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In the present study, using a combination of theoretical discussions, practical examples, and case studies, we sought to gain a comprehensive understanding of how numerical solutions could be used to improve the design and optimize the thermal efficiency of a heat exchanger that utilizes wastewater to reduce the domestic consumption of hot water. To this end, we developed a validated numerical model. We also carried out simulations and experiments, the results of which are presented in this paper. The novelty of this work derives from our use of a new heat exchanger design for a domestic shower, and from the presented experimental–numerical evidence that proves its efficiency. We found that use of our newly designed appliance improved thermal efficiency from 14% to 27%. Moreover, we estimated that the cost of manufacturing and installing such a device did not exceed that of a widely available drain grid. Using our newly designed exchanger, a family of four living in Poland could save EUR 38 (at 2022 values) and reduce CO2 emissions by 192 kg. An average European family could save EUR 68 and reduce CO2 emissions by 76 kg. [ABSTRACT FROM AUTHOR]

To achieve carbon neutrality and address global energy supply issues by 2050, there is active progress in the industrial sector for waste energy recovery and commercialization projects. It is necessary to consider both the energy recovery efficiency and economic feasibility based on the production volume for the resource utilization of waste energy, along with eco-friendly processing methods. In this study, a waste heat recovery system was designed to recover a large amount of thermal energy from high-temperature exhaust gases of gas engines for power generation by using biogas produced from organic waste in industrial complexes. Types and sizes of components for a waste heat recovery system that were suitable for various engine sizes depending on biogas production were designed, and the energy recovery efficiency was analyzed. The waste heat recovery system consisted of a smoke tube boiler that generated superheated steam at 161 °C under 490 kPa of pressure from the exhaust gas as the heat source, along with two economizers for heating both supply water and hot water. Heat exchangers that were suitable for three different engine sizes were configured, and their performance and energy flow were calculated. In particular, when operating two engines with a power output of 100 kW, the boiler showed the highest steam production efficiency, and the superheated steam production from high-temperature exhaust gas at 600 °C was designed to be 191 kg/h, while hot water at 58 °C was designed to be produced at 1000 kg/h. In addition, further research on the heat exchanger capacity ratio confirmed that it was within a certain range despite the difference in heat exchanger capacity and efficiency depending on the engine size. It was confirmed that the heat exchange capacity ratio of the boiler was important as an optimal-capacity design value for the entire system, as it ranged from 46% to 47% of the total heat exchanger size. [ABSTRACT FROM AUTHOR]

A study of heat recovery in a façade ventilation unit was carried out under laboratory conditions using a climate chamber that allowed stable outdoor and indoor conditions to be simulated. The unit, equipped with a reversible fan and a chamber for the heat exchanger, controlled by an automation control system, was designed to exchange air in the room by alternating supply and exhaust cycles of specific durations. Three types of heat exchangers were tested, which were filled with different phase change materials, in order to estimate the efficiency of the façade ventilation unit in terms of its heat recovery capability. The efficiency of the unit was determined based on the temperature efficiency of heat recovery for 144 setting combinations. The best efficiency results between 73.56% and 76.29% were obtained with a solution using a heat exchanger consisting of cylinders with an external diameter of 10 mm and a wall thickness of 1 mm filled with jojoba oil in a one minute cycle. The tests confirmed that the heat exchangers, which are part of the façade ventilation unit, fulfil their function and allow heat recovery from the exhaust air to pre-heat the supplied air. The study complements the existing scientific knowledge on the efficiency of heat exchangers filled with phase change material, operating in winter conditions with work cycles up to 5 min. [ABSTRACT FROM AUTHOR]

The technique of air sterilization by thermal effect was revisited in this work. The impact of incorporating a high efficiency heat recovery exchanger to a sterilizing cell was especially assessed. A mathematical model was developed to study the dynamics and the steady state of the sterilizer. Computer simulation and reported data of thermal inactivation of pathogens permitted obtaining results for a proof-of-concept. The simulation results confirmed that the incorporation of a heat recovery exchanger permits saving more than 90% of the energy needed to heat the air to the temperature necessary for sterilization, i.e., sterilization without heat recovery consumes 10–20 times the energy of the same sterilization device with heat recovery. Sanitization temperature is the main process variable for sanitization, a fact related to the activated nature of the thermal inactivation of viruses and bacteria. Heat recovery efficiency was a strong function of the heat transfer parameters but also rather insensitive to the cell temperature. The heat transfer area determined the maximum capacity of the sterilizer (maximum air flowrate) given the restrictions of minimum sanitization efficiency and maximum power consumption. The proposed thermal sterilization device has important advantages of robustness and simplicity over other commercial sterilization devices, needing practically no maintenance and eliminating a big variety of microorganisms to any desired degree. For most pathogens, the inactivation can be total. This result is not affected by the uncertainties in thermal inactivation data, due to the Arrhenius-like dependence of inactivation. Temperature can be adjusted to achieve any removal degree. [ABSTRACT FROM AUTHOR]

