History

In the early 1960's, as extension Agricultural Engineer, William J. Roberts was working with some growers who were using low-cost polyethylene film on simple wooden frames to construct greenhouses used primarily for spring transplant production and for bedding plant operations. One early concern was the tendency for a single layered roof to collect condensation that dripped on the small seedlings causing problems. To ameliorate this problem, a second layer of film was added by fastening it to the underside of the frame creating an airspace, and keeping the inner layer warmer. This was a cumbersome process so the next step was to install a single layer over the frame and fasten it to the rafters with 2x2 spacers then add on a second layer on top of the 2x2's and fasten it down with a 1x2. This was an improvement but still required two fastening steps for every rafter.

There had been some experimentation at Rutgers University on using a fan to create a bubble house from a single sheet of plastic with the edges buried in the ground. Recognizing the importance of the double layer covering on a greenhouse to reduce condensation, Bill tried fastening two sheets together around all four edges and used a small, low-pressure fan to inflate the space between. The first greenhouse on which this concept was successfully applied is now a national landmark. It is a wooden frame structure designed for the width of polyethylene sheeting available at the time. It was quickly noted that not only was there a significant reduction in the required construction materials and labor time, but the tension in the film due to the slight air pressure reduced the film flexing and flapping in the wind, reducing the likelihood of tearing the film, thereby increasing structural reliability and extending film life.

This concept was next applied to a portion of a large, gutter-connected commercial greenhouse in Allentown NJ, (Kube-Pak, Inc., then managed by Aart VanWingerden). Several companies then developed frame structures for multi-span and single span structures. Probably first among these were VanWingerden with steel frames and PolyGrower with aluminum. Bill also designed wooden greenhouse frames of several sizes to match available film widths, as well as a pipe frame structure and pipe bender to assist hand bending of the hoops. He developed the engineering plans and drawings for these easy-to-construct greenhouses. These plans were made available through the extension plan service. The early popularity of these designs and their rapid commercial acceptance was due primarily to their low-cost relative to conventional greenhouses glazed with glass or fiberglass. It was also noted that the insulation properties of the inflated air space reduced heat requirements by over a third, which became a more highly appreciated advantage in the years of the energy crisis from 1973 on. Finally, it should be noted that this development could not have taken place when it did nor have spread into commercial practices so rapidly and broadly without the contributions of commercial growers who took the early risks. The leaders among these would be Aart VanWingerden and Kenneth Bryfogle, who also started the first companies to provide gutter connected structures of steel and aluminum, respectively, and Frank Stuppy who developed the first extruded aluminum film fastener.

David Mears worked with Bill Roberts on some aspects of the early research and contributed the engineering analysis of film stress as related to the geometry and size of the coverings and the structural loads. Later, David and a series of graduate students advised by him and Bill did further research on structural aspects, energy conservation and management, film properties and other issues associated with a series of advances in the technology. The key to all of these advances was the simplicity and functionality of the concept of using air to inflate the space between the layers, a concept that was developed by Bill.

Impact

In 1999, approximately 9,250 ha (23,125 acres) of air-inflated double-layer polyethylene greenhouses were in production in the US (682,050 ha or 1,705,125 acres worldwide) (Takakura and Fang, 2002, Climate under Cover, 2nd edition, 190 pp.). Approximately 65% of all commercial greenhouses in the United States use the air-inflated system. While the total area for greenhouse production may seem small, production in these greenhouses occurs year-round, often producing multiple high-value crops. Therefore, the production on an area basis is much higher in greenhouses compared to field production. In addition, especially in lower-income countries, the AIDLPG is the only economic alternative for year-round production helping local farmers secure living wages and providing the local population with affordable produce even when adverse weather conditions prevent outdoor production. The total worldwide area in 2003 is estimated at 3,000 square miles (the quoted 1999 data plus 12%, representing a 4% annual increase which was the annual increase reported in the reference between 1991 and 1999) or roughly the size of the states of Delaware and Rhode Island combined. The development of the AIDLPG at Rutgers University set in motion successive developments of the movable thermal insulation screens and in-floor root zone heating systems. All three of these developments were included in the list of outstanding achievements of the 20th century in Agricultural Engineering as described in Resource Magazine (March 2000, pg 19). While much of the research at Rutgers University leading to subsequent advances in greenhouse engineering has been conducted in other facilities, the original AIDLPG structure has also been used continuously for a variety of research studies. It was also the first unit in which developments in floor heating and movable insulation/shade curtains were attempted. The method of fastening the film has been upgraded from the original wooden strip fastening method (Roberts and Mears, 1969) to various designs involving aluminum extrusions. Otherwise, the AIDLPG frame is the original to this date.

Historic Landmark

The Society for Engineering in Agricultural, Food, and Biological Systems (ASAE) has named the important development of the first ever air-inflated double-layer polyethylene greenhouse (AIDLPG) as an ASAE Historic Landmark. To date, only 42 such landmarks have been dedicated in the United States. The first AIDLPG was developed by Professor Emeritus William J. Roberts in the early 1960’s at Cook College, Rutgers University. Following the development of the AIDLPG, members of the former Bioresource Engineering Department were able to conduct studies investigating greenhouse film material characteristics, greenhouse heating system design and management, and optimization of environmental control strategies. These studies led to the development of movable thermal curtains and root zone heating systems, two developments that were highlighted in ASAE’s Resource Magazine (March 2000) as one of the outstanding achievements in agricultural engineering during the 20th century. The original AIDLPG greenhouse structure is still located on Cook Campus, just behind the Extension Conference Center.