More light in the period October through March. That is the objective of the Winterlight greenhouse in the Netherlands, which was officially opened by Secretary of State Martijn van Dam.
In winter, product prices are generally high, so that during this period, a large production is important. But precisely in wintertime the natural light is the limiting factor for growth and production. The "Winterlight greenhouse" therefore is a new greenhouse concept with maximum light transmission during the winter period, possibly at the expense of the summer. The concept consists of a number of innovations with respect to greenhouse cover, cover materials, greenhouse structure and the screen to be able to improve the overall yield of the natural winter light by more than 10% for the crop in the period October through March.
Energy is saved by making use of additional sunlight.
In the first phase of the project, a ray-tracing model has been used to calculate many greenhouse concepts in respect of slope of the roof, orientation, asymmetry, arched covers, Anti-Reflective (AR) coatings and selection of materials for the greenhouse structure. AR-coatings have been an indispensable aid in order to obtain an as high as possible transmission of a material. Most coatings increase transmission at low angles, but in winter high angles of incidence (relative to perpendicular) are important, even when taking a reduced transmission at low angles into account.
Based on the calculated transmission values and hourly radiation (direct and diffuse) the daily sums of natural light in winter are calculated. Conclusions were, among other things, that tunnel greenhouses (both convex and concave) are in respect of light transmission very unfavorable. For a traditional Venlo roof an East-West gutter direction is more favorable than a North-South direction. Here, a roof pitch of between 20° and 25° is optimal, both for clear and diffuse glass. It is remarkable that an asymmetrical sawtooth roof cover has more light gain with clear glass but not with diffuse glass. And by designing an AR coating at high angles of incidence theoretically a 7% light gain can be achieved. The production of such a coating still remains a challenge. In the overall concept the influence of the glass, clear or diffuse, and the degree of dispersion of light, have a substantial effect on the end result. Because the greenhouse structure is responsible for at least about 8% of light interception, reflection and dispersion of light of the materials has been looked into. By making the construction highly reflective a light gain of about 6% can be achieved.
Also condensation can strongly influence the light transmission of glass (+2 to -5%), but still little is known about it. In Bleiswijk a test arrangement has been constructed to measure the level of light directly and continuously of different types of (diffuse) glass in dry and wet condition. The results will be included in the final design of the winterlight greenhouse.
Before the greenhouse is built, a validation of computational results is needed. To facilitate this, a few scale models have been built from which the transmission in open field and over a longer period have been measured and have been compared with standard calculations. Preliminary results show a good match between calculated and measured results and this gives confidence in the methodology in order to achieve an integrated design of cover and materials with the highest light sum. The greenhouse will be designed and realized by a consortium in the IDC Energy in Bleiswijk. This greenhouse will be used for a cucumber variety that is most suitable for winter cultivation, achieving at least a 10% more efficient use of the available natural light.
This project is funded by the program Greenhouse as source of Energy, the innovation and action program of LTO Glaskracht Netherlands and the Ministry of Economic Affairs (EZ). The implementation will take place in cooperation with BOM greenhouse construction, Glascom, Bayer CropSience and Ludvig Svensson.
Source: Kas als Energiebron