Increase In Inlet Temperature Research Articles

A Study of Thermoelectric Generation Coupled with Methanol Steam Reforming for Hydrogen Production

Waste heat recovery was considered as a promising candidate for energy conservation and emission reduction. Methanol steam reforming was considered to be an effective means for hydrogen production because of its advantages. In this work, a micro reactor was constructed and thermoelectric generation coupled with hydrogen production from methanol steam reforming was innovatively used to recycle waste heat, which was simulated by hot air from a hot air gun. The waste heat was converted into electricity and hydrogen at the same time. The characteristic of thermoelectric generation coupled with methanol steam reforming was investigated. It was experimentally verified that both the hydrogen production rate and methanol conversion increased with the increasing inlet temperature, but thermal efficiency increased firstly and then decreased with the increasing temperature. The methanol steam reforming could effectively maintain cold side temperature distribution of thermoelectric generation. In the case of the thermoelectric module (1), the highest temperature difference of 37 °C was determined and the maximum open circuit voltage of 2 V was observed. The highest methanol conversion of 64.26% was achieved at a space velocity of 0.98 h−1 when the temperature was 543 K, comprehensively considering the CO content and thermal efficiency.

THERMODYNAMIC PERFORMANCE ANALYSIS OF HUMAN CARDIOVASCULAR SYSTEM FOR NORMAL AND CARDIOMEGALY CONDITIONS

This study aims to determine the thermodynamic performance of the cardiovascular system for healthy and cardiomegaly conditions where work done by the heart and lung is considered as input expenses. Analysis based on the second law of thermodynamics was performed under three distinct physiological conditions: resting, moderate, and excessive activity levels. From this analysis, it is noted that, exergy transfer rate increases with the increase in the physiological activity level. The exergetic efficiency of the cardiovascular system is about 55% at resting and changes by about 4% with physical activity level. The human cardiovascular system efficiency is increased with an increase in breathing air inlet temperature and relative humidity irrespective of physiological conditions. The work also quantifies the efficiency of the heart and lung as individual systems. For a fixed breathing air inlet temperature and relative humidity, the cardiovascular system performance increases with a decrease in physical activity levels.

Investigation on oxy-fuel biomass integrated gasification combined cycle system with flue gas as gasifying agent

A biomass integrated gasification combined cycle with oxy-fuel combustion technology is established to realize negative CO2 emissions in power plants. CO2 and H2O from flue gas are adopted as gasifying agents in the proposed system. The effects of gas turbine pressure ratio and inlet temperature with increasing CO2/C on the system performance are studied using Aspen Plus. The results show that as CO2/C increases, the concentration of CO in the syngas first increases and subsequently decreases. The concentration of H2 and the lower heating value of syngas decrease monotonously as CO2/C increases. The system efficiency increases when the CO2/C value, gas turbine pressure ratio, and gas turbine inlet temperature increase, and the influence of the inlet temperature is more significant than that of pressure ratio. When CO2/C, gas turbine pressure ratio, and inlet temperature are 1.0, 20, and 1150 °C, respectively, the energy efficiency reaches 29.1%, and the corresponding exergy efficiency and net power output are 28.4% and 116 MW. Levelized cost of electricity is estimated to be 0.17 $/kWh in the system. The performance results obtained in this study can provide theoretical support for the application of biomass integrated gasification combined cycle with oxy-fuel combustion technology.

Experimental study on the forced convection heat transfer characteristics of airflow with variable thermophysical parameters in a circular tube

The heat transfer of fluids in pipelines under conditions with variable thermophysical parameters is ubiquitous in engineering applications, but effective methods to predict the heat transfer parameters are scarce. To address this issue, an experimental method was proposed to measure the heat transfer parameters in a circular tube by considering the effect of the insufficient development of turbulence in the inlet region and a Robin boundary on the wall. The variation of the Nusselt number of the inner wall was calculated by measuring the temperature distribution of the outer wall, and a temperature-related Gnielinski correlation was developed for conditions under which the thermophysical parameters vary with the fluid temperature. The results demonstrated that the deviations between the experimental Nusselt numbers and the values predicted using the traditional Gnielinski correlation became larger and larger as the flow distance increased. The temperature-related correlation generally improved the prediction accuracy by more than 25%, and the accuracy was better with increasing inlet temperature and decreasing airflow velocity. This study provides an important empirical reference for the prediction of the convective heat transfer parameters in pipelines and related engineering applications under strong heating or cooling conditions.

