Off-design Performance Research Articles

Optimization of Centrifugal Pump Based on Impeller-Volute Interactions

The design and off-design performance of a centrifugal pump largely depends on geomechanical parameters. This study aims at enhancing the performance by optimizing three geomechanical parameters of impeller-volute interactions. The present optimization is carried out using the Taguchi method combined with a numerical approach. A comparison between the base and optimized pumps is presented under the design and off-design conditions based on numerical and experimental analyses. The numerical results reveal that, compared to the base pump, the optimized pump shows the improved performance through uniform pressure distribution in the impeller, the reduced low-pressure region towards a blade’s leading edge, and the stable total pressure at the impeller-volute interaction zone. The experimental results suggest that the optimized pump covers a wider range of operation, and its best efficiency point (BEP) is 10%, 5%, and 12% higher in flow rate, head, and efficiency, as compared to the base one.

Off-design modeling of a microturbine combined heat & power system

There is increasing interest in decision support software tools that evaluate the techno-economic potential of distributed energy system technologies for deployment in combined heat and power (CHP) applications, such as commercial buildings, microgrids, and connected communities. This trend is particularly evident among end-users looking to integrate renewable resources with more traditional CHP technologies for on-site generation and enhanced resiliency. However, many of these software tools do not account for part-load and off-design characteristics of prime mover and heat recovery equipment, resulting in lower accuracy for both the optimal selection and sizing and the economic value proposition of integrated hybrid renewable energy-CHP systems. Microturbine technology is an attractive distributed energy resource whose performance is sensitive to both part-load and off-design (i.e., excursions of ambient conditions away from the design point temperature and pressure) operating conditions but is widely unreported. In the present work, we address this deficiency by developing a detailed, thermodynamic model of a commercial 200 kW microturbine CHP system and paramaterizing results for incorporation into the Renewable Energy Optimization (REopt) tool developed by the National Renewable Energy Laboratory. The details of the microturbine model, including model benchmarking and validation, are presented. Microturbine off-design modeling includes part-load analysis, assessment of ambient sensitivities, and mapping heat recovery heat exchanger response to hot water grade requirements. The model is then exercised in several illustrative examples to depict the numerous factors which alter electric power, electric efficiency, and hot water recovery. Model results show, for example, predictions of a 4 percentage point electric efficiency range occurring between intake air temperatures of -18 to 50 °C. Likewise, the model closely predicts the 25 % loss in heat recovery while at -18 °C and the 8 % gain in heat recovery while at 35 °C. An additional case shows how heat recovery may increase or decrease by 9.5 % of the design value when changing the return water temperature plus-or-minus 22 °C. In particular, the impact of ambient conditions on best-possible microturbine performance is conveyed through hourly simulations of the system subjected to the weather of different geographic locations within the United States. • A model of a 200 kW microturbine configured for hot water recovery is developed. • Part-load modeling approach and simulation results for both electric efficiency and waste heat recovery are detailed. • Off-design performance as functions of ambient temperature and pressure is explored. • Heat recovery performance maps as functions of return and supply hot water temperatures are developed.

A review on integrated design and off-design operation of solar power tower system with S–CO2 Brayton cycle

The solar power tower (SPT) system integrated with supercritical CO2 (S–CO2) Brayton cycle is a potential flexible power output station to balance supply and demand in the future power system with high renewable energy penetration, so as to maintain the reliability of power supply. Reasonable design and accurate parameter adjustment are crucial to the efficient, low-cost, and reliable operation of the SPT system with S–CO2 Brayton cycle. Many works have studied the design optimization and operation control strategy of the system. But the research focuses and conclusions are various, and there is a lack of important summary in this field. This paper provides a review on the integrated design and off-design operation of SPT system with S–CO2 Brayton cycle. In terms of system integrated design, the performance criteria, subsystem selections, and key parameter optimizations are summarized. Furthermore, an improved system design method based on off-design operating performance is introduced and recommended. In terms of system off-design operation, the fluctuating factors, parameter control strategies, and the actual operation results are introduced. The conclusions and study prospects for this promising technology in the aspects of design and operation are presented.

