Abstract:
An airflow window system that includes at least three glazing layers positioned roughly parallel to each other to define at least two internal airflow cavities within the window system. A first of the glazing layers is adjacent a first of the airflow cavities, a second of the glazing layers is adjacent a second of the airflow cavities, and a center glazing layer is between the first and second glazing layers and separates the first and second airflow cavities. Airflow cavity openings are located adjacent the uppermost and lowermost extents of each airflow cavity, and airflow is enabled through the first airflow cavity between the openings thereof and enabled through the second airflow cavity between the openings thereof. The window system operates as a crossflow heat exchanger capable of supplying fresh outdoor air to an enclosed indoor space, while thermally tempering the incoming fresh air with outgoing indoor air.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/807,732, filed Jul. 19, 2006, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     It has been reported that individuals in the USA spend up to about 90% of their time indoors. Because poor indoor air quality has been linked to respiratory illnesses, allergies, asthma and sick building syndrome, adequate ventilation and indoor air quality are important for the health, well-being, productivity and thermal comfort of building occupants. However, heat gains and losses through infiltration and ventilation are believed to account for a significant amount of the energy required to maintain comfortable conditions within buildings. Consequently, in an effort to save energy by reducing shell heat gains and losses, the construction of the building envelope has become increasingly tighter. Increased airtightness of buildings results in less ventilation, with the result that the benefits of lower energy requirements are generally obtained at the expense of adequate indoor air quality.  
         [0003]     For commercial buildings, indoor air quality can be regulated by air systems that supply air to the indoor space by mixing fresh outdoor air with return air from the indoor space. In residential buildings, however, outdoor air typically enters the space through doors, operable windows, and infiltration. During the heating and cooling seasons, ventilation is usually limited to infiltration because residential air systems typically use only recirculated air and residential hydronic systems heat air through convection with no direct air exchange. The low ventilation associated with these systems can increase indoor pollutant levels because and indoor air pollutants (for example, emissions from indoor sources) are not able to escape the home, and insufficient outdoor air is available to dilute indoor air pollutants.  
         [0004]     In view of the above, measures for providing adequate fresh air to residential buildings are being explored, with particular emphasis on achieving improvements in indoor air quality with minimal energy usage. In recent years, integrated sustainable design concepts have been adapted that can improve indoor air quality in buildings while conserving energy. For instance, ventilated building facades are currently being integrated into commercial buildings. However, this technology has not been utilized as frequently in residential buildings because of expense and because multistory facades may not be applicable to residential designs. Another approach is windows having a ventilation capability. An example is an airflow window, which as the name implies differs from a conventional window by the existence of internal airflow, in the form of free or forced convection through an airflow cavity between two layers of glass (glazing). The potential for using airflow windows in residential construction has been explored because they are not as complicated as ventilated facades and have the potential for improving indoor air quality and conserving energy for heating and cooling while also allowing daylight to enter a room.  
         [0005]     The airflow cavity of an airflow window is usually combined with a double-glazed insulated unit (two layers of glass spaced apart and hermetically sealed with an air space therebetween), resulting in a triple-paned construction. However, various combinations of single panes or double-glazed insulated units can be used to form an airflow window. Four main modes of operation have been reported for airflow windows: supply, exhaust, indoor air curtain, and outdoor air curtain. These modes are respectively represented in  FIGS. 1 through 4 , which depict outside air being to the left of each window  100 ,  200 ,  300 , and  400 , respectively, and the inside air being to the right of each window. Typically used during the heating season, the supply air window  100  ( FIG. 1 ) draws air from the outdoor space (e.g., outside)  102  to the indoor space (e.g. a room)  104  through an airflow cavity  106  between an outside glass pane  108  (represented as a single pane) and an inside glass pane  110  (represented as a double-glazed insulated unit). Conversely, during the cooling season, the exhaust air window  200  shown in  FIG. 2  exhausts air from the indoor space  204  to the outdoor space  202  through an airflow cavity  206  between an outside glass pane  208   110  (represented as a double-glazed insulated unit) and an inside glass pane  210  (represented as a single pane).  FIGS. 3 and 4  show the indoor and outdoor air curtain windows  300  and  400 , respectively, as having airflow cavities  306  and  406  that define airflow paths from inside to inside and outside to outside, respectively. In all cases, airflow is typically from bottom to top as a result of the configurations making use of the thermal buoyancy effects as air increases in temperature. It has been reported that the exhaust air window  200  may also be used during the heating season with airflow from top to bottom.  
