A dynamic analysis of the global warming potential associated with air conditioning at a city scale: an empirical study in Shenzhen, China
Introduction
The construction sector is a major contributor to carbon dioxide equivalents (CO2 eq.) emissions, accounting for about 20%–30% of the total greenhouse gas (GHG) emissions (IPCC, 2018). The majority of these GHG emissions, otherwise referred to as the global warming potential (GWP), is generated during the operational stage of a buildings' life cycle (Zhang and Wang, 2016). The primary source of GWP attributed to air conditioning (AC) units derives from operational energy consumption and the release of refrigerants, which in combination accounts for 10–40% of the total GWP of buildings (DENA, 2018; EIA, 2018). Previous studies have quantified the GWP associated with energy consumption and refrigerant release of AC units (Calm, 2002, 2006; Li, 2014).
Tackling the energy demands of AC units (EAC) is a logical approach and opportunity to promote building energy conservation and reduce GHG emissions. Reliable EAC data can be used to study the shortcomings that exists in the current operational management practices, and a detailed analysis of this data can support measures to improve the efficiency of AC systems. Many developed countries are equipped with detailed statistics on building energy consumption, including EAC data. For example, the U.S. Energy Information Administration (EIA) distributes thousands of questionnaires and measures consumption data from energy suppliers to provide detailed statistics on the energy demands in U.S. buildings. The most recent EIA survey showed that the EAC of the U.S. accounted for about 17% of the total energy consumption of residential buildings (EIA, 2018). Similarly, the German Energy Agency (DENA) reports annual energy consumption in the construction sector, providing a comprehensive overview of the energy efficiency of buildings. The 2018 DENA building report estimated that the energy consumption of AC refrigeration in residential buildings accounted for 0.3% of their total buildings' energy consumption in Germany (DENA, 2018). In 2012, the Australian Department of the Environment and Energy (DEE) also published a report of the energy consumption and GHG emissions for Australian commercial buildings which showed that EAC accounted for 43% of the total energy consumption of commercial buildings (DEE, 2012).
Research relating to AC performance in residential buildings should also been considered in building energy efficiency measures (Park et al., 2007). Taking Mexico as an example, Davis and Gertler (2015) presented a relationship between an increase in EAC and temperature, as well as a correlation between EAC and residential income. The results illustrated the significant environmental impacts potential due to the use of AC systems. The findings highlighted the clear link between regional climate and EAC (Jadhav and Lele, 2015; Li et al., 2014). Yuan et al. (2011) developed a compatible analysis of energy efficiency between different air conditioning products in China, and the study found that AC units with the same energy efficiency grade can have different thermodynamic characteristics and performance efficiencies.
In addition, emissions from refrigerants should be considered in environmental impact assessments of AC systems (Duan et al., 2018; Xue et al., 2017a, Xue et al., 2017b). Duan et al. (2018) recently evaluated the release of refrigerants from AC units in China, and found that if current AC management practices continue without change, the total GWP of refrigerants waste would peak at 135 Mt. by 2025 in China. This highlights an urgency to improve the management of refrigerants in AC units in China. Xue et al., 2017a, Xue et al., 2017b conducted a dynamic analysis of the GWP impacts of refrigerants and the stock flow of refrigerants from AC units in Japan. However, these studies paid little attention to GWP from AC systems at a city level, despite their rigorous calculations and analysis from a national perspective. Therefore, there is a need to conduct research at a city level, as it is a necessary step to inform policy change, especially a city with a distinctive climate.
There is clear direct and indirect contributions of AC systems to GHG emissions due to both operational energy consumption and refrigerant release (Hitchin et al., 2015; IPCC, 2014). Therefore, an integrated and comprehensive assessment of GWP impacts due to refrigerant release (direct) and energy consumption (indirect) is undertaken for AC units need to be considered in research studies.
Section snippets
Goal and scope
The scope of this study has been defined as the city of Shenzhen, a sub-tropical city located in Southern China. Shenzen is approximately 2000 km2 in area and has a population of 13 million, equivalent to 13% of the total population of the Guangdong province (Liu et al., 2017). The city of Shenzhen is presented in Fig. 1. The city is divided into six primary regions or districts, of which two (Baoan and Longgang) is broken into sub-districts. Its sub-tropical climatic consist of a hot summer
Energy consumption
The EAC of residential buildings in Shenzhen is presented in Fig. 2a. It shows an increase from 1.7 ± 0.3 billion kWh in 2005 to 4.5 ± 0.7 billion kWh in 2017, representing an average 8.5% increase in energy consumption per annum. This accounts for almost 32% of the total energy consumption of residential buildings in Shenzhen. The trend is correlated to a rapid increase in the area of urban residential buildings, with upgrades of more energy efficiency AC systems taking place during this time.
Discussion
Compared with the BAU scenario, it was found that Scenarios 2–4 present a great potential for reducing GWP impacts in Shenzhen (see Fig. 6). The CO2 mitigation rates stabilize for each Scenario at 60%, 50%, and 40% respectively by 2030. The trend relates to peak GWP emissions for the BAU in 2030. The cumulative mitigation of GWP impacts could ahieve reductions of 26.7–42.0 Mt. between 2018 and 2030.
This demonstrates the potential impact of the CO2 mitigation potential in China, if this was
Conclusions and implications
This study focuses on the environmental impact of residential AC units in Shenzhen, and a dynamic analysis model is developed to account for the GWP burdens associated with energy consumption and the release of refrigerants. The results showed that despite the government's efforts to promote a low-carbon society, there is still a major gap for achieving the national government's short-term goal of 2020. According to these results, the main results obtained from this study follow as:
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AC systems
Declaration of Competing Interest
None.
Acknowledgments
This work was financially supported by the Guangdong Provincial Key R&D Program (No. 2019B110209001) and the National Science Foundation of Guangdong Province (2017A030313438).
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