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The pursuit of sustainable district heating solutions has driven a growing interest in ultra-low temperature district heating (ULTDH) systems, where booster heat pumps (BHPs) play a pivotal role despite challenges posed by their efficiency limitations under large temperature glide conditions. This paper investigates the potential of drop-in R-1234yf/R-32 zeotropic mixtures in BHPs compared to a baseline R-134a system, within the context of a ULTDH framework. This study focused on the viability of the mixtures of R-1234yf/R-32 with the composition ratio of 80 %/20 % and 90 %/10 %. The investigation reveals disparities in compressor efficiency and heat exchanger pressure drop at the component level. Device-level analysis unveils increased COP for R-1234yf/R-32 mixtures, alongside with maximum second-law efficiencies reaching 0.32. A remarkable enhancement in heating capacity up to 58 % was found. System-level analysis demonstrated exergetic efficiencies and identified preferable district heating temperatures. Exergetic efficiencies of 0.47, 0.55, and 0.59 were achieved for domestic hot water preparation at district heating supply temperatures of 30 °C, 35 °C, and 40 °C, with a subsequent shift in optimal district heating temperatures as central heating station efficiency decreased. Temperature profile analysis underscored challenges stemming from excessive subcooling, highlighting the need for configuration refinements. [Display omitted] • Drop-in application of zeotropic mixtures in booster heat pumps are studied. • Coupling effects between district heating and heat pump parameters are analyzed. • Heating capacity and COP improvements with mixture refrigerants are demonstrated. • Temperature profile analysis identified the challenges for efficiency improvement. [ABSTRACT FROM AUTHOR]

Here, to resolve the open problem of the second-law efficiency evaluation for heat exchangers, an efficiency-effectiveness method is developed from pure convection theory, where the entropy transfer efficiency defined as the outlet temperature ratio of cold and hot fluids exchanging entropy and local entropy generation number can measure the global and local irreversibilities of heat transfer. The efficiency and effectiveness are related by the inlet temperature ratio τ and capacity rate ratio φ , and their maxima subject to the local entropy generation number are determined for different flow arrangements. When τ = φ 2, the effectiveness maxima are 0.5/(1 + φ) and 1/(1 + φ) while the highest efficiencies are φ and unity for parallelflow and counterflow arrangements, respectively. The optimum operating points of counterflow exchangers, at which the equal outlet temperature τ T 1 ∞ (T 1∞ represents the hot inlet temperature) of two fluids with a critical capacity rate ratio τ induces uniform irreversibility, the effectiveness limit of 1 / (1 + τ) , and the efficiency limit of unity, are obtained. A linear temperature difference method is also developed based on the redefined convective and overall heat transfer coefficients, invariably yielding an effectiveness equal to the number of heat transfer units. This method has potentials to be extended to wet heat exchangers with phase-change flows. • The open question of second-law efficiency evaluation of a heat exchanger is resolved. • The heat transfer effectiveness and entropy transfer efficiency limits are presented. • The global and local irreversibilities of exchangers are measured by N S and N S,l. • Two optimum design criteria of exchangers are obtained subject to N S,l. [ABSTRACT FROM AUTHOR]

In the process of biomass gasification syngas waste heat recovery, water and organic compounds in the syngas will condense at the same time, resulting in immiscible condensates adhering to the wall, which can reduce the heat exchanger heat transfer efficiency and working life. Changing the wettability of the wall surface can affect the condensate flow state on the wall surface, thus affecting the condensation heat transfer performance. Water and organic cyclohexane were used as immiscible working fluids in this paper, the condensation flow and heat transfer characteristics of the mixed vapors on hydrophilic, super-hydrophilic, and super-hydrophobic wall surfaces were experimentally investigated. The results showed that cyclohexane in immiscible condensates existed as a liquid film on hydrophilic, super-hydrophilic, and super-hydrophobic wall surfaces, while the water existed in droplets on the hydrophilic and super-hydrophobic wall surfaces, and there were two forms of channels and droplets on the super-hydrophilic wall surface. The condensation heat transfer coefficient on the super-hydrophobic wall surface was higher than that on the hydrophilic wall surface and the super-hydrophilic wall surface, with a maximum of 34% higher than hydrophilic and 45% higher than super-hydrophilic, which was related to the smaller droplet departure diameter on the super-hydrophobic surface. In addition, the super-hydrophilic wall surface had better heat transfer performance than the hydrophilic wall surface only when the immiscible condensate was a "film-channel" flow pattern. The results can provide guidance for enhancing the condensation heat transfer of the water and organic compounds binary mixed vapors. • Wall wettability can change the immiscible condensates flow behavior and thus affect condensation heat transfer. • The smaller the condensate droplets departure diameter on the wall, the greater the condensation heat transfer performance. • The condensation heat transfer coefficient on the super-hydrophobic wall surface was the highest. [ABSTRACT FROM AUTHOR]

