Waste Heat Recovery Applications Research Articles

Roadmap for Power Generation of Industrial Hot Extruded Bi2Te3-Based Thermoelectric Device in Real-World Heat Sources.

Recovering waste heat through thermoelectric (TE) technology is critical for enhancing energy efficiency. However, commercial devices often require n- and p-type thermoelectric units to have similar cross-sectional areas and conductivities, complicating optimization of their figure of merit (ZT). Moreover, real-world heat sources are not constant temperature sources, and only heat flux can be measured. Given these constraints, selecting appropriate parameters for TE devices to maximize their output power for specific heat sources is of significant scientific and practical importance. In this work, we studied a series of n- and p-type Bi2Te3-based TE materials, prepared using commercial hot extrusion techniques, with average conductivities of approximately 650 S cm-1, 820 S cm-1, 1050 S cm-1, and 1300 S cm-1 over the temperature range of 50-250 °C. These materials were assembled into four different thermoelectric devices (HE1, HE2, HE3, and HE4). Finite element simulations for devices HE1, HE2, HE3, and HE4 (TE leg: 1.4 mm × 1.4 mm × 1.6 mm, device area: 40 mm × 40 mm) showed maximum power densities of 3953.5 W m-2 and 1300.8 W m-2 at 67,000 W m-2 and 33,500 W m-2 heat fluxes, respectively. The fabricated devices achieved maximum power densities of 2907.1, 3014.0, 2910.9, and 2525.4 W m-2 at 67,000 W m-2 heat flux, with discrepancies attributed to interface thermal resistance. Based on these findings, we have plotted output performance roadmaps for TE devices with different average electrical conductivities under various heat flux densities (67,000 W m-2 and 33,500 W m-2) as functions of external load and TE leg height. These roadmaps enable rapid design of TE devices tailored to specific external heat source conditions and cost requirements, thereby maximizing waste heat recovery. This study's use of electrical conductivity as a performance indicator for final device output power density offers direct guidance for practical waste heat recovery applications.

Research on the Basic Properties and Preparation Methods of Heat Conversion Materials

The purpose of this literature review is to discuss the research progress and application prospects of high-efficiency thermoelectric conversion materials. In recent years, high-efficiency thermoelectric materials have garnered extensive attention due to their significant applications in waste heat recovery and energy conversion. This paper reviews various types of thermoelectric materials, including metal chalcogenides, silicides, and skutterudite materials, and analyzes their synthesis methods, properties, and advantages and disadvantages. The findings indicate that chemical vapor deposition (CVD), hydrothermal synthesis, and hot pressing are the primary methods for preparing thermoelectric materials, each with distinct advantages and limitations. Nevertheless, by optimizing the preparation processes and material compositions, high-efficiency thermoelectric materials are poised to play a crucial role in the field of sustainable energy in the future. This review summarizes the key findings of current research and suggests potential directions and challenges for future studies.

Interface kinetic manipulation enabling efficient and reliable Mg3Sb2 thermoelectrics

Development of efficient and reliable thermoelectric generators is vital for the sustainable utilization of energy, yet interfacial losses and failures between the thermoelectric materials and the electrodes pose a significant obstacle. Existing approaches typically rely on thermodynamic equilibrium to obtain effective interfacial barrier layers, which underestimates the critical factors of interfacial reaction and diffusion kinetics. Here, we develop a desirable barrier layer by leveraging the distinct chemical reaction activities and diffusion behaviors during sintering and operation. Titanium foil is identified as a suitable barrier layer for Mg3Sb2-based thermoelectric materials due to the creation of a highly reactive ternary MgTiSb metastable phase during sintering, which then transforms to stable binary Ti-Sb alloys during operation. Additionally, titanium foil is advantageous due to its dense structure, affordability, and ease of manufacturing. The interfacial contact resistivity reaches below 5 μΩ·cm2, resulting in a Mg3Sb2-based module efficiency of up to 11% at a temperature difference of 440 K, which exceeds that of most state-of-the-art medium-temperature thermoelectric modules. Furthermore, the robust Ti foil/Mg3(Sb,Bi)2 joints endow Mg3Sb2-based single-legs as well as modules with negligible degradation over long-term thermal cycles, thereby paving the way for efficient and sustainable waste heat recovery applications.

