Low-grade Waste Heat Recovery Research Articles

Performance of thermoelectric generator system to generate electrical output from a low-grade waste heat temperature under linear and swirling waste heat streams

A thermoelectric generator (TEG) module converts heat directly into electrical energy. Waste heat from a process is a viable heat source for TEG modules to support the energy sustainability agenda. A module was designed to recover low-temperature waste heat from a 1 kW fuel cell stack used in a mini hydrogen vehicle. The module was constructed using a single TEG cell that receives heat directly on its surface from the waste heat stream, coupled with a heat pipe connected to a finned heat sink to effectively cool the cold junction of the TEG cell. This research presents a comparison of the module characteristics when it is operated under direct impinging jet flow and swirl flow waste heat streams at 60°C, while the cold junction of the cell is cooled under stationary vehicle conditions (natural convection cooling) and cruising conditions (forced convection cooling) at an air speed of 5 m/s. Results indicate that the swirling effect increases the maximum power point (MPP) by 60%. The introduction of swirl to the heating stream is a viable approach to significantly enhance the recovery of low-grade waste heat using TEG. However, the MPP shows a greater increase of 70 to 80% due to the forced cooling effect, indicating that cold junction cooling has a more significant influence on the MPP compared to the swirl effect.

Experimental research on chemisorption energy storage performance for industrial waste heat recovery and conversion

This study investigates a solid chemisorption energy storage system utilizing a multi-component chloride salt composite adsorbent with a mass ratio of NH4Cl, CaCl2, MnCl2, and expanded natural graphite treated with sulfuric acid (ENG-TSA). The system features a shell-and-tube heat exchanger and analyzes refrigeration and heat storage performance under varying industrial low-temperature waste heat (60–110 °C) conditions, with condensation and evaporation temperatures of 0–25 °C and discharging temperatures of 40–60 °C. The results indicate that the multi-salt composite adsorbent reduces hysteresis between adsorption and desorption stages. The highest coefficient of performance (COP) and specific cooling power (SCP) were 0.461 and 328 W/kg, respectively, at an evaporation temperature of 25 °C and a charging temperature of 110 °C. Excessively long cycle times negatively affect system efficiency, with optimal performance achieved at a 40-min cycle. The findings highlight the system's effectiveness for industrial low-grade waste heat recovery, providing a solid foundation for future reactor design and material selection.

Colloidal dispersions of cobalt ferrite nanoparticles in EMIM TFSI, propylene carbonate and their mixtures

Liquid thermocells are promising devices for the recovery of low-grade waste heat and its conversion into electricity. The present study proposes dispersing charged magnetic nanoparticles in liquids to improve thermoelectric conversion, due to their thermodiffusive and magnetic properties. Core@shell cobalt ferrite@maghemite nanoparticles (NPs) of various sizes with three different kinds of coatings are tested for dispersion in EMIM TFSI (ethyl-methylimidazolium bistriflimide), an ionic liquid (IL) of high electrical conductivity. Among the tested ligands, PAC6MIM±Br− (1-(6-hexylphosphonate) 3-methyl imidazolium bromide) emerges as the most efficient due to its phosphonic group. This ligand leads to dispersions of clusters of a few dozen NPs in water and a few NPs in EMIM TFSI. For improving both the viscosity and the electrical conductivity, dispersions in binary mixtures of propylene carbonate (PC), a polar medium, and EMIM-TFSI are further investigated with the sample based on NPs of ∼ 9 nm diameter. Through a pumping and heating procedure, stable dispersions are obtained with a subsequent reduction of the size of the NP clusters, down to 1 to 2 NPs in EMIM TFSI and a few NPs in PC and their binary mixtures. The mixture with an IL mole fraction ∼0.2–0.3 and maximal electrical conductivity is a promising candidate for further thermoelectric investigations.

