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In buildings, glazing plays an important role in influencing energy demands and lighting requirements. The interrelation between fenestration design and thermal/visual performance has been receiving much attentions. As a matter of fact, extensively glazed buildings from commercial complexes to residences are common in modern architecture for their lightness and good exterior appearance. But this design trend intensifies the solar heat gain and therefore the air-conditioning electricity consumptions. Most buildings in the warm climate are single glazed, with clear glass offered for residential developments and absorptive/reflective glass for commercial applications. Alternatively, ventilated double-glazing with a stream of air flowing through the glazing cavity can be used. For the warm climate region, solar heat absorption by the air stream has no direct application value. Instead, solar heat absorption via a liquid water stream can be readily transferred to a domestic hot water system. In this study, the system performance of liquid-filled double glazing integrated with a water-heater has been investigated. Figure 1 shows the working principle. The cavity in a double-pane glazing is incorporated with water inlet and outlet headers at its lower and upper ends. Together with a water-to-water heat exchanger at the top and the interconnecting tubing at the side (all are housed in the window frame), this forms a fully-enclosed water circuit. The heat conversion to another water stream via the heat exchanger allows the full utilization of the absorbed solar heat.

Figure 1: Water-flow double-pane window



Experimental rigs were designed and constructed for the investigation. At first, a small experimental unit able to compare directly the solar transmission and water heat gain of two alternative glazing options was made use of. The gained experiences then facilitated the production of a full scale prototype tested at the external wall of a climate chamber, where thermal and visual comforts were fully investigated, as in Figure 2. A numerical model validated by the experimental data was incorporated into the whole-building simulation software for studying the integrative performance under different conditions of building use, glazing properties, orientation, occupancy, seasons, and the like. Solid technical information was gathered on the best use of this innovative system, and on the energy/cost saving potential of alternative arrangements. Our results show that the use of absorptive glass panes can allow better water heat gain. The incorporation of clear glass pane to a low-emissivity integrated glass unit allows better visibility and daylight utilization. The economic payback period is around four years. This makes this innovative technology very good application potential in the private construction industry, like in hotels, health clubs, sport centers, swimming pools, elderly homes, residences, and any domestic commercial complex with a stable hot water demand. The situation is different from many photovoltaic product applications which rely heavily on government subsidies as in many developed countries.

Figure 2: Dr Tin-tai Chow and his PhD student
Chunying Li at the full-scale water-flow window experimental system

Dr Tin-tai CHOW
Division of Building Science
and Technology
City University of Hong Kong