Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    We will mail you new results for this query: keywords==Air permeability
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A method to quantify the energy-saving performance of greenhouse screen materials
Hemming, S. ; Baeza Romero, E.J. ; Breugel, A.J. van; Mohammadkhani, V. - \ 2018
Acta Horticulturae 1227 (2018). - ISSN 0567-7572 - p. 221 - 229.
Air permeability - Dynamic greenhouse model - Emission - Energy saving - Greenhouse screens - Thermal infrared transmission - Water vapour permeability

Screens used in practice are made from various material compositions (woven fabrics, knitted fabrics, foils, open or closed structures, transparent, diffuse, aluminized, various colours) and for various purposes (energy saving, reduction in light sum, or diffuse light obscuration). An important goal of the use of screens in Dutch greenhouses is to save energy. Unfortunately, to date, there is no objective method to determine the energy-saving performance of a material under standardized conditions. Energy-saving rates are estimated by manufacturers using different methods. Growers have no way of comparing material performances independently in order to make a proper investment decision. In the current research, the goal was to develop a method to quantify the energy-saving performance of greenhouse screen materials under standardized conditions. The method is based on the scientific literature and expertise of different screen producers and growers. The research focused on the three main aspects that affect energy saving of a screen: 1) thermal radiation losses, determined by the emissivity and reflectivity for thermal infrared radiation; 2) air permeability, which determines heat convection losses, characterized at a wide range of air velocities to account for velocities by buoyancy through materials as well as for velocities by forced convection caused by internal fans; and 3) water vapour permeability, which determines latent heat losses, determined under temperature, humidity and air velocity conditions normally encountered in commercial greenhouses. For all aspects, different measurement methods were compared to choose the best method based on reproducibility, accuracy and practicability. Screen material properties were then fed into both steady-state and validated dynamic greenhouse climate models to calculate overall screen energy saving under well-defined conditions. In the current research, different screen materials from different producers were investigated. The paper describes the methodology developed and shows data on different screen materials.

Characterization of air velocities near greenhouse internal mobile screens using 3D sonic anemometry
Baeza Romero, E.J. ; Hemming, S. ; Breuge, A.J. van; Mohammadkhani, V. ; Jansen, H. ; Kempkes, F. - \ 2018
Acta Horticulturae 1227 (2018). - ISSN 0567-7572 - p. 159 - 164.
Air permeability - Fans - Greenhouse screens - Vents - Wind

In Dutch greenhouses, different screen types are used for different purposes (shading, energy saving, black-out, light emission, etc.). In order to quantify the energy and mass transfers through screens, characterization of air permeability through the screens is required. In the case of energy-saving screens, it is an essential parameter to estimate the energy saving of each screen. Air permeability can be measured under defined conditions in a laboratory. In order to select the appropriate equipment for air velocity measurements, the air velocity vector near screens in a practical situation in a greenhouse needs to be identified by measurements. Sonic anemometry techniques have been used extensively in different types of greenhouses: a) to study natural ventilation, with and without insect screens, and in different positions; b) to study airflow patterns in greenhouses with mechanical ventilation/pad and fan systems; c) to study airflow patterns induced by different types of heating systems, and d) for the estimation of crop evapotranspiration (i.e., eddy covariance). However, to the best of our knowledge, no research has been carried out to study the airflow near different types of screens in a greenhouse. Many Dutch growers are increasingly using various types of fans with different positions in the greenhouse for dehumidification and improved climate uniformity purposes. The effect of such fans on the air velocity near screens, and therefore the effect on energy and mass transfer, is unknown. For this purpose, air velocities near different types of screens in commercial greenhouses were measured using ultrasonic 3D anemometers. The results show that, in the absence of fans, air velocity near the screens is affected by vent opening. With vents closed, air velocities are hardly ever above 0.2 m s-1. Therefore, a simple air-suction device can be used to characterize permeability of screens at a very low Reynolds range.

An app to quantify radiative heat loss from greenhouse crops
Zwart, H.F. de; Baeza Romero, E.J. ; Breuge, A.J. van; Mohammadkhani, V. - \ 2018
Acta Horticulturae 1227 (2018). - ISSN 0567-7572 - p. 69 - 76.
Air permeability - Dynamic greenhouse model - Energy saving - Thermal infrared transmission - Thermal screens

Deploying a thermal screen in the night gives a significant reduction in radiative heat losses from the crop and heat losses of the greenhouse in general. The reduced radiative heat loss gives a smaller vertical temperature gradient in the crop. Deployment of a thermal screen results in increases in top-leaf temperatures of 1-2°C, which allows for a higher humidity set point without risk of wet leaves, even at higher humidity in the greenhouse. This increment in tolerance of humidity is the second contribution of thermal screens to energy saving. Both aspects of thermal screens have made increased screening one of the main pillars of “next-generation cultivation”, a term referring to growing strategies that reduce energy consumption while promoting crop production. In order to support knowledge on screens and to stimulate growers to apply the benefits of next-generation cultivation, an app was developed that quantifies the effect of screens on leaf temperature and transpiration. On top of that, the app computes the net radiation from the crop, a figure that has gained attention as more and more growers install net radiation sensors in their greenhouse. The effect of screens is, of course, dependent on the outside and inside climate conditions, the crop, the greenhouse covering material and the type of screens used. The app enables the user to select the screen and covering materials from a number of options and to select from a number of crops. Among the screens, a selection can be made from partly open shading screens to transparent energy screens and completely blocking blackout screens. Also, the effect of artificial light can be shown. The app solves the steady-state energy balance of the greenhouse to calculate the promptly presented output. With the output, a quick exploration of the effect of screens on radiative losses and crop vertical temperature profile can be made, to learn from this for practical use.

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