Fixed and tracking PV systems for Net Zero Energy Buildings: Comparison between yearly and monthly energy balance

The energy needs of a Net Zero Energy Building (NZEB) are fully met by renewable energy sources. Nevertheless, the time span of the energy balance can affect the effective energy self-sufficiency of the building. The aim of this paper is to demonstrate the lack of reliability that a yearly energy balance has in characterizing an NZEB. At this scope, comparisons between two different photovoltaic systems (fixed and tracking) and two kinds of energy balance (yearly and monthly) are made for a real NZEB building located in South-Italy. The investigation was carried out through dynamic energy simulations after validating the building model. The results show that the PV surface which attains the NZEB target on yearly basis not always achieves the same objective as the monthly balance. Furthermore, considering a monthly balance, the size of the photovoltaic system with biaxial solar tracking is 50% minor than the fixed one, thanks to a steadier energy production. Also, solar tracking systems show a significantly lower reduction of operational and embodied CO2 emissions. A final technical-economic analysis shows, however, that design solutions that satisfy NZEB target not always can meet the financial interest of the private stakeholder, showing a high discounted payback period.