Conversation with Alan Rushforth on April 2, 2008

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Alan Rushforth from Rushforth LLC comments:

Demand Requirements

  • Load Requirements / Assumptions
    • Design Heating Season = Nov 15 - March 15 ... AR: This is pretty radical to get to Nov.15 with no heat, but reportedly this was well calculated with super insulation.
    • Total Solar Heating Load = Space Heating Load + Domestic Hot Water
      • Space Heating Load = 22 million BTU per heating season; 184k BTU/day - AR: Again, by normal standards this is at least an order of magnitude low, but ...we did discuss your super-insulation plan.

Solar Heating Schematic Overview

  • Use Drainback w/o Heat Exchangers, Low pressure - I would recommend skipping the heat exchanger on the collector loop, and directly pumping the water in the 3,500 gallon tank through the collectors using a drainback design. It will take a few more watts of pumping power, but the cost and inefficiency of the collector loop heat exchanger would be eliminated. If I were doing the job, I would want to pump the tank water directly through the collectors, no heat exchanger, all draining back to the big tank.
  • Maximum tank temp = 160F - I would also be inclined to keep the high limit more like 160F rather than 175F for longest liner life. Likely the only time it would get the tank to 175 would be in the summer, when you would not need the extra heat then anyway. My feeling is the key and most useful temperature range for space heating, will be from 80F to 130F. I doubt solar will get it much over that in the winter. It just occured to me, if the geothermal is going to dump heat in the tank, during the winter, that will cut the efficiency of the solar collectors, maybe significantly. This warrants more consideration.
  • Eliminate Small Domestic Hot Water Tank - The small solar tank that preceeds the instantaneous heater, can probably be eliminated.
  • Geothermal Concerns
  • Mixing the geothermal into the solar thermal tank gives me the most concern.

Solar Collector Design

  • Use Flat Panels - because of longer life via simpler design. The prices are about the same (square foot cost of discount Chinese evacs can be roughly equal to flat plates.) but the evacuated tube seals break. He has 8 tube seals broken on one of his installations right now.
  • Prevent Overheating - Option 1 - cover the panels...would like to see someone use the greenhouse tarp screens to rig something that could be manually controlled or controlled automatically from the ground. Option 2 - use a heat dump...a radiator or sauna. You can't just leave the panels empty (drained) as the high heat build up would damage the panel. If you use a drainback, you have a little more leeway to handle occasional stagnations (no glycol to turn acidic), but I would still recommend some overheat protection - could be a good greenhouse/tarp-roll arrangement manually raised and lowered over part of the array - or a heat dump of some sort.
  • Array Configuration
    • Connect the panel headers/footers in series and have the water flow through the panels in parallel - this minimizes the amount of tubing on the roof. If the collectors are mounted in one straight row and connected with high temp silicone heater hose (clamped on) (no solder joints or unions) that would keep things more simple and efficient.
    • Low pressure - with drainback it is at atmospheric pressure +/-. With glycol, probably 15 to 25 psi.
    • Flow Rate - when you are dealing with a drainback with no heat exchanger on the solar loop side, I feel 0.5 gpm/collector is adequate. I feel a full 1 gpm is wasting watts and adding wear and tear on the copper collector piping)
    • Delta T - you will probably find your T in vs. T out is under 15 especially if you have more than about .6 gpm/collector.)
    • Panels are butted up against one another w/ very short, high temp silicon hoses (1-1/8") connecting the headers/footers on the panels.
    • Panels can be portrait or landscape
    • Use insulated pex-al-pex to run/return between the panels and the storage.
    • Use differential controller - Stucca (located at the storage tank) w/ $10 sensors to determine when to send the water through the panels. Pleace the sensors in the space at the top of the collector...have it go on when it is > 95F.
  • Collector Positioning
    • I would recommend 5 or 8 degrees West of magnetic South to slightly favor the afternoon sun when it is warmer with lower delta T.
    • 10 panels at 55 degrees will probably create a lot of summer overheating. My first thought for this application is more like 65 to 70 degrees.
  • Collector Manufacturers
    • We are now shifting from SolarHot collectors to Solene Cromagen - slightly better numbers and equally good pricing - just under $800 for a 4x10. My local distributor, Hickory Ridge Radiant could probably help if there is no one in your area.

Thermal Storage

  • Storage Size...Bigger is Better
    • With space heating in the equation, the way I see it, the bigger the better, with the limitation being your available space and the tank cost. On the extreme end would be seasonal storage. 3,500 gallons does not remotely approach that size. I think 3,500 is an ok start, but if you had space and budget for 5,000 to 7,500 gallons, I would endorse the extra storage. If the tank is site built, the extra gallons do not have to add proportionally to the cost.
    • I agree more storage is better - especially in the shoulder seasons.
    • Hold four days of heat (Rushforth LLC uses 2+ days... and gets much better results than one day)
  • Tank Manufacturers/Construction
    • Have used STSS...good tanks
    • Build your own tank - With experience, it is possible to site-build tanks similar to STSS, but bigger and better insulated in the $2/gallon or less range could get the plans and might be willing to help build it.
  • Well Insulate the Tank...R30ish
    • Insulation: R-30 around tank - Unless every inch of space is supercritical, I would suggest more insulation.
    • This may be overkill to insulate the room the tank is in. If the tank itself is well insulated, there is very little heat loss from it.
  • Use direct Thermal Exchange when possible...
    • Use direct exchange when possible...lowers cost and raises efficiency. Only use exchangers when they are needed
    • How about having the radiant floor not have heat coils but just go directly in and out of the tank? AR: This might be a good idea but may ask the contractor to do something they are not accustom to.
  • Use One Tank...Yes - one tank not only keeps heat loss area minimized, but it keeps plumbing simpler. With the DHW preheat coil, the instantaneous heater, and the mixing valve, I see no need for the small hot water tank.
  • Questions on Connecting Geothermal
    • Off-peak cooling storage for the geothermal? AR: To use the tank for cooling half the year, and heating the other half, changes everything. You loose the summer hot water. My initial feeling is to nix that idea, or have one hot tank and one cold tank - probably not practical.
    • Heating: I have a concern about intermingling geothermal heat and solar heat in the same tank. The solar collectors work best at low temperatures. Any geothermal heat added into the tank will diminish the efficiency of the solar collectors. Can the geothermal heat go directly into pex lines in the slab and skip the tank entirely?
  • Other Points
    • Should consider a floor drain near the tank.
    • If there is headroom to raise the floor, has Bob Ramilow's 'High Mass' slab floor storage been considered? This involves and insulated sand bed with pex solar lines, covered with a concrete slab - in his book 'Solar Water Heating

Radiant Floors

  • Though not strictly solar advice, instead of a wood floor, I would consider foam insulation and then a second concrete slab (with pex tubing) over the foam - more thermal mass, more thermal storage. Possibly off peak geothermal summer cooling could integrate into the slab.
  • What are the areas of most risk in our design? Wood flooring on the concrete to me seems like a potential problem area – termites, rot, labor intensive, etc. Is there ways to mitigate these risks? I like concrete over foam – simple, sturdy, good thermal mass, good for pex.

Geothermal

  • Mostly sounds good to me although mixing the geothermal into the solar thermal tank gives me the most concern.

Suggested Reading

  • Tom Lane Lessons Learned Solar Thermal heating (Florida)
  • Chuck Marken - Home Power Magazine