Temperature Difference In Heat Exchanger Research Articles

Enhancing performance of multi-pressure evaporation organic Rankine Cycle/Supercritical Carbon Dioxide Brayton cycle through genetic algorithm and Machine learning optimization

The Supercritical Carbon Dioxide Brayton combined cycle is one of the most important ways to enhance the performance of the cycle. Since the considerable exergy loss resulting from large temperature differences in the heat exchangers of combined cycles, the multi-pressure evaporation concept was adopted from Organic Rankine cycle research. Two novel multi-pressure evaporation configurations, namely parallel evaporation Brayton combined cycle and series evaporation Brayton combined cycle, are designed with the purpose of enhancing SCO2 Brayton cycle performance. The performance of the two new cycles was compared with that of the baseline cycle. Moreover, Printed Circuit Heat Exchangers were employed to improve the overall compactness of the cycles. The results from model calculations were utilized to train an Artificial intelligence neural network model using machine learning techniques which allowed to simplify the cycle and improves the computational speed. Subsequently, genetic algorithm was employed to conduct multi-objective optimization on the three cycles. Finally, a detailed exploration was conducted from the perspective of thermal and exergy to elucidate the reasons for the advantages and differences between the two new cycles compared to the baseline cycle. The results indicate that the series Brayton combined cycle exhibits the highest performance, and the combined cycle of multi-pressure evaporation helps to improve the performance of the combined cycle. Compared to the baseline cycle, the combined cycle efficiency of both series and parallel combined cycles has increased from 45.54% to 46.18% and 46.29%, respectively, while the bottom cycle efficiency has improved from 3.81% to 4.93% and 5.03%. The ratio of heat exchange area to net output power increased from 0.6051 to 0.6614 and 0.6740 for the parallel and series cycles, respectively. Thermal analysis shows that, the fundamental purpose of changing the layout is to improve the specific power per unit of working medium in the cycle. Exergy analysis shows that, the two new combined cycles improve cycle efficiency by reducing exergy losses in the intermediate heat exchanger. Compared to the parallel version, the series version's staged compression allows for a lower temperature difference in intermediate heat exchanger 1, resulting in smaller exergy losses and higher cycle efficiency. This work will contribute to an enhanced understanding of Supercritical Carbon Dioxide combined cycles within the academic community and the introduction of multi-pressure evaporation will further improve the performance of these important cycles.

Practical design guidelines for heat transfer enhancement of condensers

Enhanced condensation is of great practical interest to further improve and miniaturize power generation and refrigeration systems. Studies in the literature that investigate enhanced condensation focus on increasing the heat flux at high vapor qualities; however, many thermodynamic cycles such as vapor compression and the Rankine cycle require complete condensation for cycle operation, and in such cases, low-quality condensation can be the limiting factor. Performance evaluation criteria (PEC) can be used to assess the relative improvement achieved by different enhancement methods, but most PEC are only applicable to single-phase flows. The present study therefore develops local design guidelines for condensing flows to determine the regions of the condenser that most need heat transfer enhancement. This is achieved by comparing the relative benefits and penalties associated with enhancing low-quality, medium-quality, and high-quality condensation equally. The effects of varying heat transfer coefficient, frictional pressure drop, temperature reduction due to pressure drop, driving temperature difference, and coupling fluid thermal resistance are considered. Three enhancement techniques are then analyzed with these approaches to determine their relative benefits and disadvantages. Cases that consider fixed geometries, fixed flow areas, and variable flow area are considered, along with comparing the relative benefits of size reduction, increased heat duty, and reduced temperature difference. Results show that heat transfer enhancement is achieved when the increase in heat exchanger conductance exceeds the reduction in the heat exchanger temperature difference induced by condensation pressure drop. Additionally, it is found that enhancement is typically most needed in the low vapor quality region of the condenser, except in situations where temperature pinching occurs along the heat exchanger.

