A thermal storage is made of a granite rock bed of 10 m3 which is heated to

Question:

A thermal storage is made of a granite rock bed of 10 m3 which is heated to 425 K using solar energy. A heat engine receives heat from the bed and rejects the waste heat to the ambient surroundings at 290 K. During the process, the rock bed cools down and as it reaches 290 K, the engine stops working. The heat transfer from the rock is given as QH = ΔE. Determine
(a) The magnitude of the heat transfer (QH) and the maximum thermal efficiency (ηth) at
(b) The beginning
(c) The end of the process.