• Chemical equilibrium was introduced and a novel two-stage in-cylinder steam thermodynamic model was proposed. • A theoretical thermodynamic model considering in-cylinder steam assist and exhaust gases waste heat recovery was established. • The effects of the different working fluids on the in-cylinder steam assist cycle were revealed. • The effects of in-cylinder steam assist on thermal efficiency optimization under different working fluids were evaluated. Improving efficiency and reducing emissions are essential to achieving carbon peaking and carbon neutrality in transportation sector. As a carbon neutral fuel, hydrogen is widely considered the most promising fuel for future zero-carbon internal combustion power, while iso -octane, represented by gasoline, symbolizes the current hydrocarbon fuel utilized in modern internal combustion engines. In this study, the maximum indicated thermal efficiency of both hydrogen and iso -octane utilizing air (blending O 2 and N 2), oxy-fuel (blending O 2 and CO 2) and argon power cycle (blending O 2 and Ar) were investigated theoretically based on in-cylinder steam assist cycle. A theoretical thermodynamic model combining in-cylinder steam assist and exhaust gases waste heat recovery was established to explore the thermodynamic process of in-cylinder steam assist and systematically analyze the effects of different working fluids. Results show that the utilization of in-cylinder steam assist improves thermal efficiency under O 2 , N 2, and CO 2 working fluids, but limits the improvement of thermal efficiency under Ar. In addition, the maximum achievable steam temperature and mass are limited by the exhaust gases temperature and heat exchanger efficiency. When running with H 2 under 400 K steam temperature, the optimized water-to-gas ratios were 1.58, 1.13, 1.62, and 1.13 for O 2 , Ar, N 2 , and CO 2 , respectively, the maximum thermal efficiencies increased by 35.80 %, 0.28 %, 28.21 %, and 62.79 % compared with baseline condition. Meanwhile, the competition of overall specific heat ratio and working fluid mass under different working fluids leads to different thermal efficiency enhancement, such as Ar, the decrement of overall specific heat ratio under limited steam mass leads to thermal efficiency deterioration, which can be further improved as steam mass increased. [ABSTRACT FROM AUTHOR]

Improving the energy efficiency in heat exchanger networks (HENs) remains a significant industrial problem, specifically in energy-intensive operations. A particular method for such an objective is the modification of HENs at the equipment-use level, where structural changes take place and units within the network are moved, replaced and/or removed. This practice is usually known as retrofit. The objective of a retrofit is to maximize the heat recovery using the minimum modifications possible and minimum retrofit cost. Traditional retrofit techniques would normally consider one type of heat exchanger (based on the original network) with no additional design features (i.e., heat transfer enhancement technologies). The expansion of such alternatives is limited by practical use and availability of theoretical methods. In this context, the inclusion of high-performance heat exchangers such as plate heat exchangers (PHEs) has not been widely explored, even when their design and operational advantages are known. In this work, a new step-by-step automated HENs retrofit approach based on Pinch Analysis is proposed. The approach is possible to identify the best modification, its location within the network, and its cost simultaneously. Moreover, to increase energy savings, this work presents a strategy that seeks to utilize high efficiency heat exchangers such as plate heat exchangers for retrofit. A distinctive feature of this new method is the ability to handle different minimum approach temperatures, given the different types of exchangers, within the optimization of HENs. Three cases are studied using this methodology to quantify the potential benefits of including PHEs in HEN retrofits, via the analysis of the retrofit cost. Results are compared with a baseline consisting in the same network, where only Shell-and-Tube-Heat-Exchangers (STHXs) are used. In addition, the results demonstrate that this methodology is flexible enough to be applied in a wide range of retrofit problems. [ABSTRACT FROM AUTHOR]

Wastewater has significant potential as a source of clean energy. This energy can be used both within external sewer networks and on the scale of individual residential buildings, and the use of shower heat exchangers appears to be the most reasonable solution. However, in the case of Poland, the problem is still the unwillingness of society to use this type of solution, caused mainly by the lack of space for the installation of vertical drain water heat recovery (DWHR) units and the low efficiency of horizontal units. In response to this issue, the efficiency of a new compact shower heat exchanger designed to be mounted below the shower tray, as well as its linear counterpart, was investigated under various operating conditions. In addition, the financial efficiency of using the compact DWHR unit with average water consumption for showering was evaluated. For this purpose, discount methods were used to estimate the financial efficiency of investments. The study showed that the compact shower heat exchanger has higher efficiency than its linear counterpart. Depending on the temperature of cold water and the flow rate of both media through the heat exchanger, it achieves efficiencies ranging from 22.43% to 31.82%, while the efficiency of the linear DWHR unit did not exceed 23.03% in the study. The financial analysis showed that its use is particularly beneficial when the building uses an electric hot water heater. The investment's sensitivity to changes in the independent variables is small in this case, even with low water consumption per shower. The only exceptions are investment outlays. Therefore, the compact DWHR unit is a clean energy device, which in many cases is financially viable. [ABSTRACT FROM AUTHOR]

