Low Refrigeration Research Articles

Investigation on hourly characteristics of solar absorption refrigeration cycle integrated with a dual-cooling grade-compression cycle

Solar absorption refrigeration cycle consumes almost no electricity, but its low refrigeration efficiency and intermittent operation issues need addressing. Based on the viewpoint of cold energy grade production matching air-conditioning cooling load grade, a solar absorption refrigeration cycle integrated with a dual-cooling grade-compression cycle (SARC-DGC) is proposed to boost the solar utilization and achieve all-weather cooling, which includes solar absorption refrigeration (SAR) subsystem, subcooled water circulating (SWC) subsystem and dual-cooling grade-compression refrigeration (DGR) subsystem. The simulation models of proposed cycle are developed by lumped-parameter and distributed-parameter methods, and the simulation results are validated with experimental results. The hourly characteristics of SARC-DGC cycle on a typical day are evaluated and compared with those of the conventional solar absorption refrigeration (SAR) cycle and air-cooled compression refrigeration (ACR) cycle. The results show that the hourly cooling performance of SARC-DGC cycle is well matched with the building cooling load demand, so as to realize the cold energy grade production and utilization. The working hours of SAR subsystem are 42.86 % longer than that of conventional SAR cycle, with a 31.92 % increase in daily cooling capacity, while the daily power consumption of the proposed cycle is on average 27.31 % lower than that of conventional ACR cycle.

Thermoeconomic assessment of the air-cooled tropical data center through energy-saving strategies

Rapid developments in the electronic information industry drive the increased energy usage and carbon emission of data center buildings, prompting the focus on the energy efficiency and environmental sustainability. Expanded operation envelopes of tropical data centers is assessed to analyze the potential for the building energy savings and carbon emission reduction through collaborative analysis of operation modes (OMs), supply air temperature (SAT), and outdoor air temperature (OAT). The OMs of compression vary with the setpoints of SAT, in which the average exergy efficiency of compressors at alternate operation mode is 6.8 % and 8.0 % lower than that of double and single compression operations. As SAT rises from 20 °C to 32 °C, the system exergoeconomic factor increases from 5.4 % to 8.0 %, and the average carbon cost decreases by 36.5 %. Additionally, with just an 8.5 % increase in exergy cost (i.e., Case 8) at OAT rising from 30 to 34 °C, the high SAT and low refrigerant charges provide considerable exergy cost advantages versus resisting the OAT fluctuations. Dynamic operation strategies are also proposed and compared to cope with the impacts of tropical environments. Compared to the 26 °C SAT baseline, the average energy savings are 9.1–14.7 %, indicating the ability to fully utilize outdoor and indoor conditions.

Flow Patterns in Wire-on-Tube Heat Exchangers Based on Various Low Refrigerant Mass Flow Rates

Flow Patterns in Wire-on-Tube Heat Exchangers Based on Various Low Refrigerant Mass Flow Rates

Natural refrigerant mixtures in low-charge heat pumps: An analysis of the potential for performance enhancements

The study investigated the characteristics of different natural refrigerants and their mixtures in a residential heat pump with low refrigerant charge. Three main evaluation criteria were utilized for comparing different mixtures: Coefficient of Performance (COP), Volumetric Heating Capacity (VHC), and a newly proposed indicator, the heating capacity at charge limit. Propane was used as a reference refrigerant. It was found that some mixtures significantly improved both COP and the heating capacity at charge limit while maintaining similar volumetric heating capacity and operating conditions. Alternative multi-criteria decision-making techniques were adopted to rank the best refrigerant mixtures. Mixtures rich in Dimethyl Ether (DME), such as DME-CO2 [0.96-0.04] and DME-Propylene [0.75-0.25] were found consistently among the best options. Those were followed by Propylene-rich mixtures such as Propylene-CO2, Propylene-Isobutane and Propylene-Butane. The levelized cost of heat (LCOH) could be improved by up to 12 %. To accelerate the transition to natural refrigerants, without compromises on efficiency and costs, further research and certification activities on non-fluorinated refrigerants based on Dimethyl Ether (R-E170), CO2 (R-744) and Propylene (R-1270) are recommended.

