Design Criteria of Greenhouse for Cooling and Heating Purposes

A greenhouse is a structure covered with transparent material to admit natural light for plant growth. It provides a controlled environment to optimize growing conditions for crops. When designing a greenhouse for cooling and heating, several key criteria must be considered to ensure optimal temperature control, energy efficiency, and plant health.

Design Criteria for Construction

Site Selection and Layout:

• Land Requirement: Choose a site larger than the immediate need to allow for future expansion. The ultimate size of the greenhouse range should be estimated by considering the crops to be grown and the area required per plant. Additional space for service buildings, storage, access drives, and parking should be considered.
• Service Building Area: Small firms require service buildings to be about 13% of the greenhouse floor area, decreasing to about 10% as the firm size increases. Service buildings should be centrally located in a nearly square layout to minimize the movement of plants and materials.
• Door Dimensions: Doors between service buildings and the greenhouse should be wide enough to utilize the full width of corridors. Common dimensions are at least 3.1 m wide and 2.7 m high.
• Height and Clearance: Greenhouse gutters should be at least 3.7 m above the floor to accommodate automation systems, thermal blankets, and allow space for future innovations.

Cooling Systems in Greenhouses Greenhouses require effective cooling systems to maintain optimal temperatures, especially during the summer. Two primary cooling systems are used:

Natural Ventilation:

• Overview: Natural ventilation relies on the natural movement of air to cool the greenhouse. It is a passive system that doesn’t require mechanical devices. In tropical regions, greenhouses often have open sides to maximize airflow, primarily acting as rain shelters.
• Ventilator Design: Ventilators are positioned on both roof slopes adjacent to the ridge and on both side walls. Typically, ventilators on the roof and side walls each account for about 17-10% of the total roof area.
• Seasonal Adjustment:
  • Winter Cooling: The south roof ventilator is opened in stages for cooling. If more cooling is needed, the north ventilator is also opened.
  • Summer Cooling: The south ventilator is opened first, followed by the north ventilator to optimize airflow.
• Chimney Effect: As air enters the greenhouse, it is warmed by sunlight and internal heat sources. Warmer air becomes lighter and rises, exiting through the roof ventilators, creating a chimney effect. This movement draws cooler air through the side ventilators, creating a continuous cycle of air circulation.
• Limitations: Natural ventilation may not provide sufficient cooling on extremely hot days. Water can be sprayed on the interior walls and floor to enhance cooling efficiency.

Forced Ventilation:

• Overview: Forced ventilation uses mechanical devices, such as fans, to expel warm air and bring in cooler outside air. It provides uniform cooling and precise temperature control.
• Components and Types:
  • Propeller Blade Fans: Low-pressure, medium-volume fans are typically used.
  • Air Intake and Exhaust: Fans are placed at one end of the greenhouse opposite the air intake. Air intakes are covered with gravity or motorized louvers to control airflow.
  • Motorized Louvers: Louvers are synchronized with fan operation to prevent wind from affecting airflow, especially when heating is active.
  • Wall Vents: Wall vents should be continuously placed across the end of the greenhouse to avoid hot spots in the crop zone.
• Cooling Methods:
  • Evaporative Cooling (Fan-and-Pad System): Fans are installed on one wall, and cooling pads are on the opposite wall. As air passes through the wet pads, it cools through evaporation before entering the greenhouse. Common pad materials: excelsior (wood fiber), aluminum, glass fiber, plastic, and cross-fluted cellulose. Best suited for low-humidity environments.
  • Fogging Systems: Fogging systems create a fine mist to cool the air through evaporation. They require clean water free of soluble salts to avoid clogging nozzles. Used as a secondary cooling method when natural ventilation is insufficient.
• Hybrid Ventilation Systems:
  • Passive and Active Systems Combined: Passive (natural) ventilation is the first cooling stage. Fan-pad evaporative cooling is activated when passive ventilation is inadequate. Initial costs are higher, but operational costs are minimized in the long run due to energy savings from natural ventilation.

Heating Systems in Greenhouses Heating is essential to maintain optimal temperatures during colder months. Several systems are used for greenhouse heating:

Unit Heater System:

• Description: Most common and least expensive heating system.
  • Warm air is blown from unit heaters with self-contained fireboxes. Heaters consist of:
    • Firebox for fuel combustion.
    • Thin-walled metal tubes for heat transfer.
    • Fan to circulate heated air in the greenhouse.
• Operation: Exhaust gases pass through metal tubes, transferring heat to the metal walls. Greenhouse air is drawn in, heated, and circulated back into the environment.

Central Heating System:

• Description: A central boiler produces steam or hot water distributed throughout the greenhouse. More efficient for large greenhouse ranges compared to unit heaters. Multiple boilers may be used in large setups.
• Heat Distribution:
  • Overhead Pipe Coil: Pipes run across the greenhouse, radiating heat.
  • In-Bed Pipe Coil: Embedded pipes in the plant zone.
  • Concrete Floor Coil: Pipes embedded in the floor for radiant heating.

Radiation Heating System:

• Description: Gas is burned within overhead pipes, radiating heat to plants directly. Uses low-intensity infrared radiant heaters, saving up to 30% fuel.
• Advantages: Lower air temperatures are needed since plants and the root substrate are heated directly. Multiple heaters can be used in tandem for uniform heat distribution.

Solar Heating System:

• Description: Solar heating is used as a partial or total alternative to fossil fuel systems. Rarely used in commercial greenhouses due to high costs.
• Components:
  • Solar Collector: Flat plate collector absorbs solar energy.
  • Heat Storage Facility: Water or rock bed stores heat.
  • Heat Exchange System: Transfers stored heat to greenhouse air.
  • Backup Heater: Activated when solar energy is insufficient.
  • Control System: Manages the transition between solar and backup heating.
• Efficiency: Depending on location, solar systems can provide 20-50% of heating requirements.

Water and Rock Storage:

• Water Storage: Water can store 4.23 kJ of heat per kg for each 1°C rise in temperature.
• Rock Storage: Rocks store heat in air spaces and release it when temperatures drop. Typically used with solar heating systems for heat retention.