A.M. Abdel-Ghany

Human Thermal Comfort and Heat Stress in an Outdoor Urban Arid Environment: A Case Study

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 (, ), natural wet bulb temperature (), and globe temperature () 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.

Covering Materials Incorporating Radiation-Preventing Techniques to Meet Greenhouse Cooling Challenges in Arid Regions: A Review

Cooling greenhouses is essential to provide a suitable environment for plant growth in arid regions characterized by brackish water resources. However, using conventional cooling methods are facing many challenges. Filtering out near infra-red radiation (NIR) at the greenhouse cover can significantly reduce the heating load and can solve the overheating problem of the greenhouse air. This paper is to review (i) the problems of using conventional cooling methods and (ii) the advantages of greenhouse covers that incorporate NIR reflectors. This survey focuses on how the cover type affects the transmittance of photosynthetically active radiation (PAR), the reflectance or absorptance of NIR and the greenhouse air temperature. NIR-reflecting plastic films seem to be the most suitable, low cost and simple cover for greenhouses under arid conditions. Therefore, this review discusses how various additives should be incorporated in plastic film to increase its mechanical properties, durability and ability to stand up to extremely harsh weather. Presently, NIR-reflecting covers are able to reduce greenhouse air temperature by no more than 5°C. This reduction is not enough in regions where the ambient temperature may exceed 45°C in summer. There is a need to develop improved NIR-reflecting plastic film covers.

Evaluation of PE Films Having NIR-Reflective Additives for Greenhouse Applications in Arid Regions

Linear-low-density-polyethylene- (LLDPE-) based formulations with several near-infrared- (NIR-) reflective pigments were prepared by melt blending technique and their subsequent films were prepared by blown film extrusion process. Thermal properties of these films were evaluated using differential scanning calorimetry (DSC). The results showed that the melting and crystallization temperatures ( and , resp.) of these formulations were almost similar to that of control resin. The melt viscosity was measured by stress-controlled rotational rheometer and melt flow index (MFI) instruments. Rheological measurements indicated that the blend formulations with NIR-reflective additive have similar melt viscoelastic behavior (storage modulus and dynamic viscosity) to the control resin. The mechanical test performed on NIR-reflective films showed similar values of tensile strength for blend samples as that of control resin. The spectral radiometric properties of the blend films were evaluated in the solar wavelength range of 200–1100 nm and found to be improved over the control sample without having NIR-reflective pigment.

Modeling Approach for Determining Equivalent Optical Constants of Plastic Shading Nets under Solar Radiation Conditions

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 determine 𝑛eq and 𝜎eq of 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 for 𝑛eq as a function of the net porosity and determined an average value for 𝜎eq. Once values of 𝑛eq and 𝜎eq were determined, the solar radiative properties of a net could then be calculated from 𝑛eq and 𝜎eq for 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.

On the Emissivity and Absorptivity of Plastic Shading Nets under Natural Conditions

It is impossible to directly measure the thermal radiative properties of shading nets because of the nets perforated structure and the difficulty to distinguish the transmitted, reflected, and emitted radiation on the net surface. In addition, Kirchhoffs 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.

Effect of the Evaporative Cooling on the Human Thermal Comfort and Heat Stress in a Greenhouse under Arid Conditions

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.