Waste Heat Recovery Applications Research Articles

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.

THERMODYNAMIC ANALYSIS OF RECUPERATIVE AND REHEAT BASED ON ORGANIC RANKINE CYCLE FOR HIGH-TEMPERATURE WASTE HEAT RECOVERY

Heat recovery from available waste heat is an essential method in renewable energy processes due to the ability to generate electricity by using Organic Rankine Cycles (ORC). The process model is developed with the Cycle Tempo software. Fluid properties were analyzed with Reference Fluid Properties (REFPROP) software. This research presents a thermodynamic comparison of the Basic Organic Rankine Cycle (BORC) and Recuperative Organic Rankine Cycle (RORC) for waste heat recovery applications using pure refrigerants R-141b, R-245fa, R-123, and R-21. By performing simulations for heat source high-temperatures ranging from 160 to 200 °C. The comparison between BORC and RORC performance parameters and refrigerant properties was evaluated on the heat source temperature. Research shows that RORC produces higher thermal efficiency but has lower irreversibility than BORC. RORC has a maximum thermal efficiency of 12.67%, and BORC has a thermal efficiency of 11.49% for refrigerant R-141b at a flue gas temperature of 160 °C. The thermal efficiency of the ORC increases as the temperature of the heat source increases.

Comparative Assessment of sCO2 Cycles, Optimal ORC, and Thermoelectric Generators for Exhaust Waste Heat Recovery Applications from Heavy-Duty Diesel Engines

This study aimed to investigate the potential of supercritical carbon dioxide (sCO2), organic Rankine cycle (ORC), and thermoelectric generator (TEG) systems for application in automotive exhaust waste heat recovery (WHR) applications. More specifically, this paper focuses on heavy-duty diesel engines applications such as marine, trucks, and locomotives. The results of the simulations show that sCO2 systems are capable of recovering the highest amount of power from exhaust gases, followed by ORC systems. The sCO2 system recovered 19.5 kW at the point of maximum brake power and 10.1 kW at the point of maximum torque. Similarly, the ORC system recovered 14.7 kW at the point of maximum brake power and 7.9 kW at the point of maximum torque. Furthermore, at a point of low power and torque, the sCO2 system recovered 4.2 kW of power and the ORC system recovered 3.3 kW. The TEG system produced significantly less power (533 W at maximum brake power, 126 W at maximum torque, and 7 W at low power and torque) at all three points of interest due to the low system efficiency in comparison to sCO2 and ORC systems. From the results, it can be concluded that sCO2 and ORC systems have the biggest potential impact in exhaust WHR applications provided the availability of heat and that their level of complexity does not become prohibitive.

Enhanced zT due to non-stoichiometric induced defects for bismuth telluride thermoelectric materials

Thermoelectric materials have potential applications in energy conversion and waste heat recovery. Thermoelectric materials with non-stoichiometric composition may exhibit improved thermoelectric properties compared to the stoichiometric composition material. Bi2Te3 is one of the best-known thermoelectric materials having optimum performance near room temperature. In the present study, non-stoichiometric bismuth telluride (Bi2±x Te3), x = (0, 0.01, 0.02, 0.03), was synthesized using the co-precipitate method to create imperfections and strains in the crystal structure of the material. The crystal structure of the synthesized bismuth telluride revealed significant changes in the c direction when examined using X-ray diffraction (XRD). Scanning electron microscopy (SEM) revealed the formation of micro-particles. Energy dispersive spectroscopy (EDX) also confirmed the non-stoichiometric elemental composition of Bi and Te rich bismuth telluride. Thermogravimetric analysis (TGA) was performed in a non-inert environment, and an oxidation temperature of 685 ​K was recorded in non-stoichiometric samples. The post-oxidation of stoichiometric and non-stoichiometric Bi Rich and Te Rich bismuth tellurides was also confirmed by XRD and TGA comparison. The thermal conductivity was reduced by the imperfections and defects for all non-stoichiometric compositions, and a zT value of 0.54 at 420 ​K was obtained in the Tellurium Rich bismuth telluride, which is about a 12% improvement compared to the pure sample.