Thermo-economic analysis and comparative study of different thermally integrated pumped thermal electricity storage systems

Pumped thermal electricity storage (PTES) is a cost-effective and little geographic confinement technology for large-scale energy storage. In this work, a comprehensive analysis is conducted on four different types of thermally integrated pumped thermal electricity storage systems, including thermodynamic, economic, and optimization aspects. The results are compared and evaluated to determine the most efficient and cost-effective system. Parameter analysis and optimization are conducted on four systems to determine the effects of various factors on system performance. The factors considered include thermal storage temperature (Tts), evaporation temperature of the heat pump cycle (Teva,hp), pinch point temperature difference of heat exchanger (Tpp), flash pressure (Pf), and temperature of the heat pump cycle (Teva,hp). Under the effects of Tts on system performance, among the four systems, the thermally integrated pumped thermal electricity storage systems with regenerators, which consists of a heat pump cycle and an organic Rankine cycle, shows the highest power-to-power efficiency (ηPTP), with a maximum value of 79.81%. Furthermore, based on the results of multi-objective optimization, it can be concluded that the thermally integrated pumped thermal electricity storage systems with regenerators, which consists of a heat pump cycle and an organic flash cycle is the most suitable system, and the final solution of the system is obtained by considering the multi-objective optimization results. The exergy destruction and economic analysis of the thermally integrated pumped thermal electricity storage systems with regenerators (including a heat pump cycle and an organic flash cycle) are conducted in the optimal working conditions. For the thermally integrated pumped thermal electricity storage systems with regenerators, which consists of a heat pump cycle and an organic flash cycle, the component with the greatest exergy destruction is the evaporator, and the cost of the expander is the highest, followed by the evaporator, and it has the optimal system performance.

Dual-objective optimization of a novel hybrid power generation system based on hydrogen production unit for emission reduction

In this study, a biomass based power generation system is proposed. This system includes a BIG (BIG) system to produce syngas, a proton exchange membrane (PEM) type fuel cell (FC), a gas turbine (GT), and an organic Rankine cycle (ORC). In order to evaluate the system function, first a parametric study is conducted and the effect of the design variables on the production power, exergy efficiency, and the total cost rate (TCR) of the system is investigated. Design variables include biomass moisture content, gasification temperature, compressor pressure ratio, air heat exchanger temperature difference, pressure of FC, current density of FC, LP Stage PPTD, and HP stage pressure. It is observed that the determining factors in the TCR of the system are more affected by the cost of the gasifier and PEM FC system. It is also observed that the ORC plays a greater role in recovering wasted heat and generating power compared to GT. Finally, it is observed that in optimal operating conditions, the exergy efficiency of the system are 33.19% and TCR is 693 $/h, respectively.

Finite-time thermodynamic and economic analysis of Rankine Carnot battery based on life-cycle method

Rankine Carnot battery (R-CB) has a great potential in renewable energy-to-grid integration, and the theoretical thermodynamic performance is inaccurate because of the dynamic heat transfer processes and system performance. In this paper, an R-CB system consisting of a heat pump, a heat engine cycle, and a sensible heat storage tank was established. The finite-time thermodynamic model considering the irreversible loss of energy transfer and conversion processes was proposed. Based on the combination of finite-time thermodynamic model and life-cycle method, the levelized cost of energy storage model was established for the combination of economic and energy performance. Furthermore, evaporation temperature, heat storage temperature, temperature difference of shared heat exchanger, and shared heat transfer area were adopted as main variables to conduct the global optimization and parametric analysis of the system. The simulation is based on an R-CB system with a storage capacity of 10 MW/6h, and the results showed the optimized system roundtrip efficiency was 78.1%, and the levelized cost of storage (LCOS) was 0.329 $/kWh with an evaporation temperature of 100 ℃. The roundtrip efficiency increased from 25.6% to 78.1% with the evaporation temperature rising from 60 to 100 ℃, while LCOS decreased by 45.8 %. Reducing the highest heat storage temperature effectively increased the roundtrip efficiency and reduced the heat storage cost, while decreasing the LCOS. The increase in roundtrip efficiency, which was obtained by reducing the shared heat exchange temperature difference and increasing the shared heat transfer area, was the primary factor in reducing the LCOS. The combination of finite-time thermodynamics and the life-cycle method in this paper may provide a more realistic evaluation approach for measuring the technical and economic potential of R-CB systems.

