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Advantages:
·Low energy requirements
·Unused greenhouse can be easily
cooled in the summer
·No ventilation restrictions
on length of greenhouse
·Very high ventilation rates
are possible
·All side doors can be open
in the summer
·Quiet internal working and
shopping environment
·Air temperature can be maintained
very close to outside air
·Low temperature gradients across
greenhouse
Disadvantages:
·Can easily be designed improperly
(especially under-designed)
·Pad cooling cannot be used
(fogging possible)
·Micro insect screening is difficult
(but researchable)
·Must be incorporated with some
form of shade system (for most crops)
·Low light plants below open
vents can be sun scorched
·Ventilation rate is highly
dependent on wind speed and wind direction
·Vents are subject to wind damage
·Plants near a side vent can
be wind damaged
Design Considerations:
·Roof vent opening should be
15% of floor area and open leeward
·Minimum windward side vent
should equal one roof vent
·For large multi-spans, windward
side vent should be greater than 3% of floor area
·Windward side vent should be
low enough to prevent air short-circuiting to first top vent
·Higher gutter heights are generally
better if windward side is aerodynamic
·A 50% shading system is necessary
for summer growing of most crops
·Internal shapes should allow
upward and outward convective flows (no traps)
·Computer control is essential
for best climate control and wind damage protection
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Most naturally ventilated
greenhouses have been designed by experience, with little
engineering input. This is due to a previous lack of real
data. Engineers at Ohio State have been aerodynamically modeling
some of the newest double poly, multi-span greenhouses. These
same engineers have also tested designs in a commercial grower
greenhouse with some very positive results.
How Natural Ventilation
Works
Natural ventilation in greenhouses functions primarily by
wind blowing in one side and out the other. Wind can also
create a vacuum pressure along the roof to “suck”
the air out while letting air in the same vent or into the
side vents. A secondary, much smaller effect is that of buoyancy,
which predominates on hot, low wind days. In all cases, it
is important to have at least one very effective inlet with
multiple outlets; and that the air moves from inlet to outlet
through the plants for good ventilation. For gutter-connected
multi-spans, a combination of windward side vents and continuous
leeward roof vents tends to result in the most effective ventilation
design. For retractable roof designs, open windward side vents
are as important as the open roof area to achieve mid-summer
cooling.
The Role of Buoyancy
Buoyancy has many examples, such as wood rising to the surface
of water because it is less dense that the water. Also, most
know from experience that hot air rises because it is less
dense that cool air. However, most are surprised to learn
that MOIST AIR RISES BECAUSE IT IS LESS DENSE THAN DRY
AIR! The reason for the confusion is that water in its
liquid form is very dense; water as a vapor is less dense
that the surrounding air and rises to form clouds in the atmosphere.
Therefore, the combination of hot moist air in a naturally
ventilated greenhouse must have a smooth path up and out.
The slightest entrapment will stall the natural ventilation
process on a low wind day.
The Role of Wind
Wind with a little buoyancy is the ultimate system driver.
The ultimate test of a natural ventilation system design its
response to a “no wind” day. Most people can think
of moments on hot days when it feels as though there is “no
wind.” Such days have been recorded for nearly 20 years
on precision weather stations in Ohio by taking hourly data
16 feet above the ground. From this data, Ohio State engineers
have found most of the “no wind” cases occurred
when the outside temperature was less than 75 degrees Fahrenheit.From
75-80 degrees Fahrenheit, there was approximately one hour
in t five years of “no wind” and above 80 degrees
Fahrenheit, there was always a measurable wind above 1 mile/hour
with the average being approximately 5 miles/hour. Therefore,
“no wind” ventilation tests were designed by the
engineers to be 1.0 miles/hour.In other words, if a greenhouse
ventilates well at 1.0 miles/hour, it is a very good design
and will ventilate better at higher wind speeds. (At 1.0 miles/hour
near plants, one will tend to see very slight movements of
plant leaves.)
