Testing of the condensation characteristics and light transmissions of different plastic film covering materials

Transcription

1 Polymer Testing 24 (2005) Material Performance Testing of the condensation characteristics and light transmissions of different plastic film covering materials Bilal Cemek*, Yusuf Demir Agricultural Structures and Irrigation Department, Ondokuz Mayis University, Samsun, Turkey Received 1 October 2004; accepted 20 November Abstract In this study, eight model pitched roof greenhouses for the investigation of optical transmission of agricultural plastic films were designed and constructed in Samsun, Turkey. Light transmissions associated with time and condensation characteristics were determined in the experimental greenhouses covered with UV stabilised polyethylene (UVCPE), IR absorber polyethylene (IRCPE), polyethylene with no additives (PE) and double layer polyethylene films (D-Poly). Comparison of light transmission of the films over a period of 3 months showed that all films in a dry state had higher transmission than that in a wet state. Light transmission of D-Poly was found to be the lowest while that of PE was the highest. The average loss in transmission due to dirt was approximately 9 15%. Drop size and water condensation were highest in IRCPE followed by UVCPE and PE. Comparative analysis implied that results of our research, conducted with vegetation, corroborated the findings of previous studies conducted without vegetation. q 2005 Elsevier Ltd. All rights reserved. Keywords: Cover material; Condensation; Light transmission 1. Introduction The change in greenhouse cover materials and associated design affects the greenhouse environment. Greenhouse air temperature, humidity, and leaf temperature, are in turn affected by the light transmission of the cover material. Changes in the greenhouse environment factors may have significant effects on growth, development and productivity of crops. The most important factor affecting the growth, development and yield of crops is light transmission. Photosynthetic rates of plants in greenhouse are reduced at low light transmission and it is generally assumed that the loss of light will lead to a proportional loss in productivity of crops. * Corresponding author. Tel.: C address: bcemek@omu.edu.tr (B. Cemek). An important property of plastic covering materials relates to their condensation characteristics. Water condensation on the inner surface of the covering material influences the light transmission and the heat transfer through the cover. The structure of the patterns created by the condensed water vapour on the cover depends on the wetting tension of the covering material. One can distinguish different kinds of condensation behaviour. On most untreated plastic materials, condensation appears in small droplet form, which reduces light transmission considerably due to multiple reflections of the incoming solar radiation inside the drops [1]. A crucial problem with plastic covering materials is drop-wise condensation on the interior surface of the covers. A number of undesirable effects may results from these water droplets: they can reduce the light transmission due to total internal reflection of incident light; they can act as lenses and burn the plant tissue by focusing the incident /$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi: /j.polymertesting

2 B. Cemek, Y. Demir / Polymer Testing 24 (2005) light; and the coalescence of small drops into larger ones will cause dripping and provide a high humid atmosphere for a long period within the greenhouse, which favours the development of fungal diseases [2 4]. As a result of the important role of condensation on the greenhouse climate, several studies, such as those of Walker and Walton and de Halleux et al. [5,6], have been devoted to the interaction between condensation and heat transfer though greenhouse covering materials [7]. Another source of loss in transmissivity of covering materials is the accumulation of dust and dirt on the material. Jaffrin and Morisot [8] have described the effect of dust and condensation on plastic films. They reported solar radiation losses of about 40% under combined effect of dust and condensation. Geoola et al. [2] investigated optical transmission and ageing of agricultural films. Comparison of light transmissions of the films in their study, over a period of 3 months, showed that in a dry state all films had higher transmissions than in a wet state. The loss of transparency of plastic films due to dirt during the period of their experiment was between 9 and 10%. Many authors have investigated the influence of condensation on the transmission. The results of experimental studies on real greenhouse covers often are very difficult to analyze because of large number of variables which affect the forming of condensation and the light transmission and which cannot be controlled, such as the outside climate, the position of the sun, etc. [9]. Most of the earlier studies have been focused on the effects of the condensation and light transmission of cover materials in model greenhouses without any vegetation. The present study was addressed to measuring the light transmission of plastic films in wet and dry states and the amount of condensed water of different plastic covering materials by examination of photographs, under a microscope, of the covering materials with vegetation inside greenhouses. 2. Experimental details 2.1. Experimental greenhouse The experiment was carried out in eight independent mini-greenhouses oriented East West. The eight greenhouses of the same size (9!3!2.5 m, L!W!H) and shape were constructed (Fig. 1) at Samsun (428N), Turkey Materials and procedures The cover materials of the greenhouses used to test condensation characteristics and light transmission were UV stabilised polyethylene (UVCPE), IR absorber polyethylene (IRCPE), polyethylene with no additives (PE) and double layer polyethylene films (D-Poly), with a thickness of 150 mm. Ni quenchers were used as UV stabilizers in UV stabilised polyethylene. According to the commercial description, light transmissions of the used covering materials were 89 92, 86 92, and 92 93% for PECUV, PECIR, and PE, respectively. On the other hand, transmission of ,000 nm wavelength radiation of the covering materials were 56, 33 and 54% for UVCPE, IRC PE and PE, respectively. The experiments were carried out for a period of 3 months starting in June 2001, with vegetation inside the greenhouse. In order to determine the condensation characteristics of greenhouses, sidewall and roof of greenhouses were photographed at the same distance and magnification. The condensation characteristics such as area, volume, diameter, number of the condensation drops obtained from both the sidewall and roof of greenhouses were determined by a stereobinoculer microscope (Leica) using a 2.5!2.5 cm area of photograph under 10! magnification. The mean diameter of 15 condensation drops was accepted as the covering material s condensation drop diameter. For calculation of the volume, all drops were accepted as being spherical in shape. Fig. 1. Description of the dimensions greenhouses (solid areas represent side and roof ventilation gaps).