Exergy analysis of reliquefaction system based on Reverse Brayton cycle (RBC), which is used to condense boil-off gas (BOG) generated in LNG carrier ships, was presented with its basic configuration in Part 1. In this paper, three modifications of the basic system are evaluated. One modification is based on intercooling and precooling of BOG before compressor suction. Others involve ambient compression of BOG. Effects of pressure ratio of RBC, mass flow rate of nitrogen, thermal sizes of heat exchangers and entry temperature of BOG to reliquefaction system on each configuration have been evaluated. Sensitivity analysis shows that 7.8% improvement of rational exergy efficiency is obtained for systems with ambient compression. When BOG enters as superheated gas, modified configurations could keep up their performance while basic system deteriorated. Higher adiabatic efficiencies of compressors and turbines improve the performance of the reliquefaction systems across the board without any additional weight or space. Design and operating parameters have been optimized with the aid of in-built optimiser tool of Aspen HYSYS® V8.6. It resulted in an improvement of rational exergy efficiency of configurations by 1%–3% and decrease of total UA by 5–7% compared to best values predicted by sensitivity analysis. • Evaluates and compares improved configurations for BOG reliquefaction. • Detrimental effects of higher BOG entry temperature effectively circumvented. • Nondimensionlization of parameters helps to apply results to any size of LNG ship. • Evaluated effects of higher adiabatic efficiency of compressor and turbine. • Optimises rational exergy efficiency with reasonable heat exchanger sizes. [ABSTRACT FROM AUTHOR]

Abstract This paper presents an air-cooled waste heat-driven absorption refrigeration cycle with a non-adiabatic absorber using R290/oil as working fluid. The effect of various operating parameters (generator temperature, absorber outlet solution temperature, evaporator and condenser temperature, absorption efficiency and solution heat exchanger effectiveness) on the system performance has been discussed based on the first and second law of thermodynamics. In order to reveal which components have a poor utilization of the input exergy, theexergy destruction coefficient has been defined and calculated under considered working conditions. The simulation results show that the system can run steadily under air-cooled conditions with an appreciable COP of 0.4696 and exergetic efficiency of 0.1293. So R290/oil mixture is a highly potential working fluid for absorption refrigeration. The application of the air-cooled non-adiabatic absorber improves both the coefficient of performance and the exergetic efficiency of the system, which also contributes to the miniaturization and cost reduction of air-cooled systems. And the solution heat exchanger and generator are the major contributors to the total exergy destruction of the system. More efforts should be made to optimize these components for higher exergetic efficiency of the system. Highlights • A waste heat-driven absorption refrigeration system with R290/oil is analyzed. • The superiority of using non-adiabatic absorber in practical application is discussed. • Energy and exergy analysis under different working conditions are carried out. • The exergy loss of each component is calculated and optimization methods are discussed. [ABSTRACT FROM AUTHOR]

Abstract Because of the complex relationship of the characteristics of the liquefied natural gas (LNG) processes, the use of multi-objective optimization algorithms seems rational, for the reason that these methods have the ability to tradeoff between different objectives. The unit energy consumption (UEC); the sizes of the LNG heat exchangers; and the liquefaction rate (LR) are chosen as the objective functions of the MOPSO algorithm. Optimizations were performed under two separate circumstances: initially, the optimization in the early stages of the plant design was considered; and consequently the optimization in the operational conditions was implemented. A fuzzy-based clustering method is then used to choose the optimum solution from the resulting points of the Pareto fronts derived from the MOPSO algorithm. The performance of this optimization is compared against the performances of a base case and the optimization performed on the same process using the genetic algorithm (GA). The utilized algorithm improves the objectives values, reduces the exergy destructions in the process and enriches the performance of the LNG heat exchangers. In addition, it is confirmed that the optimization performed at the early stages of the process design yields better results and cost savings. Highlights • Multi-objective optimization has been performed using the MOPSO algorithm. • Optimizations are performed in two cases: operating condition and design condition. • Unit energy consumption, liquefaction rate, and UA are the objective functions. • The objective functions values have been improved in both of the two optimization cases. • Optimization improved exergy efficiency and the performance of the heat exchangers. [ABSTRACT FROM AUTHOR]

