Climate Battery

A greenhouse on a bright cold day throws away more warmth than it can use, then goes cold after dark. A climate battery stores that surplus in the ground directly under the floor. Fans push the hot air gathered near the roof down through pipes buried in the soil, which takes the heat out of it, and at night the same fans draw air back through the warmed ground and return it to the growing space. It is a store and not a source: nothing arrives that the glazing above did not collect, and the mass gives back only what the day put into it.

Two things decide whether it performs, and fans are the first: they are the machine rather than an accessory - one field trial found 25-watt fans moved far too little air and settled on about 300 watts of fan moving 210 cubic metres (7,500 cubic feet) of air a minute. Because the fans work hardest at night, when a solar supply is producing nothing, a battery bank is part of the build rather than an addition to it. The second is coverage. Spreading pipe across the whole floor matters more than burying it deep: the same trial laid 90 perforated pipes at two levels under a tunnel of 9 by 29 m (30 by 96 ft) and found nothing gained below about 1 m (3 ft).

The common mistake is expecting it to cool. In that trial summer cooling could not be measured at all with the tunnel closed up, and the sides still had to be opened to stop it overheating - ordinary ventilation doing the work. Ground charged all summer to around 21 C (70 F) has little cool left in it by the time cooling is wanted. Water condensing in the buried pipes has to be drained, and the drainage is laid while the trench is open or not at all, because the buried half can never be reached again; its sizing and layout are decided once and for good. That half then asks almost nothing, and the running cost is the battery bank - rated at 12 years or 750 discharges to half capacity. No published standard or design specification covers the system.