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Plant residues consist of biodegradable organic materials and minerals that are well suited for compost production. The main prerequisite for compost to be safely applied into the soil is a high de...

The effect of external roof shading on the spatial distribution of air temperature and relative humidity in a greenhouse (Tin and RHin) was evaluated under the arid climatic conditions of Riyadh City, Saudi Arabia. Two identical, evaporatively-cooled, single-span greenhouses were used in the experiment. One greenhouse was externally shaded (Gs) using a movable black plastic net (30% transmissivity), and the other greenhouse was kept without shading (Gc). Strawberry plants were cultivated in both greenhouses. The results showed that the spatial distribution of the Tin and RHin was significantly affected by the outside solar radiation and evaporative cooling operation. The regression analysis showed that when the outside solar radiation intensity increased from 200 to 800 W m–2, the Tin increased by 4.5°C in the Gc and 2°C in the Gs, while the RHin decreased by 15% in the Gc and 5% in the Gs, respectively. Compared with those in the Gc, more uniformity in the spatial distribution of the Tin and RHin was observed in the Gs. The difference between the maximum and minimum Tin of 6.4°C and the RHin of 10% was lower in the Gs than those in the Gc during the early morning. Around 2°C difference in the Tin was shown between the area closed to the exhausted fans and the area closed to the cooling pad with the external shading. In an evaporatively-cooled greenhouse in arid regions, the variation of the Tin and RHin in the vertical direction and along the sidewalls was much higher than that in the horizontal direction. The average variation of the Tin and RHin in the vertical direction was 5.2°C and 10% in the Gc and 5.5°C and 13% in the Gs, respectively. The external shading improved the spatial distribution of the Tin and RHin and improved the cooling efficiency of the evaporative cooling system by 12%, since the transmitted solar radiation and accumulated thermal energy in the greenhouse were significantly reduced.

The sky thermal irradiance (Rsky) is an important factor in meteorological, climatic, and energy balance studies. There are several empirical models for predicting Rsky; however, these are based on the meteorological parameters measured in non-arid climatic conditions and an empirical model for predicting Rsky in arid climates is unavailable. Therefore, this study presents two empirical models for predicting the Rsky under clear sky conditions, common in arid regions such as the Arabian Peninsula. One is based on the sky equivalent emissivity (esky-model) and the other on the sky equivalent temperature (Tsky-model). Meteorological parameters (i.e., air temperature, relative humidity, solar radiation, and downward Rsky) were measured over 3 years and used to develop and validate the proposed models. The results showed that the proposed esky- and Tsky-models had the highest predictive accuracy with a coefficient of determination of 0.86 compared with previously published models. In addition, the proposed esky- and Tsky-models resulted in low values of root mean square and mean absolute percentage errors (7.2 and 7.8 W·m−2 and 1, 2 %, respectively); they were able to precisely predict Rsky at any time in a clear arid climate compared with the prohibitive and time-consuming measurements.

Hexane (Hex) extracts of fresh navel orange (citrus sinensis) and lemon (citrus limon) peel were prepared to isolate and identify toxic bioactive secondary metabolites for potential use as natural ecofriendly biopesticides against the cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae). Hex extracts were fractionated by column chromatography. The fractions that showed activity against the larvae, were identified and quantified by gas chromatography combined to mass spectrometry(GC/MS), after checking component profile by thin layer chromatography (TLC). The results revealed that F2 and F3 fractions were the toxic active fractions of both lemon and orange peel Hex extracts, but lemon fractions were more toxic than orange ones. F2 and F3 fractions of orange extract causd 60 and 20% larval mortality, respectively. Both lemon extract fractions caused 100% morality. Orange fractions had approximately equal dlimonene content (≈86%) as illustrated by mass spectral fragmentation pattern. However, F2 fraction contained slightly higher amounts of linalool, α-terpinol and citral than F3 fraction. The active fraction of both lemon and orange peel extracts contained d-limonene, α-pinene, linalool, citronllal and citral. Limonene oxide, geranial and terpineol were only detected in orange peel extracts. The results indicated that secondary metabolites quality and quantity differ according to the citrus spp., and the quantity of d-limonene is much lower (≈15%) in lemon than orange extracts in spite of lemon peel was more toxic than the orange. However, d-limonene was a major compound in both extracts. The results suggest that citrus extract contains many toxic compounds, other than limonene, and /or they could potentiate each other to give the observed toxicity. It could be concluded that the quantity of d-limonene is not a determinant factor, but it is probable that other metabolites confer toxicity allover citrus extract depending on the species.

Huge quantities of palm tree residues are available annually in Saudi Arabia, they are often disposed of by burning, leading to large environmental pollution and health problems. Enclosed composting may be a successful method for utilizing these residues. Effective composting in a rotary bioreactor depends on the composting materials and the frequencies of the rotation&ndash, aeration process. Here, we attempted to determine the optimal method for composting these residues in a bioreactor. Four identical pilot-scale bioreactors (D1&ndash, D4) were used. The respective aeration&ndash, rotation frequencies were continuous (D1), and 20 min per 1 h (D2), 2 h (D3), and 3 h (D4), the rotation speed of the bioreactors was 3 rpm. Three periods were analyzed, their names and their respective lengths for D1, D2, D3, and D4 were as follows: the lag period was 10 h for each, the mesophilic periods were 13, 14, 89, and 231 h, and the thermophilic periods were 0, 55, 71, and 17 h. The peak compost temperatures (Tc, max) for D1, D2, and D3 were 65.2, 57.2, and 46.9 &deg, C, and were achieved at 36, 71, and 108 h of operation, respectively. In D1, Tc quickly declined after reaching Tc, max due to limited microbial activity at Tc &gt, 60 &deg, C. In D2, a Tc of 50&ndash, 57.2 &deg, C was maintained for 61 h, and the highest organic matter degradation (OMD) of 0.30 was achieved. During the composting period, the carbon to nitrogen (C/N) ratio and moisture contents (MC) remained in the optimal ranges for microorganisms and did not affect the composting process. The results indicated that combined aeration&ndash, rotation for 20 min every hour (D2) gave the best results for composting palm tree residues, it ensured the hygiene safety of the end product, and it also consumed less power for aeration and rotation than D1.