         [0006]     In general, the working principle of an airflow window is to entrain the solar heat that has been captured by the airflow window and direct the solar energy indoors or outdoors, depending on the operating mode of the window. Captured solar energy is used to preheat outdoor air in the supply mode of  FIG. 1 , and reheat indoor air in the indoor air curtain mode of  FIG. 3 . This working principle is ideal for use during the heating season. For the exhaust and outdoor air curtain modes of  FIGS. 2 and 4 , airflow is used to remove solar energy by convecting away the excess heat during the cooling season and decreasing conductive heat losses during the heating season. The supply air window  100  can also be used for night cooling.  
         [0007]     Airflow through the supply airwindow  100  is mainly driven by buoyant effects. Solar energy absorbed by the window  100  heats the air inside the airflow cavity  106 . The heated air rises, causing the air in the cavity  106  to stratify and move in an upward direction. The strength of the buoyant forces is governed by the vertical temperature differences in the airflow cavity  106 , which is influenced by the height of the window  100 . In general, the taller the window  100  and/or the greater the temperature difference, the greater the buoyant force. To ensure airflow into the room when buoyant forces are weak, the supply air mode requires that the room  104  in which the window  100  is located be kept at a slightly negative pressure. Airtight construction in the rest of the room  104  is also essential for achieving airflow only through the window  100 .  
         [0008]     As compared to a conventional window, the exhaust air window  200  can improve thermal comfort conditions by tempering and then exhausting room air between the two glass panes  208  and  210 . This is beneficial during both the heating and cooling seasons because the airflow cavity  206  is respectively warmer or cooler. The decrease in temperature difference between an occupant of the room  204  and the surface of the inside glass pane  210  decreases the radiation exchange and improves thermal comfort. The temperature of the airflow cavity  206  also helps to reduce conduction losses through the window  200 . Air can be exhausted by natural effects or mechanical effects by positively pressurizing the room  204 .  
         [0009]     Although the air curtain modes cannot be used to improve indoor air quality or meet ventilation requirements, they offer benefits related to energy consumption and thermal comfort. The outdoor air curtain  400  of  FIG. 4  is most beneficial on a sunny day during the cooling season. Warmer outdoor air is driven upward through the airflow cavity  406  because of buoyancy effects. As the air is heated in the cavity  406 , it is drawn to and exhausted from the top of the window  400 , which in turn causes air to be drawn from the outdoor space  402  into the airflow cavity  406  through an opening at the bottom of the cavity  406 . In this way, the daylighting benefits from solar radiation can be enjoyed without overheating the window  400  and subsequently increasing the temperature of the room  404 . By helping to prevent overheating in the airflow cavity  406 , the temperature difference between the outdoor space  402  and indoor room  404  is minimized, which reduces heat transfer through the window  400  into the room  404  and consequently decreases the amount of energy needed to cool the room  404 .  
         [0010]     The indoor air curtain window  300  of  FIG. 3  works in a similar fashion during the heating season. Solar energy is absorbed by the air within the airflow cavity  306 , causing the air to become heated and rise through the cavity  306 , and finally convected to the indoor space/room  304  through an opening at the top of the window  300 . The rising air within the cavity  306  causes cooler air to be drawn from the room  304  into the airflow cavity  306  through an opening at the bottom of the cavity  306 .  
         [0011]     Airflow windows are most effective when installed on the south facade of a building because the increased incident solar radiation on the west and east facades can promote overheating of the window. On the other hand, an airflow window installed on the north facade may not receive enough incident solar radiation during the winter months to effectively temper air supplied to the building. Therefore, for most climates, airflow windows are limited to installation on the south facade.  
