Author: Zhao, Wenxuan
Title: Planning and design optimization of air-conditioning systems for high-tech cleanrooms of enhanced energy efficiency under full-range climate conditions
Advisors: Wang, Shengwei (BEEE)
Degree: Ph.D.
Year: 2024
Subject: Clean rooms
Air conditioning
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Environment and Energy Engineering
Pages: 195 pages : color illustrations
Language: English
Abstract: Over the past few years, numerous countries have rapidly expanded the number and size of their high-tech clean fabrications, to keep pace with electronic devices and Internet needs. Moreover, extremely strict dry-bulb temperature, relative humidity, and particle concentration requirements cause high-tech cleanrooms to consume enormous energy, which is nearly three times the average energy consumed by data centers. Reducing energy use and ensuring the required built environment for normal production have become key challenges in high-tech clean manufacturing, and gradually attracted public and academic attention.
However, different from extensive research and review studies conducted on data centers, there is a lack of correct understanding and sufficient awareness towards high-tech cleanrooms, and relevant research on their energy aspects is rather rare. The performance and applications of existing environmental control systems for high-tech cleanrooms, i.e., air-conditioning systems, are still not fully investigated. These air-conditioning systems also exist serious problems including large cold-heat offset and low cooling efficiency, which are generally neglected during the design stage but often are noticed during the subsequent operation stage. There is still a lack of readily available energy model, directly usable outdoor air ventilation strategy, and effectively applicable design optimization method for air-conditioning systems of high-tech cleanrooms.
Therefore, this study aims to conduct a comprehensive evaluation and analysis on commonly used air-conditioning systems, develop a generic energy model for easy and quick energy assessments, develop an optimal outdoor air ventilation strategy for air-side free cooling, and develop an optimal air-conditioning system that fully avoids cold-heat offset and corresponding design optimization method considering uncertainties. Simultaneously, the following questions can be addressed, which are not mentioned in existing research studies.
o What is the most efficient air-conditioning system for high-tech cleanrooms, encompassing diverse climate conditions and application scenarios?
o How the climate conditions affect the energy performance of high-tech cleanrooms and what are geographically favorable regions to accommodate large-scale high-tech clean fabrications?
o How can high-tech cleanroom air-conditioning systems avoid cold-heat offset and maximize their “free cooling” potentials and what are effective design methods in uncertain conditions?
The commonly used air-conditioning systems for high-tech cleanrooms are categorized into three typical types. Their energy and economic performance, and applicable situations are systematically assessed and analyzed under full range of climatic conditions. Results show that the “fully decoupled” MAU (make-up air handling unit) +DCC (dry cooling coil) +FFU (fan filter unit) system is the most energy-efficient and cost-efficient system, which can achieve 39.2–79.6% energy savings compared with the conventional coupled systems. This system also presents higher energy-saving potential in hot and mild climates, with short dynamic payback periods (<0.7 year) for retrofitting existing coupled systems.
An ANN-based generic energy model is proposed for quick location and technology assessments of high-tech cleanrooms at planning stage. The model is developed based on energy data of a typical cleanroom under all climates in China and all application scenarios by detailed simulations. The correlation of model parameters with climates and application scenarios is identified using a regression method and an ANN method. The model is validated in 22 worldwide cities under 420 application scenarios, which shows satisfactory accuracy. A comprehensive energy assessment study on high-tech cleanrooms in 31 major Chinese cities is conducted and results show that proper location selection could provide potential energy savings of 30% for large-scale clean fabs.
A novel outdoor air ventilation strategy that enables maximum air-side free cooling potential is proposed for high-tech cleanrooms under full ranges of weather conditions and application scenarios. This strategy resorts to theoretically formulating “energy differential” (change of cleanroom energy use per unit of outdoor air volume increase) to determine the optimal outdoor air volume. The energy and economic performance of the proposed strategy are evaluated on eight actual semiconductor clean fabrications. Results show that annual free cooling hours are 662–2,537h for the traditional “fully coupled” AHU (air handling unit) system in 31 major Chinese cities. Up to 8% energy saving is achieved in transition months and average 514.2kWh/m2 electricity and 1.8GJ/m2 primary energy are saved in a year.
An upgraded optimal air-conditioning system is proposed for high-tech cleanrooms by integrating heat recovery and free cooling techniques at air-side and water-side, respectively. The proposed system can fully eliminate cold-heat offset, simultaneously reduce cooling/heating loads and enhance cooling efficiency. By detailed modeling and simulations, the proposed system is validated and tested under various indoor cooling loads, ventilation rates, and surrounding weather and climate conditions. Results show that 2.3–33.1% energy savings are achieved and up to 15.8GJ/m2 annual primary energy is saved, compared with the conventional design. It is also observed that cities in cold and mild climates have higher energy-saving potentials than those in hot climates.
A multi-objective optimal design method is proposed for air-conditioning systems of high-tech cleanrooms considering load uncertainty and equipment degradation. The conventional method of designing the air-conditioning systems would lead to significant deviations in the indoor environment because it fails to meet the cooling, heating, and humidification demands adequately. The proposed design method can determine the optimal capacities of main air-conditioning equipment for high-tech cleanrooms, which can achieve a good balance between two conflicting objectives, i.e., minimizing cleanroom unmet hours and minimizing life cycle costs. By making a mere 5–7% increase in life cycle costs, a remarkable 92% reduction in annual unmet hours could be achieved.
This PhD study can be regarded as a seminal work in the research field, providing substantial and valuable insights and references for the planning, design, and operation of high-tech cleanrooms. Additionally, it has the potential to catalyze the comprehensive development of related high-tech industries, especially semiconductor industry.
Rights: All rights reserved
Access: open access

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