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material classes, the focus of the technology classes is also on application. The technologies are divided into two major groups: the first is devoted to building op- erational systems such as HVAC, lighting, and plumb- ing systems, and the second is devoted to building con- struction systems such as structural, drainage, and ver- tical circulation systems. The specifications for the build- ing operational systems are almost entirely supplied by manufacturers.
PROPOSED CLASSIFICATION SYSTEM FOR SMART MATERIALS
The introduction of smart materials into architecture poses a challenge to the normative classification system. A smart material may be considered as a replacement for a con- ventional material in many components and applications, but most smart materials have inherent “active” behaviors, and, as such, are also potentially applicable as technolo- gies. For example, electrochromic glass can be simultane- ously a glazing material, a window, a curtain wall system, a lighting control system, or an automated shading system.
The product would then fall into many separate categories, rendering it particularly difficult for the architect to take into consideration the multimodal character and perfor- mance of the material. Furthermore, many smart mate- rials are introducing unprecedented technologies into the field of design, and are also making more commonplace many technologies, such as sensors, which previously had only limited application in highly specialized functions.
Table 1 describes a proposed organization in which smart materials establish a sequential relationship between ma- terials and technologies. The proposed organization also maintains the fundamental focus on application of the traditional classification system.
Table 1. Proposed Classification System for Smart Materials and Systems
Category Fundamental Material Characteristics Fundamental System Behaviors
Traditional materials: Materials have given properties Materials have no or limited Natural materials (stone, wood) and are “acted upon” intrinsic active response
Fabricated materials (steel, capability but can have good
aluminum, concrete) performance properties
High performance materials: Material properties are designed
Polymers, composites for specific purposes
Smart materials: Properties are designed to Smart materials have active Property-changing and energy-exchanging respond intelligently to varying responses to external stimuli and
materials external conditions or stimuli can serve as sensors and actuators
Intelligent components: Behaviors are designed to Complex behaviors can be Smart assemblies, polyvalent walls respond intelligently to varying designed to respond intelligently
external conditions or stimuli in and directly to multimodal demands discrete locations
Intelligent environments Environments have designed Intelligent environments consist interactive behaviors and of complex assemblies that often intelligent response—materials combine traditional materials and systems “act upon” the with smart materials and
environment components whose interactive
characteristics are enabled via a computational domain TAXONOMY OF SMART MATERIALS
Four fundamental characteristics are particularly relevant in distinguishing a smart material from the traditional materials used in architecture: (1) capability of property change (2) capability for energy exchange, (3) discrete size/location, and (4) reversibility. These characteristics can potentially be exploited either to optimize a material property to match transient input conditions better or to optimize certain behaviors to maintain steady-state condi- tions in the environment.
Smart Material Characteristics
Property Change. The class of smart materials that has the greatest volume of potential applications in architec- ture is the property-changing class. These materials un- dergo a change in a property or properties—chemical, thermal, mechanical, magnetic, optical, or electrical—in response to a change in the conditions of the material’s environment. The conditions of the environment may be ambient or may be produced via a direct energy input. In- cluded in this class are all color-changing materials, such as thermochromics, electrochromics, and photochromics, in which the intrinsic surface property of the molecular spectral absorptivity of visible electromagnetic radiation is modified by an environmental change (incident solar radiation, surface temperature) or an energy input to the material (current, voltage).
Energy Exchange. The next class of materials predicted to have a large penetration into architecture is the energy- exchanging class. These materials, which can also be called
“first law” materials, change an input energy into an- other form to produce an output energy in accordance with the first law of thermodynamics. Although the energy
ARCHITECTURE 61 converting efficiency of smart materials such as photo-
voltaics and thermoelectrics is typically much less than those of conventional energy conversion technologies, the potential utility of the energy is much greater. For exam- ple, the direct relationship between input energy and out- put energy renders many of the energy-exchanging smart materials, including piezoelectrics, pyroelectrics and pho- tovoltaics, excellent environmental sensors. The form of the output energy can further add direct actuating capa- bilities such as those currently demonstrated by electrore- strictives, chemoluminescents and conducting polymers.
