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Underground Pit Storage

4 min read

An underground pit store is an excavation lined with a waterproof membrane, filled with water or a gravel-and-water mix, and capped and walled with thick insulation. A 50 m³ cube of water with 1 m of straw (k = 0.096 W/m·K) keeps about 56% of its heat above the surroundings after 180 days, and 74% with a drier, better insulator of 0.05; a gravel-and-water fill holds only 68% as much heat per cubic metre, so it needs 1.5 times the volume. Keep it above the water table and the insulation dry.

Part of Seasonal Storage

A pit store is a lined, insulated hole in the ground that holds summer’s solar heat in water or gravel until winter.

What The Parent Page Specifies

Excavate a pit of 10-50 m³ minimum; line it with a waterproof membrane of clay or plastic sheeting; fill with water or gravel; insulate the top and sides with at least 50 cm of insulation; charge all summer from collectors; draw heat in winter by heat exchanger or direct plumbing.

How Much Heat Does It Keep?

Treat the store as a cube at uniform temperature losing heat through its insulation to surroundings at a fixed temperature. Its time constant is τ = C ÷ UA, where C = volume × 4.18 MJ/m³·K and UA = k × surface area ÷ insulation thickness. The excess over the surroundings then decays as e^(−t ÷ τ).

Inputs: V = 50 m³; side a = 50^(1/3) = 3.68 m; surface area = 6 × 3.68² = 81.4 m²; insulation thickness t = 1.0 m; k = 0.096 W/m·K (corpus value for straw, Natural Insulation Materials) or 0.05 (the hay-like value in Thermal Insulation Basics); storage period 180 days.

  1. Capacity C = 50 × 4.18 = 209 MJ/K.
  2. k = 0.096: UA = 0.096 × 81.4 ÷ 1.0 = 7.8 W/K; τ = 209,000,000 ÷ 7.8 = 2.67 × 10⁷ s = 309 days; fraction kept after 180 days = e^(−180 ÷ 309) = 0.56, or 56%.
  3. k = 0.05: UA = 4.07 W/K; τ = 594 days; fraction kept = e^(−180 ÷ 594) = 0.74, or 74%.
  4. With only 0.5 m of straw (k = 0.096): UA = 15.6 W/K, τ = 155 days, fraction kept = 0.31.

So the parent page’s “50 m³, 1 m of straw, supplements winter heating for a single home” is plausible but marginal: about half the heat is gone by midwinter if the straw is dry and a quarter if the insulation is better. These are simple, assumption-laden figures: they ignore the ground warming around the store (which reduces the loss over time) and assume dry insulation (which wet insulation ruins). Large stores do better; see Scale Requirements.

Gravel Or Water?

A gravel pit filled with water has porosity of about 35% (assumed). Its heat capacity: 0.35 × 4.18 + 0.65 × 2.10 = 1.46 + 1.37 = 2.83 MJ/m³·K, which is 68% of water’s 4.18. For the same energy it needs 4.18 ÷ 2.83 = 1.48 times the volume, about 1.5 times. Gravel carries the lid and any load on top; water is cheaper per kilojoule but needs a membrane tank.

What You Need

  • A site above the water table, in stable soil
  • A waterproof membrane that tolerates the store’s temperature
  • Dry insulation for the top and sides, kept dry by a membrane and drainage
  • A heat exchanger or direct plumbing
  • A load-bearing cover and a vent
  • A fence or lid that keeps people out

Method: Build The Pit

Step 1: Dig with sloped sides or shoring; keep spoil away from the edge.

Step 2: Line and test the membrane by filling it with cold water first.

Step 3: Fill with gravel or water, then insulate the top and sides. Drain the surroundings so water cannot enter the insulation.

Step 4: Charge over the summer; read temperatures at two depths.

Step 5: Discharge through the exchanger in winter.

Common Mistakes

  • Wet insulation. Water in straw conducts many times more than dry straw.
  • A pit below the water table. Moving groundwater carries the heat away.
  • A membrane not rated for the temperature. Some plastics and clays degrade when held warm.

Safety

Excavation collapse, scalding water, and a lid people walk on

Walls of a deep pit collapse without warning and can bury a person: slope or shore the sides and never work alone below ground level. A store of hot water scalds fatally; a lid that fails over it drops a person into scalding water. Build the cover to carry a person, fence it, vent the store to prevent pressure, and keep children away.

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