Earthen Heat Storage
I’m going to start with a LTM Sunspace and list the costs and benifits. Then add the next sensible low-cost upgrade to see where the cost/benifit ratio tails off. EARTHEN-STORED SOLAR HEATING explains these concepts and authors experience with them in great detail.
Is there waste stream polystyrene that could reduce costs? 6″ thick polystyrene seedling trays sequestered from the silviculture (forestry) industry waste-stream. Also Insulation Depot
Low Thermal Mass Sunspace
Costs: 7’ 2x double stud walls 16’ of vertical glazing wall 16’x10’ roof glazing corrugated or twinwall polycarbonate 16’x7’ insulated + cork floor
Benefits: As a rough guide, under full sun conditions, LTM sunspaces will deliver about 160 to 180 BTU per hour per sqft of glazed area depending on season.
In less sunny conditions (600 watt/sm), the delivery rates falls to 80 to 100 BTU per hour per sqft of glazing – still a very useful amount of heat. ref
7’ x 16’ + 10’ x 16’ = 279 ft2 collector area (this is a high estimate)
80 x 279 = 22,320 Btu/h under 600 watt/sm sun 180 x 279 = 50,220 Btu/h under full sun
Risks: The house max heat load is 6720 Btu/h, so even with “less sunny conditions” the sunspace will produce 3x that and overheat the house.
Todo: Find solar data and estimate how much useable heat per month
Upgrade 1: Underslab Rock Bin Heat Storage
Costs:
- 2’ more depth of cinder block foundation. No longer FPSF.
- $60 - Move the R-26.4 insulation from the horizontal under-slab position to a 4’ vertical perimeter insulation just inside the foundation. It only uses 96 ft2 more insulation.
- $180 - (288 ft2 - 192 ft2) x 3 layers x $0.625/ft2 Substitute the smaller crushed stone under the slab for 4” stone - final layer of the smaller crushed stone between vapor barrier and slab to smooth surface for slab pour. 1’-2’ tall rock bin with no addtional cost
- ~$200 - 90vdc horizontal airflow fan - inline duct boosters and inline axial fans also work for flow rates under 1000 cfm - Patrick Amos uses 4cm inline axial fan
- $100 - a layer of polyethylene vapor barrier above and another below the 4” stone
- $? - 6”-12”? trunk duct to carry air between sunspace and rock bin
Benefits:
- 12’x16’x4” Slab 64(cf)x26.6(Btu / cf / °F)x(85°F-68°F) = 28,940.8 Btu ref
- 20’x16’x2’? 4” Rock Bin Storage = 640(cf)x20(Btu / cf / °F)x(85°F-68°F) = 217,600 Btu
- 20’x16’x0.5’ Ground Under Rock Bin 160(cf)x0.191x97(Btu / cf / °F)x(85°F-68°F) = 50,393 Btu ref ref dry sand
- Total Storage Capacity: 296,933 Btu
Risks:
- Mildew smell from rocks bin for a few days when starting to use in the fall. (could mitigate by having sunspace vents open while circulating air in rock bin in first days of fall)
- Heat loss from uninsulated bottom
Upgrade 2: Annualized Geo Solar
Costs:
- $200 - 4’ of perimeter insulation
- $100 - 4’ of perimeter drainage.
- $0 - A few extra hours with the backhoe/excavator - Dig a trench 8’ to 10’ deep 20’ long 2’-ish wide under the house
- $? - 2x 4” diameter 40’ long polyethylene flex pipes laid in the trench, on each side as far apart as possible.
- $0 - A few more hours filling and compacting soil back into the trench
- $0 - the intake manifold and fan from Upgrade 1 switched to this pipe during the non-heating season.
- $100 - A few simple temperature and humidity sensors placed down 1” pipes in strategic locations to monitor heat gain and adjust if overheating is experienced
Benefits:
- Slowly heat soil under the house to ~85°F over the summer. It takes 3 or more summers to reach full temp.
- (12’+1’+1’+1’+7’+4’+4’)x(16’+1’+1’+4’+4’)x10’x(85°F-70°F)x(0.191x97((Btu / cf / °F) = 2,167,659 Btu Storage. That’s a very high estimate. But some fraction like 1/2 or 1/4 is still a lot of heat.
- Soil around rock bin is warmer so it will loose less heat
Risks:
- Increase risk of ground settling under house where the trench is dug
- Ground water could rise at some point during the year and take a large part of the heat away with it.
- A couple more feet of perimeter insulation might be needed to meet the minium of 20’ distance between ducts and edge of perimeter insulation. Todo: check 3D model
Upgrade 2b: 4’ EARTHEN-STORED SOLAR HEATING
See EARTHEN-STORED SOLAR HEATING
Costs:
- Excavate 4’ “basement” instead of only trench
- $600 - R26.4 Horizontal insulation layer at bottom of 4’ “basement floor” - 20’x16’x3 layers x $0.625/sf
- Put a series of tubes 6” from the bottom
- Another series of tubes or 4” stone at the top (under slab)
- Fill with soil taken out
- Overall costs similar to building a basement. pretty minimal for 4’
Benefits:
- Mitigates risk of ground water rising and taking away a large part of heat stored
- 2’ Ground Under Rock Bin 20’x16’x2’ 640(cf)x0.191x97(Btu / cf / °F)x(85°F-68°F) = 201,573 Btu
- With slab and rock bin Total: 448,113 Btu
Risks:
- Easier to overheat
- Less heat is stored around the peremeter
Upgrade 2c: Additional 1’-6’ Depth of Earth-Storage
Costs
- Additional Excavation
- Additional Cinder Block
- $45 = (20’+20’+16’+16’)x1’x$0.625/sf Additional Insulation
Benefits
- Each additional 1’ earth under Rock Bin 20’x16’x1’ 320(cf)x0.191x97(Btu / cf / °F)x(85°F-68°F) = 100,786 Btu
With slab, rock bin, and 6’ of earth storage total: 851,261 Btu
Upgrade 4: Insulate just below top rock bin
From EARTHEN-STORED SOLAR HEATING
Below this upper layer of hydronics we placed 1″ thick insulation across the central areas of floor, but left a few feet around the interior’s perimeter un-insulated. This as a way to ensure the heat running through the upper loops didn’t migrate downward into the mass battery below. But the un-insulated perimeter still allowed the heat from below to conduct upwards through the floor slab – ala Don Stevens’ approach.
Costs: $75 - 10’x12’x 1 layer x $0.625/sq ft