Ivan Pollet

PAR transmittances of dry and condensate covered glass and plastic greenhouse cladding.

Abstract In this study, the angular transmittances over the photosynthetically active radiation (PAR) range of two types of glass (single glass (SG) and low emissivity glass (LEG)) and three types of plastic films (low density polyethylene (LDPE), anti-drop condensation polyethylene (ADCPE) and photoselective diffusive polyethylene (DPE)) were determined experimentally in the laboratory. The measurements were performed on dry as well as on condensate covered materials, since condensate frequently occurs on a greenhouse cladding material. Based on the angular measurements, the transmittance of diffuse PAR incident on a 25° inclined slab was calculated. For the dry state, it was found that the PAR transmittance decreased with increasing photoselectivity of the materials. When covered with condensate, the PAR transmittance of the glass plates was only reduced at incidence angles higher than 15° by at most 20% on a relative scale, while the transmittance of non-anti-drop condensation films was especially reduced at small angles of incidence by at most 25%. Due to the addition of anti-drop condensation agents to a plastic film, the PAR transmittance of the film was not affected by the condensate and, therefore, showed the highest PAR transmittance in the wet state. For the photoselective cladding materials, the spectral dependency of the transmittance was only slightly changed by the condensate.

Laboratory measurements of PAR transmittance of wet and dry greenhouse cladding materials

Abstract A laboratory set-up which consisted of two climate rooms (cold box/hot box) was used to assess the effects of condensation on the PAR transmittance of a vertical greenhouse cladding material under controlled climate conditions for several incidence angles. It was observed that the absolute transmittance difference between dry and wet polyethylene decreased gradually from 23% to about 0% for increasing incidence angles from 0° to 75°. Results for radiation incident in the horizontal and the vertical planes agreed within 3% over all incidence angles. The diffuse radiation transmittance for a wet polyethylene film was 15% lower than the one for a dry polyethylene film.

IMPACT OF WATER DROPS ON THE VISIBLE RADIATION TRANSMITTANCE OF GLAZINGS UNDER OUTSIDE RADIANT CONDITIONS

Abstract Experimentally determined directional-hemispherical transmittances for visible radiation were used to calculate the effect of condensate on the hemispherical–hemispherical transmittance values under diffuse radiant conditions for single glass and polyethylene. The hemispherical–hemispherical transmittances were determined in the horizontal plane for different inclinations of both glazing materials assuming a completely overcast sky and a clear sky. In all cases, condensate was found to reduce the transmittance. Under an overcast sky, the transmittance of single glass was reduced by the presence of condensate from 8 up to 13% (on a relative scale) for increasing inclination angles of the glass plate, while for the polyethylene film, the transmittance reduction due to condensate decreased from 20 down to 13%. Under a clear sky, depending on date, time, and orientation of the glazing material, the transmittances of single glass and polyethylene were reduced by between 6 and 15% and by between 8 and 21%, respectively, due to the presence of condensate. These results revealed that the impact of the presence of condensate on the transmittance of a glazing cannot be omitted from design and performance calculations of solar energy systems.

Measurements and simulation on the comfort of forklifts

Abstract In order to determine the influence of some parameters of a forklift such as the road profile, the tyre characteristics, the riding comfort, etc., measurements carried out on a forklift with different tyres and seats were evaluated using different standards and methods. In addition, a simulation model was developed and used to investigate the influence of these parameters. Simulations and test run results showed good agreement. The comparison of the results obtained with several methods of comfort evaluation and a series of tests showed that they nearly all resulted in the same classification. However, the results obtained with different methods could not always be compared among themselves. Solid tyres were found to be more comfortable than pneumatic ones because of their high damping. The negative influence of higher stiffness was smaller than the positive influence of higher damping. The simulations pointed out that for a global general investigation about comfort, the influence of the horizontal tyre stiffness and damping can be neglected. Also the seat characteristics could be linearized. When the stability of the forklift has to be investigated, the horizontal forces must also be considered.

Angular dependence of forward scattering induced by condensate on greenhouse cladding materials

The forward scattering properties of four cladding materials, namely low-density polyethylene (LDPE), anti-drop condensation polyethylene (ADCPE), diffusive polyethylene (DPE), and single glass (SG) were measured in the dry state and when covered with condensate at three different incidence angles, namely 0, 15, and 30°. In the dry state, single glass behaved as a quasi non-diffusive material in contrast to the three plastic films which scattered the radiation due to their surface roughness, bulk heterogeneities or the addition of diffusive agents. With the exception of the anti-drop condensation polyethylene, the forward scattering of the materials was broadened by the presence of condensate. For these materials, the scattering pattern shifted towards the normal to the cladding with increasing incidence angle. This meant that less radiation was scattered to the opposite side of the directly transmitted, unscattered beam. At an incidence angle of 30°, a distinct shoulder in the scattering pattern, situated between the direct beam and the normal, was found on wet single glass in contrast to the plastic films.