In industry, the thermal flow reverse reactor (TFRR) is considered to be an effective means to reduce ventilation air methane (VAM) from fossil fuel operations. The purpose of VAM combustion is either to reduce the emission of greenhouse gas, to recover energy, or both these subjects. The heat exchanger tubes have been embedded on both sides of high-temperature zone of the regenerative oxidizer for heat recovery in this study to improve economy. The mathematical model is established, as well as the effects of inlet methane concentration and air flow rate on dynamic heat extraction characteristics of heat exchanger tubes under periodic conditions are investigated. The results show that sustainable and stable heat recovery can be achieved when the concentration is greater than 0.6 vol.% and the air flow rate is less than 596 m3/h. Heat extraction mainly depends on downstream heat exchange tubes. The asymmetry of heat transfer process between upstream and downstream heat exchanger tubes is analyzed. It is worth noting that the asymmetry is improved at high inlet methane concentration and low air flow rate. The heat recovery efficiency by the bilateral heat exchanger tubes is 61.72% at most, which provides a reliable theoretical basis for the heat extraction mechanism of the heat exchanger embedded in TFRR. [ABSTRACT FROM AUTHOR]

High-efficiency microchannel heat exchangers (MHEs) can remarkably meet the thermal requirements in many areas of technology. Several active and passive techniques can improve the efficiency of heat exchangers; the improvement enables engineering processes to decrease operational costs through waste heat reduction. In the present article, two different passive techniques are applied to improve the heat transfer efficiency of a counter-flow heat exchanger. To do this, four various microchannels with airfoil fins, circular fins, trapezoidal fins, and triangular fins are designed. First, the temperature distribution and effectiveness for each microchannel are calculated and compared with each other. Results of the comparison showed that the optimum structure belongs to the microchannel with trapezoidal fins. Second, BeO ceramic is utilized as the microchannel heat exchanger's material. At a given mass flow rate of 30.5 kg/h, the superior thermal features of BeO lead to more uniform temperature distribution, and also it makes an improvement of 219% in the effectiveness of MHE with trapezoidal fins compared to alumina-made MHE with simple channels. [ABSTRACT FROM AUTHOR]

With the continuous worldwide energy use increase, energy efficiency is gaining high importance. Consequently, many methods have been investigated for potential energy savings. One of these methods is the use of heat recovery systems. These systems basically re-use waste heat and reduce energy consumption. Also, they are increasingly used to reduce heating and cooling demands of buildings. Their main feature is to provide fresh air to the place which is heated by the exhaust air with the help of a heat exchanger (HEX) working between two different temperature sources. The most commonly used types of heat exchangers in ventilation systems are cross-flow and counter-flow heat exchangers. Cross-flow heat exchangers have a thermal efficiency in the range of 50-75% while counter-flow heat exchangers have 75-95%. Many studies have been carried out to increase the efficiency of this type of heat exchangers. In this study, different designs of crossflow and counter-flow exchangers are compared using ANSYS Fluent software. The aim is to determine how the plate surface geometry affects heat transfer and pressure drop. It is aimed to find the optimum design with maximum efficiency, high heat transfer and low pressure drop for heat exchangers. As a result, it has been observed that thermal efficiency increased from 18% to 60% when changing from cross flow to counter flow in flat plate design, while it increased from 25% to 77% in enhanced plate designs. For enhanced designs, counter flow heat exchanger is 52% more efficient than cross flow heat exchanger. Also, improvements to increase the surface area and turbulence in both flow types have increased heat transfer and thermal efficiency. [ABSTRACT FROM AUTHOR]

