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A more sustainable curtain wall system: Analytical modeling of the solar dynamic buffer zone (SDBZ) curtain wall
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Building and Environment 44 (2009) 1–10 www.elsevier.com/locate/buildenv
A more sustainable curtain wall system: Analytical modeling of the solar dynamic buffer zone (SDBZ) curtain wall R.C. Richman, K.D. Pressnail Department of Civil Engineering, University of Toronto, 35 St. George Street, Toronto, Ontario, Canada M5S 1A4 Received 25 April 2007; received in revised form 9 January 2008; accepted 10 January 2008
Abstract Given the increases in both the environmental and economic costs of energy, there is a need to design and build more sustainable and low-energy building systems now. Curtain wall assemblies are engineered wall assemblies that are used widely in both high-rise as well as low-rise construction. These assemblies show great promise—with the minimal modification outlined in this paper they can be built better now. Often ignored, spandrel panels that comprise a part of curtain wall assemblies can be natural solar collectors. By using a new, simple, low-cost method such as a solar dynamic buffer zone (SDBZ), solar energy can be efficiently gathered or excluded using the movement of air. Such a method can be used in both retrofit as well as new construction. This paper will introduce and outline a proposed SDBZ curtain wall system and present the results of analytical modelling. Using these results, a SDBZ system will be shown to be a more sustainable option for traditional curtain wall assemblies. r 2008 Elsevier Ltd. All rights reserved. Keywords: Energy efficiency; Sustainability; Curtain wall; Solar air collectors
1. Introduction There are compelling reasons why building design and construction should adopt more sustainable, more energyefficient systems. From ‘peak oil’ or ‘peak natural gas’, to the emergence of ‘developing’ countries attaining ‘western’ standards of living and consumption, to climate change and global warming, many experts agree that the present system of growth and consumption of hydrocarbon-based energy is not sustainable. The need to build better now is ever increasing. A recent paper presented quantitative economic analysis showing that it was economically more favorable to build residential buildings to a higher standard at the time of construction rather than retrofitting in the future [1]. Tomorrow’s buildings should be built today. Since building energy consumption accounts for approximately 19% of all energy use in Canada [2], it seems logical to focus on minimizing this demand.
Corresponding author. Tel.: +1 416 978 5964; fax: +1 416 978 6813.
E-mail address: [email protected] (R.C. Richman). 0360-1323/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2008.01.006
Humans have always tried to create more efficient separations between the interior and exterior environments. What started as the utilization of natural shelters has evolved into elaborate assemblies of highly complex manufactured components. Buildings were once clad using primarily solid structural masonry walls; now cladding can comprise a diverse range of systems and materials, including intricate non-load bearing skins. These skins, hung from the structure like a curtain, are complicated assemblies that are slight in thickness, contain multiple materials and must necessarily incorporate a multitude of interfaces and joints. The modern exterior wall assembly is exposed to same stresses as its historical counterpart —air pressure gradients (stack, mechanical systems, etc.), extreme thermal gradients (during peak summer and winter loads), vapor pressure gradients, driving rain, and wind-induced pressures. As occupant expectations of what constitutes a comfortable indoor environment have risen, and as buildings have become taller, the intensity of these stresses have increased and more demand is placed on today’s exterior wall assemblies.
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Nomenclature Ag As Aw Cp G hc1 hc2 hc,ext
hc,int
hk1 hr1
hr,air
area of the glass surface, m2 effective area of the receiving surface, m2 cross-sectional area of the wall components (i.e. insulation and metal back pan), m2 specific heat of air, kJ/kg 1C incoming radiation normal to the glass, W/m2 convective heat transfer coefficient between interior glass surface and cavity air, W/m2 K convective heat transfer coefficient between collector surface and cavity air, W/m2 K convective heat transfer coefficient between exterior glass surface and exterior environment, W/m2 K convective heat transfer coefficient between interior metal back pan surface and interior, W/m2 K conductive heat transfer coefficient between the interior and exterior glass surfaces, W/m2 K radiative heat transfer coefficient between collector surface and interior glass surface, W/ m2 K radiative heat transfer coefficient between exterior glass and exterior air, W/m2 K
The quest for improved curtain wall systems has led design engineers to concentrate their efforts on the management of moisture and control of heat flow. In terms of heat flow, the general focus in recent years has been on the vision areas of the curtain wall, not the spandrel areas. Development of high-performance glazing units that include multiple air spaces, special coatings, ‘warm-edge’ thermal spacers, and photochromic glass have resulted from this focus. As well, mechanical shading devices have been developed. All of these developments aim to control the flow of heat through the vision areas. Yet, little work has been done on the spandrel areas. The spandrel panels deserve attention. During the heating season, heat is lost from the interior to the exterior through these panels. In the cooling season, the flow is reversed. By insulating the spandrel panels, attempts are made to minimize the heat loss or gain through the spandrel panel. However, the heat losses or gains that do occur through these panels have to be managed by the HVAC system. Given the variation of magnitude and direction of heat flow through the spandrel panels, this approach to thermal management may not be the most efficient. To date, few attempts in Canada or around the world have been made to better manage heat flow through this space, or to utilize the potential solar gain through the spandrel panel. The technology already exists for the construction of more efficient curtain wall assemblies [3]. Curtain wall
hr,int
hr,gr hr,sky _ m Ta Text T gi T go Tinlet Tint Toutlet Ts Tw U1
ag as tg
radiative heat transfer coefficient between interior metal back pan surface and interior, W/m2 K radiative heat transfer coefficient between exterior glass and ground, W/m2 K radiative heat transfer coefficient between exterior glass and sky, W/m2 K mass flow rate of cavity air, kg/s temperature of cavity air, K temperature of exterior, K temperature of interior glass surface, K temperature of exterior glass surface, K temperature of the air at the inlet opening, K temperature of the interior, K temperature of the air at the outlet opening, K temperature of the collecting surface, K temperature of the interior metal back pan surface, K conductive heat transfer coefficient between the collector surface and the interior metal back pan, W/m2 K absorptance of the glass absorptance of the collector surface transmittance of the glass
systems that provide natural daylighting can be adapted using computer-controlled air pressure zones to facilitate the provision of fresh air, while improving occupant comfort and reducing energy consumption. Dynamic buffer zones (DBZ) work by ventilating a cavity within a wall with heated exterior air to control moisture migration across the assembly [3]. By employing this principle within the spandrel panel, a solar dynamic buffer zone (SDBZ) can be utilized to create a more sustainable curtain wall system and to manage solar energy in order to reduce the costs of heating and cooling of buildings. This article reviews the fundamental design principles and the conceptual layout of an SDBZ curtain wall system designed, developed and currently being tested at the Building Science Laboratory (Department of Civil Engineering) at the University of Toronto, Toronto, Canada. This review will be followed by a presentation and discussion of the results from analytical modelling. Finally, the research directions that emerge from modelling and discussion will be outlined.
2. The building blocks—previous research The SDBZ synthesizes three discipline areas of civil engineering: solar architecture, dynamic buffer zones, and curtain wall design. A brief summary of recent research pertaining to the SDBZ is essential prior to discussion of the proposed system.
