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3 – Theoretical Background and System Analysis The temperature gradient between the water reservoirs is denoted the thermocline, and the extent of the thermocline zone is mainly depending on the initial temperature difference (TS-TCW) and the duration of the tapping and charging periods. Figure 3.27 shows the principle of a thermocline in a DHW tank, where VTC-CW is the city water volume that has been heated by the DHW reservoir, VTC-S is the DHW volume that has been cooled by the city water reservoir, VTC is the total volume of the thermocline zone and Tm is the average thermocline temperature. Figure 3.27 Principle of the thermocline, i.e. the temperature gradient, between the hot and cold water reservoirs in a DHW tank. DHW tank VTC Thermocline zone, height HTC Temperature V'S TS VTC-S VTC-CW V'CW TCW TCW Tm TS VTC-CW and VTC-S are identical as long as there is no mixing of hot and cold water, the heat transfer through the tank walls is neglected and the density, the specific heat capacity and the thermal conductivity of water are regarded independent of temperature. At temperatures ranging from 10 to 80oC, these properties will deviate by less than ±1.5%, ±0.2% and ±6.5% from their average values, respectively (NIST, 2000). The total volume of the thermocline zone is calculated as: VTC = (VTC−CW + VTC−S )= ADHW ⋅HTC (3.27) were ADHW is the cross-sectional area of the DHW tank and HTC is the total height of the thermocline zone. The general conduction equation (Kreith and Black, 1980) can be used to determine the transient temperature gradient between the hot and cold water reservoirs, and ultimately the extent and the average temperature of the thermocline zone: 60PDF Image | Residential CO2 Heat Pump System for Combined
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