Pioneering computational insights into the structural, magnetic, and thermoelectric properties of A3XN (A=Co, Fe; X=Cu, Zn) anti-perovskites for advanced material applications

This study pioneers the utilization of computer-controlled simulations to elucidate the structural, elastic, electronic, and thermoelectric properties of A3XN anti-perovskites (A = Co, Fe; X = Cu, Zn). Starting with the development of a detailed structural model, structural optimization calculations identify the stable Pm-3m cubic phase, aligning with experimental findings. Also, these optimizations indicate the ferromagnetic phase stability of A3XN anti-perovskites, further supported by positive Curie-Weiss constants of 60 K, 80 K, and 100 K for Co₃CuN, Co₃ZnN, and Fe₃CuN, respectively. The spin-dependent electronic properties, including band structure and density of states, are explored using multiple exchange-correlation (XC) functionals—GGA, GGA + U, and GGA + mBJ. The results uphold the metallic nature of the materials, accompanied by significant spin magnetic moments. Specifically, the calculated values of magnetic moments for Co3CuN, Co3ZnN, and Fe3CuN are 5.08 μB, 3.70 μB, and 7.61 μB, respectively. Furthermore, we compute the Curie temperatures for each compound, 942.48 K for Co3CuN, 692.7 K for Co3ZnN, and 1400.41 K for Fe3CuN, which are substantially elevated, indicating robust ferromagnetic behavior that persists well above ambient conditions. Mechanical properties, including stability, deformation behavior, and ductility, are validated through elastic calculations, with various mechanical anisotropy factors examined. From an application perspective, the thermoelectric characteristics of Co3CuN, Co3ZnN, and Fe3CuN are meticulously evaluated, focusing on parameters like the Seebeck coefficient, electrical conductivity, thermal conductivity, power factor and figure of merit (zT). The analysis highlights promising power factor values of 3.0×1011(W/cm.K2), 4.0×1011(W/cm.K2), and 6.0×1011(W/cm.K2) for Co3CuN, Co3ZnN, and Fe3CuN, respectively, suggesting potential for enhanced efficiency in thermopower generation, particularly in waste heat recovery applications. Furthermore, the study provides a comprehensive analysis of the ground-state optical properties of A3XN, including the dielectric constant, photoconductivity, reflectivity, refractive index, absorption coefficient, and extinction coefficient. The reflectivity spectra demonstrate high reflectivity of 45 % across visible and ultravioletregions, suggesting suitability for applications in coatings designed to mitigate solar heating effects.

Operation characteristics and limitations of small-diameter two-phase closed thermosyphon

Two-phase closed thermosyphon (TPCT) is a latent heat-based, high-efficiency heat transfer device primarily utilized in heat pipe heat exchangers in waste heat recovery (WHR) applications. Recently, there has been increasing interest in small-diameter TPCT due to thermal confinement issues and space and cost limitations of heat transfer devices. The nature of the two-phase flow and the resulting thermal performance of a small-diameter thermosyphon differ from that of a large-diameter thermosyphon, which is called the “confinement effect” of TPCT. In the present study, we experimentally explored the internal flow and heat transfer characteristics of TPCT with a very small inner diameter of 5 mm. Based on the Laplace length, which means the characteristic bubble size, four working fluids were considered: water, acetone, ethanol, and the HFE-7000. As a result, except for a specific condition where the degree of confinement and heat flux were very low, the shear force between the falling liquid condensate and upward vapor prevented the smooth circulation of working fluid inside the small-diameter channel. This caused local dry-out of the evaporator, resulting in the effective thermal conductivity of the TPCT being lower than that of the copper block.

