Thermal Efficiency Of System Research Articles

Thermo-economic analysis of a pumped thermal energy storage combining cooling, heating and power system coupled with photovoltaic thermal collector: Exploration of low-grade thermal energy storage

As renewable energy penetration grows, traditional power systems face significant challenges due to their intermittency and volatility. Pumped thermal energy storage (PTES) is a potential energy storage technology that has a low specific cost and geographical restriction. In this paper, a PTES system which is coupled with solar photovoltaic thermal (PVT) collectors is proposed to satisfy the demand for cooling, heating and electricity supply, and achieve energy cascade utilization. An ejector refrigeration cycle and the turbine extraction heating are accompanied with a PTES system. Thermodynamic analysis of the system is performed by evaluating the influence of key parameters on system performances for 1 MW power output. The economic analysis adopted levelized cost of energy storage (LCOS), which mainly includes investment costs, operation and maintenance costs and system depreciation. The results show that the maximum amplification of roundtrip efficiency with PVT is 12.04 % compared to the system without PVT. Moreover, the maximum average power generation benefit of PVT is 4.85 %. The increase in cold energy storage tank temperature can effectively improve the roundtrip efficiency of the system. The lower cold energy storage tank temperature and higher hot energy storage tank temperature have a negative impact on system thermal efficiency (ηthermal) but benefits for LCOS. Multi-objective optimization is carried out to obtain the optimal design performance that ηthermal and LCOS are 51.06 % and 0.533$/kWh respectively.

Carbon-Enhanced Hydrated Salt Phase Change Materials for Thermal Management Applications.

Inorganic hydrated salt phase change materials (PCMs) hold promise for improving the energy conversion efficiency of thermal systems and facilitating the exploration of renewable thermal energy. Hydrated salts, however, often suffer from low thermal conductivity, supercooling, phase separation, leakage and poor solar absorptance. In recent years, compounding hydrated salts with functional carbon materials has emerged as a promising way to overcome these shortcomings and meet the application demands. This work reviews the recent progress in preparing carbon-enhanced hydrated salt phase change composites for thermal management applications. The intrinsic properties of hydrated salts and their shortcomings are firstly introduced. Then, the advantages of various carbon materials and general approaches for preparing carbon-enhanced hydrated salt PCM composites are briefly described. By introducing representative PCM composites loaded with carbon nanotubes, carbon fibers, graphene oxide, graphene, expanded graphite, biochar, activated carbon and multifunctional carbon, the ways that one-dimensional, two-dimensional, three-dimensional and hybrid carbon materials enhance the comprehensive thermophysical properties of hydrated salts and affect their phase change behavior is systematically discussed. Through analyzing the enhancement effects of different carbon fillers, the rationale for achieving the optimal performance of the PCM composites, including both thermal conductivity and phase change stability, is summarized. Regarding the applications of carbon-enhanced hydrate salt composites, their use for the thermal management of electronic devices, buildings and the human body is highlighted. Finally, research challenges for further improving the overall thermophysical properties of carbon-enhanced hydrated salt PCMs and pushing towards practical applications and potential research directions are discussed. It is expected that this timely review could provide valuable guidelines for the further development of carbon-enhanced hydrated salt composites and stimulate concerted research efforts from diverse communities to promote the widespread applications of high-performance PCM composites.

Exploration of heat transfer and entropy generation in an irregular wall cavity filled with Al2O3-water nanofluid

