It has been proposed that the very hot air trapped inside the attic of a house in the summer may be used as the energy source for a passive water heater installed in the attic. Energy costs associated with heating the cool water and air conditioning the house are both reduced. Ten thermosyphons, similar to that of Problem 10.68, are inserted in the bottom of a well-insulated water heater. Each thermosyphon has a condensing section that is L_c = 50 mm long, an insulated section that is of length L_i = 40 mm, and a boiling section that is L_b = 30 mm long. The diameter of each thermosyphon is D = 20 mm. The working fluid within the thermosyphons is water at a pressure of p = 0.047 bars.

  (a) Determine the heating rate delivered by the 10 thermosyphons when boiling occurs at 25% of the CHF. What are the mean temperatures of the boiling and condensing sections?

  (b) At night the attic air temperature drops below the temperature of the water. Estimate the heat loss from the hot water tank to the cool attic, assuming losses through the tank insulation are negligible and the stainless steel tube wall thickness of each thermosyphon is very small.