"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.
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.
The word "Walipini" comes from the Aymara Indian language and means "place of warmth". They've been able to grow banannas at 14,000 feet elevation in the Andes."
The Walipini PDF file