Waste Heat Recovery Applications Research Articles

Performance boost for bismuth telluride thermoelectric generator via barrier layer based on low Young’s modulus and particle sliding

The lack of desirable diffusion barrier layers currently prohibits the long-term stable service of bismuth telluride thermoelectric devices in low-grade waste heat recovery. Here we propose a new design principle of barrier layers beyond the thermal expansion matching criterion. A titanium barrier layer with loose structure is optimized, in which the low Young’s modulus and particle sliding synergistically alleviates interfacial stress, while the TiTe2 reactant enables metallurgical bonding and ohmic contact between the barrier layer and the thermoelectric material, leading to a desirable interface characterized by high-thermostability, high-strength, and low-resistivity. Highly competitive conversion efficiency of 6.2% and power density of 0.51 W cm−2 are achieved for a module with leg length of 2 mm at the hot-side temperature of 523 K, and no degradation is observed following operation for 360 h, a record for stable service at this temperature, paving the way for its application in low-grade waste heat recovery.

Research of the thermal storage properties of thermally conductive carbon fiber-reinforced paraffin/olefin block copolymer composite phase change materials with thermotropic flexibility

This study focuses on the fabrication and evaluation of paraffin (PA)/olefin block copolymer (OBC)/carbon fiber (CF) composite phase change materials (CPCMs) prepared using the melt impregnation process. The resultant PA/OBC/CF CPCMs exhibit superior thermal stability below 200 °C and maintain excellent shape stability at 60 °C. These characteristics are attributed to the alternating soft and hard segments of OBC, wherein the hard segments prevent liquid leakage from PA. The distinct structure of OBC further lends flexibility and recyclability to the PA/OBC/CF CPCMs. The latent heat of these CPCMs ranges from 169.1 to 197.6 J/g. Notably, the CPCMs variant (PA/OBC/CF-1) with 2 wt% CF addition retains 95 % of its initial latent heat even after 100 thermal cycles, thereby demonstrating superior cycling stability. The thermal conductivity of CPCMs with 10 wt% CF is 1.3 times that of pure PA. This significant improvement in thermal conductivity is attributed to the heat conduction pathways provided by the fibrous CF. These results indicate that the fabricated CPCMs offer substantial promise for utilization in low-temperature waste heat recovery applications.

Facile, catalyst-free, and high enthalpy PEG-based phase change composite for use in photothermal conversion and waste heat recovery

The poor shape stability, low thermal storage density, and complex process confronted in fabricating high-performance phase change composites (PCCs) via the chemical cross-linking technology. Herein, PEG-based shape stability PCC (SSPCC) with high enthalpy and good shape stability was obtained using a facile one-pot method without additional catalysts. The reliable three-dimensional cross-linked structure formed by the interaction of epoxy resin and curing agent binds the movement of PEG molecular chains, which simultaneously in coupled with the stronger hydrogen bonding between PEG and cross-linked products to endow the as-prepared SSPCC with excellent shape stability, thermal stability, durability, and high thermal storage density (160.8 J/g). Moreover, the Raman spectra clearly presented the interaction force and phase morphology distribution between PEG and the skeleton structure. SSPCC with the high relative enthalpy efficiency and melt enthalpy can play an essential role in photothermal applications, while the facile manufacturing process free from additional organic solvents or high temperature, highlighting its green and sustainable features. Additionally, the photothermal transport and thermostatic effects of the SSPCCs show great potential for application in waste heat recovery, which can maintain higher temperatures for a long time. Totally, such SSPCC with reliable and distinguished phase change properties ensure a wide range of potential applications, while the easy-to-prepare and low-cost fabrication method provides an inspiring strategy for alternative SSPCC development.