Off-design and flexibility analyses of combined cooling and power based liquified natural gas (LNG) cold energy utilization system under fluctuating regasification rates

The Liquified Natural Gas (LNG) regasification rate tends to fluctuate during the diurnal period due to the varying demand of natural gas. This fluctuation leads to the energy mismatch between LNG cold energy utilization system and users. Consequently, the actual LNG cold energy utilization efficiency deteriorates significantly. In this study, a novel cold storage-based combined cooling and power system has been designed to improve the efficiency and versatility of LNG cold energy utilization under fluctuating regasification rates. The system incorporates a CO2 based Rankine cycle, a direct expansion unit, and cooling recovery units. The waste heat (90 °C) harnessed from gas power plants instead of seawater (15 °C) is employed as the heat source to improve the Rankine cycle performance. Further, cold energy storage tanks are adopted to decouple the power and cooling outputs from the fluctuating regasification rates. Off-design performance of the system is investigated under the different regasification rates. Key results have revealed that the power output profile from the conventional system is monotonous and the valley-to-peak ratio of power is as low as 0.05, resulting in potential downtime for downstream users. In contrast, the proposed system implements a versatile power and cooling strategy to satisfy different users’ demands. Results from two case studies demonstrate that the valley-to-peak ratio of power can be markedly improved to values spanning 0.4 to 1. The daily cold recovery rate, exergy efficiency, and reducing CO2 emission approach 53.45%, 25.26%, and 645.13 tons/day, respectively, for a 3 megatons per annum regasification LNG plant. The payback period for the proposed system is 0.67 years, which is almost half of the conventional system’s period.

Optimal solar thermal retrofit for geothermal power systems considering the lifetime brine degradation

Geothermal power plants usually suffer from performance deterioration during the operation due to the geothermal resource's degradation. Combining solar thermal energy to retrofit the system is considered an effective way to alleviate this problem. This study explores the off-design performance of organic Rankine cycle-based hybrid system throughout a 30-year life span, rather than a typical single year as the previous research. Then several interesting questions are consequently discussed: (1) Which year is most suitable for system retrofit? (2) How many times should the system be retrofitted? (3) How much is the difference between the 30-year-based results and the conventional single-year-based analysis? Results indicate that the single-year-based approach could greatly underestimate the Levelized cost of electricity (LCOE) by up to 18% and also significantly overestimate the increment in electricity generation (Etot). Thus a life-span-based approach is highly recommended for geothermal-based hybrid plants. Besides, the retrofit year has huge impacts on both thermodynamic and economic performance, and the optimal retrofit should be carried out in 6th year. Moreover, this study reveals that multiple retrofits can indeed further increase the Etot but are not cost-effective. Therefore, a single solar thermal retrofit is recommended for existing geothermal power systems rather than multiple retrofits.

Off-Design Performance of 9F Gas Turbine Based on gPROMs and BP Neural Network Model

Off-Design Performance of 9F Gas Turbine Based on gPROMs and BP Neural Network Model

Design and Off-Design Performance Analysis of a Zigzag Channeled Pre-Cooler for Indirect Cooling System of Supercritical Co2 Recompression Cycle

Design and Off-Design Performance Analysis of a Zigzag Channeled Pre-Cooler for Indirect Cooling System of Supercritical Co2 Recompression Cycle

Experimental Characterization of an Adaptive Supersonic Micro Turbine for Waste Heat Recovery Applications

Micro turbines (<100 kWel) are commercially used as expansion machines in waste heat recovery (WHR) systems such as organic Rankine cycles (ORCs). These highly loaded turbines are generally designed for a specific parameter set, and their isentropic expansion efficiency significantly deteriorates when the mass flow rate of the WHR system deviates from the design point. However, in numerous industry processes that are potentially interesting for the implementation of a WHR process, the temperature, mass flow rate or both can fluctuate significantly, resulting in fluctuations in the WHR system as well. In such circumstances, the inlet pressure of the ORC turbine, and therefore the reversible cycle efficiency must be significantly reduced during these fluctuations. In this context, the authors developed an adaptive supersonic micro turbine for WHR applications. The variable geometry of the turbine nozzles enables an adjustment of the swallowing capacity in respect of the available mass flow rate in order to keep the upper cycle pressure constant. In this paper, an experimental test series of a WHR ORC test rig equipped with the developed adaptive supersonic micro turbine is analysed. The adaptive turbine is characterized concerning its off-design performance and the results are compared to a reference turbine with fixed geometry. To create a fair data basis for this comparison, a digital twin of the plant based on experimental data was built. In addition to the characterization of the turbine itself, the influence of the improved pressure ratio on the energy conversion chain of the entire ORC is analysed.

