1、Omar Abdelaziz, Som Shrestha and Jeff Munk are Research and Development Staff, and Randall Linkous is Technical Staff in Building Technologies Research and Integration Center, ORNL, TN, USA. Evaluation of Alternative Refrigerants for High Ambient Applications in a Mini-Split AC Unit Omar Abdelaziz,
2、PhD Som Shrestha, PhD Jeff Munk Member Member Member Randall L. Linkous ABSTRACT The Montreal Protocol has a stringent timeline for the phase-out of HCFC refrigerants, including R-22, in both developing and developed nations. However, in most developing nations, high ambient temperatures limit the u
3、se of alternative refrigerants due to performance and safety concerns. Furthermore, the developed worlds transition through higher global warming potential (GWP) refrigerants like HFC and HFC blends resulted in significant direct CO2 equivalent emissions. It is imperative to develop a bridge for dev
4、eloping nations to avoid the transition from HCFC to HFC and then from HFC to alternative lower GWP refrigerants. In this paper, we summarize an experimental campaign on alternative refrigerant evaluation for an R-22 mini-split system. The experimental evaluation was performed according to ANSI/ASHR
5、AE Standard 37 and the performance was rated according to ANSI/AHRI 210-240 standard. Furthermore, extended test conditions were evaluated at outdoor ambient temperatures of 46C, 52C, and 55C. R-22 alternative refrigerants included propriety refrigerant blends (ARM-20b, DR-3, and N-20b). The unit pe
6、rformance was first verified using the baseline refrigerant and then drop-in refrigerant evaluation followed including soft optimization to ensure refrigerant performance is adequately represented. The soft optimization included: 1) charge optimization, 2) lubricant, and 3) flow control. The paper p
7、resents the relative performance (efficiency and capacity) of the alternative refrigerants compared to the baseline refrigerant at the different operating conditions. We conclude with remarks about the alternative refrigerants for R-22 applications in high ambient temperature regions. INTRODUCTION H
8、CFCs (hydrochlorofluorocarbons) are currently used in the refrigeration, foam, solvent, aerosol and fire suppression sectors as a transitional substance to substitute CFCs especially in developing countries listed under article A5 of the Montreal Protocol. HCFCs were introduced in the 1990s as alter
9、natives for CFCs and added to the list of substances controlled by the Montreal Protocol (UNEP OzonAction). HCFCs were considered transitional refrigerants while technology utilizing more sustainable refrigerants that pose no ozone depletion potential was being developed. According to UNEP OzonActio
10、n, the global HCFC production was 34,400 ODP tonnes and approximately 75% of global HCFC use is in air-conditioning and refrigeration sectors in 2006. The main HCFC used is R-22. Figure 1 below shows the current schedule for the phase-down of HCFCs (UNEP OzonAction) Figure 1. Phase-down schedule of
11、HCFCs based on the Montreal Protocol (UNEP OzonAction). On the other hand, hydrofluorocarbon (HFC) refrigerants are non-ozone-depleting fluids that are used as working fluids in air conditioning and refrigeration equipment. They are currently in wide use as alternatives to ozone-depleting substances
12、 in non-article 5 countries. Their significant global warming potential (GWP100), which is 1400 4000 times that of Carbon Dioxide, resulted in increased concern over their use and the development of alternative lower GWP refrigerants. As shown in Figure 1, non-article 5 countries have already begun
13、the transition from HCFC to HFC and are in the transition from the high GWP HFC to alternative lower GWP HFOs (hydrofluoroolefen) and HFO/HFC blends. Every time the industry goes through such transition it has to incur significant cost for retooling and system design modifications. As such, there is
14、 an ongoing effort to bridge the transition from HCFC to lower GWP refrigerants in article 5 countries thereby limiting the cost endured by the industry. However, it is still questionable if lower GWP refrigerant will be capable to perform adequately at higher ambient temperatures which would preval
15、ent in most article 5 countries. In this paper, we present an experimental campaign to evaluate the performance of alternative lower GWP refrigerants as drop-in replacements to R-22 in a mini-split Air Conditioning (AC) system designed for high ambient temperature applications. The drop-in replaceme
16、nt study was subject to oil change, charge optimization, and capillary tube length optimization. EXPERIMENTAL FACILITIES AND EQUIPMENT We performed drop-in tests for a baseline mini-split system: a 5.25 kW (1.5 TR) R-22 system inside the Multi-zone Environmental Chambers, shown in Figure 2. This fac
17、ility provides the capability for testing the performance of multi-zone electric or gas HVAC systems for residential and light commercial use. The “outdoor” chamber is 6.14.6 m (2015 ft.); the 8.5 m (28 ft.) square “indoor” chamber can be divided into up to four spaces controlled at different condit
