Back Work Ratio Research Articles

Design and optimization of power conversion system for a steady state CFETR power plant

A promising magnetic fusion steady state reactor is Chinese Fusion Engineering Testing Reactor (CFETR) and a different power conversion system is required than tokamak reactor. The control and utilization of high outlet temperature and thermal power generated by the fusion reactor efficiently are the main challenges in the field of thermo-fusion technology. An efficient and optimized power conversion system is an urge for the thermo-fusion energy system with a candidate working fluid under high temperature and pressure. Five cycle topologies based on the Brayton cycle have been designed such as multi stage expansion and recompression cycle with intercooling, recompression cycle with intercooling, partial expansion with multistage compression, partial expansion and partial recompression with cooling across range of turbine and compressor inlet temperatures. Brayton cycle with different system configurations have been simulated with He-gas and supercritical Carbon Dioxide (CO2) to conceive high outlet temperature of CFETR about 500 °C and thermal power of 200 MWth for better thermal performance by using REFPROP, EES and analytical model has been developed with MATLAB. The supercritical CO2 has better thermal performance about 36.18 % as compared to He-gas has 29.81 % under same thermal conditions. The Schematic-I with addition of preheat and recompression enhance the thermal performance about 2 % and has been proposed for CFETR. The effect of change in pressure of the system, effect of the working fluid, isentropic efficiency, heat rate and back to work ratio on the thermal performance and network output have been analyzed and optimized based on analytical model with the MATLAB. The system net power is increased from 69.20 MW to 79.71 MW, heat rejected by the system is decreased from 134.46 MW to 127.06 MW and thermal performance is increased about 34.62 %–39.85 % with pressure from 24 MPa to 28 MPa. Using multi stage expansion and recompression cycle with intercooling instead of other topologies can improve the thermal performance up to 6.1 % depends upon the operation conditions and working fluid. The heat rate of the system decreased while back work ratio increased with the pressure and it reveal that calculations are correct.

Theoretical and experimental study on the selection of pure and mixed refrigerants for a power-cooling system driven by ultra-low-grade heat

This study theoretically and experimentally evaluates the performance of a system that combines an organic Rankine cycle (ORC) and a vapor compression cycle (VCC) driven by ultra-low-grade heat (ULGH). A systematic method is developed for the selection of pure refrigerants for the efficient performance of ORC over a temperature range between 50°C to 100°C. Binary and ternary mixtures are developed followed by sensitivity analyses and composition optimization to determine the optimal performance of these mixtures. Several experimental tests are conducted to ensure the operability of the system with the developed refrigerants. The results show that the use of ternary and binary mixtures enhances the performance of the system with lower GWP and ODP compared to pure refrigerants. Several mixtures are developed with energy efficiencies higher than 9% at a heat source temperature of 75°C. A mixture of R142b/R152a/R600 improves the energy efficiency of the system by 22.80%, reduces the back work ratio by 19.40% with an increase in the evaporation capacity by 13.25%. The methodology and results presented herein will be useful in the development, design, and optimization of power and cooling systems driven by ULGH with pure and mixed refrigerants.

Control strategies of pumps in organic Rankine cycle under variable condensing conditions

The working medium pump and cooling water pump are important components in organic Rankine cycle system. However, they have not received as much attention as the expander and working medium. Simultaneously, the dramatic changes in condensing environment deviate the organic Rankine cycle working point from the designed operating conditions. Hence, this paper focused on the performance improvements and control strategies of working medium pump and cooling water pump in small-scale organic Rankine cycle under variable condensing conditions. A model was established to predict the performances of an organic Rankine cycle system with R245fa, especially the pumps in the system, under variable condensing conditions. The results showed that the overall efficiency of multi-stage centrifugal pump was between 14% and 39% at an estimated speed of approximately 1600 RPM to 2328 RPM. In winter conditions, the back work ratio increased from 0.05 to 0.19 with increasing evaporation pressure and decreasing mass flow rate. In other words, it is unnecessary to pursue excessively high evaporation temperature at low rotational speed of multi-stage centrifugal pump. As for the cooling water pump, it is feasible to reduce the power consumption of the cooling system through changing the rotational speed of water pump. The water pump can be operated with high efficiency (between approximately 58.4% and 65.3%, corresponding to an estimated speed range of 1374 RPM to 1813 RPM) when the system run in optimum operating conditions. It is also found that the performance of organic Rankine cycle was strongly influenced by fluctuating ambient temperature, with the net efficiency and net power increase of 3.36 % and 5.11 kW in winter compared to summer.