This article presents experimental analysis of heat removal capabilities of a wavy finned tube heat exchanger operating under natural convection conditions Two types of heat exchangers are considered in this work: standard construction exchangers, most often found in waste heat rejection devices and exchangers based on a novel geometry, which supports intensification of heat transfer processes under natural convection. The aim of the research was to experimentally verify the novel conception and to determine the thermal efficiency of heat exchangers under the influence of selected design parameters. The factors analyzed in the research include: temperature difference between the heat transfer surface and the surrounding air, different angles of the units transferring heat and the height of the exchanger casing. Four variants of the casing height were considered in this work. The conducted research also allowed to determine the influence of the fin spacing on the performance of the heat exchangers under natural convection conditions. The test results obtained show that the novel geometry allows for significance improvement of heat transfer efficiency. The proposed solution allowed to increase the density of the heat flux from 12% in (fin spacing of 2.8 mm) up to 24% (fin spacing of 5.6 mm). It was also observed that the height of the casing has significant impact on the heat transfer efficiency. For each of the analyzed cases, the heat exchanger without a casing was characterized by the lowest thermal efficiency, while the highest heat flux was removed from the exchanger with the highest casing (up to four times higher heat transfer rate). It was determined that the largest amount of heat was removed from exchangers located horizontally, while the exchanger with an angle of 60°, was characterized by the lowest rate of heat removal. Another factor which was determined to have significant impact on the performance of the heat exchangers is the spacing between the fins. The research results obtained shows that increasing the spacing of the finned surface from 2.8 mm to 5.6 mm resulted in an average twofold increase in thermal efficiency in the case of four-row exchangers and an almost threefold increase in the case of two-row exchangers. The obtained results can be used as basic guide for the development of finned tube heat exchangers operating under natural convection conditions. [ABSTRACT FROM AUTHOR]

• A novel air-flue gas total heat exchanger is proposed, which can solve the problem caused by flue gas nonlinearity. • The limit efficiency of the novel total heat exchanger is up to 100 %. • Heat transfer model of the proposed heat exchanger is established by employing CFD simulation. • Efficiency of the novel total heat exchanger is 87 %–92 %. • Thermal performance of the novel heat exchanger has been compared with a double tower system and an enthalpy wheel. Because of high humidity and nonlinearity of flue gas, waste heat from flue gas is hard to recovery. Boilers equipped with vapor-pump system is developed to solve the problem caused by high humidity. In this system, double spray towers subsystem is equipped to realize total heat waste heat recovery. However, caused by nonlinearity, limited waste heat recovery efficiency is just 83 % (1 segment) and 93 % (2 segment). Further, based on boilers equipped with vapor-pump (BEVP) system, enthalpy wheel system is developed to solve the problem caused by nonlinearity. However, enthalpy wheel system cannot solve the problem completely. In this article, a novel air-flue gas total heat exchanger is put forward to achieve full waste heat recovery. In this system, waste heat recovery efficiency limit is up to 100 %. Then, the limit condition of total heat transfer process is discussed. Performance of the total heat exchanger is discussed and compared to double spray towers system and enthalpy wheel system. As the result, considering heat transfer temperature difference, the total heat exchanger total heat transfer efficiency of the total heat exchanger is 7 % higher than 2-segment BEVP system and 10 % higher than enthalpy wheel system. [ABSTRACT FROM AUTHOR]

This paper presents an exergy analysis of marine waste heat recovery CO2 closed-cycle gas turbine system. Based on the operating parameters obtained in system exploitation, it is performed analysis of each system component individually, as well as analysis of the whole observed system. While observing all heat exchangers it is found that combustion gases-CO2 heat exchangers have the lowest exergy destructions and the highest exergy efficiencies (higher than 92%). The lowest exergy efficiency of all heat exchangers is detected in Cooler (51.84%). Observed system is composed of two gas turbines and two compressors. The analysis allows detection of dominant mechanical power producer and the dominant mechanical power consumer. It is also found that the turbines from the observed system have much higher exergy efficiencies in comparison to compressors (exergy efficiency of both turbines is higher than 94%, while exergy efficiency of both compressors did not exceed 87%). The whole observed waste heat recovery system has exergy destruction equal to 6270.73 kW, while the exergy efficiency of the whole system is equal to 64.12% at the selected ambient state. Useful mechanical power produced by the whole system and used for electrical generator drive equals 11204.80 kW. The obtained high exergy efficiency of the whole observed system proves its application on-board ships. [ABSTRACT FROM AUTHOR]

To ensure low-energy consumption in new generation energy-efficient houses, the technology of a ground heat exchanger with a heat recovery system is used almost everywhere. However, this technology has not been widely disseminated in Ukraine. The work is aimed at presenting insights from research on the combination of ground heat exchangers with a heat recovery system for building ventilation by analyzing the operational and techno-economic indicators obtained. Current studies permit revealing the optimal configuration of a ground heat exchanger with a heat recovery system for ventilation in a residential building in order to analyze the efficiency of ground heat exchangers with a heat recovery system for ventilation of a residential building in comparison with several conventional ventilation options to assess the main price/ performance ration of the process of constructing a ground heat exchanger with a heat recovery system and to determine the duration of the technological process, the labor-intensive characteristics, and the estimated cost of the technology. [ABSTRACT FROM AUTHOR]

