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Sensible vs Latent Heat

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Sensible heat raises a material’s temperature, while latent heat is absorbed or released at constant temperature when a material melts, freezes, boils or condenses. Melting 1 kg of ice takes 334 kJ, the same as warming 1 kg of water by 80 K, and condensing 1 kg of steam releases 2,257 kJ, which is why steam burns far worse than boiling water. For fuel-free heat storage, sensible storage in water, rock or masonry is the practical choice.

Part of Thermal Mass Fundamentals

Sensible heat changes temperature; latent heat changes the state of a material at a fixed temperature.

Latent Heat In Numbers

Standard values (recalled, not read from this wiki): latent heat of melting ice 334 kJ/kg at 0°C; latent heat of boiling water 2,257 kJ/kg at 100°C; specific heat of water 4.18 kJ/kg·K.

  1. Ice melting: 334 ÷ 4.18 = 80 K. The heat that melts 1 kg of ice at 0°C would warm 1 kg of liquid water from 0°C to 80°C.
  2. Boiling: 2,257 ÷ 4.18 = 540 K. The heat that boils 1 kg of water would warm it by 540 K if it stayed liquid.
  3. Boiling water cooling to 20°C releases 4.18 × 80 = 334 kJ/kg. Steam condensing at 100°C releases 2,257 kJ/kg. The ratio is 2,257 ÷ 334 = 6.8.

The parent page says latent heat storage needs specific materials and engineering. Salt hydrates and waxes melt near a useful temperature: sodium sulphate decahydrate (Glauber’s salt) melts at about 32°C and absorbs roughly 250 kJ/kg (recalled handbook values), so 1 kg stores about as much heat as warming 1 kg of water by 60 K but at a nearly constant 32°C. Beeswax melts at around 62-64°C (recalled). Such materials cycle poorly: salt hydrates tend to separate, and supercooling holds them liquid below the melting point.

Why Sensible Storage Is The Practical Choice

Water and rock need no special material, no sealed container for a phase change, and do not degrade with cycling. They need more mass for the same energy but are available everywhere and do not fail in a way that loses the stored heat all at once. Use latent storage only where a nearly constant output temperature matters and you can find and test the material.

Ice is the latent store for cold: 334 kJ/kg for each kilogram melted (Drying Sheds & Ice Houses covers storing winter ice).

What You Need

  • A thermometer that reads the plateau at the melting point
  • A closed, leak-proof container that tolerates the swelling of the phase-change material
  • Water or rock as the default store

Method: Recognise The Plateau

Step 1: Heat a pot of ice slowly, stirring, with a thermometer in it.

Step 2: The temperature stays at 0°C while ice remains, then rises. The plateau is the latent heat.

Step 3: Repeat at the boil: the temperature stays at about 100°C (at sea level) while water boils away, however hot the fire (Why Boiling Temperature Is Enough).

Common Mistakes

  • Treating a plateau as a stall. The energy is going into the phase change.
  • Assuming latent stores are always better. They hold more per kilogram only within a narrow temperature range.
  • Sealing a phase-change store completely. Materials expand on melting.

Safety

Steam releases more than six times the heat of boiling water

Steam condensing on skin releases 2,257 kJ/kg against 334 kJ/kg for boiling water cooling to 20°C, a ratio of 6.8, so a steam burn is far deeper than a boiling-water scald of the same mass. Keep hands and faces away from steam outlets and open lids away from you. Cool burns with running water (Thermal Injury Care).

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