Simulating cool roofs with Energy3D

Fig. 1: Compare solar energy absorption of different colors.

Cool roofs represent a simple solution that can save significant air-conditioning cost and help mitigate the urban heat island effect, especially in hot climates. Nobel Prize winner and former Secretary of Energy, Steven Chu, is a strong advocate of cool roofs.

With Version 4.0 of Energy3D, you can model cool roofs and evaluate how much energy you can save by switching from a dark-colored roof to a light-colored roof. All you need to do is to set the colors of your building blocks. Energy3D will automatically assign an albedo value for each color.

Fig. 2: Compare dark and white roofs.

Figure 1 shows five blocks in different gray colors (upper) and their thermal view (lower). In this thermal view, blue represents low energy absorption, red represents high energy absorption, and the colors in-between represents the energy absorption at the level in-between.

Now let's compare the thermal views of a black roof and a white roof of a cape code house, as shown in Figure 2. To produce Figure 2, the date was set to July 1st, the hottest time of the year in northern hemisphere, and the location was set to Boston.

How much energy can we save if we switch from a perfectly black roof (100% absorption) to a perfectly white roof (0% absorption)? We can run the Annual Energy Analysis Tool of Energy3D to figure this out in a matter of seconds. The results are shown in Figure 3. Overall, the total yearly energy cost is cut from 6876 kWh to 6217 kWh for this small cape code house, about 10% of saving. Figure 3 shows that the majority of saving comes from the reduction of AC cost.

Fig. 3: Compare heating and AC costs (blue is white roof).

Of course, this result depends on other factors such as the U-value and thermal mass of the roof. The better the roof is insulated, the less its color impacts the energy cost. With Energy3D, students can easily explore these design variables.

This new feature, along with others such as the heat flux visualization that we have introduced earlier, represents the increased capacity of Energy3D for performing function design using scientific simulations.