Difference between revisions of "Green Garage Solar Heating Design"

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(Alan Rushforth Comments)
(Alan Rushforth Comments)
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=== Alan Rushforth Comments ===
 
=== Alan Rushforth Comments ===
 
Solar Heating Schematic Overview:  
 
Solar Heating Schematic Overview:  
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 you will eliminate the cost and inefficiency of the collector loop heat exchanger. 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 to 175 would be in the summer, and you would not need the extra heat then anyway.  My feeling is your 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.  
+
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.  
 
+
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.
 +
The small solar tank that preceeds the instantaneous heater, can probably be eliminated. 
  
 
Thermal Storage
 
Thermal Storage

Revision as of 23:29, 30 March 2009

return to Solar Heating Design

Demand Requirements

  • Load Requirements / Assumptions
    • Targeting meeting 90% of space heating load
    • 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 ...

Solar Heating System Overview

Green garage Solar Heating Schematic - V5


Solar Thermal Collectors

  • Goal = highest Heat Output / Total Life-cycle Cost
    • Should BTU generated per dollar invested...why not just buy little more of a slightly lower efficient panel that is much less expensive and get same total energy.
    • where Heat Output (Q-out) = Temp Rise x Volume Flow Rate x Specific Heat of Water
  • Basics Guidelines
    • Solar thermal (liquid) panels are up to 80% efficient ... 5 to 10 times that of PV panels (AR: I would suggest thermal is normally more like 35% to 70% efficient and about 3 or 4 times the efficiency of PV)
    • The efficiency varies greatly with delta T (Tin - Tout)the greater the difference the lower the efficiency
  • Solar Collectors
    • Flat Panel vs. Evacuated Tube
      • Choose: Flat panel because of lower cost (approx 1/3 the cost) and higher durability (twice the life)..performance usually better in the system vs. on the testing bench (AR: I second most of the above, except square foot cost of discount Chinese evacs can be roughly equal to flat plates.)
    • Manufacturer
      SRCC Data for SunEarth - Empire
  • Preliminary Specifications
    • Size: 4ft x 10ft
    • Number: 10 = area 400sf
    • Positioning
      • On the annex building flat roof
      • Vertical Angle: 55 degrees per SRCC guide the panels should be (Latitude + 15 degrees) for winter heating driven systems (AR: 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)
      • Horizontal Angle: solar "true" south - determine at site...slightly west of magnetic south...estimated to be approx + 4degrees degrees for Detroit. +/- 15 degrees is ok. Site for sunrise/sunset data (AR: I would recommend 5 or 8 degrees West of due South to slightly favor the afternoon sun when it is warmer with lower delta T)
      • Configuring
        • Portrait orientation
        • Parallel connection (AR: For lowest cost installation, and best efficiency with least exterior piping, I would recommend all 10 panels in one straight row, in series - Feed emerges from roof at one end. Return enters roof at other end - could not be more efficient)
        • One feeder pipe to two groups of five panels in parellel (i.e. five per manifold.) ** Thermal Capacity:
      • 23 Million BTU per heating season (Nov15 - Mar 15) See GG Solar Thermal Workbook (AR: Kudos if you can achieve this - I will take your word for it that you can.)
      • SunEarth Thermal Capacity
      • Assume
        • Full Sun: 750 BTU/sf Winter; BTU/sf Summer
        • Partly Sunny: 560 BTU/sf Winter; BTU/sf Summer
        • Cloudy: 375 BTU/sf Winter; BTU/sf Summer
    • Flow Rate: 1g/m per panel (12 g/m max): Total 10g/m...max at 30 - 40g/min (AR: When you are dealing with a drainback with no heat exchanger on the solar loop side, I feel .5 gpm/collector is adequate. I feel a full 1 gpm is wasting watts and adding wear and tear on the copper collector piping)
    • Pressure: 160 psi (AR: What is this? Max. test pressure? With drainback it is at atmospheric pressure +/-. With glycol, probably 15 to 25 psi.)
    • Temp: 15 - 25F delta T for T in vs. T out; (AR: You will probably find your T in vs. T out is under 15 especially if you have more than about .6 gpm/collector.) Max - can boil...control with the flow. Min: - above storage temp or radiant floor or indoor temp
      • Overheating / Heat dump (AR: 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.)
          • He has covered the panels...often recommends doing so
          • paint the panels with poster paint that washes off (AR: Of the 3, I would have least confidence in this measure)
          • uses radiator coils with fans to dump heat
  • Open Issues
    • Drainback option that eliminates glycol (AR: If possible, and I think it is, I would recommend drainback.)
    • Thermal capacity calcs...w/ domestic hot water...how many additional panels 2 vs. 3.
    • Review other panel manufacturers
    • Insulation of panels?