18、ions to represent separate zones. Dry-bulb temperature can be controlled at 23 to 55C (10 to 131F) and relative humidity at 30 to 90%. Utilities include 480 V, 3-phase power at 225 A with step-down to single phase 120 V. In this project, the indoor side was split into 2 chambers, each 8.54.25 m and
19、the unit under test was evaluated in one of those chambers. Outdoor ChamberMulti-Zone Indoor ChamberFigure 2. Multi-zone environmental chambers. An experimental test facility was designed and built to comply with ANSI/AHRI Standard 210/240-2008, and ANSI/ASHRAE Standard 37-2009. The Air Enthalpy met
20、hod is used to evaluate the performance of the indoor unit and the Refrigerant Enthalpy Method is used as a secondary means of evaluating the system performance in order to establish energy balance and assess measurement accuracy. Figure 3 and Figure 4 provide an overview of the experimental test se
21、tup, with the measurement locations indicated. The unit under test was well instrumented. For air-side capacity, the air flow was determined using a nozzle designed and manufactured to the ANSI/ASHRAE 51-07 specifications (ANSI/ASHRAE 51-2007), the dry-bulb temperature was measured using a calibrate
22、d thermocouple grid and thermocouple tree (0.3C), the dew-point temperature was measured using a chilled mirror hygrometers (0.2C) and the condensate was weighed using an electronic precision scale. The refrigerant-side capacity was determined using refrigerant flow rate measurements (Coriolis mass
23、flow meter, 0.1% reading), calibrated thermocouples (0.3C), and precision pressure sensors (0.08% BSL). The overall capacity uncertainty analysis was 3.6%. To determine the unit efficiency, electric powers were measured using high precision power meters (0.2% reading) yielding an overall COP uncerta
24、inty of 3.64%. Figure 3. Top view of the R-22 baseline unit experimental setup showing both the indoor and outdoor sides along with instrumentation locations and design of the air enthalpy tunnel. For line legend please refer to Figure 4. Figure 4. Side view of the air enthalpy tunnel showing additi
25、onal details and legend for the lines. The unit under test was well instrumented. For air-side capacity, the air flow was determined using a nozzle designed and manufactured to the ANSI/ASHRAE 51-07 specifications (ANSI/ASHRAE 51-2007), the dry-bulb temperature was measured using a calibrated thermo
26、couple grid and thermocouple tree (0.3C), the dew-point temperature was measured using a chilled mirror hygrometers (0.2C) and the condensate was weighed using an electronic precision scale. The refrigerant-side capacity was determined using refrigerant flow rate measurements (Coriolis mass flow met
27、er, 0.1% reading), calibrated thermocouples (0.3C), and precision pressure sensors (0.08% BSL). The overall capacity uncertainty analysis was 3.6%. To determine the unit efficiency, electric powers were measured using high precision power meters (0.2% reading) yielding an overall COP uncertainty of
28、3.64%. Alternative Refrigerants Five alternative refrigerants were evaluated; their ASHRAE safety Class and environmental characteristics are shown in Table 1. Table 1. Baseline and Alternative Refrigerant Data for the R-22 Unit Refrigerant ASHRAE Safety Class GWP a, AR4 GWP a, AR5 R-22 (Baseline)aA
29、1 1810 1760 N-20BbA1 947 874 DR-3bA2L 256 255 ARM-20BbA2L 363 365 L-20A (R-444B)bA2L 407 408 DR-93bA1 1130 1048 a Sources: IPCC AR4, 2007 ; IPCC AR5, 2013 bGWP values for refrigerant blends not included in IPCC reports are calculated as a weighted average using manufacturer-supplied compositions. Te
30、st Conditions Testing of all refrigerants was performed at each of the environmental conditions described in Table 2. These were selected to include both U.S. conditions (AHRI), international conditions (T3), and extended conditions representing hot climates (T3*, Hot, and Extreme). Table 2. Test Co
31、nditions Test Condition Outdoor Dry-Bulb Temp., C (F) Indoor Dry-Bulb Temp., C (F) Wet-Bulb Temp, C (F) Dew Point Tempa, C (F) Relative Humidity, % AHRI Bb27.8 (82) 26.7 (80.0) 19.4 (67) 15.8 (60.4) 50.9 AHRI Ab35.0 (95) 26.7 (80.0) 19.4 (67) 15.8 (60.4) 50.9 T3* c46 (114.8) 26.7 (80.0) 19 (66.2) 15
32、.8 (60.4) 50.9 T3 46 (114.8) 29 (84.2) 19 (66.2) 13.7 (56.6) 39 Hot 52 (125.6) 29 (84.2) 19 (66.2) 13.7 (56.6) 39 Extreme 55 (131) 29 (84.2) 19 (66.2) 13.7 (56.6) 39 aDew point temp and RH at 0.973 atm (14.3 psi), typical barometric pressure in the lab bPer AHRI Standard 210/240 cT3* is a modified T
33、3 condition where the indoor settings are similar to the AHRI conditions. EXPERIMENTAL PROCEDURE We followed ANSI/ASHRAE Standard 37-2009 during the unit evaluation. The following steps were taken to evaluate the equipment and refrigerant combinations: Perform charge and capillary tube optimization
