Thermal Power Conversion Research Articles

Evaluation of hybrid forecasting methods for organic Rankine cycle: Unsupervised learning-based outlier removal and partial mutual information-based feature selection

The construction of organic Rankine cycle (ORC) system model is the key to system performance analysis and prediction. However, traditional analysis methods have obvious limitations in constructing strong coupling relationship between operating parameters and performance due to the complex thermal power conversion process of ORC system. First, this study systematically analyzes the nonlinear relationship between ORC system operating parameters and performance by using unsupervised learning and bilinear interpolation algorithm. Compared with the traditional thermodynamic modeling method, the artificial neural network (ANN) has obvious advantages in constructing the mapping relationship of ORC system. However, the ORC system prediction model still has the defects of low accuracy, poor robustness, and high time cost due to the absence of outlier removal and feature dimensionality reduction. A hybrid algorithm for ORC system prediction model construction is proposed on the basis of the data characteristics, information theory and unsupervised learning. This algorithm can remove outliers and reduce the dimensionality of features in ORC system simultaneously. Then, the effectiveness of outlier removal, feature dimensionality reduction, and overall performance of the hybrid algorithm is verified. The mean squared error and mean absolute percentage error of the model is 1.64 × 10−11 and 5.1 × 10−3%. Compared with other algorithms, the hybrid algorithm suitable for ORC system has improved in accuracy and time cost. The accuracy of the hybrid algorithm is improved by 5.56% at least. The time cost of the hybrid algorithm is reduced by at least 17.05%. The hybrid algorithm can provide direct guidance for constructing ANN model of ORC system.

Numerical study on the wall-impinging diesel spray mixture formation, ignition, and combustion characteristics in the cylinder under cold-start conditions of a diesel engine

The combustion of wall-impinging diesel spray is a typical phenomenon in a diesel engine due to the limited in-cylinder space, especially for small and medium-sized diesel engines under the cold-start condition. To investigate the wall-impinging diesel spray combustion process at low-temperature and low-speed conditions, the mixture formation, ignition, and combustion characteristics of the fuel were quantitatively analyzed using a computational fluid dynamics model. It was discovered that the diesel spray was guided by the piston surface and the limited in-cylinder space, resulting in more vapor-phase fuel flowing into the region between the two spray paths, Thus, high-temperature ignition (HTI) occurred in this region. During a cold-start, in-cylinder HTI also occurred in the region between the two spay paths with different injection timings. The flame then propagated first to the region with the higher equivalence ratio near the piston surface and finally to the region with the lower equivalence ratio in the center of the combustion chamber. Because an earlier injection timing resulted in both a leaner mixture in the cylinder at top dead center and the transition period from the low-temperature reactions to the high-temperature reactions, there was a limited ability to adjust the HTI and thermal power conversion by controlling the injection timing. The optimal control of in-cylinder combustion during the cold start of a diesel engine depends on the HTI, the center of combustion, and the deposited fuel mass. The optimal of injection timing during a cold start of the diesel engine in this study was −13 °CA ATDC.

Polarity regulation for stable 2D-perovskite-encapsulated high-efficiency 3D-perovskite solar cells

There have been a few types of passivators used to improve the efficiency and stability of perovskite solar cells. It seems that strong interactions between the passivator and the [PbI6]4- octahedron are beneficial not only for defect passivation but also for suppressing ion migration. Here, we used molecular induction to design model passivators including ortho-fluorinated phenethylamine (2-F-PEAI) and ortho-fluorinated benzylamine (2-F-PMAI) to attain an optimized effect by regulating the interaction between the passivator and the [PbI6]4- octahedron. The first-principals study and experimental results including FTIR NMR and TGA show that the larger molecular polarity of the passivator, the stronger the interactions between the passivator and the perovskite surface. These were used to systematically scrutinize the polarity regulation to establish a general passivation model with parameters including the passivator molecular structure, polarity, thermal decomposition temperature and power conversion efficiency (PCE). Consequently, the solar cell efficiency is significantly improved to as high as 23.5% with very good reproducibility. The excellent long-term stability is manifested by 82.5% of the initial PCE remaining after 2000 h of aging under relative humidity (RH) 40% at ambient temperature. Finally, large-area devices (1 cm2) with PCE over 20% have been prepared to show the uniformity of the perovskite film for further upscaling.

