Low Global Warming Potential Refrigerants Research Articles

District heating utilizing waste heat of a data center: High-temperature heat pumps

Data centers are energy-intensive facilities with substantial low-grade waste heat. High-temperature heat pumps can be critical in boosting the data center’s waste heat for district heating, improving the system-level energy efficiency of data centers, and reducing CO2 emissions in district heating. This study built thermodynamic models to assess high-temperature heat pumps with six configurations using low global warming potential refrigerants to supply heat up to 120 °C. The heat pump configurations include single-stage or two-stage cycles with advanced components, such as internal heat exchanger, economizer, flash tank, or parallel compressor. The refrigerants include R1234ze(Z), R1233ed(E), R1224yd(Z), R600, and R600a, and R245fa is used as a reference. A case study was carried out to recover the waste heat from the Frontier high-performance computing data center and provide hot water for district heating at the US Department of Energy’s Oak Ridge National Laboratory campus. The optimized performance of high-temperature heat pumps is characterized with various effectiveness of internal heat exchangers, and the operating parameters of economizer or flash tank, as well as their combination. The results show that the configurations of two-stage cycles with internal heat exchanger + flash tank and internal heat exchanger + economizer/parallel-compressor provide the highest coefficient of performance under scenarios of the maximum allowable value and a fixed value (0.3) of the internal heat exchangers’ effectiveness, respectively. R1234ze(Z) and R600a are the most promising refrigerants, considering trade-offs between the coefficient of performance and the volumetric heating capacity. The single-stage cycle with internal heat exchanger + economizer/parallel-compressor using R1234ze(Z) is recommended for utilizing Fronter’s waste heat in district heating. A one mega-watt high-temperature heat pump will reduce 33,100–33,200 metric tons of CO2 emission annually, corresponding to 85.4 %-85.6 % of equivalent CO2 emissions from natural gas boilers. This study provides good guidelines for designing and deploying high-temperature heat pumps to support sustainable data centers and decarbonize district heating in the US.

Energy, environmental and economic analysis of low GWP refrigerant heat pumps for simultaneous heating and cooling applications

Industries such as hotels, pharmaceuticals, dairy and food and beverage units require heating and cooling for their functional requirements. These industries mostly use fossil fuel-fired boilers for heating (hot water at 60 to 90 °C) and vapor compression refrigeration systems for cooling (−30 to 5 °C), which results in higher primary energy consumption and CO2 emissions. Heat pumps (HPs) are a sustainable option for meeting the simultaneous heating and cooling demands. However, HPs use synthetic refrigerants with high global warming potential (GWP). Therefore, this research aims to evaluate the potential of low and medium-temperature HPs using low GWP refrigerants such as R744, R290, R600a, R161, R1234yf, R1234ze(E) and R1233zd(E). The best refrigerant for the HPs is proposed based on energy efficiency, environmental impact and economic feasibility as critical factors, compared with the conventional high GWP refrigerant (R134a). The occurrence of pinch points across the gas cooler/condenser is evaluated in the analysis. Life cycle climate performance (LCCP) is assessed to study the environmental impact. Also, the operating costs of these HPs are estimated and compared with the conventional systems used in the industry. Results show that R744 HP is an energy-efficient and clean solution for simultaneous heating and cooling applications, primarily at higher temperature lift (lower Tw,i and higher Tw,o) conditions.

Transitioning to low GWP refrigerants in automobiles: A numerical simulation study on R1234yf/R152a’s heat transfer characteristics in porous microchannels

