Air movement is crucial for Next Generation Growing

Ventilation and air circulation within protected cropping systems is one of the most important but often under-estimated areas of greenhouse design and operation. Traditionally it was simply a matter of opening the vents, but as commercial greenhouses have got larger (and higher) and management techniques have moved towards Next Generation Growing (NGG) and semi-closed environments, the complexities of air movement have also changed, writes Richard Crowhurst.

In fact controlling ventilation to improve crop growth lies at the heart of NGG recommendations. The GrowSave initiative from AHDB Horticulture and NFU Energy summarises the seven key tips for NGG as:

  1. Achieve an even temperature horizontally in your glasshouse, and ensure the right vertical temperature profile to help keep the plant in balance.
  2. Keep an active climate by promoting good air movement.
  3. Improve your humidity control, including accepting more moisture in the air under high radiation conditions.
  4. Use both vent sides to minimise average vent position and allow venting above closed screens.
  5. Reduce the radiative cooling effect on the plants by shielding them from clear skies at night.
  6. Maximise photosynthesis by reducing vents and improving CO2 uptake, misting rather than shading when conditions are too warm.
  7. Keep the plant in balance in consideration of the ratio of light sum achieved to 24-hour average air temperature.

It is easy to see the importance of ventilation, with two of these statements relating directly to ventilation and another three being controlled by the air movement within the greenhouse. GrowSave point out that, ‘Prior to the development of NGG techniques, conventional industry practice was to rely mainly on leeside venting to remove humidity and excess temperature from the glasshouse. However, above a closed screen, large leeside vent angles would be required to exhaust sufficient moisture through the screens.’ As a result, it is usually advised to use both windside and leeside vents, something that was found to provide climate benefits in the early stages of the NGG technique in Holland. Using windside ventilation enables growers to use much lower vent openings compared to leeside ventilation alone, and this creates greater resistance between the glasshouse climate and that outside. This results in more stable temperature and humidity in the glasshouse, as well as noticeably higher CO2 concentrations because less CO2 is lost to the outside.

Whether you are using NGG or traditional growing and venting techniques, you still need to ensure adequate air movement within the greenhouse, and this is where fans come in. “The aim of the fan layout design is to cause the whole body of air in the greenhouse to move and circulate,” explains Max Manning of Hortisystem UK. “All this air has a significant mass, and therefore inertia, and so it can take a while after switching on the fans before the air is circulating fully. It is therefore often better to leave fans running continuously during any critical phases of the crop than to just switch them on when a problem is seen. In any event, by running the fans continuously, issues of temperature and CO2 uniformity and of disease control will be mitigated or avoided altogether.”

“Keeping the air moving within the greenhouse is important to maintain that active climate that means that the conditions around the plant are conducive to growth and that transpiration can continue,” adds Jon Swain of NFU Energy. He points out that in most greenhouses the conditions around the plant are different from what the sensors in the measuring box are recording in terms of humidity and temperature because of the difficulties in measuring microclimatic effects at the leaf level. “It’s important to keep air moving to ensure that the conditions that you want and that you are seeing are actually the conditions that the plant wants and is seeing,” he points out. “By promoting active air movement you can make sure that the active climate around the plant exists as well as the general aerial environment.”

In their advice, GrowSave stress that, ‘Achieving an even glasshouse climate is the cornerstone of NGG. An even climate ensures crops grow and produce uniformly, while saving energy, as control is more appropriate for the whole environment, not just the worst areas of the glasshouse.’ The screen strategy, and in particular not gapping the screens, is one key part of achieving this, but using fans is another important tool to even out the climate by increasing air movement. ‘It also has the effect of keeping an active climate, which improves plant conditions,’ continue GrowSave. ‘An active climate is essential for nutrient uptake. Keeping the plant active will ensure that calcium, which is only transported by water in the plant and is essential for new cell development, reaches the growing tips. A key technique for ensuring an active plant is to create air movement around the important developmental areas.’ As modern greenhouses become increasingly well insulated, and screen gapping and pipe heat is not used to encourage upward air movement, the successful implementation of NGG relies on creating air movement, and this often means using fans.

Max Manning points out that circulation fans in greenhouses can have a number of different roles, the importance of which varies depending on circumstances and the particular requirements of the crop. “These roles can include maintaining uniform temperature and humidity conditions over the whole house, or correcting temperature differences between sides facing the sun and in shade or between internal and external sides for example,” he explains. “Fans can also correct a temperature difference due to a change in level across the structure, as well as improving heat distribution and raising CO2 levels by replacing depleted air.”

Another key role of fans is in reducing temperature stratification. “At the most basic level, heat rises and it is possible for the crop to require heating even when there is warm air at a higher level,” adds Max.

“It might be better sometimes to look at vertical air movement systems that actually stir the air up at ground level, especially for ornamental crops being grown in soil,” agrees Jon. “Moving air at speed tends to be less efficient and you don’t necessarily need air to be moved at speed. In fact sometimes that can be counterproductive as the plant only needs a small amount of airspeed across it. High volume, low speed, low pressure systems work well, but what’s really important is that if you have high level horizontal fans, make sure they are delivering the air movement that you want, otherwise you might just be blowing air around the top level of the greenhouse and not influencing the plant level and therefore not being energy efficient with your air movement.”

“Fans hung in the greenhouse and directed horizontally have long been the standard approach,” points out Max. “They offer the ability to move air around large areas at relatively low cost and if positioned correctly will give good results. Our Multifan Greenhouse Circulation Fans meet the latest EU Eco-design directives with high efficiency new generation fan motors. Even though the fans are primarily blowing horizontally, air is also drawn from the plant zone and mixed with that in the rest of the structure. Fans positioned at the right distance from the ends and corners will aid movement of air from these spots to minimise the chance of stagnant air in the structure.”

Vostermans Ventilation, who manufacturer the Multifan range, have also developed a vertical fan: the V-FloFan. “The Multifan V-FloFan draws air vertically upwards through the crop and then turns the airflow out and downwards thus reducing humidity in the crop zone and mixing the warmer air which accumulates at higher level,” explains Max. “An extra benefit is the redistribution of the heat generated by supplementary lighting. The design of the Multifan V-FloFan means that this mixing and effective humidity reduction can be achieved with relatively low airspeeds through the crop canopy.”

Vostermans have also been involved in Dutch research looking at allowing the screens to be left closed while a small fan introduces dry air from above the screen and introduces it with the output from a Multifan V-FloFan. In accordance with the principles of NGG, such a system gives the possibility of reducing energy use while providing an improved growing environment with reduced humidity.

If installing the right combination of horizontal and vertical fans in a glasshouse helps implement NGG techniques, can the same thing be done in a polytunnel? “Glasshouse ventilation has generally been vertical (through the roof) whereas polytunnels have historically relied on horizontal ventilation through the side walls,” points our Nigel Carr of Northern Polytunnels. “By using polytunnel roof fans we are now able to apply the benefits of both systems to polythene clad structures. When the two systems are employed together, warm air is exhausted through the roof whilst cool air is drawn-in through the sides.”

He points out that adding automatic control enables growers to get the most from such a system: “By setting the thermostat on the side vents a couple of degrees lower than the roof vents you allow the sides to open first. If there is no natural air movement from outside and the temperature continues to rise, then this causes the roof-mounted extraction fans kick-in.  Instead of relying on convection alone, heat is forced out of the structure through the roof fans creating a negative pressure inside, which then pulls cool air through the side vents.”