Compressor Discharge Temperature Research Articles

Performance analysis of a novel air source ammonia/water cascade heat pump for heating buildings in subarctic climate

Performance of available air-source heat pumps (ASHP) for buildings is extremely influenced by the outdoor air temperature (OAT). When OAT falls below –20 °C, especially in subarctic climate, most commercially available ASHPs lose their defined functionality. To address this issue, a novel cascade heat pump (CHP) is proposed that utilizes two natural (with zero global warming index) refrigerants: ammonia, aka R717, as the low stage (LS) refrigerant, and water, aka R718, as the high stage (HS) working fluid. The proposed system is equipped with two rotary vane compressors for LS and HS. To desuperheat the compressor discharge temperatures and increase the HS heating capacity, liquid refrigerant is injected into the chamber of both LS and HS compressors. The wet compression is thermodynamically modeled and validated against experimental data available in the literature. Optimum design parameters such as the maximum allowable temperatures during compression and the minimum vapor quality after injection are obtained where the COP is maximized. Total exergy destruction, number/location of injectors and the injected amounts are also investigated. The proposed CHP can deliver 6.35 kW heating load at 50 °C with a COP of 1.985 when OAT is –40 °C. From a broader perspective, the proposed CHP not only proves its efficacy in extremely cold subarctic climates but also emerges as a pivotal player in addressing the global warming issue. This is accomplished through a dual approach to decarbonization—firstly, by electrifying heating processes and secondly, by substituting phased-out refrigerants with environmentally friendly natural alternatives.

Fault diagnosis for cross-building energy systems based on transfer learning and model interpretation

Timely and accurate fault diagnosis in building energy systems is crucial for improving the energy efficiency of equipment and realizing energy savings. With the widespread use of building automation systems, data-driven deep-learning fault diagnosis methods have gained popularity due to their flexibility and accuracy. However, they have certain associated challenges, such as limited extrapolation capabilities and their black-box nature. To address these issues, this study focuses on transfer learning to improve the energy efficiency of building energy system equipment, exploring the influencing factors and behaviours of transfer learning fault diagnosis performance and model interpretation results. A transfer-learning strategy is employed, utilizing feature-level SHAP and visual interpretation methods to understand the inner workings and diagnostic mechanisms of transfer-learning models. The study constructs five transfer-learning strategies for two typical building energy systems. The results demonstrate that the proposed fine-tuning approach achieves the highest accuracy, of 94.32 %, surpassing non-transferred and shallow approaches. The interpreted methods reflect the joint diagnosis of refrigerant leakage faults by compressor discharge temperature and condenser inlet and outlet water temperature in the target chiller. The findings from both the transfer mechanism and interpretation of the deep-learning model provide valuable guidance for the development of practical deep transfer-learning models and corresponding interpretation methods. This research contributes to improving fault diagnosis accuracy and understanding the diagnostic mechanisms in building energy systems, ultimately leading to enhanced energy efficiency and energy savings.

A Water-to-Water Heat Pump Model with Experimental Validation

An experimental validation of a steady-state model for water-to-water heat pumps is conducted on a 10 kW test bench. The objective of the model is to predict the capacity and the required compressor power, based on the inlet conditions of the secondary fluids in the evaporator and condenser. Detailed manufacturer performance maps based on the AHRI 540-2020 standard are utilized to model the fixed-speed scroll compressor. A new semi-empirical model for the thermostatic expansion valve incorporates condensing temperature effects on superheating prediction. Sub-models for individual components, including detailed representations of the evaporator and condenser, are integrated into a global model, resulting in a set nonlinear equation solved using an equation solver with appropriate guess values. The validation of the model is conducted in an experimental test facility equipped with two precisely controlled secondary fluid loops. The heat pump is instrumented to measure condensation and evaporation pressures, the compressor discharge temperature, compressor power, superheating, and sub-cooling. The results are divided into three sub-sections: the first validates the complete heat pump model by comparing its power consumption and COPs in heating and cooling; the second compares the predicted and measured operational conditions; finally, it is shown how the model can be used to predict the non-operational conditions of the heat pump for specific scenarios.

