Greenhouse Technology - Open Roof Design

William J. Roberts
Rutgers University,
New Brunswick, NJ, USA

Introduction

Greenhouse design technology has made significant progress in the last few years. Perhaps one of the areas where this is most evident is in the recent innovation of open-roof greenhouse design. There is a revolution sweeping the greenhouse industry in the form of open-roof structural designs. The pictures on this page indicate only one type of structure which is available to growers. These structures offer cooler growing temperatures and the opportunity for growing during some seasons and selected parts of the day in direct sunlight potentially increasing the photosynthesis in the crop
CCEA, the Center for Controlled Environment Agriculture at Rutgers has constructed a plastic greenhouse research facility on campus. This was one of the final assignments of the author and a very gratifying and exciting experience. The author was continually amazed at the lower temperature in the MXII greenhouse nearly equal to outside at all times. Results to date on this project will be illustrated later in the paper.

Growers have known for many years the value of growing outside in the spring to harden off plants and to make room for crops which need the more valuable greenhouse space. Cold frames were an important but labor-intensive part of that system. Plastic greenhouses gave growers a low-cost option of producing spring plants. The problem with plastic greenhouses as well as other glazed structures is the adverse effect of high temperatures on the crops during times when it is difficult to cool the greenhouses.

The next development in hardening-off plants was developed by Jack Van de Wetering at Ivy Acres in Long Island and consisted of a system of transportable benches which moved the crops in and out of the greenhouse. During the day, weather permitting in the spring, the plants would be moved outside on the transportable benches. At night they would return to the greenhouse space. Figure 3 shows the benches at Allen Van Wingerden’s greenhouses in Pompton Plains, NJ. In most instances another crop was grown on the floor using soil heating in the same greenhouse space. The difficulty came when there was a weather problem and the plants under the bench would suffer from lack of light for a few hours. Another problem was the amount of labor and attention required to move the plants in and out of the greenhouse. Some of these growers have now substituted the open-roof design for the transportable bench system to eliminate the handling labor demands.

Recent innovations in greenhouse design have provided for retractable roof greenhouses. With these greenhouses it is possible to harden off the crop inside the greenhouse without moving it. Another advantage is that during part of the day during the season the crop can grow inside the greenhouse in full PAR (photosynthetically active radiation) light at approximately the same temperature as outside.

The open roof experiment began with the use of thermal screens, which were installed in greenhouses without glazing. The screen was open during the day to allow the crop to grow in full sunlight. Some of these structures use horizontally moving thermal screens and others use a structure similar to a greenhouse where the actual screen material follows the shape of a traditional a-framed greenhouse. There are at least three manufacturers of this type of open-roof structure.
Mr. Aart Van Wingerden, a pioneer in our American Plasticulture Society and in the development of the controlled environment agriculture industry, in 1990, demonstrated a greenhouse which he called an MX greenhouse, patterning the name after the operation of missile silos. In this first design the roof opened only on one side from the ridge, hinging at the gutter. This concept was the precursor to the fully articulating roof greenhouse MX-II illustrated on the page one.

The open-roof design creates an exciting concept of having a greenhouse with all of its controlled environment and growing system features in addition to a roof which fully opens for environmental control. This also presents the opportunity of growing crops in direct sunlight during part of the growing season. There are at least 6 companies which manufacture this type of open-roof greenhouse. Four of these use fully articulating roof sections which hinge at the gutter and open to various stages of opening from the ridge. Two of the manufacturers utilize articulating roof sections which hinge at the ridge and open at the gutter line and move across the greenhouse bay powered by a rack and pinion mechanism as indicated in Figure 4.

In addition to the articulating roof greenhouse there is the design which utilizes moving polyethylene roof sections which roll up, similar to a window shade mechanism. There are at least two manufacturers who feature this design. The plastic film is wrapped around tubing which extends the length of the structure. As the tubing is rolled with a hand or motor-driven crank the roof section opens. There is a temptation to leave the greenhouse open and not move it on a daily basis because of the difficulty in opening and closing the greenhouse. Some growers use these greenhouses as either open or closed structures, not utilizing the open-close feature.

