Glasshouse technology

The EU's latest energy-efficiency drive will impact fans being used in greenhouses, Sally Drury explains.

Fans: circulate air to reduce the difference in air temperature and moisture content
Fans: circulate air to reduce the difference in air temperature and moisture content

The Energy Related Product (ErP) Directive affects fans, including those used in greenhouses. The drive to improve energy-efficiency performance is being accelerated by new regulations that apply across the EU.

"ErP is being introduced in stages as part of the global commitment to reduce energy consumption. Stage one came into effect in 2013, with a further update in 2015 and more to follow in 2020," explains Hydor group technical director Alan Macklin. Established more than 50 years ago, Hydor is a specialist manufacturer and supplier of ventilation, heating and control equipment for horticulture.

Plants grown in greenhouses or other contained environments benefit from the movement and changing of air. "In much the same way as animals do, plants expel waste products during respiration that are toxic to themselves and these, along with the build-up of non-toxic by-products such as oxygen, reduce plants' ability to efficiently operate, so compromising production," explains Unigro development director Angus Padfield.

Reasons for venting

However, while these products must be exchanged, not least to help prevent disease, there are other reasons why we vent and move air. Many growers use air movement continually to help build a stronger and more physically resilient plant as the stems become broader as the canopy above is moved.

But the most common reason for increasing the movement of air in a growing environment is to enable the plants to have a more consistent thermal and moisture environment on a micro and macro scale.

"Despite the best efforts of glasshouse designers, the environments in production glasshouses are not ubiquitous," says Padfield. "Helpfully, the plants are able to combat the disparities by using their own systems of homeostasis to maintain the correct internal conditions, including temperature. The use of fans to circulate the air helps to reduce the differences in air temperature and moisture content."

Although it is simple enough for glasshouse managers to monitor air temperature and moisture content and for control systems of varying complexities to react to changes measured, historically such feedback systems have not simulated the plants' responses. This can mean that the plant is working hard to cool itself but for economic reasons, such as a cold weather forecast for the coming night, the grower - or his control system - may not be keen to vent the day's gained heat.

Just as animals loose heat to the surrounding air by sweating or panting, so a plant that is too warm will try to transpire to evaporate moisture from the stomata. Some of the energy required for this process will come from the leaf dropping its temperature. However, it may be the case that if the glasshouse air temperature is below that of the plant, then this required transpiration will become efficient enough to cool the plant if there is air movement over the leaf - the air movement ensuring a constant supply of cooler air to the open cells for this transpiration to occur.

Evapotranspiration efficiency

"By moving the air rather than venting it, the grower can retain the heat for the following colder night," Padfield confirms. "This evapotranspiration efficiency for a given location relies largely on a relationship between this difference in leaf-to-air temperature, air velocity and the air moisture content.

"As with animals, if the air moisture content increases, more energy is required to evaporate the moisture and remove the heat so despite increased air velocities this moisture needs to be removed from the environment. When plants are subjected to extremely high temperatures and low moisture contents, the moisture can be lost at an almost uncontrollable rate from fresh green plants and this can mean that plants require significant levels of minerals to replace these losses."

The relationship between moisture and air velocity is complex and can be difficult to manage correctly when drivers such as the solar gain and temperature increase can fluctuate significantly and build to unnatural levels in a protected environment. Additional moisture in an overheated environment can reduce the air temperature but excessive moisture that cannot be ventilated away will prevent the plant from being able to transpire and will cause long-term damage. This process is accelerated considerably by the use of fans and their uncontrolled use can cause significant damage.

Traditionally, fans creating additional air change rates have been controlled to the temperature within the crop, the fans increasing the effects of convection and venting the heat from the roof space. Growers may have left fans on throughout the year, continuously working to move air above the canopy to enable destratification. In winter this has the effect of mixing the air to spread heat that has gathered at a high level due to convection. In summer they operate to aid evapotranspiration.

Use of fan coil units

Today more commercial and research applications are turning to fan coil units because they are able to extract greater amounts of energy from a heat exchanger when high volumes of air are pulled across them. The fan coil is positioned to be able to disperse this heat around the house and is often used without the heating for destratification.

"The increased output from fan coil units means that while control is more accurate a far smaller area of heat exchangers is required for the same heating capacity and significant boiler operating efficiencies can be achieved," Padfield points out. "In some applications these same units can be used for cooling in addition to heating."

The ErP directive aims to ensure that, among other criteria, all motor and fan combinations sold within Europe meet the criteria to increase energy efficiency by 20 per cent. Fans are rated to take into account the motor and impeller efficiencies and should this figure, referred to as the N grade, fail to meet the minimum requirement under the regulation, then a CE mark will not be assigned to the product.

Future fan units and ventilation products will need to comply with the regulation. The only circumstance where a non-compliant product can be supplied is where spare parts are being provided for a fan that was originally shipped before 1 January 2015.

"In such a case it is possible to supply spare parts required to affect a repair," says Padfield. "The only exception is spare impellers, which cannot be supplied as non-compliant variants. If a replacement impeller is required for a fan supplied before 1 January 2015, then the customer should instead purchase a new and fully compliant fan."

Many manufacturers serving a variety of industries will only supply fans with integral speed-control devices to ensure their complete ranges comply with the regulation. Some manufacturers providing fans only may supply them without casings, complying to the ErP regulation by denoting the final efficiency when used with a speed-control device but not making that device part of the product.

For manufacturers, the new regulations have meant research and development followed by changes in production. That may sound alarm bells in terms of price, but Padfield suggests products will improve.

"Fan speed control can be achieved in a number of ways, but it will be increasingly common to purchase the manufacturer's proposed solution for the control and, in order to achieve the compliance, these controllers will be built into the motors," he says. "While the added cost of these controllers is significant compared to that of mass-produced fans, manufacturers have been using the opportunity to redesign equipment as fan and design technologies have improved."

Bringing down energy use

Both Padfield and Macklin believe the higher costs necessary to improve the energy efficiency of fan motors and impellers will be matched by reduced energy consumption.

"The major investments we are making to achieve a high level of energy-efficient fans will continue to benefit our horticulture customers ahead of the continual tightening of legislation," says Macklin. "Improved efficiencies will provide significant savings in electricity bills for growers, which will more than cover the additional costs of the new motors within a short payback period."

Padfield agrees. "Coupled with the correct control system, using variable speed fans for control in many applications will see a payback in terms of electricity in a very short period," he maintains.

Kit for ventilation and air movement

Roof vents to release excessive heat build-up without the need to open the structure's doors.

Side vents to release excessive heat and stale or humid air - usually in plastic structures.

Air-intake louvres are meant to be used in conjunction with ventilation or exhaust fans.

Extraction fans are designed to exhaust excessive heat from glasshouses and tunnels.

Ventilation fans are suitable for removing stale or humid air from large greenhouses and are usually used in association with air-intake louvres.

Air-recirculation fans provide fresh air and reduce temperatures in summer while also removing humidity and reducing condensation.

Air tubes and fans are positioned beneath the crop and designed to create a slow, vertical air movement up through the crop.

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