Off-design Working Conditions Research Articles

Numerical analysis of scroll compressor performance with pre-discharge valve for low-temperature heat pumps

Scroll compressors are a crucial component of automotive air conditioning systems. When operating at off-designed working conditions, over-compression can occur in the scroll compressor. A pre-discharge valve (PDV) is proposed to enhance the performance of the scroll compressor under off-design working conditions. The performance of the scroll compressor with a PDV for a low-temperature heat pump system is simulated. The results show that the scroll compressor with the PDV structure has lower pressure in the compression chambers and improved over-compression compared to conventional scroll compressors. The use of the PDV also leads to a reduction in the temperature of the refrigerant gas in the compression chambers and a decrease in the leakage between the compression chambers. As the evaporating temperature increases from −25 °C to −5 °C, the mass flow rate and volumetric efficiency increase, and the discharge temperature decreases. At an evaporating temperature of −5 °C, the discharge temperature at the center discharge port of the scroll compressor with the PDV is reduced by 2.4 °C compared to the conventional scroll compressor. The input power of the scroll compressor is reduced by 165.4 W–897.7 W. The volumetric efficiency and the isentropic efficiency are improved by 0.13 % and 6.63 % respectively.

Comparative study on the organic rankine cycle off-design performance under different zeotropic mixture flow boiling correlations

Zeotropic mixture utilization enhances the performance of the Organic Rankine cycle (ORC) significantly. For geothermal ORC power plants, the zeotropic mixture flow boiling correlation plays a vital role in both evaporator design and system performance evaluation. Based on the screened zeotropic mixture R1224yd(Z)/R1234ze(E) (0.452/0.548), a comparative analysis of 4 flow boiling correlations is performed to assess their impact on both evaporator design and system performance under off-design conditions. The results underscore a substantial impact of these correlations on determining the plate length, with the largest relative difference being 53.8%, consequently resulting in uncertainties of 31.7% and 23.4% for the evaporator exergy loss and cost, respectively. Under off-design working conditions, the relative difference in net power output varies from 16.43% to 7.67%. The exergy efficiency variation is up to 11.82% and the electricity production cost (EPC) uncertainty is up to 11.2% at the working condition with the maximum difference in net output work. This study provides valuable insights for selecting the appropriate zeotropic mixture flow boiling correlation in practical evaporator design and evaluating the off-design performance of ORC systems.

Research on operational characteristics of coal power centrifugal fans at off-design working conditions based on flap-angle adjustment

The coal power centrifugal fan (CF) is one of the most significant equipment in coal power plants, which usually works at off-design conditions according to the demand of electricity consumption. Consequently, the efficiency of CFs is far lower than the designed maximum efficiency under this condition, causing large amounts of potential energy wasted. A novel flap-angle adjustment (FAA) method is proposed to efficiently solve this problem in the CF system. This manuscript establishes a theoretical model of the flap-angle adjustable centrifugal fan (FAACF). The operating economy of FAACF at both design and off-design conditions is comprehensively studied. The internal flow mechanism of efficient operation is analyzed, which reveals the more stable flow state inside CF impellers at off-design conditions. By comparing with the traditional inlet guide vanes CF (IGVCF), the application of the FAA improves the energy efficiency of the CF system significantly. A wider high-efficiency range is obtained, which is superior to the IGVCF. The numerical results effectively reveal the mechanisms by which flow loss occurs in the FAACF at the outlet of blades. In particular, the energy-saving rate of the G4-73-11 type CF system based on the FAA can reach 20.4 % when the relative flow ratio decreases to 74 %, which proves the positive adjustment potential of energy efficiency and indicates great energy-saving benefits of the FAA application in the CF system.

