Waste Heat Recovery Technologies Research Articles

Optimally integrated waste heat recovery through combined emerging thermal technologies: Modelling, optimization and assessment for onboard multi-energy systems

This paper investigates waste heat (WH) valorisation on maritime vessels from a systemic perspective. It aims to demonstrate how a set of innovative waste heat recovery (WHR) technologies can synergistically work together to valorise waste heat in multiple ways. It proposes and develops a dedicated techno-economic analysis framework and model, surpassing previous literature that focused on individual technologies or specific combinations. Employing a Mixed Integer Linear Programming (MILP) approach, this study evaluates the dynamic and flexible operation of the WHR system throughout the round-trip journey of a vessel. Identifying the most profitable WHR system layout, determining the capacity of technologies, optimising the interconnections between technologies, and establishing strategic WH dispatching are all among the key objectives of the study.An average-scale vessel with a 36 MW diesel engine, is selected for this study which involves a 17-day journey with multiple stops in Northern Europe. The vessel adequately represents the complexity of onboard energy systems in terms of types and variability of demands, as well as WH availability. The proposed WHR system, which is tailored for the selected vessel, integrates three active technologies: an isobaric expansion engine (IEE) to contribute to mechanical power demand, a sorption system for providing cooling, and an advanced Organic Rankine Cycle (ORC) for trigeneration of power, heating, and cooling. All technologies are supported by the passive WHR technology of Thermal Energy Storage (TES). The results show that deployment of the optimised WHR system onboard the selected vessel enhances energy efficiency by 5 to 7.5 percentage points and reduces fuel consumption by 13%. The study also explores economic key performance indicators (KPIs), such as the Internal Rate of Return (IRR) which is found at about 15%, clearly evidencing a compelling solution to ship owners. Additionally, the discussion includes a detailed analysis of the contribution of individual technologies in covering onboard demands, as well as their synergistic interactions.This work clarifies the role, value, and benefit of WHR technologies onboard, advancing the understanding from individual WH recovery interventions to a system-level approach. This is especially valuable in practice, considering its adaptability across various vessel types within the global fleet.

Thermo-economic feasibility study to utilize ORC technology for waste heat recovery from Indian nuclear power plants

A feasibility study is essential before installing a power generation system to recover waste heat from nuclear power plants, but not yet been done. Hence, nine possible waste heat locations of Indian nuclear power plants are identified and two feasible cases (case-1 and case-2) are deduced to use the organic Rankine cycle (ORC) as a bottoming cycle. Energy, exergy, economic and environmental performances are evaluated for feasible cases with three working fluids. Capital investment in alternator has been correlated based on market data and applied in economic analysis. The economic study is based on Net present value, Discounted payback period, Levelized cost of energy, Internal rate of return, Profit and per unit build-up cost of ORC to suggest whether the installation is worth to venture or not. The result shows that below 41 °C condenser temperature, case-2 based bottoming ORC should be the priority to install and at the availability of sufficient funds, both cases can be installed, which will cumulatively produce 486.2 kW of electricity from waste energy with a total of 201.14 thousand USD annual profit from the total capital investment of 723.74 thousand USD. In 40 years of the project's lifetime, a total of 1.317 million USD will be used if both cases are employed and the invested cost will be returned in 8.3 years.

CCUS-assisted electricity-chemical polygeneration system for decarburizing coal-fired power plant: Process integration and performance assessment

In the context of global carbon neutrality, carbon capture and storage (CCS) technology has become an important transition pathway to decarbonizing coal-fired power plants (CFPP). However, CCS technology has strict requirements for geological storage and the potential risk of CO2 leakage. Meanwhile, the deployment of the CCS technology in the CFPP could drastically reduce plant efficiency. To address the aforementioned issues, a novel carbon capture, utilization and storage (CCUS)-assisted electricity-chemical polygeneration process (i.e., ECPP) was proposed to produce electricity, liquid fuels, and high-calorie synthetic natural gas simultaneously. To reduce the efficiency loss caused by the consumption of internal steam and electricity, heat integration and Organic Rankine Cycle (ORC) technologies were adopted to fully recover the available waste heat and achieve the cascade utilization of the internal energy. Meanwhile, a detailed techno-economic assessment was conducted to further determine the benefits of the process integration. The results indicated that the application of heat integration and ORC technologies improves plant efficiency by 6 %. Moreover, it also reduces the cost of electricity and CO2 conversion cost by 17 and 15 %, respectively. In addition, compared with the traditional CFPPs retrofitted with CCS technology, the CO2 utilization and waste heat recovery technologies in ECPP enhance net electricity output and plant efficiency by 19 and 34 %, respectively. Therefore, the proposed CCUS-assisted ECPP achieves the efficient utilization of the waste CO2, and reduces the efficiency penalty for retrofitting the CFPPs. Overall, the proposed ECPP is an essential alternative for the retrofit of the existing CFPPs, and provides a feasible strategy for establishing a clean and sustainable power polygeneration system.

