Abstract

Due to high solar intensity, the internal temperature of greenhouses in subtropical regions is so high that it is difficult for crops to survive. There are two types of shading nets that are applied to reduce the solar intensity entering the greenhouse. The purpose of this research is to develop a simple greenhouse model to describe the effect of these shading nets on the inside temperatures of a greenhouse. The thermal condition was assumed as the steady state. The detailed microclimate data of an experimental greenhouse with internal and external shading nets were collected during various weather conditions. The model was validated using experimental data collected from various conditions. The prediction accuracy of the model for air temperature was about 1.5°C. It is concluded that this model could be applied to evaluate the performance of shading nets and serve as a tool for the design of a subtropical greenhouse.

Key words: Subtropical, greenhouse climate model, shading nets.

Abbreviation

Notation: a, constant; A₁,  surface area of the upper layer included the roof and partial wall, m²; A₂, wall area of the lower part, m²; A_f, floor area, m²; A_op, area of opening, m²; A_s, area of internal shading nets, m²; AR, natural air exchange rate of the greenhouse, m³s^-1; C_d, discharge coefficient value; C_p;  air specific heat, J kg^-1°C^-1; F_1s, shape factors for the sky as seen from the upper layer; F_2s, shape factors for the sky as seen from the lower layer; g, gravitational acceleration, ms^-2;H, opening height, m; h_t, constant; I_S, Incoming shortwave radiation, Wm^-2; LAI, leaf area index; M₁, ventilation rate of mechanical devices in the upper part, m³s^-1; M₂,ventilation rate of mechanical devices in the lower part, m³s^-1; M₁₂, ventilation rate between the upper and lower layer due to the mechanical ventilation of the upper layer, m³s^-1; M₂₁, ventilation rate between the upper and lower part due to the mechanical ventilation of the lower layer, m³s^-1; n, number of data; P_ave, predictive performance, °C; Pf, Canopy area index; P_i, predictive error, °C; P_ws, air saturated vapor pressure, kPa; RH, relative humidity, %; T₁, upper layer temperature, °C; T_1k,absolute air temperature of the upper layer air, K; T₂, lower layer temperature, °C;T_2k, absolute air temperature of lower part, K; T_a, ambient temperature, °C; T_ak,ambient air temperature, K ; T_ave, average temperature of greenhouse, °C; Tr,  transpiration rate of tomatoes, kgm^-2s^-1; T_sky, sky temperature, K; U₁₂, heat transfer coefficient of internal shading nets, Wm^-2°K^-1; U_k2, heat transfer coefficient of cover materials, Wm^-2°K ^-1; U_w1, heat transfer coefficient of greenhouse cover materials, Wm^-2K^-1; V₁, volume of the upper layer, m³; V₂, volume of the lower layer, m³; X_i, predicted temperature by thermal model, °C; Y_i, measurement temperature, °C; , absolute values of predictive error; α, absorptance of internal nets; ρ, air density, kgm^-3; σ, Stephan-Boltzman constant; ε, long ware thermal transmittance of cover materials; τ₀,  transmittance of external shading nets; τ₁, transmittance of internal shading nets; λ, latent heat of vaporization, kJkg^-1; γ,  thermodynamic psychrometric constant, kPa K^-1