Twin-screw Expander Research Articles

Modelling and off-design performance optimisation of a trilateral flash cycle system using two-phase twin-screw expanders with variable built-in volume ratio

This research work presents a numerical chamber model of a two-phase twin-screw expander and its further integration in a one-dimensional model of a Trilateral Flash Cycle (TFC) system for low-grade heat to power conversion applications. The novel feature of the expander is the capability of changing the built-in volume ratio (BIVR) of the machine through a sliding valve in the casing that opens an additional suction port. Lowering the BIVR from 5.06 to 2.63 results in an improvement of the volumetric efficiency from 53% to 77% but also in a reduction of the specific indicated power from 4.77 kJ/kg to 3.56 kJ/kg. Parametric analysis on several degrees of freedom of the full TFC system concluded that expander speed and BIVR are the variables that mostly impact the net power output of the unit. An optimisation study enabled the net power output of the TFC system, at design point, to increase from 81 kW to 103 kW.

Power Recovery in a Gas Pressure Reduction Station using 3D CFD Modeling of a Twin Screw Expander

Power Recovery in a Gas Pressure Reduction Station using 3D CFD Modeling of a Twin Screw Expander

A renewable energy scenario for a new low carbon settlement in northern Italy: Biomass district heating coupled with heat pump and solar photovoltaic system

In the framework of the building sector de-carbonization, the case of a new nearly zero-energy district, located in the Milan urban area (Italy), is presented. In particular, the scope of the work is to demonstrate that the proposed energy concept, based on the combination of low-energy building design and high-efficiency technical systems, allows the reduction of the final energy uses. After the evaluation of the energy needs, a low-temperature and small-size wood biomass district thermal plant was designed to be integrated with groundwater heat pumps (GWHP) and solar photovoltaic (PV) systems, taking up the challenge to design an almost full-renewable urban district by means of a Multi-Energy System. The core is a biomass boiler coupled with a small combined heat and power (CHP) unit with a twin-screw steam expander (TSSE). The heat produced by the CHP satisfies a consistent fraction of space heating and domestic hot water (DHW) needs during the winter season. GWHPs coupled with PV satisfy remaining thermal needs in winter and the entire thermal needs in mid-season and in the summer period. The obtained outcomes prove the benefits of the combination of a wood biomass CHP with GWHPs and PV with a significant share of renewable energies. • Provide an overview of the evolution of district heating networks. • Propose a solution for energy decarbonization of new settlements. • Show how biomass CHP, PV systems and heat pumps may be successfully integrated. • Propose an effective control strategy to enhance local-RES self-consumption.

Advanced ORC architecture for geothermal combined heat and power generation

Several approaches to enhance the performance of Organic Rankine Cycles (ORC) have been investigated in literature, including architecture, working fluid selection and part-load optimization together with combined heat and power (CHP) generation. To contribute to these developments, the design and the first operation of an advanced ORC-CHP architecture is being presented in this study. This architecture extends the state of the art by a two-stage concept with turbine bleeding and a regenerative direct contact preheater. The aim of this architecture is to increase the heat source utilization, flexibility and part-load efficiency. To evaluate the performance of this concept, a test rig has been constructed. It is heated with a 200 kW electrical heater and a twin-screw expander is being used. The low-GWP fluid R1233zd(E) is applied as working fluid. In order to analyze the system in its full operating range, experiments are conducted with varying heat loads of the district heating network. Furthermore, an operating strategy of the system is being developed. With this first operation, a very high operational range of the novel ORC-CHP architecture down to a minimum load of 15.3% is being demonstrated. Furthermore, is could be proven that thermal efficiency increases during part-load operation.