Shading greenhouses in a hot and sunny climate is essential to reduce the inside greenhouse air temperature. However, the type and location of nets need to be addressed properly to determine the shading method that provides a better cooling effect on the greenhouse air. This study was conducted to evaluate the effects of five different shading methods on greenhouse microclimates, and to investigate the cooling potential of each method. Four greenhouse models covered with 200-&micro, m thick plastic film were used for the study: one was kept as control (C), one was whitened with slaked lime (L), and two were shaded with white and black nets (50% shading factor) deployed simultaneously at two locations, in contact (WC, BC) and at 20-cm distance from the cladding film (WD, BD). The microclimatic parameters were measured inside and outside the greenhouse models, and the cooling potential (CP) was predicted for each shading method. The results showed that the black net at 20-cm distance (BD) is desirable as it provides high CP with a reasonable solar radiation transmission. The maximum CP was estimated as 8.5 &deg, C, 8 &deg, C, 6 &deg, C, 3.2 &deg, C, and 2.1 &deg, C for L, BD, BC, WD, and WC, respectively. Shading with white nets is not recommended because their cooling potential is very low. Based on these results, we developed correlations for predicting the CP for each shading method as a function of the transmitted solar radiation flux (Si). These correlations depend on Si. Accordingly, they can be used for small greenhouses, whitewashed with a slaked lime of any concentration, or shaded with a black net having any shading factor.

In arid regions, limitation of protected cultivations in greenhouses is mainly due to the overheating of the inside air during hot and long summer seasons. The greenhouse accumulates around 10 MJ m-2 of heat energy per day; this requires an expensive, energy consuming cooling system to remove the heat and to achieve favorable conditions for crop growth. Cooling greenhouses are facing several challenges in arid regions due to the excessive solar irradiance, extremely high ambient air temperature, as well as water scarcity and salinity. A new concept (net-house) as a naturally-ventilated alternative agricultural structure, covered with low-cost, plastic nets was introduced. The proposed net-house does not require cooling and ventilation systems, and provides a uniform distribution of solar radiation and a suitable environment for crop growth and the production costs will be significantly decreased.

This study is to describe the degradation behavior of the optical constants (refractive index $$ \bar{n} $$ and extinction coefficient $$ \bar{\sigma } $$ ) of PE-LD sheets during exposure to an arid climate. Values of $$ \bar{n} $$ and $$ \bar{\sigma } $$ are used to determine the solar radiative properties of any PE-LD cover at any time without the need of measurements. Different greenhouse models were covered with PE-LD films and exposed for 1 year to an arid climate, and values of $$ \bar{n} $$ and $$ \bar{\sigma } $$ were investigated. Exposing films to the arid climate for 1 year increased the values of $$ \bar{n} $$ from 1.58 to 2.86 and $$ \bar{\sigma } $$ from 3.86 to 8.1 mm−1, resulting in a decrease in transmittance from 0.42 to 0.12. Two correlations for predicting the degradation rate of $$ \bar{n} $$ and $$ \bar{\sigma } $$ for PE-LD covers were developed.

Rotary drum composters are used to produce high-quality, pathogen-free compost without weed seeds. Insulation is usually applied to small-scale composters to warm up the composted materials and enhance metabolic reactions to produce stable and mature compost within a short time. However, the relationship between the composter size and the heat loss rate is still unclear. In this study, the relationship between the composter size (designated as the ratio of surface area to volume, As/V) and heat loss was analyzed and identified. To show the effect of insulation on the composting performance, two identical rotary drum bioreactors (each of As/V = 9) were used to compost tomato plant residues, one insulated and the other kept without insulation. Results showed that insulation increased the overall resistance against heat loss from the bioreactor from 0.37 (m2 °C W−1) to 1.12 (m2 °C W−1), quickly increasing the compost temperature, and a temperature of 55⁻67 °C could be achieved and remained for three days. Therefore, mature, stable, well-aged, and high-quality compost was obtained. In the non-insulated bioreactor, the compost temperature did not exceed 37 °C; this caused a decline of microbial activity and the composting process temperature was only in the mesophilic range, leading to a high risk of the existence of weed seeds and pathogens in the final immature compost. Insulation is necessary for laboratory-scale and small pilot-scale bioreactors (As/V ≥ 6), because heat loss is high as As/V is high, whereas it is not necessary for commercial full-scale bioreactors (As/V ≤ 4), because heat loss is minor as As/V is low. For larger pilot-scale bioreactors (As/V: 4⁻6), insulation cost must be considered when comparing the impact of energy saving on the composting process.

The transmitted radiation into the conventional net-houses is very low in the early morning and late afternoon that may not fulfill the crop growth requirements. To solve this problem, two types of net-house models (polygon and curved-arch net-houses) were newly designed, each having seven surfaces made up of different net types. The spectral radiative properties of 32 nets were examined and three nets were selected to cover the surfaces of each model. The two designs showed high transmittance in the morning and afternoon, and low at around noon (U shape). The polygon style was scaled-up and an experiential net-house was constructed with a floor area of 28 m2. An evaporatively-cooled greenhouse with a floor area of 28 m2 was used for comparison; each floor was planted with 500 potted Chrysanthemum plant. The results showed that the PAR and microclimate in the net-house and cooled greenhouse were similar. The net-house reduced water consumption by 13 kg m−2 day−1 in summer and by 0.94 kg m−2 day−1 in winter; and reduced electric energy consumption by 0.26 kw-h m−2 day−1 in summer and by 0.18 kw-h m−2 day−1 in winter compared to the greenhouse.