Investigation of environmentally friendly new generation refrigerant R471A instead of R404A in low and medium temperature commercial refrigeration system

Hydrofluorocarbon (HFC) refrigerants (such as R134a and R404A), which are widely used in commercial refrigeration systems, contribute greatly to global warming. Because the global warming potential (GWP) values of HFC refrigerants are high. With increasing concerns about the environment (global warming, climate change, etc.) various legal regulations have been made to limit or phase out the use of refrigerants with high GWP values. To use the R471A refrigerant developed by Honeywell in 2022 as an environmentally friendly new-generation refrigerant in the commercial refrigeration system, the performance of the R471A refrigerant should be comprehensively evaluated. In this study, energy, exergy, and environmental analyses of using R471A refrigerant instead of R404A in low and medium-temperature commercial refrigeration systems were carried out and presented. Evaporator temperatures of − 30 °C, − 25 °C, − 20 °C, − 15 °C and − 10 °C and condenser temperatures of 30 °C, 40 °C, and 50 °C were selected to represent low and medium cooling temperatures and hot and cold country seasons. It was seen that R471A presents a lower mass flow rate (between 67 and 71%) and cooling capacity (between 61 and 68%) than R404A however has higher COP (between 6% and 14%) and R404A has exergy efficiency value of 6–14% lower than R471A. Also, from an environmental point of view, R471A has better environmental results than R404A.

Analysis and optimization of effect factors of refrigeration performance of portable micro-refrigerator

The objective of this study was to ascertain an efficient portable cold container, considering the constrained space, low airflow velocity, and uneven heat distribution of a micro-refrigerator, which lead to in insufficient heat dissipation and low refrigeration efficiency. A thermoelectric micro-refrigerator model with a volume of 9083.8 cm3 was established under forced convection heat transfer conditions. Through single-factor and response surface experiments, the impact of thermoelectric elements configuration on refrigeration efficiency was identified. These elements include the input current of the Thermo Electric Cooler (TEC), the derating factor (actual power) of the cooling fan, and the height of the heat sink fin.The results indicate that optimizing the refrigeration conditions significantly enhances the refrigeration efficiency of the micro-refrigerator. At an ambient temperature of 25℃, with a fan derating factor of 0.77 (corresponding to an actual power of 0.8218 W), a heat sink fin height of 59.15 mm, and a TEC input current of 5.09A, under stable conditions,the cold side temperature can reach −11.3 ℃. The hot side temperature is 38.2 ℃, consistent with the response surface experiment result. Numerical simulation under these conditions reveals a cold side temperature of −12.4 ℃ and a hot side temperature of 41.9 ℃, showing a 9.7 % difference from the experimental results. The simulation data further demonstrate that the vertical arrangement of the cooling fan and heat sink can achieve a cold side temperature of −12.9 ℃, effectively maintaining the hot side temperature below 40 ℃. The thermoelectric system investigated in this paper holds promising potential for applications in portable fresh-keeping and refrigeration.

Seasonal performance assessment and experimental investigation of R32 drop-in and BPHX size in an R410A chiller when using two compressor modulation strategies