Performance Investigation of Cylindrical Cavity Receiver Using Roughened Surfaces

Solar cavity receiver is an integral part of the solar parabolic dish collectors used for absorbing concentrated solar irradiation. The performance of the receivers can be enhanced utilizing different types of heat transfer fluids (HTF) and modified geometries. The thermal performance of a redesigned cavity receiver with solid semi-circular tube attached to its surface was investigated using CFD three-dimensional simulation. The influence of operating and geometric parameters (i.e., inlet mass flow rate and inlet temperature) were investigated. The results obtained shows that, redesigned solar cavity receiver has thermal efficiency of 8.49-10.27% higher than a normal cavity receiver with an inlet flow rate of 0.01-0.04 kg/s under the same environmental and operating conditions. The temperature of outlet of HTF increases with increases in inlet temperature for Therminol66 oil. The comparison of thermal performance using different HTF (i.e., Therminol66, Ethylene Glycol, Water) is also conducted and it is found that Therminol66 oil gives 9% and 17% higher outlet temperature than ethylene glycol and water respectively at intake flow rate of 0.02 kg/s.

Medium-entropy (Me,Ti)0.1(Zr,Hf,Ce)0.9O2 (Me = Y and Ta): Promising thermal barrier materials for high-temperature thermal radiation shielding and CMAS blocking

With continuous enhancement of gas-turbine inlet temperature and rapid increase of radiant heat transfer, thermal barrier coating (TBC) materials with a combination of low thermal conductivity and good high-temperature thermal radiation shielding performance play vital roles in ensuring the durability of metallic blades. However, yttria-stabilized zirconia (YSZ), as the state-of-the-art TBC and current industry standard, is unable to meet such demands since it is almost translucent to high-temperature thermal radiation. Besides, poor corrosion resistance of YSZ to molten calcia-magnesia-alumina-silicates (CMAS) also impedes its application in sand, dust, or volcanic ash laden environments. In order to improve the high-temperature thermal radiation shielding performance and CMAS resistance of YSZ and further reduce its thermal conductivity, two medium-entropy (ME) oxide ceramics, ME (Y, Ti)0.1(Zr, Hf, Ce)0.9O2 and ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2, were designed and prepared by pressureless sintering of binary powder compacts in this work. ME (Y, Ti)0.1(Zr, Hf, Ce)0.9O2 presents cubic structure but a trace amount of secondary phase, while ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2 displays a combination of tetragonal phase (81.6 wt.%) and cubic phase (18.4 wt.%). Both ME (Y, Ti)0.1(Zr, Hf, Ce)0.9O2 and ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2 possess better high-temperature thermal radiation shielding performance than YSZ. Especially, the high-temperature thermal radiation shielding performance of ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2 is superior to that of ME (Y, Ti)0.1(Zr, Hf, Ce)0.9O2 due to its narrower band gap and correspondingly higher infrared absorbance (above 0.7) at the waveband of 1 to 5 μm. The two ME oxides also display significantly lower thermal conductivity than YSZ and close thermal expansion coefficients (TECs) to YSZ and Ni-based superalloys. In addition, the two ME oxides possess excellent CMAS resistance. After attack by molten CMAS at 1250 °C for 4 h, merely ∼2 μm thick penetration layer has been formed and the structure below the penetration layer is still intact. These results demonstrate that ME (Me, Ti)0.1(Zr, Hf, Ce)0.9O2 (Me = Y and Ta), especially ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2, are promising thermal barrier materials for high-temperature thermal radiation shielding and CMAS blocking.

Multi-objective optimization of a small-scale solar-hot-spring-geothermal brackish water direct contact membrane distillation system

A small-scale solar-hot-spring-geothermal brackish water membrane distillation (MD) system is proposed by combining hot-spring and solar components to reduce fossil energy consumptions. A performance parameter framework and carbon footprint model is developed to perform an analysis on whole life cycle of the proposed system. The results show that the solar subsystem improved heat recovery with the water generation ratio increased by 22–42 %. As the feed-side inlet temperature increases, the total Specific Energy Consumption (SEC) contribution rate of renewable geothermal and solar energies increases from 12.34 % to 20.45 %. The cold and hot sources are responsible for the largest exergic losses at 1.79 kJ/S and 0.48 kJ/S. The unit exergy cost of the solar subsystem is the highest of 0.037 $/h in terms of economic benefits. One of effective ways to improve the carbon emission reduction rate is to develop a cogeneration process for the water purification. A multi-software co-simulation methodology is proposed to explore innovative MD systems of optimal configurations, improve desalination efficiency and reduce the energy consumptions. The Non-dominated Sorting Genetic Algorithm (NSGAII) and improved NSGAII genetic algorithms (NSGAII-A and NSGAII-B) are utilized for multi-objective optimization of the MD system. Finally, the Technique for Order Preference by Similarity to Ideal Situation (TOPSIS) method based on entropy weight is employed to obtain the Pareto frontier solution set. It is indicated that NSGAII-B based on the four objective-optimal scheme is the best for the Multi-objective optimization of the MD system.