Off-design performance and control strategies of sCO2 recompression power systems considering compressor operating safety

The supercritical carbon dioxide (sCO2) power cycle is a promising power cycle in the field of thermal power generation. The system is often operated under off-design conditions, which would lead to the unsafe or unstable operation of the sCO2 power system. The performance of the sCO2 recompression cycle system under off-design working conditions by different control strategies is investigated in this paper. A one-dimensional design prediction model for centrifugal compressors is introduced to predict off-design system performance and the operating conditions and safety of centrifugal compressors under the 5 control strategies are simulated and compared. To show the difference between the control strategies, the thermal efficiency was compared, the highest thermal efficiency of the system is 11.2 % by the inventory control method, and the lowest thermal efficiency is 5.45 % by the turbine bypass control method. The compressor backflow control method makes the main compressor has a choking risk and the recompressor has a surging risk when the output power of the system is small. The speed variations of centrifugal compressors are 39.55 % (MC) and 34.71 % (RC) using the inventory control method. A hybrid control strategy (the compressor backflow and inventory control strategies) is proposed based on the performance maps of centrifugal compressors. The turbine efficiency is improved by 4.5 % and the system thermal efficiency is improved by 7.1 % by using the hybrid control strategy. The hybrid control strategy can solve the problem of dangerous working conditions of centrifugal compressors. The results of this study can guide the system control and operational safety of sCO2 recompression cycles.

Photothermal conversion potential of full-band solar spectrum based on beam splitting technology in concentrated solar thermal utilization

It is still a great challenge for the concentrated solar thermal (CST) technology to promote the utmost out of solar energy further because the CST's receiver generally discards the longer wavelength spectra to prevent radiative heat loss due to high temperatures. Spectrum splitting is a well-known technology generally used in multi-energy cogeneration devices, such as Photovoltaic/thermal (PV/T) systems, aiming to improve photoelectrical efficiency. In this study, the solar spectrum splitting technology is employed in the CST system to take full advantage of solar radiation through enhanced absorptions and conversions of piecewise solar radiation. The exergy efficiency of a beam-splitting photothermal (BSPT) system is used to evaluate its effectiveness. Here, the mechanisms of beam splitting under different splitting wavelengths (λsp) and concentration ratios (Cr) are investigated. Furthermore, the influence of the split number is evaluated, and the performance of the dual-segment BSPT system is compared with traditional photothermal systems with real and assumed coatings. It shows that beam splitting can play an excellent role in improving exergy performance. The exergy efficiency improvement ratio of the BSPT stabilizes at around 9% when λsp is 1300 nm, and the proposed dual-segment BSPT system can maintain an excellent performance even under off-design working conditions.

Advances in aerodynamics of power turbines for marine and aviation applications

The use of power turbines has the advantage of flexible output load adjustment, and is a major configuration of marine gas turbines and aviation turboshaft/turboprop engines. Marine gas turbines and aviation turboshafts and turboprop engines generally have multiple working states and multiple working conditions change laws, which cause the aerodynamic performance of power turbines to fluctuate greatly with changes in working conditions, and different variable working conditions have a severe impact on the turbine loss characteristics. Therefore, it is necessary to deeply understand the internal flow mechanism of power turbines under off-design working conditions in different power turbine application modes, and carry out researches on the design methods of power turbines under wide working conditions. This paper summarizes and analyzes the recent advances in the field of aerodynamics of power turbines for marine and aviation applications. This review covers the following topics that are important for power turbine designs: (1) turbine flow loss prediction models under multiple operating conditions, (2) aerodynamic design and flow mechanism of power turbines for marine gas turbines, (3) aerodynamics of variable speed power turbines for turboshaft/turboprop engines, and (4) other issues about power turbines for marine and turboshaft/turboprop engines. The emphasis is placed on the aerodynamic design and flow mechanism of marine and aviation power turbines. We also present our own insights regarding the current research trends and the prospects for future developments.

Dökme yük gemisi için Rejeneratif Organik Rankine Çevrimi Sisteminin Dekarbonizasyon Üzerindeki Etkisinin Araştırılması

Shipping has a very important share in world trade. However, it has an inevitable effect on global greenhouse gas emissions. Therefore, there is a great motivation for the reduction of fuel consumption and exhaust emissions. Waste heat recovery systems based on Organic Rankine Cycle (ORC) technology have a significant potential to reduce fuel consumption and exhaust emissions. In this study, the optimization of the regenerative ORC was carried out for a bulk carrier. Multi-objective optimization was performed using a Grey Wolf Optimization algorithm that is a powerful and novel algorithm. Thermo-economic evaluations were carried out by considering the design and off-design working conditions of the ship. In addition, the impact of the optimized ORC system on decarbonization was investigated. The results showed that the annual average Wnet was determined as 372.78 kW. The annual average fuel saving and the annual average CO2 reduction were calculated as 522.83 tfuel/year and 1628.09 tCO2/year, recpectively. The findings indicated that using the RORC system on ships is a promising solution for increasing emission restrictions and environmental concerns.

