Techno-economic analysis of DC power distribution in commercial buildings

Abstract

Improvements in building end-use efficiency have significantly reduced the energy intensity of new buildings, but diminishing returns make it a challenge to build very-low energy buildings cost-effectively. A largely untapped efficiency strategy is to improve the efficiency of power distribution within buildings. Direct current (DC) distribution with modern power electronics has the potential to eliminate much of the power conversion loss in alternating current (AC) building distribution networks that include photovoltaics and DC end uses. Previous literature suggests up to 15 percent energy savings from DC power distribution in very energy efficient buildings with onsite generation and battery storage. This paper extends prior energy modeling of DC versus AC distribution in buildings, to consider the cost of implementing DC systems on a life-cycle basis. We present a techno-economic analysis framework that evaluates the cost-effectiveness of DC systems in U.S. commercial buildings, based on commercially available products for various PV and battery storage capacities. We use Monte Carlo simulation to account for uncertainty and variability in the cost inputs, and compute the payback period (PBP) and lifecycle cost (LCC) savings of DC versus AC distribution systems. We also conduct a sensitivity analysis to evaluate how future efficiency improvements in power converters and changes in electricity tariffs may affect LCC savings. This analysis shows that DC systems can be cost-effective in all scenarios that include large capacities of battery storage and onsite solar, whereas for systems without storage, DC distribution is generally not cost-effective due to lower energy bill savings.

Cost Analysis Framework for Comparing AC and DC Design Alternatives for Building Electrical Distribution Systems

“…• Facilitate cost comparison of distribution design alternatives • Support sensitivity analysis and optimization. This report synthesizes the prior work in these areas and, in particular, relies heavily on the comprehensive cost discussion provided by Vossos et al (2018).…”

“…At present, it is often assumed that the design cost of DC distribution systems is high compared to traditional AC distribution due to low DC distribution penetration rates (Vossos et al 2018). It is also likely that DC system permitting and commissioning costs are higher now than they will be in the future, again due to the lack of familiarity requiring additional review and system verification steps.…”

“…Much of the published literature focuses on efficiency comparisons between AC and DC distribution systems. Although savings estimates vary depending on system topology and other assumptions, recent studies have concluded that energy savings of 3%-8% are achievable for buildings without on-site PV and 5%-15% for buildings with PV, with higher savings possible if on-site battery storage is present (Vossos, Garbesi, and Shen 2014;Fregosi et al 2015;Glasgo, Azevedo, and Hendrickson 2016;Gerber et al 2018;Vossos et al 2018;2020). However, efficiency gains by themselves do not guarantee financial savings when other system costs are considered.…”

“…Sannino, Postiglione, and Bollen (2003) perform a general cost analysis for commercial DC distribution with battery backup using fixed cost assumptions. Thomas, Azevedo, and Morgan (2012); Glasgo, Azevedo, and Hendrickson (2016), and Vossos et al (2018) propose stochastic, quantitative models for AC and DC distribution cost comparisons based on life-cycle cost (LCC) analysis. They apply their proposed models to several case studies using Monte-Carlo simulation: lighting for an office building in Pittsburgh, PA (Thomas, Azevedo, and Morgan 2012); residences in Austin, TX (Glasgo, Azevedo, and Hendrickson 2016); and several commercial building types in multiple climate zones (Vossos et al 2018).…”

“…Nevertheless, most major cost categories (capital for hardware and materials, installation labor, energy, and O&M) are covered by at least one publication. The "soft costs" of design, permitting, and the like are not well-addressed in the existing cost models, although Vossos et al (2018) do provide a qualitative discussion of soft costs. Demolition and disposal costs or cost recovery (via salvage) are not addressed in the existing literature, nor does the literature explore cost differences between new construction and retrofits.…”