A greenhouse traps solar energy, rapidly converting light into heat, which can quickly lead to dangerous temperatures for plants. Managing this environment without electrical systems requires a strategic approach that harnesses natural physical principles. Passive cooling utilizes methods that prevent heat from entering, remove it efficiently, or stabilize the internal temperature. This involves manipulating solar radiation, air movement, and the thermal properties of materials to maintain a healthy growing space.
Minimizing Heat Entry Through Shading
The most effective defense against overheating is blocking intense solar radiation before it enters the structure. External shading is significantly more effective than internal methods because it intercepts sunlight and prevents the conversion of light energy into heat inside the greenhouse. Once radiation passes through the glazing, it converts to long-wave infrared radiation, which is then trapped, creating the greenhouse effect.
Shade cloths are a common external option, typically woven polyethylene or polypropylene materials stretched over the exterior glazing. They are rated by the percentage of light they block, with 30% to 50% being common for warm-weather vegetable growing. White or aluminized cloths reflect a greater portion of the sun’s energy away from the structure.
A simple, temporary alternative is applying shading compounds, often called whitewash, directly to the exterior glazing. These liquid formulas are sprayed or painted onto the surface in the spring to create a reflective coating. The coating gradually wears off due to rain or can be removed with a cleaner in the fall.
Another non-structural shading method involves planting deciduous trees on the south and west sides of the greenhouse. In the summer, the fully leafed trees provide a natural canopy, casting shade during the hottest parts of the day. As the seasons change, the leaves drop, allowing maximum light penetration to warm the structure during the cooler winter months.
Maximizing Air Exchange Through Passive Ventilation
Once heat enters the structure, the objective is to remove it efficiently through passive ventilation. This method relies primarily on the stack effect and wind pressure to move air without fans. Hot air is less dense than cool air, causing it to rise naturally toward the highest points of the enclosure.
To exploit thermal buoyancy, the system requires large, high-level openings, typically ridge vents, and corresponding low-level openings, such as side vents or roll-up sidewalls. As hot air escapes through the ridge vents, it creates negative pressure, drawing cooler, denser outside air through the lower vents. For optimal performance, the total area of the vents should be at least 15% to 20% of the greenhouse floor area.
The difference in height between the inlet and outlet vents directly relates to the strength of this air movement; the taller the structure, the more pronounced the stack effect becomes. Strategically orienting the greenhouse with its length perpendicular to the prevailing summer winds can also encourage cross-breezes, assisting the air exchange. This wind-driven ventilation works in tandem with the stack effect to ensure a continuous and vigorous flow of air.
To automate this process without electricity, mechanical vent openers use a hydraulic cylinder filled with temperature-sensitive mineral wax. As the greenhouse temperature rises, the wax expands, pushing a piston that mechanically opens the vent. When the temperature drops, the wax contracts, and a heavy spring pulls the vent closed, providing automatic temperature regulation without external power.
Utilizing Water and Earth for Temperature Stabilization
Supplementary methods focus on stabilizing temperatures using the thermal properties of water and the ground. Evaporative cooling exploits the principle that water absorbs latent heat when it changes phase from liquid to gas. Simply wetting the floor, pathways, or benches provides a large surface area for evaporation, drawing heat out of the immediate air.
A low-tech approach involves placing large containers, like barrels of water, inside the greenhouse to act as a thermal mass. Water has a high specific heat capacity, meaning it can absorb and store a large amount of heat energy from the air during the day without a significant rise in its own temperature. This stored heat is slowly released back into the space at night, buffering temperature swings. Concrete floors and stone structures also function as thermal mass.
For a subterranean solution, earth tube systems, also known as ground-coupled heat exchangers, utilize the constant temperature of the earth a few feet below the surface. A network of buried pipes draws hot air from the top of the greenhouse and runs it underground. The cooler surrounding soil absorbs the heat, and this pre-cooled air is then drawn back into the greenhouse.
The system works passively by connecting the earth tubes to the low-level air intake, allowing the stack effect to pull the cooled air into the structure. As the warm, humid air cools within the tubes, moisture condenses out, slightly dehumidifying the air while transferring heat to the soil. Proper installation requires the pipes to be perforated and sloped to allow condensation to drain away.