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RESPONSES TO COMMON GREENHOUSE GLAZING QUESTIONS or Coffee Table
Conversation about Greenhouse Glazings Dr. Gene A. Giacomelli* Department of Bioresource Engineering Cook College Rutgers University New Brunswick, New Jersey, USA June 1, 1999 There are at least 2000 concerns to be dealt with before the year 2000 but here are a few that are more readily understood and hopefully solvable. For more information watch for the next Educational Workshop from CCEA on greenhouse glazing light transmission. CCEA, The Center for Controlled Environment Agriculture 732 932-9753 Ph; -7931 FAX giacomel@bioresource.rutgers.edu or look http://www.cook.rutgers.edu/~ccea ___________________ *Dr. Giacomelli is professor in the Bioresource Engineering Department and Associate Director CCEA, at Rutgers University. NJAES Paper No. J-03232-19-99 1 The inside of a glass covered greenhouse always seems brighter than a double plastic ……and since light is important……is there really a choice to be made? The key words are “seems brighter”. Be cautious, with “eye-witness” measurements. An electronic sensor which measures the specific portion of the light that the plant requires is a much better means to determine light intensity. The eye of the grower is great for plant care, but glazing transmission can offer deceiving perceptions. To really know, measure with a sensor. In general, for any glazing, a single layer allows more light to pass than two layers, and plastic coverings diffuse the light more than clear glass. Our eyes and most sensors cannot compare direct and diffuse light. The greater amount of diffuse light passing from plastic covers can give the impression of reduced light conditions in the greenhouse. 2 Is PAR [light] the most important factor for choosing a glazing? It is a very important one! PAR photosynthetically active radiation, is a measurement of the intensity of a portion of solar radiation that is useful for plant growth. Special quantum sensors are able to interpret the light intensity, similar to the plant leaf. Many other factors affect PAR reaching the plant, which can be more important than even a 5% difference in transmission among types of glazings. 3 Besides purchase price and light transmission, what other reasons are there for selecting a film over a rigid plastic panel or glass as a greenhouse cover? A continuous film glazing has less edges and connections on its roof and side walls, which reduces the amount of uncontrolled outside air infiltration. This means less heating costs in winter, but also higher inside air humidity resulting from the water vapor of plant transpiration. A “tight” greenhouse can result in reduced carbon dioxide in the air [important for plant growth] when not regularly venting. 4 The specific crops to be grown have little to do with the selection of my glazing? The plants have everything to do with glazing selection, as well as, all the other greenhouse systems and the structure itself. Consider the relationship within the overall greenhouse crop production system, and the grower’s plant production experience with the potential systems selections. 5 Glass is traditional, why should new plastics be considered as glazings? All glazing material commonly sold for the industry offers strength, consistency, durability, manufacturing quality control, and safety in use. Solar radiation transmission and heat energy conservation should be considered, and ease of attachment is important for glazings which must be removed and replaced as they age. Most plastic coverings are affected by weathering. Glass is quite inert, in contrast to plastic, and can provide effective use for many decades. Glass is non-combustible, resistant to UV radiation and air pollutant degradation, and it maintains its initial radiation transmission throughout its life. The most predominant drawback of glass may be its vulnerability to catastrophic losses caused by hail. 6 Do glazings perform the same at all locations? The best glazing may still not allow sufficient light in the winter season for some crops. The short-duration, cloud-covered days of late fall, winter and early spring contribute to the primary cause for reduction of plant growth…too little solar radiation, or “light”. Subsequent difficulties in crop production from diseases, nutritional deficiencies and reduced product quality may also result. Whatever the amount of natural solar radiation available for the plant, it must first reach the greenhouse, then pass through the glazing and beyond overhead structural framework before reaching the plant canopy. Therefore it is important to consider a southerly exposure which is free from nearby buildings, groves of trees and other obstructions to light. Note that an unobstructed northern exposure is important as well, since on cloudy, diffuse days, a significant portion of light for plant growth does enter the greenhouse from the north. 