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  1. #1
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    Heatpump in a greenhouse = efficiency?

    I'd welcome anyone's thoughts concerning building a small greenhouse for a heatpump to increase its efficiency. I'm just now reading-up on the science behind a greenhouse, so anyone good with numbers willing to run 'em by this concept is welcome.
    Think of it! It's 10 degrees f. outside, but the heatpump is in a 60 degree f. greenhouse and even benefits at night through re-radiation from the stored heat in the waterwalls?

  2. #2
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    The green house won't be 60 inside if you have a heat pump in it absorbing all the heat.
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  3. #3
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    Quote Originally Posted by beenthere View Post
    The green house won't be 60 inside if you have a heat pump in it absorbing all the heat.
    That's where "running the numbers" comes in. Obviously if the greenhouse is big enough it will be 60 because we are absorbing more btu's than we are using. And how about downtimes wherein the heat would just getting stored?
    I'm still reading up on greenhouses and waterwalls to try and figure out how big it would have to be.

  4. #4
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    How big of a green house, would depend on how big of a heat pump you had.
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  5. #5
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    Will you tear the greenhouse down in the summer. If not the heat pump efficiency will drop because it is 130 degrees in the greenhouse all summer. :-)

    Seems like a lot of work for very little benefit.

    Why not just buy a more efficient system?
    Remember, Air Conditioning begins with AIR.

  6. #6
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    Don't know about very little benefit. It could have louvers in summer for proper air flow with maybe just a plastic roof to keep the sun off the area. This is sunday, things have been slow, and this seemed like a good topic for discussion. Heck, they're using passive solar systems to maintain houses comfort levels without even having an hvac system.
    On a zero degree day and it's even 40 in the greenhouse spells more than a little benefit in my opion. Especially in some unique situations.

  7. #7
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    What about if you put some thermalite blocks in there to absorb and help retain the heat? You know the kind you find in storage heaters. Or was you thinking about putting a pond in there and having a water cooled condenser fed from the pond as a ground source. I suppose you could dig out the hole and line the pond with clay, and additionally line half way upwards to the top with these blocks or some other kind of insulated block, you would probably have to water proof them. Then at night time or when the ambient drops any ground heat source would be free to conduct into the water from below the blocks without the insulating effect of the blocks stopping the heat to conduct. You could fit a ball cock piped from your water mains to fill it up, then you could help maintain it's level with water piped from the top water from your gutters and let it run off through an overflow, then the ball cock will act as back up.

    Put a gantry type of removable floor grill in there, and have insulating blinds/louvres cover the green house windows powered via small motors controlled by one or more photocells and outdoor/indoor temp sensors via a controller to help cut down any heat from escaping. Then for warmer weather a secondary matrix with a fan could be put in parallel with the pump and condenser water tank and be isolated from the pond with a sol valve controlled by a thermostat if the water gets too warm. And the rejected heat could be ducted out or used to supplement a secondary heat system. Also to cut down on solar gain the blinds could be closed and ventilation louvres opened for via the sensors and controller.

    One of the questions asked whilst sitting the City and Guilds 6078 exam was about a deep garden pond having a theoretical temperature of 4 deg C at the bottom and -3 deg C on the frozen surface as an average. Would it be suitable for a groundsource for a heat pump system with a submersed condenser with water fed from the pond. That was one of the easier questions, But I don't know how deep the pond should be to attain this temperature in the real world. Or if anyone has actually done it and found it was worth the effort and expense.

    A factory where I once worked had a bore hole system with a pump submersed 300' below the ground that fed water through a 12' pipe up through the ground to holding wells in the floor of the factory, then pumped it to various processes. The holding wells at a guess were about 0 - 3 deg C in winter time within an ambient temperature of about 10 - 14 deg C inside, and about -5 - 0 deg C outside, but when you felt the water temp from the bore hole pump it was noticeably higher, maybe about 2-3 deg C higher than the wells.

    That's one deep hole to obtain for a few degrees of water temperature, even if you had a larger hole and not as deep, would it be worth the investment, and how long would it take to break even? I suppose it would make the heat pump operate much better than just relying on the varying outdoor ambient temperature. But it would be interesting to see or hear if any one has achieved it.
    Martyn

    50 & 60 hz but 100's worse

  8. #8
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    Chillywilli: A few more people like you and we'll have this idea perfected. Thanks for the imput. I'm thinking this could actually work. We just have to work out some of the numbers. Hell, they are seriously planning on life on mars-I'm sure we can work out a decent greenhouse for a heatpump.

  9. #9
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    chilliwilly.
    What size is this guys greenhouse gonna be. in order to have any room left in it to use it as a green house also.
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  10. #10
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    You would be far better off with solar thermal panels and a coil.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  11. #11
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    Nov 2007
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    My Dad uses his greenhouse to provide supplemental heat for his house. No heat pump though. He put old sliding glass door panels on top of his black shingle greenhouse roof. He modified his thermostat, so that it turns on a (filtered) blower on that pushes air from the ceiling of the greenhouse into the living room of the house.

    If that doesn't keep up, it turns on the central gas furnace.

    As a bonus, the house smells nice

    Of course, Texas winters don't get as cold as some places
    Technical incompetence is NOT a sales tool....

  12. #12
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    Quote Originally Posted by tipsrfine View Post
    Chillywilli: A few more people like you and we'll have this idea perfected. Thanks for the imput. I'm thinking this could actually work. We just have to work out some of the numbers. Hell, they are seriously planning on life on mars-I'm sure we can work out a decent greenhouse for a heatpump.
    Yes a bit of food for thought, if I had the time I would look into sizing up and cost feasibility.

    Labour carried out by myself of course, and any materials ponsed out of a skip (dumpster).
    Martyn

    50 & 60 hz but 100's worse

  13. #13
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    Quote Originally Posted by beenthere View Post
    chilliwilly.
    What size is this guys greenhouse gonna be. in order to have any room left in it to use it as a green house also.
    I ain't talkin' 'bout using it as a greenhouse BT! Just housing to increase a heatpumps efficiency so it can be more practical in the colder climates that get a lot of sun. If I can just maintain 35 f in the greenhouse when its zero ambient the efficiency gain is great enough to at least consider this crazy idea.

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