34、to maximize COP at the AHRI A conditions. At the optimum charge, evaluate the performance at the T3 condition. If adequate subcooling and superheat are available, proceed with testing. Otherwise, adjust the charge to ensure 100% liquid entering the capillary tube and avoid compressor flood back. Run
35、 the test matrix (each refrigerant at each test condition). Collect steady-state data for 30 min at each condition. To ensure system performance is maintained over the test period, the unit is retested with the baseline refrigerant to verify the system performance stability after finishing all alter
36、native refrigerant tests. Process for Soft Optimization The soft optimization process consisted primarily of adjusting the capillary tube and performing adequate charge optimization. The baseline capillary tube had an inner diameter (ID) of 2.00 mm (0.079”), an outer diameter (OD) of 3.2 mm (1/8”),
37、and a length of 508 mm (20”). Unfortunately, we were only able to find capillary tubes with an ID of 1.65 mm (0.065”) at that time. As such, a new baseline capillary tube configuration was established, and a 254 mm (10”) capillary tube was selected as the baseline for the R-22 unit. The soft optimiz
38、ation process used was as follows: 1. Size capillary tubes using appropriate correlations and fabricate. (2”, 1”, and exact size per calculations) 2. Charge the system with Mopt,ref#= Mopt,ref#* ref#, liq/ R-22, liq3. Run charge optimization procedure at the AHRI A condition: collect steady-state da
39、ta for 10 minutes at each condition a. Mopt,ref#and exact capillary tube length b. Mopt,ref#and 1” shorter capillary tube length; if better performance, proceed to c), else proceed to d) c. Mopt,ref#and 2” shorter capillary tube length, skip to f) d. Mopt,ref#and 1” longer capillary tube length; if
40、better performance, proceed to e), else proceed to f) e. Mopt,ref#and 2” longer capillary tube length f. Add/subtract refrigerant charge (approximately 2 oz. at a time), go back to a). 4. Run the unit with Mopt,ref#and the selected capillary tube at T3 conditions to ensure adequate subcooling and su
41、perheating; if not, adjust the charge accordingly (10 min of steady-state data collected). 5. Evaluate the system performance for all test conditions listed in Table 2. RESULTS AND DISCUSSION As discussed in the experimental procedure section, 30 minute data were used to evaluate the performance of
42、alternative refrigerants at the different test conditions. A summary of the test data is shown in Figures 5 and 6 below. The system COP is evaluated based on the air-side capacity and power measurements since the refrigerant-side capacity might be inaccurate as the refrigerant exiting the condenser
43、or the evaporator approaches saturation or becomes 2-phase. It is worthy to note that the condensate measurement was used as a method for latent capacity evaluation since at higher ambient conditions the difference in dew point temperature between the inlet and outlet of the indoor unit was small re
44、sulting in significant uncertainty. As shown in Figure 5, the COP drops from 3.1 in the AHRI A condition to 1.8 in the extreme condition, a 42% drop in COP, using the baseline refrigerant and oil (R-22 with mineral oil). When the oil was changed from mineral to POE, the unit performance degraded by
45、6%. Using POE oil was imperative since the alternative refrigerants are not quite miscible in mineral oil. The change in unit performance when moving from the mineral oil to the POE oil can be attributed to the difference in oil viscosity and refrigerant oil miscibility. When comparing the alternati
46、ve refrigerants to the baseline refrigerant and oil, it can be seen that the R-444B has a unique behavior. The percentage reduction in COP between the baseline refrigerant/oil and R-444B seems to diminish as the ambient temperature increases. At the AHRI B condition, the difference is 13%, while at
47、the Extreme condition the difference is only 7%. With regards to cooling capacity, as shown in Figure 6, the baseline refrigerant/oil has a capacity of 6.1 kW (20.8 kBtu/hr) at the AHRI A condition and drops to 4.8 kW (16.3 kBtu/hr) at the Extreme conditions. This results in only 21% reduction in ca
48、pacity. It also indicates that the unit is designed to maintain capacity better than maintaining efficiency. From Figure 6, we can deduce that both ARM-20B and the R-444B are promising candidates. Once again, R-444B shows the same interesting behavior of reducing the gap in performance with the base
49、line at higher ambient conditions. In Figure 7 below, we summarize the performance of the alternative refrigerants compared with baseline refrigerant/oil (R-22/mineral oil) at both the AHRI A conditions (left) and the Extreme conditions (right). As shown in Figure 7, the performance of the unit (R-22 POE vs. R-22 POE rerun) did not change after carrying the entire test campaign which lasted for more than 6 weeks. Also, the figure shows the relative positions change for R-444B indicating that it gets closer to the baseline performa
