Multi-vane Expanders Research Articles

Application of the Multi-Vane Expanders in ORC Systems—A Review on the Experimental and Modeling Research Activities

This paper reviews the applications of the multi-vane expanders in ORC (organic Rankine cycle) systems. The operating principle and design of the ORC systems are addressed in the introduction. Then, there is a brief review of the expanders applied in small-power and micro-power ORCs, and a discussion of the multi-vane expander design and operating principle as an introduction to a comprehensive review on the applications of the multi-vane expanders in ORC systems. The different features of the multi-vane expanders—i.e., the design of the expander, its geometrical dimensions and operating conditions, durability, applied working fluid, obtained power output, and efficiency—are analyzed in this paper. This review clearly indicates that multi-vane expanders are a promising alternative to the different types of the expanders applied in ORC systems.

Influence of the Applied Working Fluid and the Arrangement of the Steering Edges on Multi-Vane Expander Performance in Micro ORC System

Micro-power domestic organic Rankine cycle (ORC) systems are nowadays of great interest. These systems are considered for combined heat and power (CHP) generation in domestic and distributed applications. The main issues of ORC systems design is selection of the expander and the working fluid. Thanks to their positive features, multi-vane expanders are especially promising for application in micro-power ORC systems. These expanders are very simple in design, small in dimensions, inexpensive and feature low gas flow capacity and expansion ratio. The application of multi-vane expanders in ORC systems is innovative and currently limited to prototype applications. However, a literature review indicates the growing interest in these machines and the potential for practical implementation. For this reason, it is necessary to conduct detailed studies on the multi-vane expanders operation in ORC systems. In this paper the results of experimental and numerical investigations on the influence of the applied working fluid and the arrangement of the steering edges on multi-vane expander performance in micro ORC system are reported. The experiments were performed using the specially designed lab test-stand, i.e. the domestic ORC system. Numerical simulations were proceeded in ANSYS CFX software (ANSYS, Inc., Canonsburg, PA, USA) and were focused on determining the expander performance under various flow conditions of different working fluids. Detailed numerical analysis of the arrangement of the machine steering edges showed existence of optimal mutual position of the inlet and outlet port for which the multi-vane expander achieves maximum internal work and internal efficiency.

Experimental investigation on multi-vane expander operating conditions in domestic CHP ORC system

Small- and micro-power ORC systems designed for domestic use by prosumers should be cheap and reliable. The main part of the ORC system cost is the price of heat exchangers and the expander. Multi-vane expanders are positive displacement volumetric machines which are nowadays considered for application in micro-power domestic ORC systems. The multi-vane expander design is very simple, which translates into low production costs. Compared to the other types of volumetric machines and micro turbines, which are adopted in micro-power domestic ORC systems, multi-vane machines feature a lower gas flow capacity, lower expansion ratios, and an advantageous ratio of the power output to the external dimensions. These machines are insensitive to the negative influence of the gas-liquid mixture expansion. Moreover, the multi-vane expander can be easily hermetically sealed, which is one of the key issues in the ORC system design. Multi-vane expanders feature power outputs of several hundred W to approximately 5 kW. Maximum gas pressure on the inlet to multi-vane expander reaches approximately 10 bar. The issues concerning the application of multi-vane expanders in such systems are innovative and not fully scientifically described. The solution of these problems requires comprehensive study and experimental analysis.This paper reports the results of experimental investigations carried out on domestic CHP ORC system adopting multi-vane expander under varied operating conditions. The experimental results showed that the expander indicated work varies in the range of 0.96—4.18 kJ/kg while its internal efficiency varies in the range of 17.2—58.3 % depending on the experimental conditions. Moreover, the results of numerical investigations on multi-vane expander operating conditions are presented. The numerical model of the expander was validated using the data obtained from the experiment and analyses were performed in ANSYS CFX software. The results proved that multi-vane expanders are suitable for domestic ORC systems and are a promising alternative to the other types of volumetric expanders used in such systems.

Numerical modelling of multi-vane expander operating conditions in ORC system

Multi-vane expanders are positive displacement volumetric machines which are nowadays considered for application in micro-power domestic ORC systems as promising alternative to micro turbines and other volumetric expanders. The multi-vane expander features very simple design, low gas flow capacity, low expansion ratios, an advantageous ratio of the power output to the external dimensions and are insensitive to the negative influence of the gas-liquid mixture expansion. Moreover, the multi-vane expander can be easily hermetically sealed, which is one of the key issues in the ORC system design. A literature review indicates that issues concerning the application of multi-vane expanders in such systems, especially related to operating of multi-vane expander with different low-boiling working fluids, are innovative, not fully scientifically described and have the potential for practical implementation. In this paper the results of numerical investigations on multi-vane expander operating conditions are presented. The analyses were performed on three-dimensional numerical model of the expander in ANSYS CFX software. The numerical model of the expander was validated using the data obtained from the experiment carried out on a lab test-stand. Then a series of computational analysis were performed using expanders' numerical model in order to determine its operating conditions under various flow conditions of different working fluids.

