Thermal Exergy Efficiency Research Articles

Effects of intake air temperature on energy, exergy and sustainability analyses in an RCCI engine fueled with iso-propanol and n-heptane

In this study, performance and emission tests were carried out in a diesel engine operating with RCCI combustion mode by varying the amount of fuel taken into the cylinder and the air temperature in one cycle. Thermodynamic performance was determined by making energy, exergy and sustainability analyzes using the data obtained in the tests. The effects of change in intake air temperature on thermal efficiency, exergy efficiency, exhaust exergy, exergy destruction, and sustainability index were calculated. As the temperature of the air taken into the cylinder increases, the lost exergy decreases and the exergy efficiency increases. At 2500 rpm engine speed, when the amount of fuel taken into the cylinder is 20 mg/cycle, if the inlet temperature is 30°C, 50°C and 70°C, the exergy destruction is 9.873 kW, 9.760 kW and 9.394 kW, respectively. The highest thermal and exergy efficiencies were calculated as 31.58 % and 28.7 %, respectively, at an engine speed of 2000 rpm, when the fuel taken into the cylinder per cycle was 20 mg and the intake air temperature was 70°C. Moreover, the sustainability index was found to be 1.4 under the same conditions. The thermodynamic results show that RCCI mode can be used as alternative combustion mode.

Energy and Mass Matching Characteristics of the Heat-Absorbing Side of the Ammonia Energy Storage System under Nonuniform Energy Flow Density.

Ammonia thermochemical energy storage is based on a reversible reaction and realizes energy storage and utilization by absorbing and releasing heat. Under different energy flow densities, the efficiency of an ammonia reactor composed of multiple ammonia reaction tubes is different. Based on the coupling model of light, heat, and chemical energy of an ammonia decomposition reaction system, taking a 20 MW solar thermal power plant as the research object, this paper proposes a new model of ammonia energy storage system, which places the ammonia decomposition side in a low-pressure environment and the ammonia synthesis side in a high-pressure environment. The effects of different inlet temperatures, inlet flow rates, flow distribution, and energy flow density distribution on the ammonia energy storage system were studied. The results show that the increase of inlet temperature and the decrease of inlet flow rate are beneficial to the improvement of thermal efficiency and exergy efficiency of the system to a certain extent, but when the inlet temperature increases or the inlet flow rate decreases to a certain extent, the efficiency of the system will decline. Under the condition of nonuniform energy flow density and nonuniform inlet flow distribution, more ideal system thermal efficiency and exergy efficiency can be obtained.

Effects of Installing Different Types of Cooling Fins on the Cold Side of a Thermoelectric Power Generation Device on the Thermal Efficiency and Exergy Efficiency of Power Cable Surface Waste Heat Recovery.

This study investigates the thermal efficiency and exergy efficiency of a thermoelectric power generation device for recovering power cable surface waste heat. Numerical simulations are conducted to analyze the impact of different types of cooling fins on the system's performance. The results demonstrate that the installation of cooling fins improves heat transfer efficiency and enhances the thermoelectric power generation device's output power. Among the various fin designs, the system equipped with cooling fins with 17 teeth exhibits the highest performance. These findings highlight the importance of fin design in optimizing the system's thermal efficiency and exergy efficiency. This study provides valuable insights for the development and improvement of thermoelectric power generation systems for power cable surface waste heat recovery applications.

Comparative Analysis of Transient Thermodynamic Performance for the Parabolic-Trough Photothermal Conversion Technology

The intermittent nature of solar energy poses a great challenge to the parabolic trough collector (PTC) technology. Although PTC technology is mature in application, its thermodynamic performance in transient processes is in need of a more comprehensive understanding. Here, a transient photothermo-hydraulic model of a parabolic-trough collector loop (600 m) was established with the finite volume method (FVM). A comprehensive comparative analysis for the transient photo-to-thermal characteristics (transient entropy generation and thermal-exergy efficiency) of PTC loop under a main weather disturbance [direct normal irradiance (DNI)], main control parameter (the mass flow rate of the heat transfer fluid mHTF), and geometry-dependent parameters (the collector width a and the absorber tube diameter douter) was carried out. The research shows that increasing DNI increases the entropy generation rate of each part of the absorber tube (up to 45.52%) and decreases the energy and exergy efficiency during the transient process, and vice versa. Transient entropy generation can be reduced by regulating the operating parameters (increase mHTF rapidly) and changing the geometric parameters (a or douter) of PTC (up to 87.58%, 38.03%, and 32.92%, respectively). Moreover, the influencing mechanism of mHTF (a or douter) on the transient entropy generation of the absorber tube is different from that in steady state. The transient thermodynamic response analysis is of great significance to the control and design optimization of PTC loop in the actual system.