Design, computational and experimental investigation of a small-scale turbopump for organic Rankine cycle systems

The hydraulic design, computational analysis, and experimental investigations of a high-speed small-scale turbopump for mobile waste heat recovery applications based on organic Rankine cycle systems are presented in this paper. Such applications demand high-pressure rise, lightweight, and compact pumping systems with simple construction. The investigated turbopump features an unshrouded 37.75 mm tip diameter single-stage centrifugal pump equipped with eight radial blades, eight splitters blades, and a rectangular axisymmetric volute. The pump should deliver 0.28 kg/s of mass flow rate with a pressure rise of 20 bar at a designed rotational speed of 25,000 rpm. Due to uncertainties observed in employing state-of-the-art 1-d methods that are valid for much larger machines, computational fluid dynamics is utilized to obtain a design meeting the specifications. The pump’s performance is evaluated experimentally at different rotational speeds, mass flow rates, and impeller tip clearances. The excellent agreement between experimental data and predictions from computational fluid dynamics validates the design methodology and computational results. The turbopump’s characteristics are then utilized to estimate the possible performance improvement of a target organic Rankine cycle using the novel turbopump instead of a commercial multi-stage centrifugal pump. The comparison suggests that the novel turbopump increases the efficiency of the target organic Rankine cycle by 0.3 % points and decreases its back-work ratio by nearly 50 %. The novel turbopump is approximately ten times more compact compared to commercial systems.

Experimental Analysis of Partial Evaporation Micro-ORC for low-temperature Heat Recovery

In this paper, we present an experimental assessment of the performance of a partial evaporating organic Rankine cycle (PE-ORC) power system. The system converts low temperature heat into electrical energy, with a power size around 1 kW, thus suitable for micro-generation in the residential sector. Although the test bench was designed for operating with superheated vapour at the expander inlet, it has demonstrated to be able to work with the expansion occurring entirely in two-phase condition. Since the direct measurement of the vapour quality is not possible using the sensors installed in the test rig, the state of the fluid in the two-phase condition is estimated by means of the thermal balance at the heat exchangers, so the thermodynamic cycle can be evaluated. Temperatures of the heat source in the range between 40 °C and 75 °C have been tested, and for each temperature value the vapour quality at the expander inlet has been varied by regulating the feed-pump rotating speed. Experimental data are provided regarding the performance of the overall cycle, of the heat exchangers, of the expander and of the feed-pump. It was observed that the effectiveness of the evaporator and the efficiency of the pump are improved with respect to the operation with superheated vapour at the expander inlet. However, the overall performance is lower, especially due to the high ratio of the pump consumption over the expander produced power, commonly called back work ratio (BWR). The latter, under some boundary conditions, has resulted higher than the unit, meaning that the system is not able to produce net electrical power. The aim of the paper is to identify the design characteristics required by a micro-ORC energy system in order to enhance its performance in the PE operating mode.

Thermodynamic optimization of a linear thermomagnetic motor

Thermomagnetic motors can be applied to recover low-grade thermal energy waste and convert it into mechanical energy, therefore, used as an energy harvester. The present work proposes a mathematical model to simulate the heat transfer and the thermodynamic cycle of a linear thermomagnetic motor coupled to a spring mechanism. The simulation results are the input information in a subroutine to optimize the motor design based on two objective functions: to minimize the total entropy generated and to minimize the back work ratio, defined as the ratio between the pumping power and the total produced power. The mathematical model solves coupled the energy equations for the fluid and solid phases, and the entropy generation model includes four contributions: interstitial heat transfer (finite temperature difference), axial conduction in the fluid and solid phases, and viscous dissipation. Gadolinium is used as soft magnetic material due to the availability of properties data. Four design and operating conditions constraints are taken into account: heat exchanger length, spring constant, mass flow rate, and warm stream temperature (heat source), and their optimized values are, respectively, 52 mm, 1560 N/m, 80 kg/h at any warm stream temperature ranging from 310–330 K. At these combinations, the optimum thermomagnetic motor is able to produce a net power of 5.00 ± 0.15 W with a minimum back-work ratio of about 12%. The proposed methodology can be used to optimize the design of novel thermomagnetic motors or to optimize geometric parameters and operating conditions of state-of-the-art concepts.

