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♦ The transient temperature development in the water reservoirs near the insulating plate was a result of radial heat transfer between the water and the tank wall as well as axial heat transfer across the insulating plate. Despite the larger initial temperature difference at the hot water side of the plate, the temperature gradient changed more rapidly at the cold water side during the first hour of operation. This was due to the fact that the thermal diffusivity of the stainless steel balancing weight was about 6 times higher than that of the XPS plate. The growth rate at each side of the insulating plate decreased gradually during the test period (ref. Figure 5.57). ♦ In comparison with the static thermocline experiment (ref. Figure 5.56), the application of an insulating plate resulted in a considerable reduction in both the extent and the average temperature change for the thermocline zones. As an example, after a detention period of 4 hours, the transferred energy between the hot and cold water reser- voirs for the two experiments was approximately 2100 kJ and 400 kJ, respectively, which means that the insulating plate reduced the average heat transfer rate by roughly 80%. In a real DHW tank, the movable insulating plate will contribute to a further reduction of the thermodynamic losses, since it will eliminate the mixing of hot and cold water during the tapping mode. ♦ In the experiments, the initial temperature of the insulating plate was about 20oC. However, for a real DHW tank equipped with a movable insulating plate, the initial surface temperature at the hot water side of the plate will be in the region of the hot water temperature, whereas the surface temperature at the cold water side will be determined by the thermal resistance of the plate, the hot water temperature and the previous heat transfer process. By alternatively mounting the balancing weight at the upper side of the plate, the temperature changes at both sides of the plate will be reduced, since the thermal diffusivity of the balancing weight is about 5 to 10 times higher than that of the insulating plate. 5.2.2.2 Hydrostatic Balance and Functionality The two insulating XPS plates, which were tested in the transparent 200 litre ID 500 mm tank, had a thickness of 50 mm and a diameter of 490 and 495 mm. Although the mass and volume of the balancing weights were accurately tuned, it was necessary to make minor weight adjustments in order to compensate for the effects of water absorption, thermal expansion 5 – Experimental Results 167PDF Image | Residential CO2 Heat Pump System for Combined
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