Greenhouse Energy
Chris Callahan, PE
Vermont Vegetable and Berry Growers Association
Annual Meeting
February 1, 2010
Montpelier, VT
2010 02 01
UVM Greenhouse Energy Project
Slide 1
Agenda
• What is a BTU, anyway?
• Review of Greenhouse Energy Flows
• Some simple examples
• Survey results (prelim)
• Plans and next steps
2010 02 01
UVM Greenhouse Energy Project
Slide 2
What is a BTU?
• A "British thermal unit" (BTU or Btu)
is a measure of the heat content of
fuels. Amount of energy.
• Defined as: the quantity of heat
required to raise the temperature of 1
pound of liquid water by 1°F
• One Btu is approximately equal to the
energy released in the burning of a
wood match.
• One hundred BTU is about the energy
content of a mug of hot water.
• One million Btu equals about 8 gallons
of motor gasoline.
1
BTU
100
BTU
Based on information from US DOE EIA:
http://tonto.eia.doe.gov/energyexplained/index.cfm?page=about_btu
2010 02 01
UVM Greenhouse Energy Project
Slide 3
What is a kWh?
• A Watt is a measure of power equal to a 1
ampere of current running at 1 volt at an
instant in time.
• A Watt-hour is 1 Amp at 1 Volt running
for an hour.
• A kiloWatt-hour, kWh or kW-hr is a
cumulative measure of power (i.e. energy)
which is 1000 Watt-hours.
• 1 kWh is a 10 Amp circuit run for 1 hour.
2010 02 01
UVM Greenhouse Energy Project
Slide 4
Greenhouse Energy Overview
(-) leakage
or ventilation
(-) inefficient
space
+ solar
radiation
(-)
convective
and
radiant
losses
(-) pumps (-) product
mov’t
water mov’t
+ electrical
feed
(-) fans
air mov’t
+ burn
fuel
(-) infiltration,
ventilation,
makeup air
2010 02 01
(-)
conductive
ground
losses
UVM Greenhouse Energy Project
Slide 5
Electrical Energy
VT average commercial rate: $0.13/kWh
• Any time a motor runs, electricity is used
– In a new or well maintained motor:
• Approx. 80% is for useful purposes
• Approx. 20% is lost
• Consumers: fans, pumps, lights, heaters
• Improvements: optimize control (lower ontime), variable speed drives (multi speed pumps
or blowers), maintenance, bearing care.
• Estimated $/yr
= On time (hrs) x Volts x Amps / 1000 x $/kWh
(estimate, or
see next slide)
2010 02 01
(from nameplate)
UVM Greenhouse Energy Project
(W -> kW) (from bill)
Slide 6
Electrical Energy
Sample motor
nameplate
V = 115 volts
A = 3.0 amps
$17
2010 02 01
Assume it runs
15 min every
hour for 3
months. (540
hours)
$80
UVM Greenhouse Energy Project
540 h x 115 V x 3.0 A x
0.13 $/kWh /1000 =
$24.22
$80
Slide 7
Electrical Energy
• Sometimes there are hidden costs…
• Ventilation fans when run unnecessarily,
cost you twice:
– Electrical energy
• motor
– Thermal energy
• exchanging warm air for cold
2010 02 01
UVM Greenhouse Energy Project
Slide 8
Thermal Energy
• To maintain a temperature difference
between inside and out, energy must be
added.
– Solar gain (daytime in clear weather)
– Thermal input (propane, oil, wood, other)
• How much?
– Depends on the greenhouse “envelope”
– … and temperature difference needed.
2010 02 01
UVM Greenhouse Energy Project
Slide 9
Thermal Energy
• Envelope
– R-value and U-Value
are inverses of each
other
– R-value is resistance to
heat flow
• °F-ft2-hr / BTU
– U-value is ease of heat
flow
• BTU / hr-ft2-°F
2010 02 01
UVM Greenhouse Energy Project
Heat Loss
BTU/hr =
U-value x Area x Temp Diff
Slide 10
Thermal Energy
• Two houses
– both in Montpelier (outside temp = 10 °F)
– Inside temp = 45 °F (temp diff = 35 °F)
• 20’x40’x10’ hoop single poly cover
– “Heat transfer area” = 1256 ft2
– U-value = 1.1 BTU/hr-ft2-°F
– Q = BTU/hr = 1.1 x 1256 x 35 = 48,356 BTU/hr
• 20’x80’x10’ hoop double poly cover
– Heat transfer area = 2512 ft2
– U-value = 0.7 BTU/hr-ft2-°F
– Q = BTU/hr = 0.7 x 2512 x 35 = 61,544 BTU/hr
2010 02 01
UVM Greenhouse Energy Project
Note, this is a
simplistic
model. It only
accounts for
losses from
the exposed
covering.
Twice the
growing area
for 27% more
heat addition.
Slide 11
Low to no cost improvements
• Envelope
–
–
–
–
–
Repair rips and tears in cover
Door closure and sealing
Row covers if feasible for your operation
Segment house if growing diverse crops
Check seals of louvers, fans, etc.
• Inputs
–
–
–
–
2010 02 01
Clean and tune furnace or boiler annually
Confirm thermostat settings
Optimize ventilation schedule / control
Clean screens / filters
UVM Greenhouse Energy Project
Slide 12
More significant improvements
• Envelope design trade-offs
– When building new, consider using design
tools to look at trade-offs.
• USDA Virtual Grower – Software lets you build a
“virtual” greenhouse and run it through a growing
season. Compare various designs. Free
download.
• Alternative inputs
– Are there other readily available and
sustainable fuels you can better utilize?
2010 02 01
UVM Greenhouse Energy Project
Slide 13
UVM Greenhouse Energy Project
• Contacts
Vern Grubinger
UVM Extension
(802) 257-7967
[email protected]
Chris Callahan
Callahan Engineering
(518) 677 5275
[email protected]
• Periodic updates
http://www.uvm.edu/vtvegandberry/
2010 02 01
UVM Greenhouse Energy Project
Slide 14
VT Greenhouse Grower Energy
Survey – Prelim Review
• 54 respondents (THANK YOU!)
• Expansion: 71% plan new houses in the future
• Grow location: 29% in ground, 50% on benches,
21%hanging
• Cover: 76%Double poly, 18% Single poly, 4%
Polycarbonate, 2% Glass
• Insulation: 42% uninsulated, 22% insulated knee walls,
20% insulated endwalls, 13% perimeter, 3% underfloor.
• Fuels: 45% Propane, 7% Wood, 9%Oil, 39%Other
• Using renewables: 39%Yes, 52%No, but considering it,
9%No, not interested
2010 02 01
UVM Greenhouse Energy Project
Slide 15
Appendices
2010 02 01
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Slide 16
Fundamentals – Energy and Power
• Energy
– Ability to do work
– Neither created nor destroyed
– Cumulative
– Think “fuels”
• Power
– Rate of energy conversion
– Always <100% efficient
conversion
– Instantaneous
– Think “equipment”
• Units
• Units
• Examples
• Examples
– BTU, kBTU, MBTU, quad,
therm, calories
– kW-hr, kWhr, kWh, Joule, kJ, MJ
–
–
–
–
2010 02 01
Gallon of gasoline
Tons of coal
Cords of wood
Cubic feet of Natural Gas
– BTU/hr, kBTU/hr, MBH
– Watts, W, kW, J/s
– Tons of AC
– Burning a fuel (gph)
– Light bulb emitting light (e.g. 32
Watt CFL)
UVM Greenhouse Energy Project
Slide 17