Synthesis and characterization of carbon dots from coconut shell by optimizing the hydrothermal carbonization process

Coconut shell is one of the major agro-by products vis-a-vis agro-waste generated by coconut processing units. At present, Coconut shells are largely utilized as feed material for thermal power conversion by various allied industrial sectors, which is a highly energy inefficient and ecologically unfriendly process. The present study aimed to generate activated carbon dots/ carbon nanomaterials with a wide range of potential applications through a relatively less energy dependant hydrothermal carbonization process. Hydrothermal carbonization is a one-step, simple, low cost and environmental friendly approach to obtain carbon dots. The findings demonstrate that coconut shells when subjected to hydrothermal carbonization process at 250ᵒC for 6 h produced uniform-sized, stable, negatively charged and amorphous forms of carbon dots. Characterization of carbon dots using High-Resolution Transmission Electron Microscopy (HR-TEM), Scanning Electron Microscopy (SEM), Selected Area Electron Diffraction (SAED), X- ray Diffractometer (XRD), UV- Visible Spectroscopy, Particle Size Analyzer (PSA), Brunauer–Emmett-Teller (BET) Analyzer, Elemental Dispersive X-ray (EDX) analyzer and Fourier Transform Infrared Spectroscopy (FTIR) had conclusively confirmed the versatility of the carbon dots generation process and were able to achieve stable 2 nm-sized, spherical shaped carbon dots with numerous downstream applications. The study will help the conversion of agro-waste coconut shells into useful bio-based fluorescent carbon dots.

Parametric analysis and system optimization of a novel steam production system by synthetic cascade utilization of industrial waste heat

A great deal of waste heat is released in many industrial procedures, resulting in not only serious energy waste but also heat pollution. To alleviate this phenomenon, in-depth analyses of a steam generation system, whose novelty lies in synthesis of mechanical vapor recompression and thermal power conversion processes for dealing with low-grade waste heat utilization, are performed. Determination of organic Rankine cycle (ORC) working fluid and thermal power conversion cycle are conducted in detail. Then, parameter analyses and optimization are investigated. The simulation results show that the ORC–based system performs well in efficiency improvement and cost reduction with 69.41% higher exergy efficiency and 9.66% lower cost per ton of steam than the transcritical carbon dioxide-based system. In addition, there is an optimum flow rate of ORC working fluid for thermodynamic performance. A lower steam compression ratio and a higher heat source temperature are beneficial to improve techno-economic performance. At last, according to optimized results, the steam production yield and exergy efficiency of the ORC–based system are 2.50% and 44.31% under the 100 °C heat source condition, as well as the cost per ton of recycled steam is 7.67 $/ton. The optimizations of the system, working fluid, and parameters provide valuable information for improving system performance.

Design and performance experiment of radial inflow turbine expander for organic Rankine cycle system

Abstract Expander is the core component of organic Rankine cycle (ORC) system. Taking R245fa as the organic working fluid, the miniaturization design of radial inflow turbine expander with theoretical maximum output power 20 kW and rotational speed of 30 000 r/min is carried out. The deflector and the moving impeller are simulated and designed. The ORC system with preheater and superheater is built for performance analysis of the designed turbine expander. The effects of temperature, pressure and rotational speed on the output power, temperature drop and efficiency of turbine expander are studied experimentally. The experimental results show that with the increase of the inlet temperature, the efficiency of the turbine expander has a minimum value, but the output power keeps increasing gradually. The analysis shows that the lowest efficiency is caused by the deterioration of expansion due to the rapid increase of flow rate. The turbine expander has the maximum efficiency of 0.886 at the inlet temperature 393.15 K and the inlet pressure 1.2 MPa. When the rotational speed increases from 12 000 r/min to 28 000 r/min, the temperature drop increases from 17.2 K to 32.7 K, and the output power increases from 7.04 kW to 17.3 kW. With the increase of rotational speed, the thermal power conversion capacity and temperature drop increase, so the output power increases. The system efficiency increases with the increase of turbine expander efficiency and has the same change rule.