Optimization Design of the Organic Rankine Cycle for an Ocean Thermal Energy Conversion System

This study selects five parameters as decision variables for the optimization design of an ocean thermal energy conversion system, including the evaporating temperature, the condensing temperature, the pinch-point temperature difference between the evaporator and condenser, and the working fluid flow rate. The optimization goal is to maximize the net output power per unit area and the exergy efficiency. The final scheme is comprehensively screened out from the Pareto solution set through some evaluation indexes. Finally, this study also analyzes the effects of four decision variables on the optimization objectives and the evaluation indexes. This study finds that evaporating temperature and condensing temperature have similar effects on the two objective functions. However, the pinch-point temperature difference has different effects on them. The back work ratio is obviously affected by the condensing temperature. A small pinch-point temperature difference is beneficial and improves the performance of an ocean thermal energy conversion system. The effects of evaporating temperature and condensing temperature on the investment cost per unit net output power are roughly similar to those on the net output power per unit heat exchange area. However, the effects of the pinch-point temperature difference on the two performance aspects are inconsistent.

Design and Operational Control Strategy for Optimum Off-Design Performance of an ORC Plant for Low-Grade Waste Heat Recovery

The applicability of organic Rankine cycle (ORC) technology to waste heat recovery (WHR) is currently experiencing growing interest and accelerated technological development. The utilization of low-to-medium grade thermal energy sources, especially in the presence of heat source intermittency in applications where the thermal source is characterized by highly variable thermodynamic conditions, requires a control strategy for off-design operation to achieve optimal ORC power-unit performance. This paper presents a validated comprehensive model for off-design analysis of an ORC power-unit, with R236fa as the working fluid, a gear pump, and a 1.5 kW sliding vane rotary expander (SVRE) for WHR from the exhaust gases of a light-duty internal combustion engine. Model validation is performed using data from an extensive experimental campaign on both the rotary equipment (pump, expander) and the remainder components of the plant, namely the heat recovery vapor generator (HRVH), condenser, reservoirs, and piping. Based on the validated computational platform, the benefits on the ORC plant net power output and efficiency of either a variable permeability expander or of sliding vane rotary pump optimization are assessed. The novelty introduced by this optimization strategy is that the evaluations are conducted by a numerical model, which reproduces the real features of the ORC plant. This approach ensures an analysis of the whole system both from a plant and cycle point of view, catching some real aspects that are otherwise undetectable. These optimization strategies are considered as a baseline ORC plant that suffers low expander efficiency (30%) and a large parasitic pumping power, with a backwork ratio (BWR) of up to 60%. It is found that the benefits on the expander power arising from a lower permeability combined with a lower energy demand by the pump (20% of BWR) for circulation of the working fluid allows a better recovery performance for the ORC plant with respect to the baseline case. Adopting the optimization strategies, the average efficiency and maximum generated power increase from 1.5% to 3.5% and from 400 to 1100 W, respectively. These performances are in accordance with the plant efficiencies found in the experimental works in the literature, which vary between 1.6% and 6.5% for similar applications. Nonetheless, there is still room for improvement regarding a proper design of rotary machines, which can be redesigned considering the indications resulting from the developed optimization analysis.

Isobaric Expansion Engines Powered by Low‐Grade Heat—Working Fluid Performance and Selection Database for Power and Thermomechanical Refrigeration

A database is developed for the most suitable working fluids for Worthington‐type and Bush‐type isobaric engines based on their performances for a wide range of heat source temperature, from 40 to 300 °C, and operating pressures from 1 to 100 bar. Thermodynamics models are developed and simulated to study the effects of different operating temperatures and pressures on the efficiency and back work ratio of both engines. Results show that in temperature range from 40 to 60 °C, the achieved efficiency is less than 4% for most cases, suggesting that practical applications in this range are very limited. Ammonia and R32 show the highest efficiencies (≈11%) at high pressure of 50 bar for the temperature range of 100–300 °C. The refrigerant R161 has high performance for pressures between 10 and 50 bar for the full range of temperatures from 80 to 300 °C, which makes R161 the choice fluid for a wide range of applications. Novel applications are introduced, which integrate these isobaric engines with vapor compression refrigeration systems. The thermodynamic cycles of the heat driven compressor with the selected working fluids as in the current study and the simplified technological solutions are crucial components of the study novelty.