Ferrimagnetic half Heusler alloys for waste heat recovery application - First principle study using different exchange–correlation functionals

ZrMnSi and HfMnSi half Heusler alloys with magnetic ground state are investigated through density functional theory. The different exchange–correlation functionals (GGA, LDA, PBESol and WC) are used to study these alloys. The equilibrium lattice constants and corresponding ground state energies are calculated using these four approximation methods. From GGA functional, the obtained band gap for ZrMnSi is 1.268 eV and HfMnSi is 1.311 eV. The reported alloys are predicted to be half metals from different potentials, where those alloys show semiconductor character in minority spin channel and also exhibit metallic character in majority spin channel. The studied alloys are mechanically stable and ductile in nature. The total magnetic moment of −3μB is obtained in the reported ferrimagnetic alloys. The obtained thermoelectric figure of merit for ZrMnSi and HfMnSi is 0.46 at 1200 K and 0.31 at 1200 K respectively, it shows that these alloys are the potential candidates for waste heat recovery application, and they are also useful for spin flip device application.

Synergetic effect of Bi and Al co-doping in GeTe-based thermoelectric materials leading to optimized carrier concentration tuning and high ZT

Improving the performance of thermoelectric (TE) materials that can convert waste heat into electricity is becoming increasingly important. Accordingly, the dimensionless figure of merit, ZT needs to be increased. Al, a group-III element, has not been widely used as a dopant in GeTe-based thermoelectric materials because it acts as a p-type dopant and thus further increases the hole carrier concentration of GeTe, which has already a high intrinsic hole concentration due to inherent Ge vacancies. Therefore, Al as a dopant in GeTe has been considered to have detrimental effects on the resulting thermoelectric performance. In this study, we aim to enhance the suitability of Al doping in GeTe-based materials through synergistically co-doping it with Bi. In this case, co-doping Al and Bi optimizes the carrier concentration and induces a high Seebeck coefficient from an additional increased effective mass and low total thermal conductivity arising from the numerous phonons scattering centers and decreased electronic contribution to heat transport. Consequently, a maximum ZT of ∼1.95 at 673 K with an increased average ZT is achieved. These results indicate that the p-type dopant Al, co-doped with Bi in GeTe-based thermoelectric materials is one of the very effective dopants which can eventually improve the overall TE performance of these materials for useful waste heat recovery application and cooling.

Exergy and energy analysis of Organic Rankine Cycle integration in the carbon black industry using pinch technology

Industrial Carbon black production is a petrochemical process that has high level of waste heat and emissions. Main points of CB process include furnace and stack lines. Also, the use of different feedstocks can be effective in level of waste heat and amount of emission pollutants. So the main objective of this research use from the pinch technology in CB process in order to minimize the level of waste heat by utilization of waste heat recovery (WHR) applications for power generation. By numerical investigation and modelling of WHR in furnace and stack points, can be defined best location for power generation in CB process. Moreover, through the analysis of Organic Rankine Cycle (ORC) systems integrated with WHR from both energy and exergy perspectives, second target to enhance their thermal efficiency. The results indicate that the presence of blowers on stacks leads to an increase in RoHR and RoHC. In order to determine the optimal location for power generation cycle installation, an ORC energy and exergy analysis was conducted. The results showed that the energy and exergy values at the SLC stack were higher than those at the HLC point. Also, numerical results showed the points that have suitable thermal efficiency and rate of exergy of WHR where have higher temperature gradient than other points. So SLC (ĖxWHR=166.7kW) and HLC (ĖxWHR=136.1kW) stacks integrated with blower are the optimal location for WHR utilization in the CB production process.

Data-driven identification and fast model predictive control of the ORC waste heat recovery system by using Koopman operator

Organic Rankine Cycle (ORC) has received its wide application in low-grade waste heat recovery (WHR) technology for its significant performance and easy access to its components. Given the highly fluctuating nature of the waste heat source, Model Predictive Control (MPC) is usually utilized to realize the reasonable adjustment of ORC based WHR systems and has performed satisfactorily. In order to apply MPC, a relatively precise model should be built up for the ORC system which ensures acceptable control performance. However, popular modeling methods based on the mechanism of the ORC establish the high-dimensional nonlinear model and suffer from computational costs when utilizing complicated nonlinear MPC. Even if linear MPC is employed for the purpose of reducing the calculation amount, the model obtained by linearization near the operating points usually makes it valid locally, thus bringing about suboptimal or unstable control performance. To address this problem, the Koopman operator is introduced for the data-driven identification and MPC of the ORC system. Koopman identification constructs a linear model in the lifted space in which the ORC system possesses nearly global linear evolution and enables the prediction of the nonlinear ORC dynamics from measurement data. In view of the possible online calculation burden increment caused by the rise of variable dimension through lifting action, the fast Koopman MPC (FKMPC) algorithm is thus proposed based on the invariance of the Hessian matrix of the optimization problem to shorten the computing time. Simulations on setpoint tracking and disturbance rejection are performed to verify the established model accuracy and the control effectiveness of the proposed strategy in comparison with other approaches.

