Solar Radiation Transmission through Plastic Shading Nets

Transcription

1 Solar Radiation Transmission through Plastic Shading Nets I.M. Al-Helal and A.M. Abdel-hany epartment of Agricultural Engineering College of Food and Agriculture Sciences King Saud University PO box 2460, Riyadh Saudi Arabia Keywords: shading net, solar radiation, transmission, radiative properties Abstract Plastic shading nets are being used extensively in hot and sunny regions to protect plants from intense solar radiation. However, the choice of net to fulfill specific shading requirements often depends on empirical or economic criteria, not on scientific considerations because parameters used to characterize the different types of nets is not available. Solar radiation transmission is the most important parameter characterizing nets. A simple measuring method is presented and used to quantify global, diffuse and direct beam solar radiation transmission trough seven plastic shading nets. Nets with colors and nominal shading factors that are most commonly used in the Arabian Peninsula were selected for the study. The results showed that the behaviors of the plastic nets and their radiative properties under solar radiation conditions were similar to translucent materials. Accordingly, investigating equivalent optical constants (i.e., refractive indexes and extinction coefficients) for plastic nets similar to translucent materials is possible. etermining these constants can help to predict the radiative properties of the net without the need of measurements. Solidity and color of the net had significant effects on the radiative properties. The shading factor of a plastic net depends on the time of the day and on the design and meteorological parameters. aily integral of the shading factor, solidity and the total radiative properties are appropriate parameters describing a net whereas, nominal shading factors provided by the producers cannot be used. INTROUCTION Plastic nets made from high-density polyethylene are widely used in various agricultural applications. The most important application of the plastic nets in hot and sunny regions is to shade plants from high solar radiation levels to improve the microclimate in the net houses. Plastic nets offer many advantages and environmental benefits (Castellano et al., 2006, 2008; Briassoulis et al., 2007a, b). This include: (i) Reduction or elimination of energy consumption used for heating or cooling greenhouses. (ii) Reduction of crop transpiration as well as water consumption for irrigation in arid regions. (iii) Improvement in environmental and human-health conditions by reducing the need for pesticides. (iv) Scattering of solar radiation diffusively allowing plants to receive light from all sides. In particular, shade netting can enhance productivity, quality and homogeneity of plants and fruits throughout the year in hot and sunny regions. Although various types of plastic nets are widely used by growers in Arabian Peninsula, growers choose the nets based on empirical and economic criteria rather than on scientific criteria. This is largely because technical information to describe the locally produced plastic nets such as the radiative properties is not available. Transmission of solar radiation is the most important factors used for characterizing nets. Various research groups have investigated the characteristics of metallic thermal screens and insect screens. However, little information is available on the radiative properties of different types of plastic nets. Castellano et al. (2006) examined the influences of the net porosity and color on the spectral transmittance of different plastic net-covered structures. Briassoulis et al. (2007a, b) provided testing methods for measuring the mechanical properties of plastic nets commonly used in European countries. Hemming et al. (2008) Proc. XXVIII th IHC IS on reenhouse 2010 and Soilless Cultivation Ed.: N. Castilla Acta Hort. 927, ISHS