Tibetan medicine has a long history to serve as a national medicine second only to Chinese medicine. Fresh Tibetan medicine cannot be directly used as medicine. Drying process is essential as a preliminary processing step. The heat pump drying method can be suitable for this case, due to its energy saving, environmental protection, and low cost. In this study, a novel drying system mode of heat recovery and air-jet enthalpy heat pump was proposed to improve the drying characteristics of Tibetan medicine under low-temperature environments. A systematic experiment was conducted to explore the stability and energy efficiency for the continuous operation of system, thereby to obtain the changing characteristics of continuous operation on the heat pump in different load modes at low temperature conditions. In order to explore the performance characteristics of heat pump drying for the Tibetan medicine at low temperature environment in winter, a drying oven was built with the size of 3.9m ×2.1m ×2.3m (length ×width ×height)and a volume of about 15m³, as well as a rated load of 1.5 t. An ordinary ring scroll compressor,and an air jet enthalpy compressor in a heat pump system were selected to carry out the load experiment test at the ambient temperature of -11.4−18.3 and -13.39−16.69°C. The ordinary scroll compressor in a heat pump system worked in the segmented drying mode, and the continuous drying operation mode was used to dry Tibetan medicine. The air-jet enthalpy-increasing heat pump system performedday and night for the continuous drying. Results were as follows: Under the severe cold and large temperature difference (30°C) in the winter plateau, the average efficiencyof heat transfer in the air-jet enthalpy-increasing heat pump system (-13.39−16.69°C) was 9.56%, and that in the ordinary scroll compressor heat pump system (-11.88−18.29°C) was 13.58%. It infers that the efficiency of heat transfer in a heat exchanger was relatively low, as the ambient temperature decreased.The compressor selection has no significant effect on the heat exchange efficiency of heat exchanger. The minimum coefficient of performance (COP) of air-jet enthalpy-increasing heat pump system(-13.39−16.69 ℃) increased by 51.5%, compared with the ordinary scroll heat pump system (-11.4−18.3 ℃). The average value of COP increased by 16.9% during the whole progress, indicating effectively improved the heating energy efficiency of a system at low temperatures. The placement and thickness of amaterial presented a significant influence on the drying effect. When the loading capacity of drying box was about 100% and stacking thickness was 20 cm, the final product behaved bright color, uniform drying, and the high specific moisture extraction rates (SMER). Compared with the traditional air-drying method, the drying time of a heat pump was only 1/9 of conventional dry method for the same amount of materials, while,the drying cost was reduced by 1 160 yuan/t. The recovery and air-jet enthalpy heat pump system can meet the energy supply that required for the drying of Tibetan medicinal materials under the low-temperature conditions. [ABSTRACT FROM AUTHOR]

Tree-shaped fin exhibits an excellent performance in balancing the strong and weak heat flow in latent heat exchangers, which renders higher solidification efficiency and more uniform temperature distribution. Based on the enthalpy-porosity method, a transient model of the solidification heat transfer in the heat exchangers with five tree-shaped fins are established and numerically analysed. The solidification behaviours of the phase change material (PCM), including liquid fraction and temperature evolution, in tree-shaped heat exchanger are investigated and compared with corresponding ones. The results indicate that the tree-shaped fin introduces more uniform solidification front and yield a higher solid fraction. Compared with the traditional longitudinal fins, the efficiency of heat exchanger with four-level tree-shaped fins increases by 53%. In terms of solidification efficiency, the tree-shaped fin configuration with four-level is considered as the optimal fin configuration. The initial temperature difference and the thermal conductivity of fins largely determine the solidification trend of the PCM in the heat exchanger. Compared with the width ratio, the heat exchanger with larger length ratio stores more energy during the phase change process. • Tree-shaped fin is introduced to optimize heat transfer performance of heat exchanger. • The solidification behaviour of PCM in the heat exchanger with five fin configurations are analysed. • The efficiency of heat exchanger with tree-shaped fin increases by 53%. • The tree-shaped fins render the solidification front more uniform and yield a higher solid fraction. [ABSTRACT FROM AUTHOR]

The inverse Brayton cycle is a potential technology for waste heat energy recovery. It consists of three components: one turbine, one heat exchanger, and one compressor. The exhaust gas is further expanded to subatmospheric pressure in the turbine, and then cooled in the heat exchanger, last compressed in the compressor into the atmosphere. The process above is the reverse of the pressurized Brayton cycle. This work has presented the strategy on performance improvement of the inverse Brayton cycle system for energy recovery in turbocharged diesel engines, which has pointed the way to the future development of the inverse Brayton cycle system. In the paper, an experiment was presented to validate the numerical model of a 2.0 l turbocharged diesel engine. Meanwhile, the influence laws of the inverse Brayton cycle system critical parameters, including turbocharger speed and efficiencies, and heat exchanger efficiency, on the system performance improvement for energy recovery are explored at various engine operations. The results have shown that the engine exhaust energy recovery efficiency increases with the engine speed up, and it has a maximum increment of 6.1% at the engine speed of 4000 r/min (the engine rated power point) and the full load. At the moment, the absolute pressure was before final compression is 51.9 kPa. For the inverse Brayton cycle system development in the future, it is essential to choose a more effective heat exchanger. Moreover, variable geometry turbines are very appropriate to achieve a proper matching between the turbocharging system and the inverse Brayton cycle system. [ABSTRACT FROM AUTHOR]