High-performance tellurium-free thermoelectric module for moderate temperatures using α-MgAgSb/Mg2(Si,Sn)

α-MgAgSb is a promising thermoelectric material with an average figure of merit above unity between room temperature and 573 K, making it attractive for low temperature waste heat recovery or temperature regulation applications. We demonstrate here the first α-MgAgSb/Mg2(Si,Sn)-based thermoelectric module, where Mg2(Si,Sn) was chosen as n-type counterpart due to its excellent thermoelectric properties, low weight, abundant, and eco-friendly constituents. By using this new material combination, a maximum conversion efficiency of 6.4% and maximum power of 0.68 W were achieved for a temperature difference of 275 K, superior to commercially available (Bi,Te,Sb)-based thermoelectric modules. The Constant Property Model was used to check the validity of the measurement results and to identify room for further improvement. The characterization of the prototype shows a relatively good match between measurement and calculation for most of the thermoelectric module performance parameters; however, the measured internal electrical resistance is around 10% higher than the predicted values, indicating opportunities for further enhancement.

A smart and sustainable energy approach by performing multi-objective optimization in a minichannel heat sink for waste heat recovery applications

Minichannel heat sink is widely used in waste heat recovery systems for their compactness and ability to recover heat effectively from high heat flux applications. However, the heat recovery efficiency is constrained by the flow configurations resulting in flow maldistribution. Numerous neural network combined evolutionary algorithms have been used to reduce pressure drop and flow maldistribution factors in the literature. But it is very challenging to assign appropriate weights to these parameters with no physical significance between them for optimization studies. To overcome this, TOPSIS-based optimization studies have been used in the current work to reduce the flow maldistribution factor (ϕ) and increase the Nusselt number (Nu) with ribs and inclined structures. Four Minichannel designs are studied to assess the channel heat recovery efficiency from small-scale incinerators using water and Graphene oxide (GO) nanofluid for three different volume fractions of GO-0.02%, GO-0.07%, and GO-0.12%. The motive is to determine an optimal nanofluid volume fraction and a suitable Minichannel configuration for the given heat flux. The TOPSIS method handles five criteria, including the combination of weightage for the maldistribution factor and Nusselt number. For criteria I ((ϕ)min: (Nu)max = 0.0:1.0) maximum weightage is given to heat transfer, the ribbed channel has gained a higher performance score for GO-0.07% nanofluid volume fraction. For criteria V ((ϕ)min: (Nu)max = 1.0:0.0) maximum weightage is given to maldistribution reduction, the ribbed inclined channel has gained with significantly higher performance score for all the studied nanofluid volume fractions. Further, the study is extended to determine the heat recovery efficiency, and it is found that with the increase in mass flow rate and nanofluid volume fraction, the heat recovery efficiency increases significantly. In particular, the maximum heat recovery efficiency of 66% was obtained for ribbed Minichannel using GO-0.12% nanofluid.

Progress and perspectives in thermoelectric generators for waste-heat recovery and space applications

The direct conversion of thermal energy into electrical current via thermoelectric (TE) effects relies on the successful integration of efficient TE materials into thermoelectric generators (TEGs) with optimized characteristics to ensure either optimum output power density or conversion efficiency. Successfully employed for powering deep-space probes and extraterrestrial rovers since the 1960s, the development of this technology for waste-heat-harvesting applications faces several key issues related to the high temperatures and oxidizing conditions these devices are subjected to. This Perspective provides a brief overview of some prospective thermoelectric materials/technologies for use in radioisotope thermoelectric generators utilized in space missions and highlights the progress made in the field over the last years in the fabrication of TEGs. In particular, we emphasize recent developments that enable to achieve increased power densities, thereby opening up novel research directions for mid-range-temperature applications. In addition to showing how using lower quantities of TE materials may be achieved without sacrificing device performance, we provide an outlook of the challenges and open questions that remain to be addressed to make this technology economically and technologically viable in everyday-life environments.