This study investigates 2-D natural convection heat transfer within an irregular wall cavity filled with nanofluid, employing a computational approach. The rough profile of the cavity surface is generated using the Weierstrass-Mandelbrot (W-M) function, and various parameters such as Rayleigh number ( 10 3 ≤ Ra ≤ 10 6 ), Knudsen number ( 0 ≤ Kn ≤ 1.5 ), solid volume fraction ( 0 ≤ ϕ ≤ 0.15 ), and amplitude of the irregular rough wall ( 0 ≤ A 1 ≤ 0.1 ) are varied. The nanofluid consists of water as the base fluid and aluminum oxide (Al2O3) nanoparticles. Heat transfer characteristics are analyzed using the Nusselt number (Nu), while entropy generation is quantified to optimize thermal system efficiency. Grid sensitivity analysis and validation against existing literature are conducted to ensure the accuracy of the numerical methodology. Notably, the study demonstrates that increasing the slip velocity at the cavity wall enhances convection within the cavity, leading to a substantial increase in Nu. At Ra = 106, the Nu increases by 53.52% when Kn changes from 0 to 1. Furthermore, the study highlights the significant influence of nanoparticle volume fraction on Nu, showing that as the volume fraction increases, Nu also increases, indicating improved convective heat transfer performance. The Nu for a cavity with an amplitude of 0.1 is roughly 2.5 times higher compared to a smooth-walled cavity at Ra = 106. Increased surface irregularity enhanced turbulence intensity and mixing, leading to stronger convective currents. The novelty of this study lies in examining natural convection in irregular shaped cavities with wall-slip conditions, which are common in practical applications such as solar collectors, microelectronic devices, energy storage units, industrial furnaces, etc. This study offers valuable insights into the thermal behavior of rough enclosures filled with nanofluids, which will facilitate the optimization of heat transfer systems to minimize energy losses.

Influence of PCHE size and types on thermodynamic and economic performance of supercritical carbon dioxide Brayton cycle for small modular reactors and its optimization

The supercritical CO2 Brayton cycle is a potential technology in small modular reactors (SMRs), and the overall system efficiency can be significantly improved by optimizing the PCHE design parameters. This study develops a thermodynamic-economic analysis model for a supercritical CO2 simple Brayton cycle cooled SMR, with consideration of the detailed parameters of PCHEs. The impact of the length, channel number and channel type for the recuperator and the precooler on system thermal efficiency and levelized cost of electricity (LCOE) are analyzed. A dual-objective optimization study is conducted at the system level to identify the optimal design of size parameters and types for the heat exchangers. The results indicate that the influence of PCHE size parameters variations on system performance varies across different channel types. Additionally, the selection of the recommended recuperator type is influenced by the recuperator inlet Reynolds number. At the optimum design, the recommended channel type for recuperator and precooler are both S-shaped, resulting in a system efficiency of 39.12% and LCOE of 0.0456 $/kWe. The findings are valuable for enhancing energy utilization and reducing the power generation cost of SMRs.

Experimental and numerical assessment of thermal characteristics of PCM in a U-shaped heat exchanger using porous metal foam and NanoPowder

The utilization of Latent Heat Thermal Energy Storage (LHTES) has gained significant attention to address the disparity between energy supply and demand. One of the key advantages lies in the use of phase change materials (PCM). The purpose of this research is to overcome this obstacle by focusing on enhancing the thermal efficiency of an advanced thermal energy storage system specifically designed for solar domestic and industrial application. to overcome this obstacle by focusing on enhancing the thermal efficiency of an advanced thermal energy storage system specifically designed for solar domestic and industrial application. Through computational and experimental studies, a novel and small LHTES system with parallel U-shaped heat exchanger (USHX) has been created and investigated. To improve performance, two approaches are employed: optimizing thermal efficiency by dispersing nano-sized graphite powders into the paraffin material, and/or incorporating metal foams. The PCM is RT35HC, and the hot/cold heat transfer fluid is H2O, which travels via the U-shaped tube. The model incorporates the enthalpy-porosity technique to account for phase change phenomena. After comparing the numerical outcomes with the experiments herein run, data are shown in terms of liquid fraction, temperature evolution, stored energy, and a dimensionless parameter that characterizes the phase change process. The findings suggest that the proposed methods for enhancing heat transfer can enhance the thermal efficiency of systems. The outcomes illustrate that by addition of all methods, reduces the melting time by 13.39 %, 60.77 %, and 71.93 %, when compared to system with pure PCM.