EN Analysis of temperature modes and temperature differences in heat exchangers of solar-powered absorption refrigeration systems

Solar cooling technologies include a solar power plant as a heat source and heat-using refrigeration machine as a cooling production. The article presents the analysis of temperature regimes and temperature differences in heat exchange devices of absorption refrigeration systems from the standpoint of ensuring the workability and high efficiency of processes. The analysis was performed on the example of a single-effect water-ammonia absorption machine. The analysis is based on the following conditions: the presence of the system limit cycle. That is, the interdependence of the temperature potentials of three heat sources, two of which are chosen arbitrarily, the third is a function of the first two; temperature regimes and temperature differences in the heat exchangers are given in interaction with the temperatures of external energy sources .The presence of a system total temperature differences, consistent with the limit cycle at known temperature levels of the heat sources, was established. The total head takes into account: the generator temperature range, the temperature at the generator hot edge, the temperature at the solution heat exchanger cold edge, the temperature differences at the absorber cold edge, at the condenser smallest temperature differences, water heating in the condenser. The issue of optimizing temperature differences in system elements is proposed to be solved by considering the variable component of capital costs associated with the cost of each heat exchanger depending on the temperature differences. For the practical implementation of cycles with the considered temperature regimes and solar powered generator on the basis of energy saving, cycles of absorption systems "with an extended degassing zone" are proposed. The cycles are implemented under conditions of different nature of temperature changes of energy sources and working fluid in the heat exchange processes between them. Solving problems is recommended by creating complex schemes and cycles of hybrid water-ammonia refrigeration systems

Method to delay frost formation under high relative humidity by choosing proper heat exchange temperature difference and air velocity

To delay frost formation and maintain high heat transfer capacity of an evaporator, a closed-loop wind tunnel was designed and constructed to investigate the effect of heat exchange temperature difference (ΔT) and air velocity on frost formation characteristics, refrigerating capacity and water removal of air. The closed-loop wind tunnel provided flowing air at temperature of −18 °C, relative humidity of 85% and velocity from 2 to 5 m/s. During experiment, three obvious stages of frost thickness were found when air initial velocity was 3–5 m/s. With ΔT changing from 7 to 3 °C, the water removal of air through the evaporator decreased by 3–5 times and the high efficient operation time ratio (HETR) increased by 2–3 times. Meanwhile, the average frost mass reduced by about half at any fixed air velocity. When ΔT was 7 °C and air velocity was 2 m/s, the average refrigerating capacity was almost equal to that when ΔT was 3 °C and air velocity was 5 m/s. The combined results showed that smaller ΔT and larger air velocity was useful to forbid frost formation, and to maintain stable high refrigerating capacity and relative humidity.

Pilot test of water-lean solvent of 2-(ethylamino) ethanol, 1-methyl-2-pyrrolidinone, and water for post-combustion CO2 capture

Water-lean solvent for post-combustion CO2 capture is promising to significantly reduce regeneration energy. It hasn’t been put into commercial application due to unsolved challenges including water balance issue, increased viscosity, and high volatility. Even though numerous water-lean solvents were developed in laboratory, rare of them were tested in pilot scale. Water-lean solvent of 2-(ethylamino) ethanol (EMEA) dissolving into 1-methyl-2-pyrrolidinone (NMP) and water was tested in a CO2 capture pilot with flue gas rate of 280 m3/h. Effects of liquid/gas ratio and reboiler steam rate on CO2 removal efficiency, regeneration energy, lean/rich heat exchanger performance, and volatile amine emission were investigated. Water-lean EMEA achieved CO2 removal efficiency greater than 90 % and the lowest regeneration energy of 2.3 GJ/tCO2, which is 43 % lower than that of 5 M MEA. Water-lean EMEA showed significantly reduced latent heat, greater CO2 capacity, and lower reaction heat (secondary amino) than aqueous MEA and amine blends. It was found that water-lean EMEA regeneration in stripper had extremely lower lean loading (0.1–0.5 mol CO2/L) than aqueous MEA and amine blends, which enables a faster CO2 absorption rate in absorber and a greater CO2 capacity, but also causes larger volatile amine emission. Volatile amine emission after water wash was 110–380 mg/m3. 13C NMR spectrum suggested amine carbamate dominated CO2 absorption products. Due to that increased viscosity decreases heat transfer coefficient (h), lean/rich heat exchanger temperature difference (ΔT) for water-lean EMEA (6.5–7.5℃) was greater than MEA (3.5-5℃) and amine blends (5-6℃), yet smaller than biphasic solvent (8-9℃). A negative power (-0.614) function relation between h and viscosity was fitted and can be further used to normalize sensible heat.

