Theoretical Framework of Performance Consistency Index and Its Application in Operational Diagnostics for Air-Source Heat Pumps

In the operational diagnosis of air-source heat pumps, the conventional coefficient of performance faces limitations in distinguishing environmental effects from equipment performance degradation due to its dependency on operating conditions. To address this, this study proposes performance consistency index, based on the second law of thermodynamics. By normalizing the thermodynamic perfectibility under actual operating conditions against its rated value at nameplate conditions, this index establishes an energy efficiency diagnostic method with both operational robustness and design bench-marking capabilities. The study demonstrates that the performance consistency index, inherits the stability and comparability of thermodynamic perfectibility, effectively decouples the impacts of environmental parameter fluctuations and equipment performance degradation, remains independent of operating conditions, and accurately characterizes equipment status. By setting 1 as the design benchmark, deviations between actual performance and design objectives are quantified. Validation through operational diagnosis of four units confirms that the performance consistency index, successfully identifies partial-load energy efficiency degradation and low-temperature performance degradation. The performance consistency index provides a theoretical tool for long-term energy efficiency conservation of air-source heat pumps, enabling precise identification of inefficient units through threshold settings and supporting scientific decision-making for equipment renewal in "coal-to-electricity" projects.