Endpoint Use Efficiency Comparison for AC and DC Power Distribution in Commercial Buildings

Arthur Santos,Daniel Gerber,Gerald Duggan,Daniel Zimmerle,Stephen Frank

doi:10.3390/en14185863

Abstract

Advances in power electronics and their use in Miscellaneous Electric Loads (MELs) in buildings have resulted in increased interest in using low-voltage direct current (DC) power distribution as a replacement for the standard alternating current (AC) power distribution in buildings. Both systems require an endpoint converter to convert the distribution system voltage to the MELs voltage requirements. This study focused on the efficiency of these endpoint converters by testing pairs of AC/DC and DC/DC power converters powering the same load profile. In contrast to prior studies, which estimated losses based on data sheet efficiency and rated loads, in this study, we used part load data derived from real-world time-series load measurements of MELs and experimentally characterized efficiency curves for all converters. The measurements performed for this study showed no systematic efficiency advantage for commercially available DC/DC endpoint converters relative to comparable, commercially available AC/DC endpoint converters. For the eight appliances analyzed with the pair of converters tested, in 50%, the weighted energy efficiency of the DC/DC converter was higher, while, for the other 50%, the AC/DC converter was. Additionally, the measurements indicated that the common assumption of using either data sheet efficiency values or efficiency at full load may result in substantial mis-estimates of the system efficiency.

Highlights

Since the “war of the currents” in the 19th century, alternating current (AC) has been the predominant method for electrical power transmission and distribution due to the ease of voltage conversion using transformers and the practical advantages of three-phase systems in electrical machines [1]
This study considered endpoint conversion, which exists in both AC and direct current (DC) distribution system architectures, using commercially available products for both AC/DC and DC/DC converters
The measurements performed here indicated that commercially available DC/DC converters did not exhibit systematically better efficiency that comparable AC/DC converters in practice, and any efficiency advantage was substantially reduced when weighted by realistic load profiles

Summary

Introduction

Since the “war of the currents” in the 19th century, alternating current (AC) has been the predominant method for electrical power transmission and distribution due to the ease of voltage conversion using transformers and the practical advantages of three-phase systems in electrical machines [1]. Advances in power electronics have increased direct current (DC) voltage conversion efficiency and lowered its cost, increasing interest in DC distribution at all scales. Because low-voltage DC power distribution (≤60 VDC in the United States (U.S.)) does not require all cable runs to be in conduit, it can potentially be implemented at a lower cost than AC distribution in commercial buildings. Commercial building loads include a large number of electronic devices that are classified as “miscellaneous electric loads” (MELs), i.e., loads that are not related to the building’s core functions—lighting or heating, ventilation, and air conditioning (HVAC). In 2017, MELs represented 40% of the electrical load in commercial buildings in the U.S.; this is projected to increase to 49% by 2040 [3]

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Conclusion