Increase In Coefficient Of Performance Research Articles

Thermodynamic study of zeolite and graphene nanoplatelet-based composites for adsorption cooling systems

The consolidated composite adsorbents containing AQSOA-Z02 zeolite and graphene nanoplatelets (GNPs), recently developed and exhibiting excellent thermophysical and water adsorption characteristics, are found to overcome the above-mentioned limiting factors. However, rigorous thermodynamic analysis has not yet been studied, which is crucial in the simulation of adsorption characteristics, system design, and ACS analysis. Therefore, this study aims to conduct a rigorous thermodynamic investigation and the assessment of performance parameters for various AQSOA-Z02 – GNPs–based composites/water pairs. In the thermodynamic analysis, the heat of adsorption (Qst) and the adsorbed phase-specific heat capacity (cp,a) are investigated theoretically. The study reveals that Qst decreases with adsorption uptake, while cp,a exhibits the opposite trend. The incorporation of H25-GNPs into zeolite leads to a reduction in both Qst and cp,a, contributing favorably to the design of an energy-efficient ACS. Additionally, the performance parameters such as specific cooling effect (SCE) and coefficient of performance (COP) are theoretically investigated with variations in desorption and evaporation temperatures. Both SCE and COP increase with evaporation and desorption temperatures. The composite containing 90 wt% AQSOA-Z02 and 10 wt% H25-GNPs exhibit the maximum COP value of 0.351 and the maximum volumetric cooling effect of 581.621 MJ/m−3(−|-), which are almost 20 % and 41 % improvement over parent AQSOA-Z02 zeolite, respectively. All the findings of this study will greatly assist in the design of an energy-saving and eco-friendly adsorption cooling system.

Study on Waste Heat Recovery of Fuel‐Cell Thermal Management System Based on Reinforcement Learning

Fuel cells are very sensitive to temperature. To enhance the temperature stability of fuel cells, in this article, a reinforcement learning control method is proposed to regulate the speed of the cooling water pump, and a waste heat recovery structure is designed based on this. First, the model's accuracy is validated by comparing it with experimental data. Subsequently, in the simulation results, it is shown that reinforcement learning control improves average temperature regulation capability by 5.6% compared to proportional‐integral‐derivative (PID) control. Following the incorporation of waste heat recovery, the energy consumption of the pump is reduced by 0.04 kWh in comparison to PID control. Similarly, for the air‐conditioning system, the energy consumption of the compressor is reduced by 0.63 kWh. For the same driving distance, hydrogen consumption decreases by 80 g. Additionally, the coefficient of performance increases by 1.5% in the waste heat recovery mode.

Experimental and simulation study on the performance of a solar assisted multi-source heat pump drying system in Zhengzhou area

Zhengzhou of Henan province is an important agricultural production base. To improve the performance of the combined drying system and promote the development of agricultural products in Zhengzhou area, a solar assisted multi-source heat pump (SMSHP) drying system was proposed based on a traditional solar assisted air source heat pump (SASHP) drying system. The simulation models were developed for the air source heat pump (ASHP), SASHP, and SMSHP drying system respectively. The coefficient of performance, energy consumption and specific moisture extraction rate of the drying systems under different seasons were analyzed. Moreover, to validate the simulation models, an experimental platform of the combined drying system was designed and set up, and the experimental tests were carried out on the ASHP, SASHP, and SMSHP drying system. The simulation results revealed that the SMSHP drying system had lower energy consumption, higher coefficient of performance and specific moisture extraction rate than that of the SASHP drying system. The most significant performance enhancement was observed in spring, where the SMSHP system demonstrated a 22.69 % increase in coefficient of performance, a 19.2 % increase in specific moisture extraction rate, and a 16.11 % decrease in energy consumption compared to the SASHP drying system. Comparing with the experimental data, the simulation model errors of the energy consumption, coefficient of performance, and specific moisture extraction rate of the SMSHP drying system were 3.19 %, 0.28 %, and 2.27 %, respectively. Therefore, the accuracy of the simulation results was confirmed. To better calculate the heat provided by the solar collection system, the concept of the tank heating guarantee rate was first introduced, and it was found to be more scientific than the solar energy guarantee rate. At last, The economic and environmental benefit of the drying systems were analyzed during the life cycle, and the SMSHP drying system exhibited better comprehensive performance than the SASHP drying system. This paper provides valuable insights into the comprehensive utilization of solar energy and the control logic of solar assisted air source heat pump combined drying system under different seasons.

