Transforming a passive house into a net-zero energy house: a case study in the Pacific Northwest of the U.S

Highlights: • A modeled Passive House (PH) has been converted into a net-zero energy house (NZEH). • Energy conservation measures (ECMs) are key to transforming a PH to NZEH. • Integrating PV panels is the most cost effective approach for meeting remaining loads. • PV panels pay back in less than 10 years compared with 28 years for solar water heating. • Energy use intensity of 26 kWh/m² is a good target for transforming a PH into a NZEH. - Abstract: This paper presents the end-use energy performance analysis of the eastern unit of a duplex built based on Passive House Standard requirements in Portland, OR, USA. The energy consumption evaluation showed that this unit has 7.75, 34, 91 kWh/m² year of heating demand, total site consumption, and primary energy consumption, respectively. A detailed energy analysis was used to explore pathways through which the house could be transformed into a net-zero energy house (NZEH). A model of the eastern unit was calibrated against measured data to perform a detailed energy analysis not only for heating energy, but also including energy used for cooling, water heater, plug loads, electric appliances, and kitchen appliances. Based on this analysis we conclude that two steps are required in order to transform a passive house into a NZEH. First, opportunities for energy conservation measures (ECMs) must be identified. In this study, changes in natural ventilation and occupant behavior were the most effective measures. Modeled implementation of these measures led to energy consumption reduction of 814 kWh/year (∼17%), and 0.1 tons of avoided CO₂/year. The second step was to investigate the most appropriate renewable energy systems; photovoltaic (PV) panels and solar water heating were evaluated. Integrating PV panels over an area of 26 m² was sufficient to balance the remaining annual energy demand (3936 kWh/year) by producing 4053 kWh/year with a payback period of 15.4-years. On the other hand, using thermal solar water heating was found to reduce the energy consumption by 3047 kWh/year (64%); the remaining energy demand can be overcome by integrating a total of 13 m² of PV panels. The latter approach has around a 20-year payback period and saves 0.6 tons of CO₂/year. We therefore conclude that the first approach of using PV panels is more cost-effective for transforming a passive house to a ZEH for the climate of the northwest US.