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10:30   Session 11: Screw expanders
Chair: Andreas Brümmer
10:30
20 mins 		System-search SCREW EXPANDERS IN ORC APPLICATIONS, REVIEW AND A NEW PERSPECTIVE
Henrik Ohman, Per Lundqvist
Abstract: Performance of Organic Rankine Cycles is sensitive not only to the entry temperature ratio between heat source and heat sink, but also to the temperature degradation of the heat source flow caused by the heat transfer to the process in pre-heater, evaporator and super heater. In order to adopt the cycle to the great variety of heat sources a multitude of fluids are required. Alternatively fluid mixtures, trans-critical or supercritical fluid conditions can be used to match the process temperatures with the heat source. Screw expanders offer an alternative approach to the matching problem of ORC’s as they allow for multi-phase expansion. Hereby the vapour fraction of the ORC evaporator exit can be used to partially match the temperatures of the process to a particular heat source. To provide a perspective on the use of screw expanders in ORC-systems previous experimental and commercial experience have been reviewed and discussed as well as some of the fundamental challenges in understanding screw expander characteristics. Screw expanders are versatile machines used for the production of mechanical work in power ranges from 3kW to 1.5MW. As the functional characteristics differ significantly from dynamic expanders the explanatory models used to generalise results are different. For dynamic expanders analysis plenty of academic research has resulted in a wide flora of publications and well generalized explanatory models. For screw expanders similar explanatory models exist mainly in commercially confidential environments. A few public sources disclose test data, often analysed using thermodynamic models suitable for dry gas expansion. With minor adaptations to the specifics of the screw expander such analysis typically gives reasonable result, though seldom suitable for detailed understanding of the process. In applications with 2-phase expansion theory used to simulate functional characteristics is entirely insufficient. This is mainly due to the fact that common dynamic expanders are seldom used for multi-phase expansion. This paper describes a review of multi-phase screw expander experiences and explains why a unique theory is required to model its characteristics. In the absence of such a unique theory a correlation is presented, allowing estimation of screw expander efficiency in multi-phase conditions, based on empirical data. By using this correlation measured dry expansion efficiency, or such efficiency simulated, can be used to estimate adiabatic efficiency with expansion entry vapour fractions ranging from 0 to 1. Hence estimating expansion efficiency during multi-phase expansion is simplified, allowing for better optimisation of the ORC-systems.
10:50
20 mins 		System-search DYNAMIC MODEL FOR THE PERFORMANCE PREDICTION OF A TWIN SCREW EXPANDER IN AN ORC
Iva Papes, Joris Degroote, Jan Vierendeels
Abstract: The Organic Rankine Cycle (ORC) is well known and proven technology for waste heat recovery. The current generation of twin screw expanders used for low-grade heat recovery are in fact compressors working in the opposite sense. In this paper a mathematical model for calculating the performance of a twin screw expander is presented. The model is based on geometrical parameters which describe volume and leakage areas for every angular position. With these functions the entire design of a screw expander is determined. The differential equations used in the model are derived from the mass and energy conservation laws and are solved together with the appropriate Equation of State in the instantaneous control volumes. Since R245fa is selected as a working fluid, the Aungier Redlich-Kwong Equation of State has been used. The results of the mathematical model are compared to the 3D Computational Fluid Dynamics (CFD) calculations of the same twin screw expander using the same working fluid. To calculate the mass flow rates through the leakage paths formed inside the screw expander, flow coefficients are considered as constant and they are derived from 3D CFD calculations. The outcome of the mathematical model is the P-V indicator diagram which is compared to CFD results of the same twin screw expander. It is shown that the developed model accurately predicts the performance of the expander.
11:10
20 mins 		System-search COMPARISON OF ORGANIC RANKINE CYCLE SYSTEMS UNDER VARYING CONDITIONS USING TURBINE AND TWIN-SCREW EXPANDERS
Matthew Read, Ahmed Kovacevic, Ian Smith, Nikola Stosic
Abstract: 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.
