A system for controlled recovery of thermal energy and conversion to mechanical energy. The system collects thermal energy from a reciprocating engine, specifically from engine jacket fluid and/or engine exhaust and uses this thermal energy to generate a secondary power source by evaporating an organic propellant and using the gaseous propellant to drive an expander in production of mechanical energy. A monitoring module senses ambient and system conditions such as temperature, pressure, and flow of organic propellant at one or more locations; and a control module regulates system parameters based on monitored information to optimize secondary power output. A tertiary, or back-up power source may also be present. The system may be used to meet on-site power demands using primary, secondary, and tertiary power.