COP Reduction Research Articles

Study on performance of a multi-heat source heat pump coupled energy storage system for plant factory heating system

CO2 air source heat pump (CASHP) faces challenges of performance degradation caused by the high return water temperature and the low ambient temperature for building heating. This study proposed a novel multi-heat source heat pump system (MHSHP) that combined with a CO2 air source heat pumps (CASHP) and ground source heat pumps (GSHP) with the implementation of a thermal storage tank. The performance and CO2 emissions of both CASHP and MHSHP systems were investigated by applying a plant factory located in Beijing, China. The effects of user-side parameters and ambient temperature were explored with the storage water temperatures ranging from 20 °C to 40 °C. The results demonstrated that the heating performance of the CASHP improved with an increase of user water flow rate and ambient temperatures, while it decreased as the storage water temperature increased. At the flow rate of 1.1 m3/h, the increase of the compressor frequency from 40 Hz to 60 Hz led to a significant improvement in heating capacity by 53.3 %, and a reduction in COP by 23.3 %. When the user-side water flow ratio decreased or the total flow rate increased, the COP of the MHSHP system showed improvement and outperformed that of the CASHP system, with an increase ranging from 46.9 % to 61.5 %. Additionally, it exhibits reduced sensitivity to ambient temperature fluctuations, resulting in a decrease in COP variability from 33.3 % to 20.6 %. Moreover, the CO2 emissions of the system decrease as the COP increases, with significant reduction of 69.2 %.

Experimental study on the operating performance of the air source heat pump (ASHP) with variable outdoor airflow rate under the standard frosting condition

Frosting is a common phenomenon of the ASHP under the heating mode in winter, and the outdoor air flow rate flowing through the evaporator of the ASHP was always thought to be a major contributor. In order to validate its contribution, effects of outdoor fan airflow rate on the performance of air source heat pumps (ASHPs) were investigated under the winter heating condition. The experiment was conducted in a laboratory at the standard 2 °C air dry bulb temperature (DB)/ 1 °C air wet bulb temperature (WB) frosting condition, which enabled the analysis of the operating performance, frosting performance, and heating performance of the ASHP unit by changing the airflow rate of the outdoor fan. Results showed that as the airflow rate of the outdoor fan reduced from 100% to 36%, the operating performance decline and the elevated frosting-defrosting loss were observed. Meanwhile, both the frosting rate and the operating efficiency during frosting-defrosting cycles showed an increasing trend then followed by decreasing tendency. The maximum frosting rate and operating efficiency were 0.92 g/m2.min and 2.92, respectively, which were observed at 74% airflow rate of the outdoor fan of the ASHP unit. The observation implied the existence of the “minimum frosting suppression airflow rate”. At 36% airflow rate of the outdoor fan of the ASHP unit, however, the performance of the ASHP unit was attenuated greatly, with the frosting-defrosting efficiency loss coefficient of 0.47, the heating capacity and COP reduction by 51.5 and 38.8%, respectively. These findings provided significant references to the optimization of ASHPs performance with variable airflow rate of the outdoor fan under frosting conditions.

Drop-in analysis of R450A and R513A as low-GWP alternatives to R134a in a water-to-water heat pump

This paper discusses the results of an experimental analysis of the use of R450A and R513A as drop-in alternatives to R134a in a water-to-water heat pump. The heat pump is tested setting the rotational frequency of the compressor shaft, the vapour superheating at the evaporator outlet and the mass flow rates and temperatures of the secondary fluids at the heat exchangers outlets to the desired conditions. Compared to R134a, the use of R450A leads to a capacity reduction, that lays in the range 6.59% to 15.02%, and to a COP variation, that is within the range −2.48% to +2.99%, whereas with R513A the variations of the heating capacity and COP lay in the range −8.78% to +3.16% and −7.48% to +2.22% respectively. The increase in the rotational frequency of the compressor shaft to bring the heating capacity back to the R134a values is discussed too. The required increase is in the range 10% to 16% with R450A and 2% to 8% with R513A, but a COP reduction in the range 0.14% to 5.22% with the former refrigerant and 2.34% to 6.48% with the latter consequently arises.

Common fault effects on a natural refrigerant, variable-speed heat pump

The European heat pump market is growing every year as European legislation moves towards the use of natural refrigerants that have less global warming potential than conventional refrigerants. Even though heat pumps are very efficient, the appearance of hard-to-detect faults increases energy consumption. The present study emulated some of the most common faults on a variable-speed heat pump charged with propane. The faults tested were evaporator fouling, compressor valve leakage, liquid line restriction and refrigerant overcharge. The effect of these faults on performance and several heat pump features are described. Evaporator fouling was the fault with least COP degradation and compressor valve leakage with the most (7% and 56% COP reduction, respectively). The results are compared with the results from a comparable study. The heat pump covered the demand generated by nearly all the faults at the expense of increasing the power consumption. The features of each fault are summarized in a chart, which could be used as table for diagnosing faults.