Loss analysis of oil-free twin-screw expanders for recovering energy in fuel cell systems by means of p-θ diagrams

Twin-screw expanders extend their application and become more and more favored in fuel cells. For twin-screw expanders in fuel cells, pressure loss is a key factor to affect their performance, and thus is investigated by means of the indicator diagram in this paper. Hence, a series of pressure transducers were arranged in consecutive positions of the casing to measure the pressure variation inside working chamber of three expanders with different suction port area and rotor length. By analyzing the measured p-θ diagrams, the features of working process are identified. Moreover, the relevant influence mechanisms of rotating speed and suction back pressure on losses including suction loss and mechanical loss are discussed. Results show that the suction throttling pressure loss and leakage shows a significant influence on the expander performance. The suction port area is suggested to be larger than the maximum value of filling area, and it had better to set both radial and axial filling port in order to decrease the pre-expansion pressure loss. Finally, a method to judge the operating state of expanders according to the measured p-θ diagrams is proposed. Results indicate that the rotor length of 60 mm may be just suitable for the developed expanders.

Compressed air energy storage facility with water tank for thermal recovery

The paper presents the prototype of the first Romanian Compressed Air Energy Storage (CAES) installation. The relatively small scale facility consists of a twin-screw compressor, driven by a 110 kW threephase asynchronous motor, which supplies pressurized air into a 50m3 reservoir, of 20 bar maximum pressure. The air from the vessel is released into a twin-screw expander, whose shaft spins a 132 kW electric generator. The demonstrative model makes use of a 5m3 water tank acting as heat transfer unit, for minimising losses and increasing efficiency and the electric power generated. Air compression and decompression induce energy losses, resulting in a low efficiency, mainly caused by air heating during compression, waste heat being released into the atmosphere. A similar problem is air cooling during decompression, lowering the electric power generated. Thus, using a thermal storage unit plays an essential role in the proper functioning of the facility and in generating maximum electric power. Supervisory control and data acquisition is performed from the automation cabinets. During commissioning tests, a constant stable power of around 50 kW with an 80 kW peak was recorded.

Experimental study on the performance of oil-free twin-screw expanders for recovering energy in fuel cell systems

Fuel cell systems are considered to be one of the best alternative power sources for automobiles to replace internal combustion engines. For the Proton Exchange Membrane fuel cell operated at the high pressure, adding an expander downstream the stack to recover the pressure energy of air exhausted from the stack is an effective way to reduce the actual power consumption of the compressor and improve overall system efficiency. This paper aims to explore operating characteristics of the twin-screw expander under different operating conditions and geometric construction, and thus find out proper design parameters. Hence, a detailed contrast experiment is performed on three expanders to investigate the effect of the suction port area and rotor length on the expander performance. Besides, the influences of rotating speed and suction pressure on the performance of expanders including mass flowrate, filling factor and recovered power are analyzed. By analyzing the experimental results, the relevant influence mechanisms of operating conditions on the expander performance are explained according to the equations of these performance parameters. Meanwhile, the reasons why the developed twin-screw expanders have unsatisfactory performance are recognized. Finally, the directions for further improvement and optimization of the oil-free dry twin-screw expanders used in fuel cells in the future work are pointed out based on the research results.

Comparison between single and cascaded organic Rankine cycle systems accounting for the effects of expansion volume ratio on expander performance

Compared to single-stage organic Rankine cycle (ORC) systems, cascaded ORC systems, in which a high-temperature topping cycle and low-temperature bottoming cycle are coupled together, could have advantages in terms of removing the potential for sub-atmospheric condensation conditions and improving expander performance as the expansion process is effectively divided across two stages. Moreover, reducing the expansion volume ratio could facilitate the use of volumetric expanders, such as twin-screw expanders, which, in turn, could facilitate two-phase expansion to be utilised in one, or both, of the cycles. The aim of this paper is to compare single-stage and cascaded ORC systems, accounting for the effect of the expander volume ratio on expander performance. To investigate this, thermodynamic models for single-stage and cascaded ORC systems are developed, which include variable efficiency expander models for both radial turbines and twin-screw expanders that can estimate the effect of the expansion volume ratio on the expander isentropic efficiency. Using this model, three different scenarios are compared for different temperature heat-source temperatures, namely: (i) single-stage ORC systems with vapour-phase expansion obtained using a turboexpander; (ii) single-stage ORC systems operating with a twin-screw expander, with the possibility for two-phase expansion; and (iii) cascaded cycles with either vapour- or two-phase expansion. The results from this comparison are used to identify applications where cascaded ORC systems could offer performance benefits.