In-situ phase separation to improve phase change heat transfer performance

Phase change is widely applied in thermal-power conversion, cooling and thermal management, influencing efficiency, safety and cost of energy systems. The complicated mass, momentum and energy exchanges between two-phases causes large pressure drops, critical heat fluxes and flow instabilities. Here, a review on phase separation principle for phase change heat transfer is provided. The phase separation principle is presented in a theoretical form. Design and operation of phase separation for evaporator, condenser and heat pipe are commented. Because phase separation assigns pathways for liquid and vapor, it improves pool boiling performance and eliminates flow instability for micro-evaporator. For micro-condenser, the lined pin fins array creates liquid and vapor passages. The expanded liquid passage adapts increased flow rate to increased flow passage area for liquid. Pin fins are effective for filmwise condensation to increase heat transfer coefficients at low cost of pumping power. Phase separation for macro-condenser was fulfilled by suspending mesh screen membrane tube in condenser tube to confine vapor in annular region, yielding enhanced thin film condensation. Phase separation was commented on smart heat pipe. Superhydrophilic evaporator and superhydrophobic condenser generate nucleate mechanism, achieving increased heat transfer coefficients when heat loads increase. Perspective of phase separation was discussed.

Analysis and prediction of the performance of free- piston Stirling engine using response surface methodology and artificial neural network

Free piston Stirling engine is a popular area of research in high-efficiency thermal power conversion technology. However, owing to its strong coupling, nonlinearity, and parameter interactions, building an effective model to predict the performance is of great importance. This study was to investigate and derive the prediction models of a nonlinear free-piston Stirling engine using response surface methodology and artificial neural network. The interactive influences of thermodynamic and dynamic parameters which have significant effects on the amplitudes of the displacer and piston, operating frequency, and output power were illustrated in detail. Also, error analyses were then performed between the simulated and predicted values for both methods by comparing the mean absolute percentage errors, mean-squared errors, and correlation coefficients. The results indicated the correlation coefficients for the four output parameters from the response surface methodology as 0.9998, 0.9998, 0.9999, and 0.9994, and approximately 95% of the output parameter data were predicted with < 5% errors during verification, indicating that the response surface methodology model had good predictability compared with the artificial neural network model. Therefore, this research provides an effective approach to predict performances and can be applied to optimise the Stirling engine performance accurately and quickly.

Application of Green Power Generation Technology for Distributed Energy

This article discusses and analyzes the technical strengths and weaknesses of the green power generation that can be used for distributed system (power generation) power generation, for instance, solar power generation, wind power, hydrogen fuel cells, biomass power generation, and small gas turbines. The key to the discussion is to apply the technical distributed power generation of solar power stations. In addition, it also discussed the use of "focusing solar power generation high-temperature solar thermal power conversion system software" technical completion of distributed system power distribution. Low-cost, high-temperature solar thermal power generation is selected as the power generation solution medium, the power generation is technically low consumption and high-efficiency, the volume and power generation methods are conveniently equipped, the stability is high, and the economic development is environmentally friendly.