Automobile air conditioning systems significantly contribute to vehicle energy consumption. The transition to new, environmentally sustainable refrigerants in place of traditional ones is a critical trend shaping the future of this industry. The prevalent refrigerant R134a, used in automotive air conditioning, is environmentally detrimental, necessitating the development of low Global Warming Potential refrigerants as viable long-term alternatives. This study examines the flow heat transfer characteristics of the proposed refrigerant blend R1234yf/R152a in porous microchannel tubes via numerical simulation. It assesses the impact of heat flux, mass flow rate, saturation temperature, and wall superheat on the Jakob number, vaporization core density, and boiling heat transfer coefficient. Furthermore, it compares these parameters with those of R134a and R1234yf to evaluate the potential of R1234yf/R152a as a substitute. The findings indicate that the Jakob number differences among the three refrigerants are negligible. Notably, R1234yf/R152a consistently exhibits higher vaporization core density and the highest boiling heat transfer coefficient, coupled with the lowest pressure drop, thereby affirming its suitability as a heat transfer-efficient alternative. These insights could inform the enhancement and gradual transition from R134a in automotive air conditioning systems.

Pool boiling heat transfer characteristics of low-GWP refrigerants in a horizontal tube bundle configuration

Heat transfer enhancement techniques have been adapted on the shell side to improve the overall performance of the flooded evaporators, such as finned tubes. In recent years, it has been demonstrated that the metal foam structure can offer enhanced heat transfer performance under pool boiling conditions. However, there are not much research in the open literature that examine the feasibility of metal foam embedded tubes in horizontal tube bundle configurations. Therefore, this paper proposed a novel metal foam embedded tube (i.e., foam embedded outside the tube) to improve the heat transfer behavior of flooded evaporators. The experiments were performed on a horizontal tube bundle with a staggered arrangement. Moreover, the performance of low global warming potential (GWP) refrigerants (R-1234yf and R-1234ze(E)) is compared against R-134a for both plain tubes and metal foam tubes with porosities of 81%, 75%, and 62%. The results showed that a metal foam tube with a porosity of 62% showed a maximum heat transfer coefficient (HTC) enhancement of 291% compared to the plain tube. As compared with R-134a, the HTCs of R-1234yf and R-1234ze(E) are nearly 10% higher and 5% lower, respectively.

Flow boiling heat transfer of R454B in a 24-port microchannel tube

R454B is a relatively low Global Warming Potential refrigerant, and it has emerged as a promising substitute for traditional refrigerants. Understanding the heat transfer and fluid dynamics characteristics of R454B becomes important for applying it. An experimental setup was built for measuring heat transfer coefficient (HTC) and pressure gradient (dP/dz) in a 24-port microchannel tube. The test condition covers mass fluxes of 100–250 kg-m-2s−1, heat fluxes of 2–8 kW-m−2, and saturation temperatures of 10–30 °C. The mass flux had a significant impact on the HTC, particularly at higher vapor quality. Additionally, the heat flux also played a crucial role in determining the HTC, with a stronger influence observed at lower vapor quality. The saturation condition had a relatively minor effect on HTC, showing a positive impact at low vapor quality but a negative impact at moderate and high vapor quality. The dP/dz tended to increase with vapor quality. Both the mass flux and saturation condition exhibited strong effects on dP/dz. However, the heat flux had a negligible impact on the dP/dz. The study also compared the performance of R454B with R32, R1234yf, and three R32/R1234yf mixtures (85/15, 50/50, and 15/85 by mass). The results showed that R454B had a HTC close to R1234yf and the 85/15 mixture. However, it had the lowest dP/dz compared among the compared fluids. The results were compared to existing correlations and those with less error were recommended. This study pioneered the examination of R454B flow boiling in microchannel tubes, and employed Pearson correlation coefficient analysis which presents a first-of-its-kind quantitative analysis of the impact of operational conditions on HTC and dP/dz, marking a significant advancement in thermal systems optimization.