Physics-based machine learning optimization of thermoelectric assembly for maximizing waste heat recovery

With the increasing energy consumption of data centers around the world, the recovery of low-grade waste heat in data centers has become increasingly important. To maximize the waste heat recovery of thermoelectric modules in data centers, physics-based machine learning optimization of the thermoelectric assembly was carried out. A theoretical model and machine learning model of a fan-heat sink-thermoelectric generator system for waste heat recovery were proposed for predicting the performance of a thermoelectric assembly. Genetic algorithms were used to find the best heat sink geometric parameters for the possible greatest power generation. Given a temperature difference constraint of 75 °C between the heat source and the environment, the greatest output power from the thermoelectric assembly was 3.836 (watts), with the optimal plate fin height of 60 (mm), 50 fins, and 1.7 (mm) of fin pitch over a 120 mm × 120 mm area under the determined parameters of the thermoelectric module with a 120 mm cooling fan. In addition, by comparing different machine learning algorithms, the results revealed the superiority of random forest regression in solving optimal problems. This work provides a convenient and practical means for the fast optimization of thermoelectric assembly.

Performance analysis of compression-assisted absorption refrigeration-heating system for waste heat recovery of liquid-cooling data center

The power consumption for cooling of data centers increases continuously contributed to the enlargement of heat dissipation demands. Waste heat recovery is regarded as a promising solution for energy conservation and carbon reduction. A compression-assisted absorption refrigeration-heating system aimed at waste heat utilization of liquid-cooling data center is proposed. The effects of heat sources, auxiliary compression process and split ratios on the proposed system performance are discussed, and the analysis of application that synchronously produces chilled water for liquid-cooling data center and hot water for domestic use is conducted. The compression process can provide effective compensation for the system performance that weakens the demands on the quality of cooling source and driving heat source. As the compression ratio raises from 1.067 to 2.0, the minimum generation temperature decreases from 52.0 °C to 27.2 °C. The system can completely satisfy the refrigeration need, of 4.29 kW for liquid-cooling data centers and provide additional thermal energy, of 1.37 kW for heating. A quantitative relationship between the flow split ratios and circulation ratios is found out that can be a wise presentation for the adjustment of the refrigeration and heating loads. This research results can provide theoretical basis and guidance for low-grade waste heat recovery in liquid-cooling data center based on the absorption heat pump technology.

Working fluid pair selection of thermally integrated pumped thermal electricity storage system for waste heat recovery and energy storage

Global issues such as the energy crisis and carbon emissions impulse the development of waste heat recovery and energy storage technologies. In most practical industrial scenarios, the electricity supply and consumption cannot be perfectly matched and effective utilization of waste heat is in urgent need. In the present study, we develop a mathematical model to evaluate the thermally integrated pumped thermal electricity storage (TI-PTES) system to achieve off-peak electricity storage along with low-grade waste heat recovery. A double-layer optimization for screening working fluid pairs with high round-trip efficiency is carried out from 24 fluids of the heat pump and 21 fluids of the Organic Rankine cycle (ORC). In the first-layer multi-objective optimization, 3 types of working fluid pair combination strategies are compared and the great improvement of round-trip efficiency by using zeotropic fluids is proved. Among 7 energy storage temperatures covering from 393.15 K to 423.15 K with an increment interval of 5 K, the highest round-trip efficiency of 101.29% is achieved by adopting the zeotropic fluid pair [90Diethyl ether_10Pentane - 80Butane_20Pentane] at 398.15 K. Furthermore, in the second-layer single-objective optimization, the thermo-economic performance indicators of TI-PTES is evaluated and compared under different designing weighting factor groups, which effectively contributes to the screening of working fluids according to designer's trade-off. Finally, through varying energy storage temperatures and designing weighting factors, optimal working fluid pair recommendations including pure fluids and zeotropic ones were proposed to the fluid selection of TI-PTES.

