New Research in Natural Ventilation

Ohio State engineers are changing the "guess & try" practice of naturally ventilated greenhouse design by aerodynamically modeling some of the newest double poly, multispan greenhouses. They have computer modeled both multispan sawtooth and curved roof designs with top vents at different locations. They are leasing highly sophisticated computer software that you may have seen in aerodynamic car advertisements on TV. The technique essentially places a greenhouse cross-sectional profile into an electronic wind tunnel.

How Natural Ventilation Works

Naturally ventilated greenhouses rely primarily on wind pressure blowing in one side and out the other. Wind can also create a vacuum pressure along the roof to "suck" air out while letting air in the same vent or in the side vents. A secondary, much smaller effect is that of buoyancy which predominates only on hot, low wind days. In all cases, it is essential that there is at least one very effective inlet with multiple outlets; and air must move from inlet to outlet through the plants to have good ventilation. Thus 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.

Grower Tests

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, double poly production and retail facility. The greenhouse used a combination of windward (west) side vent and leeward opening roof vents. During the summer of 1998, temperature, humidity, wind, and solar radiation sensors were placed inside and outside the greenhouse.

For westerly winds, 90% of the air came in the west side vent and 10% the first roof vent. The outlet percentages were 3% for roof vent 1, 13% for roof vent 2, 30% for roof vent 3, and 54% for roof vent 4. Amazingly, uniform temperatures were measured throughout the entire greenhouse at all times. The volumetric air exchange for this period was 0.9 air changes per minute with inside temperatures never exceeding outside by more than 5 degrees F. In most cases, the inside temperature was within 2 degrees F of 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 outlet percentages were 2% for roof 3, 7% for roof 2, 41% for roof 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 F above the outside. Average air exchange for an easterly wind was half that of a westerly wind at the same wind speed. 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.

Current Research

More work is being 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. Internal retractable shading with nets are also being studied for both humid and arid desert climates.

The Ohio State engineers expect that future greenhouse designers will be able to have an "instant" aerodynamic evaluation of both fan and naturally ventilated systems!

Natural Ventilation Check List
(GrowerTalks; Summer 1998 Edition)

Advantages

• low energy requirements
• all side doors can be open in the summer
• quiet internal working and shopping environment
• unused greenhouse can be easily cooled in summer
• no ventilation restrictions on length of greenhouse
• air temperature maintained very close to outside air
• very high ventilation rates are possible
• low temperature gradients across greenhouse

Disadvantages

• can easily be designed improperly
• must be incorporated with some form of shade system
• pad cooling cannot be used
• may not allow for micro insect screening
• plants near the side vent can be wind damaged
• is dependent on wind speed and wind direction
• vents are subject to wind damage
• low light plants below open vents can be sun scorched

Design considerations

1. roof vent opening should be 15-20% of floor area and open leeward to wind
2. minimum windward side vent should equal one roof vent
3. for large multispan, wind-ward side vent should be greater than 3% of floor area
4. windward side vent should be low enough to prevent air short circuiting to first top vent
5. higher gutter heights is generally better if windward side is aerodynamic
6. a 50% shading system is necessary for summer growing
7. internal shapes should allow upward and outward convective flows (no traps) computer control is essential for
8. climate control and vent protection from high wind