Shading greenhouse is an effective method to achieve a suitable environment for crop growth and to enhance crop productivity and quality in hot and sunny regions. This article reviews common shading methods used for greenhouses in summer; their cooling effects on greenhouse air were explored to determine the optimum shading technique for arid regions. Effects of these methods on the overall microclimate as well as the distribution of the microclimatic parameters in greenhouses were discussed. The survey revealed that combining a shading method (e.g., whitewash or shade netting) with ventilation and/or evaporative cooling showed a significant effect on improving overall microclimate as well as the distribution of microclimatic parameters. This consequently reduces the energy and water consumptions and increases the crop productivity and its quality. In sunny regions, incorporating shading with a cooling method is able to maintain greenhouse air temperature at 5–10 °C lower than the outside temperature, increases the relative humidity by approximately 15–20% and decreases the transmitted solar radiation by 30–50% compared to a greenhouse without shading. In cold regions, shading materials act as insulators; significantly reduce heat loss from greenhouses at night. Thus, greenhouse air temperature can be maintained at 5 °C higher than outside with saving approximately 15–20% of the energy used for heating. The information in this review would be useful for farmers, researchers and greenhouse designers to select the suitable cooling or heating strategy for greenhouses.

In hot and sunny regions, extensive solar radiation transmitted into greenhouses makes it essential to select a greenhouse cover with specific characteristics. Reflection and diffusion are the most important properties of covers for reducing solar heating load and scattering solar beams for better growth of plants. Three types of plastic films that are commonly used for covering greenhouses in arid climate were selected for this study. These were reflective-diffusive (RDF), diffusive (DF), and locally produced (CF) films. The films were used to cover three identical twin-span greenhouse models; the radiation components, air temperature (Ti), and relative humidity (RHi) were measured in each model. Transmission of solar radiation through each cover was characterized by determining: (i) The beam that is diffused during transmission, and (ii) the unscattered beam that is transmitted directly through the film. The results show that the diffuse radiation transmitted through the DF, RDF, and CF covers was enhanced, respectively by 77%, 85%, and 109% as a result of diffusing 34%, 33%, and 43% of the transmitted beam radiation by the DF, RDF, and CF covers, respectively during transmission. The diffusive nature of the tested covers increased the ratio of diffuse to direct beam radiation (D/B) from 0.3 outside the greenhouse to 0.77, 0.69, and 0.95 inside a greenhouse covered with DF, RDF, and CF, respectively. At around noon, the CF cover decreased Ti by about 5−10 °C and increased RHi by about 3%−5% compared to those under the DF and RDF covers. However, DF and RDF covers showed almost similar effects in Ti and RHi. The low-price CF cover showed higher diffusive-radiative properties than DF and RDF covers and can serve effectively in arid climate as an alternative covering material.

Plastic films are widely used to cover greenhouses in arid regions. Unlike predicting the total radiative properties of films, predicting the deterioration behavior of the spectral radiative properties is quite difficult during the service life of the films. These properties should be of great interest for crop morphogenesis studies. This study was to investigate the degradation behavior of spectral transmittance, $$ \tau_{\lambda } $$ , and reflectance, $$ \rho_{\lambda } $$ (in the solar spectrum range), of a 200 µm thick, LDPE film-covered greenhouse model that was exposed for 1 year to an arid climate. Every 30 days, $$ \tau_{\lambda } $$ and $$ \rho_{\lambda } $$ were measured for six samples taken from the six surfaces of the greenhouse cover and averaged to obtain the cover properties. Based on the collected data, nonlinear correlations (each as a function of the wavelength, λ, and the exposure time, t) were obtained to predict $$ \tau_{\lambda } $$ and $$ \rho_{\lambda } $$ at any time during the service life of the cover under such climatic conditions. Results showed that 1 year exposure drastically reduced the spectral and total transmittance of the cover film to global and PAR solar radiation by about 32 % and increased the spectral and total reflectance by about 19 % compared to new film. Degradation of the radiative properties of the film did not affect the light quality or the transmission ratios of light into the greenhouse. The transmitted PAR was reduced from 115 W·m−2 to 27 W·m−2 after 1 year exposure.

Energy produced from plant residue composting has stimulated great interest in heat recovery and utilization. Composting is an exothermic process often controlled through temperature measurements. However, energy analysis of the overall composting system, especially the rotary bioreactors, is generally not well known and very limited. This study presents detailed energy analysis in a laboratory-scale, batch-operated, rotary bioreactor used for composting tomato plant residues. The bioreactor was considered as a thermodynamic system operating under unsteady state conditions. The composting process was described, the input generated and lost energy terms as well as the relative importance of each term were quantitatively evaluated, and the composting phases were clearly identified. Results showed that the compost temperature peaked at 72 h of operation reaching 66.7 °C with a heat generation rate of 9.3 W·kg−1 of organic matter. During the composting process, the accumulated heat generation was 1.9 MJ·kg−1 of organic matter; only 4% of this heat was gained by the composting material, and 96% was lost outside the bioreactor. Contributions of thermal radiation, aeration, cylindrical, and side-walls surfaces of the reactor on the total heat loss were 1%, 2%, 69%, and 28%, respectively. The information obtained is applicable in the design, management, and control of composting operations and in improvement of bioreactor effectiveness and productivity.

