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Laboratory and field testing of a residential heat pump retrofit for a DC solar nanogrid

Aaron H. P. Farha, Jonathan P. Ore, Elias N. Pergantis, Davide Ziviani, Eckhard A. Groll, Kevin J. Kircher

TL;DR

The paper addresses the challenge of powering large residential loads with direct current by validating a DC distribution approach for a common heat pump retrofit. It uses a combination of steady-state lab tests and dynamic field testing to show device-level performance is comparable between AC and DC operation, then couples these results with a calibrated nanogrid model to estimate annual bill savings of $12.5\%$ to $16.7\%$ under DC distribution. The key contributions are the first laboratory results for heating-mode DC heat pump operation, the first field demonstration of a DC heat pump retrofit, and a real-data-driven nanogrid model illustrating tangible economic benefits. This work supports the viability and economics of residential DC micro/nanogrids and points to practical pathways for higher-efficiency, DC-based building energy systems.

Abstract

Residential buildings are increasingly integrating large devices that run natively on direct current (DC), such as solar photovoltaics, electric vehicles, stationary batteries, and DC motors that drive heat pumps and other major appliances. Today, these natively-DC devices typically connect within buildings through alternating current (AC) distribution systems, entailing significant energy losses due to conversions between AC and DC. This paper investigates the alternative of connecting DC devices through DC distribution. Specifically, this paper shows through laboratory and field experiments that an off-the-shelf residential heat pump designed for conventional AC systems can be powered directly on DC with few hardware modifications and little change in performance. Supporting simulations of a DC nanogrid including historical heat pump and rest-of-house load measurements, a solar photovoltaic array, and a stationary battery suggest that connecting these devices through DC distribution could decrease annual electricity bills by 12.5% with an after-market AC-to-DC heat pump retrofit and by 16.7% with a heat pump designed to run on DC.

Laboratory and field testing of a residential heat pump retrofit for a DC solar nanogrid

TL;DR

The paper addresses the challenge of powering large residential loads with direct current by validating a DC distribution approach for a common heat pump retrofit. It uses a combination of steady-state lab tests and dynamic field testing to show device-level performance is comparable between AC and DC operation, then couples these results with a calibrated nanogrid model to estimate annual bill savings of to under DC distribution. The key contributions are the first laboratory results for heating-mode DC heat pump operation, the first field demonstration of a DC heat pump retrofit, and a real-data-driven nanogrid model illustrating tangible economic benefits. This work supports the viability and economics of residential DC micro/nanogrids and points to practical pathways for higher-efficiency, DC-based building energy systems.

Abstract

Residential buildings are increasingly integrating large devices that run natively on direct current (DC), such as solar photovoltaics, electric vehicles, stationary batteries, and DC motors that drive heat pumps and other major appliances. Today, these natively-DC devices typically connect within buildings through alternating current (AC) distribution systems, entailing significant energy losses due to conversions between AC and DC. This paper investigates the alternative of connecting DC devices through DC distribution. Specifically, this paper shows through laboratory and field experiments that an off-the-shelf residential heat pump designed for conventional AC systems can be powered directly on DC with few hardware modifications and little change in performance. Supporting simulations of a DC nanogrid including historical heat pump and rest-of-house load measurements, a solar photovoltaic array, and a stationary battery suggest that connecting these devices through DC distribution could decrease annual electricity bills by 12.5% with an after-market AC-to-DC heat pump retrofit and by 16.7% with a heat pump designed to run on DC.

Paper Structure

This paper contains 21 sections, 9 equations, 16 figures, 8 tables.

Table of Contents

  1. Introduction
  2. Related work
  3. DC power systems and the DC Nanogrid House
  4. Residential DC appliance experiments
  5. Residential DC nanogrid modeling
  6. Contributions of this paper
  7. Experimental methodology
  8. Steady-state laboratory test descriptions
  9. Dynamic field test descriptions
  10. Experimental results
  11. Steady-state laboratory test results
  12. Dynamic field test results
  13. System-level nanogrid modeling
  14. General nanogrid model
  15. AC and DC nanogrid configurations
  16. ...and 6 more sections

Figures (16)

  • Figure 1: The DC Nanogrid House is a 1920s-era, 208 m$^2$, two-story, detached single-family house in West Lafayette, Indiana.
  • Figure 2: DC distribution deployments in the United States and Canada from 2022 vossos_adoption_2022. Reproduced with permission.
  • Figure 3: Piping and instrumentation diagram of the heat pump cycle in cooling mode.
  • Figure 4: Piping and instrumentation diagram of the heat pump cycle in heating mode.
  • Figure 5: Wiring of the outdoor unit to the 350 VDC bus at the DC Nanogrid House using a lockout/tagout system.
  • ...and 11 more figures