Section 1: Introduction
For most climates, there exists at least a period of time during the year where the ambient temperatures outside are too low for crop production, or the low temperatures that occur would result in significantly reduced crop productivity. This is the primary reason behind greenhouse-based agricultural production, whether it is for ornamental plants, forestry, or vegetable crops. Therefore, providing heat energy to maintain optimal temperatures within the greenhouse (or the hotbed, Growth Chambers, etc.) is a critical function in greenhouse management.
Heat energy may be measured as calories, joules, or even horsepower. However, in the greenhouse industry (especially in the U.S.) it is most often measured as British thermal units (Btu). A Btu is the amount of heat energy required to raise 1 pound of water 1° F.
In greenhouse heating, heat energy must be added to the greenhouse at the rate that it is lost from the greenhouse in order to maintain the established temperature. The higher the rate of heat loss from the greenhouse, the higher the amount of heat energy that must be generated and added back to the greenhouse. There are 3 ways that heat is lost from greenhouses:
- Conduction is the loss of heat energy through the glazing, metal purlins, barcaps, doors, and fans (i.e. like heat energy moving from hot coffee or soup through a spoon sitting in the coffee or soup). The vast majority of conductive heat loss is through the glazing, and most heat loss from greenhouses occurs through conduction.
- Infiltration/exfiltration is the heat energy loss through cracks between or around glass panels, doors, and fans by mass airflow. Even in a well-designed "tight" greenhouse, up to 10% of total heat loss may be by infiltration/exfiltration. In older or less “tightly sealed” greenhouses, a much higher amount of heat loss may occur through infiltration/exfiltration.
- Radiation is the heat energy loss due to the emission of radiant energy from a warm body (greenhouse) to a cold object (outside objects) with little warming of the air. Glass, vinyl, plastic, FRP, and water do not readily allow the passage of radiant energy, whereas polyethylene film readily allows the passage of radiant energy.
