Specific Heat Capacity

Leave a metal spoon in hot soup for ten seconds and it burns your hand. A wooden spoon in the same soup is barely warm. Both absorbed heat from the same source, but the metal heats up far faster per gram. The property that governs this difference determines why water is the universal coolant in power generation.

Specific heat capacity is the energy required to raise one kilogram of a substance by one degree Celsius, measured in J/(kg·K). The formula: Q = mcΔT, where Q is heat energy, m is mass, and ΔT is the temperature change. Water's specific heat (4,186 J/(kg·K)) is roughly 10 times that of steel (450 J/(kg·K)). Water absorbs enormous amounts of heat with modest temperature rise, which is why cooling water carries waste heat away from nearly every thermal power plant on Earth.

This same property makes water the working fluid of choice for steam turbines in coal, nuclear, and concentrated solar plants. It also explains why molten salt (specific heat ~1,500 J/(kg·K)) is used in thermal storage: it operates at much higher temperatures than water without boiling, storing more total energy per unit volume.

Worked Example

A thermal storage system uses 10,000 metric tons of molten salt heated from 290°C to 565°C (a 275°C swing).

• Calculate stored energy. Q = mcΔT = 10,000,000 kg x 1,500 J/(kg·K) x 275 = 4.125 x 10¹² J = 4,125 GJ.
• Convert to MWh. 4,125 GJ / 3.6 GJ per MWh = 1,146 MWh.

How long could this storage run a 100 MW turbine?

1,146 MWh / 100 MW = 11.5 hours. Enough to carry a concentrated solar plant from sunset through the night. The combination of mass, specific heat, and temperature range determines how much energy you can bank.

Specific heat capacity is why power plants are sited near water, why thermal storage works, and why ocean coastlines have milder climates than inland areas.

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