MULTIOBJECTIVE OPTIMIZATION OF CASCADE REFRIGERATION SYSTEM USING NH3-CO2 AND NH3-N2O REFRIGERANT PAIRS

In this paper a comparative analysis based on multiobjective optimization of a cascade refrigeration system using NH<sub>3</sub>-CO<sub>2</sub> and NH<sub>3</sub>-N<sub>2</sub>O refrigerant pairs has been done. Exergetic effciciency and overall cost rate have been taken as the two conflicting objective functions, and evaporator temperature, condenser temperature, LTC condenser temperature, and cascade heat exchanger temperature difference are the four design variables. The optimization work has been carried out using genetic algorithm, which results in a nondominated optimal solution represented in the form of pareto-optimal curve, and the technique for order performance by similarity to ideal solution (TOPSIS) method is used to select a unique solution. The results show that NH<sub>3</sub>-N<sub>2</sub>O is a better alternative as compared to NH<sub>3</sub>-CO<sub>2</sub> refrigerant pair from a multiobjective optimization point of view. The corresponding optimal operating conditions have also been suggested for both the refrigerant pairs.

Thermodynamic analysis and parameter optimization of a Chemical Looping Electricity Storage system

In this study, a novel Chemical Looping Electricity Storage (CLES) system which integrated thermochemical energy storage into the Pumped Thermal Energy Storage (PTES) system was investigated and the corresponding thermodynamic models considering the temperature difference between charge and discharge were developed. Then, the exergy flow distribution characteristics of the Chemical Looping Electricity Storage system using Ca(OH)2 and CaO working pair were revealed. The influence of the key thermodynamic parameters on the system's round-trip efficiency was investigated. Finally, approaches to further improve the round-trip efficiency were briefly discussed. The results show that the existence of the charge-discharge temperature difference will reduce system round trip efficiency as a result of the changing of the heat balance between the charge and discharge processes. Taking Ca(OH)2/CaO working pair as an example, when the charge temperature ranges from 750 to 850 K and the discharge temperature ranges from 650 to 750 K, the system can achieve a round-trip efficiency of 43.38–61.45 %, lower than that of the Pumped Thermal Energy Storage system at the same temperature range. The discharge temperature variation will influence the round-trip efficiency more significantly than the charge temperature. Adoption of a turbomachine with high isentropic efficiency, reducing the pressure loss within the reaction vessel and the heat exchange temperature difference within the recuperator, and optimizing the temperature ratio can all effectively improve the system round-trip efficiency.

Heat Transfer Modeling on High-Temperature Charging and Discharging of Deep Borehole Heat Exchanger with Transient Strong Heat Flux

High-temperature charging and discharging by deep borehole heat exchanger is typical of a large heat exchange temperature difference and transient strong heat flux. Simulation of this problem is not only computationally expensive, but it is also challenging in terms of robustness and stability for numerical methods. This paper formulates a generic and efficient heat transfer model with two distinctive novelties: Firstly, it highlights unsteady- and quasi-steady-state modeling strategies for heat transfer outside and inside a borehole. Secondly, this model provides analytical solutions for the heat front propagation and heat flux density distribution for unsteady-state heat transfer in the rock zone. These analytical formulations prove to be generic and critical to relieve computational effort in the face of strong heat flux. This model is validated by a typical high-temperature heat storage case from the literature, as well as the pilot demonstration project in China. It was discovered that a large prediction error of the heat transfer model only exists in very short operation days during the initial unsteady stages of charging and discharging. Both relative errors under charging and discharging phases are within 5% during the steady-state period. A comparison of the simulation cost with OpenGeoSys software demonstrates its high efficiency. It proves that this heat transfer model achieves an acceleration ratio of 30 times relative to the fully numerical method. In general, the heat transfer model has four advantages: generic applicability, good accuracy, easy implementation, and high efficiency, but it is limited to the heat transfer of a single deep borehole heat exchanger under pure heat conduction.

