The transparent areas of the building envelope, normally built with glazed systems, have a relevant impact on the final building energy performance, both due to the thermal transmittance and the energy gains related to the sun radiation transmitted into the building spaces. Compared to the common glazing, the BIPV glazing include a few new elements that may cause different phenomena. First and more obvious are the new photovoltaic solar cells, or alternatively photovoltaic thin film coatings, and the elements for the electrical connection like the ribbons. This introduces an impact on the optical properties that leads to a higher radiation absorption, affecting the operating temperature of the PV glazing. The potentially different operating temperature may impact on several aspects. From the U value viewpoint, it affects the gas cavity temperature and convection properties, the radiation heat fluxes between cavity surfaces, the heat transfer coefficients of external and internal surfaces and the direction of heat flux from outer to inner glass layers in some moments. As the G value depends on thermal resistances used for U value, the G value is also affected by these phenomena. Moreover, G value calculation could be affected by the PV conversion effect because a part of the absorbed solar radiation is transformed into electricity instead of heat. In addition, the temperature may impact on the ageing and pressure of the gas cavity, ageing of materials and mechanical properties of the laminated glass.
Other new elements like the junction box (JB) may impact on the optical and thermal properties of the PV glazing. The JB introduces an additional gas cavity in the thermal system that may affect the total thermal transmittance; it also can absorb a part of the radiation if located in the transparent area of the PV module. The framing is often used to hide the cables, so its thermal transmittance might be affected by holes required, that can communicate different chambers of the frames.
This work is mainly focused on glazing BIPV modules. The aim of this work is to clarify whether specific factors related to BIPV products can impact the energy economy procedures as defined in current construction standards and, if required, propose adaptations in the assumptions and boundary conditions to correctly evaluate the calculation models of the thermal performance of the BIPV systems.