Flow boiling heat transfer of R1234ze(E) in a horizontal mini-channel at medium and high saturation temperatures

This study reports experimental results of flow boiling heat transfer characteristics of R1234ze(E) (a low global warming potential refrigerant) at moderate and high saturation temperatures. The study was performed in a single horizontal circular smooth tube of 2.07 mm internal diameter. Heat transfer characteristics and flow pattern visualization through high-speed camera were investigated for saturation temperatures ranging from 60 to 95 °C, corresponding to reduced pressures from 0.35 to 0.75, mass velocities from 300 to 500 kg/m2s, heat fluxes from 30 to 50 kW/m2 and vapor qualities from 0 to 1. The pre-dryout data (214 data points) measured were compared against different prediction methods from the literature developed under different theories. The study revealed that the saturation temperature has a significant effect on heat transfer coefficient whereas the mass velocity presented a less pronounced or negligible effect. Furthermore, the heat transfer coefficient was observed to decrease slightly to a minimum in the low vapor quality region and then levels off or increases slightly with increasing vapor quality until dryout occurs. As a result, nucleate boiling was observed to be the dominant mechanism controlling the heat transfer. It was observed that the higher the saturation temperature, the earlier the vapor quality at which dryout incipience occurs. Considering the 214 pre-dryout data points analyzed, the methods proposed by Kew and Cornwell (1997), Lazarek and Black (1982) and Choi et al. (2007) were the most accurate, predicting the entire pre-dryout database with mean absolute errors (MAEs) of 13.7%, 13.9 % and 14.4 %, respectively.

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.

A novel approach for designing efficient and sustainable cooling cycles with low global warming potential refrigerants

With the current legislation on the phase-out of 3rd generation refrigerants, new cooling cycles are expected to operate with 4th generation refrigerants, hence, the need for optimal system performance with these new working fluids. In this study, an innovative optimization approach was applied for the design of cooling cycles with 4th generation refrigerants based on integrating statistical analysis design of experiments (DoE) with molecular modeling using the polar soft-SAFT model. First, a detailed multi-criteria assessment was applied for the selection of 4th generation refrigerants in medium–low temperature applications. Results demonstrated the potentiality of trans-1,3,3,3-tetrafluoroprop-1-ene (R1234ze(E)) or cis-1,2,3,3,3-pentafluoroprop-1-ene (R1225ye(Z)) in terms of environmental, safety, and technical criteria in basic cooling cycles under specific operating conditions. Multi-objective optimization guided using statistical analysis tools were implemented to study potential enhancement in cooling cycle performance using these promising refrigerants in terms of energy and exergy efficiency. This included evaporator and condenser temperature, fluid choice, compression yield, superheating/subcooling degrees, and cycle type. The evaporation and condenser temperature, and the compressor efficiency are shown to monopolize more than three-quarters of the cumulative contribution on system efficiency for those operated with R1234ze(E) in advanced liquid-to-suction line heat exchangers (LL/SL-IHX). Further optimization through examining the effect of heat transfer phenomena assisted in determining optimal operating conditions for new systems operating with low GWP refrigerants, instilling annual cost savings of $1.59 K and reduction in CO2 emissions by 1.02 tCO2-eq compared to the baseline system.

In tube condensation of low global warming potential refrigerants in an axial micro-fin aluminum tube

Environmental regulations have driven the development of refrigerants with low global warming potential (GWP). To design heat exchangers using these new refrigerants, data are needed concerning the heat transfer coefficient and pressure drop in two-phase flow. Another change is the increasing use of aluminum tubes rather than copper tubes to reduce heat exchanger cost. Hence, this study presents an experimental investigation of flow condensation using an expanded axial micro-fin aluminum tube with a fin-tip diameter of 5.96 mm. The experiments included single compounds R-32, R-1234yf, and R-1234ze(E), zeotropic mixtures with low glide (R-454B), and zeotropic mixtures with high-glide (R-454C and R-455A). Experiments were conducted at condensation temperatures ranging from 40 °C to 50 °C, reduced pressures ranging from 0.21 to 0.55, and mass fluxes ranging from 150 to 350 kg/(m2 s). Data obtained for these refrigerants constitute one of the first reports for high-glide refrigerants using axial micro-fin aluminum tubes. An evaluation of heat transfer degradation of zeotropic mixtures due to mass transfer resistance at the liquid/vapor interface is presented. This information can be used to design heat exchangers for next generation air conditioning and refrigeration systems.

