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Fig.3 shows the schematic of the packed-bed TES testing system, which consists of a blower, an air heating furnace, the packed-bed TES device mentioned above, regulating valves and some testing apparatus. During the charging process, the ambient air pumped from the blower can be heated to 900°C by the 60 kW electric heating furnace, and then the hot air enters from the top of the TES device, flows downwards through the packed bed to charge the ceramic balls. Charging from the top allows the full use of the air buoyancy effect to create and maintain thermal stratification inside the packed bed, keeping the hottest region at the top and the coldest bottom. While during discharging, the direction of the flow is reversed as cold air is circulated through the tank from the bottom. During the testing processes, the temperatures both of air throughout the system and ceramic balls in packed bed are measured by 43 K-type or S-type thermocouples, of which 30 are arranged inside the packed bed, vertically 5 measurement sections in every 0.5-m high, and radially 6 thermocouples in each measurement section, notably that the thermocouples at the height of 2.5 m are not covered with ceramic balls, shown as Fig.4. In addition, other measurements include: ambient air temperature, the air flow rates at the inlet and outlet of TES system, and pressure drop when the air flowing through the packed bed. The thermocouples have an accuracy of ±0.5%. The flow meters installed at the inlet of system is a volumetric flow meter with a measuring range of 15~300m3/h and outlet a vortex-shedding flow meter with range of 80~800m3/h, both of which share a common accuracy of ±0.5%. (a) (b) FIGURE 4 Distribution of testing sections in packed bed (a) and testing points in each section (b) NUMERICAL MODEL Hypothesis and Modeling Since the transient temperature of air and ceramic balls inside the TES device is the key parameter indicating the TES performance, the energy balance analysis for air and ceramic balls is to set up with governing equations for the temperature of both phases. Shown in fig.4 is a one dimensional control volume in size dz in the packed bed. Several suitable assumptions based on the actual experimental conditions are typically made to simplify the analysis of the TES process: 1. The radial distribution of air and ceramic balls in the TES tank is assumed to be uniform, thus the problem is simplified to be a one-dimensional one along the height of the tank. 2. The Biot number of the transient heat conduction in a single ceramic ball is small enough (Bi<0.1) that lumped heat capacitance method is applicable to the heat conduction in solid phase. 3. Contacts between ceramic balls are point contacts, thus heat conduction between ceramic balls are negligible. 4. The heat conductivity of air is so low that axial heat conduction of fluid phase is negligible. 080027-3PDF Image | packed-bed thermal energy storage device
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