Experimental study on optimized using activated agricultural wastes at hemispherical solar still for different types of water

The subsequent work aims to improve freshwater production and reduce the cost of generated water with little energy expenditure. Thermo-economic performance, hourly and daily production were evaluated by the experimental study on a hemispherical solar still in several configurations. The two floating agricultural waste materials utilized in the experiment were charcoal derived from guava tree wood (CHL) and carbonized corncobs (CCC). To enhance photothermal properties of the utilized primary materials, both have undergone physical and chemical activation by HCL or NaOH under specific conditions. In addition, the experiment was conducted using both seawater and lake water. Each modified distiller's performance was compared to that of a conventional unit in terms of thermal efficiency, exergy efficiency, daily output, and cost of freshwater in the climate of Kafrelsheikh, Egypt (31.09679°N, 30.94945°E) from 9:00 A.M. to 5:00 P.M. Therefore, this study aims to enhance the photothermal properties of charcoal and corn cobs by chemical and physical activation, which lead to enhance the efficiency of the hemispherical solar still by using this modified agricultural waste in its base compared to the traditional way. The daily productivity, thermal, and exergy efficiencies for H.CCC.LW were 4455 mL/m2,41.19%, and 3.85% showing improvement compared to a conventional hemispherical solar still by 41.2%, 40.72%, and 59.09%, respectively. At approximately 0.014 $/L (a savings of 26.70%), H.CCC.LW was shown to be the most cost-effective source of freshwater.

Thermodynamic and economic assessment of two-stage series organic Rankine cycle (TSORC) and regenerative TSORC (R-TSORC) for a solar project in hot, non-temperate climate

This paper provides a thermodynamic and economic assessment of two-stage series organic Rankine cycle (TSORC) and regenerative TSORC (R-TSORC) for a solar project in hot, non-temperate climate. The objective thermodynamic parameters include the net power output (Pnet), the thermal efficiency (ηth) and exergy efficiency (ηex), while the objective economic parameters include the electricity production cost (EPC), the payback period (PBP) and the rate of return on investment (ROROI). The total length of the receiver tube has been also considered as objective parameter. The two solar ORCs were compared on the basis of the same thermal potential generated by the solar system. The results show that, for the same thermal potential of energy source, the R-TSORC requires more mass flow rate of heat transfer fluid, but with a total length of receiver tube less than the TSORC. However, the Pnet, ηth, ηex, EPC, PBP and ROROI of R-TSORC are more competitive than those of the TSORC. In addition, it was found that the EPC of R-TSORC is lower than the electricity self-generation costs for industrial companies in Cameroon. Therefore, the R-TSORC can be effectively exploited in the micro solar plants of hot, non-temperate regions where the electricity issues remain problematic.

Thermo-Economic Analysis of Simple Cycle Steam Power Plant

Thermo-Economic Analysis of Simple Cycle Steam Power Plant

Thermal evaluation of different integration schemes for solar-nuclear hybrid systems

Solar-nuclear hybrid systems have been proposed to meet the growing energy demand and mitigate the effects of global warming caused by fossil fuels. To achieve optimal design for these systems, an appropriate integration scheme is crucial. This article proposes three integration schemes to couple the solar system with the nuclear power plant and evaluates their performance based on energy and exergy evaluations. A reference design of the hybrid system is used, consisting of a 77.04 MWth solar tower system coupled with a 330 MWth System-Integrated Modular Advanced Reactor. Numerical results show that heating partial feedwater and saturated steam with solar energy is the best-integrated scheme, resulting in the maximum thermal efficiency and exergy efficiency for the solar-nuclear hybrid system. Comparing the standalone reactor power system to the solar-nuclear hybrid system reveals that using solar energy to heat the feedwater and saturated steam can greatly improve the energy efficiency of the power plant, increasing it to 34.97%. The solar heat to nuclear heat ratio is 25.01%, while the solar power to nuclear power ratio is as high as 44.42%, indicating effective utilization of solar heat contribution. Exergy loss distributions of the solar-nuclear hybrid system reveal that exergy losses mainly occur within the nuclear reactor and solar field, taking possession of 70% of all exergy losses.

