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Active Façades 

demand-responsive programs | active load management window strategies

Smart windows and shading systems have optical and thermal properties that can be dynamically changed in response to climate, occupant preferences and building energy management control system (EMCS) requirements. These include motorized shades, switchable electrochromic or gasochromic window coatings, and double-envelope macroscopic window-wall systems. "Smart windows" could reduce peak electric loads by 20-30% in many commercial buildings and increase daylighting benefits throughout the U.S., as well as improve comfort and potentially enhance productivity in our homes and offices. These technologies will provide maximum flexibility in aggressively managing demand and energy use in buildings in the emerging deregulated utility environment and will move the building community towards a goal of producing advanced buildings with minimal impact on the nation's energy resources. Customer choice and options will be further enhanced if they have the flexibility to dynamically control envelope-driven cooling loads and lighting loads.

Demand-responsive programs 

A variety of different strategies have been implemented by utilities and other their customers in attempts to manage and reduce electric load. Most have been voluntary, with various economic incentives associated with the strategies. Demand responsive programs provide a means to economically incent customer's participation to shed load or use alternate energy sources during critical periods of high demand. In the recent context of the 2000-2001 California energy crises, the emphasis of such programs has been on a near immediate response to curtail energy loads to avoid impending electricity outages. In the long-term, there is a need to increase customer participation in managing finite regional and nationwide energy resources to reduce price volatility and improve system reliability (Kueck et al. 2001). Demand responsive programs and utility rate structures such as time-of-use (TOU) and real-time pricing (RTP) schedules cause customers to directly experience the time-varying costs of their consumption decisions and therefore act as an incentive for customers to actively manage their loads.

Many of the simple curtailment strategies utilized in California during the past summer enabled customers to shed load without incurring additional capital costs for existing as-is facilities. However, in some cases, these strategies significantly impacted the comfort and potentially the health and productivity of the building tenants. Strategies included increasing temperature set points in occupied spaces, reducing fan speed or run-time, switching off lighting, reducing outside air intake volume, and pre-cooling the building during off-peak hours. Drawbacks of such strategies include thermal and visual discomfort; potential increases in CO2 levels, and possible degradation of indoor air quality depending on the severity of the load response required. Preferable strategies are those that can provide significant load shed with minimum negative impacts to building tenants.

Utility load management programs have historically been aimed at reducing demand during critical times (such as summer or winter peak) using either direct load control (utility operates customer's equipment) or interruptible load programs (customer implements method of load shed). The critical summer peak for the commercial sector occurs in the afternoon and is driven predominantly by weather: hot temperatures and high solar gains. For example, the California statewide commercial building sector peaked at 23,000 MW at 2 PM, an increase of 15,000 MW from nighttime usage. Together, interior lighting and air-conditioning in the commercial sector make up 25% or 12,476 MW of the total 1999 California statewide peak load for all electricity use sectors (Brown and& Koomey 2002). Cooling loads are dominant in all large commercial building types and more than one-third is due to lighting and another one-third to solar heat gains through windows (Franconi and& Huang 1996).

Therefore, for interruptible load programs, strategies involving daylighting and window solar heat gain management offer significant demand reduction potential without the negative drawbacks of occupant discomfort. Peak daylight availability coincides with summer peak periods enabling reduction of lighting and cooling demand, given careful control of solar heat gains in perimeter zones. This is a critical concept associated with the strategies listed in the next section. Many people recognize that control of solar heat gains during peak periods can be accomplished by simply blocking all solar radiation before or just after it enters the window. People also recognize that admitting daylight (solar radiation) reduces the need for electric lighting. Determining the optimum energy balance between solar heat gains (increased cooling) and daylight (decreased lighting and cooling) is a critical issue and is key to optimizing window and lighting peak demand reductions during the summer. Other long-term opportunities not normally associated with window systems are those that allow windows to become part of the space-conditioning solution. Natural ventilation, heat extraction, and nighttime cooling strategies using operable windows reduce a building's dependence on mechanical cooling or shifts the load to off-peak hours.

Active load management window strategies 

Demand responsive (DR) strategies below have been loosely defined as solutions that provide a 1-2 hour response or a 24-hour response to requests for load shed. Short-term solutions are those that can be implemented within existing buildings. Long-term solutions are those that are more cost-effective and practical to implement in new buildings or in buildings that are being extensively renovated.

GSW operable vertical shades, Berlin, Germany (click for link to case study)

Short-term strategies for existing buildings 

A. Occupants voluntarily close interior shades on all windows. Lighting is curtailed. Request is made over a central public address system or by email notification system. Flyers distributed before event could explain manual strategy. 1-2 hour notification. 

B. Motorized interior or exterior shades are closed automatically by the facility manager during a load shed event. Lighting is curtailed. 1-2 hour notification.

Long-term strategies for new or renovated buildings 

C. Automated exterior or interior shading systems combined with daylighting controls to reduce cooling and lighting loads. 1-2 hour notification. 

D. Automated switchable windows controls (e.g., electrochromics) combined with daylighting to reduce cooling and lighting loads. 1-2 hour notification. 

E. Heat extraction double-envelope façades with automated venting during peak periods to reduce cooling loads. Lighting loads could also be reduced with daylighting controls. 1-2 hour notification. 

