True Analytics™ - Energy Savings, Comfort, and Operational Efficiency
Thanos Tzempelikos, M.A.Sc., Ph.D.
Driven by technological advances in transparent building facades and the general motivation for high quality green buildings, facade design alternatives have shifted to utilizing dynamic fenestration and shading systems for optimal control of daylight and solar gains. The concept of dynamic facades (installing controllable elements on the building envelope) is not new; however, it is only during the last few years that architects and engineers have started to trust these systems and use them in buildings.
Dynamic building envelopes include advanced window technologies, innovative fenestration systems and automated shading control, all of which characterize the new “intelligent” buildings generation (together with efficient HVAC control systems). Although a great idea, the design and implementation of such systems is a quite complex task.
Each building requires a different design approach, depending on the type of use, climate, orientation and transparency. During the early design stage, the building design team has to choose from a wide variety of design options, for many of which the evaluation of their impact on building performance could be difficult –especially for innovative technologies. Inevitably, the selection of final design solutions often involves many subjective factors. The fragmented nature of the building process, in which no member of the design team considers the overall optimization of the indoor environment, further compounds the problem. Therefore, traditional passive designs are often suggested as the “safe” traditional solution in the final stage.
Nevertheless, the advantages of dynamic elements on the building envelope are obvious. For example, glare can be controlled efficiently if automated shading is used in perimeter zones. Roller shades move automatically so as to block direct sunlight and allow diffuse light into the room, thereby eliminating glare and creating a pleasant luminous environment; horizontal (venetian) blinds will re-direct natural daylight deep into the space and improve lighting uniformity even in open plan offices; automated operable windows will allow for natural ventilation in order to reduce overheating and bring fresh air in the building.
It is the fact that the design of a dynamic façade equipped with all the above is complex that does not allow for every building to be designed in this way. The performance of the building envelope relates to different aspects of the buildings’ operations (heating, cooling, lighting) and human comfort (thermal and visual). Consequently, an integrated approach should be followed from the early design stage in order to achieve optimal results, with architects, engineers and specialized building energy consultants providing input during the design process.
Recent studies have shown that appropriate fenestration/shading design and control, linked with simultaneous control of electric lighting and HVAC components, could significantly reduce peak cooling load and energy consumption for lighting and cooling, while maintaining good thermal and lighting indoor conditions.
Regarding the benefits of automated building envelope components, there are some basic issues. In office and institutional buildings, the designers have to deal with protection from glare before anything else. That means that the shades would have to close in order to prevent direct sunlight falling on the occupants –that would also prevent overheating. If not automated, it is up to the person seated near the window to open/close the shade; but this would not contribute to reducing energy consumption for cooling/lighting since it is a random and subjective process. It has been observed that at least 30% of the people would leave the shades closed during cloudy days (minimizing useful daylight transmission) and open during clear days (increasing the cooling demand). And what about when there is no one in the room (lunch breaks, weekends, etc.)? Automated operation can solve all these problems without compromising comfort (with appropriate control). Individual needs can also be met with manual override control.
But then comes the question: “What should the properties of the shades be”? We still want natural daylight into the space (and view to the outside) but no glare. Therefore the shade should allow some daylight (diffuse) but at the same time block sunlight. In the end, it all comes down to the balance between positive and negative effects of solar radiation (and daylight). It is now known that shades with transmittance higher than 5% are bound to cause glare under a very clear sky. Therefore 5% is often a recommended value. However, the color of the shade affects the view to the outside and the absorbed heat from the sun. Dark colors allow better view but absorb more so they will increase air temperature. In other words, the color of the shade could improve visual comfort but also increase thermal discomfort. There are solutions to this problem as well. The fabric could be dark on the interior surface and light on the exterior (reflecting outside instead of absorbing). And, of course, the possibility of horizontal blinds (louvers) which rotate automatically according to the sun’s position can ensure optimal performance –however, this system has to be designed carefully. The location of the shading device is important: exterior devices outperform any interior one, but usually shades are placed inside for aesthetic and maintenance reasons.
When the task of shading design and control has been solved (at least partly at the early design stage), there is still more that can be done. The evolution of lighting controls now allows for cost effective and efficient automated light dimming (or switching). The lights will turn off (or dim to a minimum level) when there is enough daylight in the space and they will be controlled accordingly when electric lighting is needed. Occupancy sensors will ensure that no extra energy will be consumed when there is no one in the room. Light sensors are now equipped with sophisticated control algorithms that only require two-three sensors per building facade. The lights can be also controlled based on type of task and user preferences.
Now it’s just a matter of integrating lighting control operation with automated shading in order to achieve the best possible performance. How can this be done? By considering shading and lighting control, the “total lighting system”, as an integral part of envelope (and perimeter zone) design from the early design stage. The shades will respond to the continuously changing outdoor conditions and the lights will be dimmed accordingly -not necessarily as a separate system but depending on the shading properties, position and control. This may sound simple but it requires that there must be some sort of inter-communication between the two systems –and the technology is here today.
The last part of the integration of dynamic facades with the other building systems relates to the HVAC system. The impact of automated shading and lighting control on the HVAC system design and control has two parts: (i) reduce the chiller size due to effective shading, therefore reducing capital cost and (ii) reduce cooling energy consumption due to decreased internal gains (lights) and solar gains (shading). These two points should be an essential part of the early design stage because this is when the critical decisions are being made. Except for reducing the system size, appropriate dynamic temperature control in perimeter zones (taking into account shading and lighting control) could significantly reduce energy consumption. Other dynamic envelope options such as operable windows for natural ventilation are also important and need to be taken into account when designing a facade and the mechanical system requirements.
Working in projects with dynamic facade technologies has shown that, for an average size building (10-15 stories, 50,000 sq. ft) with glazed facades, the energy savings for cooling due to automated shading and lighting control could reach 40%, while the lighting energy consumption can be reduced by more than 60%, compared to a passive envelope design. Peak cooling load can also be reduced by 20-40%. These make dynamic envelope technologies very attractive, since they contribute to reduction in energy demand, reduction in greenhouse gas emissions and healthier building environments.
About the Author
Thanos Tzempelikos, M.A.Sc., Ph.D., (email@example.com) is an independent façade energy consultant and a researcher within the Solar Buildings Research Network (www.solarbuildings.ca). He has been doing research and working on projects related to optimization of perimeter zones and integrated building design (dynamic facades, daylighting, shading design and control and integration with electric lighting systems and HVAC components) for the last 5 years.
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