Comparative performance analysis of different hydrogen power composite cycles with waste heat recovery

ABSTRACT Hydrogen energy has been under close scrutiny by countries around the world. At present, the main technologies for utilizing hydrogen energy in transport power systems are hydrogen internal combustion engine (HICE) and proton exchange membrane fuel cell (PEMFC). Since both of them generate a large amount of waste heat, and waste heat recovery technology is one of the most promising means of utilizing waste heat to improve efficiency, this paper makes a comprehensive comparison of the two technologies in composite waste heat recovery (WHR) system in terms of modeled maximum efficiency, full operating condition efficiency and future predicted maximum efficiency. Based on the thermal efficiency analysis, the advantages, disadvantages, and development prospects of the two technologies in composite the WHR system are reevaluated. The results demonstrate that the efficiency of the HICE and PEMFC can be improved by 6.83% and 2.81%, with the use of WHR system, respectively. Under the full range of operating conditions, the HICE-WHR composite system has obvious efficiency advantage in middle-high load working condition, but PEMFC-WHR composite system has obvious efficiency advantage in low load working condition. Both of them will be very competitive and potential technology routes of hydrogen energy utilization for land transportation.

Research on Boiler Energy Saving Technology Based on Internet of Things Data

This paper proposes a heat demand prediction model that analyzes the heat behavior of heat users, and analyzes the heat behavior data of heat users through a clustering algorithm to predict their heat demand. This paper is based on the supply-demand balance strategy to reduce the heat loss during the transmission of heat medium, and then improve the energy-saving efficiency of the boiler. The traditional boiler energy-saving and consumption reduction method is to optimize the boiler combustion parameters, improve the fuel combustion efficiency and waste heat recovery technology through the Internet of Things and big data technology. The method of balancing the heat-using end with the load of the boiler at low usage frequency is seldom considered. Therefore, this paper predicts the heat demand of the heat-using end by analyzing its thermal behavior, and balances the heat demand and boiler heat supply under the condition of meeting the heat demand. Finally, through simulation experiments, the validity of the model is verified, and the trend and data can be well predicted in the short-term heat demand prediction.

The Development and Research of Waste Heat Recovery Technology

In the context of advancing societal and technological landscapes, the global escalation in energy demand has underscored the looming threat of dwindling energy resources. Over recent years, China has consistently showcased its commitment to assuming the role of a responsible major nation by actively advocating for green and low-carbon development. This commitment is tangibly reflected in a series of green initiatives that progressively prioritize and incentivize enterprises to partake in the recovery and utilization of waste heat. This paper offers a comprehensive review of extensively researched waste heat recovery technologies within China, elucidating their operational principles and highlighting contributions made by relevant scholars. Currently, waste heat utilization stands as a focal point for intensive research, with simultaneous benefits including heightened energy efficiency, resource conservation, and the mitigation of thermal energy wastage, thereby addressing environmental concerns. The implications of such advancements extend significantly to the enhancement of China's societal, ecological, and economic spheres.

