FORUM Testing inverter type air conditioners for field performance Dr Satya Mavuri (BE MSc PhD), M.AIRAH, Vipac Engineers and Scientists Ltd. ABSTRACT Considerable technological advancement in domestic air conditioner (AC) designs over the past few decades has seen recent developments in variable-speed compressor (inverter) operation. Current test standards allow inverter AC (IAC) manufacturers to “lock” the compressor speed for a desired “rated” capacity. However, it is proposed that rating the inverter under “locked” mode is unrealistic, as it does not reflect the way inverters operate in real operation. In this study, an “unlocked” mode test method is discussed for testing IAC for “field” behaviour, and the results are compared to “locked” laboratory performance. For almost all inverter units tested, efficiency degradation was recorded in “unlocked” modes compared to when tested in “locked” mode for similar output capacity. It is suggested a new test method be considered for IAC unit performance in “unlocked” mode offering an alternative and more realistic field behaviour assessment. KEY WORDS Air conditioner, test standard, calorimeter, inverter, performance, locked, unlocked IACs An AC is a mechanism designed to change and control the air temperature and humidity within an area (in Australia, these are used primarily for cooling and sometimes heating). The operation is typically performed by a simple refrigeration (vapour compression) cycle, shown in Figure 1 below. COMPRESSOR INLET SUCTON LINE COMPRESSOR OFF COMPRESSOR OUTLET COMPRESSOR DISCHARGE LINE conditioning system to regulate energy required to achieve the desired indoor space temperature conditions. It functions like the accelerator of a car, gently increasing or decreasing power. It maintains the desired temperatures without wild fluctuations. Figure 2 shows room temperature during the operation of a fixed-speed AC and IAC unit. TEMP. INTRODUCTION SET TEMP. CONDENSER CONDENSER FAN WARM AIR OUT AMBIENT (OUTSIDE) AIR IN COOLS A ROOM IN LESS TIME COMPRESSOR OFF SMALL TEMP. DIFFERENCE START WARM RETURN AIR IN COOL SUPPLY AIR OUT EVAPORATOR BLOWER EVAPORATOR METERING DEVICE LIQUID LINE DRIER A typical refrigeration cycle uses an electric motor to drive a compressor. The two most common types of compressors are the “fixed-speed” type and the “variable-speed” (inverter) type. With conventional fixed-speed air conditioners, the compressor is either on (working to 100% capacity) or off. An inverter in an air conditioner is used to vary the compressor motor speed in order to drive variable refrigerant flow in an air E CO L I B R I U M • A P R I L 2 0 1 4 TEMPERATURE TOO HOT TEMPERATURE TOO COLD INVERTER TYPE NON-INVERTER TYPE Figure 2: Illustration of IAC operation, courtesy: www.daikin.com.au REFRIGERANT FLOW Figure 1: AC operation principle, courtesy: www.air-conditioning-and-refrigeration-guide.com 44 COMFORTABLE TEMPERATURE TIME DURING COOLING OPERATION IAC manufacturers claim that IACs are more powerful and energy-efficient, and provide uninterrupted comfort, significant savings on running costs, and are also quieter compared to the fixed-speed unit operation. IAC and MEPS ACs sold in Australia must be tested to Australian New Zealand test standard AS/NZS 3823, comply with the Minimum Energy Performance Standards (MEPS) and be registered in a government database1. The Greenhouse and Energy Minimum Standards determination (GEMS1) regulates vapour compression-type appliances (ACs and heat pumps). FORUM ZONE 3 ZONE 1 COIL REMOVAL AIR CONDITIONER HEAT FLOW LOSSES OUT SIDE 2 2 ROOM SIDE Figure 4: Cooling test power balance. ZONE 3 1 COIL REMOVAL HEATING INPUT HEATING INPUT Calorimeter test method The calorimeter provides a method for determining the capacity of an AC simultaneously on both the indoor side and the outdoor side. ZONE 4 ZONE 2 LOSSES Australian statistics2 suggest that more than 80% of the registered AC products are single-phase, non-ducted, reverse-cycle and inverter-type units. Over the years, there is a clear domestic market shift from fixed speed units towards inverter-type, primarily due to their technological advancement in operation over the fixed-speed ACs. Realistic test results are important for the consumer’s benefit and in estimating and meeting the overall energy minimum targets. 