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2.1. Solar architecture Aspects of solar architecture pertaining to the SDBZ include: passive heating/cooling, thermal mass concepts, Trombe walls and ventilation techniques. Solar architecture is an extensively researched field. A number of principles from this research were broadly applied during the conceptual analysis of the SDBZ. Kimura [4] has studied solar architecture for over 25 years. His paper ‘Solar Architecture for the Happiness of Mankind’ provides a comprehensive summary of solar architecture research throughout the world. Kimura has studied many aspects of solar architecture including: design and construction of ‘solar houses’, building integrated photovoltaic systems, thermal comfort, natural illumination and passive heating/cooling methods. His research provided the foundation for solar architecture principles applied to the SDBZ. 2.2. Dynamic buffer zone (DBZ) The DBZ concept consists of ventilating a cavity within a wall to control heat movement and moisture migration across the assembly. Stemming from a 30-year-old notion of ventilated walls during winter operation, the DBZ typically introduces heated outside air containing little moisture into an interstitial cavity within the wall assembly [5]. By pressurizing the cavity with respect to both the interior and exterior air pressure, air leakage can be controlled across the wall assembly. Further, given the relatively small amounts of moisture and consequently low relative humidity of the warmed exterior air, any cavity air leakage towards the interior or the exterior poses little or no threat in the way of interstitial condensation. Early notions of the DBZ can be traced in Canada [6]. In his paper, Garden proposes the use of DBZ walls in hospitals to combat the problem of interstitial condensation common to such buildings with extreme interior climates (i.e. continuous high humidity and temperature). It appears that Garden was ahead of his time; the hospital engineering industry has gradually recognized the benefits of incorporating DBZ technology. One of the first DBZ systems was designed and installed as a retrofit measure in the Canada Life Building in Toronto, Ontario in 1997. This installation spawned a monitoring study and a supporting laboratory-testing program. This program was performed by Pasqualini [3] at the University of Toronto. Although historically the DBZ concept has focused on the problem of concealed condensation, the 1999 study focused on the effect of the DBZ and internal surface temperatures. Prior to the study, it was a common belief that the introduction of DBZ cavity air (while heated, it is generally significantly cooler than interior air) would result in lower interior surface temperatures—leading to issues of thermal comfort and unnecessary heat loss. The testing demonstrated that the DBZ air temperature equilibrates quickly with the boundary conditions because of the small
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heat capacity of air [5]. Pasqualini went on to state ‘‘in general, the relatively large variation in average cavity temperatures observed did not significantly affect warm side surface temperatures of the wall’’. This research opened the door for use of the DBZ in terms of thermal control. It has also been recognized that the use of the DBZ system can lead to ‘dynamic insulation’ [7] or the effect of overcoming some of the heat that would otherwise be lost through the wall [5]. This promising area should be pursued further in future research. The SDBZ borrows from the traditional concept of the DBZ to manipulate the transmission of heat across the spandrel cavity assembly. 2.3. Curtain wall design The initial concept of the SDBZ focused on creating an ‘interactive’ curtain wall system. As such, an exhaustive review of the literature pertaining to innovative curtain wall systems was required. Based on a substantial review of academic literature, it appears that little research has been focused on modern curtain wall systems. Although large amounts of research have been published on solar air collectors, this application to curtain walls is yet to be documented. Curtain wall research can basically be segregated into two main areas: (i) focus on the spandrel (opaque) areas and (ii) focus on the glazing areas that will not be discussed here. Research focusing on the spandrel areas includes the use of photo-voltaic (PV) cells in the exterior skin of the system. Benemann identifies the first use of buildingintegrated photovoltaics (BIPV) in 1991 in Aachen, Germany [8]. Since then building integration is one of the fastest growing market segments in photovoltaics. An impressive solar installation, the Academy Mont-Cenis in Herne (Germany), with 410,000 m2 of PV integrated into the roof and fac- ade, was finished in March, 2001 [8]. Photovoltaics provide an economic means of harnessing the solar energy reaching the building fac- ade otherwise lost to the exterior. The SDBZ attempts to exploit this same energy by using the existing curtain wall components in a near passive manner. Perhaps the most beneficial previous work relating to the SDBZ was completed by Behr [9] at the University of Missouri-Rolla, USA. Several spandrel panels at sites across the United States were monitored to record the micro-environmental conditions, including glass surface temperature, glass edge temperature, air temperature and relative humidity within the enclosed air space behind the spandrel glass panel and interior spandrel surfaces. Most impressive were the reported interior cavity still air temperatures (maximum sustained of approximately 45 1C) and interior glass surface temperatures (maximum of 82.1 1C). As the key aspect of the SDBZ is to harness the solar energy entering the spandrel cavity during the heating season, Behr’s work provides quantitative data that the micro-environments within the spandrel cavities support such a concept. Based on correspondence with the author,
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R.C. Richman, K.D. Pressnail / Building and Environment 44 (2009) 1–10
Fig. 1. Typical curtain wall system.
In general, these systems are designed to minimize the transmission of heat across the assembly all year. During the heating season, most solar energy that enters the spandrel cavity is not transmitted to the interior is rather rapidly ‘lost’ back to the exterior primarily by radiative and convective effects. This is due in part to the relatively low thermal mass and high emmissivities of the spandrel components. In an effort to ‘capture’ as much of the solar gain as possible, two options exist: (i) install a low-e coating on the spandrel glass to minimize the radiative losses to the exterior and (ii) replace traditional back pan materials with high thermal mass materials. These options have been discussed further by Richman and Pressnail [10]. Once captured, the solar heat must be brought into the building. Ordinarily the spandrel insulation greatly reduces this heat gain component. One simple way of bringing in this captured solar heat is by drawing air through the spandrel cavity. In its preliminary stage, the SDBZ can act to preheat cold exterior air during the daytime. Fig. 3 is a schematic representation of how air can be used to bring in the solar heat. In a module-based SDBZ curtain wall system, the transfer of the preheated cavity air can be controlled by a system of small fans. As such, the SDBZ can be a module based, nearly passive system that requires the use of a relatively simple, low-cost fan. This approach would serve to deliver the air in close proximity to the SDBZ module, therefore reducing (i) the pressure drop associated with long runs of duct work and (ii) the overall energy required to transport the air. It may be more desirable to link the SDBZ to the building’s air handling system as part of its fresh air intake circulation loop. By addressing such practical issues as minimizing additional duct work, connection points for optimal heat exchange and air filtering, it is possible that no additional energy is required to move the SDBZ air.
Fig. 2. Conventional spandrel cavity.
Behr has no knowledge of further work in this area of research since his original study (Fig. 1). 3. The SBDZ—a conceptual layout Fig. 2 shows the typical operating conditions for a southfacing curtain wall assembly during a bright day in the northern hemisphere. In terms of thermal considerations, solar gain is desirable in the heating season only. A conventional spandrel cavity is shown in Fig. 2. This assembly consists of: a spandrel face (e.g. glass, stone, etc.), the spandrel cavity, semi-rigid insulation and a metal back pan. For buildings requiring supplemental energy during the heating or cooling seasons, a drawback of conventional systems is that they are not designed to gather solar heat, but rather, they are designed to reduce heat transfer.
Fig. 3. The conceptual SDBZ.
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An efficient design has the potential to minimize the overall energy used for a building’s fresh air intake. To maximize the performance as a solar absorber, the collecting surface (i.e. typically semi-rigid mineral wool insulation) can be painted or otherwise colored black. For this analysis, the SDBZ construction included standard mineral wool (U-value ¼ 0.54 W/m2 K) with a 6 mm clear spandrel glass light. 4. Analytical model Based on the conceptual description of the SDBZ model described above, Fig. 4 presents the thermal characteristics of the model. This is used as a reference base for developing the governing heat balance equations for the system. Based on the thermal behavior presented in Fig. 4, the governing heat balance equations were developed using the following assumptions: (i) We assume steady-state conditions. (ii) Thermal storage by the SDBZ components was negligible.
(iii) Minimal stratification of surface temperatures existed. As such, average surface temperatures were utilized. (iv) Cavity air is completely mixed. (v) The heat flow to the sides is negligible. (vi) The heat flux is taken to be unidirectional in the system (i.e. perpendicular to the direction of air flow). (vii) The glass layer is opaque to infrared radiation. (viii) The short-wave radiation absorbed in a glass layer can be apportioned equally to the two surfaces. (ix) The inlet temperature is equal to the ambient air temperature. (x) The sky temperature is approximated by the bliss correlation. (xi) The ground temperature is equal to the ambient air temperature. Since the boundary conditions change relatively slowly compared to the reaction of the SDBZ components, steady state is justified. Further, preliminary experiments confirmed: (i) the SDBZ system reacted rapidly to changes in radiation, cavity airflow or exterior temperature and (ii) the
Toutlet
G
OUTLET DUCT CONNECTION VARIES DEPENING ON SYSTEM DESIGN
· Cp m
hc ext
hc1
hc int
hc2
hr sky hr 1
hr air
hr int
hr gr
Text
Tg o
EXTERIOR SPANDREL GLASS
Tg i
Ta CAVITY
5
Ts
Tw
INSULATION
BACK PAN
Tint INTERIOR
Tinlet
Fig. 4. Schematic model of SDBZ showing heat transfer coefficients.
