One of the main principles involves embedding the greenhouse in the earth to take advantage of the earth’s constant temperature, to store the solar energy collected during the day. Water barrels can also be used to store the thermal heat and carry it through the night or cloudy days (which are not as cold). Water is a much better thermal mass storage mechanism than soil. This is perfect for us preppers and hardcore gardeners, I would also take advantage of this and maybe install an acquaponics system in this space too or a small fishery.
The solar gain comes through a light-permeable material such as plastic, Visqueen, polycarbinate. The angle of the panels is designed to be 90-degrees to the Winter Soltace sun (Dec. 21 / June 21, depending on hemisphere). The the upper portion of the walls are insulated down past the frost line. to learn more about underground greenhouses.