An energy storage system is proposed to absorb thermal

Chapter , Problem 11.52

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An energy storage system is proposed to absorb thermal energy collected during the day with a solar collector and release thermal energy at night to heat a building. The key component of the system is a shelland-tube heat exchanger with the shell side filled with n-octadecane (see 8.47). (a) Warm water from the solar collector is delivered to the heat exchanger at Th,i 40C and 2 kg/s through the tube bundle consisting of 50 tubes, two tube passes, and a tube length per pass of Lt 2 m. The thin-walled, metal tubes are of diameter D 25 mm. Free convection exists within the molten n-octadecane, providing an average heat transfer coefficient of ho 25 W/m2 K on the outside of each tube. Determine the volume of noctadecane that is melted over a 12-h period. If the m total volume of n-octadecane is to be 50% greater than the volume melted over 12 h, determine the diameter of the Ls 2.2-m-long shell. (b) At night, water at Tc,i 15C is supplied to the heat exchanger, increasing the water temperature and solidifying the n-octadecane. Do you expect the heat transfer rate to be the same, greater than, or less than the heat transfer rate in part (a)? Explain your reasoning.