Energy performance evaluation of R600a/MWCNT-nanolubricant as a drop-in replacement for R134a in household refrigerator system

Babarinde, T. O; Akinlabi, S. A; Madyira, D. M Article Energy performance evaluation of R600a/MWCNT- nanolubricant as a drop-in replacement for R134a in household refrigerator system Energy Reports Provided in Cooperation with: Elsevier Suggested Citation: Babarinde, T. O; Akinlabi, S. A; Madyira, D. M (2020) : Energy performance evaluation of R600a/MWCNT-nanolubricant as a drop-in replacement for R134a in household refrigerator system, Energy Reports, ISSN 2352-4847, Elsevier, Amsterdam, Vol. 6, Iss. 2, pp. 639-647, https://doi.org/10.1016/j.egyr.2019.11.132 This Version is available at: https://hdl.handle.net/10419/243945 Standard-Nutzungsbedingungen: Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden. Sie dürfen die Dokumente nicht für öffentliche oder kommerzielle Zwecke vervielfältigen, öffentlich ausstellen, öffentlich zugänglich machen, vertreiben oder anderweitig nutzen. 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If the documents have been made available under an Open Content Licence (especially Creative Commons Licences), you may exercise further usage rights as specified in the indicated licence. https://creativecommons.org/licenses/by-nc-nd/4.0/ Available online at www.sciencedirect.com ScienceDirect Energy Reports 6 (2020) 639–647 www.elsevier.com/locate/egyr The 6th International Conference on Power and Energy Systems Engineering (CPESE 2019), 20–23 September 2019, Okinawa, Japan Energy performance evaluation of R600a/MWCNT-nanolubricant as a drop-in replacement for R134a in household refrigerator system T.O Babarindea,c, S.A Akinlabib,c, D.M Madyiraa,∗ aDepartment of Mechanical Engineering Science, University of Johannesburg, Johannesburg, South Africa bDepartment of Mechanical & Industrial Engineering Technology, University of Johannesburg, Johannesburg, South Africa cDepartment of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria Received 8 October 2019; accepted 23 November 2019 Abstract Vapour compression refrigerator systems working with R134a are associated with high energy demand and environmental problems and refrigerator consumers’ prioritise energy consumption in choosing their choice of refrigerator systems. Therefore, this research work investigated R600a in MWCNT-nanolubricant (0.4 g/L and 0.6 g/L) as a drop in replacement for R134a refrigerant in a household refrigerator system with varied mass charge of R600a (50, 60 and 70 g). The refrigerator was instrumented at the inlet and outlet of the refrigerator compressor, condenser, expansion valve and evaporator. Two bourdon type pressure gauges were connected to the compressor inlet and outlet of the system. The results were taken and evaluated and compared with the result obtained through R134a refrigerant in the system. The result showed that R600a perform better in terms of COP, power consumption and cooling capacity compared to R134a in the system with lower evaporator temperature of −11 ◦C and power consumption of 0.0639 kW and highest COP in the system. Therefore, R6000a/MWCNT- nanolubricant can serve as a drop in replacement for R134a in the household refrigerator. c 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 6th International Conference on Power and Energy Systems Engineering (CPESE 2019). Keywords: GWP. R134a; R600a; MWCNT; COP; Power consumption 1. Introduction Major energy demand in homes and commercial buildings is as a result of refrigeration, heating, ventilation and air-conditioning systems. Hence, energy researchers are looking into improving the energy consumed by comfort systems such as the household refrigerators and air-conditioning systems [1]. Introducing nanofluids into vapour compression systems is capable of giving that solution to the high energy consumption associated with refrigeration systems. Choi et al. [2] found out that the use of nanoparticles as additives in refrigerator in nanolubricant and nanorefrigerant has the potential to enhance its thermal properties. When nanoparticles are dispersed in the lubricant ∗Corresponding author. E-mail address: [email protected] (D.M. Madyira). https://doi.org/10.1016/j.egyr.2019.11.132 2352-4847/ c 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 6th International Conference on Power and Energy Systems Engineering (CPESE 2019). 