Screw Expander Research Articles

Off-design optimisation of organic Rankine cycle (ORC) engines with different heat exchangers and volumetric expanders in waste heat recovery applications

Organic Rankine cycle (ORC) engines in real applications experience variable heat-source conditions. In this paper, the off-design performance of small- to medium-scale ORC engines recovering heat from stationary internal combustion engines (ICEs) is investigated. Of particular interest are the employment of screw vs. piston expanders, and two heat exchanger (HEX) architectures. Unlike previous studies where the performance of the expander and HEX are assumed fixed during off-design operation, here we consider explicitly their varying and interacting characteristics within the overall system. Nominal sizing results reveal indicated isentropic efficiencies > 80% for twin-screw and > 85% for piston expanders. Following nominal design, the ORC engine operation is optimised for ICE part-load (PL) operation. Although the heat transfer coefficients in the evaporator decrease by up to 30% at PL, the effectiveness in this HEX increases by 20% due to the larger temperature differences across the component. The screw expander efficiency reduces by up to 3% at off-design operation, whilst that of the piston expander increases by up to 16%. Optimised off-design maps indicate that the ORC engine power output reduces to 77% (piston) or 68% (screw) of its full-load value when the ICE operates at 60% PL, and that ORC engines with plate HEXs generate 5–11% more power than those with double-pipe HEX designs. Under variable ICE operation, smaller ORC engines with piston expanders generate more power than larger engines with screw expanders, highlighting the resilient off-design operation of piston machines. The modelling tool developed here can predict ORC performance over a wide operating envelope and provides performance maps that can be used by operators to optimise ORC engine operation in variable conditions and by ORC vendors to inform component design decisions.

Analysis of the thermodynamic disequilibrium loss of the two-phase expansion process and its influence on the performance of a reciprocating expander

The screw expander and the reciprocating expander were considered as the suitable technologies to be employed as two-phase expanders of the Trilateral Flash Cycle (TFC) systems. The two-phase expansion process in the screw expander could be approximately viewed as the thermodynamic equilibrium process due to the strong disturbing and mixing between the two-phase working fluids in the chamber volume. However, for the reciprocating expander, the situation was different. The flashed vapor in the reciprocating expander cylinder was separated from liquid in the flash chamber, which would result in a significant thermodynamic disequilibrium loss. To assess this loss, the thermodynamic equilibrium model (TEM) and the flash vaporization model (FVM) were simultaneously proposed in this paper. Water was used as the two-phase working fluid. The two-phase expansion processes, the isentropic efficiencies and the output expansion works of the two models were obtained. Predicted results were analyzed and compared under different rotation speeds and injection temperatures. The results showed that when rotation speed of the expander was increased from 50 r/min to 1000 r/min, the thermodynamic disequilibrium loss was approximately increased from 5.3% to 6.3%, and when injection temperature was decreased from 523.15 K to 473.15 K, the thermodynamic disequilibrium loss was approximately increased from 4.5% to 6.2%.

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.

Thermo-economic assessment of a novel trigeneration system based on coupling of organic Rankine cycle and absorption-compression cooling and power system for waste heat recovery

A novel trigeneration system consisting of organic Rankine cycle (ORC) and absorption-compression cooling and power system (ACCPS) is proposed to recover diesel engine waste heat. A compressor is used between evaporator and absorber in ACCPS. Moreover, a screw expander is applied in ACCPS to increase power generation and domestic hot water (DHW) is prepared by DHW heat exchanger (HX) in ORC. Exergoeconomic analysis using the specific exergy costing (SPECO) method is implemented for system assessment. Base case and parametric analysis as well as the effects of using expander and compressor in ACCPS on total performance of the system are examined. The results show that the best thermo-economic performance of the system is achieved when ORC condensation temperature and generator outlet temperature have respectively the lowest (85 °C) and the highest (80 °C) values. It is difficult to select the proper ORC evaporation temperature, because the lower this temperature is, the higher net power output is obtained while higher ORC evaporation temperature leads to less total cost rate and more total energy efficiency of system. Moreover, system exergy efficiency has an optimum point versus variation of ORC evaporation temperature. By increasing expander inlet steam quality, ACCPS coefficient of performance (COP) and total energy efficiency of system decrease; however, due to increase in net power output and system exergy efficiency respectively equal to 18.23% and 3%, total system performance improves from exergy viewpoint. By utilizing a compressor in ACCPS and increasing its pressure ratio, the performance of ACCPS improves from energy viewpoint; however, system thermo-economic performance decreases.

