Comparison of Organic Rankine Cycle Systems under Varying Conditions Using Turbine and Twin-Screw Expanders

Abstract

Abstract:A multi-variable optimization program has been developed to investigate the performance of Organic Rankine Cycles (ORCs) for low temperature heat recovery applications using both turbine and twin-screw expanders when account is taken of performance variation due to changes in ambient conditions. The cycle simulation contains thermodynamic models of both types of expander. In the case of the twin-screw machine, the methods used to match the operation of the expander to the requirements of the cycle are described. The performance of turbine expanders in a superheated ORC has been modelled using correlations derived from operational data for single stage reaction turbines to predict the turbine efficiency at "off-design" conditions. Several turbine configurations have been considered including variable nozzle area and variable speed. The capability of the cycle model has been demonstrated for the case of heat recovery from a steady source of pressurized hot water 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. A case study is presented for a low temperature heat recovery application with system electrical power output of around 100 kWe at design conditions. The results indicate that similar overall performance can be achieved for ORC systems using either type of expander.

Hybrid Electric Vehicle Performance with Organic Rankine Cycle Waste Heat Recovery System

“…These vehicles not only consume fuel in an excessive manner, but also produce great amounts of hazardous emissions, including NOx and CO 2 gases. The implementation of a waste heat recovery system in such vehicles can offer energy recovery benefits in various forms that can further increase the efficiency of the powertrain by approximately 10-15% and also reduce harmful emissions [4,9,14,25].…”

“…As far as the safety and environmental aspects are concerned, the toxicity and flammability of the fluid concerns engineers, while the global warming potential and ozone depletion danger are the major dangers for the environment. The working fluids should compromise among several criteria specified below [10,11,20,21,25,[30][31][32][33][34] Considering all of aforementioned requirements, R245fa was selected as the working fluid in this study entirely considering prior experience and potential for widespread use. This organic fluid has no ozone impact (ODP), low global warming impact (GWP), it is non-flammable, and its thermodynamic properties fulfill the above criteria.…”

Numerical Modelling and Experimental Validation of Twin-Screw Expanders

“…The thermal efficiency of ORCs can generally be increased by allowing a higher mean temperature of heat addition (in accordance with Carnot's principle) or by reducing the mean temperature of heat rejection. By allowing expansion to take place within the two-phase region, the ORC system can achieve a higher mean temperature of heat addition to increase cycle efficiency and to avoid the requirement to desuperheat the working fluid before the condenser, thereby reducing the mean temperature of heat rejection [3]. Allowing two-phase conditions at the expander inlet also reduces the Energies 2020, 13, 4700 2 of 13 constraints due the heat exchanger minimum temperature difference, leading to better temperature matching of the heat source and working fluid.…”

Numerical Analysis of an Organic Rankine Cycle with Adjustable Working Fluid Composition, a Volumetric Expander and a Recuperator

“…The primary effect of this is that the pressure ratio at the expander inlet is reduced compared to the ideal case. A preliminary investigation assuming a 5% pressure drop in each heat exchanger, as was carried out in [34], indicating that in a recuperative cycle this would cause an 18% decrease in pressure ratio, and a 14% decrease in expander power, while the evaporator heat load remained relatively constant. For a non-recuperative cycle, the pressure ratio decreased by 14%, causing a 10% drop in expander power.…”

“…The isentropic efficiency of these devices drop off sharply if a pressure ratio different to that defined by an adiabatic expansion process between these volumes is imposed [32][33][34]. This pressure ratio is hereafter referred to as the "designed pressure ratio".…”