The Influence of the Heat Source Temperature on the Multivane Expander Output Power in an Organic Rankine Cycle (ORC) System

Organic Rankine Cycle (ORC) power systems are nowadays an option for local and domestic cogeneration of heat and electric power. Very interesting are micropower systems for heat recovery from low potential (40–90 °C) waste and renewable heat sources. Designing an ORC system dedicated to heat recovery from such a source is very difficult. Most important problems are connected with the selection of a suitable expander. Volumetric machines, such as scroll and screw expanders, are adopted as turbine alternative in small-power ORC systems. However, these machines are complicated and expensive. Vane expanders on the other hand are simple and cheap. This paper presents a theoretical and experimental analysis of the operation of a micro-ORC rotary vane expander under variable heat source temperature conditions. The main objective of this research was therefore a comprehensive analysis of relation between the vane expander output power and the heat source temperature. A series of experiments was performed using the micropower ORC test-stand. Results of these experiments are presented here, together with a mathematical description of multivane expanders. The analysis presented in this paper indicates that the output power of multivane expanders depend on the heat source temperature, and that multivane expanders are cheap alternatives to other expanders proposed for micropower ORC systems.

The Application of Rotary Vane Expanders in Organic Rankine Cycle Systems—Thermodynamic Description and Experimental Results

Small (10–100 kW) and micro (0.5–10 kW) Organic Rankine Cycle (ORC) power systems are nowadays considered for local and domestic power generation. Especially interesting are micropower applications for heat recovery from dispersed low potential (85–150 °C) waste and renewable heat sources. Designing and implementing an ORC system dedicated to energy recovery from such a source is difficult. A proper working fluid must be selected together with a suitable expander. Volumetric machines can be adopted as a turbine alternative in small-capacity applications under development, like, e.g., domestic cogeneration. Scroll and screw expanders are a common choice. However, scroll and screw expanders are complicated and expensive. Vane expanders are mechanically simple, commercially available and cheap. This paper documents a study providing the preliminary analysis of the possibility of employing vane-expanders in mini-ORC systems. The main objective of this research was therefore a comprehensive analysis of the use of a vane expander for continuous operation with a low-boiling working fluid. A test-stand was designed and set up starting from system models based on thermodynamic analysis. Then, a series of experiments was performed using the test-stand. Results of these experiments are presented here, together with a model of multivane expanders and a thermodynamic-based method to select the working fluid. The analysis presented in this paper indicates that multivane expanders are a cheap and mechanically simple alternative to other expansion devices proposed for small-capacity ORC systems.

Multi-Vane Expanders as Prime Movers in Low-Grade Energy Engines

Multi-vane expanders possess many advantages, over turbines and other positive-displacement machines, as prime movers for low-grade energy engines of small power outputs. It has been found that the performances of such expanders can be improved considerably by optimizing their design and operating variables. For an organic Rankine-cycle engine, utilizing low-temperature heat as the input, a multi-vane expander with isentropic efficiencies exceeding 73 per cent at rotational speeds up to 3000 r/min was designed, built and demonstrated.

Selection of operating conditions and optimisation of design parameters for multi-vane expanders

Multi-vane expanders possess many advantages as prime movers for organic Rankine-cycle engines utilising low-grade heat as the energy inputs. The performances of these devices can be improved considerably by the proper selection of the operating conditions for existing units and by optimising their designs proposed for specified applications. Computer programs, composed for predicting the behaviours of multi-vane expanders, were employed to ascertain the most suitable operating conditions for two existing designs. A simple formula for selecting the optimal working pressure ratios of the expanders, using R-113 as the working fluid, is presented. Optimal values of the design parameters of the expanders were determined for selected operating conditions. The performances of the optimised designs of the considered expanders are relatively insensitive to changes in the operating conditions of the machines in the vicinity of their optimised values. This is a desirable feature of prime movers in Rankine engines utilising fluctuating low-grade energy sources, such as solar energy.

Influences of vane design and lubricant on a multi-vane expander's performance

Multi-vane expanders possess advantages as prime movers for organic Rankine-cycle engines stimulated by low-grade heat, particularly for low-power output applications. The behaviours of the vanes in these devices influence radically their overall performances. In this study, the effects of the percentage of the lubricating oil mixed with the working fluid, the arrangement of the pressurising slots machined into the vanes and the vane material upon the performance of a multi-vane expander are investigated. The experimental results indicate that the performances and operation reliabilities of multi-vane expanders can be improved considerably by optimising these design and operational variables.

Optimal design and operating conditions for a multi-vane expander

Multi-vane expanders (MVEs) possess several advantages over turbines and other positive-displacement machines (i.e. reciprocating or screw expanders) as prime movers for low grade energy engines, particularly for systems with low (< 10 kW) power outputs. The performances of such devices can be improved considerably by controlling their operating conditions and by optimising their designs. A suite of computer programs developed for simulating a MVE's behaviour was employed to predict the most appropriate operating conditions and design for a non-circular MVE. A prototype of the optimised expander design was built and operated. It exhibited isentropic efficiencies in excess of 73 per cent at rotational speeds up to 3000 rpm. This is comparable with those of the most advanced rival prime movers for the considered ranges of speed and power output.

