How to select a climate file?
By Christoph Reinhart
ClimateStudio provides EPW files for three sites in San Francisco
ClimateStudio now directly connects users to over 30,000 climate files from a series of sources including EnergyPlus.net and Climate.OneBuilding.org. This data serves as input for the expanded site analysis workflow as well as daylighting and thermal analysis simulations. Users can search by city name to potentially find several EPW files from nearby weather stations. For example, in the case of San Francisco, the user will see files from the International Airport, Presidio and Treasure Island. The image above shows that the three sites represent weather stations south of the city, in downtown San Francisco and on an island in the bay. In a city known for its pronounced microclimates, which one of these files should you select for your latest SF project?
Climate file selector in ClimateStudio
To go through this exercise, imagine that your site was on Treasure Island but no EPW file for this location was available. In that case should you use the Presidio or the San Francisco Airport? Does it even matter? To answer this question, let’s consult our new climate summary tab below for all three EPW files.
Summary tables for three SF sites in ClimateStudio
While Koppen and ASHRAE climate zones as well as annual solar radiation are the same or very close for all files, the Presidio has somewhat lower wind speeds than the airport. More significantly, the Presidio heating and cooling degree days are 40% higher and 57% lower than at the airport, respectively. Compared to these two files, the heating and cooling degree days on Treasure Island are closer to the airport data. The wind varies somewhat between the three sites but comes in all cases mostly from the West.
Wind roses for three SF sites in ClimateStudio
How much do these differences matter for an annual daylight availability analysis? The figure below shows a spatial daylight autonomy simulation for the top floor of a three story office building, similar to the US DOE Medium Office. In all three cases, the top floor would receive one LEED daylighting credit with sDA300lux/50% values ranging from 49.4% to 50.8% for regularly occupied spaces. Annual Solar Exposure (ASE1000lux/250h) values range from 13.7% to 14.4%. These differences are within the margin of error of the simulation program.
Spatial daylight autonomy simulations for three SF sites using ClimateStudio
What about energy use intensity (EUI)? The figure below shows that the cooler climate in the Presidio area translates into about 28% less cooling energy for the investigated building, an enormous difference.
Energy Use Intensity comparison between three sites in SF
Conclusion
These results show that the choice of local climate file can be of significant consequence for an energy or thermal comfort analysis and should not be rushed through. The million-dollar question is of course how, in the absence of the Treasure Island EPW file, the user could have known that the microclimate on Treasure Island is more similar to that of the San Francisco Airport than that of the Presidio. The answer is that – without local knowledge of the area – he or she could not have predicted this. Instead it might have been useful to consult with locals and compare coincident measurements from nearby weather stations via Weather Underground. For the daylighting analysist, this procedure is less crucial.
