Storing Heat Energy

Heat capacity is the ability of a body to absorb heat without actually getting hot, and is defined as the amount of heat energy that must be supplied to a body to raise it's temperature by 1°C. Bodies with higher heat capacities can absorb more heat. The specific heat capacity of a body or material is the amount of heat per kg needed to raise the temperature by 1°C. The table below shows the specific heat capacities of some substance.

The specific heat capacities of metals are low in general compared to other materials. Water has quite a high specific heat capacity. It needs a lot of energy to heat up, but once heated, it can act as a store of a lot of energy. It is for this reason that a hot water bottle is so useful in keeping you warm in bed. If an equal weight of mercury were used, it would only store one thirtieth the amount of energy. For the same reason, water is the best liquid to use in a central heating system, and the cooling systems of vehicle engines and power stations. The high specific heat capacity of water helps to keep our body temperature stable. The UK has a much more temperate climate than Siberia, despite being at the same latitude. The sea around the UK acts as a sink and store of heat, able to absorb heat when it gets hot and release it when it gets cold. Siberia has no such heat store, and quickly heats up and cools as the weather dictates.

Night storage heaters use concrete blocks to store heat. The blocks are heated using cheap electricity at night and the stored heat energy is slowly released during the day. Though concrete has a lower specific heat capacity than water, it is more more dense, so energy stored per unit volume is not so different.