A Modeling Toolkit for Comparing AC and DC Electrical Distribution Efficiency in Buildings

Abstract

Recently, there has been considerable research interest in the potential for DC distribution systems in buildings instead of the traditional AC distribution systems. Due to the need for performing power conversions between DC and AC electricity, DC distribution may provide electrical efficiency advantages in some systems. To support comparative evaluations of AC-only, DC-only, and hybrid AC/DC distribution systems in buildings, a new modeling toolkit called the Building Electrical Efficiency Analysis Model (BEEAM) was developed and is described in this paper. To account for harmonics in currents or voltages arising from nonlinear devices, the toolkit implements harmonic power flow, along with nonlinear device behavioral descriptions derived from empirical measurements. This paper describes the framework, network equations, device representations, and an implementation of the toolkit in an open source software package, including a component library and graphical interface for creating circuits. Simulations of electrical behavior and device and system efficiencies using the toolkit are compared with experimental measurements of a small office environment in a variety of operating and load configurations. A detailed analysis of uncertainty estimation is also provided. Key findings were that a comparison of predicted versus measured efficiencies and power losses in the validation testbed using the initial toolkit implementation predicted device- and system-level efficiencies with reasonably good accuracy under both balanced and unbalanced AC scenarios. An uncertainty analysis also revealed that the maximum estimated error for system efficiency across all scenarios was 3%, and measured and modeled system efficiency agreed within the experimental uncertainty in approximately half of the scenarios. Based on the correspondence between simulation and measurement, the toolkit is proposed by the authors as a potentially useful tool for comparing efficiency in AC, DC, and hybrid AC/DC distribution systems in buildings.

Verification of a Modeling Toolkit for the Design of Building Electrical Distribution Systems

“…DC distribution technologies and standards continue to increase power limits and support more types of building systems (e.g., LED lighting). Traditional AC distribution requires AC/DC converters for building equipment and MELs (e.g., phone chargers, laptop chargers, LED drivers), often leading to power losses [10,[13][14][15]. A single AC to DC conversion stage can exhibit losses ranging from 4% to 15% of the input power [3].…”

“…In 2020, NREL developed the Building Electrical Efficiency Analysis Model (BEEAM), a Modelica toolkit that facilitates the modeling of building electrical systems in a graphical environment, and the simulation of their energy use via harmonic power flow analysis [10]. The harmonic power flow method was chosen to strike a reasonable balance between accuracy, computational time, and model development time for whole-building simulations [10] and allows for predicting harmonic content, simulation of highly unbalanced loading conditions, and scalability for simulating large networks [32,33]. The BEEAM toolkit comprises many families of models, each representing a specific type of equipment that is commonly found in building electrical systems.…”

“…The accuracy of the BEEAM toolkit has thus far not been extensively verified. The developers of the toolkit performed an uncertainty analysis that estimated that the maximum simulation error for system efficiency across different system architectures was 3% [10]. Notably, that analysis, as well as the initial modeling approach comparison study, reported errors only at the system level and not at the more granular device level.…”

Advances in the Co-Simulation of Detailed Electrical and Whole-Building Energy Performance

“…Although greatly simplified, Equations ( 1) and ( 2) are sufficient to describe the relationships needed for co-simulation (see Section 2.1.3). Othee et al [25] provided a full mathematical treatment of the BEEAM, including equations for voltage-current relationships, power balance, transformer ratios, and converter losses (AC/DC, DC/DC, and DC/AC).…”

“…In this article, we describe an advance in co-simulation capability that combines the existing strengths of EnergyPlus ® [24] for whole-building energy simulations with the Building Electrical Efficiency Analysis Model (BEEAM), a newly developed Modelica library for the harmonic power-flow simulation of building electrical distribution systems [25,26]. The co-simulation capability leverages the FMI standard and is compatible with any Modelicabased co-simulation workflow.…”