Analysis-Based Key Components Selection of Latent Heat Thermal Energy Storage System Along With Active Heat Transfer Enhancement

Abstract Energy storage is an effective approach to bridging the gap between energy supply and demand created due to the sporadic nature of solar energy. Thermal performance enhancement is a key research subject for effective energy storage using latent heat thermal energy storage (LHTES) systems. This paper focuses on the analysis-based design of suitable LHTES system components for solar absorption-based cooling applications with a working temperature of up to 200 °C. Initially, the medium-temperature range (80 °C to 200 °C) phase change material (PCM) is selected using the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS). Further, a suitable heat transfer fluid (HTF) is selected along with the design of a geometrical assessment and an appropriate LHTES system. Finally, the effect of the stirrer on the thermal performance of the LHTES system has been discussed. The melting time of PCM reduces by 58% while input energy increases by 20 kJ with an increase in HTF inlet temperature from 180 °C to 190 °C. However, input energy increases faster with a further increase in HTF inlet temperature while melt time does not reduce significantly. Therefore, selecting optimum HTF inlet temperature is an important criterion for efficient LHTES system design. Implanting a rotating stirrer at 200 RPM inside a PCM tube decreases the net-input energy by 73 kJ. Using back-of-the-envelope calculations, the analysis-based selection of key components of the LHTES system will pave the way forward to designing an application-specific LHTES system. Further, this study can be instrumental in theoretically scrutinizing the stirring effect on PCM charging before experimental analysis.

Numerical study of high-temperature cascaded packed bed thermal energy storage system

The thermal energy storage system (TES) in the form of packed bed with encapsulated phase change materials (EPCMs) can further improve the thermal performance of ordinary TES. This study presents a numerical model for a three-dimensional cascaded packed bed thermal energy storage system (PBTES), in which an effective thermal conductivity model is used by consisering the natural convection effects in each EPCM based on its volume average liquid fraction. To examine the system-storage thermal energy and exergy in PBTES accurately, a thermodynamic analysis approach is also given. The high-temperature PCMs are grouped as KNO3, NaNO3, and NaNO2 according to the phase transition temperature from high to low in the packed bed stage. The effects of element stages on the thermal performance of PBTES are studied. Results show that the total thermal energy storage of cascaded-PBTES is 38.5% higher than that of single-PBTES (KNO3), and the total thermal exergy storage is 30.8% higher than single-PBTES. The average exergy efficiency possessed by cascaded-PBTES is 5% higher than that of single-PBTES. Furthermore, the effects of heat transfer fluid (HTF) inlet temperature and mass flow rate on the thermal performance of cascaded-PBTES are discussed. The increase of inlet temperature and mass flow rate can promote the charging process and influence the exergy efficiency. However, increasing the inlet mass flow rate will not lead to an increase in maximum thermal energy and exergy storage capacity.

Experimental study on thermal performance of phase change heat storage device with rectangular shell structure

To solve the imbalance between energy supply and demand, it is necessary to explore the heat exchange performance of large-scale phase change heat storage devices in line with engineering applications. Based on the principle of low-cost enhanced heat transfer, a shell and tube phase change heat storage device with a rectangular shell with annular fins was designed in this paper. Using paraffin as phase change material (PCM) and water as heat transfer fluid (HTF). The effects of different flow rates and inlet temperature on the temperature distribution, heat, power, and efficiency of phase change materials during melting and solidification were studied experimentally. It is found that when the flow rate is 4 L/min, the total heat release was 61.06 MJ and the minimum heat release rate was 81.23%. The increase in flow rate and inlet temperature will shorten the heat storage time. The influence of inlet temperature on the heat storage time is more obvious than flow rate. Along with the raise of flow rate the shortening range of heat storage time decreases gradually. Additionally, the increase of the flow rate not only causes the enhancement of the heat storage and release power, but also results in the reduction of the average efficiency. Therefore, to improve the heat exchange speed in practical application, priority should be given to changing the inlet temperature.