The environment inside laying hen houses has an important effect on hen productivity, health, and well-being. Heat exchangers (HE s) can recover waste heat in ventilation exhaust to reduce supplemental heating needs while maintaining a greater fresh air exchange rate. For laying hen houses, there is limited information on the effect of heat recovery ventilation (HRV). Thus, the objective was to evaluate an air-to-air HE for manure belt drying ventilation in aviary laying hen housing. Temperature (T), relative humidity, ammonia, and manure dry matter (DM) content were characterized during a 4-wk period in October 2018. In weeks 2 and 4, the HRV was shut down and compared to weeks 1 and 3 when the HRV was operational. Average (±SD) ambient T was 10.6°C ± 4.0°C, similar for the 4-wk period. Heat exchanger efficiency was 75.07% ± 9.4% with the average supply temperature increased by 10.0°C ± 3.4°C and an average of 93.94 ± 31 kW heat recovered. Average indoor T (23.1°C ± 0.5°C) was warmer as a function ambient T and daily average T range was lower with HRV (1.8°C ± 0.7°C) compared to without HRV (22.2°C ± 1.2°C; 3.1°C ± 1.1°C). Seven days after manure removal, final average manure DM was 40.6% ± 3.1% (without HRV) and 60.0% ± 3.3% (with HRV). Implementation of HRV positively influenced indoor thermal environment by maintaining less dynamic diurnal fluctuations and greater temporal T uniformity. [ABSTRACT FROM AUTHOR]

• Recuperative ORC (RORC) presents a higher efficiency (+20 %) and power (+18 %) than ORC. • RORC works with a lower mass flow rate than ORC for a given evaporating temperature. • RORC needs a lower heat input thus increasing the operating time in absence of solar radiation. • The higher operating time involves a higher yearly electricity energy production (+37 %). • Hot water and coolant temperature play the most important effect on RORC performance. Organic Rankine Cycle -based microcogeneration systems that exploit solar sources to generate electricity and domestic hot water simultaneously are promising solutions for reducing CO 2 emissions in the residential energy-intensive sector. Such systems can be assisted by thermal energy storage reservoirs in which water is heated by solar energy and cooled by the direct use of thermal energy for domestic hot water production and acting as heat source of the power plant. An interesting plant layout optimisation involves the adoption of a recuperative heat exchanger to preheat the working fluid before it enters the heat recovery vapour generator, which is fed with the same working fluid leaving the expander. Despite the positive effect of recuperative heat exchanger adoption on the efficiency of the entire plant, the impact on electrical energy production at this microscale is not straightforward to assess as the heat source is represented by the water inside the reservoir and it is not a continuous high- or medium-temperature stream. The lack of experimental analyses in the literature on these applications makes this a debatable question. Thus, to fill this gap and quantitatively assess the benefits introduced by the adoption of a recuperative stage, a wide experimental comparison between recuperative and not-recuperative Organic Rankine Cycle-based power unit layout was performed. To support the experimental analysis, a comprehensive theoretical model of the Organic Rankine Cycle-based plant was developed and validated against experimental data. The experimental campaign demonstrated that the recuperative Organic Rankine Cycle-based unit required a lower working fluid flow rate for a given expander inlet temperature and pressure ratio. Consequently, a higher electrical power can be produced, requiring lower thermal power from a heat source. The reduction in the thermal power absorbed from the heat source, produced by a higher temperature incoming working fluid entering the Heat Recovery Vapor Generator, had a positive effect, allowing a longer-in-time working condition with a higher conversion efficiency intrinsically produced by the recuperative heat exchanger. Considering the annual electricity consumption of 2700 kWh of a family living in a city in central Italy, the contribution given by the recuperative Organic Rankine Cycle plant operating with a reservoir fed by 15 m2 of flat solar panels was 330 kWh, whereas without the recuperative stage, the production was 240 kWh. The cost of electricity saved justified the adoption of a recuperative heat exchanger. [ABSTRACT FROM AUTHOR]