640 T.O. Babarinde, S.A. Akinlabi and D.M. Madyira / Energy Reports 6 (2020) 639–647 Nomenclature VCRS Vapour compression refrigeration system MWCNT Multiwall carbon nanotube POE Polyester Oil TEM Transmission electron microscope COP Coefficient of performance Qevap Cooling capacity (W) WCCompressor power input (W) hEnthalp (kJ/kg) Subscripts 1Evaporator outlet 2Compressor oulet 3Condenser outlet 4Evaporator inlet or refrigerant they are called Nanolubricants or Nanorefrigerants. Part of the advantages of nanoparticles in the compressor lubricant is that they enhance the thermal conductivity and reduce friction and wear which in turns cause an enhancement in the heat transfer rate and reduce the pumping power required by the compressor for the effective operation of a system [3]. However, the performance of nanoparticles in the base fluid largely depends on the size, shape, and concentration of the nanoparticles [4]. In the past, some researchers have studied the use of nanolubricants in cooling systems such as vapour compression system (VCRS) in order to enhance its performance. Nabil et al. [5] evaluated water/ethylene glycol (EG) mixture in terms of heat transfer performance with TiO2 nanoparticles within a range of 30 ◦C to 80 ◦C. The result obtained show a 15.4% improvement at 60 ◦C. Wang et al. [6] compared fullerene (C70) and NiFe2O4 nanolubricant with the base lubricant in an automobile air-conditioning (A/C) system. The result showed that COP of the A/C system was increased by 23%. Also, a reduction in the coefficient of friction was achieved which means a reduction in wear. Melnyk [7] tested Al2O3 and TiO2 nanoparticles with R600a refrigerant using mineral oil in VCRS. The result indicated that R600a/Al2O3 and R600a/TiO2 performances improved in terms of thermal conductivity and viscosity in the system. Babarinde et al. [8] also carried out an experiment on the use of R600a as a direct replacement for R134a in a VCRS, the result showed that R600a/TiO2 performed better compared to R134a in terms of COP and power consumption in the system. Sun et al. [9] evaluated the performance of the heat transfer of MWCNT-OH/R141b and MWCNTCOOH/R141B in a cooling system by varying the volume concentration (0.059, 0.117 and 0.17655%vol) at vapour quality range of 0.2–0.7. The result showed that heat transfer coefficient of MWCNT-COOH/R141B was higher compared to MWCNT-OH/R141b. Mahmoud [10] also investigated the performance of Single-Wall Carbon Nanotube (SWCNT) in refrigeration system using R134a as working fluid. The nano concentration was varied from 1% to 5 vol% within the temperature range of 300 K to 320 K. The result showed an increase in thermal conductivity and specific heat transfer of SWCNT/R134a in the system compared to pure R134a. Hung and Gu [11] also carried out research on MWCNT Nanofluid in a hybrid energy system. The MWCNT was added as an additive in water with a weight fraction of 0.125%, 0.25% and 0.5% to produce a Nanofluid. The 0.125% Nanofluid showed a 5% increase in the heat transfer behaviour compared to the water. From the existing literature, MWCNT nanoparticles are capable of increasing the thermal conductivity, and heat transfer rate when used as additives in the base fluid. Currently, there is more literature on the use of MWCNT in halogenated refrigerant and water. Furthermore, MWCNT nanoparticles have not been investigated in hydrocarbon refrigerants of which R600a is one of such. Today, household refrigerator designed to work with