Effect of inclination angle on the light transmittance of condensate covered homogeneous thin transparent materials

Abstract The inclination angle of a transparent material exposed to solar radiation strongly varies depending on the application. It is well known that the inclination angle can highly influence the shape of condensation drops—which are sometimes unavoidable in solar thermal applications—via its effect on the receding and advancing contact angles of the drops. By means of a laboratory set-up, the effect of condensate on the transmittance of 25° and 90° (i.e., vertical) inclined single glass, polyethylene and anti-drop condensation polyethylene was measured for incidence angles up to 45°. No significant impact of the inclination angle on the transmittance of the three transparent materials in the wet state was observed. This means that transmittance results obtained on wet materials can be applied irrespective of the inclination angle of the material.

Forward scattering induced by water drops on a transmissive substrate

In this study the scattering of radiation by condensation drops deposited on a single glass plate is dealt with. Experiments were carried out in the visible radiation range by means of a laboratory measuring unit as a function of three parameters, namely, the phase of the condensation process, the wavelength of the incident radiation, and the radiation incidence angle. The experiments indicated that during the condensation process a steady state in the scattering pattern of single glass occurred after a transition phase. Owing to the condensate, more than 80% of the transmitted visible radiation was scattered. The scattering slightly diminished with increasing wavelength, from 400 to 700 nm, and the asymmetry of the scattering pattern enlarged with increasing incidence angle of the radiation.

Wettability and high incidence angle light transmittance of greenhouse claddings.

The effects of condensate on the transmittance of a vertical standard polyethylene film and a vertical standard glass plate were investigated to verify whether condensate on a greenhouse cladding material could increase the transmittance at high angles of incidence as predicted in literature. At an incidence angle of 80°, a mean transmittance increase of about 4% was measured when condensate was present on the polyethylene film. On the glass plate, however, a transmittance decrease of about 5% was measured under similar circumstances. Therefore, it was concluded that the transmittance loss or gain due to condensate at high incidence angles depended on the wettability of the cladding material.

Design of an Earth Tube Heat Exchanger System for Greenhouses

Publisher Summary This chapter discusses the design of an earth tube heat exchanger system for green houses. Greenhouses are primarily built to increase the air and plant temperatures, due to the so called greenhouse effect. Under certain circumstances, however, optimal plant production requires the greenhouse to be cooled. An earth tube heat exchanger-storage system is designed and installed in a polyethylene clad double wall greenhouse, to reduce under Western European summer conditions (the inside air temperature to 14–17°C). The thermal mass of the soil was used as a heat buffer. The system consisted of nonperforated corrugated plastic drainage pipes, 12 m in length and 20 cm in diameter, buried at a depth of 2 m underneath the greenhouse. Groups of five pipes are gathered in a collector equipped with a fan. In this way, the air can be circulated internally in the greenhouse—pipes system. The system allows one to transfer excessive heat from the greenhouse air to the soil during hot summer days and to protect the greenhouse plants from freezing during winter.

Transmission properties of dry and wet greenhouse cladding materials and relation to crop photosynthesis

Solar transmission properties of dry and condensate covered single glass and a common polyethylene film (PE) were measured as a function of incidence angle. The experiments revealed that scattering by dry glass was negligible, in contrast to dry PE which scattered 47% of the transmitted radiation. With the presence of condensate, single glass scattered 81 to 86% of the totally transmitted radiation, depending on incidence angle. At an incidence angle of 30degrees, condensate lowered the total transmittance by 4%. The scattered fraction of the radiation transmitted by the PE film was increased to 78-83% by the condensate drops, but the total amount of transmitted radiation was reduced by 22-23%. To assess the effect of scattering on plant growth, net photosynthesis rates were simulated for a tomato crop. The photosynthesis rate was predicted to be 16% higher under dry polyethylene when compared with dry glass due to the scattering proper-ties of the plastic film. The presence of condensate gave rise to a 13% higher photosynthesis rate under single glass and a reduction of 6% under polyethylene when compared with the respective materials in the dry state.