This paper presents an exergoeconomic and enviroeconomic analyses of partially covered photovoltaic thermal (PVT) flat plate collector (FPC) integrated solar distillation system known as PVT-FPC active solar distillation system. The analysis is based on experimental studies for composite climatic condition of New Delhi. The hourly thermal, exergy, electrical, overall exergy and overall thermal efficiency have been evaluated and the results have been compared with the results obtained by earlier researchers. It has been observed that the proposed PVT-FPC active solar distillation system can meet the daily requirement of potable water as well as DC electrical power during sunshine hours.

Most of thermal models used to estimate the natural ventilation of greenhouses ( m ˙ a ) use an average value for the cover transmittance (τc) to estimate the input solar energy to the greenhouse. These models failed to predict m ˙ a at low solar radiation flux due to the spatial variation of τc in the greenhouse and the thermal inertia of soil. This study is to develop and validate a thermal model predict m ˙ a precisely. All modes of energy were treated, under the unsteady state conditions, at the outer surface of the cover to avoid the sources of error. The required parameters were measured for a naturally ventilated glasshouse. The predicted values of m ˙ a were in the range between 0.36 kg s−1 and 1.65 kg s−1 and showed good accordance with measured and simulated values in the literature. The results also confirmed that outside wind at a speed m ˙ a and the temperature difference of air between inside and outside the greenhouse (ΔT) is the main driving force of ventilation. A correlation between m ˙ a and ΔT was provided, it can be used to estimate m ˙ a that required to maintain the air temperature in the greenhouse at a desired level.

Determining convective heat is essential for analyzing energy exchange with shading nets. Unlike solar and thermal radiation, the convection (Qn−a) and the convective coefficient (hn−a) have never been estimated or measured for plastic nets. In this study, an energy balance was applied to a horizontal net under outdoor conditions to predict Qn−a and hn−a. Therefore, the net was tacked onto a wooden frame; energy balance was applied below and above the net surfaces and above a black substrate underneath the frame. The wind speed (V), net-air temperature difference (ΔT), downward and upward solar and thermal radiation fluxes were measured, on sunny days, below and above the net to be used for solving the energy balance equations. Accordingly, the time dependence of Qn−a and hn−a were investigated. Nets with different porosities and colors were used. For each net tested, hn−a was expressed as a function of V and ΔT. The results showed that hn−a depends mainly on the texture structure of the net rather than its color or porosity. Therefore, a similar net texture would have nearly the same hn−a value. These expressions can be used to estimate hn−a for nets having similar texture structures, different colors, and different porosities.

To determine the energy transfer through a plastic net-covered structure, the temperature of the net ( T n ) is an essential parameter. The perforated nature of nets and the effects of solar and thermal radiation, during the daytimes, make it is impossible to measure T n correctly by using most types of thermometer. Therefore, T n has never been theoretically estimated or experimentally measured in the presence of solar and thermal radiation. In this study, T n was simulated by determining the emissive power of the net ( E n ) and the net emittance ( e n ). To determine E n and e n , the net was tacked onto a wooden frame; thermal radiation balance was applied below and above the net surfaces and above the substrate underneath the frame. The downward and upward thermal radiation fluxes were measured below and above the net to be used in the simulation. A simple measuring method was proposed to predict T n using a thermocouple junction of 0.3 mm in diameter (copper constantan, type-T) inserted into the net texture. The measured values of T n were in agreement with those predicted by the simulation. The mean absolute difference between the measured and the simulated values of T n ranged from 0.75 °C to 1.9 °C for all the nets tested. For nets with low porosity, the inserted junction should keep exposing to the surrounding environment. However, for nets with high porosity, the inserted junction should be shielded from radiation to improve measurements in the correct direction.

To protect humans from heat stress risks, thermal comfort and heat stress potential were evaluated under arid environment, which had never been made for such climate. The thermal indices THI, WBGT, PET, and UTCI were used to evaluate thermal comfort and heat stress. RayMan software model was used to estimate the PET, and the UTCI calculator was used for UTCI. Dry and wet bulb temperatures (Td,Tw), natural wet bulb temperature (Tnw), and globe temperature (Tg) were measured in a summer day to be used in the calculation. The results showed the following. (i) The thermal sensation and heat stress levels can be evaluated by either the PET or UTCI scales, and both are valid for extremely high temperature in the arid environment. (ii) In the comfort zone, around 75% of individuals would be satisfied with the surrounding environment and feel comfortable during the whole day. (iii) Persons are exposed to strong heat stress and would feel uncomfortable most of the daytime in summer. (iv) Heat fatigue is expected with prolonged exposure to sun light and activity. (v) During the daytime, humans should schedule their activities according to the highest permissible values of the WBGT to avoid thermal shock.

Thermal sensation and heat stress were evaluated in a plastic greenhouse, with and without evaporative cooling, under arid climatic conditions in Riyadh, Saudi Arabia. Suitable thermal comfort and heat stress scales were selected for the evaluation. Experiments were conducted in hot sunny days to measure the required parameters (i.e., the dry and wet bulb temperatures, globe temperature, natural wet bulb temperature, and solar radiation flux) in the greenhouse. The results showed that in the uncooled greenhouse, workers are exposed to strong heat stress and would feel very hot most of the day time; they are safe from heat stress risk and would feel comfortable during night. An efficient evaporative cooling is necessary during the day to reduce heat stress and to improve the comfort conditions and is not necessary at night. In the cooled greenhouse, workers can do any activity: except at around noon they should follow a proposed working schedule, in which the different types of work were scheduled along the daytimes based on the heat stress value. To avoid heat stress and to provide comfort conditions in the greenhouses, the optimum ranges of relative humidity and air temperature are 48–55% and 24–28°C, respectively.