Efficient harvesting of renewable evaporative energy from atmospheric air through hierarchical nano/microscale shaping of air-water interface

Renewable evaporative energy from atmospheric air is a recently emerged research field that demonstrates the great potential for significant energy savings in the air conditioning of buildings, data/big-data/integrated-big-data centers, agricultural storage facilities, and greenhouses. Here, we develop a method for an essential increase in the harvesting efficiency of renewable air evaporative energy by enhancing the water evaporation rate through the shaping of the air-water interface (AWI) by the menisci formed in superhydrophilic hierarchical surface nano/microstructures. Using this method, we achieve a 10-fold rise in the evaporation rate for the microscale AWI shaping. For the first time, we discover the superevaporation effect that provides an unprecedented 130-fold rise in the evaporation rate at the hierarchical nano/microscale AWI shaping. Furthermore, based on these findings, for the first time, we elaborate a novel material with an engineered AWI for a significant increase in the harvesting of renewable air evaporative energy in the dew point (Maisotsenko cycle) indirect evaporative air conditioning systems whose operation is fundamentally based on utilizing renewable air evaporative energy. The dew point cooling device fabricated using this novel material shows significantly superior cooling performance, validating the substantially enhanced harvesting of the air's evaporative energy. Our findings can essentially advance the emerging field of renewable air evaporative energy and, besides the air conditioning technology, can provide significant efficiency enhancements in water purification/desalination, thermal management, gas turbine power generation, and waste heat recovery technologies.

Research on the coal saving and emission reduction potential of advanced technologies in China's iron and steel industry

The coal-based energy consumption structure has led to high coal consumption and carbon emissions in China's iron and steel industry, and the task of coal saving and carbon reduction in the steel industry is difficult and urgent. The economic viability of various coal-saving and emission-reduction technologies, the extent of their coal-saving and emission-reduction potential, and the guidance on the adoption of advanced technologies are crucial issues that need to be addressed in the Chinese steel industry. This article employs the Energy-Production (E-P) analysis method and the Conservation Supply Curve (CSC) model incorporating coal consumption ratio coefficients to systematically analyze coal consumption and carbon dioxide emissions in the overall iron and steel industry and its sub-processes under different scenarios. It calculates the coal-saving emission reduction costs of 32 technologies, evaluates them comprehensively based on their coal-saving and emission-reduction potentials and cost-effectiveness, and categorizes them accordingly. According to the cost of coal saving and emission reduction as the demarcation point, a total of 19 technologies are cost-effective; according to the coal-saving emission reduction potential and cost division, 32 technologies are divided into 6 categories, among which 6 technologies such as sintering waste heat recovery technology can be used as key technologies for popularization and application. Seven technologies, such as efficient preheating technology of ladle, have high cost and low potential, which can be gradually eliminated or improved and upgraded. After the comprehensive promotion of the 32 technologies, the maximum coal saving potential is about 3.453 billion GJ, the CO2 emission reduction potential is 395 million tons, the total coal consumption of the steel industry can be reduced to about 13.904 billion GJ, and the carbon dioxide emissions can be reduced to 1.179 billion tons.

Complementary waste heat utilization from data center to ecological farm: A technical, economic and environmental perspective

The waste heat recovery technology of data center has attracted the attentions for energy conservation and emission reduction in recent years. This paper presents a new scheme which uses heat recovery technology to link the data center and the agricultural sector. The detailed system design, technical validation, and environmental and economic analysis show that the project can fully absorb a large amount of waste heat in the data center, and use the air source heat pump (ASHP) to transport heat to an ecological farm, reducing the consumption of electric energy and promoting the development of agriculture and animal husbandry. In a case study of China Brain Green Data Center, the dynamic payback period is about 3.4 years, and the cumulative net present value will reach $666 k. In terms of environmental impact, the proposed ASHP-based complementary utilization scheme can save 676 MWh of electricity for the data center and 88 MWh of electricity for the ecological farm in each year, which is equivalent to reducing coal by 230 tons and reducing CO2 emissions by 168 tons. Therefore, from the perspective of economic and environmental benefits, this new complementary waste heat utilization technology realizes a win-win development.

A three-heat source segmented heating control strategy based on waste heat recovery technology for electric vehicles

The control strategy of thermal management systems is crucial for electric vehicles to ensure thermal comfort of the cabin and thermal safety of the battery and motor. However, this will also impact the energy consumption of the vehicle. In this study, a three-heat source segmented heating control strategy was proposed to reduce the heating energy consumption of electric vehicles, with the three sources denoted motor waste heat, air, and positive temperature coefficient (PTC) heaters. Specifically, a simulation model of an integrated thermal management system for heat pump air conditioning, motor thermal management, and battery thermal management systems was first established and validated using published experimental results. Subsequently, the performance of three cabin heating modes was investigated based on the standard driving cycle. The optimal opening and closing strategies for each mode were discussed to ensure the temperature requirements of the cabin, and the optimal method for using motor waste heat and the PTC heater to heat the battery at low temperatures was explored. The segmented heating control strategy was developed by determining the heating priority for the battery and cabin, as well as the heating capacity of heating modes using air source. Compared to the traditional heating strategy of the cabin and battery, the proposed control strategy could reduce energy consumption by 18.49 % while running at an ambient temperature of − 10 °C for 2.5 h.

