Performance analysis of an R744 ground source heat pump system with air-cooled and water-cooled gas coolers

In order to decrease the earth's energy imbalance degree, an R744 ground source heat pump system (GSHPS) with air-cooled and water-cooled gas coolers was designed, and the performances of this system were analyzed numerically. The results show that, for the proposed R744 system in different regions, the earth's energy imbalance degree can always be decreased to zero by optimizing the cooling load proportion of air-cooled gas cooler. Comparing with the system without air-cooled gas cooler, the investment cost of the proposed R744 system is decreased by 14.5–25.5% due to the decreasing cost of ground heat exchanger, and the coefficient of performance is increased. With the increase of ground heat exchanger depth, the performance deteriorates, and the ground heat exchanger with shorter depth is preferred in the GSHPS. The investment and operation costs of the proposed R744 system are lower than that of the existing R134a system.     

An experimental study on heat transfer and pressure drop characteristics of carbon dioxide during gas cooling process in a horizontal tube

The heat transfer coefficient and pressure drop during gas cooling process of CO₂ (R744) in a horizontal tube were investigated experimentally. The experiments are conducted without oil in the refrigerant loop. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and a gas cooler (test section). The water loop consists of a variable speed pump, an isothermal tank, and a flow meter. The refrigerant, circulated by the variable-speed pump, condenses in the inner tube while water flows in the annulus. The gas cooler of tube diameter is 6000 mm in length, and it is divided into 12 subsections.The pressure drop of CO₂ in the gas cooler shows a relatively good agreement with those predicted by Blasius's correlation. The local heat transfer coefficient of CO₂ agrees well with the correlation by Bringer–Smith. However, at the region near Pseudo-critical temperature, the experiments indicate higher values than the Bringer–Smith correlation. Based on the experimental data presented in this paper, a new correlation to predict the heat transfer coefficient of supercritical CO₂ during in-tube cooling has been developed. The majority of the experimental values are within 18% of the values predicted by the new correlation.     

R-744 gas cooler model development and validation

A first principles-based model was developed for a transcritical CO₂ gas cooler, using a finite element method. The model uses published correlations for refrigerant and airside heat transfer and pressure drop. Experimental results are presented at 48 operating conditions. The model predicted the gas cooler capacity within ±2% and pressure drop on the R-744 side well within the range of experimental error. The model's usefulness is demonstrated by analyzing alternative circuiting and multi-slab designs.     

The natural fluid nitrous oxide—an option as substitute for low temperature synthetic refrigerants

A theoretical investigation was performed concerning the coefficient of performance (COP) of cascade refrigerating systems using N₂O as refrigerant for the low temperature cascade stage and various natural refrigerants like ammonia, propane, propene, carbon dioxide and nitrous oxide itself for the high temperature stage. The basis of the comparison was a conventional R23/R134a-cascade refrigerating system for heat rejection temperatures of +55, +35 and +25 °C for air cooling, cooling tower water cooling and city water cooling, respectively. It can be stated that such an application of N₂O at the primary stage and ammonia or hydrocarbons as refrigerants at the secondary stage in refrigerating systems achieves similar COP-values compared to the R23/R134a-cascade refrigerating system, whereas CO₂ and N₂O in a transcritical cycle in general perform worse.An application of N₂O in a two-stage compression cycle with interstage injection and city water cooling at low and high interstage temperatures has a nearly equal COP as a conventional R23/R134a-cascade refrigerating system and is an interesting alternative for small laboratory refrigerating systems.     

CO_2非共沸混合工质制冷系统的理论分析

在设定工况条件下,采用3组CO2非共沸混合工质(R744/R22、R744/R1270、R744/R600a).对制冷系统进行了热力学理论分析和计算.研究了系统制冷量、压缩机功耗、制冷COP,和冷凝压力随CO2质量配比的变化关系.结果表明:在相同工况下,R744/R600a的冷凝压力最低,比R744/R22平均低22.9%,比R744/R1270平均低18.8%;R744/R1270具有较好的综合性能.     

