tank was not enough to service my needs given the size of the collector system.

        I'm still optimistic that the one good tank can heat the glycol in the other 2 tanks, using a recirc .pump appropriately sized. It adds another dimension to the project, and based on your comments, I think it will work. Maybe not to my expectations but at this point I'm kinda out of options. I'm not in a position to add 2 more new 120 gallon tanks. I gotta go with what I have.

        Thanks for your help. Your comments are valuable. Please clarify the external shell and tube heat exchanger.
        You're most welcome.

        I'd respectfully suggest some serious education before you do anything. Not my house/project/life/money/business, but look before you leap. Then look some more. For starters, in spite of what you may think, 300 gal. ~ = 2,500 lbm of H2O is not much storage for 1800 ft^2 heated space, particularly in a colder climate. Maybe a few hours worth of heating or less, depending on building heat loss.

        The heat load I'm referring to is how much heat it takes to maintain a comfortable inside temp. when the outside design temp. is reached. Other building energy loads such as lights are loosely related to that building heat load. How that load is supplied is a separate question to what the load actually is. So what's the design heat load, and what's the annual heat load ? Hard to design a dwelling without knowing what those #'s are.

        Meeting that building heat load with resistance heating - that is electric heating elements - is very inefficient and wasteful - kind of like cutting butter with a chain saw. Do that as a very last resort, except for MAYBE spot heating, like electric blankets, or under desk foot warmers.

        Be optimistic all you want, but without some information and ability to understand some basics about heat loss, insulation, solar thermal engineering and building heat loss/gain load calculations you will be spinning your wheels, probably in the wrong direction.

        BTW, adobe is a pretty good material for thermal mass, but a relatively poor insulator. You want a building with a low heat loss rate, not necessarily a thermally massive building. The two are not the same. Because a building will stay warm for a long time does not make it energy efficient. It'll take as long to heat a cold building as cool a warm one.

        As for an external heat exchanger, it's just that, external to a storage tank. Warm fluid goes in one "side" - the sides being either hot or cold - and colder fluid goes through the other "side". Both fluids are usually pumped.Heat is transferred to the colder fluid mostly by forced convection of the fluids and thermal conduction through the separating wall between the fluids. Both fluids are pumped through the HX, usually, but not always in "opposite" directions. Shell & tube is one simple design of probably hundreds of types. See Google.

        If I lived in CO @ 7,300 ft., Id think about a passive, solar tempered, highly insulated, high mass design, but you have what you have, and opinions vary as do choices.

        Good luck.

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        • #5
          Thanks.
          I'm researching your suggestions

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          • #6
            Originally posted by Stephentk View Post
            Thanks.
            I'm researching your suggestions
            You are most welcome.

            Last thing: It is not wise way beyond any economic reasons to use equipment that cannot pass pressure testing.

            Comment

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