A novel dual condensers heat pipe photovoltaic/thermal (PV/T) system: Numerical dynamic modelling and effect factors analysis in dual-function operation mode

This paper presents a dual condensers heat pipe PV/T system with multiple operating modes, including hot water, space heating, and dual-function. To exploiting the system's potential of energy supply, this paper proposes a dynamic model of the system in dual-function mode on the basis of the verified dynamic heat transfer model and condenser distributed parameter model. Several effect factors on the system performance have been analyzed, including water tank capacity, air-cooled condenser area and the dual condensers installation height difference. The simulated results show that the above parameters have little effect on the system's total thermal efficiency but a significant effect on single condenser heat gain distribution. When the water tank capacity increases from 100 L to 200 L, the thermal efficiency of the water-cooled condenser increases from 10.68 % to 17.82 %, while the air-cooled condenser's decreases from 48.12 % to 40.72 %. As the air-cooled condenser area is changed from model 1 to 4, the total efficiency increase from 58.13 % to 71.41 %. The system's thermal efficiency falls from 58.66 % to 47.50 % and its overall efficiency falls from 71.41 % to 60.40 % when the height difference rises from 0 m to 0.7 m.

Techno-economic analysis of ammonia cracking for large scale power generation

The increasing interest in leveraging green ammonia to mitigate carbon emissions in fertilizer production is paralleled by an expanding acknowledgment of its potential as a fuel for decarbonizing the electricity sector, particularly in high-efficiency gas turbine power plants. Co-firing ammonia with hydrogen presents a promising method for integrating ammonia into existing infrastructures. Within this context, the development of efficient technology for ammonia cracking presents a potential avenue for deploying ammonia in gas turbines. The objective of this study is to conduct a preliminary techno-economic evaluation and uncertainty analysis of two cracking technologies namely a membrane reactor and a conventional FTR (Fired Tubular Reactor) for the co-firing of ammonia with hydrogen in a CCGT (Combined Cycle Gas Turbine) plant. The integration of a membrane reactor during the cracking stage demonstrates a remarkable improvement in the system's thermal efficiency, surpassing traditional approaches by over 25%. Additionally, it brings about an approximate 10% reduction in the levelized cost of hydrogen (LCOH), despite a higher initial capital expenditure (CAPEX). At the CCGT level, the discrepancy in levelized cost of electricity (LCOE) narrows, as it is strongly influenced by the cost of ammonia constituting 80% of the LCOE. Beyond LCOE, the widespread adoption of these systems also faces challenges due to material scarcity. Analysis reveals that revamping just 1 GWe of CCGT assets using membrane reactors would for example necessitate approximately 0.11% of the global palladium supply, and 10% of the global ruthenium production. Considering the limited availability of these resources, coupled with their high demand across multiple sectors and the possibility of external factors such as geopolitical tensions, this strategy seems unfeasible. To tap into this market, future research should prioritize the exploration of alternative membrane materials, such as carbon molecular sieves, and catalysts, like nickel.

Integrating computational models and machine learning for corrugated absorber plate solar collector thermal predictions

This research provides a computational analysis, development, and empirical testing of a Corrugated Absorber Plate Solar Collector (CAPSC) with fins inserted Thermal Energy Storage (TES) system outlet temperature and thermal efficiency, which is then predicted employing machine learning techniques. For computational analysis, the governing equations of the CAPSC device have been generated and integrated into Computational Fluid Dynamics (CFD) software. Several experimental tests were conducted in both summer and winter seasons during the year, with varying mass flow rates ranging from 0.014 to 0.064 kg/s. The maximum average temperature difference among computational and experimental CAPSC outlet temperatures has been 1.72 °C at 0.044 kg/s. However, to enhance the thermal efficiency forecast of the CAPSC with fins inserted TES system, machine learning techniques such as Multiple Linear Regression (MLR), Random Forest (RF), K-Nearest Neighbors (KNN), eXtreme Gradient Boosting (XGBoost), and Artificial Neural Network (ANN) have been used. A substantial empirical dataset of 288 samples was utilized to analyze the machine-learning approaches. The observed statistical error and regression values in all techniques indicate the adequacy of the approaches presented. When compared to MLR, KNN, and RF techniques, ANN, followed by XGBoost, provides the most accurate forecast. Notably, XGBoost demonstrated superior accuracy due to its ability to handle complex non-linear relationships and interactions within the dataset. Furthermore, to improve the thermal efficiency forecast of the CAPSC with fins inserted TES system by integrating the computational model with the machine learning techniques by utilizing computational CAPSC output temperature as one of the inputs. This CFD with machine learning integration technique considerably enhanced the thermal performance forecast accuracy.

