Low-grade Waste Heat Recovery Research Articles

Advanced exergo-economic schemes and optimization for medium–low grade waste heat recovery of marine dual-fuel engine integrated with accumulator

Facing the multiple pressures of energy reserves decline, ecological environment deterioration and transportation industry prosperity, in order to enhance the marine performance, improve the enterprises economic benefits, and achieve the purpose of energy conservation and emission reduction, the revolution of green and efficient waste heat recovery technology is imperative. Therefore, the exergy-based configuration design of marine cascade waste heat recovery system is proposed, including supercritical Brayton cycle, organic Rankine cycle, organic flash cycle, organic trilateral cycle and absorption refrigeration cycle. Firstly, the mathematical models of three cycle-configurations are established based on thermodynamic equations, and the authority of simulation results is preliminarily verified. Then, the relationship between working-fluid, matching mode and operation parameters with cycle-configuration performance is analyzed in turn. After the multi-objective optimization, the thermodynamic performance and economic benefits of the cycle-configuration are comprehensively evaluated. Finally, the positive influence of the accumulator on the refrigeration system is explored. The numerical results prove that the proposed cycle-configuration has significant thermodynamic performance and economic benefits, reflects the good performance of energy saving and emission reduction, and can eliminate the negative impact of latent heat fluctuation on the refrigeration system. It provides a feasible idea for researchers and manufacturers who are committed to comprehensively improve the performance of cascade waste heat recovery, and fills the research gap in the latest investigation.

Experimental investigation on the flow instability of supercritical CO2 in vertical upward circular tube in trans-critical CO2 Rankine system

The trans-critical CO2 Rankine cycle is a promising low-grade waste heat recovery technology. The research about the heat transfer deterioration of supercritical CO2 in tube has been attracted more and more attentions recently, but there is not much attention that it produces the flow instability of supercritical CO2 under a certain range of working conditions as well in the trans-critical CO2 Rankine system. The present experimental work is aimed at the investigation of flow instability of supercritical CO2 in vertically upward flow tube. The experiments are conducted at the conditions of the inlet temperature from 10 to 20 °C, the inlet pressure from 7.5 to 8.5 MPa, the mass flux from 315 to 477 kg/m2·s and the heat flux from 59.95 to 90.05 kW/m2. The influences of the operating parameters on the heat transfer deterioration and the flow instability are discussed. For the upward flow, it is observed that the flow instability occurs with the continued increasing of the heat flux after the heat transfer deterioration. Moreover, the flow instability occurs in the transition from the heat transfer deterioration to the heat transfer recovery, and the flow remains stability after heat transfer recovery with the increasing of the heat flux, pressure, inlet temperature or mass flux. Compared with the upward flow, the downward flow is stable without any oscillation. It follows from analysis that the oscillation in the upward flow tube results from the buoyancy effect with the variation of heat transfer mode. The experimental results also show that the periodical oscillation of local temperature occurs within the region where the density variation is greatest. The growth and collapse of the fluid particle between the near-wall flow and the core flow makes the volumes of the fluid vary greatly, resulting in the occurrence of oscillation phenomenon.

Thermo-economic optimization of small-scale Organic Rankine Cycle: A case study for low-grade industrial waste heat recovery

This work is focused on a case study of a small-scale Organic Rankine Cycle (ORC) adopted for electricity production from low-grade industrial waste heat recovery. This kind of applications raises a great interest due to the high amount of low-grade waste heat recoverable within industrial processes, but lacks of in-depth experimental investigations on the topic. The main reason is the difficulty to reach profitable small-scale projects, so more cost-effective solutions are being explored in the literature through thermo-economic optimizations. Nonetheless, the results obtained cannot be discussed with respect to actual operating data. In light of this, this paper proposes to conduct the thermo-economic optimization on the basis of an experimental application. In this manner, a comprehensive model of the facility is developed, calibrated, and validated from actual operating data. The model is used to conduct the thermo-economic optimization, revealing the influence of the organic fluid, cycle architecture, geometric parameters of main components, or control strategy used to obtain the best cost-effective solution. The main results show that, by means of a multivariable optimization using cost-effective ratios as objective function, a cheaper and powerful solution adapted to each specific project may be designed.