         [0012]     The airflow window designs described above have several limitations. For instance, only the supply air mode offers the potential for improving indoor air quality by drawing fresh air from an outdoor space  102  into the room  104 . Several limitations to the implementation of these airflow windows also arise from the design of their airflow cavities  106 ,  206 ,  306 , and  406 , which are open and as a result raise issues concerning security, acoustics, air quality, cleaning and maintenance, thermal comfort and/or condensation. For some building locations, conventional windows are useful to attenuate outdoor noise, whereas the airflow cavities  106 ,  206 ,  306 , and  406  of the airflow windows  100 ,  200 ,  300 , and  400  may provide a channel for outdoor noises to enter the indoor space  104 ,  204 ,  304 , and  404 , potentially causing acoustic problems. The ability to filter outdoor air before it enters a building in the supply air window  100  is important when considering indoor air quality. However, filters can hinder the effectiveness of natural ventilation. Airflow in the airflow cavities  106 ,  206 ,  306 , and  406  of all airflow window modes can also promote the collection of dirt and dust on the interior surfaces of the window. Though offering the benefit of preheating air that enters a building during the day during the heating season, the supply air window  100  can contribute to heat losses during the night when the temperature of the inner pane  110  can drop, affecting the thermal comfort of the building occupants. Finally, condensation in an airflow window may occur if the surface temperature of a glazing layer falls below the dew point temperature of the air it contacts. Moisture can accumulate at the base of the window, which can lead to damage of the materials used to construct the window. Additionally, high outdoor humidity levels can increase the humidity indoors and decrease thermal comfort.  
         [0013]     Other shortcomings of airflow windows are due to their added complexity as compared to a conventional window. The initial cost of purchasing an airflow window is likely higher, though strongly dependent on the type of airflow window and exact construction, as well as the availability of the product in relation to the building location. However, the use of airflow windows may reduce the size of the HVAC system required to heat and cool and building, providing a significant trade-off for the increased cost of an airflow window.  
         [0014]     In view of the foregoing, though airflow window technology offers significant potential benefits including improved indoor air quality and reduced heating/cooling loads, current airflow windows have a number of limitations and as such further improvements in their construction and effectiveness would be desirable.  
       BRIEF SUMMARY OF THE INVENTION  
       [0015]     The present invention provides airflow window systems capable of drawing fresh outdoor air into an indoor space to improve air quality within the indoor space, and also tempering the incoming outdoor air with outgoing indoor air, thus reducing the heating/cooling demands associated with introducing the outdoor air to the indoor space.  
         [0016]     The airflow window system generally includes at least three glazing layers positioned roughly parallel to each other to define at least two internal airflow cavities within the airflow window system. A first of the glazing layers is adjacent a first of the two internal airflow cavities, a second of the glazing layers is adjacent a second of the two internal airflow cavities, and a center glazing layer is between the first and second glazing layers and separates the first and second internal airflow cavities. Airflow cavity openings are located adjacent the uppermost and lowermost extents of each airflow cavity, and airflow is enabled through the first internal airflow cavity between the openings thereof and enabled through the second internal airflow cavity between the openings thereof.  
         [0017]     A significant advantage of this invention is the ability to employ the center glazing layer as a heat transfer medium between two air flows, one drawn from an outdoor space and supplied to an indoor space and the second drawn from the indoor space and exhausted to the outdoor space. In this manner, the window system operates as a crossflow heat exchanger capable of supplying fresh outdoor air to an enclosed indoor space, while reducing the thermal load resulting from the import of fresh air by thermally tempering the incoming fresh air with the outgoing indoor air.  
         [0018]     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIGS. 1 through 4  are schematic cross-sectional representations showing four modes of operation for prior art airflow windows: supply, exhaust, indoor air curtain, and outdoor air curtain, respectively.  
         [0020]      FIGS. 5 and 6  are schematic cross-sectional representations showing two modes of operation, supply and exhaust, respectively, for airflow windows in accordance with two embodiments of this invention.  
         [0021]      FIG. 7  schematically represents a frontal view of the airflow window of  FIG. 5 .  
         [0022]      FIGS. 8 and 9  are schematic cross-sectional representations of the airflow windows of  FIGS. 5 and 6  with optional features in accordance with additional embodiments of the invention.  
         [0023]      FIGS. 10 and 11  are overviews of the convection and radiation effects, respectively, on the airflow window of  FIG. 5 .  
         [0024]      FIGS. 12 and 13  are graphs plotting data obtained from simulations to assess the performance of airflow windows with the supply and exhaust operating modes represented in  FIGS. 5 and 6 , respectively.  
         [0025]      FIGS. 14 and 15  are graphs plotting data obtained from simulations to assess the winter and summer performance of the supply-mode airflow window of  FIG. 5  under varying solar radiation conditions.  
         [0026]      FIGS. 16 and 17  are graphs plotting data obtained from simulations to assess the winter and summer performance of the supply-mode airflow window of FIG.  5  under varying combinations of solar radiation and wind conditions.  
         [0027]      FIGS. 18 and 19  are graphs plotting data obtained from simulations to assess the winter and summer performance of the supply-mode airflow window of  FIG. 5  at different airflow rates.  