Design and thermo-economic comparisons of large scale solar absorption air conditioning cycles

Sustainable cooling absorption technologies, using renewable energy sources, and climate-friendly cooling liquids gain more and more attention. In this work, design and thermo-economic analyses are presented to compare between two different collector types (parabolic trough and evacuated tube) by Water-Lithium Bromide absorption systems. Generally, the main component of the entire system is the absorption refrigeration chiller, the plant will then consist of three subsystems. The first subsystem is the solar system for thermal power conversion. The second subsystem is represented by the condenser and chiller absorber that need to be cooled, using a cooling tower. The third subsystem is represented by the cold circuit and the terminals of the useful cooling power produced. A case study for a sports arena with 700–800 kW total cooling load is also presented. Results reveal that parabolic trough collector combined with H2O–LiBr (PTC/H2O–LiBr) gives lower design aspects and minimum rates of hourly costs (5.2$/h), while ETC/H2O–LiBr configuration (5.6$/h). H2O–LiBr thermo-economic product cost is (0.14$/GJ). The cycle coefficient of performance COP was in the range of 0.5–0.9.

Thermomechanically Induced Precipitation in High-Performance Ferritic (HiperFer) Stainless Steels

Novel high-performance fully ferritic (HiperFer) stainless steels were developed to meet the demands of next-generation thermal power-conversion equipment and to feature a uniquely balanced combination of resistance to fatigue, creep, and corrosion. Typical conventional multistep processing and heat treatment were applied to achieve optimized mechanical properties for this alloy. This paper outlines the feasibility of thermomechanical processing for goal-oriented alteration of the mechanical properties of this new type of steel, applying an economically more efficient approach. The impact of treatment parameter variation on alloy microstructure and the resulting mechanical properties were investigated in detail. Furthermore, initial optimization steps were undertaken.

Derivation and comparison of thermodynamic characteristics of endoreversible aircraft environmental control systems

An aircraft environmental control system (ECS) ensures a healthy and thermally comfortable environment for passengers and has a significant influence on the energy consumption of the aircraft. Existing simulations rely on complex iterations to determine the thermodynamic performance of ECSs under specific conditions, yielding results that are not universal. Therefore, to avoid repeated modeling and calculations, analytical solutions for the coefficient of performance (COP) of the ECS were derived based on the endoreversible thermodynamic analysis model. In this study, the solutions for the COPs of five types of ECSs were obtained. We found that each type of ECS had a unique solution for COP, reflecting the thermodynamic characteristics of the system. Results showed that the high-pressure water removal module does not affect the thermal performance of the ECS under dry conditions. The thermal performances of the three-wheel and four-wheel ECS were the same. The effects of different flight conditions, pressure ratio of the compressors, and heat exchanger effectiveness on the COP of the ECSs were also analyzed, reflecting the external and internal responses of the ECSs. The solutions reveal the mechanism of thermal power conversion in the ECS, which is helpful for component selection, ECS integration, and aircraft conceptual design.

Super-Planckian radiative heat transfer between macroscale metallic surfaces due to near-field and thin-film effects

In this study, we demonstrate that the radiative heat transfer between metallic planar surfaces exceeds the blackbody limit by employing the near-field and thin-film effects over macroscale surfaces. Nanosized polystyrene particles were used to create a nanometer gap between aluminum thin films of different thicknesses from 80 nm to 13 nm coated on 5 × 5 mm2 silicon chips, while the vacuum gap spacing is fitted from the near-field measurement with bare silicon samples. The near-field radiative heat flux between 13-nm-thick Al thin films at 215 nm gap distance is measured to be 6.4 times over the blackbody limit and 420 times to the far-field radiative heat transfer between metallic surfaces under a temperature difference of 65 K with the receiver at room temperature. The experimental results are validated by theoretical calculation based on fluctuational electrodynamics, and the heat enhancement is explained by non-resonant electromagnetic coupling within the subwavelength vacuum gap and resonant coupling inside the nanometric Al thin film with s polarized waves. This work will facilitate the applications of near-field radiation in thermal power conversion, radiative refrigeration, and noncontact heat control where metallic materials are involved.

