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Comparative Exergetic Assessment for the Modernization of Synthetic Fluids by Natural Refrigerants Used in Residential and Commercial Cooling and Air Conditioning Applications
Written By
Eraldo Cruz dos Santos, Carlisson Arnaud de Azevedo, Caio Luiz de Carvalho Macêdo, Luiz Felipe Pinheiro Oliveira, Fernando Lucas Alves da Silva, Luis Gabriel Brito Mota and Ismael dos Santos Marinho
Submitted: 30 July 2024Reviewed: 20 August 2024Published: 19 December 2024
Currently, in the world, there is a real race, involving several manufacturers that search for more efficient refrigerant fluids, with the need to develop new refrigerants to replace the fluids currently being used in air conditioning and refrigeration systems, aiming to maintain or improve the performance of such systems, reducing their electrical energy consumption. Therefore, this work intends to be another contribution to this scenario and aims to carry out an exergy analysis comparative of the operating characteristics of a residential Chiller system, in which the natural fluid R-290 was used, operating at 60 Hz, and at 70 Hz. The HC R-290 was compared with the HCFC fluid R-22 in the same indirect expansion refrigeration system. For comparison purposes, the main operating parameters of the systems were observed, such as pressures and temperatures, electrical energy consumption, and exergy of the system components. When working with both fluids at 60 Hz, the only negative point for the HC was the higher suction pressure, indicating that the chilled water temperatures were lower with R-22. Under normal conditions, the results expected in this work were obtained, in which HC presented satisfactory behavior, with energy and exergy efficiency values higher than those obtained for HCFC, for the same local psychrometric conditions, being an excellent alternative for replacing the R-22 fluid in air conditioning and refrigeration systems.
Federal University of Pará - UFPA, Belém, Pará, Brazil
Carlisson Arnaud de Azevedo
Federal University of Pará - UFPA, Belém, Pará, Brazil
Caio Luiz de Carvalho Macêdo
Federal University of Pará - UFPA, Belém, Pará, Brazil
Luiz Felipe Pinheiro Oliveira
Federal University of Pará - UFPA, Belém, Pará, Brazil
Fernando Lucas Alves da Silva
Federal University of Pará - UFPA, Belém, Pará, Brazil
Luis Gabriel Brito Mota
Federal University of Pará - UFPA, Belém, Pará, Brazil
Ismael dos Santos Marinho
Federal University of Pará - UFPA, Belém, Pará, Brazil
*Address all correspondence to: eraldocs@ufpa.br
1. Introduction
This study was produced and developed at the Climatization and Thermal Comfort Laboratories – ClimatConT and Refrigeration of the Federal University of Pará – UFPA and aims to analyze types of refrigerant fluids used in air conditioning and refrigeration systems, replacing the current fluids used commercially, aiming to maintain the safety conditions of the systems, with the lowest environmental impact and evaluating their performance and thermal efficiency indexes.
In a refrigeration or air conditioning system, a set of refrigerant fluids are used, many of which are already discontinued or have ceased production, as is the case of hydrochlorofluorocarbons – HCFCs, such as the R-22 synthetic refrigerant fluid, but there is still a large quantity of these fluids in stock and that continues to be used by the systems. This work proposes to provide support for decision-making on system modernization, taking as a reference the energy and exergy analyses of the system components.
Over the past 60 years, due to global economic development, the use of refrigerants has increased considerably in many countries. In order to prevent environmentally harmful substances from was continuing to be used by countries, especially in air conditioning and refrigeration systems, numerous meetings and gatherings have been held in various countries around the world.
A highlight was the creation of the Montreal Protocol (1987) in Canada, which restricted the use of chlorofluorocarbon refrigerants (CFCs, such as R-11), leading to COP 26 in Scotland in 2021, which discontinued the use of carbon dioxide (CO2) in such industrial systems.
Long before the Montreal Protocol, several studies were carried out, and in them, the destruction of the ozone layer and the increase in global warming were proven. To monitor the evolution, indicators were created such as, the Global Warming Potential (GWP) and the Ozone Depletion Potential (ODP). Since then, several fluids have been used, analyzed, and those that have been proven to present some type of damage, or that contribute to global warming, have been discontinued, had their production terminated, or were banned in air conditioning, heating and refrigeration systems.
Even so, nowadays, the indiscriminate use of some fluids such as HCFCs (R-22) still continues to harm the environment, as when these substances are released into the atmosphere during maintenance processes, for example, there is an increase in global warming, or simply, they destroy the earth’s protective ozone layer.
It is necessary to know which quantities, by mass, of the R-22 refrigerant fluid are greater than those of the R-290 fluid because although the two fluids are compatible, when it comes to the compressor lubricating oil, the working pressures and temperatures of the system, are different parameters, which implies a loss of system efficiency.