Oxy-fuel combustion-based blue hydrogen production with the integration of water electrolysis

Blue hydrogen is gaining attention as an intermediate step toward achieving eco-friendly green hydrogen production. However, the general blue hydrogen production requires an energy-intensive process for carbon capture and storage, resulting in low process efficiency. Additionally, the hydrogen production processes, steam methane reforming (SMR) and electrolysis, emits waste heat and byproduct oxygen, respectively. To solve these problems, this study proposes an oxy-fuel combustion-based blue hydrogen production process that integrates fossil fuel-based hydrogen production and electrolysis processes. The proposed processes are SMR + SOEC and SMR + PEMEC, whereas SMR, solid oxide electrolysis cell (SOEC), and proton exchange membrane electrolysis cell (PEMEC) are also examined for comparison. In the proposed processes, the oxygen produced by the electrolyzer is utilized for oxy-fuel combustion in the SMR process, and the resulting flue gas containing CO2 and H2O is condensed to easily separate CO2. Additionally, the waste heat from the SMR process is recovered to heat the feed water for the electrolyzer, thereby maximizing the process efficiency. Techno-economic, sensitivity, and greenhouse gas (GHG) analyses were conducted to evaluate the efficiency and feasibility of the proposed processes. The results show that SMR + SOEC demonstrated the highest thermal efficiency (85.2%) and exergy efficiency (80.5%), exceeding the efficiency of the SMR process (78.4% and 70.4% for thermal and exergy efficiencies, respectively). Furthermore, the SMR + SOEC process showed the lowest levelized cost of hydrogen of 6.21 USD/kgH2. Lastly, the SMR + SOEC demonstrated the lowest life cycle GHG emissions. In conclusion, the proposed SMR + SOEC process is expected to be a suitable technology for the transition from gray to green hydrogen.

Thermodynamic analysis of photovoltaic/thermal heat pump based on phase change slurry

To explore the feasibility and relative advantages of phase change slurry (PCS) as operating working medium, a solar hybrid vapor compression heat pump based on phase change slurry (PCS-PV/T-VCHP) is constructed by directly combining PCS as operating fluid with evaporator. Furthermore, performance of PCS-PV/T-VCHP under different initial conditions is studied and demonstrated by simulation method. PCS as operating medium has higher PV/PT and total efficiencies, which respectively reach 15.46%/83.17% and 98.63%. PCS has multiple advantages for VCHP applications due to high heat storage density and stable heat release. Under given initial conditions, PV/PT efficiencies are 15.15%–15.77%, 67.63%–84.03% respectively, and the maximum solar efficiency are 98.81%. The minimum and maximum values of VCHP coefficient of performance (COP) reach 2.23 and 5.82 respectively, and total COP is distributed in the range of 3.02 and 23.71. The maximum electrical exergy efficiency and thermal exergy efficiency are 15.77% and 7.57% respectively, and total exergy efficiency ranges from 16.61% to 23.25%. PCS-PV/T-VCHP shows good heating performance in typical cities, COP reaches 1.99–5.87 in selected cities. The feasibility of using PCS as operating fluid to supply heat has been confirmed, and PCS-PV/T-VCHP has significant performance advantages, low emissions and good economy.

Multiobjective optimization of a pressurized water reactor cogeneration plant for nuclear hydrogen production

In this paper, energy, exergy, economic analysis and multiobjective optimization of a pressurized water reactor (PWR) nuclear cogeneration plant for hydrogen production through high-temperature steam electrolysis (HTSE) are carried out. HTSE requires energy in the form of both heat and electrical work. A novel parameter, namely the heat/total energy ratio, is defined, and used as a decision variable in optimization. In addition to energy ratio, hydrogen production capacity, reactor thermal power, live steam temperature, reheating mass flow rate ratio, reheating temperature and steam extraction location are considered as the decision variables to simultaneously optimize the thermal efficiency, thermal-to-hydrogen efficiency, utilization factor, exergy efficiency and total revenue of the cogeneration plant. The analysis and optimization focus on the secondary cycle of the PWR and the effects of hydrogen and electricity prices and ambient conditions are also taken into account since these prices have a significant impact on the optimum design. For a hydrogen price of 4 $/kg and an electricity price of 0.1 $/kWh, when equal preference is given to all objective functions, the optimum production capacity is 6.778 kg/s. The energy ratio has an optimum value if the optimization focuses exclusively on the thermal efficiency and total revenue.

