Stephen Frank

A comparative study of DC and AC microgrids in commercial buildings across different climates and operating profiles

Bosch has developed and demonstrated a novel direct current (DC) microgrid system that maximizes the efficiency of locally generated photovoltaic energy while offering high reliability, safety, redundancy, and reduced cost compared to equivalent alternating current (AC) systems. Several demonstration projects validating the system feasibility and expected efficiency gains have been completed and additional ones are in progress. This paper gives an overview of the Bosch DC microgrid system and presents key results from a large simulation study done to estimate the energy savings of the Bosch DC microgrid over conventional AC systems. The study examined the system performance in locations across the United States for several commercial building types and operating profiles. It found that the Bosch DC microgrid uses generated PV energy 6%-8% more efficiently than traditional AC systems.

Estimation of induction motor equivalent circuit parameters from nameplate data

The induction motor equivalent circuit parameters are required for many performance and planning studies involving induction motors. These parameters are typically calculated from standardized motor performance tests, such as the no load, full load, and locked rotor tests. However, standardized test data is not typically available to the end user. Alternatively, the equivalent circuit parameters may be estimated based on published performance data for the motor. This paper presents an iterative method for estimating the induction motor equivalent circuit parameters using only the motor nameplate data.

Temperature-Dependent Power Flow

In conventional power flow, the system branch resistances are assumed to be constant despite the fact that they are sensitive to temperature, and therefore to branch loading and losses. When the accurate calculation of losses is important, temperature correction of branch resistance can improve the accuracy of the power flow calculation. This paper introduces a temperature-dependent power flow algorithm: a novel approach which integrates an estimate of branch temperatures and resistances with the conventional power flow equations. This methodology relies on the creation of a set of coupled temperature and power flow equations which are solved using the Newton-Raphson solution method for nonlinear equations. The core methodology is developed first, then extended via several decoupling techniques.

An introduction to optimal power flow: Theory, formulation, and examples

ABSTRACT The set of optimization problems in electric power systems engineering known collectively as Optimal Power Flow (OPF) is one of the most practically important and well-researched subfields of constrained nonlinear optimization. OPF has enjoyed a rich history of research, innovation, and publication since its debut five decades ago. Nevertheless, entry into OPF research is a daunting task for the uninitiated—both due to the sheer volume of literature and because OPFs ubiquity within the electric power systems community has led authors to assume a great deal of prior knowledge that readers unfamiliar with electric power systems may not possess. This article provides an introduction to OPF from an operations research perspective; it describes a complete and concise basis of knowledge for beginning OPF research. The discussion is tailored for the operations researcher who has experience with nonlinear optimization but little knowledge of electrical engineering. Topics covered include power systems modeling, the power flow equations, typical OPF formulations, and common OPF extensions.

Extracting Operating Modes from Building Electrical Load Data

Empirical techniques for characterizing electrical energy use now play a key role in reducing electricity consumption, particularly miscellaneous electrical loads, in buildings. Identifying device operating modes (mode extraction) creates a better understanding of both device and system behaviors. Using clustering to extract operating modes from electrical load data can provide valuable insights into device behavior and identify opportunities for energy savings. We present a fast and effective heuristic clustering method to identify and extract operating modes in electrical load data.

Estimation of electricity consumption in commercial buildings

For many engineering studies involving commercial buildings, quick but reliable estimates for building electrical load characteristics are helpful. Often, little information about the building electrical characteristics is known, so such estimates must be drawn from general engineering knowledge or historical data. In the United States, the Commercial Buildings Energy Consumption Survey and the Department of Energy Commercial Reference Buildings provide historical and simulation data, respectively, that may be used to predict the electricity consumption of commercial buildings. This paper develops a methodology for using these data sources to develop typical load characteristics, including building average power, electrical energy consumption, peak demand, load duration curves, and typical load profiles. Numerical results for office buildings are presented.

Electrical Distribution Systems for Commercial Reference Building Models

AbstractCommercial and residential buildings consume approximately 40% of the total energy in the United States and use approximately 70% of the electricity. The electrical energy efficiency of the buildings sector thus plays a vital role in reducing energy use, peak demand, and greenhouse gas emissions. Maximizing the efficiency of electricity use is a key priority for high-performance buildings; however, to date, electrical distribution systems have received little attention. Rather, the focus has been on broad categories of end-use devices. As a result, a relatively small body of literature relates to the energy efficiency of electrical distribution in commercial buildings. Understanding this system and its components will help us examine its efficiency in significantly more detail. This paper describes the determination of an electrical distribution system for a commercial reference building model (CRBM). The purpose of the CRBM project was to develop standard or reference building energy models for t...

Reevaluation of induction motor loss models for conventional and harmonic power flow

In conventional and harmonic power flow analysis, three-phase induction motors are typically modeled using constant parameters (power or impedance). Such models do not fully capture the impact of voltage deviation, unbalance, and harmonics on motor losses. This paper discusses simple frequency-domain induction motor modeling techniques suitable for conventional and harmonic power flow analysis, including an optimally fitted fundamental frequency ZIP model, several harmonic impedance scaling methods, and a temperature correction procedure. The proposed models improve power flow accuracy without substantially increasing algorithm complexity.

Common Faults and Their Prioritization in Small Commercial Buildings: February 2017 - December 2017

This study documents faults that are commonly found in small commercial buildings based on a literature review and discussions with building commissioning experts. It also provides a list of prioritized faults based on an estimation of the prevalence, primary energy impact, and financial impact of each fault. A total of 39 faults were analyzed for this paper and classified by location, stage, and type. The annual primary energy impact (AEIprimary) and annual financial impact (AFI) of each fault were estimated based on available information. Based on these estimates, 20 top priority faults were identified. Eight out of the 20 top priority faults occur in vapor compression systems such as airconditioning, heat pump, and refrigeration equipment. Nonstandard refrigerant charging, condenser fouling, and evaporator fouling are the most important faults for this type of equipment.