Biomethane grid injection or biomethane liquefaction: A technical-economic analysis

The gradual reduction of subsidies for electricity production from biogas and the raising interest of bio-methane as an integration to natural gas market force the biogas plant owners to choose alternative solutions for biogas exploitation.In this study, two solutions for biomethane distribution have been compared: biomethane liquefaction and grid injection. The analysis was carried out as a function of gas connection cost, electric tariff, selling price and type of expander adopted in the liquefaction cycle (radial turbines or screw expanders). A nitrogen Joule-Brayton reverse cycle was considered for liquefaction. A detailed model of the cycle was developed in Aspen Hysys and optimized to minimize the energy specific consumption. Results show that expander efficiency has a key role in the liquefaction scenario. Screw expanders lead to a specific consumption 1.45 times higher than radial turbines but reduce capital costs by a factor of 1.39.Biomethane grid injection is the preferable solution in terms of investment risk if the connection cost is below 500 k$, independently on the electricity price. As far as it concerns the profit, liquefaction with radial turbines is preferable up to electricity price of 0.23 $/kWh. A sensitivity analysis on product selling prices shows that biomethane grid injection is always the most profitable solution when connection costs are low. For higher connections cost, liquefaction with radial turbines is the best solution for minimizing the investment risk and maximizing the profit for most of the combination of selling prices.

Comparison of different ORC typologies for heavy-duty trucks by means of a thermo-economic optimization

This study focuses on the design phase of ORC systems recovering the heat wasted from two of the sources available on a Heavy-Duty Truck (HDT): the exhaust and recirculated gases. From these heat sources and their combinations, 5 possible architectures are considered. The main components (i.e. the heat exchangers, the pump and the expander) of the WHR systems are investigated and modeled. Plate type heat exchangers are considered for both the hot and cold sides of the system. Regarding the expansion devices, 5 positive displacement machine technologies, the scroll, screw, piston, vane and roots expanders, are considered and modeled while, among the turbo-expanders, the radial-inflow turbine is taken into consideration. A semi-empirical model is proposed to simulate a volumetric pump. The models of components are first confronted with experimental data. The validated models are then used as references for the design of the new components, which is achieved following similitude rules. This ultimately leads to 30 typologies that will be used with 6 of the various investigated working fluids. In order to identify the most promising system(s), a 3-step optimization tool is developed. First, the most suitable conditions are identified for the design of the ORC systems using a simplified model of an expansion machine. In the second step, the design phase, using more detailed models for the expanders and a proposed economic model for the overall system, a thermo-economic optimization is performed. In the third step, the output power for each of the obtained system models is maximized, optimizing the evaporating pressure and the superheating degree for various off-design conditions. The average power weighted using the frequency distribution of the gas operating conditions is computed and used to compare the 180 systems. Finally, because power is not the only criterion to select the most suitable system topology, additional criteria are taken into consideration.

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.

Modeling and energetic-exergetic evaluation of a novel screw expander-based direct steam generation solar system

This paper proposes an innovative solar electricity generation system based on the steam Rankine cycle, which employs screw expanders and parabolic trough collectors, with water as heat transfer and working fluid. Owing to its good efficiency at low-medium heat source temperatures, the solar generation system can meet the need to design renewable energy power plant with simple technical requirements in heat storage and collection. Screw expander-based solar thermal electricity systems have further advantages: avoidance of overheating, satisfactory applicability in energy conversion with liquid-steam mixtures, low operating pressure and good applicability for distributed power generation. In this paper a thermodynamic mathematical model on the polytropic expansion process is first investigated, revealing variation of isentropic efficiency with the operating pressure ratio and built-in volume ratio of the screw expander. Basic criteria are then provided to evaluate the energetic and exergetic benefits of such a power plant, developing a macro balance to define optimum operating conditions. Water evaporation temperature and condensation pressure were thus optimized with respect to heat-to-power conversion efficiency, solar thermal power generation efficiency and exergetic efficiency. Our results indicate that the optimum evaporation temperature ranges from about 204 °C to 276 °C with increasing solar radiation, whereas the corresponding maximum solar thermal power efficiency and exergetic efficiency are around 12.6% and 17.8%, respectively.

Simulation-based investigation of spontaneous condensation in steam-driven screw expanders

Condensation and its effects on turbomachinery operation are well understood and have been widely investigated. However, only little scientific work on condensation in positive displacement machines has been published. Although, depending on machine type, high expansion rates and, as a consequence, significant supersaturation can be achieved for working fluids with a negative saturation vapour curve. In this paper, the effects of spontaneous steam condensation in screw expanders are discussed. Classical nucleation theory is used for the thermodynamic simulation of operational behaviour. The study shows at which point during the expansion phase spontaneous condensation can be expected and typical nucleation rates are determined. The impact of released latent heat during expansion on chamber states is depicted. Furthermore, a comparison of purely metastable expansion with equilibrium expansion is provided in order to show the full-range discrepancies. Additionally, the influence of internal leakage and throttling effects during the inlet phase on the course of spontaneous condensation and droplet growth is analysed. Typical operating parameters are widely varied in simulation so as to identify the impact of steam parameters and expander parameters on the condensation process.