GA−BP Prediction Model for Automobile Exhaust Waste Heat Recovery Using Thermoelectric Generator

Thermoelectric generator (TEG) has important applications in automotive exhaust waste heat recovery. The Back propagation neural network (BP) can predict the electrical generating performance of TEG efficiently and accurately due to its advantage of being good at handing nonlinear data. However, BP algorithm is easy to fall into local optimum, and its training data usually have deviation since the data are obtained through the simulation software. Both of the problems will reduce the prediction accuracy. In order to further improve the prediction accuracy of BP algorithm, we use the genetic algorithm (GA) to optimize BP neural network by selection, crossover, and mutation operation. Meanwhile, we create a TEG for the heat waste recovery of automotive exhaust and test 84 groups of experimental data set to train the GA−BP prediction model to avoid the deviation caused by the simulation software. The results show that the prediction accuracy of the GA−BP model is better than that of the BP model. For the predicted values of output power and output voltage, the mean absolute percentage error (MAPE) increased to 2.83% and 2.28%, respectively, and the mean square error (MSE) is much smaller than the value before optimization, and the correlation coefficient (R2) of the network model is greater than 0.99.

Design, computational and experimental investigation of a small-scale turbopump for organic Rankine cycle systems

The hydraulic design, computational analysis, and experimental investigations of a high-speed small-scale turbopump for mobile waste heat recovery applications based on organic Rankine cycle systems are presented in this paper. Such applications demand high-pressure rise, lightweight, and compact pumping systems with simple construction. The investigated turbopump features an unshrouded 37.75 mm tip diameter single-stage centrifugal pump equipped with eight radial blades, eight splitters blades, and a rectangular axisymmetric volute. The pump should deliver 0.28 kg/s of mass flow rate with a pressure rise of 20 bar at a designed rotational speed of 25,000 rpm. Due to uncertainties observed in employing state-of-the-art 1-d methods that are valid for much larger machines, computational fluid dynamics is utilized to obtain a design meeting the specifications. The pump’s performance is evaluated experimentally at different rotational speeds, mass flow rates, and impeller tip clearances. The excellent agreement between experimental data and predictions from computational fluid dynamics validates the design methodology and computational results. The turbopump’s characteristics are then utilized to estimate the possible performance improvement of a target organic Rankine cycle using the novel turbopump instead of a commercial multi-stage centrifugal pump. The comparison suggests that the novel turbopump increases the efficiency of the target organic Rankine cycle by 0.3 % points and decreases its back-work ratio by nearly 50 %. The novel turbopump is approximately ten times more compact compared to commercial systems.

Macroencapsulated carbonate eutectic salt phase change material with high durability for high temperature heat storage

Carbonate eutectic salt is a kind of promising phase change material (PCM) for high temperature thermal storage application, which unfortunately constrained by its significant corrosiveness and low thermal conductivity. The encapsulation of salt PCM with stable inorganic shell to make capsules of millimeter to centimeter size can solve these problems, which is unfortunately technologically difficult to be prepared. In this work, a double shell strategy was proposed to coat the salt macrospheres with inner expanded graphite (EG) layer and outer ceramic kaolin layer. All raw materials were powders, which were shaped to spheres and capsules with the help of organic binder, pressed by isostatic pressing and finally sintered at high temperature for capsule formation. In a comparative case, the single kaolin shell coated capsules leaked when they were heated over the melting point of salt since that the relatively porous kaolin shell could be infiltrated with liquid salt because of their good wettability. However, the double shells coated capsules could prevent the leakage of salt since that the inner EG layer was repellent to liquid salt. The double shells coated macrocapsules showed good stability and high durability for long-term use, as confirmed by a cycling test over 300 cycles. The eutectic salt (Na2CO3/Li2CO3 = 57/43 in weight) core PCM presented a high latent heat of 313 J/g and melting point of around 498 °C. The macrocapsules with a size of 10@1.5@3 showed a high heat storage capability of 380 J/g in a temperature range of 400–600 °C. The macroencapulated carbonate eutectic salt with good heat storage capability and good thermal cycling durability can be extensively used for high-temperature heat storage applications in solar thermal power plant and industrial waste heat recovery.