Theoretical performance evaluation of ejector and economizer with parallel compression configurations in high temperature heat pumps

Abstract Climate change evolution urges us to take action to reduce greenhouse gas emissions. As one of the main contributors, the industrial sector requires cleaner methods of heat production, such as high temperature heat pumps (HTHP) with the highest energy efficiency. Facing this challenge, this paper provides a performance comparison of the ejector and economizer with parallel compression configurations in HTHPs for low-grade waste heat recovery. A single-stage cycle has been used as a reference configuration to compare the proposed options. The internal heat exchanger (IHX) is included in all the configurations to extend the analysis and illustrate the influence of this component in high temperature applications. Moreover, alternative low-GWP refrigerants have been analyzed in order to replace the reference working fluid HFC-245fa. Ejector and economizer with parallel compression configurations with IHX provide a coefficient of performance (COP) improvement of up to 36% and 72.5%, respectively, using HFC-245fa for a heating temperature production of 140 °C. Moreover, the volumetric heating capacity (VHC) increases around 36% for the ejector cycle and 80% for the economizer with parallel compression cycle with IHX. HC-601, HFO-1336mzz(Z), and R-514 have the most significant COP improvement compared to HFC-245fa, along with the highest VHC drop. Multi-objective evaluation illustrates that single-stage and ejector configurations with IHX have similar results, whereas the economizer with parallel compression cycle with IHX presents a significant COP and VHC improvement. HCFO-1233zd(E) and HCFO-1224yd(Z) presents the proper trade-off between COP and VHC.

3-D printed gradient porous composite electrodes improve anodic current distribution and performance in thermally regenerative flow battery for low-grade waste heat recovery

A thermally regenerative flow battery (TRB) with 3-D porous Cu/C composite electrodes is constructed and its anodic current distribution is investigated with 5 anode electrode segments. It is demonstrated that there is a markedly uneven current distribution in a TRB with uniform porous electrodes (TRB-M), thereby limiting the maximum power generation. The contribution of each electrode segment to the total current decreases along the flow direction due to the high resistance of mass transfer and the increased consumption of ammonia during the reaction. The gradient porous electrodes have a significant influence on the distribution of the anodic current and the power generation of the TRBs. A TRB using a gradient porous electrode with decreasing pore diameter (TRB-LS) results in a more uniform anodic current distribution and a 17% increase in the maximum power compared to that of a TRB-M. Further improvements in the current distribution and performance of a TRB-LS are made by increasing the concentration of ammonia and the electrolyte flow rate by effectively enhancing mass transfer.

Double-section absorption heat pump for the deep recovery of low-grade waste heat

Absorption heat pump is an effective option for industrial waste heat recovery. However, significant conflict between low evaporation temperature and high efficiency exists in conventional absorption heat pump, which limits its application in deep recovery of low-grade waste heat recovery. To address this issue, this paper presents the use of double-section absorption heat pump for low-grade waste heat recovery. Both theoretical and experimental researches are carried out to analyze the features of double-section LiBr-water absorption heat pump. Driven by hot water with temperature of 120.1 °C, the experimental prototype was able to extract thermal energy from the waste hot water by reducing its temperature from 45.0 °C to 15.9 °C, realizing a large temperature decrease of 29.1 °C for waste hot water. Meanwhile, the experimental prototype could heat water up to 52.5 °C for domestic heating application with capacity of 8666.4 kW and high coefficient of performance of 1.77. The excellent performance is attributed to the stepped generation/evaporation processes with gradient temperatures, which makes the double-section absorption heat pump achieve both higher efficiency for internal circulation and reduced irreversible loss for external circulation comparing with the conventional single-section absorption heat pump. This makes the double-section absorption heat pump an efficient way to realize the deep recovery of low-grade waste heat.