R134/POE oil are still in use, especially in the developing countries. Therefore, there is a need to replace them with improved R600a/MWCNT-Mineral oil. This work presents the application of R600a/MWCNT using mineral oil as the lubricant to replace R134a in the household refrigerator system. T.O. Babarinde, S.A. Akinlabi and D.M. Madyira / Energy Reports 6 (2020) 639–647 641 2. Experimental procedures The household refrigerator used as test rig for this experiment was primarily designed to work with POE-Oil and 100 g R134a. Therefore, R134a is used as a baseline for this experiment. The refrigerator was evacuated with the aid of vacuum flasher before use and after each experiment. In this present study R600a/MWCNT-nanolubricant is used as a drop on a replacement for R134a in a household refrigerator. R600a is considered due to its Zero ODP and very low GWP compared to R134a [12]. The R600a refrigerant was varied within the range of 50 g–70 g. The lubricant used for R600a refrigerant was 1 L lubricant (mineral oil). MWCNT-nanolubricant was selected for this experiment because of its good, energy storage, thermal properties and very large surface area. The MWCNT nanolubricant used has a specification of 10 nm ±1 nm ×4.5 nm ±0.5 nm ×3–∼6µm as stated by the manufacturer in Table 2. The transmission electron microscope of the MWCNT nanoparticles is shown in Fig. 2. The MWCNT nanolubricant was prepared with 1 L of lubricant (mineral oil). Different samples of 0.4 g/L and 0.6 g/L MWCNT-nanolubricant concentrations were prepared. Digital charging scale was employed to measure the nanoparticles before dispersed into the lubricant. The mixture of the MWCNT and the lubricant was stirred with the help of magnetic stirrer for 45 min. Also, ultrasonic homogeniser was used to homogenised the MWCNT-Lubricant mixture together for 3 h within a temperature range of 15 ◦C–20 ◦C [13]. Each MWCNT-nanolubricant was tested for 50 g, 60 g and 70 g of R600a. Fig. 3 shows the preparation flow chart of the MWCNT-nanolubricant while Table 4 shows the properties of the POE and the mineral oil used for the experimental study. The 100 g of R134a refrigerants was first introduced into the system with the aid of digital charging scale. The system was evacuated and flushed after. Then R600a refrigerants were charged into the system. The temperature readings were taken at the inlet and outlet of each refrigerator components (compressor, condenser, expansion valve and evaporator) for each experiment with K type thermocouples connected to the inlet and outlet of each component. Also, two Bourdon type pressure gauges were connected to the inlet and out of the compressor to measure the inlet and outlet pressure of the compressor. A digital wattmeter was used to measure the power consumed by the refrigerator compressor. The experiment was carried at an average environmental temperature of 27 ◦C. The experimental set-up, measuring instruments, uncertainties, experimental condition and range are shown in Fig. 1, Tables 3–4. The experiment outputs were evaluated and compared with the performance of R134a. Performances such COP, power consumption, cooling capacity of the system were evaluated using Eqs. (1)–(3) (see Table 1). The refrigerator system performance was calculated using equations below = ˙m(h1−h4) (kW) (1) = ˙m(h2−h1) (kW) (2) C O P =Qevap Wc (3) Table 1. Characteristics of the refrigerants. S/N Refrigerants Normal boiling point (◦C) ODP GWP Critical Temp. (◦C) Safety group Molar mass 1 134a −26.07 0 1300 101.06 A1 102.03 2 R600a −12 0 20 135 A3 56.12 Table 2. Specification of the nanoparticles. Table Nanoparticles Information S/N Type Specification Manufacturer 1 Multi-Wall carbon nanotube 10 nm ±1 nm ×4.5 nm ±0.5 nm ×3–∼6µm Aldrich 3. Results and discussion The comparison of the performance of R134a, R600a in the bsae lubricant (mineral oil) and MWCNT- nanolubricant concentrations is being discussed considering evaporator air temperature, COP, power consumption and cooling capacity. 