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system showed no significant capacity for thermal storage as it is constructed for this research. Assuming negligible heat flow to adjacent panels is valid for SDBZ areas in large installations where many units are installed next to one another. For any given elevation, the average temperature in adjacent spandrel cavities and framing will be the same. This assumption can become invalid at points of transition between cladding systems; for example, where a SDBZ curtain wall meets an architectural stone veneer or precast concrete wall. Further complication may be encountered when modelling the SDBZ in a module-based system, where actual field performance will differ based on the thermal regime surrounding the perimeter framing. Since this research is focusing on the initial stages of development, it is acceptable to make this assumption based on the premise of large installations. The remaining assumptions (i.e. (vii) through (xi)) are common to similar principle-based air collecting systems published in the literature [11–15]. Keeping the notation of heat in (left side of equation) and heat out (right side) for each element, the governing heat balance equations are then as follows: Glass cover–exterior: GAg ag þ hk1 Ag ðT gi T go Þ ¼ hr;gr Ag ðT go T gr Þ 2 þ hr;sky Ag ðT go T sky Þ þ hr;air Ag ðT go T ext Þ þ hc;ext Ag ðT go T ext Þ.
The boundary conditions for the Toronto January design day are outlined below. Figs. 5 and 6 present the performance of the system: solar radiation values for January 21 (431 latitude) assuming bright, sunny conditions (near optimal conditions), exterior ambient air temperature of 20 1C, exterior relative humidity of 80%, interior ambient air temperature of 21 1C, interior relative humidity of 25%.
(2)
5.2. Seasonal simulation
(3) Collector surface: GAs tg as ¼ hc2 As ðT s T a Þ þ hr1 As ðT s T gi Þ (4)
Metal back pan surface—interior: U 1 Aw ðT s T w Þ ¼ hr;int Aw ðT w T int Þ (5)
Relationship between inlet, outlet and cavity air temperatures: T a ¼ 0:75T outlet þ 0:25T inlet .
5.1. January design day
_ p ðT outlet T inlet Þ þ hc1 Ag ðT a T gi Þ. hc2 As ðT s T a Þ ¼ mC
þ hc;int Aw ðT w T int Þ.
Two separate simulations were conducted to study the performance of the SDBZ curtain wall: (i) a January design day and (ii) a typical heating season, both for Toronto, ON, Canada. A brief description of boundary conditions and input sources follow. Based on values from the literature [11] for average flows in fresh-air preheating systems, the design flow used during the simulations was 60 m3/m2 h. It should be noted the efficiency of the SDBZ is sensitive to the cavity flow. As such, results would vary both in a positive and negative manner when different design flows are utilized.
(1)
Cavity air:
þ U 1 Aw ðT s T w Þ.
5. Results
Glass cover–interior: GAg ag þ hr1 As ðT s T gi Þ þ hc1 Ag ðT a T gi Þ 2 ¼ hk1 Ag ðT gi T go Þ.
relationships published in the literature [11,12,16,17]. Table 1 summarizes the heat transfer coefficients and model input used to obtain the results.
(6)
The relationship between inlet and outlet temperatures stated in Eq. (6) represents a common validated assumption found in the literature [11–15]. The above six equations include six unknowns (knowing Text and Tint as boundary conditions for a simulation) and can be solved using a standard Newton–Rhapson method. The heat transfer coefficients were solved using accepted empirical
To gain a better understanding of the actual performance of the SDBZ, the analytical model was simulated using weather data from an average heating season for Toronto. Weather data was obtained from a standard Canadian Weather for Energy Calculation (CWEC) file and manipulated to satisfy the required input of the analytical model. The CWEC file serves to simulate actual weather conditions on an hourly basis, including both bright and overcast days and combinations of the two. Based on values reported in the Canadian Climate Normals [18], the typical heating season for Toronto was found to be between September 21 and April 21. Fig. 7 presents the performance of the SDBZ over most of the heating season. In this period from October 1 to March 31, the average overall seasonal efficiency based on the model was found to be 35%. 6. Discussion 6.1. Performance Figs. 5–7 graphically show that the SDBZ concept is feasible. With average efficiencies of approximately 35%,
ARTICLE IN PRESS R.C. Richman, K.D. Pressnail / Building and Environment 44 (2009) 1–10 Table 1 Summary of model input and heat transfer coefficients Item
Equation or value
Source
sF gr ðT 4go T 4gr Þ Radiation to hr;gr ¼ ground T g T gr
[16]
sF sky bðT 4go T 4sky Þ Radiation to h ¼ r;sky sky T g T sky
[16]
sF air ð1 bÞðT 4go T 4ext Þ Radiation to hr;air ¼ air T g T ext
[16]
s Radiation to hr ¼ ðT 2 þ T 2b ÞðT a þ T b Þ ð1=a Þ þ ð1=b Þ 1 a interior
[17]
o
o
Nu l dh
hc ¼
Laminar Nusselt
Nu ¼ Nu1 þ
Turbulent Nusselt Exterior convection Interior convection Conduction
All areas Cavity depth U1 Glass absorb Collector surface absorb Glass transmittance
[12] aðRePrðDh =LÞÞm 1 þ bRePrðDh =LÞn
Nu ¼ 0:0158Re0:8 þ ð0:00181Re þ 2:92Þ
[12]
[12]
expð0:03795Lc =d h Þ
hc;ext ¼ 2:53Rf
T g T air 1=3 PV z 1=2 o þ 9:482 A 7:238
hc;int ¼ 1:31jT w T int j1=3 hk ¼
1 ð1=C 1 Þ þ ð1=C 2 Þ þ ð1=C 3 Þ þ þ ð1=C n Þ
required fresh-air intake for an average square mid-rise commercial building with 88% office area and 12% reception, corridor and amenities, when it is installed on the south, east and west elevations. To implement the SDBZ curtain wall system in a building, practical issues surrounding installation and synergy with the air handling system must be addressed. 6.2. Application
o
Convection in cavity
7
[16]
[16] [17]
0.5505 m2 0.093 m 0.54 W/m2 K 0.154 0.6
– – [17] [17] [17]
0.7705
[17]
this system outperforms many standard passive solar collecting systems with published efficiencies ranging between 20% and 40% [19]. During an average Toronto heating season, the SDBZ analytical model predicts energy savings of 205 kWh/m2 of spandrel area. This value lists the SDBZ in the upper range of published energy savings (150–210 kWh/m2) for fresh air pre-heating solar air collectors [11]. The results show that the SDBZ can significantly contribute to reduce a building’s heating cost. These results are limited to the effectiveness of the model. Uncertainties such as collector surface absorbtance, convective heat transfer and glass transmittance were based on reported values. All stated results require further validation in a laboratory or field setting. In addition to reducing heating cost, the SDBZ acts to supplement a building’s fresh-air intake required for ventilation. Using the requirements set out in ASHRAE Standard 62.1 [20], the SDBZ can supply 100% of the
The analytical modelling performed at this stage of the research shows promise. At a fundamental level, this modelling has shown that the SDBZ can directly reduce a building’s heating load. The system, as it is presently modelled, could be implemented within low and mid-rise residential and commercial buildings. In these buildings, daytime heating is typically required due to their relative inability to retain and generate large amounts of internal heat as a result of smaller volume to surface area ratios. The SDBZ can be used as an effective retrofit to older buildings with dated mechanical systems. Further, the SDBZ can be coupled with advanced HVAC systems in modern buildings to create a complete system reducing the overall energy consumption of the building. As building height increases, the need for additional solar heat decreases. Due to the larger volume to surface area ratio for this building type, high-rise buildings tend to require less heating. During the daytime, heating is provided by occupants and their use of electrical equipment. However, at night, additional heating may still be required. A means to store the heat gained during the day and release it during the night may still be required. By utilizing thermal mass within the SDBZ or between the SDBZ and the building interior (e.g. a thermally massive plenum), a lag can be introduced between the SDBZ and the interior space in order to provide the thermal heat energy when the building needs it. Finally, during cooling periods a dynamic buffer zone can still be used in tall buildings to exclude unwanted solar heat. These possibilities are currently being investigated by the authors. 6.3. Rainwater control It is widely accepted among curtain wall manufacturers that traditional face-sealed systems show poor performance compared to a pressure-moderated system in terms of moisture management. Although the main focus of the SDBZ is to harness solar energy, the system can act to mitigate exterior rain penetration as well. The SDBZ cavity can be pressurized with respect to the exterior environment when required (i.e. during rain storms). By minimizing (or eliminating) the driving force of rain penetration coupled with smart design to minimize rain entry by non-pressure driven methods (i.e. gravity, capillarity, etc.), the SDBZ can be adapted to minimize the potential of rain penetration. This method can be used
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500
January Design Day Performance
450 Incident Solar Radiation SDBZ Collected Power (Model)
400
Power (W h)
350
Total average efficiency = 36.5%
300 250 200 150 100 50
1: 00 2: AM 00 3: AM 00 4: A 00 M 5: AM 00 6: AM 00 7: AM 00 8: A 00 M 9: AM 0 10 0 A :0 M 11 0 A :0 M 12 0 A :0 M 0 1: P 00 M 2: PM 00 3: P 00 M 4: PM 00 5: PM 00 6: PM 00 7: PM 00 8: PM 00 9: PM 0 10 0 P :0 M 11 0 P :0 M 12 0 P :0 M 0 AM
0
Time Fig. 5. January design day performance of SDBZ model results.