Recovery and Utilization of Low-Grade Waste Heat in the Oil-Refining Industry Using Heat Engines and Heat Pumps: An International Technoeconomic Comparison

We assess the technoeconomic feasibility of onsite electricity and steam generation from recovered low-grade thermal energy in oil refineries using organic Rankine cycle (ORC) engines and mechanical vapour compression (MVC) heat pumps in various countries. The efficiencies of 34 ORC and 20 MVC current commercial systems are regressed against modified theoretical models. The resulting theoretical relations predict the thermal efficiency of commercial ORC engines within 4–5% and the coefficient of performance (COP) of commercial MVC heat pumps within 10–15%, on average. Using these models, the economic viability of ORC engines and MVC heat pumps is then assessed for 19 refinery streams as a function of heat source and sink temperatures, and the available stream thermal energy, for gas and electricity prices in selected countries. Results show that: (i) conversion to electrical power with ORC engines is, in general, economically feasible for heat-source temperatures >70 °C, however with high sensitivity to energy prices; and (ii) steam generation in MVC heat pumps, even more sensitive to energy prices, is in some cases not economical under any conditions—it is only viable with high gas/low electricity prices, for large heat sources (>2 MW) and higher temperatures (>140 °C). In countries and conditions with positive economics, payback periods down to two years are found for both technologies.

A combined cooling and power cogeneration system by coupling duplex free-piston stirling cycles and a linear alternator

Abstract We propose a novel cooling and power cogeneration system based on duplex free-piston thermoacoustic-Stirling cycles and an oscillating linear alternator. In such a cogeneration system, a free-piston Stirling heat engine produces acoustic power via the thermoacoustic power cycle, and a free-piston Stirling refrigerator produces cooling power via the thermoacoustic refrigeration cycle; an oscillating linear alternator is used to couple the free-piston heat engine and refrigerator and simultaneously produces electricity. The configuration and operating principles of the cogeneration system are described along with the thermoacoustic energy transformations and transmission mechanisms. An experimental prototype system was designed, built and successfully operated for the first time. In the preliminary experiments, thermoacoustic oscillation startup typically occurred at heating temperatures around 50 to 60 °C, and a cooling power of about 230 W at -20 °C and an electrical power of more than 6 W were obtained at a heating temperature below 190 °C. The alternator-coupled system has potential applications in heat-driven cogeneration systems for low-grade waste heat recovery and solar energy utilization.

Multi-objective optimization and off-design evaluation of organic rankine cycle (ORC) for low-grade waste heat recovery

Organic Rankine Cycle (ORC) is a technically feasible way for low temperature waste heat recovery. Multi-objective optimization of the ORC using R245fa is conducted considering both thermodynamic performance and economic factors simultaneously, by means of Non-dominated sorting genetic algorithm-Ⅱ. The optimum operating parameters were achieved by sorting the Pareto-optimal solutions using the technique for order preference by similarity to ideal situation. A comprehensive analysis on the off-design performance of the optimized ORC system is examined. Rls is defined as the ratio of latent heat to sensible heat of the input heat to investigate the relation between the distribution of the input heat and the system performance. The effects of hot water temperature and mass flow rate, cooling water temperature and mass flow rate, superheated degree, subcooling degree and working pump rotational speed, on thermal efficiency, exergy efficiency, net power output, investment cost per unit power and Rls were investigated. The fitted correlations between Rls and the system performance indexes were derived. The novelties of the present paper are the multi-objective optimization for the further off-design behavior analysis and the quantitative relation investigation between Rls and system performance indexes. It is indicated that the direction of system optimization is to reduce Rls.

Enhanced seebeck coefficient and low thermal conductivity of Cu2SexTe1-x solid solutions via minority carrier blocking and interfacial effects

Abstract The investigation of state-of-the-art copper chalcogenides (Cu2-yX; X = S, Se or Te) based thermoelectric (TE) materials can direct to improve the performances of TE devices. Herein, we report the synthesis of Cu2SexTe1-x solid solutions prepared by two-step ball-milling method. The effect of Se concentration on their TE performance as a function of temperature, was investigated. XRD and HRTEM analyses confirmed the size reduction and the presence of amorphous regions with polycrystalline behavior of the samples. To evaluate their TE performance, as-prepared powder samples were cold-pressed and annealed at 773 K. The high figure of merit was obtained for Cu2Se0·7Te0.3 solid solution as 0.3 at 500 K due to the synergy of high Seebeck coefficient (174 μVK−1 at 500 K) and low thermal conductivity (1.16 Wm−1K−1 at 500 K). The high Seebeck coefficient with optimum carrier concentration and low thermal conductivity was achieved due to wide range of phonon scattering. The results suggested that, Cu2SexTe1-x solid solutions were efficient for low grade (300–500 K) waste heat recovery.

