Effect of climate change on the energy performance and thermal comfort of high-rise residential buildings in cold climates

Buildings now produce more than a third of global greenhouse gases, making them more than any other sector contributing to climate change. This paper investigates the effect of climate change on the energy performance and thermal comfort of a high-rise residential building with different energy characteristic levels, i.e. bylaw to meet current National Energy Code of Canada for Buildings (NECB), and passive house (PH) under two climate zones in British Columbia, Canada. SRES A2, RCP-4.5 and RCP-8.5 emission scenarios are used to generate future horizon weather data for 2020, 2050, and 2080. The simulation results show that for both bylaw and PH cases, the heating energy consumption would be reduced while cooling energy consumption would be increased. As a result, for the bylaw case, the total energy consumption would be decreased for two climate zones, while for PH case, the total energy consumption would be increased for zone 4 and decreased for zone 7. In addition, the number of hours with overheating risks would be increased under future climates, e.g. doubled in 2080, compared to the historical weather data. Therefore, efforts should be made in building design to take into account the impact of climate change to ensure buildings built today would perform as intended under changing climate.


Introduction
The greenhouse gases (GHG) are one of the main causes for climate change [1]. The building sector produces 23% of greenhouse emissions in Canada [2]. Therefore, Canada, specifically British Columbia (BC), set the target to reduce the GHG from building sector by building all new buildings with net-zero energy consumption by 2032. However, the common practice of assessing the energy performance of buildings is to use historical weather data (Typical Meteorological Year TMY), while NASA data showed that there are significant changes in the current climate compared to the historical weather [3]. Therefore, the energy consumption of energy efficient or net-zero energy buildings may be significantly affected by these undesirable changes. Intergovernmental Panel on Climate Change (IPCC) has been established since 1988 to investigate the causes and impact of climate change and measures to mitigate the effects of climate change.
In recent years, research has been carried out on the impact of climate change on energy performance of buildings based on Third or Fourth IPCC scenarios [4][5][6][7][8]. For instance, Shen concluded that the total energy use of residential and office buildings in USA would increase by 14% and 3%, respectively in 2050 in warm climate zones [7]. In Canada, Robert et al. [8] found that the net-zero energy buildings would not be able to maintain the net-zero status by 2050 as a result of climate change. However, climate change will not only affect the energy performance of buildings, but also the thermal comfort of occupants. UK has released studies on overheating in buildings under future climate and how to mitigate the climate change effect on indoor thermal comfort. A number of European studies showed that under the current climate the overheating risks have been increased to 44% in bedrooms and 29% in living rooms compared to the historical weather [9][10][11]. However, there are very limited studies on this subject in Canada. This paper aims to assess the energy performance and thermal comfort of a high-rise residential building in British Columbia that is built based on the current and future energy codes under historical and future climatic conditions.

Case study building
To study the climate change effect on the energy performance of residential buildings, a high-rise multi-unit residential building is selected. Figure 1 shows the sketch up of the high-rise residential building. The archetype building is the same building that used in the BC Hydro Power Smart guide [12] to represent a typical multi-unit high-rise residential building built in recent years in BC. The selected building has 30-storey with a window wall ratio of 40%. The gross floor area of the building is 27,871 m 2 and its length to width ratio is 1:1 and wall to roof ratio is 10.8, respectively.

Building envelope components
The high-rise building chosen is a pour-in-place concrete construction with exterior insulation. The configurations of the building envelope assemblies that adopted from BC Hydro Power Smart report [12] are used as bylaw case and varying insulation thickness are  used for PH cases. The overall thermal transmittance of building envelope assemblies are listed in Table 1 for bylaw and PH case. Bylaw design refers to building envelope assembly meets the current NECB building code requirement for the specific climate zone, while PH design refers to building envelope assembly to meet the energy performance of PH standard requirement.

Generation of future weather data
The building is simulated under the historical (TMY) and predicted future horizon data of 2020, 2050, and 2080 for two climatic zones, i.e. Vancouver (zone 4) and Fort Nelson (zone 7), using DesignBuilder. The SRES A2, RCP 4.5 and RCP 8.5 emission scenarios are used to generate future climate. CCWorldWeatherGen tool, which uses the morphing method for temporal and spatial downscaling, is used to generate future horizon years based on HadCM3 General Circulation Models (GCM) for A2 scenario. While, WeatherShift tool, which also uses the morphing method for temporal and spatial downscaling, is used to generate future horizon years based on 14 GCM for RCP 4.5 and 8.5 generation.

