Torhildur Kristjansdottir

Life cycle emissions analysis of two nZEB concepts

The net-zero emissions building (nZEB) performance is investigated for building operation (EO) and embodied emissions in materials (EE) for Norways 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 buildings life. The main drivers for the CO2 equivalent (CO2e) emissions are revealed for both building concepts through a detailed emissions calculation. The influence of the CO2e factor for electricity is emphasized and it is shown to have significant impact on the temporal evolution of the overall CO2e emissions balance. The results show that the criterion for zero emissions in operation is easily reached for both nZEB concepts (independent of the CO2e 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.

Comparative emission analysis of low-energy and zero-emission buildings

ABSTRACT Different designs and concepts of low-energy and zero-emission buildings (ZEBs) are being introduced into the Norwegian market. This study analyses and compares the life cycle emissions of CO2 equivalents (CO2e) from eight different single-family houses in the Oslo climate. Included are four ZEBs: one active house, two passive houses, and a reference house (Norwegian building code of 2010). Monthly differences in CO2e emissions are calculated for the seasonally sensitive Norwegian context for electricity generation and consumption. This is used to supplant the previous applied symmetric weighting approach for CO2e/kWh factors for import and export of electricity for the ZEB cases. All the ZEBs have lower use-stage emissions compared with the other buildings or the reference case. Embodied impacts are found to be 60–75% for the analysed ZEB cases, confirming the importance of embodied impacts in Norwegian ZEBs. The lowest total emissions were from the smallest ZEB, emphasizing area efficiency. The highest emissions were from the reference case. By abandoning the symmetric approach, a new perspective was developed for assessing the performance of ZEBs within the Norwegian context. One of four ZEB cases managed to balance out its annual energy-related emissions.

Life Cycle GHG Emissions from a Wooden Load-Bearing Alternative for a ZEB Office Concept

A major contributor to global greenhouse gas emissions is the production of concrete and steel for the construction industry IPCC (2007). To combat global warming, innovative solutions are needed in the construction industry to reduce emissions from both energy and material use in buildings. In a previous study the first phase of a GHG emissions analysis for a Norwegian ZEB office concept was presented. The aim of which was to achieve a zero emission balance where operational and material emissions are accounted for ZEB OM. The results from the first phase showed that the load bearing system accounted for a large share of the embodied emissions. In addition, the ZEB OM ambition level was not met, thus emphasizing the need for further work on alternative solutions and material choices. This paper presents the results of a comparative study between this original office concept study and a predominantly wooden alternative loadbearing structure consisting of wood trusses, glue laminated beams and columns. The wooden alternative is comparable since it has been dimensioned to fulfil the same technical requirements for bearing capacity, sound and fire resistance. In addition, the system boundary was extended to include three alternative end-of-life scenarios. It was found that the wooden alternative structure almost halved the emissions compared to the original concrete and steel ZEB office concept model. This trend is the same in the cradle to gate and all three end-of life scenario’s. The analysis clearly shows that emissions from the production process outweigh any emissions from the material’s end-of-life treatment. This means that the material choice plays a major role in embodied emissions, as well as it being crucial to reduce the required construction material quantity.