US Patent 7610964 Positive displacement pump

A submersible pumping system for use downhole that includes a housing containing an expandable fluid, that when expanded pushes a piston that in turn pumps wellbore fluid to the surface. The expandable fluid can be a silicon based heat transfer fluid with a coefficient of thermal expansion of at least about 0.0005 in3/in3/° F. The expandable fluid is expanded upon exposure to heat. A heat source is selectively activated for expanding the fluid.