Numerical study on dynamic ash deposition and heat transfer characteristics of radiant syngas cooler

Ash deposition greatly influences the performance of Radiant Syngas Cooler (RSC) which plays a pivotal role in the heat recovering process in coal gasification technology. Therefore, a numerical model of ash deposition coupled with heat transfer is established in present study to predict the ash deposition of RSC, and the effects of inlet temperature and division wall structure on deposition characteristics are further discussed. The simulation results indicate that the particle impact mainly occurs in the transition region of the jet flow to plug flow, while the particle deposit mainly occurs in the recirculation region. The deposition of inner wall and division wall shows a malignant growth pattern successively with the width increase of division wall. The division wall of 400 mm width is more conducive to the safe and stable operation of the RSC than those of 250 mm and 650 mm. The deposition becomes heavy with the increase of inlet temperature, and the deposition of division wall belongs to the slow growth, the large-scale detaching and the malignant growth pattern at the inlet temperature of 1573 K, 1673 K and 1773 K, respectively. Present result may provide a reference for the structural design and operational adjustment of RSC.

Micro-Mixing Combustion for Highly Recuperated Gas Turbines: Effects of Inlet Temperature and Fuel Composition on Combustion Stability and NOx Emissions

Abstract The micromix combustion concept offers an elegant compromise between premixed and nonpremixed combustion. By mixing the fuel and air at the smallest scale possible, one can achieve NOx emissions comparable to premixed combustion while removing the risks of auto-ignition and flashback. Current literature reports multiple micromix designs that achieve low NOx emissions (<10 ppm) with hydrogen or hydrogen-rich fuels at combustor inlet temperatures (CIT) representative of low to medium pressure ratio gas turbines (<650 K). This paper seeks to bridge the gap between current literature and the design requirements of highly recuperated ceramic gas turbines, which should allow low NOx operation with various fuels at combustor inlet temperatures upwards of 900 K. To this end, micromix injection nozzles were designed and tested at small scale to investigate the effects of fuel composition and inlet temperature on combustion stability and NOx emissions. The nozzles were additively manufactured in Inconel 625 having hundreds of holes as small as 0.25 mm. An axial swirler is used to induce recirculation of the products behind the nozzle, which helps stabilize combustion with hydrocarbon fuels due to their longer reaction times and slower flame speeds. Experimental results show that NOx emissions can be decreased down to premixed levels if the jet Damköhler number is kept under a critical value, which requires increasingly smaller holes or higher jet velocities as the inlet temperature increases. Combustion instabilities are observed at low inlet temperatures with hydrocarbons, which are also correlated to the jet Damköhler number.

Experimental investigation on combustion characteristics and influencing factors of PODE/methanol dual-fuel engine

PODE/methanol dual-fuel combustion mode was conducted on a four-cylinder common-rail engine with PODE direct injection and methanol intake fumigation. This study is aimed to investigate the effects of methanol substitution ratio (MSR), injection timing and inlet temperature on in-cylinder combustion, emission characteristics and fuel economy of dual-fuel engine. Results show that with the increase of MSR, the ignition delay of dual-fuel engine is prolonged, and the COV increases. At medium and small loads, the peak combustion pressure decreases, and the combustion phase is delayed. At high load, the peak values of heat release rate and combustion pressure increase, combustion duration is shortened, and slight knock occurs at 40% MSR. Dual-fuel mode has benefits in reduction of NOx and soot, while increases NO2 and HCHO emissions. With the delay of injection timing, combustion duration is first prolonged and then shortened. The excessive delay of injection timing leads to increased fluctuation of combustion pressure, and partial misfire occurs. With the increase of inlet temperature, the peak values of heat release rate and combustion pressure increase significantly, combustion duration is shortened, and the COV increases. At the inlet temperature of 65 °C, methanol spontaneous combustion occurs near TDC, and thermal efficiency increases to 44.6%.

