Propagation/Greenhouse ManagementUnit 1.3 | Part 1 – 115c) Micro-climate heating: In the form of bottom heat, whether electric mats or hot water
tubing directly on the bench tops, can be the most energy efficient because the grower
does not necessarily strive to heat the air of the entire greenhouse, but rather the soil/
root zone and by extension the leaf canopy through conduction. Heat mats are normally
electric, must be plugged in to a power source and generally are used for smaller-scale
operations. Closed loop, bottom heat hot water tubing, such as the Biotherm system,
can be powered by electricity, gas, or be connected to a solar hot water system to
efficiently heat the root zone. This type of system can be particularly useful for heat-
loving crops such as Solanums and Cucurbits.
- Cooling mechanisms
Cooling mechanisms are required for summer greenhouse production in all but the
mildest environments. In virtually all other growing environments, trapped solar radiation
can create an environment too hot for most seedlings. The importance of active cooling
mechanisms cannot be overstated unless you are only producing heat-loving crops.
Depending on the crops you grow, the size of your facilities, and the nature of your climate,
different cooling mechanisms may be available to you
a) Evaporative Cooling
i. Fan and pad systems combined with exhaust fans are commonly used in actively
managed commercial greenhouses
- Fan and pad systems are electrically powered and are made up of corrugated
cellulose pads housed on one wall of the greenhouse. A water reservoir and pump
system saturates the pads, and a fan evaporates the water in the saturated pads.
Air coming into the greenhouse is cooled via the heat energy absorbed as the
water evaporates. - Fan and pad systems, combined with exhaust fans, must be appropriately sized
for the greenhouse structure, and the environmental conditions that need
modification - These systems work most efficiently in drier climates; in high humidity
environments, systems should be over engineered by approximately 20% to
compensate for the inefficiency of evaporative cooling in already water-saturated
air - While highly effective, fan and pad systems can be costly to operate during peak
electricity rate periods, which coincide with the times/conditions when the
systems are most needed - Typical fan and pad systems operate at about 85% efficiency and have a
temperature differential of as much as 7–10ºF because cooling is centralized at the
fan and pad, and depends on the fans and exhaust system for distribution across
the structure
b) Swamp coolers work on the same principle as fan and pad systems, but are usually
installed in smaller structures, often without active exhaust fans. Instead, the
evaporatively cooled air is moved across the structure by strong fans within the swamp
cooler; warmed air exits the structure passively through ridge and end wall venting.
c) Fog systems also work on the same evaporative cooling principles, but distribute fog
across the entire greenhouse through careful placement of atomizing nozzles
i. Results in nearly 100% cooling efficiency, with temperature differentials at no more
than 1ºF
ii. Can be used in greenhouses with only natural ventilation and/or mechanical
ventilation
iii. Operate under high pressure, with water forced through very fine-aperture fog
nozzles. Clean water and regular maintenance are required to keep the systems
operating properly.
Lecture 3: Greenhouse Climate Control Systems