Impacts of urban microclimate on summertime sensible and latent energy demand for cooling in residential buildings of Hong Kong

Highlights

• The UHI and UMI effect among various LCZs in Hong Kong are compared.

• The nocturnal UHII and UMII during summer both decreases with the increasing LCZ grades.

• The impact of UMI effect on latent cooling demand is investigated in Hong Kong.

• The combination of UHI and UMI effect leads to an increase in electricity demand for urban residential buildings.

Abstract

The urban heat island (UHI) and urban moisture island (UMI) effect can be significant in Hong Kong due to its high-density land utilization, and this can strongly affect building energy performance. While the UHI’ energy impact has been rather intensively studied recently, the UMI effect on latent energy is still underexplored, especially for humid subtropical climate like Hong Kong. This study investigated the intensity of UHI and UMI in Hong Kong, and its impacts on the sensible and latent cooling demand of residential buildings in summer. Firstly, a ten-year weather dataset from 2004–2013 for the six stations selected based on the local climate zone (LCZ) scheme was analysed. The results show that the urban area of Hong Kong appears as both a heat and moisture island during summer nights but as cooling and dry islands during daytime, and the nocturnal UHI and UMI intensity vary significantly with different LCZs. Furthermore, the energy performance of a typical residential building in Hong Kong was simulated with measured weather data for the selected stations as an input. The urban building shows a higher sensible cooling demand, approximately twice that of the comparative rural one, and the latent cooling demand could be up to 96% higher. Both sensible and latent cooling energy demand decrease with increasing LCZ grades. Our study highlights that both UHI and UMI effect should be considered in the estimation of building energy in Hong Kong due to their significant impacts on the cooling energy demand.

Introduction

Globally, the significant increases in populations in urban areas over the recent decades due to rapid urbanization have induced the formation of the local climate change. One of the major factors in this change is the urban heat island (UHI) effect, which describes the excess warmth of the urban atmosphere in comparison to the rural areas [1]. In recent decades, a number of UHI studies have been conducted for the quantification of UHI effect in large cities [[2], [3], [4]]. The magnitude of UHI in different cities mainly depends on anthropogenic heat, building density, the presence of green land and water bodies within the city, and thermal storage in the building fabric [5]. The spatial and temporal variability of the UHI in a city are also very evident [5,6]. It has been shown that UHI effect could result in the increased energy consumption and in summertime heat-related mortality [7].

The impact of UHI on building energy demand have been extensively studied in many large cites using physics-based [8,9] and empirical statistical modelling [10,11]. The UHI studies indicate that the impact of UHI is generally positive for heating load and negative for cooling load [[12], [13], [14]]. Li et al. [15] summarised existing literature and found that the UHI could result in a median increase of 19.0% in cooling energy consumption and a median decrease of 18.7% in heating energy. The magnitude of the specific energy impact of UHI effect depends on many factors such as local microclimate, the intensity of urban overheating, and the type and characteristics of buildings [13,15,16]. Toparlar et al. [17] found that cooling demand even of buildings with good thermal insulation and lower infiltration rates, can increase by 48% once moved from the rural location to an urban location of Antwerp, Belgium. Zinzi et al. [18] showed that the UHI caused an increase in cooling consumption up to 74% for residential building and up to 53% instead for the office building.

In addition to the UHI effect, local climate change caused by rapid urbanization in large cities is projected to create significant humidity differences between urban and rural areas. Previous studies have shown that urban centres are generally drier than their surroundings, and this phenomenon is termed as urban dry island (UDI) effect [1,[19], [20], [21]]. As examples, urban absolute humidity was found to be lower than that in rural areas in the Matsuyama Plain by the field measurements and the UDI intensity was significant in daytime [20]. In Milan, the urban area shows lower absolute humidity than rural surroundings for the most time and the interquartile range is between −1.44 g/kg and −0.27 g/kg, with a median of −0.67 g/kg [21].

However, urban moisture island (UMI) effect, in which the urban humidity level is higher than rural, has been also reported in some other studies [[22], [23], [24]]. Deosthali et al. [22] reported that the core of Pune City appeared as both heat and moisture islands at night but as heat and dry islands at the time of sunrise. Richards [24] found that UMI effect was observed on fine nights in Vancouver by comparing humidity data at urban and rural sites. In summary, UMI effect is normally observed during night-time as the continued evaporation in the city centre during the night is responsible for the excess nocturnal moisture. Several studies have discussed the main causes for the humidity difference between urban and rural areas, but there are only a few quantitative studies on the development for UMI effect. It is acknowledged, however, that evaporation, condensation, and anthropogenic moisture emissions contribute to the formation of urban and rural humidity differences [25].

Urban microclimate (UHI and UMI in current study) has a significant impact on building energy performance and could result in positive or a negative feedback on energy demand. While the UHI’ energy impact has been quite intensively studied over recent years, the UMI impact on building energy performance is still little researched. To the best of our knowledge, UMI impact on the latent cooling energy has only been investigated as a preliminary case study on climatic conditions in Milan. Simulation results showed the latent cooling load for dehumidification in urban built environment was 74%–78% lower than that for rural buildings [21].

Hong Kong, however, has specific features in terms of local climate and urban morphological characteristics in compared to Milan which could result in different UHI and UMI intensities. It is for the reason that the impacts of urban microclimate on the energy performance of buildings in Hong Kong require specific study. Firstly, Hong Kong has a monsoon-influenced humid subtropical climate throughout the year. Summers are very hot and humid, the average air temperature is approximately 28 °C and the average relative humidity is more than 80% [26]. Secondly, Hong Kong has unique urban morphological characteristics due to its high-density land utilization with a large population size of around 7.2 million and high urbanization rate [27]. The released anthropogenic heat and moisture therefore is very high in the metropolitan of Hong Kong and could have a significant impact on building energy performance, which really warrants detailed study.

The aim of this study is to analyse the UHI and UMI effect in Hong Kong and then to quantify their impacts on the energy performance of residential buildings. Most previous studies related to urban microclimate only consider a pair of urban and non-urban stations [[28], [29], [30]]. In this study, we selected six weather stations in Hong Kong based on local climate zone (LCZ) scheme. The spatial characteristics of UHI and UMI effect in different LCZs of Hong Kong are assessed by analysis of the data from a ten-year period of monitoring temperature and relative humidity. Furthermore, the measured weather data for various LCZs are used as input for building energy simulation to evaluate their impacts on energy demand, including sensible and latent cooling demand. As over 90% of people live in high-rise apartment in Hong Kong [31], we selected a representative high-rise residential building as a case study.