Elsevier

Biosystems Engineering

Volume 113, Issue 2, October 2012, Pages 120-128
Biosystems Engineering

Research Paper
Light distribution in multispan gutter-connected greenhouses: Effects of gutters and roof openings

https://doi.org/10.1016/j.biosystemseng.2012.06.014Get rights and content

To ventilate large, multispan gutter-connected greenhouses effectively in warm regions, both side and roof openings are usually required. Since the roof openings and gutters are constructed of opaque structural elements, and since the openings are generally covered with dense insect-proof screens, they partially block penetration of solar radiation into the greenhouse. Experiments were carried out during summer and winter campaigns in three different multispan greenhouses, each of about 0.1 ha, to characterise the disturbance induced by the roof openings and gutters to the intensity and distribution of light reaching the plants. Results show that the mean daily PAR level directly below the cover of the greenhouses was 58–66% of the external PAR; above the crop, the daily mean PAR level along a 10-m transect was 39–51% of the outside level. This further decrease of light transmission was mainly caused by structural elements, gutters and roof openings. The largest drop in radiation was measured at midday, in the region below the roof openings. This drop was larger by 15–28%, depending on the greenhouse type, than the drop measured at the centreline of the greenhouse span. There was good agreement between spanwise variations in summer and winter.

Highlights

► PAR distribution in a multispan, gutter-connected, greenhouse is not homogeneous. ► Near span centreline, there is negligible effect of gutters and roof vents on PAR. ► The largest mean daily reduction of PAR is below the gutters and roof openings. ► PAR change from summer to winter mainly due to outside intensity not incident angle.

Introduction

It is well accepted that light is a dominant factor in the greenhouse environment; it directly affects plant physiological processes (Hemming, 2011). As indicated by Albright, Both, and Chiu (2000), the primary reason for growing crops in greenhouses is to achieve better control over the environments that affect plant growth and development. With this regard, Albright et al. (2000) stated that temperature and PAR are the two most important components of the plant environment and, furthermore, their results showed that dry mass accumulation was proportional to the light integral and that a consistent daily light integral was essential for consistent production. Transpiration, another physiological process, is also affected by light because it directly depends on the light level experienced by the plants; it is inversely proportional to the stomatal resistance of the leaves, which in turn is strongly dependent on light intensity at leaf level (Thevenard, Zhang, Jewett, & Shipp, 1999). In greenhouses that are enriched with CO2, the response of the plants to the enhanced CO2 concentration is limited mainly by the light level.

Because of the non-ideal transmission of light by the cladding material, the light level inside a greenhouse is always lower than that outside when there is no use of artificial light. In some cases the properties of the cladding material and the roof structure can even reduce the light level to below its desired value, and change its spatial distribution and spectrum. The reduction in light level is generally due to three main effects: i) the characteristics of the cover material and accumulation of dust and dirt on the cover; ii) structural elements, internal environment-control systems, roof openings and screens (insect-proof and/or shading); and iii) water vapour condensation on the inner surface of the cover. In addition, the amount of light reaching the plants depends on the radiation angle of incidence on the greenhouse cover.

Light-transmission losses caused by multispan gutters, treated as infinitely long black circular cylinders, were addressed by Critten (1983), whose analytical study predicted the losses closely, and was in good agreement with experimental measurements. A few years later Critten (1987a) indicated that there was some difficulty in choosing the diameter of the cylinder to represent the gutter, because of the complex cross section of the latter. To improve the model he proposed a more accurate approach in which the gutters were represented by two adjacent horizontal cylinders; he then used the improved model to determine light transmission in multispan greenhouses under diffuse and direct skylight conditions Critten (1987a, 1987b).

Giacomelli, Ting, and Panigrahi (1988) studied the availability of global solar radiation and PAR inside a greenhouse by placing sensors at fixed positions: above the crop, at truss level, and outside the greenhouse. They showed that the transmittance through a polyethylene film was equal for both global solar radiation and PAR, and its value was about 67%. The hourly average transmittance of PAR at the canopy level was 44.8%, which represented a 33% reduction of radiation between values measured at the truss level and those just above the canopy. Giacomelli et al. (1988) argued that this reduction was primarily due to shading by structural members and internal environment-control systems.