Investigation of Transcritical Carbon Dioxide Power Generation System Based on Vortex Tube

In this paper, a transcritical carbon dioxide power generation system based on a vortex tube is studied, which has the advantage of the self-condensation of carbon dioxide. The thermodynamic performance of the system was investigated by establishing a mathematical model. The results showed that under fundamental working conditions, the system could output a net power of 271.72 kW, and the thermal efficiency as well as the exergy efficiency of the system could reach 7.38% and 27.09%, respectively. Exergy analysis showed that the turbine had the greatest exergy loss among the system’s components, followed by the vortex tube, pump, heater and cooler. Parameter analysis showed that increasing the outlet pressure and inlet temperature of the vortex tube can improve the thermal efficiency and exergy efficiency of the system. In addition, the improvement in the turbine component’s efficiency is the most beneficial to the system’s performance, among which the turbine’s efficiency has the greatest impact. Carbon dioxide can be effectively liquified by expanding it in the vortex tube, and its liquefaction ratio increases with the decrease in the vortex tube’s inlet temperature and the increase in the vortex tube’s inlet pressure.

Numerical analysis of a modified combined volumetric filtered concentrating PV/T system using MCRT and FVM coupled method

To solve the problem that solar cell temperature limits high-quality thermal output from a traditional concentrating PV/T (CPV/T) system, this paper proposes a linear CPV/T system with a vacuum tube receiver incorporated with a combined volumetric filter. In which a solid absorption glass along with liquid filter is used as the combined volumetric filter for the silicon CPV module. However, the entire photovoltaic/photothermal conversion process in the combined filtered CPV/T system is complicated. To achieve more accurate predicted performance, a three-dimensional numerical model of this system is presented and the combined Monte Carlo Ray-Tracing (MCRT) method with the Finite Volume Method (FVM) is employed. The stated optical models are validated against the experimental results from an indoor test under one sun. And the coupled method of MCRT and FVM is also verified. The optimal filter combination is obtained by optical simulation using LightTools software based on the MCRT method firstly. Then, the solar energy flux distribution on the receiver achieved from LightTools is adopted as a boundary condition for CFD simulation. Based on these models and the MCRT-FVM coupled method, parametric study is conducted to quantify optical efficiency, temperature distribution, energy and exergy efficiency of the system. The results show that the optimal combined filters for silicon PV module are HB700+water and HB700+propylene glycol (PG). Because the optical absorption of water differs from that of PG, the PV maximum temperature is 308.36 K using HB700+water and it is 321.94 K using HB700+PG, whereas the outlet temperature of water and PG exceeds 352 K. The greater thermal and total exergy efficiencies can be achieved at higher inlet temperature and lower mass flow rate, while the electrical exergy efficiency shows slight change. To further reduce heat loss, an indium tin oxide (ITO) coating with low emissivity is applied on the receiver surface. The results reveal the receiver with 240° ITO coating achieves the highest total exergy efficiency of 14.92% and optical efficiency of 88.94%.

Energy, exergy and economic (3E) study on waste heat utilization of gas turbine by improved recompression cycle and partial cooling cycle

ABSTRACT As the bottom cycle of gas turbine waste heat utilization, the S-CO2 cycle can enhance the power output capacity of the system. Based on the energy analysis of the recompression cycle and the partial cooling cycle for gas turbine waste heat utilization, the two cycles are improved from the matching of heat exchange flow of regenerator and heater in this paper. Under the same parameters, compared with the cycle before improvement, increase in heat source utilization rate and net power output of improved cycles is 45.69% and 39.31%, 43.54 MW and 35.43 MW, and the power consumption cost is decreased by 0.6106 $/kWh and 0.6289 $/kWh. However, as part of heat recovery inside the system is replaced by the heat of the heat source, the thermal efficiency is decreased by 10.43% and 8.7%, respectively. Variable parameter study is conducted on the two improved cycles to analyze the cycle characteristics from the perspective of energy, exergy, and economy (3E). The results showed that the improved recompression cycle has a high thermal power conversion capacity, higher system thermal efficiency and exergy efficiency and lower power generation cost, while the improved partial cooling cycle has higher waste heat utilization potential. However, the area of heat exchangers in both cycles tends to increase, which is not conducive to the compactness of the system.

