P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-A-DRV January 12, 2002 18:53 56 AIRCRAFT CONTROL, APPLICATIONS OF SMART STRUCTURES Combined effectiveness Leading edge Trailing edge Dynamic pressure Active aeroelastic wing: Blending of leading and trailing edge effectiveness Performance index Figure 22. Achievable roll performance by combining leading and trailing edges. for example, by an all-movable aerodynamic surface that has adaptive rotational attachment stiffness. This also pro- vides high effectiveness at low speeds, and excessive loads from diverging components or flutter instabilities at high speeds can be avoided. The usable aeroelastic effectiveness for conventional concepts is rather limited between take-off and cruise speed. Aileron reversal usually occurs between the cruise speed and limit speed, and too high an effectiveness of lead- ing edge surfaces must be avoided at the limit speed. On the other hand, adaptive all-movable concepts can provide high effectiveness at all speeds and avoid excessive loads at the high end of the speed envelope, as indicated in Figs. 23 and 24. This means, for example, that a stabilizer surface can be built smaller than would be required by “rigid” aero- dynamic low-speed performance. NEED FOR ANALYZING AND OPTIMIZING THE DESIGN OF ACTIVE STRUCTURAL CONCEPTS Of course, active materials and structural components, together with the stimulating forces, need a correct Active aeroelastic concepts Range of aeroelastic effectiveness on conventional designs Dynamic pressure Dynamic pressure Effectiveness Effectiveness Rigid aircraft Range of effectiveness for advanced active aeroelastic concept 1.0 Rigid aircraft Figure 23. Aeroelastic effectiveness of conventional and adaptive-all-movable active aeroelastic concepts. For conventional active aeroelastic concepts Usable range of aeroelastic effectiveness by flight envelope For advanced active aeroelastic concept Dynamic pressure Dynamic pressure Rigid aircraft V min V C V D V D Effectiveness Effectiveness 1.0 V min V C Rigid aircraft Figure 24. Usable range of aeroelastic effectiveness for conven- tional and advanced active aeroelastic concepts. description in theoretical structural or multidisciplinary analysis and optimization (MDAO) models and methods. Once this is provided, the actively deforming structure needs another approach for static aeroelastic analysis. The deflections of selected control surfaces of an aircraft that has conventional control surfaces can be predescribed for aeroelastic analysis. For an actively deformed structure, initial deformations without external loads first need to be determined, for example, by static analysis. As described before, the deformations achievable in con- junction with the distribution of external aerodynamic loads are essential for the effectiveness of active structural concepts for aircraft control. This requires efficient tools and methods for simultaneous, multidisciplinary analyti- cal design. The best design involves optimizing r external shape, r arranging the passive structure (topology), r sizing the passive structure, r placing and sizing the active elements, and r a control concept for the active components. The aims of this approach are the optimum result for the objective function (minimum weight, aerodynamic perfor- mance), fulfillment of all constraints like strength, and also optimization of additional objectives, such as minimum en- ergy. As depicted in Fig. 25 for the optimization of a passive structure that has different constraints for the required rolling moment effectiveness, the energy required to actu- ate the control surface can be considerably reduced, even if the required (low) roll rate is already met. MDO does not mean combining single discipline ana- lytic tools by formal computing processes. It means first a good understanding of what is going on. This is essential for a conventional design. Only from this understanding can the creative design of an active concept start. It is also very important to choose the proper analytic methods for individual disciplines. Usually, not the high- est level of accuracy is suitable for the simulation of impor- tant effects for other disciplines. This also refers to refin- ing the analytic models, where local details are usually not P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-A-DRV January 12, 2002 18:53 AIRCRAFT CONTROL, APPLICATIONS OF SMART STRUCTURES 57 Structural weight Rolling moment effectiveness Baseline static design 