642 T.O. Babarinde, S.A. Akinlabi and D.M. Madyira / Energy Reports 6 (2020) 639–647 Table 3. Specifications of the measuring instrument and uncertainty. S/N Parameter Specification Range Uncertainty 1 Temperature Digital thermocouple K −50 ◦C–700 ◦C±3◦C 2 Pressure Bourdon gauge 0–2000 kPa ±1% 3 Power consumption Digital Watt/Watt-h-metre 1−2000 W (0.0001−999.9 kWh) ±1% Fig. 1. Experimental setup. Fig. 2. Transmission electron microscope image of MWCNT nanoparticles. Figs. 4–6illustrate the pull-down time of R134a and R600a in 0.4 g/L and 0.6 g/L MWCNT-lubricant. It can be observed that R134a has an evaporator temperature of −8 at a pulldown time of 210 min while R600a in 0.4 g/L of MWCNT-nanolubricant concentration has the shortest pull-down time and lowest evaporator air temperature. The evaporator air temperature of −6◦C, −11 ◦C and −3◦C at a pulldown time of 180, 150 and 90 min was recorded for 50, 60 and 70 g of R600a in 0.4 g/L MWCNT-nanolubricant concentration. Also, evaporator air temperature of −8◦C, −11 ◦C and −5◦C at a pulldown time of 210, 240 and 210 min was recorded for 50, 60 and 70 g of R600a in 0.6 g/L MWCNT-nanolubricant respectively. The improvement is as a result of enhancement in heat transfer due to the increase thermal conductivity of the system. T.O. Babarinde, S.A. Akinlabi and D.M. Madyira / Energy Reports 6 (2020) 639–647 643 Fig. 3. MWCNT nanolubricant flow chart. Table 4. Experimental conditions. S/N Parameter Range 1 Refrigerant mass charge 50, 60 and 70 g 2 Refrigerant R600a 3 Compressor lubricant Pure-lubricant, MWCNT nanolubricant 4 MWCNT 10 nm ±1 nm ×4.5 nm ±0.5 nm ×3–∼6µm 5 Nano-lubricant concentration 0.4, 0.6 g/L 6 Ambient Temp. 27 ◦C 7 Capillary length 1.5 m 8 Condenser Air-cooled 9 Evaporator capacity 70 L 10 Density at 15 ◦C (POE) 0.980 11 Density at 15 ◦C mineral oil 0.914 12 Kinematic viscosity at 40 ◦C/100 ◦C (POE) 32 cSt/5.8 cSt 13 Kinematic viscosity at 40 ◦C/100 ◦C mineral oil 30 cSt/4.4 cSt Fig. 7 shows the cooling capacity of R134a and R600a in MWCNT-nanolubricant in the refrigerator system. The cooling capacity first slightly decrease with 0.4 g/L of MWCNT-nanolubricant for 50 g of R600a then continue to improve with the introduction of the MWCNT-nanolubricant into R600a. The cooling capacity of R600a increases with increase in the concentration of MWCNT-nanolubricant from 0.4 g/L to 0.6 g/L in the system until it attained its optimal mass charge. The cooling capacity of R600a in MWCNT-nanolubricant increases within the range of 3.5%–12.9%, 11.90%–14.30% and 3.20%–14.90% for 50 g, 60 g and 70 g of R600a in MWCNT-nanolubricant respectively The 60 g of R600a in 0.4 g/L MWCNT-nanolubricant has the highest cooling capacity of 0.1893 kW with an evaporator air temperature of −11 ◦C. This general increase in the cooling capacity of MWCNT-lubricant is due to the increase in thermal conductivity of the R600a in MWCNT-nanolubricant compared to the R134a. Fig. 8 depicts the power consumption of the system and the effect of MWCNT-nanolubricant on the power consumption of the system. The power consumption of R600a in MWCNT-nanolubricant concentration decreases with the introduction of MWCNT nanoparticles into the lubricant until it reaches its optimal concentration and mass charge for R600a in the MWCNT-nanolubricant. The power consumption of 0.4 g/L and 0.6 g/L MWCNT - nanolubricant is lower within the range of 0,0612 kW–0.0719 kW, 0.0639 kW– 0.0728 kW and 0.0761 kW–0.0814 kW for 50 g, 60 g and 70 g of R600a in 0.4 g/L and 0.6 g/L of MWCNT-lubricant respectively with 60 g of R600a in 0.6 g/L of MWCNT-nanolubricant having the lowest evaporator and lower power consumption of 0.0639 kW at −11 ◦C compared to R134a. This is as a result of a reduction in the pumping power of the compressor due to the tribological behaviour of MWCNT to reach a lower evaporator temperature. Fig. 9 shows the effect of MWCNT-nanolubricant