The perforated nature of nets makes it impossible to measure the thermal radiative properties (emittance, εn, transmittance, τn, and reflectance, ρn) correctly by using any measuring device under natural conditions. In this study, a theoretical model was developed and validated to predict εn, τn, and ρn precisely by solving the model equations simultaneously. The net was tacked onto a wooden frame; thermal radiation balance was applied below and above the net surfaces and above a black substrate underneath the frame. The downward and upward thermal radiation fluxes were measured below and above the net to be used as input parameters to the simulation. Nets with different porosities (ϕ) and colors were used for the study. The results showed that the estimated εn ranged from 0.41 to 0.82 and τn ranged from 0.16 to 0.55 for the nets tested, whereas the reflectances were very low (ρn≤0.08). The color and porosity together affect the properties of the net. Even though under equilibrium conditions of a net with the surrounding environment, the emittance of the net is equal to its absorptance. However, the absorbed thermal radiation by the net dose not equal to the emitted radiation and the difference is a convected heat.

The long-wave radiative properties of a plastic shading net are essential to determine the thermal performance of a particular structure covered with this net. Nets are non-homogeneous, three-dimensional and semi-transparent materials, and therefore the radiative properties of a net cannot be measured in a laboratory using instruments designed for homogeneous materials. In addition, net radiometers cannot be used to measure these properties directly due to radiation emitted from the net surfaces. No suitable technique is presently available for determining the long-wave properties of shading nets. This paper describes a simple method for determining the total hemispherical long-wave radiative properties (i.e., transmittance, reflectance and absorptance or emittance) of nets. Nets with colors, porosities, texture type and nominal shading factors that are most commonly used in hot regions were selected for testing. The measurements were performed under equilibrium conditions (i.e., nighttime, steady-state and uniform sky thermal irradiance). The results show that the behaviors of plastic nets under thermal radiation conditions were different from those of translucent homogeneous materials. Net temperature and porosity were more important than texture type and color in determining transmittance and reflectance. The net temperature drop at night (6–10.5 °C) caused shrinkage in the net. This resulted in a decrease in porosity, which in turn caused a decrease in transmittance and an increase in reflectance. For the nets tested, transmittance decreased from 1.2% to 6.3% per 1 °C decrease in temperature. The reduction in transmittance was equal to the increase in reflectance. Emittance of the net depends on net temperature, color, porosity and texture type, although, at low temperature, none of these effects was significant.

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. Determining 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. Daily 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.

The radiative properties of several plastic shading nets were measured under natural solar radiation conditions. We found that the plastic nets behave as homogeneous translucent materials (e.g., plastic film, plastic sheets, and glass). Based on this behavior, we suggest that it is possible to treat plastic nets as translucent materials and to characterize them with equivalent optical constants (i.e., equivalent refractive indexes,neq, and equivalent extinction coefficients,σeq). Here a physical model to determineneqandσeqof plastic nets was described in analogy to homogeneous translucent materials. We examined three groups of nets based on their color (black, black-green, and beige). Each group consisted of nets with four or five different porosities. Nets of each group had almost the same texture structure. For each group, we derived an equation forneqas a function of the net porosity and determined an average value forσeq. Once values ofneqandσeqwere determined, the solar radiative properties of a net could then be calculated fromneqandσeqfor any incident angle of solar beam radiation without the need of measurements. The present model was validated by comparing the calculated with the measured radiative properties of three nets at different incident angle of solar beam radiation. The calculated radiative properties reasonably agreed with measured values.

For the greenhouse thermal analysis, it is essential to know the amount of solar radiation that was utilized by the greenhouse and consequently converted into sensible and latent heat via convection and evapo-transpiration. Parameters are frequently used for this purpose are: utilization efficiency factor (π), and sensible and latent heat factors (η and δ). Previous studies considered these factors as constants, although they depend on the time of the day, the environmental conditions inside and outside the greenhouse, plants and soil characteristics and the greenhouse structure, orientation and its location. These factors in addition to a proper correlation to estimate the evapo-transpiration rate in a greenhouse have not yet been investigated under the arid climatic conditions of the Arabian Peninsula. This study was to investigate π, η and δ factors; conversion of solar radiation into sensible and latent heat; and the evapo-transpiration rate (ET) in the greenhouse under arid conditions. A polyethylene-covered, evaporatively cooled, planted greenhouse with a floor area of 34 m2 was used. The parameters required for the analysis were measured in a hot summer and sunny day. The results showed that value of π was almost constant (≅0.75); whereas values of η and δ strongly depended on the net solar radiation over the plants (Sn1). ET depended on the vapor pressure deficit (VPD) much more than Sn1. Correlations to estimate ET, as a function of VPD and Sn1 and to estimate η and δ factors as functions of Sn1, were investigated. At a plant density corresponding to a leaf area index (LAI) of 3, about 54% of the integrated solar radiation that was utilized by the greenhouse was converted to sensible heat and about 46% converted to latent heat via evapo-transpiration.

For a greenhouse thermal analysis, it is essential to know the energy partition and the amount of solar and thermal radiation converted into sensible and latent heat in the greenhouse. Factors that are frequently needed are: efficiency of utilization of incident solar radiation (π), and sensible and latent heat factors (η and δ). Previous studies considered these factors as constant parameters. However, they depend on the environmental conditions inside and outside the greenhouse, plants and soil characteristics, and structure, orientation and location of the greenhouse. Moreover, these factors have not yet been evaluated under the arid climatic conditions of the Arabian Peninsula. In this study, simple energy balance equations were applied to investigate π, η and δ; energy partitioning among the greenhouse components; and conversion of solar and thermal radiation into sensible and latent heat. For this study, we used an evaporatively cooled, planted greenhouse with a floor area of 48 m2. The parameters required for the analysis were measured on a sunny, hot summer day. The results showed that value of π was almost constant (≅0.75); whereas the values of η and δ strongly depended on the net radiation over the canopy (Rna); and could be represented by exponential decay functions of Rna. At a plant density corresponding to a leaf area index (LAI) of 3 and an integrated incident solar energy of 27.7 MJ m−2 d−1, the solar and thermal radiation utilized by the greenhouse components were 20.7 MJ m−2 d−1 and 3.74 MJ m−2 d−1, respectively. About 71% of the utilized radiation was converted to sensible heat and 29% was converted to latent heat absorbed by the inside air. Contributions of the floor, cover and plant surfaces on the sensible heat of the inside air were 38.6%, 48.2% and 13.2%, respectively.