Exergoeconomic evaluation of an organic Rankine cycle integrated with a compression refrigeration to select the best working fluid for waste heat recovery application: a case study of Isfahan, Iran

Exergoeconomic evaluation of an organic Rankine cycle integrated with a compression refrigeration to select the best working fluid for waste heat recovery application: a case study of Isfahan, Iran

Dynamic performance analysis of transcritical power cycle with preheater and regenerator using CO2-based binary zeotropic mixtures

The CO2 transcritical Rankine cycle is considered as an ideal solution in the application of waste heat recovery due to its safety, environmental friendliness and compactness characteristics. In this work, for the heat recovery from internal combustion engine, a dynamic model of an improved CO2 transcritical Rankine cycle system containing both a preheater and regenerator with CO2-based binary zeotropic mixtures as working fluid is developed. The performance of CO2 transcritical power cycle with preheater and regenerator was compared and analyzed for mixture working fluids with different proportions under the disturbance of heat source conditions and system input parameters, which can provide references for system control strategy to ensure efficient and stable operating conditions when selecting control variables. The results show that there is a certain pump speed to obtain the optimal value of the net output work and thermal efficiency of the system. And the net output power and thermal efficiency of system with CO2-based binary zeotropic mixtures at optimum point are higher 6. 34 kW and 2.19% respectively than pure CO2 as working fluid by adjusting pump speed.

Investigations of microstructures and thermoelectric properties of TiNiSn half-Heusler compounds with micro- and nano-scale copper additions

The TiNiSn-based half-Heusler compounds have been considered as a promising thermoelectric material system for waste heat recovery applications. There have been numerous studies dedicated to improving its intrinsic low thermoelectric performance, and one strategy is to incorporate certain types of metal dopants into the host materials. Only a few research works have been focused on the effects of the dimensions of metal particles on the thermoelectric properties of materials. In this study, we investigate the roles of micro- (M) and nano-scale (N) Cu powders in affecting the microstructures and thermoelectric properties of TiNiCuxSn (x = 0−0.1) compounds. Owing to the ease of oxidation of Cu nanoparticles and the tedious preparation procedure, TiNiCux, NSn shows lower carrier concentration and electrical conductivity compared to the TiNiCux, MSn with the same nominal Cu content. The high-temperature Seebeck coefficient of TiNiCux, NSn gradually deteriorates as the temperature rises, partly due to the impurities-induced bandgap narrowing effect. The maximum power factor achieved for TiNiCux, NSn is about 20.2% lower than that for TiNiCux, MSn. Even though the reduced contribution of electronic component leads to a lower total thermal conductivity at low temperatures, the highest zT obtained in TiNiCux, NSn is markedly lower than that in the corresponding TiNiCux, MSn. These findings highlight the importance of investigating the dimensions of metal dopants in studies of the thermoelectric properties of materials.

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.

Enhancing sustainable energy conversion: Comparative study of superheated and recuperative ORC systems for waste heat recovery and geothermal applications, with focus on 4E performance