Temperature data taken at DeGroot’s greenhouse, Figure 5, a local grower with this type of greenhouse located in Pompton Plains New Jersey confirm these data. This commercial greenhouse range has several conventional double poly greenhouses adjacent to a new MX-II type. The temperatures in the conventionally naturally ventilated greenhouse can be as high as 100F (5.5^oC) warmer than the adjacent MX-II greenhouse.

The spring season for those using the open-roof greenhouse design has been excellent. Good temperature and increased lighting to the crop without having to move the crop have produced good crops as anticipated. Grower acceptance around the country has been phenomenal. One greenhouse manufacturer indicated to the author that 3 of the 4 inquiries that come to the company, via phone, fax or e-mail are about the open roof designs. This is also true among other manufacturers.
CCEA has set up a comprehensive research effort to observe the performance of the MX-II design which is pictured in Figures 1 and 2. Preliminary data indicate the temperature within the MX-II during warm weather periods ranges about 2 - 40F (1.1-2.2 ^oC) above outside conditions.

As indicated earlier the author worked on the erection of the CCEA research greenhouse with Steven Kania, Horticultural Engineeer in the Bioresoure Engineering Department at Rutgers. The summer of 1999 was exceptionally hot at times as indicated in Figure 6 which is a record of July 6, 1999, a day when outside temperatures reached 1040F. Throughout the construction period and after completion the greenhouse temperature was nearly the same as outside.

The graph indicates data taken in the experimental MX-II greenhouse, pictured earlier, located on the Cook College campus at Rutgers University on July 6, 1999. These data were taken with no crop in the greenhouse so no transpiration cooling was involved. During construction of the MX-II greenhouse this summer, temperatures were typically 2-4oF above outside temperatures. The fan ventilated greenhouse, designed for a ventilation rate of one volume per minute, located adjacent to the MX-II traditionally was 10-15 degrees F. higher than outside temperatures as indicated in Figure 6. Solar radiation data indicates it was a perfectly clear solar day with the temperatures peaking a few hours after solar noon. During the night hours all of the temperatures came together.

The greenhouse is glazed with double polyethylene film. Covering the greenhouse was a challenge. Normally multi-span greenhouses are covered with polyethylene tubing which is placed over the roof bars and secured in both gutters. Since this greenhouse has to open at the ridge a different approach was needed. The roll of 14’4” tubing required for covering was cut in half. The roof sections were opened fully and the now folded plastic film was attached at the ridge aluminum extrusion as the roll was unrolled on edge standing in the gutter. Three men worked in the gutter, the first holding the roll upright, the second attaching the film at the top extrusion and the third fastening the roll at the gutter. This arrangement proved to be very easy with rapid application of the film.

Figures 7 and 8 show data taken in April and May, 2000. It is evident that the Argus computer environmental control system can maintain the set point temperature very accurately until outside conditions become to warm. The April data shows exact control and the May data indicates that the inside temperature is the same or lower than outside. During this time a crop of lilies.

We are excited about the success to date of this plastic glazed, cutting edge greenhouse and look forward to a successful research project.

Note
New Jersey Agricultural Experiment Station paper # P03130-09 00.

Figure 1 View showing the 58’ by 60’ CCEA MX-II research greenhouse at Rutgers University

Figure 2 Interior view of the MXII with the roof in the nearly fully open position

Figure 3. View of crop at Allen Van Wingerden’s range in Pompton Plains, NJ. The crop is on transportable benches that have been rolled out from the greenhouse to receive maximum light and cooler growing conditions. The vigor and uniformity of the crop is clearly seen

Figure 4 Two types of articulating open-roof greenhouse designs.

Figure 5. View of DeGroot’s commercial MX-II greenhouse. The direct sunlight and shadow caused by the roof sections can be seen in the photo.

Figure 6. Graph showing the outside temperature, two temperatures within the MX-II greenhouse and the temperature in a nearby fan ventilated greenhouse. Solar radiation is also shown indicating it was a perfectly clear day.

Figure 7. Graph showing the conditions on April 1, 2000

Figure 8. Graph showing data on May 8, 2000 when the outside temperature reached 93 0F at solar noon and it became cloudy during the afternoon