Plastic nets with opaque threads are frequently used for shading agricultural structures under high solar radiation conditions. A parameter that is often used to define a net is the net porosity (Π). Value of Π is usually estimated by one of three methods: image processing, direct beam transmittance, or solar radiation balance (hereafter radiation balance). Image processing is a rather slow process because it requires scanning the net sample at high resolution. The direct beam transmittance and radiation balance methods greatly overestimate Π because some of the solar radiation incident on the thread surfaces is forward scattered and add a considerable amount of radiation to that transmitted from the net pores directly. In this study, the radiation balance method was modified to estimate Π precisely. The amount of solar radiation scattered forward on the thread surfaces was estimated separately. Thus, the un-scattered solar radiation transmitted from the net pores directly, which describes the net porosity, Π could be estimated. This method, in addition to the image processing and the direct beam transmittance methods were used to estimate Π for different types of nets that are commonly used for shading structures in summer. Values of Π estimated by using the proposed method were in good accordance with those measured by the image processing method at a resolution of 4800 dpi. The direct beam transmittance and the radiation balance methods resulted in overestimation errors in the values of Π. This error strongly depends on the color of the net. The estimated errors were +14% for a green net and +37% for a white net when using the radiation balance method, and were +16% and +38%, respectively, when using the direct beam transmittance method. In the image processing method, a resolution of 2400 dpi is sufficient to estimate Π precisely and the higher resolutions showed no significant effect on the value of Π.

Plastic shading nets are being used extensively in hot and sunny regions to protect plants from intense solar radiation. Different varieties of these nets are commercially available. However, the choice of net to fulfill specific shading requirements often depends on empirical or economic criteria and not on scientific considerations because essential information used to characterize the different types of nets is not available. In order to replace expensive measuring facilities that use artificial lighting to investigate the radiative properties of plastic shading nets, a simple method was presented to investigate these properties under global, diffuse and direct beam solar radiation conditions. Measurements were performed on clear sunny days, (December 28, 2008–February 10, 2009). Nets with colors and shading factors that are most commonly used in hot regions were selected for the study. The results showed that the behaviors of the plastic nets under solar radiation conditions were similar to those of translucent materials. Accordingly, it is possible to (i) treat plastic nets as translucent materials and (ii) investigate the equivalent optical constants (i.e., refractive indexes and extinction coefficients) for plastic nets as functions of the net solidity, texture type and color. Solidity and color of the net had significant effects on the radiative properties and the effect of color was much more than the effect of solidity. The shading factor of a plastic net, is a function of the daytime, depends on several design and meteorological parameters, so it cannot be used to describe a net. However, the net solidity together with color, and the daily integrals of the shading factor and of the radiative properties are appropriate parameters for describing a net.

Most of solar radiation incident on a greenhouse is absorbed by greenhouse components (i.e., the cover, humid air, plants and soil) and the remaining portion is lost to outside the greenhouse. It is essential to know the absorbed and lost energy terms for any thermal analysis of greenhouses. Existing greenhouse thermal models use the radiative properties of the greenhouse components to directly determine the absorbed energy terms. However, these models neglect the lost energy term and neglect the effects of the multiple reflections of solar radiation between the greenhouse components. The present study describes the general relations for estimating the amounts of solar energy absorbed by the greenhouse components and lost to outside the greenhouse. The relations take into consideration the interrelations as well as the multiple reflections of solar radiation between these components. Thus, the greenhouse system was treated as a solar collector having an absorber plate (i.e., the greenhouse soil) and a cover system consisting of three semi-transparent parallel layers (i.e., the greenhouse cover, the humid air, and the plants). Superposition theory and ray tracing technique were used for the analysis. The presented relations were applied to an experimental plastic-covered greenhouse with a floor area of 34 m2. The greenhouse, located in Riyadh, Saudi Arabia, was planted with tomatoes with a leaf area index (LAI) of 3.0 and was cooled by a wet pad and fan system. Results of the presented relations were accurate and more realistic comparing to results of other relations reported in the literatures. Absorption of solar radiation by water vapor in the greenhouse was negligible. The presented relations can estimate the absorbed and lost energy terms for a greenhouse precisely with a max possible error of +1.8% on each term if the LAI was less than 1.5. The error is significantly decreased to less than +0.7% if the LAI in the greenhouse is increased to 5.

Solar radiation transmission through plastic shading nets is the most important parameter used to characterize the net type. The diffusion characteristics of solar radiation through nets are still unknown because nets are non-homogeneous materials. In this study, transmission of solar radiation through a plastic net was characterized by determining: (i) the beam that is diffused during transmission, (ii) the unscattered beam that is transmitted through the net pores and (iii) the beam that is reflected forward on the thread surfaces specularly. Experiments to measure radiation parameters for determining these components were conducted on clear sunny days in Riyadh, Saudi Arabia. Nets with colors and porosities that are most commonly used were selected for the study. The results showed that the diffuse radiation transmitted through the selected nets was enhanced by 17-170% depending upon the structure of the net textures rather than colors. In the tested nets, the contributions of the diffused beam, the unscattered beam and of the forward specularly reflected beam to the total beam transmitted were in the ranges 0.02-0.2, 0.29-0.83 and 0.11-0.52, respectively. These values depended on the colors, texture structures and porosities of the nets.

