This paper addresses the early conceptualization of a system for reversible heat/work conversion based upon the heat engine cycle, developed in 1833 by John Ericsson, in combination with utility scale thermal energy storage in particulate mineral (e.g., gravel) and the development and test of the first prototype. Using these test results, mathematical modeling of the engine/heat pump has yielded improved second and third prototypes. Design of the second prototype and its behavior under test is discussed. Extant test results are used to extrapolate to the predicted performance of utility scale equipment.