Thermal Performance Analysis of a Linear Fresnel Concentrating Solar Collector Absorber Tube

Linear Fresnel solar collectors are suitable for a wide range of applications, which include steam generation for power generation, industrial process heat, solar cooling, institutional and domestic hot water production etc. The thermal performance of a linear Fresnel solar collector absorber tube was investigated in this study for different fluid mass flow rates, heat flux intensities, heat transfer fluid inlet temperatures, ambient air temperatures and external loss heat transfer rate. Turbulent flow liquid water heat transfer fluid and uniform circumferential heat flux distribution boundary were considered under steady-state conditions. The fluid flow was considered incompressible. This study was limited to a first order model approximation for thermal performance of a linear Fresnel solar collector absorber tube. The heat transfer fluid and absorber tube material properties were considered constant and independent of temperature. The results indicated that the absorber tube outlet heat transfer fluid temperature decreased with an increase in the Reynolds number and increased with the heat flux intensities. Thermal efficiency increased with an increase in the heat transfer fluid mass flow rate and however, decreased with increase in pressure drop and the consequent pumping power requirement. It was also found that the thermal efficiency of the collector decreased with an increase in the fluid bulk inlet temperature and decreased with an increase in the external loss heat transfer rate.

Numerical Investigation of Heat Transfer Performance and Structural Optimization of Fan-Shaped Finned Tube Heat Exchanger

Latent heat storage technology is widely used in solar power generation. Aiming to enhance the energy utilization rate to a greater extent, an innovative fan-shaped structure has been proposed to construct the metal fins of the shell-and-tube thermal storage device. The enthalpy method is used to simulate the heat storage process and focuses on the influence of inlet conditions on heat transfer. The influence of the fin structure on the melting properties of phase change material has been studied. The results show that increasing inlet temperature and inlet flow rate is a convenient and effective way to improve energy efficiency. As the inlet temperature is increased from 343 K to 358 K, the total heat storage and energy efficiency are improved by 13.4% and 10.2%, respectively, and the melting time is reduced by 36.2%. As the flow rate is increased from 3 L/min to 9 L/min, the complete melting time is reduced by 33.4%. Energy efficiency peaks at a flow rate of 5 L/min. Reasonable optimization of the fin structure can enhance the natural convection circulation during the melting process and further improve the energy efficiency. The research results can guide the design and structural optimization of the finned tube heat storage device.

Waste Heat Recovery from Air Using Porous Media and Conversion to Electricity

This paper presents a numerical study of waste heat recovery from a fluid stream using thermoelectric devices. The system consisted of a square section duct with spherical porous media placed in its central region. Hot air circulates continuously through the duct and exchanges energy with the solid matrix and subsequently with the thermoelectric modules. The mathematical model of the system was solved using ANSYS/FLUENT software, requiring the implementation of user-defined functions (UDFs) and user-defined scalars (UDS) regarding porous media and thermoelectric generation modelling. From the simulations carried out, global efficiency and electrical power values were obtained in the range of [0.11–4.51] [%] and [0.01–12.77] [W], respectively. Furthermore, for the set of variables analysed, it was observed that the performance of the system is favoured by an increase in the fluid inlet temperature and speed, as well as by a higher external heat transfer coefficient.

Bubble behaviors and flow boiling characteristics in open microchannels with large aspect ratio

Large aspect ratio of microchannels enables the corresponding heat sink possess higher heat transfer capability, however, the generated bubbles tend to rapidly extend forward and backward due to the large aspect ratio of the channels, thereafter blocking the channels, deteriorating the heat dissipation and increasing the pressure loss. Open microchannels have been reported to inhibit the two-phase flow instability due to the existence of extra gap for vapor expansion. However, the visualization of bubble dynamics and characterization of flow boiling behaviors of open microchannels with large aspect ratio are limited. In this study, the open microchannel heat sink with large aspect ratio was proposed and flow boiling tests were conducted at two mass fluxes of 153 and 229 kg/m2·s with the effective heat flux ranging of 180.81–2165.15 kW/m2. The inlet temperatures of the test module were set in the range of 5-85 °C. Visualization results showed that the flow patterns can be classified into bubbly flow, combined bubble flow and stratified slug flow. The flow mode tended to transition to stratified flow as the inlet temperature increased, and the combined bubble flow was found to be absent at high inlet subcooling. The size of the nucleated bubbles increased with the increase of inlet temperature during bubbly flow. The length to width ratio, survival time of nucleated bubbles and the bubble dynamic behaviors were respectively discussed. The heat transfer characteristics and pressure drop were strongly dominated by the two-phase flow patterns. The optimum flow boiling heat transfer was achieved when the flow pattern transitioned to combined bubble flow and early stage of stratified slug flow, during which bubble nucleation and thin liquid film evaporation jointly contributed to the heat dissipation. The pressure drop increased with the increase of inlet subcooling because of the greater liquid viscosity, and the sudden spurt of pressure drop was observed due to the occurrence of stratified slug flow. This work provides a preliminary investigation of two-phase flow boiling performance in large aspect ratio open microchannels, which may provide important insights of bubble dynamics or interfacial mechanics in open microchannel heat sinks.