Experimental Investigation on Thermal Performance of Solar Air Heater using Nano-PCM

Herein, the performance of a solar air heater (SAH) is experimentally investigated, utilising an array of tubes as the absorbent part. The study evaluates the impact of incorporating Al2O3-paraffin wax as a non-PCM storage medium in comparison to a traditional flat-plate solar air heater, specifically under Najaf-Iraq climate conditions. The SAH is positioned at an inclination of 32.1 degrees with respect to the horizon, allowing it to align optimally with the solar direction. The results reveal notable differences in thermal performance characteristics among the various models. The highest thermal efficiency values are observed for distinct configurations: the proposed model achieves about 55.2%, the wax-supported model reaches 55.9%, and the nano-PCM-reinforced model attains 57.7%, while the traditional model lags at 48.2%. Furthermore, an analysis of different air mass flow rates highlights a crucial finding. Specifically, an air mass flow rate of 0.01 kg/s results in a higher temperature exiting from the system compared to a flow rate of 0.02 kg/s. This is attributed to the extended interaction time between the passing air and the absorbing surface, facilitating enhanced heat exchange. Consequently, the system's thermal efficiency experiences an increase. The study underscores the superior thermal performance and efficiency of the tube array nano-PCM collector type under Najaf city-Iraq climate conditions.

Experimental Study on Ultra-Low Concentration Methane Regenerative Thermal Oxidation

As a major coal country, China faces the issue of significant gas emissions during the coal mining process. This study aims to improve the utilization efficiency of mine gas, reduce greenhouse gas emissions, and promote the low-carbon and green transformation of the coal industry. A 10 kW gas regenerative thermal oxidizer (RTO) experimental system was constructed. The effects of initial methane concentration, low-temperature flue gas proportion, and operating load on combustion temperature, methane oxidation rate, high-temperature flue gas energy, and system thermal efficiency were studied. The results show that when the combustion temperature is below 600 °C, the CH4 combustion reaction cannot proceed effectively, and the system temperature continuously decreases and cannot be maintained stably. The experiment determines the stable operating methane concentration range of the RTO. In this experimental system, the lower limit of the initial methane concentration is 0.28%, corresponding to an 86% methane oxidation rate. As the initial methane concentration decreases, the combustion temperature also decreases, and the methane oxidation rate follows suit. The higher the low-temperature flue gas proportion, the higher the combustion temperature, and the system’s thermal efficiency and output heat decrease with the increase in the low-temperature flue gas proportion. This experiment explores multiple factors affecting regenerative thermal oxidation, providing a basis for ensuring the safe and stable operation of the system and its optimization. Improving the thermal insulation and heat exchange performance of the storage body can expand the lower limit of the initial methane concentration, thereby increasing the stability and thermal efficiency of the system.

Performance Prediction and Optimization of Nanofluid-Based PV/T Using Numerical Simulation and Response Surface Methodology.

A numerical investigation was carried out in ANSYS Fluent® on a photovoltaic/thermal (PV/T) system with MXene/water nanofluid as heat transfer fluid (HTF). The interaction of different operating parameters (nanofluid mass fraction, mass flow rate, inlet temperature and incident radiation) on the output response of the system (thermal efficiency, electrical efficiency, thermal exergy efficiency, and electrical exergy efficiency) was studied using a predictive model generated using response surface methodology (RSM). The analysis of variance (ANOVA) method was used to evaluate the significance of input parameters affecting the energy and exergy efficiencies of the nanofluid-based PV/T system. The nanofluid mass flow rate was discovered to be having an impact on the thermal efficiency of the system. Electrical efficiency, thermal exergy efficiency, and electrical exergy efficiency were found to be greatly influenced by incident solar radiation. The percentage contribution of each factor on the output response was calculated. Input variables were optimized using the desirability function to maximize energy and exergy efficiency. The developed statistical model generated an optimum value for the mass flow rate (71.84 kgh-1), the mass fraction (0.2 wt%), incident radiation (581 Wm-2), and inlet temperature (20 °C). The highest overall energy and exergy efficiency predicted by the model were 81.67% and 18.6%, respectively.

