A system for controlled recovery of thermal energy and conversion to mechanical energy. The system collects thermal energy from a reciprocating engine (for example, from engine jacket fluid) and may also collect further thermal energy from a natural gas compressor (for example, from compressor lubricating fluid). The collected thermal energy is used to generate secondary power by evaporating an organic propellant and using the gaseous propellant to drive an expander in production of mechanical energy. Secondary power is used to power parasitic loads, improving energy efficiency of the system. A supplementary cooler may provide additional cooling capacity without compromising system energy efficiency.