         [0028]      FIGS. 20 and 21  are graphs plotting data obtained from simulations to assess the winter and summer performance of the supply-mode airflow window of  FIG. 5  for different airflow cavity widths.  
         [0029]      FIGS. 22 and 23  schematically represent perspective views of the airflow windows of  FIGS. 5 and 6  with optional features in accordance with additional embodiments of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     The present invention provides embodiments for an airflow window system that defines two separate airflow paths in an arrangement that has the potential for providing energy savings and improving indoor air quality within a building in which the window system is installed. Two embodiments are schematically represented in  FIGS. 5 and 6 . In each embodiment, the dual airflow path configuration is believed capable of offering benefits over the conventional airflow windows of  FIGS. 1 through 4 . In particular, the dual airflow path configuration has the advantage over the supply air window  100  ( FIG. 1 ) of tempering incoming outdoor air with outgoing indoor air, thus reducing the heating/cooling demands associated with introducing outdoor air to an indoor space, and has the advantage over the exhaust air window  200  ( FIG. 2 ) and air curtain windows  300  and  400  ( FIGS. 3 and 4 ) by drawing fresh outdoor air into an indoor space, thus improving air quality of the indoor space.  
         [0031]     The airflow window systems  10  and  50  of  FIGS. 5 and 6  are shown as being a triple glazed unit, i.e., three glass layers or panes  18 ,  19 , and  20 , that define two parallel airflow cavities  16   a  and  16   b  through which air is allowed or forced to flow (because of the similar construction and sharing of basic components, identical reference numbers are used to identify the individual components of the window systems  10  and  50  in the Figures). The window systems  10  and  50  provide two different modes of operation, referred to as supply and exhaust, respectively. In the supply mode embodiment depicted in  FIG. 5 , outdoor air (OA) from an outdoor space  12  enters an opening  22  at the lower end of an outer airflow cavity  16   a  defined between the outer pane  18  and the center pane  19 , flows upward through the cavity  16   a , and is discharged as tempered fresh air (TFA) into an indoor space  14  by passing through an opening  24  at the upper end of the cavity  16   a . Simultaneously, indoor air (IA) enters an opening  26  at the upper end of an inner airflow cavity  16   b  defined between the center pane  19  and the inner pane  20 , flows downward through the cavity  16   b , and is discharged as exhaust air (EA) into the outdoor space  12  by passing through an opening  28  at the lower end of the cavity  16   b . In the exhaust mode embodiment depicted in  FIG. 6 , airflow directions through the airflow cavities  16   a  and  16   b  are reversed. Outdoor air (OA) from the outdoor space  12  enters through the opening  24 , flows downward through the outer airflow cavity  16   a , and is discharged through the opening  22  as tempered fresh air (TFA) into the indoor space  14 , and indoor air (IA) simultaneously enters the opening  28 , flows upward through the inner airflow cavity  16   b , and is discharged through the opening  26  as exhaust air (EA) into the outdoor space  12 .  
         [0032]     In each embodiment, exhausted indoor air is used to temper incoming outdoor air, thus reducing hearing/cooling demands of the indoor space  12  while providing fresh air to the indoor space  12 . The exhausted indoor air flows through the inner airflow cavity  16   b  of each window system  10  and  50 , so that the temperature of the inner pane  20  stabilizes relatively close to the air temperature within the indoor space  12  to promote the thermal comfort of occupants of the indoor space  12 . Other operational aspects and efficiencies associated with these different modes will become apparent in the following discussion.  
         [0033]     As can be seen from the airflow schematics in  FIGS. 5 and 6 , the openings through which air enters the window systems  10  and  50  ( 22  and  26  in  FIG. 5 ;  24  and  28  in  FIG. 6 ) are positioned adjacent the openings through which air exits the window systems  10  and  50  ( 24  and  28  in  FIG. 5 ;  22  and  26  in  FIG. 6 ). To reduce short-circuiting tempered fresh air (TFA) into the indoor air (IA) stream, the window systems  10  and  50  can be configured so that the width of the upper and lower extent of each window system  10  and  50  is divided (perhaps equally) between the openings serving as inlet and outlet, as represented in  FIG. 7  for the supply mode embodiment of  FIG. 5 . Due to this positioning of the inlets/outlets in the supply mode of  FIG. 5 , airflow  40  through the outer airflow cavity  16   a  is generally diagonally upward from the opening  22  to the opening  24 , and airflow  42  through the inner airflow cavity  16   b  is generally diagonally downward from the opening  26  to the opening  28  (flow directions are reversed for the exhaust mode of  FIG. 6 ). With this configuration, each of the window systems  10  and  50  performs as a crossflow heat exchanger with solar energy recovery.  
