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Rock Bed Design

4 min read

A rock bed of 2 m³ with 35% air gaps holds 2.73 MJ for each kelvin it is warmed, so heating it from 20°C to 80°C stores 164 MJ (45.5 kWh), and only the part above room temperature is usable. That is below the 280 MJ the parent page quotes for the same bed, which would need a 103 K swing; size the bed from your own swing and your daily heat need, with stones of one size around 5-10 cm, and at least 15 cm of insulation on every side.

Part of Rock & Sand Heat Stores

A rock bed is a box of uniform loose stones, insulated on all sides, through which hot air flows to charge it and room air flows to discharge it.

Capacity: Derive It From Your Own Numbers

Heat stored = bed volume × volumetric capacity × temperature swing.

Inputs: bed volume 2 m³; stone density 2,500 kg/m³ and specific heat 0.84 kJ/kg·K (parent page’s stone figures); void fraction 35% (the parent page asks for 30-40%).

  1. Bulk density = 2,500 × (1 − 0.35) = 1,625 kg/m³.
  2. Capacity per kelvin = 2 m³ × 1,625 kg/m³ × 0.84 kJ/kg·K = 2,730 kJ/K = 2.73 MJ/K.
  3. Swing 20°C to 80°C, 60 K: 2.73 × 60 = 164 MJ = 45.5 kWh.
  4. The parent page’s 280 MJ would need 280 ÷ 2.73 = 103 K, e.g. 20°C to 123°C. It fits a solid 2 m³ of stone only if the swing is about 67 K (280 ÷ 4.2 = 67), and a bed with air gaps holds less.

Usable heat is smaller still. If the room is kept at 20°C and the air leaving the bed must be at least 30°C to be worth using, the usable swing is 80 − 30 = 50 K, which is 2.73 × 50 = 137 MJ, 38 kWh.

Does it last 24-48 hours? 164 MJ over 24 hours is an average 164,000 kJ ÷ 86,400 s = 1.9 kW; over 48 hours, 0.95 kW. So the parent page’s range holds for a room needing about 1-2 kW, using the whole swing.

Surface Area And Stone Size

A bed of spheres of diameter d with void fraction ε has 6 × (1 − ε) ÷ d square metres of stone surface per cubic metre of bed (geometry, not a handbook figure).

  • d = 5 cm: 6 × 0.65 ÷ 0.05 = 78 m²/m³.
  • d = 7.5 cm: 52 m²/m³.
  • d = 10 cm: 39 m²/m³.

Smaller stones give more surface for heat exchange but resist airflow more (Airflow Channels). The parent page’s 5-10 cm stones are a reasonable balance. Uniform size keeps the voids open; fines (dust and small chips) block the gaps and are the usual reason a bed starves for airflow.

What You Need

  • A box or pit 60-100 cm deep (parent page), tall and narrow rather than wide and flat, so the hot front moves down in order
  • Clean, dry stones of one size
  • Perforated pipes or open channels at the bottom, and a plenum at the top
  • At least 15 cm of insulation on all sides and the bottom (parent page)
  • A fan or a strong draft, and a thermometer at top and bottom
  • A vapour barrier under the bed to keep ground water out

Method: Build The Bed

Step 1: Build the box and insulate the sides and base before anything goes in.

Step 2: Lay the bottom channels and cover them with the largest stones to keep them open.

Step 3: Wash the stones free of dust, dry them thoroughly and fill the bed in layers.

Step 4: Cover the top with a plenum and a lid so air enters and leaves at one controlled point.

Step 5: Charge slowly over a day or two the first time to drive off moisture.

Common Mistakes

  • Using the maximum quoted capacity as a plan. Compute with your own swing.
  • Mixed stone sizes. Small pieces fill the gaps between the large ones.
  • Insulation on the top only. Heat leaks through the sides and floor.

Safety

Fire hazard, carbon monoxide and bursting stone

A bed at 80°C or hotter ignites nothing by itself, but a bed fed by a stove can reach hundreds of degrees; keep combustible framing and insulation away from the hot side, and use heat-tolerant insulation near the inlet. Never charge a bed in an occupied space with flue gas, which contains carbon monoxide. Wet or river stone heated quickly can burst; dry it first.

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