Difference between revisions of "Conversation with Alan Rushforth on April 2, 2008"

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* '''Eliminate Small Domestic Hot Water Tank''' - The small solar tank that preceeds the instantaneous heater, can probably be eliminated.   
 
* '''Eliminate Small Domestic Hot Water Tank''' - The small solar tank that preceeds the instantaneous heater, can probably be eliminated.   
 
=== Solar Collector Design ===
 
=== Solar Collector Design ===
[[Image:Image:Alan Solar Collector design.jpg|thimb|400px|right]]
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* '''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.
* '''Use Flat Panels''' - because of longer life via simpler design.  The prices are about the same but the evacuated tube seals break.  He has 8 tube seals broken on one of his installations right now.
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* '''Prevent Overheating''' - Option 1 - cover the panels...would like to see someone use the green house screens to rig something that could be controlled 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.  
 
* '''Prevent Overheating''' - Option 1 - cover the panels...would like to see someone use the green house screens to rig something that could be controlled 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.  
 
* '''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.
 
* '''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.

Revision as of 12:52, 11 April 2009

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.
  • 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.

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 green house screens to rig something that could be controlled 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.
  • 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.
    • Low pressure
    • 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.

Thermal Storage

  • Storage Size...Bigger is Better
    • 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.
  • Questions on 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?
  • Alternatives
    • 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

Suggested Reading

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