Research on the Influence of High-Low Pressure-Volume Ratio and Discharge Pressure on the Heating Performance of Transcritical CO2 Single- Machine Two-Stage Compression Heat Pump Water Heater

Background: In order to reduce global carbon emissions, a cross-critical CO2 singlemachine two-stage compression system was proposed. Due to the characteristics of cooling on the high-pressure side of the cross-critical cycle and bipolar compression on the single-stage, the discharge pressure and the high-low pressure volume ratio have a great influence on its heating performance. Objective: The purpose of this paper is to explore the influence of high-low pressure-volume ratio and discharge pressure on the heating performance of the system, so as to obtain the optimal operating conditions of the system. Methods: By constructing theoretical calculation formula of discharge pressure, high-low pressurevolume ratio and intermediate pressure, combined with experiments. Results: Under certain working conditions, with the increase of the volume ratio of high-low pressure, the intermediate pressure increases, the heating capacity of the system increases due to the increase of the discharge temperature and the increase of the heat exchange temperature difference on the hot water side, while the heating performance coefficient decreases slowly because the rate of the increase of the power consumption of the compressor is greater than the rate of the increase of heating capacity. With the increase of discharge pressure, the increasing rate of CO2 discharge temperature will gradually decrease at the high-pressure side, so the heating capacity of the system increases rapidly first and then tends to slow down, and the heating performance coefficient increases first and then decreases. There is an optimal value, that is, when the evaporation temperature is -20 °C, the maximum heating capacity can reach 3 kW. The system heat production performance coefficient has an optimal value of 3.29, and the optimal discharge pressure is 8.5 MPa. The optimal discharge pressure will increase with the increase of evaporation temperature. Conclusion: The selection of reasonable high-low pressure volume ratio and the control of discharge pressure can effectively improve the heating performance of cross-critical CO2 single-machine twostage compression heat pump water heater.

Concise analytical solution and optimization of compressed air energy storage systems with thermal storage

The evaluation of compressed air energy storage (CAES) system mostly focused on system efficiency and cost, while less attention has been paid to energy density in the past, and each performance expression was complex, making it difficult to obtain clear variation law of multiple indexes with key parameters, as well as the optimal coupling relationship among them. In view of the above problems, the research on concise analytical solution of CAES systems with thermal storage (TS-CAES) is carried out in this paper, in which a dimensionless pressure coefficient K and the parameter Z to describe thermal storage characteristics are established. Based on the above parameters and simplified model, the concise analytical expression of system efficiency and energy density for constant-volume and constant-pressure CAES systems are established. Based on the analytical expression, the influence law of each key parameter on the system evaluation indexes is revealed, and the optimal matching relationship of key parameters is obtained. It is revealed that without considering the heat exchange temperature difference, the system efficiency is only related to the thermal storage temperature, K and the efficiency of compressor/expander. The greater the temperature and K are, the higher the system efficiency is. The higher the temperature and pressure are, the higher the energy density of the two systems is. The energy density is more sensitive to the value of Z with lower temperature and higher pressure. It is also found that there is an optimal K under a certain temperature and pressure to maximize the energy density for constant-volume CAES, and the optimal value of K corresponding to 200 bar and 100 °C is 0.77.

Performance and economic evaluation of a solar-air hybrid source energy heating system installed in cold region of China

This paper proposes a solar-air source energy storage heating system (SASES-HS), which can solve the problems of high energy consumption and difficult defrosting when the ambient temperature is low. By coupling solar energy,air energy and phase change energy, the system heats the end of the user through a two-stage heat pump. In order to analyze the feasibility of the system, the mathematical model of the system is established, and an experimental platform is built. A typical experimental condition was chosen to analyze the performance of the system in detail. The results show that the system can continuously provide high-temperature hot water of nearly 60°C for buildings in a severe cold environment of −20°C for 24 h, and the heat exchange temperature difference is as high as 80°C. In addition, the system operates with a higher coefficient of performance than a single-stage heat pump system. The COP can reach the level of 4.1 during the day and as low as 2.5 at night. The operating cost of the system only accounts for 65.8% and 88.4% of the coal-fired boiler and the air source phase change heat pump, respectively. And the CO2 emissions of this system only account for 57.5% and 88.4% of the coal-fired boiler and the air source phase change heat pump, respectively. In view of the analysis of operational performance and economic and environmental benefits, it is found that the system is worth popularizing and using in the northern cold regions.

