Profitability of active retrofitting of multi-apartment buildings: Building-attached/integrated photovoltaics with special consideration of different heating systems

Multi-apartment buildings comprise almost half of the European housing stock and are for the most part old and energy inefficient, making active retrofitting an important topic. The objective of this paper is to determine the profitability and optimal size of different technology portfolios for renewable building energy. A mixed-integer linear programming optimisation model is developed in Matlab with the objective of maximising the Net Present Value over a time horizon of 20 years. To examine multiple use cases, a modular approach is used for realising different multi-apartment building set-ups. Building-attached and building-integrated photovoltaic systems on different parts of the building skin already achieve break-even. Heat pumps, pellet and district heating can hardly compete with gas heating yet. However, heat pumps have synergy effects with PV systems, thus reinforcing their implementation, as does a tenant portfolio with a good correlation with the sunshine hours. The profitability gap between investment costs for passive building renovation and resulting energy cost savings is significant. However, it is the smallest for buildings with quality standards. In conclusion, governmental subsidies and financial incentives such as the true cost pricing of CO2 emissions are necessary to trigger investments in reasonable combinations of passive and active retrofitting measures.