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
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.