In many engineering applications, it is mandatory to know separately the solar radiation diffuse and direct components. Examples regard the assessment of the energy potentially exploitable by a system of solar thermal or photovoltaic panels and, in general, all the cases where it is necessary to calculate the radiative solar power collected by a surface. In fact, radiation components will differently project on the surface of interest and will weigh in a different manner, depending on the surface orientation, in the computation of the effective incident radiation. To perform this decomposition starting from data relative to a horizontal plane, two non-dimensional quantities, namely, the diffuse fraction, kd, and the clearness, kt, are usually put in mutual relation by correlating experimental data on a graphical ground rather than using physical considerations. In the present study, some insights are given on the shape of this correlation starting from geometric and physical considerations. It is shown that many results and graphs presented in literature have not physical meaning; rather they are simply artifacts due to geometrical or other constraints. These evidences open the way to a new approach to solar radiation decomposition founded on physical-based correlations.

The advantages of the solar assisted heat pumps are well-known but their diffusion on the market is still very limited. One of the main causes is associated to their complexity and almost always working under transient regime which can lead to frequent blocks especially when a low level of control and automatization is present. From the point of view of conceptual design, solar assisted heat pumps tend to be more subjected to blocks due to their serial asset. The present work illustrates the case study of the solar assisted heat pump pilot plant at Palacus, Genoa, equipped with a heat pump bypass which can directly connect the solar thermal field to either the domestic hot water or the space heating systems when the temperature in the circuit exceeds a fixed set point. This configuration allows therefore the use of the solar field either independently from the heat pump or interfaced with it, according to the external conditions. A numerical model within TRNSYS environment has been developed basing on the information collected on the pilot plant by the data acquisition system. A simulation of the working of the pilot plant along a typical year has been performed, evaluating its increased performance and optimisation thanks to the bypass.