Reducing energy use - glasshouse innovations

European researchers are finding innovative ways to cut energy use in glasshouses, Gavin McEwan reports.

Solar collector - cold water pipes draw off extra heat in summer to be recirculated - image: HW
Solar collector - cold water pipes draw off extra heat in summer to be recirculated - image: HW

Both German and Dutch governments have invested heavily in recent years in projects to demonstrate lower energy use in glasshouse horticulture and several of these were showcased at last month's IPM show in Essen, Germany.

The German Committee for Technology & Structures in Agriculture, or KTBL, chief executive Dr Heinrich de Baey-Ernsten explained the importance of the topic. "Despite a slight decline in energy prices over the past two years, price rises are inevitable in the future," he said. "Horticultural businesses must make every effort to cut their energy costs and so secure their future."

The array of prestigious institutions involved in projects in the two countries and the resources made available to them dwarf any research elsewhere in Europe. Among them, ZINEG, a five-year collaboration between Berlin's Humboldt University, the universities of Munich and Hannover and other research and technology bodies, has a budget of £5m.

Leading the movement

So-called closed glasshouses, which harvest excess warmth for later use, have for some time been in the vanguard of moves towards carbon-neutral growing and Humboldt University is attempting to refine the system with its Solar Collector Glasshouse, which began operations in May last year.

Adjacent to the 300sq m by 6m-high closed Venlo-type glasshouse stands an equal-sized and similarly equipped conventional glasshouse growing the same tomato crop to serve as a control.

The collector glasshouse has a network of water-filled cooling pipes under the roof to draw off excess daytime heat. The warmed water is then stored in the adjacent 300cu m storage tank, then recirculated to provide heating when later required.

According to project leader Professor Uwe Schmidt: "So far we have only used a conventional rainwater tank - the next stage will be to insulate it. There are other questions to resolve, such as the optimal placing of the pipes."

He adds: "An innovative aspect is the integration of plants. Through plant transpiration, a lot of energy is liberated in the form of water vapour. So the temperature of the plants goes down and the air is cooled by the leaf surfaces. In a closed glasshouse, the energy can be drawn off and stored using a heat pump." This yields a surprising amount of energy - 120MW/h between May and October last year.

To control the plants' transpiration function and to minimise stress on them, so-called phytomonitors have been deployed to detect and process physiological signals from the plants. "The research is as much about incorporating these signals into the glasshouse's control systems as about the minimal input of energy," says Schmidt.

Fellow researcher Ingo Schuch adds: "In a closed glasshouse system like this you get a lot of humidity caused by the plants' transpiration - 60 per cent of the irrigation water comes back that way. So you also need to dehumidify."

But even with higher humidity levels than the control, the tomato yield has remained comparable, the team has found. "We have had a lot of visits to the glasshouse," says Schmidt. "But it will take four to five years to work out the commercial value."

Government funding

In the Netherlands, the government - together with the Horticultural Production Board - has also been investing in energy-saving technology as part of its ongoing Kas Als Energiebron (Glasshouse as Energy Source) programme. Wageningen University & Research Centre (WUR) has used funding from this to run two projects at its Innovation & Demo Centre (IDC) in Bleiswijk, near Rotterdam.

Firstly, the Venlow Energy Kas (glasshouse) harnesses glass technology developments to reduce energy use. Twin panes are coated on three sides with an anti-reflective coating, giving relatively high transparency, while the fourth is treated to give a low emission rating (a "U-Value", or heat transfer coefficient, of 1.1), to keep heat in.

The glass was developed by Dutch firm Scheuten, which happens to be based in the town of Venlo that gave its name to the classic glasshouse design with its "zig-zag" profile roof. Though just 3mm thick, the Venlo glass is toughened, allowing panes of up to 3.5x1.6m, though the extra weight has entailed a stronger roofing framework.

The recurring problem of excess humidity in the closed glasshouse is overcome by using air pumps from Climeco Engineering. These "blow in dry outside air - otherwise the humidity is too high", explains WUR greenhouse technology research team leader Dr Silke Hemming.

Though only one year into the two-and-a-half-year project, already the team is claiming a 50 per cent energy saving on tomatoes and a 65 saving on cucumbers. This marks an improvement on WUR's previous Flow Deck glasshouse format, of which fellow researcher Frank Kempkes reports: "The results in terms of energy savings were impressive but disappointing in terms of crop production."

Specialised technology

A second, one-and-a-half-year project at IDC will require construction of another 500sq m glasshouse, expanding on a successful 40sq m prototype at WUR's headquarters.

Both are described as Fresnel (pronounced "freNELL") glasshouses because of the optical technology used in the roof. Originally and still used in lighthouses, the Fresnel lens is named after its 18th century French inventor and has a partly serrated cross-section, giving it optical characteristics of a much bulkier standard lens that could not be mounted on roof panels.

Made from a moulded plastic sheet, the lenses are sandwiched between panes of anti-reflective glass on the south-facing roof sections. This concentrates incoming solar radiation onto a single strip, where a moveable water pipe is positioned, arrayed with photovoltaic cells.

Its position is automatically adjusted according to the sun's trajectory to keep it at the focus of the beam. "The water in the pipe is heated to 60-80 degsC and can then be stored underground to provide heat in winter," says Hemming.

"You can export the surplus energy, which would otherwise be wasted, to the grid. But it still allows diffuse light, on occasions such as a cloudy day, into the greenhouse. Or the receptor can be moved out of the way entirely." The format has already been used in houses growing six different ornamental crops with a range of lighting requirements, including Phalaenopsis and Dracaena.

According to lead researcher Dr Piet Sonneveld: "The ambition is a greenhouse that produces at least 35kW/h of electricity per square metre and 240kW of heat per square metre annually. The horticultural sector is showing great interest in this type of greenhouse with such an installation - growers as well as architects see perspectives in the technology." Hemming adds: "The aim with all this is to have a five-year payback time. Otherwise, there's no point."

LIGHTING UP THE FUTURE

Making its first appearance at IPM, Dutch electronics giant Philips signalled that LED technology was moving into mainstream horticulture production. It reported that it had installed lighting systems at 27 growers in the Netherlands alone.

"At the show we have had a lot of interest from international growers," said Philips marketing director Udo van Slooten. "We are now making this commercially available, but we never just offer the product without looking at the requirements of the individual grower."

Danish Kalanchoe grower Knud Jepsen, which markets under the Queen brand, installed Philips GreenPower LEDs last year underneath its mobile benches to provide lighting to plants as they were being moved around the 10ha glasshouse complex. Now, their growth can be controlled during this phase by the use of specific light frequencies.

According to managing director Frands Jepsen: "The results I have seen convince me that it is possible to grow plants with LEDs as the sole source of light and I am beginning to think that I might have built my last greenhouse.

"The future will provide other means of expansion and, as the LED light becomes cheaper and energy becomes more expensive, this day draws closer."


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