7 Are there other factors which combine to modify the amount of PAR transmitted to the plants? These are: day of year, and hour of day; latitude; local weather conditions; predominance of direct or diffuse solar radiation; cover material properties (at installation and as affected in time by weathering, air pollutants, moisture condensation, and dust and dirt accumulation). As well as, the greenhouse physical structure, including: angle and shape of the roof; the number and width of spans (distance from gutter to gutter, if multi-span or ground to ground, if single span); height of end walls; length to width ratio of the structure, and compass orientation. So do not immediately contact your glazing salesperson when your crop displays a need for more light. 8 Is a north-south or east-west greenhouse orientation the best? In short, a N-S oriented greenhouse is best for year-round production, while an E-W greenhouse is best for winter production. Greenhouse compass orientation not only affects the total light entering each day, but also its distribution within the greenhouse. An east-west oriented ridge will have a large south-facing wall and roof area for collecting the low sun angle winter sunlight. Thus it will provide the most total daily light during the winter season. However distribution of the light may not be uniform to all plants within the greenhouse. Non-uniform light patterns causes irregular plant growth. Those on the north side may receive less light. This is especially a problem for tall crops, especially if they are grown in rows which are aligned with the east-west ridge. A freestanding, single-bay greenhouse [ground to ground], generally provides more light inside to the plants than a gutter-connected, multi-bay greenhouse. There is less overhead structure in the single-bay greenhouse, and the relatively narrow span allows for more glazing area. A long, narrow [less than 25 feet wide] freestanding east-west greenhouse for bedding and potted plants, or hydroponic lettuce will offer the best light conditions for the winter months. However, a tall crop, grown within a gutter-connected, multi-bay greenhouse should be oriented with its gutters [or ridges] in the north-south direction. The reduction in total light entering the greenhouse is offset by an even more important factor, the improvement of light uniformity to all locations within the growing area. The benefit of the north-south ridge orientation is the “movement” of the shadows caused by the overhead structures as the day progresses from an eastern to western sun location. No single location of the greenhouse remains in shadow throughout the day. A non-moving shadow will occur within an east-west oriented greenhouse, and it is most pronounced during the winter months. 9 Single layer versus two layers….if one is good is two better? Transmission of light is directly affected by the number of glazing layers. The number of layers of glazing is generally more important than the type of glazing material in determining the amount of light entering the greenhouse. Double-layer materials always reduce light transmission more than single layer. Rigid double-layer materials reduce transmission more than film double-layer glazings for low sun angles, that is, whenever the sun is not directly above or nearly overhead. This is primarily caused by the additional interior surface used to create the channels of the double-walled rigid panel glazings. Multiple layer glazings have improved winter season heat energy savings, and they create a strong, yet light-weight greenhouse cover. 10 What is Infrared [IR] Barrier, or Thermal Film glazing? Glazings are generalized as a thermal glazing if they reduce radiation heat loss. Infrared radiation is one means to lose heat from the greenhouse, and it is dependent on the total glazing surface area and the physical properties of the glazing. Thermal polyethylene [PE] films reduce radiation heat loss more than non-thermal PE films, because they contain an additive which can absorb and return the heat which tries to escape from the greenhouse. In this case, it is not the visible light radiation [PAR] which is of concern, but the non-visible infrared [IR] and thermal radiation [longwave]. 11 If Thermal films will keep the greenhouse warmer on cold nights, then will they keep them warmer on hot days?…and make air temperature higher in the summer? If there are no plants in the greenhouse to evaporate water by transpiration [a cooling process], then the greenhouse air temperature will increase slightly more in a thermal film covered greenhouse. However, during crop production, most of the energy from sun will be absorbed by the plant canopy and used to evaporate water from the leaf. With proper air ventilation exchange, there should be little increased effect of thermal films on daytime air temperature under full crop conditions. 