Author: Bao, Yani
Title: Developing a novel dedicated outdoor air system (DOAS) for energy efficiency and environmental health
Advisors: Lee, Wai Ling (BSE)
Degree: Ph.D.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Air conditioning -- Efficiency
Air conditioning -- Energy conservation
Department: Department of Building Services Engineering
Pages: xxiii, 149 pages : color illustrations
Language: English
Abstract: A novel dedicated outdoor air system (DOAS) for better energy efficiency and environmental health objectives is proposed in this study. It consists of a multi-stage variable speed direct expansion (DX) air-conditioner as the central system to generate extra-low temperature (XT) outdoor air (OA) to handle the entire space cooling load and a mixing chamber as the terminal system to mix XT OA and return air (RA) to become supply air (SA). For the successful application of the proposed system (XT-DOAS), the major concern is how the multi-stage DX air-conditioner must be designed to achieve the desired air conditions and better energy efficiency under highly variable indoor and outdoor operating conditions. As a performance evaluation of the XT-DOAS as compared to a conventional system, simulation studies were conducted based on actual equipment performance models and realistic building and system characteristics. The results showed that XT-DOAS as compared to conventional system, was superior for achieving the desired relative humidities; could better achieve the desired thermal comfort conditions; saved 22.6% annual energy use for air-conditioning; and reduced the annual cumulative number of non-comfortable hours by 31%. To further enhance the performance of XT-DOAS, the optimum number of cooling stages and treated OA temperature need to be determined. As the thermodynamic states of moist air entering and leaving individual cooling stages will affect the performance of XT-DOAS, the optimization process requires the development and validation of a coil performance model that takes into account the extra-high and extra-low entering air temperature at the first and the last cooling stages. Factory test data and field measurement data were collected for the model development. Based on the developed model, energy and exergy analyses confirmed that the optimum configuration for XT-DOAS is two cooling stage with a treated OA temperature of 7 °C. As the terminal system of XT-DOAS is of VAV control, the drawback is the risk of overcooling due to the minimum air flow fraction requirement of VAV system. To maintain space air temperature and to avoid overcooling, conventional VAV (Con_VAV) systems are provided with reheating coil because of the uncertainties in the possible variations in indoor and outdoor conditions thus the fluctuations of cooling demand. To confirm the effective use of the optimized XT-DOAS (without reheat provision) operating under a highly variable indoor and outdoor conditions, a probabilistic approach based on Monte Carlo simulations with 10,000 iterations was adopted to investigate the overcooling risk of XT-DOAS as compared to Con_VAV. The results showed that the potential annual overcooling hours using XT-DOAS were 29 hours, which is equivalent to 0.31% long-term percentage of dissatisfied (LPD). These results, as compared to 426 overcooling hours and 1.69% LPD achieved by the typical Con_VAV system, confirmed the excellent performance of XT-DOAS. From the above, the investigation of XT-DOAS in this thesis can be summarized into four major aspects: the effective use for achieving the desired air conditions and better energy efficiency; the development of a realistic coil performance model; the determination of the optimum configuration; and the assessment of overcooling risk.
Rights: All rights reserved
Access: open access

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