         [0034]     The performance of the window systems  10  and  50  were investigated both using computational methods (computational fluid dynamics, or CFD) and experiments to confirm the computational methods. The CFD simulations employed FLUENT®, a commercial CFD software program, to model convection, conduction, and radiation within the window systems  10  and  50 . Because the window systems  10  and  50  are intended for use in residential buildings, the computational and experimental investigations were based on a window height of about 1.22 meters (about four feet) and a window width of about 0.92 meter (about three feet), which are within common ranges for residential window dimensions. The thickness of each pane  18 ,  19 , and  20  was set at 3 mm. Because mixed-mode heat transfer is present in the window systems  10  and  50 , the effects from conduction, convection and radiation must be considered when developing a window model. The following is an overview of the three main modes of heat transfer as they relate to the window systems  10  and  50 .  
         [0035]     Due to a temperature difference on either side of each glass pane  18 ,  19 , and  20 , conduction occurs through each pane  18 ,  19 , and  20 . Because conduction through the glass panes  18 ,  19 ,  20  is intended, a double-glazed insulated unit is not believed to be necessary or preferred for any of the panes  18 ,  19 , and  20 , particularly the center pane  19 . To the contrary, conduction across the center pane  19  is desirable because of the intended heat exchanger effect between the air flows in the two airflow cavities  16   a  and  16   b . As such, heat transfer between the two air flows can be improved to some extent by manufacturing the center pane  19  from a material having a relatively high thermal conductivity coefficient, for example, greater than the materials of the inner and outer panes  18  and  20 . Notable highly conductive materials include metals such as aluminum alloy, pure copper, and pure silver with conductivities of about 170, 401 and 429 W/m 2 , respectively. Disadvantages of metallic materials include poor transmittance of light and susceptibility to corrosion. Transparent/translucent materials such as polymers tend to be less conductive than glass, for example, acrylic, polycarbonate, and polyethylene have conductivities that range from about 0.13 to 0.30 W/m·K. Therefore, glass is believed to be preferred for the center pane  19 , though it is within the scope of this invention that other materials could be used, especially transparent/translucent materials that are more thermally conductive than glass.  
         [0036]     The linear temperature profile across each pane  18 ,  19 , and  20  is small when compared to the more parabolic temperature profile due to convection. The resistance to conduction (R cond ) is defined as: 
 
 R   cond   =L/kA  
 
 where L is the thickness of the pane  18 ,  19 , or  20 , k is the thermal conductivity of the pane material, and A is the surface area of the pane  18 ,  19 , or  20  perpendicular to heat transfer. For a 3 mm thick glass pane with a conductivity of about 1.4 W/m·K, the resulting resistance to heat transfer is small. As a result, the temperature difference across each glass pane  18 ,  19 , and  20  is relatively small. It was therefore assumed that the surface temperatures of each pane  18 ,  19 , and  20  are the same across the thickness of the pane  18 ,  19 , and  20  at each position for the computational simulations and experiments. 
 
         [0037]     Convective heat transfer effects are present within and around the window systems  10  and  50  due to the airflow over the glass panes  18 ,  19 , and  20 , as represented in  FIG. 10 . Convection can be due to natural or forced effects. On the outer surface of the outer pane  18 , wind is the main driving force. Therefore, windy and calm conditions should be considered. Per design conditions listed in the ASHRAE Fundamentals Handbook (2001), a windy condition indicates an outside air velocity of about 6.7 m/s, whereas calm conditions are similar to indoor airflows far from a diffuser, or about 0.2 to about 0.3 m/s. On the interior surfaces of the panes  18 ,  19 , and  20 , i.e., those defining the airflow cavities  16   a  and  16   b , convective heat transfer effects are present as a result of natural convection as the air within the cavities  16   a  and  16   b  becomes more or less buoyant as a result of an increase or decrease in temperature, as the case may be. Natural convection within the cavities  16   a  and  16   b  has different influences on the performance of each window system  10  and  50  because of their different operating modes: supply and exhaust ( FIGS. 5 and 6 ). Depending on the season, each configuration would align the airflow paths between the indoors/outdoors with the direction of dominant buoyancy forces. The supply mode ( FIG. 5 ) would appear to be most effective during the heating season, when exhausted indoor air cools and sinks in the inner airflow cavity  16   b , driving the air to the outdoor space  12 , while cold outdoor air is heated and rises within the outer airflow cavity  16   a , driving the air to the indoor space  14 . Conversely, the exhaust mode ( FIG. 6 ) would appear to be most useful during the cooling season, again because the airflow patterns within the window system  50  work with the naturally prevailing buoyancy forces.  