12 With a thermal film greenhouse cover, an energy blanket [thermal screen] is not necessary? Depends. To know for sure, the net radiation of the greenhouse is necessary for evaluating the greenhouse energy situation, and thus the potential energy savings with an IR glazing. Net radiation is the difference between the energy received and energy lost in the greenhouse by radiation. During the day, the sun which provides a large amount of radiation assures a net gain of energy, because the heat losses from the greenhouse are much smaller than the gains. This net gain of energy causes the greenhouse air temperature rise. However, at night, the warm materials within the greenhouse (earthen floor, concrete paths, metal benches, plants, etc) will have significant radiation heat losses to the colder outdoor environment, if the glazing allows. The net energy loss is caused by transmission of infrared and thermal radiation through the cover. The amount of this radiation energy loss depends, not only on the properties of the cover, but also on the temperature of the cover, and the sky conditions (water vapor/clouds, carbon dioxide, and ozone content). Generally thermal glazings have proven cost-effective where heating energy costs are a significant operating expense, even for well-designed and maintained greenhouse operations.The addition of a thermal blanket or screen between the crop and the glazing can further reduce the heat loss by radiation, and in combination with a thermal glazing can offer additional energy savings. 13 Can shading the crop with another [movable] cover above the glazing help keep them cool? Yes, by shading, the amount of energy entering the greenhouse is reduced, thus preventing the plant leaves and interior components from increasing in temperature as much. However, there is a potential loss of plant growth from reduced PAR. It is best to shade to intensities and/or daily duration which benefit cooling without seriously reducing plant growth. Shade material on a moving system above the glazing is best for cooling, but is subjected to the weather. Shade system inside does not reduce air temperatures as well, however, it can double as a thermal screen [heat blanket] in the winter. All shade systems should be automated for better control. 14 Condensation on the Glazing….good or bad? Both! Condensation may seem to be an unwanted event in the greenhouse, however, it is an important indication of the environment responding to plant transpiration of moisture into the greenhouse air. Essentially it cannot be stopped, but its potential negative effects can be minimized. Transpiration [good], the evaporation of water from the plant leaves to the greenhouse air, is critical for natural leaf cooling, nutrient uptake and growth of the plant. The plant will not even stand upright unless it continually transpires water. Condensation of water vapor from the warm moist air onto the cool surface of the greenhouse covering material is the primary means of reducing the greenhouse air humidity whenever ventilation is not occurring [good]. Water vapor in the air is converted to liquid water on the glazing. It is also a method for heat loss from the greenhouse [bad]. The change of water vapor to liquid results in a release of energy called latent heat, which is then lost through the glazing. Water droplets remaining on the glazing is an undesirable situation. They will fall to cause potential crop damage by localized over-watering or encourage disease infestation [bad]. In addition, droplets remaining on the glazing become another obstruction for light to pass [or to be reflected away], prior to entering the greenhouse [bad]. Efforts to incorporate inhibitors to droplet formation during manufacturing, particularly on plastic coverings, have somewhat reduced this problem. Water movement along the roof surface should not be impeded by the structural supports of the glazing, unless at a specific location for the drip gutter to safely carry the water away. Water vapor from the air can condense inside the multi-layer film and rigid panel glazings. Where a small fan is used for double-polyethylene film greenhouses, it is very important to use outside air, not greenhouse air for inflation. 15 What are co-extrusion or multi-layer films? Techniques of co-extruding films and multi-layering of films during the extrusion process have been developed. “Designer films and rigid panels” are the result, and they offer even further opportunities for improved performance of the covering material and ultimately improved crop production. The most recently developed films have included an infrared (IR) barrier, condensate control, and/or wavelength selective properties. 16 Can open-roof greenhouse become the new “better” greenhouse? The potential of open-roof greenhouse structures has only begun to be explored. Currently there are several greenhouse designs available where the entire roof can be mechanically opened and closed. The result can be improved plant lighting and air temperature reduction. |