Novel compound waste heat-solar driven ejector-compression heat pump for simultaneous cooling and heating using environmentally friendly refrigerants

This work investigates the theoretical performance of a novel compound waste heat-solar driven ejector-solar assisted heat pump for simultaneous heating and cooling purposes. The system utilizes waste heat from three different sources, PV/T (photovoltaic thermal) with flat plate collector, milk pasteurization process, and condenser exchange heat. Real weather data is employed to determine the hourly performance of the system in three European cities, Valencia (Spain), Berlin (Germany), and Stockholm (Sweden) representing warm, middle and cold climates, respectively. Moreover, R450A and R513A alternative refrigerants are compared to the greenhouse gas R134a. The results show that the proposed system improves cooling COP by 7% using R450A in comparison with a conventional R134a vapor compression system. Furthermore, the utilization of waste heat recovery enhances the system COP from 3.7 to 4 in the absence of solar intensity. On the other hand, the results of the heating mode show that the system with PV/T and using R450A has a COP increase over the adopted solar time, ranging from 2 to 5% in Valencia; 2 to 2.5% in Berlin, and1 to 1.5% in Stockholm. However, R513A shows a COP reduction of about 5% in comparison to R134a. Regarding the heating mode with waste heat utilization from milk pasteurization, the system based on R450A results in the highest system COP, increasing COP up to 75% compared to the R134a baseline scenario. Overall, the proposed R450A systems show the highest equivalent carbon dioxide emission reduction and, therefore, it is recommended from an environmental point of view.

Thermodynamic analysis of the optimal operating conditions for a two-stage CO2 refrigeration unit in warm climates with and without ejector

The natural refrigerant R744 seems to be the long term solution to be imposed in the food retail industry, where low and medium refrigeration temperatures are usually required, despite the technical difficulties related to high the pressure, especially in warm climates, leading to trans-critical operation with the consequent COP reduction. To overcome such difficulties, different approaches have been proposed in literature, being very popular the use of parallel compressor or its combination with ejector expansion devices. The efficient operation of trans-critical R744 systems requires the control of the gas cooler pressure, as well as the pressure of the flash-tank. In the case of ejector expansion devices, the ratio of entrained mass flow rate must be also included in the control variables and, at the same time, its use implies additional restrictions. The current paper presents the optimization methodology for the operating conditions of a two-stage CO2 refrigeration unit for both cases, with and without ejector. The curves of optimal combinations of gas cooler and flash-tank pressures, understood as operational control laws, are provided together with the COP achieved. The operating conditions and performance of the ejector case are compared showing COP improvements of up to 13%. The results also show that the operating region of the control variables is limited due to the use of the ejector.

Comparative energy and exergy analysis of a subcritical cascade refrigeration system using low global warming potential refrigerants

This study examines a variety of evaluation criteria such as various operating and design parameters for thermodynamic performance of a cascade refrigeration system (CRS). R41 is used in low-temperature circuit (LTC), whereas R1243zf, R423A, R601, R601A, R1233zd (E) and RE170 are used for the first time in this study and are adapted for a high-temperature circuit (HTC). This study with four aims are conducted: (1) to analyse refrigerant pairs with low global warming potential (GWP) to further contribute to the literature; (2) to investigate the impact of various operating parameters on system performance; (3) to suggest potential alternative refrigerant to R423A; (4) to also compare the performance of the refrigerant pairs used in this study with those analysed by researchers in the literature. The results demonstrate that condenser and evaporator temperature ranges have a significant effect on the performance parameters studied in the system designed for six refrigerant pairs. The greatest reduction in COP occurs in R41-R423A with about 43.85% in a change of 10°C in the condenser temperature. A change of 10°C in the evaporator temperature causes the COP of the CRS to rise by approximately 19% for all refrigerant pairs. For applications that desire low temperatures, R41-R423A exhibits the lowest COP and exergy efficiency with 1.105 and 33.93%, respectively, whereas R41-RE170 presents the highest COP and exergy efficiency with 1.210 and 37.18%, respectively. Furthermore, exergy destruction in HTC compressor is made up almost to one-third of total exergy destruction. RE170 is proposed as an alternative refrigerant for R423A by the reason of its low environmental detriment and superior performance. Furthermore, there is a maximum COP improvement of 13.05% compared to studies in the literature.