The decrease in fossil fuel reserves and high energy consumption by HVAC&R equipment require the use of compressors with better efficiencies and capacity modulation strategies. Usually, seasonal performance studies focus on compressor capacity modulation methods, however the current study explores the idea of improving the seasonal performance of two compressor modulation techniques: a two-stage and a variable speed compressor with different capacity modulation ranges by using larger condensers in an R410A water ethylene glycol chiller system. The analysis shows that oversizing the heat exchangers has larger benefits with the compressor modulation strategy with lower range. Additionally, the seasonal performance estimated according to AHRI 551/591 (2020) of a two-stage compressor with oversized condenser is higher than a basic single speed compressor with a better motor indicating that oversizing brazed plate heat exchanger (BPHX) might yield higher performance than improving the compressor motor efficiency. Due to the high global warming potential (GWP) of R410A, there is a focus on replacing it with low GWP refrigerants such as R32 to reduce CO2 emissions. This study would also compare the performance when using R32 as a drop-in refrigerant in the two different R410A compressor capacity modulation strategies with comparable cooling capacity. When the refrigerant was changed from R410A to R32, the integrated part load value (IPLV.SI), an efficiency metric for variable speed compressor remains relatively constant at 5.5 while it increases from 3.9 to 4.4 for the two-stage compressor. It is hypothesized that low refrigerant mass flow rate (m˙ref) provided by the variable speed compressor when matching the load at the lower part load conditions leads to refrigerant maldistribution in BPHX and reduced IPLV.SI.

Influence of compressor speed on the performance of low pressure vapor-injected refrigeration systems

For the problems of high compressor discharge temperature and system performance decay during the operation of cold storage, a parallel flow heat exchanger based low pressure make-up gas refrigeration system experimental bench was designed and built, and the changes of refrigeration system performance were analyzed under different compressor speeds. The results show that when the compressor speed increases from 2500 rpm to 4500 rpm, the compressor discharge temperature increases, the refrigeration capacity increases by 39.53% and the compressor power increases by 38.89%, in addition, as the speed increases, the system COP shows a trend of first increasing and then decreasing, with the best value of 2.71 at 3500 rpm.

Experimental study on refrigeration system performance of pure electric refrigerator car based on medium pressure air supply

In view of the low refrigeration efficiency of the refrigeration system of the pure electric logistics car under the high temperature environment, an experimental platform of the refrigeration system based on medium pressure air supply of the pure electric refrigerator car was built with R404A as the refrigerant, the experiment results show that the system refrigerating capacity, COP and EER decrease with the increase of external environment temperature under the same internal environment temperature, the compressor power and exhaust temperature increase with the increase of the external environment temperature. When the external environment temperature is 43 °C, the medium pressure air supply technology is obviously better than the non-air supply technology, the refrigerating capacity increased by 4.1–12.73 %, the compressor power and EER changed little, the COP increased by 1.3–6.38 %, and the exhaust temperature reduced by 3.4 °C-7.11 °C, especially at -3 °C, the system refrigerating capacity decreases by 30.1 %, the compressor power increases by 35.6 %, the COP and EER distribution decreases by 51.24 % and 42.68 %.

A proposal for a non-flammable, fluorine-free, CO2-based mixture as a low TEWI refrigerant

A thermodynamic analysis of a simple inverse cycle is proposed as a means to evaluate mixtures of CO2 and four organic substances (propane, iso-butane, dimethyl-ether and propylene) as working fluids. The cycle features an internal heat exchanger and is designed to cool down a finite heat capacity flow on the cold side. The analysis covers trans-critical as well as sub-critical cycles. For each organic substance, the composition is optimized. Flammability is accounted for. Dimethyl-ether (DME) turns out to be the best option, as a relatively low mass fraction of this compound is sufficient for reaching the thermodynamic optimum. In this way, a simple, sub-critical refrigeration system may be obtained featuring a non-flammable working fluid and a significant COP increase with respect to pure CO2.

Development of the double two-phase thermosyphon loops used in low temperature Stirling refrigerator

With the growing of demand for the low temperature pharmaceutical cold chain, the Stirling cooler has been attractive for years. However, the heat transfer inside the refrigerator could be ineffective by natural convection due to the small area of the plug-in Stirling cold finger. In this paper, the double two-phase thermosyphon loops (DTPTLs) used for low temperature Stirling refrigerator is developed. The characterization of heat transfer and pressure drop in the DTPTLs is investigated by theoretical calculation for various working refrigerants. Then the basic structure of the DTPTLs is modified and improved for dragging and moving convenience. Moreover, the comparison of the temperature distribution inside the refrigerator container between the DTPTLs and the plug-in cold finger methods is demonstrated by numerical simulation. The simulation result indicates that a well temperature distribution inside the container can be obtained by using DTPTLs. In the end, an experimental test is conducted at room temperature. It's shown that the no-load cooling temperature could reach −80 °C in a few minutes. Over steady-state operation, the maximum temperature difference all inside the container is less than 5 °C, which proved the positive heat transfer effect of the DTPTLs on the cooling performance of the low temperature Stirling refrigerator system.

