How Does a Building Use Energy Inputs?

A conventional building constantly interacts through its outer “envelope” (skin), windows, and ventilation system with the ever-changing outside world. The portions of the ambient temperature, fresh air and lighting needs of the occupants that are not provided by the building’s natural response are supplied by energy-driven thermal, ventilation and lighting systems. Any other energy needs of its occupants, such as to run computers, must also be met.

A building is therefore by definition a “whole” physical object, and it also behaves as a “whole” dynamic system, both internally and in the larger coordinate system that includes its direct and induced interactions with the natural world. (See Box 1.) Of course, a building does not actu-ally care what temperature it is, or whether it is light or dark inside. The goal is to provide for the comfort and productiv-ity of its occupants.

The advent of advanced heating, cooling, ventilation and lighting technologies means that a building can now use en-ergy to counteract its own intrinsic response to environmen-tal changes. The internal thermal needs are basically met by heating and cooling systems that mitigate the natural response of the building: as the structure loses heat in winter, heat must be reintroduced to maintain a comfortable temperature for the occupants. And or as the building absorbs excessive heat in summer, heat must be rejected to maintain comfort. The building’s internal lighting systems compensate for in-adequate natural lighting, while shades and blinds compen-sate for glare or overheating. And the heat from the bodies of the occupants, from all the lights, and from energy-using devices (computers, copy machines, and so on) can put addi-tional strain on the building’s cooling system. All this energy-consuming compensation for the natural state of build-ings goes on constantly and simultaneously.

The productivity of occupants, which defines a building’s economic value to the building’s owners (whether those are developers, store owners, or school districts) is not determined merely by thermal comfort or sufficient lighting. It is increas-ingly understood that the quality of the space enhances its economic value. And it is becoming clear that the perceived quality of the space derives in part from the user’s ability to have control over comfort and lighting conditions. Thus one of the great gifts of passive solar buildings, daylit buildings, and energy-efficient climate-responsive buildings is that the very design practices that deliver energy efficiency improve-ments also create conditions that improve the quality of the space and the performance or productivity of the occupants.

How Can Inputs Be Reduced from Within a Building?

That all these activities actually interact through physical feedback has led to energy-saving approaches, such as energy management system (EMS) computers that constantly ana-lyze sensor inputs to reveal the state of each energy system, and that seek to optimize that state and minimize adverse interactions. In this sense, an EMS seeks to manage a building’s functions as a single “whole” system.

Research over the years has led to innovations that have dramatically reduced both the energy demand of buildings and the magnitude of internal energy-consuming interactions within them. Equally important has been the research and years of experience that now enable designers to select materials and design building envelopes (shells), windows, and interiors that respond naturally to meet the comfort requirements of their occupants. In this case, the building’s own mechanical and lighting systems become backups, “touching up” conditions only when necessary, or over a much reduced range of demand, or for less frequent or shorter times.

This has turned out to be a much more certain way to accom-plish energy efficiency than by trying to force an efficient result through the mere use of efficient components and “smart” central energy management systems. Too often we put “smart” brains into architecturally “dumb” buildings, lead-ing to far lower energy reductions than could be delivered by buildings designed and assembled to respond in more com-fortable ways internally to changing conditions outside.

How Can Inputs Be Reduced from Outside a Building?

Designing buildings to respond compatibly to the natural environment also means providing opportunities to use en-vironmental resources directly. This includes a host of pos-sible design strategies, such as passive solar heating for residences and small commercial office buildings, solar air pre-heated through ventilated building skins on commercial build-ings, solar water heating, daylighting, and even on-site electricity production.

It also includes a portfolio of possible natural cooling and ventilation techniques, including: shade from nearby trees, overhangs, or porches; light-colored or otherwise heat-reject-ing exterior surface coatings; natural cooling ventilation (either fan-forced or through operable windows); nighttime flushing of heat accumulated and stored during the day in building interior mass elements; or evaporative cooling as-sist. New building component technologies are greatly en-hancing these results, including window coatings that block unwanted heat gains in hot climates while still letting in natu-ral light, and radiant barriers to reduce heat radiation to the interior from opaque surfaces.

Daylighting (which uses solar energy for its light, rather than heating, value) is a valuable resource both for diminishing the direct (illumination) and indirect (cooling) energy demands of lighting and for enhancing the quality and beauty of any space and improving the productivity of its users. And finally, exciting developments in photovoltaics (PVs) mean building components — which can be “building-integrated” into roofing, glass, or spandrel panels or can be separate PV arrays that can generate electricity, so buildings can now use the significant surface areas available. This, in turn, can con-tribute to energy-saving goals, while the buildings themselves contribute economic value to the utility grid as “distributed utility” generators and peak-load shaving resources during the daytime.

Larger commercial buildings can use building-integrated PVs as shading devices in synergy with daylighting control require-ments. When any electricity generated is delivered to the building’s internal distribution panel, owners can reduce and manage peak load demands and charges caused by the other buildings systems. PV skylights, shingles and roofing tiles, and glass curtain wall components are now also on the market. Transparent PV windows are well along in the development stage in the laboratory.

This description of new technology options for reducing build-ing energy use by capitalizing on available environmental resources at the building site also reinforces the need to take a whole buildings perspective in the application of multiple energy-saving strategies. This is because passive solar heating can deliver up to six times more energy per square foot of area, and solar water heating can deliver up to three times more, than solar electricity. Recent federal and public excitement for “solar roofs” must be tempered by careful analy-ses to use all building components in combination so that the greatest energy and cost saving potential is realized by the overall, integrated design.

This means that buildings should be designed to be intrinsi-cally low energy users first, to use the thermal energy poten-tial of solar energy second, and then to meet a desired fraction of electricity needs through solar electric devices third. (This is set by a combination of costs and available unshaded sur-face area.) Experience has shown that a careful integration of passive solar design and daylighting into buildings, however, usually leaves ample space for the production of electricity by solar energy as well. This condition need not cause design incompatibilities, provided that heat, light and electricity from the sun are simultaneous design goals right from the start. And new products just coming on the market today integrate the two functions of electricity production and water or air heating into single devices, which further reduces the build-ing surface area that is required.

The advances in technology described here highlight the complexity of the relationships between building components, energy consumption, and building design. This can lead to inappropriate strategies in final building design due to igno-rance of the importance of the interactions. A sure way to guarantee such an unfortunate result is to select components or design elements one by one according to their individual capacity to save energy, rather than to appraise the perfor-mance of the combination of all potential measures in their actual interactive roles. This is incredibly difficult and well beyond the capacity of any one designer or even industry group. It requires a large coordinated effort as well as the avail-ability of fast, accurate, user-friendly design tools. Such tools are now beginning to emerge (such as Designing Low-Energy Buildings with ENERGY-10 software). This need for an inte-grated approach is behind calls for leadership and the contin-ued involvement of the federal government.

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