The effect of changing temperature and mass flow rate on the charging and discharging time in a horizontal packed bed passive space heating system
Research into rock-beds for residential use has declined over the past three decades. Although rock-beds were constructed for agricultural and solar home applications in the 1970’s and 1980’s, very little research has been undertaken to establish their viability as a Thermal Energy Storage (TES) option in the residential sector. Therefore, there is insufficient up-to-date research that provides overall information on the utilisation of horizontal uni-directional rock-beds for domestic space heating. The intention of this study was to investigate the use of a horizontal packed bed with a uni-directional flow, as a method of storing thermal energy for residential use. The quantitative research method was selected for this study. Both an empirical and a numerical research design were included. The empirical research design was comprised of the proposed horizontal, uni-directional packed bed, followed by the construction phase. The most important performance metrics of the packed bed were identified, as well as how they were measured and what instruments were used. Tests were conducted for inlet temperatures of 40°C, 45°C, 50°C and 55°C and mass flow rates of 150 kg.h-1, 200 kg.h-1, 250 kg.h-1 and 300 kg.h-1. The numerical design was developed by using the STAR-CCM+® Computational Fluid Dynamics (CFD) package to model the empirical design. The horizontal, uni-directional packed bed provided a good platform for space heating in the residential sector. The results indicated that an increase of 50 kg.h-1 in the mass flow rate and 5°C of the inlet temperature resulted in an average reduction in the charging time of 750 and 500 seconds, respectively. It was established that the packed bed could be charged to more than 90% of its charging capacity in two-and-a-half hours of the total eight-hour charging time. After a 16-hour discharge cycle, the packed bed retained, on average, 70.211% of its maximum stored thermal energy. The results showed that the heating table that was developed as the input to the numerical model, correlated well with the empirical inlet temperature data, with an average percentage error of 0.138%. It was concluded that the heating table of the numerical model was a suitable method for mimicking the empirical inlet curve. The final temperature states of the space region, for all the tests that were conducted, had a maximum error of 2.973% and a minimum error of -0.051%. The numerical model proved to be an efficient and accurate method for predicting the final temperature state of the space region, although the trends were not accurately followed during the charging and discharging phases throughout the packed bed. The energy flow balance of the horizontal, uni-directional packed bed displayed very low efficiencies; however, if the system were completely passive and continuously made use of the retained energy in the packed bed for each charging cycle, it could become a viable option to be implemented in the residential sector.
- Engineering