Heat and Mass Transfer in a Large Evaporative Cooled Greenhouse Equipped with a Progressive Shading

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1 Heat and Mass Transfer in a Large Evaporative Cooled Greenhouse Equipped with a Progressive Shading Th. Bartzanas and C. Kittas University of Thessaly, School of Agricultural Sciences, Department of Agriculture, Crop Production and Agricultural Environment, Fytokou St., N. Ionia, GR-38446, Magnisia Greece Keywords: Evaporative cooling, shading, temperature gradients, crop transpiration Abstract An experimental study was conducted in a large commercial greenhouse equipped with a fan and pad evaporative cooling system and a half-shaded plastic roof. Experimental measurements were carried out at four different locations inside the greenhouse: (i) next to the cooling pads, (ii) 15 m away from the cooling pads (unshaded section), (iii) 45 m away from the cooling pads (shaded section) and (iv) before the extracting fans. The experimental results showed strong climate heterogeneity along the air stream from cooling pads to extracting fans. The overall efficiency of the system was quite high (~ 80%) and in combination with the dry and warm outside climate conditions enabled the air to enter the greenhouse with a temperature even 10ºC lower than that of outside air temperature. However, due to the long air path through the greenhouse, air exits the greenhouse even 8ºC warmer than the inlet air. Air vapour pressure deficit varied from 0.5 kpa (after the cooling pads) to 2 kpa (extracting fans). Transpiration rate was found larger at the second (shaded) part of the greenhouse mainly due the higher vapour pressure deficit found at this section of the greenhouse. Crop-to-air temperature difference was smaller at the second part of the greenhouse (45 m) indicated that the crop in this part was cooler than in the first one as a result of the higher transpiration rate at this section. INTODUCTION The current trend in greenhouse cultivation is to extend the production season, in order to maximize the use of the greenhouse equipment, extend the export season, increase the annual yield per unit area and increase the profitability. Nevertheless, in many Mediterranean greenhouses such a practice is limited because the cooling method used (mainly ventilation and shading) does not provide the desired conditions, especially during the hot summer months. Natural ventilation and roof shading are the most common techniques. Ventilation reduces greenhouse overheating, but it may even enhance the risk of water stress because it often increases crop transpiration (Seginer, 1994). Kittas et al. (2001) reported that high ventilation rates were not, a priori, the best solution for alleviating crop stress in greenhouses during summer conditions. Shading screens mounted externally or internally, may be used to reduce radiation inside the greenhouse but the effective temperature reduction is not really proportional to the shading rate. Willits and Peet (1993) showed that externally mounted black polyethylene films were less than 50% effective in reducing energy and temperature gains compared to their commercially given values, while white shading cloths were only slightly more effective. If the greenhouse air temperature has to be kept near or below outside ambient temperatures, some form of cooling must be provided. Evaporative systems for cooling greenhouses have been developed to provide the desired growing conditions in the greenhouse during the hot period of the year. These systems are based on the conversion of sensible heat into latent heat. It can be done by spraying water droplets in a naturally ventilated building (by low or high pressure fog systems), or by forcing ambient air through wet pads. Both produce a temperature drop with an absolute humidity rise in the greenhouse, which contributes to decrease the vapour pressure deficit and moderate the transpiration demand (Katsoulas et al., 2001). Various works on evaporative cooling Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS

2 systems applied to horticulture, mainly fog systems, were already published, and, among others, those by Montero et al. (1981, 1990) and Giacomelli et al., (1985). Most of these works analyse the thermodynamic efficiency of the system and its climatic effects. Seginer (1994) found that evaporative cooling systems are mainly effective when crop transpiration is low, and Fuchs (1993) reported that a highly transpiring crop combined with a proper ventilation rate is the most effective mechanism to keep leaf temperatures moderate. A theoretical study was conducted by Arbel et al. (1999) to evaluate an evaporative cooling system for greenhouses by installing uniformly distributed fog generating nozzles in the space over the plants. Landsberg et al., (1979), proposed a theoretical model for the efficiency of evaporative cooling in different physical conditions and Bowen ratios. One limitation of this model is the a priori specification of the sensible heat to latent heat ratio rather than its deduction from actual crop behaviour. Moreover, this model was not tested against experimental data. More recently, Willits (2000) proposed a model to predict air and crop temperatures as a function of ventilation rate and external temperature and Kittas et al. (2003) present and validate a model to predict temperature gradients in an large evaporative cooled greenhouse. The main disadvantage of fan and pad systems is the lack of uniformity of the climatic conditions, which are characterized by rising temperature and falling humidity along the length of the structure and in the airflow direction. To overcome these problems fan and pad systems are usually combined with shading. Aim of the present study was to experimentally investigate the microclimate in a large evaporative cooled greenhouse combined with partial roof shading. MATERIALS AND METHODS The greenhouse (Fig. 1), 50 m wide and 60 m long, is made of eight adjacent 6.25 m wide arches covered with double inflated plastic Polyethylene Ethylene Vinyl Acetate (PE-EVA) films. Ambient air is forced through a 61 m 2 area of 10 cm thick cooling pads set on the 50 m wide south wall. These corrugated cellulose pads offer a 200 m 2 / m 3 specific exchange surface for evaporation. After crossing the pads, air travels a 60 m distance before being extracted by nine fans situated on the opposite north wall. Each fan generates a flow rate of about 9.3 m 3 /s under a 3 mm water column pressure. At the last 30 m downstream the greenhouse roof was shaded by a whitewash shading screen. The greenhouse was commercially operated for cut rose production. Rose plants (cv First Red, from Nirp International) were cultivated in deep soil under a straw mulch and were irrigated and fertilised with drippers. The plantation lines were 1.5 m apart, parallel to the airstream. Temperature and humidity were recorded with ventilated psychrometers (Delta-T devices, UK, accuracy ±0.2 C for temperature and ±3% for humidity) at four different locations inside the greenhouse: one just next to the cooling pads, one 15 m away from the pads (middle point of unshaded section), the third one 45 m away from the pads (middle point of shaded section) and the fourth one just before the fans. For the determination of the light transmission of the roof, the global solar radiation was measured by two pyranometers (model CM-6, Kipp and Zonen, Delft, Holland, accuracy ±5 Wm -2 ) located in the middle of the first and of the second half of the greenhouse. The crop temperature was monitored by means of copper-constantan thermocouples (type T, accuracy ±0.2 C), with wire diameter of 0.1 mm. Transpiration rate was measured by means of two weighing lysimeters at two different locations: one 15 m away from the pads and one 45 m away from the pads. Simultaneously to the measurements of inside climatic variables, outside variables were recorded: outside air temperature and humidity, global solar radiation and wind speed and direction (Vector instruments, Delta-T devices, UK). The measurements of all the above sensors were collected on a data logger system (DL3000, Delta-T devices, UK) with a sample frequency of 30 s and the 30 min average values were recorded. Finally evaporative cooling efficiency was calculated according to Kittas et al. (2003). 626

