Greenhouse Climate Control

TEMPERATURE CONTROL:

few hot or cold spots; small temperature difference between hottest and coldest spots
manage/minimize temperature gradients in greenhouse

thermostatic control is a must; shield from direct sun; thermostat in a representative location (near crop, at crop height)
manual control must be available in case of system failure (may not be possible with heating)
both heating and cooling should occur in stages

hot air rises
ethylene (frequently found in combustion exhaust) negatively impacts most flowers; combustion directly into greenhouse needs: pure fuels, proper adjustments.
inventory of cool or hot spots should be made; can be used to speed up and slow down portions of crops.
uniformity can be enhanced with horizontal air flow (HAF)

piped through greenhouse in metal pipes, sometimes with fins
temperature of water can vary; the hotter - the less pipe is needed
water is moved with circulating pump
control: water temperature, number of circuits, circulating pump speed or on/off

piped through greenhouse in metal pipes, sometimes with fins
temperature of steam will not vary much; steam carries a lot more heat than hot water; less piping is needed.
heating coils change temperature very rapidly
steam can be moved over long distances
control: number of circuits, motorized valves open/closed.

air is heated by burning natural gas; exhausted to outside (otherwise air pollutants and ethylene can cause damage)
Horizontal discharge unit heaters

mounted either overhead or on the ground
measures should be taken to distribute the hot air uniformly: perforated tube attached to heater is used to blow hot air to all areas of the greenhouse. Heater should be adjusted properly to prevent melting of tube.

consists of small heater which fires directly into a steel tube suspended over the plants (reflector behind tube).
just as light is diminishes as it passes through the canopy, so does infrared radiation. Consequently there is a problem with any crops with high LAI or large canopy (plenty of heat at the top; not enough at the bottom).
studies have shown reduced quality for some pot crops (pot roses, easter lilies) resulting in taller plants
suitable in some situations (e.g. propagation) or in conjunction with other heating systems (e.g. bottom heat)
heat object without heating air; air temperature can be somewhat cooler
thermostat must be shielded from direct infrared rays

warm or hot water is pumped through tubes immediately below plants; heats root-zone, rising warm air heats foliage from below.
root-zone dries out much faster
most growers raise pots so they are not in direct contact with the tubes to prevent accidents and having root zone temperature much higher than foliage temperature.

roof or side vents - a must for every greenhouse
used to replace air inside with air from the outside which has a more desirable temperature.
if outside temperature is extremely cold, care should be taken not to let blast of cold air hit plants directly
control: degree to which vents are open, number of vents opened
useful only for small temperature reductions; only if outside air is significantly cooler than desired temperature.
direction of prevailing winds will affect effectiveness
cooling of up to 10F can be achieved, slow without wind

Exhaust fans pull air out of greenhouse; outside air enters through open vents.
control: fan motor speed, number of fans on.
if outside temperature is extremely cold, care should be taken not to let blast of cold air hit plants directly; mix cold air into greenhouse air
use perforated tube attached to intake vent to mix and distribute air uniformly.
cooling of up to 20F (maximally) can be achieved
there will be a gradient if side vents are used exclusively

in addition to exhaust fans, walls on opposite side are replaced with pads through which the air enters the greenhouse. When the pads are wetted, evaporation cools this air.
roof vents and doors should be closed
cooling degree depends on relative humidity; i.e. less effective at night
control: same as forced-air plus on/off for pump delivering water to pads.
there will be a gradient; should be a design criterion: path traveled by air should be short enough so that gradient is less than 7F
pads extend full length of wall - no gaps.
material should be durable
cooling: 20F cooler than with just vents, can be achieved

Greenhouse Pads. Water flows over these as an evaporative cooling method

Exterior of pads where air intake occurs. Note the vents that can be opened or closed to control air flow.

high pressure generated water droplets; evaporate before striking plant or ground
operates in cycles (pulses)
evaporation results in cooling - resulting humid air must be removed when relative humidity raises above 90%
potential problems with water on foliage - disease
control must be very precise; water must be very clean
mist (1000 micron diameter) vs fog (40 micron diameter)

inside greenhouse, used to shade plants from direct sun, reducing heat-load on plants
automated curtains, thermostatically controlled

Shade curtains inside a greenhouse

Exterior shade cloth