Searching for the Most Suitable Loss Model Set for Subsonic Centrifugal Compressors Using an Improved Method for Off-Design Performance Prediction

This work investigates loss model sets based on empirical loss correlations for subsonic centrifugal compressors. These loss models in combination with off-design performance prediction algorithms make up an essential tool in predicting off-design behaviour of turbomachines. This is important since turbomachines rarely work under design conditions. This study employs an off-design performance prediction algorithm based on an iterative process from Galvas. Modelling of ten different loss mechanisms and physical phenomena is involved in this approach and is thoroughly described in this work. Geometries of two subsonic compressors were reconstructed and used in the evaluation of individual loss correlations in order to obtain a suitable loss model. Results of these variations are compared to experimental data. In addition, 4608 loss model sets were created by taking all possible combinations of individual loss estimations from which three promising candidates were selected for further investigation. Finally, off-design performance of both centrifugal compressors was computed. These results were compared to experimental data and to other loss model sets from literature. The newly composed loss model set No. 2137 approximates experimental data over a 21.2% better in relative error than the recent Zhang set and nearly a 36.7% better than the outdated Oh’s set. Therefore, set No. 2137 may contribute to higher precision of centrifugal turbomachines’ off-design predictions in the upcoming research.

Modeling and Off-Design Performance Analysis of a Screw Expander-Based Steam Pressure Energy Recovery System in a Combined Heat and Power Unit.

A screw expander-based heating system is proposed based on a 330 MW combined heat and power unit to recover the extraction steam pressure energy. EBSILON Professional software was applied to model the proposed system, and the thermal performance of the traditional system and the new system was compared under different operating conditions. The results show that under the designed heating condition, the standard coal consumption rate of the new system is reduced by 4.74 g/kWh, and the exergy efficiency of the heating process is increased by 17.29%. When the extraction steam pressure changes with the load within a range greater than 0.1 MPa, there is an optimal operating range for the new system, which is related to the built-in pressure ratio of the screw expander and the extraction steam pressure difference, and the distribution of the optimal operating range varies with the supply-water temperature. The heating process exergy efficiency of the two systems shows opposite trends under off-design conditions. Additionally, under the condition of meeting the extraction steam pressure, the method for increasing the output power of the screw expander by adjusting the butterfly valve to increase the extraction steam pressure only increases the standard coal consumption rate of the system. Furthermore, through an application case study, the annual economic benefit of the new system is expected to be ¥ 4.15 million, and the payback period is 5.02 years.

Turbine Design and Optimization for a Supercritical CO2 Cycle Using a Multifaceted Approach Based on Deep Neural Network

Turbine as a key power unit is vital to the novel supercritical carbon dioxide cycle (sCO2-BC). At the same time, the turbine design and optimization process for the sCO2-BC is complicated, and its relevant investigations are still absent in the literature due to the behavior of supercritical fluid in the vicinity of the critical point. In this regard, the current study entails a multifaceted approach for designing and optimizing a radial turbine system for an 8 MW sCO2 power cycle. Initially, a base design of the turbine is calculated utilizing an in-house radial turbine design and analysis code (RTDC), where sharp variations in the properties of CO2 are implemented by coupling the code with NIST’s Refprop. Later, 600 variants of the base geometry of the turbine are constructed by changing the selected turbine design geometric parameters, i.e., shroud ratio (rs4r3), hub ratio (rs4r3), speed ratio (νs) and inlet flow angle (α3) and are investigated numerically through 3D-RANS simulations. The generated CFD data is then used to train a deep neural network (DNN). Finally, the trained DNN model is employed as a fitting function in the multi-objective genetic algorithm (MOGA) to explore the optimized design parameters for the turbine’s rotor geometry. Moreover, the off-design performance of the optimized turbine geometry is computed and reported in the current study. Results suggest that the employed multifaceted approach reduces computational time and resources significantly and is required to completely understand the effects of various turbine design parameters on its performance and sizing. It is found that sCO2-turbine performance parameters are most sensitive to the design parameter speed ratio (νs), followed by inlet flow angle (α3), and are least receptive to shroud ratio (rs4r3). The proposed turbine design methodology based on the machine learning algorithm is effective and substantially reduces the computational cost of the design and optimization phase and can be beneficial to achieve realistic and efficient design to the turbine for sCO2-BC.