1 ZONE 1 COIL REMOVAL ZONE 4 ZONE 2 AIR CONDITIONER HEAT FLOW COIL REMOVAL 4 3 HEATING INPUT HEATING INPUT 3 1 Outer chamber − Controlled temperature air space 3 Air sampler Figure 3: Typical balanced ambient calorimeter chamber courtesy: www.gzlans.com The balanced ambient room-type calorimeter, shown in Figure 3, is based on the principle of maintaining the temperature surrounding the particular compartment equal to the temperature maintained within that compartment, providing stable room conditions, reduced losses from each room, and achieving lower uncertainties in the test results. The equipment is installed in a manner similar to a normal installation. Each room reconditioning unit (RRU) works on “automatic” controls by adjusting the active load to achieve the desired room test conditions. Two test conditions are shown in Table 1. Indoor side Outdoor side ROOM SIDE 2 Room reconditioner 4 Typical split, non-ducted air-conditioner with connected refrigerant piping Rating “Air-on” test conditions LOSSES OUT SIDE LOSSES 4 T1 Cooling H1 Heating Dry-bulb 27°C 20°C Wet-bulb 19°C 15°C Dry-bulb 35°C 7°C Wet-bulb 24°C 6°C Table 1: AS/NZS 3823 – standard test conditions Figures 4 and Figure 5 show a schematic of the balanced ambient calorimeter system and power fluxes during a capacity test. In steady-state conditions, the capacity of an AC (indoor or outdoor side) is measured as power that each chamber has to supply to the system to keep the same temperature conditions equivalent to the power removed or added by the AC in each chamber. Figure 5: Heating test power balance. Need for “locked” mode IAC tests in a calorimeter with “normal” RRU controls IACs work as fixed capacity units if their operating speed is locked. When the IAC unit is tested without locking its compressor speed, inverter controls and “normal” RRU controls work against each other. A fluctuating AC power input from a variable-speed compressor and the corresponding fluctuating capacity output may have counter-responding RRU controls. This may result in non-stable room temperature conditions. Hence, the compressor speed is locked during the testing and IAC is rated at the corresponding capacity. The current air conditioners test standards requires IAC manufacturers to fix the compressor speed and rate the units based on “locked” tests. The “locked” instructions for fixing the compressor speed are supplied by the manufacturer. During capacity tests, input power is locked and capacity is determined from the calorimeter principles as a typical fixed-speed AC test. This “locked” method of testing IAC units is found to be repeatable to within ±3%. IAC – Laboratory performance The capacity of a typical IAC in “locked” mode is directly proportional to the compressor speed, but its efficiency drops with capacity as shown in Figure 6. An IAC unit’s performance is typically labelled at a desired rated capacity, which is normally lower than the “full-load” capacity. The regulations also allow inverter units to register by complying with 95% MEPS requirements at “rated’ efficiencies and at “part-load” efficiencies, which need to be between 50–100% of rated-load capacities. In most IAC units, the unit controls appear to be optimised for higher efficiencies under test conditions with “locking” instructions. A P R I L 2 0 1 4 • E CO L I B R I U M 45 FORUM It is also difficult to conduct “part-load” performances when no “locked” instructions are available. Efficiency (kW/kW) 5.0 50% of Rated-load 4.5 Inverter Efficiency ‘Locked’ 4.0 Rated-load 3.5 Full-load 3.0 Part-load 2.5 2.0 25% 50% 75% 100% Rated Capacity 125% 150% Figure 6: Typical performance of IAC unit in “locked” or laboratory mode. CHOICE3 mentions that the current test standard method of allowing the IAC compressor speed to be fixed at a certain “rated” capacity is unrealistic. Other Issues (i) Current test standards When there are no “locked” instructions from the manufacturers, current test standards allow a deviation from the “locked” method, by testing IAC units at peak cooling and heating settings. But this method produces “full-load” capacities, which may be different to “rated-load” results. 