Performance analysis of automotive air conditioning systems with electric compressors using R1234yf refrigerant: Insights into power consumption, cooling capacity, and energy efficiency

R1234yf, a low global warming potential (GWP) refrigerant, offers a promising alternative to traditional refrigerants like R134a. This study analyzes automotive air conditioning systems (AAC) equipped with electric compressors and utilizing R1234yf refrigerant. The interrelationships between refrigerant charge, compressor speed, power consumption, cooling capacity, and energy efficiency are examined. The results show that increasing refrigerant charge and compressor speed leads to higher power consumption due to increased compressor work. However, higher refrigerant charges and compressor speeds improve cooling capacity in line with heat transfer principles. As refrigerant charge and compressor speed increase, the energy efficiency, measured by the Coefficient of Performance (COP), tends to decrease, indicating a trade-off between cooling capacity and energy efficiency. Optimizing refrigerant charge and compressor speed is crucial to strike a balance between cooling performance and energy efficiency. R134a outperforms R1234yf, with an average percent difference in COP approximately 25% higher at all compressor speeds. This study contributes to the understanding of AAC system performance and guides the development of energy-efficient automotive air conditioning. Future research can further explore system components and conditions to enhance energy efficiency and advance air conditioning technology.

Study of vapor compression refrigeration system with suspended nanoparticles in the low GWP refrigerant.

The rising global temperatures, attributed to the high global warming potential (GWP) of conventional refrigerants, necessitate the adoption of low-GWP alternatives in HVAC systems. However, these low-GWP refrigerants often exhibit high toxicity and flammability, limiting their usage. To address these challenges, compact heat exchangers incorporating blended refrigerants have been introduced to enhance HVAC system performance. Researchers have also made significant strides in improving HVAC system efficiency by introducing the concept of suspending nanolubricants and nanorefrigerants within the system. This review paper seeks to comprehensively assess the potential of alternative refrigerants containing suspended nanoparticles, commonly referred to as nanorefrigerants. The paper reviews various mechanisms and potential combinations of different nanorefrigerants employed to enhance refrigeration system effectiveness and efficiency. A detailed examination of key heat transfer parameters and the performance predictions of low-GWP refrigerants, including those from the hydrofluoroolefin (HFO) and hydrocarbon (HC) classes, is conducted through energy and exergy analyses. Commercial refrigerants like R-134a, R-290, R-600, R-600a, R-123, R-125, R-22, R-141b, R-152, R-11, R-113, R-404a, R-407c, R-502, R-600a, R-507a, R-1234yf, R-1234ze, 1336mzz(Z), and R-410a are evaluated in conjunction with suspended nanoparticles, considering their specific properties. The findings indicate that the utilization of nanorefrigerants leads to notable improvements in overall system performance, characterized by reduced compressor workloads and increased heat transfer rates. Consequently, the integration of blended nanoparticles into refrigerants holds significant promise for advancing the HVAC field.

Heat exchanger circuitry optimization using an enhanced integer permutation-based genetic algorithm in low-GWP reversible heat pump applications

Tube-fin heat exchangers (HXs) are key components in air-conditioning and heat pump applications. Advanced heat exchanger design optimization tools are desired to adapt to the industrial transition from conventional refrigerants to low Global Warming Potential (low-GWP) refrigerants. The performance of heat exchangers is strongly influenced by the refrigerant circuitry. This research presents an enhanced integer permutation-based Genetic Algorithm (IPGA) to optimize the circuitry of tube-fin heat exchangers. To remedy the limitation of the previous IPGA, which cannot generate designs with splitting and merging of circuits, a new chromosome, as well as new generic operators, are developed. The enhanced IPGA uses effective chromosome representations and GA operators to guarantee the chromosome (genotype) can be mapped to valid heat exchanger designs (phenotype) and incorporates real-world manufacturability constraints to ensure the optimal designs are manufacturable with the available tooling. Previous circuitry optimizations focus on improving component-level performance under a specific operating condition, i.e., the heat exchanger either works as a condenser or an evaporator. The optimal circuitry obtained under air conditioning (AC) mode cannot guarantee optimal performance when used in heat pump (HP) mode. To improve the dual-mode performance of reversible heat pumps, a bi-objective optimization problem formulation is implemented to achieve optimal system performance in both AC and HP modes. A case study using six refrigerants, i.e., R410A, R452B, R454B, R32, D2Y60, and L41a, shows that optimal heat exchangers yield up to 6.1 % energy efficiency ratio (EER) improvement under AC mode and up to 8.5 % coefficient of performance (COP) improvement under HP mode. The use of this design approach can ensure a preferable performance of reversible heat pumps during both cooling and heating modes.