A Data-Driven Tip Flow Loss Prediction Method for a Transonic Fan Under Boundary Layer Ingesting Inflow Distortion

Abstract In a boundary layer ingesting (BLI) propulsion system, the fan blades need to operate continuously under large-scale inflow distortion. The distortion will lead to serious aerodynamic losses in the fan, degrading the fan performance and the overall aerodynamic benefits of the aircraft. Therefore, in the preliminary design of a BLI propulsion system, it is necessary to evaluate the influence of the fuselage boundary layer under different flight conditions on the fan aerodynamic performance. However, a gap exists in the current computational methods for BLI fan performance evaluations. The full-annulus unsteady Reynolds-averaged Navier–Stokes (URANS) simulations can provide reliable predictions but are computationally expensive for design iterations. The low-order computational methods are cost-efficient but rely on the loss models for accurate prediction. The conventional empirical or physics-based loss models show notable limitations under complex distortion-induced off-design working conditions in a BLI fan, especially in the rotor tip region, compromising the reliability of the low-order computational methods. To balance the accuracy and cost of loss prediction, the paper proposes a data-driven tip flow loss prediction framework for a BLI fan. It employs a neural network to build a surrogate model to predict the tip flow loss at complex non-uniform aerodynamic conditions. Physical understandings of the flow features in the BLI fan are integrated into the data-driven modeling process, to further reduce the computational cost and improve the method’s applicability. The data-driven prediction method shows good accuracy in predicting the overall values and radial distributions of fan rotor tip flow loss under various BLI inflow distortion conditions. Not only does it have higher accuracy than the conventional physics-based loss models but also needs much less computational time than the full-annulus time-accurate simulations. The present work has demonstrated a significant potential of data-driven approaches in complex aerodynamic loss modeling and will contribute to future BLI fan design.

One-Dimensional Optimization Design of Supercritical Carbon Dioxide Radial Inflow Turbine and Performance Analysis under Off-Design Conditions

In the present study, the one-dimensional optimization design and the performance analysis based on a three-dimensional model were performed for a 2.1 MW supercritical carbon dioxide radial inflow turbine. Firstly, an in-house code was developed with MATLAB language for one-dimensional optimization design to maximize the total-to-static efficiency. Then, the three-dimensional radial inflow turbine model was constructed based on the one-dimensional optimization design result. Finally, the flow field and the aerodynamic performance were studied using the commercial software NUMECA. It is shown that the total-to-static efficiency obtained from three-dimensional simulation under the nominal design condition is 85.77%, with a relative deviation of 0.55%, as compared with that from the one-dimensional optimization design. Furthermore, the static temperature and pressure from the turbine inlet to the outlet drop uniformly, and there is no obvious flow separation under the nominal design condition. The values of total-to-static efficiency are always higher than 75% at the 80–110% relative rotating speed and the expansion ratio of 1.75–4.48, which also demonstrates good performance under the off-design working conditions.

Numerical analysis of environment-friendly solar aided coal-fired supercritical thermal power plant cycle integrated with dual heat exchangers

The hybrid solar-coal supercritical steam thermal system has been developed in current research work. The proposed system uses a double-tank heat storage system to increase the stability of the solar tower system, and two heat exchangers to transfer heat from molten salt in the solar tower system to steam in a supercritical coal-fired thermal system. TRNSYS software has been used as a platform to build a model of the solar coal-fired hybrid supercritical steam thermal system. Local weather data is imported to simulate the characteristic year’s operation of a hybrid system under design and off-design working conditions. The variation of heat transferred from solar system to supercritical steam thermal system, power generated from supercritical steam thermal system, solar-electrical generation capacity, efficiency and proportion of solar power generation are compared and analyzed. The results show that after the system is integrated, the tower solar steam production mechanism will increase the output power of supercritical steam thermal system and reduce thermal efficiency of the integrated system. When boiler steam flow rate is kept at less than reference flow rate after joining with solar steam flow rate, total steam flow rate exceeds the reference flow rate, while solar steam flow rate increases.

