A giant dehumidifier in cherry tomato grower Robert van Koppen’s greenhouse in Kwintsheul (Westland, the Netherlands) keeps the RH below the critical value of 95%. The principle is simple: when moist air in the greenhouse is sucked along the cold ribs of the dehumidifier, the water vapour condenses, just like the condensation that forms on a cold bottle of beer when you take it out of the fridge. The dry, slightly warmer treated air goes back into the greenhouse.
Robert van Koppen grows cluster cherry tomatoes on four hectares. “I’m only a small grower in terms of surface area, so I want to make my company stand out. We grow ‘Delight’ cherry tomatoes which are very sweet. They have a fruit weight of 8 grams and are 30 mm in diameter. We’re constantly looking for ways to improve the climate and save energy, but quality and flavour need to be guaranteed. Saving energy means tolerating moisture because we keep the screens closed for longer. The DryGair dehumidifier prevents the humidity from rising too high.”
Van Koppen heard about the device from his Dutch supplier Royal Brinkman and, starting in January this year, decided to rent one and set up a trial in a 1,500 m2 section which can be ventilated and heated separately.
The grower is very happy with it. “We extract 1,000 litres of water from a surface area of 1,500 m2 every day. This lowers the RH by 6% and the difference in RH and temperature from one end to the other is less than 1%.” But it’s still to early to put a definite price tag on the benefits.
According to Eef Zwinkels, technical account manager at Royal Brinkman, the device is the best way to get rid of the excess moisture generated by plants transpiring in the greenhouse. Seen in that light, this technology is a good fit for Next Generation Growing, which advocates using less energy and screening more.
He shows it on the Mollier diagram, which is also used on the Next Generation Growing course. The bottom horizontal axis shows the amount of water in the air in g/kg. The vertical axis shows the temperature. The graph contains a series of RH curves from 10 to 100% and indicates how much absolute humidity (AH) the air contains at a certain temperature. If the screens are closed in the afternoon, both the RH and the AH rise. “If you actively eliminate absolute humidity from the greenhouse, the RH doesn’t rise when the temperature drops and you don’t move into the danger zone,” Zwinkels says.
The manufacturer recommends using one unit per 1,500 to 5,000 m2, depending on the amount of transpiration from the crop and how much space there is above it. It goes without saying that a fully grown tomato crop transpires more than a pot mum on an ebb and flow system.
Besides extracting water actively from the air, the units also circulate about 22,000 m3 of air in the greenhouse per hour. This makes for a homogeneous climate, which in turn translates into a uniform crop. In addition, air movement is responsible for plant activity, which aids calcium uptake.
Zwinkels installed the dehumidifier last year in a large number of nurseries with a wide range of crops such as patio plants, pot plants, tomatoes, mother plants, geraniums, lettuce in gutters and anthuriums, he reports. The units work best between 10 and 30°C.
Van Koppen has seen a number of benefits in the crop in the few months since the trials began. “Dehumidifying the air has given us a more generative crop status, which is producing more fruit. We try to grow the sweetest cluster cherry tomatoes. We don’t go for quantity but quality. The more generative crop status also helps produce a better flavour.”
Due to the lower RH, the pressure from diseases is lower, especially from fungal attacks. "Over the past five years we have constantly been cutting our energy consumption, so the risk of fungal attacks has been increasing. That’s why I focus on reducing moisture. You can see that there is less moisture in the air now.”
An additional benefit is that the grower is keeping more of the CO2 inside the greenhouse because he has to vent less. That promotes the growing process. And because the crop is slimmer, there is a better balance between roots and leaves and Van Koppen has no problems with high root pressure. He is going to continue to use the dehumidifier, although he has not yet decided how many units to buy and when.
Cherry tomato grower Robert van Koppen is trialling a dehumidifier in a 1,500 m2 greenhouse section. Warm, moist greenhouse air condenses against the cold ribs of the dehumidifier. Not only does he get 1,000 litres of moisture out of the air every day, which reduces the RH by 6%, the RH and temperature are also more evenly distributed. Other benefits of the lower RH include a more generative crop status, less disease pressure and lower energy consumption.
Pot mum grower Ruud Nederpel:
“Efficient dehumidifier eliminates need for minimum vent position and drying the air with heat”
Brothers Theo and Ruud Nederpel from Wateringen in Westland, the Netherlands, grow pot mums on 4 hectares. They use an ebb and flow system on concrete floors. Production takes 9 to 10 weeks and the crop is under a blackout screen for 13 hours a day for 7½ weeks. Because the RH rises quickly under the blackout and energy screens in winter, in early December 2016 they decided to rent a dehumidifier for an 8,000 m2 section of the greenhouse to see whether it would make a difference.
"In our case, the RH was rising to more than 93% in the winter. This causes problems with diseases such as Botrytis and rust because the plants are no longer active. In the other sections we are still using a minimum pipe at 45ºC, leaving gaps in the blackout and energy screens and venting. This costs energy. In the trial section with the dehumidifier, the RH is down to between 83 and 86% and we no longer have to remove any moisture,” says Ruud Nederpel. He removes about 54 litres of water per hour. The benefit is likely to be greatest in the autumn and winter when there is a lot of dark, dull weather.
The device is also very user-friendly, the grower says. “It’s easy to install. You simply plug it into the wall socket and the condenser removes the water. You can easily route the water to the return water in the ebb and flow system and re-use it on the crop.”
Having had such positive results with the dehumidifier, the brothers have now bought the first one they rented. With winter behind them, they are thinking about whether to invest in dehumidifiers for the other sections in the autumn. "It's simply about the humidity in the autumn and winter as it’s drier and hotter in the summer. There is a price tag issue: how much energy will it cost me to remove the moisture? How many pests and diseases will it prevent? How much will it improve crop quality? It’s different for every grower. It’s an attractive solution for us because we want to keep the greenhouse completely closed in winter. Removing the moisture keeps the plants more active. And an active plant is more resilient than a plant that is stressed.”
The Nederpel brothers are reckoning on a payback time of three to four years. "This is an assumption. We don’t yet know how much energy we will save compared with previous years.”
Text and images: Marleen Arkesteijn.