Multi-vane expanders: Vane dynamics and friction losses

Mechanical friction and viscous drag represent major causes of power loss in multi-vane expanders. So a mathematical model has been developed to describe these loss phenomena. The constructed dynamic model was employed to investigate the loss of contact between the vanes and the stator-cylinder, which results in significant internal leakage losses. Two existing designs of multi-vane expander were considered in detail when using R-113 as the working fluid. The composed computer sub-routines were used to predict the effects of the different design parameters and operating conditions on the mechanical efficiencies of the expanders. The predictions obtained indicate that most of the frictional power loss in a multi-vane expander occurs due to the rubbing of the vanes against the stator-cylinder. Either the operating conditions for an existing expander can be controlled, or the optimal design parameters of a proposed expander for a particular application can be selected, in order to maximise the appropriate multi-vane expander's mechanical efficiency.

Multi-vane expanders: Geometry and vane kinematics

A multi-vane expander (MVE) offers considerable promise as a prime mover for organic Rankine-cycle engines utilising solar energy or waste heat to provide the power input. 1 Therefore, mathematical models describing an expander's behaviour have been constructed; these models are applicable to a wide range of MVEs. For two existing designs, the geometrical characteristics and the vane kinematics have been evaluated using the computer sub-routines composed. The results indicate that the neglect of the vanes' thickness when evaluating the geometrical characteristics of the expanders can lead to considerable errors in their predicted volume expansion ratios, especially when numerous vanes and small inlet angles are employed. The expected effects of the geometrical parameters of the expanders on their performances are discussed.

Multi-vane expander performance: Breathing characteristics

The throttling of the working fluid flow at the inlet and exhaust port openings of multi-vane expanders leads to power losses. Therefore, a mathematical model has been developed to analyse their breathing characteristics. In particular two existing designs of multi-vane expanders, using refrigerant 113 as the working fluid, were considered. The results obtained indicate that multi-vane expanders exhibit easy-breathing behaviours. Improved breathing characteristics can be achieved by the wise selection of operating conditions for any existing design or by optimising the design parameters for a specific application.

Performances of multi-vane expanders

Multi-vane expanders possess many advantages as prime movers for organic Rankine-cycle power systems utilising low grade energy as the heat inputs, particularly for low power outputs (i.e. <10kW). The performances of these devices can be improved considerably by optimising their design parameters and by controlling their operating conditions. To achieve this, mathematical models were developed and computer codes composed for predicting the performance characteristics of multi-vane expanders, as functions of their design and operating variables. The constructed computer programs were applied to two existing designs. Also, a series of tests was carried out on the two considered expanders, using the low grade energy organic Rankine-cycle test rig at Cranfield Institute of Technology. The testing of the expanders was conducted in parallel with the theoretical analysis in order to determine the operational constraints and practical coefficients required for the computer simulation and to verify the validity of the models developed. The analytical predictions of the performance characteristics of the expanders have been compared with experimentally obtained observations. Due to the nature of the simplifying assumptions, a power correction factor was needed to match the analytical predictions with the experimental results to within a ±22 per cent relative deviation zone, over the investigated range of operating conditions.

Multi-vane expanders: Internal-leakage losses

Secondary flows of the working fluid via internal leakage paths in an MVE contribute significantly to its efficiency reduction. So the major leakage channels were indentified, and a mathematical model for predicting the flow rates via them has been developed. The behaviours of two existing designs of MVEs, using R-113 as the working fluid, were considered. Computer sub-routines were composed and employed to predict the effects of changes in the design and operating variables on the leakage characteristics of the expanders. The predictions indicate that the major internal-leakage flows, in the two MVEs considered, occur past the tips of the vanes. By maintaining continuous contact between the tips of the vanes and the stator cylinder, leakage efficiencies of over 75% are expected for the non-circular expander. The predictions also show by how much the leakage characteristics can be improved by running an existing expander at its optimal operating conditions. Alternatively for a proposed application, there will be an optimal choice of design parameters for an MVE so that the maximum efficiency can be achieved.

Multi-vane expanders as prime movers for low-grade energy organic Rankine-cycle engines

The behaviour of the prime mover in a low-grade energy Rankine-cycle engine is one of the most important factors affecting the overall system performance. A survey of published data concerning various types of expander in use in Rankine engines and an analysis based on the concept of similarity shows that, for the low power outputs, positive-displacement expanders have potential advantages when compared with turbines. However, the multi-vane expander (i.e. the MVE) offers considerable promise as the most appropriate prime mover for organic Rankine-cycle engines utilizing solar energy or waste heat as energy inputs, especially in the developing countries. The features and characteristics of the appropriate MVE's are discussed, together with the operational problems remaining to be solved.