Greenhouse building is seeing something of a boom all over the world as higher crop prices fuel grower confidence along with the increasing demand for food. In Holland, millions of euros are being spent expanding greenhouse facilities while here in the UK a sixth greenhouse has been confirmed for Thanet Earth. Each will be fitted out with yieldand quality-enhancing technology along with energy-saving systems.
Today's greenhouses tend to be clad with glass, polyethylene or polycarbonate sheeting, but what will we be using in the future? We can already manipulate wavelengths entering a structure and perhaps we may see materials that can also generate energy to make protected cropping more sustainable.
We have access to a wide variety of coatings that can be applied to glazing and cladding materials. Some are used for summer shading. ReduSol, for instance, part of the ReduSystems product portfolio, is a shading agent that reflects high levels of solar energy and is intended to protect crops against excess heat and light. It provides up to 80% shading but also gives higher light transmission during rainfall. It can be used on glass, acrylate, polycarbonate and plastics films.
But there are also coatings such as ReduSystems' ReduHeat designed to enhance photosynthetically active radiation - the part of the spectrum that plants use - while reflecting most of the heat radiation. Then there are coatings such as ReduFuse that diffuse light as it enters the greenhouse, enabling it to penetrate deeper into the crop to improve production and quality.
There are also selective coatings that work like the filters in plastics films. ReduFlex Blue transmits less blue light than red so is useful for producing cut roses, while ReduFlex Green reduces green and yellow light in the house to lower plant and greenhouse temperatures.
Glass remains popular as a cladding material yet is far from perfect. For starters, transmission is not 100% even on the cleanest glass. Only part of the light will be transmitted through to the crop. Part of the sunlight is absorbed by the cladding and reflected back off normal greenhouse glass and never reaches the crop at all. The situation is made worse when we consider the angle of the light.
When light falls perpendicular to a glass roof, transmission rates can be 90%. But how often does that happen? Much of the light will come from other angles. The lower the angle, the lower will be the transmissions, while the rate of reflection goes up. This means that in north-west Europe reflection can be significant in winter when angles are at their lowest.
"A large part of the year, light is the limited factor for production under glass. More light is therefore very welcome," says Mardenkro national account manager Paul van Gils. "With fruits and light-loving ornamental plants the rule '1% more light equals 1% more production' counts."
When a new house is built, anti-reflective glass can be specified so the amount of reflection is reduced and light transmission through to the crop improved. But now there is a solution for existing glasshouses. The latest product from Mardenkro, the Holland-based manufacturer of ReduSystems products, is a coating called AntiReflect.
Van Gils says: "AntiReflect is a real breakthrough in the field of light transmission. It allows more light throughout the whole year and this leads not only to more production but also better quality. The extra light also provides a saving on energy due to more solar energy inside the house."
AntiReflect is the result of three years of research. It works because light changes direction when it passes from the air into glass, the extent of the directional change being determined by the refractive index of the material, which in turn determines reflection. The bigger the difference in the indices, the greater the reflection.
The refractive index of air is approximately 1, whereas for glass it is 1.5. It amounts to more than 4% of the light being reflected as it enters the glass and again as it exits on the other side. AntiReflect reduces the refractive index to 1.3, so reducing the reflection of all angles of incidence by at least 3%. Measurements taken at the Wageningen University LightLab in Holland show that light gain through an AntiReflect coating is highest at 60 degs - the angle of incidence particularly common in north-west Europe and Canada. The light gain delivered by AntiReflect is also found to be a transmission gain across the entire light spectrum.
The next step in the evolution of the greenhouse may be to see the cladding, rather than adjacent ground-fixed panels or turbines, generate energy that can be used for climate control inside the structure. Protected cropping, by controlling climate, water use and plant nutrition, enables out-of-season production on a relatively small area of land compared to traditional outdoor production. Nevertheless, greenhouses are expensive to build, maintain and run.
