Selective reflection of near infrared radiation in shading screens for cooled greenhouses

Shading screens are used in greenhouses to prevent excessively high indoor temperatures and light intensities. Such screens reduce the transmissivity for solar radiation, both for near infrared radiation (NIR) and photosynthetically active radiation (PAR), reducing the heat load in the greenhouse. However, the reduction in PAR transmissivity also reduces the amount of PAR available to the crop, influencing growth and production. New materials with selective reduction of NIR transmissivity are available, but they often have a lower PAR transmissivity. This study quantified the benefits of shading screens with selectively low NIR transmissivity, weighing them against the drawbacks of a reduction in PAR transmissivity. In particular, the study considered the dual effect of low PAR transmissivity in cooled greenhouses controlled according to a radiation-temperature ratio (RTR). In such cases, lower PAR transmissivity reduces the incoming energy load, but also lowers the target temperature for cooling, resulting in a complex effect on the cooling needs. The study used model simulations of a sweet pepper greenhouse in the Arabian Peninsula, equipped with a pad and fan cooling system, and focused on the effect of the screen on the reduction of water use for irrigation and cooling. The performance of a typical reference screen was compared to screens with various values for PAR and NIR transmissivity. The results indicated that in general, any improvement in the screen (higher NIR reflectivity, higher PAR transmissivity) improved the performance of the greenhouse in terms of water use efficiency. However, the screen use strategy also played an important role, and any reduction in the frequency of screen use directly improved the potential production and the water use efficiency. This suggests that in greenhouses with sufficient cooling capacity, controlled according to RTR, and managed so that production is close to the potential, it is best to use shading screens as little as possible.