Reversibility/Directionality.Some of the materials in the two previous classes also exhibit the characteristic of ei- ther reversibility or bidirectionality. Many of the electricity converting materials can reverse their input and output energy forms. For example, some piezoelectric materials can produce a current from an applied strain or can de- form from an applied current. Materials that have a bi- directional property change or energy-exchange behav- ior can often allow further exploitation of their transient change rather than only of the input and output energies and/or properties. The energy absorption characteristics of phase changing materials can be used either to stabilize an environment or to release energy to the environment, de- pending on the direction in which the phase change is tak- ing place. The bidirectional nature of shape-memory alloys can be exploited to produce multiple or switchable outputs, allowing the material to replace components composed of many parts.
Size/Location.Regardless of the class of smart material, one of the most fundamental characteristics that differen- tiates smart materials from traditional materials is the discrete size and direct action of the material. The elimi- nation or reduction in secondary transduction networks, additional components, and, in some cases, even packaging and power connections allows minimizing the size of the active part of the material. A component or element com- posed of a smart material can be much smaller than a simi- lar construction using traditional materials and also will require less infrastructural support. The resulting compo- nent can then be deployed in the most efficacious location.
The smaller size coupled with the directness of the prop- erty change or energy exchange renders these materials particularly effective as sensors: they are less likely to in- terfere with the environment that they are measuring, and they are less likely to require calibration.
Relevant Properties and Behaviors
Architectural materials are generally deployed in very large quantities, and building systems tend to be highly integrated into the building to maintain homogeneous in- terior conditions. Materials and systems must also with- stand very large ranges of transient exterior conditions.
The combination of these two general requirements tends to result in buildings of high thermal and mechanical in- ertia. Therefore, even though the typical building uses
several different materials for many functions, there are only a few areas in which the characteristics of smart mate- rials can be useful. The transient environmental conditions experienced by most buildings often results in oversizing systems to accommodate the full range of the exterior en- vironmental swing. The swings may be instantaneous, as in the case of wind, diurnal, or seasonal. These conditions include those that affect both heat transfer and daylight transmission through the building envelope (also known as the building fac¸ade or exterior skin) as well as those that create dynamic loading on the building’s structural support system. For the building envelope, the property-changing class of smart materials has the most potential application, whereas the energy-exchanging class is already finding ap- plication in building structural systems.
Buildings consume two-thirds of the electrical energy generated in the United States, and the majority of that electrical energy is used to support the building’s ambi- ent environmental systems, primarily lighting and HVAC (heating, ventilating, and air conditioning) systems. The intent of these systems is to effect a desired state in the interior. That state may be defined by a specified illumi- nance level or by an optimum temperature and relative humidity. Because conditions are generally maintained at a steady state, the primary need is for more efficacious control. Energy-exchanging materials have potential ap- plication as discrete sources, particularly for lighting deliv- ery systems, and also as secondary energy supply sources.
The most significant applications of smart materials in buildings, however, has been and will continue to be as sensors and actuators for the control systems of these am- bient environmental systems.
Smart Material Mapping
The material properties and/or characteristics that are most relevant to architectural requirements are mapped in Table 2 against examples of smart material applications.