1.0 Aileron hinge moment [kNm] Rigid 50.0 10.0 Figure 25. Optimization of the rolling moment and hinge mo- ment of a trailing edge aileron of a low aspect ratio fighter wing. interesting for interactions. It is more important to keep the models as versatile as possible for changes in the de- sign concepts and to allow the simulation as many variants as possible. This also means an efficient process for gener- ating models, including the knowledge of the user for this process. Fully automated model generators can create ter- rible results, if the user cannot interpret or understand the modeling process. Any improvement in a technical system is often referred to as an optimization. In structural design today, this ex- pression is mainly used for formal analytic and numerical methods. Some years after the introduction of finite ele- ment methods (FEM) for analyzing aircraft structures, the first attempts were madetousethese tools in anautomated design process. Although the structural weight is usually used as the objective function for optimization, the major advantage of thesetools is the fulfillmentof aeroelastic con- straints, not the weight saving. Other than static strength requirements, which can be met by adjusting the dimen- sions of individual finite elements, the sensitivities of the elements to aeroelastic constraints cannot be expressed so easily. In the world of aerodynamics, the design of the required twist and camber distribution for a desired lift at minimum drag is also an optimization task. Assuming that minimum drag is achieved by an elliptical lift distribution along the wingspan, this task can be solved by a closed formal so- lution and potential flow theory. More sophisticated nu- merical methods are required for the 2-D airfoil design or for Euler and Navier–Stokes CFD methods, which are now maturing for practical use in aircraft design. Formal optimization methods have been used for con- ceptual aircraft design for many years. Here, quantities such as direct operating costs (DOC) can be expressed by rather simple equations, and the structural weight can be derived from empirical data. Formal methods such as op- timum control theory are also available for designing the flight control system. So, one might think that these individual optimization tasks could easily be combined into one global aircraft op- timization process. The reasons that this task is not so simple is the different natures of the design variables of in- dividual disciplines and their cross sensitivities with other disciplines. The expression multidisciplinary optimization (MDO) summarizes all activities in this area, which have intensified in recent years. It must be admitted that today most existing tools and methods in this area are still single discipline optimization tasks that have multidisciplinary constraints. To design and analyze active aeroelastic aircraft con- cepts, especially when they are based on active materials or other active structural members, new quantities are re- quired to describe their interaction with the structure, the flight control system, and the resulting aeroelastic effects. SUMMARY, CONCLUSIONS, AND PREDICTIONS In the same way as it was wrong in the past to demand that an aircraft design to be as rigid as possible, it’s wrong now to demand a design that is as flexible as possible. It is sometimes said that smart structural concepts can completely replace conventional control surfaces. But this looks very unrealistic, at least at the moment. The major difficulties for successful application are the limited defor- mation capacity of active materials, as well as their strain allowables, which are usually below those of the passive structure. However, this can be resolved by proper design of the interface between the passive and active structures. But the essential difficulties are the stiffness and strain limitations of the passive structure itself. It cannot be ex- pected that the material of the passive structure just needs to be replaced by more flexible materials without an exces- sive weight penalty. It is also not correct to believe that an active aeroelastic concept becomes more effective, if the flexibility of the structure is increased. Aeroelastic effec- tiveness depends on proper aeroelastic design, which needs certain rigidity of a structure to produce the desired loads. A very flexible structure would also not be desirable from the standpoints of aerodynamic shape, stability ofthe flight control system, and