A coordinated approach of multi‐energy system considering the off‐design characteristics of the devices in energy hub

As essential components in the energy hub, the multi-energy devices are diverse, and their working conditions are complex. The relevant energy efficiency level of devices in the energy hub is also an important factor for the long-term operation of the energy systems. The device efficiency is always considered constant in modeling the energy hub. However, it is necessary to study the efficiency variation and optimization of devices under off-design working conditions. In this paper, we propose a coordinated dispatch method of the multi-energy system considering the device off-design characteristics in the energy hub. The energy efficiency optimization is included in the dispatch process. Then, we use the analysis target cascade (ATC) to obtain the optimal solution. The method is tested in the system, including distribution network, natural gas network, and energy hub. Compared to the traditional method, the proposed method can describe the operation of the whole system more authentically and thereby prove the effectiveness of energy efficiency optimization. The corresponding nonlinear constraints are linearized by piecewise linearization. Besides, we discuss the results of energy efficiency optimization at different load levels and capacity combinations, which is of guiding significance to optimize system working conditions.

Combined use of volumetric expanders and Scheffler receivers to improve the efficiency of a novel direct steam solar power plant

This research proposes an innovative solar thermal plant able to generate mechanical power through an optimized system of heliostats with Scheffler-type solar receivers coupled with screw-type steam expanders. Scheffler receivers appear to perform better than parabolic trough collectors due to the high compactness of the focal receiver, which minimizes convective and radiative heat losses even at high vaporization temperatures. At the same time, steam screw expanders are volumetric machines that can be used to produce mechanical power with satisfactory efficiency also by admitting two-phase mixtures and with further advantages compared to steam turbines: low working fluid velocities, low operating pressures, and avoidance of overheating. This study establishes a mathematical model to assess the energetic advantages of the planned solar thermal power system by evaluating the solar-to-electricity efficiency for different off-design working conditions. For this purpose, a numerical model on the Scheffler receiver is initially investigated, thus assessing all the energy losses which affect the heat transfer phase. A thermodynamic model is then developed to evaluate the energy losses and performance of the screw expander under real working conditions. Finally, parametric optimization of the solar energy conversion is performed in a wide range of operating conditions by establishing thermodynamic formulations related to the whole solar electricity generation system. Water condensation pressure and vaporization temperature are so optimized with respect to global energy conversion efficiency which, under the best operating conditions achieved in this research, rises from 10.9% to 14.4% with increasing solar irradiation intensity. Hence, the combined use of screw expanders and Scheffler receivers for solar thermal power system application can be a promising technology with advantages over parabolic dish concentrators. Novelty statement This research proposes an innovative direct steam solar power plant based on an SRC, with water utilized as both heat transfer and working fluid, equipped with Scheffler solar receivers as a thermal source and screw expanders as work-producing devices. Technical studies and energy assessments of this kind of SEGS at part-load operation do not exist in scientific literature; after reviewing the literature, it was determined that volumetric expanders have been rarely combined with Scheffler receivers for solar thermal power system application. In effect, combined use of screw expanders and Scheffler-type solar concentrator in a direct steam solar power system represents a completely new plant configuration; however, as a promising DSG solar system, at present numerical model of this new sort of SEGS is lacked in literature and the optimum operating conditions have yet to be defined. For this reason, the chief objective of this paper is to define a first parametric optimization of all thermodynamic variables involved to maximize global efficiency of the proposed solar thermal power generation system for ordinary working conditions.

Model based control of the inlet pressure of a sliding vane rotary expander operating in an ORC-based power unit