A review of research status of internal leakage in scroll expander

With the development of the world, the energy crisis has become increasingly prominent, which has greatly affected the sustainable development of the global economy. To ease the energy crisis and move towards sustainable development, the recovery of low-grade waste heat has gained increasing attention. Through a literature tracking strategy, this report provides the current state of research on internal leakage of the scroll expander, a key component of the organic Rankine cycle system. The structure, working principle, heat transfer and expansion efficiency of the scroll expander are introduced. The types of internal leakage, calculation model of leakage, influencing factors, influencing factors and solutions are analysed in detail. The main influencing factors of internal leakage are axial clearance, radial clearance, speed, air pressure, lubricating oil and so on.

Triple effect absorption refrigeration systems for the deep recovery of low grade waste heat

With regard to energy conservation, low grade waste heat recovery technology is attracting significant interest. The absorption refrigeration can significantly utilize waste heat. The current researches have devoted little attention to the recovery of waste heat at lower temperature. This paper presents three triple effect absorption refrigeration systems for the deep recovery of low grade waste heat, which can make use of more waste heat as low as 60 °C. The involvement of absorption heat transformer cycle enables the proposed systems to exploit waste heat at lower temperature. The models of three systems are established using EES software. Energy, exergy and economic analysis of three systems are conducted. The results indicate that the best of three systems can achieve a coefficiency of performance of 0.178 and the maximum exergy efficiency is 0.190. The triple effect absorption refrigeration systems not only generate more cold energy, but also demonstrate better economic performance than the previous cascade absorption refrigeration system. Except for the first system, the other two systems achieve reductions of 8.7% and 15.2% in the cost of unit cold energy respectively. This paper provides three novel approaches for producing high grade cool energy from lower temperature of waste heat.

High performance dual-mode module for power generation and cooling based on optimized BiSbTe

Bismuth telluride is currently the most outstanding thermoelectric (TE) material near room temperature, and the high demand for solid-state refrigeration and low-grade waste heat recovery means there is an urgent need to improve its TE performance. This study investigates the enhanced TE performance of Bi0.4Sb1.6Te3 through the addition of Ag2Se. The introduction of Ag promotes an acceptor doping effect, leading to increased carrier concentration, while inhibiting lattice thermal conductivity together with Se doping. As a result, a notable enhancement in the figure of merit (ZT) to 1.35 at 373 K is achieved, with an average ZT of 1.17 spanning temperatures from 300 K to 523 K. More significantly, a 7-pair TE module was integrated using p-type Bi0.4Sb1.6Te3-0.00125Ag2Se and n-type extruded BiTeSe for cooling and power generation. A competitive conversion efficiency of 5.54 % at a temperature difference of 230 K and a maximum cooling temperature difference of 63.1 K at Th = 300 K are achieved, demonstrating the enormous practical application potential of this dual-mode module.

High Thermoelectric Performance Sheds New Light for Low-grade Waste Heat Recovery

High Thermoelectric Performance Sheds New Light for Low-grade Waste Heat Recovery

Thermoeconomic analysis of a novel cogeneration system for cascade recovery of waste heat from exhaust flue gases

In low-grade waste heat recovery scenarios, an absorption refrigeration cycle (ARC) or organic Rankine cycle (ORC) can be used because of its widespread acceptance in combined cooling and power applications. This paper introduces a cogeneration configuration employing a cascade waste-heat recovery architecture (including an ARC, ORC, and intermediate heating loop) to harness waste heat from the flue gas of a gas turbine (GT). The results demonstrated that the first-law efficiency and total power generation were largely influenced by the ARC, whereas the second-law efficiency and total available energy were primarily affected by the ORC. When the ARC generation temperature difference and ORC evaporation temperature were adjusted, the first- and second-law efficiencies exhibited divergent trends, suggesting the absence of a unified strategy to enhance both efficiencies simultaneously in the current system. The system achieved its highest first-law efficiency of 54.84 % and maximum total power generation of 1866.10 kW at an ORC evaporation temperature of 145 °C and a heat-source inlet temperature of 160 °C. Furthermore, at an ORC evaporation temperature of 150 °C and a heat-source inlet temperature of 200 °C, the system reached its highest second-law efficiency of 34.57 % and maximum total available energy of 1233.3 kW. The case study revealed that the GT system (including air compressor, air preheater, combustion chamber, and gas turbine) accounted for the highest level of irreversibility, comprising 74 % of the overall system. Specifically, the combustion chamber in the GT system produced a significant irreversibility due to chemical reactions. Compared with traditional combined cooling and power systems, the proposed system can increase the first-law efficiency and lower the unit energy cost of the output.

