Maximum Coefficient Of Performance Research Articles

Comparative thermodynamic study of CO2 transcritical supermarket booster refrigeration system combined with vapor injection and parallel compressionÉtude thermodynamique Comparative du système de réfrigération transcritique de supermarché au CO2 combiné à l'injection de vapeur et à la compression parallèle

The use of CO2 refrigerants in supermarket refrigeration has recently demonstrated great potential owing to their excellent thermophysical properties and environmental protection. To further improve the efficiency of transcritical CO2 refrigeration systems, a parallel compression cycle with vapor injection system (SRC) is proposed. Thermodynamic model is established to investigate the influence of working condition parameters and compared with basic booster refrigeration system (BRC) and parallel compression refrigeration system (PRC). In addition, the effect of vapor injection parameters on the characteristics of the proposed system is investigated. The results show that the medium pressure has little effect on BRC, while the optimum value exists for both PRC and SRC. The performance of the three systems can be significantly enhanced by optimizing the discharge pressure and medium pressure. The optimal value of the relative vapor injection pressure is constant at about 1.15. The maximum coefficient of performance (COPmax) of the SRC is significantly improved by 23.99%∼32.14% and 14.44%∼20.07% at disccussed working conditions compared with BRC and PRC, respectively. In summary, this paper confirms the performance enhancement potential of the proposed cycle and provides theoretical support for its practical implementation.

Experimental comparison of cycle modifications and ejector control methods using variable geometry and CO2 pump in a multi-evaporator transcritical CO2 refrigeration system

To reduce the direct global warming impact of refrigerants in HVAC&R applications, low-global warming potential (GWP) refrigerants, including natural refrigerants, have been extensively investigated as alternatives to hydrofluorocarbon (HFC) refrigerants. Among the natural refrigerants, Carbon Dioxide (CO2) offers several advantages, such as excellent transport and thermo-physical properties, being neither toxic nor flammable, and having a low price and high availability around the world. However, the high critical pressure and low critical temperature of CO2 often lead to transcritical operation, resulting in lower efficiency due to the additional compressor power necessary to achieve transcritical operation relative to subcritical HFC cycles. Therefore, a number of cycle modifications are used to enhance the coefficient of performance (COP) of transcritical CO2 cycles to meet or surpass those of HFC cycles. This paper provides a systematic experimental investigation of four such cycle architectures by employing the same multi-stage, two-evaporator CO2 refrigeration cycle test stand, 3 of these configurations in transcritical and 1 in subcritical conditions. The four cycles architectures included intercooling, open economization, an internal heat exchanger and two different ejector control approaches. Specifically, a variable-diameter motive nozzle and a variable-speed liquid CO2 pump located directly upstream of the ejector motive nozzle inlet were analyzed. Based on the experimental data, the maximum COP improvements are 4.64 % and 9.47 % when the ejector and the internal heat exchanger are used, respectively. The CO2 pump, once successfully stabilized, can control the ejector, increase its efficiency by up to 15 % and increase the cooling capacity to a maximum of 6.2 %. Nevertheless, a reduction in COP is measured when the pump is in use; however, unlike the other three different configurations, it was only analyzed under subcritical conditions.

Experimental study on the operating characteristics of multiple collector direct expansion solar-assisted heat pump

Solar thermal application is one of the most important ways to replace fossil energy. The discontinuous heating character and low heating efficiency are two key factors that limit the large-scale promotion of solar thermal application. Large-scale direct expansion solar heat pump systems (DX-SAHP) can provide high efficiency and low-cost continuous heating. However, the characteristics of multiple collector arrays and the system's performance still need more research and exploration. In this paper, a DX-SAHP system with multiple solar collectors was constructed and the work process of the system under different operation conditions was expressed. An experimental platform with two DX-SAHP system was built and studied, and each system contained a 10HP compressor, 16 set solar collectors and one fin evaporator in parallel. The results show that the coefficient of performance (COP) of the system running with solar collectors was nearly 30 % higher than that with the fin evaporator under constant frequency mode, and nearly 45 % under variable frequency mode, indicating that such a system has better performance under variable frequency operation mode. The refrigerant in the collector/evaporators were distributed uniformly in equal-path connection mode, which was beneficial in promoting the system's performance. The heating performance of the system under sunny and cloudy weather conditions was analyzed and discussed, respectively. The test results indicated that the COP of the system on sunny or cloudy days was higher than 4.0, and the maximum hourly COP can reach 11.3 and the maximum hourly heating capacity was 27 kW.

