Abstract

This paper presents an analysis of how the design of a photovoltaic (PV) system influences the greenhouse gas emission balance in a net zero emission building (nZEB). In a zero emission building, the emissions associated both with the energy required in the operation of the building (operational emissions) and the energy used to produce the building materials (embodied emissions) are offset by renewable energy generated on-site (avoided emissions). The analysis is applied to a nZEB concept for a single-family building, developed by the Norwegian Research Centre on Zero Emission Buildings. Previous analyses have shown that the installation of a PV system accounts for a significant share of the embodied emissions of a nZEB. The objective of this paper is to assess how the PV system design choices influence the embodied and avoided emissions, in order to determine how the environmental impact can be minimised. Four different PV technologies (Si-mono, poly-Si and CIS, and high-efficiency Si-mono) in four different system designs for flat roofs are evaluated using two different grid emission factors. The installations are compared by means of net avoided emissions, greenhouse gas payback time (GPBT), greenhouse gas return on investment (GROI), and finally the net emission balance of the building. The results show that the system with the largest area of high-efficiency Si-mono modules achieves the best lifetime emission balance, but that the greenhouse gas return on investment is highest for the optimally oriented CIS modules.

Abstract

The net-zero emissions building (nZEB) performance is investigated for building operation and embodied emissions in materials for Norway’s cold climate. An nZEB concept for new residential buildings was developed in order to understand the balance and implications between operational and embodied emissions over the building’s life. The main drivers for the CO2 equivalent (CO2eq) emissions were revealed for the building concept through a detailed emissions calculation.
Previous investigations showed that the criterion for zero emissions in operation is easily reached by the nZEB concept (independent of the CO2eq factor considered). Nevertheless, embodied emissions from materials appeared significant compared to operational emissions. It was found that an overall emissions balance, including both operational and embodied energy, is difficult to reach and would be unobtainable in a scenario of low carbon electricity from the grid i.e. low CO2eq factor for electricity. In order to make these conclusions robust, a sensitivity analysis was performed on the dominant sources of CO2eq emissions, as well as, on how it impacts the emission balance during the building lifetime. In the baseline work, embodied emissions were evaluated using the EcoInvent database in order to get a consistent life cycle assessment (LCA) method for all the building materials. The first step of this sensitivity analysis is therefore performed to compare embodied emissions when specific Norwegian Environmental Product Declarations (EPD) were used instead of generic data from EcoInvent thus making data more representative for the Norwegian context. In addition, the photovoltaic (PV) system, which supplies renewable electricity to the building, also contributes significantly to the embodied emissions. The second step of the analysis evaluates different PV system design options in order to find the one with highest net emissions reduction. Finally, since the building concept was based on a highly-insulated building envelope, the dominant source of emissions during building operation turned out to be electric appliances. The third step of the analysis thus discusses the energy consumption of electric appliances and how it could be reduced through more efficient products, especially the so-called hot-fed machines (i.e. washing machines, tumble dryer and dishwasher).

Abstract

The net-zero emissions building (nZEB) performance is investigated for building operation (EO) and embodied emissions in materials (EE) for Norway's cold climate. nZEB concepts for new residential and office buildings are conceived in order to understand the balance and implications between operational and embodied emissions over the building's life. The main drivers for the CO₂ equivalent (CO_2e) emissions are revealed for both building concepts through a detailed emissions calculation. The influence of the CO_2e factor for electricity is emphasized and it is shown to have significant impact on the temporal evolution of the overall CO_2e emissions balance. The results show that the criterion for zero emissions in operation is easily reached for both nZEB concepts (independent of the CO_2e factor considered). Embodied emissions are significant compared to operational emissions. It was found that an overall emissions balance including both operational and embodied energy is difficult to reach and would be unobtainable in a scenario of low carbon electricity from the grid. In this particular scenario, the net balance of emissions alone is nonetheless not a sufficient performance indicator for nZEB.

Abstract

The design and assessment of net-zero buildings commonly focus exclusively on the operational phase, ignoring the embodied environmental impacts over the building life cycle. An analysis is presented on the consequences of integrating embodied impacts into the assessment of the environmental advantageousness of net-zero concepts. Fundamental issues needing consideration in the design process – based on the evaluation of primary energy use and related greenhouse gas emissions – are examined by comparing three net-zero building design and assessment cases: (1) no embodied impacts included, net balance limited to the operation stage only; (2) embodied impacts included but evaluated separately from the operation stage; and (3) embodied impacts included with the operation stage in a life cycle approach. A review of recent developments in research, standardization activities and design practice and the presentation of a case study of a residential building in Norway highlight the critical importance of performance indicator definitions and system boundaries. A practical checklist is presented to guide the process of incorporating embodied impacts across the building life cycle phases in net-zero design. Its implications are considered on overall environmental impact assessment of buildings. Research and development challenges, as well as recommendations for designers and other stakeholders, are identified.