|Achieving even frosting on the outdoor coil along the airflow direction in a space heating air source heat pump for improved energy efficiency and occupant thermal comfort
|Wei, Minchen (BEEE)
You, Ruoyu (BEEE)
Deng, Shiming (BSE)
|Air source heat pump systems
Hong Kong Polytechnic University -- Dissertations
|Department of Building Environment and Energy Engineering
|xxvii, 182 pages : color illustrations
|Air source heat pump (ASHP) is an important clean energy technology for building space heating and cooling. ASHP systems are widely accepted because of high energy efficiency and low initial and operating costs. Over the past decades, a growing number of countries have promoted the development and / or a wider application of ASHP technology by introducing favourable policies, so as to alleviate the energy crisis and environmental degradation. Therefore, with the continuous technology developments, future extensive applications of ASHP systems can be expected.
However, frosting on the surface of the outdoor coil in an ASHP unit significantly limits its efficient operation. It is well understood that when the surface temperature of an outdoor coil in an ASHP unit is lower than the dew point of humid ambient air and freezing point of water, frost will form on the outdoor coil surface. Frosting may lead to a series of negative impacts on the operation of ASHP units, including increased heat transfer resistance between ambient air and refrigerant, increased air pressure drop in an outdoor coil, reduced output heating capacity and coefficient of performance (COP) of the ASHP unit and the necessity of periodical defrosting operations. Therefore, frosting is an unwanted phenomenon that should be suppressed as much as possible for a higher heating operating efficiency and better indoor thermal comfort. Up to date, a great number of studies have focused on developing effective frosting-suppression technologies and efficient defrosting methods, with however inadequate attentions paid to the uneven frosting of outdoor coils, a commonly seen phenomenon for space heating ASHPs. In particular, uneven frosting along the airflow direction, i.e., more frost deposited on the windward side than that on the leeward side in a traditional single-fan outdoor coil, has been commonly observed. As a result, frost deposited on the windward side may rapidly block the airflow path, causing a rapid deterioration in the heating performances of an ASHP unit, the energy waste during defrosting, and frequent defrosting operations. Although there have been efforts to deal with uneven frosting along the airflow direction, it cannot be effectively avoided in ASHP units where traditional single-fan outdoor coils with a fixed airflow direction are used. To effectively alleviate uneven frosting along airflow direction, a novel dual-fan outdoor coil (DFOC) that can alternately reverse the airflow direction has been proposed and studied, experimentally and by modelling, in the research project reported in this Thesis.
This Thesis begins with reporting the development of an experimental novel DFOC having two identical fans, and an experimental ASHP unit using the DFOC(ASHP/DFOC). The two fans, Fan A and Fan B, may operate alternately to reverse airflow direction based on a pre-set value of time interval for alternate operation (tTIAO). An experimental ASHP / DFOC setup was established. The setup included the experimental ASHP / DFOC unit and an environmental chamber where there were two rooms, one for a simulated indoor space and the other for simulated outdoor environment. A computerised measuring and monitoring system that can collect real-time data for all key operating parameters in the experimental setup has been installed. The availability of the setup would facilitate all the experimental work required in the research project reported in this Thesis.
Secondly, the Thesis presents an experimental study on even frosting characteristics of the experimental ASHP / DFOC unit in terms of the frosting / defrosting duration, the total output heating capacity and COP at different dual-fan operating modes. There were seven experimental cases including both single and multiple frosting-defrosting operation cycles. The experimental results demonstrated that when using the experimental DFOC in Cases 3, 4 and 7, the heating / frosting duration was lengthened by 23.6%, 34.5% and 46.4%, and the averaged output heating capacity, q , was increased by 5.5%, 9.5% and 9.6 %, and averaged COP, COP by 6.7%, 10.5% and 11.1% respectively, and the defrosting frequency was decreased by 33.3% in Case 7, when compared to using the traditional single-fan outdoor coil in other cases. Besides, as tTIAO values were decreased from 15 mins to 10 mins for both fans, and to 6 mins for Fan A and 8 mins for Fan B, heating / frosting operation duration was increased by 8.9% and 19.4%, respectively, at a higher q and COP, as compared to those using the traditional single-fan outdoor coil. The experimental results clearly demonstrated that the use of the experimental DFOC can help achieve evener frosting along airflow direction, and hence a longer heating operation duration and a higher operating efficiency and output heating capacity.
Thirdly, the Thesis reports on the development and experimental validation of a dynamic mathematical model for the experimental ASHP / DFOC unit, in order to more effectively and comprehensively study the operating characteristics of the ASHP / DFOC unit at different fan operating modes, with different configurations and under different operating ambient conditions. With the validated model, a follow-up modelling study for different operating modes of the two fans, fin pitches and operating ambient conditions was carried out. The modelling results demonstrated that by optimizing fan operating mode, the difference in frost thickness between the windward and leeward sides of DFOC can be reduced by up to 77.3%, the averaged output heating capacity and COP improved by up to 11.1% and 12.7%, respectively, and the number of switching operation of the two fans decreased by 55.6%. Besides, the recommendations for using the DFOC at different operating modes of the two fans, different fin pitches and operating ambient conditions were also made.
Finally, the Thesis presents the development of a GRNN model-based optimal control strategy that enables the efficient operation of the experimental ASHP / DFOC unit at varying ambient conditions. A database for developing the GRNN model was established first, and a new coefficient of performance-related index, which was the change rate of averaged COP(CRAC), was proposed after analysing the database. Then, based on the developed database and CRAC, a GRNN model was developed and validated, and then used to predict and search the optimal dual-fan operating modes when the experimental ASHP / DFOC unit was operated at different ambient air conditions. Lastly, an optimal control strategy for the experimental ASHP / DFOC unit based on those identified optimal dual-fan operating modes was developed and experimentally validated. The experimental results demonstrated that the use of the developed optimal control strategy led to a higher heating efficiency, a longer frosting-defrosting duration and a fewer number of dual-fan switching for the experimental ASHP / DFOC unit, with its averaged COP improved by 9.9 % and 6.8 %, CRAC decreased by 17.1 % and 10.5 %, defrosting frequency reduced by 22.2% and 12.5%, and averaged frosting-defrosting duration extended by 32.8% and 19.4%, respectively, when compared to the use of traditional single-fan operating mode and a fixed dual-fan operating mode at varying ambient conditions.
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