Experimental entropy generation, exergy efficiency and thermal performance factor of CoFe2O4/Water nanofluids in a tube predicted with ANFIS and MLP models

The thermal entropy generation, frictional entropy generation, and exergy efficiency of CoFe2O4/water nanofluids flow in a tube have been analyzed experimentally and the obtained data is predicted with ANFIS and MLP algorithms. The CoFe2O4 nanoparticles were developed through the chemical coprecipitation procedure and then characterized with XRD and TEM instruments. The thermophysical properties of CoFe2O4/water nanofluids were analyzed with various instruments in the temperature range from 20 °C to 60 °C and in the volume concentration range from 0.25% to 1.25%, respectively, and then entropy and exergy efficiency experiments were conducted, while the CoFe2O4/water nanofluids flow in a tube. Results show, the thermal entropy generation is lowered by 31.80% and the frictional entropy generation is larger by 240.17%, and the exergy efficiency is higher by 17.58% at φ = 1.25% and at a Reynolds number of 19301, over base fluid. Results also shows, the ANFIS model predicts very high accuracy for the experimental data compared to the MLP-ANN model with a correlation coefficients of 0.99798, 0.9944, and 0.99427 for thermal entropy, frictional entropy and exergy efficiency, respectively. The new thermal entropy, frictional entropy and exergy efficiency correlations were presented from the experimental data.

Thermocline in packed bed thermal energy storage during charge-discharge cycle using recycled ceramic materials - Commercial scale designs at high temperature

ReThink Seramic – Flora is an innovative ceramic material made from 100 % recycled materials. Due to its affordability, suitable thermal performance, and low pressure drop in packed bed thermal energy storage (TES), it is considered as a promising storage material option for high-temperature TES applications including concentrated solar power (CSP) plants. In the present study, a validated CFD model is used to analyze the thermal performance of the sustainable ceramic at high temperature (1000°C) for a commercial-scale packed bed. The paper focuses on modeling the charging/discharging processes and cyclic behavior based on threshold temperature, where dispersion thermal conductivity was considered. The effective thermal conductivity is affected when the dispersion thermal conductivity and the radiation heat transfer are taken into account in the model. The addition of dispersion conductivity leads to a variation in effective thermal conductivity in the range of 28.9–39.9 (W/m.K) for temperature range of 0–1000°C, while the addition of radiation heat transfer leads to a small variation in effective thermal conductivity based on two correlations from literature. The results show excellent thermal performance, with thermal exergy efficiency of 88 % for a full charge/discharge cycle. The net exergy efficiency for seven repeated cycles increased from 73.4 to 88.7 %, where thermal and pressure drop exergy losses are considered. The performance of ReThink Seramic – Flora is compared to the one of a commercial alumina-based TES material product. Thermal exergy efficiency for alumina is higher; however, the alumina-based product showed high pressure drop losses, leading to lower net exergy efficiency.

Exergy, exergoeconomic, and sustainability analyses of a diesel engine using biodiesel fuel blends containing nanoparticles

The current paper investigated in detail the influence of titanium dioxide (TiO2) and silver oxide (Ag2O) nanoparticles additives into biodiesel fuel obtained from cottonseed oil in terms of performance and emissions. The fuel blends formed by nanoparticles with biodiesel fuel were evaluated from a different perspective with energy, exergy, and exergoeconomic analyses by utilizing the data from the experiments. Thermal efficiency and exergy efficiency increase when nanoparticles were mixed to the biodiesel fuel. Total exergy losses in fuel blends decrease with the nanoparticle additives. When the engine torque was 40 Nm, the total exergy losses for C100, CAg-75, and CTi-75 test fuels were 14.49 kW, 13.91 kW, and 12.17 kW, respectively. The total exergy loss in D100 fuel was calculated as 12.04 kW under the same conditions. The sustainability indexes for D100 and CTi-75 fuels at an engine torque of 40 Nm were 1.626 and 1.620, respectively. Due to the high price of nanoparticles, test fuels with nanoparticles have a higher cost per unit exergy for engine work than pure biodiesel fuel. Hence, it is essential to decrease the cost of nanoparticle production to expand the using of nanoparticle additives in biodiesel.