Efficiency enhancement in a heat-driven single-unit thermoacoustic refrigeration system

Heat-driven thermoacoustic cooling technology provides promising alternatives for eco-friendly refrigeration systems due to its high reliability and minimal environmental impact, meanwhile facing limited efficiency compared to current prevailing cooling technologies. The present work introduces an innovative bypass configuration in heat-driven thermoacoustic refrigeration systems aimed at enhancing the energy coupling between the engine and refrigerator, thereby enhancing system efficiency, particularly when the sources temperature is high. The acoustic power diversion mechanism of the bypass tube is firstly explored in an ideal thermoacoustic core unit through theoretical analysis. This core unit is subsequently studied in Sage platform and integrated into an actual single-stage looped traveling-wave thermoacoustic refrigeration system, respectively, and the effects of the bypass tube dimensions and operating conditions on the system are investigated. Furthermore, a comparative analysis is conducted between the proposed bypass tube-based system and the conventional bypass tube-free system, considering aspects such as system performance, key parameter distribution, and exergy loss analysis. The results demonstrate a significant efficiency enhancement, with a 68.8% increase in Coefficient of Performance (COP) and a 6.2 percentage point boost in relative Carnot efficiency compared to the system without a bypass tube. Additionally, the impact of the bypass dimensions and the DC flow introduced by the bypass tube on the system performance is further discussed at a numerical level. These findings offer valuable insights and serve as a reference for the future development of more efficient thermoacoustic refrigeration systems.

Enhancing the efficacy of humidifier-dehumidifier desalination in humid regions through the use of an absorption refrigeration cycle (an economic and experimental investigation)

This experimental study investigates the performance of a novel humidifier-dehumidifier (HDH) desalination system integrated with a vapor absorption refrigerator (VAR) and photovoltaic (PV) panels for humid regions. The effects of air mass flow rate (m˙a) and inlet air temperature on key performance indicators were evaluated. Increasing m˙afrom 0.0039 kg/s to 0.0071 kg/s led to a 73 % increase in freshwater output, a 35 % increase in Coefficient of Performance (COP), a 100 % increase in Gain of Output Ratio (GOR), and a 75 % increase in Recovery Ratio (RR), but a 20 % decrease in dehumidifier efficiency. The maximum freshwater production rate of 870 g/h and a GOR of 1.4 were obtained at m˙a= 0.0071 kg/s and inlet air temperature of 50 °C, representing a 117 % increase compared to the lowest temperature and flow rate tested. The cost of freshwater for solar and electric operation modes was 0.075 and 0.094 USD/kg, respectively. The system's novelty lies in the integration of HDH desalination with a VAR and PV system, enabling continuous freshwater production even under cloudy or nighttime conditions. The findings demonstrate the potential of the proposed system for sustainable freshwater production in humid areas.

Numerically investigating the effect of using nanofluids on the thermal performance and coefficient of performance of a U-tube deep borehole ground source heat pump

In this study, a deep-borehole ground source heat pump was simulated using computational fluid dynamics (CFD) method. The thermal performance and coefficient of performance (COP) of the vertical ground source heat pump simulated with accuracy using CFD for water and 4 different nanofluids as working fluids were investigated. Two nanofluids contained silver and copper metallic nanoparticles and the other two nanofluids contained copper and aluminum oxide nanoparticles. For each of these nanofluids as well as water, the heat exchange rate, total exchanged heat, outlet temperature and COP were calculated and the results were compared together. The results showed that using nanofluids leads to increased exchanged heat. Nanofluids based on aluminum oxide, copper oxide, copper and silver nanoparticles resulted in 4.83, 5.76, 6.9, 7.69 percent increase in total exchanged heat, respectively. In addition to increased exchanged heat, using nanofluids increased the outlet temperature. Also, the results showed that the maximum and minimum increase in COP were 36.5% and 5.2% which were related to using silver and aluminum oxide nanoparticle-based nanofluids, respectively.