F. Pre-cooling of thermal mass using nighttime natural or mechanical ventilation through windows. 24-hour notification.

Strategy A involves a request to all building personnel via email notification, flyers, or the public address system to voluntarily close their window shades so that the entire window surface is blocked. If the shade allows one to modulate daylight (such as Venetian blinds or louvers), the occupant is asked to tilt the blind angle so that incoming daylight is sufficient to meet task lighting levels. Occupants near windows are also asked to switch off unnecessary lighting. This strategy can be applied to most commercial buildings without additional expenditures. Its effectiveness is dependent on the level of voluntary cooperation. Effectiveness is also dependent on baseline shade usage, shade type and reflectance, properties of existing window glazing, and window size and orientation. For example, white shades can reflect solar radiation back through the window if the window glazing has a high transmittance. Impacts on occupants are limited to annoyance at the disruption. Impacts on demand can be as much as 3 W/ft² -floor in perimeter zones. If two- or three-stage fluorescent light switching exists in the building, demand reductions may be less.

Strategies B and C are similar to A in that interior or exterior shades are used to reduce solar heat gains and manage daylight admission during critical peak periods. In this case, it is assumed that the facility manager through a central control system deploys the shades automatically. Lighting is curtailed either manually or automatically. Strategy B assumes that such a system exists within the building, which is admittedly unlikely for the majority of the commercial building stock. Retrofitting motors to existing static shades is costly. Strategy C assumes that new motorized shades are installed in new or existing buildings. If the building is new, the shades could be coupled to work with dimmable lighting using an integrated control system to achieve better reliability and energy efficiency. With existing buildings, switchable or dimmable lighting would need to be installed. Impacts on demand are similar to strategy A: as much as 3.5 W/ft² -floor in perimeter zones. Occupant disruption is likely as well; however, occupant override should be allowed during non-critical peak periods.

Peak electric demand (W/ft²) for a west-facing perimeter zone (please click on the picture for high resolution image)

Strategy D uses low-maintenance, non-mechanical means to regulate solar heat gains and daylight. Switchable windows include electrochromic or gasochromic glazings, which can be modulated from a clear to a dark tinted state (similar to switchable sunglasses) with either a small-applied voltage (3-5V DC) or a minute influx of gas (e.g., hydrogen). Electrochromic glazings are commercially available in limited quantities in Germany. U.S. products are anticipated to enter the market in 2003. Gasochromic glazings are still under development. Competitively priced products are dependent on volume and on how quickly products get adopted into the marketplace. Electrochromic windows with dimmable daylighting controls regulate peak demand in a similar fashion to strategy C and are slated for new construction. Demand reductions can be as much as 4.75 W/ft² -floor in the perimeter zone.

Heat extraction double-skin façades (strategy E) rely on sun shading located in the intermediate space between the exterior glass façade and interior façade to control solar loads. The concept is similar to exterior shading systems in that direct solar radiation loads are blocked before entering the building, except that heat absorbed by the between-pane shading system is released within the intermediate space then drawn off by ventilative means. Cooling load demands are diminished with this strategy. During peak conditions, mechanical ventilation can be used to extract heat if natural means (via thermal buoyancy or stack ventilation) are insufficient. Impacts on peak demand are difficult to quantify due to the complexity of the heat exchange. Occupant impacts are minimal; again, override on shade use should be allowed during noncritical periods. Thermal comfort may be improved, compared to some façade systems, due to a reduction in the interior window surface temperature if designed correctly.

During the summer and in the some climates where there is sufficient variation in diurnal outdoor temperatures, nighttime ventilation (strategy F) can be used to cool down the thermal mass of the building interior and reduce air-conditioning loads. Heat gains generated during the day are absorbed by furnishings, walls, floors, and other building surfaces then released over a period of time in proportion to the thermal capacity of the material. Removal of these accumulated heat loads can be achieved with a variety of cross-ventilation schemes that rely on wind-induced flow, stack effect, and/or mechanical ventilation. Deployment of such a strategy for peak demand reductions must be implemented given a 24-hour notification. In recent years, the concept of radiant cooling has been coupled with traditional cross ventilation schemes. For some climates and building types, this strategy can be used to completely eliminate the need for mechanical air-conditioning. Heavyweight thermal mass is strategically located in exposed concrete ceilings. This mass is "activated" or cooled at night using outdoor air directed to flow over its unobstructed surface. During the day, occupants exposed to this chilled thermal mass perceive a cooler environment due to a radiative exchange with the low surface temperature of this thermal mass. Peak demand reductions can be significant particularly if all central cooling requirements are eliminated and if rules for proper daylighting are observed (see strategy C). 

References 

Brown, R.E. and J.G. Koomey. 2002. "Electricity Use in California: Past Trends and Present Usage Patterns." Submitted to Energy Policy for publication. LBNL-47992, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA. 

Franconi, E. and Y.J. Huang. 1996. "Shell, System, and Plant Contributions to the Space Conditioning Energy Use of Commercial Buildings." In Proceedings for the ACEEE 1996 Summer Study on Energy Efficiency in Buildings, Asilomar Conference Center, Pacific Grove, CA. Washington, D.C.: American Council for an Energy-Efficient Economy. 

Kueck, J.D., B.J. Kirby, J. Eto, R.H. Staunton, C. Marnay, C.A. Martinez, and C. Goldman. 2001. Load as a Reliability Resource in Restructured Electricity Markets. ORNL/TM2001/97 and LBNL-47983. Oak Ridge, Tennessee: Oak Ridge National Laboratory. 

Lee, E.S., S.E. Selkowitz, M.S. Levi, S.L. Blanc, E. McConahey, M. McClintock, P. Hakkarainen, N.L. Sbar, M.P. Myser. 2002. "Active Load Management with Advanced Window Wall Systems: Research and Industry Perspectives". Accepted for presentation and to be published in Proceedings from the ACEEE 2002 Summer Study on Energy Efficiency in Buildings: Teaming for Efficiency, August 18-23, 2002, Asilomar, Pacific Grove, CA. Washington, D.C.: American Council for an Energy-Efficient Economy.

Question/Information: eslee@lbl.gov