Design and Thermo-Economic Comparisons of an Absorption Air Conditioning System Based on Parabolic Trough and Evacuated Tube Solar Collectors

Solar absorption cycles for air conditioning systems have recently attracted much attention. They have some important advantages that aid in reducing greenhouse gas emissions. In this work, design and thermo-economic analyses are presented in order to compare between two different collector types (parabolic trough and evacuated tube) by water–lithium bromide absorption systems, and to select the best operating conditions. Generally, the system consists of three major parts. The first part is the solar field for thermal power conversion. The second part is the intermediate cycle, which contains a flashing tank and pumping system. The third part is the water lithium bromide absorption chiller. A case study for a sports arena with 700–800 kW total cooling load is also presented. Results reveal that a parabolic trough collector combined with H2O–LiBr (PTC/H2O–LiBr) gives lower design aspects and minimum rates of hourly costs (USD 5.2/h), while ETC/H2O–LiBr configuration give USD 5.6/h. The H2O–LiBr thermo-economic product cost is USD 0.14/GJ. The cycle coefficient of performance COP was in the range of 0.5 to 0.9.

Design and Thermo-Economic Comparisons of an Absorption Air Conditioning System Based on Parabolic Trough and Evacuated Tube Solar Collectors

Solar absorption cycles for air conditioning systems have recently attracted much attention. They have some important advantages that aid in reducing greenhouse gas emissions. In this work, design and thermo-economic analyses are presented in order to compare between two different collector types (parabolic trough and evacuated tube) by water–lithium bromide absorption systems, and to select the best operating conditions. Generally, the system consists of three major parts. The first part is the solar field for thermal power conversion. The second part is the intermediate cycle, which contains a flashing tank and pumping system. The third part is the water lithium bromide absorption chiller. A case study for a sports arena with 700–800 kW total cooling load is also presented. Results reveal that a parabolic trough collector combined with H2O–LiBr (PTC/H2O–LiBr) gives lower design aspects and minimum rates of hourly costs (USD 5.2/h), while ETC/H2O–LiBr configuration give USD 5.6/h. The H2O–LiBr thermo-economic product cost is USD 0.14/GJ. The cycle coefficient of performance COP was in the range of 0.5 to 0.9.

Effect of regenerator on the direct steam generation solar power system characterized by prolonged thermal storage and stable power conversion

Abstract The direct steam generation (DSG) solar power system using two stage accumulators and cascade steam-organic Rankine cycle (RC-ORC) has remarkably enlarged storage capacity. It can facilitate stable power generation and address the challenges of conventional DSG systems. Regenerator is generally an issue worthy of discussion in organic Rankine cycle (ORC) systems. However, its influence on the newly proposed DSG system has not been investigated yet and is expected to be appreciable. Introducing a regenerator affects not only the ORC efficiency, RC-ORC efficiency, heat exchanger area, but also heat storage capacity, discharge duration, discharge efficiency, aperture area of collectors and the net profit ( Δ P ). Detailed performance comparison between the DSG systems without/with regenerator is carried out in this paper. The results indicate that at a given power output, aperture area is reduced by the regenerator especially for MM, R365mfc and pentane due to the increment in ORC, RC-ORC and discharge efficiencies, as well as the decrement in heat input. Discharge duration is shortened by 0.01–1.78 h depending on ORC fluids. R365mfc exhibits the maximum Δ P (4.19 ∼ 6.48 million USD), followed by MM and pentane. On the contrary, Δ P is negative for benzene (−5.61 ∼ -4.31 million USD).

Investigation of heat transfer in a steam generator bayonet tube for the development of PbLi technology for EU DEMO fusion reactor

In the frame of the EUROfusion roadmap for the development of the DEMO power plant, a research activity was carried out to develop a Lithium–Lead/water heat exchanger. The component should be capable to remove nuclear heat deposited in the liquid metal of the Dual Coolant Lithium Lead breeding blanket and feeding a steam turbine, ensuring an efficient thermal power conversion to electricity. One of the selected configurations is the steam generator bayonet tube. The HERO test section is an experimental mock-up in a relevant scale of this steam generator, consisting of a bundle of seven double-wall bayonet tubes with a leakage monitoring system. This test section, developed by ENEA at Brasimone Research Center and installed in the main vessel of the CIRCE pool facility, aims to investigate the thermal-hydraulic features of the system, providing a database for thermal-hydraulic system codes validation. An experimental campaign was carried out to demonstrate technological feasibility and performances of the prototypical heat exchanger, suitable as steam generator for the PbLi loop of the Dual Coolant Lithium Lead and Water Cooled Lithium Lead breeding blankets. A post-test analysis has been realized with RELAP5-3D and RELAP5/Mod3.3 codes in order to evaluate code capability in simulating heat transfer in liquid metal side.