transmission of static loads. Because large control surface deflections are required at low speeds, where aeroelastic effects on a fixed surface are small, it is more realistic to use conventional control surfaces for this part of the flight envelope and use active aeroelastic deformations only at higher speeds. This would still save weight on the control surfaces and their actua- tion system due to the reduced loads and actuation power requirements. To produce usable deformations of the structure also at low speeds, all-movable aerodynamic surfaces that have a variable attachment stiffness are an interesting option. This concept relies on development efforts for active de- vices that have a wide range of adjustable stiffness. The reasons that we have not seen more progress to date in successfully demonstrating smart structural concepts in aeronautics may be that r specialists in aircraft design do not know enough about the achievements in the area of smart mate- rials and structures, and r smart materials and actuation system specialists, who try to find and demonstrate applications in P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-A-DRV January 12, 2002 18:53 58 AIRCRAFT CONTROL, APPLICATIONS OF SMART STRUCTURES aeronautics, do not know or care enough about real- world conditions for airplane structures. What we need is more awareness on both sides, as well as stronger efforts to learn from each other and work together. Although there are strong doubts about useful applica- tions of smart structures for aircraft control, it should al- ways be remembered how often leading experts have been wrong in the past in their predictions, in many cases even on their owninventions. Norman R.Augustine quotes some of them in his famous book “Augustine’sLaws” (46): r “The [flying] machines will eventually be fast; they will be used in sport but they should not be thought of as commercial carriers.”–Octave Chanute, aviation pioneer, 1910. r “The energy produced by the breaking down of the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine. – Ernest Rutherford, physicist, ca. 1910. r “Fooling around with alternative currents is just a waste of time. Nobody will use it, ever. It’s too dan- gerous it could kill a man as quick as a bolt of lightning. Direct current is safe.”–Thomas Edison, inventor, ca. 1880. Also quoted by Augustine (46), the eminent scientist Niels Bohr remarked: “Prediction is very difficult, especially about the future.” At the moment it looks more realistic that new hybrid, concentrated active devices, positioned between a passive but properly aeroelastically tailored main aerodynamic surface and the corresponding control surfaces are showing the like Hopefully this article will inspire useful applica- tions of smart structures and prevent some unnecessary research. BIBLIOGRAPHY 1. O. Wright, How We Invented the Airplane. Dover, Mineola, NY, 1988. 2. T.A . Weisshaar, Aeroelastic tailoring—creative use of unusual materials. AIAA-87-0976-CP 3. W. Schwipps, Schwerer als Luft—die Fr ¨uhzeit der Flugtech- nik in Deutschland. Bernhard & Graefe Verlag, Koblenz, Germany, 1984. 4. M.I. Woods, J.F. Henderson, and G.D. Lock, Aeronaut. J. (2001). 5. E. Pendleton, B. Sanders, P. Flick, and O. Sensburg. Int. Forum Aeroelasticity Struct. Dynamics, Madrid, Spain, 2001. 6. J.J. 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SCHODEK Harvard University Cambridge, MA INTRODUCTION An inextricable link has existed historically between a building’s characteristics—form, appearance, and function—and the characteristics of the different materi- als that were available and suitable for construction. As exemplified by historical building traditions in stone and wood, early architects sought to understand intuitively the intrinsic physical behavior of commonly available materials to exploit their properties in designing and constructing buildings. Conversely, later innovations in the type and availability of materials strongly impacted the development of new architectural forms as architects began to respond to changing societal demands and new building functions emerged. This trend is illustrated by the development of steel in the nineteenth century and the related emergence of long-span and high-rise building forms. Today, architects are beginning to look forward to using the developments in smart materials to bring new solutions to long-standing problems and also to exploit the potential of smart materials in developing new building functions, forms, and