COMPARISON OF ORGANIC RANKINE CYCLE SYSTEMS UNDER VARYING CONDITIONS USING TURBINE AND TWIN-SCREW EXPANDERS

asme-orc2015 Tracking Number 131

A multi-variable optimization program has been developed to investigate the performance of Organic Rankine Cycles (ORCs) for low temperature heat recovery applications. This cycle model contains detailed thermodynamic models of the system components, and the methods used to match the operation of the expander to the requirements of the cycle are described. Two types of ORC system are considered; one containing a turbine to expander dry saturated or superheated vapour, and one with a twin-screw machine allowing expansion of partially evaporated fluid [1]. Modelling of the ORC system with a twin-screw expander has been described in previous papers [2, 3]. The performance of the turbine in the superheated ORC has been modelled using available operational data for single stage, reaction turbines, where correlations have been used to estimate the efficiency of the turbine at ‘off-design’ conditions using either fixed or variable nozzle geometries. The capability of the cycle model has been demonstrated for the case of heat recovery from a source fluid at 120°C. The system parameters are optimised for a typical operating condition, which determines the required size of heat exchangers and the expander characteristics. Performance at off-design conditions can then be optimized within these constraints. This allows a rigorous investigation of the effect of air temperature variation on the system performance, and the seasonal variation in net power output for the turbine and twin-screw ORC systems is estimated. REFERENCES [1] H. Leibowitz, I.K. Smith, N. Stosic, “Cost Effective Small Scale ORC Systems for Power Recovery from Low Grade Heat Sources”, Proceedings of IMECE2006, ASME, p. 521-527, 2006. [2] M. Read, I.K. Smith, N. Stosic, “Multi-Variable Optimisation of Wet Vapour Organic Rankine Cycles with Twin-Screw Expanders”, 22nd International Compressor Engineering Conference at Purdue, Paper 2359, 2014. [3] M. Read, I.K. Smith and N. Stosic, “Effect of Air Temperature Variation on the Performance of Wet Vapour Organic Rankine Cycle Systems”, Transactions of the 38th GRC Annual Meeting, Portland OR, 2014.