New Glass Coatings for High Insulating Greenhouses without Light Losses - Energy Saving, Crop Production and Economic Potentials

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 8283% for traditional single glass) and a k-value of 3.6 W m K (compared to7.6 W m K of a traditional single glass). Other double glasses were produced using a combination of anti-reflection and modern low-emission coatings, reaching an even lower k-value of ≈2.4 W m K, however, showing a slight light loss (78.5% for hemispherical (diffuse) light). Calculations of greenhouse climate (temperature, humidity, CO2) and energy consumptions year-round were carried out with a validated dynamic climate model. Additionally the effects on tomato production (dry matter) were calculated for the different prototypes of coated and insulated glass. Double materials show the highest energy saving with 25-33%, depending on the composition but also low-emission coatings on single glass decrease the energy use with 15-20%. Economic calculations with current tomato and energy prices showed that single and double glasses with anti-reflection coating currently have the highest potential. INTRODUCTION With 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, 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 a [email protected] Proc. IS on High Technology for Greenhouse Systems GreenSys2009 Ed.: M. Dorais Acta Hort. 893, ISHS 2011 218 as high energy savings year-round. MATERIALS AND METHODS Covering Materials In a pre-study several glass types were evaluated: greenhouse glass, greenhouse glass 90+, greenhouse glass low-iron. Glasses were covered with different anti-reflection coatings by three different producers: SA, CS and GG, applied by sputtering or etching. Double glasses were produced from all glasses; their optical properties were determined using modern light measurement equipment. The materials used are shown in Table 1 and Figure 1. In a follow-up study different prototypes of covering materials produced by GG were evaluated in order to study their energy saving potential, and their plant performance. Glasses were covered with an anti-reflection coating having partly near infrared (NIR) reflective properties, others were combined with a low-emission coating for a higher NIR-reflection. All single glasses had a thickness of 4 mm, double glasses had a distance of 8 mm (split). The materials used are shown in Table 2 and Figure 2. Optical Properties The optical properties of the glasses described above were determined at Wageningen UR Greenhouse Horticulture laboratory in The Netherlands. The total light transmission in the PAR range (τPAR in 400-700 nm) of samples with the size of 50×50 cm was measured with a large and a small integrating sphere (port opening 40× 40 cm or 8×8 cm). Data were gathered by means of a diode-array spectrophotometer with a resolution of 1 nm. The PAR transmission for perpendicular light (τPAR p) and the PAR transmission for hemispherical (diffuse) light (τPAR h) were determined following NEN 2675. The total solar spectrum (300-2500 nm) was measured on a Perkin Elmer spectrophotometer. The emission coefficient was determined following EN12898. All relevant data are shown in Table 1 and 2 and Figures 1 and 2. From measured optical data the amount of PAR energy (400-700 nm) and the amount of NIR energy (7002500 nm) entering the greenhouse was calculated. For a clear sky the radiation energy per nanometer wavelength is defined by CIE 85 (1989). Multiplying the global radiation per wavelength (or spectral range) with the measured spectral transmission of a covering material gives the fraction of the energy entering through the material into the greenhouse. Dynamic Climate Model Model calculations of greenhouse climate and energy consumption were carried out with the KASPRO model developed by de Zwart (1996). The dynamic simulation model KASPRO can simulate a full-scale virtual greenhouse based on the construction elements, greenhouse equipment, different covering materials and their properties (transmission, reflection, emission), set points for inside climate and the outside climate of a given location. Output are several climate parameters, such as air temperature, relative humidity, CO2-concentration and energy consumption. The model is based on the computation of relevant heat and mass balances (Bot, 1983). The heat balances describe both the convective and radiative processes. The mass balances are constituted from exchange processes through leakage and ventilation (de Jong, 1990). They include canopy transpiration (Stanghellini, 1987) and condensation at cold surfaces. The mass balances around the CO2-concentration are based on losses of CO2 by ventilation and photosynthesis, and gains of CO2 by dosing and respiration. Greenhouse climate is controlled by a replica of commercially available climate controllers. A standard Venlo glass-greenhouse with a trellis bar of 9.6 m carrying two roofs of 4.8 m is assumed with a distance between two trellis of 5 m for all calculations. Three glass panes of 1.675 m are in between two trellis bars. A standard energy screen is installed inside the greenhouse. The total set of differential equations is solved numerically (de Zwart, 1996). Tomato is chosen as model crop. Plant datum is 8 December, last harvest takes place on 25