The current definition of the evaporative cooling efficiency (ηc) can be applied only for a greenhouse with a wet pad-fan system, in which no sensible or latent heat is added to the air during the cooling process. However, ηc of the wet pad-fan system cannot be applied for a greenhouse with a fogging system, in which sensible and latent heat are added to the air during the cooling process. The purpose of this study is to define ηc correctly for a greenhouse with a fogging system and to describe the fogging process. For this purpose, the concept of un-cooled air was developed, which is the greenhouse air gaining sensible and latent heat from the cover, the floor and the plant surfaces. The dry bulb temperature of un-cooled air (Tu) was defined based on the sensible heat added to the air. Sensible heat strongly depends on the convective heat transfer coefficients in the greenhouse (i.e., between the air and the cover, the floor, the plant surfaces). Therefore, different relations expressing each coefficient were examined. ηc was defined using Tu and the measured values of the dry and wet bulb temperatures in the greenhouse with fog. Values of Tu and ηc were estimated in a greenhouse located in the Tokyo area, Japan, and cooled by intermittent fogging on a hot summer day. The results show that value of Tu was found to be not very sensitive to the selection of convective coefficients, with maximum relative error of 4% observed using different expressions for the convective coefficients from the literature. The assumption that the fogging process takes place in the greenhouse air at a constant wet bulb temperature is practically acceptable.

Shading the greenhouses is necessary in summer to reduce the solar radiation load. This however generates a considerable amount of thermal radiation heat load that needs to be removed via cooling systems. This study aimed to evaluate the effect of different shading configurations on the solar and thermal radiation in a greenhouse. Nets at four different locations were employed to shade the roof and side-walls of a polycarbonate, mechanically ventilated greenhouse. The spectral radiative properties of all these plastic materials were measured in short and long wave spectrum bands. The net solar and thermal radiations and air temperature were measured outside and inside two identical shaded and unshaded greenhouses. The results showed that external roof-shading is desirable, as it reduced the generated thermal radiation in the greenhouse by 21% and 15% during the day and night time, respectively and reduced the greenhouse air temperature during the day. The internal shading (roof and side walls) is undesirable, since it drastically increased the generated thermal radiation in the greenhouse by 147% and strongly increased the greenhouse air temperature during the day. Shading the side-walls is not recommended because it significantly reduces the transmitted solar radiation in the morning and afternoon (when the outside irradiance is low) and is useless at around noon when the outside irradiance is extremely high.

It is impossible to directly measure the thermal radiative properties of shading nets because of the net's perforated structure and the difficulty to distinguish the transmitted, reflected, and emitted radiation on the net surface. In addition, Kirchhoff's identity was derived for an object in thermal equilibrium and exchanges only thermal radiation with its surrounding, and the assumption that is valid for an object exchanging solar and thermal radiation and convection with the surrounding is unclear. This study was to (i) develop a theoretical model to predict the absorptivity, α n; transmissivity, τ n; reflectivity, ρ n; and emissivity, ∊ n, of plastic nets and (ii) examine the equality (∊ n = α n) under natural conditions. Nets with different porosities, texture structures, and colors were tacked onto a wooden frame, fixed horizontally over a black substrate. Thermal radiation balance was applied to the net-substrate system. The model input parameters (i.e., the thermal radiation fluxes below and above the net; the net and substrate temperatures) were measured on sunny days. The estimated values of α n and τ n were in the ranges 0.41–0.82 and 0.16–0.55, respectively, whereas ρ n ≤ 0.08 for the nets tested. Under the steady-state natural conditions, ∊ n = α n for a net in thermal equilibrium. However, the solar and thermal radiation absorbed by the net did not equal the emitted radiation.

In a greenhouse cooled by a fogging system, the fraction of fog that evaporates by absorbing sensible heat from the greenhouse air, β , is an essential parameter to be determined for evaluating the system performance. Recent studies estimated β under indoor conditions by collecting the non-evaporated fog that fell on a plastic sheet during a specified time and β was the difference between the amount of sprayed fog and the non-evaporated fog divided by the amount of sprayed fog. Using this method in the greenhouses causes an overestimation of β due to the evaporation of the fog that fell on the plant leaf/floor surfaces affected by solar and thermal radiation and the difficulty of collecting the non-evaporated fog that falls on the plant leaf surfaces. This paper presents a method for simulating β and for analyzing the fog evaporation based on the heat and water vapor balance of the greenhouse air. The conditions of the un-cooled air in the greenhouse were investigated to be used essentially in the simulation. An experiment to determine parameters for the simulation was conducted on a hot sunny day (August 9, 2004) in the Tokyo area to measure the environments inside and outside a naturally ventilated greenhouse with a floor area of 26 m2. The greenhouse was cooled intermittently at a fogging rate of 10 g s−1 for five different fogging durations (i.e., fogging time–interval time) of 1–3, 0.5–1.5, 1.5–4.5 , 0.5–1 and 1–2 min, respectively. Evapo-transpiration rate of 150 potted tomato plants was estimated using reported correlations and the resulting values could be corrected based on the water vapor balance of the greenhouse air. The results showed that β had a certain pattern along with the fogging and interval time. The values of β estimated from the heat balance were identical to those estimated from the water vapor balance. The fogging duration of 1–3 min showed relatively high evaporation rate, in which, the integrated value of β over a working time of 41 min was 0.36.