At the University of California's UC Riverside research facility, a greenhouse is being built with a translucent roof that generates solar power as the sunlight passes through to the crop. The technology, developed by UC Santa Cruz, involves a fluorescent dye and the theory behind it is that a portion of the light spectrum is used to make electricity, while the red light needed to grow plants is magnified as it passes through the panels.
The project sees land used simultaneously for power and crop production, with the research specifically looking at solar energy production, plant growth and the costs of growing food in such a system. Commercial growers at two sites in California and one in Canada are also trialling the panels. It could mean that one day crops will be grown in greenhouses without using an external mains supply or generator to power the lighting and fans. As well as providing climate-neutral greenhouse structures, perhaps the panels could be used on fields for crop production to exist at the same time as power generation.
In Australia, an energy-harvesting glass has been developed whereby the glass is embedded with nanoparticles that work to collect ultraviolet and infrared light from the sun and transfer the rays to solar cells on the edge of the panels. The technology, developed at Edith Cowan University's Electron Science Research Institute in Perth, allows 70% of visible light to pass through, enabling plant production.
Inside today's greenhouses, computers control the environment to ensure optimum conditions for growth, quality and yield. Engineering developments mean that ventilation can be supplied as and when required and in some cases the whole roof opened up, as in the case of Naturelight's greenhouse open-roof system and Rovero's Roll Air structure. Last year, after many years of development, the new Plantopia house was completed at Plantopia Nurseries in Lancashire.
The Plantopia All Climate Commercial Greenhouse allows all the benefits of an outdoor climate while affording the full protection of a greenhouse as required. "This means the greenhouse does not overheat. Plus the full light spectrum, including ultraviolet rays, provides nature's growth regulant," says Plantopia company director Chris Haley.
The structure has a fully automated roof consisting of aluminium construction with rigid, injection moulded polycarbonate panels that are electronically operated, complete with control system and capable of opening in under two minutes to allow in natural rain and sunshine.
Bringing the outside in by using opening greenhouses and manipulating light transmission by the use of nanotechnology are just two of the ways we may see greenhouse structures alter in the future. But perhaps, with pressures on land use, eventually we may also see production underground with LEDs being used to replace sunlight altogether.
Biomass case study: Bridge Greenhouses
Bridge Greenhouses has installed a 995kW Justsen Woodchip boiler at Roundstone Nurseries. Supplied by Core Biomass, it burns woodchip with higher moisture content than many others and is certified to burn high-grade waste wood if required. When ordered with a ceramic lining, the boiler can burn wood with moisture content up to 50%.
The boiler is inside the glasshouse and is fed by a Javo Toploader sitting on the road outside. It is protected by a polytunnel roof bolted to the frame, so overcoming the necessity of a new fuel store building. The Toploader provides hands-free fuel handling. Fuel is delivered in walking-floor trucks and then scraped up by the large rake within the Toploader, eliminating the need to move wood with a telehandler.
Boiler control is via the greenhouse computer to only produce the amount of heat required. If the boiler cannot supply sufficient heat in cold weather, gas pumps are automatically ramped up. Load on the biomass is always optimised, increasing running hours, saving on fossil fuel and boosting Renewable Heat Incentive income.
Firms offering renewable heating systems include Bridge Greenhouses (01243 641789), Ebtech Glasshouse Systems (01964 621726), Ecovision (01666 501580), Manco Energy (01430 828660), Rural Energy (020 3189 0654), Talbott's Biomass Energy Systems (01785 213366) and VG Energy (01563 829990).
Suppliers of commercial greenhouses in the UK include:
Bridge Greenhouses 01243 641789
Cambridge HOK 01430 449440
Debets Schalke 00 31 174 245321
Ebtech Glasshouses 01964 621726
Jaro Glasshouse Projects 00 31 165 346490
Keder Greenhouses 01386 49049
KG Greenhouses 00 31 105 121644
Naturelight Greenhouses 01282 863040
New Century 01262 608831
Olsthoorn 00 31 174 244344
Plantopia 01204 841811
SNRG Horticulture Structures (Rovero) 00 353 419 806676
- For details of suppliers of ReduSystems coatings and AntiReflect, call Mardenkro on 00 31 135 075399.