Modelling and preliminary design of a variable-BVR rotary valve expander with an integrated linear generator

The Organic Rankine Cycle (ORC) is currently one of the most suitable technologies to convert waste heat into me- chanical work or electricity. While large and medium scale systems are widely available on the market for various temperature and power ranges, small-scale ORCs below 50 kWe are still in a pre-commercial phase because of the relatively high specific cost per kW and the lack of technologically mature and high efficient expanders. Small-scale ORC installations for automotive applications operate at variable heat source profiles combined with the fluctuating power demand from a vehicle. The prediction of an optimum operating point is challenging. Exhaust gases are a limited heat source, therefore the more heat is recovered at an optimal cycle efficiency level, the more power is produced. By using advanced cycle architectures (e.g. trilateral ORCs, partial-evaporating ORCs, zeotropic mixture ORCs, etc.) and the right fluids, an optimum can be found. An expander with a variable built-in volume ratio (BVR) can allow to operate at optimal conditions within the whole range of pressures imposed by the variable heat source and heat sink. Adjustable expanders are known but mainly limited to large-scale applications. Neither a positive displacement expander, nor a turbine can provide an optimal expansion of a working fluid in a wide range of operation conditions. As a response to this challenge, the concept of a variable-BVR piston expander with an integrated linear generator is proposed in this paper. The internal part-load control is based on a rotary valve which controls the suction and discharge processes in the expander. An analytic model has been developed to relate the position of the valve with the motion of the piston. By means of a deterministic model, the influence of the main design parameters is investigated. A preliminary design based on the expander model results is described and the predicted performance over the operating range of interest is discussed