CATEGORIES OF APPLICATIONS
One of the major difficulties in incorporating smart mate- rials into architectural design is the recognition that very few materials and systems are under single environmen- tal influences. For example, the use of a smart material to control conductive heat transfer through the building en- velope may adversely impact daylight transmission. Fur- thermore, because most systems in a building are highly integrated, it is difficult to optimize performance without impacting the other systems or disrupting control system balancing. As an example, many ambient lighting systems include plenum returns through the luminaires (lighting fixtures) that make it particularly difficult to decouple HVAC from lighting systems. The following discussion es- tablishes four major categories of applications for smart materials and takes into account the material/behavior mapping described in Table 2 but also considers the com- plex systems that are affected. The four categories—
glazing materials, lighting systems, energy systems, and
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Table 2. Mapping of Smart Materials to Architectural Needs
Architectural Need Relevant Material Characteristic Smart Material Application
Control of solar radiation Spectral absorptivity/transmission Electrochromics
transmitting through the building of envelope material Photochromics
envelope Liquid crystal displays
Suspended particle panels Relative position of envelope material Louver control systems
r exterior radiation sensors (photovoltaics)
r interior daylight sensors (photoelectrics)
r controls (shape-memory alloys)
Control of conductive heat Thermal conductivity of envelope Thermotropics
transfer through the building envelope material Phase change materials
Control of interior heat generation Heat capacity of interior material Phase change materials Relative location of heat source Fiber-optic systems
Thermoelectrics Lumen/watt energy conversion ratio Photoluminescents
Light-emitting diodes Secondary energy supply systems Conversion of ambient energy to Photovoltaics
electrical energy
Optimization of lighting systems Daylight sensing Photovoltaics
Illuminance measurements Photoelectrics Occupancy sensing
Relative location of source Fiber optics Electroluminescents
Optimization of HVAC systems Temperature sensing Pyroelectrics
Humidity sensing Hygrometers
Occupancy sensing Photoelectrics
CO2and chemical detection Biosensors Relative location of source Thermoelectrics
and/or sink Phase change materials
Control of structural vibration Euler buckling Piezoelectric
Inertial damping Magnetorheological
Electrorheological Shape-memory alloys
Strain sensing Fiber optics
monitoring/control systems—are also intended to be con- sistent with the more normative and identifiable classifi- cation systems of architecture.
Glazing Materials
Whether serving as windows or as glass curtain walls, glazing materials are extensively used on the building en- velope. Originally incorporated and developed during the twentieth century for aesthetic reasons, the current use of glazing materials also considers the delivery of daylight into the building’s interior. The majority of developments in high-performance glazing materials have focused on ther- mal characteristics—spectral selectivity to reduce radiant transmission to the interior or low emissivity to reduce ra- diant loss to the exterior. Glazing introduces the problem- atic condition in which, depending on the exterior envi- ronmental conditions, performance criteria that have been optimized for one set of conditions may be undesirable in a matter of hours or even moments later. The ideal glaz- ing material would be switchable—managing the radiant transmission between exterior to interior to transmit so- lar radiation when the envelope is conducting heat out
(typical winter daytime condition) and reflect solar radi- ation when the envelope is conducting heat into the build- ing (typical summer daytime condition). Photochromics, thermochromics and thermotropics have been proposed as switchable glazing materials, although only thermotropics are currently being developed commercially for this appli- cation. The basic operation of these materials is that ei- ther high incident solar radiation (photochromic) or high exterior temperature (thermochromic or thermotropic) produces a property change in the material that increases its opacity, thereby reducing radiant transmission to the interior. When incident solar radiation lessens or when the exterior temperature drops, the material reverts to a more transparent quality, allowing more solar radiation to trans- mit to the interior.
There are numerous circumstances, however, for which this type of switching is neither desirable nor useful. Di- rect solar radiation into the building can create over- heated zones in particular locations, even in the dead of winter. Winter sun altitude is also much lower, thereby significantly increasing the potential for glare if solar radiation is not controlled. During the summer, reduc- ing the radiant transmission may increase the need for
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Human perceptions and actions
External stimuli (Light level)
Direct user control e.g., switches
Liquid crystal film
Laminations
(Film laminated between glass layers)
Sensor control (Light level sensor) Interface
Building enclosure element (wall) with controllable transparency
Enabling technologies
Figure 1. Typical current use of a smart material in architecture. Only a single behavior is controlled.
interior lighting systems and, because all electrically gen- erated light has a lower lumen/watt ratio than daylight, might exacerbate the building’s internal heat gains. As a result, the majority of efforts to develop smart glazing have focused on the electrically activated chromogenics—
electrochromics, liquid crystal panels, and suspended particle panels (see Fig. 1). By using an electrical in- put to control transparency, these materials can be more easily incorporated into the control schemes for energy management systems and/or lighting control systems. The optimum balance among lighting needs, heating/cooling re- quirements, and occupant comfort can be determined, and the transparency can be adjusted to meet these demands in highly transient conditions.