Abstract Hydrothermal reservoirs commonly exist in association with fault activities. This study discusses the geothermal energy exploitation strategies in a fault-controlled geothermal system based on the Zhacang geothermal field in the northeastern Tibetan Plateau. The influence of well placement (including production depth, injection depth and well distance) and operation parameters (including injection temperature and production backpressure) on the heat production performance are evaluated by coupled thermal–hydraulic numerical modeling. Results indicate that a geothermal energy production of 1.67 MW can be obtained in the Zhacang geothermal field in 50 years, when the injection rate is 2.5 kg/s and injection temperature is 10 °C, with the injection depth at 1500 m, production depth at 300 m and well distance at 300 m. This optimized strategy is weakly affected by the terrestrial heat flux, because the heat transport in permeable fault zone is dominated by convection rather than conduction. Heat production increases with permeability in the conductive fault zone. The relationship between geothermal energy production and the associated natural and anthropogenic factors discussed in this study can help optimize heat extraction management in the other fault-controlled hydro-geothermal reservoirs. Highlights • General heat production strategies are discussed for a fault-controlled geothermal reservoir. • Performance assessment of the geothermal system is based on realistic geological conditions. • Sensitivities of heat production to natural and anthropogenic factors are investigated. [ABSTRACT FROM AUTHOR]

In this study, a parabolic dish concentrator with two types of cavity receivers was investigated. The designed experimental setup included a parabolic dish concentrator, cavity receivers, a heat exchanger system, and hydraulic circuit unit. Two optimized shapes of cubical and cylindrical cavity receiver were made and studied. Also, numerical modeling was developed for predicting the cavity receiver performance. The numerical results of the cavity receivers show a good agreement with the experimental results. The results indicated that the receiver heat gain and thermal efficiency of the cavity receivers had a similar trend compared to the temperature difference of the heat transfer fluid between the inlet and outlet of the cavity receivers. The results also clarified that the thermal efficiency of the cubical cavity receiver was higher than the thermal efficiency of the cylindrical cavity receiver in the steady-state period. The average thermal efficiency of the cubical and cylindrical cavity receiver was obtained as 65.14% and 56.44% in the steady-state period, respectively. The cubical cavity receiver can be recommended for an efficient heat gain, in comparison with the cylindrical cavity receiver based on the conducted experimental tests on 11 October, and 26 September 2016. [ABSTRACT FROM AUTHOR]

Cryogenic natural gas liquefaction plant has huge capital and operating expenses corresponding to operating equipment and energy utilization. Considering ever-increasing energy price, therefore, minimization of energy consumption rate for a better profit is highly required. However, any un-engineered energy cut off would result in larger surface area of heat-exchanger and hence bigger capital cost. Here, the net profit of establishing a mini 50 ton/day liquefied natural gas facility, operating for 25 years, is optimized via Genetic Algorithm technique. Poly Refrigerant Integrated Cycle Operations (PRICO) process is simulated in HYSYS environment and linked to MATLAB software for subsequent maximization. The simulation resulted in total consumed power, heat exchanger area and total profit by 2745.33 kW, 3285.58 m 2 and 1266.64 million$, respectively. In order to determine unit efficiency and plant irreversibility rate, exergy analysis is performed on individual equipment. Basically, thirteen independent variables are considered for optimization of objective function. Sensitivity analysis for objective function is considered by altering each variable. Final results indicate 9.26% rise in total profit (1383.95 million$) by 59% reduction in energy utilization (1127.68 kW) and 37.50% in heat-exchanger size (2053.7 m 2 ). Meanwhile, the total and heat-exchanger exergy losses are decreased by 65.8% and 80.7%, respectively. [ABSTRACT FROM AUTHOR]

The current study presents an experimental analysis for integrating a phase change material (PCM) in a typical direct flow evacuated tube solar water heater with a U-tube heat exchanger (HX). Each evacuated tube is filled with 0.8 kg of paraffin wax to store the absorbed incident solar energy. As water flows through the U-shape copper tube inside the PCM, the stored energy is transferred to the water through a combination of conduction and convection. The proposed system is investigated under two configurations; un-finned and finned HX to investigate the effectiveness of adding the fin. Outdoor experiments are carried out to demonstrate the thermal performance of the purposed systems under various scenarios including the charging phase and the overnight heat loss as proposed by Chinese National Standard CNS 7277-12558 [1]. Also, the thermal performance during the discharging phase is evaluated under various loads based on the experimental design approach. The results show that the natural convection is the main heat transfer mechanism during the charging phase, with a higher system efficiency for the un-finned collector by about 14% due to the high average temperature of the PCM. During the discharging phase, the presence of the fin helps to overcome the poor thermal conductivity of the solidified layer of the PCM by offering another path for the energy transfer from the PCM to the water, and subsequently enhance the total effective energy discharged. [ABSTRACT FROM AUTHOR]