Multiple Valence Bands Convergence and Localized Lattice Engineering Lead to Superhigh Thermoelectric Figure of Merit in MnTe.

MnTe has been considered a promising candidate for lead-free mid-temperature range thermoelectric clean energy conversions. However, the widespread use of this technology is constrained by the relatively low-cost performance of materials. Developing environmentally friendly thermoelectrics with high performance and earth-abundant elements is thus an urgent task. MnTe is a candidate, yet a peak ZT of 1.4 achieved so far is less satisfactory. Here, a remarkably high ZT of 1.6 at 873 K in MnTe system is realized by facilitating multiple valence band convergence and localized lattice engineering. It is demonstrated that SbGe incorporation promotes the convergence of multiple electronic valence bands in MnTe. Simultaneously, the carrier concentration can be optimized by SbGeS alloying, which significantly enhances the power factor. Simultaneously, MnS nanorods combined with dislocations and lattice distortions lead to strong phonon scattering, resulting in a markedly low lattice thermal conductivity(κlat ) of 0.54Wm K-1 , quite close to the amorphous limit. As a consequence, extraordinary thermoelectric performance is achieved by decoupling electron and phonon transport. The vast increase in ZT promotes MnTe as an emerging Pb-free thermoelectric compound for a wide range of applications in waste heat recovery and power generation.

Selection maps of dual-pressure organic Rankine cycle configurations for engine waste heat recovery applications

Dual-pressure organic Rankine cycles are widely used for engine waste heat recovery. Moreover, several dual-pressure organic Rankine cycle configurations exist. The configuration design and parameter optimisation of an optimal dual-pressure organic Rankine cycle system under different engine waste heat source conditions is difficult. Thus, an efficient and accurate selection method is required. To select system parameters for different waste heat sources quickly and accurately, this study proposes a method for constructing system selection maps. To use this method and verify its correctness, we applied it to a practical example of a dual-pressure organic Rankine cycle system selection process. The selection maps demonstrate that the best dual-pressure organic Rankine cycle system can increase the net output power and thermal efficiency by 8.34 %–28.52 % and 8.43 %–32.38 %, respectively, compared to the dual-pressure organic Rankine cycle system with the lowest net output power and thermal efficiency. After obtaining selection maps, the key parameters affecting the distribution of each selection map were analysed, as well as the map itself. The selection maps considered in this study provide a reference for appropriately selecting dual-pressure organic Rankine cycle systems for engine waste heat recovery. This research method can be extended to other areas (geothermal and solar) for waste heat recovery and other cycle configurations for optimum selection and design. In engineering applications, the selection map allows for quick and accurate determination of the best matching system for the target waste heat source, which reduces the capital and time costs of technical advice.

First experimental characterization of CaCl2 coated heat exchanger for thermochemical heat transformer applications in industrial waste heat recovery

This study aims at evaluating the performance of a heat exchanger coated with CaCl2 (PVA polyvinyl alcohol used as binder) for Thermochemical Heat Transformers (THT) applications in industrial waste heat recovery.For this purpose, an experimental test bench has been built allowing to test operating temperatures up to 160 °C. A model heat exchanger plate is used to test two configurations: a thick layer of CaCl2 to model a packed bed reactor (used as reference) and a CaCl2-PVA deposit to model a coated reactor.For the packed bed configuration, results comparable to that of the literature are obtained with a specific power of 180 W/kg obtained for a temperature lift of 60 K. A high sensitivity to temperature lift is identified with a decrease of the specific power down to 38 W/kg for a lift of 77 K. The coated heat exchanger configuration achieves significantly higher performance with a specific power up to 341 W/kg for a temperature lift of 63 K. However, the analysis of the repeatability tests highlights a rapid change in temperature profiles highlighting a change in properties of the CaCl2/PVA compound material. Next steps will consist in enhanced material formulation and change in deposit protocol and to move to work at small reactor scale to assess system performance.