Blends of carbon dioxide and HFCs as working fluids for the low-temperature circuit in cascade refrigerating systems

This paper describes an analysis on the performances of a cascade refrigeration cycle operated with blends of carbon dioxide (CO₂, or R744) and hydrofluorocarbons (HFC) as the low-temperature working fluid. The aim of this work was to study the possibility of using carbon dioxide mixtures in those applications where temperatures below CO₂ triple point (216.58 K) are needed. The analysis was carried out by developing a software based on the Carnahan–Starling–De Santis (CSD) equation of state (EoS) using binary interaction parameters derived from our experimental data. The properties of the investigated blends (R744/R125, R744/R41, R744/R32, R744/R23) were used to simulate the behavior of a cascade cycle using ammonia (R717) as the high-temperature-circuit working fluid and operating at evaporating temperatures down to −70 °C. The use of a suction–liquid heat exchanger on the low-temperature side of the circuit was also investigated. Results show that the R744 blends are an attractive option for the low-temperature circuit of cascade systems operating at temperatures approaching 200 K.     

Experimental evaluation and performance enhancement prediction of desiccant assisted separate sensible and latent cooling air-conditioning system

CO₂ and R410A desiccant wheel (DW)-assisted separate sensible and latent cooling (SSLC) air-conditioning systems were tested under the AHRI standard. At a 50 °C regeneration temperature, the coefficient of performance (COP) of the vapor compression cycles improved only 7% from the respective baseline systems for both refrigerants. This paper proposed the idea of applying divided condensers (or gas coolers) to the R410A (or CO₂) SSLC system to enhance its performance. It was found that the application of divided heat exchangers to the SSLC system provided sufficiently hot airflow for regenerating the desiccant wheel at both a reduced high side pressure (from 10.4 MPa to 9.7 MPa for CO₂, from 3.46 MPa to 3.45 MPa for R410A) and a reduced discharge temperature from the condenser (gas cooler) (4 K lower for both refrigerants). The COP improvement is 36% and 61% to R410A and CO₂ baseline systems, respectively.     

Analysis and optimisation of different two-stage transcritical carbon dioxide cycles for heating applications

Increased interest in the environmental impact of refrigeration technology is leading toward design solutions aimed at improving the energy efficiency and use of eco-friendly refrigerants with low GWP. The aim of this paper is to theoretically analyse R744 air to water heat pump cycles for heating applications up to 80 °C. This work studies the following cycle configurations: two-stage with injection (with and without intermediated cooling between compressors) and a single-stage circuit coupled with an auxiliary circuit. Internal heat transfer among the different streams of refrigerant is included, and the cycles have been optimised with regards to COP in terms of the intermediate conditions and gas cooler pressure. Finally, these cycles have been compared and analysed among each other and with a subcritical injection cycle working with R134a and a single-stage R744 cycle. The improved cycle with R744 can represent a global improvement of 15% in terms of COP.     

Effect of heat conduction through the fins of a microchannel serpentine gas cooler of transcritical CO2 system

This paper presents results of an experimental study to investigate the effect of conduction through the fins on the capacity of a serpentine gas cooler. The gas cooler was a part of a transcritical CO₂ system which was operated in A/C mode. The capacity of the gas cooler was carefully measured in the chamber which simulated the outdoor condition with the original heat exchanger. In order to experimentally validate the conduction effect on the capacity, some sections of the fins, where the conduction was most significant, were cut by EDM (Electrical Discharge Machining). The capacity of the heat exchanger, after cutting fins, was measured in the same chamber at nearly identical test conditions as before cutting. Gas cooler capacity was improved up to 3.9% by cutting the fins, and temperature difference between refrigerant exit and air inlet for the gas cooler was reduced by 0.9–1.5 °C. The maximum uncertainty in the capacity measurements was 2.5% and the accuracy of temperature measurements was 0.1 °C. It was shown by system simulation that system COP could be improved by 5% by eliminating this severe conduction effect, as was done in this experiment. The tube surface temperature at some points of the gas cooler was measured and infrared images were taken to show the conduction effect before and after cutting fins.     

Modified neural network correlation of refrigerant mass flow rates through adiabatic capillary and short tubes: Extension to CO2 transcritical flow

This paper presents a modified dimensionless neural network correlation of refrigerant mass flow rates through adiabatic capillary tubes and short tube orifices. In particular, CO₂ transcritical flow is taken into account. The definition of neural network input and output dimensionless parameters is grounded on the homogeneous equilibrium model and extended to supercritical inlet conditions. 2000 sets of experimental mass flow-rate data of R12, R22, R134a, R404A, R407C, R410A, R600a and CO₂ (R744) in the open literature covering capillary and short tube geometries, subcritical and supercritical inlet conditions are collected for neural network training and testing. The comparison between the trained neural network and experimental data reports 0.65% average and 8.2% standard deviations; 85% data fall into ±10% error band. Particularly for CO₂, the average and standard deviations are −2.5% and 6.0%, respectively. 90% data fall into ±10% error band.     

