Researchers from Purdue University in the United States claim to have demonstrated that commercially available residential air-source heat pumps designed for conventional AC systems can run directly on DC power after a few simple hardware modifications, with little loss in performance.
The scientists explained thtam, unlike conventional AC systems, DC nanogrids minimize unnecessary power conversions, potentially improving efficiency by 5–15%, reducing distribution losses, and enhancing voltage regulation. They also offer greater resilience by allowing buildings to operate independently during grid outages. However, widespread adoption remains limited by high installation costs, the lack of unified standards, and technical challenges related to DC protection and equipment availability.
To address these barriers, the research team developed the DC Nanogrid House, a renovated 208 m² single-family home equipped with solar PV, battery storage, dual AC/DC wiring, and a custom energy management system. The house enabled direct comparison of AC and DC operation under real-world conditions. A key milestone was retrofitting its variable-speed heat pump to operate on either AC or DC, as the heat pump accounts for more than two-thirds of the home’s annual energy use. This experimental platform supported the long-term evaluation of fully DC-powered residential buildings.
Experimental setup
The academics retrofitted a variable-speed residential split-system heat pump with a 14 kW cooling capacity was to operate on both AC and 350 V DC by replacing its standard AC input. The unit was calibrated using R-410A refrigerant before testing in both AC and DC modes. Laboratory experiments were conducted in controlled psychrometric chambers, where performance was measured under standardized steady-state conditions with variable-speed operation, and results were validated through air–refrigerant energy balance checks within 6% error.
Field experiments were then performed at the DC Nanogrid House in Indiana using a similar heat pump, where only the outdoor unit, responsible for over 90% of energy use, was retrofitted to DC, while the indoor unit remained on AC for safety reasons. The system was tested under real operating conditions with continuous monitoring of temperatures, pressures, and electrical power using calibrated sensors and data acquisition systems.
Both AC and DC configurations were evaluated using comparable weather and setpoint conditions, though slight differences in outdoor temperatures were noted between datasets. Results showed close agreement between air-side and refrigerant-side measurements, with deviations within 6%. The experimental setup enabled a robust comparison of AC and DC operation in both laboratory and real-world environments under variable-speed conditions, according to the research team.
Results
The laboratory experiments showed that the heat pump capacity is nearly identical under AC and DC, with the coefficient of performance (COP) varying slightly with temperature lift. Field data, meanwhile, showed that power consumption scaled predictably with outdoor conditions, with no statistically significant difference between AC and DC operation.
Moreover, simulations of AC, hybrid DC, and fully DC systems showed the biggest gains come from moving away from a purely AC architecture, while full DC adds only small additional benefit. Overall, DC configurations reduce annual electricity costs by about 12.5–16.7%, mainly due to lower conversion losses. These savings mainly arise from reduced power conversion stages and improved converter efficiency.
However, the scientist warned tha the study is limited by simplified DC retrofit implementation, weather normalization challenges, and simplified converter modeling, as well as omission of real-world DC grid effects such as stability and protection. Future work should include full field-scale DC nanogrids, improved converter and compressor testing, and more detailed cost analyses to assess real-world deployment feasibility.
PV performance
The researchers emphasized that the economic benefits of DC nanogrids are strongly dependent on PV penetration levels, particularly in cold climates such as Indiana.
“I would expect our savings are quite sensitive more-so to PV penetration levels for a cold climate such as we have here in Indiana,” corresponding author Aaron Farha told pv magazine, noting In such systems, winter daytime PV overproduction can be stored and shifted to later use, especially since building heating demand typically does not align with peak solar generation. “The peak thermal load of a building is typically completely outside the peak PV production in the winter, which can enhance the value of storage and increase overall savings in highly electrified homes where heat pumps dominate consumption.”
Simulation results further showed that system design significantly affects PV utilization. The PV array produced a total of 15.2 MWh over the whole year, while under AC operation net exports totaled to 6.8 MWh, compared to about 7 MWh under DC configurations. “This means that the AC heat pump self-consumption was 54.6%, and the DC heat pump self-consumption was 53%, meaning we had more energy overall to export,” Farha said.
Operationally, PV variability was found to have limited impact on heat pump behavior because system stability was maintained electronically. However, integration constraints remain a challenge: “We weren’t able to run the PV or battery on our DC Nanogrid while the heat pump was running, largely due to the absence of a full bus stabilization controller,” he went on to say.
Finally, the authors noted that system scale plays a key role in economic viability. While the studied home yielded only modest savings, the estimated annual net bill savings from our DC retrofit were only about $60, larger systems are expected to benefit more significantly. In particular, for larger commercial or industrial systems, the 12-16% savings we predict would be more significant, suggesting that DC nanogrids may become more economically attractive as system size and PV penetration increase.
Their findings were presented in the study “Laboratory and field testing of a residential heat pump retrofit for a DC solar nanogrid,” which was recently pubished in Applied Energy.
The post How to retrofit residential air-source heat pumps to run on DC appeared first on pv magazine Global.
