In this study, performance assessment of the variable speed compressor-based air source heat pump (ASHP) system as a domestic retrofit technology instead of fossil fuel-based heating technologies for the 1900s Mid terraced house is investigated. The assessment was conducted considering operating mode of control and heat supply temperature impact of the system. In the literature, ASHP system experimental development with variable speed mode (VSM) of control in comparison to fixed speed mode (FSM) of control at low to medium and high heat supply temperature in the context of UK was found with very limited number of studies, but without considering retrofit application. The focus of the earlier studies was on the individual components and performance improvement. The designed heat pump (HP), developed, and tested at constant heat load, simulating the real domestic heat demand under the controlled laboratory conditions and numerical modeling is utilized for the analysis purposes. The HP performance, energy demand, carbon emissions, and cost varies significantly due to changing heat supply temperature (35 °C, 45 °C, and 55 °C), control mode and accordingly the carbon emission and cost savings are achieved. The oil and gas boilers ranges from conventional to highly efficient type and evaluated in terms of annual running cost, energy consumptions, and carbon emissions in comparison with the HP system. Additionally, a comparative study with the existing retrofitted very high temperature ASHP inside the house is conducted. The developed HP at 55 °C could not defeat the very high heat supply temperature HP system (75 °C supply temperature) in performance and cost savings but become attractive at low supply temperature (35 °C). The HP system in VSM at low heat supply temperature instead of gas boiler (90% efficiency) could cut the annual carbon emissions by 59% but with additional 6% running cost for the Mid terraced test house in Belfast climatic conditions.
Decarbonization of the UK residential heating sector is crucial to cut the carbon emissions and meet the legal binding of the Climate Change Act, 2008. The current progress with residential building sector carbon neutrality is slow and, hence, acceleration in action is required. The heat pump (HP) technology was found to be a potential candidate for sustainable development instead of fossil fuel-based oil/gas boilers, but improvement in its coefficient of performance (COP) is essential to compete with the lower gas/oil unit energy cost. The number of studies found in the literature were very limited, with the customized prototype development in the context of Northern Ireland, but without considering the simultaneous impact of heat supply temperature and operating mode of control for performance improvement in different property types. It is evidenced in the literature that the variable speed capacity control approach could improve the annual performance, but the literature has not looked into the compressor efficiencies challenges. In this study, steady state testing with a range of fixed constant heat loads (3–18 KW), done by varying compressor speed and its impact on COP, compressor efficiencies, and inverter losses, was established. The HP performance was measured and evaluated at low (35 °C)-to-medium (45 °C) and high (55 °C) heat supply temperature levels under the controlled laboratory conditions over the experienced ambient temperature. According to the result the COP values varies according to heat supply temperature, ambient temperature conditions, and heating capacity. The HP annual performances with Irish housing stock were evaluated in two modes of control and three case studies (C1, C2, C3) based on the experimentally validated model. The heat load demand in five property types with four age periods were considered in the analysis. The system could meet the required heat load demand for all property types in VSM with different percentage improvements in performance in comparison to FSM depending on the considered case level of the heat supply temperature (C1, C2, C3).
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