Zeotropic Mixture Selection for an Organic Rankine Cycle Using a Single Screw Expander

The organic Rankine cycle (ORC) is a popular and promising technology that has been widely studied and adopted in renewable and sustainable energy utilization and low-grade waste heat recovery. The use of zeotropic mixtures in ORC has been attracting more and more attention because of the possibility to match the temperature profile of the heat source by non-isothermal phase change, which reduces the irreversibility in the evaporator and the condenser. The selection of working fluid and expander is strongly interconnected. As a novel expander, a single screw expander was selected and used in this paper for efficient utilization of the wet zeotropic mixtures listed in REFPROP 9.1 in a low-temperature subcritical ORC system. Five indicators, namely net work, thermal efficiency, heat exchange load of condenser, temperature glide in evaporator, and temperature glide in condenser, were used to analyze the performance of an ORC system with wet and isentropic zeotropic mixtures as working fluids. The calculation and analysis results indicate that R441A with an expander outlet temperature of 320 K may be the suitable zeotropic mixture used for both open and close type heat source. R436B may be selected with an expander outlet temperature of 315 K. R432A may be selected with an expander outlet temperature from 295 K to 310 K.

High-Performance Thermoelectric Generators for Field Deployments.

Thermoelectric power generation is a reliable energy harvesting technique for directly converting heat into electricity. Recent studies have reported the thermal-to-electrical energy conversion efficiency of thermoelectric generators (TEGs) up to 11% under laboratory settings. However, the practical efficiency of TEGs deployed under real environments is still not more than a few percent. In this study, we provide fundamental insight on the operation of TEGs in realistic environments by illustrating the combinatory effect of thermoelectric material properties, device boundary conditions, and environmental thermal resistivity on TEG performance in conjunction with the module parameters. Using numerical and experimental studies, we demonstrate the existence of a critical heat transfer coefficient that dramatically affects the design and performance of TEGs. Results provide a set of concrete design criteria for developing efficient TEGs that meet the metrics for field deployments. High-performance TEGs demonstrated in this study generated up to 28% higher power and 162% higher power per unit mass of thermoelectric materials as compared to the commercial module deployed for low-grade waste heat recovery. This advancement in understanding the TEG operation will have a transformative impact on the development of scalable thermal energy harvesters and in realizing their practical targets for efficiency, power density, and total output power.

Analysis of a supercritical organic Rankine cycle for low-grade waste heat recovery

The organic Rankine cycle (ORC) is an important method of producing electricity from industrial waste heat and renewable energy resources. This paper presents a thermo-economic analysis of a supercritical ORC for low-grade waste heat recovery using four working fluids. Based on the first law of thermodynamics, R152a was found to be the best performing fluid, producing the highest values for net power production and thermal efficiency. When the mass flow of waste heat is about 1 t/h and at an evaporation temperature of 136°C, the petroleum saved is 12·3 m3/year and carbon dioxide emissions are reduced by 41·2 t/year.

Exergoeconomic performance comparison, selection and integration of industrial heat pumps for low grade waste heat recovery

Mechanical heat pumps, absorption heat pumps, and absorption heat transformers are typical technologies for low-grade heat upgrading to save energy. However, there are few guidelines on the selection and integration of heat pumps in industrial processes, and different heat pumps need the input energy with several types or grades (i.e., mechanical work, high or medium grade heat), the coefficient of performance is not suitable to evaluate different heat upgrading technologies. In this work, an exergoeconomic criterion (i.e., exergy loss per total capital investment), measuring the exergy performance of each type of heat pumps and considers economic impact, is introduced to assist the screening of industrial heat pumps. The process models of heat pumps are developed using Aspen Plus. A systematic method for heat pump integration into an industrial process is presented, relying on Pinch Analysis of a given heat exchanger network. The impacts of different waste heat temperatures and temperature lifts on the selection of heat pumps are analyzed. A case study of a catalyst reforming unit in a petroleum refinery is used to demonstrate the applicability of the proposed method, and the energy-saving and economic performance of three types of heat pumps at different waste heat upgrading options are compared. Results show heat pump selection based on the exergoeconomic criterion can achieve better thermodynamic (exergy-based) and economic performance than that based on conventional one. The introduced guide map can simplify the heat pump integration process, and the proposed method of heat pump integration can be further extended to other industrial processes for low-grade waste heat recovery.

