Enhancing the Energy and Non-Energy Performance of Existing Buildings: a Multi-Objective Approach

A wide selection of energy efficiency measures (EEMs) are technically available to improve the energy performance of existing buildings, each of which can be applied to a different extent. The definition of the best combination of retrofit strategies is generally pursued through the balance of economic and energy targets. The so-called cost-optimal approach proposed by the recent European Union legislation, takes into account those two perspectives by minimizing the net present value, a utility function that accounts for both investment and operational costs along the lifespan of the renovated building. What this approach does not consider are some different performance aspects, although related to the same EEMs, and in particular the occupant well-being. In this framework, the definition of the best retrofit strategies should consider three objectives characterized by a competing nature: the energy savings, the economic advantage and the indoor thermal comfort. The aim of this work is to evaluate a consistent number of possible EEMs in order to define the optimal combinations considering also the effect of the government subsidies. The multi-objective analysis has been implemented by means of a Genetic Algorithm coupled with a simulation code. The analysis has been conducted on a set of reference buildings obtained by introducing some variations to an uninsulated single-storey residential module of 100 m2. The windows orientation and the ratio between dispersing envelope and conditioned volume are modified in order to simulate different reference buildings and to study their relation with the optimal solutions. Moreover, two different climates have been considered as representative of the Southern Europe: Milano and Messina. The indoor air temperature is controlled during the winter season, while during summer season the air temperature is free floating. The evaluation of the long-term comfort performance is defined by means of the Discomfort Weighted Time (WDT) index as proposed by the Standard EN 15251:2007. The number of occupied hours during which the actual operative temperature lays outside the specified comfort range are weighted by a weighting factor which depends on by how many degrees the operative temperature is outside the range. The definition of the acceptable ranges depends on the presence of HVAC system: when the HVAC system works the actual operative temperature is compared with a fixed operative temperature comfort range, established according to the Standard, while when the air is not conditioned the assessment of the indoor thermal comfort is implemented considering the adaptive comfort approach. The assessment of the thermal comfort during the summer season has been used to evaluate the impact of the adopted solutions. The entity of the government subsides required to improve the economic profitability of the Pareto solutions has been evaluated and the impact of different incentives scenarios on the composition of the Pareto front have been then tested repeating the Multi-objective analysis