Off-design operation of the dry-cooled supercritical CO2 power cycle

The supercritical CO2 recompression cycle (the ‘sCO2 cycle’) is highly efficient, scalable, simple, and compatible with dry cooling. Two scenarios where dry-cooled sCO2 cycles may operate off design are (1) during periods of high ambient temperature, and (2) when operating at part-load output (in load-following roles). Understanding sCO2 cycle off-design performance is essential for financial feasibility assessment, detailed design, and providing control system setpoints. Off-design performance can be maximised by exploiting the cycle’s pressure, temperature, and turbomachinery speed degrees of freedom. Several previous studies arbitrarily constrain these degrees of freedom, giving sub-optimal performance predictions. Other studies fail to consider CO2’s steep and non-linear property variations, leading to inaccurate results. To address these limitations, this work presents a detailed sCO2 cycle model, which captures property changes and component off-design behaviour. The model is used to systematically explore the off-design operating space, thus showing novel off-design performance trends and best operating strategies. Elevated ambient temperatures reduce power output more than efficiency, but this effect can be mitigated with appropriate operating strategies. At an ambient temperature of 40 °C (10 °C above design), design-point power output can be maintained with an 8.7% minimum turbine pressure increase, alongside compressor speed control. Alternatively, at this same temperature, design-point power output can be maintained with only a 4.4% (2 percentage point) efficiency reduction and a slightly higher (13.7%) turbine pressure increase. During turn-down, design-point efficiency can be maintained at 44% of design-point power output by using pressure and compressor speed control. Main compressor speed control alone is effective for above-design ambient temperatures, while dual compressor speed control provides significant benefits during turn-down. Compared to past sCO2 cycle studies, this work reports better off-design performance, which demonstrates the importance of off-design operating strategies and highlights the flexibility and potential of the cycle.

Development of a fully deterministic simulation model for organic Rankine cycle operating under off-design conditions

A fully deterministic simulation model that requires only external boundary conditions as input parameters is newly developed for predicting the off-design performance of an organic Rankine cycle. Accurate prediction of the evaporation and condensation pressures without using any assumptions has been of major concern. In this context, the actual pressure formation characteristics are reflected into the model to identify the high- and low-pressures. The mass balance of the system is realized by applying the proposed passive design of the liquid receiver. The sub-models of each component are integrated into a three-stage solver which iterates under the law of mass and energy conservation. 77 sets of experimental data were collected from a 1 kW scale testbed using R245fa as the working fluid. The developed model is verified by examining the thermodynamic states of the working fluid and the energy balance error of the simulation results is less than 0.3%. The simulation results are compared to the experimental results and are validated within 8% error range. Also, the computational time reduced from 412 h to 93 s after applying the meta-models with only 0.03% difference in thermal efficiency. The effects of various modelling methods are compared to each other which emphasizes the importance of the newly proposed reality-based logics. The simulation model can detect several operational failure scenarios and accurately predict the off-design performance of the system. The developed model is fully predictive without imposing internal assumptions and has the potential to be utilized in various applications without conducting excessive experiments.

Off-Design of a Pumped Thermal Energy Storage Based on Closed Brayton Cycles

Abstract The growth of renewable energy source requires reliable, durable, and cheap storage technologies. In this field, the pumped thermal energy storage (PTES) is drawing some interest as it appears not to be affected by geographical limitations and use very cheap materials. PTES is less efficient than pumped hydro and batteries, but it could achieve satisfactory efficiencies, show better economic performance, and be characterized by negligible environmental impacts. A PTES stores the electric energy as thermal exergy in solid packed beds, by operating two closed Brayton cycles, one for charging and the other one for discharging. Although PTES thermodynamical behavior is well understood, the interaction between the components is rarely investigated. This study investigates the impact of packed-bed behavior on turbomachines operating conditions. In this way, PTES off-design and part-load performance are estimated. A control strategy especially suited for closed Brayton cycles, i.e., the inventory control (IC), is used to control the system. As it resulted, PTES is characterized by an excellent part-load performance, which might be a significant advantage over the competing technologies. However, the off-design operation induced by the packed-bed thermal behavior might significantly reduce the system performance and, in particular, that of the discharge phase.