3 286 B. Cemek, Y. Demir / Polymer Testing 24 (2005) Table 1 Monthly average inside and outside temperature in greenhouses covered with different covering materials Outside Inside UVCPE IRCPE D-Poly PE Min. Max. Mean Min. Max. Mean Min. Max. Mean Min. Max. Mean Min. Max. Mean June July August Average The numbers of condensation drops were counted in the 2.5!2.5 cm area. The condensation area was calculated based on the diameter of the condensation drops. The condensation characteristics of the D-Poly covering greenhouse was not investigated because the amount of condensation was very low compared to other plastic films. Light transmission was measured twice daily: once in the morning (07:00) when the films were wet, and once in the afternoon when the films were dry (14:00). A data logger using sunscan analyses (Delta-T) recorded light measurements under each film and outside simultaneously. Once these data were available, the instantaneous values of transmissivity of each plastic film were calculated by dividing the corresponding readings of the sun scan analyses inside and outside the greenhouses. Finally, instantaneous transmissivities of each plastic film were computed. In order to ease the comparision of transmission loss at different dates, it was decided to use the relative transmission losses for each covering material. Loss in transmission and relative loss in transmission are defined as: Loss in transmissionzt new KT Relative loss in transmissionz(t new KT)!100/T new where, T new is light tranmission of new material, and T is light transmission after a given period of time [2]. 3. Results and discussion The monthly outside and inside temperature and relative humidity are presented in Tables 1 and 2. The average ambient minimum and maximum outside temperatures were 21.0 and C, respectively, whilst those for inside temperatures with UVCPE, IRCPE, D-Poly and PE were approximately 21.9 and 30.2, 23.1 and 31.2, 24.5 and 32.3, and 21.9 and C, respectively. The outside relative humidity was, on average, approximately 74%, varying between 59.9 and 85.1%. The minimum and maximum values of inside relative humidity of greenhouses with different covering materials were approximately 34 and 82.9% for UVCPE, % for IRCPE, % for D-Poly, and % for PE, respectively. As can be seen in Table 3, greenhouses with high drop number showed smaller drop diameter, while the reverse was the case in greenhouses with low drop number. High drop diameter in the IRCPE greenhouse might be due to the cover surface temperature of this greenhouse. The total drop area calculated based on the drop number in a 6.25 cm 2 area on the roof of greenhouses for UVCPE, IRCPE and PE were 2.36, 1.80 and 2.90 cm 2, respectively. When focusing on the sidewall of greenhouses total drop area became smaller. These were 1.42 cm 2 on UVCPE, 1cm 2 on IRCPE and 2.98 cm 2 on PE. Fig. 2 shows sidewall and roof photographs of greenhouses with different plastic covering materials. It was clear based on the photographs that water condensation was the highest on IRCPE followed by PE and UVCPE. Condensation of covering materials may be affected by the chemical structures of covering materials. Similarly, Dilara and Briasoulis [11,12] pointed out that chemical structures of covering materials affect water condensation. Drop number was highest in PE followed by UVCPE and IRCPE covered greenhouses, while drop diameter was Table 2 Monthly average inside and outside relative humidity in greenhouses covered with different covering materials Outside Inside UVCPE IRCPE D-Poly PE Min. Max. Mean Min. Max. Mean Min. Max. Mean Min. Max. Mean Min. Max. Mean June July August Average

4 B. Cemek, Y. Demir / Polymer Testing 24 (2005) Table 3 Area, volume, diameter, number of the condensation drops in the sidewall and roof of greenhouses with different plastic covering materials Drop number Drop diameter (cm) Drop volume (cm 3 ) Drop area (cm 2 ) Sidewall Roof Sidewall Roof Sidewall Roof Sidewall Roof UVCPE IRCPE PE highest in IRCPE. Similar drop diameter, drop volume and area were found on UVCPE and PE greenhouses (Table 3). From the total drop area determined on 6.25 cm 2 of the roofs, it was calculated that the condensation covered approximately 38% of the area for UVCPE, 29% for IRCPE, and 46% for PE. The results of the present study showed that all covering films in a dry state had higher light transmission than in a wet state. These findings are in agreement with the results of study conducted by Jaffrin and Makhlouf [10]. Geoola et al. [2] also pointed out that all sheets with an anti drop layer had higher transmissions in a wet state than in a dry state. In the present study, the transmission of polyethylene films was always higher than that of other films. From comparison of the transmissions of the plastic films in June with those in July and August, a general reduction was noticeable. Fig. 2. Photographs of the condensation characteristics of greenhouses with the different plastic covering materials (a) sidewall, (b) roof.

