| Author: | Liu, Shengnan |
| Title: | Experimental and modeling studies on developing a condensing-frosting performance map for a variable speed air source heat pump unit for frosting suppression |
| Advisors: | Wei, Minchen (BEEE) You, Ruoyu (BEEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Air source heat pump systems Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building Environment and Energy Engineering |
| Pages: | xxiv, 202 pages : color illustrations |
| Language: | English |
| Abstract: | Air source heat pumps (ASHPs) can be used to help effectively achieve carbon emission reduction and alleviate the global energy crisis, and have been hence widely used in many parts of the world. However, for a space heating AHSP unit, frost may form on its outdoor coil surface at certain operating and ambient conditions. The accumulation of frost would adversely affect the normal operation of the ASHP unit, and regular defrosting was therefore required, leading to a low operating efficiency and poor indoor thermal comfort. Therefore, effective frosting suppression was crucial to the further wider applications of ASHPs. However, existing frosting suppression measures may not be applicable in practice due to their various inadequacies. In addition, variable speed (VS) technology has been increasingly used in air conditioning (A/C) systems and ASHPs. Previous studies on the operating characteristics of a VS direct expansion (DX) A/C system and the development of its operating performance maps were reported. The study results suggested that simultaneously changing compressor speed and air fan speed of the VS DX A/C system could cause the changes in its evaporating temperature. Therefore, for a VS ASHP unit, likewise, changing both speeds of its compressor and outdoor air fan can also cause the changes in its evaporating temperature, thus changing its outdoor coil surface temperature that affects frosting performances directly. However, this would also impact the heating performances of the VS ASHP unit. At present, there have been no systematic studies on the effects of changing both speeds of compressor and outdoor air fan of a VS ASHP unit on its frosting and heating performances, so as to provide essential guidelines for the design, operation and control of VS ASHPs. Therefore, in this Thesis, a study on developing, experimentally and by modeling, condensing-frosting performance maps for an experimental VS ASHP unit to comprehensively evaluate its frosting suppression and heating performances under different speed combinations of the compressor and outdoor air fan is presented. A novel control strategy that can be used to achieve effective frosting suppression while meeting the heating requirements, based on the developed performance maps, is also presented in the Thesis. Firstly, an experimental setup is detailed. It included an experimental VS ASHP unit and an environmental chamber. The experimental VS ASHP unit was placed inside the environmental chamber, where there were a simulated indoor heated space and a simulated outdoor space. The experimental setup could provide different test conditions required for the intended experimental study. All operating parameters of the experimental VS ASHP unit and the environmental chamber could be real time monitored and recorded using high-precision measuring devices. With the availability of the experimental setup, the operating characteristics of the experimental VS ASHP unit could be experimentally studied, an established mathematical model for the experimental VS ASHP unit experimentally validated, and finally the developed novel control strategy for the experimental VS ASHP unit tested. Secondly, an experimental study on the detailed relationships between frosting suppression performances and the total output heating capacities of the experimental VS ASHP unit under different speed combinations of its compressor and outdoor air fan is reported. The condensing-frosting performance maps obtained in the experimental study demonstrated that for the experimental VS ASHP unit, changing the speeds of its compressor and outdoor air fan would result in not only different outdoor coil surface temperatures, but also different total output heating capacities. A higher outdoor coil surface temperature and thus a better frosting suppression performance may be achieved, but usually at the expenses of losing some of its total output heating capacity. Furthermore, the experimental results also suggested that when the experimental ASHP unit was operated at different speed combinations that can lead to the same or similar total output heating capacities, its operating characteristics in terms of outdoor coil surface states can be significantly different. The obtained condensing-frosting performance maps may be used as an essential guidance for the design, operation and control of ASHPs for frosting suppression operation. Thirdly, a modeling study on developing a mathematical model for the experimental VS ASHP unit is presented. The model was developed by referring to previously published mathematical models for VS A/C systems and experimentally validated using the experimental data obtained in the experimental study. The average relative errors between the measured and predicted total output heating capacity and COP were at less than 4.0 %, and the average error between the measured and predicted outdoor coil surface temperature at 0.2 °C. A follow-up modeling study was then carried out using the validated model to obtain condensing-frosting performance maps for the experimental VS ASHP unit having different outdoor coil surface areas and at different outdoor operating conditions. The modeling study results suggested that by increasing outdoor coil surface area of the experimental VS ASHP unit from 50 % to 150 %, its surface temperature on average was increased by more than 1.37 °C, which was inducive to a better frost suppression performance, and the total output heating capacity by more than 11.07 %, but at a higher initial manufacturing cost. Besides, at a fixed outdoor relative humidity, the maximum output heating capacity of the experimental VS ASHP unit under frost-free operation was significantly increased as the outdoor air temperature was increased. Furthermore, at a fixed outdoor air temperature, as the outdoor air relative humidity was decreased, the outdoor coil surface temperatures and total output heating capacities were slightly reduced, but the frost-free region on the condensing-frosting performance map was enlarged, mainly due to the change in outdoor air dew point temperature. The modeling study results also suggested that the developed model can be a useful tool in studying the characteristics of VS ASHPs during both frosting and frost-free operations. Finally, the Thesis reports the development of a novel control strategy based on the obtained condensing-frosting performance maps for the experimental VS ASHP unit to achieve effective frosting suppression during operation. The developed novel control strategy was experimentally tested using the experimental VS ASHP unit. The test results suggested that the experimental VS ASHP unit under the developed novel control strategy could not only achieve effective frosting suppression, but also output the required heating capacity at high COP values whenever possible. By using the developed novel control strategy, the heating duration in one frosting-defrosting operation cycle could be increased by up to 157.68 %, and the daily defrosting frequency, duration and energy consumption reduced by up to 59.09 %, 36.30 % and 32.96 %, respectively. The proposed novel control strategy was straightforward and simple to implement, and could contribute to the further developments of frosting suppression technology for ASHPs. |
| Rights: | All rights reserved |
| Access: | open access |
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