Thermodynamic analysis of high temperature cascade heat pump with R718 (high stage) and six different low-GWP refrigerants (low stage)

Decarbonizing industrial heating globally is a significant challenge, driving the integration of high-temperature heat pumps for heat recovery and electrification in the industrial sector. However, current high-temperature heat pumps with low global warming potential refrigerants face limitations in delivering high-temperature heat due to their restricted compressor discharge temperature or require several compressors/intercoolers. To address these issues, a novel high-temperature cascade heat pump system is proposed, utilizing water (R718) in the high stage, while selecting one among six low GWP refrigerants (R1234ze(Z), R1233zd(E), R1336mzz(Z), R600, R600a, R601) in the low stage, with single compressor at each stage. To manage high discharge temperatures of high stage, liquid water injection is applied during high stage compression, modeled for both wet and dry steam. The model is validated against available experimental data with maximum error less than 5 %. Further, the effectiveness of the internal heat exchanger in low stage cycle is assessed for all investigated low stage refrigerants. Results reveal that R718 + R600 offers the optimal refrigerant working pair, achieving a maximum COP × VHC of 2895.1 kJ m−3 for delivering high temperature heat at water condensation temperature of 190 °C and recovering heat from low temperature waste energy of 50 °C.

Experimental investigation on condensation heat transfer coefficient and frictional pressure drop of low GWP refrigerant R1234yf on a wavy fin surface

Refrigerant R1234yf is an ideal substitute to replace R134a due to its lower value of global warming potential (GWP). The thermophysical properties of R1234yf are similar with R134a and thus it is considered as a potential replacement for R134a in thermal fluid systems. Further, it was noticed that there is a lack of analysis on the condensation of R1234yf in a brazed plate fin compact heat exchanger (CHE) employing wavy fins. In the present work, condensation heat transfer coefficient and frictional pressure drop of refrigerant R1234yf are investigated in a brazed plate fin compact heat exchanger with wavy fin surfaces. The test condenser with wavy fin surface was selected to examine the condensation heat transfer coefficient and frictional pressure drop using refrigerants R134a and R1234yf for varying mass flux and saturation temperatures. The present study also discusses the dependency of specific kinetic energy of refrigerant on frictional pressure drop. Further, using the experimental data of wavy fin surfaces, suitable correlations for the condensation heat transfer coefficient and frictional pressure drop of R1234yf were proposed. The present paper also reports the comparison made between refrigerants R134a and R1234yf. Comparison between R1234yf and R134a shows that condensation heat transfer coefficient and frictional pressure drop of R1234yf is 9–16 % and 10–17 %, respectively, lower than of R134a in the mass flux range 15–44 kg/m2s.

Energy and emission performance of enhanced vapor injection air source heat pump system using low global warming potential refrigerants in different climate regions

