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

principlesC. Factors affecting

heating– Q = Qc + Qi– Qc = U x A x (ti-to)

– Qi = .018 x V x N x (ti-to)

Q - heat loss

Qc - conduction & radiation heat loss

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 Natural gas

Oil Propane

– Minor:

Wood chips Straw

Wheat Sawdust

2. Electricity

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 system1) 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 + (CH2-Ch2, CO, SO2)

–Remedy • 1 sq in/2000BTU/hr for air inlet

3. Infrared Heatera. 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 heatCan 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