Conventional greenhouses rely on natural gas designs to keep them warm in the winter. Although heat can be felt during the winter months in an unheated greenhouse, the ground will still become very cold and most plants will hibernate or die.
Solar greenhouses use thermal banking designs to create a sustainable approach to growing all year long. Thermal banking is simply storing energy (heat) that is available and using it in various ways at a later date. A thermal banking greenhouse uses passive solar design and other innovative techniques to trap and save the excess heat that is normally ventilated out of a greenhouse and transfers it through a ventilation system to heat the greenhouse in different ways.
Steven Schwen from Lake City, Minnesota has created a thermal banking greenhouse at his Earthen Path Organic Farm. He collects and stores the heat from the sun and uses it to heat the ground underneath his garden beds during the cold winter months. Using one long pitched glazed roof that allows the hottest air to be trapped at the highest point of a ridged compartment, he can control if the heat is ventilated or suctioned out and into his ground heating system. Digging 3 foot deep trenches allowed him to place the heating source right under his garden beds keeping them warm all winter long. This has allowed him to use the sun to his advantage all year and not have to rely on unsustainable heating sources.
One other passive solar techniques that Schwen used was solar orientation. He knew that if he built his greenhouse with the long pitched glazed roof oriented East to West, he would be able to take advantage of the solar heat absorption all year long, even at the sun’s lowest point in the winter. Conventional greenhouses orient their placement North to South which only benefits the house fully in the warmer season.
Schwen also used recycled materials to fill in some of the trenches and cover the heating tubes. The soil used to fill in the beds was mixed with composted manures from his farm, peat moss and sand for the perfect water and air percolation. Soil thermometers that reach up to 20 inches deep will monitor the temperature of the soil to keep track of the heat during a regular season and try to understand how to mimic the same patterns in the winter months. A thermometer was also placed in the hot air compartment to tell the fan when to start blowing the air if it raised past a certain temperature.
Schwen’s greenhouse was partly funded by SARE’s Farmer-Rancher Grants program and has allowed him to prolong his growing season while lowering his energy costs. It is a great example of a sustainable greenhouse design. -KATIE FLYNN