3 RESULTS AND DISCUSSION The experiments were carried out during summer 2001 and the results of three consecutive days with similar outside climate conditions were selected for the diagrammatic representation in this paper (vertical columns in the graphs show the transition from one day to another). Owing to the proximity of these three days, it was assumed that there were no significant changes in the leaf area and in the physiological status of the crop. Table 1 gives the values of the outside climate variables during these three days (average over the period h local time). Outside solar radiation varied from 160 to 970 Wm -2 with its maximum values been reached at 13:30 local time. Due to the shading at the second section of the greenhouse the intensity of solar radiation at this section was reduced (Fig. 2). Using the radiation measurements inside and outside the greenhouse, the transparency of the greenhouse covering was found to be about 0.64 in a distance of 15 m from cooling pads (non-shaded part of the greenhouse) and 0.5 in a distance of 45 m from cooling pads (shaded part of the greenhouse). The experimental data showed that the internal temperature was 10 C below outside air, even during hot afternoons (temperatures up to 35 C), owing partly to the low outside humidity levels and to the high efficiency of the cooling system (near 80%). However, due to the long air path through the greenhouse (60 m), strong thermal gradients were observed. Figure 3 shows the temperatures at various positions inside the greenhouse: a gradual temperature rise, along to pads-to-fans axis, reaching 8 C at noon when solar radiation reached the maximum level. The thermal gradients were more pronounced in the first no-shaded part of the greenhouse, while being much lower in the second shaded part of the greenhouse. The daily variation of vapour pressure deficit (VPD) at different positions inside the greenhouse during the three continuous days is presented in Fig. 4. From this.ure, it appears that VPD increases along the no-shaded and shaded parts of the greenhouse and that VPD is much lower in the no-shaded part compared to the shaded part. The VPD values observed were lower than 1.8 kpa, a value which correspond to no-stress conditions since an index for crop stress is a VPD value higher than 2 kpa (Baille et al., 1994). Furthermore, although very low values of VPD were observed at the first meters after cooling pads, no condensation was observed in the leaves since their temperatures were steadily higher than the dew point temperature of air. Crop temperature and its relation with air temperature is a crucial parameter affecting growth, yield and quality. Peet et al. (1997), found that increasing mean daily canopy temperatures from 25 C to 26 C reduced tomato fruit weight, number and seed content nearly as much as increasing from 28 C to 29 C. Similarly, in several cultivars of chrysanthemum; the inverse of time-to-flower was shown to decrease linearly with increasing mean daily temperature above an optimum of approximately 18 C (Pearson et al., 1993; Willits and Bailey, 2000). The crop to air temperature difference 15 m (noshaded portion) from the cooling pads and 45 m (shaded portion) from the cooling pads is presented in Fig. 5. Crop was cooler than air at the shaded part of the greenhouse. Mean crop to air temperature difference at the first no-shaded part of the greenhouse was 2.8 C, whereas at the shaded part, this difference was reduced to the half (1.4 C). This can be attributed not only to the shading but to the higher transpiration rate at this portion as well. The relation between the transpiration rate and the outside solar radiation 15 m and 45 m from cooling pads is presented in Fig. 6. Although the second part (30-60m) of the greenhouse was shaded transpiration rate was higher at this section. For a typical value of outside solar radiation around noon of 800 Wm -2 transpiration rate was 157 W m m away from cooling pads and 208 W m m away from cooling pads. In the first half of the greenhouse (0-30 m), the low VPD levels allowed the plants to maintain higher stomatal conductance (Katsoulas et al., 2001) and, despite the greater solar radiation load, to have, for the same incident solar radiation, a stronger transpiration rate in the shaded part of the greenhouse than in the no-shaded one. 627

