Development of urban building energy models for Wellington city in New Zealand with detailed survey data on envelope thermal characteristics

The urban building energy model is increasingly applied in energy conservation research to understand building energy utilization on a city scale. Typical models in most studies use designed values of building envelopes but do not consider the changes in thermal transmittance that result from aging factors. This study introduces a degradation model for building envelopes derived from surveys of 30 buildings. The resulting distribution curve was used to develop an urban energy model, analyzing its impact on building energy consumption. First, the envelope U-values of 30 buildings were measured using the heat flux meter (HFM) method, and a degradation model of the U-values was developed. Two scenarios were established: one incorporating the U-values from the degradation model and the other employing identical initial U-values as used in the design, which served as the baseline model. A case study was conducted on 68,532 buildings in Wellington, New Zealand. The GIS dataset of the buildings was analyzed to create 16 archetypes, and OpenStudio and AutoBPS were used to create the building energy models in EnergyPlus. For buildings from 1978 to 2022, 720 EnergyPlus models were generated in conjunction with the degradation model. The results showed a significant variation between actual and initial U-values in buildings constructed before 2004, particularly those over 40 years old, in contrast to minimal discrepancies in buildings built after 2004. Annual energy consumption for detached houses, townhouses, and apartments in residential buildings increased by 23 kWh/m2, 31 kWh/m2, and 35 kWh/m2, respectively. Meanwhile, the impact on commercial buildings was relatively minor. From an urban perspective, these increases contributed to an overall 8 % higher energy consumption in the residential sector compared to the base model, while commercial building consumption remained unchanged. The UBEM used in this study estimated the city's total carbon emissions to be 283,688 t CO2-e, with residential buildings contributing 152,250 t CO2-e and commercial buildings 131,438 t CO2-e. The carbon emissions from older housing stock are substantially higher than those from newer buildings. Therefore, it is recommended that the impact of building age on envelope thermal transmittance in residential building modeling be considered.