Study of the physical properties of the half-heusler XBrH with (X= sr, Ca and mg) compounds

Half-Heusler alloys are among the most promising thermoelectric materials for medium- and high-temperature waste heat recovery applications. This study investigates the physical properties of Half-Heusler compounds XBrH, with X = Sr, Ca, and Mg. We explored the structural, electronic, optical, and thermoelectric properties of these compounds using density functional theory (DFT) as implemented in the Wien2k package. The Generalized Gradient Approximation (GGA) as proposed by Perdew-Burke-Ernzerhof (PBE) and the modified Becke-Johnson (mBJ) functional were employed to calculate the exchange-correlation interactions. After doing structural optimization for a range of parameters, we discovered that the SrBrH compound is more stable than the CaBrH and MgBrH compounds. Our results show that the density of state calculations indicate semiconductor behavior for the XBrH with (X = Sr, Ca, and Mg) compounds. We also evaluated the real and imaginary parts of the dielectric tensor, the refractive index, the absorption coefficient, the electron energy loss, and the real and imaginary components of the optical conductivity, among other optical parameters. Additionally, we investigated the figure of merit (ZT), electronic and lattice thermal conductivity, Seebeck coefficient, and electrical conductivity. The MgBrH, CaBrH and SrBrH compounds showed p-type behavior, with positive values for the Seebeck coefficient and the greatest value of ≈180 μV/K, according to the thermoelectric data. At 1000 K, the ZT reach their highest values for CaBrH, SrBrH, and MgBrH, respectively, at 2.9, 2.8, and 0.9. The physical properties of the XBrH with (X = Sr, Ca, and Mg) compounds have been studied in detail.

Thermoelectric Generator Applications in Buildings: A Review

With growing concerns about building energy consumption, thermoelectric generators (TEGs) have attracted significant attention for their potential to generate clean, green, and sustainable power. This comprehensive review explores the applications of thermoelectric generators (TEGs) in building systems, focusing on recent advancements from 2013 to 2024. The study examines TEG integration in building envelopes, including façades, walls, windows, and roofs, as well as non-integrated applications for waste heat recovery and HVAC systems. Key findings highlight the potential of TEGs in energy harvesting and thermal management, with façade-integrated systems generating up to 100.0 mW/m² and hybrid LCPV/T-TEG systems achieving overall efficiencies of 57.03%. The review also identifies critical parameters affecting TEG performance, such as solar intensity, thermoelectric arm length, and PCM melting temperature. Despite promising results, challenges remain in improving overall system efficiency, cost-effectiveness, and scalability. Future research directions include developing more efficient thermoelectric materials, optimizing system designs for various climatic conditions, and exploring integration with smart building management systems. This review provides valuable insights for researchers and practitioners working towards more energy-efficient and sustainable building designs using TEG technology.

Part-load analysis and preliminary annual simulation of a constant inventory supercritical CO2 power plant for waste heat recovery in cement industry

The present work investigates the part-load performance of a MW-scale sCO2 power plant designed as waste heat recovery unit for an existing cement plant located in Czech Republic, in the framework of the H2020 funded project CO2OLHEAT. The study first presents the selected power plant configuration and then focuses on the evaluation of its part-load operation due to variation of flue gas mass flow rate and temperature. The range of flue gas conditions at the outlet of the upstream process is retrieved from a preliminary statistical analysis of historical trends obtained through the cement plant monitoring. The numerical model developed for this study aims at providing realistic results thanks to the adoption of turbomachinery performance maps provided by the turbomachinery manufacturer of the project. Moreover, heat exchangers have been modelled through a discretized approach which has been validated against manufacturer data, while piping inventory and pressure losses have been assessed through a preliminary sizing that considers the actual distances to be covered in the cement plant. Performance decay is estimated for the whole range of flue gas conditions, reporting the most significant power cycle parameters, and identifying the main causes of efficiency loss. The part-load analysis is carried out considering a constant CO2 inventory, in order to reduce the system complexity and capital cost and simplify plant operation. Results show that the operation entails minor variation of the compressors operative points in the whole range of operating conditions of the cement plant, avoiding the risk of anti-surge bypass activation. Moreover, the plant is able to work close to the nominal thermodynamic cycle efficiency (20.5 %–23.0 %) for most of the year and benefits from part-load operation in terms of overall performance. In the last part of the work, a preliminary techno-economic analysis of the plant is also presented to highlight the potential advantages of sCO2 technology for waste heat recovery applications. The results of the part-load performance of the plant are combined with the flue gases data obtained from the preliminary statistical analysis and the cement plant historical monitoring. An annual electricity production equal to 13′909.7 MWh is obtained, corresponding to 6560 equivalent hours and a system capacity factor of 74.9 %. The investment cost of each CO2OLHEAT plant component is estimated by means of cost correlations obtained from literature and the non-discounted payback time is computed as a function of the electricity selling price. The results show that, even considering electricity prices before 2022, the payback time of the CO2OLHEAT plant is estimated to be lower than 8 years, justifying the industrial interest in the proposed technology.