Thermodynamic modeling of a residential heat pump for combined space and water heating using binary CO2-based mixtures

The residential sector answers for a considerable share of the energy consumption worldwide. An alternative to meet technical and environmental requirements of space heating and hot water demanded by this sector uses CO2 mixtures as a refrigerant in compression-based cycles. However, while CO2 is nonflammable and has low GWP and ODP, it performs poorly due to its low specific refrigerating effect and large throttling losses. Differently, mixtures with a low-GWP refrigerant and CO2 could enhance the performance of these cycles while attending the environmental requirements. Therefore, this study considered the use of a combined heat pump for space and water heating using binary mixtures of CO2 and several refrigerants. Through a computational model, three configurations were tested, along with different ratios of space heating to hot water demands, and different CO2 mass fractions on the mixture. The heat pump configuration with the best performance range was the one with three gas coolers, where the low temperature gas cooler preheats the water, the intermediate equipment heats the space, and the high temperature gas cooler heats the already preheated water to the operating temperature. For this arrangement, heat pumps primarily dedicated to space heating benefit from mixtures with low mass fractions of CO2, approximately 10%. In contrast, heat pumps primarily attending water heating perform better for high mass fractions of CO2 (∼ 90%). Furthermore, in this combined configuration, about 87% of the mixtures analyzed could have higher coefficient of performance than pure fluids, indicating an improvement of CO2 heat pumps.

Experimental and dynamic simulation study of the Joule-Thomson cryocooler working at liquid hydrogen temperature

With advantages of long-life time, low level vibration and continuous refrigeration, precooling Joule-Thomson cryocooler (PJTC) working at liquid hydrogen temperature has the potential to be applied to liquid hydrogen storage system in space. For the large-cooling-power PJTC working at liquid hydrogen temperature, its dynamic response characteristic to heat load has a great importance on liquid hydrogen storage system, especially in aerospace applications. In order to study dynamic characteristics of PJTC working at liquid hydrogen temperature, an experimental setup for an open-cycle PJTC is built and its dynamic model is established by MATLAB software. Cool-down process and temperature variation characteristic with a heat load of 10.19 W are simulated and verified with experimental results. Meanwhile, the effect of precooling power on the performance of JT cryocooler is analyzed. It is found that if the precooling power is lower than 50% of the original precooling power, the recovery time of JT cryocooler will increase significantly.

Improving Thermal Performance of Rectangular Microchannel Heat Sinks using Porous Layer: CFD Simulation and Optimization

Microchannel heat sinks are very widely used due to their high heat transfer coefficients and low refrigerant requirements. Nevertheless, microchannel heat sinks still perform sub-optimally when it comes to thermal performance. Therefore, this paper investigates the individual and combined impacts of different characteristics of porous media on the thermal performance of microchannel. Four porosity values are considered: 0.8, 0.85, 0.9, and 0.95. The evaluation is based on three-dimensional computational fluid dynamics simulations. Due to the large number of degrees of freedom in this study, Constructal Theory and Design of Experiments are employed. In this study, the response surface type is Genetic Aggregation, while the Latin Hypercube Sampling algorithm is used for data sampling and Genetic algorithm is used for optimization. Combining porous layers with microchannel heat sinks reduces maximum temperatures about 3K. It is also observed that a lower maximum surface temperature is achieved in the cases with less porosity. Furthermore, the optimal geometry and size of the microchannels with porous layers are determined.

