An ammonia oil-free linear compressor could increase home heat pump SCOP, says new study

April 15, 2024

A new study has found that an ammonia (R717) oil-free linear compressor could increase the seasonal COP of residential heat pumps in comparison to those equipped with inverter-driven compressors, which cycle on/off more frequently as a building’s thermal demand lessens.

The study, “A Small Ammonia Heat Pump Using Linear Compressor,” was written by a team of researchers, including Nibin Qian, Chunhui Yang and Zhaohua Li, and published in the journal Energy. The heat pump model is based on a previous study by Liang et al. on a linear electromagnetic-drive oil-free refrigeration compressor using R134a. A numerical model was established to evaluate the baseline performance of a similarly designed ammonia heat pump.

According to the study, one of the main challenges in the widespread adoption of domestic heat pumps is the dilemma between sizing for peak heating demand and achieving optimal efficiency. Heat pumps sized for peak demand often run at partial load during most of the heating season since peak conditions are relatively rare.

“Frequent on/off cycling, even when inverter-driven compressors are present, can decrease the annual efficiency,” the study said.

Inverter-driven heat pumps can reduce the frequency of on/off cycling by nearly 50% during the cold season. However, as the building’s thermal demand lessens, the incidence of on/off cycling can increase, reaching up to 60 cycles per day when the ambient temperature hits 10°C (50°F). This cycling can lead to a reduction in the seasonal COP of inverter-driven heat pumps by as much as 12%, the study found.

The study found that at an ambient temperature of 0°C (32°F), the ammonia oil-free linear compressor had a COP of 2.

“Given that the numerical study demonstrated very attractive features of the proposed ammonia heat pump using a linear compressor, our future work will be prototyping a 2kW (0.56TR) linear compressor and experimentally evaluating the heat pump performance,” the authors wrote.

The study notes that while linear compressors are on the market today, there are currently none available that can scale to the needs of a residential heat pump, which was pegged at greater than 2kW of heating capacity. There are also very few residential heat pumps on the market today that use ammonia as a refrigerant.

U.S.-based OEM Stone Mountain Technologies has recently begun sales of its Anesi-branded ammonia-absorption heat pump in North America, which provides both domestic hot water (DHW) and space heating. The units are located outdoors and have a charge of 11lbs (5kg) of ammonia. They use propane (R290) or natural gas to provide the heat that powers the absorption cycle. Italian manufacturer ROBUR said it has seen increased interest in its ammonia-absorption heat pump from U.S. utilities over the past five years. ROBUR’s heat pump is designed for commercial and large residential properties and can provide DHW, space heating and cooling.

The model

The research underscores a direct relationship between ambient temperature and key performance metrics such as specific power input, heating capacity and COP. The capacity modulation of linear compressors, achieved through stroke adjustments, represents a significant improvement over the traditional frequency adjustment method.

The model fixed the condenser’s temperature at 50°C (122°F) and the ambient temperature at 10°C. With a pressure ratio of 3.5, it was observed that increasing the stroke length of the linear compressor from 7 to 11.5mm (0.27 to 0.45in) raised the current from 2.4 to 5.6A and the voltage from 52 to 120V. With the stroke increase, the power input grew from 30 to 272W.

“The potential for capacity modulation by adjusting the compressor stroke offers significant advantages over traditional methods, including on/off control and variable frequency given that heat pumps operate at part load most of the time,” the study said.

The compressor design

The compressor utilizes a moving magnet linear motor and is based on the Oxford-type design but introduces a flexure instead of the conventional coil spring. This adjustment allows for easier scaling of the device to capacities of 2kW or more, according to the study.

“Another unique feature is that the motor windings are kept outside the pressure vessel so that there is no contact between the working fluid and the copper coils, making it possible for R717 to be used,” the authors wrote.

Here the researchers have developed a linear compressor design that uses a free piston, driven by a linear motor, that moves along a straight path. This design has crucial components, including a linear motor, two reed valves, a flexure spring and a piston/cylinder assembly.