Design and Analysis of a Valveless Impedance Pump for a Direct Sodium Borohydride–Hydrogen Peroxide Fuel Cell

Abstract A valveless impedance pump is designed and applied for the first time to supply the liquid fuels for a direct sodium borohydride–hydrogen peroxide fuel cell (DBHPFC). This valveless pump consists of an amber latex rubber tube, which is connected at both ends to rigid stainless-steel tubes of different acoustic impedance, and a simple actuation mechanism with a small direct control (DC) motor and a cam combined. The cam is activated by the motor and periodically compresses the elastic tube at a position asymmetrical from the tube ends. The traveling waves emitted from the compression combine with the reflected waves at the impedance-mismatched rubber tube/stainless-steel tube interfaces. The resulting wave interference creates a pressure gradient and generates a net flow. When connected to a DBHPFC with an active area of 25 cm2, the pump can deliver the fuel at a maximum pumping rate of 30 ml/min, resulting in corresponding DBHPFC maximum power and a current of 13.0 W and 25.5 A, respectively. The specific power, volumetric power density, and back work ratio of the DBHPFC with this pumping method have been proven superior to those of the other pumping configuration with peristaltic pumps. This valveless impedance pump is mechanically simply and less susceptible to corrosion, and it can reduce the volume and weight of fuel cell systems to a measurable extent. The experimental results demonstrate the feasibility of the device for practical DBHPFC applications.

Performance Analysis of a Multistage Centrifugal Pump Used in an Organic Rankine Cycle (ORC) System under Various Condensation Conditions

In an organic Rankine cycle (ORC) system, the working fluid pump plays an important role in the system performance. This paper focused on the operating characteristics of a multistage centrifugal pump at various speeds and condensation conditions. The experimental investigation was carried out to assess the influence of the performance of the pump by the ORC system with special attention to actual net power output, thermal efficiency as well as back work ratio (BWR). The results showed that an increase in the pump speed led to an increase in the mass flow rate and expand in the operating range of the outlet pressure. The mass flow rate decreased nonlinearly with the increase of the outlet pressure from 0.22 to 2.41 MPa; the electric power consumption changed between 151.54 and 2409.34 W and the mechanical efficiency of the pump changed from 7.90% to 61.88% when the pump speed varied from 1160 to 2900 r/min. Furthermore, at lower pump specific speed the ORC system achieved higher thermal efficiency, which suggested that an ultra-low specific speed pump was a promising candidate for an ORC system. The results also suggested that the effects of condensation conditions on the pump performance decreased with the pump speed increasing and BWR was relatively sensitive to the condensation conditions, especially at low pump speed.

Comparative study of RDE and conventional plant for moderate scale power generations

Electricity generation processes from coal burning at conventional plant and nuclear at Reaktor Daya Eksperimental (RDE) plant are compared using Cycle-Tempo. RDE is a conceptual demonstration nuclear plant with thermal capacity 10 MWth to generate steam and turbine to expand into condenser pressure and electricity generation. In reactor vessel, operating conditions are maintained on pressure drop 0.25 bars, inlet temperature 250 °C, and outlet temperature 700 °C. Environment conditions are set in pressure 1.013 bars and temperature 35 °C. Objective of present paper is to analyses performance of those plants on moderate scale power. In analysis, power plants are simplified into available components within code Cycle-Tempo, and approximated into steady-state flow. Mathematical formulation method is applied to calculate mass and energy balances on each component. Results show that RDE has values of generator power output 2863.8 kW, thermal efficiency 27.051 %, turbine efficiency 88.81 %, and back work ratio 0.87; conventional plant has values of generator power output 2863.8 kW, thermal efficiency 25.022 %, turbine efficiency 80.34 %, and back work ratio 0.869. It is concluded that RDE is properly used for utilizing nuclear heat source on moderate scale power plant.