Effect of the refrigerant charge on transcritical CO2 direct evaporation ice-making system

The low global warming potential and zero ozone depletion potential of CO2 make it highly suitable for application in ice rinks. The refrigerant charge prediction model of the transcritical CO2 direct evaporation ice-making system is proposed and compared. According to the predictive results, the performance of the ice-making system was experimentally studied at three predictive refrigerant charges. The results show that the ice-making system with the refrigerant charge predicted by the comprehensive model with GC + ECH (gas cooler refrigerant charge model and evaporator refrigerant charge model with Hughmark correlations) presents the optimum performance. The refrigerant charge in the pipeline accounts for 46.6 %–57.2 % of the total refrigerant charge, so proper pipeline arrangements are crucial to reduce the overall refrigerant charge, especially before and after the throttle valve. The experiment data shows that the optimal high-side pressure of the ice-making system is between 7.9 MPa and 8.3 MPa, and the optimal COP (coefficient of performance) is between 1.34 and 1.75. Excessive or insufficient refrigerant charge leads to overall superheating or overall unsaturation of the refrigerant at the outlet of the evaporator. At the proposed refrigerant charge, both cooling capacity and COP increase by 8.3 %–38.3 % and 21.5 %–30.6 % respectively compared to other charges tested. In addition, the ice-making time at the proposed refrigerant charge can be saved by 10.0 %–15.6 %.

Experimental study and exergy analysis of electric vehicle single-stage compression secondary throttling CO2 air-source heat pump system in cold climate

A novel CO2 air-source heat pump system (ASHPs) with a secondary throttling based on a single-stage compression was proposed to further improve the heating energy efficiency of electric vehicles (EVs) for heating in cold climates. The heating characteristics of the secondary throttling system (STS) and conventional throttling system (CTS) at different indoor air supply temperatures were compared. The STS performance characteristics were investigated under varying ambient temperature, outdoor air velocity, and exergy analysis was performed on the system and its components. A fitting correlation equation for optimal discharge pressure was obtained based on experimental data. The results demonstrate that implementing STS significantly reduces CO2 ASHPs energy consumption (Wcom), enhances the coefficient of performance (COP), and reduces exergy destruction. When the indoor air supply temperature reaches 45 °C, using STS can result in an 8.1 % reduction in Wcom, a 9.4 % increase in COP, and a 6.1 % improvement in the exergy efficiency (Exη) compared to that of CTS. Furthermore, the advantages of STS become more pronounced with higher indoor target air supply temperatures. As the ambient temperature decreases, the exergy efficiency of the STS reaching the target air supply temperature decreases. The velocity of outdoor air significantly affects the STS performance, and increasing outdoor air velocity can effectively enhance the Exη of STS. The optimal discharge pressure fitting equation for the STS was obtained through multiple linear regression analysis. These findings can provide ideas for architectural optimization and design of CO2 ASHPs for EVs, which are important for further improving system performance.

Optimizing R152a/R600 and R290/R600 mixtures for superior energy performance in vapor compression systems: Promising alternatives to Isobutane (R600a)

This study presents an experimental investigation into the optimal composition of R152a/R600 and R290/R600 mixtures as potential alternatives to isobutane (R600a) in a single-stage vapor compression cycle. The research involved testing eight different mass compositions for R152a/R600 and ten mass compositions for R290/R600, comparing their performance with that of R600a. After careful analysis, the “optimum” mass compositions were identified as (10/90)% for both R152a/R600 and R290/R600, achieving a balanced trade-off among coefficient of performance (COP), volumetric cooling capacity (VCC), and cooling output (Q̇o). Further tests were conducted on the “optimum” compositions at nine different secondary fluid temperature inlets. The results indicated an average COP increase of + 7.3 % for the R152a/R600 mixture, with decreases of −11.4 % in VCC and −9.7 % in Q̇o. Similarly, the R290/R600 (10/90)% mixture showed a COP increase of + 10.3 %, accompanied by VCC and Q̇o decreases of −8.8 % and −6.6 %, respectively. Notably, both mixtures exhibited superior energy performance compared to isobutane while maintaining similar thermodynamic properties, particularly the R290/R600 (10/90)% mixture. These findings suggest that the R152a/R600 and R290/R600 mixtures could serve as long-term, high-efficiency alternatives to R600a in vapor compression cycles.