Analysis of the exergy transfers in subcritical and transcritical CO[formula omitted] ejectors and their connection with the performance of the refrigeration cycle

Ejector refrigeration systems (ERSs) operating with carbon dioxide are notoriously challenging from both design and operation perspectives. In particular, the ejector itself gives rise to special attention since it directly affects the performance of the global system. In this study, the connections between local exergy transfers within the ejector and global cycle performance are investigated by using a novel multi-scale numerical approach. Computational Fluid Dynamics (CFD) is used to generate accurate predictions of the ejector operation with access to the local flow and heat transfer features. The exergy tube analysis is applied onto the simulation results, linking local transfers within the ejector to the results at the component- and cycle-scale. A state-of-the-art ejector thermodynamic model is then calibrated onto the CFD results, and employed to determine the physically possible operation of the ERS. In addition, the metastability impact on the exergy transfers and overall system performance is investigated by comparing the CFD results of the classical Homogeneous Equilibrium Model with those obtained with the Homogeneous Relaxation Model. Notably, it is found that at fixed ejector inlet conditions, there exits an outlet pressure that maximizes the exergy transfers from primary to secondary streams. Interestingly, this work shows for the first time that the maximum exergy gain of the secondary stream appears to correlate with the maximum coefficient of performance of the system, independently of the on- or off-design operation of the ejector. Moreover, metastability generally deteriorates the performance at both scales. The results of the present study serve to pave the way towards better ejector design objectives.

Comparative performance assessment of different halide composites in sorption cooling system: A dynamic approach

The adsorption technique presents a promising approach for cooling, heat storage, and gas storage. This study focuses on the viability of adsorption in cooling applications, specifically utilizing various halide salts in conjunction with thermal energy, highlighting its potential as an energy-efficient solution for modern energy demands. A dynamic numerical model is developed and used to study the performance of transverse finned reactor-based adsorption cooling system (ACS) with different halide composite salts. The simulations are carried out at different supply pressures ranging from 3.55 bar to 7.28 bar corresponding to saturation pressure of ammonia at an evaporator temperature ranging from −5 ℃ to 15 ℃ during adsorption. The desorption process is simulated at different regeneration temperatures with a required desorption pressure of 17.28 bar corresponding to saturation pressure at condenser operating temperature of 40 ℃. The average cooling power produced in first 60 min of system operation at an evaporator temperature of −5 ℃ is 4.89 W, 207 W, 206 W and 925 W with the use of BaCl2, CaCl2, SrCl2 and MnCl2 composites respectively. The theoretical maximum coefficient of performance (COP) obtained are 0.31, 0.365, 0.308 and 0.552 with BaCl2, CaCl2, MnCl2 and SrCl2 respectively. The cooling power (CP) is increasing with increase in evaporator temperature. The shortest adsorption completion time is observed at 15 ℃ of evaporator temperature with all metal halides used. The lowest adsorption time is observed with MnCl2 composite salt and highest with BaCl2 composite salt. The regeneration time of all composite salt beds is decreasing with increase in regeneration temperature.

Elastocaloric Heat Pump by Twist Induced Periodical Non-Linear Stress for Low Hysteresis and High Carnot Efficiency.

Elastocaloric cooling is one of the most promising solid-state cooling approaches to address the issues of energy shortage and global warming. However, the cooling efficiency and cycle life of this technology need to be improved, and the required driving force shall be reduced. Here, a novel elastocaloric heat pump by periodic non-linear stress is developed by employing fiber twisting and separated cooling and heating media. The non-linear stress generated by fiber twisting yields a hierarchical, rigid-yet-flexible architecture and a periodic entropy spatial distribution, which result in a low mechanical hysteresis work and a high cooling efficiency (a maximum material coefficient of performance (COP) of 30.8 and a maximum Carnot efficiency of 82%). The torsional non-linear stress inhibits crack propagation and results in a highly extended cycle life (14752 cycles, more than ten times of fiber stretching). The heat pump exhibits a maximum average temperature span of 25.6 K, a maximum specific cooling power of 1850W Kg-1, a maximum device COP of 19.5, and a maximum device power of 5.0W, under each optimal condition.

