The study of storage-enhanced heat recovery room air-conditioner (SEHRAC) for combined space cooling and water heating in residential buildings in Hong Kong

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The study of storage-enhanced heat recovery room air-conditioner (SEHRAC) for combined space cooling and water heating in residential buildings in Hong Kong

 

Author: Jia, Jie
Title: The study of storage-enhanced heat recovery room air-conditioner (SEHRAC) for combined space cooling and water heating in residential buildings in Hong Kong
Degree: Ph.D.
Year: 2016
Subject: Dwellings -- Heating and ventilation.
Air conditioning.
Heat recovery.
Heat exchangers.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xxii, 157 pages : illustrations (some color)
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2890612
URI: http://theses.lib.polyu.edu.hk/handle/200/8476
Abstract: In subtropical climates such as Hong Kong's, air-conditioning and domestic water heating are two dominant energy end-uses in residential buildings. Local statistics show that out of the total energy consumption of the residential sector, 23% goes to air-conditioning and 19% goes to water heating. These figures clearly show that a technology that can simultaneously reduce energy use for space cooling and water heating is highly valuable. Water heating utilizing condenser heat rejected from a refrigeration cycle is considered to be an energy efficient measure for domestic hot water production due to the associated heating efficiency is always higher than unity. On this basis, heat pump water heater is widely investigated for use in residential buildings. However, a majority of previous studies focused only on its heating capacity at the condenser. The cooling performance at the evaporator under various operation conditions was seldom studied. This is because heat pump water heaters are often used as stand-alone water heating appliances with evaporator leaving air discharged to outdoors and thus the associated cooling effect is unexploited. Economic barrier also exists to limit its use in residential buildings, since a considerable investment must be made in the heat pump by the user. Therefore, a technology that allows a room air-conditioner to serve the combined purpose of space cooling and water heating is preferred, due to the fact that air-conditioners are commonly equipped in Hong Kong households to combat the hot and humid summer. However, considering the differences between a heat pump and a room air-conditioner, relevant research works are limited in public domain. In view of this, a storage-enhanced heat recovery room air-conditioner (SEHRAC) was proposed. Besides space cooling, this installation can also provide water heating utilizing recovered condenser heat. In this study, the effectiveness, energy performance, operation characteristics, and performance improvement of SEHRAC when used for combined space cooling and water heating were investigated. For the application of SEHRAC, a major concern is whether or not the amount of recoverable heat can meet the household water heating demand. The concern is particularly prominent in Hong Kong because most local residential units are small but the occupancy density is high (average living space is 12.9 m{194}/person). The small space cooling load and thus the small amount of recoverable heat may not be sufficient to offset the high water heating demand. To address the concern, a typical residential estate in Hong Kong was chosen for a supply and demand analysis. The analysis was based on hour-by-hour simulations using actual building characteristics and realistic equipment performance data. It was found that if a water tank was used for thermal energy storage, the daily cumulative recoverable heat far exceeded the daily household water heating demand, confirming the effective use of SEHRAC in Hong Kong. Further, for optimal design of the water tank of SEHRAC, its design requirements including the minimum required heat storage capacity and heat transfer effectiveness were also determined and discussed. Room air-conditioners are often sized for satisfying peak cooling loads. At part load conditions, most air-conditioners operate intermittently to regulate their cooling outputs for maintaining the indoor set-point temperature. Intermittent operation of SEHRAC can lead to significant fluctuations in operating parameters on the refrigerant side. Adding that capillary tube is often used as the expansion device to magnify the fluctuations, whether SEHRAC can still operate satisfactorily in practice is of concern. Further, as operation of SEHRAC is affected by outdoor temperature and space cooling load, their influences on the energy performance of SEHRAC also need to be ascertained. In view of this, a prototype SEHRAC was designed and setup for laboratory experiments. The experimental results confirmed the satisfactory operation of SEHRAC. Its overall coefficient of performance (COP) was also found better than conventional room air-conditioner for a range of outdoor temperatures and space cooling load conditions. On this basis, a prediction model was developed for evaluating the sole use of SEHRAC for water heating in the entire air-conditioned season. The potential water heating energy saving on wider application of SEHRAC was estimated to be 9.1% of the overall energy consumption of the residential sector in Hong Kong.
Considering the significant refrigerant pressure fluctuations during the operation of SEHRAC, the selection of a suitable type of expansion device that can function properly despite the fluctuations is very important. In this study, the use of capillary tube (CT) and thermostatic expansion valve (TEV) in SEHRAC were side-by-side compared with the aim of optimizing the system configuration of SEHRAC. A prototype SEHRAC, which could be switched between the CT and TEV systems, was used for two identical sets of experiments. For each set of experiments, a series of tests under different outdoor temperatures were conducted. It was found that TEV could better regulate the refrigerant flow, maintain a stable evaporator superheat, and minimize the rise in refrigerant vapor content at the evaporator inlet during the operation of SEHRAC. The space cooling and water heating capacities for the TEV system were on average 16.3% to 19.4% and 18.5% to 23.4%, respectively, larger than the CT system. Its overall COP was also found 12.5% to 20.9% higher for the range of outdoor temperatures. Accumulation of heat in the water tank of SEHRAC can result in an increase in tank water temperature, which will unavoidably affect the space cooling and water heating performances of SEHRAC. Measures to enhance the heat storage capacity of the water tank will thus be beneficial to the overall performance of SEHRAC. For achieving this objective, the use of phase change material (PCM) in the water tank was proposed in this study. To confirm and quantify the resultant performance improvement, laboratory experiments were conducted with and without the use of PCM (abbreviated as wPCM and woPCM scenarios). It was confirmed that the PCM could function as an internal heat sink to lower the rate of water temperature increase and to enhance the heat storage capacity of the water tank. As a result, the space cooling and water heating capacities for the wPCM scenario, as compared to the woPCM scenario, were 5.4% and 16.1% larger, respectively. The overall COP was therefore found 6.9% to 9.8% higher. It was also revealed that the use of PCM could extend the heat retention time of the tank water by 21.1%. From the above, the academic contributions of this thesis can be summarized into four aspects. First, the effectiveness of SEHRAC in satisfying the daily space cooling and water heating demands of Hong Kong households was confirmed. Second, the energy performance and operation characteristics of SEHRAC in comparison with conventional room air-conditioner under different outdoor temperatures and space cooling load conditions were examined. Third, the system configuration of SEHRAC was optimized by identifying a suitable type of expansion device. Last, the use of PCM to improve the overall performance of SEHRAC was proposed and confirmed.

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