Sensitivity analysis of operation parameters on the system performance of organic rankine cycle system using orthogonal experiment

In the present study, a 1 kW-scale experimental system which can be switched between BORC (basic organic Rankine cycle) and RORC (organic Rankine cycle with a regenerator) is employed. In order to analyze the influence of working parameters on the system performance quantitatively, the range analysis based on orthogonal design is adopted. The selected three system parameters are: the temperature at the inlet of the expander, temperature at the outlet of the condenser and the plunger travel of working fluid pump. For the system performance, six indices are selected, including the thermal efficiency, the back-work ratio, output power of the expander, pump work consumption, network and the mass flow of the working fluid. The optimal and worst level constitutions of factors for the six indices are concluded, the orders of the three factors' sensitivity to the six indices are also obtained. The results show that the temperature at the inlet of the expander is the dominant factor of the thermal efficiency, the back-work ratio, output power of the expander, and network. While the plunger travel of the working fluid pump is the dominant factor of pump work consumption and the mass flow of the working fluid.

Matching and operating characteristics of working fluid pumps with organic Rankine cycle system

The present paper focuses on the comparative experimental operating characteristics of working fluid pumps used in the organic Rankine cycle (ORC) system with R245fa, including multistage centrifugal pump, hydraulic diaphragm metering pump, and roto-jet pump. Effects of mass flow rate and outlet pressure are investigated systemically on the performance of the three pumps. Further theoretical studies are conducted on the effects of various types of working fluid pumps on the system overall performance, such as exergy destruction rate, back work ratio (BWR), heat absorption rate, thermal and exergy efficiency, and net power output. Results show that the maximum actual efficiency of the three pumps can reach 58.76%, 55.26%, and 30.51%, respectively, when outlet pressure ranges from 0.4 to 1.1 MPa. A significant difference was observed between actual and theoretical BWR, which illustrates that the pump power cannot be ignored and lower pump efficiency has a negative impact on the increase in the net power output of the ORC system. Additionally, the selection of working fluid pumps has significant effects on the heat absorption rate. Consequently, the paper provides detailed and valuable insights into the selection of working fluid pumps for the ORC system with different heat capacities.

Development of portable power unit with catalytic micro-combustor

A prototype of the portable power unit with coupling catalytic microcombustor, thermo-electric modules and air/fuel supply devices was developed. The developed power unit was completely self-standing; the air supply system was driven by a part of electricity generated at the generator unit. When the thermal input from the combustor was 12W, the total electricity is 330 mW and the net output electricity was 225 mW, which corresponded to the back work ratio of 32%. The final conversion efficiency of the total system reached 1.87%, which was order of world record.

Experimental study and performance analysis of a hydraulic diaphragm metering pump used in organic Rankine cycle system

In this paper, a performance test of a hydraulic diaphragm metering pump using R123 is conducted. The interaction relationships of key parameters of the pump and its influence on the performance of the ORC system are analyzed. The application feasibility of the pump has been proved by comparison to previous studies. The results indicate that the mass flow rate of the pump varies from 0.23 t/h to 2.06 t/h and is mostly independent of outlet pressure. Both power input and actual efficiency of the pump increase with stroke; the actual efficiency reaches up to a maximum of 88.27%. The specific speed decreases with the increase of outlet pressure. Moreover, the actual net power output and thermal efficiency of the ORC system increase with evaporating temperature. Actual net power output is more sensitive than thermal efficiency to stroke. The thermal efficiency presents a nonlinearly rapid decline trend with the increase of specific speed. Back work ratio (BWR) can reach up to a maximum of 0.93. Thus, the power input of working fluid pump is not negligible in the ORC system and the assumptions of actual efficiency of pump should be dependent on experimental results according to various operating conditions.