Flat-plate solar collector thermal performance assessment via energy, exergy and irreversibility analysis

This paper presents a detail exergy analysis of a flat-plate solar collector based on irreversibility rates. The governing equations of the flat-plate collector are obtained by writing energy and exergy conservation equations for glass cover, absorber plate and working fluid. The computed results of this study are in concordance with previous studies experimental data. Our findings reveal that 84.97% of total collector irreversibility are linked to the absorber plate. Increasing the flow rate to 0.0056 kg/s decreases the irreversibility rate in the absorber plate to a minimum of 579.20 W and increases the irreversibility rate in the working fluid to it maximum of 158.83 W. The maximum exergy efficiency was 33.21% for a flow rate of 0.0056 kg/s and inlet fluid temperature of 30 °C. An increase in inlet fluid temperature from 20 to 65 °C decreases the absorber plate irreversibility by 129.13 W, meanwhile the working fluid and glass cover irreversibility increase by 30.37 W and 99.39 W, respectively. An increase in number of absorber tube from one to sixteen increases the energy and exergy efficiencies respectively by 34.61% and 92.94%. Meanwhile the absorber plate irreversibility rate increases by 97.49% and the irreversibility rates of the working fluid and glass cover reduce by 72.06% and 89.01%, respectively.

Thermal analysis to investigate the effects of operating parameters on conventional cupola furnace efficiency

Background: Thermal analysis was performed on a conventional 6.5 m high dome with a 0.65 m diameter at Nigeria Railway Corporation. Energy and thermal analyses were performed on the furnace to improve efficiency and minimize energy losses in the system. Results: The energy efficiency recorded was 55%. The losses in the furnace amount to 45%, the highest value being the heat loss to the combustion gas at 34%. A parametric study was examined to see the effect of some design and operational parameters on furnace efficiency through energy savings and the study found a positive trend. The parameters include the flue gas temperature, the combustion air inlet temperature, the thickness, and the emissivity of the refractory wall. A decrease in the temperature of the flue gases increased the efficiency of the furnace to 73%, an increase in the air inlet temperature increased the efficiency of the furnace to 61%, and an increase in the thickness of the refractory wall increased the efficiency of the furnace. Oven to 56% and a decrease in emissivity of the refractory wall increased the efficiency of the furnace to 56%. Conclusions: The studied parameters were further combined and their net effect showed a possible increase in furnace efficiency from 55% to 78%. The maximum efficiency of 78% was determined with respect to the minimum/maximum estimated values of the parameters studied, as more metals are melted with the combined energy saved from a variation of these parameters. This implies that cupola furnace or similar furnace designers or operators should be guided on the preheating conditions, thickness, and emissivity of the refractory wall to be used with other parametric combinations to be adopted to ensure energy efficiency and improvement in such furnaces.

Thermodynamic analysis of CO2–SF6 mixture working fluid supercritical Brayton cycle used for solar power plants

To overcome the poor cycle thermal efficiency of supercritical CO2 Brayton cycle caused by the high rejection temperature used for solar power plants, mixture of CO2–SF6 as working fluid for the Brayton cycle system is proposed and evaluated. Detailed thermodynamic analysis is conducted to the CO2–SF6 mixture Brayton cycle compared with the referenced s-CO2 cycle. Influence of MC inlet temperature and mole fraction of SF6 on the thermodynamic performances is discussed. Moreover, exergy efficiencies and exergy loss distribution of the components in the CO2–SF6 mixture Brayton cycle are analyzed. The results suggest that the system cycle thermal efficiencies of CO2–SF6 mixture cycle and s–CO2 cycle both decrease with the increase of the MC inlet temperature, however, the thermal efficiency of CO2–SF6 mixture cycle is significantly higher than that of s-CO2 cycle. An optimal mole fraction of SF6 exists in the CO2–SF6 mixture for the CO2–SF6 mixture cycle. The optimal value is 0.356 in this context. The heliostat field and solar receiver has the lowest exergy efficiency among all the components of the CO2–SF6 mixture cycle, which is as low as 31.59%.