Lighting Systems
Most high efficiency lighting systems—fluorescent, HID (high intensity discharge)—are relatively unsuitable for low-level lighting or task lighting. Furthermore, the typical ambient lighting system requires enormous infrastructure for support: electronic control systems, ballasts, integrated cooling, light diffusers/distributors (often part of the lumi- naire or lighting fixture). The efficiency and economics of these systems drop as the overall lighting requirements be- come smaller or more discrete. Ambient systems are also difficult to dim and to focus, so that very low-efficiency in- candescent/halogen systems are still widely used for task or discrete lighting requirements. The low efficiency of the typical lighting system results in producing a substantial amount of heat and can be responsible for as much as 30%
of a commercial building’s cooling load. The development of fiber-optic lighting systems allows decoupling the deliv- ered light from the primary energy conversion processes for generating light. This has the dual advantage of allowing light delivery to any location in a building, which is much more efficacious than using ambient lighting systems to de- liver light, as well as removing the heat source from the oc- cupied space. Current applications for fiber-optic systems
include many museums and retail display areas, where the removal of the heat source can profoundly improve the en- vironmental conditions of the objects under display and the discrete nature of the light allows better highlighting and focusing.
Ambient lighting systems are generally designed to pro- vide a standard illuminance level throughout a space at a specified height (usually three feet above the floor). The human eye, however, responds to the relative luminance contrast between surfaces in the field of vision. A light- ing level of 100 footcandles may be too low for reading if the surrounding surfaces provide little contrast and may be too high if the surfaces provide high contrast. The di- vision of light into smaller and more discrete sources al- lows optimizing contrast within the field of vision. Fur- thermore, the design of lighting for managing contrast enables using lower levels of lighting. Sources produced by the various luminescents—chemo, photo, electro—are starting to find application in architectural interiors, par- ticularly as emergency lighting systems, because they have low and in some cases no input power requirements. LED (light-emitting diode) systems are also being developed as low energy lighting delivery systems. The latest develop- ments in polymer LED technology have produced lighting fixtures that have precise color control. They provide ex- cellent color rendition and also allow for color variation—
features that are difficult to achieve in standard lighting systems.
Energy Systems
The majority of buildings in the United States are con- nected to a utility grid and as such have little need for primary energy conversion on-site. There are numerous circumstances, however, where secondary energy conver- sion can be quite useful, including back-up power genera- tion, peak demand control, and discrete power for remote needs. For these situations, photovoltaic energy systems are increasingly becoming popular because they can be
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readily deployed on roofs or integrated directly into the building envelope to take advantage of the incident so- lar radiation. Two other developments in smart materials hold greater promise for managing energy needs within a building. The large interior heat loads of most build- ings coupled with a diurnal exterior temperature swing has encouraged investigation into thermal mass systems for maximum exploitation of a building’s thermal iner- tia. Although theoretically sound, thermal mass systems have three major problems: (1) very slow response time, (2) the inability to switch off the phenomenon when it is not desirable, and (3) the large embodied energy re- quired to provide the necessary mass of material. Phase change materials offer the advantages of thermal mass and very few of its disadvantages. The materials can be tuned to particular temperatures and can have very rapid responses. Much less mass is required, and therefore, the materials can be packaged and distributed throughout the building much more efficiently and strategically. By lay- ering phase change materials and other smart materi- als, such as electrochromics or thermotropics, there may be a potential to add switching capability that allows ac- tivating or deactivating of the inertial behavior of the materials.