Considerations on alternative organic Rankine Cycle congurations for low-grade waste heat recovery

When organic Rankine cycles (ORC) are employed to convert waste heat into work, the thermal efficiency is not recommended as a key performance metric because waste heat recovery and power output are not generally maximized at the point of peak efficiency. In such an application, maximization of the net power output should be the objective. Two alternative cycle configurations that can increase the net power output from a heat source with a given temperature and flow rate are analyzed and compared to a baseline ORC. These cycle configurations are: an ORC with two-phase flash expansion (TFC) and an ORC with a zeotropic working fluid mixture (ZRC). A design-stage ORC model allowed a consistent comparison of multiple ORC configurations with finite capacity of the source and heat sink fluids. Simulation results indicated that the TFC offered the most improvement over the baseline ORC, but required a highly efficient two-phase expansion. The ZRC shows improvement over the baseline as long as the condenser fan power requirement is not negligible. At the highest estimated condenser fan power, the TFC is no longer beneficial. Finally, a partial-evaporating ZRC (PE-ZRC) has also been considered to reduce the volume ratio requirements of a flash expansion.

Thermoelectric Properties of Minerals with the Mawsonite Structure

Synthetic copper sulfides have emerged as promising nontoxic and low-cost materials for thermoelectric power generation in low-grade waste heat recovery systems. Similarly to tetrahedrite and colusite, mawsonite Cu6Fe2SnS8 exhibits a modified corner sharing Cu–S tetrahedral network which usually leads to p-type character and low thermal conductivity. In order to explore the applicative potential of mawsonite, we studied the band structure, the phonon dispersions, the electronic and transport coefficients, as well as the effect of isovalent substitutions of Fe, Sn, and S. The combined analysis of electronic and vibrational properties highlights the role of the weakly bonded copper component in achieving a very low thermal conductivity. We also demonstrate that the Cu–S bond builds a 2D conductive network where the contribution from other elements is negligible. Magnetic calculations point to an antiferromagnetic ground state substantially affected by the covalency of the bonds with the conductive plane. The chemical substitution of Fe with Ni leads to nonmagnetic metals whereas Cu6Fe2SnSe8, Cu6Fe2PbS8, and Cu6Fe2GeX8 with X = S, Se, and Te are semiconductors.

Thermoelectric performance of Cu-doped MoS2 layered nanosheets for low grade waste heat recovery

Transition Metal Dichalcogenides (TMDCs) are the potential thermoelectric materials due to their high thermal stability as well as tunable electronic and phonon transport properties. In this work, we investigate theeffect of copper doping in thermoelectric properties of layered MoS2 nanosheets. X-ray Diffraction analysis and HRTEM micrographs show the reduction in interatomic spacing which confirms the interaction of copper ions with MoS2 lattice. Downshift in X-ray Photoelectron Spectroscopy (XPS) of Cu doped samples proves the shift of Fermi energy near to the valence band and it results in the enhancement of electrical conductivity of Cu doped MoS2 samples from 0.75 Sm−1 to 14.20 Sm−1 (for 7.5 wt% Cu doped sample). Thus, obtained maximum power factor of 0.313 µWcm−1K−2 for 7.5 wt% Cu doped MoS2 at 420 K.

