Abstract: A high efficiency shell and tube heat exchanger coupled to a hydraulic thermal engine includes an insulated cylindrical shell having first and second ends for conveying a coolant and a plurality of tubes passing through at least one of the ends for conveying a working fluid. A plurality of spaced-apart generally-transverse baffles are disposed in the shell. Each baffle is truncated along one edge defining a slot for passage of coolant along the shell inner wall and past the baffle. Successive of the baffles are rotated with respect to the longitudinal axis of the shell to cause the coolant flowing therethrough to follow a non-axial path. Multiple coolant inlets and outlets may be provided. The heat exchanger is optimized for a use wherein the second working fluid is liquid CO2 in a supercritical fluid state. The heat exchanger tubes have an inside diameter of 0.26 inches or less.