Thermal Storage

  • Demand Requirements
    • Space heating and domestic hot water. We're investigating cooling ideas.
    • Goal is, practically speaking, 90% with a retro fit...no opportunity for below floor storage.
    • 10 gallons ( or approx 1 cu ft) for every 1 sf of solar panel if you want to get to near 95% of heating demand
    • Hold four days of heat (Rushforth LLC uses 2+ days... and gets much better results than one day)
  • Basic Guidelines
    • Place storage indoors...not outdoors, because heat loss indoors helps heat the building.
  • Manufacturer
  • Preliminary Specifications
    • Location of Storage
      • Ground floor slab in addition bldg
      • Create insulated room w/ R-25 walls + R-25 floor + R-25 ceiling
    • Thermal Architecture
      • All heat generators (i.e. solar and geothermal) connected to the storage
      • All heat consumers (i.e radiant heat and domestic hot water) can draw heat from storage via heat exchangers.
    • Size of storage
      • Store 2 million BTU to start (3,500 gallons)
      • Number of tanks: 1 - 3,500 gal
      • Assume 1,500 gal
    • Size: 3,500 gal = 10 ft dia x 7ft tall
    • Capacity: 470cf
    • Thermal Capacity:
      • Winter: 3500 gals = 470cf = 2.3 mBTU: equivalent to approx 13 average winter days (almost two weeks)
      • Summer: TBD
    • Thermal Loss: R-19 around tank
    • Flow Rate: Max 15g/m ... geothermal connection
    • Pressure: Sealed, non-pressurized
    • Fluid: Water...no additive
    • Temp: Max: 175F Min: above radiant floor or indoor temp
  • Open Issues
    • Number of tanks...prefer 1 tank due to cost of heat exchangers
    • Min temp from a systems design standpoint...not a tank material standpoint?
    • What about using it for off-peak cooling storage for the geothermal?

Radiant Floor

  • Demand Requirements
  • Basic Guidelines
    • Only 10% of heat is lost through the floor...42% through the roof
    • Radiant heats up to about 7ft from the floor
  • Manufacturer
  • Preliminary Specification
    • Zones: approx 1100sf; 8 - Historic; 2 - Annex
    • Circuits: 6 per zone
    • Plex: 1/2in
    • Spacing: 8in
    • Flow Rate: 4.5 g/m
    • Temp: normal winter operating 90F; max = 130F
    • Thermal Capacity: 38,000 BTU/hr per zone
    • Floor architecture
      • Historic
        • Vapor barrier or sealer (10mm?)
        • Two layers of 1" XPS with seams staggered and sealed. (2")
        • Reflective radiant barrier (could be foil on the XPS?) (mm)
        • Sleepers (1.5") with metal heat extenders
        • Plywood? Wood Flooring (1")
        • All screwed together, run trenches
  • Open Issues
    • Normal operating temps winter...summer? Winter = 90F max...summer = 68F min
    • Can radiant floor be used for cooling? Yes see ASHRAE report ... min 68 degrees
    • Plex sizing in Michigan code? Appears we can use 3/4"...1/2 min

Geothermal

  • Preliminary Spec
    • Location: Basement
    • Connect Geothermal to with open loop to the mass storage
  • Manufacturer
    • Water Furnace...Envision
  • Open Issues
    • Geothermal - operating temps (min-max)

Alan Rushforth Comments

Solar Heating Schematic Overview: 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. 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. The small solar tank that preceeds the instantaneous heater, can probably be eliminated.

Thermal Storage Demand Requirements Space heating and domestic hot water. We're investigating cooling ideas. Goal is, practically speaking, 90% with a retro fit...no opportunity for below floor storage. (AR: 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') 10 gallons ( or approx 1 cu ft) for every 1 sf of solar panel if you want to get to near 95% of heating demand (AR: 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) Basic Guidelines Place storage indoors...not outdoors, because heat loss indoors helps heat the building. Manufacturer STSS Communications with Brad @ STSS Tank Sizes Other: (AR: With experience, it is possible to site-build tanks similar to STSS, but bigger and better insulated in the $2/gallon or less range ). American Solar Solutions Comes on a pallet and you assemble...you can get it in the basement. Comes in 800 gallon tanks with heat exchangers. Design Tank Fiberglass tanks comes in sections Preliminary Specifications Location of Storage Ground floor slab in addition bldg Create insulated room w/ R-25 walls + R-25 floor + R-25 ceiling (AR: This may be overkill. If the tank itself is well insulated, there is very little heat loss from it. ) Thermal Architecture All heat generators (i.e. solar and geothermal) connected to the storage All heat consumers (i.e radiant heat and domestic hot water) can draw heat from storage via heat exchangers. Size of storage Store 2 million BTU to start (3,500 gallons) Number of tanks: 1 - 3,500 gal Assume 1,500 gal (AR: ??? I like 3,500 gallons much better) Size: 3,500 gal = 10 ft dia x 7ft tall (AR: That is a very do-able size tank) Capacity: 470cf Thermal Capacity: Winter: 3500 gals = 470cf = 2.3 mBTU: equivalent to approx 13 average winter days (almost two weeks) Summer: TBD Thermal Loss: R-19 around tank (AR: Unless every inch of space is supercritical, I would suggest another inch or two of insulation here.) Flow Rate: Max 15g/m ... geothermal connection (AR: no comment - no experience with geothermal) Pressure: Sealed, non-pressurized Fluid: Water...no additive Temp: Max: 175F Min: above radiant floor or indoor temp Open Issues Number of tanks...prefer 1 tank due to cost of heat exchangers (AR: Yes. One tank not only keeps heat loss area minimized, but it keeps plumbing simpler). Min temp from a systems design standpoint...not a tank material standpoint? What about using it for 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)

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