Off-design operation of ORC engines with different heat exchanger architectures in waste heat recovery applications

Abstract Organic Rankine cycle (ORC) engines in waste-heat recovery applications experience variable heat-source conditions (i.e. temperature and mass flow-rate variations). Maximising ORC system performance while accounting for off-design operation in response to such variations is of crucial importance for the financial viability and wider adoption of this technology. In this paper, the off-design performance of an ORC engine is investigated in a heat-recovery application from a stationary internal combustion engine (ICE), employing a screw expander and two heat exchanger (HEX) architectures. Unlike previous studies where the ORC expander and HEX performance are assumed fixed during off-design operation, here we consider explicitly the time-varying characteristics of these system components. Nominal system sizing results indicate that the screw expander isentropic efficiency exceeds 80%, and that the heat transfer area requirements of plate HEXs (PHEXs) are 50% lower than those of double-pipe HEXs (DPHEX). Following nominal system design, the ORC engine operation is optimised at conditions relating to part-load (PL) operation of the ICE. Although, the heat transfer coefficients in the evaporator decrease by 30% at PL, the HEX effectiveness increases by up to 20% due to higher temperature differences between the working fluid and the heat source, and the performance of the PHEX appears less sensitive to off-design operation conditions. The optimum PL screw expander efficiency maps reveal only a minor reduction of the expander efficiency (by 2%) relative to the design conditions. Optimum off-design maps indicate that the power output of the ORC engine reduces to 72% of full-load for an ICE at 60% of full-load, and that ORC engines with PHEXs generate slightly more power for the same heat-source conditions. Overall, the tool developed predicts ORC performance over an operating envelope and allows the selection of optimal HEX and expander designs. The findings can be used by ORC plant operators to optimise the ORC engine power output given varying on-site heat-source conditions, and by ORC vendors to inform HEX and expander design decisions.

Economic Analysis of Organic Rankine Cycle Using R123 and R245fa as Working Fluids and a Demonstration Project Report

The organic Rankine cycle (ORC) is a popular technology used in waste heat recovery and low-grade heat utilization, which are two important measures to solve the problems brought by the energy crisis. The economic performance of ORC system is an important factor affecting its application and development. Therefore, the economic analysis of ORC is of great significance. In this study, R123 and R245fa, two frequently-used working fluids during the transition period, were selected for calculating and analyzing the economic performance of an ORC used for recovery of waste heat with a low flow rate and medium-low temperature. Five traditional economic indicators, namely total cost, net earnings, payback period, return on investment, levelized energy cost, and present value of total profit in system service life, which is a relatively new indicator, were used to establish the economic analysis model of ORC. The variation effects of evaporation temperature, condensation temperature of working fluid, flue gas inlet temperature, and mass flow rate of flue gas on the above six economic indicators were analyzed. The results show that the optimal evaporation temperature of R123 is 125 °C, the optimal condensation temperature is 33 °C, and the optimal heat source temperature is 217 °C. For R245fa, the optimal evaporation temperature is 122 °C, the optimal condensation temperature is 27 °C, and the optimal heat source temperature is 177 °C. The economic performance of an ORC demonstration project was reported and used for comparison with the estimation and analysis. It was found that the single screw expander has an excellent economy performance, which greatly reduces the proportion of expander cost in the ORC system.

Compressed Air Energy Storage Installation for Renewable Energy Generation

Compressed Air Energy Storage (CAES) installations are used for storing electrical power, under the form of potential energy from compressed air. The heat generated during compression can be stored to improve the efficiency of compression-expansion cycle. The solution presented consists of a 100 kW screw compressor driven by a 110 kW asynchronous three-phase motor. The compressor supplies air into vessels which store it until a high electrical energy demand arises. At that time, the compressed air is released into a 132 kW screw expander whose shaft spins a 132 kW asynchronous generator, producing electric power and supplying it into the electrical grid. Before expansion, the air must be preheated in order to avoid the freezing of expansion equipment. If the heat generated during compression is used for air preheating before expansion, the process is adiabatic. A demonstrative model of the installation is currently being developed, with the expander part being completed so far. The maximum power to be produced was calculated to be around 100 kW. During expander commissioning tests with air supply from a 250 kW high pressure compressor, a maximum generated power of 49.7 kW was attained, expected to be higher when releasing air from the reservoirs.