Thermo-economic performance enhancement in batch evaporation process via mechanical vapor recompression system coupled with steam accumulator

Energy consumption in batch evaporation process has aroused widespread concern around the world. In this investigation, a mechanical vapor recompression system coupled with steam accumulator (MVR-SA) was proposed to reduce the start-up time and enhance energy efficiency during the batch evaporation process. Mathematical models were established considering the first and second laws of thermodynamics and relevant economic theory. Performance analysis was carried out from energy, exergy and economic viewpoints. Experimental results showed that the MVR-SA system could achieve distinctly faster intermittent start-up than conventional MVR. Energy efficiency ratio was experimentally obtained as 6.25, the input exergy, output exergy, exergy destruction and exergy efficiency were 21.7 kW, 2.04 kW, 19.66 kW and 9.4%, respectively. Simulated results indicated that increasing the heat transfer temperature difference of heat exchanger and liquid level height of steam accumulator, or decreasing evaporation pressure, could increase the system input exergy and exergy destruction, which reduced the system exergy efficiency. The optimum compression ratio and heat transfer temperature difference were obtained to achieve the minimum total cost considering annual investment, operation and environmental costs. Finally, an economic performance comparison confirmed greater superiority for the novel MVR-SA compared to conventional single effect system in the batch evaporation fields.

An integrated energy storage system consisting of Compressed Carbon dioxide energy storage and Organic Rankine Cycle: Exergoeconomic evaluation and multi-objective optimization

In this paper, an integrated energy storage system consisting of Compressed Carbon dioxide Energy Storage (CCES) and Organic Rankine Cycle (ORC) was proposed. Four criteria (system exergy efficiency, total cost rate of exergy destruction, total product unit cost, and total exergoeconomic factor) were defined to evaluate the system performance from exergy and exergoeconomic points of view. The influence of key parameters on system performance was analyzed, and multi-objective optimization of the system was conducted. The results showed that for the base case, the net power output, system exergy efficiency, and total product unit cost were 27.736 MW, 66.64%, and 20.34 $/GJ, respectively. Recuperator had the largest exergy destruction (39.17% of the total exergy destruction) and a higher value of investment cost rate, signifying its necessity of optimization. Sensitivity analysis demonstrated the monotonic effects of compressor inlet temperature, turbine inlet temperature, and minimum temperature difference in heat exchangers on system performance, but for pressure ratio or pump2 outlet pressure, there was an optimal value for the system performance within the range of values studied. Finally, multi-objective optimization recommended a 72.6% for system exergy efficiency, 452.35 $/h for total cost rate of exergy destruction, and 18.49 $/GJ for total product unit cost.

Numerical and experimental investigation of enhanced heat transfer radiator through air deflection used in fuel cell vehicles

Proton exchange membrane fuel cells are an ideal power source for modern energy vehicles owing to their high efficiency, fast response, and zero emission. Because of the low heat exchange temperature difference and high power load, the cooling system in internal combustion engine vehicles can no longer satisfy the heat dissipation demand of fuel cell vehicles (FCVs). Thus, the thermal management of FCVs must be improved, for which the compact radiator is a crucial component that discharges most of the waste heat outside the FCV. By inserting metal elbows into the airside of a compact radiator, we propose a novel structure aimed at promoting the thermal performance of a traditional compact radiator. The thermal and flow characteristics of the two types of radiators (traditional and optimised radiator) are compared, and the effect of the inserted windward bend structure is investigated numerically and experimentally. The Colburn j-factor and Fanning friction f-factor are calculated to evaluate the airside performance of the two radiators. The results indicate that the heat exchange is improved by 17.1%, 19.6%, 19.3%, and 22.5% accompanied with a modest increase in the pressure drop of 0.1%, 18%, 21%, and 18%, respectively, under four different operating conditions.