Investigation into the effects of surface condensation in steam-driven twin screw expanders

During the operation of twin screw expanders with slightly superheated vapours or even two-phase fluids, surface condensation on machine parts occurs during the filling period and the expansion phase when the working fluid is in contact with cooler inner surfaces. This heat exchange from the working fluid to adjacent machine parts effects the working cycle and the efficiency of these machines. Short time scales and the periodicity of the process indicate the condensation process is best described by models for dropwise condensation. In this paper the effects of surface condensation on the operation of twin screw expanders are initially discussed in a simulation-based investigation. Chamber model simulation coupled with a thermal analysis is used for the thermodynamic simulation, whereby heat transfer coefficients are systematically varied. It is found that during the inlet phase condensate emerges on the inner surfaces of the machine being substantially cooler than the working fluid. This results in a higher mass being trapped within the working chamber and, thus, an increasing mass flow rate of the machine. An increase in power output is, however, not observed. The results obtained from chamber model simulations are finally compared against experimental data of a screw expander prototype.

One-Dimensional Modelling of a Trilateral Flash Cycle System with Two-Phase Twin-Screw Expanders for Industrial Low-Grade Heat to Power Conversion

This paper provides an overview of a one-dimensional modelling methodology for equipment and systems for heat to power conversion based on a staggered grid space discretization and implemented in the commercial software GT-SUITE®. Particular attention is given to a newly developed modelling procedure for twin-screw machines that is based on a chamber modelling approach and considers leakage paths between cells and with the casing. This methodology is then applied to a low-grade heat to power conversion system based on a Trilateral Flash Cycle (TFC) equipped with two parallel two-phase twin-screw expanders and a control valve upstream of the machines to adapt the fluid quality for an optimal expander operation. The standalone expander model is used to generate performance maps of the machine, which serve as inputs for the TFC system model. Parametric analyses are eventually carried out to assess the impact of several operating parameters of the TFC unit on the recovered power and cycle thermal efficiency. The study shows that the most influencing factors on the TFC system’s performance are the inlet temperature of the heat source and the expander speed. While the first depends on the topping industrial process, the expander speed can be used to optimize and control the TFC system operation also in transient or off-design operating conditions.

Research on structural parameter optimization and thermal performance of twin-screw expander

Abstract This paper analyzes rotor profile and meshing law of twin-screw expander. Elementary area is calculated by meshing sequence method, and elementary volume is solved by integration. The influence of geometric characteristics such as torsion angle and internal expansion angle of male rotor on the thermal performance of twin-screw expander is studied. The results show the following: reasonably reducing internal expansion angle and positive rotor torsion angle can increase isentropic adiabatic power of twin-screw expander. Taking isentropic adiabatic efficiency as optimization goal, sequential quadratic programming algorithm is used to optimize the structural parameters of twin-screw expander, and the influence of leakage on its functional force is analyzed. Considering leakage effect, actual shaft power of twin-screw expander is 16.7 kW, the isentropic adiabatic power under ideal conditions is 21.3 kW, and the adiabatic efficiency is 78.71%.

Experimental and numerical investigation of direct liquid injection into an ORC twin-screw expander

The Organic Rankine Cycle (ORC) is a thermal engine, which is used to convert low temperature heat to electrical power using organic working fluids. It is an established technique for waste heat recovery and for the utilization of renewable heat. This study presents a novel operational strategy of an ORC, which allows for reliable control of process parameters while simultaneously ensuring high power output. Preheated liquid working fluid is injected directly into a volumetric screw expander at an intermediate pressure level. The injected mass flow bypasses the evaporator and can be controlled by a valve. Thus, direct liquid injection into the expander reduces the exhaust temperature, leading to a lower risk of thermal damages in case of a hermetic or semi-hermetic expander. This strategy is analyzed experimentally and compared with a system simulation. The experimental and simulation results show that the exhaust vapor temperature can be reduced by approx. 40 K for the investigated operational conditions. This enables the expander to run at higher live vapor conditions by simultaneously ensuring sufficient cooling of the generator and thus allows for up to 40% higher power production depending on the operational conditions.

