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Load Side Heat Exchange System for Solar Assisted Radiant

Best to look before leaping!

After waiting 5 years for the real estate down to up, I'm finally able to again build.  I've got a multi-family lot in a heating climate (Mt. Shasta, CA).  No natural gas available, it's expensive propane or electric for backup energy.   I'm building multi-family rental.

Looking at a mass storage bed solar thermal design (see image).  I find precious little data on this design's operational characteristics.  I've communicated with several people who have built and live with this design,  and they uniformly love it.  Still, I'm nervous, so I'll build a smaller building first and test this design.  I can bury sensors to test and measure.

The design uses a Simple Drainback system for the heat exchange tank, with evacuated tube collectors.  The Simple Drainback system is a load side heat exchange system, meaning water in the tank is collector fluid.  I want to take the collector fluid directly to the floor and back to the tank.   I am also planning a wood stove loop to supplement the solar (<a href="http://www.hilkoil.com/">www.hilkoil.com</a>).  Everything will be plumbed in thermosiphon.

I'm going with 'rules of thumb' design for the solar assist (2 s.f. of solar collector for every 10' of floor space, rounding down). 

And so, I'll post my results as more is learned.  Should anyone care to comment, I'd love to hear ideas/criticism or comment.


  • Ironman
    Ironman Member Posts: 6,900
    Open Loop System...

    Conceptually speaking, I think you're on a good track. But you're headed for problems if make the floor part of an open loop by connecting it directly to the drain back tank. Use an internal coil or heat ex. for the floor so it's can be a pressurized, closed loop.

    If you're going with evac tube, use Calleffi or some other European / American made. Stay away from the chinese made stuff.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Gordan
    Gordan Member Posts: 891
    I'm confused

    It looks like the thermal storage is thermally connected to the living area. What's to stop the first floor from terribly overheating?

    Since this is new construction, you'll certainly want to use a system that places tubing above the subfloor for the upper floor. These work on much lower supply temperatures.

    Not sure how you plan to incorporate the sand bed into the system.

    A cute thought experiment would be to use the concrete slab as the source-side heat exchanger for the upper floor, so that it gets heated from the storage and the upper floor just gets heated from water circulated through the slab and the upper floor emitters. I can't imagine how you'd control the flow of heat from the storage below the slab, though...
  • Zman
    Zman Member Posts: 7,375
    More data

    I would start with a heat loss calc on the house.I would estimate your hot water needs.

    Next I would look at estimating the panel output. It is important to compare summer and winter outputs. Generally you are going to produce 2-3x more in the summer. The last place you want to store that extra energy in the summer is under a slab. The house will get hot.

    I would do the math 2 ways. Look at the heating and dhw needs in the winter and compare that number to the proposed panel output. I would then look at summer dhw needs and compare the proposed panel output to that. There should be a sweet spot where you are not throwing away too much energy in the summer and are offsetting much of your winter heat loss.

    Your insulation looks fantastic. I would steer clear of rule of thumb estimations

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Gordy
    Gordy Member Posts: 9,514
    Thermal storage under slab

    I agree with Gordan on that the thermal storage under the slab would be a control nightmare with over heating issues. I would insulate between the concrete, and sand bed. Then install pex in the concrete for better control. Or use another type of storage large insulated tank. The heat transfer from the tubing to the sand is not great. If you go to the beach on a hot sunny day the sand is hot to walk on but just below is much cooler.
  • zacmobile
    zacmobile Member Posts: 211
    as above

    Definitely need to thermally separate storage from living space. These guys have a system down for just what you're thinking of: http://www.icax.co.uk/interseasonal_heat_transfer.html
This discussion has been closed.