January Design Day - Exterior and Outlet Temperatures 10 Texterior air Toutlet air
5
Temperature (C)
0 -5
Preheated Air (Model)
-10 -15 -20
1:
00 2: AM 00 3: AM 00 4: AM 00 5: AM 00 6: AM 00 7: AM 00 8: AM 00 9: AM 0 10 0 A :0 M 11 0 A :0 M 12 0 A :0 M 1: 0 P 00 M 2: PM 00 3: PM 00 4: PM 00 5: PM 00 6: PM 00 7: PM 00 8: PM 00 9: PM 0 10 0 P :0 M 11 0 P :0 M 12 0 P :0 M 0 AM
-25
Time Fig. 6. Predicted temperature rise of the preheated air (January design day).
to couple the SDBZ with modern high performance pressure moderated curtain wall system construction. By slightly tightening the pressure moderating rainscreen, the benefits of natural ventilation in the SDBZ cavity (through conventional pressure moderated design) will be recognized while still capturing solar gain. The added benefit in coupling the SDBZ with pressure-moderated design is that the mitigation of rain penetration through pressure equalization (or even positive pressurization with respect to the exterior) will be more easily achieved due to the ‘head start’ provided by the pressure-moderating design and construction.
6.4. Perceived drawbacks Although the SDBZ shows promise, designers may be skeptical since it involves creating discontinuities in the air barrier. Such an approach appears to contradict traditional building science practice. In answer to this perceived discrepancy, it should be understood that the SDBZ can be used to carefully control the location and amount of air movement across the envelope. One may argue that if air is allowed to infiltrate from the exterior, then interior warm moist air will at times be able to exfiltrate towards the exterior at uncontrolled rates/times. If this occurs, the
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Monthly Average Daily Performance 300
Direct Solar + Diffuse (Simulated Weather) SDBZ Collected (Model)
250
Power (W h)
200
150
100
50
7:00 AM 9:00 AM 11:00 AM 1:00 PM 3:00 PM 5:00 PM 7:00 PM 8:00 AM 10:00 AM 12:00 PM 2:00 PM 4:00 PM 6:00 PM 8:00 AM 10:00 AM 12:00 PM 2:00 PM 4:00 PM 6:00 PM 8:00 AM 10:00 AM 12:00 PM 2:00 PM 4:00 PM 6:00 AM 8:00 AM 10:00 AM 12:00 PM 2:00 PM 4:00 PM 6:00 PM 6:00 AM 8:00 AM 10:00 AM 12:00 PM 2:00 PM 4:00 PM 6:00 PM
0
Oct
Nov
Dec Jan Time of Day (Month)
Feb
Mar
Fig. 7. SDBZ model performance for a simulated heating season in Toronto.
psychrometric chart shows us the potential exists for condensation on cold days. However, by coupling the SDBZ to the building’s mechanical system, a natural air barrier can be formed by pressurizing the spandrel cavity or surrounding ducts. By raising the cavity (or duct) air pressure slightly more than the building’s interior air pressure, an effective air barrier is created. In addition to the theoretical argument, this concept has been confirmed in many DBZ applications across Canada [3]. 7. Future research The SDBZ curtain wall system is currently being tested in the Building Science Laboratory at the University of Toronto (Department of Civil Engineering), Canada. These laboratory tests aim to increase the precision of the analytical model and explore the impact on system performance of particular variables such as incident radiation, collector surface characteristics and cavity flow rate. 8. Conclusions Using an analytical model and seasonal simulation, the writers have modelled the performance of a SDBZ within a curtain wall system. It is possible to capture solar energy from the spandrel panels to the interior. Given the example design conditions, the analysis showed that the SDBZ can act to replace up to 90% of required fresh air for a typical mid-rise commercial building while significantly reducing the cost by preheating during daytime hours. The savings
will vary depending on system design, environment and geographical conditions. However, more research is needed, including the practical restraints of incorporating such a system into a building’s air handling system. The current research shows that an opportunity exists to improve upon curtain wall cladding systems. With such improvements, these systems may continue to dominate the market place as they become a more sustainable choice. Society is at a point where energy will not flow as easily or as inexpensively as before. The age of squandering inexpensive energy is rapidly coming to an end. Every effort to reduce energy consumption will have to be made. Systems such as the SDBZ offer an opportunity to build a more sustainable curtain wall systems. Acknowledgments The authors wish to acknowledge the Natural Sciences and Engineering Research Council (NSERC), Halcrow Yolles and the Ontario Graduate Scholarship (OGS) for funding this research. References [1] Gray S., Richman R., Pressnail K., Dong B., In: Low energy homes: evaluating the economic need to build better now, 33rd annual general conference of the Canadian society for civil engineering, vol. GC-336. CSCE, Toronto, Ontario, Canada, 2005. p. 1–9. [2] Cuddihy J, Kennedy C, Byer P. Energy use in Canada: environmental impacts and opportunities in relationship to infrastructure systems. Canadian Journal of Civil Engineering 2005;32:1.
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[3] Pasqualini P, The control of moisture movement in building using the dynamic buffer zones, Thesis, University of Toronto, Toronto, Ontario, 1999. [4] Kimura K. Solar architecture for the happiness of mankind. Solar Energy 1999;67(4–6):167–79. [5] Pasqualini P, Pressnail KD. The DBZ and wall surface temperature. Canadian Architect 2002;40. [6] Garden GK, The building envelope to 2001. In: Tenth congress of the International Federation of hospital engineering, Edmonton, Alberta, 1988. [7] Timusk J, The dynamic wall, In: Fourth conference on building science and technology, February 18–19, Ottawa, Canada, 1998. [8] Benemann J, Chehab O, Schaar-Gabriel E. Building-integrated PV modules. Solar Energy Materials and Solar Cells 2001;27:345–54. [9] Behr RA. On-site investigations of spandrel glass microenvironments. Building and Environment 1995;30(1):61–72. [10] Richman R, Pressnail K, More sustainable curtain wall systems: using a solar dynamic buffer zone (SDBZ) to reduce energy costs, In: The 2006 annual general conference of the Canadian Society for Civil Engineering, Calgary, Alberta, Canada, 2006. [11] Eicker U. Solar technologies for buildings. West Sussex, England: Wiley; 2003.
