Daniel H. Willits, Professor

Research Summary

Environmental Control of Greenhouses

The North Carolina Greenhouse Industry

The traditional `greenhouse industry' of NC can be divided into three reasonably distinct groups: 1) flower producers, 2) vegetable growers and 3) landscape nurseries. Only the first two actually depend upon greenhouses as primary production facilities, the latter using them primarily as rooting and overwintering structures. Floricultural production in NC is healthy and growing. Vegetable operations constitute a much smaller segment; however, the number of growers has been increasing steadily over the last 10 years.

Greenhouse Cooling

Considerable activity in the 70's and 80's was devoted to reducing heating costs; however, the nearly decade long decline in fuel prices has diminished heating concerns, elevating cooling to the top of the problem list. As opposed to heating, cooling concerns are not driven by cost, but rather by a lack of appropriate alternatives. Traditional cooling systems depend upon exhaust fans to remove heated air from the interior of greenhouses, replacing it with air from outside. In hotter regions of the world, NC included, evaporative pads are used at the air inlets to reduce the dry bulb temperature of the air entering the greenhouse. Even so, it is quite common to find interior temperatures exceeding 32 C (90 F) during the day. Temperatures this high interfere with floral development, causing delay in bud initiation, malformed flowers and flowers of reduced size. They also contribute to poor fruit set in tomato and pepper and bolting in lettuce.

NCARS Project NC06240

The work under this project is devoted exclusively to greenhouse cooling concerns. The primary areas of emphasis are described below:

Night Cooling

Excessive temperatures are a serious problem for mum producers in the southeastern U.S., as they are for the producers of a number of floral crops in warm climates. High temperatures delay production schedules, reduce floral quality and generally reduce summertime productivity. For most of the year, chrysanthemum schedules are very regular, with timing ranging from 11 weeks for some cultivars to 12 weeks for others. High temperatures can delay flowering by several weeks and cause some buds not to open at all. Variations in timing due to changing weather patterns can mean varying amounts of plant material available for market at any given time. Excessively high night temperatures are generally a bigger problem than excessively high day temperatures.

Greenhouse have some inherent characteristics that suggest the use of mechanical refrigeration for night cooling: 1) greenhouses typically possess little thermal mass so that cooling load is almost exclusively a function of solar load; 2) relative humidities in greenhouses are generally higher (85-95%) than commercial or residential buildings (~50%), thus heat transfer in the evaporator, and therefore overall performance, should be enhanced by the condensation of water vapor; and 3) the required night-time temperature differential (between inside and out) is generally small, only 3 to 7 C, yielding a typical nighttime cooling load for an acre of greenhouses in Raleigh in July/August as little as 0.8 MWh, with a peak demand of only about 0.1 MW.

It is also true that the night cooling period approximately corresponds to typical off-peak periods used in time-of-use rates (e.g., Carolina Power and Light uses 10:00 pm to 10:00 am while Duke Power uses 10:00 pm to 12:00 am). Electrical rates during the off-peak periods are typically half of the normal rates, which considerably reduces the cost of operating air conditioners at night. The economics of night cooling are expected to depend heavily on time-of-use rates.

Our current work is attempting to model the use of heat pumps for winter heating and summer nighttime cooling. Heat pumps are rarely used in greenhouses because the high heating demand makes them too expensive to consider under normal circumstances. If restricted to off-peak periods, however, the cost the heat provided drops dramatically. If sufficient utilization of the heat pumps can be made during these periods to justify the investment, it may be possible to justify their use for night cooling in the summer.

See for example:

Peet, M. M., D. H. Willits and R. Gardner. 1997. Response of Ovule Development and Post- Pollen Production Processes in Male Sterile Tomatoes to Chronic, Sub-Acute High Temperature Stress. J. Exp. Bot. 48:101-112.

Willits, D.H. and M.M. Peet. 1998. The effect of night temperature on greenhouse grown tomato yields in warm climates. Agr. and For. Meteor. 92:191-202.

Willits, D.H. and D.A. Bailey. 2000. The effect of night temperature on chrysanthemum flowering: heat tolerant vs. heat sensitive cultivars. Scientia Hort. 83(3):325-330.

Shade Cloths

Shade cloths, either internal or external, have been used for many years to limit the amount of solar radiation entering greenhouses; however, work at NCSU has shown that most function at about 50% efficiency. For example, a shade cloth rated at 50% shade (blockage) can be expected to reduce total energy by only about 25%.