Thus, starting from the contextualization of the use of refrigerant fluids in refrigeration systems, where new substitutes were sought for the fluids currently used, this work was designed and carried out with the objective of observing the behavior and evaluating whether it would be possible to use natural fluids, with a low degree of contribution to global warming, in a water and air cooling system, simulating a commercial air conditioning system, which was set up on a Chiller test bench in the laboratory, where comparative exergy results were obtained between two types of refrigerant fluids, in order to serve as input in the selection of refrigerants for air conditioning and refrigeration systems.
In this work, what is expected is to present a set of comparative characteristic curves of the parameters used in the energy and exergy analyses of the synthetic fluids R-22 and the natural fluid R-290, operating in a refrigeration system at frequencies of 60 Hz and 70 Hz.
Since the 1950s, researchers from various international organizations have realized that the use and release of refrigerants into the local atmosphere has a direct impact on our planet, causing global warming and the destruction of the ozone layer. As a result, annual monitoring of the global effects caused by these fluids has been created. Since then, with the popularization, initially of refrigeration systems and, later, of air conditioning systems, the massive use of these industrial equipment has worsened the conditions of the Earth’s protective layers, such as the Ozone Layer [1].
Numerous world events were held and, at each event, solutions were sought for this scenario, which is getting worse every day, with the sharp increase in residential, commercial and industrial refrigeration, and with the air conditioning of environments: residential, commercial, industrial and automotive, the use of refrigerant fluids has grown significantly, but without effective actions to control the maintenance of these systems.
According to the International Energy Agency – IEA, with the pollution generated, as societies grow technologically, in recent years, there has been an increase in global emissions of carbon dioxide – CO2 related to energy, which are around 33 Gigatons, and the concentration of CO2 in the atmosphere is greater than 420 parts per million – ppm [2].
According to Ref. [1], in Brazil, in the areas of air conditioning and refrigeration, it is common to obtain, from the responsible installation technicians, information on the modernization of systems or on the replacement of refrigerant fluids used in refrigeration and air conditioning equipment, but without informing the criteria or parameters used in the process.
Companies in the air conditioning and refrigeration sectors, which produce refrigerant fluids, are looking for fluids to replace hydrochlorofluorocarbons – HCFCs, such as R-22 and Hydrofluorocarbons – HFC, such as R-410A, considering that production and the commercialization of HCFCs have already been interrupted in Brazil, and HFCs are being discontinued from 2024 onwards, even so, there are still large stocks of fluids produced and stored for use. In this sense, it is necessary to identify or prepare new fluids, in order to use parameters to maintain the energy and exergy efficiency of the systems.
These are CO2 emissions which it is generated directly as a product of a reaction; however, other chemical compounds used in industry and commerce are also as polluting as CO2 and, with pollution factors, hundreds to thousands of times more intense than carbon dioxide. This factor is called Global Warming Potential – GWP, to account for its pollution potential (global warming) in relation to CO2, which has a GWP equal to 1 [1].
Among these chemical compounds are the refrigerant fluids used in heating, refrigeration and air conditioning systems, such as trichlorofluoromethane, known as refrigerant R-11 and dichlorofluoromethane, or R-12. These fluids were mass produced interrupted in Brazil in the 1990s [3].
In Brazil (2023), the refrigeration and air conditioning markets account for approximately 82% of the consumption of refrigerant fluids, which will have less availability as deadlines for discontinuation or production stoppage approach, as is the case with R410a fluid. Therefore, refrigerant changes during a retrofit or new installation are certain, and everyone interested in the planet needs to understand the paths that the market is proposing [1, 4].
Thus, all these developments described above, in summary, imply transformations for the HVAC-R market as a whole. Designers, installers, suppliers and customers will need to conduct updates (or even component replacements) on their already installed systems.
However, there are already several reports of serious accidents involving technicians who believed they were working with a type of fluid, such as HFC R-134A, whose ASHRAE flammability and toxicity classification is A1, that is, a non-flammable and non-toxic refrigerant, in a refrigeration system, when in fact some other technician had already replaced the system fluid with R-290, whose flammability and toxicity classification is A3, that is, this is a flammable and non-toxic refrigerant fluid [1, 3].
These refrigerants have, respectively, GWP values equal to 4750 and 10,910, which means that releasing 1.0 kg of R-11 into the atmosphere represents pollution caused by the release of approximately 5000 kg of CO2. For R-12, this value represents almost 11,000 kg of CO2 released into the atmosphere for each kg of this refrigerant [1, 3].
Table 1 shows the types of refrigerant fluids most used in air conditioning and refrigeration systems, regarding their composition, ODP, GWP and ASHAE Safety Classification [1, 4, 5].
Types of refrigerant fluids used in air conditioning systems.