[12] Duffie JA, Beckman WA. Solar engineering of thermal processes. Hoboken, NJ: Wiley; 2006. [13] Hernandez V, Morillon D, Best R, Fernandez J, Almanza R, Chargoy N. Experimental and numerical model of wall like solar heat discharge passive system. Applied Thermal Engineering 2006;26: 2464–9. [14] Goyal R, Tiwari G, Garg H. Effect of thermal storage on the performance of an air collector: a periodic analysis. Energy Conversion Management 1996;39(3/4):193–202. [15] Hirunlabh J, Kongduang W, Namprakai P, Khedari J. Study of natural ventilation of houses by a metallic solar wall under tropical climate. Renewable Energy 1999;18:109–19. [16] (DOE) Department of Energy, US, 2006, EnergyPlusTM engineering reference manual, LBNL. [17] Hutcheon N, Handegord G. Building science for a cold climate. Ottawa, Canada: National Research Council of Canada; 1995. [18] Canadian Climate Normals, 2006, /http://www.climate.weatheroffice. ec.gc.ca/climate_normals/index_e.htmlS, accessed 2/20/07. [19] Yeh H, Lin T. Efficiency improvement of flat-plate solar air collectors. Energy 2005;21:435–43. [20] ASHRAE, 2004, Standard 62.1, Atlanta, GA.
Building and Environment 44 (2009) 1–10 www.elsevier.com/locate/buildenv
A more sustainable curtain wall system: Analytical modeling of the solar dynamic buffer zone (SDBZ) curtain wall R.C. Richman, K.D. Pressnail Department of Civil Engineering, University of Toronto, 35 St. George Street, Toronto, Ontario, Canada M5S 1A4 Received 25 April 2007; received in revised form 9 January 2008; accepted 10 January 2008
Abstract Given the increases in both the environmental and economic costs of energy, there is a need to design and build more sustainable and low-energy building systems now. Curtain wall assemblies are engineered wall assemblies that are used widely in both high-rise as well as low-rise construction. These assemblies show great promise—with the minimal modification outlined in this paper they can be built better now. Often ignored, spandrel panels that comprise a part of curtain wall assemblies can be natural solar collectors. By using a new, simple, low-cost method such as a solar dynamic buffer zone (SDBZ), solar energy can be efficiently gathered or excluded using the movement of air. Such a method can be used in both retrofit as well as new construction. This paper will introduce and outline a proposed SDBZ curtain wall system and present the results of analytical modelling. Using these results, a SDBZ system will be shown to be a more sustainable option for traditional curtain wall assemblies. r 2008 Elsevier Ltd. All rights reserved. Keywords: Energy efficiency; Sustainability; Curtain wall; Solar air collectors
1. Introduction There are compelling reasons why building design and construction should adopt more sustainable, more energyefficient systems. From ‘peak oil’ or ‘peak natural gas’, to the emergence of ‘developing’ countries attaining ‘western’ standards of living and consumption, to climate change and global warming, many experts agree that the present system of growth and consumption of hydrocarbon-based energy is not sustainable. The need to build better now is ever increasing. A recent paper presented quantitative economic analysis showing that it was economically more favorable to build residential buildings to a higher standard at the time of construction rather than retrofitting in the future [1]. Tomorrow’s buildings should be built today. Since building energy consumption accounts for approximately 19% of all energy use in Canada [2], it seems logical to focus on minimizing this demand.
Corresponding author. Tel.: +1 416 978 5964; fax: +1 416 978 6813.
E-mail address: [email protected] (R.C. Richman). 0360-1323/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2008.01.006
Humans have always tried to create more efficient separations between the interior and exterior environments. What started as the utilization of natural shelters has evolved into elaborate assemblies of highly complex manufactured components. Buildings were once clad using primarily solid structural masonry walls; now cladding can comprise a diverse range of systems and materials, including intricate non-load bearing skins. These skins, hung from the structure like a curtain, are complicated assemblies that are slight in thickness, contain multiple materials and must necessarily incorporate a multitude of interfaces and joints. The modern exterior wall assembly is exposed to same stresses as its historical counterpart —air pressure gradients (stack, mechanical systems, etc.), extreme thermal gradients (during peak summer and winter loads), vapor pressure gradients, driving rain, and wind-induced pressures. As occupant expectations of what constitutes a comfortable indoor environment have risen, and as buildings have become taller, the intensity of these stresses have increased and more demand is placed on today’s exterior wall assemblies.
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2
Nomenclature Ag As Aw Cp G hc1 hc2 hc,ext
hc,int
hk1 hr1
hr,air
area of the glass surface, m2 effective area of the receiving surface, m2 cross-sectional area of the wall components (i.e. insulation and metal back pan), m2 specific heat of air, kJ/kg 1C incoming radiation normal to the glass, W/m2 convective heat transfer coefficient between interior glass surface and cavity air, W/m2 K convective heat transfer coefficient between collector surface and cavity air, W/m2 K convective heat transfer coefficient between exterior glass surface and exterior environment, W/m2 K convective heat transfer coefficient between interior metal back pan surface and interior, W/m2 K conductive heat transfer coefficient between the interior and exterior glass surfaces, W/m2 K radiative heat transfer coefficient between collector surface and interior glass surface, W/ m2 K radiative heat transfer coefficient between exterior glass and exterior air, W/m2 K
The quest for improved curtain wall systems has led design engineers to concentrate their efforts on the management of moisture and control of heat flow. In terms of heat flow, the general focus in recent years has been on the vision areas of the curtain wall, not the spandrel areas. Development of high-performance glazing units that include multiple air spaces, special coatings, ‘warm-edge’ thermal spacers, and photochromic glass have resulted from this focus. As well, mechanical shading devices have been developed. All of these developments aim to control the flow of heat through the vision areas. Yet, little work has been done on the spandrel areas. The spandrel panels deserve attention. During the heating season, heat is lost from the interior to the exterior through these panels. In the cooling season, the flow is reversed. By insulating the spandrel panels, attempts are made to minimize the heat loss or gain through the spandrel panel. However, the heat losses or gains that do occur through these panels have to be managed by the HVAC system. Given the variation of magnitude and direction of heat flow through the spandrel panels, this approach to thermal management may not be the most efficient. To date, few attempts in Canada or around the world have been made to better manage heat flow through this space, or to utilize the potential solar gain through the spandrel panel. The technology already exists for the construction of more efficient curtain wall assemblies [3]. Curtain wall
hr,int
hr,gr hr,sky _ m Ta Text T gi T go Tinlet Tint Toutlet Ts Tw U1
ag as tg
radiative heat transfer coefficient between interior metal back pan surface and interior, W/m2 K radiative heat transfer coefficient between exterior glass and ground, W/m2 K radiative heat transfer coefficient between exterior glass and sky, W/m2 K mass flow rate of cavity air, kg/s temperature of cavity air, K temperature of exterior, K temperature of interior glass surface, K temperature of exterior glass surface, K temperature of the air at the inlet opening, K temperature of the interior, K temperature of the air at the outlet opening, K temperature of the collecting surface, K temperature of the interior metal back pan surface, K conductive heat transfer coefficient between the collector surface and the interior metal back pan, W/m2 K absorptance of the glass absorptance of the collector surface transmittance of the glass
systems that provide natural daylighting can be adapted using computer-controlled air pressure zones to facilitate the provision of fresh air, while improving occupant comfort and reducing energy consumption. Dynamic buffer zones (DBZ) work by ventilating a cavity within a wall with heated exterior air to control moisture migration across the assembly [3]. By employing this principle within the spandrel panel, a solar dynamic buffer zone (SDBZ) can be utilized to create a more sustainable curtain wall system and to manage solar energy in order to reduce the costs of heating and cooling of buildings. This article reviews the fundamental design principles and the conceptual layout of an SDBZ curtain wall system designed, developed and currently being tested at the Building Science Laboratory (Department of Civil Engineering) at the University of Toronto, Toronto, Canada. This review will be followed by a presentation and discussion of the results from analytical modelling. Finally, the research directions that emerge from modelling and discussion will be outlined.