The Mechanism behind the High Thermoelectric Performance in YbCd&lt;sub&gt;2-x&lt;/sub&gt;Mg&lt;sub&gt;x&lt;/sub&gt;Sb&lt;sub&gt;2&lt;/sub&gt;

YbCd&lt;sub&gt;2&lt;/sub&gt;Sb&lt;sub&gt;2&lt;/sub&gt; is a promising Zintl compound for waste heat recovery applications due to its low thermal conductivity, originating from its complex crystal structure. Many strategies such as alloying or doping have been suggested to further reduce the thermal conductivity of YbCd&lt;sub&gt;2&lt;/sub&gt;Sb&lt;sub&gt;2&lt;/sub&gt; to improve its thermoelectric performance. However, the effects of alloying or doping on the electronic transport properties of YbCd&lt;sub&gt;2&lt;/sub&gt;Sb&lt;sub&gt;2&lt;/sub&gt; have not been evaluated in detail. Here, previously reported thermoelectric properties of YbCd&lt;sub&gt;2-x&lt;/sub&gt;Mg&lt;sub&gt;x&lt;/sub&gt;Sb&lt;sub&gt;2&lt;/sub&gt; (&lt;i&gt;x&lt;/i&gt; = 0, 0.2, 0.4) with drastic thermal conductivity suppression were evaluated using the Single Parabolic Band (SPB) model and Callaway von Bayer (CvB) model. The SPB and CvB models evaluate any changes in electronic band parameters and phonon scattering strength, respectively, due to Mg alloying. Based on the SPB model, Mg alloying deteriorated the weighted mobility, mostly due to non-degenerate mobility reduction. However, the magnitude of point-defect phonon scattering significantly increased with Mg alloying, as evaluated by the CvB model. As a result, the maximum &lt;i&gt;zT&lt;/i&gt; is achieved when x = 0.4 at 700 K despite the decreased electronic transport properties from Mg alloying. Our work suggests that carefully designed alloying can improve the thermoelectric performance of the Zintl compound even when it changes its electronic and thermal transport properties in opposite directions.

Thermoacoustic cascade engine free from resonance length

The high reliability and long lifespan of thermoacoustic generators (TAG) make them ideal for waste heat recovery applications. However, the size and weight of these generators is still a challenge. This paper introduces a traveling-wave thermoacoustic generator that is free from a wavelength long loop. The new generator consists of an acoustic driver (linear motor), thermoacoustic engine and a linear alternator. The linear motor introduces the initial acoustic power into the thermoacoustic engine which amplifies it, then the amplified acoustic power is converted into electricity through the linear alternator. The electric power obtained from the alternator exceeds the initial input (“seed”) power consumed by the motor to kick-start the thermoacoustic amplification, thus providing the net power gain. The advantage lies in decoupling the operation from the reliance on the acoustic resonance. A two-stage engine can be built with half a meter length, while a three-stage engine can be less than a meter long including a coiled inertance pipe. The three-stage TAG can generate 120.5 W of electricity, at a thermal to acoustic efficiency of 34.2% and a total efficiency of 20.5%.

Thermal Energy Storage Performance Enhancement of Microencapsulated n-Tetracosane with a Polymorph-Modulated Calcium Carbonate Shell

Microencapsulated n-tetracosane as a phase change material with an inorganic calcium carbonate (CaCO3) shell has been synthesized through a self-assembly process with varying pH value of a precursor solution, and the resultant materials were systematically investigated. Spherical microcapsules with a mean diameter of less than 3 μm were seen by microscopic observation. X-ray diffraction analysis revealed the presence of three CaCO3 polymorphs such as calcite, aragonite, and vaterite in the shell, and these compositions depended on the solution pH. This fact was also confirmed by Fourier transform infrared spectra and reflected in the surface morphology of microcapsules. The characterization of the phase change behavior has shown no significant change in the phase change temperatures and more than 170 mJ/mg of phase change enthalpies with the microcapsules synthesized at pH = 1. This enthalpy corresponds to 69.2% of encapsulation efficiency which is improved from the value reported by other microcapsules with CaCO3 shells. Thermal cycling analysis exhibited good reliability and durability of phase change phenomena in the long term. These results demonstrate a promising potential of microcapsules for developing thermal energy storage materials at a mild temperature range, which can be applied for waste heat recovery applications.

