Using Heat as a Long Term Energy Store

9th Sep 2011


In an earlier article titled ‘A Strategic Local Energy Generation System’ I emphasised the need for back-up sources if renewable power sources such as wind and solar were ever going to provide us with a substantial proportion of our total energy requirements. By using these sources to drive a heat pump, short term differences between electricity supply and demand could be accommodated by simply switching these on and off, since the thermal mass of the building acts as an energy store to smooth out any short term temperature variations.  


Unfortunately, the thermal mass wouldn’t be enough to accommodate longer term storage. This constitutes a serious barrier for high penetration of intermittent supply driven renewable power sources such as wind turbines and photovoltaic collectors. Diversifying and distributing these over a wide area and connecting them with long distance high-voltage inter-connectors may not be sufficient to cope with fluctuations in demand, especially when large anticyclones cover entire continents resulting in low wind speeds and fog. In fact, for a reliable supply of electricity, the power grid would have to cope with the worst imaginable situation. This means that long term energy storage systems, or on-demand generators would be required. However these generators would be still acting as surplus plant during periods of low demand which adds to the overall cost.


Perhaps a better method of obtaining a standby reserve, is to store or extract heat from the bedrock either underneath or surrounding a building. This has another advantage. If the thermal mass is large enough to cover the heating or cooling requirements for an entire winter or summer season respectively, then further efficiency gains can be achieved. Normally heat pumps vent off hot or cold air to the environment when cooling or heating of the building is required.  However if the hot air vented in summer could be stored to heat the building in winter, and the cold air vented in winter stored to cool the building in the summer then the overall system efficiency could be nearly doubled.  This is on top of the already high efficiencies normally achieved by heat pumps relative to direct heating systems.


One possible design is illustrated in the figure below. The heat pump operates whenever surplus wind energy is available, shifting heat from one area of bedrock to another to create a permanent hot and cold store.  These stores can then be accessed for heating or cooling on demand. This design would provide a complete carbon free strategic heating and cooling system throughout the year.  More importantly it would overcome the principal barrier to mass penetration of wind and solar power, that of intermittency by the means of a suitably large but cheap energy store.




Using a Heat Pump and thermal mass of Bedrock to store Heat for Space Heating  



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