46 Test standards define a circumvention device as any control, control device, software, component or part that alters the operating characteristics during any test procedures, resulting in measurements that do not represent an appliance’s true characteristics that may occur during normal use under comparable conditions. Generally, circumvention devices save energy during an energy test, but not during normal use. Hence, the inverter speed “locking” instructions can be inferred as a circumvention device. The standards, however, specify that any AC shall not contain any circumvention device. (ii) Verification tests Verification (check) tests are likely on registered AC products, which helps maintain the quality of registered products being sold in Australia and New Zealand. In most instances, the GEMS regulator does not have enough information on “locked” instructions. Sometimes these instructions are performed through special software/hardware, and may require authorised personnel to operate it. Obtaining information for locking the inverter speed from manufacturers prior to or during check-tests, may compromise the veracity of the check tests, delay the process and open up other complications. Currently during check tests, there are no requirements to match the compressor speed to registered value or to measure the compressor speed and report. The mycoolingtower.com.au website has been developed by AIRAH as part of a collaboration between industry groups, water corporations and the Victorian government providing best practice information and tools to help businesses operate cooling towers water and energy efficiently. E CO L I B R I U M • A P R I L 2 0 1 4 FORUM IAC unit 2 Currently, there is no accepted test method available in the public domain to test the IAC units under “field” behaviour. In this study, a new method for “unlocked” mode tests on IACs is discussed and the associated efficiency was evaluated. This study highlights the findings from various inverter products tested at Vipac using an “unlocked” mode and compares the results to the traditional ‘locked’ mode. When tested in an “unlocked” mode, the indoor test room is given an “almost” fixed thermal load, and the inverter unit under test adjusts its speed or the power input to meet the load. Because the RRU generated a “fixed” effective heating or cooling capacity, the indoor room temperatures can fluctuate depending on operation of the test IAC. The IAC test operation is adjusted with the remote controller to achieve a close match to the indoor-unit room temperatures required by the test standard. Temperature resolution on remote (usually ±1°C) could affect the stability of the final indoor room conditions. The final remote temperature setting may show a different value to the actual room condition. Outdoor room conditions are maintained at the same as the standard test conditions. For example, when 50% of the rated-load cooling experiment in “unlocked” mode was conducted using a rated 4kW IAC unit, with RRU controls first, both the indoor-unit room as well as the outdoor-unit room were set to the desired test conditions. After the temperatures of the indoor-unit room and outdoorunit room were stabilised, IAC was operated at the set room temperatures. The RRU in the indoor-unit room was locked to produce effective heating output of 2kW (50% of the rated cooling capacity) to obtain the designated 50% of rated load. The AC unit would tend to make the indoor room temperature lower or higher than the required temperature conditions. Then, the remote controller would need to be reset in order to keep the indoor room at the required condition as per the test standard. Once stable operation was obtained, the IAC unit capacity was calculated from the Calorimeter principles. IAC Unit 1 The Annual Energy Efficiency Ratios (AEER) or Annual Coefficient of Performance (ACOP) was calculated from measured capacity and electrical energy consumption including the non-operating power. MEPS compliance in accordance with AS/NZS 3823 was evaluated from the calculated AEER and ACOP values. Cooling capacity T1 test-rated load Registered value Lab test “locked” Field test “unlocked” Cooling capacity (kW) 4–10 - 6.7% - 0.1% Power input (kW) >2 + 3.7% + 25.4% >3 - 10.0% - 20.3% PASS FAIL FAIL AEER (kW/kW) Compliance to MEPS Table 2: IAC-1 cooling: “laboratory” vs “field” behaviour. Cooling capacity T1 test-rated load Registered value Cooling capacity (kW) 4–10 - 0.8% + 0.6% Power input (kW) <3 + 0.5% + 10.1% >3 - 1.0% - 8.2% PASS PASS FAIL AEER (kW/kW) Compliance to MEPS Lab test “locked” Field test “unlocked” Table 3: IAC-2 cooling: “laboratory” vs “field” behaviour. IAC unit 3 Cooling capacity T1 test-rated load Registered value Lab test “locked” Cooling capacity (kW) <4 - 0.9% + 0.6% Power input (kW) <1 - 1.8% + 56.9% >4 + 1.0% - 35.7% PASS PASS FAIL AEER (kW/kW) Compliance to MEPS Field test “unlocked” Table 4: IAC-3 cooling: “laboratory” vs “field” behaviour. Heating capacity H1 test-rated vs full load Registered value Heating