A novel data center free radiating system driven by waste heat and wind energy system (DCFRWWs) and its operation performance analysis

Refrigeration equipment for data centers consumes a significant portion of global electricity, resulting in high Power Usage Effectiveness (PUE) values and increased entropy of the data center. To address these challenges, this study proposes a novel data center free radiating system driven by waste heat and wind energy (DCFRWWs). This approach recovers low-grade heat energy from the hot air heated by the central processing units (CPUs) and transform it into work. Unlike conventional cooling systems, the DCFRWWs do not require additional energy to meet CPU cooling demands and can effectively suppress the entropy increase of the data center. The Bernoulli equation was used to establish the mathematical model for DCFRWWs, which explained the underlying mechanisms of pipeline wind speed and the influence of ambient wind speed and pipeline inlet temperature on system operating characteristics. In addition, this study employed pipeline height and diameter as decision variables, while maximizing exergy efficiency served as the optimization objective. To obtain the optimal pipeline size (height and diameter) that is effective under real-world experimental conditions, the optimization calculations were conducted using weather data from Qinhuangdao. To verify the reliability and accuracy of the model, an experimental system was constructed, and the simulation results indicated that the relative error rates for the three key parameters are as follows: pipeline wind speed (33.09%), pipeline inlet temperature (0.59%), and CPU temperature (3.85%). A single factor analysis was performed on 14 different operating conditions to analyze the operating characteristics of the DCFRWWs. Moreover, the applicability verification conducted in three distinct cities across different regions suggest that the system is indeed suitable for diverse settings. The DCFRWW system, utilizing the dual forces of the chimney effect of waste hot air in pipelines and the extraction force generated by unpowered roof ventilator, achieves the objective of free heat dissipation. Furthermore, the DCFRWWs can not only be applied in the field of data center cooling but also serves as a thermodynamic reference for low-grade waste heat recovery in other sectors, thereby further expanding the scope of direct utilization of wind energy.

Thermal-driven osmosis utilizing hollow fiber membranes: Sustainable dye water treatment and electricity extraction

Excessive discharges of chemical dyes have led to severe contamination of seawater, resulting in significant ecological damage. Thermo-osmotic energy conversion technology, which purifies wastewater and generates electricity under temperature gradients, has garnered considerable interest from researchers. However, the operational efficiency of thermo-osmotic energy conversion is inherently limited by the effective membrane area, and conventional systems typically employ flat membranes as separators, resulting in suboptimal performance for the same floorage. In this study, we introduce a novel system that combines thermo-osmotic energy conversion technology with hollow fiber membranes that possess a high effective membrane area, thereby achieving optimized performance. Driven by the temperature gradient, the water in the dye solution condenses outside the membrane and converted from vapor to liquid. The continuous accumulation of condensed water generates pressure and drives a water turbine for sustainable power generation. Our results demonstrate that at temperature gradient of 40 °C, the system can achieve the power output of 16.75 Wm−3 and the freshwater production rate of 70.29 kg m−3 h−1 under ideal conditions. This study presents a novel water-electricity co-production system based on low-grade waste heat recovery for dye solution treatment, which extends the applicability of thermo-osmotic energy conversion technology and improves its practicality.

Developing a Multiband Electronic Band Structure Model and Predictive Maps for Bismuth-Rich Mg3(Sb1-xBix)2 Thermoelectric Materials.

