What can British growers learn from recent Dutch research on glasshouse sustainability?

From 2020 all new Dutch greenhouses must by climate- and carbon-neutral, which has thrown down a challenge to the country's horticulture research establishment.

Image: E Dronkert (CC BY 2.0)
Image: E Dronkert (CC BY 2.0)

The most recent Dutch greenhouse horticulture energy monitor, published in June, said the sector’s annual CO2 emission has decreased by 1.97 Mtonnes between 2010 and 2015, due to smaller area of cultivation, intensification of production and increased use of sustainable energy as well as lower energy use overall, which accounted for around 30% of the total decrease.

A separate overview report published earlier this month by Wageningen University & Research, the country’s main horticultural research establishment, looking at glasshouse heating within "a post-fossil energy infrastructure", said that to this end: "There are two types of primary energy sources, namely green electricity and residual heat/geothermal energy, each requiring other technical solutions depending on the type of cultivation."

It concluded: "If greenhouses are heated by electricity, then a heat pump in combination with heat-cold storage, is the best solution both energetically and economically." And it noted: "Energy systems in a fossil-free infrastructure are characterized by high fixed costs and low variable costs."

But it said that further investigation is required to determine the economic and energy-use case for the different options available in each form of crop cultivation. "For example, what can be expected of less vigorous varieties, lower-temperature varieties, better glasshouse insulation, moisture-proof screens, lighter glasshouses, efficient irrigation systems for non-substrate crops, low temperature heating systems, or behavioural change among growers?"

Meanwhile at the end of July the Dutch government announced it would invest a further €100m in what is called "not-yet-mature submarkets within the energy transition", covering 80% of the costs of research and development in geothermal energy, energy saving, energy storage and biomass.

The government-backed Glasshouse as Energy Source (Kas als Energiebron, KaE) research programme has already launched six new studies, together worth nearly €200,000, to address issues in harnessing geothermal energy, while Wageningen this week also released a geomorphological map of the Netherlands as open data, which it says marks a world-first.

An outcome of KaE research at Wageningen on dehumidification was also published this month. It calculates that dehumidifying a glasshouse by ventilation costs 8-10m3 of natural gas per m2 to compensate for the lost heat, and looks at options to reduce this.

Using heat exchangers or the Hinova VentilationJet system can reduce ventilation by 25%, while latent heat in the exhaust air can be used to heat the incoming outside air to glasshouse temperature. "Heat exchangers with 100% efficiency to do that are available", it says, while the heat latent in the water vapour leaving the greenhouse can also be recovered by means of a condensating wall, then stored in a water tank to use in heating pipes.

Another approach is to skip ventilation altogether and instead use a set of a cold and warm heat exchangers connected to a heat pump or to use a salt crystal moisture absorber, which "opens the possibility of using solar or wind energy to dehumidify the glasshouse", it says.

This "New Growing" (Het Nieuwe Telen) approach recently featured in a series of online promotions by the Dutch branch of discount retailer Lidl. In one, Venlo herb grower Thijs Gipmans explains the various sustainability features employed in production of fresh Basil for Lidl.

Glasshouse sector body LTO Glaskracht Nederland representative Stijn Laagland says: "The story is a very nice example of Het Nieuwe Telen in practice, and illustrates how the market embraces and uses the concept as a sales argument."

Other recent research has looked at more everyday tweaks to improve glasshouse energy use. Following on from similar work in tomato, an experiment by Wageningen researchers on high-wire cucumber last year compared yields from plants with 0%, 16% of 33% leaf removal in the head, in two planting densities. The success of this strategy was mixed, they found. "Leaf removal led to a slightly higher abortion, a lower development rate and therefore less axils, and a lower LAI [leaf area index] with less light interception, and therefore produced less fruits and kilos. This, however, can be compensated with a higher stem density. On the other hand, labour and the chance on too small fruits will increase," their report concludes.

"The recommendation for growers would be: remove some small leaves, combine this with a relatively high stem density, and save energy in winter" by using leaf removal to reduce the need to dehumidify, they advise.

Further recent Wageningen research on supplemental lighting for tomatoes has found that the application of red LED light at 30 µmol per m2 per second gave an 11% boost in yield. The trials on the Soranzo variety took place at grower Prominent’s Groeneweg Innovation Centre in the Westland region, where the lighting was combined with conventional high-pressure sodium lighting, and a comparison made of red LED as interlighting at different intensities and as top lighting, against a control without any such lighting.

"Growers wanted to know whether the same effect would be achieved with fewer red lamps and thus less electricity, and whether the lamps should be suspended above or in the crop," Wageningen researchers Anja Dieleman en Kees Weerheim said. But the lower-intensity LED trials gave a yield uplift of just 1-2%, they found. And while the tomatoes grown under higher intensity lighting had a superior flavour in January, this difference disappeared by the end of the supplemental lighting period in April.

Cleaner outflows

Dutch growers faced with a new law on discharge water from the start of next year are having to take measures to ensure such discharges from their operations are free from plant protection products.

In a joint project between two of the nine "top sectors" in the Dutch economy prioritised for research, horticulture and water, Wageningen has worked with water research body KWR to develop a means of achieving this using powdered activated carbon.

This highly porous form of carbon, already used commercially in a number of purification processes, is added to the discharge water, left a while and then filtered out again, after which the treated water can be discharged to the sewer system.

WUR project leader Jim van Ruijven says: "This method will be feasible for growers that only irregularly discharge small amounts of water, or will be strongly decreasing the amount of discharge water in the coming years under influence of the nitrogen emission standards."

A pilot installation is now being developed with industry partners to prove the technology in practice, with regulatory approval also being sought by the end of the year.

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