AC-to-DC Heat Pump Retrofit Overview

• AC-to-DC heat pump retrofit is a method that converts standard AC heat pumps to operate natively on DC nanogrids, eliminating inefficient conversion stages.
• Laboratory and field tests show performance parity between AC and DC modes within ±6%, ensuring reliable operation under modified configurations.
• Simulation and economic analysis indicate up to 16.7% annual cost savings by removing redundant AC/DC conversion processes and optimizing overall system efficiency.

Current residential buildings increasingly incorporate large DC-native loads such as solar photovoltaic (PV) arrays, stationary batteries, electric vehicles, and heat pumps utilizing DC motors with variable-frequency drives. Despite this, most device interconnections operate over conventional alternating current (AC) infrastructures, resulting in energy losses due to repeated AC/DC and DC/AC conversions. An AC-to-DC heat pump retrofit, as established by recent laboratory and field testing, converts a standard residential heat pump designed for AC operation to run natively on a building’s high-voltage DC nanogrid with minimal hardware intervention and negligible change in operational performance. The retrofit eliminates redundant rectification stages and capitalizes on direct DC bus connection, contributing to measurable system-level efficiency and cost improvements (Farha et al., 23 Nov 2025).

1. Heat Pump Architecture and Underlying Principles

Modern residential heat pumps incorporate variable-frequency drives (VFDs) that internally rectify AC input to DC before re-inverting it to variable-frequency AC for motor speed control. This intrinsic architecture enables heat pumps to function as DC machines after modest circuit modifications. In the referenced study, a 14 kW (4 ton) scroll-compressor, air-source heat pump (refrigerant: R-410A) was retrofitted to operate on a nominal ±350 V DC bus by bypassing its onboard AC rectifier and feeding the internal inverter directly. Only the outdoor unit (compressor, heat exchanger, outdoor fan) was shifted to DC; the indoor unit (fan and auxiliary resistance heating) remained AC-fed to conserve original relay controls and minimize rewiring risk.

2. Retrofitting Workflow and Hardware Modifications

The retrofit process centers on substituting the outdoor unit’s AC breaker with a three-pole DC switch, enabling direct DC input to the pre-existing inverter. Safety protocols include a lockout/tagout panel for mode selection and a 30 A DC fuse for bus protection. Existing control circuitry—thermostats, safety interlocks—remains untouched, ensuring system integrity. The DC bus utilizes AWG 8 copper conductors (rated ≥50 A), and metering is handled via Hall-effect watt transducers (DC side: 350 V, ±20 A, ±12.5 W full-scale uncertainty), complemented by inductive wattmeters for AC components.

Major Modification	Specification	Rationale
DC supply to outdoor unit	±350 V DC, up to ~15 A continuous	Supports 5 kW peak compressor + fans
Rectifier bypass	Bus ties directly to inverter’s DC link	Eliminates AC→DC conversion losses
DC fuse	30 A, bus-protection	Fault mitigation

3. Laboratory and Field Performance Evaluation

Performance testing adheres to AHRI Standard 210/240 across ten steady-state load points. Instrumentation encompasses refrigerant-side thermodynamic states, Coriolis mass-flow, and air-side enthalpy grids. The key performance metric is the coefficient of performance (COP):

$$\mathrm{COP} = \frac{|\dot{Q}_\text{indoor} + \dot{Q}_\text{in,fan}|}{\dot{W}_\text{in,fan} + \dot{W}_\text{out,fan} + \dot{W}_\text{comp}}$$

Lab results (partial):

Test	Q̇_th (kW)	Ẇ_comp (kW)	Ẇ_out,fan (kW)	Ẇ_in,fan (kW)	COP (AC)	COP (DC)
A2	12.75	3.474	0.372	0.000	3.40	3.49
H32	10.32	4.292	0.572	0.000	2.12	2.19

Both AC and DC configurations displayed performance parity within ±6%, fitting within the uncertainty bounds specified by AHRI. Field deployment over one month in a 208 m² residential structure (West Lafayette, IN, climate 5A) found no statistically significant differential in daily power consumption between AC and DC modes (Welch’s t-test, $p = 0.18$, α = 0.05).

4. Nanogrid Simulation and System Modeling

A full-year, hourly simulation scenario modeled DC nanogrid integration for the test house, incorporating empirical load traces, PV array output, and battery storage dynamics:

• PV spec: Four sub-arrays with a total 14.3 kW DC, tilts 32–50° and azimuths 90–270°, I–V modeled via pvlib and OikoLab data.
• Battery: 20 kWh capacity ($\overline{x}$), max power 12.5 kW ($\overline{b}$), self-dissipation $\tau = 1600$ h, efficiency $\eta = 95\%$.
• Heat pump: Field-measured power demand, data-matched for AC and DC modes.
• Household loads: Historical sub-metered data including auxiliary heating.

Nanogrid bus balance is governed by:

$p_\text{export}(k) = s(k) - d(k) - b(k)$

with dispatch logic: solar allocation to heat pump, surplus charging battery, remainder exported; deficit met by battery, then grid import.

Converter efficiency curves adopted manufacturer peak values:

• MPPT: $98\%$
• DC–DC: $98\%$
• Inverter: $95\%$
• Rectifier: $95\%$
• Heat pump inverter: $97\%$

5. Economic Impact and Cost Reduction Analysis

Net-metered electricity billing at $0.14$/kWh was applied to simulated import/export profiles. The annual cost summary demonstrates the efficacy of the DC retrofit:

Configuration	Annual Bill (USD)	Relative Savings (%)
AC nanogrid	367.4	Baseline
DC retrofit	321.6	$-12.5\%$
Ideal DC heat pump	306.2	$-16.7\%$

The savings are computed by $$\mathrm{Savings}(\%) = \frac{Cost_\mathrm{AC} - Cost_\mathrm{DC}}{Cost_\mathrm{AC}} \times 100$$. The reduction is primarily attributed to eliminated conversion stages and higher end-to-end converter efficiencies. No formal sensitivity analysis was performed; a plausible implication is that further reduction is contingent on scaling PV capacity, battery size, and optimizing tariff structures.

6. Practitioner Guidance and Future Research Directions

Best practices for retrofit implementation include:

• Retaining original variable-frequency inverter, bypassing only the input rectifier contingent on DC-link voltage compatibility (350–400 V).
• Maintaining auxiliary resistive heating on AC unless full DC relay conversion can be effected safely.
• Ensuring refrigerant charge and subcooling validation across AC and DC operation for equitable evaluation.
• Deploying Hall-effect DC wattmeters and inductive AC meters for accurate consumption monitoring.
• Applying part-load converter efficiency curves in system models over static values for realism.

Prospective work involves full DC nanogrid demonstrations extending to all major appliances, EV charging, and lighting. Such installations are expected to inform refined cost-benefit analyses, reveal grid-protection and power quality challenges, and guide standardization for residential low-voltage DC distribution systems.

7. Context and Significance in Residential Electrification

The direct retrofit of AC heat pumps for DC operation substantiates an incremental pathway for integrating DC-native devices and building nanogrids, sidestepping systemic AC/DC conversion inefficiencies. Field and laboratory parity of operational performance, coupled with consistent energy cost reductions, frames AC-to-DC retrofits as a viable enhancement compatible with legacy hardware and variable renewable generation scenarios. This suggests the technique is a promising candidate for future electrification efforts—pending further validation under comprehensive DC distribution environments and emerging standardization initiatives (Farha et al., 23 Nov 2025).