capacity (kW) <4 - 0.1% + 24.4% Power input (kW) <1 - 0.7% + 70.4% >3 + 0.7% - 26.8% PASS PASS FAIL ACOP (kW / kW) Compliance to MEPS Lab test “locked” Field test “unlocked” Table 5: IAC-3 heating: “laboratory” vs “field” behaviour. Rated COOLING Test – T1 Conditions - ‘Unlocked’ Mode Indoor & Outdoor – Temperature, Relative Humidity and Power Input 100 1400 90 80 1200 70 1000 60 800 50 600 40 30 20 Power (Watts) Under current laboratory test conditions, the IAC unit is operated at a locked compressor speed (hence fixed power input), but in field conditions (i.e. the consumer’s home/office), the unit operates to match the room load. This field performance can be different to the laboratory operation. Hence the “locked rating” can mislead consumers, as the test results reflect laboratory performance rather than field performance. Temperature (ºC) and RH (%) Test method for field behaviour 400 200 10 0 8:52 a 9:07 a Indoor Drybulb 9:21 a 9:36 a Outdoor Drybulb 9:50 a 10:04 a 10:19 a Time Indoor RH Outdoor RH 10:33 a 0 10:48 a Unit power input Figure7: IAC-3 cooling: rated-load “unlocked” mode A final 21°C remote setting produced 26.7°C of room air-on temperature (27°C is required, see Table 1, T1). A P R I L 2 0 1 4 • E CO L I B R I U M 47 FORUM a matching load) and the corresponding efficiency under simulated field conditions were measured. COOLING Test – T1 Conditions - Part-load ‘Unlocked’ Mode Indoor & Outdoor – Temperature, Relative Humidity and Power Input 100 350 90 250 200 50 150 40 30 20 100 50 10 0 0 2:52 p 3:21 p 3:50 p Indoor Drybulb Outdoor Drybulb 4:19 p Indoor RH 4:48 p 5:16 p Outdoor RH 5 Unit power input A final 26°C remote setting produced 27.4°C of room air-on temperature (27°C is required, see Table 1, T1). HEATING Test – H1 Conditions – ‘Unlocked’ Mode Indoor & Outdoor – Temperature, Relative Humidity and Power Input 100 1400 90 1200 70 1000 60 800 50 600 40 30 20 400 200 10 11:02 a 11:31 a Outdoor Drybulb 12:00 p Indoor RH 12:28 p 12:57 p Outdoor RH TIme Unit power input Figure 9: IAC-3 heating: full-load “unlocked” mode. A final 23°C remote setting produced 20°C of room air-on temperature (20°C is required, see Table 1, H1). HEATING Test – H1 Conditions – Part-load ‘Unlocked’ Mode Indoor & Outdoor – Temperature, Relative Humidity and Power Input 100 700 70 60 40 30 20 0 100 2:24 p Indoor Drybulb 7:12 p Outdoor Drybulb 12:00 a Indoor RH 4:48 a Outdoor RH 0 9:36 a Time 2:24 p Unit power input Figure 10: IAC-3 heating: part-load “unlocked” mode. 25% 50% 75% 100% 125% 150% Rated Capacity Figure 11: Observed IAC performance – “locked” vs “unlocked” tests. When the compressors were varying their speed repeatedly at a certain interval, energy consumption is high compared to the “locked” mode operation. In some inverters, inefficient and faulty hot-gas bypass controls may have resulted in lower efficiencies in field behaviour. • Any calorimeter laboratory can adopt the “unlocked” test method to conduct the performance tests on IAC units for field behaviour. This can also be applied to psychometric or air-enthalpy type (AS/NZS 3823) test methods. • There is no need for “locked” instructions from the manufacturer or any “special” assistance during the tests. A final 18°C remote setting produced 19.9°C of room air-on temperature (20°C is required, see Table 1, H1). • Field performance can be obtained for any suitable thermal load and corresponding unit behaviour (power input and efficiency) in meeting that load. DISCUSSION • Use of any circumvention device in the air conditioner system can be more easily identified. Tested cooling or heating capacity, within ±1% of registered (or desired) values was obtained during the “unlocked” tests. IAC power input (which was responded by the unit to obtain 48 2 There are distinct advantages to the proposed “unlocked” mode of testing the IAC units compared to the existing “locked” mode test method: 300 9:36 a Part-load Advantages of proposed test method 200 10 Full-load 3.0 500 400 50 Rated-load 3.5 600 Power (Watts) Temperature (ºC) and RH (%) 80 Inverter Efficiency ‘Unlocked’ 4.0 Thus, the efficiencies of the IACs will fit to a lower curve relative to the “locked” mode condition. 800 90 Inverter Efficiency ‘Locked’ As displayed, at the maximum compressor speed, the full-load capacity and efficiency would be the same in either “locked” or “unlocked” modes. However, the gap was noticed to widen towards the lower capacity range. 