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NATURAL VENTILATION OF GREENHOUSES IN DESERT CLIMATE A,. Arbel, I. Segal, 0. Yekutieli, N. Zamir Institute of Agricultural Engineering, Agricultural Research Organization, Bet Dagan, Israel. Abstract Desert weather is usually characterized by clear skys, very low precipitation and low air humidity. These conditions are favorable for greenhouse horticulture to produce winter cropping. Development of greenhouses during recent years in these regions in Israel is very rapid. Development started with utilizing low and high tunnels, the latter showing significant horticultural advantages. The Institute of Agricultural Engineering was involved in developing greenhouses to assist in the specific needs of the desert areas in achieving the following: 1. Low cost of construction and maintenance. 2. Simple construction and covering procedures. 3. Large continuous growing space. The result is a no-gutfeer, roof ventilated ('French system), 5000m2 square structure. Ventilation of this structure is achieved by opening the four sides and 'wedges' in the roof (total 15)1 of horizontal area). Theoretical considerations taking into account natural ventilation and dry weather, forecasted acceptable1 climate inside the greenhouse during winter and spring days. Comparison of temperature and humidity in the low structure with a conventional (30 m length,- f| m gutter height with openings on two sides and gables) showed small differences in temperature while relative humidity in the low greenhouse was higher - closer to required humidity. Climate and horticultural experiences of the first year will be presented with description of essential structures and ventilation details. Acta Horticulturae 281, 1990 Greenhouse Construction, Design Introduction Climate in desert regions in Israel (specifically the Arava region) is specified by very low precipitation and high solar radiation (less than 60 mm - about 10-15 cloudy days per year). Relative humidity is very low and daily ambient temperatures are relatively high (Figs. 1,2). These are favourable conditions for growing horticultural products for export during winter. The development of greenhouses in desert areas is one of the major efforts of Israeli agriculture. The Institute of Agricultural Engineering has initiated development of low cost wide span low silhouette gutterless greenhouse for desert climate. These greenhouses were designed with the purpose achieving: 1) Low cost of construction. 2) Minimum maintenance requirements and simple procedures replace covering films. 3) Large continuous growing space. of to The commercial version is a 70x70m greenhouse with a minimum height of 1.8-2.2m. Ventilation is achieved by opening edges in the roof (so called 'French' system) and curtains in the four sides. Total openings are about 15% of floor area. One of the main forseeable disadvantages of the new type, low wide span (LWS) greenhouse is natural ventilation ability. However, preliminary calculations taking into account the large contribution of evaporative cooling due to plant transpiration in dry climates promised acceptable climate conditions during autumn and spring. The use of conventional and non-conventional evaporative cooling systems is also very promising in dry climates. Direct spraying or fogging could be the solution for cooling of low wide span (LSW) greenhouses when vegetation is not a sufficient source for water evaporation. Investigation of the climate conditions in greenhouses in the Arava desert region is described in the following paper. Methods and Materials During the first year of investigation manual measurement of climate conditions (temperature, relative humidity, wind velocity) was carried out during mid-day hours (11-14) starting 30 November 1987 up to 8 May 1988. These measurements were carried out in 17 different commercial greenhouses in the Arava region. These 17 greenhouses can be represented by four 231» groups: (Fig. 7.) 1) Walk in tunnels (almost semi-circle- Gl). Width 8.5 m, height 3.5 m, side curtains and end opening. Total openings 25?! of floor area. 2) Low Wide Span Greenhouse (-G12) Gutterless structure roof ventilated (French system) operated centrally by cables on pipes. Minimum height 1.8 m, maximum 3.0 m, 7.12 m span, total area 5,000 m2 (70x70m approx). Max. opening 5% of floor area (5% on roof). 