on the COP of the refrigerator system. The COP of R600a in MWCNT-nanolubricant is higher than R134a. The COP of R600a in the 0.4 g/L and 0.6 g/L of MWNT-Lubricant ranges from 2.6–2.8, 2.6–2.9 and 1.84–2.5 for 50 g, 60 g and 70 g of R600a respectively. The 60 g of R600a in 0.6 644 T.O. Babarinde, S.A. Akinlabi and D.M. Madyira / Energy Reports 6 (2020) 639–647 Fig. 4. Effect of MWCNT nanolubricant on the pull-down time of 50 g of R600a. Fig. 5. Effect of MWCNT nanolubricant on pull downtime of 60 g of R600a. g/L of MWCNT-nanolubricant has the highest COP. The improvement in the COP can be linked to the influence of the MWCNT nanoparticles on the system which led to a decrease in the friction and wears in the compressor which - caused a decrease in power consumption. Also, it can be attributed to enhancement in the heat transfer which led to higher cooling capacity compared to the R134a Fig. 10 depicts the discharge pressure of the system. The compressor discharge pressure of R600a in MWCNT- nanolubricant is lower than R134a. The reduction in discharge pressure is also responsible for the reduction in the power consumption of the system. This can be credited to a reduction in friction and wear as a result of the introduction of MWCNT-nanolubricant into the system. One of the benefits of reduction in the discharge pressure of the refrigerator compressor is that it prolongs the life span of the compressor. In this research it was noticed that for every mass charge of R600a refrigerant [see Table 5] MWCNTLubricant performed better in terms of pull-down time which established the enhancement in heat transfer rate, increase in thermal conductivity and most importantly the improvement in the energy efficiency of the system T.O. Babarinde, S.A. Akinlabi and D.M. Madyira / Energy Reports 6 (2020) 639–647 645 Fig. 6. Effect of MWCNT nanolubricant on the pull-down time of 70 g of R600a the system. Fig. 7. Effect of MWCNT nanolubricant on the cooling capacity of the system. Table 5. Summary of the experimental result of MWCNT-nanolubricant in the system. S/N Refrigerant mass charge (g) Nanolubricant concentration (g/L) Pull downtime (min) Evaporator temp. (◦C) COP Power consumption (kW) Cooling capacity (kW) Discharge pressure (MPa) 1 100 (R134a) – 210 −8 1.84 0.0900 0.1656 0.81 2 Pure 210 −5 2.1 0.0821 0.1736 0.75 4 50 0.4 180 −7 2.8 0.0612 0.1714 0.59 5 0.6 210 −8 2.6 0.0719 0.1870 0.72 6 Pure 90 −3 2.1 0.0822 0.1679 0.68 7 60 0.4 150 −11 2.6 0.0728 0.1893 0.60 8 0.6 240 −11 2.9 0.0639 0.1853 0.60 9 Pure 150 0 1.6 0.0861 0.1410 0.75 10 70 0.4 90 −3 2.1 0.0814 0.1709 0.74 11 0.6 210 −8 2.5 0.0761 0.1902 0.66 646 T.O. Babarinde, S.A. Akinlabi and D.M. Madyira / Energy Reports 6 (2020) 639–647 Fig. 8. Effect of MWCNT nanolubricant on the power consumption of the system. Fig. 9. Effect of MWCNT nanolubricant on the COP of the system. 4. Conclusion In this present study, the experimental analysis of R600a in MWCNT-nanolubricant concentrations was compared with the performance of R134a in a household refrigerator. The different mass charge of R600a refrigerant (50 g, 60 g and 70 g) was tested in different MWCNT-nanolubricants of 0.4 g/L and 0.6 g/L. Based on the experimental findings, and considering the performance of the refrigerator, these following conclusions were made for R600a in MWCNT-nanolubricant concentration. •The 60 g mass charge of R600a in 0.6 g/L MWCNT Nano lubricant obtained the best result in terms of COP, with evaporator temperature −11 ◦C at 150 min which meets the ISO 8187 standard temperature of −3◦C at 180 min. Also, power consumption is lower compared to R600a in the pure lubricant. Therefore, it is considered as a drop-in replacement for the R134a refrigerant in a household refrigerator. •The MWCNT-nanolubricant concentrations of 0.4 g/L and 0.6 g/L offer reduced power consumption throughout the experiment with the 50 g charge of R600a in 0.4 g/L MWCNT-nanolubricant with the lowest power