Influence of key parameters on the performance of a helium cryogenic system in refrigeration and liquefaction modes

At present, medium-scale helium cryogenic system is commonly based on modified Claude cycle with two expanders in series. In this study, considering the minimum allowable heat exchanger temperature difference (MHTDamd), the influence of key parameters (including compressor discharge pressure (Pcd) , temperature at the inlet of expander1 (Tin-EXP1) and temperature at the inlet of Joule-Thomson valve (Tin-JT)) on the system performance (including refrigeration capacity (RC)/liquefaction rate (LR), specific energy consumption (SEC) and total UA of the heat exchangers (UAtotal)) of the helium cryogenic system is investigated and analyzed in refrigeration and liquefaction modes, respectively. The results show that in refrigeration and liquefaction modes, the optimal ranges of Pcd, Tin-EXP1, and Tin-JT are within 8 ∼ 10 bar, 30 ∼ 45 K and 5.2 K ∼ 7.2 K, respectively. Furthermore, the constraint relation between UAtotal and SEC is obtained. By using genetic algorithm (GA), it is found that keeping constraint SEC within 100%∼115% of the chosen SEC from the optimal range can achieve a relatively economic trade-offs between SEC and UAtotal. In the end, a comparison of T-S diagram obtained from design and experimental values in refrigeration and liquefaction modes is carried out, respectively. The maximum temperature deviation is within 5%, which verifies the accuracy of the calculation models. The optimized results in this study are also applicable to the same processes with mass flow rate of compressor lower than 100 g/s.

Experimental study on the effect of heat transfer temperature difference on frosting characteristics of air-cooled heat exchanger

In this paper, the frosting experiment was carried out by the wind tunnel experimental device in the single experimental section of the closed-mouth return flow. Through the experimental study on the influence of heat exchange temperature difference on the growth characteristics of the frosting layer in the finned evaporator under the experimental conditions of different initial head wind speeds, the following conclusions were drawn: At 2m/s, the frost thickness of finned tube evaporator increases gradually with the frosting time at different heat exchange temperature differences. At 3, 4, 5m/s initial face wind speed, the frost thickness of finned tube evaporator grows in a three-stage pattern at different heat exchange temperature differences. The cooling capacity of the evaporator at different face wind speeds and different heat transfer temperature differences increases first and then decreases with the frosting time.

Indirect mechanical heat pump assisted humidification‐dehumidification desalination systems

In this study, four new indirect heat pump assisted mechanical vapor compression humidification-dehumidification (HDH) systems are proposed and their superiorities over the reference system are demonstrated from thermodynamics and thermoeconomics viewpoints. The proposed models are configured based on an HDH unit and a simple cascade heat pump, an HDH unit and a heat pump with an ejector, an HDH unit and a cascade heat pump with an ejector, and an HDH unit and a vapor injection heat pump. Although employing a heat pump with cascade and ejector configurations improved gain-output-ratio (GOR) and specific power consumption (SPC) values in comparison with the base system, the performance metrics of such layouts are still inferior to those of the HDH unit coupled with the vapor injection heat pump. It is found that the desalination unit with vapor injection mechanism has the highest GOR and the lowest SPC values of 9.22 and 71.29 kWh/m3, respectively, under constant electrical power of 31.26 kW. Overall, proposal of the cascade heat pump, heat pump with an ejector, cascade heat pump with an ejector, and a vapor injection heat pump instead of the conventional heat pump improved GOR by 4.83%, 10.03%, 11.52%, and 14.25%, respectively. Meantime, the SPC of the HDH unit coupled with a simple heat pump decreased by 4.6%, 9.1%, 10.26%, and 12.46% when a cascade heat pump, a heat pump with an ejector, a cascade heat pump with an ejector, and a vapor injection heat pump were used instead of the conventional heat pump, respectively. Also, it is found that a maximum improvement of 14.13% can be achieved in exergy efficiency with a cost penalty of only 1.59 $/m3. Also, it was found that with the use of a simple cascade heat pump instead of the simple heat pump, the exergy efficiency can be enhanced by 4.9%, while the unit cost of the distilled water (UCDW) was degraded by 51.32%. Likewise, the HDH unit coupled with the cascade heat pump with an ejector was resulted in high UCDW compared to the base system, while its exergy efficiency was not enhanced so significantly. Variation of exergy efficiency versus the intermediate temperature had a maximum value of 1.93% (for the HDH unit coupled with a simple cascade heat pump) and 2.05% (for the HDH unit coupled with a cascade heat pump with an ejector) at Tint = 315 K. Moreover, it was found that the UCDW can be decreased with the rise of the intermediate temperature and cascade heat exchanger temperature difference as the fresh water cost rate decreases as a result, while the exergy efficiency can be increased with the rise of the cascade heat exchanger temperature difference.