Advanced Heat Exchangers for Waste Heat Recovery Applications

The incentive for industrial waste heat recovery, which has attracted much research interest in recent years, has been twofold: the obligation to reduce greenhouse gas emissions in line with climate change targets and the need for processes to reduce overall energy consumption in order to remain commercially competitive [...]

High-Performance Skutterudite/Half-Heusler Cascaded Thermoelectric Module Using the Transient Liquid Phase Sintering Joining Technique.

Thermoelectric (TE) materials have made rapid advancement in the past decade, paving the pathway toward the design of solid-state waste heat recovery systems. The next requirement in the design process is realization of full-scale multistage TE devices in the medium to high temperature range for enhanced power generation. Here, we report the design and manufacturing of full-scale skutterudite (SKD)/half-Heusler (hH) cascaded TE devices with 49-couple TE legs for each stage. The automated pick-and-place tool is employed for module fabrication providing overall high manufacturing process efficiency and repeatability. Optimized Ti/Ni/Au coating layers are developed for metallization as the diffusion barrier and electrode contact layers. The Cu-Sn transient liquid phase sintering technique is utilized for SKD and hH stages, which provides a high strength bonding and very low contact resistance. A remarkably high output power of 38.3 W with a device power density of 2.8 W·cm-2 at a temperature gradient of 513 °C is achieved. These results provide an avenue for widespread utilization of TE technology in waste heat recovery applications.

Enhancement of electrical transport properties in CrSi2-NbSi2 composite synthesized by the AM-SPS technique

Waste heat can be harnessed into electrical energy by employing thermoelectric power generation devices. Chromium disilicides are being considered as p-type thermoelectric material which is used in thermoelectric devices for waste heat recovery applications in the mid-temperature regime owing to their low cost, stability and constitute of earth-abundant elements. In the current work, spark plasma sintering was used to develop CrSi2-NbSi2 composite materials via arc melting. The surface morphology of all the synthesized samples was examined using a field emission scanning electron microscope and the elemental mapping confirms the homogenous distribution of both the CrSi2-NbSi2 phases. The phase structure of each sample was identified using X-ray diffraction. The Seebeck coefficient and the resistivity was determined for all the samples between 300 K and 800 K. A significant power factor value of ≃ 1.44 × 10-3 W/mK2 was achieved at 423 K in an optimized composition of CrSi2/2 wt% NbSi2 material in comparison to pristine ≃ 0.92 × 10-3 W/mK2 sample. It is primarily caused by the metallic NbSi2 phase that was incorporated into the CrSi2 matrix, which increased electrical conductivity and leads to the enhancement of thermoelectric performance of CrSi2.

Definition of a general performance map for single stage radial inflow turbines and analysis of the impact of expander performance on the optimal ORC design in on-board waste heat recovery applications

