A self-sustaining greenhouse is able to "run itself," in terms of temperature regulation and energy production. Many passive greenhouses meet this requirement and aren't any more expensive or difficult to operate than those dependent on the grid. In fact, without dependence on technology, they can be simpler to maintain and cheaper to operate. To make a passive greenhouse, set it up in an area with full sun and orient the long side of the greenhoues so it faces south. Choose a clear cover that allows maximum light transmission and doesn't yellow over time from UV expsosure. Certain covers also offer radiant protection which prevents the interior heat from passing back out, a great advantage in cold winter climates.
Take advantage of thermal mass to warm your greenhouse over cool nights and keep it cool in the summer. The easiest thermal masses to use in greenhouse are containers of water (large barrels or reused milk jugs, for example). Painted black and exposed to sun during the day, the water inside will heat up from the sun, then gradually release the heat in the greenhouse overnight. In summer, move the masses to a shaded area during the daytime, where they will absorb warmth from the air in the greenhouse. Keep your most heat-sensitive plants close to the cooling effect of the water containers. Significant heat can be added during winter by placing compost bins within the greenhouse. One centrally placed bid may keep a smallish greenhouse toasty, will larger structures may require several bins distributed throughout the greenhouse.