Study on Pressure Loss of Inlet and Exhaust Flow in Single Screw Expander With Large Internal Volume Ratio

Study on Pressure Loss of Inlet and Exhaust Flow in Single Screw Expander With Large Internal Volume Ratio

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

Experimental analysis of a micro-ORC driven by piston expander for low-grade heat recovery

In this paper, a full experimental characterization of a micro-scale ORC system is presented. The facility under investigation is driven by a piston expander prototype, made of three cylinders arranged radially around the drive shaft. The system is rated for a thermal input around 30 kW, being suitable for residential, tertiary sector or small industry applications. It is conceived for exploiting low temperature heat sources, such as solar collectors, biomass boilers, geothermal energy or waste heat streams. The facility was provided with an electric boiler as heat source, which warms water up to 90 °C, and cold water at ambient temperature as heat sink. A test campaign was performed varying the hot source temperature and the organic fluid feed pump velocity, in order to characterize the system behavior at different off-design working conditions. The electric consumption of the ORC feed pump was measured, in order to quantify the actual impact of the auxiliaries on the overall efficiency. Moreover, the number of electric loads connected to the generator was varied, changing the equivalent phase impedance value, for evaluating the effect on the expander rotating speed and power output.The experimental analysis demonstrated that small reciprocating expander is suitable for exploiting low enthalpy heat sources, with quite good performances compared to other architectures like scroll and screw expanders, more applied within low temperature sources. The results show that the gross electric power output varied between 250 W and 1150 W, depending on the expander speed and on the number of electric loads activated. The expander total efficiency showed a barely constant trend around 40%. The pump total efficiency varied between 10% and 20%, increasing with the pump rotational speed. The maximum ORC gross and net efficiency were 4.5% and 2.2% respectively, confirming that the auxiliaries impact cannot be considered negligible in such type of systems.

Thermo-economic evaluation of an innovative direct steam generation solar power system using screw expanders in a tandem configuration

Tandem screw expander (SE) technology is a promising solution for large volume ratio situations. Tandem SE driven direct steam generation (DSG) system is first proposed for distributed solar thermal power generation. Steam accumulator is adopted for storage due to its moderate heat source temperature. Compared with the cascade steam-organic Rankine cycle (SORC) system, the novel tandem system has simpler structure, easier control strategy and lower technical requirements, less operating and maintenance fees, more stable power output and higher security. Thermodynamic analysis and economic evaluation of the novel system are conducted based on some parameters of a recently constructed tandem SE project. Steam Rankine cycle (SRC) efficiency of 18.49% is achieved by employing built-in volume ratio (rv,b) of 3 for the high-pressure SE (SE1) and rv,b of 7 for the low-pressure SE (SE2). The cost-effectiveness of the system is improved as the power capacity and heat storage time increase. Levelized electricity cost (LEC) of 0.118 $/kWh and payback period (PP) of 10.48 years are obtained for 1 MW tandem plant with 6.5 h heat storage. The cost of parabolic trough collectors accounts for nearly half of the total investment while the accumulators occupy less than 7%. The cost of SE2 is approximately seven times that of SE1 due to the larger design outlet volume flow rate and rotor diameter.

Steady and transient operation of an organic Rankine cycle power system

Abstract The Organic Rankine Cycle (ORC) driven by low-grade energy can operate in off-grid mode. Here, we report the steady and transient characteristics of an ORC system. A single screw expander and a synchronous generator convert mechanical power to electricity. A lamp array simulates the variable external loads. The quality parameters including electric frequency, electric power and effective voltage are measured by a power meter. We show that at setting frequency of 50 Hz, the maximum system efficiency can be reached when the external load is not larger than the ORC power capacity. The expander isentropic efficiency is above 80% for the vapor superheating of 5–10 K at expander inlet. A control strategy is proposed to achieve better system performance under varied heat source and environment conditions. By use of residual frequency and adjusting pumping frequency, the vapor pressure at expander inlet can be automatically adjusted to reach the optimal value. Transient tests indicate that the electric quality is insensitive to the step change of external loads and heat/cold sources parameters. The thermohydraulic parameters are self-adapted to the change of external parameters. This work provides a self-adapting strategy for ORC operation at varied heat/cold source parameters.

Experimental study on the net efficiency of an Organic Rankine Cycle with single screw expander in different seasons

The present paper experimentally investigates the thermal performances of an Organic Rankine Cycle (ORC) using a self-developed single screw expander. Experiments were conducted to study the net efficiency of the ORC under different cooling water flows in different seasons. Taking the power consumed by the circulation pump, cooling water pump, cooling tower fan and lubricant oil pump into account, the experimental results showed that the maximum value of produced net power and net efficiency achieved 3.27 kW and 3.04%, respectively, obtained at a cooling water flow of 12 m3 h−1 in winter. The experimental results also reported that the shaft power and shaft efficiency of the expander increased gradually with growth of cooling water flow. Under the same evaporating temperature, the performances of the ORC system deteriorated with the increase of ambient temperature, and the net efficiency of the system in summer was decreased by more than 16.45% than that in winter. In addition, the power consumption by the cooling system was the largest factor restricting the net efficiency than the power consumption by other auxiliary machines. Therefore, reducing the power consumption by the cooling system can effectively enhance the net efficiency of the ORC system.