10/26/2012
Energy: ’Tis the Season for Savings
Kurt Parbst
Any fan of HBO’s “Game of Thrones” knows that
the motto of House Stark,
the family who rules
the North region, is, “Winter is coming.”
The threat is as chilling as it sounds.
The first cool days of autumn always remind me to look into
the update of
the Department of Energy’s Outlook report
for an indication of fuel prices and expenditures, which consider anticipated wea
ther
for the upcoming
season.
Energy expenses are among
the highest expenses
for greenhouse operators and become a top concern as our days begin to shorten. With technological advances in drilling in shale, natural gas supply will be in abundance, which will keep
the prices relatively low. Although newer demands, such as
the use of natural gas to offset coal
for the generation of electrical power, will continue, it’s expected that natural gas prices will be low
for some years to come.
If you have
the ability to lock in prices
for an extended period, it’s probably a great idea to do so. With spot prices
for natural gas around $3.00 per million BTU
for the commodity,
they’re less than half
the price of just a few winters back. Growers in some regions aren’t
fortunate enough to have access to a natural gas pipeline.
For comparison’s sake, propane prices are around $13.00 per million BTU, and No. 2 heating oil is around $24.00. (Keep in mind that equipment and distribution efficiencies should be noted when comparing fuel prices.)
Still, prices
for natural gas should be relatively stable
for some years to come, so investments in gaining access to natural gas should receive some attention.
Likewise, energy efficiency ef
forts, including
the consideration of heat retention curtains, should be a top priority
for growers who are heating with propane or oil.
Greenhouse curtain function
The utility of
the greenhouse energy curtain
for retaining heat during
the night is well documented by
the agricultural engineering community in not only
the U.S. and Canada, but also in Europe and Asia. Heat is transferred from
the greenhouse to
the atmosphere via several modes. Important modes include radiation, infiltration and condensation. Radiation is
the transfer of heat from
the crop to
the sky, and its rate is dictated by
the temperature difference between
the two objects.
A clear sky is 18 to 45F (10 to 25C)—lower than
the outside ground temperature. In
the sou
theastern U.S., if a typical spring night is characterized by a clear sky and a 41F (5C) ground temperature,
the sky temperature could be in
the neighborhood of -4F (-20C). If we assume a greenhouse set point temperature of 68F (20C),
the temperature difference can be 72F (40C). This large temperature difference makes radiative heat loss
the most significant mode of heat loss.
More than 70% of heat transmission is through
the greenhouse cover, while 5% to 20% is due to infiltration/exfiltration heat loss (see drawing above depicting radiative barrier and attic creation).
The greenhouse curtain is designed to be an opaque heat transfer barrier to slow
the radiation heat loss. It creates an attic that reduces
the infiltration effect.
When greenhouse vents don’t seal tightly, wind creates a pressure that draws heated air out of
the greenhouse. With
the creation of an attic,
the wind displaces unheated air ra
ther than heated air. Heated air also carries moisture.
The heat that moves water from
the leaf surfaces in
the air becomes latent heat. Latent heat is lost not only with infiltration/exfiltration, but from
the greenhouse cover when water vapor condenses on
the underside of
the roof. This is an under-appreciated mode of heat loss. Latent heat released via condensation on
the roof increases heat transfer by about 10%.
With
the majority of
the surface area of
the heated greenhouse being
the roof,
the heat transfer barrier makes a tremendous impact on nighttime heating requirements as
these three important modes of heat loss are disrupted.
While usual
savings in
the heat requirements attributed to energy curtains have been reported in
the neighborhood of 50%,
savings of as much as 90% have been observed by
the use of a highly insulated, reflective night curtain in a greenhouse with a
thermally massive floor.
For most operations that are growing a spring crop, an annual
savings in
the range of 25% to 40% is readily attainable. Realizing
the potential
savings is dependent upon
the curtain selection, hours of use and
the quality of installation.
The attic must be sealed off well from
the growing zone.
The investment and important resources
Return on investment
for a greenhouse energy curtain depends on growing
season, target plant temperature, location, heating equipment and available fuel. Most systems pay back in less than three years, but some installations are recovered in less than one year.
The USDA has a very user-friendly energy use calculator called Virtual Grower 3.0. This calculator can be used to enter one’s greenhouse zone geometry, crop-heating schedule and heating system details.
The program uses historical wea
ther data
for your region to estimate
the required heat input. This in
formation, coupled with an estimate of annual
savings, is sufficient to provide valuable investment decision guidance.
Although
the economic per
formance is very attractive, governments and utilities are currently encouraging investments in energy saving technologies.
The most widely available program is
the USDA Rural Energy
for America Program (REAP) Grants and Loan Guarantees. Contact your local office of Rural Development (
www.rurdev.usda.gov/recd_map.html)
for assistance in filing an application.
The U.S. Department of Energy’s Energy Efficiency & Renewable Energy office, in conjunction with
the North Carolina Solar Center and
the Interstate Renewable Energy Council, has created a continually updated database of federal and state incentives
for renewables and energy efficiency (
www.dsireusa.org). When searching your state, see opportunities
for energy efficiency and agricultural equipment.
The opening strategy is related to both light and temperature
The main effects of closing a curtain are:
The reduced light transmission into
the greenhouse and, thus, loss of photosyn
thesis; and increased insulation and
the associated gain from fuel conservation.
The best time to cover and uncover
the curtain is when
these two effects are equal. That is,
the economic gain from your addition of biomass is equivalent to
the cost to heat
the space.
Whereas this can be estimated and managed based on a combination of time and temperature,
the best technique is to establish solar radiation intensity thresholds. I. Seginer and L.D. Albright demonstrated that
for their conditions in upstate New York, along with some assumptions
they made in balancing of
the photosyn
thesis and heat loss functions,
the best level
for the whole year on a year-round crop was about 33 watts (W) per m2.
Researchers studying tomatoes in
the Ne
therlands (J.A. Dieleman and F.L.K. Kempkes, 2006) considered
the curtain-triggering thresholds of 5 and 50 W/m2 and found that
the higher threshold reduced energy consumption by 3.5% at a cost of only 0.34% of radiation.
With such a small cost associated with
the light loss, it makes it easy to delay
the uncovering in
the morning and make more
thermally productive conditions
for the crop by not dropping cold attic air.
The higher threshold allows more time
for the sun to heat
the attic air closer to
the growing zone temperature. This, of course, leads us to
the real value of having a moveable
thermal barrier between
the crop and
the glazing—to make a more productive climate
for the crop. But that’s a different story … winter is coming.
GT
Kurt Parbst is president of Ludvig Svensson Inc. He can be reached at (704) 357-0460 or
kurt@svenssonamericas.com.