2 used a light model, developed mainly for greenhouses, to analyze the radiometric performance of a structure covered with plastic nets. They also measured the transmittance of different plastic nets in the laboratory under direct beam radiation. However, measuring the transmittance of a net-covered structure does not reflect the actual transmittance of the net itself, but indicates the transmittance of the whole structure, which is completely different. Characterization of plastic nets according to different specific purposes is still lacking and more research is needed to measure and quantify solar radiation transfer through plastic nets. In order to achieve a better understanding about the characteristic of solar radiation transfer as well as the radiative properties of plastic shading nets, a simple method for measuring the net transmission and reflection under natural conditions of global, diffuse and direct beam solar radiation was developed. Seven different plastic shading nets that growers commonly use for shading agricultural structures in the Arabian Peninsula in summer seasons were selected for testing. MATERIALS AN METHO Shading Net Materials Two groups of plastic shading nets, locally produced by Saudi Yarn And Knitted Technology Factory (SYNTECH-ISO 9001), defined by the manufacturer as nets-50 and nets-80 that mean the shading factors are 80 and 50%, respectively. Solidity of each net was measured using image-processing method (Table 1). Nets of each group have different colors (i.e., white, green, black for nets-50 and blue, beige, orange and darkgreen for nets-80). Measuring lobal and iffuse Radiation Components Two experiments for testing each net sample under global and diffuse solar radiation conditions were conducted on the roof of the building of the Agricultural Research and Experiment Station, Agriculture Engineering epartment, King Saud University (Riyadh, Saudi Arabia, E, longitude and N, latitude). Each sample was tested on two consecutive clear sunny days in the period from ec 28, 2008 to Feb 10, 2009, one day for each experiment. The incident, transmitted and reflected components of solar radiation fluxes were measured and used to estimate the radiative properties related to the global, direct beam and diffuse solar radiation for each net. Two wooden frames were used for measuring the global radiation components (1.25 m width 2.25 m length 0.2 m height) and for measuring the diffuse components ( m). The net samples were tacked on the frames (Figs. 1 and 2); layout dimensions and locations of solarmeters used to measure the different solar radiation components under global and diffuse radiation conditions are illustrated, not to scale, in Figures 1 and 2, respectively. The floor under each frame was covered with a black plastic mulch to eliminate upward reflections. Locations of the solarmeters were decided based on several trails were done before starting the actual measurements. The arrangements in Figure 1 allow the specular component of the reflected solar radiation from the net surface to reach the inverted solarmeter at a range of incidence angles from 0 up to 80. At times when the incidence angle was higher than 80, the incident and reflected radiation were usually diffuse and the reflected portion could be detected by the inverted solarmeter. Because diffuse radiation transmits and reflects diffusively, the locations of the solarmeters in Figure 2 did not affect the accuracy of measurements. The arrangement in Figure 2 was continuously shaded by moving a shading system positioned a few meters away from the frame to allow the net to receive diffuse solar radiation from all directions of the hemisphere. The shading system in Figure 2 was a m black-painted wooden plate. The plate was mounted on a structure constructed from black-painted steel tubes ( m in diameter). The junctions of the structure are rollers that make it possible to tilt the plate around its horizontal axis to generate enough shade to cover the frame and the solarmeters in Figure 2. Solarmeters used in the two experiments were CMP3 (Kipp & Zonen B.V. Inc., USA), having a wavelength range of nm. The measured global 732

3 components are: the incident (S i ), transmitted (S t ), and reflected (S r ) radiation flux in W m -2. The measured diffuse components are: the incident ( i ), transmitted ( t ) and reflected ( r ) radiation flux, in W m -2. The measured parameters were recorded every 5- minute in a data logger (LI-1400, LICOR, Inc.). etailed of the experimental set up and measuring procedures is reported by Al-Helal and Abdel-hany (2010). Estimating the Solar Radiative Properties of the Nets Before mounting the net samples on the frames of each experiment, one day of measurement was carried out to estimate correction factors representing the upward reflectance from the floor and from the inner surfaces of the panels. These factors are given by: F S S and F (1) r i r i withoutn net without net The radiative properties of the nets were considered as equivalent properties describing the net as a translucent homogeneous layer. The transmitted radiation through the net is multiply reflects between the upper surface of the panel and the lower surface of the net. Equations could be deduced describing the net transmittance and reflectance related to global radiation (τ and ρ ) and those related to diffuse radiation (τ and ρ ) in the form: S t t 1 F and 1 F (2) S S S r i i 2 F 1 F and r i i 2 F 1 F By solving Eqs. (2) and (3) simultaneously, values of (τ and ρ ) and (τ and ρ ) could be obtained at each time interval. Properties related to direct beam solar radiation (τ R and ρ R ) were estimated as: N 1 N and N 1 N (4) R where N is the diffuse index (N = i / S i ). The shading factor of the net (SF) is not constant value; it changes with the daytime, incidence angle of solar beam radiation, net orientation and its location and is equal to (1- τ ). RESULTS AN ISCUSSION Values of the radiative properties related to the global and diffuse radiation [Eqs. (2) and (3)] and the corresponding values related to the direct beam radiation [Eq. (4)] were estimated at 5-minute intervals and averaged over 30-minute intervals. Figures 3a and b depicts the time courses of the global and diffuse transmittances (τ and τ ) for the seven tested nets. In this figure, the inflection of τ values in the morning and afternoon was due to the increase of the diffuse radiation percentage in the global solar radiation significantly at around the times of sunrise and sunset. Nets having low solidity (white- 50, green-50 and black-50 nets) showed higher transmittances than those having high solidity (nets-80). Increasing the net brightness significantly increased the net transmittance to global and diffuse solar radiation (τ and τ ) due to the increase of the downward reflection of incident radiation on the net threads as affected by the brightness colors. Therefore, the net color showed a stronger effect on the values of τ and τ much more than the net solidity. The changes of τ and τ with the time of day were similar to those of different translucent materials (e.g., plastic films, glass and rigid plastic sheets such as polycarbonate and acrylic) that were measured under natural conditions of solar radiation (Abdel-hany et al., 2001; He et al., 1991). The effects of incidence angle of solar beam radiation on the direct beam transmittance and reflectance (τ R and ρ R ) of the tested nets are illustrated in Figures 4a and b, respectively. However, values of τ R and ρ R at low values of incidence angle could not be represented because of the latitude and the time of the year at which the R (3) 733