Operational characteristics and controlling strategies of a novel dual-return-air dehumidification evaporative cooling system (DDEC)

Dehumidification evaporative cooling (DEC) systems exhibit superior environmental adaptability compared to conventional evaporative cooling systems. However, the dehumidification process in DEC is coupled with its cooling process, presenting challenges in adapting to varying environmental conditions. To address this issue, this study proposes a novel dual-return-air dehumidification evaporative cooling system (DDEC) that utilizes two return airflows to control both dehumidification and cooling efficiency. A thermodynamic model based on COMSOL LiveLink with MATLAB for the DDEC has been established and validated, enabling an analysis of the impact of operational parameters on its performance and the identification of key parameters. The results show that there are an optimum inlet flow rate (1100 m3/h) and a third flow rate control factor (0.2) that maximize the coefficient of performance (COP), giving the maximum COP of 0.87 and 0.35 respectively. Moreover, the first and second air flow rate control factors only have a significant influence on the supply air humidity of the DDEC. A higher regenerative temperature can reduce both the temperature and humidity ratio of the supply air but tends to decrease COP. Finally, a single-directional controlling strategy is proposed and analyzed for the DDEC application in different environments.

Experimental studies of activated carbon/graphite composite adsorbent for improved performance of packed and coated bed adsorption cooling system

In this study, composite adsorbents are developed by using various compositions of activated carbon (AC)/polyvinyl alcohol (PVA) with graphite powder (GP). The optimal adsorption material chosen for the adsorption cooling system (ACS) is found to be composite A, which comprises 93 % AC, 5 % graphite powder, and 2 % PVA. This selection is made based on the physical characterization of the composite materials by BET, FESEM, adsorption capacity, desorption capacity and thermal conductivity tests. The specific cooling power (SCP), coefficient of performance (COP), and exergetic COP of the adsorption chiller are experimentally evaluated. By maintaining the optimal desorption and condenser temperatures (Tdes = 70 °C and Tcon = 35 °C), the maximum COP, SCP and exergetic COP of the system are found to be 0.582, 206 W/kg and 0.19 for packed bed and 0.738, 261.37 W/kg and 0.25 for coated bed, when the adsorption temperature (Tads) = 30 °C and evaporation temperature (Teva) = 25 °C. The Freundlich isotherms exhibit a better correlation with the vapour pressure of methanol. The samples exhibited no deformations after 17 adsorption/desorption cycles, confirming the durability of the activated carbon. The more uniform coating thickness and smooth surface structure could ensure long-term stability and performance of the adsorbent.

Experimental Study and Performance Analysis of Thermoelectric Cooler Using Solar Photovoltaic Energy

Thermoelectric coolers need electrical energy to create temperature differences between the hot and cold sides; however, photovoltaic systems immediately convert solar radiation into electrical energy. The study is a combined (PV-TEC)—experimental study on a thermoelectric cooler operating by the Peltier effect to analyze and develop the TEC. One TEC was used, and its dimensions were (40*40*3.4) mm. The current required by the TEC is 6A and 12V DC. The thermoelectric cooler is electrically powered by a solar system consisting of two 660W solar panels, a solar charger and two 12V batteries. The results revealed a relationship between the coefficient of performance and the input energy, as the COP showed an increase as the input energy decreased, which is an essential factor for the cooling process. The temperature difference, which was the difference between ambient and cold temperatures, is related to the COP. The COP rises as the temperature difference decreases until it becomes stable. Moreover, the consumption current is an important factor in electronic devices, so the study focused on demonstrating the effect of the cold side temperature on the current, and an empirical equation was found between them. It has been found that a decrease in the temperature of the cold side leads to a sharp reduction in the consumption current until it stabilizes. The maximum coefficient of performance was 3.7436, obtained at the current 1.4 and cold side temperature of zero degrees centigrade. This high value of the coefficient of performance resulted from using curved fins to improve heat dissipation from the hot side.

