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Article Excerpt
Abstract: A Transparent Water Storage Envelope (TWSE) is an innovative curtain wall system being developed to improve the overall energy efficiency of commercial buildings by means of minimizing the cooling load in summer and heat loss in winter as well as remaining transparent for facilitating direct utilization of natural light in the daytime. Based on theoretical thermal analysis, a Convection Baffle Layer (CBL) system was originated. A typical TWSE prototype with CBLs has been investigated, especially from the perspective of incorporating thermal mass benefit into thermal performance in summer. Several optical characteristics of TWSEs are discussed in this paper as a guide for more specific technical designs. This paper mainly reports on the development of TWSEs through theoretical analyses on its thermal performance and optical properties.
Keywords: Convection baffle layers, Optical properties, Thermal performance, Transparent insulating walls, Transparent water storage envelopes (TWSE)
Introduction
Buildings, especially large-scale commercial buildings, have become and are continuing to become more and more complex. In order to acquire certain modern architectural appearances as well as to create a comfortable indoor environment, various glass curtain wall and HVAC systems have been widely used around the world.
Although pure glass boxes have been heavily rebuked since the energy crisis of the 1970s, the enthusiasm of architects and engineers for transparent envelopes has never faded. Double Skin Facade (DSF) and even Multiple Skin Facade (MSF) have been developed for reducing the annual energy consumption of transparent buildings. The major technical characteristic of DSF and MSF is the greenhouse effect produced by their cavities, which can cause natural ventilation for cooling and effectively prevent heat loss. However, recent studies have revealed that the greenhouse effect is not always favorable. In summer, if the cavity of a DSF or MSF is poorly ventilated, the envelope is at risk of overheating (Oesterle, Lieb, Lutz & Heuslet, 2001), which can subsequently compromise the original purpose for achieving an energy efficient building.
Is there any new technical paradigm besides DSF for transparent energy efficient building envelopes? This paper will demonstrate that there is another alternative for the transparent energy efficient building envelope, and again a form of transparency is not the contradiction of the need of a low energy building if appropriate new technologies can be applied according to the climate and site.
Transparent Water Storage Envelope (TWSE) is an upgraded water wall, a convertible building facade system with seasonal dimorphism, which is developed with the goal of improving the overall energy efficiency of commercial buildings, to be specific, reducing energy consumption for heating, cooling and electrical lighting by creating transparent envelopes with two distinct thermal stages. These are exploiting the thermal mass benefit of water for enhancing the thermal stability of envelopes in summer and minimizing the heat loss through them in winter. This is by generating air gaps instead of a water interlayer, as well as keeping an optical transparent stage all the time to enable transmission of visible light through them directly in the daytime. Theoretically speaking, the function of TWSEs mainly depends on thermal and optical characteristics of the filling materials in its cavity, in which any convection should be prohibited as thoroughly as possible. This point is one of the major differences between TWSE and DSF, which mainly function on dynamic fluid.
The concept of TWSE originates from the idea of integrating the technique of water walls with Transparent Insulating Walls (TIW) (Xia, Xia & Shi, 2001) in order to meet the need of annual outdoor environmental changes. In the conceptual developing process, physical analyses focus on the thermal performance and optical properties of TWSEs. Through the comparison with other building envelopes, the outstanding thermal performance can be fully manifested by some representative thermal parameters. These include U-value, thermal inertia index, surface thermal admittance, damping factor and heat lag which can provide preliminary objective criteria for evaluating the thermal effectiveness of TWSE systems. The intrinsic transparent appearance of TWSEs can also bring about new technical designs based on its unique optical properties, for example, the appropriate assembling forms and shading devices for taking full advantage of natural light while preventing excessive solar gain, the liquid form sunlight diffusers, and water prism systems. Although the research work on TWSEs is still ongoing it has already shows great potential to be an exceptional envelope for green buildings.
Theoretical Analyses of the Thermal Performance of TWSEs
The proposed TWSE for commercial buildings consists of modular transparent water containers, purified water (summer stage) or air (winter stage) filled in them and other facilitating accessories such as pipes and valves for connecting with the water treatment systems in these buildings.
TWSEs differ from conventional water walls. Conventional water walls can be defined as building components consisting of a volume of water filled in certain containers, which are generally made of steel, plastic or reinforced concrete. Though water has excellent heat storage capacity, which is shown by the fact that the specific heat of water is 4.18 KJ/kg K, larger than that of reinforced concrete (the value of reinforced concrete is 0.92 KJ/kg K in the same condition) and most other construction materials. Thermal wave through conventional water walls still may be easier than that through solid concrete walls with similar thicknesses and other boundary conditions due to potential convection occurred in the water domain (Xia, Xia & Shi, 2001). In order to become an upgraded water wall, thermal performance of the water interlayer in TWSE should be improved.
The water interlayer in TWSE is a unique part in comparison with most curtain walls. To evaluate its thermal performance in the practical condition an idealized physical model, i.e., the enclosed rectangular fluid domain with opposite vertical parallel isothermal planes needs to be established first. The heat transfer between the opposite planes is a complex process incorporating conduction, convection, and radiation effects together. The three actions should be investigated separately and then be integrated as a whole to describe the overall thermal performance of the fluid domain.
Actually, in the vertical water interlayer pure conduction only occurs when the viscous stress dominates and stabilizes the fluid domain (Guo, 1992). In heat engineering the thermal conductivity [lambda] of steady water in the normal condition is approximately 0.58 WlmK, resembling the value of cavity brick, not being as good as that of steady air under the same circumstance, whose thermal conductivity can adopt 0.029 W/mK (Ye, 1996). It implies that water is not a good heat insulating material, and so the best way to enhance the thermal insulation of TWSE in winter is to discharge water and substitute it for air.
Physical Analysis with Consideration of Natural Convection in the Water Interlayer
In gravitational field temperature differences between the inside and outside isothermal planes can vary the densities of water in the boundary layers near both planes. If the viscous force cannot restrain the buoyancy caused by the density difference, natural convection will occur consequentially. One of the decisive factors describing natural convection is non-dimensional Grashof number.
Gr = g[alpha][DELTA]t[l.sup.3]/[v.sup.2] (1)
In the Grashof number, the numerator portion indicates the buoyant effect and the denominator portion is the viscous term. The bigger Gr number the fluid has, the more intensive natural convection will develop in it.
The Prandtl number is used to describe convection in dynamic thermal physics. It is another specific governing factor for convection that indicates the ratio of momentum transfer ability to heat transfer ability in fluid. The Prandtl number of water in 20[degrees]C is 7.02 (Zhang & Ren, 2001).
Pr = v/a (2)
The correlation of convection in the enclosed rectangular fluid domain can be expressed as follows:
[Nu.sub.[delta]]=[C([Gr.sub.[delta]] Pr).sup.m] [([delta]/H).sup.[1/9] (3)
In formula (3), the non-dimensional Nu (Nusselt number) expresses the ratio of convection effect to conduction effect and the geometrical character of the fluid domain is described by the ratio of width [delta] to height H. C, m and n are all constants for specific cases which can be derived from experiments (Zhang & Ren, 2001).
The empirical correlations for the enclosed rectangular fluid domain with opposite vertical parallel isothermal planes can be expressed as follows:
Nu=1 (4)
The range of applicability for formula {4): [Gr.sub.[delta]] [less than or equal to] 2000:...
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