Enhancing cooling performance and economic analysis of a vertical earth air heat exchanger (VEAHE) through geometric shape optimization

AbstractThe cooling and heating sector is responsible for the highest energy consumption in the building sector, comprising approximately 30% of the total. Extensive research has been conducted to address this issue and minimize energy consumption through the implementation of innovative technologies. Among these technologies, the passive earth‐air heat exchanger (EAHE) has proven highly effective in reducing energy usage in the cooling and heating sector. This research focused on optimizing U‐shaped EAHE systems and examined their functional and thermal‐fluidic parameters through numerical analysis. The simulation employed COMSOL Multiphysics software, and the results obtained were in excellent agreement with experimental data. The study investigated a base case, as well as five optimized cases with varying inlet velocities, to evaluate performance. The findings revealed that increasing the working fluid's inlet velocity led to a decrease in the system's thermal efficiency. However, at higher velocities, the economic parameters for energy production showed improvements. Specifically, the system generated a maximum energy output of 9132 W in the fifth case, operating at a velocity of 2 m/s. Additionally, the system achieved an impressive performance coefficient of approximately 5.13 in the same case, with an inlet velocity of 0.46 m/s. Notably, the lowest recorded output temperature of the system was 22°C at the specified inlet velocity.

Investigation of factors affecting photovoltaic thermal system efficiency

This experimental study investigates the effects of ambient temperature (Tamb) and solar irradiance on the efficiency of photovoltaic panels (ηPV). Experiments have shown that increasing these parameters, which affect ηPV, also raises panel Tcell, leading to decreased electrical energy production. A photovoltaic thermal (PV/T) system was created to enhance the ηPV by reducing Tcell. The excess heat generated in the cells is stored as hot water in this system. In the experiments, water was used as the heat transfer fluid (HTF) to lower the temperature of the PV panel. A closed loop with a 25-liter tank volume circulated the water at a constant mass flow rate of 0.0161 kg/s. The heat transferred from the panel cells to the HTF was accumulated in a 50-liter water tank. The ηth of a standard PV panel and a PV/T system, with and without a fan-cooled heat exchanger, was assessed. The results showed that the ηelec of the system without a fan-cooled heat exchanger increased by 2%. However, for systems designed for maximum efficiency, the presence of the fan-cooled heat exchanger caused a 13% reduction in ηth. Additionally, the temperature of the water in the tank increased by 50%. The efficiency of the designed PV/T system was analyzed without the use of a fan-cooled heat exchanger. The 8-hour average thermal efficiency was calculated to be 66.53%, with an electrical efficiency of 3.42%. The results are presented in graphs for better data visualization.

Impact of double-axis tracking on thermal performance of the linear Fresnel Collector: An experimental study

Due to the concerning trend of fossil fuels destructive effect on the environment, renewables, especially solar energy, can be a promising alternative to meet the world's energy demands. Linear Fresnel Collectors (LFCs) are solar thermal collectors that can effectively provide the required thermal energy for various applications. Despite the low capital cost and non-complicated structure of the LFCs, they have some drawbacks, such as end loss effects that reduce their optical and thermal efficiencies. The current research introduces a mechanical tracking system to enable the system to rotate in the secondary axis to decrease the end loss effect. Four experimental tests at laminar and turbulent flows in two single and double-axis tracking modes have been performed to investigate the non-illuminated length in each scenario. Moreover, a theoretical study was conducted, and the results were compared with the experiments. The results showed that the thermal efficiencies were 8.67 %–22.03 % and 40.97 %–63.5 % for the double-axis tracking mode in laminar and turbulent, respectively. The double-axis tracking system enhanced the system's thermal efficiency by 18.91 % and 6.42 % for laminar and turbulent flows on average, respectively, compared to the single-axis tracking. Furthermore, the difference between the experimental and theoretical non-illuminated length values was calculated as 4.14 % and 8.44 % on average for laminar and turbulent flows, respectively. Finally, according to the economic analysis, 4.78 years was calculated for the payback time of the investigated system.