Matching Characteristics of Refrigerant and Operating Parameters in Large Temperature Variation Heat Pump

Characterizing the optimal operating parameters for a heat pump with a specific refrigerant is paramount, as it provides valuable guidance for refrigerant selection. The temperature mismatch between cold and hot fluids in the evaporator and condenser can lead to degraded thermal performance in heat pumps with large temperature variations. To address these two key issues, we selected several pure refrigerants with varying critical temperature levels for use in a large temperature variation heat pump configuration. The corresponding thermal performance was then investigated using the Ebsilon code under fixed temperature lift conditions as the operating temperature varied. It indicates that the maximum coefficient of performance (COP) is typically achieved when the deviation factors of temperature and pressure from their critical parameters fall within the ranges of 0.62~0.71 and 0.36~0.5, respectively. Our research recommends the binary refrigerant mixture of R152a/R1336mzz(z) (COP = 3.54) for the current operating conditions, as it significantly improves thermal performance compared to pure R1336mzz (z) (COP = 2.87) and R152a (COP = 3.01). Through research on the impact of the compositional ratio of R152a/R1336mzz(z) on the thermal performance of the heat pump, we found that that the optimal ratio of R1336mzz(z) component to R152a component is 0.5/0.5. This study offers valuable guidance for selecting the most suitable refrigerants for heat pumps in practical engineering design scenarios.

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 composite/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.

IoT-driven monitoring and controlling to improve performance of thermoelectric air conditioning system

Thermoelectric module-based air conditioners (TEM-AC) are scalable, reliable, and have the potential to replace conventional air conditioners. However, the coefficient of performance (COP) of TEM-AC is still relatively low. Improving the COP requires a combination of temperature control, voltage control, heat recovery, airflow control, and material selection. The present study focuses on enhancing COP of the TEM-AC for building space cooling. To control and monitor the parameters of the TEM-AC, a low-cost, IoT-enabled monitoring and controlling system (MCS) is developed. The MCS helps in TEM-AC system designing as it can apply different current pulses to improve cooling capacity and control the temperature difference between both surfaces. MCS also measure all the required parameters of the TEM-AC to calculate the COP of the system and helps in reducing the calculation time for each experiment. Using a developed technique and TEM-AC design, a performance comparison of eight experimental cases has been done. Experiments have been performed in normal and control modes with different heat sinks and airflow of TEM-AC. The TEM-AC system is installed over a 16-inch × 12-inch enclosed space with a TEC-12706 module. The study indicates the COP of the system is enhanced by controlling the cold side temperature and air flow rate. For this study, maximum COP is achieved around 0.44 with control operation. The developed IoT-based monitoring and controlling system offers a promising solution for improving the performance of thermoelectric modules in building space conditioning applications.

Selection of pure and binary working fluids for high-temperature heat pumps: A financial approach

High-temperature heat pumps are increasingly being recognized for decarbonizing industrial heat supply up to 200 °C. In the open literature, the operating conditions, working fluids and configurations of high-temperature heat pumps are typically optimized for achieving a maximum Coefficient of Performance (COP). This work however presents a method that optimizes the operating conditions, working fluid and configuration of the heat pump, based on a combined financial and technical appraisal. In this regard a financial model to calculate the levelized cost of heat (LCOH) of a heat pump unit is developed. The model screens a large set of working fluids and therefore allows for subcritical, transcritical and supercritical operation and includes (zeotropic) binary mixtures. The methodology is applied to a large set of generic temperature profiles, with process temperatures between 160 °C and 200 °C and heat source temperatures between 80 °C and 120 °C. The results indicate that designing a heat pump solely based on maximum COP may lead to a sub-optimal financial solution. Overall, binary mixtures, either zeotropic or azeotropic, of natural working fluids often performed best. The benefits of zeotropic mixtures are twofold: improved temperature matching and flexibility in their thermophysical properties. However, for industrial processes with high temperature glides, pure fluids operating in the transcritical regime proved to be the most promising. The study also highlights the sensitivity of the financial performance to the electricity price, annual operating hours and heating capacity.

Thermodynamic optimization of heat exchanger circuitry via genetic programming

In this study, an evolutionary thermodynamic technique was developed for the optimization of heat exchanger circuitry. The proposed technique is capable of handling the unrestrained implementation of genetic operators while ensuring circuitry feasibility and basic manufacturability. The optimization tool was used to clarify the optimal heat transfer features in relation to the characteristics of the refrigerant and to provide a thermodynamic interpretation of the optimization results to extract general design guidelines. Evaporator circuitry optimization was conducted under given cooling capacity, superheating degree, and boundary conditions representative of air-conditioning applications. The consistency between the minimum entropy generation and the maximum coefficient of performance (COP) was demonstrated under these settings. Accordingly, heat exchanger configurations that take maximum advantage of the thermodynamic benefits of each refrigerant are proposed by optimizing the distribution of friction and heat transfer irreversibility. Consequently, the evaporator outlet pressure increases, thus lowering the compression ratio and maximizing the COP. The developed optimization method maximizes the benefits of low-GWP alternative refrigerants and shows that zeotropic mixtures may exhibit performance analogous to that of R32 and higher than that of R410A by approaching a Lorenz cycle operation.