Parametric analysis for nanolubricant in vapor compression refrigeration cycle

With the escalating global energy crisis and the consequential surge in energy consumption, particularly attributed to air conditioning (AC) systems in buildings, there arises a pressing need for sustainable solutions to mitigate this escalating demand. Vapor compression refrigeration (VCR) cycles, ubiquitous across residential, commercial, and industrial sectors, represent a significant portion of global electricity consumption, exceeding 10%. This demand is further exacerbated in regions characterized by hot climates. Consequently, there is a critical imperative to address the environmental impact of these systems, particularly in terms of reducing their Global Warming Potential (GWP). In response to this imperative, this study aims to develop a comprehensive numerical model for VCR systems, focusing on the implementation of nanolubricants. The primary objective is to assess the impact of integrating Al2O3 nanoparticles into the lubricating oil (referred to as nanolubricant) on the performance of the VCR cycle. The scope of the study encompasses the investigation of nanolubricants within refrigeration systems, aiming to enhance their efficiency and environmental sustainability. Utilizing the Engineering Equation Solver (EES), a numerical model of a VCR cycle utilizing R-410A refrigerant is developed. The model specifically emphasizes the compressor component, which is flooded with a polyolester (POE) oil base. The study explores various parameters, including ambient conditions, refrigerant-to-oil dissolving ratio, and mass concentration of nanoparticles, to evaluate their influence on system performance. The results indicate a significant enhancement in performance when flooding the base oil before introducing Al2O3 nanoparticles, with further improvements observed upon nanoparticle integration. Specifically, the coefficient of performance (COP) of the VCR system exhibits a notable increase compared to the base case configuration. Additionally, the adoption of eco-friendly refrigerants, such as R-32, demonstrates promising COP values following nanoparticle implementation by up to 4%. Moreover, the study reveals a reduction in compressor discharge temperature by up to 20% with higher oil mass concentrations during nanolubricant implementation, indicating enhanced durability and longevity of the VCR system’s compressor. These findings underscore the potential of nanolubricants to contribute to both the efficiency and environmental sustainability of refrigeration systems. In conclusion, the findings of this study offer valuable insights into the utilization of nanolubricants in VCR cycles, highlighting their potential to enhance system performance and mitigate environmental impact. These advancements hold promise for the development of more sustainable refrigeration technologies in the face of mounting energy challenges.

Theoretical analysis of R1234yf and R1234yf/R125 mixture as replacement of R134a in vapor compression system

Abstract In this article, in order to reduce the climate hazards caused by global warming, a new environmentally friendly mixed refrigerant, R1234yf/R125 (with a mass ratio of 95:5), is introduced. Alongside R1234yf, the article conducts theoretical research to assess the feasibility of substituting R134a in vapor compression systems. The results show that coefficient of performance (COP) and compressor discharge temperatures of R1234yf and R1234yf/R125 are ~5.5% and 11°C lower than those of R134a, respectively. The volumetric cooling capacity of R1234yf is on average 8% smaller than that of R134a. The volumetric cooling capacity of R1234yf/R125 is similar to that of R134a, making it highly suitable for direct charging. R1234yf/R125 is basically nonflammable, with a minimal temperature glide, ozone depletion potential of 0 and a global warming potential of 144. It adheres to F-gas regulations and serves as an outstanding replacement for R134a.

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’s performance of the refrigerator truck using R404A refrigerant

A refrigerated system experimental bench using R404A as the refrigerant was built to address the current issues of high compressor discharge temperature and poor refrigeration performance of refrigerated systems for cold storage. The effects of compressor speed and temperature changes within the storage on the main refrigeration performance parameters of the system were studied and analyzed. The results show that when the rotational speed is increased from 2500-4500 rpm under the conditions of 32?C outside and 0?C inside, the highest system exhaust temperature is 84.3?C, which is lower than 90?C, and the maximum system COP is 3.86, and when the compressor speed is increased from 3500 rpm under the conditions of -5?C to 5?C, the system exhaust temperature is lower than 90?C. The COP can reach 2.74, especially at -5?C.

Performance Evaluation of Vapour Compression Cascade Refrigeration System for Storing Blood Plasma