responses. The wide variety of smart materials available has great potential for use within the field, but, in this area, their applications remain only marginally explored. MATERIAL CONSIDERATIONS IN ARCHITECTURE Unlike materials used for specific applications or products such as in refractory linings or engine blocks that are fundamentally chosen on the basis of performance crite- ria and cost, the choice of materials for architectural use has always been based on very different types of criteria. Performance and cost obviously play a role, but the final selection is often based on appearance and aesthetics, ease of constructability in terms of labor skill, local or regional availability, as well as the material used in nearby exist- ing buildings. The multimodal nature of the selection pro- cess coupled with the wide-ranging array of building types, uses, and locales has resulted in a material palette that en- compasses all of the major material classes. TRADITIONAL MATERIAL CLASSIFICATIONS IN ARCHITECTURE The Construction Specification Institute (or CSI) devised a classification system in 1948 that is used throughout the architectural design and building construction indus- tries. The classification system is bipartite: the first half is devoted to the broad classes of materials typically used in buildings, including paint, laminate, and concrete, and the second half categorizes standard building components such as doors, windows, and insulation. The emphasis in both major groupings is on application, not on fundamental behavior or properties. For example, in Division 6 the char- acteristics of wood are discussed in relationship to their relevance to the intended application: the grade of wood suitable for load-bearing roof structures or the type of wood suitable for finish flooring. The CSI index serves as a template for communication among architects, contractors, fabricators, and suppliers. After the preliminary design of a building is completed and approved, architects prepare construction documents (known as CDs) that will serve as the “instructions” for constructing the building. Accompanying each set of CDs are the “Construction Specifications”: a textual document that defines each building element documented in the CDs and specifies the material or component. The Construction Specifications serve as a binding contract that construc- tion professionals and contractors must follow. Trade asso- ciations and manufacturers of building products routinely write their material and product specifications in CSI for- mat to streamline the specification process for architects, and many architectural firms maintain an internal set of Construction Specifications that is used as the baseline for all of their projects. TRADITIONAL TECHNOLOGY CLASSIFICATION IN ARCHITECTURE The CSI index also categorizes the technologies used in architectural design and construction. Unlike the standard technology classifications used in engineering sciences that categorize according to process and product, the CSI specifications categorize by system. As in the CSI P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-A-DRV January 12, 2002 18:53 60 ARCHITECTURE 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 intoarchitecture 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 P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-A-DRV January 12, 2002 18:53 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 loadingon the building’sstructural 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 P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-A-DRV January 12, 2002 18:53 62 ARCHITECTURE 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 CO 2 and 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 developmentsin 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 P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-A-DRV January 12, 2002 18:53 ARCHITECTURE 63 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 lightingsystems 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 P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-A-DRV January 12, 2002 18:53 64 ARCHITECTURE 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. [...]... Mn2 O3 Mg + 2MnO2 + H2 O → Mn2 O3 + Mg(OH )2 Zn+2MnO2 → ZnO + Mn2 O3 Zn + HgO → ZnO + Hg Zn + 0.5O2 → ZnO 2Li + 2SO2 → Li2 S2 O4 Li + MnO2 → LiMnO2 1. 6 2. 8 1. 5 1. 34 1. 