Optimization of Triple Effect Vapour Absorption Refrigeration System: A Statistical Approach

Abstract The present manuscript optimized the First and Second Law performance of the triple effect vapour absorption refrigeration systems (TE-VARS) using statistical techniques like Taguchi, Taguchi-based GRA, and RSM-based GRA methods, which provide the most accurate and optimized results. Liquified pertrolium gas (LPG) and Compressed natural gas (CNG) are considered as the source of energy to operate TE-VARS, as the system requires significantly higher generator temperature. Also, volume flow rate of these gases along with the annual operating cost to drive the system have been presented. A thermodynamic model was first formulated using EES software for the computation of the coefficient of performance (COP) and exergetic efficiency (ECOP). The most influential parameters like temperature in the main generator, concentration and pressure at different components are studied and determined using ANOVA and Taguchi methods. The optimum parameters were determined based on the mean effect plot of S/N ratios for COP and ECOP. It has been found that the maximum COP and ECOP were calculated to be 1.915 and 0.15, respectively under the Taguchi method. Furthermore, Taguchi-GRA was used for the simultaneous optimization of the operating parameters and performance of the system. It is observed that the absorber temperature is the most influential parameter for affecting COP and ECOP. Moreover, a RSM-based GRA method was also applied and developed regression models that yield most optimum COP and ECOP as 1.963 and 0.1606, respectively. Comparison shows that the RSM-based GRA method provide the most optimum conditions, which is one of the key finding of the present study. Also, rate of exergy destruction at each component of TE-VARS has been plotted under optimized operating conditions. The optimum volume flow rate for LPG and CNG comes out to be 0.057 and 0.177 m3/s, while the minimum operating cost (yearly) are 299.827$ and 183.293$, respectively.

Limits of coefficient of performance of adsorption cooling cycle based on refrigerant’s type

This research work discusses how the thermodynamic properties of refrigerant constrain the upper limit of the coefficient of performance (COP) of adsorption cooling (AC) cycle. A mathematical model is developed based on the mass and energy balance across the adsorption bed and used to estimate the maximum COP. Langmuir model is used to derive the ideal COP. Besides, Carnot COP and exergy efficiency are derived to evaluate the adsorption cooling cycle further. These new parameters are applied to the available cycles in the open literature to assess how much room for improvement is still left in the adsorption technology based on the refrigerant type. Water, ammonia, methanol, ethanol, Butane, R134a, R32, CO2 and R1234ze are used in this study. The maximum cooling COP cannot exceed 1.0 for cooling applications and is always less than 2.0 for heat pumps. Results show that AC cycles using water or ethanol as refrigerants achieve the highest exergy efficiency. The AC cycle using water as an adsorbate and silica gel as an adsorbent has the highest relative COP, about 80 % at 85 °C regeneration temperature. In turn, CO2 exhibits the lowest one. This work is merely an attempt to save experimental effort and time by identifying the most suitable refrigerant based on its physical and thermal properties and the cycle operating conditions.

Seasonal COP of a residential magnetocaloric heat pump based on MnFePSi

The performance of a magnetocaloric heat pump (MCHP) consisting of active magnetocaloric regenerators (AMR) of 12 layers of MnFePSi magnetocaloric materials (MCM) with a linear distribution of Curie temperatures was investigated using a 1D numerical model. The model predicted the heating power and coefficient of performance (COP) of the AMR for a fixed temperature span of 27K, between 281K and 308K, and variable flow rate and AMR cycle frequency. A maximum applied magnetic field strength of 1.4T was used. A well-insulated house with a maximum heating power demand of 3kW (under quasi steady state conditions) was considered. Ambient temperature in The Netherlands was taken as a reference for the estimation of the seasonal heating power demand. Without optimizing the design of the AMR, the model predicts a maximum single-AMR heating power equal to 43.5W when the AMR operates at 3Hz and 3Lmin-1, and a maximum COP equal to 5.8 when it operates at 1.5Hz and 1Lmin-1 Considering the maximum heating power of a single AMR, approximately 69 AMRs are needed to provide the design heating power demand of the house. It was found that it is possible to achieve an AMR seasonal COP of 5.6 by continuously adjusting the flow rate and frequency of operation of the MCHP along with the ON/OFF switching of some groups of AMRs in order to adjust the heating power of the MCHP to the heating power demand of the house.