As can be seen in Table 1, many of the refrigerant fluids used in air conditioning systems seek to present themselves as ecological fluids, which do not harm the environment where they are used; however, fluids such as R-22, R-134a and R-410a and its likely replacement R-449a, have a strong contribution to global warming, not living up to what is disclosed by resellers and manufacturers of these products [1].
A viable solution to this scenario would be the collection of these discontinued or banned refrigerant fluids, by resellers, technicians and engineers, who work directly in the areas of air conditioning and refrigeration, through appropriate collection and storage procedures, so that companies’ recyclers, created in each state in Brazil, can give a new destination to the current fluids used.
This is a slow solution process, and many technicians, due to lack of financial resources, knowledge and technical training, still today many professionals release fluids from air conditioning and refrigeration systems into nature, without worrying about their consequences, this being a common practice. And, even Brazil, having signed the Kigali amendment, in 2024, there are still no concrete actions for control, storage and reuse or disposal of these types of products in the various Brazilian states.
Precisely because of these two factors mentioned above, the refrigerant fluid research and development industry has been seeking to improve systems that use refrigerant fluids, whether for heating, refrigeration and air conditioning, with a focus on reducing electricity consumption, combined with the use of fluids that are less aggressive to the environment. Some of these fluids are hydrocarbons – HC; among them, the most used are Propane (R-290) and Isobutane (R-600a), with the main advantage being the low GWP value which is equal to 3.0 for both fluids [6, 7, 8].
In addition to this advantage of having a low level of pollution (global warming), according to [3], several studies indicate that mechanical vapor compression systems that operate with these HCs present better energy performance when compared to the synthetic fluids currently used.
For the development of the experiment, a test bench was built with a Chiller system for air conditioning environments on the premises of the UFPA Refrigeration Laboratory to carry out applied research, consisting of a condenser unit and an evaporator unit, with a set of temperature sensors and pressure, for measuring all local psychrometric parameters and during tests.
3.1 Test bench
For the experiment, an Elgin condensing unit with 1.5 hp. of power, with a capacity of 1.6 kW of refrigeration for the R-22 fluid, was used to cool an insulated container with a volume of water equal to 40 liters, as shown in Figure 1(a)–(d), a water pump with a flow rate of approximately 6.0 l/min and a forced air heat exchanger for circulating water at low temperatures to simulate thermal load.
Figure 1.
Chiller test bench: (a) Condenser unit, (b) Evaporator unit, (c) Cooling system and (d) Water circulation system.
The condenser unit of Chiller test bench, Figure 1(a), and the water cooler evaporator, Figure 1(b) have the equivalent length of 15 meters of 5/8″ internal diameter copper piping, without fins, as shown in Figure 1(c).
The circulation system shown in Figure 1(d) for the secondary evaporator consists of a 1/3 cv water pump with a fixed flow rate of 6.0 l/min.
To exchange the heat of the secondary fluid with the ambient air, a split-type evaporator unit was used, initially sized for refrigerant fluid and with a nominal capacity of 1.6 kW, with an airflow of 350 m3/h.
3.2 Refrigeration cycle
The water-cooling system is carried out by the mechanical vapor compression refrigeration system, shown in Figure 2; the evaporator coil is submerged in the water reservoir, in which fluid circulates at low pressure and temperature in a mixed state, absorbing the heat from the liquid while the liquid is circulated by a centrifugal pump to a heat exchanger between air and water to obtain an air-conditioned environment with lower temperature and humidity. The first part of the experiment was testing the R-22 fluid in the simulator; no adaptation to the condensing unit was necessary as the equipment was designed to work with this fluid.
Figure 2.
Schematic of the water-cooling system.
All fluid that was initially in the condensing unit was collected and treated, after which the main and secondary fluid evaporator was installed, pressurizing the entire system with dry nitrogen for 24 hours at the end of the procedure to ensure that there were no leaks.
The test bench set up for the experiment can be represented by the refrigeration cycle diagram shown in Figure 2, for the ambient temperature and pressure, of the place to be air-conditioned, that is, T0 and p0:
With the verification completed, the system evacuation process began to remove moisture and other non-condensable gases that could cause damage to the system and consequent loss of performance.
For the cooling test, the R-22 load used was 1.2 kg, which was the load necessary until the system’s liquid sight glass showed that only fluid in the liquid phase existed at the inlet of the expansion device.
After collecting the synthetic fluid, the system was completely cleaned and 0.650 kg of R-290 was loaded. The procedure for operating the system with R-290 fluid was the same as R-22 [4].