The removal of heat generated in a building is becom- ing an increasing concern as point loads from lighting, computers, and other electrical equipment escalate. Am- bient HVAC systems do not distinguish between human- generated and equipment-generated cooling needs. The ability to manage and remove the heat generated by a point load without affecting the ambient environmental system could improve the operation of the ambient system and significantly reduce the energy requirements. Ther- moelectrics are currently being explored for their potential to manage point loads discretely. Already serving as heat sinks in the majority of microprocessor cooling packages, thermoelectrics could be incorporated into integrated cool- ing for many other types of point sources. Although the devices are not practical for cooling air directly because of their low coefficient of performance (COP), they are ideal for managing the conjugate heat transfer that is charac- teristic of most nonhuman heat sources encountered in a building.
Monitoring and Control Systems
The increasing push to reduce the energy used by build- ing HVAC systems has led to tighter buildings to reduce infiltration and to larger resets for the control equipment.
This combination of an impermeable building envelope and more variable interior conditions has led to an increase in occupant complaints and indoor air quality problems.
Many of the strategies intended to reduce energy can im- pact human health adversely, and much discussion of the appropriate compromise between the two requirements continues. One solution that holds promise is DCV, or “de- mand controlled ventilation.” DCV adjusts interior venti- lation depending on the presence of occupants; it reduces ventilation when no occupants are in a room or zone and increases ventilation as more occupants enter. Because the
human need for fresh air is linked to activity, simple occu- pancy sensors are not enough. The level of carbon dioxide in a room has been proposed as a good surrogate for the amount of fresh air needed in a space, but many concerns have arisen in regard to other chemical contamination, such as finish material outgassing, that is not connected to occupancy. Chemical sensing for building monitoring has previously been too expensive to incorporate and too slow to be useful. New developments in smart sensors for environmental monitoring, particularly biosensors, hold great promise for optimizing the controls of ambient HVAC systems.
The need to control various kinds of motions and, in particular, vibrations in a structure appears in many forms. At the level of the whole building structure, ex- citations resulting from seismic or wind forces can re- sult in damage to both primary structural systems and nonstructural elements. User discomfort can also result.
Many pieces of delicate equipment in buildings also need to be protected from external vibrations by using similar strategies. Alternatively, many pieces of equipment used in buildings can produce unwanted vibrations that can prop- agate through buildings. In response to these needs, meth- ods of mitigating structural damage have been proposed that seek to control overall structural responses via con- trollable smart damping mechanisms used throughout a structure. Several smart base isolation systems for miti- gating structural damage in buildings exposed to seismic excitations have also been proposed. These dampers are based on various electro- or magnetorheological fluids or piezoelectric phenomena. Piezoelectric sensors and actua- tors, for example, have been tested for use in vibrational control of steel frame structures for semiconductor manu- facturing facilities.
Active control can be used to modify the behavior of specific structural elements by stiffening or strengthening them. Structures can adaptively modify their stiffness properties, so that they are either stiff or flexible as needed.
In one project, microstrain sensors coupled with piezo- ceramic actuators were used to control linear buckling, thereby increasing the bucking load of the column several- fold.
Several new technologies provide capabilities for dam- age detection in structures. Various kinds of optical-fiber sensors have been developed for monitoring damage in ma- terials as diverse as concrete and fiber-reinforced plastic composite laminate structures. Optical fibers are usually embedded in the material. Strain levels can be measured via wavelength shifts and other techniques. Crack devel- opment in structures made of concrete, for example, has been monitored via optical-fiber sensors, and special dis- tributed systems have been developed for use in the struc- tural health monitoring of high-performance yachts. Dis- tributed fiber-optic systems have also been proposed for leak detection in site applications involving infrastructure systems. Other site-related structural applications include using optical-fiber sensors for ground strain measurement in seismically active areas. Other applications where smart materials serve as sensors include the use of embedded temperature sensors in carbon-fiber structures.