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Various approaches are used to determine direct, sky diffuse, and reflected contributions to the solar irradiance on building surfaces. Among these are ray tracing, pixel counting, and projection and clipping. Projection and clipping is closest to an exact solution but it is often considered as too slow. This paper presents an improved projection and clipping method to determine the direct and the sky diffuse contribution to the irradiance. In a second step a projection and clipping approach is applied to calculate the view factors between building surfaces, so that the reflected contribution can be obtained. The presented approach delivers results which are comparable to those obtained by EnergyPlus in acceptable computation time. This shows that modern building simulation tools can benefit from an approach that applies fewer approximations than common raytracing or pixel counting techniques.

The objective of this work was to develop a CFD model designed to study the night-time climate of an unheated greenhouse. A three-span sloped-roof greenhouse was considered for the simulations. For the first set of simulations, a single layer of polyethylene (PE with a thickness of 0.2 mm) was used as a cladding material. A second set of simulations were run for a greenhouse with the same dimensions but with an internal horizontal curtain of 0.2 mm thick PE. The CFD model used the Boussinesq approach and included a radiation sub-model, so that sky thermal radiation could be simulated. On clear nights, greenhouse air temperature was 2.5 K cooler than outside air. It was also observed that the roof temperature was up to 4.4 K cooler than outside air. On overcast nights, greenhouse air was 3.6 K warmer than outside air. The effect of the internal PE curtain was significant: in all cases, greenhouse air temperature was higher than that of outside air. According to simulations, using the curtain can induce increases in air temperature of around 2.8 K on clear nights. While roof temperature was not significantly affected by the curtain, curtain temperature was clearly higher than roof temperature. The increase in temperature of the curtain may have the practical effect of reducing dew formation and dripping over the crops. Further simulations are presently being conducted to assess the potential for reducing thermal losses in greenhouses using other materials and methods.