2. The building blocks—previous research The SDBZ synthesizes three discipline areas of civil engineering: solar architecture, dynamic buffer zones, and curtain wall design. A brief summary of recent research pertaining to the SDBZ is essential prior to discussion of the proposed system.
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2.1. Solar architecture Aspects of solar architecture pertaining to the SDBZ include: passive heating/cooling, thermal mass concepts, Trombe walls and ventilation techniques. Solar architecture is an extensively researched field. A number of principles from this research were broadly applied during the conceptual analysis of the SDBZ. Kimura [4] has studied solar architecture for over 25 years. His paper ‘Solar Architecture for the Happiness of Mankind’ provides a comprehensive summary of solar architecture research throughout the world. Kimura has studied many aspects of solar architecture including: design and construction of ‘solar houses’, building integrated photovoltaic systems, thermal comfort, natural illumination and passive heating/cooling methods. His research provided the foundation for solar architecture principles applied to the SDBZ. 2.2. Dynamic buffer zone (DBZ) The DBZ concept consists of ventilating a cavity within a wall to control heat movement and moisture migration across the assembly. Stemming from a 30-year-old notion of ventilated walls during winter operation, the DBZ typically introduces heated outside air containing little moisture into an interstitial cavity within the wall assembly [5]. By pressurizing the cavity with respect to both the interior and exterior air pressure, air leakage can be controlled across the wall assembly. Further, given the relatively small amounts of moisture and consequently low relative humidity of the warmed exterior air, any cavity air leakage towards the interior or the exterior poses little or no threat in the way of interstitial condensation. Early notions of the DBZ can be traced in Canada [6]. In his paper, Garden proposes the use of DBZ walls in hospitals to combat the problem of interstitial condensation common to such buildings with extreme interior climates (i.e. continuous high humidity and temperature). It appears that Garden was ahead of his time; the hospital engineering industry has gradually recognized the benefits of incorporating DBZ technology. One of the first DBZ systems was designed and installed as a retrofit measure in the Canada Life Building in Toronto, Ontario in 1997. This installation spawned a monitoring study and a supporting laboratory-testing program. This program was performed by Pasqualini [3] at the University of Toronto. Although historically the DBZ concept has focused on the problem of concealed condensation, the 1999 study focused on the effect of the DBZ and internal surface temperatures. Prior to the study, it was a common belief that the introduction of DBZ cavity air (while heated, it is generally significantly cooler than interior air) would result in lower interior surface temperatures—leading to issues of thermal comfort and unnecessary heat loss. The testing demonstrated that the DBZ air temperature equilibrates quickly with the boundary conditions because of the small
3
heat capacity of air [5]. Pasqualini went on to state ‘‘in general, the relatively large variation in average cavity temperatures observed did not significantly affect warm side surface temperatures of the wall’’. This research opened the door for use of the DBZ in terms of thermal control. It has also been recognized that the use of the DBZ system can lead to ‘dynamic insulation’ [7] or the effect of overcoming some of the heat that would otherwise be lost through the wall [5]. This promising area should be pursued further in future research. The SDBZ borrows from the traditional concept of the DBZ to manipulate the transmission of heat across the spandrel cavity assembly. 2.3. Curtain wall design The initial concept of the SDBZ focused on creating an ‘interactive’ curtain wall system. As such, an exhaustive review of the literature pertaining to innovative curtain wall systems was required. Based on a substantial review of academic literature, it appears that little research has been focused on modern curtain wall systems. Although large amounts of research have been published on solar air collectors, this application to curtain walls is yet to be documented. Curtain wall research can basically be segregated into two main areas: (i) focus on the spandrel (opaque) areas and (ii) focus on the glazing areas that will not be discussed here. Research focusing on the spandrel areas includes the use of photo-voltaic (PV) cells in the exterior skin of the system. Benemann identifies the first use of buildingintegrated photovoltaics (BIPV) in 1991 in Aachen, Germany [8]. Since then building integration is one of the fastest growing market segments in photovoltaics. An impressive solar installation, the Academy Mont-Cenis in Herne (Germany), with 410,000 m2 of PV integrated into the roof and fac- ade, was finished in March, 2001 [8]. Photovoltaics provide an economic means of harnessing the solar energy reaching the building fac- ade otherwise lost to the exterior. The SDBZ attempts to exploit this same energy by using the existing curtain wall components in a near passive manner. Perhaps the most beneficial previous work relating to the SDBZ was completed by Behr [9] at the University of Missouri-Rolla, USA. Several spandrel panels at sites across the United States were monitored to record the micro-environmental conditions, including glass surface temperature, glass edge temperature, air temperature and relative humidity within the enclosed air space behind the spandrel glass panel and interior spandrel surfaces. Most impressive were the reported interior cavity still air temperatures (maximum sustained of approximately 45 1C) and interior glass surface temperatures (maximum of 82.1 1C). As the key aspect of the SDBZ is to harness the solar energy entering the spandrel cavity during the heating season, Behr’s work provides quantitative data that the micro-environments within the spandrel cavities support such a concept. Based on correspondence with the author,
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Fig. 1. Typical curtain wall system.
In general, these systems are designed to minimize the transmission of heat across the assembly all year. During the heating season, most solar energy that enters the spandrel cavity is not transmitted to the interior is rather rapidly ‘lost’ back to the exterior primarily by radiative and convective effects. This is due in part to the relatively low thermal mass and high emmissivities of the spandrel components. In an effort to ‘capture’ as much of the solar gain as possible, two options exist: (i) install a low-e coating on the spandrel glass to minimize the radiative losses to the exterior and (ii) replace traditional back pan materials with high thermal mass materials. These options have been discussed further by Richman and Pressnail [10]. Once captured, the solar heat must be brought into the building. Ordinarily the spandrel insulation greatly reduces this heat gain component. One simple way of bringing in this captured solar heat is by drawing air through the spandrel cavity. In its preliminary stage, the SDBZ can act to preheat cold exterior air during the daytime. Fig. 3 is a schematic representation of how air can be used to bring in the solar heat. In a module-based SDBZ curtain wall system, the transfer of the preheated cavity air can be controlled by a system of small fans. As such, the SDBZ can be a module based, nearly passive system that requires the use of a relatively simple, low-cost fan. This approach would serve to deliver the air in close proximity to the SDBZ module, therefore reducing (i) the pressure drop associated with long runs of duct work and (ii) the overall energy required to transport the air. It may be more desirable to link the SDBZ to the building’s air handling system as part of its fresh air intake circulation loop. By addressing such practical issues as minimizing additional duct work, connection points for optimal heat exchange and air filtering, it is possible that no additional energy is required to move the SDBZ air.
Fig. 2. Conventional spandrel cavity.
Behr has no knowledge of further work in this area of research since his original study (Fig. 1). 3. The SBDZ—a conceptual layout Fig. 2 shows the typical operating conditions for a southfacing curtain wall assembly during a bright day in the northern hemisphere. In terms of thermal considerations, solar gain is desirable in the heating season only. A conventional spandrel cavity is shown in Fig. 2. This assembly consists of: a spandrel face (e.g. glass, stone, etc.), the spandrel cavity, semi-rigid insulation and a metal back pan. For buildings requiring supplemental energy during the heating or cooling seasons, a drawback of conventional systems is that they are not designed to gather solar heat, but rather, they are designed to reduce heat transfer.
Fig. 3. The conceptual SDBZ.