In order to meet the space heating requirement of residential buildings in low-temperature areas, the performance of the enhanced vapor injection (EVI) air source heat pump (ASHP) system and single-stage compression heat pump system (BASE) using low global warming potential (GWP) working fluids in low-temperature environment are studied. The thermodynamic and emission characteristic models of air source heat pump are developed and optimized, and further compared with traditional heating solutions when used in five different typical cities throughout the world. The results indicate among the selected working fluids, R152a achieves the highest COP of 3.91 among all of the selected low GWP working fluids. When the ambient temperature is 0°C, the maximum COP of the EVI system is 2.51 when CO2 is adopted, and the corresponding optimal discharge pressure and intermediate pressure are 10.57 MPa and 3.83 MPa, respectively. By exploring the changes of HSPF in five typical cities, the HSPF of the EVI system using CO2 is the most significant, which is 17.13%–26.69% higher than the BASE system. The most significant reduction of LCCP in EVI system using CO2 is 15.34%–26.66% compared with BASE system. For SO2 and NOX, the EVI system using R152a has a better emission reduction effect, which is 3.73%–64.73% and 3.72%–66.04% lower than the other solutions, respectively. This study can provide a theoretical reference for the application of low GWP heat pumps with EVI technology.

Study on the carbon footprint of cold storage units using low-GWP alternative refrigerants

The increasing contribution of refrigeration systems to global warming has led to widespread demand for low global warming potential (GWP) refrigerants as alternatives. This paper proposed a comprehensive environmental assessment method for evaluating the carbon footprint of units charged with low-GWP refrigeration. Experimental and numerical studies were used to investigate the carbon footprint impact of using alternative refrigerants at different ambient temperatures in China. Initially, the model was validated through experimental case studies on the use stage of cold storage units. Subsequently, a comparison was made between the cooling capacity (CC) and energy efficiency ratio (EER) of cold storage units charged with different refrigerants. Additionally, the impact of alternative refrigerants in the use stage was analyzed by numerical calculations, and the adaptability of the cold storage unit to the ambient temperature and the national energy structure was also studied. The results indicate that among the four stages of the cold storage unit, the use stage is the hotspot of carbon footprint due to the large amount of energy consumption and the existence of refrigerant leakage and other behaviors. Utilizing low-GWP alternative refrigerants could significantly reduce the carbon footprint of the cold storage units' use stage by 10.56%–16.39%, thereby lowering the whole stage carbon footprint. Furthermore, the ambient temperature and national energy structure could also effectively reduce the carbon footprint of cold storage units' whole stage by 1%–27.17%. Furthermore, after adopting low GWP refrigerants, the life-cycle cost (LCC) of the units have also decreased by 6.1%–12%. Taking into account both environmental impact and economic considerations, it is recommended to directly replace R404A with R448A in the units in regions where the average ambient temperature is above 15 °C.

Condensation heat transfer and pressure drop of low-global warming potential refrigerants in smooth aluminum tubes

In this work we report condensation heat transfer coefficient and pressure drop data for R-454B, R-32, and R-454C. These refrigerants are low Global Warming Potential (GWP) replacements for R-410A. We obtained these data in a smooth 7.2 mm outer diameter (5.94 mm inner diameter) aluminum tube. In these experiments, the key variables were condensation temperature (35 °C ≤Tcond≤50 °C) and refrigerant mass flux (151.1≤G≤347.3 kg m−2 s−1). In general, for similar nominal operating conditions, we measured the highest heat transfer coefficients for R-32 and the lowest for R-454C. Among the correlations we used to predict the heat transfer data for all three refrigerants, the one developed by Thome et al. (2003) predicted the data with the greatest accuracy. Deviations between experimental data and predictions of this correlation were 21.7% for R-454B (with SBG correction), 22.0% for R-32, and 18.8% for R-454C (with SBG correction). Experimental pressure gradient data were predicted by the Friedel correlation Friedel (1979) with deviations of 13.8% for R-454B, 9.8% for R-32, and 10.2% for R-454C. In this work, we also compare the predictions of the Cavallini et al. correlation Cavallini et al. (2006) for R-454B and R-32 in smooth copper versus smooth aluminum tubes. To improve this correlation’s predictive capability for aluminum tubes, we have proposed some modifications. After incorporating these modifications, the deviations between the correlation’s predictions and our data were 10% for R-454B (with SBG correction), 8.1% for R-32, and 5.2% for R-454C (with SBG correction). These data and findings will assist the HVAC and refrigeration industry to implement low-GWP refrigerants in condensers using smooth aluminum tubes.