The primary reason for this inefficiency is that a significant amount of the energy blocked by the cloth is transferred into the greenhouse via radiation and conduction, rather than being transferred into the atmosphere. When the shade cloths are mounted on top of the covering, water evaporation can be a relatively inexpensive means of increasing heat transfer to the air (assuming the availability of inexpensive water), considering that approximately 2300 kJ of energy is transferred for every kg of water evaporated. This suggests that keeping shade cloths wet might improve overall efficiency.

Recent studies have shown that misting does indeed increase shade cloth performance, by as much as a factor of two. Some of the studies, which have included misted and unmisted black vs. white shade cloths, have provided interesting insights into the performance of shade cloths. It now seems reasonable to expect that an accurate computer model can be developed to predict shade cloth behavior. Dr. Mary Peet, Dept of Hort Sci, has been serving as a cooperator on this project.

See for example:

Willits, D. H. and M. M. Peet. 2000. Intermittent Application of Water to an Externally Mounted, Greenhouse Shade Cloth to Modify Cooling Performance. Trans of the ASAE Vol. 43(5):1247-1252.

Willits, D. H. 2000. The effect of ventilation rate, evaporative cooling, shading and mixing fans on air and leaf temperatures in a greenhouse tomato crop. ASAE Paper no. 00-4058, pp 18.

Cooling Model

A finite difference cooling model has been developed based upon energy balances of three prmary components: 1) cover/structure, 2) canopy and 3) ground. The model predicts air, leaf and ground temperatures as a function of distance along the greenhouse. It has been used successfully to check data collected in various cooling studies conducted since its development. It has also provided insight as to how plants control leaf temperature via transpiration.

The model has been improved and extended several times since initial development and will likely continue to improved as new information becomes available. Simulation runs with the model have already suggested several experiments of interest to be pursued when funding is available.

See for example:

Willits, D.H. 2000. Constraints and limitations in greenhouse cooling: Challenges for the next decade. Acta Hort. 534: pp 57-66.

Transpirational Cooling

We have recently completed a 3 year project to better understand how plants cool themselves and to recommend ways of improving their cooling efficiency. The first year was devoted to examining the differences in transpiration cooling with different canopy densities. We also looked at measurements of chlorophyl fluorescence and photosynthesis as a means of objectively determining when plants are undergoing temperature or irrigation stress. We found that plants in a sparse canopy transpire more per unit leaf area than do plants in a dense canopy. We were also able to link fluorescence readings of single leaves to irrigation stress (and apparently temperature stress). We were not able to separate temperature stress from irrigation stress with the experimental plan we used.

With regard to transpirational cooling, we have established that there is little advantage to increasing air flow rates above about 0.04 m3/m2 s (8 cfm/ft2) when evaporative pad cooling is not used, but when it is increasing air flow rates to as high as 0.087 m3/m2 s (17 cfm/ft2) can still provide significant cooling. With evaporative cooling, the largest reductions in temperature with increased air flow rate were at the exit end of the house. The increased air flow extended the cooling effect of the evaporative pad further down the length of the greenhouse, reducing both air and leaf temperature gradients in the process. This suggests that current guidelines for limiting air flow rates to 0.04 m3/m2 s (8 cfm/ft2) are not appropriate for evaporative pad cooling and that designers of greenhouses to be used in hot climates should consider increasing air flow.

Other interesting features of the work will be discussed in future publications. This information will be released here as soon as it becomes available.

See for example:

Willits, D. H. 2000. The effect of ventilation rate, evaporative cooling, shading and mixing fans on air and leaf temperatures in a greenhouse tomato crop. ASAE Paper no. 00-4058, pp 18.

Seginer, I., D. H. Willits, M. Raviv and M. M. Peet. 2000. Transpirational cooling of greenhouse crops. BARD Final Scientific Report IS-2538-95R, Bet Dagan, Israel.

Willits, D.H. 1999. The effect of canopy density, ventilation rate and evaporative cooling on the transpiration of a greenhouse tomato crop. ASAE Paper no. 99-4227, pp 18.

Willits, D.H. and Peet, M.M. 1999. Using chlorophyll fluorescence to model leaf photosynthesis in greenhouse pepper and tomato. Acta Hort. 507, pp 311-315.

Willits, D. H. and M. M. Peet. 2001. Measurement of chlorophyll fluorescence as a heat stress indicator in tomato: laboratory and greenhouse comparisons. J. of the Am. Soc. For Hort. Sci. (in press).