0 0 50% of Rated-load 4.5 2.5 Power (Watts) Temperature (ºC) and RH (%) 80 Indoor Drybulb In general, IAC units used much higher electric power than the “rated” or “locked” value when subjected to a thermal load equivalent to the rated capacities under field behavior. Some units also failed to meet MEPS under “unlocked” mode operation. Time Figure 8: IAC-3 cooling: part-load “unlocked” mode. 10:33 a Some IAC units have produced fluctuating behaviour in “unlocked” tests, but they ran with a very stable power input, and provided stable room conditions under “locked” mode operation. Efficiency (kW/kW) Temperature (ºC) and RH (%) 70 60 Power (Watts) 300 80 E CO L I B R I U M • A P R I L 2 0 1 4 • Rating will show performance in field conditions, which will greatly benefit consumers. FORUM Backward curved centrifugal fans • Creative IAC units receive the credit they deserve and the poor designs would be eliminated. • Check testing can be quicker, independent and with more veracity. • The field performance results obtained with this method could be applied to check MEPS compliance ratings. • This method will encourage IAC manufacturers to focus on “real” performance rather than the laboratory performance. Suggested considerations in the current test standards and regulations • Remove the “locked” operation allowance when testing IAC units. • Adopt the proposed “unlocked” test method and account for the possible transient behaviour. • Include a method of proportioning the calculated results when the achieved indoor room temperatures are different to the required standard test conditions. • In most of the IAC units, the rated-load is lower than the fullload capacities. It is recommended to consider performance of IAC units at full-load, part-load and also at a “desired” rated-load when determining the MEPS compliance. behaviour in a laboratory environment. Correct field test rating and labelling results would benefit consumers. It is suggested that the AS/NZS Standards be revised for testing inverter units by considering the possible performance deterioration in “unlocked” conditions. ❚ REFERENCES 1) Australian/New Zealand Standards AS/ NZS 3823, Performance of electrical appliances – Airconditioners and heat pumps 2)www.energyrating.gov.au; • • • • • • • • Impeller diameter: 190 to 710 mm Max. Capacity: up to 6,500 l/s Max. Static: up to 1,800 Pa High power density with most optimized sound level 100% speed controllable EC-option available Easy installation and maintenance Requires up to 50% less input power than comparable AC fans 3)www.choice.com.au http://www.choice.com.au/reviews-andtests/household/heating-and-cooling/ home-cooling/small-air-conditionersreview-2009/page/results.aspx http://www.choice.com.au/reviews-andtests/household/heating-and-cooling/ home-cooling/large-air-conditioners2008-review/page/results.aspx 4) Air conditioner test facilities at VIPAC, Melbourne: Vipac is NATA accredited for testing various AC products, having six environmental chambers up to 540m³. The balanced ambient calorimeter is rated to 18kW. Lab circuits are of 1MW heating and 1MW cooling using recirculated water loops and dedicated glycol and DX systems to multiple chambers. www.vipac.com.au CONCLUSIONS There has been significant technological advancement in domestic AC designs over the past few decades, such as variable speed compressor operation. Current test standards allow IAC manufacturers to fix the compressor speed for a certain “rated” capacity. However, we believe rating the inverter under “locked” method is unrealistic, as it does not reflect the way inverters operate in real life. In this study, the IAC “unlocked” field testing method was discussed. Vipac testing shows that IAC units under “unlocked” conditions used much higher electric power than the “rated”’ values when subjected to a similar thermal load equivalent to the “rated” or “locked” capacities. Some units failed to meet MEPS under the new proposed test methodology. Using proposed “unlocked” test method, IAC units can now be tested for “field” About the Author Dr Satya Mavuri, M.AIRAH, is a senior testing and numerical modelling engineer at Vipac. His role includes testing, design and development of air conditioners, heat pump hot water systems, heat exchangers, numerical modelling and R&D consulting for a range of thermodynamic systems. Meet us at Stand 457 Satya has had more than 12 technical papers published in journals and for conferences. Email: [email protected] A P R I L 2 0 1 4 • E CO L I B R I U M 49
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