5 288 B. Cemek, Y. Demir / Polymer Testing 24 (2005) Table 4 Light transmission loss of plastic films in the wet state (07:00) June July August Transmission (%) Transmission (%) Relative loss (%) Transmission (%) Relative loss (%) UVCPE IRCPE D-Poly PE Table 5 Light transmission loss of plastic films in the dry state (14:00) June July August Transmission (%) Transmission (%) Relative loss (%) Transmission (%) Relative loss (%) UVCPE IRCPE D-Poly PE The highest transmission loss was observed in greenhouses covered with D-Poly, followed by IRCPE, UVCPE and PE, respectively. Similar results were also reported by others [2 10]. This transmission reduction could be attributed in part to dust deposits on the plastic films. Jaffrin and Marisot [8] reported solar radiation losses of about 40% under combined effect of dust and condensation. The monthly values of transmission and transmission loss of the plastic film in dry and wet states are presented in Tables 4 and 5, respectively. The data presented for each month are averaged over the last week of that month. The average percentage losses in transmission for a 2-month period, for dry and wet states, were approximately 9 and 15%, respectively. High condensation area and volume caused reduction of light transmission of covering materials in this study which is in agreement with findings of Faaehnrich et al. [13] and Jaffrin and Marisot [8]. 4. Conclusions The results of the present study show that the light transmission in a dry state was higher than that in a wet state for all types of plastic. There was a relationship between condensation and light transmission. High condensation area and volume led to the reduction of light transmission of covering materials. Light transmission of D-Poly was found to be the lowest while that of PE was the highest. The average loss in transmission was about 9 15% under combined effect of dust and condensation. It is clear based on the above evidence that our research findings are in line with the results of previous research conducted without vegetation. References [1] B. Von Elsner, D. Briassoulis, D. Waaijenberg, A. Mistriotis, Chr. Von Zabeltitz, J. Gratraud, G. Russo, R. Suay-Cortes, Review of structural and functional characteristics of greenhouses in European Union countries, part I: design requirements, Journal of Agricultural Engineering Research 75 (2000) [2] F. Geoola, U.M. Peiper, F. Geoola, Outdoor testing of the condensation characteristics of plastic film covering materials using a model greenhouse, Journal of Agricultural Engineering Research 57 (1994) [3] F. Geoola, Y. Kashti, U.M. Peiper, A model greenhouse for testing the role of condensation, dust and dirt on the solar radiation transmissivity of greenhouse cladding materials, Journal of Agricultural Engineering Research 71 (1998) [4] F. Geoola, Y. Kashti, A. Levi, R. Brickman, Ouality evaluation of anti-drop properties of greenhouse cladding materials, Polymer Testing 23 (2004) [5] J.N. Walker, L.R. Walton, Effect of condensation on greenhouse heat requirement, Transactions of the ASAE 14 (2) (1971) [6] D. de Halleux, J. Nijskens, J. Deltour, S. Coutisse, A. Nisen, The effect of condensation on the heat transfers of greenhouse covering: glass or polyethylene?, Plasticulture 66 (1995) [7] J.G. Pieters, J.M.J. Deltour, M.J.G. Debruyckere, Onset of condensation on the inner and outer surface of greenhouse covers during night, Journal of Agricultural Engineering Research 61 (1995) [8] A. Jaffrin, A. Morisot, Role of structure, dirt and condensation on the light transmission of greenhouse covers, Plasticulture 94 (1) (1994) [9] J.G. Pieters, Interaction effects in simulating the light transmission through condensation drops on greenhouse covers, Transactions of the ASAE 40 (5) (1997)

6 B. Cemek, Y. Demir / Polymer Testing 24 (2005) [10] A. Jaffrin, S. Makhlouf, Mechanism of light transmission though wet polymer films, Acta Horticulture 281 (1990) [11] P.A. Dilara, D. Briassoulis, Degradation and stabilization of low-density polyethylene films used as greenhouse covering materials, Journal of Agricultural Engineering Research 76 (2000) [12] D. Briassoulis, A. Aristopoulou, M. Bonora, I. Verlodt, Degradation characterisation of agricultural low-density polyethylene films, Biosystems Engineering 88 (2) (2004) [13] I. Faehnrich, J. Meyer, Chr. Von Zabeltitz, Influence of condensation on light transmission and heat transfer through greenhouse covering materials, Plasticulture 84 (4) (1989)