At high irradiation levels, diffuse greenhouse coverings result in better light distribution, lower crop temperature, decreased transpiration, and increased photosynthesis and growth. Various greenhouse coverings (plastic films, glass panes and temporal coatings) can be used to transform direct light into diffuse light. However, light diffusing properties of materials are hardly known. Optical properties of a wide range of materials are being currently investigated, including direct light transmission under different angles of incidence, transmission for hemispherical light and haze. The potential of diffusing greenhouse covering materials is estimated by analyzing the global radiation data for different climatic regions: marine winter climate (The Netherlands), Mediterranean mild winter climate (Italy) and semi-arid climate (Arizona). The required optical properties differ for the various climates. With modern material technology, the optical properties (haze and light transmission) can be altered to meet the requirements for different climatic regions in order to optimize crop performance in the future.

More than 90% of the Dutch greenhouse area is covered with single glass. Energy losses through the covering are high during the heating period (winter) but energy requirements are also high during the cooling period (summer) in the case of semi-closed greenhouses. Until now, light losses of insulating coverings prevented growers from using double glass or plastic film. However, increasing energy prices allow new developments. Wageningen UR Greenhouse Horticulture studied the possibilities to use modern glass coatings to increase light transmission and save energy. Several glass types (standard glass, 90+ glass, low-iron glass) were covered with different anti-reflection coatings from different producers. Double glasses were produced; their optical properties were determined. It was possible to produce double glasses with new coatings having a higher light transmission than traditional single greenhouse glass (83-85% for hemispherical (diffuse) light, compared to 82-83% for traditional single glass) and a k-value of 3.6 W m INTRODUCTIONWith increasing energy prices the need for energy saving is high in horticulture. The energy saving potential of double layered covering materials for greenhouse applications have been pointed out in many research studies before (e.g., Zhang et al., 1996; Andersson and Nielsen, 2000; Bot, 2001; Villeneuve et al., 2005). However, until now suitable greenhouse covering materials combining both a high transmission and a high insulation value for greenhouse applications are missing. Though many studies focussed on the development of modern materials in order to save energy and/or achieve a better cooling of greenhouses (e.g., Swinkels et al., 2001; Waaijenberg et al., 2004; Hemming et al., 2006 Hemming et al., , 2007, the optimum combination of materials' properties is still not found. Since more than 90% of the Dutch greenhouse area is covered with single glass, energy losses through the covering are high during the heating period (winter) but also during the cooling period (summer) in semi-closed greenhouses. This research will show the future potentials of recently developed glass coatings (anti-reflection and lowemission) for single and double materials in order to have a high crop production as well

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De doelstellingen van het project waren het kwantifi ceren van in kassen gebruikte scherm eigenschappen (emissie en transmissie voor warmtestraling, luchtdoorlaatbaarheid en vochttransport) en de bepaling van de totale energiebesparing onder gedefi nieerde omstandigheden om de prestaties van verschillende schermen (en leveranciers) met elkaar te kunnen vergelijken. Dit helpt telers om meer informatie te krijgen over relevante scherm eigenschappen en stelt hen in staat om een weloverwogen keuze voor een investering te doen. De resultaten tonen aan dat ondoorlatende schermen en schermen met lage emissie en transmissie voor warmtestraling de hoogste energiebesparing geven. Doorlaatbare schermen geven de hoogste vochtafvoer en de laagste luchtvochtigheid tijdens scherm gebruik zonder de noodzaak voor extra mechanische ontvochtiging. AbstractThe objectives of the project were the quantifi cation of the greenhouse screen properties (emissivity and transmissivity for thermal infrared radiation, air permeability and humidity transport) and the determination of the total energy saving under defi ned conditions in order to be able to compare the performance of different screens (and suppliers) with each other. This helps growers to understand more about screen properties and allows them to make an informed choice of investment. The results show that impermeable screens and screens with low emissivity and low transmissivity for thermal infrared radiation give highest energy saving. Permeable screens give highest transport for humidity and lowest air humidity during screen usage without the need for additional mechanical dehumidifi cation.

Nets are commonly used for agricultural applications. However, only little is known about the radiometric properties of net types and how to influence them. In order to investigate the influence of net construction parameters on their radiometric properties, a set of radiometric tests were performed on 45 types of agricultural nets. Laboratory tests on large size net samples was performed using a large and a small integrating sphere. Open field radiometric test were carried out by means of an experimental set up (120x120x50 cm) and a full scale shade house. Small differences (less than 5%) occurred between laboratory and open field tests. Results highlighted that the porosity and the mesh size, combined with the colour and secondarily, with the fabric and the kind of threads of the net influenced the shading performance of the net. The colour influenced the spectral distribution of the radiation passing through the net absorbing its complementary colours. Since nets are three-dimensional structures the transmissivity of direct light under different angles of incident of solar radiation changes when installed in the warp or weft direction. Transmissivity could be considered one of the main parameters involved in the agronomic performances of the netting system

