Why Partial Occupancy in Office Buildings Can Actually Increase the Energy Savings from Automated Window Shading

Before COVID-19, commercial office buildings were seen as a prime opportunity for reducing U.S. greenhouse gas emissions via energy-efficiency retrofits. Not only do office buildings account for a substantial fraction of total greenhouse gas emissions, but some energy-efficiency retrofits actually provide a payback period short enough to appeal to building owners/operators.

But today, with average office building occupancy at only about 50% of pre-pandemic levels, the interest in energy-efficiency retrofits seems to have diminished. That’s unfortunate, because some energy-saving technologies can be even more valuable in partially-occupied buildings—and automated window shading is a prime example.

How Automated Shading Saves Energy in Office Buildings

Automated shading can save energy in two ways: by adjusting the shading to minimize loads on the HVAC system, and by adjusting the shading to maximize glare-free natural illumination (which can then be “harvested” by an auto-dimming lighting control to reduce lighting energy consumption).

However, these two savings mechanisms aren’t necessarily additive: a shading setting that will minimize HVAC consumption might actually increase lighting consumption, and vice-versa. Fortunately, it’s possible to craft an automated shading control protocol that balances these two mechanisms to reduce net energy consumption.

But there’s another issue: a shading setting that will minimize net energy consumption won’t always be acceptable to building occupants: there might be too much glare, or not enough privacy, or an excessively obscured outward view. Since occupant acceptance is crucial for the success of any building technology, the automated shading control protocol must prioritize occupant satisfaction over energy savings (for example, with a user interface that enables occupants to override the automatic settings):

Despite these constraints, automated shading can still provide significant year-round energy savings—but not nearly as much as if the shading setting could be optimized just for energy savings.

However, while that’s not possible in occupied spaces, it is possible in unoccupied spaces: when a space is unoccupied, there’s no need for artificial lighting and no need to accommodate occupants’ shading preferences, so the shading can be optimized just for HVAC savings.

Factors Determining Impact of Partial Occupancy on Relative Savings from Automated Shading

When HVAC consumption significantly exceeds lighting consumption (as it does in most office buildings in the U.S.), the impact of partial occupancy on the energy savings achievable through automated shading will depend mainly on three variables:

• Whether the HVAC system attempts to adapt to partial occupancy (either automatically or via deliberate scheduling/programming).
• How the partial occupancy is manifested in terms of HVAC zones, i.e. whether
  • all the HVAC zones in the building are either fully occupied or fully unoccupied, or
  • at least some of the HVAC zones in the building are partially occupied.
• The ratio of each HVAC zone area to the area of each window equipped with automated shading, specifically whether
  • the HVAC zone area is no greater than ~3X the window area, or
  • the HVAC zone area is much greater than ~3X the window area.

The following figure shows the impact of these variables on the relative energy savings typically provided by automated shading in partially occupied buildings.

This can be summarized as follows: automated shading will save more energy when a building is partially occupied than fully occupied unless (1) the HVAC system is optimized for partial occupancy, and (2) all the perimeter HVAC zones are either fully occupied or fully unoccupied, and (3) each perimeter HVAC zone is no larger than about 3X the window area in that zone.

Few operating office buildings meet all three criteria mentioned above, so partial occupancy generally will increase the potential energy savings from automated shading.

The amount of that increase will depend on too many variables to generalize. However, it can easily reach 100 percent in buildings with a relatively high ratio of HVAC to lighting energy consumption.

Many office buildings will be permanently decommissioned in the aftermath of the pandemic, but for those that aren’t, automated shading could play an important role in minimizing operating costs in what appears to be the “new normal” of partial occupancy.