By using a model to show the energy balance in a greenhouse you can see at a glance where the energy losses are likely to arise. Evaporation uses expensive energy and this rushes out the vents when a grower opens them to reduce the relative humidity. This can be done differently say Dutch researchers of Wageningen UR Greenhouse Horticulture. The more uniform the greenhouse climate the more the relative humidity can be allowed to rise.
First turn on the heating, then open the screens and then ventilate. This was the common method used by every grower to remove moisture from the greenhouse in order to reduce the relative humidity (RH). In the latest greenhouses with single or double screens this happens the other way round: First ventilate above the screen, then make a gap in the upper screen and then a gap in the lower screen. Only then turn on the heating.
However, this correct method of dehumidification meets some difficulties. Nevertheless it is virtually the most important measure to keep the crop healthy and active. Also, the careful and balanced removal of moisture can save a lot of energy. Researchers Feije de Zwart and Marcel Raaphorst of Wageningen UR Greenhouse Horticulture have calculated the year round energy balance in the greenhouse. Although this is a theoretical calculation it can help to give growers more understanding about managing energy in the nursery.
Raaphorst explains the basis of the energy balance. The researchers have assumed a modern greenhouse with two energy screens which as much as possible are controlled according to the principles of the Next Generation Growing. Energy consumption is based on 26 m3/m2 for heating the crop, an average figure for energy-efficient cultivation of an unlit vegetable crop.
On one side of the equation is the energy input from the heating and sun. It’s noteworthy that the sun is responsible for 82% of the input (3600 MJ) and the heating for just 12% (800 MJ). On the other side of the equation is the output. Due to reflection, 18% (800 MJ) of the input doesn’t even reach the plant. The largest loss, 39% (1700 MJ), is heat that leaves the greenhouse directly via the walls and roof. Ventilation via the windows adds another 11.5% (500 MJ). A further 3.5% (150MJ) disappears via biomass, so the crop itself. And finally 28% (1250 MJ) of the energy ‘evaporates’ via water vapour.
Raaphorst explains that several factors are difficult to influence. “But a reasonable amount can be done with evaporation and dehumidification. A substantial amount can be saved with sensible management.”
Ventilation and evaporation
The model goes further. The 1250 MJ in water vapour converts into 500 litre per m2 per year. Of this, 350 litres leaves the greenhouse through ventilation and 150 litres through dehumidification. In addition, another 150 litres/m2 evaporates and condenses on the roof. The condensation heat (360 MJ) that is released on the roof mostly escapes to the outside. To avoid double counting condensation heat is not included in the equation.
Back to the 150 litres/m2 that has to be removed by dehumidification. In total 360 MJ, in other words 11 m3 gas/m2, is needed to remove this amount of moisture. Some of this energy is needed for the evaporation process itself. Evaporation costs energy. Another part is involved in warming the cold air that is required to remove the moisture. Therefore reducing evaporation can save a lot of energy, for example, by using one or more screens.
Double energy screens
Hugo Plaisier of screen supplier Ludvig Svensson sees the interest in double energy screens rising. These screens help to insulate the greenhouse but they also have an impact on the RH. Double screens are already used in cucumber and pepper crops, where a high RH is generally accepted and the 24-hour temperature is higher than that for tomatoes. Tomato growers are starting to show more interest but in most cases they use just one screen and a fixed foil.
Plaisier: “In practise we see that the RH in the greenhouse increases under two screens. Then it may be necessary to remove extra moisture, if this exceeds a certain limit. Then the best strategy is to ventilate above the screen first and then make a gap in the screen. First make a gap in the top screen and then in the lower screen. In this way you maintain a stable greenhouse and a uniform climate.”
Sometimes growers think that fire-resistant screens allow less moisture to penetrate than the older screens, but he says that this idea is purely fictional. “This is not so. The advent of fire resistant screens came at about the same time as the new way of cultivation, the consequence of which was a higher RH,” he says.
Accept high RH
Raaphorst encourages growers to accept a higher RH. “I’m convinced that it’s possible to grow at an RH of 99%, providing that no condensation occurs on the crop.” Perhaps for growers this seems remote because then the horizontal temperature differences in the greenhouse would have to be at a minimum. Yet in practise the researcher sees more growers embracing this idea.
A few years ago an RH of 85% was the maximum allowed (88% at night), and now the limit has moved upwards, to well over 90%. Growers accept a higher RH by using a double screen, so that the top remains warmer and the fluctuations in temperature decrease. “I know growers who allow a much higher RH and still have little disease pressure,” he says.
The calculation model Kaspro shows that allowing 1% extra RH year round saves 0.65 to 1 m3 of gas per m2. Therefore 5% higher RH saves 3.2 to 5 m3 gas per m2. In addition, a second screen installation brings a saving of 3.5 m3 of gas per m2. If this is dehumidified with external air, the heating required to warm this air is about 6 m3/m2 per year. By using heat recovery it is possible to save 70% of this, or 4 m3/m2 per year.
Condensation on screen
Allowing a higher relative humidity does have its limits. One of the risks of growing at a high RH is a screen that drips because it has been saturated with condensation. Raaphorst: “Under certain conditions the evaporation is greater than the amount of moisture that can be transported through the screen, which results in the screen becoming wet. By using double screens we can learn to deal with this better.”
To prevent dripping the cold upper screen has to be the most permeable, or a grower has to sometimes make a gap in it. “Thanks to the upper screen the lower screen remains a little warmer so less condensation occurs.”
Growing at a higher RH seems more obvious for vegetable growers than for example, roses grown in winter under a lot of light. In this situation it is very difficult to keep the horizontal temperature differences small so the chance of moisture on the crop can arise in certain corners of the greenhouse. But for these crops too, managing the climate differently can lead to large savings.
A model of the energy balance in the greenhouse shows that dehumidification accounts for one third of energy losses. Therefore changing the method of moisture removal can save a lot of energy. By using a double screen it is possible to reduce the fluctuations in the climate conditions. This makes it possible to work with a higher relative humidity.
Text/photos: Pieternel van Velden