Automated fault detection and diagnosis of airflow and refrigerant charge faults in residential HVAC systems using IoT-enabled measurements

While automated fault detection and diagnosis (AFDD) in residential heating, ventilation, and air-conditioning (HVAC) using smart thermostat data is gaining increasing attention in recent times, it still requires in-depth investigation for market adoption, especially with real-life data. This paper proposes an Internet of Things (IoT) - based approach that adds a smart sensor to the smart thermostat data to carry out AFDD. The approach uses a model which predicts enthalpy change across the evaporator and compares the prediction to the measured enthalpy change. Deviations which exceed analytically determined thresholds then signal faults in the HVAC system. The faults detected are either installation related or degradation related. Experimental tests were carried out in four homes located in Norman, Oklahoma. From the tests, installation issues like indoor/outdoor mismatch were detected in two homes, while a 30% low charge and low indoor airflow rate were detected in one home. The results show that the proposed AFDD algorithm was able to successfully detect two prevalent faults, namely low indoor airflow and low refrigerant charge. Unlike most of the smart thermostat-based approaches, the proposed IoT-based approach can detect and diagnose both faults but only require one additional sensor which is provided by smart thermostat manufacturers.

Thermal performance analysis and burning questions of refrigerant direct cooling for electric vehicle battery

Refrigerant direct cooling technology has been gaining attention due to its advantages of simple system composition and high cooling efficiency, but its temperature control performance does not have advantages over mature liquid cooling in industrial applications, reasons for which should be discovered. This paper proposes a novel discrete model based on an electro-thermal coupling method and thermal resistance network to analyze the local temperature control performance of refrigerant direct cooling and liquid cooling systems under real operating scenarios with acceptable computing time and accuracy. The results show that the phase change of the refrigerant in the refrigerant direct cooling plate significantly affects the temperature control performance. The vapour-liquid two-phase zone has a high heat transfer capability, which can dissipate heat well and maintain a small temperature difference with the maximum temperature of the battery reduced by 28.3% compared with the liquid cooling system. There is a serious heat transfer deterioration in the superheated gaseous zone, causing a significant increase in the local battery temperature, and the maximum temperature difference is much higher than that in the liquid cooling system. When the refrigerant switches from two-phase to single-phase under 2C charging condition, the heat transfer coefficient reduces by roughly 73.6%, which causes the accompanying battery unit temperature to rise by about 12 ∼ 14 °C. Besides, the low refrigerant temperature will cause a large temperature difference in the vertical direction of the battery, which restricts the application of the refrigerant direct cooling system in practice. When the evaporation pressure increases from 300 kPa to 500 kPa, the temperature difference will decrease from 14.1 °C to 8.8 °C which is a 37.6% reduction. Through an in-depth analysis of the local temperature distribution of battery units, two burning questions were identified which deteriorate the temperature control performance of the refrigerant direct cooling system and results can provide directions for performance improvement.

Condensation heat transfer of superheated vapour of R1234ze(E) and R134a inside a brazed plate heat exchanger

The recent restrictions on the use of refrigerants with high Global Warming Potential (GWP) have pushed the research to consider alternative solutions, such as the HydroFluoroOlefins, as possible replacements. Even though they are generally mildly flammable, their GWP is significantly lower compared to those of the HydroFluoroCarbons. This paper presents experimental measurements of R1234ze(E) and R134a heat transfer coefficients during condensation inside a brazed plate heat exchanger. Experimental tests during condensation heat transfer have been conducted considering different degrees of superheating, subcooling and outlet vapour quality. The condensation temperature varies between 34.6 °C and 42.3 °C and the refrigerant mass flux is between 9 kg m−2 s−1 and 49 kg m−2 s−1. The results showed that the heat transfer coefficients measured with R134a are between 4% and 8% higher than those of R1234ze(E). Complete condensation experiments showed that an increase in the liquid subcooling degree significantly reduces the thermal performance at low refrigerant mass velocities. In some cases, the plate area occupied by liquid refrigerant reached almost 30% of the overall heat transfer area, thus decreasing by 2.7 times the average heat transfer coefficient when passing from 3 K to 8 K subcooling degree. A comparison with the predictions of some empirical models is also presented to assess which one can better predict the experimental data.