A cascade refrigeration system consists of two cycles to get the greatest possible refrigeration effect for applications requiring extremely low temperatures. The experimental examination of the cascade refrigeration system utilized for quickly freezing and storing blood products, notably plasma, was the main emphasis of this paper. This system is able to manage and apply cold storage for plasma at temperatures as low as -35oC. R410A (low temperature cycle) and R404A (high temperature cycle) are the working fluids in this system respectively, and their selection was based on their reasonable boiling point differences and rapid freezing times. These working fluids have very little ozone depletion and global warming potential, and they are non-toxic to the environment. This system performance evaluation determines the cascade refrigeration system's parameters ranging effects of variation in evaporator temperature, condenser temperature and temperature difference in cascade condenser. As the evaporator temperature increases from – 35oC to -70C at interval of 7oC, the suction pressure increased from 0.22 MPa to 0.64 MPa. The decrease in compressor discharge temperature of 30oC to 26oC resulted to decrease in discharge pressure ranging 1.62 to 1.49MPa in the high temperature cycle at constant condenser temperature of 40 0C. At varying heat exchanger temperature, all parameters reduced with exception of discharge temperature in both cycles and evaporator heat. A substantial increase in the coefficient of performance of comparable systems was found to be 3.04 (LTC) and 7.7 (HTC). At the high temperature cycle condenser, there was a total heat rejection of 6228 kJ, which could then be recycled for heating.

Modeling and optimization of two-stage compression heat pump system for cold climate applications

When an air source heat pump runs at very low ambient temperatures, several issues could arise and thus limit its applications. These issues include lower coefficient of performance (COP), deteriorated heat output and higher compressor discharge temperature resulting from an increased pressure ratio. Therefore, there is a need to develop more efficient heat pump systems for low ambient temperature (∼-30 °C) applications. Under this circumstance, it becomes advantageous to use devices in which the compression of refrigerant vapor occurs in two or more stages. Two-stage vapor compression heat pumps can operate more efficiently at a greater difference in temperatures between evaporator and condenser and improve their performance under cold climate conditions. In this study, a model for a two-stage compression heat pump system is developed with the system configuration containing a closed economizer cycle and inter-stage refrigerant injection. In the two-stage heat pump, the intermediate temperature and injection pressure (or displacement ratio) between low-pressure compressor and high-pressure compressor are important parameters that influence the operating performance. In this paper, the optimal intermediate parameters corresponding to the maximum possible COP have been determined. The performance of the two-stage heat pump using R410A and R290 are modeled and compared. It is shown that the two-stage heat pump achieves a COP of 2.85 and a COP of 2.7 at an ambient temperature of −30 °C using R290 and R410A, respectively. In addition, the optimized total fin surface areas of the evaporator and the condenser heat exchangers are calculated from the developed model.

Thermal design and optimization of high-temperature heat pump integrated with district heating benchmarked in Denmark for process heat supply

This work aims to assess and optimize the performance of cascade high-temperature heat pump (HTHP) integrated with district heating (DH) to produce 1 MW steam at 160 °C for the industrial processes. The heat available in the primary loop of the DH network at 80 °C is considered as the heat source; which is cooled down 70 °C through the HTHP evaporator, before supplying the DH secondary loop. The use of alternative hydrocarbons in the low-temperature loop are examined; and considering the gas compressor limitation, the HTHP performance of using each refrigerant is optimized and compared to each other.The optimization results reveal that pentane- hydrocarbon with the highest critical temperature- is the most promising refrigerant to be paired with steam in the high-temperature loop, reaching the highest COP of 2.66. However, concerning safety and compressor sizing issues, butane is an excellent candidate; with volumetric heating capacity (VHC) of about two times more than that of pentane, in the expense of just about 4 % reduction in the HTHP COP. In addition, water injection theoretically controls the steam compressor discharge temperature successfully, with just 0.9 K superheating at the compressor outlet; and reduces its power consumption and the HTHP COP up to 4.3 % and 1.7 %, respectively. Moreover, techno-economic analysis demonstrates that the HTHP technology shows a better business case compared to the conventional natural gas and electric boilers.

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.

An Experimental Study on the Cooling Performance of Refrigerant (R134a/R1234yf) in Automobile HVAC System

The present work deals with the experimental investigation on the performance evaluation of R134a and R1234yf as automotive HVAC refrigerants. As the usage of R134a increases environmental issues due to its higher global warming potential (GWP), it is necessary to find an alternative without compromising the performance of the HVAC system. R1234yf having low GWP is one such drop-in alternative. But the COP of the system using R1234yf is observed to be low. Hence this work investigates on the effects of using an internal heat exchanger (IHX) with the conventional automotive HVAC system in the performance enhancement. The results indicate acceptable range of cooling capacity, COP and compressor discharge temperature for R1234yf when integrated with IHX and a high efficiency evaporator. COP of the R1234yf IHX system is observed to higher than the COP of R134a baseline system. The results of cooling capacity and compression discharge temperature of R1234yf IHX system also shows an enhancement and matches with the corresponding values of R134a baseline system.

