1. Introduction
Greenhouses create a managed microclimate that allows crop production beyond the limits of the outdoor environment. Proper temperature control—both preventing overheating in summer and providing sufficient heat in winter—is central to crop health, yield and energy cost control. Design must therefore consider passive measures (orientation, glazing, insulation, thermal mass) together with active systems (ventilation, evaporative cooling, forced heating) and modern control strategies.
2. Heat transfer fundamentals and climatic inputs
Design decisions hinge on three thermal processes: (1) solar and internal heat gains, (2) conductive/convective/radiative losses, and (3) ventilation/infiltration exchange with outdoor air. Local climate (peak summer temperatures, winter minima, solar intensity, humidity and prevailing winds) sets baseline loads; the greenhouse must be sized and detailed to handle these loads while minimizing energy use.
- Daily and seasonal solar radiation at site
- Design cold and hot outdoor temperature extremes
- Desired inside setpoint ranges for the crop
- Acceptable humidity limits
3. Glazing, coverings and insulation
Material selection: glazing choices (single/double glass, twin-/multi-wall polycarbonate, polyethylene films) determine light transmission, U-value (thermal transmittance), durability and initial cost. Multi-wall polycarbonate gives improved insulation over single glass while maintaining high diffuse light transmission and impact resistance; double-pane glass provides high clarity but is heavier and can be costlier. Choose glazing based on the balance of light needs, thermal performance, maintenance and wind/snow loads. :contentReference[oaicite:0]{index=0}
Insulation and thermal screens: Night curtains or energy/thermal screens reduce nocturnal radiative and convective losses by trapping a layer of still air. Well-fitted thermal curtains commonly reduce winter heating need substantially and can also function as movable shading during hot periods. Use automated deployment to maximize effectiveness when temperatures change rapidly. :contentReference[oaicite:1]{index=1}
4. Heating system design criteria
- Sizing for extreme conditions: heating capacity should be sized to meet the worst-case design cold day for your location plus a margin for infiltration and distribution inefficiencies.
- Distribution uniformity: use under-bench heating, perimeter radiant tubes, or low-velocity warm-air distribution to avoid cold pockets at plant level. Radiant systems reduce air temperature stratification and can be efficient in larger houses.
- Control and modulation: modulating burners or variable flow hydronic systems reduce cycling and save energy compared to simple on/off control. Integrate thermostat, sensor clusters and automated curtains for coordinated response.
- Sealing and airtightness: reduce uncontrolled infiltration by sealing frames, doors and service penetrations—this directly lowers heat load and fuel consumption.
5. Cooling system design criteria
Ventilation: Venting is the fundamental cooling method in most greenhouse designs. For natural ventilation systems, roof (ridge) vents together with sidewall vents are used to create stack effect and wind-driven flow. A commonly used design guideline is that combined vent area (roof and side) should be a significant fraction of floor area—practical guidance in engineering literature often cites roof/vent area on the order of 15–20% of floor area as a useful rule of thumb for effective natural ventilation; continuous ridge vents and adequate sidewall venting should be sized and located to encourage cross flow. Automatic vent actuation gives much better temperature control than manual systems. :contentReference[oaicite:2]{index=2}
Forced ventilation & circulation: In climates or seasons where natural ventilation is insufficient, use exhaust fans sized to provide required air changes (expressed in CFM per ft² or m³/h per m²). Internal circulation (HAF—horizontal airflow fans) keeps air mixed and prevents hot strata near the roof while maintaining a uniform crop environment.
Evaporative cooling: Wet-pad/fan systems and high-pressure foggers can lower greenhouse air temperature in dry climates by evaporative cooling. They can produce substantial temperature drops but raise relative humidity; excessive canopy wetness increases disease risk, so system selection must follow the crop’s disease tolerance and ambient humidity constraints. Mist systems are better for spot cooling; whole-house pad-and-fan systems are preferred for controlled humidity and whole-space cooling in arid regions. :contentReference[oaicite:3]{index=3}
6. Passive thermal storage (thermal mass)
Incorporating thermal mass—water barrels, sealed containers, masonry walls or phase change materials—smooths daily temperature swings. Water is commonly used because of its high heat capacity; when placed where it receives direct or diffuse sunlight, it stores daytime heat and releases it overnight, lowering night heating demand. Phase change materials offer higher storage density per volume but at greater cost. Use thermal mass in combination with good insulation and thermal screens to be effective. :contentReference[oaicite:4]{index=4}
7. Integration: controls, deadbands and coordination
Design heating and cooling not as isolated subsystems but as coordinated elements under a control strategy. Important items:
- Deadbands: set non-overlapping temperature ranges for heating and cooling (deadband) to prevent simultaneous operation and short cycling.
- Coordinated shading: energy curtains and retractable shade screens should be sequenced with ventilation and evaporative systems to balance light, temperature and humidity needs.
- Sensor placement: place multiple sensors at canopy height and at various locations to capture gradients and avoid local over-reaction to a single sensor reading.
8. Crop-specific and site-specific considerations
Crop type sets allowable temperature, humidity and light ranges; ornamentals, leafy greens and fruiting vegetables differ in their sensitivity to high humidity, leaf wetness and night temperature. Similarly, local site factors—wind exposure, shading from adjacent buildings or trees, and the seasonal pattern of solar radiation—must be integrated into the final design choices.
9. Practical sizing and rules of thumb (summary)
- Natural vent area: aim for combined roof/side vent areas in the order of 15–20% of floor area as a starting guideline for effective natural ventilation (adjust for local wind and crop load). :contentReference[oaicite:5]{index=5}
- Thermal screens: install an energy curtain sized to fully cover the glazed area at night—this reduces radiative losses and can cut winter heating demand substantially when used properly. :contentReference[oaicite:6]{index=6}
- Evaporative cooling selection: choose pad-and-fan systems for whole-house cooling in dry climates and reserve mist/fogging for spot or short-duration cooling where leaf wetting is acceptable. :contentReference[oaicite:7]{index=7}
- Thermal mass: consider water barrels or masonry east/north walls for passive storage; combine them with night curtains to get measurable fuel savings. :contentReference[oaicite:8]{index=8}
10. Maintenance, durability and economics
Durable materials, easy access for maintenance and a life-cycle cost approach are essential. Higher upfront costs for insulating glazing, automated screens, or variable-output heating can be justified by fuel savings and improved crop quality—run a site-specific cost/benefit and payback analysis before committing to expensive options.
11. Conclusion
Successful greenhouse thermal design blends passive strategies (orientation, glazing, insulation, thermal mass) with active systems (ventilation, evaporative cooling, heating) under a carefully tuned control scheme. Follow proven sizing guidelines (vent areas, curtain coverage), use crop-appropriate cooling methods, and always design for local climate extremes. Thoughtful integration minimizes energy use, stabilizes crop environment, and increases profitability.
