An engine testing system comprises a rotary absorber that provides a variable resistance to an engine under test and heats coolant from a cold reservoir. A hot reservoir coupled to rotary absorber stores the heated coolant for later (or concurrent) use. Moreover, an organic Rankine cycle turbine-generator device is coupled to the hot reservoir, which converts heat from heated coolant into electrical power. A conditioning system is coupled to the organic Rankine cycle turbine-generator device that cools the coolant for storage in the cold reservoir. The available captured waste energy may be augmented with waste energy that is also available during engine testing. The additional waste energy may be in the form of exhaust gases, thrust, heat from engine coolant systems, residual engine heat, radiant or convective waste heat, friction etc. Variations of the above system may replace the primary energy re-capture from other than a rotary absorber.