(Digital Presentation) Synergistic Enhancement of Thermoelectric Performance through One-Dimensional Hybrid Nanocomposites: Wrapped Nickel Oxide-Decorated Multi-Walled Carbon Nanotubes with Polypyrrole

Given the drawbacks of organic materials’ thermoelectric (TE) conversion efficiency, an approach has been made in this study to synthesize a hybrid organic-inorganic material based on polypyrrole (PPy), multi-walled carbon nanotubes (MWCNTs) and nickel oxide nanoparticles (NiO). The nanocomposite is formed through three steps: MWCNTs functionalization via diazonium salt grafting of 5-amino-1,2,3-benzene tricarboxylic acid; in situ generation on their surfaces of NiO nanoparticles with a homogenous distribution; the chemical polymerization of pyrrole using methyl orange as templating and dopant to wrap the MWCNTs-(COOH)3-NiO. To explore the structural and physical properties of the fillers and the nanocomposites, X-ray diffraction (XRD), Transmission electron microscopy (TEM), Raman spectrometer, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used. Thereby investigating the electrical conductivity (𝜎), Seebeck coefficient (S) and thermal conductivity (κ) at room temperature, a promising improvement was observed in PPy-MWCNTs-(COOH)3-NiO compared to pure PPy. The observed enhancements in TE properties (ZTPPy-MWCNTs-(COOH)3-NiO = 1.51×10-2) are attributed to the presence of NiO, which acts as a dopant and improves the charge carrier density in the nanocomposite; this result could be increased at higher temperatures. Overall, these results demonstrate the potential of our approach for developing high-performance TE materials with applications in energy harvesting and waste heat recovery. Furthermore, this approach offers the potential for scalability and reproducibility, making it a promising method for large-scale fabrication of high-performance composites. Figure 1

An Organic–Inorganic Superlattice with Nanocrystal‐Amorphous Composite Nanolayers for Ultrahigh Thermoelectric Performance

Thermoelectric materials play a crucial role in converting heat into electricity, offering significant potential for applications in waste heat recovery and cooling. Herein, an innovative approach that combines an organic–inorganic hybrid superlattice structure with nanocrystal‐amorphous composite nanolayers is introduced. The nanocrystal‐amorphous composite enhances the Seebeck coefficient resulting in a notable twofold improvement in the power factor. The superlattice, alternating self‐assembled organic monolayers and inorganic nanolayers, effectively reduces lattice thermal conductivity by creating multiple interfaces that scatter phonons effectively. The integration of the nanocrystal‐amorphous composite nanolayers into the superlattice provides a dual advantage, simultaneously boosting the power factor and suppressing thermal conductivity. This synergistic effect leads to exceptional thermoelectric performance in the 4‐mercaptophenol/Sb2Te3 superlattice, with achieved figure of merit (ZT) values of 3.48 at 300 K and reaching a peak ZT value exceeding 4.0 at 400 K while surpassing 2.5 over the temperature range from 300 to 500 K. These results suggest that this innovative approach paves the way for the development of highly efficient thermoelectric materials, propelling efforts toward more energy‐efficient and environmentally friendly solutions.

