Shaping High-Rise Towers to Meet Future Energy Codes, by Stantec
Large commercial developments regularly require multi-phase build-outs. Each phase needs to comply with the energy code at the time of permit application which, in British Columbia, can mean different stages of the Energy Step Code or the City of Vancouver Zero Emissions Building Plan. These standards are performance-based codes that could be met by thousands of design solutions. In order to identify suitable and desirable design options, we’ve developed a parallel simulation and data sensitivity analysis workflow to explore a vast number of potential choices quickly.
Historically, it was not uncommon to design a tower’s shape based on non-energy requirements only — such as views, apartment size, aesthetics and more — and then calculate the levels of required envelope performance and mechanical systems performance. Instead, in this study, we focus on how these performance levels are impacted by the shape of the floor plate, the orientation, the Window-to-Wall Ratio (WWR), and the location of the core in the floor layout. In order to reduce the annual energy use with more confidence, we use large-scale data analysis to support informed decision making. Some key recommendations derived from the study are summarized here.
The study uses the following workflow and scripts to investigate the intersections of architectural design, parametric modeling, building performance simulation and evaluation (see Figure 1). In this study, the building geometries are set up in Rhino/Grasshopper, then simulated in Honeybee using the EnergyPlus. The model does not include HVAC systems but uses “ideal air loads”. Heating recovery ventilation is set as 65% as common in the current market. Outdoor air “economizer mode” is turned off, meaning that the potential for “free” cooling through increased outside air rates when temperatures allow, is not considered in this study. Parametric simulations include:
The study evaluates a high-rise design so roof and floor thermal performance has a negligible impact on overall energy needs, though they still are important for other reasons (comfort, durability). Other simulation settings, such as heating/cooling schedule and setpoint, comply with the City of Vancouver modelling guidelines version 2.0, as required by the BC Building Code and Vancouver Building By-Law. The building performance metrics and simulation outputs include Thermal Energy Demand Intensity (TEDI), cooling energy, and Total Energy Use Intensity (EUI or TEUI).
The energy model simulates all combinations for the simulation parameters listed above, generating thousands of individual energy model simulations and the corresponding large dataset of results; in this case, 5760 individual energy models. To process the data, it is represented in parallel coordinates, an interactive browser-based visualization, and is analyzed using statistical methods (see Figure 9 and 10).
With a huge amount of simulated data, we can work backwards from energy code targets to find the best matches. The recommendations for Step Code Compliance Analysis are summarized in Table 1.
Step 2 of the Step Code can be met by most floor plan, shape and envelope construction choices, but Steps 3 and 4 result in significant limitations. Step 3 eliminates the lower insulation options from the articulated square shape and the rectangle shape with the central core. Step 4 favours a more compact floor type within the square shape or the more strategic floor layout with the core on the north side of the building.
Floor types significantly impact energy performance. The articulated square floor plan has more envelope area for the same floor area of living space (shape factor). This results in poor energy performance due to increased heat transfer area. If the floor shapes are well-designed however, comparing the articulated square and the rectangle, we find that a higher shape factor does not necessarily result in less optimal energy performance. The impact of different floor layouts can be understood by comparing the rectangular shape with a central core option between another option where the core is on the North side of the building. The rectangular floor plan with the core in centre can only meet Step 4 requirements with two variations of very high requirement in all other parameters. But the rectangular floor plan with the core on the North façade, conversely, can meet Step 4 with the largest range of options. This is largely due to the fact that the stair core has a lower heating setpoint, and its thickness has a high thermal mass, while not limiting passive solar heating on the south façade.
The results from the Multivariate Sensitivity Analysis are summarized in Table 2. It shows that to effectively decrease the TEDI, a decrease in window U-value and WWR is much more effective than adding insulation to the wall when the R-value is already relatively high. Also, the depth of shades does not appear to significantly affect the TEDI, although this is some loss of solar gain to decrease cooling energy needs, increased shading depths or lower WWRs are most effective. This contributes to reducing the EUI.
Based on this analysis, it is recommended that the design of a tower consider the interactions of floor plate, WWR, shade depth at an early stage to produce in a more cohesive design and avoid facing extreme insulation or window performance (and costs) requirements later. As the study shows, the more stringent requirements of the energy codes do not necessarily exclude a high WWR or moderate wall R-value options. In some cases, less insulted walls (R-10) could still be used with an appropriate choice of floor plate type.