Greenhouse Climate

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Greenhouse Climate

• Objectives
• Heat in winter
• Cool in summer
• Cool in winter?
• Temperature control Balance
• Inputs
• Solar Heat Input
• Heat System Input

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Greenhouse Heating

• Objectives
• Understand Heat Loss
• Understand Heat Loss Coefficients
• Relate Heat Loss to Covering Materials/Frames

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Modes of Heat Loss

• Conduction
• Infiltration/Convection
• Radiation

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Conduction

• Heat loss through the covering of the greenhouse
  and the structural units exposed to the air.

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Infiltration/Convection

• Heat loss through the movement of air to cracks
  and crevices in the greenhouse structure.
• Movement of outside air into the greenhouse
  through laps in the glass, holes and other small
  openings.

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Radiation

• Heat loss from already heated objects such as
  plants, soil and other masses (as radiant heat)
  that are part of the greenhouse growing system.
• The heat is or can be transferred to other
  objects in the greenhouse and become lost further
  by way of conduction and/or convection/infiltratio
  n.

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Heat Source

• Central Heat System
• Central Boiler Room
• Separate Location
• Hot Water or Steam
• Piping System
• Localized Heat Unit(s)
• Unit Heaters (Forced Air)

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Providing Heat

• How much?
• Depends on
• Covering and understructure
• Curtain wall construction
• Minimum inside to outside temperature difference
  allowed
• Average wind velocity

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Units of Heat

• BTU British Thermal Unit
• Defined as Amount of heat required to raise 1
  of water 1 degree F.
• Calorie Amt. of heat required to raise 1 gram of
  water, 1 degree C.

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Heat Calculation

• Determine heat req. for a 30 x 100 GH. Glass
  and steel structure. Also has a 2 CW made of 4
  concrete block. 6 sidewalls sit on CW.
• Ave. wind vel. 15 mph, with a 60º in out temp
  difference, w/0º as outside low and 60º F as
  inside temp.
• Refer to tables in Chapter 4

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Forms of fuel

• Solid - examples
• Requires storage, not as clean burning.
• Liquid - examples
• May require pre-ignition devices, not clean
  burning.
• Gas - examples
• No or less storage, depending on type of gas,
  Cleaner burning.

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GH Fuel Consumption

• How much coal will be required?
• How much heating oil will be required?
• How much natural gas will be required?
• If natural gas is used, what will be our monthly
  heating cost?

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GH Fuel Consumption

• Natural gas consumption based on amount of therms
  combusted.
• What is a therm?

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1 Therm

• 100,000 BTU

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Cost of Heating?

• Amount of natural gas combusted
• Convert to therms used per hour
• Multiply by cost per therm
• Multiply by hours used per day
• Multiply by days in the month

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Example

• Cost per therm 0.60
• You run the heaters for 12 hours per day (night)
  combusting 391K BTU /hr.
• This is done for 30 days in that month.
• How many therms have you combusted?
• What is the monthly heating bill?

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Solution

• 391,000 BTU / 100,000 BTU 3.91 Therms
• 3.91 x 12 46.92 Therms combusted per day
• 46.92 x 30 1407.6 Therms combusted per month.
• 1407.6 x 0.60 844.56 (monthly bill)

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Heat Distribution

• Hot Water
• Steam
• Forced air
• Infrared

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Heat Distribution

• Hot Water at 180º F
• 2 pipe, yields 160 BTU/ft. /hr.
• At 203º F
• 2 pipe yields 200 BTU/ft./hr.

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Heat Distribution

• A greenhouse requiring 160,000 BTU/hr. will need
  1000 ft of pipe.
• 160,000 BTU/hr divided by 160 BTU/hr./ft. 1,000
  ft. of 2 pipe.

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Heat Distribution

• If finned pipe is used, the amount of pipe can
  be reduced by four times.

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Heat Distribution

• How much pipe is required if steam pipe is used?
• Steam at 215º F in a 1.5 pipe yields 210 BTU/ft.
• In a 1.25 pipe, yields 180 BTU/ft.

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Heat Distribution

• EPDM Tubing, also known as Biotherm
• Used for root zone heating
• Propagation
• General growing
• Used for floor heating

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Heat Distribution

• Unit Heaters
• Forced air heat using warm air distribution.
• May be associated with a convection tube.
• May also be associated with the use of horizontal
  air flow fans.

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Heat Distribution

• Infrared Heating
• Device used to heat objects in the greenhouse
  using long wave radiation.
• Air is not warmed, only the final object in its
  path.
• Cannot have any obstructions.

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Heat Distribution

• Other associated devices
• Thermostat
• Sensor
• Air aspirators
• Emergency heaters and generators