Performance analysis of a direct-expansion solar-assisted heat pump for water-heating application in South Africa

Abstract

This study investigated the potential application of direct expansion solar-assisted heat pump water heating (DX-SAHPWH). The system is a confluence of heat pumps and solar thermal water heating systems. This is normally achieved by replacing the conventional evaporator of a heat pump with a solar-thermal collector, which results in several unique advantages such as higher efficiencies, extracted from two energy sources, i.e. from both solar and ambient air, and improved collector lifetime, amongst others. However, the primary shortcoming is that the system demonstrates an erratic collector-evaporator load due to diurnal and seasonal swings in meteorological conditions. Therefore, depending on the geographical location ,the DX-SAHP system performance will vary. Consequently, to discern whether it is a suitable technology for South Africa, an analysis is therefore required to evaluate the performance of the DX-SAHPWH system prior to implementation. A theoretical model was developed based on an analytical steady-state approach, which was used to evaluate its thermal performance characteristics of the DX-SAHPWH system. It consisted of two bare/unglazed solar collectors with a combined area of 4.2 m2, a 680 W rotarytype hermetic compressor using R22 as the refrigerant, and a 150 L cylindrical heat storage tank with an immersed helical coil condenser. The analysis was conducted for typical summer and winter days and for an annual simulation, using the meteorological data in Stellenbosch in Western Cape for the year 2018. To assess the performance, the following metrics were used: coefficient of performance; collector efficiency; solar fraction; and heating time. Moreover, a parametric analysis was conducted to evaluate the effects of the meteorological and operational parameters on the performance of the system. The daily results indicated that the system performance is generally better on summer days than in winter days due to the higher ambient temperature, initial water temperature and solar radiation. Moreover, the specific results indicated for the summer condition, the instantaneous energy performance showed that the system is capable of producing 1700 to 4000 W of thermal energy and a compressor consumption from 550 to 750 W based on a 24-hour average. The condenser heat gain is 2000 W, collector heat gain is 1400 W and compressor power consumption is 632 W. The coefficient of performance (COP) and collector efficiency varied from 1.83 - 5.65 and 45-51 % respectively. The solar fraction and heating time ranged from 0 - 82 % and 94 to 403 minutes. In the winter condition, the results showed that the system is capable of producing 1100 to 1800 W of thermal energy and a compressor consumption from 550 to 585 W. Based on a 24-hour average, the condenser heat gain is 1320 W, collector heat gain is 772 W, and compressor power consumption is 547 W. The COP and collector efficiency ranged from 1.07–2.80 and 45–60 % respectively. The solar fraction and heating time ranged from 0–74 % and 300 to 480 minutes. The synthesis of the daily performance further indicated that the system should ideally run during the day when solar radiation is abundant and that the COP could increase as additional ambient energy heat gains are expected. The annual performance results indicated that minimum (min) and maximum (max) COP expected are 1.4 and 5.75 respectively, with an annual average of 2.3. The COP is higher during the periods with higher ambient temperatures, solar radiation and higher initial water temperatures which is the converse for the colder seasons. The min and max collector efficiency is 0 and 59.96 % respectively; it can be seen that the collector efficiency remains fairly constant throughout the year. The consequence is that the temperature gradient between the collector and the ambient temperatures does not vary annually. The min and max solar fraction is 0 and 86.47 % with an annual average of 34.81 %. Higher solar fractions are expected during periods that exhibit higher solar radiation values compared to other seasons. The heating time min and max are 92 and 570 minutes with an annual average of 355 minutes. It was found that periods with lower ambient temperature, initial water temperatures and solar radiation values exhibited higher heat times. The parametric analysis aimed to investigate the effects of meteorological conditions. The results indicated that the effect of ambient temperature (for the range 0–40°C) results in a linear increase in both the COP and collector efficiency by 77 and 81 % respectively. In addition, an increase in the solar fraction and a decrease in the heating time was found to be 95 % and 25 % respectively. The effect of the solar radiation was also investigated for a range of 200–1200 W/m2. The results indicated an increase in solar radiation, resulting in an increase in COP and a decrease in collector efficiency by 73 % and 48 % respectively. In addition, an increase was shown in the solar fraction and a decrease in the heating time by 88 % and 43 % respectively. Lastly, the wind speed was investigated for a range 1–10 m/s. As the wind speed increases, the COP and collector efficiency decreased by 15.6 % and 34 % respectively . In addition, a decrease was noted in solar fraction and heating time of 3.7 % and 37 % respectively. This research aimed to provide an initial benchmark for the performance of a DX-SAHPWH system in South Africa. Moreover, the research undertaken was conducted as a preliminary stage and was meant to theoretical gauge the feasibility of the specific DX-SAHPWH system.

The thermal performance metrics such as COP, collector efficiency, heating time and solar fraction provided a quantitative measure of how these systems will perform for the specific geographical location. Although the analytical modelling approach is not entirely novel and complete, it does provide a guide to the theoretical analysis of the DX-SAHPWH system, which can be used for design and manufacture purposes. The parametric analysis further provides performance insight, so that one might understand how the meteorological conditions affect performance and where the system will best be implemented. Furthermore, it must be highlighted that the results presented were only for the simulated models and thus a prototype must be correctly design, manufactured and tested to obtain accurate results. Based on the results of this dissertation, the author highly recommends the implementation of the DXSAHPWH system in South Africa.