3.3 System data collect
Data collection on the constructed test bench was carried out at intervals of 10 minutes, for a total period equivalent to 150 minutes, where the following information was collected:
Suction, discharge, expansion device inlet and outlet pressures and evaporator coil inlet and outlet pressures,
Heat exchanger air inlet and outlet temperatures,
Temperatures of suction, discharge, compressor body, inlet and outlet air passing through the condenser,
Temperatures of the chilled water reservoir and exchanger return,
Electric current and voltage during operation.
From these data, the following were calculated: the mass flow, the real power consumed by the system; overheating; subcooling; the cooling capacity of the system; the cooling effect; the COP; the work of the compressor; the capacity of the evaporator; the capacity of the condenser; exergies and exergy efficiency.
3.4 Thermodynamic analysis of the basic refrigeration cycle
Applying steady-state conditions to the energy equations for each stage of the cycle, shown in Figure 2, the following parameters were obtained [4]:
ẆCP=ṁf·h1−h2E1
where Eq. (1) represents the work performed by the compressor, in kW, in an isentropic process, during its operation, with ṁf being the mass flow of refrigerant fluid, in (kg/s) and h1 is the value of the enthalpy at the inlet of the compressor, in (kJ/kg) and h2 is the enthalpy at the compressor outlet, in (kJ/kg).
Eq. (2) represents the rate of the amount of heat rejected in the condenser – Q̇CD, in kW, where h2 is the value of the enthalpy at the condenser inlet, in (kJ/kg) and h3 is the enthalpy at the condenser outlet, in (kJ/kg), the unit of the amount of heat [1, 4]:
Q̇CD=ṁf·h2−h3E2
For Ref. [2], the mass flow rate – ṁf of refrigerant fluid, in kg/s, can be calculated by Eq. (3):
ṁf=Q̇0h1−h4E3
where Q̇0=Q̇EV represents the refrigeration capacity of the cycle, in (kW), and this value can be calculated by Eq. (4):
Q̇41=Q̇0=Q̇EV=ṁf·h1−h4=ṁL·h6−h5E4
Eq. (4) represents the rate of quantity of heat absorbed in the evaporator, h4 is the enthalpy value at the evaporator inlet, h1 is the enthalpy value at the evaporator outlet, ṁL mass flow of water from the cooler, in kg/s, h5 is the heat exchanger inlet enthalpy and h6 is the heat exchanger outlet enthalpy.
The energy of the expansion device is obtained by an isenthalpic process, that is, h3 = h4.
Eq. (5) represents the efficiency of the ideal cycle, that is, the efficiency of the First Law of Thermodynamics [1]:
COP1°Law=Q̇0ẆCP=ṁL·h5−h6ṁf·h2−h1E5
However, the real system is different from the ideal, so the theoretical system was idealized, starting from an adaptation of the Carnot cycle to real conditions, namely:
Modification of the compressor entry point, which changed from admitting the fluid still in the mixing phase to a more advanced point where the fluid is already completely saturated vapor due to the constructive characteristic of the compressors used since the liquid phase is practically incompressible.
During compression, even if it occurs isentropically, there is an increase in the temperature of the fluid; thus, the efficiency of the cycle is also lower than that observed in the Carnot cycle.
The isentropic expansion process is ideal but is not used in reality due to the high cost of components. These are just some modifications that make the theoretical cycle different from the Carnot cycle and justify its lower efficiency.
3.5 Exergetic analysis of the basic refrigeration cycle
For Ref. [9], energy analysis evaluates how energy is used in the system, based on the first law of thermodynamics; however, there is no accounting for spontaneous processes and irreversibilities that may occur during the operation of the system, such as the balance of energy does not consider the way in which energy is degraded. At this point, exergy analysis is considered, which complements the system’s energy information by using the second law of thermodynamics as a basis, that is, it takes into account the entropy in each process, allowing the calculation of the real efficiency of the equipment, informing the amount of work that can be performed by the system.
Exergy efficiency, in one of its many definitions, is the following: “is the ability to produce the desired effect without loss or with minimal waste of the resource used.” Being calculated by Eq. (6) [9, 10]:
ψ=Generated exergyExergy suppliedE6
where ψ is the exergy generated and the value in the analyzed component and the exergy supplied is the total value of exergy that the system obtained for operation.
The exergy analysis carried out by [11, 12, 13] applied to a refrigeration system is calculated for each component separately. The specific exergy – ex, in kJ/kg, of the system components was obtained using Eq. (7):
ex=hx−h0−T0·sx−s0E7
where hX and sX are, respectively, the enthalpy and entropy at the observed point, h0 and s0 are, respectively, the enthalpy and entropy in relation to the ambient temperature, and T0 is the temperature of the refrigerated environment [1, 4].
