Peter R. Armstrong

Analysis and Evaluation of DC-Link Capacitors for High-Power-Density Electric Vehicle Drive Systems

In electric vehicle (EV) inverter systems, direct-current-link capacitors, which are bulky, heavy, and susceptible to degradation from self heating, can become a critical obstacle to high power density. This paper presents a comprehensive method for the analysis and comparative evaluation of dc-link capacitor applications to minimize the volume, mass, and capacitance. Models of equivalent series resistance that are valid over a range of frequency and operating temperature are derived and experimentally validated. The root-mean-square values and frequency spectra of the capacitor current are analyzed with respect to three modulation strategies and various operating conditions over practical ranges of load power factor and modulation index in EV drive systems. The modeling and analysis also consider the self-heating process and resulting core temperature of the dc-link capacitors, which impacts their lifetimes. Based on an 80-kW permanent-magnet (PM) motor drive system, the application of electrolytic capacitors and film capacitors has been evaluated by both simulation and experimental tests. The inverter power density is improved from 2.99 kW/L to 13.3 kW/L, without sacrificing the system performance in terms of power loss, core temperature, and lifetime.

Estimation of variable-speed-drive power consumption from harmonic content

Nonintrusive load monitoring can be used to identify the operating schedule of individual loads strictly from measurements of an aggregate power signal. Unfortunately, certain classes of loads present a continuously varying power demand. The power demand of these loads can be difficult to separate from an aggregate measurement. Variable-speed drives (VSDs) are industrially important variable-demand loads that are difficult to track non-intrusively. This paper proposes a VSD power estimation method based on observed correlations between fundamental and higher harmonic spectral content in current. The technique can be generalized to any load with signature correlations in harmonic content, including many power electronic and electromechanical loads. The approach presented here expands the applicability and field reliability of nonintrusive load monitoring.

Detection of Rooftop Cooling Unit Faults Based on Electrical Measurements

Nonintrusive load monitoring (NILM) is accomplished by sampling voltage and current at high rates and reducing the resulting start transients or harmonic contents to concise “signatures.” Changes in these signatures can be used to detect, and in many cases directly diagnose, equipment and component faults associated with rooftop cooling units. Use of the NILM for fault detection and diagnosis (FDD) is important because (1) it complements other FDD schemes that are based on thermo-fluid sensors and analyses and (2) it is minimally intrusive (one measuring point in the relatively protected confines of the control panel) and therefore inherently reliable. This paper describes changes in the power signatures of fans and compressors that were found, experimentally and theoretically, to be useful for fault detection.

Variable-speed heat pump model for a wide range of cooling conditions and loads

A modular variable-speed heat pump model is developed from first principles. The system model consists of steady-state evaporator, compressor, and condenser component sub-models. The compressor model currently implemented accounts for re-expansion, valve pressure drop, and back leakage using empirical coefficients obtained from tests covering a wide range of pressure ratio and shaft speed. Variations in heat transfer coefficients with refrigerant and secondary fluid flow rates are modeled over a wide range of capacity. Pressure drops in piping and heat exchangers are also modeled. The resulting heat pump model is flexible and fast enough for use in finding optimal compressor, fan, and pump speeds and optimal subcooling for any specified capacity fraction and operating condition. To confirm the models accuracy, simulation results are compared to experimental data with condenser inlet air temperatures ranging from 15°C to 45°C, evaporator inlet air (dry) from 14°C to 34°C, and cooling capacity from 1.1 kW to 3.9 kW. The refrigerant charge balance has not been modeled; instead, it is assumed that a liquid receiver maintains the necessary charge balance. Over this wide range of conditions, the coefficient of performance prediction errors are found to be ±10%. An example of configuring the HPM with a different evaporator demonstrates the benefits of a modular approach.

Predictive pre-cooling of thermo-active building systems with low-lift chillers

This article describes the development and experimental validation of a data-driven model predictive control algorithm that optimizes the operation of a low-lift chiller, a variable-capacity chiller run at low pressure ratios, serving a single zone with a thermo-active building system. The predictive control algorithm incorporates new elements lacking in previous chiller pre-cooling control optimization methods, including a model of temperature and load-dependent chiller performance extending to low-pressure and part-load ratios and a data-driven zone temperature response model that accounts for the transient thermal response of a concrete-core radiant floor thermo-active building system. Data-driven models of zone and concrete-core thermal response are identified from monitored zone temperature and thermal load data and combined with an empirical model of a low-lift chiller to implement model predictive control. The energy consumption of the cooling system, including the chiller compressor, condenser fan, and chilled-water pump energy, is minimized over a 24-h look-ahead moving horizon using the thermo-active building system for thermal storage and radiant distribution. A generalized pattern-search optimization over compressor speed is performed to identify optimal chiller control schedules at every hour, thereby accomplishing load shifting, efficient part-load operation, and cooling energy savings. Results from testing the systems sensible cooling efficiency in an experimental test chamber subject to the typical summer week of two climates, Atlanta, GA, and Phoenix, AZ, show sensible cooling energy savings of 25% and 19%, respectively, relative to a high efficiency, variable-speed split-system air conditioner.

