Transcript XIV. INCREASING TEMPERATURE

X.
INCREASING TEMPERATURE HEATING
A. Heating system requirements
– Optimum inside temperature
– Uniform temperature
– Prevent hot air on plants
– Low cost
– Fuel available
– Automated
Energy Loss from Greenhouse
B. Heating terminology
– refer to physical
principles
C. Factors affecting
heating
Q - heat loss
– Q = Qc + Qi
Qc - conduction &
radiation heat loss
– Qc = U x A x (ti-to)
– Qi = .018 x V x N x (ti-to) Qi - infiltration heat loss
U - heat transfer coef.
A - area of coverings
ti-to - inside set pt –
coldest temp
1. House surface area vs volume
– Surface area
– Reducing surface area
• lower eaves
• ridge and furrow
• shape of house
perimeter & surface area correlated
2. Temperature differential
– (ti -to)
3. Covering: Number of layers
• Heat transfer coeffieicnt
–2 layers 40% less energy than 1
–3 layers 16% less energy than 2
4. Types of coverings
• Heat transfer coefficient
5. Air leakage
• Tight house vs loose house
• Leaks around fans, doors, vents
• Thermal radiation
6. Side walls
• Heat transfer coefficient
7. Structure - conductional heat loss
• 8% more loss through metal than
wood
• Frame on double layer not exposed
to outside
8. Wind
• Sweep away boundary layer
D. Sources of heat
1. Fossil fuels
– Major:
Coal
Oil
– Minor:
Wood chips
Wheat
2. Electricity
Natural gas
Propane
Straw
Sawdust
3. Other possible sources
• Generating plants
• Natural gas compression stations
• Ethanol plants
• Geothermal
–hot springs
–ground water
–underground caverns
Greenhouse heat: Gas from Landfill
Mine Air Heated Greenhouse
E. Types of Heating Systems
– hydronic
– forced air
– Infrared
1. Hydronic - water or steam
a. heating process
–conduction
–convection
–radiation
b. Steam vs hot water
–boiler
–steam higher pressure
–steam cools faster
c. boiler
• fuel - gas,
coal, propane
• operation and
maintenance
• manual or
automatic
control
d. distribution system
1) sidewalls, under benches,
above benches
2) circulate air
natural
forced convection
3) finned pipe
2/3 along side wall,
1/3 under
benches
Greenhouse Heat: Hot water or steam
2. Forced Air - Unit Heater
a. Types
• hot water or steam
– boiler required
• fuel fired unit heaters
– Fuel burned in house
– Air distribution
• forced convection
• electricity
b. distributing heat from unit heaters
–Polytube
–Heater fan and HAF
c. problems arising from heat distribution
– Hot air on plants
– Uneven temperatures
– Incomplete combustion
CH3-CH2-CH2- + O2 -----> CO2 + H2O + (CH2Ch2, CO, SO2)
– Remedy
• 1 sq in/2000BTU/hr for air inlet
3.
Infrared Heater
a. Principles
• Energy not absorbed by air
• Leaves, etc., absorb energy
– Increase in temp.
• Air warmed
– conduction - leaves, etc., to air
– convection - air rises
b. Possible less condensation
• Plants warmer than air at night
– Air up to 7 deg cooler
• Other systems - plants cooler than air at
night
– Radiation heat loss
– Transpiration
Infrared Heating
c. Energy savings
• 30-70%
• Fuel combustion 90%
• Less temperature differential
– Air up to 7 deg cooler
– Less energy loss
• Do not use circulation fans
– Less electricity
• Installation cost higher
4. Bottom heat
Can provide 25-50% of heat during winter
a. Root system warmer
b. Natural air currents
c. Water
•
•
•
small rubber tubes on bench or floor
Finned pipe under bench
Plastic pipe in floor
d. Electricity
•
Resistance coil
e. Advantages
• Uniform
temperature
• Crop time reduced
• Reduced disease
–Root rot- soil
dries faster
–Foliar - leaves
warm less
condensation
• Crop uniformity
• Compact plants
• Zone flexibility
Bottom Heating
Bottom heating: tube placement
Bottom heating: Biotherm (tube)
F. Special Heating Needs
1. Propagation
• Warm bottom temperatures
• Cable, pad, pipe under bench
2. Sterilization/pasteurization
• Steam best if available
G. Using Less Energy
1. Conservation
• Seal cracks
• Burner efficiency
• Insulation - side walls, north wall
• Double layer
• movable curtains
• Foam between polyethylene
• Styrene beads
2. Management practices
• Optimum space utilization
–progressive spacing
–movable benches
–grow under benches
–hanging baskets
• Reduce container size
• Improved varieties
–faster production
–cooler temp. requirements
2. Management practices (cont)
• Supplemental lighting
• CO2 increase
• Reduce crop losses
• Reduce night temperature
Reduce Energy Use: Management Practices
Reduce Energy Use: Use space more efficiently
Supplemental heating: collect and
store solar radiation