Solar Radiation &
Shading
Solar radiation drives the whole natural ventilation process,
including the outside wind. Solar radiation provides the heat
to the internal greenhouse air and forces the plants to transpire
water vapor. Excess solar radiation above 50% of clear day
noon levels, however, requires some form of shading for most
climates. Often the shading is put on the outside as a paint
or shade net. It can also be done with a porous horizontal
shade screen or net inside the greenhouse. With internal shades,
it is very important that the ventilation air enters the side-wall
openings and travels up through the shade-screen to carry
the heat out the roof vents.
Aerodynamic Models
Ohio State agricultural engineers have computer modeled both
multi-span sawtooth and curved roof designs with top vents
at different locations. They are leasing sophisticated computer
software similar to what has been used in aerodynamic car
designs. The technique essentially places a greenhouse cross-sectional
profile into an electronic wind tunnel. With an emphasis on
multi-spans, variables studied have been greenhouse widths,
roof and side vent locations, vent opening widths, windward
side profiles, internal and external shading systems, internal
temperature profiles, and benching for various wind speeds
and directions. With the highest speed PC computers available,
the calculation process is so complex that a single run may
take anywhere from 8-48 hours of continuous calculations!
An On-Farm Test
The author of this article helped a small Ohio grower (Quailcrest
Farm, Wooster, OH) consolidate Quonset houses and plan a four
and one-half span, gutter connected, naturally ventilated,
production and retail facility. The greenhouse uses a combination
of windward (west) side vent and leeward roof vents. During
the summer of 1998, temperature, humidity, wind, and solar
radiation sensors were placed inside the greenhouse by a graduate
student to evaluate the design. The greenhouse was modeled
aerodynamically and evaluated for both westerly and easterly
wind flows. For westerly winds, 90% of the wind came in the
west side vent and 10% the first roof vent. The outlet percentages
were 3% for roof vent1, 13% for roof vent 2, 30% for roof
vent 3, and 54% for roof vent 4. Uniform temperatures were
measured throughout the entire greenhouse at all times. The
volumetric air exchange for this period was predicted to be
0.9 air changes per minute with inside temperatures never
exceeding outside by more than 5 degrees Fahrenheit. In most
cases, the inside temperature was within 2 degrees Fahrenheit
of the outside. For easterly winds (reverse flows), 95% of
the air came in the east side roof vent 4, 4% came in roof
vent 3, and 1% came in roof vent 2. The outlets were 2% for
roof vent 3, 7% for roof vent 2, 41% for roof vent 1 and 50%
for the west side vent. Again, uniform temperatures were measured
throughout the entire greenhouse with no temperature being
more than 5 degrees Fahrenheit above the outside. Average
air exchange for an easterly wind was predicted to be half
that of a westerly wind at the same velocity. East winds,
however, tended to have higher velocities making the actual
air exchanges similar to west winds.
Retail buyer and grower responses
to the Quailcrest Farm greenhouse have been extremely positive.
All doors are typically open on warm and hot days allowing
easy access for browsing customers and plant toting employees.
While the greenhouse was sometimes 5 degrees warmer than outside
during the hottest part of the day, the greenhouse was still
more comfortable than outside due to the 50% or more shading
from direct solar radiation.
Future Work
More work should be done to size and specify full-scale greenhouses,
including retractable roof designs. Side vent placement to
prevent plant damage and short-circuiting to the first roof
vent is still being evaluated. Alternative shading techniques
such as internal nets are also being studied for use in naturally
ventilated greenhouses in both humid and arid desert climates.
While span width studies have been limited by computer memory
and speed, this problem is changing with each new computer.
The engineers and horticulturalists
also want to improve the aerodynamic models to account for
plant influences. The possibility exists that growers and
manufacturers will eventually be able to give the Ohio State
engineers a sketch or layout of a proposed greenhouse design
and get an “instant” aerodynamic evaluation of both
fan and naturally ventilated systems.
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