Cost Analysis Framework for Comparing AC and DC Design Alternatives for Building Electrical Distribution Systems
In recent years, in response to the changing nature of building load, direct current (DC) distribution systems for buildings have been proposed as alternatives to traditional alternating current (AC) systems. DC distribution offers a closer match to the types of loads and generation sources found in modern buildings, the majority of which use DC electricity internally either natively or as a power conditioning stage. The proposed benefits of DC distribution compared to AC distribution within buildings include higher efficiency, lower installation cost, lower operating cost, higher reliability, improved communication and control, and simplicity. Most recent DC distribution research has focused on quantifying the efficiency advantage of DC distribution over AC distribution. However, energy savings alone do not guarantee cost savings; a more complete cost accounting is required to establish financial benefit. This report provides a framework for cost analysis and comparison of building electrical distribution systems, including common variants for both AC and DC distribution systems. The framework includes all major cost categories, including up-front costs (hardware, installation labor, soft costs, retrofit-specific costs) and long-term costs (energy, operations and maintenance); The proposed cost model excludes costs and benefits associated with reliability because, at present, there is no accurate way to evaluate these costs in the context of building electrical distribution system design alternatives. However, the report discusses this capability gap in relation to recent research and proposes future work to address it. The proposed cost model also excludes financing and removal costs associated with project end of life. (Demolition, disposal, and salvage of existing systems in the context of retrofits is included.) The report proposes standard formulas for calculating costs for each of the categories shown in Figure ES-1 and discusses sources of cost modeling data. The report also briefly defines and discusses three cost metrics often used to evaluate alternative designs: net present value, lifecycle cost, and simple payback period. Finally, the report provides a plan for integrating the proposed cost analysis framework into the OpenStudio® building energy analysis ecosystem. While this report provides a framework for comparison of AC, DC, and hybrid distribution system design alternatives, it does not provide quantitative cost comparisons or draw conclusions about whether AC or DC distribution systems are more cost-effective. The authors recommend future research to refine the cost models presented in this report, develop high-quality quantitative cost estimates for the cost categories identified, and integrate the modeling approach into other building energy modeling tools such as OpenStudio.