Efficient Low-Lift Cooling with Radiant Distribution, Thermal Storage, and Variable-Speed Chiller Controls— Part I: Component and Subsystem Models

Component and subsystem models used to evaluate the performance of a low-lift cooling system are described. An air-cooled chiller, a hydronic radiant distribution system, variable-speed control, and peak-shifting controls are modeled. A variable-speed compressor that operates over 20:1 speed range and pressure ratios ranging from one to six is at the heart of the chiller. Condenser fan and chilled-water pump motors have independent speed controls. The load-side distribution is modeled from the refrigerant side of the evaporator to the conditioned zone as a single subsystem controlled by chilled-water flow rate for a specified instantaneous cooling load. Performance of the same chiller when operating with an all-air distribution system is also modeled. The compressor, condenser fan, and chilled-water pump motor speeds that achieve maximum coefficient of performance (COP) at a given condition are solved at each point on a grid of load and outdoor temperature. A variable-speed dehumidification subsystem is modeled and simulated as part of a dedicated outdoor air system to condition the ventilation air. A companion paper evaluates the annual cooling system energy use and potential energy savings to be gained by integrating radiant cooling, cool storage, and variable-speed compressor and transport motor controls.

Gauss-Seidel accelerated: implementing flow solvers on field programmable gate arrays

Non-linear steady-state power flow solvers have typically relied on the Newton-Raphson method to efficiently compute solutions on todays computer systems. Field programmable gate array (FPGA) devices, which have recently been integrated into high-performance computers by major computer system vendors, offer an opportunity to significantly increase the performance of power flow solvers. However, only some algorithms are suitable for an FPGA implementation. The Gauss-Seidel (GS) method of solving the AC power flow problem is an excellent example of such an opportunity. In this paper we discuss algorithmic design considerations, optimization, implementation, and performance results of the implementation of the Gauss-Seidel method running on a Silicon Graphics Inc. Altix-350 computer equipped with a Xilinx Virtex II 6000 FPGA

Efficient Low-Lift Cooling with Radiant Distribution, Thermal Storage, and Variable-Speed Chiller Controls—Part II: Annual Use and Energy Savings

This paper evaluates the cooling efficiency improvements that can be achieved by integrating radiant cooling, cool storage, and variable-speed compressor and transport motor controls. Performance estimates of a baseline system and seven useful combinations of these three efficient low-lift inspired cooling technologies are reported. The technology configurations are simulated in a prototypical office building with three levels of envelope and balance-of-plant performance: standard-, mid- and high-performance, and in five climates. The standard performance level corresponds to ANSI/ASHRAE/IESNA Standard 90.1-2004 Energy Standard for Buildings Except Low-Rise Residential Buildings (ASHRAE 2004a). From the savings estimates for an office building prototype in five representative climates, estimates of national energy saving technical potential are developed. Component and subsystem models used in the energy simulations are developed in a companion paper.

Performance of a 100 kWth Concentrated Solar Beam-Down Optical Experiment

An analysis of the beam down optical experiment (BDOE) performance with full concentration is presented. The analysis is based on radiation flux distribution data taken on Mar. 21st, 2011 using an optical-thermal flux measurement system. A hypothetical thermal receiver design is used in conjunction with the experimental data to determine the optimal receiver aperture size as a function of receiver losses and flux distribution. The overall output of the plant is calculated for various operating temperatures and three different control strategies namely, constant mass flow of the heat transfer fluid (HTF), constant outlet fluid temperature and real-time optimal outlet fluid temperature. It was found that the optimal receiver aperture size (radius) of the receiver ranged between (1.06 and 1.71 m) depending on temperature. The optical efficiency of the BDOE ranged from 32% to 37% as a daily average (average over the ten sunshine hours). The daily average mean flux density ranged between 9.422 kW/m2 for the 1.71 m-receiver and 20.9 kW/m2 for the 1.06 m-receiver. Depending on the control parameters and assuming an open receiver with solar absorptivity of 0.95 and longwave emissivity of 0.10. The average receiver efficiency varied from 71% at 300 °C down to 68% at 600 °C. The overall daily average thermal efficiency of the plant was between 28% and 24%, respectively for the aforementioned temperatures. The peak of useful power collected in the HTF was around 105 kWth at 300 °C mean fluid temperature and 89 kWth at 600 °C.

Energy dissipation distributions and dissipative atomic processes in amplitude modulation atomic force microscopy

Instantaneous and average energy dissipation distributions in the nanoscale due to short and long range interactions are described. We employ both a purely continuous and a semi-discrete approach to analyze the consequences of this distribution in terms of rate of heat generation, thermal flux, adhesion hysteresis, viscoelasticity and atomic dissipative processes. The effects of peak values are also discussed in terms of the validity of the use of average values of power and energy dissipation. Analytic expressions for the instantaneous power are also derived. We further provide a general expression to calculate the effective area of interaction for fundamental dissipative processes and relate it to the energy distribution profile in the interaction area. Finally, a semi-discrete approach to model and interpret atomic dissipative processes is proposed and shown to lead to realistic values for the atomic bond dissipation and viscoelastic atomic processes.