Performance of Stirling-type non-magnetic and non-metallic co-axial pulse tube cryocoolers for high-Tc SQUIDs operation

A set of Stirling-type non-magnetic and non-metallic co-axial pulse tube cryocoolers, intended to achieve portable cryogen-free systems with very low interference for high-T_c SQUIDs operation, have been designed and tested in TIPC/CAS. The key feature is that all cooler components in the vicinity of SQUIDs pick-up loops are made of non-magnetic and non-metallic materials, in order to eliminate complicated interference and realize direct couple with SQUIDs. The cooling options, cooler interference and corresponding solutions are reviewed briefly, and then we focus our attention on the cryogenic design and selection of the materials. Over 30 cooler samples have been fabricated and tested systematically. A typical cooling power of over 100 mW at 80 K with 70 W input electrical power has been achieved. Detailed cooling performance and elementary interference characteristics of the coolers are also analyzed and evaluated.     

Multi-ejector R744 booster refrigerating plant and air conditioning system integration – A theoretical evaluation of energy benefits for supermarket applications

The multi-ejector rack is the most promising technology to push the so-called “CO₂ equator” further south and improve the global energy efficiency of R744 supermarket refrigeration systems.This paper theoretically compares the energy consumption of a CO₂ refrigerating plant equipped with a multi-ejector unit with that of a R404A direct expansion system (DXS), of a conventional CO₂ booster configuration and of two CO₂ solutions using parallel compression. The energy benefits related to the adoption of low temperature (LT) overfed display cabinets were also assessed. Furthermore, various scenarios involving different sizes of the supermarket, integration and capacity of the air conditioning (AC) system and efficiency of the parallel compressors were investigated. The evaluations were carried out by considering different locations in Southern Europe. The results showed that, as a function of the selected boundary conditions, energy savings ranging from 15.6% to 27.3% could be accomplished with the multi-ejector concept over DXS.     

The effects of non-condensable gases in domestic appliances

It is well known that the presence of non-condensable gases inside a compression vapour refrigerating circuit introduces an additional thermal resistance at the condenser, which can significantly decrease the energy efficiency of the system. However, this problem so far has been investigated mainly for shell and tube condensers of large capacity and limited information is available on small systems, as is the case for household appliances where the internal volumes are extremely reduced and therefore a very small amount of non-condensable gas has large effect. Moreover, non-condensable gas behaves differently when condensation takes place outside tubes (shell and tube condensers) or inside tubes (condensers of small appliances); in the first case all heat transfer area is wrapped by a gas layer, whereas in the second case non-condensable gas is collected at the end of the tube. The effect of non-condensable gas in this work is experimentally investigated by injecting controlled amounts of air into a refrigerating circuit and by recording the thermal and electric variables during different modes of operation (steady state and cyclic running). The tested refrigerating circuits are part of two appliances on the market, a household refrigerator and a vertical freezer. The presence of non-condensable gas was found to spoil energy efficiency, since it brings about an increase in condensing pressure and a concomitant decrease in evaporating temperature, although larger liquid subcooling partially compensates for the first negative effects: the reason for this behaviour is the clogging action of bubbles of gaseous mixture (air and refrigerant vapour) that enter the capillary tube.     

具有独立气路的液氦温区G-M型二级脉管制冷机性能研究

研究了一台具有独立气体回路的液氦温区G-M型二级脉管制冷机的制冷性能.目前的实验装置由两套独立的单级双向进气型脉管系统构成,第一级冷头对第二级进气的预冷通过安装在第二级回热器中部的换热器与一级冷头之间的热联接来实现.研究表明,该制冷机采用4He为工质,分别以Leybold CP4000和RW2氦压缩机来驱动第一级和第二级,可以获得2.18 K的最低无负荷制冷温度,4.2 K提供的最大制冷量为595 mW.     

