Screen suppliers each have their own methods of determining the energy-saving performance of their screens, making it very hard for the grower to compare them. But this is about to change. Wageningen University & Research in the Netherlands has been working with screen manufacturers to develop an objective measurement method.
There are many different types of screen fabrics: from fully transparent to completely dark, from woven to knitted, with open or closed structures and with or without aluminium components. Screens are also used for different purposes: saving energy, reducing solar radiation, diffusing light or as a blackout. But whatever your reason for screening, as soon as you close a screen you start saving energy.
You would think it would be difficult to compare the energy-saving credentials of such a wide variety of screens. But that’s not the case. How much energy can be saved depends on a number of physical phenomena which can be measured in all screens. However, if all manufacturers apply their own criteria, the results will always be difficult to compare.
This problem has been around in the sector for some time. A previous project by two Dutch research institutes, Wageningen University & Research and TNO in Delft, and two screen manufacturers resulted in a method of objectively determining screen emissivity, but this method is not suitable for quantifying the screen’s energy-saving performance.
“The amount of energy saved by a screen fabric depends on three components: radiation exchange (transmissivity and emissivity), air permeability and water vapour transport through the fabric. If you can measure these three properties, you can compare screen fabrics,” says Silke Hemming, head of the WUR Greenhouse Technology research team. Hemming headed up the project which, besides Wageningen University & Research (WUR), also involved Cultilene, Ludvig Svensson, Low & Bonar and Novavert.
“The manufacturers sent us 29 different fabrics,” Hemming explains. “Because the aim of the project was to develop a measurement protocol, we selected seven that were very different, ranging from very dense to highly permeable, for example. We used these to develop effective measurement methods.”
This resulted in a quest for which the WUR LightLab even had to build its own equipment in order to determine one of the properties. Hemming: “We started off with radiation exchange. There was already a device available for this: the TNO emissivity meter. The total emissivity (radiated heat loss) depends on the transmissivity of thermal radiation through the fabric, the reflectivity of radiation on the underside and the absorption and emissivity of radiation on the upper surface.”
Some screen fabrics allow high levels of thermal radiation through, whereas a highly aluminised screen does not. The combination of low thermal radiation transmissivity and low emissivity on the upper surface traps the radiated heat on the inside, maximising energy savings and minimising cooling of the crop.
The second part of the measurement protocol is air permeability. If it is warm inside the greenhouse and cold outside, air permeability always means a loss of energy (sensible and latent heat). This property was particularly tricky to measure. Hemming: “First we investigated what air speeds were commonly found in greenhouses. They turned out to be very low, which meant that we were unable to carry out measurements in a wind tunnel. So the LightLab developed a piece of equipment especially for us: an air suction device, or permeability meter.”
This consists of a round steel tube in which small discs of the screen fabric can be clamped. Air is sucked through the fabric at different speeds, causing a slight pressure difference. By measuring the air pressure on both sides of the fabric you can quantify its permeability.
Water vapour transmission
There is no standard for the two properties mentioned above (emissivity and air permeability). However, there is one for the third property, water vapour transmission: ASTM E96. This standard originates in the textile industry, where it is used for applications such as developing breathable clothing. Water vapour transmission is determined using the cup method. “But this standard turned out to be irrelevant to greenhouses. In a greenhouse, not only is there a difference in moisture concentrations on either side of the screen, there is always a temperature difference as well. The cup method therefore underestimates water vapour transmission in greenhouse situations. And then there is the issue of condensation forming on the fabric. So we brought in the Swedish institute Swerea, which can measure all relevant properties of water vapour transport through textiles,” the project leader explains.
Screen fabrics differ quite considerably. Some let a lot of moisture through, which benefits growers who want to use this method to remove moisture. On the other hand, a screen that is completely impermeable to moisture saves a lot of energy. “But that means you need another way to remove moisture, such as a mechanical dehumidification system,” Hemming says.
The three fabric properties are not entirely independent of one another. A completely wet screen is impermeable to thermal radiation, for example, and this was taken into account in the project. The main aim of the project was to develop replicable measurement methods. If manufacturers were to start using these methods as a standard and pass on the measurement results to their customers, every grower would be better placed to decide which screen to use. Simply making a more informed choice can deliver energy savings of 5%.
Once they had developed the measurement methods, the researchers entered the results into the KASPRO climate and energy model to work out the overall energy saving performance for each screen. The model crop was tomato, and the screens were only used at night to enable all the fabrics to be compared under standard conditions. “The highest scoring screen saved around 26% year round compared with the reference (no screen), while the worst performing screen only saved around 9% as it was highly permeable to light and moisture. Looking at the savings achieved during the night-time screening hours alone, the highest scoring screen scored almost 50% and the lowest 13%,” she says.
These figures relate to the fabric properties. The actual energy savings to be achieved in practice depend very much on how the grower uses the screen (the number of screening hours and the time at which the screen is closed) and on the type of screening system they use. “If you only look at the energy savings, the best fabric is one that allows no thermal radiation, moisture or air through at all. But without adjustments this would simply result in condensation on the crop. The grower will always have to find a balance between saving energy and the amount of moisture they want to remove through the screen,” Hemming says.
The researchers are calling on screen manufacturers to routinely include five fabric properties in their product information: emissivity, thermal radiation permeability, air permeability, water vapour transport and a performance indicator for total energy savings. At present, product brochures tend to only give the highest figure: the energy that can be saved on the coldest night of the year. The new method makes it possible to present a realistic and comparable indicator for energy-saving performance.
Hemming expects the new measurement protocol to have many benefits: it will enable growers to make better informed choices and manufacturers will gain new insights into aspects such as air permeability, which was previously poorly quantified. They will then be able to incorporate these findings in their product development work.
Researchers have been working with screen manufacturers to develop an objective method of determining the energy saving performance of screen fabrics. The method looks at three parameters: radiation exchange, air permeability and water vapour transport through the fabric. If manufacturers elevate this method to a standard, it will enable growers to make more informed choices when buying screens.
Text: Tijs Kierkels. Images: Wilma Slegers.