Techno-economic comparison of different cycle architectures for high temperature waste heat to power conversion systems using CO2 in supercritical phase

Bottoming thermodynamic systems based on supercritical carbon dioxide as working fluid (sCO2) are a promising technology to tackle the waste heat to power conversion at high temperature levels and that might outperform the conventional power units based on Organic Rankine Cycles. In fact, CO2 is an inexpensive, non-toxic, non-flammable, thermally stable and eco-friendly compound. Moreover, CO2 in its supercritical state shows an extreme increase in density that allows turbomachinery downsizing and a high cycle efficiency due to the reduced work required by the compression stage. In addition, supercritical CO2 permits a better temperature glide matching within the heat source which increases the overall efficiency of waste heat utilization. With the aim of identifying pro and cons of different sCO2 cycle layouts, this paper investigated four design Joule-Brayton configurations at increasing complexity: simple regenerative, with recompression, with reheating and with recompression and reheating. The research methodology is based on 1st and 2nd laws thermodynamic analyses and includes correlations to estimate the investment costs of the equipment. With reference to a high temperature industrial waste heat source, performance, costs and exergy losses in the different cycle layouts are compared. Furthermore, a parametric analysis regarding the effects of the cycle pressure ratio on net power output and back work ratio is carried out.

Design of Radial Inflow Turbine for 30 kW Microturbine

Microturbines are small gas turbines that have the capacity range of 25-300 kW. The main components of microturbine are compressor, turbine, combustor and recuperator. This research paper focuses on the design of radial inflow turbine that operates in 30 kW microturbine. In order to operate the 30 kW microturbine with the back work ratio of 0.5, the radial inflow turbine should be designed to produce power at 60 kW. With the help of theory of turbo-machinery and the analytical methods, the design parameters are derived. The design results are constructed in 3D geometry. The 3D fluid-geometry is validated by computational fluid dynamics (CFD) simulation. The simulation results show the airflow path, the temperature distribution, the pressure distribution and Mach number. According to the simulation results, there is no flow blockage between vanes and no shock flow occurs in the designed turbine.

Operation characteristic of a R123-based organic Rankine cycle depending on working fluid mass flow rates and heat source temperatures

The test and operation characteristic of an organic Rankine cycle using R123 and a scroll expander have been investigated. The steady-state operation characteristic is addressed with the varying working fluid mass flow rates ranging of 0.124–0.222kg/s and heat source temperatures ranging of 383.15–413.15K. The behaviors and detailed discussion for those four major components (pump, evaporator, expander and condenser) are examined. The experimental results show that the environmental temperature presents a higher influence on the pump behaviors. The range of pump power consumption, isentropic efficiency and back work ratio are 0.21–0.32kW, 26.76–53.96%, and 14–32%, respectively. The expander isentropic efficiency presents a slight decrease first and then a sharp increase with mass flow rate, while a degree of superheating more than 3K is necessary to avoid expander cavitation. The expander isentropic and generator efficiencies are in range of 69.10–85.17% and 60–73%, respectively, while the respective heat transfer coefficients for evaporator and condenser are ranging of 200–400 and 450–2000W/m2K. The maximum expander shaft power and electrical power are 2.78kW and 2.01kW, respectively, while the maximum system generating efficiency is 3.25%. Moreover, the tested thermal efficiency presents a slight decrease trend with mass flow rate.

Determination of Optimized Parameters for the Flexible Operation of a Biomass-Fueled, Microscale Externally Fired Gas Turbine (EFGT)

Biomass as a source of renewable energy is a promising solution for current problems in energy supply. Olive waste is considered as an interesting option, especially for Mediterranean countries. Within this paper, a microscale externally fired gas turbine (EFGT) technology is presented as a decentralized power plant, within the range of 15 kWth, based on olive residues. It was modeled by Aspen Plus 8.6 software to provide a sufficient technical study for such a plant. Optimized parameters for pressure ratio and turbine air-mass flow have been mapped for several loads to provide information for process control. For all cases, mechanical output, efficiency curves, and back-work ratio have been calculated. Using this information, typical plant sizes and an example of power production are discussed. Additionally, achievable energy production from olive waste is estimated on the basis of this data. The results of this study show that such a plant has an electrical efficiency of 5%–17%. This variation is due to the examination being performed under several combustion temperatures, actual load, heat exchanger temperatures, and heat transfer efficiency. A cost estimation of the discussed system showed an estimated capital cost of 33,800 to 65,300 € for a 15 kWth system.