Testing inverter type air conditioners  for field performance ABSTRACT F O R U M

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: satyam@vipac.com.au
A P R I L 2 0 1 4 • E CO L I B R I U M
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