         [0038]      FIGS. 8 and 9  depict modified versions of the embodiments of  FIGS. 5 and 6 , in which the airflow through the cavities  16   a  and  16   b  is supplemented with fans  30 , whose size and efficiencies can be selected to ensure that the indoor space  12  can be supplied with sufficient outdoor air to improve indoor air quality. For the following investigations, forced convection using fans was studied in detail. In part, the concern was that the experiments on test prototypes were to be conducted in an indoor test facility, and without exposure to solar radiation or a radiation source of the same intensity, buoyancy forces would be too small to accurately measure and airflow may be flowing in several directions at the inlets/outlets of the window system. Therefore, it was concluded that the validation of a CFD model by experimental measurements would only be possible if a mechanically ventilated (forced convection) window was evaluated. ASHRAE Standard 62.2-2004 specifies a minimum 10 L/s (20 cfm) per person of outdoor air in residential areas. For the investigations discussed below, flow rates of about 10 to about 20 L/s (about 20 to about 40 cfm) per window were evaluated, based on the premise of two occupants in a room with two windows.  
         [0039]     Three radiation interactions are present in the window systems  10  and  50 : radiation to the indoor space  14 , radiation between each pane  18 ,  19 , and  20 , and solar radiation as represented in  FIG. 11 . Each type of radiation plays a role in the performance of the window. The energy from solar radiation was estimated for each pane  18 ,  19 , and  20  based on a survey of typical meteorological (TMY2) solar data and calculations from the FLUENT® solar load calculator was conducted for several locations in the United States. This data suggested about 1000 W/m 2  as a suitable approximation for the average solar radiation flux during a sunny day, with about 800 W/m 2  as direct radiation and about 200 W/m 2  from atmospheric diffusive radiation and ground reflection. Likewise, a cloudy day was estimated to provide no direct radiation, but 200 W/m 2  diffusive radiation. The absorptivity of each pane  18 ,  19 , and  20  was estimated using data from the ASHRAE Fundamentals Handbook (2001) for a clear-clear-clear triple glazing unit. In general, about 12% of solar radiation was estimated as being absorbed by the outer pane  18 , about 8% by the middle pane  19 , and about 5% by the inner pane.  20 . At solar noon on a vertical south facade, the actual incident solar radiation is dependent on the angle of the sun. For winter computations, the sun angle was presumed to be about 350 from horizontal, and for summer computations this angle was presumed to be about 75° from horizontal. From these angles, the solar radiation flux values were adjusted accordingly. During sunny days in the winter and summer, incident radiation was estimated to be about 820 and about 260 W/m 2 , respectively.  
         [0040]      FIGS. 8 and 9  further show the window systems  10  and  50  equipped with optional louvers  34  located in their outer airflow cavities  16   a . The louvers  34  can promote the absorption of solar radiation, and are therefore of interest to the invention. If configured to be rotated, the louvers  34  can also be used to effectively obstruct the flow of airflow through the airflow cavity  16   a . While within the scope of the present invention, the presence and possible effect of the louvers  34  was not included in the simulations and experimental investigations.  
         [0041]     Taking into consideration the above factors, CFD simulations were performed based on the configurations of the window systems  10  and  50  described above. The summer indoor and outdoor temperatures used for the simulations were 24° C. and 37° C., respectively, and the winter indoor and outdoor temperatures used for the simulations were 22° C. and 2° C., respectively. Due to their complexity, the CFD simulations will not be described in any detail here, other than to report that the results provided numerous temperature data for each panel  18 ,  19 , and  20  and for each opening  22 ,  24 ,  26 , and  28  under steady-state conditions, and that these results suggested that significant benefits could be obtained with the window systems of  FIGS. 5 and 6 . Therefore, validation of the simulation data was pursued with experimental testing of actual hardware.  