Effect of system configuration on the performance of a hybrid air conditioning system based on R-1234yf

The study presents theoretical results on a hybrid air conditioning system in which the sensible and latent loads are separately managed. The problem studied is significant in view of the growing demand for air conditioning systems globally, and also due to the adverse impact of these systems on global warming. In order to reduce the environmental impact of the air conditioning systems, this study proposes a hybrid system comprised of a vapour compression heat pump and a desiccant wheel. Due to restrictions on high global warming potential (GWP) refrigerants, R-1234yf, a low GWP refrigerant is considered as the working fluid for the heat pump. An enthalpy wheel is included to reduce the ventilation load on the system. The proposed hybrid system’s performance is evaluated using the first and second laws of thermodynamics and psychrometric principles. Results are compared with that of a conventional air conditioning system. Based on the calculated values of component and process wise irreversibility, further modifications to the hybrid system are proposed. These modifications consist of addition of a liquid-to-suction heat exchanger and use of a split condenser. Results show that depending upon the latent load on the conditioned space, the compressor power input can be reduced by as much as 47% using the proposed hybrid system. Compared to the conventional system, the modified hybrid system shows excellent performance under both high as well as low latent loads, whereas the simple hybrid system performs very well in applications involving high latent loads. The work presented is novel as it considers both first and second laws of thermodynamics to arrive at an optimum hybrid system configuration using an environment friendly refrigerant. It is expected that the study will be useful in the development of energy efficient and environment friendly air conditioning systems that operate equally well under low and high latent loads.

Recent progress in pool boiling heat transfer of low GWP refrigerants with the effect of POE lubricant oil

The recent research on pool boiling heat transfer of low global warming potential (GWP) refrigerants under the effect of polyolester (POE) lubricating oils are summarized in this article. Recently, several low GWP refrigerants have been developed as alternatives to the high GWP refrigerants such as hydrofluorocarbons (HFCs). However, it is crucial to comprehend how lubricating oil affects the performance of low GWP refrigerants in terms of compatibility and pool boiling heat transfer (commonly seen in flooded evaporators). The overview suggests that the low GWP refrigerants are compatible with POE oil. Whereas, its heat transfer performance depends on the mass concentration of oil present in the refrigerant. In addition, the pool boiling performance of the low GWP refrigerant is also affected by other factors such as evaporator pressure, saturation temperature, heat flux, and thermophysical properties of refrigerant and oil mixture. The analysis of available heat transfer coefficients (HTCs) for refrigerant-oil mixture shows that the heat transfer performances of the enhanced surfaces are very sensitive in the presence of oil; whereas, it is less severe on a smooth surface. On smooth surfaces, a refrigerant-oil mixture (3% mass concentration) could augment the HTCs as compared to the pure refrigerants subject to evaporator pressure/saturation temperature. In contrast, the simulation studies show that the oil additives always deteriorate the HTCs compared to the pure refrigerant.

Flame Propagation in Blends of R-152a, R-134a, and R-1234yf with Air

ABSTRACT Some new, low-global warming potential refrigerants will be flammable, and the laminar burning velocity is a useful parameter for quantifying fire risk. Laminar burning velocity measurements have been made using a constant volume experiment with dry air and the refrigerant R-152a (CH3CHF2), pure and blended with R-134a (CH2FCF3), or R-1234yf (CF3CFCH2). The resulting burning velocity data deduced from the pressure rise in the chamber are presented for a range of fuel air equivalence ratio and loading of the less flammable refrigerant, for unburned gases at 298 K and 101 kPa as well as at 375 K and 253 kPa. For comparison, the 1-D, planar laminar burning velocity was numerically simulated using a recently developed kinetic mechanism that includes a wide range of refrigerants with air. The predicted burning velocities agree reasonably well with the experimental values, and the numerical results are used to understand the kinetic mechanism of the reaction of the refrigerants. Uncertainties in the experimental data from radiation heat losses as well as extrapolation to ambient conditions are explored.