Compressed air external combustion engines

I personally feel that the genius of the external combustion engines should not be underestimated. air, water as the working fluids is something we can all appreciate.

This site gives an interesting coffee-table type discussion on why the swtch to compressed air ext. combustion engines would be great.

I myself have been thinking about large engines with tapering combustion chambers. Fill gallons of water into an enormous tank, heat it up with a jolt of high power-density raw heat (from the sun, naturally), perform your rankine cycle, and then dump the water back into a water body. Great thing to have by a lake, I would imagine. Would love to hear peoples' thoughts on this one.