Integrated optimization of the building envelope and the HVAC system in nZEB refurbishment

The research presents the method for optimization of a building with its heating, ventilation and air-conditioning (HVAC) system in the scope of achieving nearly zero energy building (nZEB) in the context of Energy Performance of Buildings Directive 2010/31/EU. The method is based on coupling the dynamic simulation models created in Trnsys software with the non-dominated sorting genetic algorithm (NSGA-II), as the engine which performs an optimization process to minimize global cost (GC) and primary energy consumption (PEC). The optimization comprises energy efficiency measures (EEM) on a building envelope with a parametrized model of the HVAC system. The procedure implements building simulations under design weather conditions for heating and cooling to establish the connection between building design and the HVAC system size. The analysis is performed using discrete equipment sizes and specifications according to manufacturers' data, ensuring the selection of actual products with their sizes and costs. In continuation annual simulations using referent year data were performed. Several HVAC systems were considered, ranging from conventional natural gas-based systems to systems based on renewable energy sources (RES). The method is applied in the case study of the refurbishment of a representative hotel building on the Croatian Adriatic. The results reveal a large gap in terms of PEC between the nZEB system (biomass boiler, solar thermal collectors and absorption chiller, 138.9 kWh/m(2)) and the cost-optimal system (natural gas boiler and compression chiller, 269.6 kWh/m(2)). The most promising solution for a hotel with year-round operation character is water-to-water heat pump system using seawater as the heat source and sink (170.2 kWh/m(2)). When the use of seawater is not possible, or when cooling energy prevails over heating energy use, air-to-water heat pump system appears to be suitable solution (176.4 kWh/m(2)). It is found that in addition to HVAC system with low PEC, the application of solar photovoltaic (PV) system is required to reach Croatian nZEB target for a renewed hotel (115 kWh/m(2)). Further analysis of the results shows that the optimal parameters of EEM vary depending on applied HVAC system. The results indicate that optimal solutions are achieved when the HVAC system size matches the design load. Therefore, excessive oversizing of the HVAC system may have negative impact on GC. Based on the results and the findings, conclusions and general guidelines for the refurbishment of a hotel building on the Adriatic coast are established.