Thermal Model Setup
Once you have designed a well daylit and visually comfortable building with an electric lighting system in place, it is time to turn your attention towards evaluating and improving the overall energy performance of your building. In this exercise, you will first create a simple baseline thermal model of your building and then adjust occupancy, electric lighting and equipment loads to fit your future program.
Task A: Geometry
To get started, you have to convert your architectural/lighting model into a multi-zone thermal model. While the former is based on surfaces, the latter consists of touching volumes/thermal zones that fill up the overall building volume (Figure 1).
Figure 1: Architectural and thermal models of the DOE Medium Office Building (Source C. Reinhart, Climate-Driven Design)
Figure 2: World divided according to ASHRAE climate zone classification (Source: C. Reinhart, Climate-Driven Design)
The division of the overall building volume into these discrete zones follows several rules. Generally, you need different zones for different floors since internal gains (occupancy, equipment and lighting loads) are inputted as densities per floor area. This means that, if you were to combine two identical floors into one zone, the internal gains would be a factor of two too low. It is further customary to divide floors with a single usage type into a core zone, that is more than 5m away from any façade, and perimeter zones. The latter are further broken up by adjacent façade orientation (see the ClimateStudio documentation). Window surfaces have to be co-planar with their containing walls or roof surfaces. For any surfaces in direct contact with the ground, draw an additional surface that is coplanar with the surface and mark it as a ground boundary conditions. Similarly, you need to add external shading elements such as overhangs and neighboring buildings as shading surface to your thermal model. To start building a thermal model out of your architectural model, please go over the simulation workflow described in this video tutorial and/or consult this slide deck.
Task B: Assign Zone Settings
As also shown in the above linked tutorial, zone templates should be assigned by climate zone and program type. For North America, templates are available for all ASHRAE climate zones and key commercial building types. Simulation assumptions are based on the US Department of Energy’s Commercial Reference Buildings. If your building is located outside of the US, you may refer to the map below to get the corresponding ASHRAE climate zone and assign your template accordingly. Note that tis procedure is only recommended if construction practices at your location are similar to those in North America. Otherwise, you need to should work with custom templates.
Before running the thermal model, specify the carbon emissions and costs for heating, cooling, hot water and electricity under Zone Settings (Figure 3).
Task C: Adjust Internal Gains Schedules
Occupancy Schedule
Occupancy density, lighting and equipment loads in your building may be significantly different from the DOE default values with obvious consequences for the energy use of your design. To determine the correct peak occupancy, look at a typical floorplan layout and count the number of workspaces. The example floorplan in Figure 5 has 112 work spaces in total. However, it is highly unlikely that all of these workspaces including the interior conference room will ever be occupied at the same time. A more realistic peak occupancy may be 100 occupants. Given that the overall floor area is 1661m2, the peak occupancy density is 100p/ 1661m2 = 0.06p/m2.
Another question is how often all workspaces will be occupied over the course of the year? This step requires some deliberate design thinking and discussion with the future building occupants (if possible). How often will employees come in? Is hot-desking an acceptable option? Describe your thinking and report what occupancy peak and schedule you select for your building. To create an annual occupancy fraction schedule, follow the ClimateStudio Documentation.
Figure 3: Adjust carbon emissions and costs under Zone Settings
Figure 4: Thermal model and annual energy use for the baseline model
Run an annual thermal simulation and report energy use, costs and carbon emissions (EUI). This is your building’s baseline energy use (Figure 4).
Lighting Schedule
To adjust the lighting schedule use the peak lighting power densities from the Electric Lighting design exercise as described in this video tutorial. Note that if the occupancy schedule has significantly changed during the previous step, you should make sure that the electric lighting schedule reflects these changes, i.e. the system should be always “on/activated” when somebody is in the building.
Equipment Schedule
As a last step you should adjust the equipment peak load and schedule. For the former, compile a list of devices that your building occupants are going to use such as printers, laptops, desktops, monitors, projectors, 3D printers etc. For simplicity, you may assume that your equipment and lighting schedules trail building occupancy, i.e. equipment and lighting are used proportionally to occupancy. (This assumption ignores any standby loads.) Some further information on managing equipment plug loads can be found here. Rerun the thermal simulation and show annual energy use for your baseline and adjusted schedules. Submit the comparison graph (Figure 6) and comment on your findings. For example, in Figure 6 the reduced loads in the DOE Medium Office reduce the cooling loads but increase annual heating energy use.
Figure 5: Typical office floorplan with 112 workspaces including the conference room
Figure 6: Annual energy use of the DOE Medium office building for baseline and adjusted internal gain schedules