Waste heat recovery integration options for commercial bakeries in a thermo-economic-environmental perspective

In commercial bakeries, a substantial amount of heat is exhausted which is not only a waste of useful resource, but also contributes to higher fuel consumption and carbon emissions, if not recovered. In this study, waste heat from a single oven is considered and five potential heat recovery options are investigated in a techno-economic-environmental perspective to provide essential results for integrating an appropriate technology for waste heat recovery in the commercial bakeries sector. Waste heat recovery options were selected considering the temperature profile, the waste heat source, quality and quantity of heat and the heat energy demand for the various processes in commercial bakeries. Thermodynamic, economic, and environmental models are developed to assess the heat recovery performance, cost savings and emission reduction at both design and off-design conditions. Results show that up to 286 kW of waste heat can be recovered and reused in the case of air pre-heater, which can save up to 161.93 t/year of natural gas and an equivalent cost and emission savings of $ 93,594/year and 412.5 tCO2e/year, respectively. Moreover, the earliest payback period of 0.77 years was estimated for the air pre-heater option with an estimated capital investment cost of $71,631, whereas a maximum payback period of 4.59 years was estimated for the electricity generation by the organic Rankine cycle having an estimated capital investment cost of $304,040. These results reveal that air preheating is the most energy-efficient and cost-effective option to recover the waste heat from the ovens in the bakery industry.

Waste heat upgrading with high-temperature heat pumps for assisting steam generation in ships: Performance, cost and emissions benefits

An alternative waste heat recovery technology is examined based on a high-temperature heat pump (HTHP) for heat upgrading in ships. This solution exploits the low-temperature heat of the cooling water of marine engines at 85 °C with two integration options, for either preheating the water before entering the auxiliary (steam) boiler or directly generating 6-bar steam for a vessel’s needs. An electric-driven heat pump cycle is used with an economizer for increasing the Coefficient of Performance (COP) and the heating capacity, while reducing the discharge temperature of the compressor. An ultra-low GWP refrigerant (R1233zd(E)) with a high enough critical temperature is considered. The sizing parameters of the HTHP are examined with the use of a validated numerical model attempting to find the best match between high performance and short discounted payback period (PBP). The latter is estimated with appropriate cost correlations that relate the component sizing with the equipment cost, in order to calculate the total capital cost. The operating costs on the other hand depend on the net fuel savings, i.e. fuel savings minus the fuel required to generate the electricity for the heat pump, the fixed operating cost such as for maintenance and service and the fuel price. The net emissions savings of the HTHP are also examined in a variety of ship types and sizes, based on their typical specific factors. The results show that the proposed solution leads to significant net fuel savings especially in oil tankers and cruise ships, reducing the boiler’s fuel consumption by up to 2.5%. But due to the low COP of the steam generation HTHP case of around 2, a very long PBP is achieved of over 20–30 years, whereas the preheat case always leads to a very short PBP of 2–6 years in all ship types, even though its capacity is low. Moreover, the net total emissions savings are significant especially for the preheat case, although additional fuel is consumed for producing electricity to drive the HTHP, while NOx and CO emissions are increased with the steam generation HTHP.

A novel optimization method for transcritical-subcritical parallel organic Rankine cycle under variable engine loads and cooling water temperature

Organic Rankine cycle (ORC) is an effective technology for engine waste heat recovery. However, when engine loads and cooling water temperature are variable, the optimization of ORC system will be difficult and time-consuming, especially for complex system like transcritical two-stage ORC. In this study, a novel optimization method is proposed for optimizing the transcritical-subcritical parallel ORC (TSPORC) system, and the system operates under variable engine loads and cooling water temperature. The key idea of the optimization method is a combination of artificial neural network (ANN) and non-dominated sorting genetic algorithm (NSGA-II), the ANN is used to establish a fast and accurate prediction model of the system performance to reduce computing time significantly, and the NSGA-II is furtherly used to optimize system performance fast and accurately based on the ANN prediction model. To improve the accuracy of ANN model, three different artificial neural network models are optimized and compared. To satisfy different objective optimization for application, both single-objective and multi-objective optimization are conducted. The results demonstrate that the optimization method can achieve real time optimization for TSPORC system operating under variable engine loads and cooling water temperature fast and accurately.