• Experimental analysis on Sliding Vane Rotary Expander in ORC plant. • Development of a new approach for volumetric losses calculation. • Inlet pressure control strategy based on expander speed variation. • Effectiveness assessment of the proposed control strategy. • Performance optimization through the proposed regulation strategy. Sliding vane rotary expanders (SVREs) are widely used in organic Rankine cycle (ORC)-based power units for low-grade heat recovery because of their capability to deal with severe off-design working conditions. In particular, the speed of SVREs is a very effective operating parameter, together with the speed of the pump, to regulate the recovery unit and to lead the involved components in an acceptable operating behaviour when they are far from the design conditions. In this study, a control strategy based on the variation in revolution speed of a SVRE was developed, where the inlet pressure of the expander is the main controlled property, which must be verified when the flow rate of the working fluid is changed to match the thermal power recovery at the hot source. In fact, pressure level control is a key point of the recovery unit for thermodynamic reasons and for the safety and reliability of the expander and, more generally, of the whole recovery unit. The proposed control strategy is based on an original theoretical procedure that relates the expander speed, inlet pressure, volumetric efficiency, and working fluid mass flow rate in an analytical form. This analytical formulation is widely nonlinear and is simplified for use as a tool for the model-based control of the inlet expander pressure. An experimental activity performed on a SVRE operating in an ORC-based power unit, fed by the exhaust gases of a supercharged diesel engine, was the base of the analytical formulation. This provided the possibility of deriving a simplified model-based control of the expander inlet pressure and assessing its effectiveness and limits during off-design conditions. Higher expander global efficiencies were obtained (up to 45%), allowing a greater mechanical energy recovery (up to 2 kW).

Investigation on improvement potential of ORC system off-design performance by expander speed regulation based on theoretical and experimental exergy-energy analyses

In this paper, the potential of expander speed regulation for optimization of ORC system adaptability to off-design working conditions has been explored through experiments. According to the obtained results, it has been found that there existed two different optimal speeds: optimal power speed (rop), optimal efficiency speed (roee/rote) which could endow the ORC system with maximum output power or highest thermoelectric/exergy efficiency. For working conditions of tw = 70, 73, 80 °C, power of expander at rop has been increased by 199.1%, 137.4%, 6.5% compared with the power output at design speed (rds). The thermoelectric efficiency at rote was 11.1%, 27.7% higher than that at rop and rds. To further investigate the influences of expander speed on the performance, energy and exergy analyses were conducted based on theoretical calculation and experiment results within the range of 400–1600 r/min. The results showed that actual exergy destruction of expander and working fluid pump was closely related to expander speed. Gaps between theoretical and actual exergy destruction rates proved that optimization of expander and working fluid pump would be more effective for the ORC system performance.

Comparison on energy consumption evaluation methods for CHP plants under off-design working conditions

Comparison on energy consumption evaluation methods for CHP plants under off-design working conditions

Influence of the limited room space on inlet conditions and fan operation at the rotating test rig

The limited space of the laboratory room allows maintaining the selected ambient air parameters in the required ranges. It can be, however, a source of many problems in the case of rotating test rigs that blow a large amount of air. The aim of the paper is to examine the influence of limited space and its shape on the working conditions of axial fans test stand. The focus was particularly on the conditions prevailing at the inlet to the test rig and their potential impact on the tested fan operation. The article presents the results of CFD simulations for two rooms of different volumes and the results of experimental measurements carried out for the smaller one. Good qualitative agreement between the simulations and the experiment was obtained. The limited space around the test rig causes the unsteadiness of the axial velocity profile and introduces an unsteady swirl at the inlet. This leads, in turn, to local changes in the angle of attack of the fan blades and the resulting consequences related to the off-design working conditions. The paper proves the need to minimise the effects of the induced airflow in the test room and proposes some methods to accomplish this task.

Influence of Inlet Angle of Guide Vane on Hydraulic Performance of an Axial Flow Pump Based on CFD

Axial flow pump has been widely used in hydraulic engineering, agriculture engineering, water supply and sewerage works, and shipbuilding industry. In order to improve the hydraulic performance of pump under off-design working conditions, the influence of the inlet segment axial chord and inlet angle adjustment of the guide vane on the pump segment efficiency and flow filed was simulated by using the renormalization group (RNG) k − ε turbulent model based on the Reynolds-averaged Navier–Stokes equations. The results indicate that the inlet segment axial chord and inlet angle adjustment of guide vane have a strong influence on the pump segment efficiency. Considering the support function and hydraulic loss of the guide vane, the inlet segment axial chord is set to 0.25 times the axial chord of guide vane. On the basis of the inlet angle of the guide vane under design conditions, when the inlet segment angle is turned counterclockwise, the pump segment efficiency is improved in the lower flow rate region; moreover, the pump segment efficiency is improved in the larger flow rate region when the inlet segment angle is turned clockwise. As the conditions deviate from the design working conditions, the influence of the guide vane inlet angle on the pump segment efficiency increases. If the inlet segment angle is properly adjusted under off-design working conditions, the flow pattern in the guide vane is improved and the hydraulic loss is decreased, because the inlet segment angle matches with the flow direction of impeller outlet; consequently, the pump segment efficiency is increased.