65 3 .1 3 .1 22 4 2 71 22 4 19 0 658 379 28 6 Based only on active cathode and anode materials ies, chemical reversibility on the electrodes is crucial to maintaining good capacity retention Raw materials and fabrication costs, cell safety, and. .. Oxygen 1. 8 Li1−xNi0.85Co0 .15 O2 1. 6 Li1−xCoO2 0.0 0 .2 0.4 0.6 Lithium content 0.8 1. 0 Figure 10 Variations of the oxidation state of the transitionmetal ions and oxygen content as lithium content varies in Li1−x CoO2−δ and Li1−x Ni0.85 Co0 .15 O2−δ into the O:2p band rather than the Co:3d band during the electrochemical extraction of lithium Introduction of a significant amount of holes into the O:2p band... (19 98) 26 W Li and J Curie, J Electrochem Soc 14 4: 27 73 (19 97) 27 X.Q Yang, X Sun, and J McBreen, Electrochem Commun 2: 10 0 (20 00) 28 L Croguennec, C Pouillerie, and C Delmas, J Electrochem Soc 14 7: 13 14 (20 00) 29 A.N Mansour, X.Q Yang, X Sun, J McBreen, L Croguennec, and C Delmas, J Electrochem Soc 14 7: 21 0 4 (20 00) 30 M Balasubramanian, X Sun, X.Q Yang, and J.Mcbreen, J Electrochem Soc 14 7: 29 03 (20 00)... Electrochem Soc 14 5: L53 (19 98) 41 S Choi and A Manthiram, J Electrochem Soc 14 7: 16 23 (20 00) 42 R.J Gummow, D.C Liles, and M.M Thackeray, Mat Res Bull 28 : 12 49 (19 93) 43 A.R Armstrong and P.G Bruce, Nature 3 81: 499 (19 96) 44 I.J Davidson, R.S McMillan, and J.J Murray, J Power Sources 54: 20 5 (19 95) 45 M.M Doeff, T.J Richardson, and L Kepley, J Electrochem Soc 14 3: 25 07 (19 96) 46 Y.U Jeong and A Manthiram,... eg band For LiNi0.85 Co0 .15 O2 , the electrons will be removed fr band for (1 − x) > 0 .15 Because the eg band lies w the O:2p band, this system does not lose oxygen a lower lithium content The band diagrams in F consistent with the recent spectroscopic evidenc introduction of holes into the O:2p band rather the Co:3d band in LiCoO2 (20 , 21 ) and into the N in Li1−x NiO2 and Li1−x Ni0.85 Co0 .15 O2 (29 ,30)... O1 structures by sliding of oxide layers c Intensity (arbitrary uni Li0 .23 Ni0.85Co0 .15 O2 Li0.09Ni0.85Co0 .15 O2 Li0.002Ni0.85Co0 .15 O2 10 20 30 40 50 60 70 80 Cu Kα 2 (degree) Figure 13 X-ray diffraction patterns of Li1−x Ni0.85 Co0 .15 O2−δ that were synthesized by chemical delithiation lower lithium content (1 − x) compared to that in Li1−x CoO2 permit a higher capacity in the Li1−x Ni0.85 Co0 .15 O2... State Lett 2: 4 21 (19 99) 47 J Kim and A Manthiram, Nature 390: 26 5 (19 97) 48 J Kim and A Manthiram, Electrochem Solid State Lett 2: 55 (19 99) (19 97) 56 C Lampe-Onnerud, J.O Thomas, M Hardgra Yde-Andersen, J Electrochem Soc 14 2: 3648 (19 9 57 A Manthiram and J B Goodenough, J Power Sour (19 89) 58 A.K Padhi, K.S Nanjundaswamy, and J.B G J Electrochem Soc 14 4: 11 88 (19 97) 59 N Imanishi, Y Takeda, and O Yamamoto,... absence of oxygen the maintenance of the initial O3 structure to Intensity (arbitrary unit) Li0.35CoO2 Li0 .16 CoO2 - O1 Li0.02CoO2 - P3 10 20 30 40 50 60 70 80 Cu Kα 2 (degree) Figure 11 X-ray diffraction patterns of Li1−x CoO2−δ that were synthesized by chemical delithiation A A A C B A A C B B c C B A C C B A A A C C B C C B B A c B B B A A A O3 P3 O1 Figure 12 Schematic of the transformation of O3... Layer T1 O Layer T2 Li Layer c b a Figure 15 Schematic representation of the diffusion processes of nickel ions in Li1−x Ni0.85 Co0 .15 O2 Dotted and solid squares refer to tetrahedral site and lithium-ion vacancy, respectively T1 and T2 refer to tetrahedral sites at (0, 0, 0 . 12 5) and (0, 0, 0.375), respectively Although LiCoO2 and LiNi0.85 Co0 .15 O2 are attra didates, both Co and Ni are expensive and. .. Electrochem Soc 14 7: 29 03 (20 00) 31 R.V Chebiam, F Prado, and A Manthiram, J Electrochem Soc 14 8: A49 (20 01) 32 J E Huheey, Inorganic Chemistry: Principles of Structure and Reactivity Harper & Row, NY, 19 72, p 29 4 33 M.M Thackeray, Prog Solid State Chem 25 : 1 (19 97) 34 A Manthiram and J Kim, Chem Mat 10 : 28 95 (19 98) 35 A Manthiram and J Kim, Recent Res Dev Electrochem 2: 31 (19 99) 36 S.J Wen, T.J Richardson, . ZnO · Mn 2 O 3 1. 6 22 4 Magnesium Mg MnO 2 Mg + 2MnO 2 + H 2 O → Mn 2 O 3 + Mg(OH) 2 2.8 2 71 Alkaline MnO 2 Zn MnO 2 Zn+2MnO 2 → ZnO + Mn 2 O 3 1. 5 22 4 Mercury Zn HgO Zn + HgO → ZnO + Hg 1. 34 19 0 Zinc–air. PbO 2 Pb + PbO 2 + 2H 2 SO 4 → 2PbSO 4 + 2H 2 O 2 .1 120 Nickel–cadmium Cd NiOOH Cd + 2NiOOH + 2H 2 O → 2Ni(OH) 2 + Cd(OH) 2 1. 35 18 1 Nickel–hydrogen H 2 NiOOH H 2 + 2NiOOH → 2Ni(OH) 2 1. 5 28 9 Nickel–metal. 19 0 Zinc–air Zn O 2 Zn + 0.5O 2 → ZnO 1. 65 658 Li–SO 2 Li SO 2 2Li + 2SO 2 → Li 2 S 2 O 4 3 .1 379 Li–MnO 2 Li MnO 2 Li + MnO 2 → LiMnO 2 3 .1 28 6 a Based only on active cathode and anode materials. or