According to Ref. [2] for the evaporator, exergy relates to the entry and exit of fluids in it, that is, ṁL=ṁ5=ṁ6 and ṁf=ṁ1=ṁ4, following the numerical sequence in Figure 2, we arrive at Eqs. (8) and (9):
EEV,Ent=ṁf·e1−e4−Q̇0·1−T0TEVE8
or
EEV,Q=Q̇0·1−T0TEV−ṁL·e5−e6E9
where ṁL·e5−e6 represents the water vapor exergy, T0 is the ambient temperature and TEV is the evaporation temperature, both in (K).
The irreversibility of the evaporator – IEV, in kW, can be calculated by Eq. (10):
IEV=T0·ṁL·s5−ṁf·s1−ṁf·s4−ṁL·s6E10
Next, the component evaluated was the compressor, with the total exergy of the compressor being defined – ECP, in kW, is obtained by Eq. (11):
ECP=ṁf·e2−e1−ẆCPE11
where e2−e1 is the difference between the specific input and output exergies of the compressor, ẆCP is the electrical power consumed by the compressor, in kW.
For the refrigeration cycle condenser, the exergy equation – ECD is obtained by Eq. (12):
ECD=ṁf·e2−e3+Q̇CD·1−T0TCDE12
where Q̇CD is the thermal power rejected in the condenser, in (kW) and TCD is the condensation temperature, in (K).
And for the expansion device, since there is no heat generation or power consumption, we have Eq. (13) for total exergy of the expansion device – EDE:
EDE=ṁf·e4−e3E13
Therefore, using Eq. (14), the exergy efficiency can be calculated – ηEx applied to a refrigeration system can be calculated as: [1, 4, 9].
ηEx=η2°Law=COP1°Law·1−T0TCD=ṁL·e5−e6ṁf·e2−e1E14
In this way, by applying the data obtained during the tests to these equations, information related mainly to the effective performance of the equipment can be obtained.
3.6 Results
Two tests were carried out on the refrigeration system with the R-22 fluid, the first at a frequency of 60 Hz, two tests with the R-290 fluid at a frequency of 60 Hz and two tests with the R-290 fluid at a frequency of 70 Hz, all lasting 150 minutes in order to observe how the system behaves according to ambient conditions, the operation before and after the chilled water circulation pump was compared.
The main cause of this event is linked to the fact that the thermostatic expansion valve works with a fluid in the bulb calibrated to the R-22 response, and because R-290 absorbs more energy per mass, the device was unable to maintain a continuous flow, which ended up harming the temperature reduction, since the evaporation temperature suffered directly from this fluctuation in values.
Figures 3 and 4 show the variations in suction and discharge pressures, respectively, for the two refrigerant fluids and for the evaluated frequencies.
Figure 3.
Refrigerant fluid suction pressures over time.
Figure 4.
Refrigerant fluid discharge pressures over time.
For both the 60 Hz and 70 Hz frequencies, in Figure 3, there was a variation between pressures of 57 to 61 psig, and for R-22, the variation with temperature stabilization was between pressures of 58.5 to 59.5.
It can be seen in Figure 4 that for the frequency of 60 Hz, the lowest pressure of R-290, the average pressure value was 237 psig, which shows an advantage over R-22, which had an average pressure of 255 psig, indicating a reduction of the compression ratio, since the suction pressure values were similar, with the reduction in discharge pressure, the mechanical efforts on the compressor are reduced in addition to the reduction in electrical energy consumption.
For the frequency of 70 Hz, the value of the average discharge pressure of the refrigerant R-209 was 265 psig, exceeding that of R-22; this fact is associated with the compressor operating outside the working speed, which is fixed at 60 Hz, which caused an increase in temperature in general and thus an increase in condensation pressure.
The graph in Figures 5 and 6 shows how the mixture saturation temperature varied during the tests.
Figure 5.
Fluid saturation in the evaporator temperatures over time.
Figure 6.
Fluid saturation temperatures over time: (a) and (b) in the condenser.
As seen in the graph in Figure 5, the Chiller system running with R-22 fluid, with the compressor for R-22 and the expansion device calibrated for R-22, the lowest evaporation temperatures were reached, with values varying minimally around 0.5°C, while the system, after changing the R-22 fluid to R-290, was unable to reduce the evaporation temperature below 2°C, even with the increase in volumetric displacement caused by the operating frequency of 70 Hz.
It can be seen in Figure 6 that for the frequency of 60 Hz, in the same way, that the saturation temperature in evaporation was high compared to R-22, the fact is repeated for the natural fluid now in condensation, with temperatures of saturated fluid varying at values close to 50°C, for R-22, the temperature was close to 48°C even though the pressure at this point was above the values collected for R-290.
Figures 7 and 8 lists the compressor powers and the mass flow rate for the two types of refrigerant fluids and at different frequencies.
Figure 7.
Electrical energy of Compressor powers consumption.
Figure 8.
Mass flow rate.