Abstract In this work, we experimentally demonstrate a commercial ceramic membrane heat exchanger (CMHE) with an average pore size of 4 nm for novel heat recovery in post-combustion carbon capture. The CMHE shows superior performance over a conventional stainless steel heat exchanger (SSHE) with the same dimensions in recovering heat from the stripped gas mixture (H 2 O(g)/CO 2) on top of the stripper in a monoethanolamine-based rich-split carbon capture process. Due to the coupled mass and heat transfers of water vapor through the membrane, the CMHE has higher heat flux, heat recovery and overall heat transfer coefficient than the SSHE. Liquid water transfer dominates the mass transfer mechanism in the CMHE. Thermal conduction contributes to more than 80% of the total heat transfer, dominating the heat transfer through the membrane heat exchanger. Our study demonstrates that membrane heat exchangers can be excellent candidates for heat recovery in post-combustion carbon capture. In further research, more types of membranes with higher thermal conductivities (e.g., porous metal membranes with lower porosity and smaller thickness) should be fabricated and tested for further performance enhancement. Graphical abstract fx1 Highlights • We experimentally test a membrane heat exchanger for heat recovery in carbon capture. • Membrane heat exchanger shows superior heat recovery than conventional heat exchanger. • Liquid water transfer dominates mass transfer in the membrane heat exchanger. • Thermal conduction dominates heat transfer in the membrane heat exchanger. • More types of membranes with higher thermal conductivities should be explored. [ABSTRACT FROM AUTHOR]

Herein, the effect of the power capacity, W net , on the performance of supercritical carbon dioxide (sCO 2) coal-fired power plants is investigated, with W net in the range of (100–1000) MWe. A comprehensive model was established wherein the sCO 2 cycle was coupled with the models of various components. For the sCO 2 boiler, the total thermal load was assigned to various heaters, and the pressure drop in each heater was calculated. Owing to the strong penalty effect of the pressure drop, both total flow mode (TFM) and partial flow mode (PFM) were applied to the sCO 2 boiler. A fluid network integrating the recuperator units was established for heat recovery in the system. A thermal-hydraulic model was proposed for a single unit and an integration package. Various losses were considered in the prediction of the efficiencies of axial flow turbines and compressors. The thermal efficiency increases, attains a maximum, and then decreases with increase of W net. This parabolic distribution results from the tradeoff between the decreased efficiency owing to pressure drops in the heat exchangers and the increased efficiency of the turbomachinery. The maximum thermal efficiency occurred at W net of 300 MWe and 200 MWe when using the PFM and TFM, respectively. Based on the results, PFM is found to eliminate the penalty effect of pressure drops. Our study provides guidelines for the design and operation of sCO 2 coal-fired power plants. [ABSTRACT FROM AUTHOR]

This study firstly develops a novel combined cycle system consisting of a supercritical CO 2 recompression Brayton cycle and a transcritical CO 2 refrigeration cycle to recover waste heat from a marine turbine for both power generation and refrigeration. The pressure drop of the total heat exchanger in the whole cycle is considered in the thermodynamic and economic modeling process. Then, the influence of crucial system parameters on the system performance and economics is investigated through parametric sensitivity analysis. Finally, the system is optimized parametrically by multi-objective optimization. The results show that the higher inlet pressure of the high-pressure compressor helps to improve the thermal efficiency but reduces the exergy efficiency; the low-temperature heat exchanger plays a decisive role in the overall system exergy destruction; the HRHE accounts for a more significant proportion of the system cost, and the pressure drop has the most significant impact on the network of the system. The optimal solution is COP = 3.059, LCOE = 18.348$/(kW/h), and η Whr = 0.651 when waste heat recovery efficiency, COP, and LCOE are considered optimization objectives. • A new combined Supercritical/Transcritical CO 2 cycle is proposed for waster heat recovery. • Effects of heat exchanger pressure drop and parameter sensitivity are analyzed. • Thermal efficiency with reducing exergy efficiency of the combined cycle was assessed. • Genetic algorithm and TOPSIS methods are applied for optimization of the combined cycle. [ABSTRACT FROM AUTHOR]

Waste heat recovery in automobiles is a practical technique to reduce emissions and fuel consumption. The extent of waste heat recovery is directly proportional to the efficiency of the heat exchanger. However, the change in internal geometry has become a convenient method to extract the maximum heat from the exhaust heat exchanger while producing a minimum pressure drop. In the current study, the performance of the exhaust heat exchanger with twisted ribs has been studied for the six different operating points of the diesel engine. Three distinct geometric parameters of twisted tape i.e., pitch ratio, twist ratio, and tilt angle have been studied in detail. Experiments were repeated for various geometric parameters to arrive at the best specification of the twisted tape. The results show that the pitch ratio of 8, twist ratio of 4, and the tilt angle of 60° yield the highest power output. The results obtained were compared with the internal flat heat exchanger. It has been observed that the system equipped with a twisted rib produces more net power than the smooth surface system. Moreover, a comparison of fuel consumption with the commonly used alternator has been performed and the proposed system has the potential to improve the BSFC by 0.65%. Image 1 • The effect of geometric parameters of twisted tape on power output is investigated. • The comparison of proposed system with the conventional alternator is performed. • The highest power output of the system yields with pitch ratio of 8, twist ratio of 4 with tilt angle of 60°. • The system has the potential to improve the BSFC by 0.65%. [ABSTRACT FROM AUTHOR]

