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ambient temperature can be greatly lowered with no reduction in yield:
Textiles that heat roots of plants can lower energy costs in greenhouse
The textiles being developed needed to have a stable temperature profile, permit variable heating from about 5 0C to 40 0C, be structurally resilient and water-resistant and not degrade in soil.
The temperature around the roots is a crucial factor in germination (seeds), sprouting (bulbs), plant development (generative phase) and plant growth (vegetative phase). That is why, when these high-tech textiles are used in the greenhouse, they help save energy, because the ambient temperature can be greatly lowered with no reduction in yield. Furthermore, the heating elements can be used to control the vegetative phase very accurately to optimise the harvest. Last but not least, because the heating textiles provide safe and reliable frost protection, they enable more tender plants to be grown even in difficult climatic conditions.
When these high-tech textiles are used in the greenhouse, they help save energy, because the ambient temperature can be greatly lowered with no reduction in yield.
The research project, sponsored by the Central Innovation Programme for SMEs (ZIM), was based on an extensive list of specifications in order to meet the various requirements. Among other things, the textiles being developed needed to have a stable temperature profile, permit variable heating from about 5 0C to 40 0C, be structurally resilient and water-resistant and not degrade in soil (anti-fouling). To achieve these objectives, the project partners investigated the suitability of various knitted heating fabrics. The basic material for the fibres was always polyester, in once case galvanised with silver to make it conduct electricity. In a second material, conductivity was achieved by knitting a stainless steel yarn into the fabric. In the third test material, two stainless steel threads entwined in a criss-cross pattern provided the electrical conductor. The three fabrics were subjected to various textile technology tests at the Hohenstein Institute. These included not only mechanical stress testing but also tests to assess the resistance of the textile to destruction by microorganisms. Naturally, the Hohenstein researchers paid particular attention to analysing the heating capability of the textiles. This was judged from the germination behaviour of trial plants both in laboratory conditions and outdoors, in order to optimise the technical characteristics of the heating textile for use operationally.
Field trials in a greenhouse using the ebband- flow system after 14 weeks of heating Pelargoniums Left: heated, Right: unheated
On the basis of this work, in the winter of 2012/13 the researchers from the State College of Horticulture and Agriculture put the heating textiles into use in various experimental greenhouses. Alongside the functional analysis of germination behaviour and plant growth, the main focus of these practical tests was on possible negative influences, e.g. from plant fertilisers used in so-called ebb-and-flow hydroponics systems. In this kind of modern irrigation system in the greenhouse, the nutrient solution floods the containers on the benches and is then released again when the plants have absorbed sufficient fluid. During the first trials, the researchers had problems with salt deposits and corrosion of the heating textiles. The following winter, more experiments were carried out in real-life conditions. To alleviate the problems, the textiles had been specially modified: the researchers tested the suitability of coating films based on wax, latex, silicon and polyurethane. The polyurethane film proved to be the best at preventing the deposits of salts that caused the material to corrode.Plants with the benefit of "underfloor heating" grew significantly better than the comparison specimens, in all test conditions (laboratory, all-weather greenhouse, ebb-and-flow system).
The Hohenstein researchers paid particular attention to analysing the heating capability of the textiles. This was studied in relation to the germination behaviour of the trial plants.
The energy consumption was also measured. Depending on the temperature difference which needed to be overcome, ranging from 10 to 20 kelvin, this was from 30-80 kWh/m2 per week. For small to medium-sized areas in the greenhouse, this innovative heating textile is considerably more energy-efficient than ambient heating by conventional heating systems and therefore particularly advantageous for use during sowing or deliberate crop forcing. However, the scientists believe there is still room for improvement on the durability of the heating elements. All the different types of material suffered faults during their 14-week use in the greenhouse, as a result of damage caused by fertilisers. It is hoped that special protective encasings will help.
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