An innovative configuration for thermoacoustically-driven pulse tube coolers

Obtainable lowest temperature of a thermoacoustically-driven pulse tube cooler is generally limited by the pressure ratio provided by the thermoacoustic engine with helium as working gas. It is also known that a thermoacoustic engine filled with nitrogen can generally provide much larger pressure ratio and lower frequency than the same engine filled with helium. Here we introduce an innovative system configuration which uses an elastic membrane as the interface between the thermoacoustic engine subsystem and the pulse tube cooler subsystem. The membrane can transport acoustic work from the engine to the cooler, and meanwhile separate the working gases used in respective subsystems. Through this way, it is possible for the engine to operate with nitrogen to provide larger pressure ratio and more suitable frequency for the pulse tube cooler which can still use helium as the working gas. To test this idea, a thermoacoustically-driven pulse tube cooler was built. With the innovative configuration, the pulse tube cooler reached a lowest temperature of 139 K. On the other hand, without the membrane, the PTC only achieved a lowest temperature of 186 K when using nitrogen and 145 K with helium for both the PTC and the engine.     

内调相型脉管制冷机的理论分析

对内调相型脉管制冷机进行热力学理论分析.内调相型脉管制冷机的特点是两个冷头在脉管热端通过针阀串联,两边脉管内的压力和流量耦合,通过调节各自的输入压力相位来主动调节脉管冷端流量与压力的相位差,输入压力相位由一个具有双出口的旋转阀来实现.分析表明,该调相方法可使脉管冷端流量与压力同相,从而使制冷量最大,而且相比于传统的小孔加气库型和双向进气型脉管制冷机,其制冷效率大大提高.该结构省却了体积较大的气库,结构更加紧凑.     

Comparitive analysis of an automotive air conditioning systems operating with CO2 and R134a

This paper evaluates performance merits of CO₂ and R134a automotive air conditioning systems using semi-theoretical cycle models. The R134a system had a current-production configuration, which consisted of a compressor, condenser, expansion device, and evaporator. The CO₂ system was additionally equipped with a liquid-line/suction-line heat exchanger. Using these two systems, an effort was made to derive an equitable comparison of performance; the components in both systems were equivalent and differences in thermodynamic and transport properties were accounted for in the simulations. The analysis showed R134a having a better COP than CO₂ with the COP disparity being dependent on compressor speed (system capacity) and ambient temperature. For a compressor speed of 1000 RPM, the COP of CO₂ was lower by 21% at 32.2°C and by 34% at 48.9°C. At higher speeds and ambient temperatures, the COP disparity was even greater. The entropy generation calculations indicated that the large entropy generation in the gas cooler was the primary cause for the lower performance of CO₂.     

Theoretical and experimental analysis of a CO2 heat pump for domestic hot water

This paper describes the development of a CO₂ air/water heat pump for the production of tap hot water in a residential building. The basic design consists of a single-stage piston compressor, a coaxial type gas cooler, an electronic expansion valve, a finned tube evaporator and a low pressure receiver. The heat pump is combined with a storage tank designed to maintain internal water stratification.The gas cooler pressure optimisation in the case of fixed water delivery temperature was theoretically analysed.A new control method for the upper cycle pressure was developed to maximise the COP of the heat pump, while the water mass flow was adjusted to maintain the set water temperature at the gas cooler exit.Before commissioning, the heat pump was factory tested to verify its energy performance and to validate the high pressure control logic.     

Improving the power efficiency of air refrigeration turbomachines

One way of improving the power efficiency of air refrigerating turbo-machines (RTM) is to combine low temperature production with mechanical power generation, sea water desalination or in the processing of some technological products. A study has shown that refrigeration and electrical power generation via the combined application of RTM and gas turbine units will decrease refrigeration costs by 15–20%. In the case of regenerative systems utilizing waste heat from the RTM for sea water demineralization, the cost of low temperature production can be reduced by 15–20%. In addition, a high quality distillate is produced at the rate of 10–15 kg for every 1000 kJ of refrigeration.     

Water-side reversible CO2 heat pump for residential application

This paper presents the energy assessment of a water/water R744 chiller/heat pump, working according to a transcritical cycle, used for winter heating, summer cooling and tap water production. The different functions (heating, cooling, hot water) are managed water side. The analysis of the R744 chiller/heat pump is based on an original simplified method, which is able to predict the energy performance of the unit based only on its performance data at the nominal rating conditions. The method was validated against experimental data. A comparison with a state-of-the-art R410A unit is presented. The monthly analysis shows that the CO₂ unit is very efficient in hot water production, but penalised in heating and cooling service. The adoption of an ejector in place of the expansion valve makes the CO₂ system reach the same energy consumption as the R410A unit, despite the presence of the water loop only in the R744 lay-out.