Experimental investigation of a 10 kW-class flat-type loop heat pipe for waste heat recovery

A flat-type 10 kW-class loop heat pipe (LHP) with a box-type wick was designed and developed to handle the higher heat loads in waste heat recovery applications, such as industrial processing and internal combustion engines. Additionally, a numerical model was developed to predict the overall thermal performance of the LHP. The LHP used a stainless steel wick with a pore diameter of 2.0 µm and pure water as the working fluid. The LHP was tested using two types of cooling for the condenser (forced and natural convection), two types of evaporator orientations (horizontal and vertical), and with and without gravity assist (0.3 m and 0 m). For all the tests, the maximum heat load ranged from 5 kW to 10 kW, with the test with a gravity assist of 0.3 m, vertical evaporator orientation, and natural convection performing the best. This test sustained a 10 kW heat load at a steady-state temperature of 182 °C for the evaporator. During the same test, a maximum evaporative heat transfer coefficient of 92,000 W/m2/K at 4.5 kW and a thermal resistance between the evaporator and condenser value of less than 0.007 K/W was achieved. A numerical model was developed to compare the experimental results with the numerical temperature results for the heater block, evaporator, vapor line, condenser, liquid line, and compensation chamber. Overall, the average temperature difference for all components ranged from 7.1 °C to 10.6 °C, with the horizontal orientation without gravity assist and natural convection test predicting the best. The findings demonstrate that LHPs can handle the higher heat loads that are found in waste heat recovery applications for industrial processing and internal combustion engines.

Thermal, Energy and Exergy Analysis of Thermoelectric Generator System for Waste Heat Recovery Applications

Thermal, Energy and Exergy Analysis of Thermoelectric Generator System for Waste Heat Recovery Applications

High wide-temperature-range thermoelectric performance in GeTe through hetero-nanostructuring

GeTe is emerging as promising medium-temperature thermoelectric material due to its highly competitive performance and good mechanical properties. Strong Rashba spin splitting was harnessed to markedly improve the Seebeck coefficient and power factor of Bi and Sn codoped GeTe at low-medium temperature. Moreover, it is found that Bi-Sn-Cu doping reduces the phase-transition temperature to extend better electrical transport behavior of cubic phase to low temperature. As a result, the electrical transport properties in low-medium temperature were overall enhanced. In the meanwhile, endotaxial hetero-nanostructures efficiently scatter phonons and play a dominant role on affecting phonon propagation. The lattice thermal conductivity was reduced to 0.2 W m−1K−1 at 673 K. Drive by strengthening Rashba effect and endotaxial hetero-nanostructures, a record-high average ZT (300–823 K) of 1.6 and a high ZT of 2.1 were obtained in lead-free GeTe-based compounds. The vast increase of ZT promotes GeTe as a promising candidate for a wide range of applications in waste heat recovery and power generation.

Review of Recent Applications of Heat Pipe Heat Exchanger Use for Waste Heat Recovery

With the reduction in fossil fuels and growing concerns about global warming, energy has become one of the most important issues facing humanity. It is crucial to improve energy utilization efficiency and promote a low-carbon transition. In comparison with traditional heat exchangers, heat pipe heat exchangers indicate high compactness, a flexible arrangement, complete separation of hot and cold fluids, good isothermal operations, etc. As a result, heat pipe heat exchangers have attracted wide attention and application in various fields in recent years. This paper provides an overview of the application of heat pipe heat exchangers, with a focus on the application in waste heat recovery, and analyzes the opportunities and challenges of heat pipe heat exchanger applications based on existing publications.

Machine learning-based multi-objective optimization and thermal assessment of supercritical CO2 Rankine cycles for gas turbine waste heat recovery

Technologies for utilizing waste heat for power generation have attracted significant attention in recent years due to their potential to enhance energy efficiency and reduce greenhouse gas emissions. This research focuses on the comparative and optimization analysis of three supercritical carbon dioxide (sCO2) Rankine cycles (simple, cascade, and split) for gas turbine waste heat recuperation. The study begins with parametric analysis, investigating the significant effects of key variables, including turbine inlet temperature, condenser inlet temperature, and pinch point temperature, on the thermal performance of advanced sCO2 power cycles. To identify the most efficient cycle configuration, a multi-objective optimization approach is employed. This approach combines a Genetic Algorithm with machine learning regression models (Random Forest, XGBoost, Artificial Neural Network, Ridge Regression, and K-Nearest Neighbors) to predict cycle performance using a dataset extracted from cycle simulations. The decision-making process for determining the optimal cycle configuration is facilitated by the TOPSIS (technique for order of preference by similarity to the ideal solution) method. The study's major findings reveal that the split cycle outperforms the simple and cascade configurations in terms of power generation across various operating conditions. The optimized split cycle not only demonstrates superior power output but also exhibits enhanced net power output, heat recovery, system and exergy efficiency of 7.99 MW, 76.17 %, 26.86 % and 57.96 %, respectively, making it a promising choice for waste heat recovery applications. This research has the potential to contribute to the advancement and widespread adoption of waste heat recovery in energy technologies boosting system efficiency and economic feasibility. It provides a new perspective for future research, contributing to the improvement of energy generation infrastructure.

