Sand retains its warmth for months, just as white-hot heat can generate electricity.
Now a venue manager from Glasgow has discovered another means of producing energy, providing climate control by using the body heat of venue-goers.
To cool rooms, special ceiling-mounted devices collect heat from the air and transfuse it into fluid.
Using boreholes, this, in turn, is funnelled 200 metres underground, where it is stored until further notice.
Then, water pumps transmit it back above ground to heat rooms, or even to supply hot water.
A capacity crowd of 1250 concert-goers could generate up to 800 kilowatt hours, while the geothermal system could save 70 tonnes of CO2 annually.
These flexible generators would be useful too.