Radiation heterogeneity in protected cultivation was studied by several research groups. Wang and Boulard (2000) developed a model to predict solar radiation distribution in a greenhouse, based on the path of the sun, the greenhouse geometry, cover transmittance, and sky conditions. The model was validated by experiments in a high tunnel and then used to simulate radiation distribution at soil level in various tunnel types. The simulations considered E–W versus N–S oriented tunnels, and their results showed that in the N–S oriented tunnel the solar radiation distribution was nearly symmetrical along the tunnel axis, and the average transmittance was slightly higher than that in the E–W oriented tunnel but, on the other hand, the radiation heterogeneity in the N–S tunnel was higher than that in the E–W one. The solar radiation variability in an E–W oriented Mediterranean greenhouse was also investigated by Soriano, Hernández, Morales, Escobar, and Castilla (2004); they studied the spatial uniformity of radiation throughout the spans of multispan greenhouses with three different roof geometries, by using scale models, and found relevant variations along the transverse cross-sections of the spans.

In recent years, large multispan greenhouses became more popular among growers. In such large greenhouses, natural ventilation through side openings is not as efficient in removing excess heat and water vapour as in a small greenhouse. Consequently, natural ventilation in large greenhouses is usually based on roof openings. The configuration and area of these openings affect ventilation and may also have a significant effect on the amount of radiation reaching the plants.

Furthermore, to prevent insects from penetrating into the greenhouse, insect-proof screens are regularly installed on the roof vents and, consequently, the light level in the greenhouse is reduced. As was pointed out by Montero, Antón, Hernández, and Castilla (2001) and Möller, Cohen, Pirkner, Israeli, and Tanny (2010), light transmission of screens is proportional to their porosity, and Möller et al. (2010) recently showed that the orientation of woven screens with rectangular holes had a significant influence on light transmission. Therefore care should be taken in determining the proper installation of screens in roof vents.

Both Montero et al. (2001) and Möller et al. (2010) indicated that ageing and dirt accumulation on the screens have remarkable effects on light transmission. Montero et al. (2001) further indicated that cleaning the screens might help to improve the light transmittance in areas with low rainfall, and Möller et al. (2010) showed that light transmission decreased linearly with time during a rainless summer because of dust accumulation, but recovered after rain.

The goal of the present study was to characterise the PAR distribution and intensity at plant level, in three greenhouses with different types of roof openings. The particular aim of the experiments reported here was to determine the combined effect of gutters and screened roof openings on the light penetration and distribution in a plastic-covered greenhouse with an arched roof.

Section snippets

Materials and methods

The experiments were performed in three gutter-connected multispan greenhouses, each with an area of about 0.1 ha. The greenhouses were covered with 150-μm polyethylene sheet (AD-IR AV Diffused; Genigar, Israel) and their gutters were oriented north–south. According to the manufacturer the light transmittance of the cover material in the PAR range is 82.5%. The geometrical dimensions of the greenhouses are given in Table 1. Greenhouse A had two roof vents per span; greenhouses B and C had one

Results and discussion

Most of the results presented here are based on the more comprehensive winter campaign. Only Fig. 9, Fig. 10, Fig. 11 refer to the summer campaign and its comparison with the winter measurements.

Figure 3a–c shows the diurnal course of radiation inside and outside each of the tested greenhouses during the winter measurements. The three curves in each panel show the radiation (left-hand vertical axis) as measured on the trolley (solid line), below the ridge (dashed line), and outside the

Conclusions

The PAR distribution in a multispan, gutter-connected, plastic-covered greenhouse with an arch-shaped roof and vertical roof vents was not homogeneous. The PAR level was high at each span centreline and diminished towards the edge of the span where a gutter and roof vent were located. Near the span centreline there was practically no effect of the gutter and roof vent on PAR level, whereas directly below the gutter the daily loss of irradiance was as high as 27% relative to that near the

Acknowledgements

The authors wish to thank Mr. M. Barak, Mr. A. Antler, Mr. A. Grava, and Mr. Y. Gahali for their help with the experiments. The Chief Scientist of the Israeli Ministry of Agriculture and Rural Development financed this work under program No. 459-0251.

References (11)

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