Bidirectional interlayer cooling of 3D chip stack for temperature uniformity enhancement and hotspot mitigation

In this work, bidirectional interlayer cooling with both enhanced temperature uniformity and low flow resistance is proposed for effective 3D chip stack thermal management. Staggered uniform micropin fins and curved entrance and exit regions are elaborately designed. Experimental and numerical studies are carried out to investigate the cooling characteristics under different Reynolds numbers and heat fluxes. In comparison with common interlayer cooling solutions, counter-flow arrangement is achieved between the layers and vertical heat transfer within the chip stack is utilized. Heat fluxes of 300 W/cm2 and 100 W/cm2 are dissipated in the hotspot and background zones respectively. Total thermal resistance of 0.18 K/W is obtained. The maximum decrease in temperature difference is up to 66.2%. Both thermal resistance and coefficient of performance (COP) are considered and compared with the results from previous studies for comprehensive performance evaluation. The maximum COP of 11,523 is reached. The thermal and hydraulic performance advantages of the present design can be verified. The bidirectional interlayer cooling offers new potential for thermal management of 3D chip stack.

Assessment of booster refrigeration system with eco-friendly working fluid CO2/halogenated alkene (HA) mixture for supermarket application around the world: Energy conservation, cost saving, and emissions reduction potential

For the scope of commercial supermarkets, the demand for energy efficiency improvement and environmentally-friendly working fluid of the refrigeration system is necessary. In this study, supermarket booster refrigeration system by using eco-friendly working fluid CO2/halogenated alkene (HA) mixture is proposed, and the mixture used in systems with two evaporation temperatures and operating modes affected by ambient temperature are studied. The energy efficiency, economic performance and emission reduction potential of the whole life cycle are conducted to compare with the pure CO2 booster refrigeration system. Furthermore, the influence of climate condition is discussed when used in 40 typical cities around the world. The results show the coefficient of performance (COP) of booster refrigeration system can be significantly improved by using CO2/HA mixtures. As the ambient temperature is 33 °C, the CO2/R1234yf (93/7) operates with the maximum COP of 1.367, which is 11.59 % higher than that of pure CO2. Using CO2/HA mixtures in the booster refrigeration system can significantly improve the exergy efficiency of system. Moreover, the system using CO2/HA mixtures has higher annual performance factor and lower life cycle cost (LCC) than pure CO2. LCC of the system using CO2/R1234yf (94/6) is the lowest, and the reduction rate is 3.06–5.59 %. Meanwhile, the life cycle carbon emissions of systems in different climatic regions using CO2/R1234yf can be reduced by 2.39–5.21 %. The booster refrigeration system adopting CO2/HA mixtures is a promising alternative solution for commercial supermarket refrigeration and energy-saving.

Thermodynamic investigation of a Carnot battery based multi-energy system with cascaded latent thermal (heat and cold) energy stores

Carnot batteries store electricity in thermal form, allowing for power balancing and also multi-vector energy management as a unique asset. Cascaded thermal energy storage therefore has a vital role in Carnot battery, particularly multi-energy systems delivering electricity and thermal energy at various temperatures. Here, we propose a Carnot battery multi-energy system with cascaded latent thermal energy stores. The effects of compressor pressure ratio, total stage number, stage area and fluid velocity in tubes, on system-level coefficient of performance and total exergy efficiency are investigated. The results show that both the coefficient of performance and exergy efficiency increase significantly and then level off with the total stage number and stage area increasing, while they only increase slightly before a significant decrease with the increase of fluid velocity in tubes. The selection of compressor pressure ratios relates to the demands of cold, heat and electricity. The multi-energy system achieves a maximum coefficient of performance of 95.0% in combined cooling, heating and power mode. Although the thermodynamic performance yileds comparable with Carnot battery systems integrated with packed-bed or liquid-based sensible heat stores, the Carnot battery multi-energy system can deliver electricity and multi-grade heat and cold simultaneously, thus increasing flexibility in future energy systems.