Performance study of the supplemental combustion type compressed air energy storage system

Compressed air energy storage technology is considered to be the most promising energy storage technology, but it has not been applied commercially on a large scale, partly because of the low system efficiency, with the existing efficiency being about 70%. To improve the round trip efficiency of the system, this paper proposes a supplementary combustion compressed air energy storage system based on adiabatic compressed air energy storage. The system adds supplementary combustion equipment to increase expansion machines’ inlet air temperature by burning fuels such as syngas, hydrogen, and natural gas to increase the power generation of the system. The thermodynamic performance analysis has been conducted through the thermodynamic model of the system, and the effects of parameters such as compressor discharge temperature, supplementary combustion, and air-fuel ratio on the system performance have been further investigated. The research results show that the efficiency of the system is improved by nearly 6% compared with the conventional adiabatic compressed air energy storage system. Meanwhile, the system’s round-trip efficiency can be further increased by appropriately raising the compressor discharge temperature and supplemental combustion quantity.

Experimental Investigation of the Improvement Potential of a Heat Pump Equipped with a Two-Phase Ejector

In this paper, an experimental investigation of the performance improvement of a heat pump equipped with a two-phase ejector, called an “ejector–expansion heat pump (EEHP)”, is proposed. The system performance of the EEHP is compared with that of a vapor-compression heat pump (VCHP). The improvement potential is determined and discussed. The heat pump test system based on a water-to-water heat pump that can experiment with both the EEHP and the VCHP is constructed. A two-phase ejector with a cooling load of up to 2500 W is installed for the experiment. The results show that the EEHP always produces a higher heating rate and COPHP than the VCHP under the specified working conditions. The heating COPHP is increased by 5.7–11.6% depending on the working conditions. It is also found that, under the same heat sink and heat source temperature, the EEHP can produce a lower compressor discharge temperature and a lower compressor pressure ratio than the VCHP. This is evidence that the two-phase ejector can provide the compressor with better working characteristics, which yields a longer compressor lifetime. It is demonstrated that the expansion pressure ratio is key to the performance of the EEHP. A larger expansion pressure ratio yields greater improvement potential when compared with the VCHP.

Theoretical exploration of low GWP refrigerant mixtures as replacement to HFC-134A in a vapour compression refrigeration system

Hydro-fluorocarbons (HFCs) that are not harmful to ozone layer and are used in many appli-cations, including as refrigerants, aerosols, solvents and blowing agents for insulating foams replace chlorofluorocarbons and hydro chlorofluorocarbons (HCFCs). However, some HFCs have a high GWP, which stands for “Global Warming Potential(GWP).” Because of the grow-ing concern over global climate change, researchers are paying closer attention for alternatives to these HFCs. The main focus of this work was on the theoretical analysis of the refrigerant mixtures namely AC5, R430A, and R440A as direct replacements for HFC-134a in a refriger-ator. The performance of the refrigerator may be enhanced using heat exchanger. The com-pressor discharge temperature, COP, VCC, refrigerant mass flow rate, power consumption of a compressor and pressure ratio were used to measure the performance of a home refrigerator. The typical COP of R440A and R430A was about 2.5% and 1.4% higher than that of HFC-134, while the average COP of AC5 was 6.1% lesser than that of HFC-134a. R430A almost has the same VCC (Volumetric Cooling Capacity) as HFC-134a. The results also show that HFC-134a uses more power than AC5, R440A, and R430A. R440A and AC5 have higher compressor outlet temperatures than HFC-134a, which affects the life span of the compressor. In com-parison with all the above refrigerants, R430A gives the best overall performance and used as replacement to HFC-134a in a VCR.

Applicability evaluation of internal heat exchanger in CO2 transcritical cycle considering compressor operation boundaries

CO2 transcritical cycle has been widely promoted as a green energy utilization technology. Internal heat exchanger (IHX) is under consideration for cycle efficiency improvement. However, the interaction between IHX and compressor in CO2 cycle has not yet been fully explored. In this regard, compressor operating boundaries are introduced as constraints to IHX applicability evaluation in CO2 cycle. It indicates that considering compressor limits, IHX can either benefit or attenuate the CO2 cycle performance, depending on operating conditions. By reducing optimal discharge pressure, IHX helps CO2 cycle avoid the upper pressure limit, further improving performance by 5%. Over a wide range, however, IHX increases compressor discharge temperature, indirectly resulting in up to 50% performance degradation. The major factors affecting the duality of IHX in CO2 cycle are compressor discharge temperature limit, IHX efficiency and motor heat loss. Finally, performance-oriented IHX design guidelines for mainstream single-stage CO2 applications are provided. For automotive air conditioner, optimum IHX efficiency improves cycle performance by up to 30% in cooling mode. By contrast, IHX contribution is negligible in heating mode. For heat pump water heater and refrigeration system, both can benefit from IHX, particularly when compressor discharge temperature tolerance is high.

