Making greenhouses more efficient: a novel technology with lenses made from plexiglas molding compounds
TEMPERATURE and lighting conditions in greenhouses need to be as uniform as possible for crops to prosper. For this reason, commercial greenhouse operators generally have to spend a great deal of money on heating, climate control, and shadowing.
The Dutch company Technokas has developed a solution: their Daylight Greenhouse not only consumes less resources than a traditional greenhouse—but it harvests energy as well. The technology also eliminates the need for an additional shading system.
“Conventional greenhouses cannot utilize the entire radiation energy of the sun. A significant portion of the provided energy is lost,” says Hans van Tilborgh, one of the three managing directors of Technokas, which has been planning and implementing greenhouse construction projects, climate control systems, and business facilities in the Netherlands for 26 years.
Experts know only a portion of the sun light is used by the plants to grow. “The first thing we thus asked ourselves was, 'How do we make use of the excess energy that a greenhouse absorbs but cannot otherwise utilize?'” Van Tilborgh recalls. The solution? A canopy that collects the direct sunlight and converts it to energy. The diffuse part of the light passes through the roofing and is made available for good plant growth. The Daylight Greenhouse idea was born.
More than a decade was required for the engineers at Technokas to develop the initial idea into series-production readiness. A unique roof construction consisting of well-insulated, double-glazed panels with embedded Fresnel lenses is the heart of the new generation of greenhouses. The lenses focus the sunlight onto a collector mounted on a 2-axis sun tracker, which in turn converts the light energy to thermal energy. “The concept wouldn't work without the interaction between the various components,” Van Tilborgh explains. “The harvested energy in terms of hot water is then stored and used for heating during night hours or winter months.”
In addition to the development of a movable solar collector that tracks the movement of the sun, another technical challenge was the design of the lenses, Van Tilborgh recalls. “The lens material had to be able to collect and focus the direct sunlight, without absorbing diffuse sunlight which is required by the plants.” After testing a few different materials, the clear choice was acrylic, particularly PLEXIGLAS Solar. Injection-molded lenses made from PLEXIGLAS are manufactured by the plastics specialists at Pekago Covering Technology. “We decided on PLEXIGLAS because it transmits light extremely well and offers long-term stability—especially compared to other plastics,” Van Tilborgh explains. PLEXIGLAS Solar is a specialty molding compound that has been modified to transmit the wavelengths of light that plants need for their growth, while at the same time offering even higher UV stability. “That means it will retain its high light transmittance for decades,” notes Peter Battenhausen, Senior Business Manager at Evonik. “Plus, PLEXIGLAS is capable of reproducing surfaces with tremendous precision, and without that we wouldn't even have been able to produce the highly precise, 1,25 mm prism structures.”
Proving its merit
The first Daylight Greenhouse, which covers an area of 4,000 square meters, was completed in the Netherlands in 2014 and is operated by Ter Laak Orchids. The company was so convinced that it immediately put another greenhouse in operation in the summer of 2018—more than ten times larger than the first. “In our new greenhouse we're saving 40 percent of our heating costs, and the greenhouse produces roughly half of the energy we need all by itself,” says Richard ter Laak, Managing director of Ter Laak Orchids. “Moreover, the climate in the greenhouse is more uniform, which means we lose fewer plants to disease and fungus.” The greenhouse functions without extra shadowing which is a huge benefit in the winter months, where up to 40 percent more diffuse sunlight finds its way to the plants. “That's perfect for our orchids,” says Ter Laak.
In addition to orchids, Daylight Greenhouses are also suitable for all other plants that like the shade. “That's especially common for a lot of houseplants, for example. As opposed to vegetables like tomatoes or cucumbers, which usually need as much sunlight as they can get,” Van Tilborgh says. Daylight Greenhouses can be used anywhere in the world, “but regions where it gets relatively cold at night really showcase their advantages.” In hot regions, however, the energy they collect could also be used for absorption cooling. A new generation of greenhouses for many uses—enabled by PLEXIGLAS molding compounds.