Thermodynamic and economic analyses of modified ejector enhanced solar-air composite dual-source heat pump system in residential buildings

This research utilizes an ejector enhanced solar-air composite dual-source heat pump (EDHP) system for water heating in residential buildings. The main advantage of the EDHP system is the use of an ejector and dual-heat source to efficiently utilize solar energy, thereby enhancing heating performance. This ejector operates by utilizing a refrigerant stream from the solar collector line with higher pressure to elevate the suction pressure of the compressor. This feature enables the EDHP system to maintain stable heating performance even in adverse weather scenarios with low temperatures or limited solar radiation. Thermodynamic modeling using energetic and exergetic analysis methods is employed to theoretically evaluate the conventional air-source heat pump and EDHP systems. The energy analysis of the EDHP system using R134a refrigerant has shown significant improvements in both heating coefficient of performance (COP) and exergy performance (ECOP). Under specific conditions, such as an ambient temperature of −5 °C and an irradiance level of 250 W∙m2, and utilizing a 20 m2 PVT (Photovoltaic and Thermal) collector for a heating supply of 5 kW, the EDHP system exhibits a 23.64 % increase in COP and a 25.83 % increase in ECOP. Furthermore, when the electricity generated by the PV system is incorporated into the EDHP system, the demand for electricity from the grid can be reduced by up to 51.96 % under the same conditions. In the Beijing area and considering the current energy price system, the heating operating cost of the EDHP system is 0.206 Yuan/kWh. Compared to a standard air-source heat pump system, the payback period for EDHP system ranges approximately from 10 to 12 years, taking into account an annual heating period of 5 months. Among the alternative refrigerants to R134a in the EDHP system, R152a is found to be the most suitable. It shows COP improvements of 24.92 %–30.0 % and ECOP improvements of 24.89 %–29.96 %. This innovative approach opens up new possibilities for the application of heat pump technology under adverse weather conditions in residential and commercial buildings.

Methods for increasing the efficiency of climate control systems for electric buses

Increasing the range of an electric bus on a single battery charge is an important task facing electric bus developers. The auxiliary system that consumes the largest amount of electricity is the air conditioning system. This paper examines the main methods for increasing the efficiency of a climate control system and evaluates them using numerical methods using a verified one-dimensional mathematical model. The following methods were chosen to improve efficiency: the most suitable refrigerants, the use of an internal heat exchanger of a vapor compression unit, the recovery of heat from the air removed from the interior electric bus in the evaporator of a vapor compression unit, and the use of heat from electrical equipment in the climate control system. The greatest efficiency in cooling and heating modes is shown by the use of refrigerants R404a, R410a and R507a in the vapor compression unit. The use of an internal heat exchanger in a vapor compression unit allows an increase in coefficient of performance by 11%. The combined use of cabin heat in the evaporator of the vapor compression unit and the internal heat exchanger allows the coefficient of performance to be increased by 27%. Using the heat of electrical equipment allows you to increase the coefficient of performance in heat pump mode by 20%. The most promising is the combined use of the above methods to reduce the cost of air conditioning in the cold season.

EFFECT OF SiO2 AND ZnO NANOPARTICLES TO INCREASE REFRIGERATION MACHINE PERFORMANCE

In this investigation, the impact of silicon dioxide (SiO2) and zinc oxide (ZnO) nanoparticles on the performance of a refrigeration machine system was systematically examined. The focus was on evaluating the coefficient of performance (COP) concerning the utilization of a polyolester (POE) lubricant, R600a refrigerant, and distinct nanoparticles (SiO2 and ZnO) within the refrigeration system. The nanoparticles were individually introduced into the R600a refrigerant in masses of 0.5 g, 1.0 g, and 1.5 g. The experimental outcomes demonstrated a noteworthy enhancement in COP with the addition of nanoparticles. Specifically, the introduction of 1.5 g of SiO2 resulted in a substantial increase of 25.88% in COP, marking it as the most influential dosage. Similarly, the addition of 1.0 g of ZnO led to a significant COP increase of 13.6%, representing the optimal quantity for ZnO. Furthermore, the inclusion of 1.5 g of SiO2 brought about a remarkable reduction in energy consumption, with a decrease of 25.58%, while 1.5 g of ZnO resulted in a notable 16.28% decrease in energy consumption. The experimental configuration involved the use of 20 g of refrigerant and 500 ml of POE lubricant. Comparative analysis demonstrated that the refrigeration system incorporating nanoparticles outperformed the conventional R600a refrigeration system devoid of nanoparticles. This study contributes valuable insights into the potential enhancements in refrigeration system efficiency through the strategic incorporation of SiO2 and ZnO nanoparticles, offering a promising avenue for optimizing the performance of refrigeration technology.

