Modeling Weather Futures
By Mike Williams and Jennifer Harmer
Weather – A Foundational Input
The fundamental purpose of a building is to provide protection for the occupants from exterior conditions. Historically, this has meant that building designers – architects and engineers – look at historical typical climate conditions in the building’s location and develop their building systems to meet these conditions. In the face of climate change, the building industry has been given a new challenge: Design buildings that also can adapt to unknown future changes in the exterior environment and continue to provide occupants with thermal comfort and protection against the elements.
Building energy simulations are used to estimate the annual energy performance of a proposed building design under a given set of annual exterior climate conditions. These simulations use a set of input climate parameters known as a “weather file”. The results from these energy models are often used to inform the design of heating, ventilation and air conditioning (HVAC) systems, or to determine the effect of different design strategies on the overall building performance. Intuition coupled with research by Huang et al. highlights the importance of selecting an appropriate weather file as perhaps the most important foundational input into any energy performance analysis.
Despite this, the most commonly used weather files for buildings in Canada describe the typical climate conditions from the time period of 1959 to 1989 – data that are over 25 years old. These climate files typically are referred to as CWEC files (Canadian Weather for Energy Calculations) and are available for free download from several sites. Using Toronto, Ontario, as an example location, the graph below shows the historical heating degree day trend has shifted an entire ASHRAE Climate zone over the past 60 years. Figure 1 clearly shows that using a weather file that represents the period from 1959 to 1989, as is the case with the CWEC files, no longer provides a valid representation of current climate conditions in Toronto.
Likely with this shortcoming in mind, a private organization named White Box Technologies has developed an updated set of weather files that represent typical climate conditions over the period of 2000 to 2014. These weather files, which they have termed CN2014, are available for download from their website for a modest cost ($40 per file at time of publication). It can be seen in Figure 1 that the CN2014 weather file is a more appropriate approximation of Toronto’s current climatic conditions.
Toronto is just one example. We know that climate change is a global phenomenon and suggest that a similar study of historical degree days always be conducted to determine the appropriateness of the weather file being proposed for use in an energy model.
Modeling the Future
In addition to ensuring that a relevant weather file is being used when designing new buildings, given that the implications of climate change can already be seen in the historical weather records, it seems prudent to explore how climate change may continue to alter the weather, and how that will affect buildings moving forward. This can be accomplished using climate forecasting models that estimate how the weather may change in the future.
Continuing with Toronto as our example, we obtained data available from the Weather Research Forecast (WRF) model that was commissioned by the City of Toronto in 2011. A WRF simulation is a macro-scale computational fluid dynamics (CFD) model that is completed over an entire geographic region, in this case the Greater Toronto and Hamilton Area (GTHA). Historical weather data from climate stations within the modeled region are used to “seed” the WRF model. Running simulations of potential future scenarios provides weather data for the future time period of interest. In the case of the Toronto model the period from 2040 to 2049 was simulated and we were able to obtain hourly data for this entire period.
In Figure 2, the forecast of heating degree days from the Toronto WRF study has been added to the historical values plotted in Figure 1. It can be seen in Figure 2 that the WRF model predicts continued warming and a shift from ASHRAE Climate Zone 5 to Zone 4. To put this in context, Washington, D.C., is currently in Climate Zone 4.
Implications for Building Design
To put the projected future weather into the context of the built environment, we performed three sample building energy simulations, comparing the Toronto CWEC, CN2014 and 2040s weather files. We chose to study a typical 45,000 m2 residential building assumed to be constructed with market typical architectural, mechanical and electrical systems.
The resulting projected energy use for a typical meteorological year (TMY) presented in the table show the change in total energy was minimal. This makes sense when you considered the increased demands for cooling energy will largely be offset by decreased demands for heating energy, driven by the much warmer summers and the milder winters predicted by the 2040s weather file.
However, the number of “unmet cooling hours” predicted by the simulation is a serious concern. When a model shows an unmet cooling hour, it is an indication the mechanical cooling system is not able to meet the cooling demand in a space. The implication of an unmet cooling hour is an increased likelihood that occupants will be uncomfortable. In total the model predicts 728 unmet cooling hours in the 2040s scenario. When considering that the 728 hours are likely to occur during the middle 8 hours of the day, this equates to 91 days — or 3 full months — each year where the building will be too hot in many spaces. Such conditions would undoubtedly be unacceptable to building occupants, who in the best case would be unproductive and in the worst case would be put at health risk. Occupants likely would make modifications such as adding window mounted air condition units — the return of the window shaker! — to increase comfort in the spaces. Such “tack on” systems are not only unattractive but inefficient to run, resulting in both increased electricity use and the associated equivalent carbon emissions.
What to do?
For starters, ensure that the weather file you are planning to base your building design and performance simulations on is an accurate representation of the current climate in your project’s geographic region.
Second, consider running additional simulations of future scenarios to test potential implications to building performance objectives and occupant comfort. Although building designs need to be made based on current weather conditions, it is possible to include provisions to adapt the building allowing it continue to perform in the future. Examples of adaptable design considerations include: designing a structure that can accommodate the addition of external shading devices in the future; using phase changing glass; or designing a building that relies on high performance passive solutions rather than mechanical systems to meet occupant comfort conditions.
For further reading and more detail on the above, please refer to the White Paper: Modeling Weather Futures that fully summarizes the findings of this study.