Numerical investigations of a Trilateral Flash Cycle under system off-design operating conditions

The current research presents a comprehensive model of a 100 kWe Trilateral Flash Cycle TFC system composed of sub-modules for the plate heat exchangers and twin-screw expanders to assess and identify the operating parameters that mostly affect the TFC performance at off-design conditions. The modelling approach for heat exchangers and piping is based on 1D CFD while considering the operating maps for pump and expander. The simulation process takes into account variations of the heat source temperature and mass flow rate from the design point. The TFC performance is analysed in terms of efficiency and power output while the expander performance is discussed in terms of volumetric and isentropic efficiencies. A sensitivity analysis is carried out to assess the most suitable parameter for control purposes and system performance optimisation. The results point out a large sensitivity of the inlet quality at the expander due to the revolution speed of the machine but also the off-design behaviour of the heater.

Exergetic optimisation of twin screw expanders

Limited access to energy resources, air pollution and its subsequent detriments to ozone layer have accentuated the importance of green industry for recovering exergy from high-pressure flow to generate clean electrical power. Due to low maintenance costs, usage in two-phase fluids and longer lifespan, the twin screw expanders (TSEs) have attracted great attention though there is still much to be done to improve their performance. The complexity of computing in screw machines, as well as the time consuming of their simulation throughout the entire design space by the computational fluid dynamics (CFDs), requires the use of other complementary tools to optimise the performance of these types of machines. In this paper, based on laboratory model specification and experimental data, a TSE model was simulated and validated using CFD. Consequently, by helping design of experiments (DOE), a CFD alternative model is obtained called surrogate-based modelling (SBM) and the effect of three main geometric parameters on the expander's exergy efficiency were investigated in entire design space. Finally, a more efficient TSE with 86% exergy efficiency was obtained by using multi-objective genetic algorithm in the shortest time.

基于ANSYS Workbench的双螺杆膨胀机主箱体模态分析

基于ANSYS Workbench的双螺杆膨胀机主箱体模态分析

Exergetic optimisation of twin screw expanders

Exergetic optimisation of twin screw expanders

Performance research on a power generation system using twin-screw expanders for energy recovery at natural gas pressure reduction stations under off-design conditions

This paper has focused on the performance study of a power generation system for energy recovery at natural gas pressure reduction stations under off-design conditions. First, an experiment was carried out to test the off-design performance of a twin-screw expander prototype. Two empirical models based on the experimental data were built and validated to predict the inlet volume flow rate and the isentropic efficiency of the expander, separately. Then, a case study was presented at a natural gas pressure reduction station located in Jinan, China. The method of selecting the optimal inlet volume flow rate was presented and the thermodynamic analysis of the power generation system was performed, based on the operating data in a typical day. The results indicated that the total power output of the system based on the twin-screw expander prototype was around 147.33 kW·h in a typical day. Interestingly, high-pressure gas produced about 0.16 kW·h of electricity per cubic meter. Meanwhile, a very superior cold energy resource with a minimum temperature of −72 °C could be obtained incidentally. Moreover, the power generation system ran with high efficiency for mostofthetime, and the average energy and exergy efficiency of the system reached 96.09% and 66.00% respectively.