Comparison on thermodynamic characteristics of single- and double- reheat boilers under off-design working conditions and during transient processes

The flexibility of coal-fired power plants is urgently required to accommodate high penetration of renewable powers. Quantitatively analyzing thermodynamic characteristics of single- and double- reheat boilers could provide meaningful information to enhance the flexibility of double-reheat boilers. In this study, the single- and double- reheat boiler models were developed and validation. The thermodynamic characteristics of the single- and double- reheat boilers under off-design working conditions and during transient processes are compared from the perspective of energy and exergy. Results show that the heat absorption and exergy storage of re-heaters in the double-reheat boiler is ~1.4 and ~2.0 times those of the single-reheat boiler, respectively. The response time of the double-reheat boiler is ~400 s longer than that of the single-reheat boiler. The exergy storage change takes as high as 40% of the input exergy change that is caused by the coal feeding rate change in the double-reheat boiler, but only 25% in the single-reheat boiler. This work is expected to provide some detailed guidance of the flexible and efficient operation of double-reheat units.

Performance analysis of a solar-aided coal-fired power plant in off-design working conditions and dynamic process

Solar-aided coal-fired power generation is a promising technique to reduce cost and enhance the efficiency of concentrated solar power. Operation optimization is important but has difficulty improving the performance of solar-aided coal-fired power plants, due to the fluctuations in power load and solar energy. Therefore, the operation performance and dynamic characteristics of a solar-aided coal-fired power plant were investigated in this study. A model of the plant including trough collector system and coal-fired plant was developed. A case was studied to demonstrate the model practicability on basis of a 600 MW supercritical coal-fired plant and a trough collector system integrated in parallel with #2 and #3 high pressure heaters of regenerative system. Under different solar irradiances, operation characteristics were analyzed from 100% to 50% loads with varying ratio of feedwater to trough collector system. The solar-to-electricity efficiency under different off-design conditions was obtained. Moreover, the dynamic response characteristics of solar-aided coal-fired power plant were studied with the disturbances of direct normal irradiance and ratio of feedwater to trough collector system. Results showed that the solar-to-electricity efficiency in design condition was 25.80%. The adjustment of the ratio of feedwater to trough collector system could lead to an optimized operation. Furthermore, the dynamic characteristics of trough collector system and solar-aided coal-fired power plant were analyzed. The results are expected to guide the control optimization of solar-aided coal-fired power plant.

Optimum Organic Rankine Cycle Design for the Application in a CHP Unit Feeding a District Heating Network

Improvement of energy conversion efficiency in prime movers has become of fundamental importance in order to respect EU 2020 targets. In this context, hybrid power plants comprising combined heat and power (CHP) prime movers integrated with the organic Rankine cycle (ORC) create interesting opportunities to additionally increase the first law efficiency and flexibility of the system. The possibility of adding supplementary electric energy production to a CHP system, by converting the prime movers’ exhaust heat with an ORC, was investigated. The inclusion of the ORC allowed operating the prime movers at full-load (thus at their maximum efficiency), regardless of the heat demand, without dissipating not required high enthalpy-heat. Indeed, discharged heat was recovered by the ORC to produce additional electric power at high efficiency. The CHP plant in its original arrangement (comprising three internal combustion engines of 8.5 MW size each) was compared to a new one, involving an ORC, assuming three different layout configurations and thus different ORC off-design working conditions at user thermal part-load operation. Results showed that the performance of the ORC, on the year basis, strongly depended on its part-load behavior and on its regulation limits. Indeed, the layout that allowed to produce the maximum amount of ORC electric energy per year (about 10 GWh/year) was the one that could operate for the greatest number of hours during the year, which was different from the one that exhibited the highest ORC design power. However, energetic analysis demonstrated that all the proposed solutions granted to reduce the global primary energy consumption of about 18%, and they all proved to be a good investment since they allowed to return on the investment in barely 5 years, by selling the electric energy at a minimum price equal to 70 EUR/MWh.