Investigating thermal performance enhancement in perforated pin fin arrays for cooling electronic systems through integrated CFD and deep learning analysis

Pin fin heat sinks have garnered considerable attention within the realm of thermal management for high-heat-flux electronics systems. This study advances the understanding of perforated pin fin systems, offering novel insights into heat transfer enhancement. The research explores circular and square perforation geometries located at varying positions along the pin fin length, aiming to enhance heat dissipation and extend the lifespan of electronic components. A parametric analysis examines the impact of perforation quantity on thermal performance with a rigorous comparison against non-perforated pin fins. The investigation incorporates machine learning techniques, including artificial neural networks, to predict cooling system thermal efficiency. Using computational fluid dynamics simulations and Artificial Neural Network modeling, the obtained results indicate the optimal design mode is the use of 3 square holes on the pin fin, which leads to an increase in thermal efficiency by 16.63% compared to the case without pin fins. Additionally, the mean absolute error value calculated across all data extracted from the computational fluid dynamics simulation for predicting thermal efficiency was 2.25%. This low error value further demonstrates the accuracy of the simulation data and neural network model in predicting the heat transfer performance of the pin fin designs.

Study on the thermodynamic and economic performance of power generation cycle with small temperature difference

The low thermal efficiency and low economic benefits of power generation systems under small temperature difference conditions have always hindered their practical operation. In order to explore effective ways to achieve performance optimization of thermodynamic cycle electrical power generating systems under small temperature difference conditions, this study constructs a static analysis model of an organic Rankine cycle with small temperature difference, selects six different commonly used refrigerants, and investigates the influence of key parameters on the thermodynamic and economic performance of the system using difference working fluids. The outcomes indicate that key parameters such as evaporation temperature and condensation temperature play a critical role in the thermodynamic and economic performance of the power generation system. For the working fluids, R227ea provides the highest net power output, while ammonia (R717) is the best performer in regard to system thermal efficiency and economic performance.

Analyzing the Effect of Magnetic Drag Force on Carbon Nanotubes Suspended in Casson Fluid Within Parallel Surfaces with Heat Diffusion and Rate of Molar Reaction: An Analytical Approach

The study of hydromagnetic CNTs of Casson fluids in Poiseuille flow has significant implications for various industries and can provide valuable insights into the fundamental properties, such as viscosity and conductivity of these fluids. They can improve the heat transfer properties of fluids and enhance the overall efficiency of thermal systems and the presence of CNTs can induce a magnetic field in the fluid. The main goal of using carbon nanotubes (CNTs) in Poiseuille flow is to enhance the fluid flow properties, such as viscosity, thermal conductivity, and heat transfer and the novelty of CNTs in Poiseuille flow lies in their ability to modify the fluid flow properties by altering the structure of the fluid at the nanoscale level. The use of CNTs in Poiseuille flow has gained attention due to their unique mechanical, electrical, and thermal properties. An analytical approach to the investigation of heat transmission in hydro-magnetic forces of natural convective flow of Casson-fluid in a Poiseuille flow implanted by Darcian regime on Carbon Nanotubes with the impacts of magnetic field, heat generation, diffusion thermo, porosity, radiation and first order chemical reaction is presented. In this research, dual solutions are introduced for single-wall and multiple-wall carbon nanotubes over velocity and temperature of nanofluid by the application of various physical and they are elaborated via plane curves. The base fluid is considered for the CNTs as Engine oil. Validity of this model has established by comparing with the available previous literature and is found acceptable agreement with it. In the present study, it is found that the fluctuation in radiation and heat generation plays a significant role in CNTs. It is known that a rise in the Casson parameter and nanoparticle volume fraction enhances the fluid velocity. It is concluded that, the volume fraction of nanoparticles in Poiseuille flow can have a significant impact on the flow behaviour and properties of the fluid. This study has tremendous feasible applications in the areas related to biomedical sciences, water purification process, technology of fibers, nano-materials technology, storage of energy and various applications.