11:30
20 mins 		System-search EXPERIMENTAL INVESTIGATION OF EFFECT OF OPERATING CONDITIONS ON PERFORMANCE OF ORC SYSTEM BASED ON OIL FLOODED TWIN SCREW EXPANDER
Abhijeet Chougule, Charles Philominraj, Ramakrishna Sonde
Abstract: The paper discusses the following aspects on the experimental investigation of effect of operating conditions on volumetric expander (oil flooded twin screw expander) based ORC system. • Mechanical performance: Experimental investigation of flow induced vibrations at expander inlet and its mitigation and development of design optimization tool • Experimental investigation and analysis of oil separator pressure drop and its effect on volumetric/isentropic efficiency of the expander • Variable speed versus constant speed operation - Effect on isentropic efficiency of expander and cycle efficiency. The variable speed operation gives optimum isentropic efficiency (80 to 85%) at all loads of low potency heat recovery for power generation. • Field experience of 30 and 100 kW screw expander based ORC system
11:50
20 mins 		System-search EXPERIMENTAL STUDY ON ORGANIC RANKINE CYCLE SYSTEM WITH SINGLE-SCREW EXPANDER FOR WASTE HEAT RECOVERY FROM DIESEL ENGINE EXHAUST
Yuting Wu, Biao Lei, Wei Wang, Yeqiang Zhang, Chongfang Ma
Abstract: Two single-screw expander prototypes with 155 mm and 117mm diameter screw were developed. An ORC (organic Rankine cycle) experimental systems for waste heat recovery from diesel engine exhaust were built. Experiments with two single screw expander prototypes were conducted for different expander torque and diesel engine loads. The experimental results indicated: (1) Single-screw expander is suitable for small/medium scale ORC system, it can obtain good performance at low-medium rotational speed. The maximums of power output 10.38 kW and shaft efficiency 57.88% are gotten at 1538 rpm. (2) The maximums of volume efficiency, adiabatic efficiency and expansion ratio of single-screw expander are 90.73%, 73.25% and 4.6, respectively. (3) The performance of ORC system is affected not only by the working case of diesel, but also by the torque of single-screw expander. The biggest ORC efficiency is 6.48%, which is gotten at 250 kW diesel power output and 64.43 N.m of single-screw expander. (4) With ORC system, the specific oil consumption of diesel is effectively decreased. When the power output of diesel is 250 kW, the specific oil consumption is decreased by 3.5%, and the overall system efficiency is 43.8%, which is increased by 1.53%. (5) With the reducing of flow mass rate pumped into evaporator, the dryness of vapor is accelerated, and heat exchange quantity almost linearly decreases. Flow rate of vapor into single-screw expander increases with increase of vapor dryness but volume efficiency decreases with that. The rising of expansion rate is due to increase of inlet pressure and decease of outlet pressure with the increase of vapor dryness and the biggest expansion ratio is 4.7. With the increase of vapor dryness, torque and power output of single-screw expander are rising.
12:10
20 mins 		System-search EXPERIMENTAL CHARACTERIZATION OF SINGLE SCREW EXPANDER PERFORMANCE UNDER DIFFERENT TESTING CONDITIONS AND WORKING FLUIDS
Sergei Gusev, Davide Ziviani, Martijn van den Broek
Abstract: During the last years, one of the most popular way to recover low-grade waste heat is the organic Rankine cycle. This technology is widely studied and continuously optimized and, as a result, there are many efficient installations available on the market utilizing heat with stable parameters such as from geothermal sources or from the biomass combustion process. However, if a variable hot source in terms of either temperature or flow rate is introduced, the expansion devices have to work at non-optimal conditions, which decrease the global efficiency of ORC installations. In order to obtain the performance operation of an expander close enough to the optimum, the influence of pressure ratios, filling factor, and working fluid properties on power output is studied. In this paper, experimental results obtained on small-scale ORC test setup based on an 11 kWe single-screw expander are presented. Two working fluids are used during the tests, i.e. R245fa and SES36 (Solkatherm). These working fluids are common for ORC installations exploiting low-temperature waste heat. The waste heat source is simulated by an electrically heated thermal oil loop with adjustable temperature and flow rate. Various waste heat inlet flow rates are considered in order to find an optimal evaporation pressure and to maximize the power output with different heat source profiles. Based on the experimental data, the expander model is developed. For each working fluid, optimal working conditions are determined. In most cases, there is under-expansion due to a relatively small built-in volume ratio, causing certain losses. By means of the model, the ideal expansion process is simulated and compared with the real one obtained experimentally to quantify these losses and conclusions can be drawn whether significant benefits can be offered by using an optimized expander instead of an “off-the-shelf” reversed compressor.