Energy storage heating and cooling

Sources of natural energy are intermittent. To obtain continuous flows of energy using such sources therefore requires systems of storage. As stated earlier, this is not a new concept since, in the Middle Ages, tide mills stored water at high tide in order to release it at an appropriate rate to turn the water wheel during the ebb tide.

Energy storage offers an efficiency and cost gain in two respects. First, in buildings that optimise solar gain, surplus solar energy can be used to charge a storage facility to be used later for space heating. Second, storage can help to flatten the peaks of electricity costs by charging the store with off-peak electricity and using the stored power to reduce demand at peak periods.

The storage potential of energy is available for three purposes: heating, cooling and the storage of electricity.

Heat storage

The most straightforward method of storage is by means of a network of pipes carrying solar heated air though a reasonably dense medium such as bricks, concrete blocks or water. The storage container is heavily insulated. If sufficient space is available below a building, enough heat can be stored to supplement space heating through the whole of the heating season, hence the term 'seasonal storage'. Alternatively, off-peak or PV derived electricity may be used as the heating element. More sophisticated is the use of a phase change material such as sodium sulphate which works on the principle of the latent heat of fusion. Called eutectic or 'Glaubars' salts this medium turns from solid to liquid at around 30°C and then gives off heat as it solidifies.

Cool storage

As the automatic inclusion of full air conditioning is increasingly being questioned, the problem of space cooling enters a new dimension. Again the principle is to use spare energy, off-peak or PV electricity, to refrigerate a medium. At its crudest, the medium may be the earth beneath a building. A more practicable method is to use phase change and the latent heat of fusion as above to provide high density storage. One option called the STL storage system comprises a storage vessel containing spherical polyethylene nodules filled with a solution of eutectic salts and hydrates. This system is ideal in situations where there is cyclic demand since it facilitates cooling (or heating) when energy costs are at their lowest or a plant is shut down. In conjunction with air conditioning, this system can result in a dramatic lowering of the required capacity of the chiller unit. The system may be given a lift in efficiency by the use of heat pumps which provide either cooling or warmth on the principle of a refrigerator.

As indicated earlier, the building fabric can be a significant energy storage system on the basis of thermal mass. Heat absorbed by the structure flattens the peaks and troughs of temperature. Exposed concrete floors have been cited as an efficient storage medium for convec-tive and radiative heat transfer. It is worth noting again that it is the outer 100 mm of the fabric which comprise the effective thermal mass. The effectiveness of the underside of the floor is negated by suspended ceilings. However, a compromise solution is perforated tiles which have an open area of 42 per cent which is sufficient to allow 91.6 per cent overall heat transfer whilst concealing services.

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