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Temperature and Humidity Independent Control (THIC) of Air-Conditioning System

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Temperature and Humidity Independent Control (THIC) of Air-Conditioning System Temperature and Humidity Independent Control (THIC) of Air-conditioning System Xiaohua Liu • Yi Jiang • Tao Zhang Temperature and Humidity Independent Control (THIC) of Air-conditioning System Xiaohua Liu Yi Jiang Tao Zhang Department of Building Science Tsinghua University Beijing, People’s Republic of China ISBN 978-3-642-42221-8 ISBN 978-3-642-42222-5 (eBook) DOI 10.1007/978-3-642-42222-5 Springer Heidelberg New York Dordrecht London Library of Congress Control Number: 2013957558 © Springer-Verlag Berlin Heidelberg 2013 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publisher’s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Preface In 1995, Dr. James Freihaut from UTRC (United Technologies Research Center, the R&D base for Carrier, OTIS, and other industries) visited Tsinghua University for a meeting with me. The purpose of the meeting was to establish a cooperation project with our group in Tsinghua University for next-generation HVAC (heating, ventilation, and air-conditioning) system. “Do you have any idea on humidity independent control?” I still remember the question from James, and he tried to persuade me to do something for dehumidifying air without change of the air temperature. However, at that time, I had little idea on independent air dehumid- ification handling process. This was the time when I started to think about the concept of “temperature and humidity independent control” (THIC). Thanks to the Yung Wing funding from UTRC in 1996, a cooperation project was set up to develop a new approach for air dehumidification by a special membrane with moisture permeable feature. This was the first step for us toward the temperature and humidity independent control of HVAC system. During the first 10 years since 1996, our major effort was put into new approaches of air dehumidification. We had tried out various ways to achieve this goal, such as using moisture permeable membrane to dehumidify air by heating the air at the sink side, using two rotary desiccant wheels to recover energy from exhaust air to deeply dehumidify the inlet air, using multistage liquid desiccant air handling modules, etc. Fundamental studies were carried out at the same time to understand the real features of humidification/dehumidification process and the ideal minimum energy required for air dehumidification process and where the loss is in each stage of an air dehumidification process. Accompanied by these studies, a feeling has been getting stronger and stronger in our mind: air temperature and moisture in air (i.e., humidity ratio) are really two independent physical concepts, and unless a phase change process occurs such as evaporation or condensation, temperature and water vapor (or moisture) cannot influence each other. We should not take these two processes into account together in the indoor thermal environment control process as what engineers do in conven- tional HVAC design and operation procedures. Although both processes are affected by “heat,” the sensible heat for temperature regulation (heating or cooling) v vi Preface and the latent heat for moisture regulation (humidification or dehumidification), these two types of heat are not the same. They are not freely convertible with each other, and the latent heat only occurs with phase change. However, the misunder- standing between these two kinds of heat, i.e., the temperature regulation and humidity regulation process, may lead to insufficient or unsatisfactory indoor thermal environment in designs as well as in operations of HVAC system. For instance, a fan coil unit (FCU) with a total cooling capacity of 1 kW in conventional air-conditioning system can only provide about 50–70 % cooling if it only deals with sensible heat working under a dry condition with no air dehumid- ification. The cooling capacity for latent heat does not always accompany with that for sensible heat. Another simple example is the set point for supply air state in the all-air system: by setting the enthalpy of the supply air, the total heat removed from the conditioned space can be satisfied. However, either temperature or humidity level may be out of demand, which may result in a hot but dry condition or a cold but humid condition. If there is no condensation or evaporation in the conditioned space, the temper- ature regulation and humidity regulation are two independent processes. Why do we still put these two processes together both in design and operation of an HVAC system? It may be better to deal with these two processes separately in analysis, design, and operation. This was how we started to get into the THIC idea for HVAC system, when it was around year 2000. From then on, with the help of testing and investigating a number of air-conditioning systems in various buildings, it has been found that following the THIC concept, we could get a much clearer understanding and construct a better HVAC system during the design stage. The sensible load consisted of solar radiation, indoor devices, and so on varies completely different from the moisture load generated from occupants or other sources. If all the loads are undertaken by the air circulation, the supply air state (temperature and humidity) has to be changed all the time to accommodate to the variances of the sensible and moisture loads in order to maintain both the temper- ature and humidity for the conditioned space. This is quite a hard task. However, if the air circulation system is only used for humidity control and a radiant cooling system (or other cooling system) is adopted for removing the sensible load, things would be much easier. A radiant cooling system can remove sensible load only with little influence on humidity ratio if temperature of the cold water flowing through the radiant cooling device is higher than the indoor air dew-point temperature. In this way, both indoor temperature and humidity can be maintained well regardless of how the sensible and moisture loads change. Furthermore, as the sensible load could be removed by the cold water with a temperature higher than the indoor dew point, e.g., 16–18 C, which is also higher than the chilled water temperature in most conventional air-conditioning systems (e.g., 7 C), chiller’s efficiency will be much higher than that of conventional chiller if the chiller is specially designed to produce high-temperature chilled water for removing only sensible cooling load and regulating indoor temperature. Preface vii This is a new concept for HVAC system, which can provide more comfort thermal environment with less operation energy. This should be the principle and basic design guideline for the air-conditioning system in future. Although the key devices for the THIC system seem to be similar to those for the conventional air-conditioning system, the operating conditions and performances of devices we need in the THIC system are quite different: the air handling processor should be able to dehumidify air to a drier state for removing indoor moisture load but without a too low temperature to be supplied to the conditioned space; the chiller should provide chilled water with a higher temperature (16–18 C) and higher coefficient of performance (COP); if FCU is adopted for indoor temperature control, the FCU should work efficiently with a smaller temperature difference between circulating water and air but without the worry of condensing water; etc. These indicate that the THIC systems do need a complete set of new types of devices, a new generation of HVAC devices! We need different design approaches, we need different system components, we need different control devices with different logics, and we need different ways to operate and manage the THIC systems. To push this THIC system to the real construction market, an association for THIC air-conditioning system has been founded since 2007, with members includ- ing researchers, manufacturers, building developers, as well as operators. Thanks to the huge construction market in China, different types of THIC systems for different building functions have been adopted in many places of China since 2005. So far, there has been more than 20 million m2 of buildings with all sorts of functions designed according to the THIC concept in China.
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