First experimental results of the use of R1234yf and R1234ze(E) as drop-in substitutes for R134a in a water-to-water heat pump

In the present paper, the first results of an experimental analysis carried out to assess the performance of a water-to-water heat pump in which R1234yf and R1234ze(E) are used as drop-in substitutes of R134a are shown. The heat pump is first tested with R134a to establish a baseline performance and, then, is tested under the same working conditions, i.e. under the same water temperatures at evaporator and condenser outlets, with the above-mentioned HFO refrigerants. The results show that the heating capacity and COP of R1234yf system are up to 9.8% and 6.1% respectively lower than those obtained with R134a. On the other side, the use of R1234ze(E) leads to a capacity reduction and a COP reduction respectively up to 23.1% and 2.5%. A second set of tests is then carried out varying the rotational frequency of the compressor shaft in order to set the heat pump heating capacity to the same value found with R134a. The experimental results demonstrate that increases up to 17% and 50% are respectively needed for R1234yf and R1234ze(E), but subsequent reductions of heat pump COP up to 7.38% and 18.11% arise.

Transient performance of a Peltier super cooler under varied electric pulse conditions with phase change material

In this work, a novel technique to improve the performance of Peltier cooler under varying electric pulse conditions has been studied with the use of phase change material (PCM). The 2D heat transfer model of Peltier cooler along with heat sink has been developed and studied numerically. The results show that the cold side temperature of Peltier cooler with PCM has been reduced significantly from −14.5 °C to −17.5 °C under pulse operation as compared to the Peltier cooler without PCM under pulse operation. It is also found that, there is no significant reduction in COP of the Peltier cooler under pulse operation with PCM as compared to pulse operation without PCM. These studies have been carried out by varying the hot side heat transfer coefficient, fill volume of PCM in heat sink and the shape of the electric pulse. It has been observed that the super cooling of Peltier cooler increases with increase in fill volume of PCM. Furthermore, it has been found that the super cooling performance of the Peltier cooler is more predominant with the square pulse as compared to ramp and triangular pulses.

AN EXPERIMENTAL STUDY FOR THE EFFECT OF CAPILLARY TUBE DIAMETER ON REFRIGERATOR PERFORMANCE WITH NEW ALTERNATIVE REFRIGERANT MIXTURE TO R134A

In this work, the effect of capillary tube diameter on refrigerator performance is studied for refrigerants R134a, R290, R600a and its mixture which named: RMix1(70%R134a,20%R290,10%R600a), RMix2(70%R134a,10%R290,20%R600a), RMix3.(50%R134a,20%R290,30%R600a), RMix4 (60%R134a,20%R290,20%R600a). The three adopted capillary tubes diameter are 1.4 mm, 1.8 mm and 2.2 mm, named capillary1, capillary2 and capillary3 respectively. The experimental work show that, the refrigerator operates in 2.2 mm capillary tube's diameter with R134a gives the highest COP from other refrigerants with the same diameter, where the performance of refrigerator with capillary tube of 1.4 mm and alternative ozone friendly refrigerant RMix2 give 1.3% COP reduction from base case of R134a .

Design and Simulation of a Vapour Compression Refrigeration System Using Phase Change Material

The paper details the design and simulation of a solar powered vapour compression refrigeration system. The effect of a phase change material, in this case ice, on a vapour compression refrigeration system powered by solar panels is discussed. The battery and solar panels were sized to allow the system to function as an autonomous unit for a minimum of 12 hours. It was concluded that the presence of a phase change material in the refrigeration system caused a considerable increase in both the on and off time of the compressor. The ratio by which the on time increased was greater than the ratio by which the off time was increased. There was a 219% increase in the on time, a 139% increase in the compressor off time and a 3.5% increase in compressor work accompanied by a 5.5% reduction in COP. Thus, under conditions where there is enough load in the system to cause the initial on and off times of the compressor to be comparable, the presence of a phase change material may result in a greater on period than an off period for the compressor.

Design and Simulation of a Vapour Compression Refrigeration System Using Phase Change Material

The paper details the design and simulation of a solar powered vapour compression refrigeration system. The effect of a phase change material, in this case ice, on a vapour compression refrigeration system powered by solar panels is discussed. The battery and solar panels were sized to allow the system to function as an autonomous unit for a minimum of 12 hours. It was concluded that the presence of a phase change material in the refrigeration system caused a considerable increase in both the on and off time of the compressor. The ratio by which the on time increased was greater than the ratio by which the off time was increased. There was a 219% increase in the on time, a 139% increase in the compressor off time and a 3.5% increase in compressor work accompanied by a 5.5% reduction in COP. Thus, under conditions where there is enough load in the system to cause the initial on and off times of the compressor to be comparable, the presence of a phase change material may result in a greater on period than an off period for the compressor.