Optimization research on the off-design characteristics of partial heating supercritical carbon dioxide power cycle in the landfill gas exhaust heat utilization system

• Off-design characteristics of partial heating supercritical CO 2 cycle are studied. • A novel calculation flow chart for the system off-design performance is proposed. • Separation control and inventory control are adopted for the thermal system. • A best control scheme relied on the net power maximization can be determined. Landfill gas waste heat power generation is an effective way to promote the efficiency and economics of power plants. However, the operating performance of the power generation system needs to be improved further. In this paper, a systematic optimization research on the off-design characteristics of the partial heating supercritical carbon dioxide power system for exhaust heat utilization from gas turbine cycle are conducted. Optimal operating conditions in thermodynamics and economics obtained by genetic algorithm are determined as design conditions. Gradient descent method is used to obtain the best operating point under a certain off-design working condition. With the goal of maximizing net power output, separation control and inventory control are applied to the system. The results exhibit that the best thermoeconomic performance can be gained when the pressure ratio is 2.28 with separation ratio of 0.35 at design stage. In the off-design stage, the maximum net power shows an approximately linear growth trend from 1581 kW to 2924 kW with increasing heat source temperature and mass flow rate under the inventory control, while growth rate of net power output gradually decreases under the separation control. Finally, the best control scheme mixed with inventory control and separation control is proposed and determined quantitatively.

A holistic thermoeconomic assessment of small-scale, distributed solar organic Rankine cycle (ΟRC) systems: Comprehensive comparison of configurations, component and working fluid selection

In this paper, results from comprehensive thermoeconomic assessments of small-scale solar organic Rankine cycle (ORC) systems are presented based on weather data in London, UK, which is taken as representative of a temperate climate with modest temperature changes, mild winters and moderate summers. The assessments consider a range of: (i) solar collector types (flat-plate, evacuated-tube, and evacuated flat-plate collectors); (ii) power cycle configurations (basic/recuperative, partial/full evaporating, and subcritical/transcritical cycles); (iii) expander types (scroll, screw, and piston) and designs; and (iv) a set of suitable working fluids. All possible solar-ORC system designs are optimised by considering simultaneously key parameters in the solar field and in the power cycle in order to obtain the highest electricity generation, from which the best-performing systems are identified. Selected designs are then subjected to detailed, annual simulations considering the systems’ operation, explicitly considering off-design performance under actual varying weather conditions. The results indicate that, among all investigated designs, solar-ORC systems based on the subcritical recuperative ORC (SRORC), evacuated flat-plate collectors (EFPCs), a piston expander, and isobutane as the working fluid outperforms all the other system designs on thermodynamic performance, whilst having the highest annual electricity generation of 1,100 kW·h/year (73 kW·h/year/m2) and an overall thermal efficiency of 5.5%. This system also leads to the best economic performance with a levelised cost of energy (LCOE) of ~1 $/kW·h. Apart from the specific weather data used for these detailed system simulations, this study also proceeds to consider a wider range of climates associated with other global regions by varying the solar resource available to the system. Interestingly, it is found that the optimal solar-ORC system design remains unchanged for different conditions, however, the LCOE can drop below 0.35 $/kW·h and payback times can be shorter than 16 years in high solar-resource regions, even in the absence of incentives that would otherwise lead to even better economic performance. This work complements previous efforts in the literature by considering the full design and operational features of solar-ORC systems, thereby providing valuable guidance for selecting appropriate cycle configurations, components, working fluids and other characteristics and, for the first time, presents a comprehensive comparison of such systems in small-scale applications.