Prediction and evaluation of energy and exergy efficiencies of a nanofluid-based photovoltaic-thermal system with a needle finned serpentine channel using random forest machine learning approach

The Photovoltaic thermal (PVT) collector performance is numerically investigated considering the effect of using needle fins in the serpentine channel with Nanofluid (NF). The influence of increasing the nanoparticle concentration (φ) and Reynolds number (Re) on the energy and exergy features of the PVT device is examined. A comparison is made between the hydrothermal characteristics of the PVT with the finned and plain serpentine channels. The utilization of needle fins improves the thermal efficiency (ηth), electrical efficiency (ηel), and overall efficiency (ηel) by 8.56–10.22%, 0.13–0.24%, 5.12–5.67%, respectively, against the PVT with the plain serpentine channel. Moreover, thermal exergy efficiency (ξth), electrical exergy efficiency (ξel), and overall exergy efficiency (ξov) by 8.56–1.22%, 0.13–0.24%, and 2.61–2.79%, respectively, versus the PVT with the plain serpentine channel. Moreover, the Random Forest (RF) machine learning approach is used to develop a predictive model for ηth, ηel, ηel, ξth, ξel and ξov in terms of Re and φ. The outcomes of modeling proved that all the results were in an acceptable level of accuracy and the overall efficiency in both energy and exergy yielded superior precision in comparison with the other targets.

The Effective and Exergy Efficiency of Multi-Pass Solar Air Collector with Longitudinal Fins: Analysis and Optimization

The present work investigates the exergy and effective efficiency of the multi-pass solar air collector with longitudinal fins by analysis approach and multi-objective optimization. The effect of 0.01-0.02 kg/s air flow rate, 15-35 mm collector depth, 1-3 m collector length, and 24.21-30.67 mm fin pitch was considered. The optimization was analyzed by the Preference Selection Index (PSI) method, with three maximum criteria: thermal efficiency, effective efficiency, and exergy efficiency. Mathematical models were solved by EES software. Results indicated that the multi-pass (TPLF and DPLF) type was better than the SPWF type by three criteria. The highest exergy efficiency of the TPLF and DPLF types was 6.696% and 5.636%. The greatest effective efficiency of the TPLF and DPLF types was 69.09% and 66.17%. Furthermore, the optimization results indicated that the three efficiency criteria of the DPLF type were 58.38%, 58.22%, and 4.491% for the best case; the three efficiency criteria of the TPLF type were 60.97%, 60.85%, and 5.439% for the best case. The worst configuration was the model with a short collector length, large collector depth, and large fin pitch. The collector efficiency decreased with decreased fin pitch for the configuration with the large collector length, short collector depth, and high mass flow rate.

Performance evaluation of a hybrid thermoelectric generator and flat plate solar collector system in a semi-arid climate

This study investigates the performance of a hybrid system that combined thermoelectric generators (TEG) with a solar flat plate collector system (SFPC) to turn more renewable energy into useable energy under normal operating and environmental conditions. A multi-month case study was first time conducted to explore the performance of SFPC with and without TEGs under the cold climatic conditions of Abha, Saudi Arabia. The result demonstrates that the heat losses from the rear surface of the SFPC's absorber plate were effectively utilized by TEGs, thereby decreasing overall heat losses and increasing the SFPC's overall thermal efficiency. Further, it was found that adding TEGs at the rear surface of SFPC's absorber plate improves thermal efficiency and exergy efficiency by 8.4% and 1.3%, respectively. The hybrid TEG-SPFC system can turn waste heat into useable electricity, with a peak power output of 2.2 W at midday. Maximum power was generated around midday, when the temperature differential between the upper and lower layers of TEGs was greatest. In addition, it was noticed that the thermal efficiency of the SFPC diminishes when the inlet water temperature rises. A hybrid TEG-SFPC system's exergy efficiency depends on ambient temperature variation and solar irradiance.