• Combined inverted Brayton – organic Rankine cycle for waste heat recovery. • Pentane as working fluid gives higher system efficiency than toluene and R245fa. • Combined scheme efficiency 10% higher than organic Rankine cycle at high temperatures. • Combined scheme electricity cost 6% lower than of organic Rankine cycle. • Flue gas water condensation non-negligibly affects inverted Brayton cycle performance. Many practical cases with waste heat recovery potential such as exhaust gases of reciprocating engines, cement kilns or heat-treating furnaces, are nowadays often integrated with organic Rankine cycle to convert waste heat to the mechanical power. However, when dealing with high-temperature waste heat, organic Rankine cycle faces efficiency limit due to the physical properties of the working and thermal fluids. That gives room for further enhancement of the waste heat recovery technologies via the investigation of different non-conventional schemes as one of the possible ways. In the present work, a system introducing the combined inverted Brayton plus organic Rankine cycle is under investigation. Aspen Hysys models of both conventional organic Rankine cycle and combined cycle were designed, orienting on waste heat recovery from the heavy-load gas-fueled reciprocating engine exhaust. In this way, the performance of the combined scheme was benchmarked versus the conventional organic Rankine cycle. An assessment of the organic Rankine cycle working fluids was provided, and pentane has shown the best thermodynamic performance. The study on inverted Brayton cycle defined the remarkable effect of the water condensation in the gas duct on the inverted Brayton cycle performance. Finally, both thermodynamic and economic optimizations of the models were conducted, setting the stage for the comparison of solutions. Results have shown the 10% advantage of the combined scheme over organic Rankine cycle in generated power and system efficiency. The levelized-cost-of-energy-based optimization for variable capacity factors has highlighted above 6% advantage of the investigated solution. The analysis of the sensitivity from machines' efficiencies and heat exchangers' pinches has shown that with some sets of parameters, the studied scheme may concede to the organic Rankine cycle. [ABSTRACT FROM AUTHOR]

The cheese industry has high energy consumption, and improvements to plant efficiency may lead to a reduction of its environmental impact. A survey on a sample of small-medium Italian cheese factories was carried out in order to assess the efficiency of heat recovery of the milk pasteurization equipment for the cheese production. Then, an exergetic analysis to calculate the related exergy loss was carried out together with a cost-benefit analysis to identify the optimized value of the heat efficiency. The exergy loss reduction was determined throughout an exergy analysis that takes into account this last value and the comparison with the previous exergy losses. Finally, the feasibility and the consequent additional reduction of exergy losses were verified, if a cogeneration heat and power (CHP) combined to the pasteurization equipment is assumed. Results show a current heat recovery efficiency of 93.2% in the Italian cheese factories; a close connection between the exergetic losses and the efficiency of the heat recovery exchanger; the optimized recovery efficiency equal to 97.3% obtained from the cost-benefit analysis; a related important exergetic loss reduction of −45% in the heat exchangers, as a second result of the exergetic analysis; a similar reduction of the exergy loss (−42%) of the whole system, as a third result of the exergetic analysis; a total exergy loss reduction of 22.9 kJ kg−1milk, which corresponds to a lower environmental impact due to CO2 reduction; a further reduction of the exergy loss of −10% when the cogeneration heat and power CHP are used. [ABSTRACT FROM AUTHOR]

• Benefits deriving from MOHC-based nanofluid adoption in ORCs were investigated. • A numerical model for the assessments of the nanofluid benefits was developed. • Semi-empirical correlations for modelling MOHC desorption behaviour were defined. This paper presents an in-depth investigation of the applications of an innovative nanofluid – suspensions of nanoparticles in a base fluid- in the ORC field, based on a new class of nanoparticles – termed Metal-Organic Heat Carriers (MOHCs) – molecularly engineered to reversibly uptake and release the working fluid molecules in which they are suspended. Unlike standard nanoparticles (i.e. Al 2 O 3 , Al, ...), these MOHCs make it possible to extract additional heat from the endothermic enthalpy of desorption which can be as much as twice the level of the latent heat of vaporization of the pure fluid phase alone. The paper illustrates the development of a numerical model for assessing the MOHC-based nanofluid gain in ORC systems. More specifically, the possible combination of the base fluid R245fa with the nanoparticle MIL101, a robust Metal Organic Heat Carrier, was considered. To properly model the reversible adsorption/desorption process, experimental analyses were carried out to study the uptake of the R245fa in MIL101 at different operating conditions and departing from the experimental results, proper semi-empirical correlations were defined and adopted within the numerical model. The resulting performance of the MIL101/R245fa were compared with those of pure organic fluids, whose cycle was optimized in order to maximize the area-to-power ratio. Promising results were achieved in terms of system efficiency increase and heat exchanger area reduction. [ABSTRACT FROM AUTHOR]