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Let the Sunshine in.

Open-roof greenhouse systems cool better than fans

Open-roof greenhouses are shedding some new light on how crops are grown in controlled-environment structures.

In addition to providing cooler growing temperatures, these systems grow plants in direct sunlight part of the year, reduce energy and costs for cooling with a fan, and harden off plants during spring production. Hardening off plants prepares them for the change from a controlled to a natural environment outdoors. Temperature data collected in these greenhouses indicate that indoor temperatures can be maintained at close to 2[degrees]F to 4[degrees]F (1[degrees]C to 2[degrees]C) above outdoor temperatures on hot days.

In the past, designs for glass agricultural greenhouses have depended on roof peak vents for cooling the air inside. Without this cooling, heat from the sun builds to temperatures that are detrimental to plants.

Today, nearly 70 percent of U.S. greenhouse production is conducted in structures made of double-glazed polyethylene. The double-glazed covering system was developed at Rutgers University in 1964. The system is composed of two layers of plastic film attached to the greenhouse then separated by air -- like an air mattress. The plastic forms an insulating layer to reduce heating energy 30 percent. Fan ventilation is used in these greenhouses because it is too difficult and expensive to install and maintain peak or roof vents in them. Because the plastic is applied in large sheets, cutting and reattaching it to allow for installing ventilation devices weakens the structure. A fan ventilation system, which uses one volume air change per minute, has controls to provide optimum velocity through air inlets. These systems perform well with added cooling provided by a fan and pad, or fog system, which use evaporating water to cool greenhouse air.

Greenhouse design

Recent innovations in open-roof style greenhouses have incorporated retractable-roofs into their designs (Figure 1). In these structures a crop can be hardened off without being moved. Another advantage is that the crop can grow inside the greenhouse part of the time in full photo-synthetically active radiation (PAR) light at roughly the same temperature as outside.

Open-roof greenhouses can use thermal screens made of polyethylene with no glazing. The screens are opened during the day to allow the crop to grow in full sunlight. Some structures use horizontally sliding thermal screens that move left and right to provide protection for the crop. Other designs allow the screen material to follow the shape of a traditional A-framed greenhouse.

Articulating open-roof greenhouses

In 1990, Aart Van Wingerden of North Carolina, a pioneer in designing glazed polyethylene greenhouses and developing the controlled environment agriculture industry, developed a structure called an MX greenhouse. The name is a takeoff from the MX missile silos that house U.S. military weapons. Van Wingerden, now deceased, first designed the roof to open on one side from the peak, hinging at the gutter edge where the walls meet the roof. Next, he designed a fully articulating roof greenhouse -- the MX-II (Figure 2).

Articulating open-roof greenhouse designs are accepted and used by the industry. One of the two basic types (Figure 3) has several peaked roofs with fully articulating sections hinging at the gutters -- the low spots between the peaks. These sections open at the peak to various distances.

The other type has articulating roof sections that hinge at the peak and the gutter to open at the gutter line and move across the greenhouse bay powered by a rack and pinion mechanism.

Data from an MX-II greenhouse in Pompton Plains, N.J., (Figure 4) show that inside temperature is adequately controlled in an open-roof environment. The commercial operation has several conventional double polyethylene glazed greenhouses adjacent to the new MX-II. Temperatures in the conventional, naturally-ventilated greenhouses registered up to 10[degrees]F (5.5 [degrees]C) warmer than in the MX-II during the same time of the day.

Rutgers research

The Center for Controlled Environment Agriculture (CCEA) at Rutgers University promotes research and technology transfer in controlled environment agriculture. The program is supported by industry partners and is guided by a scientific advisory panel.

CCEA's open-roof greenhouse research facility on campus has initiated an effort to observe the MX-II's design performance. A computer-based environmental control system monitors wind velocity and closes the MX-II's roof sections when speeds exceed 35 miles (56 kilometers) per hour.

Preliminary data indicate that temperature inside the MX-II during warm weather and periods of high solar radiation ranges from about 2[degrees]F to 4[degrees]F (1.1[degrees]C to 2.2[degrees]C) higher than outside conditions. On July 6, 1999, outside temperatures reached 101[degrees]F (38.3[degrees]C). Data were taken that day in the MX-II with no crop in the greenhouse to provide transpiration cooling. Temperatures were typically 2[degrees]F to 4[degrees]F (1.1[degrees]C to 2.2[degrees]C) higher than outside temperatures, while the nearby fan-ventilated greenhouse, with a ventilation rate of one volume change per minute, was 10[degrees]F to 15[degrees]F (5.6[degrees]C to 8.3[degrees]C) hotter than outside.

The MX-II greenhouse is glazed with air-inflated double polyethylene film on its roof. The structure is being reglazed with acrylic structured sheet panels on the side and end walls. The floor has been designed and constructed to provide ebb and flood watering, and a floor heating system is installed within it. Perimeter hot water will provide the design heat load needed.

A.J. Both of bioresource engineering at Rutgers will obtain and analyze solar and temperature data to evaluate design performance in a standard production system growing flowers or vegetables.

ASAE member William J. Roberts is professor emeritus, Center for Controlled Environment Agriculture
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Title Annotation:Open-roof greenhouses
Author:Roberts, William J.
Publication:Resource: Engineering & Technology for a Sustainable World
Article Type:Brief Article
Geographic Code:1USA
Date:Jul 1, 2001
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