Multivariate Design and Analysis of Aircraft Heat Exchanger Under Multiple Working Conditions Within Flight Envelope

Abstract This paper presents a multi-condition design method for the aircraft heat exchanger (HEX), marking with lightweight, compactness, and wide range of working conditions. The quasi-traversal genetic algorithm (QT-GA) method is introduced to obtain the optimal values of five structural parameters including the height, the tube diameter, the tube pitch, and the tube rows. The QT-GA method solves the deficiency of the conventional GA in the convergence, and gives a clear correlation between design variables and outputs. Pressure drops, heat transfer, and the weight of the HEX are combined in a single objective function of GA in the HEX design, thus the optimal structure of the HEX suitable for all the working conditions can be directly obtained. After optimization, the weight of the HEX is reduced to 2.250 kg, more than 20% lower than a common weight of around 3 kg. Based on the optimal structure, the off-design performance of the HEX is further analyzed. Results show that the extreme working conditions for the heat transfer and the pressure drops are not consistent. It proves the advance of the multi-condition design method over the traditional single-condition design method. In general, the proposed QT-GA design method is an efficient way to solve the multi-condition problems related to the aircraft HEX or other energy systems.

On rotor hub design for shrouded hydrokinetic turbines

The effect of hub length and axial location on the performance of shrouded hydrokinetic turbines has been investigated using axisymmetric actuator disk simulations. Five systems with different hubs (or without a hub, for reference) are considered with a common shroud design. Flow separation on the hub was found to be detrimental to performance, and to be sensitive to rotor loading, leading to unacceptable off-design performance. When the thrust coefficient, CT, is below the optimal value of 8/9, increasing CT promotes flow separation on the central hub; continued increases beyond this value facilitate reattachment. Flow separation on the hub was avoided altogether when most of the hub was placed upstream of the rotor plane—contrary to the convention for unshrouded turbines—where the shroud geometry lends a favorable pressure gradient. This latter design achieved performance in close agreement with the open-centered design, which is recommended when feasible to avoid the risks of hub-flow separation. With a central hole diameter of 0.228D, where D is the rotor diameter, a penalty of <5% in optimal performance was found compared to an actuator disk spanning the entire shroud throat. Lastly, a discussion of the relationship between total system thrust and power extraction is offered.

Bayesian Optimization Design of Inlet Volute for Supercritical Carbon Dioxide Radial-Flow Turbine

The radial-flow turbine, a key component of the supercritical CO2 (S-CO2) Brayton cycle, has a significant impact on the cycle efficiency. The inlet volute is an important flow component that introduces working fluid into the centripetal turbine. In-depth research on it will help improve the performance of the turbine and the entire cycle. This article aims to improve the volute flow capacity by optimizing the cross-sectional geometry of the volute, thereby improving the volute performance, both at design and non-design points. The Gaussian process surrogate model based parameter sensitivity analysis is first conducted, and then the optimization process is implemented by Bayesian optimization (BO) wherein the acquisition function is used to query optimal design. The results show that the optimized volute has better and more uniform flow characteristics at design and non-design points. It has a smoother off-design conditions performance curve. The total pressure loss coefficient at the design point of the optimized volute is reduced by 33.26%, and the flow deformation is reduced by 54.55%.

An Iterative Approach towards Single stage Axial Fan Design using Off Design Prediction

A single-stage axial fan having a pressure ratio of 1.01 is designed in the current study. The design pressure ratio is chosen based on the power available from the existing motor (2.2 kW). The design space for the axial flow fan was generated by varying specific flow and geometrical parameters in suitable steps, using a program written in MATLAB. The varied flow parameters are mass flow rate, inlet Mach number, inlet flow angle, and rotor speed. The geometrical parameters that were varied are hub to tip ratio, aspect ratio, and blade solidity. Using these as the input variables and applying free vortex theory for 3-dimensional blade design, the aerodynamic design of the axial flow fan was carried out. Performance parameters like flow coefficient, stage loading coefficient, degree of reaction, diffusion factor, De Haller’s number, and blade angles were calculated at the blade’s hub, mean, and tip. Total design space of 92160 data points was obtained from the combination of input parameters. Several constraints were applied to optimise the design space based on the available power from the existing motor and in-house manufacturing limitations. The initial design space was reduced to 82 data points using these constraints. To further reduce the number of points in the design space, off-design performance was evaluated for each of these data points. Following this, one design point was selected based on the optimum performance range in off-design operation, while considering manufacturing limitations. Using Mellor charts, a suitable blade profile was chosen based on the inlet and exit blade angles. NACA 65-410 airfoil was selected with a stagger of 55 degrees and an incidence of 6 degrees for optimum performance.