Compactness, efficiency and thermal control of the heat exchanger are of critical significance for many electronic industry applications. In this view, a new concept of heat exchanger at millimeter scale is proposed and numerically studied. It consists in dynamically deforming at least one of its walls by a progressive wave in order to create an active corrugated channel. Systematic studies were performed in single-phase flow on the different deformation parameters that allow obtaining the thermo-hydraulic characteristics of the system. It has been observed the dynamic wall deformation induces a significant pumping effect. Intensification of heat transfer remains very important even for highly degraded waveforms although the pumping efficiency is reduced in this case. The mechanical power applied on the upper wall to deform it dynamically is linked to the wave shape, amplitude, frequency and outlet-inlet pressure difference. The overall performance of the proposed system has been evaluated and compared to existing static channels. The performance of the proposed heat exchanger evolved in two steps for a given wall deformation. It declines slightly up to a critical value of mechanical power applied on the wall. When this critical value is exceeded, it deteriorates significantly, reaching the performance of existing conventional systems. [ABSTRACT FROM AUTHOR]

Heat pipes are becoming increasingly popular as passive heat transfer technologies due to their high efficiency. This paper provides a comprehensive review of the state-of-the-art applications, materials and performance of current heat pipe devices. The paper is divided into four main parts; low temperature heat pipes, high temperature heat pipes, thermal modelling of heat pipes and discussion. The low and high temperature sections present an extended list with suitable working fluids and operating temperatures, along with their compatibility with casing materials. Furthermore, the sections focus on some of the most widespread industrial applications, such as solar, nanoparticles, Rankine cycles, nuclear, thermoelectric modules and ceramics, in which heat pipe technologies offer many key advantages over conventional practises. The third part of the paper consists of a thorough analysis of the thermal modelling side of heat pipes. Internal and external thermal modelling techniques, theories and methodologies are presented in this section, for various applications such as non-Newtonian fluids, nano-fluids, solar, geothermal, automotive, hybrid storage and nuclear systems. The final part of the paper discusses the limitations of heat pipes and the reasons why they are not implemented in more aspects of our lives. Operational limitations, cost concerns and the lack of detailed theoretical and simulation analysis of heat pipes are some of the point covered in this section. Finally, some of the recent and future developments in the field are discussed. [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]

The supercritical carbon dioxide Brayton cycle is one of the alternative thermal schemes for advanced energy systems. In this study, ammonia absorption Refrigeration-reheat supercritical carbon dioxide Brayton combined cycle is constructed, and the thermodynamic performance of reheat supercritical carbon dioxide Brayton cycle with ammonia absorption refrigeration cycle are analyzed. The characteristics of Brayton cycle and ammonia absorption refrigeration cycle configuring with Printed Circuit Heat Exchanger are analyzed. The result shows that under controlled freezing-point storage condition, optimized combined cycle energy and exergy efficiency is 46% and 64.98%, electricity output and refrigeration capacity is 11.4 MW and 1.78 MW. Under air-conditioning refrigeration condition, optimized combined cycle energy and exergy efficiency is 50.28% and 65.04%, and electricity output and refrigeration capacity is 11.4 MW and 3 MW. Compared with combined cycle performance before optimization, average improvement of cycle energy efficiency, exergy efficiency and exergy destruction is 4.19%, 0.73% and 3.64%, which shows obvious improvement of cycle performance after optimization. Comparative analysis of printed circuit heat exchanger applied at recuperator and subcooler shows that heat transfer performance of single-faced etched printed circuit heat exchanger in straight channel is better than that of double-faced etched printed circuit heat exchanger for same heat transfer and pressure drop. • New combined supercritical CO2 Brayton cycles with a printed-circuit-heat-exchanger are proposed. • Cycle efficiency and heat exchangers performance are assessed and quantitatively analyzed. • The combined cycles fully utilize the low-temperature heat under two working conditions. • Performance of single-side and double-side printed-circuit-heat-exchanger are compared. [ABSTRACT FROM AUTHOR]