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An efficient design has the potential to minimize the overall energy used for a building’s fresh air intake. To maximize the performance as a solar absorber, the collecting surface (i.e. typically semi-rigid mineral wool insulation) can be painted or otherwise colored black. For this analysis, the SDBZ construction included standard mineral wool (U-value ¼ 0.54 W/m2 K) with a 6 mm clear spandrel glass light. 4. Analytical model Based on the conceptual description of the SDBZ model described above, Fig. 4 presents the thermal characteristics of the model. This is used as a reference base for developing the governing heat balance equations for the system. Based on the thermal behavior presented in Fig. 4, the governing heat balance equations were developed using the following assumptions: (i) We assume steady-state conditions. (ii) Thermal storage by the SDBZ components was negligible.
(iii) Minimal stratification of surface temperatures existed. As such, average surface temperatures were utilized. (iv) Cavity air is completely mixed. (v) The heat flow to the sides is negligible. (vi) The heat flux is taken to be unidirectional in the system (i.e. perpendicular to the direction of air flow). (vii) The glass layer is opaque to infrared radiation. (viii) The short-wave radiation absorbed in a glass layer can be apportioned equally to the two surfaces. (ix) The inlet temperature is equal to the ambient air temperature. (x) The sky temperature is approximated by the bliss correlation. (xi) The ground temperature is equal to the ambient air temperature. Since the boundary conditions change relatively slowly compared to the reaction of the SDBZ components, steady state is justified. Further, preliminary experiments confirmed: (i) the SDBZ system reacted rapidly to changes in radiation, cavity airflow or exterior temperature and (ii) the
Toutlet
G
OUTLET DUCT CONNECTION VARIES DEPENING ON SYSTEM DESIGN
· Cp m
hc ext
hc1
hc int
hc2
hr sky hr 1
hr air
hr int
hr gr
Text
Tg o
EXTERIOR SPANDREL GLASS
Tg i
Ta CAVITY
5
Ts
Tw
INSULATION
BACK PAN
Tint INTERIOR
Tinlet
Fig. 4. Schematic model of SDBZ showing heat transfer coefficients.
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6
system showed no significant capacity for thermal storage as it is constructed for this research. Assuming negligible heat flow to adjacent panels is valid for SDBZ areas in large installations where many units are installed next to one another. For any given elevation, the average temperature in adjacent spandrel cavities and framing will be the same. This assumption can become invalid at points of transition between cladding systems; for example, where a SDBZ curtain wall meets an architectural stone veneer or precast concrete wall. Further complication may be encountered when modelling the SDBZ in a module-based system, where actual field performance will differ based on the thermal regime surrounding the perimeter framing. Since this research is focusing on the initial stages of development, it is acceptable to make this assumption based on the premise of large installations. The remaining assumptions (i.e. (vii) through (xi)) are common to similar principle-based air collecting systems published in the literature [11–15]. Keeping the notation of heat in (left side of equation) and heat out (right side) for each element, the governing heat balance equations are then as follows: Glass cover–exterior: GAg ag þ hk1 Ag ðT gi T go Þ ¼ hr;gr Ag ðT go T gr Þ 2 þ hr;sky Ag ðT go T sky Þ þ hr;air Ag ðT go T ext Þ þ hc;ext Ag ðT go T ext Þ.
The boundary conditions for the Toronto January design day are outlined below. Figs. 5 and 6 present the performance of the system: solar radiation values for January 21 (431 latitude) assuming bright, sunny conditions (near optimal conditions), exterior ambient air temperature of 20 1C, exterior relative humidity of 80%, interior ambient air temperature of 21 1C, interior relative humidity of 25%.
(2)
5.2. Seasonal simulation
(3) Collector surface: GAs tg as ¼ hc2 As ðT s T a Þ þ hr1 As ðT s T gi Þ (4)
Metal back pan surface—interior: U 1 Aw ðT s T w Þ ¼ hr;int Aw ðT w T int Þ (5)
Relationship between inlet, outlet and cavity air temperatures: T a ¼ 0:75T outlet þ 0:25T inlet .
5.1. January design day
_ p ðT outlet T inlet Þ þ hc1 Ag ðT a T gi Þ. hc2 As ðT s T a Þ ¼ mC
þ hc;int Aw ðT w T int Þ.
Two separate simulations were conducted to study the performance of the SDBZ curtain wall: (i) a January design day and (ii) a typical heating season, both for Toronto, ON, Canada. A brief description of boundary conditions and input sources follow. Based on values from the literature [11] for average flows in fresh-air preheating systems, the design flow used during the simulations was 60 m3/m2 h. It should be noted the efficiency of the SDBZ is sensitive to the cavity flow. As such, results would vary both in a positive and negative manner when different design flows are utilized.
(1)
Cavity air:
þ U 1 Aw ðT s T w Þ.
5. Results
Glass cover–interior: GAg ag þ hr1 As ðT s T gi Þ þ hc1 Ag ðT a T gi Þ 2 ¼ hk1 Ag ðT gi T go Þ.
relationships published in the literature [11,12,16,17]. Table 1 summarizes the heat transfer coefficients and model input used to obtain the results.
(6)
The relationship between inlet and outlet temperatures stated in Eq. (6) represents a common validated assumption found in the literature [11–15]. The above six equations include six unknowns (knowing Text and Tint as boundary conditions for a simulation) and can be solved using a standard Newton–Rhapson method. The heat transfer coefficients were solved using accepted empirical
To gain a better understanding of the actual performance of the SDBZ, the analytical model was simulated using weather data from an average heating season for Toronto. Weather data was obtained from a standard Canadian Weather for Energy Calculation (CWEC) file and manipulated to satisfy the required input of the analytical model. The CWEC file serves to simulate actual weather conditions on an hourly basis, including both bright and overcast days and combinations of the two. Based on values reported in the Canadian Climate Normals [18], the typical heating season for Toronto was found to be between September 21 and April 21. Fig. 7 presents the performance of the SDBZ over most of the heating season. In this period from October 1 to March 31, the average overall seasonal efficiency based on the model was found to be 35%. 6. Discussion 6.1. Performance Figs. 5–7 graphically show that the SDBZ concept is feasible. With average efficiencies of approximately 35%,
ARTICLE IN PRESS R.C. Richman, K.D. Pressnail / Building and Environment 44 (2009) 1–10 Table 1 Summary of model input and heat transfer coefficients Item
Equation or value
Source
sF gr ðT 4go T 4gr Þ Radiation to hr;gr ¼ ground T g T gr
[16]
sF sky bðT 4go T 4sky Þ Radiation to h ¼ r;sky sky T g T sky
[16]
sF air ð1 bÞðT 4go T 4ext Þ Radiation to hr;air ¼ air T g T ext
[16]
s Radiation to hr ¼ ðT 2 þ T 2b ÞðT a þ T b Þ ð1=a Þ þ ð1=b Þ 1 a interior
[17]
o
o
Nu l dh
hc ¼
Laminar Nusselt
Nu ¼ Nu1 þ
Turbulent Nusselt Exterior convection Interior convection Conduction
All areas Cavity depth U1 Glass absorb Collector surface absorb Glass transmittance
[12] aðRePrðDh =LÞÞm 1 þ bRePrðDh =LÞn
Nu ¼ 0:0158Re0:8 þ ð0:00181Re þ 2:92Þ
[12]
[12]
expð0:03795Lc =d h Þ
hc;ext ¼ 2:53Rf
T g T air 1=3 PV z 1=2 o þ 9:482 A 7:238
hc;int ¼ 1:31jT w T int j1=3 hk ¼
1 ð1=C 1 Þ þ ð1=C 2 Þ þ ð1=C 3 Þ þ þ ð1=C n Þ
required fresh-air intake for an average square mid-rise commercial building with 88% office area and 12% reception, corridor and amenities, when it is installed on the south, east and west elevations. To implement the SDBZ curtain wall system in a building, practical issues surrounding installation and synergy with the air handling system must be addressed. 6.2. Application
o
Convection in cavity
7
[16]
[16] [17]
0.5505 m2 0.093 m 0.54 W/m2 K 0.154 0.6
– – [17] [17] [17]
0.7705
[17]
this system outperforms many standard passive solar collecting systems with published efficiencies ranging between 20% and 40% [19]. During an average Toronto heating season, the SDBZ analytical model predicts energy savings of 205 kWh/m2 of spandrel area. This value lists the SDBZ in the upper range of published energy savings (150–210 kWh/m2) for fresh air pre-heating solar air collectors [11]. The results show that the SDBZ can significantly contribute to reduce a building’s heating cost. These results are limited to the effectiveness of the model. Uncertainties such as collector surface absorbtance, convective heat transfer and glass transmittance were based on reported values. All stated results require further validation in a laboratory or field setting. In addition to reducing heating cost, the SDBZ acts to supplement a building’s fresh-air intake required for ventilation. Using the requirements set out in ASHRAE Standard 62.1 [20], the SDBZ can supply 100% of the
