Thermal Energy Storage

Electricity Storage

Electricity storage has been described as the holy grail of the renewables industry. It is very difficult and expensive to store electricity, so every effort is made to provide electricity at the time it is needed. PV cells only generate electricity by day, and so cannot be used to meet lighting demands at night. Turbines generate electricity when the wind blows, but this does not often match times of peak demand. The problem is huge and rewards for solving it will be very large.

Thermal Energy Storage

There is an equivalent problem with heat storage. There is plenty of surplus heat on summer days, but the peak demand for heating comes on winter nights. For this reason Seasonal Thermal Energy Storage has also been described as the holy grail of the renewables industry.

Thermal Energy Storage is an integral part of ground source energy. Without the benefit of thermal energy storage provided by the thermal inertia of the ground, a ground source heat pump would have no performance advantage over an air source heat pump. An air source heat pump extracts heat from ambient air: as the air temperature falls an air source heat pump becomes less efficient. So on the coldest days, and nights, when space heating is most needed an air source heat pump is at its least efficient.

In cold conditions a ground source heat pump scores well as it can heat exchange with higher temperatures from the ground than are available from the air. In this way a ground source heat pump exploits the thermal energy storage capacity of the ground.

Enhancing the Thermal Energy Storage capacity of the ground by storing Naturally Occurring Energy

It is possible to exploit the thermal energy storage capacity of the ground by charging the ground with naturally occurring energy from the sun in the summer to enable it to yield heat more easily in winter. The temperature of the ground can be increased in summer by circulating spare heat from solar thermal collectors through the ground. It can also be charged with the heat extracted from a building in summer as a by-product of renewable cooling. Renewable cooling is more efficient than using roof mounted chillers for air conditioning – it also improves the efficiency of renewable heating the following winter. It saves on the capital cost as well, for one system can be used to provide heating in winter as well as cooling in summer.

Demand Side Response

Because it is so difficult to store surplus electricity the generating companies are now offering cheaper electricity at times of surplus production (often at night): there is thus an opportunity to use cheap electricity to transfer heat into buildings with heat pumps and store the heat in the thermal mass of buildings at night – in order to reduce the need to run heat pumps on expensive electricity by day.

To benefit from Demand Side Response any small organisation will need to work through a “Demand Side Response Aggregator” which can parcel up a large number of consumers of small amounts of electricity into sufficient volume to gain good rates from the major electricity distributors. You will also need a sophisticated control system with internet access to the Aggregator to take advantage of this opportunity. See Demand Side Response for heat pumps.