         [0042]     The experimental investigations obtained flow and temperature data with a full-scale airflow window installed in an environmental chamber facility. A preliminary investigation was performed for the forced convection supply mode ( FIG. 8 ) underwinter and summer conditions with no solar radiation. As with the CFD simulations, the glazing area was about 1.22 meters high and about 0.92 meter wide. The multiple layer construction of the window system was formed using double strength, clear glass panes with a thickness of about 3 mm. Nine thermocouples were glued on one surface of each pane for a total of twenty-seven surface temperature readings. Each of the two airflow inlets and two airflow outlets of the window system (corresponding to openings  22  and  26  and openings  24  and  28 , respectively, in  FIGS. 5, 7 , and  8 ) was monitored with three thermocouples for inlet/outlet airflow temperature measurements, for a total of twelve airflow temperature readings.  
         [0043]     The preliminary investigation was conducted for four different scenarios: winter and summer conditions with forced airflow through the airflow cavities of about 10 or about 15 L/s. Results from these experiments were found to be in good agreement with the data from the CFD simulation of the supply-mode window system. Therefore, it was concluded that the CFD simulations were of sufficient accuracy to conduct parametric studies to identify optimal values for several parameters of the window systems  10  and  50 . The parameters considered were the mode of operation (supply and exhaust), solar radiation, wind, airflow rate, and cavity width over winter and summer conditions.  
         [0044]     An optimal airflow window configuration would be expected to depend on the mode of operation and weather conditions. For example, it was conjectured that the supply mode ( FIGS. 5, 7 , and  8 ) may be most effective during winter months, while the exhaust mode ( FIGS. 6 and 9 ) may be most effective during summer months. Such configurations may make use of natural buoyancy effects to drive airflow in the window cavities, allowing for fan energy consumption to be reduced. However, for reasons previously discussed, the investigation focused on using a fan to drive the airflows through the airflow cavities. Other than where noted, the same parameters used in the original CFD simulations were used in the parametric studies.  
         [0045]      FIGS. 12 and 13  are graphs generated from a CFD simulation showing the effect that the particular mode (supply and exhaust) has on the exit temperature of the tempered fresh air supplied by the window system  10 / 50  to the indoor space. Results are presented for each window system  10  and  50  under summer and winter conditions and sunny and cloudy sky conditions, using a forced airflow rate of 10 L/s and a cavity width of 12 mm. The most desirable mode of operation would provide the highest exit temperature to the indoors during the winter and the lowest exit temperature to the indoors during the summer. For a flowrate of 10 L/s, the supply mode was slightly better during the winter and the exhaust mode slightly better during the summer. However, this difference was only about 1° C. or less, indicating that the mode of operation is not likely critical under the evaluated conditions using fan-driven airflow.  
         [0046]     A subsequent simulation with the airflow rate increased from 10 L/s to 20 L/s indicated that the performance from the supply and exhaust modes would be nearly identical. An increase in forced airflow rate is indicative of a decreased ratio of natural convection to forced convection. Because the mode of operation appeared to become less important with increasing airflow rates, it was decided that only the supply mode (the window system  10  of  FIGS. 5, 7 , and  8 ) would be evaluated with subsequent simulations.  
         [0047]     Next, the effects of solar radiation and wind were investigated with CFD simulations.  FIGS. 14 and 15  show exit temperatures to the indoor space for four combinations of solar radiation and wind under winter and summer conditions, respectively, and  FIGS. 16 and 17  show exit temperatures to the indoor space for three solar radiation conditions in winter and summer, respectively. As before, the simulation used an airflow rate of 10 L/s and a cavity width of 12 mm. During winter conditions, the exit temperature to the indoors was the highest under sunny and calm conditions. On the other hand, during summer conditions, the exit temperature to the indoors was the lowest under cloudy and calm conditions. Given the desired effect on exit temperature, these conditions provided the best performance for each season. Results indicated that solar radiation is desirable during the winter (heating) season and less desirable during the summer (cooling). Calm wind conditions were favorable for promoting less convective heat losses during the winter and less convective heat gains during the summer.  
         [0048]     CFD simulations were then conducted to evaluate the effect of airflow rate within the airflow cavities  16   a  and  16   b .  FIGS. 18 and 19  show the simulated results of airflow rate on the exit temperature during winter conditions and summer conditions, respectively. Note that the most and least desirable solar radiation and wind combinations are highlighted for each season. Again, the simulation used a cavity width of 12 mm, while the evaluated airflow rates were 10, 15, and 20 L/s. The effect of airflow rate on exit temperature can be seen to vary significantly between winter and summer conditions. During sunny, winter conditions, the largest increase in exit temperature to the indoors was achieved with the lowest flow rate. The trends also seem to indicate that the decrease in window performance with an increase in flow rate is not linear, and that window performance is most sensitive to changes at lower flow rates. However, under summer and cloudy winter conditions, flow rate appears to have little if any effect on exit temperature to the indoor space. This may have been due to the relatively small incident solar radiation simulated for sunny summer days and cloudy or sunny winter days.  