This study investigates the 4E performance of small-scale (<100 kW), low-temperature (<179 °C) Organic Rankine Cycle (ORC) technology, comparing two distinct configurations using different working fluids and heat sources. The work utilizes experimental methods to analyse the efficiency of the ORC systems, focusing on variables such as evaporation temperature and pressure drop across the heat exchanger. Various working fluids and heat sources are tested to evaluate their impact on performance using CoolProp libraries.The results reveal that the ideal evaporation temperature for maximizing overall performance and efficiency is between 140 and 150 °C. Higher pressure drop across the heat exchanger improves heat transfer, thermal efficiency, and power output, but increases pumping requirements and reduces overall effectiveness. Among the tested heat sources, the Dynalene SF exhibits the best performance, followed by therminol T72, T66, geothermal water, and Paratherm HE. The most suitable working fluids for commercial ORC units are R245fa and hexamethyldisiloxane (MM), providing excellent thermodynamic properties and low environmental impact. Comparatively, the superheated ORC system using R245fa outperforms the MM, achieving higher net efficiency (7.29%), greater network generation (20.29%), and improved exergy efficiency (20.54%). The economic viability of the recuperative system is evident from the Levelized Cost of Electricity (LCOE) range of 12.4–14.5 (cents/kWh), with an average Total Installed Cost (TIC) of 354.5 k$. These results showcase the system's competitiveness in providing cost-effective electricity generation, reaffirming its potential as a financially viable option for sustainable energy solutions.The objective is to explore the use of Organic Rankine Cycle (ORC) technology as an alternative to steam Rankine cycle for low-power and lower-temperature applications. Specifically, the study focuses on ORC systems with external heat sources, including geothermal, solid fuels and waste heat recovery (WHR).The novelty of this article lies in its updated analysis of the techno-economic-environmental feasibility of ORC systems for different applications, which goes beyond previous studies. It provides a systematic evaluation and comparison specifically tailored to small-scale and low-temperature operating conditions. The aim is to promote the integration of these two systems in various sectors, particularly for countries undergoing an energy transition. This emphasis on the technical aspects and the specific focus on small-scale and low-temperature conditions make this study unique and valuable in advancing the understanding and application of ORC technology.

Characterization by differential scanning calorimetry of thermal storage properties of organic PCMs with operating temperature of 150 °C

The involvement of Phase Change Materials (PCMs) in energy storage systems leads to more sustainable and efficient use of energy. Medium operating temperature (melting point 100–400 °C) find various applications in industrial waste heat recovery and hybrid compressed air - thermal energy storage systems. However, there is still a lack of detailed knowledge in terms of their thermal storage properties that hinders them from being more integrated in industry. In this study, thermal properties such as melting point, enthalpy and heat capacity of three medium temperature PCMs were characterized using Differential Scanning Calorimetry (DSC). The enthalpy-temperature graph for each PCM is presented that contains key information on sensible and latent heat contribution in the total energy storage. The selected organic PCMs are adipic acid with two different purities 99%, 99.5%, and its chemical equivalent PureTemp151® with a melting point of (150.5 ± 0.5 °C) and a latent heat of (230 ± 10 J/g). Furthermore, the effect of heating rate and sample mass in DSC procedures on transition temperatures and enthalpy were evaluated. Finally, thermal performance of PCMs was quantified through 150 cycles of melting and solidification. The tested PCMs show a significant storage capacity and good thermal stability with less than 10 % enthalpy loss.

Comparative assessment of the various split flow supercritical CO2 Brayton cycles for Marine gas turbine waste heat recovery

Supercritical CO2 Brayton cycle (sCO2 BC) can become easily utilized in marine gas turbine waste heat recovery applications due to their high efficiency, compact size, and low-cost advantages. In this study, the performance of the three different split flow sCO2 BCs, including turbine split flow-1 (TSF-1), turbine split flow-2 (TSF-2), and turbine split-3 (TSF-3), for the recovery of marine gas turbine waste heat is compared. The Engineering Equation Solver (EES) application is used to compare the three different split flow sCO2 BCs' performances. Moreover, to investigate the influence of important thermodynamic parameters on cycle performance, a parametric analysis is carried out. The effect of variable exhaust gas temperature, turbine input pressure, and compressor inlet pressure on net power, the energy efficiency of the system, system's exergy efficiency, and exergy destruction are examined. The results suggest that the energy efficiencies of the TSF-1 sCO2 BC, the TSF-2 sCO2 BC, and the TSF-3 sCO2 BC are calculated by 28.71%, 34.5%, and 29.42%, respectively. The TSF-2 sCO2 BC has more advantages in efficiency among all the cycle layouts while the TSF-3 sCO2 BC layout has better performance in the net power. In addition, the TSF-3 sCO2 BC has the highest exergy destruction at 99.71 kW, followed by the TSF-1 sCO2 BC at 91.83 kW and the TSF-2 sCO2 BC at 41.75 kW. It has been determined that the cycle's net power increases with rising exhaust gas temperature and turbine input pressure and decreases with compressor input pressure. Exhaust gas temperature and turbine inlet pressure have a positive effect on the performance of all split flow sCO2 BCs.

