Passive Solar Greenhouses

Figure 29: Passive solar green house design. Source: Scott Sanford

Figure 29: Passive solar green house design. Source: Scott Sanford

All greenhouses are solar powered, but the traditional greenhouse has such high heat loss, that it cannot sustain itself during cold weather without supplemental heating. Greenhouses can be optimized for winter production by orienting the length of the greenhouse east-west with double-wall glazing only on the south roof of the greenhouse tilted at about the same angle as the latitude of your location to catch the most winter sunlight. The south wall may be covered partially with double wall glazing but the north, east, and west walls, unglazed portion of the south wall and north roof area are insulated, and a thermal curtain is used to cover the glazing at night to reduce heat loss. During a sunny winter day there can be enough solar heating that the ventilation system turns on in a greenhouse. If a thermal heat sink is installed in a passive solar greenhouse, the excess daytime heat can be collected for use at night. A heat sink can be a wall of dark colored barrels filled with water, brick, or rock placed along the north wall of the greenhouse. In addition sometimes barrels, bricks or rocks can also be placed under benches.  Sand or water tanks can be installed under the greenhouse floor with ducts or pipes that collect the excess heat during the day and actively move it to the heat sink.

The temperatures in a passive solar greenhouse will need to be allowed to vary over a larger range than in a conventional greenhouse, because to get heat into the heat sink (water wall), the supply temperature (greenhouse air temperature) has to be higher than the heat sink temperature, and to remove heat from the heat sink, the air temperature (greenhouse air temperature) needs to be lower than the heat sink temperature.

Active thermal solar systems, such as for heating water, typically aren’t economical for greenhouse space heating, but can be useful for heated water.

Figure 29 shows a typical passive solar greenhouse for wintertime plant production. The north roof and north, east and west walls are well insulated. The glazing is facing south and tilted at approximately the same angle as the latitude (approximately 40-48 degrees for northern U.S). The greenhouse will contain a heat sink (typically water in barrels) to absorb heat during the day and radiate it into the greenhouse at night. The glazing is covered by an energy curtain at night to further retain heat. During the winter the sun is low in the south sky and will penetrate to the back of the greenhouse and warm the heat sink along the north wall and barrel under benches if used.