How To Build A Solar Greenhouse
Original story at Permaculture Magazine• mentions •
Controlling temperature swings in small garden greenhouses is notoriously difficult. Greenhouses are liable to overheating in full sun and, of course, most will do little to keep out a hard frost. Among those who are not keen on using electric or gas heaters in greenhouses there has been a resurgence of interest in 'solar greenhouses' using various methods to trap the solar energy striking the greenhouse and release it at night. At its simplest, the heat storage medium consists of barrels of water, but in a garden greenhouse these can take up a large proportion of the growing space.I describe here an experiment I've done, adapting an elegant idea that has been used with some success in large greenhouses and polytunnels.1,2 In the Subterranean Heating and Cooling System (SHCS), the entire mass of the soil and sub-soil under the greenhouse is used to store heat. Exchange of heat between the air and the soil is enabled by a network of buried perforated pipes through which air is forced via solar powered fans. Thus, on sunny days warm air gives up its heat to the soil as it passes though the pipes and returns to the greenhouse cooler, helping to control the temperature. At night, if the temperature falls below a set point the fans are activated again to warm the air as it passes underground. In order for this to work effectively, the sides of this heat store are insulated.