Dynamic behaviour of automobile thermoelectric waste heat recovery under different driving cycles

Different types of vehicles often run in different driving cycles, resulting in different dynamic behaviours of the automobile thermoelectric generator. The dynamic performance of a simplified automobile thermoelectric generator under different driving cycles, including HWFET, NEDC, WLTP, and CLTC, is evaluated to guide the application of automobile thermoelectric waste heat recovery by a hybrid transient CFD-TE model. The model is experimentally validated with an average output error of 10.92%, mainly caused by the instrument error. The findings indicate that the response of the dynamic output power of the automobile thermoelectric generator exhibits smoother fluctuations compared to the exhaust temperature, and also displays hysteresis in its response, while the dynamic conversion efficiency experiences sharp fluctuations. The behaviour of the automobile thermoelectric generator in dynamic situations is not entirely proportional to the vehicle speed but is instead dependent on changes in vehicle speed, more specifically, the change of exhaust heat. By comparing the steady and dynamic results, it is found that the steady-state model overestimates the output power but may underestimate the conversion efficiency. The automobile thermoelectric generator is more suitable for vehicles with frequent speed changes, such as passenger cars on urban and suburban roads, to enhance the dynamic output performance.

The Investigation of Two-Phase Expansion Performance with Indicator Diagram in a Twin-Screw Expander

Volumetric expanders are proven to be more suitable for small-scale waste heat recovery applications because of their simplicity, reliability, lower rotational speed and lower cost. Unlike turbines, volumetric expanders can work in the two-phase fluid state, which broadens their application fields. To investigate the two-phase performance of volumetric expanders, a specific twin-screw expander was chosen and modeled. The leakage loss and the suction pressure loss were primary concerns in this research. The two-phase expansion process in the expander is presented in detail using the developed mathematical model with an indicator diagram. The influence of several factors, including inlet vapor quality, rotational speed and intake pressure, are investigated. The influence mechanism of the vapor phase and the liquid phase on expander performance is clarified. In brief, this paper presents an illustrative understanding of the two-phase expansion process in twin-screw expanders.

Mathematical model establishment and optimum design of a novel cesium thermionic–thermoelectric hybrid generator

Static power sources, such as thermionic energy converters (TEC), thermoelectric generators (TEG), thermophotovoltaic converters (TPV), and alkali metal thermal to electric converters (AMTEC), have enormous potential application in aerospace, military, electronic devices, and low-grade waste heat recovery and utilization. Aiming to further improve the power output and conversion efficiency, any two of them can be cascaded according to the difference in operating temperature. In this study, a novel mathematical model of the static hybrid power generator composed of a cesium thermionic energy converter (CTEC) and a TEG was proposed. The model considered the irreversible heat loss and the additional energy barrier of the CTEC module, and the Thomson effect of the TEG module, synchronously. To obtain the temperature distribution of CTEC and TEG modules, energy balance equations were determined by nonequilibrium thermodynamic analysis. Then, the effects of the output voltage and anode work function of the CTEC, the heat source temperature, and the electric current of the TEG on the output performance were investigated systematically by means of MATLAB. The results indicate that the maximum efficiency of the hybrid generator reaches 26.82% at heat source and heat sink temperatures of 1800 K and 300 K, respectively, which is significantly higher than that of the individual CTEC module (21.28%). This reveals that the waste heat released by the CTEC is utilized effectively in the TEG module, and the increased heat source temperature will further improve the hybrid generator’s power output and efficiency. The output power and efficiency are obviously improved by reducing the anode work function. When ϕC = 1.0 eV, i = 0.005, the power reaches a maximum of 21.61 W at 0.37 V, and the efficiency reaches a maximum of 25.90% at 0.55 V. Moreover, the curve of the output power as a function of conversion efficiency could be used to determine the optimal operating region of the hybrid generator, i,e., Pη < P < Pmax, ηP < η < ηmax, thereby obtaining the thresholds of the hybrid system parameters, i.e., VP < V < Vη, T1,P < T1 < T1,η, T2,P > T2 > T2,η, T3,P > T3 > T3,η and TCs,P > TCs > TCs,η. This study provides a reference for the optimal design and application of a CTEC–TEG hybrid generator in the future.