Thermodynamic and Environmental Analysis of Novel Cascade Refrigeration Cycles with Ejector and Intercooler for Ultralow Temperatures Using Eco‐Friendly Refrigerants

The present study introduces a novel cascade ejector intercooler refrigeration cycle (CEIRC) and a novel cascade ejector booster refrigeration cycle (CEBRC), which are designed to improve ultralow temperature (ULT) refrigeration system performance based on various refrigerants in terms of energy, exergy, and environment. Through the analysis conducted with eco‐friendly refrigerants for CEIRC, the refrigerant pair exhibits the highest performance that is found to consist of RE170 for high‐temperature cycles and R170 for low‐temperature cycles. These refrigerants have very low global warming potential and are eco‐friendly. The coefficient of performance (COP) reveals improvements of 3.20–9.51%, 10.55–19.12%, and 31.96–65.97% over the CEBRC, cascade ejector refrigeration cycle (CERC), and cascade vapor compression refrigeration cycle (CVCRC), respectively, resulting in corresponding exergy efficiency improvements of 3.18–9.49%, 10.48–19.12%, and 31.87–65.98%. The total compressor power and exergy destruction are also compared, indicating that the CEIRC exhibits the best performance among the four. CEIRC achieves a COP increase of 11.6% compared with the cascade refrigeration cycle, which is considered the cycle with the highest performance operating at ultralow temperatures in the literature, which is CERC. The kg CO2 effect of the CEIRC is 11.35% lower than that of the CERC.

Numerical investigation on the thermal performance of energy tunnels under groundwater flow and tunnel airflow

The integration of underground tunnels with ground heat exchangers (GHEs), also known as energy tunnels, is a promising technology that has gained attention in energy geotechnics research. This study investigates the coupled effects of groundwater and tunnel-air flows on the energy tunnel system via 3D thermo-hydraulic numerical modelling. It is found that the combination of groundwater flow parallel to the tunnel and limited airflow velocity results in reduced operational efficiency of ground source heat pump (GSHP) due to the strong thermal interference along the tunnel. The study also highlights the importance of real-scale modelling to evaluate the thermal yield of long energy tunnels, particularly when dealing with parallel groundwater flow in geothermal tunnel design.
 The use of shallow geothermal energy is a renewable solution towards net-zero carbon emissions. In recent years, a variety of conventional geotechnical structures, such as piles, retaining walls and tunnels, have been transformed into energy geostructures to also serve as GHEs to harness renewable energy for space heating and cooling purposes [7]. The thermal performance of energy tunnels is significantly affected by the presence of groundwater flow [1, 6] and the airflow in the tunnel [2, 4]. The groundwater flow and tunnel airflow can both be present in the field, however, their coupled effects on the energy tunnel system are hardly explored in the literature. This study aims to bridge this gap via real-scale numerical investigations.
 This study uses 3D thermo-hydraulic numerical modelling developed with the finite element package COMSOL Multiphysics [3]. The numerical model was successfully validated against an energy tunnel model-scale laboratory experiment [5]. Figure 1 shows the overall geometry and boundary conditions of the numerical model. A 48 m long tunnel section is thermally activated with the embedment of HDPE pipes (outer diameter of 25 mm and 2.3 mm thickness) placed in the lining segments at a spacing of 200 mm. A single tunnel ring consists of six segments with absorber pipes joined between them. Every four adjacent rings are connected in series to form a complete loop (with a single inlet and outlet), which is then connected in parallel to a flow and return header pipe(s). The entire thermally activated section comprises six loops, with identical fluid inlet temperature and flow rates, as they are fed by the same flow header pipe. The outlet fluid from each loop is mixed along the return header pipe to the GSHP. A constant 36 kW cooling thermal load is applied, and the simulation is run for ten years to allow the system to reach a steady state condition considering the minimum groundwater flow and airflow. The thermal properties of the ground are prescribed as 2 W/(m·K) and 1100 J/(kg∙K) for thermal conductivity and specific heat capacity, respectively.
 Figure 2 shows the entering water temperature (EWT, header outlet) and the coefficient of performance (COP) of the GSHP after 10 years of operation for different groundwater flow velocities and directions for tunnel air velocities = 0.05 m/s and 2 m/s.
 Results show that an increase in and leads to a decrease in EWT and an increase in COP. However, the effect of groundwater direction and velocity is more significant when airflow is limited ( = 0.05 m/s). The parallel groundwater flow leads to a higher EWT and lower COP than when the flow is inclined and perpendicular to the longitudinal tunnel axis. Consequently, the thermal performance of the system under parallel flow with a higher can be even worse than the perpendicular and inclined flow with a lower . When = 0.05 m/s, the EWT of parallel flow increases by 15% to 20% compared to the perpendicular flow, resulting in a maximum decrease of 15% in the COP. The effect of groundwater flow is much less significant when airflow is fast ( = 2 m/s): for increases from 0.05 to 2 m/d, EWT and COP change by no more than 13% and 5%, respectively, and the impact of groundwater flow direction is minor.