The highest saturation temperature depends only on the chemical composition of the fluid. For the 70 Hz frequency, the temperature values are close to 55°C, once again indicating non-standard compressor operation, confirmed by the increase in electrical energy consumption. However, the higher condensation temperature indicated the greater temperature difference between the R-290 fluid and the external air in the heat exchange in the condenser, facilitating the conversion of the fluid from a superheated vapor state to liquid at high pressure at the condenser exit.
The consumption recorded for R-22 served as a reference for the evaluation of R-290, which for the 60 Hz frequency was lower; however, as explained in Figures 7 and 9, the saturation temperature during evaporation was higher than that of R-22, so consumption was lower due to a lower capacity to absorb energy while the system was running on natural fluid.
Figure 9.
Subcooling of liquid processes.
The average refrigeration power absorbed by R-290 at 70 Hz was 2.7 kW, while for R-22 it was 2.65 kW, a difference of 1.85%. At a frequency of 60 Hz, the average power absorbed in the evaporator was 2.5 kW, 7.41% lower than the frequency of 70 Hz and 5.66% lower than the power produced by R-22 [10, 13].
For the frequency of 70 Hz, the consumption is well above the reference value for the R-22 fluid, which agrees with the fact that the single-phase electric motor, when forced to work outside the nominal frequency, causes excessive energy losses in the form of heat, observed in the increase in electricity consumption of the refrigeration system.
It can be seen in Figure 8 that the values calculated for the R-22 fluid match the absorbed thermal load compared to R-290; the average specific mass of the synthetic refrigerant was 21.61 kg/m3, while the specific mass of the average natural fluid was 11.02 kg/m3 for the 60 Hz frequency and 11.17 kg/m3 for the 70 Hz frequency, which explains the values close to 0.0115 kg/s for R-22 circulating in the system and 0.0057 kg/s for the natural fluid. These values present a significant difference in terms of the amount of refrigerant fluid to be used within the refrigeration system.
In air conditioning and refrigeration systems, subcooling and superheating processes help maintain equipment operating conditions.
The refrigeration capacity obtained with the system running with synthetic and natural fluid and with different frequencies, at the beginning, a considerable difference was observed between the R-22 fluid and the R-290 at 60 Hz, when the R-22 yielded approximately 6.75 kW and the R-290 at 60 Hz yielded only 4.5 kW, however throughout the tests this difference reduced.
The average power absorbed by R-290 at 70 Hz was 2.7 kW, while for R-22, it was 2.65 kW, a difference of 1.85%. At a frequency of 60 Hz, the average power absorbed in the evaporator was 2.5 kW, 7.41% lower than the frequency of 70 Hz and 5.66% lower than the power produced by R-22.
Figures 9 and 10 show the results of the subcooling and superheating processes carried out during the tests.
Figure 10.
Superheating of steam processes.
This agrees with the fact that the greater the temperature difference between the condensation saturation and the ambient air temperature, the greater the amount of heat transferred from the hot fluid to the cold fluid. Therefore, the R-290 fluid, both for frequencies of 60 Hz and 70 Hz, obtained the highest subcooling values due to its higher saturated condensation temperature, since the ambient air temperature did not vary significantly between the tests.
Subcooling also ensures that the fluid that reaches the expansion device is in a liquid state, this way, only liquid at high pressure is expanded in the thermostatic valve, which guarantees the efficiency of the process in the evaporator.
In addition to the energy analysis, the exergy factors and irreversibility’s for the system were calculated as a way of complementing the explanations of the system’s characteristics and mainly the negative points that occurred when increasing the operating frequency from 60 Hz to 70 Hz.
Figure 11 shows the results of the exergy analyzes calculated for the refrigeration system condenser.
Figure 11.
Calculated exergy for the condenser.
For the condenser, the average calculated exergy values for both R-22 and R-290 at 60 Hz were close to 0.007 kW, with little difference for the exergy value for R-290 at 70 Hz which increased to 0.013 kW.
As the condenser is a component in which the variation in availability generated a low irreversibility value, that is, it was not a piece of equipment that contributed to the increase in electrical energy consumption during the tests.
The graph in Figure 12 shows the calculated exergy data in the refrigeration system evaporator.
Figure 12.
Calculated exergy for the evaporator.
For the evaporator, the calculated values showed little difference between R-22 and R-290 at 70 Hz, being in the range of 0.04 kW; however, the values for R-290 at 60 Hz were unexpected, since the value was almost four times lower, as shown in Figure 12.
It is known that some authors such as Ref. [7, 14], mention that the difference in viscosity, being much smaller in R-290 compared to R-22, can cause a more efficient heat exchange in the evaporator, which causes a reduction in the entropy of the system and reduces the final exergy value [7, 14].
