IRRADIATION ASPECTS FOR ENERGY BALANCE IN GREENHOUSES

In this paper we present irradiation aspects for the use of glass type fresnel lenses instead of typical glass or plastic covering materials of greenhouses. They can be mounted stationary on the greenhouse roof, combined with linear absorbers to receive and convert the concentrated solar radiation into heat, electricity or both. The absorbers are of small width, depending on the selected concentration ratio and are properly moved to truck the concentrated beam solar radiation, with the diffuse solar radiation not received by the absorber. The advantage of the linear fresnel lenses to separate the direct from the diffuse solar radiation makes them suitable for lighting and temperature control of the greenhouse interior space, providing light of suitable intensity level and without sharp contrasts. The incident beam solar radiation is concentrated on the absorber and can be taken away from the glazed space, achieving lower illumination level and avoiding the overheating of it. In low intensity solar radiation, the absorber can be out of focus leaving the light to come in the interior space and keep the irradiation at an acceptable level for the cultivated plants. The distribution of solar radiation on the focal plane of linear fresnel lenses and the collected solar radiation regarding absorber width and incidence angle of solar radiation are given. The study includes also design concepts for thermal and photovoltaic absorbers, emphasised to hybrid photovoltaic/thermal type linear absorbers, which convert solar radiation simultaneously into electricity and heat. INTRODUCTION Greenhouses aim to provide ideal conditions for plant growing and production throughout the year, avoiding the non-favorable weather conditions, as the low ambient temperature, the low solar radiation intensity and the high wind velocity. The genotype and the environment have a very important role in the growth and the efficiency of the cultivated plants in the greenhouses and the most restrictive factor is usually the most important one. The environment factors (irradiation, temperature, humidity, etc) define the growth of the plants because the change of the genotype is difficult and demands a lot of time. The controlling of the environmental factors in the greenhouse is very difficult, not because there are many factors simultaneous inserted, but because each one of these factors is altered unlimited and there is a continuous interaction among all of them. Each of the factors affects the plant growth individually or in combination, while the demands of the plant change continually according to its age and the existing conditions. Among them the irradiation of the greenhouse is the basic factor for the photosynthesis and plant growing and the covering materials are of significance for the transmitted amount and spectrum of the incident solar radiation. The greenhouses are studied regarding the properties and the performance of the covering materials. Among the recent works we can refer the results for the influence of covering material and shading on the spectral distribution of light in greenhouses (Kittas et al, 1999) and the review for the radiometric and thermal properties of the greenhouse covering materials, including testing methods. (Papadakis et al., 2000). Other works that can be mentioned are the measuring unit and the results for the radiation transmittance of dry and wet greenhouse cladding materials (Pollet and Pieters, 2000a) and also for a complete condensation cycle (Pollet and Pieters, 2000b). Regarding the plastic covering materials the studies on the degradation and stabilization of low-density polyethylene Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005 734 films (Dilara and Briassoulis, 2000) and the zigzag transparent plates in comparison with glass covers (Swinkels et al., 2001) give interesting results. The environmental management for plastic greenhouses in northern Greece was modeled (Spanomitsios, 2001) and results from four shade cloths for the temperature reduction in greenhouses were given (Willits, 2001). A radiometric device for a quick analysis of the photochemical reflectance index was suggested (Methy, 2000) and also overview for the factors that influence the greenhouse design for European countries were presented (Van Elsner et al., 2000a; 2000b). Several methods have been proposed for the control of the environmental factors in greenhouses depending on the climatic conditions of the application and the technical structure details of the greenhouse. Among the published works in these topics are the important factor of illumination, studying the solar radiation transmissivity by using a greenhouse scale model (Papadakis et al, 1998), the performance studies for an earth air tunnel cum greenhouse technology (Tiwari et al., 1998), the comparison of pipe and air heating methods for greenhouses (Teitel et al., 1999) and the validated model for predicting greenhouse air temperature (Trigui et al., 2001). Considering ventilation, measurements for ventilation rates in a greenhouse, by tracer gas techniques, are presented (Baptista et al, 1999), and results for natural ventilation in a tunnel greenhouse using laboratory-scale models are given (Oca et al., 1999). Recently, large scale greenhouses were measured and analyzed for natural ventilation and microclimatic performance (Demrati et al., 2001) and, investigations on the existed differences in the microclimate created by the type of ventilation regime were presented (Kittas et al., 2001). Daylight is an essential plant growth factor and greenhouses have to be built with light translucent covers in the most effective way depending on the daily and seasonal needs. Among the used materials for covering the greenhouses, glass is the most stable material with satisfactory optical and thermal properties. Plastics are cheaper than glass, but most of them are of lower performance regarding irradiation and thermal properties. An alternative transparent cover to the usual glass panes for greenhouses is the glass type fresnel lenses. The use of fresnel lenses as a transparent covering material for lighting and energy control of internal spaces has been introduced (Jirka et al., 1998) and results with fresnel lenses as second glazing in greenhouses (Jirka et al., 1999) presented. The use of fresnel lenses, instead of typical glazing on the roof of greenhouses, is a new concept aiming to improve lighting and energy needs of greenhouses. In this paper we present the fresnel lens concept for greenhouses, we give the distribution of the concentrated solar radiation on the focal plane of a linear fresnel lens and the effect of the absorber size and the incidence angle on the collected radiation. The study includes also design concepts for linear absorbers of thermal (T), photovoltaic (PV) absorbers and is emphasised to hybrid photovoltaic/thermal (PVT) type, which can be of air or water heat extraction. These new solar energy systems convert solar radiation simultaneously into electricity and heat. Recent works on PVT systems include modelling results on liquid type systems (Bergene and Lovvik, 1995) test results for an integrated PV/T system with hot water storage (Huang et al., 2001) and theoretical and experimental results from several investigated prototypes (Zondag et al., 2002, 2003). An extensive study on performance improvement of hybrid PVT systems has been done in Physics Department at University of Patras (Tripanagnostopoulos et al 2001, 2002, 2003), where new designs and results on PVT systems are presented. The hybrid PVT systems can be combined with linear fresnel lenses and can be used for space heating and cooling of the inside space of greenhouses, for plant artificial lighting and other agricultural applications. The combination of fresnel lenses with hybrid PVT absorbers is a new concept and aims to maximise the energy conversion output of fresnel lens type solar energy systems that are used as transparent material in greenhouses. GREENHOUSE EFFICIENT OPERATION DEMANDS The transparent cover of greenhouses determines the internal microclimatic conditions. The wavelength range of the solar radiation, the intensity level and internal