Improvement design and performance assessment of two supercritical carbon dioxide power cycles for waste heat recovery

Waste heat recovery (WHR) from thermal engines and industrial processes is significantly beneficial to cutting fossil fuel consumptions and carbon dioxide emissions. Recompression supercritical carbon dioxide (sCO2) power cycles have many advantages of high efficiency, compact design and environmentally friendly property, and they are regarded as the competitive and promising energy conversion technology for WHR. However, a large space is still left for recompression sCO2 power cycle to improve its capability with regard to waste heat extraction and power output. The novelty of this work is the development of two novel sCO2 power cycles based on the recompression sCO2 cycle aimed at thoroughly and efficiently reusing the waste heat. Thermodynamic and exergoeconomic models are established to perform the quantitative parametric analyses, multi-objective optimizations, exergy and exergoeconomic analyses on the proposed systems. The results indicate that the second proposed system is preferred, since it can significantly improve the net power output by 43.81% and 12.32% while reducing the total product unit cost by 7.24% and 10.39%, compared with the traditional recompression sCO2 cycle and the improved sCO2 cycle, respectively. Although the first proposed system has a better performance in waste heat extraction and net power output, it requires a higher cost.

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

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

Experimental study of boiling heat transfer characteristics of forced flow in annular tube under negative pressure

In order to investigate the recycling technology for efficiently recovering low-temperature wastewater waste heat from industrial production, we built an experimental device for forced boiling heat transfer in an annular tube. This paper takes a ring casing evaporator with an inner tube diameter of 30 mm, a length of 1450 mm, a wall thickness of 0.8 mm, and an outer tube diameter of 14.2 mm, which was used as the main object of study. We conducted experimental research on heat transfer in single-phase liquid flow and two-phase boiling phase change within annular tubes by varying the inlet circulating hot water temperature (60–85 °C), the mass flow rate of the circulating mass (0.85, 1.27, 1.69 kg m−2 s−1), and the inclination angle (0°–90°). The research results indicate that the heat transfer coefficient of boiling phase change within annular sleeves is significantly enhanced compared to convective heat transfer of single-phase liquid flow within annular tubes. This signifies that employing heat pipe technology for waste heat recovery can achieve highly efficient heat transfer processes. This study also reveals that with an increase in the inclination angle, the heat transfer coefficient on the boiling surface follows a trend of initial increase followed by a decrease. Near an inclination angle of 30° within the annular jacket tube, the heat transfer coefficient of two-phase boiling phase change reaches its maximum value. The findings of this study provide certain theoretical guidance for the application of heat pipe energy-saving technology in the field of medium- and low-temperature waste water recovery and utilization.

Study on Heat Transfer Performance and Parameter Improvement of Gravity-Assisted Heat Pipe Heat Transfer Unit for Waste Heat Recovery from Mine Return Air

One of the effective methods for energy conservation and emission reduction in coal mines is to utilize waste heat recovery technology to recover mine return air waste heat. The gravity heat pipe is widely used in mine return air waste heat recovery due to its sustainable and economic advantages, but its heat transfer is a complex process influenced by multiple parameters. A single-tube heat transfer resistance model and a heat transfer calculation model based on enthalpy difference were established for the heat exchange tubes. Four typical application cases of a low flow rate and a low number of tube rows were selected, and their heat transfer characteristics were tested onsite and analyzed. It was found that there were problems such as a low overall heat transfer efficiency, a low fresh air outlet temperature, and a risk of icing in the final tube section. The effects of the gravity heat pipe parameters on the heat transfer performance were studied, such as the tube outer diameter, tube spacing, and the finned tube outer diameter. It was found that the air-resistant force of the heat exchanger increased with the increase of the tube spacing and the finned tube outer diameter, the heat transfer resistance increased with the increase of the tube spacing and the decrease of the finned tube outer diameter, and the heat transfer coefficient first increased and then decreased with the increase of the tube outer diameter. A configuration improvement scheme with a high flow rate and a high number of tube rows is proposed here. Taking Case 2 as an example, the temperature distribution of the heat tube before and after improvement is compared and analyzed. The results show that the heat transfer performance of the heat exchange system significantly improved. Without increasing the air resistance of the heat tube, the temperature of the return air outlet after improvement was reduced to 1.1 °C, 4.1 °C lower than that before improvement, further recovering the waste heat of the mine return air. The temperature of the condensate water film was greater than 0.5 °C, avoiding the icing problem of the condensate tube section, the fresh air outlet temperature reached 5.2 °C, an increase of 7.8 °C compared to that before improvement, and the overall heat transfer efficiency increased from 56.7% to 66%.

