John Zhai

Comparative Energy Analysis of VRF and VAV Systems Under Cooling Mode

Variable refrigerant flow (VRF) or variable refrigerant volume (VRV) systems provide many benefits over traditional air-conditioning systems, with great potential to decrease energy cost and increase thermal comfort in buildings. This paper presents a method to size and select VRF systems and to compute its annual energy consumption. The study compares the cooling energy usage of a VRF system against a conventional chiller-based variable-air-volume (VAV) system and a packaged VAV (PVAV) system for a typical light commercial building. The results reveal that the peak electrical demand of the VRF system for the cooling season is about 60% of the chiller-based VAV system and 70% of the packaged VAV systems, and the operating energy usage is about 53% of the chiller-based VAV system and 60% of the packaged VAV system for the building studied.Copyright

Small Scale Mobile Hybrid Integrated Renewable Energy System (HI-RES) for Rapid Recovery and Emergency Response

The concept and inherent challenges associated with the development of a resilient, affordable, high quality integrated power generation, energy management and rapid recovery and emergency response (RR&ER) capability for providing electrical power and sustained life support operations during power disruptions is the focus of this paper. Presently, small-scale emergency mobile backup HI-RES are expensive and used primarily for military operations. As recent headlines in the news point us towards potential threats on our energy infrastructure, it becomes more imperative for us to consider alternatives to sole dependence on our electrical power grid for supplying and meeting the demands of our global and rising consumer base. Distributed Resources (DR), such as the HI-RES RR&ER Station, provide a viable option for reducing vulnerabilities of the electrical power grid while minimizing time of service to recovery while preserving focus on the human element.

Computational fluid dynamics to predict duct fitting losses: Challenges and opportunities

A shoot-out contest to determine loss coefficients using computational fluid dynamics modeling for two prescribed oval duct fittings has been conducted. The objectives of the contest were to determine if the computational fluid dynamics modeling can predict loss coefficient within 15% accuracy without previous knowledge of experimental data. The main findings of the project showed that the trends of the pressure loss coefficients were predicted correctly, while the accuracy can be improved. None of the contestants could predict the pressure loss coefficients within 15% of the measurements for all the tested cases. The prediction error varies significantly among the ten submissions (between 20% in some entries and more than 80% in most others). The reasons for this error may be attributed to several facts, including errors in the geometry, errors in the definitions of the duct fitting loss coefficients, random choice of turbulence model and near wall treatment, inappropriate modeling of wall roughness, different grid conversions, etc. This article presents an in-depth discussion of the potential influences of these factors. The article also reviews the computing requirements for such a prediction task and finds that the duct fitting flow can be simulated with acceptable computational time on a regular computing platform. Recommendations for future research are provided, including the need for and approaches to performing a systematic study to compare and improve computational fluid dynamics techniques that are capable of predicting flow in duct fittings and to further the whole duct system to within an accuracy of 15% or better of measured loss coefficients.

Expanded Microchannel Heat Exchanger: Nondestructive Evaluation

Abstract Recent theoretical developments in expanded microchannel polymer-based heat exchangers were promising, but the initial experiments underperformed simple theory. In order to understand this discrepancy, this article introduces a nondestructive methodology for characterizing polymer heat exchangers. A computerized tomography (X-ray) scan was performed to diagnose the problem. The method was tested on the expanded microchannel polymer heat exchanger to determine the variations in geometry between the theoretical and experimental heat exchanger. Channels were found to have variable heights causing flow maldistribution. The results are discussed to guide further technological development of this approach to heat exchanger design and fabrication and lays the groundwork for an advanced discretized modeling.