Winter light greenhouse project: maximum light transmission at the expense of the summer

In winter, product prices are generally high, making high production levels during this period vital. But in this season, natural light (or the lack thereof) is the limiting factor for growth and production. The "Winter light 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 the greenhouse roof, greenhouse cover, construction and screen to improve the overall yield by more than 10%. By making use of extra sunlight, energy is saved as well.

In the first phase of the project, using a ray-tracing model, a large number of greenhouse concepts were calculated. Roof slope, orientation, asymmetry, arc-shaped cover, anti-reflective (AR) coatings, and material selection of the greenhouse construction were all carefully considered. AR-coatings have been an essential tool in order to obtain an as-high-as-possible transmission of a given material. Most of the coatings increase the transmission at low angles, but in the winter, high angles of incoming light (relative to perpendicular) are important.

On the basis of the calculated transmission values and hourly radiation (direct and diffuse), the daily sums of natural light were calculated for the winter period. Conclusions state that tunnel greenhouses (both convex and concave) proof very unfavorable in relation to light. For a traditional Venlo deck, an east-west-positioned gutter (as opposed to a north-south one) is to be preferred. Here, a slope of between 20 and 25° can be considered optimal, both in clear and diffuse glass. It is striking, however, that an asymmetrical ‘saw tooth’ roof slope yields additional light with clear glass, but not with diffuse glass. And by designing a high-angle AR coating, 7% light gain can be achieved in theory.

The production of such a coating is still a challenge. In the overall concept, the influence of the glass, clear or diffuse, and the degree of light scattering on the end result large are large. As the greenhouse structure takes up at least 8% of light interception, designers looked at the reflection and light-scattering of the materials used. 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%), though little is as yet known about this issue. In Bleiswijk, a test installation was erected in which the light level is measured permanently under different types of (diffuse) glass, in dry and wet state. The results will be incorporated into the final design of the Winter Light Greenhouse.

This project is funded through the program Greenhouse as Energy Source, the innovation program of LTO Glaskracht Netherlands and the Ministry of Economic Affairs (EZ). The performance takes place in conjunction with BOM kassenbouw, Glascom, Bayer CropSience and Ludvig Svensson.

Source: Energiek2020