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Omaha, NE, United States

Li Y.,Bes Tech Inc. | Liu M.,Bes Tech Inc. | Lau J.,University of Nebraska at Omaha | Zhang B.,Bes Tech Inc.
Applied Energy | Year: 2015

As one of the significant parameters to characterize the motor performance, motor efficiency changes under variable speed and load conditions. Although there are many methods to measure the partial load motor efficiency under the full speed condition, few of them are available to estimate the efficiency under partial speed conditions. In this paper, a new and original concept of "load ratio" is brought forward to describe the motor performances. A simple method is proposed to estimate the motor efficiency under variable speeds operations and partial load conditions using load ratio, speed ratio, rated motor power and rated motor efficiency. To validate the accuracy of this method, the partial loads full speed operation data from the MotorMaster+ database and the published data under the variable speed conditions from other researcher's tests are used for demonstration. The results indicate that the relative errors using the proposed method at full speed are within ±5%. Under variable speed conditions, the relative error is within 10% when the speed ratio is above 0.5. © 2015 Elsevier Ltd. Source

Li Y.,Bes Tech Inc. | Liu M.,Bes Tech Inc. | Lau J.,University of Nebraska at Omaha
Applied Thermal Engineering | Year: 2015

Variable speed technology was applied to compressors for rooftop units (RTUs) and heat pumps in recent years. Compressor power is a critical parameter to evaluate system performance and conduct fault detection and diagnosis (FDD). However, existing power models often use numerous parameters related to the compressor geometrical dimensions and refrigerant physical properties which makes the existing models hard to implement in the field. In this paper, a semi-theoretical compressor power model was developed for single-stage RTUs equipped with a variable speed compressor and a variable speed indoor fan, based on the theoretical analysis and experimental studies. Under the normal conditions, the compressor power is correlated to the outside air temperature and compressor speed with a relative error of ±8%. This model can be employed in the field to develop a compressor power baseline for real-time FDD on the Direct Expansion (DX) RTUs. © 2014 Elsevier Ltd. All rights reserved. Source

Zhang B.,Bes Tech Inc. | Li Y.,Bes Tech Inc. | Lau J.,University of Nebraska at Omaha | Liu M.,Bes Tech Inc.
Energy and Buildings | Year: 2014

Demand control ventilation is one of the most energy efficient ways to achieve the optimum indoor air quality (IAQ). The interest in it has led to a lot of research and publications particularly for single duct VAV system with terminal reheat since the update of the ASHRAE standard 62 in 2004. However, the VAV terminal box (TBX) control algorithms have been given less attention and the influence of the VAV TBX airflow setting on the system energy use has not been quantified. Thus, this study investigated how the VAV TBX minimum airflow setting influences system energy use as well as building IAQ. First, the DOE developed benchmark building model and EnergyPlus software were used to simulate the building annual hourly thermal load. Then, the system control, energy use and IAQ models were developed for the selected building. Two VAV TBX control methods, the single maximum and dual maximum control methods, were investigated in further detail using two different minimum airflow settings from ASHRAE Standard 90.1-2010. The results indicated that when the TBX minimum airflow is reset from 30% to 20%, a reheat energy savings as great as 40% was achieved and an annual system energy savings up to 30% was reached. © 2014 Elsevier B.V. Source

Li Y.,Bes Tech Inc. | Liu M.,Bes Tech Inc. | Lau J.,University of Nebraska at Omaha | Zhang B.,Bes Tech Inc.
Energy and Buildings | Year: 2014

Fault detection and diagnosis (FDD) is an effective way to automatically detect the existing faults of a rooftop unit (RTU) and increase the reliability and availability of system. Most existing FDD methods use thermodynamic signals as fault indicators. Recently, however, major concerns have arisen in regards to whether refrigerant-related temperature sensors and intrusive pressure measurements are reliable. Electrical signal measurements provide plenty of information about a system but require the use of an electrical meter. One way to solve this issue is to install variable frequency drives (VFDs) on the HVAC system, since VFDs not only optimize the system performance, but also provide electrical parameter readings. In this study, a series of lab experiments were conducted to investigate the relationship between the electrical signals and common faults. Since environmental influences have been correlated with the system performance, the impact of several driving conditions on the electrical signals was also investigated. This study identified an electrical signature for each fault and individually separated the common faults based on three parameters: the fan power, compressor power, and the supply air temperature. The results of the study are of use to the development and implementation of a new FDD method based on combining the electrical and temperature parameter measurements. © 2014 Elsevier B.V. Source

Yu Y.,University of Nebraska at Omaha | Liu M.,Bes Tech Inc. | Li H.,University of Nebraska at Omaha | Yu D.,University of Nebraska at Omaha | Loftness V.,Carnegie Mellon University
Energy and Buildings | Year: 2012

An integrating air-handling unit (IAHU) control theory has been proposed to improve the energy efficiency in office buildings by utilizing the regional and operation differences among multiple AHUs. Unlike the conventional AHU operation, where the units are controlled as independent systems without interaction, IAHU coordinates the AHUs based on the dynamic outside air conditions and system operation modes to achieve synergized energy performance and maintain the indoor air quality. The synergization strategy allows the outside air intake and the airflows to be orderly re-allocated among the AHUs when conditions are appropriate. This paper presents the implementation methodology and performance evaluation of IAHU in an open-plan office building with multiple AHUs. The allocation of airflows among the AHUs is described first to illustrate how IAHU deals with multiple AHUs in a building. The supervisory level control algorithm is then detailed and easy-to-follow flowcharts are provided based on the decision-making schema. A two-step hourly evaluation method and the energy simulation model are introduced. An office building with multiple AHUs is selected to assess the performance of IAHU. The study concludes that the innovative IAHU with the easy-to-implement strategy can be readily implemented to achieve high energy efficiency in open space office buildings. © 2012 Elsevier B.V. All rights reserved. Source

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