$(4\mathrm{.}4)$ It has been proposed to use the thermal gradient of the ocean to drive a heat engine. Suppose that at a certain location the water temperature is $22$ at the ocean surface and $4$ at the ocean floor.$$

What is the maximum possible efficiency of an engine operating between these two temperatures?

If the engine is to produce $1$GW of electrical power, what minimum volume of water must be processed $($to suck out the heat$)$ in every second?