AVS plays key role in energy-efficient demonstration crops
Van Dijk heating’s AVS-C system plays a key role in energy-efficient demonstration crops at Wageningen University & Research (WUR) and Delphy Improvement Centre in Bleiswijk. Since December 2023, one section has been taken up by aubergines. “The annual heat input should amount to 12 m3/m2 natural gas equivalents at most,” Stijn Jochems of Delphy IC says. “This is made possible by the insulation from low-e glass, intensified screening and a heating strategy without a minimum pipe temperature. AVS-C also allows us to recover a lot of heat from the humid greenhouse air.”
Since 2018, several chrysanthemum and tomato crops have been grown in sections with AVS under full LED, and peppers have been grown without lighting for two years. Jochems is excited about the current trial at Delphy IC, in which he is working closely with Frank Kempkes, Project Lead from WUR. Financially this trial was made possible by the Kas als Energiebron (Greenhouse as an Energy Source) programme, which is supported by the Ministry of Agriculture, Nature and Food Quality and Glastuinbouw Nederland.
“Low-emission glass radiates less heat, which is of course beneficial,” notes Jochems. “The greenhouse stays warm longer and the roof doesn’t cool down as fast. This also means less moisture condenses on the inside of the roof. In order to remove moisture in a smart way – so, not through open vents, which would also cause you to lose a lot of heat and CO2 – you have to actively dehumidify the greenhouse air with air handling. We do that with an AVS-C system.”
AVS-C
AVS-C is an active air/water heat exchanger with a cooling block (C) for cooling down humid greenhouse air that is drawn in, and a central heating block for reheating the outgoing air flow. The required cold is supplied by a heat pump. The system cools the greenhouse air to below dew point, causing the water vapor that is present to condense and releasing the latent heat stored in it. This low-grade heat is removed and temporarily stored for later use.
12 m3/m2 gas per year at most
“In order to minimise the amount of fossil fuel used, we want to recover as much energy from the greenhouse as we can, as quickly as possible,” Jochems continues. “That works very well as long as the screen is closed. “On an annual basis, the external heat input should amount to 12 m3/m2 natural gas equivalents at most. This works out at half the heat consumption in the reference crop at Green Brothers, which is known as a highly energy-efficient company. The bar is therefore quite high, placing high demands on air handling technology.”
No humidity reduction with outside air
Unlike in previous trials with chrysanthemums, in which relatively dry, cool outside air was drawn in to reduce the humidity level, only greenhouse air is being recirculated in the aubergine trial. “The cooling block will have to work a bit harder, but we also recover more heat from that,” the researcher explains. “Our goal is to maximise this in order to meet a larger proportion of the heat requirement. If it isn’t needed immediately, we remove the heat and store it for a short or longer period of time. It’s only when we create real heat surpluses – harvesting more heat than necessary to heat the greenhouse – that it makes economic sense for us to use dehumidification with outside air.”
New roof
When asked about interim results and experiences, Jochems replies: “So far we have had to use slightly more external heat than planned. This is mainly due to the dark spring season we had, with a lot of rainy days and little solar radiation. In addition, there is a new roof on the greenhouse, which always leaks a bit. As a result, the greenhouse effectively ventilates continuously; we’re losing heat even when the vents and screen are closed, and the CO2 concentration gradually decreases at night instead of increasing. A small percentage of heat is bound to be lost through the cracks. That will be less in the next crop. However, we think heat consumption of 12 m3/m2 natural gas equivalents is still achievable.”
Top quality fruits
Both the researchers and the growers involved are pleased with the condition of the crop. “We’re getting top quality plants and fruits,” says Jochems resolutely. “Growers often maintain a minimum pipe temperature in the early morning to bring the fruits up to temperature before sunrise. We don’t do that, and it doesn’t seem to harm the fruits at all.”
Scoring in September
The researcher is very keen to get the crop through the summer in top condition. “September is a very important month for aubergine growers in the Netherlands,” he notes. “By then the European aubergine supply is low, which means that the fruits are generally more expensive than average. If your crop peaks then, you’re in good shape. It would be fantastic if our super-efficient crop fits into that picture, as well.”
Stijn Jochems: “The bar is high, placing high demands on the air handling technology.”