During tests at the UFPA Refrigeration Laboratory, shown in Figure 12, the evaporator generated irreversibility’s at values between two and four times greater compared to the condenser. Being the highest average value calculated for R-22, followed by the average value calculated for R-290 at 70 Hz.
The calculation of exergy in the expansion device generated values that attracted more attention since being a component developed for operation with R-22 fluid, it still generated the highest energy value when comparing fluids and operating frequencies.
When observed in the steady state, the calculated irreversibility’s were, respectively, 0.16 kW, 0.13 kW and 0.10 kW, for R-22, for R-290 at 70 Hz and R-290 at 60 Hz.
Again, possibly the lower viscosity of R-290 reduced the variation in entropy and enthalpy at the inlet and outlet of the expansion device, causing lower irreversibility. Even so, the expansion device generated exergies up to eight times greater than the evaporator.
One of the advantages of exergy analysis over energy analysis is exactly the consideration of the variation in entropy in each component, as shown in the graph in Figure 13, which shows the variation in exergy calculated for the component with the highest energy consumption of the entire system, the compressor.
Figure 13.
Calculated compressor exergy.
The main point of energy analysis is the calculation of the COP, which is the amount of power absorbed in the evaporator over the electrical power consumed by the equipment during operation. For the evaluated system, the COPs were calculated for the three tests and the data were plotted on the graphs in Figures 14 and 15.
Figure 14.
Refrigeration system performance coefficient.
Figure 15.
Exergetic efficiency of the refrigeration system.
This value agrees with the increase in electrical consumption even above the value collected for operation with R-22 and generating a COP equal to 4.21% at an evaporation temperature above that at which R-22 works, that is, the COP value would be even lower if the evaporation pressure had been reduced and the exergy value would also exceed the value of 0.52 kW, as the entropy difference would increase.
The exergy analysis also provides the system efficiency factor, making a relationship between the availability in the evaporator and the total electricity consumption of the equipment [1, 10, 13, 14].
In the steady state, as in Figure 14, the calculated COP for the R-22 was 3.491; for the R-290, with a frequency of 60 Hz, it was 3.570, an increase of 2.26% and 3.344 for the frequency of 70 Hz, a reduction of 4.21%, respectively, when compared to the synthetic fluid. Again, this indicates that the increase in frequency made the compressor less efficient, even with the increase in volumetric displacement.
Figure 15 shows the result of the exergetic analysis in the refrigeration system.
It is important to remember that even though R-290 at 60 Hz has a COP value greater than that of R-22, this difference was due to the fact that the evaporation temperature of R-290 was higher, where the average of R-290 was 2.2°C and R-22 averaged 0.8°C.
In addition to the energy analysis, the exergy factors and irreversibility of the system were calculated as a way of complementing the explanations of the system’s characteristics and mainly the negative points that occurred when increasing the operating frequency from 60 Hz to 70 Hz.
It is known that some authors, such as Refs. [7, 14], mention that the difference in viscosity, being much smaller in R-290 compared to R-22, can cause a more efficient heat exchange in the evaporator, which causes a reduction in the entropy of the system and reduces the final exergy value [7, 14].
In steady state, the exergetic efficiency values, as shown in Figure 15, were 34% for R-22 and 32% for R-290 at 60 Hz, while the efficiency for R-290 at 70 Hz was 27%, as shown in Table 2 summarizes the main properties related to the use of energy and exergy by the system.
Thermodynamic Properties
Refrigerant Fluids
R-290 (60 Hz)
R-290 (70 Hz)
R-22
Exergy in the compressor (kW)
0,37
0,52
0,33
Exergy in the evaporator (kW)
0,02
0,04
0,04
Exergy in the expansion device (kW)
0,10
0,13
0,16
Exergy in the condenser (kW)
0,007
0,013
0,007
COP
3570
3344
3491
Exergetic Efficiency (%)
32
27
34
Table 2.
Summary of exergy properties.
The observation focuses on the following detail: the pressure and temperature values for R-290 at 60 and 70 Hz had a difference of 2%, so the temperature of the chilled water was the same for both tests, which contributes to the idea that there was a drop in the electrical efficiency of the engine being driven by the frequency inverter, confirmed by the increase in condensing pressure and power consumption.
Taking into account the psychrometric conditions of the UFPA Refrigeration Laboratory, it is possible to state that the tests to obtain data and the development of energy and exergy analyses were satisfactory, as they presented the differences and similarities between the two types of refrigerant fluids analyzed, providing subsidies and conditions for replacing R-22 fluid with R-290.
The statement made above meets the needs listed by the national and international market, as the refrigerant fluid R-22 was already discontinued in 2015; even so, in Brazil, this is the fluid most used in air conditioning systems, and it is as a substitute, the R-410a fluid, as is known, will be discontinued in 2024, that is, it will be necessary to use a refrigerant fluid to replace the fluids currently used.