In recent years, bismuth-rich Mg3(Sb1-xBix)2 (x = 0.5-0.8) compositions have generated significant interest due to their excellent thermoelectric (TE) performance near room temperature, making them potential applicants for recovery of low-grade waste heat. The superior performance in these materials is due to its complex electronic band structure (EBS) with presence of multiple near degenerate bands close to the conduction band edge. The position and curvature of these bands strongly depend on the alloy composition, doping amount as well as temperature. Thus, identifying optimal material compositions to get the best TE performance depends on an understanding of the temperature dynamics of EBS and forms the objective of this work. Mg3Sb0.6Bi1.4 (x = 0.7) is chosen for this study due to its reported high near room temperature performance, and compositions with varying doping concentrations (Te used as dopant) have been synthesized. EBS parameters like effective mass and deformation potential of bands, interband separation and band gap values have been estimated using a recently developed refinement approach. Refinement results indicate that the interband separation between conduction bands to be a function of both temperature and doping concentration. Further, thermal conductivity (κ) was estimated for all of the compositions. Utilizing the EBS and κ information, predictive 3D maps indicating the variation in zT (TE figure of merit) with doping concentration and temperature have been generated. The 3D maps reveal an interesting surface topography with a broad peak zT region. This observation explains why these materials have high TE performance and are less sensitive to doping inhomogeneities. Our results provide detailed EBS information and fundamental insights on the TE properties of Mg3Sb0.6Bi1.4. Further, the proposed technique can be utilized to probe other Mg3(Sb1-xBix)2 compositions and TE materials.

Boosted Thermogalvanic Thermopower upon Solid-to-Liquid Phase Transition

Thermogalvanic cells offer scalable low-grade waste heat recovery using tunable electrode-dependent thermopower and electrolyte-dependent thermal conductivities. However, the use of single-phase electrodes thermodynamically curb the entropy difference, limiting the thermopower...

Performance analysis and multi-objective optimization of organic Rankine cycle for low-grade sinter waste heat recovery

In order to enhance the sinter waste heat recovery (WHR) rate efficiently, the organic Rankine cycle (ORC) for power production was applied to take the low-grade sinter cooling flue gas (SCFG) expelled from the waste heat boiler as the heat source. The system thermodynamic, thermo-economic and multi-objective optimization models were constructed firstly, and then R245fa was chosen as the ORC working fluid. Furthermore, the variations of system thermodynamic and thermo-economic performances with the ORC thermal parameters under different inlet temperatures of SCFG were analyzed in detail, and the ideal parameter combination was established through the multi-objective optimization approach. The results show that, the optimal inlet temperature of SCFG is determined to be 160 °C, and under the aforementioned condition of inlet temperature of SCFG, the optimal ORC thermal parameters are determined to be 95 °C for the working fluid evaporation temperature, 10 °C for the working fluid superheat degree, 28 °C for the working fluid condensation temperature and 9 °C for the pinch-point temperature difference in evaporator by considering the system thermodynamic and thermo-economic performances comprehensively. Meanwhile, the WHR rate of SCFG under the above ORC parameter combination could reach 64.86 %, showing remarkable energy-saving effect.

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.

High Performance Thermoelectric Power of Bi0.5Sb1.5Te3 Through Synergistic Cu2GeSe3 and Se Incorporations.

Bi2Te3-based alloys are the benchmark for commercial thermoelectric (TE) materials, the widespread demand for low-grade waste heat recovery and solid-state refrigeration makes it imperative to enhance the figure-of-merits. In this study, high-performance Bi0.5Sb1.5Te3 (BST) is realized by incorporating Cu2GeSe3 and Se. Concretely, the diffusion of Cu and Ge atoms optimizes the hole concentration and raises the density-of-states effective mass (md *), compensating for the loss of "donor-like effect" exacerbated by ball milling. The subsequent Se addition further increases md *, enabling a total 28% improvement of room-temperature power factor (S2σ), reaching 43.6µWcm-1K-2 compared to the matrix. Simultaneously, the lattice thermal conductivity is also significantly suppressed by multiscale scattering sources represented by Cu-rich nanoparticles and dislocation arrays. The synergistic effects yield a peak ZT of 1.41 at 350K and an average ZT of 1.23 (300-500K) in the Bi0.5Sb1.5Te2.94Se0.06 + 0.11 wt.% Cu2GeSe3 sample. More importantly, the integrated 17-pair TE module achieves a conversion efficiency of 6.4%, 80% higher than the commercial one at ΔT=200K. These results validate that the facile composition optimization of the BST/Cu2GeSe3/Se is a promising strategy to improve the application of BST-based TE modules.