Thermoelectric effect and temperature-gradient-driven electrokinetic flow of electrolyte solutions in charged nanocapillaries

A systematic theoretical study of thermoelectric effect and temperature-gradient-driven electrokinetic flow of electrolyte solutions in charged nanocapillaries is presented. The study is based on a semianalytical model developed by simultaneously solving the nonisothermal Poisson-Nernst-Planck-Navier-Stokes equations with the lubrication theory. Particularly, this paper clarifies the interplay and relative importance of the thermoelectric mechanisms due to (a) the convective transport of ions caused by the fluid flow, (b) the dependence of ion electrophoretic mobility on temperature, (c) the difference in the intrinsic Soret coefficients of cation and anion. Additionally, synergy conditions for the three thermoelectric mechanisms to fully cooperate are proposed for thermo-phobic/philic electrolytes. The temperature-gradient-driven electrokinetic flow is shown to be a nearly unidirectional flow whose axial velocity profiles vary with the axial location. Also, the flow can be regarded as a consequence of the counteraction or cooperation between a thermoelectric-field-driven electroosmotic flow and a thermo-osmotic flow driven by the osmotic pressure gradient and dielectric body force. Moreover, the Seebeck coefficient and the fluid average velocity are demonstrated to be affected by electrolyte-related parameters. The results are beneficial for understanding the temperature-gradient-driven electrokinetic transport in nanocapillaries and also serve as theoretical foundation for the design of low-grade waste heat recovery devices and thermoosmotic pumps.

Multi-objective optimization of a novel reversible High-Temperature Heat Pump-Organic Rankine Cycle (HTHP-ORC) for industrial low-grade waste heat recovery

Nowadays, a high amount of industrial thermal energy is still lost due to the lack of competitive solutions for energy revalorization. Facing this challenge, this paper presents a novel technology, based on a reversible High-Temperature Heat Pump (HTHP) and Organic Rankine Cycle (ORC). The proposed system recovers low-grade waste heat to generate electricity or useful heat in accordance with consumer demand. Compressor and expander semi-empirical models have been considered for the reversible system computational simulation, being HFC-245fa the working fluid selected. The built-in volume ratio and Internal Heat Exchanger (IHX) effectiveness have been optimized to reach the maximum energy efficiency in each operating condition. Although HFC-245fa exhibits energy performance attributes, its high Global Warming Potential (GWP) is an issue for climate change mitigation. Hence, multi-objective optimisation of the environmentally friendly working fluids Butane, Pentane, HFO-1336mzz(Z), R-514A, HCFO-1233zd(E) and HCFO-1224yd(Z) has been carried out. The results show that the system proposed, working with HFC-245fa, achieves a Coefficient of Performance (COP) of 2.44 for condensing temperature of 140 °C, operating in HTHP mode, whereas the ORC mode provides a net electrical efficiency of 8.7% at condensing temperature of 40 °C. Besides, HCFO-1233zd(E) and HCFO-1224yd(Z) are both appropriate alternatives for the HFC-245fa replacement. These working fluids provide a COP improvement of 9.7% and 5.8% and electrical net efficiency improvement of 2.1% and 0.8%, respectively, compared to HFC-245fa. This paper provides a reference study for further designs and developments of reversible HTHP-ORC systems used for industrial low-grade waste heat recovery.

Enhanced performances of ORC-based units for low grade waste heat recovery via evaporator layout optimization

The full exploitation of the upper thermal source is the key for enhanced energy performances of ORC-based units for medium and low-grade waste heat recovery. The adoption of a dual evaporation pressure cycle layout has the potential to reduce the heat exchange irreversibility at the evaporation section and to assure a higher net power available at the expander shaft, particularly in small scale units and in presence of upper thermal sources with a highly variable heat release characteristic. The adoption of the dual evaporation pressure technology to small scale recovery units represents a major technological breakthrough and an element of novelty, observing that, at present, the possibility to split the evaporation process in multiple pressure levels is considered mostly with reference to steam generators and boilers. The study investigates the potential energy and exergy advantage of a dual pressure heat recovery vapor generator, with respect to a base-single evaporation pressure layout, for a recovery unit with a mechanical power in the 1–15 kW range, for stationary (100 °C–150 °C hot source temperature) and on-board (350 °C–300 °C hot source temperature) applications. A dedicated optimization procedure allows the maximization of either the net power recovered or the cycle energy efficiency, dependently on the final application of the unit. The exergy efficiency of the heat recovery vapor generator is assessed and its dependence on the fluid characteristics and the main cycle variables discussed, along with the relationship between the energy and exergy gain for the enhanced heat exchange. A preliminary economic analysis provides a first indication of the financial merit of the dual evaporation pressure layout with respect to a base single evaporation pressure configuration.