         [0049]     Finally,  FIGS. 20 and 21  represent the data obtained from CFD simulations conducted to evaluate the effect of the width of the airflow cavities  16   a  and  16   b . Airflow rates for the simulation were again 10 L/s. Overall, it was found that smaller cavity widths improved window performance. Unlike airflow rate, cavity width appeared to have a small impact on exit temperature under winter conditions. The impact of cavity width on exit temperature for both winter and summer was about 1° C. for cavity widths over a range of 9 to 15 mm. The efficiency of the heat exchange between the two cavities  16   a  and  16   b  of the window system  10  and energy reclamation was used to measure the window performance for each of the parameters studied.  
         [0050]     Heat recovery efficiency values were assessed for the combination of parameters suggested as being optimal under the simulated conditions discussed above. As set forth in the equation below, heat recovery efficiency (ε) can be calculated by taking the absolute value of the ratio between the actual temperature change of the air in the inner cavity to the maximum temperature difference between the outdoor and indoor air temperatures. 
 
ε=|( T   out   −T   o,i )/( T   out   −T   in )|
 
         [0051]     In the equation, T out  is the outdoor temperature, T in  is the indoor temperature, and T o,i  is the average exit temperature to the indoors.  
         [0052]     Heat recovery efficiency was found to be greatest when the flow rate and cavity width are the smallest values evaluated, 10 L/s and 9 mm, respectively. During winter conditions, performance was maximized under sunny and calm weather conditions with an efficiency of 80.5%. During summer conditions, performance was maximized under cloudy and calm weather conditions with an efficiency of 23.7%. Using the same flow rate and cavity width values, the lowest heat recovery efficiencies were also calculated. During winter conditions, the lowest efficiency calculated was 34.1%, and occurred under cloudy and windy conditions. Under summer conditions, the lowest calculated efficiency was 14.7%, which occurred during sunny and windy conditions.  
         [0053]     From the above it was concluded that each of the airflow window systems  10  and  50  represented in  FIGS. 5 through 9  offer great potential for conserving energy and improving indoor air quality. Forced or natural convention airflow can be used to temper outdoor air with exhausted indoor air, thus reducing heating/cooling demands associated with providing fresh air to an indoor space year round. The window systems  10  and  50  conserve energy by operating as a cross-counterflow heat exchanger utilizing solar energy trapped by the panes  18 ,  19 , and  20  of the window systems  10  and  50 . Supply air temperatures and inner pane temperatures were closer to the indoor space temperature under all weather conditions studied, thus promoting the thermal comfort of occupants of the indoor space.  
         [0054]     Two implementations for the window system  10  of  FIGS. 5, 7 , and  8  are shown in perspective in  FIGS. 22 and 23  (the same implementations are also applicable to the window system  50  of  FIGS. 6 and 9 ). In  FIGS. 22 and 23 , the openings  22 ,  24 ,  26 , and  28  are in the form of plenums located and fluidically connected to the appropriate airflow cavity  16   a  or  16   b .  FIG. 22  shows fans  30  within the openings  28  and  24  to the airflow cavities  16   a  and  16   b , respectively, for the purpose of providing mechanical (forced) ventilation through the airflow cavities  16   a  and  16   b . Due to the symmetry of the supply and exhaust air window configurations (for example, compare  FIGS. 5 and 6 ), the supply and exhaust modes of operation can be interchanged by rotating the window system  10  about its central vertical axis  36 , as represented in  FIG. 23 .  FIG. 23  also shows the openings  22 ,  24 ,  26 , and  28  equipped with doors  38  that can be opened and closed, depending on the rotational orientation of the window system  10  and its operating mode (supply or exhaust). When open, the doors  38  can also serve as deflectors to promote natural ventilation and separation of air entering and leaving the window system  10 .  
         [0055]     While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the window systems  10  and  50  could differ from that shown, materials and processes other than those noted could be use, and more than two airflow cavities  16   a  and  16   b  could be provided. Therefore, the scope of the invention is to be limited only by the following claims.