Even so, after completing the experiments and obtaining the calculations and graphs of the refrigeration system components, some characteristics can be highlighted:
The use of flammable fluid, when operated by responsible and instructed technicians, did not generate the slightest concern during operation.
The use of natural refrigerant in a system originally designed for use with synthetic refrigerant resulted in a reduction in evaporator power of 2%, however, it increased COP by 3%.
R-290 at a frequency of 60 Hz obtained an exergy efficiency only 2% lower than R-22 operating at 60 Hz.
As it is a flammable fluid, there are risks associated with the uncontrolled use of R-290, which are minimized mainly by qualifying refrigeration technicians to respect the rules necessary to carry out maintenance and monitoring of this type of system.
In this way, this work adds to several others carried out, agreeing that the use of the natural fluid R-290 is a refrigerant with excellent thermodynamic characteristics for application in mechanical vapor compression refrigeration systems (mainly at low condensation saturation pressure), high enthalpy (which helps to reduce the amount of mass required in the system), compatibility with mineral lubricating fluid (eliminating the use of highly hygroscopic synthetic lubricating fluids) and, as made clear in this work, the reduction of electrical energy consumption.
According to the thermodynamic analysis developed for hydrocarbon fluids, it can be observed:
That natural fluids reduce the pressure levels developed in the condenser and evaporator.
Hydrocarbons have lower compressor discharge temperatures, providing longer useful life for these components.
The use of R-290 fluid and mixtures involving hydrocarbons provides a tripling of the latent heat of vaporization in relation to R-134a. This factor leads to a reduction of around 50% in the need for mass refrigerant charge in the refrigeration system for the same equipment capacity.
The coefficient of performance of the system with hydrocarbons and mixtures showed an increase of around 5% in relation to R134a. Consequently, less compression work is required for hydrocarbons in relation to R134a due to their thermophysical properties related to density in the liquid and vapor phases.
Analyzing the results of the exergy analysis carried out in the residential Chiller system, it is important to realize that the R-290 fluid, operating at a frequency of 60 Hz, presents itself as being the best condition to be used as a replacement fluid for R-22, in refrigeration and air conditioning systems, making it necessary to observe a reduction in the quantity of fluid, in mass, to be placed in the systems, in addition to the temperature and pressure conditions of evaporation and condensation, including the subcooling and superheating processes during installation of systems.
It is important to note that, in Brazil, even with the use of R-22 fluid permitted until 2015, with a total ban in 2030, there are still many air conditioning and refrigeration systems in normal operation using this HCFC. It is necessary to develop a strategic plan for recycling or definitive disposal of this and other discontinued or banned refrigerants in the world, avoiding the practice of releasing these fluids into the local atmosphere during system maintenance.
One of the viable solutions would be the creation or adaptation of companies to reuse or dispose of these fluids, in addition to providing training on the use and disposal of refrigerant fluids for technicians, engineers and other people involved in the refrigerant fluid chain in each of the States. from Brazil.
Although R-290 fluid is classified by ASHAE as a flammable refrigerant, it has a very low GWP and ODP equal to zero. This is a natural tendency to replace the current refrigerant fluids used, and this work proved that the R-290 refrigerant is one of the best substitutes for R-22. However, there is a need to carry out training for those involved with the systems, for their perfect use and destination, in air conditioning and refrigeration systems.
Thus, it is possible to use the existing compressor platform for synthetic refrigerant fluids with very few modifications for operation with natural fluid, reducing the cost of research and development, in addition to reducing the use of oils with a complex chemical composition that is quite aggressive to the environment and mainly the focus on reducing electricity consumption, which brings together the objectives of always benefiting the Environment, Sustainability and the rational use of electricity seeking to reduce pollutant emissions.
In the same way as was carried out with the R-22 fluid, other research is being carried out in the UFPA Refrigeration Laboratory in order to carry out new energy and exergy analyses of the systems, aiming to replace these fluids, such as ammonia and R-410a fluid, for other refrigerants that do not contribute to global warming or the degradation of the Earth’s Ozone layer.
The authors would like to thank the Faculty of Mechanical Engineering – FEM, the Institute of Technology – ITEC and the Postgraduate Program – PPGEM at the Federal University of Pará – UFPA.
References
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Written By
Eraldo Cruz dos Santos, Carlisson Arnaud de Azevedo, Caio Luiz de Carvalho Macêdo, Luiz Felipe Pinheiro Oliveira, Fernando Lucas Alves da Silva, Luis Gabriel Brito Mota and Ismael dos Santos Marinho
Submitted: 30 July 2024Reviewed: 20 August 2024Published: 19 December 2024
Open access peer-reviewed chapter
