Effects of anthropogenic heat due to air-conditioning systems on an extreme high temperature event in Hong Kong

Abstract

Anthropogenic heat flux is the heat generated by human activities in the urban canopy layer, which is considered the main contributor to the urban heat island (UHI). The UHI can in turn increase the use and energy consumption of air-conditioning systems. In this study, two effective methods for water-cooling air-conditioning systems in non-domestic areas, including the direct cooling system and central piped cooling towers (CPCTs), are physically based, parameterized, and implemented in a weather research and forecasting model at the city scale of Hong Kong. An extreme high temperature event (June 23-28, 2016) in the urban areas was examined, and we assessed the effects on the surface thermal environment, the interaction of sea-land breeze circulation and urban heat island circulation, boundary layer dynamics, and a possible reduction of energy consumption. The results showed that both water-cooled air-conditioning systems could reduce the 2 m air temperature by around 0.5 • C-0.8 • C during the daytime, and around 1.5 • C around 7:00-8:00 pm when the planetary boundary layer (PBL) height was confined to a few hundred meters. The CPCT contributed around 80%-90% latent heat flux and significantly increased the water vapor mixing ratio in the atmosphere by around 0.29 g kg −1 on average. The implementation of the two alternative air-conditioning systems could modify the heat and momentum of turbulence, which inhibited the evolution of the PBL height (a reduction of 100-150 m), reduced the vertical mixing, presented lower horizontal wind speed and buoyant production of turbulent kinetic energy, and reduced the strength of sea breeze and UHI circulation, which in turn affected the removal of air pollutants. Moreover, the two alternative air-conditioning systems could significantly reduce the energy consumption by around 30% during extreme high temperature events. The results of this study suggest potential UHI mitigation strategies and can be extended to other megacities to enable them to be more resilient to UHI effects. NomenclatureCOP Coefficient of performance [−] C p Specific heat of air [J K −1 kg −1 ] C w Specific heat of water at constant pressure [J K −1 kg −1 ] C sw Specific heat of sea water at constant pressure [J K −1 kg −1 ] d ai Water vapor mixing ratio of the inlet air [g kg −1 ] d ao Water vapor mixing ratio of the outlet air [g kg −1 ] e Water vapor pressure [hPa] h ai Enthalpy of the inlet air [J kg −1 ] h ao Enthalpy of the outlet air [J kg −1 ] H i out Sensible heat fluxes at each vertical level [W]

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