Cascading the Transcritical CO2 and Organic Rankine Cycles with Supercritical CO2 Cycles for Waste Heat Recovery

Concentrating solar power (CSP) applications can benefit from the superior thermophysical and chemical properties of supercritical CO2 (sCO2), which offers greater cycle efficiency in contrast to supercritical or superheated steam cycles. The transcritical CO2 (TCO2 cycle) excels in low-grade waste heat recovery (WHR), while the organic Rankine cycle (ORC) is also a potential option for WHR from sCO2 cycles. This study focuses on the thermodynamic assessment and optimization of combined power cycles, incorporating multiple bottoming cycles, to enhance overall thermal efficiency. The configurations employed include recompression, partial cooling, and main compression intercooling within the sCO2 cycle. Waste heat recovery is achieved using both TCO2 and ORC. The parametric analysis explores key variables such as turbine inlet temperature, main compressor inlet temperature, recuperator effectiveness, pressure ratios in the bottoming cycles, and condensation temperatures of the bottoming cycles. Furthermore, this research conducts performance analysis of the proposed combined cycle configurations. The results underscore the substantial enhancement of combined cycle performance with regard to thermal and exergy efficiency. A rise in the highest temperature that can occur in the sCO2 cycle leads to improved thermal efficiency. The Main Compressor Inlet Temperature (MCIT) exhibits an efficiency increase of up to 40°C–45°C, attributed to pseudocritical effects, followed by a decline. Among the various configurations, the combined partial cooling cycle generates the highest net output power. The study finds that the combined main compression model consistently outperforms other layouts, yielding a 2-2.5% increase in efficiency. Optimization of the intermediate pressure value for the partial cooling (PC) and main compression (MC) cycles at 10 MPa further contributes to enhanced performance. This study not only presents the optimized operating conditions for each combined cycle but also offers a comparative analysis of these models under different boundary conditions. The findings from this research significantly advance the utilization of waste heat and the development of clean power generation.

Development of Cu2Se/Ag2(S,Se)-Based Monolithic Thermoelectric Generators for Low-Grade Waste Heat Energy Harvesting.

With the ongoing climate and energy crises, thermoelectric conversion has slowly emerged as a clean and reliable alternative energy source for small Internet of Things (IoT) devices. Commercially available thermoelectric generators (TEGs) are typically composed of expensive and toxic Bi2Te3-based thermoelectric materials and require complicated and energy-intensive device assembly processes. As an alternative solution, we have developed a Ag- and Cu-chalcogenide-based monolithic TEG using simple, quick, and low-energy-cost device fabrication processes for low-grade waste heat recovery for energy harvesting. We used ductile Ag2S0.55Se0.45 and overstoichiometric Cu2.075Se, both possessing excellent transport properties around room temperature, with a zT value of ∼0.5 at 300 K. By optimizing the device fabrication process, we were successfully able to assemble the monolithic TEGs without any significant Ag- or Cu-ion migration and obtained a dense and robust device. Strategic optimization of the device structure was able to reduce the electrical contact resistance of the device, which resulted in increased power output. A maximum power density of 0.68 mW/cm2 at a ΔT = 30 K was obtained, which is comparable to a similar Bi2Te3-based monolithic TEG. These results show the potential of chalcogenide-based monolithic TEG as a simple and low-cost alternative to Bi2Te3-based TEGs for energy harvesting applications.