Proposal of a double ejector-two flash tank absorption refrigeration cycle: Energy, exergy and thermoeconomic evaluation

This research presents the proposal of a double ejector-two flash tank absorption refrigeration cycle, in which vapor and liquid ejectors are implemented simultaneously before the condensers and absorbers components. For the vapor ejector simulation, the shock circle approach is used. The internal environment of the vapor ejector, including the chocking phenomenon and the irreversible shock process, is also thoroughly examined. It is proposed that ejectors and flash tanks can improve the system performance. It is possible to improve the ejector entrainment ratio by incorporating a flash tank between the evaporator and condenser, which would also increase the evaporator cooling effect. Moreover, a second flash tank is installed between the generator and the absorber, which allows the generator to operate at a lower temperature and thus reduces the generator heat load. Consequently, the system's COP (coefficient of performance) increases. Furthermore, the unit cost of the final product in a wide range of operational conditions is calculated. The findings revealed up to 56.8% increase in the COP. The improvement of the maximum exergetic efficiency in the new cycle compared to the basic cycle also reaches 22.5%. Moreover, the thermoeconomic investigation show up to 34.6%. reduction in product cost.

Study of refrigeration system with minichannel condenser using R1234ze, R134a, R152a, R600a, R290 and mixture of R290/R600a (50/50)

A versatile vapor compression refrigeration (VCR) system is designed, developed and manufactured for testing different alternative refrigerants to R134a. The refrigerants such as R1234ze, R152a, R600a, R290 and mixture of R290/R600a (50/50) refrigerants are tested with an aluminum minichannel condenser which is designed for R134a. All the refrigerants are experimented for condensation temperature varying from 40 to 55°C while evaporation temperature varying from −10 to +15°C for every condensing temperature. The performance parameters like volumetric refrigerating capacity, actual work consumption of compressor, coefficient of performance (COP) and condenser capacity are compared for different alternative refrigerants. Aluminum minichannel condenser has reduced condensing temperature by 2–3°C with an increase in COP by 10%. The ratio of volumetric refrigerant capacities for R152a, R1234ze, R600a, R290 and HC mixture of R290/R600a (50/50) with respect to R134a are found to be 1.296, 1.143, 0.736, 1.749 and 1.067, respectively which is useful to select specific compressor. The ratio of compressor work for R600a, R290, HC mixture of R290/R600a, R152a and R1234ze with respect to R134a are found to be 1.81, 1.812, 1.867, 1.6 and 1.1, respectively. The ratios of condenser capacity of R600a, R290, HC mixture of R290/R600a, R152a and R1234ze with respect to R134a are found to be 1.914, 2.061, 1.939, 1.65 and 1.064, respectively. It is found that COP of R152a, R600a and R1234ze are larger by 11.53, 5.38 and 27% over R134a whereas COP of R290/R600a (50/50) and R290 are decreased by 0.75% and 14.23% over R134a, respectively. The exergy loss analysis for VCR system components is carried out at condensing temperature of 44°C while evaporating temperature ranging from −10 to +15°C for all the refrigerants and found that total exergy loss is more for R600a among all refrigerants.

