Green Garage Solar Heating Design

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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 ...)
      • Domestic Hot Water Load = 120 gals / day
  • See Green Garage - Current Design Assumptions
  • See our GG Solar Thermal Workbook

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 35% to 70% efficient ... 3 to 4 times that of PV panels
    • 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
    • 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: 57-65 degrees per SRCC guide + Alan the panels should be (Latitude + 15 degrees) for winter heating driven systems
      • Horizontal Angle: solar "true" south - determine at site...slightly west of magnetic south...estimated to be approx + 5 degrees degrees for Detroit. +/- 15 degrees is ok. Site for sunrise/sunset data
      • Configuring
        • Portrait orientation (landscape is ok, but takes up more roof area)
        • Parallel connection (see diagram Alan's Solar Collector Design)
        • 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
      • 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: .5g/m per collector: Total 5g/m. When you are dealing with a drainback with no heat exchanger on the solar loop side, .5 gpm/collector is adequate. A full 1 gpm wastes watts and adds wear and tear on the copper collector piping.
    • Pressure: 160 psi 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; Typically 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 If using a drainback approach, you have a little more leeway to handle occasional stagnations (no glycol to turn acidic), but some overheat protection is recommended - could be a good greenhouse/tarp-roll arrangement manually raised and lowered over part of the array - or a heat dump radiator of some sort.
          • Cover the panels.
          • Paint the panels with poster paint that washes off ...not sure about this one
          • 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 many additional panels 2 vs. 3.
    • Insulation of panels? Not done.
    • Framing for mounting the panels? Minimize roof roof design?

Thermal Storage

  • Demand Requirements
    • Space heating and domestic hot water. We're investigating cooling ideas.
    • Goal is, practically speaking, 90% with a retro 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: additive
    • Temp: Max: 175F Min: above radiant floor or indoor temp
  • Open Issues
    • Number of tanks? Use 1 tank due to cost of heat exchangers
    • Do you need a small tank for the domestic hot water? Probably not....except for cooling season.
    • What about using it for off-peak cooling storage for the geothermal? Haven't seen it done.

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


  • 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)