Strong plastics adhesion achieved without adhesives
POLYMERS containing plastics are essential in modern life. Being lightweight, strong and unreactive, a vast range of technologies depend on them. However, most polymers do not adhere naturally to other materials, so they need adhesives or corrosive chemical treatments to be attached to other materials. This is a problem in areas like food and medicine, where contamination must be avoided at all costs. A clean way to make industrial polymers adhesive is urgently needed. Now, a team at Osaka University has achieved just that. They have developed a suite of plasma treatments to allow vulcanized rubber and the plastic PTFE (polytetrafluoroethylene) to adhere to one another, or to other materials. The method activates the polymers' surface chemistry, as described in a study in Scientific Reports. “If you spray PTFE with a plasma of helium at 200 degrees, it can adhere to unvulcanized rubber – this is a technique we developed earlier in our lab,” says study lead author Yuji Ohkubo. “But vulcanized rubber presents a greater challenge. In our latest study, we customized a new plasma treatment for vulcanized silicone rubber, making it adhere strongly to PTFE for the first time.” The silicone in question was PDMS (polydimethylsiloxane), a well-known resin. While the key breakthrough in PTFE adhesion was the heat-assisted plasma treatment, the trick with PDMS is to bombard the surface with a plasma jet, by forcing nitrogen/air plasma through a small hole. The jet breaks the silicon-carbon bonds on the surface and converts them to silanol (Si-OH). Being more reactive than the original silicone surface, these silanol groups can bond with PTFE. Under high pressure, hydrogen bonds form between silanol and the oxygencontaining functional groups on the treated PTFE. Strong covalent bonds (C-O-Si, where C comes from PTFE and Si from silicone) further stitch the two polymers together, even with no adhesive.
Uniting the two materials allows each to enjoy the benefits of the other – the chemical resistance, dirt-repellent and slide-ability of PTFE, and the elasticity of silicone. Opaque PTFE can also be replaced by PFA (perfluoroalkoxy alkane) if transparency is needed. And that's not all – when the reverse side of the PDMS is also plasma-jetted, it can bond to copper and even glass. Like an extremely strong double-sided tape, this three-layer sandwich allows the fluoropolymers to adhere cleanly to other useful materials. “PDMS is widely used in medicine, for example in microfluidic chips,” explains co-author Katsuyoshi Endo. “There could be huge benefits in making both PTFE and PDMS more versatile for medical and food technologies through adhesive-free adhesion. Combined with the lack of any need for volatile chemicals, we hope our method will broaden the horizons for polymers in high technology.”
Alkali stress crack resistant PBT Resin for automotive applications launched by Polyplastics
THE Polyplastics Group has introduced a new polybutylene terephthalate (PBT) resin which delivers excellent alkali stress crack resistance for a range of automotive applications. The new grade, DURANEX532AR, also exhibits outstanding
- Hydrolysis and heat shock resistance and
- Electrical performance for components in the chassis and engine compartment.
Stress Cracking Resistance Properties in Alkali Environment
As the demand for durability, safety, and reliability in auto parts continues to increase, DURANEX 532AR has successfully reduced the risk of cracking in molded articles by preventing alkali from penetrating the inside of the resin, thus imparting toughness that helps generate less stress. This alkaline can arise from rust formation in metal parts.
DURANEX 532AR is treated with a hydrolysis-resistant formula and has greater durability than standard materials. Recent test evaluations show that when specimens are immersed in alkali, cracking occurs within two hours in:
- Standard material DURANEX 3300 and
- Hydrolysis- and heat shock-resistant DURANEX 531HS
While, no stress cracking occurs in DURANEX 532AR even when immersed for up to 200 hours.
Outstanding Heat Shock Resistance to Withstand Harsh Conditions
Electrical components and sensors installed near the engine are also often metal insert molded and there are cases when sudden temperature changes can cause heat shock fracture issues. This occurs because the metal and resin have linear expansion coefficients that differ by roughly a factor of 10. DURANEX 532AR also has outstanding heat shock resistance and is capable of withstanding these harsh environments. DURANEX 532AR is ideal for use in parts installed in the chassis section and lower areas of vehicles which can be splashed by water and mud and come into contact with chemicals such as snow melting agents. This grade offers a combination of high reliability and long life even in harsh environments. Further, the addition of Glass fibers dramatically boosts strength, rigidity, and heat-resistance, flame retardant allows for easy fireproofing.
Further information from: Polyplastics Marketing (India) Private Ltd. Mumbai. Tel:+91-22-6725-8661 www.polyplastics.com