The thermodynamic potential of high-temperature transcritical heat pump cycles for industrial processes with large temperature glides

Industrial heat pumps up to 200 °C are an emerging technology with the potential to reshape the industrial heating supply. For large heat sink temperature glides, transcritical cycles are able to increase the power-to-heat efficiency. Its potential is however yet to be unlocked. To examine this potential, a thermodynamic optimization model is proposed. The model includes robust cycle optimization, is able to screen a large set of working fluids, and includes proper post-processing. This model is applied to three highly relevant industrial cases, namely thermal oil heating, superheated steam drying and spray drying. The heat sink temperature glides for the respective case studies are 60 K, 81 K and 105 K. The results show that a temperature glide larger than 60 K is desired to achieve a better coefficient of performance (COP) with transcritical cycles compared to the classical subcritical cycles. Moreover, potential working fluids were identified for these high operational temperatures. For the case study with a heat sink temperature glide of 81 K, transcritical cycles allowed for a COP increase of 4.6%, whereas this increased to 7.3% for a heat sink temperature glide of 105 K. Furthermore, transcritical cycles introduce a much larger volumetric heating capacity, a lower compressor discharge temperature and a substantially lower pressure ratio. In addition, the best performing working fluids for subcritical cycles are highly flammable, which is only the case for some transcritical working fluids. Therefore, these cycles can be beneficial for temperature glides below 60 K. The compressor for transcritical cycles should however be able to cope with pressures up to 60 bar. If these compressors are available, transcritical cycles are shown to be superior compared to classical subcritical cycles.

Energetic and exergetic analysis of a Vapour compression refrigeration test rig in varying concentrations of (TiO2-SiO2/MO) hybrid nano-lubricants and R600a refrigerant charges

ABSTRACT Every residential refrigerator employs a Vapor Compression Refrigeration (VCR) technology. The performance of a VCR system is mostly determined by the input power and refrigeration effect. In present study, effects of varying concentrations of TiO2-SiO2/MO hybrid nano-lubricant (0.1, 0.2, 0.3, and 0.4 g/L), and mass charge of refrigerant R600a (80, 100, and 120 g) investigated by the authors. The performance parameters of the VCR test rig optimized at 0.2 g/L of hybrid nano-lubricant concentration using 100 g mass charge of R600a refrigerant. At this optimum condition, the percentage increase in coefficient of performance was 37.83% compared to the pure lubricant using 80 g mass charge of R600a refrigerant. The percentage reduction in input power at optimum condition was 13.46% compared to pure lubricant case using 80 g R600a, refrigerant mass charge. The maximum value of second law efficiency and the least value of exergy lost were found at 0.2 g/L concentration of hybrid nano-lubricant using 100 g charge of R600a refrigerant, with values of 77% and 96.90W, respectively. In conclusion, performance of VCR test rig altered when the concentrations of hybrid nano-lubricant and refrigerant mass concentrations changed.

Experimental comparison of R290 and R600a and prediction of performance with machine learning algorithms

The use of alternative refrigerants is among the popular topics of the refrigeration industry. In the first part of this study, thermodynamic performances of R290 and R600a gases were compared in a vapor compression refrigeration experiment setup. Although R600a caused an average of 33.44% less compressor power consumption compared to R290 refrigerant, R290 provided an average of 23.77% increase in COP (coefficient of performance), 82.55% in cooling capacity, and 20.99% increase in second law efficiency compared to R600a. In the second part of the study, the performance parameters of the refrigeration system were predicted with MLP (multi-layer perceptron), SVM (support vector machine), and DT (decision tree) machine learning algorithms. It was detected that the SVM method predicted all parameters with the least error. MAE (mean absolute error) values detected in the COP prediction with test set were 0.0317, 0.0324, and 0.0989 for SVM, MLP, and DT, respectively. Results revealed that performance of the refrigeration system increased when utilizing R290, and SVM was superior in prediction of performance indicators compared to other machine learning methods.

Numerical simulation of CO2 two-stage compression refrigeration system with external intercooler

The CO2 two-stage compression refrigeration system is a feasible alternative to conventional refrigeration systems for logistics cold storage cooling that provides high efficiency and environmental protection. In a previous study, we proposed a two-stage compression refrigeration cycle with an external intercooler and performed a thermodynamic analysis. Based on that, a more comprehensive simulation analysis is performed herein using the validated models. The performance, optimal discharge pressure, and intermediate pressure are investigated using a high-pressure compressor with both fixed and variable speeds. The results show that the cooling-water temperature has a significant positive linear correlation with the optimal discharge pressure, whereas the evaporator-inlet air temperature has little effect at a fixed high- pressure compressor speed. When the high-pressure speed is adjusted to achieve the optimal intermediate pressure and discharge pressure simultaneously, the coefficient of performance increases by 4.5% compared with that of the fixed-speed system, while the cooling capacity decreases by 8.4%. The correlation of optimal discharge pressure and optimal intermediate pressure is given.