A re-liquefier based on the reverse Brayton cycle is the first choice for the re-liquefaction process owing to its compactness, simplicity, and safety. However, due to the low efficiency of the heat exchangers, the energy consumption of the re-liquefaction process is relatively high. Therefore, a novel cascade nitrogen expansion re-liquefaction process with high heat exchange efficiency is proposed and compared with the parallel expansion process. In the designed process, the condensation and subcooling stages of the boil-off gas (BOG) are performed in different heat exchangers. The performance improvement of the designed process is analyzed using the Aspen HYSYS and the genetic algorithm is used to optimize the processes. The optimization results show that the specific energy consumption (SEC) and exergy loss rate of the designed process are 0.727 kWh/kg LNG and 148.14 kW, which are 11.7% and 16.7% lower than those of the reference process, respectively. Moreover, the exergy efficiency (EXE) of the designed process is 0.352, which is 13.2% higher than that of the reference process. Because of the higher EXE and lower refrigerant demand, the designed process saves 7.2% of the total annual costs consisting of annual capital and operating costs compared with those of the reference process. • A novel cascade nitrogen expansion BOG re-liquefaction process was designed. • The new process and the reference process were optimized by genetic algorithm. • The matching degree of composite curve of heat exchangers was evaluated. • The exergy loss and exergy efficiency of two re-liquefaction processes was analyzed. • The performance and economy of two re-liquefaction processes was compared. [ABSTRACT FROM AUTHOR]

In this study, diverse baffled longitudinal flow shell-and-tube heat exchangers (STHX) are contrasted with segmental baffle shell-and-tube heat exchanger (SG-STHX). Experimental data are obtained with municipal water served as the working fluid, and the shell-side volume flow rate ranges from 1.79 m3/h to 7.42 m3/h. The components of the shell-side pressure drop are discussed stand on different flow patterns. The maximum proportion of pressure drop in tube bundle section of rod baffle shell-and-tube heat exchanger (RB-STHX) is 12%, while it has nearly taken up 70% shell-side pressure drop for both SG-STHX and large-and-small hole baffle shell-and-tube heat exchanger (LSHB-STHX). The energy efficiency of three tested STHXs is deliberated from three perspectives, including entropy generation, exergy destruction, and efficiency evaluation criterion. The longitudinal flow pattern performed superior energy efficiency, particularly for RB-STHX with the least irreversible energy loss and the most available work. Grounded on the energy-saving potential of RB-STHX, further numerical simulations on the shell-side thermo-hydraulic performance of RB-STHX are conducted. The nexus between geometrical parameters of RB-STHX and its thermal-hydraulic performance are studied. The thermal-hydraulic performance and energy efficiency discussed in this study support further design and application of longitudinal flow STHX to retain inherent superiorities with advanced performance. • Only 12% of the total pressure drop in the shell-side of RB-STHX is derived from the tube bundle section. • RB-STHX performs 173%∼223% higher EEC than LSHB-STHX with the least irreversible energy loss. • Rod baffle shell-and-tube heat exchanger shows the best comprehensive performance. • Numerical investigation of the heat transfer enhancement. • Factors: tube arrangement, rod shape, baffle arrangement, and baffle number. [ABSTRACT FROM AUTHOR]

One promising way to improve the efficiency of borehole heat exchangers (BHEs) in shallow geothermal applications is to enhance the thermal properties of the materials involved in its construction. Early attempts, such as using metal tubes in the 1980s or the utilization of thin–foil hoses, did not succeed in being adopted by the market for diverse reasons (cost, corrosion, fragility, etc...). In parallel, the optimization of pipe size, the use of double-U-tubes, thermally enhanced grout, etc. were able to bring the measure for the BHE efficiency, the borehole thermal resistance, from 0.20 to 0.15 K / (W m) down to 0.08–0.06 K / (W m) in the best solutions today. A further improvement cannot be expected without development of new, dedicated materials, combining the versatility of plastic like PE with an increased thermal conductivity that matches the respective properties of the rock and soil. This goal was included in the Strategic Research and Innovation Agenda of the European Technology Platform on Renewable Heating and Cooling in 2013. Within an EU supported project, both BHE pipes and grouting materials have been produced prototypically in small amounts, suitable for the first tests in the intended environment. The present work explains the research pathways envisaged and the resulting sensitivity analysis to highlight the influence of some of the most critical parameters that affect the overall performance of a GSHP system. The results have allowed guiding the real development of more efficient new advanced materials for different scenarios representative of different European regions. Finally the developed materials and their properties are discussed, including a comparative assessment about their compliance with reference material properties as currently seen in the BHE market. • A vast parameter study for the development of new geothermal materials. • The objective is to obtain the best specifications for pipe and grout materials. • Representative scenarios of all simulations carried out are presented. • Assessment of combined enhancement of pipe thermal conductivity and grouting. • The implementation of the enhanced products could produce a reduction of up to 22%. [ABSTRACT FROM AUTHOR]