Thermal comfort criteria
The overheating risks are evaluated based on the operative temperature ranges for naturally conditioned spaces according to ASHRAE 55-2017 [13]. Equation 1 determines the upper limit temperature that corresponds to the acceptable operative temperature. If the operative temperature (equation 2) is higher than upper limit temperature, overheating will occur.
Upper 80% acceptable limit T c,up = 21.3 + 0.31* T om (1) Where T om is the monthly mean outdoor air dry-bulb temperature (°C), T o is the operative temperature (°C), T d is the indoor air dry-bulb temperature (°C), and T mrt is the mean radiant temperature of the zone (°C).
The overheating risk is analysed for the base-case that represents the bylaw building without natural ventilation, mechanical cooling or mechanical ventilation; and it is simulated under historical year (TMY data). In addition to the base-case, seven cases are simulated to evaluate the effect of climate change on the thermal comfort.

Settings in whole building energy simulations
The settings of whole building energy simulation are based on NEBC-2017 [14]. The heating and cooling set point are 22 °C and 24 °C, respectively, and the heating set-back is 18 °C. The COP of the heat pump is 1.7 in winter and 3.5 in summer. The light and appliances and plug loads are 5 and 5.5 W/m 2 , respectively. Mechanical ventilation is with 65% and 90% heat recovery efficiency for bylaw and PH cases, respectively. For overheating simulation with natural ventilation (free cooling simulation), the natural ventilation is activated (by opening 20% of the windows area), if the indoor temperature is higher 24°C and higher than outdoor temperature. The amount of natural ventilation is calculated based on wind speed, wind pressure, wind direction and the size of window opening.

Annual heating, cooling and total energy consumption
As shown in Table 2, the annual heating energy demand of bylaw building in 2080 would be reduced by 27-43% compared with under historical weather data in Vancouver. However, the cooling energy demand would be increased by up to 3800% and account for about 50% of the total energy demand compared to 1% under historical weather. Similar results were obtained for Fort Nelson city, where the annual heating energy demand would be reduced by 14-23%, while the cooling energy demand portion would be increased from 3% to 22% of the total energy demand. Energy efficient buildings would be affected by climate change more than bylaw buildings. The cooling energy demand of these buildings would be higher than the heating energy demand in 2080 under moderate climate (Vancouver City) and failed to meet PH requirements for the cooling energy demand. Under very cold climate (Fort Nelson City), the cooling energy demand portion in 2080 would be almost equal to the heating energy demand, but could still meet the PH requirements. Figure 2 shows the total annual energy consumption of the building, which includes the heating and cooling energy consumption of HVAC system and the light and appliances load, and the percentage difference in annual energy consumption between historical year and 2080. The total energy consumption would be reduced by 5-30% for all cases except for the PH case in Vancouver, where the total energy consumption would be increased by 16% in 2080. The increase in total energy consumption for Vancouver is primarily due to the increase of cooling energy demand and the decrease of the efficiency of heat pump system, where the COP would be 3.0 in summer of 2080 due to the higher ambient air temperature.   Figure 3 shows that for the base-case the overheating hours represent 55% (2410 hrs) and 69% (3018 hrs) of the summer time (April to September) for Fort Nelson and Vancouver, respectively. The overheating hours would be increased by 35% and 65% under the 2080 weather data for Vancouver and Fort Nelson, respectively. Improving the energy performance of the base-case to PH building would increase the overheating hours by 20% and 10% under historical weather, while by 57% and 62% under 2080 weather for Vancouver and Fort Nelson, respectively. Adding natural ventilation for the base-case can significantly reduce the overheating hours by 86% and 95% for Fort Nelson and Vancouver, respectively. However, the natural ventilation measures may not be sufficient under future climate, with 12% (542 hours) and 18% (781 hours) of summer time being overheated for Vancouver and Fort Nelson, respectively. The performance of PH building with natural ventilation would be slightly better than the base-case with natural ventilation given that the PH building has windows with lower Solar Heat Gain Coefficient. The higher number of overheating hours in Fort Nelson than that in Vancouver when natural ventilation is probably due to the slightly higher solar radiation in Fort Nelson than that in Vancouver, especially in May and June. It could be also attributed to the higher thermal resistance specified for buildings in Fort Nelson.

Conclusions
The effect of future climate on the energy performance and thermal comfort of a typical high-rise residential building built in British Columbia, Canada in compliance with bylaw and Passive House standards are studied through simulations.  Overheating risks o The climate change would increase the overheating risk, for both bylaw and PH cases by 65% compared with historical weather without any mechanical cooling, or natural ventilation. o The natural ventilation measure would significantly reduce the overheating hours, but may not be sufficient, especially for PH building where the overheating still represented 15% of the summer time. In conclusion, buildings designed based on historical weather data would perform differently under the changing future climates, and therefore more efforts should be placed on designing buildings that could adapt to climate change. Given that the cooling energy demand would be significantly increased under future years, strategies to provide proper solar control and mitigate overheating risks during summertime would become important design considerations even for cold climates.
We gratefully acknowledge the financial supports received from BC Housing as part of the Building Excellence Research & Education Grants Program, NSERC Discovery Grant, and Gina Cody School of Engineering and Computer Science, Concordia University.