Ensuring fire safety while reducing the energy need without compromising occupants' comfort is a challenge in modern-day green buildings with glass façades. One way of achieving both aspects is to construct a water wall system (WWS) as a building façade. A WWS has a water layer between two glass panes. The water layer stores the solar radiation heat incident on the façade during the daytime, reducing heat transfer to the building interior. This affects lessening the need for air conditioning to sustain the thermal comfort of the building occupants. The stored energy is released during the nighttime, reducing the heating needs. The transparency of the WWS also allows daylight to enter the building, reducing artificial lighting needs. Furthermore, the water layer acts as a fire safety mechanism in case of a fire. This study aimed at three significant facets of WWSs in modern green buildings: (i) Passive energy benefits of WWSs (ii) Fire performance of WWSs and (iii) Life cycle cost and environmental impacts of WWSs.

The thermal and energy performance of WWS is examined through building energy simulations. A mathematical model is developed to obtain the surface temperatures of WWS based on the heat balance principle of different layers of WWS. The model is then incorporated into EnergyPlus through the Energy Management Systems (EMS) feature for facilitating the direct simulation of the impact of WWS when it is attached to a building façade for the first time. The incorporated WWS model is verified against two previously published experimental studies conducted in two climatic regions. The simulation findings were found to be in good agreement with the experimental findings. The incorporated WWS model is then utilised to assess the impact of WWS compared to a single skin glass façade (SGF) on the annual building energy performance of office buildings in four climatic zones, namely (i) tropical moist climate – Singapore, (ii) dry climate – Riyadh, Saudi Arabia, (iii) Sub-tropical humid climate – Hong Kong and (iv) Mediterranean climate – Rome, Italy based on Köppen climate classification. Simulations are conducted for hot summer days, cooler winter days and annual basis. The results indicate that WWS is thermally efficient than SGF for all considered climate types. The annual energy benefit of WWS is between 40-49%, depending on the climate type, and accordingly, the energy cost-benefit is between US$15-30 per m². This study suggests that WWS is a sustainable building feature that can save considerable building energy and cost without compromising the benefits of glass façades.

The energy benefits of WWS are analysed against the double skin façade (DSF) for integration in Hong Kong buildings. As both WWS and DSF are approximately similar in aesthetic appearance, it is possible to replace DSF with a WWS or vice versa in the design stage, depending on the energy benefits. The energy simulations are performed for different building façade orientations: west, east, north and south. Simulation results show that both DSF and WWS perform better than the SGF in south orientation. WWS can save 46.1% of energy, and DSF can save 36.3% of energy compared to SGF in a hypothetical office building in the Hong Kong climate. Therefore, WWS can overtake conventional SGFs and DSFs to boost the thermal efficiency and energy efficiency of buildings in climates similar to Hong Kong. Further, WWS (0.1m gap) takes a much lower space than a DSF (0.6 m gap). This suggests that WWS is a suitable building element to be adopted in scarce space cities like Hong Kong, and it is a tremendous promise for usage in green buildings as an energy-efficient, practical building component.

The fire performance of WWSs with varying water layer thicknesses is investigated experimentally under a steady heat flux of 50 kW.m^-2 followed by a numerical analysis and compared to the fire performance of single float glass panes. WWSs have shown a better fire performance compared to single float glass panes. Besides, WWSs with 30 mm water layers sustained for a longer time than the WWSs with 10 mm and 20 mm water layers confirming that the thickness of the water layer has a substantial impact on the WWS's fire performance. In addition, a numerical model is established in ANSYS Fluent programme to obtain the glass surface temperatures, which cannot be measured during the experiments. The stress and strain developments of the glass panes were studied, employing the obtained surface temperatures to understand the fire performance of different WWSs. The results show that under this experimental condition, the glass cracks due to the stress caused by the temperature gradient in the planar direction. In WWS, the water layer helps in reducing the rate of temperature developments in the exposed glass pane as the heat incident on the glass pane is quickly transferred to the water layer. Therefore, it delays the thermal stress developments in the glass pane. Since it takes much time to develop the thermal stresses to breakage stress, the inclusion of 30 mm water within the WWS delays the cracking of the exposed glass pane compared to 20mm and 10 mm water layers under radiant heating.

Furthermore, 500 x 1000 mm² sized WWSs with 30 mm, 50 mm, and 100 mm water layer thicknesses were tested and compared to the fire performance of a 500 x 1000 mm² sized SGF. The façades were heated by a 400 x 600 mm² isopropanol pool fire. Breaking times and surface temperatures were measured and evaluated. The experimental results indicate that SGFs are more vulnerable to cracking than WWSs, but exposed glass pane fallout can easily occur in WWSs. Since the overall fire performance is dependent on the failure of the fire unexposed glass pane, WWSs are more fire-resistant than SGFs. Different water layer thicknesses significantly impact fire performance, where the WWS with a 50 mm water layer thickness resulting in a longer breakage time, increasing the façade's integrity during a fire. The experimental findings of this study are useful for developing practical guidelines for fire-safe glass façade designs.

The life cycle environmental impact and cost of WWS are analysed and compared with SGFs. It is found that the use phase has a high impact on the lifetime environmental emissions of both façade systems than the pre-use phase and post-use phase due to the energy consumed to compensate heat transferred through the façade. The results show that all WWS types considered have less environmental impacts than all considered SGF types, and fibreglass framed WWS with reflective glass as both the outer and inner layers create the lowest life cycle environmental impact. The life-cycle cost analysis shows that even the initial investment is higher for WWSs, the energy cost-benefit associated with WWS can compensate for the investment in about 4 years.

This study's findings provide a feasibility evaluation of WWS for the integration of buildings in Hong Kong. The overall research exemplifies the convergence of diverse fields, including energy engineering, fire engineering, and life-cycle evaluation, to achieve an innovative green building solution. The outcomes of the study facilitate energy efficient and fire safe WWS as a sustainable building application. Based on the study results, stakeholders and the public are encouraged to adopt WWSs in green building construction projects as an energy-efficient strategy and a fire safe structure.