A unified approach to modeling modern heat pump technologies

Heat pumps are a critical and rapidly evolving technology needed to meet decarbonization and electrification goals around the world. However, the availability of reliable performance data at conditions beyond standard ratings has not kept pace with the increasing complexity of the technology, leaving building performance simulationists with little guidance as to how to accurately characterize the equipment.

The first step towards making expanded performance data available to simulationists, has been established through the publication of ASHRAE Standard 205, “Representation of Performance Data for HVAC&R and Other Facility Equipment”. This standard defines a common format for HVAC manufacturers to transmit performance data spanning the entire range of equipment performance, however it is still early in its adoption. An intermediate approach to characterizing heat pump performance is needed to begin accurately evaluating heat pump technologies based on a more limited set of available data.

In the United States, all residential scale unitary heat pumps must be rated according to AHRI (Air-Conditioning, Heating, & Refrigeration Institute) standards to provide rated full-load heating and cooling capacities at a total of three outdoor temperatures, a Heating Seasonal Performance Factor (HSPF), a Seasonal Energy Efficiency Ratio (SEER) for cooling, and a full-load cooling Energy Efficiency Ratio (EER) at a single outdoor temperature. This is the only performance data that is independently verified. However, building performance simulation models require heat pump capacity and efficiency performance characteristics at any combination of compressor speed, outdoor air conditions, and indoor air conditions.

This paper describes the process used to establish a unified modeling approach for single-speed, two-speed, and variable-speed heat pump technologies that are rated under the AHRI standard. A major focus of the work is the data analysis of the voluntary performance data provided by manufacturers to the Northeast Energy Efficiency Partnerships (NEEP) Cold Climate Air Source Heat Pump List for over 130,000 variable capacity heat pump products. This analysis informs assumptions for the modeled performance of variable-speed units when just the verified AHRI ratings are available. For single- and two-speed heat pump systems, a congruent approach was developed based on previous analysis of a sample of manufacturer-provided expanded performance data allowing for a fair comparison across all three different compressor strategies which all have distinctly different AHRI rating procedures.

The end result is a unified modeling approach that can be adopted to reliably compare the performance of different heat pump technologies.