Quasi-dynamic model of the energy efficiency degradation coefficient for an air-to-water heat pump

Operating in part-load conditions significantly affects the energy efficiency of the air-to-water heat pumps. The currently used method of testing the devices does not provide the necessary data for estimating the degradation of the device’s efficiency in part-load operating mode. The value of the degradation coefficient Cc = 0.9 adopted in accordance with the EN 14825 standard is generally used. However, this value often does not reflect the actual degree of the COP reduction. Hence the new quasi-dynamic model and the calculation approach to the degradation coefficient were proposed. The model reflects the real and overall reduction between the full and part-load COP, not only the energy consumption in the stand-by mode. The estimation was made using the field measurement data of the air-to-water heat pump supplying heat to the low temperature installation. It has been proved that the determination of the main parameter is possible using a relatively small amount of data without losing an accuracy.

Investigation on Nano Refrigeration

Boiling heat transfer is imperative in the refrigeration and air conditioning systems. R22 is the mostly widely used alternative refrigerant in refrigeration equipment such as domestic refrigerators and air conditioners. Though the global warming up potential of R22is relatively high, it is affirmed that it is a long alternative refrigerant in lots of countries. By addition of nano particles to the refrigerant results in improvements in the thermo-physical properties and heat transfer characteristics of the refrigerant, thereby improving the performance of the refrigeration system. The results indicate that cuo nano refrigerant works normally and safely in the refrigeration system. The results indicate that heat transfer coefficient increases with the usage of nanocuo. Thus using cuo nano refrigerant in refrigeration system is found to be feasible. Objective of this project is to study the pre and post effects of addition of NRs in refrigeration and to predict COP and power consumption reduction.

Performance improvement of an ejector cooling system with thermal pumping effect (ECSTPE) by doubling evacuation chambers in parallel

The existing ejector cooling system with thermal pumping effect (S-ECSTPE) operates without consuming electricity, but has waste problems of thermal energy and chilling water. An innovative ECSTPE with double evacuation chambers in parallel (D-ECSTPE) can effectively mitigate these waste problems. The high-pressure vapor with high temperature (HPVHT) in one evacuation chamber, which is directly chilled by chilling water in the S-ECSTPE, is reused in the D-ECSTPE to pre-pressurize refrigerant in the other evacuation chamber. Performance improvement mechanisms of the D-ECSTPE are explained from both thermal energy transfer and mass transfer of the HPVHT. Case studies showed that the severer the waste problems of the S-ECSTPE, the greater COP increase and reduction in chilling water demand achieved by the D-ECSTPE. Also, the D-ECSTPE performed well with environment-friendly refrigerants (e.g., R1234yf, R161 and R1234ze(E)), which were unsuitable for the S-ECSTPE due to severe waste problems. Compared to the S-ECSTPE, the D-ECSTPE with R1234yf, R134a, R161, R1234ze(E), R1234ze(Z), R1233zd(E), R365mfc and R141b increased the COP by at least 49.44%, 26.30%, 22.33%, 19.38%, 4.39%, 3.55%, 2.14% and 1.77%, respectively, and reduced the chilling water demand by at least 29.77%, 18.04%, 15.55%, 13.86%, 3.36%, 2.76%, 1.72% and 1.37%, respectively. In all cases, the D-ECSTPE reduced the wasted thermal energy and chilling water of the S-ECSTPE by at least 65.83% and 81.14%, respectively, and its exergy efficiency was generally superior to those of the S-ECSTPE and conventional ejector cooling system. An additional increase in the number of paralleled evacuation chambers can further improve the system performance.

Modelling and experimental analysis of a GAX NH3–H2O gas-driven absorption heat pump

The experimental analysis of a gas-driven NH3–H2O heat pump whose cycle approaches the GAX concept is carried out. Full load operation is investigated by varying hot water temperatures and partial load operation is investigated by decreasing gas input down to 50% of the full load value. Numerical simulations bolster measurements accuracy and provide insight on the variation of cycle COP and Gas Utilization Efficiency (GUE), based on gross calorific value. A nearly constant GUE of about 1.5 is found for hot water temperatures lower than 50 °C. The GUE steadily decreases above 50 °C, reaching about 1.33 at 60 °C. The COP varies more smoothly, from 1.73 at 45 °C to 1.60 at 60 °C. The GUE and COP reduction at 50% of the nominal gas input is 6.8% and 6.4%, respectively. Simulations suggest that performances at partial loads can improve if active control of solution mass flow rate is implemented.

