Waste heat to power systems is an essential part of the sustainable future, allowing otherwise wasted heat to produce useful electricity. Depending on system scale, these may be steaming Rankin cycles, Stirling engines, or more commonly Organic Rankin Cycle (ORC) systems using turbines, pistons, scroll type mechanical expanders. A new rotary reverse Brayton cycle heat pump with expander isentropic efficiencies of >98% offers the potential of a new type of waste heat to power system mechanical system. The basics of this rotary heat pump, operated in reverse, are modelled as a heat engine. This simple system allows ORC and transcritical ORC heat engine systems in a single system, allowing the system to adapt and be optimized for varying heat supply. A system model is developed, and the effects of various parameters are assessed. The rotary system with one moving part is modelled with the effects of RPM, radius, geometry parameters, and heat flow. The rotary heat exchanger geometry is modelled and optimized to provide optimum heat exchange with the radius allowing changing pressure within the heat exchanger and approach temperatures of 2C. Modelling indicates the system will offer superior performance compared to current heat to power systems with second order efficiencies of 50% or greater. Initial assessment of potential mechanical arrangements and future modelling and development is discussed.