4 CONCLUSIONS The microclimate inside a large evaporative cooled greenhouse equipped with progressive shading was studied in the present work. The overall efficiency of the system was quite high (~ 80%) due to the dry and warm outside climate conditions enabling the air to enter the greenhouse with a temperature even 10 C lower than the outside air temperature. As it was expected, an increase of the air temperature from pads to fans was observed (air exits the greenhouse even 8 C warmer than the inlet air and sometimes warmer to the outside air temperature). Air vapour pressure deficit varied from 0.5 kpa at the first meters after cooling pads to 1.8 kpa near the extracting fans. In the no-shaded section of the greenhouse crop transpiration was lower than normal and does not participate much to the natural oasis effect. However at the no-shaded part of the greenhouse crop transpiration goes back to normal and contributes to the cooling process. Literature Cited Arbel, A., Yekutieli, O. and Barak, M Performance of a Fog System for Cooling Greenhouses. J. Agric. Eng. Res. 72 (2): Baille, M., Baille, A. and Delmon, D Microclimate and transpiration of greenhouse rose crops. Agric. For. Meteorol. 71: Fuchs, M Transpiration and foliage temperature in a greenhouse. International Workshop on Cooling Systems for Greenhouses, Agritech, Tel Aviv, 2-5 May Giacomelli, G.A., Ginigers, M.S., Krass, A.E. and Mears, D.R Improved methods of greenhouse evaporative cooling. Acta Hort. 174: Katsoulas, N., Baille, A. and Kittas, C Effect of misting on transpiration and conductances of a greenhouse rose crop. Agric. For. Meteorol. 106: Kittas, C., Katsoulas, N. and Baille, A Influence of greenhouse ventilation regime on the microclimate and energy portioning of a rose canopy during summer conditions. J. Agric. Eng. Res. 79 (3): Kittas, C., Bartzanas, Th. and Jaffrin, A Temperature Gradients in a Partially Shaded Large Greenhouse equipped with Evaporative Cooling Pads. Bios. Eng. 85 (1): Landsberg, J.I., White, B. and Thorpe, M.R Computer analysis of the efficiency of evaporative cooling for glasshouses in a high energy environments. J. Agric. Eng. Res. 24: Montero, J.I., Short, T., Curry, R.B. and Bauerle, W.L Influence of evaporative cooling systems on greenhouse environments. ASAE paper no Montero, J.I., Anton, A., Biel, C. and Franquet, A Cooling of greenhouse with compressed air fogging nozzles. Acta Hort. 281: Pearson, S., Hadley, P. and Wheldon, A.E A reanalysis of the effects of temperature and irradiance on time to flowering in chrysanthemum (Dendranthema grandiflora). J. Hort. Sci. 68 (1): Peet, M.M., Willits, D.H. and Gardner, A.E Response of ovule development and post-pollen production processes in male-sterile tomatoes to chronic, sub-acute high temperature stress. J. Exp. Bot. 48 (306), Seginer, I Transpirational cooling of a greenhouse crop with partial ground cover. Agric. For. Meteorol. 71: Willits, D.H and Peet, M.M The effect of evaporative cooling on the efficiency of external greenhouse shade cloths. Paper no presented to the summer meeting of the ASAE, Spokane, WA. Willits, D.H Constraints and limitation in greenhouse cooling. Challenges for the next decade. Acta Hort. 534: Willits, D.H. and Bailey, D.A The effect of night temperature on chrysanthemum flowering: heat-tolerant vs. heat-sensitive cultivars. Sci. Hort.:83:

5 Tables Table 1. Average values (period from 09:00 19:00) of outside climatic parameters during the three days selected for presentation. Temperature, C Relative humidity, % Solar radiation, W m -2 Wind speed, ms -1 Mean Standard deviation Figurese Fans Shaded 60m Unshaded Pads 50m Fig. 1. Cross sectional view of the experimental greenhouse. Filled circles indicate measurement positions. 629

6 Solar radiation, Wm :08 14:08 15:38 17:08 Local time, h 18:08 Fig. 2. Solar radiation distribution outside of the greenhouse and at the middle points of the two sections (unshaded and shaded). Out Temperatur difference, C :08 14:08 15:38 Local time, h 17:08 18:08 Fig. 3. Variation of air temperature difference between inside and outside air along the air stream direction. Pad Fans 630

7 Air vapour pressure deficit, kpa :08 14:08 15:38 17:08 Local time, h 18:08 Pad Fig. 4. Variation of air vapour pressure deficit air along the air stream direction. Crop to air temperature difference, C :08 14:08 15:38 17:08 Local time, h 18:08 Fig. 5. Crop to air temperature difference away from the cooling pads (unshaded portion) and from the cooling pads (shaded portion). Transpiration W m y = 0.23x + 24 R 2 = 0.89 y = 0.16x + 29 R 2 = Outside Solar radiation, W m -2 Fig. 6. Crop transpiration rate versus outside solar radiation 15 m away ( ) from the cooling pads (unshaded portion) and 45 m ( ) from the cooling pads (shaded portion). The straight line was obtained by linear regression. 631

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