Using compressor inlet cooling process for a supercritical Brayton cycle with various supercritical gases to recover the waste heat from a biomass-driven open Brayton cycle

The heat loss of a biomass-driven open Brayton cycle (OBC) is recuperated in the current research with the help of a supercritical Brayton cycle (SBC) and a thermoelectric generator (TEG). The temperature of the gas entering the SBC compressor is reduced through an ammonia-water absorption chiller (ACH) for performance improvement. The SBC compressor inlet cooling is examined for four supercritical gases (Nitrogen, Carbon monoxide, Air, and Helium). Moreover, the best performance of the configuration with compressor inlet cooling is compared with that of the system without compressor inlet cooling. Although compressor inlet cooling brings about the most improvement in output electricity of the SBC in the case of helium, the exergy efficiency (ηex) of the system and unit cost of the product (UCP) do not improve as much as other gases due to the reduction in the output power of the TEG in the case of helium. In addition, helium conduces to the lowest ηex (38.29 %) and highest UCP (17.62 $GJ−1) among the gases. Hence, helium is unsuitable for the SBC in total waste heat recovery applications. On the contrary, air induces the best performance with an ηex of 39.6 % and a UCP of 17.3 $GJ−1. The enhancement in ηex is 6.66 % points when the waste heat of the OBC is fully recovered.

Numerical investigation of coupled heat transfer and flow characteristics in helical coil heat exchanger for mine water waste heat recovery

To efficiently recover waste heat from mine water, this study numerically investigates the flow and heat transfer performance of a tube-shell coupled helical coil heat exchanger (HCHE), under high Reynolds number conditions. The effects of inlet water temperature, mass flow rate (m), tube diameter (d), and helix pitch (P) on the tube side, shell side, and overall performance are analyzed. Performance metrics include friction factor (f), Nusselt number (Nu), overall heat transfer coefficient (U), heat transfer rate, and the Performance Evaluation Criteria (PEC) number. Specifically, this study focuses on detailed observation and analysis of the shell-side flow and temperature fields. Findings reveal that enlarging d from 30 mm to 50 mm results in a 32.55% decrease in U, whereas increasing P from 50 mm to 200 mm boosts U by 8.02%. The highest PEC number of HCHE is 2.43, observed in conditions of P = 50 mm, d = 30 mm, and mc = 4 × 103 kg/h. Correlations for fc, Nuc, and Nush are proposed based on simulation results. This study provides a theoretical basis for the design and structural optimization of HCHE in the mine water waste heat recovery applications.

Liquid metal-enhanced thermoelectric generator for high-temperature thermal harvesting

It is a challenge to achieve high output power of the thermoelectric generator (TEG) for high-temperature waste heat recovery due to the working fluids with low thermal conductivity (such as air) or low tolerance of temperature (water or oil). In this paper, the gallium-based liquid metal (LM) with a high boiling point (>1000 ℃) and thermal conductivity (>20 W/mK) are introduced to significantly enhance heat charging and discharging at the hot/cold sides of TEG. We also developed an electromagnetic induction pumping method for the effective LM-driven flow at high temperatures. A three-dimensional coupling multi-parameter optimization simulation model of LM-based TEG was developed to reveal the internal heat transfer and power generation mechanism for clarifying the impact factors of the performance. The results have shown that the power generation capacity can be effectively improved by increasing the inlet temperature (Thot-in). It is worth noting that the temperature difference (ΔT) of the TEG module is severely limited by the thermal resistance (Rcapacity) of the LM heat capacity, such as, when the flow rate of 2 L/min, the Rcapacity accounts for 90.6 % (0.020 ℃/W). It is easier to achieve a larger open circuit voltage by increasing the height of the thermoelectric leg, but the Pmax is constantly weakened. Compared with Xceltherm 600 and Solar salt, LM heat transfer fluid has more obvious advantages in enhancing the power generation performance of the system. The LM-based TEG experiment platform was established to study heat transfer and power generation characteristics. The results have demonstrated that good consistency was proved by comparing experimental and numerical data. And the Pmax of the LM-based TEG system is 0.16 W/cm2 at ΔT of 136 ℃. LM-based TEG can open new perspectives for broad applications in high-temperature waste heat recovery.