Fast Thermal Response of Shape-Stabilized Thermal Storage Materials: The Case of Interconnected Netlike Graphene/Hexadecane/HDPE Composites.

Fast thermal response and sensitivity of organic latent heat storage materials with inherent low thermal conductivity to external temperatures is still a challenge in their practical applications. Herein, a shape-stable heat storage material composite, composed of interconnected netlike graphene, hexadecane, and commonly used high-density polyethylene (HDPE), was obtained using a convenient melt blending method. The start melting temperature, melting temperature, and end melting temperature of the obtained shape-stabilized thermal storage materials are presented, indicating that the response rate of the composites to ambient temperature could be indeed increased. This is because the interconnected netlike graphene with high conductivity intercalated into the HDPE matrix provided many conductive pathways for heat transfer. Therefore, the thermal conductivity of the composites is increased to 0.67 W/(m·K), which is about 123% higher than that of pure hexadecane. Meanwhile, the structure of polyethylene and hexadecane is similar, and hexadecane is uniformly dispersed in polyethylene and perfectly combined with polyethylene, which can effectively prevent the leakage of hexadecane during the phase transition. Thus, the obtained composites may play an important role in thermal management applications such as heat collection, transportation, thermal power conversion, and so forth.

Theoretical and Experimental Evaluation of a Thermoelectric Generator Using Concentration and Thermal Energy Storage

To improve the thermoelectric conversion efficiency of solar thermoelectric power, a concentration solar thermoelectric generator (CTEG) unit based on concentrating and storing energy is designed. A Fresnel lens is used to concentrate thermal energy, and a phase change material (PCM) is used to store thermal energy to increase the temperature difference between the hot and cold ends of thermoelectric (TE) sheets. The heat stored in the PCM container will help to generate continuous solar energy at night and improve the thermal power conversion efficiency of the TEGs. The energy conversion equilibrium equation is established for the CTEG unit. By numerical calculation, we conclude that the absorption rate of the coating surface is reduced by 0.1 and the maximum thermoelectric conversion efficiency is reduced by approximately 0.4%. By selecting different heights of the heat exchanger, the unit supplies 1.025 v, 0.4204 v and 0.299 v open circuit voltages on average for approximately 30, 44 and 53 minutes, respectively. Therefore, the height of the heat exchanger affects the rate of heat energy absorption and release. The reversible operation of the CTEG-PCM unit is conducive to the day and night operation of solar power generation and is more suitable to solving the self-supplying power problem for wireless temperature sensors in forests.

Metal Oxide Nanoparticle-Based Coating as a Catalyzer for A-TIG Welding: Critical Raw Material Perspective

Besides a wide application in corrosion protection, wear resistance increase, providing thermal properties and power conversion, oxide coatings have found an alternative application in welding technology as catalysts of the tungsten inert gas (TIG) welding process. In this paper, the novel approach of fabricating a coating containing nanoparticles based on nanosized SiO2 and TiO2 and their mixtures was applied to the austenitic stainless-steel base metal. It was found that coatings increased depths of penetration, enabling a consumable-free welding. Using this method, the use of several critical and near-critical raw materials (e.g., Si and Cr), as well as the relatively expensive Ni can be completely avoided. The most effective coating in terms of weld penetration consisted of a mixture of nanoparticles, rather than unary oxide coatings based on nanoparticles. A model for liquid weld metal flow is proposed based on the metallographic examination of recrystallized grains and microhardnesses measured near the weld metal, supporting the reversed Marangoni convection theory.