Energy Efficiency and Environmental Benefits of Waste Heat Recovery Technologies in Fishmeal Production Plants: A Case Study in Vietnam

The fishmeal production industry is essential for providing protein for animal feed in the aquaculture sector. However, the industry faces challenges related to energy consumption and environmental sustainability. This study evaluates the energy efficiency and environmental benefits of waste heat recovery (WHR) technologies in a fishmeal production plant in Vietnam. Data were collected from the plant between 2016 and 2022, and a specific energy consumption (SEC) indicator and a comprehensive methodology were utilized. Implementing an economizer as a WHR technology resulted in a 55.5% decrease in SEC compared to the state before installation. The enhanced energy efficiency also translated to reduced energy consumption per output unit. Moreover, the economizer contributed to annual energy savings of 4537.57 GJ/year and cost savings of USD 26,474.49. Additionally, carbon dioxide (CO2) emissions associated with producing one ton of fishmeal decreased by 58.37%. These findings highlight the potential for WHR technologies to improve energy efficiency and reduce the environmental footprint of fishmeal production. The study’s results provide valuable insights for practitioners and policymakers in promoting energy efficiency practices and reducing environmental impact in this and similar industries.

High-performance thermomagnetic generator controlled by a magnetocaloric switch

Low grade waste heat accounts for ~65% of total waste heat, but conventional waste heat recovery technology exhibits low conversion efficiency for low grade waste heat recovery. Hence, we designed a thermomagnetic generator for such applications. Unlike its usual role as the coil core or big magnetic yoke in previous works, here the magnetocaloric material acts as a switch that controls the magnetic circuit. This makes it not only have the advantage of flux reversal of the pretzel-like topology, but also present a simpler design, lower magnetic stray field, and higher performance by using less magnetocaloric material than preceding devices. The effects of key structural and system parameters were studied through a combination of experiments and finite element simulations. The optimized max power density PDmax produced by our device is significantly higher than those of other existing active thermomagnetic, thermo, and pyroelectric generators. Such high performance shows the effectiveness of our topology design of magnetic circuit with magnetocaloric switch.

Summary of Turbocharging as a Waste Heat Recovery System for a Variable Altitude Internal Combustion Engine.

The increase in altitude causes the decrease in internal combustion engine power and the increase in pollutant emission. Converting waste heat into more useful forms of energy through the recovery of waste heat from internal combustion engines is the most promising mechanism for improving both of these goals. This paper comprehensively reviews the development and research of waste heat recovery technology of an internal combustion engine in a variable altitude environment. It is found that exhaust gas turbocharging is the most promising waste heat recovery technology to restore high-altitude internal combustion engine power. Turbochargers are affected by low temperature and low pressure at high altitudes, resulting in poor environmental adaptability, inadequate supercharging ratios, and decreased supercharging efficiency. Therefore, it is very important to select the high pressurization system facing the plateau area and its reasonable matching characteristics. The quality of exhaust energy determines how much waste heat a turbine can recover, and only the exergy part of exhaust energy can realize heat/work conversion. The main disadvantage of turbocharging technology applied in the plateau area is that the speed ratio deviates from the design value, leading to the increase of flow loss inside the supercharger. Therefore, optimizing the internal flow field of a high-altitude supercharger is a key problem to improve the efficiency of energy recovery. The conclusion drawn from this Review is that a two-stage turbocharging system will be a key technology to improve the thermal efficiency and reduce the fuel consumption of high-altitude internal combustion engines in the coming decades. In addition, the efficient utilization of the exhaust energy of the two-stage turbine and the influence of the variable compression process of the two-stage compressor on the working medium in the cylinder will be the focus of future research.