Environmental assessment of hybrid ground source heat pump systems
Introduction
Worldwide energy use is expected to rise due to an increase in population and global warming. In Australia, electricity is the most dominant energy source used for space heating and cooling, where 38 % of households use electric heaters and 49 % use reverse cycle air conditioners (RCACs) for cooling (Australian Bureau of Statistics, 2014). The associated carbon emissions are exacerbated by the fact that 60 % of Australian electricity generated in 2018 was from coal (Department of Environment and Energy, 2019). Not surprisingly, it was reported that the electricity sector is the biggest polluter, representing 33 % of the carbon emissions in Australia (Climate Council Australia, 2018). A global analysis paper by Ang and Su (2016) reported that in 2013, Australia was the fifth most carbon-intensive electricity producer in the world, with an electricity emission factor of 0.7806 kgCO2e/kWh of electricity used. In comparison, Ang and Su (2016) estimated a worldwide average of 0.52 kgCO2e/kWh, which means that compared to the worldwide average, there is an additional 50 % carbon emissions for each kWh of electricity used in Australia.
This electricity consumption should be reduced to achieve long-term environmental sustainability and also to meet Australia’s commitment at the Paris Climate Change Conference to reduce GHG emission by around 150 Mt CO2e by 2030 (Department of Environment and Energy, 2015). Furthermore, it has been reported that emission abatement in the electricity sector is more cost-efficient compared to reducing emissions in other sectors (Climate Council Australia, 2018). One way to reduce this electricity consumption worldwide is by using more efficient heating and cooling systems, such as ground source heat pump (GSHP) systems.
Research on GSHP systems has increased in recent years and details about the systems can be found elsewhere (Brandl, 2006; Johnston et al., 2011; Sanner et al., 2003). Efficiency or coefficient of performance (CoP) of GSHP systems have been studied experimentally, analytically and numerically (Aditya et al., 2018; Huang, 2015; Ruan, 2012; Self et al., 2013; Spitler et al., 2014), and GSHP systems are typically more efficient than conventional systems (Huang, 2015; Self et al., 2013; Spitler and Gehlin, 2015; Wu, 2009). A system with a higher CoP means that less electrical energy is required to run the system. Hence, the adoption of GSHP systems instead of conventional systems can contribute to the objective to reduce greenhouse gas (GHG) emissions.
Despite all these, the uptake for GSHP systems has been relatively limited, with factors such as finance, technology and policy having been cited as the typical barriers to adopt this technology (Karytsas and Choropanitis, 2017). Several authors have indicated that installation cost is the most difficult challenge to overcome (Karytsas and Choropanitis, 2017; Lu et al., 2017). Design methods such as hybrid GSHP (HGSHP) systems have been suggested, where the lifetime costs are minimised by taking advantage of both the lower capital costs of conventional systems and the lower operational costs of GSHP systems (Aditya et al., 2019; Alavy et al., 2013; Mikhaylova et al., 2016; Nguyen et al., 2014; Cullin and Spitler, 2020; Xu, 2007). However, this hybrid system solution with the lowest lifetime costs is usually not the most optimal from the environmental perspective. GSHP systems are typically the most efficient, hence they are expected to emit the least emission compared to other heating and cooling systems.
The environmental impact of GSHP systems during the operational stage has been investigated previously. The use of GSHP and HGSHP systems has been reported to reduce GHG emissions in Ontario, Canada (Nguyen et al., 2016). Other authors estimated that a 35 % reduction in GHG emissions is possible in Sweden and Switzerland (Bayer et al., 2012; Blum et al., 2010). More recent literature reveals that the life cycle assessment (LCA) method is preferred, where environmental impact is considered throughout the life cycle of the system (Lu, 2018; Saner et al., 2010; Koroneos and Nanaki, 2017; Zhou et al., 2020; Bloom and Tinjum, 2016). The LCA method considers the lifetime environmental impact from the resources needed for the raw materials, assembly, transport, operation and disposal.
These previous works suggest that HGSHP systems have the potential to be beneficial financially and environmentally. There is also some trade-off between financial and environmental savings between the users who pay the financial costs and society who usually bear the environmental costs. Hence, a multi-objective optimisation strategy is needed, where an optimal hybrid system configuration is proposed while considering both financial and environmental benefits.
This paper built on previous work by Aditya et al. (2019), where it was reported that HGSHP systems have the potential to reduce lifetime costs in Australia. In this paper, a multi-objective optimisation method is adopted where the economic and environmental performance of HGSHP systems are investigated under Australian climatic, cost and emission conditions. Based on these design parameters, a Pareto optimal approach is then utilised to find the optimum solution set that considers both financial and environmental benefits of HGSHP systems. Finally, a sensitivity analysis is conducted to investigate the impact of potential changes in those key design parameters.
Section snippets
Data and methodology
Seven major Australian cities have been chosen to represent the different climatic conditions encountered in Australia, ranging from temperate to tropical climate (Peel et al., 2007). Those cities are Adelaide, Brisbane, Cairns, Hobart, Melbourne, Perth and Sydney.
Results and discussions
In this section,results corresponding to Sydney, Hobart and Cairns are chosen to represent a balanced climate, a heating-dominant climate, and a cooling-dominant climate, respectively.
Conclusion
In this paper, a multi-objective optimisation method is adopted where the economic and environmental performance of hybrid ground source heat pump (HGSHP) systems in seven Australian cities are investigated. Two hybrid systems are considered. The first is a combination of a ground source heat pump (GSHP) system and a reverse cycle air conditioner (RCAC) system, and the second is a combination of a gas furnace, an RCAC for cooling and a GSHP system. A Pareto optimal approach is utilised to find
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the Melbourne Research Scholarship from The University of Melbourne and by the Australian Reseach Council Linkage Project LP160101486.
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