Small-scale Organic Rankine Cycles (ORCs) represent a promising technology for on-board waste heat recovery (WHR) from internal combustion engines (ICEs) especially in innovative long-haul trucks transport applications. Despite market leader companies have already proved its effectiveness, detailed system design procedures are scarcely available in the open literature and proposed solutions are often more simplified with respect to current industrial state-of-art. The present work describes a methodology to include within the ORC design and optimization procedure an efficiency map of the turbo-expander, retrieved exploiting a reduced-order method developed in-house. The proposed approach allows therefore to design the thermodynamic cycle considering a realistic performance of the expander and to retrieve the best cycle architectures and turbine geometry depending on heat source characteristics and active constraints. First the turbine performance is defined in general terms and investigation of its applicability to different fluids and to expansions characterized by various degrees of non-ideality is provided. Then, the methodology is applied to different WHR applications highlighting for several potential working fluids the impact of adopting turbine performance maps within the cycle optimization procedure rather than assuming constant efficiency values. Results show that the cycle configuration and the main design parameters are strongly affected by the plant size and the working fluid selection, and that the adoption of a turbine performance map is particularly valuable when dealing with small size power plants adopting high density fluids. In particular, the assumption of constant turbine efficiency may lead to the definition of thermodynamic cycles extremely far from the optimal ones (e.g. adoption of supercritical cycles rather than subcritical ones) that in turn can result in an unfeasible design of the turboexpander. Moreover, this assumption may also lead to an incorrect comparison between different candidate working fluids lacking in identify those more suitable for a specific plant size. Both those limits can be solved with the adoption of the expander performance map proposed in this work. Final results show that on board WHR ORC can reach an efficiency of at least 20% in a relatively large range of internal combustion engine size. On the contrary, the choice of the best working fluid is affected by engine size and chosen hot sources: for small available thermal power the use of complex and flammable hydrocarbons (cyclopentane, cyclohexane, toluene) is the most efficient choice while the use of low GWP non-flammable halogenated hydrocarbons (R1233zd and R245fa) becomes promising only for relatively large available thermal power.

Techno-Economic Comparison of Supercritical CO2, Steam, and Organic Rankine Cycles for Waste Heat Recovery Applications

Abstract Supercritical CO2 (sCO2) is taking a growing interest in both industry and academic communities as a promising technology capable of high efficiency, flexibility, and competitive capital costs. Many possible applications are studied in the energy field, from nuclear power plants to concentrating solar power and waste heat recovery (WHR). To evaluate the competitiveness of sCO2 cycles relative to other competing technologies, mainly steam and organic fluid Rankine cycles (ORC), a specific techno-economic analysis is needed to fairly compare the different technologies for each application, in order to find the most appropriate market position of the innovative sCO2 plants, compared to the existing steam and ORC solutions. In the present study, techno-economic analysis and optimization have been conducted focusing on WHR applications, for different sizes and cycle parameters operating conditions using an in-house simulation tool. The analyzed cycles were first optimized by aiming at maximizing the net electrical power and then aiming at minimizing the specific capital cost. As a result, compared to traditional plants, we obtained that in the first case, the more complex sCO2 cycle configuration shows competitive performance, while in the second case, the simpler sCO2 cycle configuration has a lower specific cost for the same electrical power produced (with a difference of approximately −130 €/kW compared to the steam cycle). In general, while traditional technologies confirmed a good tradeoff between performance and cost, supercritical CO2 cycles show attractive characteristics for applications requiring simplicity and compactness, guaranteeing in the meantime other technical advantages such as water-free operation.

Experimental and Numerical Dynamic Investigation of an ORC System for Waste Heat Recovery Applications in Transportation Sector

ORC power units represent a promising technology for the recovery of waste heat in Internal Combustion Engines (ICEs), allowing to reduce emissions while keeping ICE performance close to expectations. However, the intrinsic transient nature of exhaust gases represents a challenge since it leads ORCs to often work in off-design conditions. It then becomes relevant to study their transient response to optimize performance and prevent main components from operating at inadequate conditions. To assess this aspect, an experimental dynamic analysis was carried out on an ORC-based power unit bottomed to a 3 L Diesel ICE. The adoption of a scroll expander and the control of the pump revolution speed allow a wide operability of the ORC. Indeed, the refrigerant mass flow rate can be adapted according to the exhaust gas thermal power availability in order to increase thermal power recovery from exhaust gases. The experimental data confirmed that when the expander speed is not regulated, it is possible to control the cycle maximum pressure by acting on the refrigerant flow rate. The experimental data have also been used to validate a model developed to extend the analysis beyond the experimental operating limits. It was seen that a 30% mass flow rate increase allowed to raise the plant power from 750 W to 830 W.