Design, modeling and performance optimization of active air terminal system

• Main Author: Ke, Ji
• Other Authors: Cai Wenjian
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Mechanical engineering::Energy conservation; DRNTU::Science::Mathematics::Applied mathematics::Simulation and modeling
• Online Access: https://hdl.handle.net/10356/90114; http://hdl.handle.net/10220/48446
• Institution: Nanyang Technological University

Air conditioning and mechanical ventilation (ACMV) system, which determines the indoor environment quality and energy efficiency of buildings, attracts increasing attentions throughout the world. In modern society, a series of problems such as the sensation of draught, energy waste arising with the massive usage of air conditioning and sick building syndrome (SBS). Prioritizing green building techniques in ACMV system can improve occupants’ fitness level and deliver dramatic energy saving. Among various ACMV schemes, the active air terminals (active chilled beam and active thermosiphon beam) have outstanding performance on energy saving, indoor environment quality improvement and space saving. However, the existing research is still inadequate and some technical difficulties stand as major obstacles for application of the air terminals especially in tropical regions. To fulfil the gaps, this thesis focuses on the performance analysis, terminal unit modeling and operating optimization of the active air terminal based systems. The contributions of this thesis include: 1. A simple yet accurate hybrid model of active chilled beam (ACB) is developed with respect to air buoyancy. The model demonstrates the air entrainment characteristics in the air chamber and the heat transfer process in the cooling coil. Compared with the existing ACB terminal unit model, the proposed model captures the effects of air buoyancy and further reduces the complexity of the cooling coil model. The ACB model includes only two equations with nine unknown model parameters that can be identified through Levenberg-Marquardt method based on experimental measurements. Experimental validation in a mock up room proves that the models can predict the supply air flow rate and heat transfer process in a wide range of operating conditions. The proposed ACB model can be further utilized in optimization and performance evaluation for the ACB system. 2. To eliminate the condensation problem and improve the heat transfer efficiency of the traditional ACB, the mechanical design of the terminal unit is optimized. Combining air entrainment effect and displacement ventilation, the active thermosiphon beam (ATB) is developed with innovative nozzle arrangement, cooling coil placement and air chamber configuration. The experimental comparisons of ATB and ACB are conducted under various operating conditions to estimate its thermodynamic and hydrodynamic performances. The comparison results indicate that 1) the cooling capacity of ATB is around 40% higher than ACB and passive displacement ventilation (PDV); 2) the ATB has better dehumidification ability with the sensible heat ratio of 0.42; 3) the initial cost of ATB system is the lowest under same cooling load requirement. More importantly, the experimental findings provide a guideline for the operation and optimization of ATB systems. 3. A model-based optimization strategy for the ATB system is developed to reduce the energy consumption and maintain indoor environment quality. The thermal models of the terminal unit and the energy consumption models of different components are established based on the experimental results. Accordingly, the global optimization strategy is formulated to search the optimal operating points of the ATB system with regard to total energy consumption under operating constraints. The experimental results indicate that the optimized operating parameters obtained by the genetic algorithm (GA) can significantly reduce the total energy consumption. The obtained findings indicate that the ATB system is a promising ACMV system in terms of initial cost, thermal comfort and energy saving for a variety of applications.