January 2024 - 16

SGAFT | BY PETER KONJOIAN
Energy Curtain Performance
The Small Greenhouse and Farm Technology (SGAFT)
project is collaborating with GPN for a series of
research reports and articles on small scale technology
application. SGAFT's mission includes providing small
greenhouse and farm operators with technology
guidance tailored to their scale of operation.
This article's research was conducted by Konjoian's
Horticulture Education Services and designed to better
understand energy curtain performance in a small
greenhouse setting. This report transitions to the SGAFT
website (sgaft.com) with additional pictures, data
graphics and discussion.
CURTAIN STUDY 2023
Six overnight data-generating sessions occurred
Feb. 25-March 19, 2023. Yes, you read that correctly,
six all-nighters were pulled over three weeks, each
thirstily stimulated by caffeine followed by a day or
more of recuperation. It was a fun experience that
generated pages of spreadsheet data. The first part
of the study covered three nights: Feb. 25, Feb. 27
and March 1. Main variables included three different
outdoor temperatures and alternating energy curtain
positions between open and closed through the night.
The greenhouse temperature was controlled by Bartlett
Instrument Company's Climate Boss system. The curtain
fabric, Svensson's Tempa 5557 (55% shade, 57% heat
retention), was part of a Wadsworth Control Systems
curtain package.
The second part of the study included a third variable
of alternating between Climate Boss and a mechanical
thermostat through the night and is presented on
the website.
The high-efficiency Modine propane fired hot air
NIGHT
Feb 25
Feb 27
Mar 1
OUTDOOR
TEMP (ºF)
12
28
35
HEAT SET
POINT (ºF)
62
63
63
START/END
P.M./A.M.
9:00/8:30
9:00/9:00
7:00/6:30
furnace and curtain in a closed position. Each furnace
cycle included three manufacturer presets: 20 seconds
between thermostat heat call and ignition, 30 seconds
between ignition and heat exchanger fan start and two
minutes post-ignition fan cool down. Total ignition (burn)
time and total fan run time varied for each cycle and were
recorded. Also recorded were the cycle start and end time
(duration) and time between cycles (frequency).
Air temperature was monitored at two locations, 18 "
above the bench and 6 " above the curtain, at the start
and end of each furnace cycle. Staged pair of sensors
monitored temperature above and below the curtain as
the curtain opened after a closed session.
The above/below curtain temperature gradient shown
is 46.0º F/62.7º F, the post-opening period called a
morning cold air dump.
Following each curtain position change (open or
closed) the first heat cycle served as a transition cycle
and was excluded from data analysis. Regarding bench
temperature, this cycle neutralized the cold air dump
following opening. Regarding above curtain air temperature,
the transition cycle allowed for cooling of the warm air after
closure. Excluding transition cycles more closely simulates
continuous curtain use through the night.
OUTDOOR TEMPERATURE
Three parameters in Figure 1 were averaged over both
curtain positions, open and closed, to present the effect
of outdoor temperature on greenhouse heating. Total
minutes of furnace burn (ignition) time and the number
of furnace cycles are presented on a per hour basis along
with the average burn time in minutes per cycle. Graphs
appear in order of descending outdoor temperature:
green-35º F, blue-28º F, and red-12º F.
CURTAIN OPEN/CLOSED POSITION (HR)
closed 1, open2/closed 2, open 1/closed 1, open 1/closed 1
closed 2/open 2, closed 1/open 1, closed 1/open 1, closed 1
closed 2/open 2, closed 1/open 1, closed 1/open 1, closed 1
Table 1. Defines parameters - date, outdoor temperature, greenhouse heat set point, night start/end time and curtain position
sequence through the night. Data courtesy of Peter Konjoian.
16
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