3) Regular Plastic Covered Greenhouse (-G3) 140x32 m, gutter height 4 m, span 7.5m, curtains 2.8m height on all four sides, total opening area 21%. k) Similar regular greenhouse (- C4) 99x4iim with roof (5%) and side curtains (18J! of floor area). Most measurements hygrometer (Hygroskop opening were made by a capacitance type ThermoGT2 by Rotronics). The readings of this instrument were compared with a conventional aspirated psychrometer by Kasela (U.K.). Wind velocity was measured by PA 4000 by Digital. Radiation was measured occasionally. However, on most relevent days sky was clear and radiation intensity is very close to "clear sky radiation" calculated by standard methods (Duffie & Beckman, Fig. 1). Results. Discussion and Conclusions Temperatures and relative humidity in the low wide span greenhouse during seasons are given in figures 3 and 4. Note that the lines do not represent continuous temperature measurements, but connect points taken every two weeks. Figures 5 and 6 show the differences betwen four major types of greenhouses and the ambient. During the beginning of the season higher temperature differences were achieved, due to partial opening of the greenhouses. Towards the end of the season with well developed plants (trained tomatoes) inside temperatures were sometimes lower than those outside due to effective evaporative cooling. Ventilation and cooling in most greenhouses during all seasons, including spring months, was sufficient. Temperatures 231» inside the greenhouses were usually not higher than required (Fig. 3). Differences between the various types were not large (Fig. 5). Evaporative cooling due to plant transpiration during spring months explains the relatively low temperatures inside the greenhouse. Humidity levels (Fig. k) in the LWS greenhouses were acceptable. The low humidity of the outside air (20-30%) during all seasons explains these acceptable conditions in the greenhouses. The Low Wide Span Greenhouse was more humid than the others. However, humidity levels were too low in the other greenhouses due to over ventilation. The following conclusions and recommendations can be drawn from this climate survey: 1) Acceptable conditions for growing can be achieved in all greenhouses by natural ventilation in the arid Arava region. The low wide span greenhouse can be used without causing overheating even during autumn and spring. 2) More investigations concentrating on the control of openings should be carried out. Also the operation of spray and fog type evaporative cooling for arid zones when plant leaf area is too small should be further investigated. 3) The advantage of arid zones enables growing during winter due to stronger solar radiation but also enables simple cooling by natural ventilation and evaporation in relatively simple low and inexpensive structures. References 1) Duffie, J.A. and Beckman, W.A. 1980. Solar Engineering of Thermal Processes. John Wiley & Sons, Inc. U.S.A. 2) Arbel, A., Zamir, N., Yekutieli, 0. and Meron, I. 1988. Mid-day Climatic Conditions in the Central Arava WinterSpring 1987/88. Internal Report (in Hebrew). 231» 11 4 B 0 > 8 7 , S 10 , 11 , le , 1 , e 3 MONTH Fig. 1. Global Solar Energy in I s r a e l Month 0 T.mox T.min O R.H.14. A R.H.OB F i g . 2. Temperatures and r e l a t i v e humidities in Arava region 231» sa 30 29 20 27 26 23 22 21 20 2B.2 out + mean 14.3 27.3 11.4 25.4 DATE 1 6 Fig. 3 . Air temp, in the LWS greenhouse and ambient ( o u t ) l o c a t i o n s 1 , 2 , 3 , 4 a r e a t height 1.2m evenly d i s t r i b u t e d along the greenhouse (North— South) DATE • out 4 mean » 1 6 2 X 3 7 4. Fig. 4 . R e l a t i v e humidity i n the LWS greenhouse and ambient. Locations 1, 2 , 3 , 4 see Fig. 3 . 231» • 03 4- 04. DATE « 012 h <31 F i g . 5 . Temperature d i f f e r e n c e s (compared with ambient) of t h e f o u r major types of greenhouses. G3 - r e g u l a r greenhouse, G4 r e g u l a r with roof opening, G12 - LWS greenhouse, G1 - walk-in tunnel DATE D G.i + G4. 6 G12 A G1 F i g . 6 . R e l a t i v e humidity d i f f e r e n c e s compared with ambient of the f o u r major greenhouses ( F i g . 5 ) . 231» 231»