Overview
We typically don’t think of the building’s envelope as having much need for automation. However the fenestrations - specifically the windows - have taken on a critical role in dealing with energy related thermal loads and lighting in building spaces. The sun provides heat and light, thus affecting the thermal loads and lighting levels in building spaces. The comfort and productivity of the occupants is very important; not only thermal comfort but also “visual comfort” such as brightness, glare and shadows. Attention to windows has resulted in automation of windows and window shading devices. What follows is an overview of shading devices, electrically switchable windows and some of the control system issues.

Interior Shading
Up until a few years ago most experience with interior shading in a typical commercial office space was primarily its impact on audio-visual presentations in meeting rooms. Oftentimes blackout shades eliminated light and glare and attendees experienced almost a movie theater type of lighting environment. Interior shading today is focused on energy conservation, specifically addressing the reduction of heat gain from the sun and daylight harvesting.

Interior shading tends to be less effective than exterior shading and window treatments for energy conservation because interior shading deals with the sun’s heat after it has already entered the building. Some of the effectiveness of interior shading depends on the color of the shade; light colors reflecting some of the sun’s heat, dark colors absorbing the sun’s heat. Aside from some energy-related benefits interior shading is best when it is filtering rather than blocking light and controlling glare.

Manual interior shading further decreases the energy effectiveness of the shading because it depends solely on a user’s behavior, which may not be so dependable. The automated interior shading basically uses motors to move the shade in place, with the control of the motor determining the overall automated functionality of the shade. Automated shades can respond to sun sensors, switches, a schedule or a specific lighting condition.

When shading is deployed building-wide the network topology of the shading devices is analogous to a BAS system; that is individual shade motors are connected to controller, which in turn are connected to a BACnet network and eventually it connects to an IP network which has a server connected just for the management and monitoring of the shades. The shade server will typical have read and write capabilities for all points and values and provide data on shade position and overall system operation.

Exterior Shading
Exterior shading comes in many flavors. Shading can be attached to the building skin or designed into the building; for example windows can be set back deeply into a wall section to provide some shading. Both methods need to address exterior aesthetic concerns. Overall, exterior shading may be more aesthetically pleasing than interior shading which tends to have shades in a variety of positions. Exterior shades can be fixed or adjustable with automated adjustable shades providing the most effective means of energy savings and occupant comfort.

Automated shades, either interior or exterior, have higher initial and ongoing maintenance costs than non-automated shades, but are much more effective in energy savings than manually operated shades. So the financial calculation that is part of the decision-making is really a comparison of additional lifecycle costs of the automation versus increased lifecycle energy savings.

Electrically Switchable Glass
Electrically switchable glass goes by many names: smart glass, smart glazing, smart windows, etc. It is basically glass or glazing or coatings that change light transmission properties when voltage is applied. There are a variety of technical means to accomplish this including electrochromic, suspended particles and liquid crystals devices with different approaches and capabilities among the technical means.

When voltage is applied to electrically switchable glass the devices or coatings change to tint and absorb light. Depending on the underlying technical means, either a one-time or constant electrical current is needed to activate. The coatings or devices return to clear when current is interrupted or polarity of the voltage is reserved.

The clear to tint or tint to clear change can occur in just seconds or a few minutes depending on the technology. The tint level can be controlled manually or automated via integration into a BAS system. The control options may vary with manufacturers; some being able to go from clear to tint back and forth, and others having some intermediate levels of tinting. Much like the motorized shades, the electrically switchable glass can be manually operated via a switch or automated based on light sensors, schedule, occupancy sensors, lighting control or thermostats.

Electrically switchable glass is not new and you may have used or seen electrically switchable glass. It’s been used in interactive displays in museums, outdoor displays, privacy glass, projection screens, and in windows on planes, trains and automobiles. The rearview mirror in your car may be using one of the underlying technologies in electrically switchable glass.

The issues with electrically switchable glass involve installation cost, the limitations on the type of windows offered by some manufacturers (i.e. not applicable to operative windows), the degree of transparency of the glass, switching speeds and the ability to control intermediate light transmission states. Prices on some of the electrically switchable windows are coming down as companies’ ramp up their manufacturing to meet what they see as a huge potential marketplace segment in energy conservation.

Automation Issues
The automation issues with shading may seem simple and straight forward but they are not. There are multiple effects we try to optimize with shading and they are interrelated. For example, daylight harvesting may allow us to dim lights but also affects heat gain and possibly occupant comfort and productivity due to increased glare or brightness. Shading done properly reduces the demand for cooling and provides a modification of the lighting to a space that improves the amount and dispersal of the lighting.

Several manufacturers of motorized shades have addressed this through shading management software, which optimizes the position of the shade based on multiple criteria. These include sun position, solar intensity, BTU load, readings from indoor and outdoor photo sensors and radiometers. Some software is able to calculate the optimum shade position every 60 seconds and position the shade accordingly.

There are three systems that need to act in tandem: lighting control systems, the HVAC system and the shading system. The lighting and HVAC control systems involve energy consumption so part of the optimal operation should be to take into account the cost of energy for both systems (i.e. are we saving more money on dimming lights than we are spending on the additional cooling due to increase heat gain?). The level of complexity increases when you also start to consider systems schedules, sun sensors, occupancy sensors, room temperature, time of day, etc. Each deployment of automated shading may have different variables to consider, thus one generic solution is unattainable. The questions to be raised and answered are what will automatically trigger shading? Will shading be used with daylight harvesting? What level of integration do we need between the systems? And finally, if we are to integrate the systems, what is the sequence of operation between the three systems?

For more information about smart buildings, technology design or to schedule a Continuing Education program, email info@smart-buildings.com
 

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