Impact of different clearance heights on the operation of a water-flooded twin-screw expander—experimental investigations based on indicator diagrams

Twin-screw expanders offer a high potential for energy conversion in the lower and medium power range, for instance in Rankine cycle systems for exhaust heat recovery. With the aim of minimising internal leakages within the expander and lubricating moving machine parts, an auxiliary liquid can be carried with the main flow or liquid working fluid can be fed to the twin-screw expander. Moreover, the operation of twin-screw expanders in Rankine cycle systems at high liquid mass fractions and in trilateral flash cycles is predicted to be a very promising application for expanders in lower power ranges. Thus, a fundamental understanding of the operation of liquid-flooded twin-screw expanders is mandatory. This paper presents results of an experimental investigation into a water-flooded twin-screw expander prototype SE 51.2 designed at the Chair of Fluidics at TU Dortmund University. On the one hand, the aspect of a two-phase working fluid is discussed considering integral characteristic numbers such as mass flow, delivery rate, and effective isentropic efficiency. On the other hand, in order to explain the influence mechanisms of a two-phase working fluid on the operating behaviour of the twin-screw expander, indicator diagrams are recorded by means of high-resolution pressure transmitters to determine indicated power. Hence, mechanical and hydraulic losses, indicated isentropic efficiency, and mechanical efficiency of the twin-screw expander can be calculated. In order to determine the influence of the narrow clearance on the hydraulic losses and the clearance sealing effect in terms of a two-phase working fluid, a systematic variation of the rotor-tip clearance height is carried out. As a result of the investigations, a water surge at the rotor tip is proved to be a significant mechanism affecting hydraulic losses in a water-flooded twin-screw expander.

The workflow of rotor machine development: design phases, steps and tools of the development process

In this work we present an overview of the workflow of rotor machine§ development, focusing on twin-screw compressors and expanders. The rotor machine development process consists of an orchestrated activity of specification, design and verification where individual phases, steps and tools contribute significantly in developing cutting-edge technology and delivering sustainable solutions. Our goal is to present those phases, steps and tools with relevant examples, highlight the development challenges and argue the inevitable deviations of real-life condition from the original specification. We believe that this contribution will shed light upon the essential elements of the rotor machine development process, something that benefits the entire community from academia and suppliers to end users.

Modeling a dry running twin-screw expander using a coupled thermal-fluid solver with automatic mesh generation

Understanding the details of the internal flow processes in screw compressors and expanders is very important for their efficient and robust design. Computational fluid dynamics (CFD) provides full access to a modeled three dimensional flow field and its variation in time. However, the application of CFD to screw compressors and expanders can be difficult because of the complicated geometries involved and the need to supply the computational grid on which the modeled equations are solved over a large number of time steps. While the majority of previous research on CFD applications to screw machines features inventive techniques for generating meshes that adequately resolve the flows in the small clearances, an alternate approach is demonstrated in this work. The screw expander SE 51.2 from TU Dortmund University is analyzed here through a CFD model which generates the grid automatically based on a modified Cartesian cut-cell approach. The grid is then adaptively refined based on local gradients of velocity and temperature. At each time step, the grid is regenerated based on the geometry motion. As opposed to resolving the flow in the clearances, a model is applied so that the cells in the clearance can remain relatively large. The detailed measurements of the screw expander are used to validate the model. The operating conditions investigated include the expansion of dry air at a four-to-one pressure ratio for four different rotational speeds. The measured internal chamber pressures are compared to the results from the model, as are the average mass flow rate, indicated power, and outlet temperature. A coupled thermal-fluid approach is used to model the rotor temperatures and corresponding thermal deformation of the rotors and housing. In this approach, the fluid and solid temperatures are solved together; to deal with the problem of the disparate time scales between the fluid and solid heat transfer, the solids are periodically solved to steady state using heat transfer coefficients and near-wall temperatures computed from an energy conserving averaging over several cycles. The effects of the various leakage flow paths in the model, including the rotor-to-rotor, rotor-tip-to-housing, and bearing leakage are demonstrated and quantified. Finally, simulation and experimental results are compared in terms of different rotor-tip-to-housing clearance heights. The model considering the appropriate thermal deformation of the rotors is shown to yield the best agreement with the measurements, however there is work remaining to reduce the model calculation time, especially at low rotational speeds.