Analysis of a rotating spool expander for Organic Rankine Cycle applications

Increasing interest in recovering or utilizing low-grade heat for power generation has prompted a search for ways in which the power conversion process may be enhanced. Amongst the conversion systems, the Organic Rankine Cycle (ORC) has generated an enormous amount of interest amongst researchers and system designers. Nevertheless, component level technologies need to be developed and match the range of potential applications. In particular, technical challenges associated with scaling expansion machines (turbines) from utility scale to commercial scale have prevented widespread adoption of the technology. In this regard, this work focuses on a novel rotating spool expansion machine at the heart of an Organic Rankine Cycle. A comprehensive, deterministic simulation model of the rotating spool expander is developed. The comprehensive model includes a detailed geometry model of the spool expander and the suction valve mechanism. Sub-models for mass flow, leakage, heat transfer and friction within the expander are also developed. Apart from providing the ability to characterize the expander in a particular system, the model provides a valuable tool to study the impact of various design variables on the performance of the machine. The investigative approach also involved an experimental program to assess the performance of a working prototype. In general, the experimental data showed that the expander performance was sub-par, largely due to the mismatch of prevailing operating conditions and the expander design criteria. Operating challenges during the shakedown tests and subsequent sub-optimal design changes also detracted from performance. Nevertheless, the results of the experimental program were sufficient for a proof-of-concept assessment of the expander and for model validation over a wide range of operating conditions. The results of the validated model reveal several interesting details concerning the expander design and performance. For example, the match between the design expansion ratio and the system imposed pressure ratio has a large influence on the performance of the expander. Further exploration shows that from an operating perspective, under-expansion is preferable to over-expansion. The model is also able to provide insight on the dominant leakage paths in the expander and points to the fact that this is the primary loss mechanism in the current expander. Similar insights are obtained from assessing the sensitivity of various other design variables on expander performance. Based on the understanding provided by the sensitivity analysis, exercising the validated model showed that expander efficiencies on the order of 75% are imminently possible in an improved design. Therefore, with sufficient future development, adoption of the spool expander in ORC systems that span a 50 kW – 200 kW range is broadly feasible