Performance characterisation of a small milk cooling system with ice storage for PV applications

The presented work proposes a calculation method to estimate the transient performance of a small on-farm milk cooling system for PV applications. The system employs 3 kg ice for fast cooling of 17 L milk in conventional 20-L-milk-cans. Two units of a commercial DC refrigerator operate at −10 °C and at 4 °C for ice production and milk preservation respectively. The development of milk temperature and energy consumption during a cooling event was studied experimentally at different ambient temperatures of 20 °C, 30 °C and 40 °C. A computational model was developed and validated where thermal resistances and COP were adjusted with experimental data. The results between 20 °C and 40 °C ambient temperature show a COP reduction of around 30% and a total daily energy consumption increase of around 100%. The specific total energy consumption of the system per litre milk was between 30 Wh/L and 58 Wh/L for the studied ambient temperature range. The suggested model can be used for the optimisation of photovoltaic stand-alone systems at specific locations.

Effect of Exhaust Gas Temperature on the Performance of Automobile Adsorption Air-Conditioner

An adsorption air conditioning unit of two beds was designed and built. Each bed was constructed from two concentric copper pipes sets, the length of the pipes in each set was 1500 mm pipe. The inner pipe was 50 mm in diameter while the outer pipe was of 100 mm diameter. The inside surface of the outer pipe and the outside surface of the inner pipe were covered by two active carbon layers which were separated from each other by a 5 mm space, to allow for methanol vapour penetration through the two layers. Each bed was inserted in a metal pipe to convey the hot stream of exhaust gases, to form a two passes heat exchanger. To complete the adsorption air conditioner cycle, an evaporator and condenser were connected to the two beds. Exhaust gases were produced by a propane burner, which can be adjusted to deliver hot gases at a temperature range of 80 to 140°C. The work showed that there was direct relationship between the hot gas temperatures that was used to heat the bed and the chilled water temperature. When the hot gases temperature exceeded 120°C, the whole cycle time increased along with a slight increase in chilled water temperature with a reduction in COP. A minimum chilled water temperature of 7.6°C was obtained. This chilled water was supplied to a fan coil unit to maintain an inside temperature of 24.2°C for a 600 Watt conditioned space load. The maximum cycle COP was about 0.31 when the hot gas temperature was 120°C.

캐스케이드 열펌프의 저단 사이클 충전량 변화에 따른 성능 특성

Abstract In this study, the optimization and performance characteristics of a cascade heat pump system wasanalyzed with the variation of low stage refrigerant charge amount. The cascade heat pump wasdesigned and constructed with R134a and R410A as the refrigerant for high stage and low stage cycle,respectively. Experiments were conducted by varying the low stage charge amount and the performancecharacteristics of the cascade heat pump were studied. The refrigerant charge amount of the low stagecycle was varied between the ranges of -15% and +10% of the optimum charge amount. Theperformance variation experienced in the cascade heat pump due to the variation of refrigerant chargeamount shows greater effect in the undercharge regions than the overcharge regions. COP reduction inthe undercharge region is larger than the decrease in the overcharge region. Some cycle variation suchas power consumption and cycle pressure according to low stage refrigerant charge amount showeddifferent trends comparing with those according to high stage refrrgerant charge amount. Therefore, theoptimum charge amount of the cascade heat pump should be determined based on the experimentaldata obtained by the variation of high and low stage refrigerant charge amount. Keywords: Cascade heat pump (캐스케이드 열펌프 ), Charge amount(충전량 ), COP(성적계수 )

The Effect of Lubricant Oil Concentration on the Performance of a Gas Cooler Using Carbon Dioxide

The effect of PAG oil concentration on the heat transfer performance and pressure drop of a fin-tube heat exchanger using carbon dioxide(CO2) heat pump during the gas cooling process of CO2 was investigated by experiment. The experimental apparatus consisted of a gas cooler, a pre-heater, a sub-cooler, a mass flow meter, pumps and a measurement system. Experiments were conducted for various conditions of inlet temperature(110°C), mass flow rate of refrigerant(50, 55, 60, 65, 70 g/s) and PAG oil concentration(0 to 2.6 wt%). The heat transfer rate decreased with the increase of the oil concentration and the decrease of the inlet pressure. And the pressure drop increased with increases of the oil concentration and the mass flow rate of the refrigerant. The COP reduction by the deterioration of gas cooler performance with oil concentration was analyzed. When the inlet pressure of the gas cooler was 100 bar, the COP reduction was estimated to be 6% under 1 wt% of oil concentration.