Maximizing energy efficiency in wastewater treatment plants: A data-driven approach for waste heat recovery and an economic analysis using Organic Rankine Cycle and thermal energy storage

Maximizing energy efficiency through waste heat recovery (WHR) processes is crucial for sustainable and eco-friendly operations across multiple industries, notably in wastewater treatment plants (WWTPs). This work proposes a comprehensive approach for assessing the WHR feasibility in WWTPs, structured in two main objectives. Firstly, an Artificial Neural Network (ANN) model is developed to accurately predict WHR based on operational data, including biogas temperature, biogas pressure, daily production in kWh, and WHR values in kWhth. The second objective focuses on economically evaluating the WHR feasibility based on the estimated WHR values obtained by the ANN model, and then realistically assessing the economic feasibility of integrating the Organic Rankine Cycle (ORC) and Seasonal Thermal Energy Storage (STES) systems. With an application to the As-Samra WWTP located in Jordan, the developed ANN model demonstrates promising results in the validation phase, with a root mean square error (RMSE) of 2206 kWh/day, a mean absolute error (MAE) of 1674 kWh/day, and an R-squared (R2) value of 68%. On the other hand, the economic analysis reveals that an optimal ORC system of 412.14 kWe capacity yields a Net Present Value (NPV) of 2.09 million US dollars, a Levelized Cost of Energy (LCOE) of 0.0749 USD/kWh, a Payback Period (PBP) of 4.8 years, and annual revenues of 428 kUSD. This work also investigates the techno-economic feasibility of integrating ORC-STES. Results indicate that the LCOE and PBP are highly affected by the ORC's capital cost, and integrating STES increases the LCOE to 0.0824 USD/kWh, rendering its integration with ORC infeasible. This study aims to advance the understanding and application of WHR in WWTPs, paving the way for more efficient and sustainable practices in the field.

Thermo-fluid dynamic modeling of a supercritical carbon dioxide compressor for waste heat recovery applications

The development of novel technical solutions for the efficient recovery of waste heat is crucial for making accessible the large amount of thermal energy released by industrial processes, thus supporting the EU energy strategy. To this end, the EU-H2020 project CO2OLHEAT aims at developing and demonstrating a novel sCO2 power unit of 2 MW capacity, recovering energy from flue gases at 400 °C. The CO2OLHEAT system features a relatively unconventional multi-shaft configuration where the sCO2 compressor is driven by a dedicated radial expander, while the electrical power is generated via a separated axial turbine.The present study focuses on the design and fluid-dynamic analysis of the CO2OLHEAT compressor. The thermodynamic optimization of the cycle led to an overall pressure ratio slightly above 2.5, delivered with a two-stage centrifugal compressor. As typically found in sCO2 power systems, the thermodynamic state of the fluid at the machine intake (P = 85 bar; T = 32 °C) is close to the critical point and to the saturation curve; therefore, the first stage of the machine demands a dedicated aero-thermodynamic design, which can account for the effects of non-ideal thermodynamics and of the potential onset of two-phase flows. The paper discusses the conceptual aero-mechanical design of the compressor and then focuses on its performance assessment over the full operating range via Computational Fluid Dynamics. Two alternative CFD models are applied, the first one based on the barotropic fluid representation and the second one featuring a complete thermodynamic model, both of them assuming homogeneous equilibrium in presence of multi-phase flows. The experimental validation and the application of the two models are presented and discussed. They are shown to provide similar outcomes, and indicate that the compressor fulfills the system requirement and guarantees wide rangeability. A thorough comparison between the CFD results highlights the implications of the underlying physical differences between the models, thus allowing to properly evaluate their use for the aerodynamic design and analysis of sCO2 compressors.