The analytical modelling performed at this stage of the research shows promise. At a fundamental level, this modelling has shown that the SDBZ can directly reduce a building’s heating load. The system, as it is presently modelled, could be implemented within low and mid-rise residential and commercial buildings. In these buildings, daytime heating is typically required due to their relative inability to retain and generate large amounts of internal heat as a result of smaller volume to surface area ratios. The SDBZ can be used as an effective retrofit to older buildings with dated mechanical systems. Further, the SDBZ can be coupled with advanced HVAC systems in modern buildings to create a complete system reducing the overall energy consumption of the building. As building height increases, the need for additional solar heat decreases. Due to the larger volume to surface area ratio for this building type, high-rise buildings tend to require less heating. During the daytime, heating is provided by occupants and their use of electrical equipment. However, at night, additional heating may still be required. A means to store the heat gained during the day and release it during the night may still be required. By utilizing thermal mass within the SDBZ or between the SDBZ and the building interior (e.g. a thermally massive plenum), a lag can be introduced between the SDBZ and the interior space in order to provide the thermal heat energy when the building needs it. Finally, during cooling periods a dynamic buffer zone can still be used in tall buildings to exclude unwanted solar heat. These possibilities are currently being investigated by the authors. 6.3. Rainwater control It is widely accepted among curtain wall manufacturers that traditional face-sealed systems show poor performance compared to a pressure-moderated system in terms of moisture management. Although the main focus of the SDBZ is to harness solar energy, the system can act to mitigate exterior rain penetration as well. The SDBZ cavity can be pressurized with respect to the exterior environment when required (i.e. during rain storms). By minimizing (or eliminating) the driving force of rain penetration coupled with smart design to minimize rain entry by non-pressure driven methods (i.e. gravity, capillarity, etc.), the SDBZ can be adapted to minimize the potential of rain penetration. This method can be used
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8
500
January Design Day Performance
450 Incident Solar Radiation SDBZ Collected Power (Model)
400
Power (W h)
350
Total average efficiency = 36.5%
300 250 200 150 100 50
1: 00 2: AM 00 3: AM 00 4: A 00 M 5: AM 00 6: AM 00 7: AM 00 8: A 00 M 9: AM 0 10 0 A :0 M 11 0 A :0 M 12 0 A :0 M 0 1: P 00 M 2: PM 00 3: P 00 M 4: PM 00 5: PM 00 6: PM 00 7: PM 00 8: PM 00 9: PM 0 10 0 P :0 M 11 0 P :0 M 12 0 P :0 M 0 AM
0
Time Fig. 5. January design day performance of SDBZ model results.
January Design Day - Exterior and Outlet Temperatures 10 Texterior air Toutlet air
5
Temperature (C)
0 -5
Preheated Air (Model)
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to couple the SDBZ with modern high performance pressure moderated curtain wall system construction. By slightly tightening the pressure moderating rainscreen, the benefits of natural ventilation in the SDBZ cavity (through conventional pressure moderated design) will be recognized while still capturing solar gain. The added benefit in coupling the SDBZ with pressure-moderated design is that the mitigation of rain penetration through pressure equalization (or even positive pressurization with respect to the exterior) will be more easily achieved due to the ‘head start’ provided by the pressure-moderating design and construction.
6.4. Perceived drawbacks Although the SDBZ shows promise, designers may be skeptical since it involves creating discontinuities in the air barrier. Such an approach appears to contradict traditional building science practice. In answer to this perceived discrepancy, it should be understood that the SDBZ can be used to carefully control the location and amount of air movement across the envelope. One may argue that if air is allowed to infiltrate from the exterior, then interior warm moist air will at times be able to exfiltrate towards the exterior at uncontrolled rates/times. If this occurs, the
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Fig. 7. SDBZ model performance for a simulated heating season in Toronto.
psychrometric chart shows us the potential exists for condensation on cold days. However, by coupling the SDBZ to the building’s mechanical system, a natural air barrier can be formed by pressurizing the spandrel cavity or surrounding ducts. By raising the cavity (or duct) air pressure slightly more than the building’s interior air pressure, an effective air barrier is created. In addition to the theoretical argument, this concept has been confirmed in many DBZ applications across Canada [3]. 7. Future research The SDBZ curtain wall system is currently being tested in the Building Science Laboratory at the University of Toronto (Department of Civil Engineering), Canada. These laboratory tests aim to increase the precision of the analytical model and explore the impact on system performance of particular variables such as incident radiation, collector surface characteristics and cavity flow rate. 8. Conclusions Using an analytical model and seasonal simulation, the writers have modelled the performance of a SDBZ within a curtain wall system. It is possible to capture solar energy from the spandrel panels to the interior. Given the example design conditions, the analysis showed that the SDBZ can act to replace up to 90% of required fresh air for a typical mid-rise commercial building while significantly reducing the cost by preheating during daytime hours. The savings
will vary depending on system design, environment and geographical conditions. However, more research is needed, including the practical restraints of incorporating such a system into a building’s air handling system. The current research shows that an opportunity exists to improve upon curtain wall cladding systems. With such improvements, these systems may continue to dominate the market place as they become a more sustainable choice. Society is at a point where energy will not flow as easily or as inexpensively as before. The age of squandering inexpensive energy is rapidly coming to an end. Every effort to reduce energy consumption will have to be made. Systems such as the SDBZ offer an opportunity to build a more sustainable curtain wall systems. Acknowledgments The authors wish to acknowledge the Natural Sciences and Engineering Research Council (NSERC), Halcrow Yolles and the Ontario Graduate Scholarship (OGS) for funding this research. References [1] Gray S., Richman R., Pressnail K., Dong B., In: Low energy homes: evaluating the economic need to build better now, 33rd annual general conference of the Canadian society for civil engineering, vol. GC-336. CSCE, Toronto, Ontario, Canada, 2005. p. 1–9. [2] Cuddihy J, Kennedy C, Byer P. Energy use in Canada: environmental impacts and opportunities in relationship to infrastructure systems. Canadian Journal of Civil Engineering 2005;32:1.
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[12] Duffie JA, Beckman WA. Solar engineering of thermal processes. Hoboken, NJ: Wiley; 2006. [13] Hernandez V, Morillon D, Best R, Fernandez J, Almanza R, Chargoy N. Experimental and numerical model of wall like solar heat discharge passive system. Applied Thermal Engineering 2006;26: 2464–9. [14] Goyal R, Tiwari G, Garg H. Effect of thermal storage on the performance of an air collector: a periodic analysis. Energy Conversion Management 1996;39(3/4):193–202. [15] Hirunlabh J, Kongduang W, Namprakai P, Khedari J. Study of natural ventilation of houses by a metallic solar wall under tropical climate. Renewable Energy 1999;18:109–19. [16] (DOE) Department of Energy, US, 2006, EnergyPlusTM engineering reference manual, LBNL. [17] Hutcheon N, Handegord G. Building science for a cold climate. Ottawa, Canada: National Research Council of Canada; 1995. [18] Canadian Climate Normals, 2006, /http://www.climate.weatheroffice. ec.gc.ca/climate_normals/index_e.htmlS, accessed 2/20/07. [19] Yeh H, Lin T. Efficiency improvement of flat-plate solar air collectors. Energy 2005;21:435–43. [20] ASHRAE, 2004, Standard 62.1, Atlanta, GA.