Characterization of air velocities near greenhouse internal mobile screens using 3D sonic anemometry

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.</p

Effect of greenhouse films on climate, energy, light distribution and crop performance – measuring film properties and modelling results

Greenhouse growers would like to choose an optimum plastic film for their greenhouse production. In parallel, greenhouse supply industry is looking for quantitative information on their product performance. All are looking for answers on the following questions: which film do I need to create an optimum greenhouse climate? Which one do I need to choose for low energy consumption, or for low summer temperatures? What will be my crop yield under the different films? Is an expensive but long-lasting film economic feasible (for my crop, my climate, etc.)? Models can help to find answers to these and other questions and are faster and often cheaper than agronomic trials.</p

A method to quantify the energy-saving performance of greenhouse screen materials

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.</p

An app to quantify radiative heat loss from greenhouse crops

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

Characterization of air velocities near greenhouse internal mobile screens using 3D sonic anemometry

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.</p

An investment order tool to guide development of greenhouse horticulture for two specific regions

Growers worldwide often lack means to find the economically soundest order of investing. Authorities face similar problems when deciding which developments to stimulate. For Dutch greenhouse horticulture, models for production, climate, revenue and costs, allow selection of an optimal investment order. This approach was widened into “Adaptive Greenhouse Methodology” which allows evaluation of worldwide climate and greenhouse technology combinations. However, running the models requires expert skills. Our goal was to deliver a simplified software tool, which would allow horticultural suppliers, researchers and growers to autonomously rank alternative investments, for a specific combination of region, greenhouse design and crop. The Investment Order Tool was developed in cooperation with selected horticultural supply companies for the regions Almeria in Spain and the Jordan Valley. In Spain, a flat roofed Parral type greenhouse was compared to an industrial multi-span greenhouse. In Jordan a single tunnel greenhouse was compared to an industrial multi-span greenhouse with passive crop based cooling. The Investment Order Tool uses a one-time run of the Adaptive Greenhouse Methodology based on local information. This data set allows further off-line calculations. All adaptions in greenhouse construction and cultivation system are defined as relative production changes from the local standard. The adaptations are provided with their specific costs and benefits. The investments compared include: reverse osmosis; substrates; nutrient dosing; climate-adapted cultivars; recirculation of drainage water; ventilation capacity; shading screens and thermal screens. The Investment Order Tool informed growers on the investment order with the highest return on investment and the investment order with the lowest demand for capital. Nursery specificity was realized by permitting user defined yield and market price level per month and by defining a first and second class for product quality. It is hoped the Investment Order Tool encourages growers and local authorities to base investment decisions on increasingly solid knowledge.</p

Exploring the boundaries of the passive greenhouse in Jordan: A modelling approach

Greenhouses are expanding fast in arid and semi-arid regions, among other reasons, because of the water savings that can be realized compared to open field cultivation. However, it is difficult for growers to recognize the optimum greenhouse design. Many competing aspects must be weighed against each other such as the structure, the cover and the climate control equipment. Obviously, the optimum design must be tailored for each specific crop and growing cycle and availability of resources (land, water, energy, labor, etc.). Simulation models can assist in this process, saving time and money. Wageningen University & Research, BU Greenhouse Horticulture has developed the Adaptive Greenhouse Methodology. It combines the use of greenhouse climate and resources simulation models, with crop growth and economic models, to solve the problem of designing the optimum greenhouse for each specific scenario in the world. In the present work we present the results of the application of this methodology to the specific case of the production of greenhouse soilless tomato in two regions in Jordan in the mid tech range: the highlands and the Jordan Valley. Results show that different mid tech designs could potentially provide yield levels of up to 35 and 27 kg m-2 in the Highlands and the Jordan Valley, respectively. The final design is similar in the two locations.</p

An investment order tool to guide development of greenhouse horticulture for two specific regions

Growers worldwide often lack means to find the economically soundest order of investing. Authorities face similar problems when deciding which developments to stimulate. For Dutch greenhouse horticulture, models for production, climate, revenue and costs, allow selection of an optimal investment order. This approach was widened into “Adaptive Greenhouse Methodology” which allows evaluation of worldwide climate and greenhouse technology combinations. However, running the models requires expert skills. Our goal was to deliver a simplified software tool, which would allow horticultural suppliers, researchers and growers to autonomously rank alternative investments, for a specific combination of region, greenhouse design and crop. The Investment Order Tool was developed in cooperation with selected horticultural supply companies for the regions Almeria in Spain and the Jordan Valley. In Spain, a flat roofed Parral type greenhouse was compared to an industrial multi-span greenhouse. In Jordan a single tunnel greenhouse was compared to an industrial multi-span greenhouse with passive crop based cooling. The Investment Order Tool uses a one-time run of the Adaptive Greenhouse Methodology based on local information. This data set allows further off-line calculations. All adaptions in greenhouse construction and cultivation system are defined as relative production changes from the local standard. The adaptations are provided with their specific costs and benefits. The investments compared include: reverse osmosis; substrates; nutrient dosing; climate-adapted cultivars; recirculation of drainage water; ventilation capacity; shading screens and thermal screens. The Investment Order Tool informed growers on the investment order with the highest return on investment and the investment order with the lowest demand for capital. Nursery specificity was realized by permitting user defined yield and market price level per month and by defining a first and second class for product quality. It is hoped the Investment Order Tool encourages growers and local authorities to base investment decisions on increasingly solid knowledge.</p