Model calculations and the few data that are available show that over 100 L water condense yearly on each square meter of a greenhouse cover. It is known that the presence of condensate reduces light transmission. This effect is suppressed to some extent by adding film-forming (anti-drop) additives to plastic film covers and surface structures or coatings on hard cover materials. There is a need, therefore to assess the effect of the surface treatment on the loss of light. On the other hand, condensation releases the energy that was used for evaporation, thereby warming-up the cover and somewhat decreasing the heating requirement of the greenhouse. The amount of condensation energy that is recovered may be expected to depend on the external and internal climate conditions. In this work we analysed the effect of condensation on light transmission and energy budget of a greenhouse, with seven different cover materials. Various internal vs external conditions were created by placing the model greenhouse (about 34 m) in a large climate chamber. Each experiment was repeated for two temperature differences between inside and outside (10 and 20°C) and two air movements in the greenhouse (7.5 and 15 cm s -1 ). Light transmissivity was reduced by 9% on average, with large differences among materials. Anti-drop coatings did suppress this effect, as did a surface structure meant to increase light diffusivity of the material. As far as energy is concerned, the overall heat transfer coefficient (U-value) of the greenhouse increased by an average of 16% (single layers) or 12% (double layer covers) when wet. Obviously there was an effect of the temperature difference on the U-value, which was found to be consistent with the heat transfer theory, whereas little effect was found of the air movement within the house. INTRODUCTIONGreenhouse covers are often wet inside from condensate, which has consequences for both the light transmission and the heat transfer. Calculations with the KASPRO simulation program (De Zwart, 1996), shows that a typical greenhouse cover is [partly] wet about 50% of the time and that total condensation amounts to some 100 litres water per square meter of the cover per year. Such an amount is confirmed by data from Van der Staaij and Douwes (1996) who collected condensate from a glasshouse for a number of weeks distributed in one year. A uniform water layer adherent to the cover may increase light transmission by a few per cent (Gbiorczic, 2003), whereas scattering and reflection on the surface of droplets will always decrease light (Pieters et al., 1997; Pieters, 2000, 2002a, b). The influence of condensate on the light transmission of the cover depends on the shape and size of the water droplets that are formed, which, in turn, depend−among others−on the surface properties of the cover (Von Zabeltitz, 1987;Jaffrin and Morisot, 1994).Condensation is also an important energy conveyor in the greenhouse, since the energy that was used for crop transpiration (the latent energy of vaporisation) is r...

ReferaatHet intensief gebruik van schermen vormt een belangrijk onderdeel van Het Nieuwe Telen. Het biedt grote mogelijkheden voor energiebesparing en leidt tot een veel homogenere temperatuurverdeling in het gewas.Vooral de temperatuur bovenin het gewas komt dichter bij de kasluchttemperatuur te liggen en daardoor wordt de kans op natslag kleiner.Om deze effecten in beeld te brengen is een rekenmodel ontwikkeld dat het effect van schermen op de temperatuurverdeling in het gewas laat zien. Het werk aan dit model is gefinancierd door de toeleverende industrie, verenigd in de Club van 100, en door het onderzoeksprogramma Kas Als Energiebron. Dit onderzoeksprogramma is een samenwerkingsverband tussen het ministerie van Economische zaken en LTOGlaskracht Nederland.De temperatuuropbouw in het gewas wordt sterk bepaald door de uitstraling en daarom heeft dit rekenmodel de naam Uitstralingsmonitor gekregen.Vergelijking van de rekenresultaten met metingen in de kas laten zien dat het model de netto uitstraling goed berekent en de temperatuuropbouw in het gewas goed voorspelt. Vooral dat het effect van het schermgebruik komt goed naar voren. Het model heeft zijn weg dan ook al gevonden naar de cursussen voor Het Nieuwe Telen. AbstractIntensive use of screens is an important part of what's called the 'Next Generation Greenhouse Cultivation'. It offers great potential for energy saving and gives a much more homogeneous temperature distribution in the crop. Especially the temperature at the top of the crop will be closer to the air temperature, and therefore the risk of condensation will decrease.To illustrate these effects, a calculation model has been developed that shows the effect of screens on the temperature distribution in the crop. The work on this model has been funded by the horticultural supply industry, united in the 'Club van 100', and through the research program 'Kas Als Energiebron'. This research program is a cooperation between the Ministry of Economic Affairs and LTO-Glaskracht Nederland.The vertical temperature distribution in the crop is strongly influenced by the longwave radiation and therefore model has been named the 'Radiation Monitor'.Comparison of the calculated values with measurements in a greenhouse shows that the model accurately calculates the net radiation from the crop and nicely predicts the vertical temperature distribution. Especially the effect of the screen application is convincingly simulated.