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

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

Experimental evaluation of split air conditioning performance using nanodiamonds particles in compressor polyester lubricant oil

Rationing energy consumption in air conditioning (AC) systems has become increasingly necessary due to the continuous global increase in energy demand and the prevailing sustainability vision worldwide. Therefore, an AC system utilizing nanodiamond/Polyester (ND/POE) lubricant oil, which shows potential to improve the performance and reduce energy consumption, was investigated. Consequently, a 5-ton refrigeration split AC unit was modified with an oil separator to assess the ND/POE nanolubricant in the compressor’s loop, in particularly. The AC unit was examined under various operating conditions up to an extreme high ambient temperature of 52˚C, with an optimal nanolubricant concentration of 0.1% as recommended from the literature, using a fully automated psychrometric testing facility. The results showed that using ND/POE instead of pure POE resulted in a 4.7% and 3% reduction in compressor discharge temperature and pressure, respectively. Furthermore, the overall system's coefficient of performance (COP) improved by up to 8% due to a reduction in power consumption of up to 3% and an increase in cooling capacity of up to 6%. Therefore, it is anticipated that the use of ND/POE will result in improved compressor durability, reliability, and longevity, as well as a significant improvement in the performance of AC systems operating for prolonged periods, such as those operating in extreme high ambient temperature applications.

Experimental Investigation of Gravity Effect on a Vapor Compression Heat Pump System

Vapor compression heat pumps have the potential to meet the future development requirements of thermal management in aerospace due to the ability to achieve high heat flux dissipation, precise temperature control, and a more compact and lightweight structure. In this study, a variable-angle test stand was constructed to investigate the effect of gravity on the performance, temperature distribution, and oil circulation rate of a vapor compression heat pump system in a ground environment. The results showed that the system could operate stably at various inclination angles, with the compressor exhibiting the lowest power consumption at an inclination angle of 90° and the maximum coefficient of performance (COP) of the vapor compression heat pump reaching 5.5. The system flow rate exhibited a sinusoidal curve with a 10.7% variation relative to the initial flow rate as the inclination angle increased. The evaporating temperature, condensing temperature, and compressor discharge temperature of the system were approximately symmetrical relative to the inclination angle of 180°. At inclination angles ranging from 90 to 270°, the oil circulation rate was highly sensitive to gravity changes and remained at a high level. The oil circulation rate agreed well with the change in subcooling/superheating, yet a rise in the oil circulation rate resulted in a decrease in COP.

Thermodynamic analysis of a novel vapor-injection Joule-Thomson refrigeration cycle with a binary hydrocarbon mixture for −86 °C freezer

This paper proposes a novel vapor-injection Joule-Thomson refrigeration cycle with mixed-refrigerant. The introduction of the compressor with vapor-injection can reduce the pressure ratio of each compression stage, thus reducing the compressor power and discharge temperature. The thermodynamic analysis results show that the performance of the novel cycle is better than that of the basic Joule-Thomson cycle. In addition, this paper studies the binary hydrocarbons applied in −86 °C freezers, and find a binary hydrocarbon R601a/R1150 with Superior performance. The mass fractions of R1150 in R601a/R1150 corresponding to the maximum COP of the novel cycle and basic cycle are 0.45 and 0.44, respectively. Moreover, the maximum COP of the novel cycle is 19.25% higher than that of the basic cycle. The comparison results demonstrate that the novel cycle has a great energy-saving potential. In addition, at different condenser outlet temperatures of 16 °C, 25 °C and 32 °C, both cycles have the optimal condensation pressure of 300 kPa, 500 kPa and 650 kPa with respect to COP. For the novel cycle, the compressor injection ratio has little effect on COP. The increase of intermediate pressure can improve cycle performance, but too high intermediate pressure is not conducive to the normal operation of the system. Furthermore, the exergy analysis result shows that the location of the largest exergy destruction rate is in the compressors for both cycles, followed by the recuperators, condensers, evaporators, and capillary tubes.