A Deep Winter Greenhouse (DWG) is a greenhouse designed to limit the amount of fossil fuel it takes to grow crops during cold winters. DWGs are passive-solar greenhouses that rely on energy from the sun to heat the building instead of more traditional heating sources.
There are a few important aspects of the design that make this possible. DWGs are built in an east-west position, with a glazing wall that faces south. This wall can be specially angled, depending on latitude, to get the most possible solar energy on the coldest day of the year. The sun heats the air inside which is blown underground with a fan and stored in a thermal mass made of rock or soil. This heated thermal mass acts as a heat battery and stores heat for when it is needed at night.
DWGs in can be used to grow crops that thrive with minimal light, providing year-round production capacity for small-scale farmers and gardeners. Crops well-suited to DWG production include a variety of lettuces, herbs, brassicas, Asian greens and sprouts.
Additional resources on building underground greenhouses can be found here (especially at the bottom of the article!)