Water stores 5.0 times as much heat per kilogram as stone (4.18 against 0.84 kJ/kg·K) but only 2.0 times as much per cubic metre, because stone is 2.5 times denser, and only 3.1 times per cubic metre of loose rock bed once 35% of the bed is air gaps. Which comparison is right depends on what limits you: weight points to water, solid volume to water by 2.0, a rock bed by 3.1, and a higher allowed temperature swing can make rock win outright.
Part of Thermal Mass Fundamentals
The heat capacity of a material can be quoted per kilogram or per cubic metre, and the two give different rankings.
Per Kilogram And Per Cubic Metre Are Different Questions
Specific heat (per kilogram) multiplied by density gives volumetric heat capacity (per cubic metre). The parent page’s table, with each row recomputed (kJ/kg·K × kg/m³ ÷ 1,000 = MJ/m³·K):
| Material | c (kJ/kg·K) | Density (kg/m³) | MJ/m³·K | Per kg vs water |
|---|---|---|---|---|
| Water | 4.18 | 1,000 | 4.18 | 1.00 |
| Stone/gravel | 0.84 | 2,500 | 2.10 | 0.20 |
| Sand | 0.80 | 1,600 | 1.28 | 0.19 |
| Clay/earth | 0.92 | 1,800 | 1.66 | 0.22 |
| Concrete | 0.88 | 2,300 | 2.02 | 0.21 |
| Brick | 0.84 | 1,900 | 1.60 | 0.20 |
| Cast iron | 0.46 | 7,200 | 3.31 | 0.11 |
Stone is 2.5 times as dense as water (2,500 against 1,000 kg/m³). Per kilogram water is ahead of every solid by 4.18 ÷ 0.84 = 5.0 (stone, brick), 4.18 ÷ 0.92 = 4.5 (earth), 4.18 ÷ 0.46 = 9.1 (cast iron). Per cubic metre the gap closes: 4.18 ÷ 2.10 = 2.0 (stone), 4.18 ÷ 3.31 = 1.3 (cast iron). Cast iron is the extreme: a poor store per kilogram, nearly as good as water per cubic metre because it is so dense.
A bed of loose rock is not solid. With 35% void (the parent page asks for 30-40% voids so air can pass), the bulk density is 2,500 × 0.65 = 1,625 kg/m³ and the volumetric capacity is 0.84 × 1,625 ÷ 1,000 = 1.37 MJ/m³·K. Water ÷ rock bed = 4.18 ÷ 1.37 = 3.1. The parent page’s “twice as much per volume” is right for solid stone and low for a bed that air must pass through. A gravel bed flooded with water falls between: 35% water and 65% stone gives 0.35 × 4.18 + 0.65 × 2.10 = 2.83 MJ/m³·K, or 68% of water (Underground Pit Storage).
Temperature Swing Changes The Verdict
Stored heat is capacity times the usable temperature swing. Open water stays below 100°C and a hot-water system is useful only above, say, 40°C, so a practical swing is perhaps 40 K. Rock and brick can be heated to hundreds of degrees.
Worked for 1 m³ (inputs assumed for the example):
- Water, 40 K swing: 4.18 × 40 = 167 MJ.
- Solid stone, 150 K swing: 2.10 × 150 = 315 MJ.
At those swings stone stores 1.9 times as much per cubic metre as water. Which to use depends on whether you can run the mass hot (a masonry heater can; a room-temperature tank cannot) and on the swing in the real system. Cheap, hot and bulky stone suits a firing; compact and movable water suits pipes and a collector.
Same Energy, Different Quantities
To store 100 MJ with a 40 K swing:
- Water: 100,000 kJ ÷ (4.18 × 40) = 598 kg, 0.60 m³.
- Solid stone: 100,000 ÷ (0.84 × 40) = 2,976 kg, 1.19 m³.
- Rock bed (35% void): same 2,976 kg of rock, 1.83 m³ of bed.
So water needs one fifth of the mass, and a rock bed needs about three times the volume.
What You Need
- The table above, kept in your notebook
- The swing you can actually achieve (measure it)
- A thermometer and a scale or volume estimate
Method: Compare Two Options
Step 1: Fix the energy you want to store.
Step 2: Compute mass = energy ÷ (c × swing) for each candidate, then volume = mass ÷ bulk density.
Step 3: Compare weight, volume, cost and how hot the material is allowed to get.
Common Mistakes
- Switching between kg and m³ mid-argument. State the basis each time.
- Using solid density for a bed with voids.
- Using the same swing for water and stone.
Safety
Hot mass hurts, and wet stone can burst
Water above about 60°C scalds in seconds and stone beds heated to hundreds of degrees burn on touch. Damp or river stone heated quickly can split with force as trapped water turns to steam; use dry stone and heat it gradually. Keep children and the elderly away from hot stores.
What’s Next
- Learn next: Rock Bed Design — applying the bed numbers
- Combine with: Sensible vs Latent Heat — storing heat without a temperature rise