4 experiments were carried out (the lowest value of was estimated to be about 40 o at solar noon). In this figure, when the incident angle was near or equal to 90 o, the beam radiation incident on the threads surfaces reflects downwards. This increased τ R to a value higher than zero and reduced ρ R to a value much lower than one. Otherwise the trends of τ R and ρ R with the variation of the incident angle are similar to those reported for translucent materials (Abdel-hany et al., 2001; He et al., 1991; uffie and Beckman, 1991). The shading factor (SF) is a time dependent, not a constant value and can be used to characterize the different types of nets. Thus, the daily integral of the shading factor (ISF) for each net was estimated in Table 1. The total (daily integral) radiative properties of the tested nets are illustrated in Table 2. The estimated values of ISF are different from the nominal values provided by the producer except the black-50 and dark green-80 nets. In agriculture applications, nets-80 with high ISF can be used for covering structures for nurseries and transplants production under hot sunny summer conditions whereas, nets-50 can be used for covering structures of mature plants, crops and fruit trees, according to the specific requirements of each type. CONCLUSIONS A simple measuring method to quantify solar radiation transmission through plastic nets was presented. Transmittance and reflectance related to global, diffuse and direct beam solar radiation were investigated for different type of nets. The behavior of nets and the trends of their radiative properties with time and with the incident angles are similar to those of homogeneous translucent materials. Accordingly, treating plastic nets as homogeneous translucent materials with equivalent optical constants is possible. The net color and its solidity have significant effects on the radiative properties and the effect of color is much more than the effect of solidity. The instantaneous shading factor of a net depends on several design and meteorological parameters and can not be used to characterize different types of plastic shading net. However, the net solidity, total radiative properties and the daily integral of shading factor are appropriate constant parameter to describe the net type. ACKNOWLEMENTS This work has been supported by the National Plan for Sciences and Technology (NPST program) by King Saud University, project number 09-ENE Authors express thank to Mr. M.R. Shady for his technical assistance during the experiments. Literature Cited Al-Helal, I.M. and Abdel-hany, A.M Responses of plastic shading nets to global and diffuse PAR transfer: Optical properties and evaluation. NJAS-Wageningen J. of Life Sciences 57: Abdel-hany, A.M., Kozai, T. and Chun, C Evaluation of selected greenhouse covers for use in regions with a hot climate. Japan J. of Tropic. Agric. 45(4): Briassoulis,., Mistriotis, A. and Eleftherakis,. 2007a. Mechanical behavior and Properties of Agricultural nets. Part II: Analysis of the performance of the main categories of agricultural nets. Polymer Testing 20: Briassoulis,., Mistriotis, A. and Eleftherakis,. 2007b. Mechanical behavior and properties of agricultural nets. Part I: Testing methods for agricultural nets. Polymer Testing 26: Castellano, S., Russo,. and Scarascia,.M The influence of construction parameters on radiometric performances of agricultural nets. Acta Hort. 718: Castellano, S., Scarascia,. M., Russo,., Briassoulis,., Mistriotis, A., Hemming, S. and Waaijenberg, Plastic nets in agriculture: A general review of types and applications. Applied Eng. in Agric. 24(6): uffie, J.A. and Beckman, W.A Solar engineering of thermal processes. John Wiley & Sons Inc. New York. 734

5 He, L., Short, T.H. and Yang, X Solar radiation transmittance of a double-walled acrylic pellet insulated greenhouse. Transactions of the ASAE 34(6): Hemming, S., Swinkels,.L.A.M., Castellano, S., Russo,. and Scarascia,.M Numerical model to estimate the radiometric performance of net covered structures (ARONETS). Paper presented at AgEng 2008 Agricultural and Biosystems Engineering for a Sustainable World, June, Crete, reece. Tables Table 1. Solidity and the shading factors of the tested nets. Net type Solidity Estimated ISF Nominal SF White reen Black Blue Beige Orange ark green Table 2. The total (daily integral) radiative properties of the tested nets. Net type lobal irect beam iffuse τ ρ τ R ρ R τ ρ White-50 reen-50 Black-50 Blue-80 Beige-80 Orange-80 ark green

6 Figurese Fig. 1. iagram of the experimental setup and locations of the solarmeters used to measure the global solar radiation components, dimensions in m, not to scale. Fig. 2. iagram of the experimental setup, shading system and locations of the solarmeters used to measure the diffuse solar radiation components, dimensions in m, not to scale. 736

7 Fig. 3. Time courses of the global (a) and diffuse (b) transmittances (τ and τ ) of the tested nets. Fig. 4. Time courses of: (a) the direct beam transmittance (τ R ) and (b) the direct beam reflectance (ρ R ) of the tested nets. 737

8 738