Enhanced thermal performance and entropy generation analysis in a novel cavity design with circular cylinder

AbstractAnalyzing fluid dynamics and heat transfer holds significant importance in the design and enhancement of engineering systems. The current investigation utilizes the finite element method to explore natural convection and heat transfer intricacies within a novel cavity containing an inner circular cylinder under steady and laminar flow conditions. The principal aim of this study is to assess the impact of Rayleigh number (Ra), Bejan number (Be), and the presence of adiabatic, hot, and cold cylinders on heat transfer, entropy generation, and fluid flow. The range of Ra considered in this investigation spans from 103 to 106, while the Prandtl number for the air is fixed at 0.71. The findings illustrate that the presence of a cylinder leads to higher Be as Ra increase, compared to scenarios where no cylinder is present. This observation suggests that buoyancy forces dominate in the absence of a cylinder, resulting in significantly enhanced convective heat transfer efficiency. However, the presence of a heated cylinder within the tooth‐shaped cavity exerts a substantial influence on the overall thermal performance of the system. Notably, the average Nusselt Number (Nu) experiences a remarkable increase of 41.97% under the influence of a heated cylinder, when compared to situations where a cold cylinder is present. This elevated average Nu signifies improved heat transfer characteristics, ultimately resulting in an overall improvement in the thermal system's efficiency.

Thermodynamic and emission characteristics of a hydrogen-enriched natural gas-fired boiler integrated with external flue gas recirculation and waste heat recovery

Hydrogen-enriched natural gas (HENG) is a low-carbon fuel and its utilization in combustion devices has been under extensive discussion recently as it is a promising way to reduce the CO2 emission during combustion. The applicability of HENG in the existing natural gas-fired combustion equipment in terms of its efficiency and emissions attracts increasing attention, especially for industrial and domestic small-scale boilers. In this study, the thermal efficiency and pollutant emissions of a 4.2 MWth HENG-fired boiler integrated with external flue gas recirculation (FGR) were respectively evaluated based on the thermodynamic and heat transfer models and the Chemical Reaction Network model. Results suggested that NOx emission rose by ∼19% as the hydrogen volumetric fraction in the fuel increased from 0 to 0.4 at a constant excess air ratio and FGR rate. To suppress the NOx rise, the FGR rate was tuned higher while the system thermal efficiency decreased subsequently. To further improve the overall system thermal efficiency, a cascade flue gas waste heat recovery strategy including sensible heat recovery using an external economizer and latent heat recovery using dual-spray heat exchangers was proposed. The thermodynamic analysis demonstrated that the external economizer improved the overall system thermal efficiency by 0.4% - 0.7% and the dual-spray heat exchangers promoted the overall system thermal efficiency by over 5%. Thus, a comprehensive performance optimization strategy was developed for HENG-fired boilers in terms of their overall system thermal efficiency promotion, NOx emission control, and CO2 emission reduction.

Design, fabrication and thermal analysis of parabolic trough collector

Parabolic trough collector (PTC) is considered environmentally friendly and one of the green technologies with net-zero emissions. In this study, performance evaluation of a developed solar PTC system is performed by conducting a series of experimentation. The study focuses on the design, development, and thermal evaluation of a PTC prototype, under hot and humid climatic conditions of Islamabad, Pakistan. Thermal efficiency of the solar PTC system is determined and effect of solar irradiance on thermal efficiency and heat gain is evaluated. Energy and exergy efficiency of the PTC systems are determined 66%, and 38%, respectively.