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1
Winter Greenhouse
Growing in Cold
ClimatesA Workshop Organized by Vermont
Technical College - Institute for Applied
Agriculture and Food Systems
Corie Pierce
Bread and Butter Farm
Shelburne, VT
Chris Callahan
UVM Extension Ag Engineering
Bennington, VT
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Workshop OutlineDAY 1
• Introductions
• Workshop Objectives
• Energy Basics
• Crop Basics
• Lunch
• Field Trip
2
DAY 2
• Recap Day 1
• Structures
• Environmental
Controls
• Case Studies
• Preparing for Severe
Weather
• Lunch
• Field Trip
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Introductions
• You and your farm, business or
organization.
• Why is winter growing important to you?
• What do you hope to learn from the
workshop?
• What have been some of your challenges
so far?
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Workshop Objectives1. Understanding the economics of 4 season growing
2. Understand different options for crops to grow
3. Understand different systems for production, e.g. low tunnels, high
tunnels, heated greenhouses and pros and cons
4. Understand planting timing and cut and come again, maximizing
offseason (summer!) sales with winter sales as priority, varieties
that are proven
5. Understand general management of high tunnels in the winter
(ventilation, soil, pest, disease, structural)
6. Have working knowledge of heating, ventilation, structures and
controls for winter growing
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Activity
• A very low capital high tunnel
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Energy Basics• Energy: The ability to do work.
– Can be stored or converted
– Cannot be created or destroyed
– Units: kWhr, BTU, Joules, Calories,
Cord, Gallons
• Power: Energy converted over time.
– Instantaneous measure
– Never 100% efficient
– Units: kW, BTU/hr, Joules/second,
Calories/day, Horsepower
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Activity
• Let’s burn something.
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Heat Transfer
• Heat will naturally flow from hot to cold
(seeking equilibrium and the “lowest
energy state”).
• This is a blessing and a curse
– We benefit from this in heating and cooling
applications (think furnaces or evaporators)
– We fight it when trying to keep a greenhouse
warm in early spring or a cooler cool in mid
summer.
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Heat Transfer
• Three modes
– Conduction – through solids
– Convection – through fluids (liquid or gas)
– Radiation – directly from one body to another
• All are proportional to temperature
difference
• …and differ by how the heat flow is slowed
(or enhanced.)
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Conduction
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Hot
(caffeinated)
liquid.
Ceramic wall
Fingers
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Convection
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Breath
Hot
(caffeinated)
liquid.
There is also
phase change
here.
Fingers
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Radiation
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No, not the
marshmallow!
The heat you feel
directly from the
fire or from the
sun.
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Heat Transfer
• The way we try to limit heat transfer in
food storage is with insulation and sealing.
– Insulation – retards heat flow through walls
– Sealing – retards air flow and infiltration
between separated spaces
• The ways we try to support heat transfer is
with immersion and air flow.
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Insulation• The rate of heat transfer is proportional to
the temperature difference and the overall
heat transfer coefficient.
• Overall heat transfer coefficient (“U”)
captures how easily heat moves from one
body or fluid to another.
– Conduction – through solids
– Convection – through fluids
– Radiation – body to body
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What Does R-Value Tell Us?
• The Rate of Heat Loss / Gain =
Surface Area times
Temperature Difference all divided by
R-Value
Area x (Tout – Tin)
Q = ----------------------------
R-value
Q = U-value x Area x (Tout – Tin)
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BTU/hr
1 1
R = -------- U = -------
U R
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Tunnel Insulation
Example
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Humidifying and Drying
• What is actually happening?
• Depends on water changing
“phase”
– Liquid
– Vapor
• That requires air, energy
flow, and temperature
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Water’s Phase Change
• What we think we know…
– Water freezes at 32 F and 0 C
– Water boils at 212 F and 100 C
• It is true….but…
• Only at standard atmospheric pressure!
• How is there water vapor in air?
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Water and Air Mixtures• When water vapor is in air, it
behaves as though it is at a
“partial pressure” or lower
pressure than atmospheric.
• Meaning, it is vapor even
though it isn’t at 212 F.
• This allows for “humidity”
below 212 F.
– And most of the weather
systems we deal with.
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Liquid Water
70 degF
Air
70 degF
14.7 psia
Water vapor
Experiences
0.4 psia @
100% RH
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Relative Humidity• The degree to which air is “saturated” with
water vapor at a certain temperature and
barometric pressure.
• Since barometric pressure is relatively
constant, RH is really a function of
temperature.
– For most agricultural applications
– Pressure’s influence is the basis of vacuum
cooling, however…
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• We don’t actually
measure Relative
Humidity (RH)
• We measure
– Dry Bulb
Temperature, and
– Wet Bulb
Temperature
• RH is a calculation
based on these
two temperatures.
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Psychrometric Charts• Relate Dry Bulb T, Wet Bulb T and RH.
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Psychrometric Charts
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Activity
• Greenhouse in a box
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Controls
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Controls - Thermostats
• Control a load based on temperature
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Controls - Thermostats
• Dramm – Accurate to 1 degC (2 deg F)
– Same model as greenhouse ones.
– Single and dual stage
– For heating
and cooling
• Different set of
contactors.
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Controls - Humidistats
• Control a load based on measured (or
calculated) RH
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Controls – Expandable Systems
• Combined Temp and RH
• Modular and expandable
• Modulated outputs as well as On/Off
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Ventilation & Airflow
• Seeking to have a well mixed storage
space.
• Avoid hot spots
• Avoid high moisture
• Strip ethylene.
• 3-5 volume changes per day is rule of
thumb.
• Higher for curing or pre-cooling.
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Measure and Monitor• “The measured variable improves.”
• Temperature AND Relative
Humidity
• Don’t assume you have the
conditions you want. Measure.
• Low tech – wall sensors, daily
checks, log book
• High tech – remote monitoring,
email alerts
• Calibration and certification
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USB Data Loggers
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www.dataq.comDATA-Q
EL-USB-2+ USB Data LoggerMeasures ambient temperature and humidity
Higher accuracy than EL-USB-2
Automatically calculates dew point
-35 to +80 °C (-31 to +176 °F) temp
measurement range
±0.3 °C (±0.6 °F) overall temp accuracy
0-100% RH measurement range
±2.0% overall RH accuracy (20-80%RH)
2 User-programmable temp alarm
thresholds
2 User-programmable RH alarm thresholds
5 minute readings = 56 days storage
1 minute readings = 11 days storage
Download data to computer
$99 (RH +/-3%)
$82 (RH +/-3%)
$125 (RH +/-2%)
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Infrared Thermometer
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$20-100
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Remote Monitoring
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Remote Monitoring
• $400-$2000 for a typical install.
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Sensaphone
• Several models
• 400 – 4 inputs
• 800 – 8 inputs
• $460 for the control
• $32 per sensor
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Mojyle
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Gateway: $300
Sensors: $30
Annual Web Fee: $300
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“The
perfect is
the enemy
of the
good.”
- Voltaire
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43
Bartok, J., & Aldrich, R. (1994). Greenhouse Engineering, NRAES - 33. Natural Resource, Agriculture and Engineering Service (NRAES). Retrieved from http://host31.spidergraphics.com/nra/doc/Fair%20Use%20Web%20PDFs/NRAES-33_Web.pdf