A physical model for analyzing the radiative and convective heat transfer in a fog cooled, naturally ventilated greenhouse was developed for estimating the overall heat transmission coefficient based on the conduction, convection and thermal radiation heat transfer coefficients and for predicting the soil heat flux. The contribution of the water vapor of the inside air to the emission and absorption of thermal radiation was determined. Measurements of the outside and inside greenhouse environments to be used in the analysis were conducted around solar noon (12:19–13:00) on a hot sunny day to provide the maximum solar radiation transmission into the greenhouse. The net solar radiation flux measured at the greenhouse floor showed a reasonable agreement with the predicted value. The net fluxes were estimated around noon. The average net radiation (solar and thermal) at the soil surface was 220.0 W m −2 , the average soil heat flux was 155.0 W m −2 and the average contribution of the water vapor of the inside air to the thermal radiation was 22.0 W m −2 . The average overall heat transmission coefficient was 4.0 W m −2 C −1 and was in the range between 3.0 W m −2 C −1 and 6.0 W m −2 C −1 under the different hot summer conditions between the inside and outside of the naturally ventilated, fog cooled greenhouse.

Temperature of the greenhouse cover T c is an essential parameter needed for any analysis of energy transfer in the greenhouse. Measuring the correct value of T c is difficult due to the transparency of the covering materials and the effects of solar and thermal radiation and air movement on the cover surface. Therefore, T c , in most cases, has been estimated theoretically by applying an energy balance to the greenhouse. In this study, a simple measuring method was proposed to predict T c using a thermocouple junction (copper constantan, type-T) adhered directly to the cover surface. A correction factor was provided in the form of a regression correlation to exclude the effect of solar and thermal radiation on the thermocouple junction. The effect of air movement on the junction was also examined for air current speeds up to 5 m s −1 . The results showed that solar and thermal radiation have a significant effect on the junction temperature and resulted in an overestimation of T c , while the air movement around the junction had no significant effect. This method, in addition to an infrared (IR) thermometer and a protected thermocouple junction with a passive radiation shield, was applied to measure the cover surface temperature of an experimental glasshouse under hot sunny and cloudy conditions. Protection of the thermocouple junction from solar and thermal radiation coming from the surroundings caused a large overestimation error in the measured value of T c under sunny conditions. Values of T c measured by the proposed method were in good agreement with those measured by the IR thermometer under cloudy and sunny conditions. The proposed method can be used to measure the greenhouse cover temperature or any surface temperature with a maximal error equal to the measuring accuracy of the IR thermometer (±1%). The different surfaces of the cover (side walls and roof) have almost the same temperature under any weather conditions.

General mathematical relations are presented for the specular exchange factors, F S , of diffuse radiation exchange between the inner surfaces of a rectangular enclosure. Three of these surfaces are specular reflectors, diffuse emitters and the fourth surface is a diffuse reflector, diffuse emitter. This enclosure can be used as a transplant production unit with artificial lighting for electric energy saving purposes. An image system and the crossed string method are used to derive these relations. The resulting expressions are conceptually simple and similar to the commonly known expressions of the exchange factors between diffuse surfaces, F . The accuracy of the presented F S relations was examined for different numbers of multiple reflections, N , on the specular surfaces and for different aspect ratios (ratio of the width, w to the height, h ). The results proved that the relations are accurate and strongly satisfy the well-known relation of the radiation exchange between enclosure surfaces and satisfy the reciprocity relation. For any aspect ratio, considering N of 150 between highly reflective surfaces ( ρ = 0.99) is sufficient to estimate the F S factors without any possible error. Using specular reflecting surfaces in such cases significantly reduces the electric energy consumption used for lighting.

A dynamic simulation model for heat and water vapor transfer in a naturally ventilated, fog-cooled greenhouse was developed to predict the temperatures of air, plant, cover and floor surface and the relative humidity in the greenhouse. Transpiration and evaporation were also predicted. An experiment was conducted on a hot summer day (Aug. 9, 2004) in the Tokyo area to measure the environments inside and outside a glass-covered greenhouse with a floor area of 26 m 2 . The greenhouse was cooled intermittently by spraying water fog at a constant rate of 0.01 kg s −1 for different fogging and interval times (0.5 min on followed by 1.5 min off; 1 min on–3 min off and 1.5 min on–4.5 min off). The system of equations of the model was solved numerically by using the predictor–corrector technique for the differential equations and the iteration procedure for the algebraic equation. The input parameters to the model were the meteorological conditions and the thermo-physical properties of the greenhouse cover, plant, air and soil. The predicted results using the present model were compared with the measured values and showed a good agreement at different fogging and interval times.

The cooling efficiency η c of wet pad and fan systems, which is currently used for fogging systems, is defined assuming that the dry bulb temperature in the greenhouse T db , i is lower than the outside dry bulb temperature T db , o and is measured directly at the outlet of the wet pad. However, in the case of η c of a fogging system, T db , i is typically measured at different locations in the greenhouse to obtain an average value. The inside air is sensibly heated mainly by solar energy transmitted into the greenhouse under sunny summer conditions. Then T db , i is often higher than T db , o , giving negative values of η c . Even though, T db , i is lower than T db , o , the current definition of η c gives unrealistic values due to the latent heat added to the inside air. In this study, η c of a fogging system was newly defined, taking into consideration the sensible and latent heat added to the greenhouse air. The definition is based on the energy and mass (water vapour) balance of the un-cooled air in the greenhouse during the operation of fogging. The value of η c was determined for an experimental greenhouse with a floor area of 26 m 2 and contained 150 potted tomato plants. The greenhouse was cooled intermittently at a fogging rate of 10 g s −1 for three different fogging times of 30, 60 and 90 s with interval times of 90, 180 and 270 s, respectively. The results showed that a fogging cycle of 60 s on, 180 s off provided a relatively higher value of η c than other cycles. Under this condition, two approximate expressions of η c could be used to estimate the η c value with an error less than ±5%.