Experimental research on vapor-injected water source heat pump using R1234ze(E)

As a low GWP alternative refrigerant to R134a, the direct use of R1234ze(E) in water source heat pumps leads to significant performance degradation. Vapor injection combined with R1234ze(E) is a possible solution to this problem. In this study, a vapor-injected water source heat pump using R1234ze(E) as the working fluid was experimentally investigated. The results indicate that vapor injection using a flash tank or internal heat exchanger can significantly increase the coefficient of performance and heating capacity of the proposed heat pump. The improvements in the heating capacity and coefficient of performance corresponded to 22.3% and 8.1% for the flash tank system, and 19.5% and 7.3% for the internal heat exchanger system compared with the no-injection system.

Comparison of R134a and R516A’s Performance at Different Air Velocities in Two Evaporator Ejector Cooling System

This study aims experimentally to investigate the performance parameters of R134a and the alternative R516A refrigerant in two evaporator ejector cooling system (DEES) at different air velocities of evaporator#1. Firstly, the tests were carried out with R134a refrigerant under steady-state conditions at different air velocities and then repeated with low GWP R516A refrigerant. As the tests were carried out with R134a, higher cooling capacity was achieved at different air velocity values. When the air velocity value was 1.1, 1.7, 2.2, and 2.7 m s-1, the COP value obtained from the tests with R134a was 1%, 2%, 5%, and 4% higher than R516A, respectively. Additionally, test results illustrate that the higher air velocity contributed to increasing performance parameters, however air velocity higher than 2.2 m s-1 had a slight effect. The study concluded that R516A performance values are slightly lower than R134a performance and can be alternatively used as a refrigerant in vapor compression refrigeration (VCR) systems.

Density and isochoric heat capacity of {x trans-1,3,3,3-Tetrafluoropropene + (1-x) Propane} at temperatures from (285.45 to 349.71) K and pressures up to 10.384 MPa

The increasingly serious global warming problem has accelerated the elimination of traditional hydrofluorocarbon (HFC) refrigerants. R1234ze(E) + R290 binary mixture as a potential alternative refrigerant are expected to overcome the deficiencies of R1234ze(E) (low volumetric refrigeration capacity) and R290 (flammability and explosiveness). In this paper, the 38 density and 119 isochoric specific heat capacity data of R1234ze(E)+R290 mixtures were measured at temperatures from (285.45 to 349.71) K using an adiabatic batch calorimeter with intermittent heating. Uncertainties of temperature, pressure, density and isochoric heat capacity were estimated to be less than 12 mK, 5 kPa, 0.30% and 1.0%. The density data were correlated using the Tait equation, and good regression results were obtained, with the AARD of 0.19% and MARD of 0.83%. A comparison was conducted between the Tait equation and REFPROP 10 and Peng-Robinson equation of state (PR EoS), the Tait equation gives the best prediction performance. In addition, the isochoric specific heat capacity data were compared with the values calculated by four models (PR model, Zhong model, Sheng model and REFPROP 10). Among them, the three models, the Zhong model, Sheng model and REFPROP 10, give the same prediction performance, with AARD of 2.88%, 2.47% and 2.51%, respectively. But for the PR model, the density data has a significant impact on the result, especially for the low-temperature region. When the density data were calculated by the Tait equation, the PR model presents the best prediction performance for the heat capacity of R1234ze(E) + R290 mixtures with the AARD of 2.00%, nevertheless, when the density data were calculated from the PR EoS, the PR model showed a large deviation in the prediction of isochoric heat capacity with the AARD and MARD of 3.74% and 11.11%. An empirical equation was correlated to represent the experimental heat capacity data with the MARD of 3.07% and the AARD of 1.15%.