MAE 276 Project Report
Real Time COP Measurement of A Rooftop Air Conditioner Unit
Team Pre-Cool : Nelson Dichter, Zhijiun Liu, Ian Sit
University of California, Davis
March 12th, 2011
INTRODUCTION
With the advent of modern technology and its subsequent cost reduction, it has become
feasible for the facilities industry to push the boundaries of building performance and efficiency. This
has become even more apparent in an age where energy resources are being understood as finite and
increasingly expensive. A major area of focus for facilities engineers is the heating, ventilation, and air
conditioning (HVAC) of their campuses; which can be considered one of the highest overhead costs
during a building’s lifetime. Research into this area has seen the development of better insulation
materials and techniques, variable frequency drives for equipment, and even global computer systems
to monitor and control climate operations for individual rooms. A recent HVAC interest involves the use
of evaporative precoolers in conjunction with air conditioning units to improve their efficiencies. An
example of these pre-coolers is shown in Figure 1.
Figure 1: Air conditioner with evaporative precooler.
Evaporative precoolers utilize atomized water droplets to be evaporated in an incoming air
stream. The evaporated water removes heat energy from this air which is then blown over the
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condenser of an air conditioning unit. Through increased finite temperature difference, more energy
should be removed from the refrigeration cycle without having to have added any additional work. This,
however, comes at the expense of the energy required to run the precooler, maintenance and water.
Additionally, there are concerns in industry that operating the condenser in a high humid environment
may damage the condenser coil over time and thus have an overall deteriorative effect on performance.
So it becomes a matter of concern as to whether or not these evaporative precoolers merit the
additional complications associated with them and thus creates the motivation for this project. The
Western Cooling Efficiency Center (WCEC) has undertaken the task of studying the effectiveness of
these precoolers, both theoretically and experimentally. As part of their experimental process, it will be
required to measure the performance of the air conditioning unit being enhanced by the evaporative
precooler. Such is the main objective of this project. The Coefficient of Performance will be used as the
measurement standard for the air conditioning unit and is given by Equation 1.
𝐶𝑂𝑃 = 𝜂
𝑊̇𝑐𝑜𝑚𝑝 (ℎ4 − ℎ2 )
(𝑊̇𝑐𝑜𝑚𝑝 + 𝑊̇𝑓𝑎𝑛 )(ℎ1 − ℎ4 )
Equation 1: Coefficient of Performance
Equation 1 gives the dimensionless Coefficient of Performance where 𝜂 is the rated efficiency of
the compressor, which is assumed as 0.95. 𝑊̇𝑐𝑜𝑚𝑝 is the compressor work, 𝑊̇𝑓𝑎𝑛 is the fan work, and
is the enthalpy of the given refrigeration point. The equation for Coefficient of Performance was
determined form the thermodynamic analysis of the refrigeration cycle as shown in Figure 2.
Figure 2: Refrigeration cycle.
The change in performance resulting from the implementation of an evaporative precooler can
be determined by monitoring the change in COP of the unit, using the COP of the refrigeration cycle
before installation of the evaporative precooler as a baseline reference.
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APPROACH
The rooftop air conditioning unit (RTU) located at the WCEC shown in figure 3 will be used as the
test bed for an evaporative precooler.
Figure 3: Rooftop air conditioning unit used for COP baseline measurements.
To determine the COP, it will be necessary to measure and report the power consumption and
pressure and temperature of the refrigerant in the RTU’s refrigeration cycle at various critical locations.
The thermal efficiency of the compressor will be taken from the specifications supplied by the
manufacturer. A National Instruments (NI) LabVIEW data acquisition system will be used to measure
power consumption of the RTU’s fan and compressor directly and the enthalpies will be measured
indirectly. From Figure 2, it can be seen that in addition to the power consumption of the fan and
compressor, there are three points of measurement in the refrigeration cycle. At each point (1, 2, and
4), a pressure and temperature measurement will be taken so that the enthalpy can be determined. The
pressure measurement at point 2 of figure 2 can be eliminated since the flow through the condenser is
an isobaric process and thus the pressure leaving the condenser is equal to the pressure at the outlet of
the compressor. This makes for 7 individual measurements to be taken by the data acquisition system
and they are listed as follows:







Compressor Outlet Temperature (T1)
Compressor Outlet Pressure (P1)
Condenser Outlet Temperature (T2)
Compressor Inlet Temperature (T4)
Compressor Inlet Pressure (P4)
Fan Workload (𝑊̇𝑓𝑎𝑛 )
Compressor Workload (𝑊̇𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 )
Each point of measurement will require its own measurement device with either analog, digital,
or pulse outputs to be acquired by the LabVIEW program. For this project, temperature measurements
will be measured by resistance temperature detectors (RTD’s), pressure will be measured using pressure
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transducers, and power consumption will be measured using current transducers and voltage taps. Each
of these devices will then be monitored by their respective NI modules. These NI modules are designed
specifically to be used with each device and its type of output. Each of these modules is then attached to
an NI USB chassis which acts as a hub for all measurement inputs to the LabVIEW computer. A detailed
list of equipment and simplified illustration of the data acquisition system can be found in Table 1 and
Figure 4 below:
Device:
Description:
Omega PR-20 RTD Probes
Used for temperature measurements.
Climacheck 35 Bar Pressure Transducers Used for pressure measurements.
CCS Wattnode Pulses
Used for power measurements.
NI 9401 C Series Module
Used for digital i/o (Wattnode).
NI 9217 C Series Module
Used for analog RTD measurements.
NI 9203 C Series Module
Used for ±20 mA current input (PT).
NI cDAQ-9174 USB Chassis
USB Chassis for C Series Modules.
Table 1: Measurement and data acquisition devices.
Computer (LabVIEW)
NI cDAQ-9174
RTD 1
PT 1
RTD 2
WattNode
(Fan)
WattNode
(Compressor)
PT 4
RTD 4
Figure 4. Schematic diagram of experiment system layout
Installation and setup of the devices and data acquisition equipment listed above is illustrated in Figure
5 below. Due to the humid environment in which the instruments must operate for an extended period
of time each measurement device was carefully insulated or enclosed in a water tight space to protect
them from water damage.
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(a)
(b)
Figure 5: Data acquisition system with (a) RTU measurement device and (b) acquisition i/o modules.
The LabVIEW program used for the data acquisition in this project was developed to run
continuously on a repeated cycle. The program allows the user to set the sampling period for the
output data and the sampling rate for the temperature and pressure measurements. Temperature and
pressure measurements are averaged over the period specified by the user and a single mean is
recorded at the end of each period. Pulse measurements taken from the WattNodes accumulate during
the sampling period and the total value is recorded at the end of each period to determine total power
during the period.
The front panel and block diagrams of the LabVIEW program can be found in Appendices A and
B, respectively. Acquisition of the temperature measurement was a straightforward measurement of
analog voltage signals using an RTD vi included with LabvVIEW. The vi incorporated conversion factors
to convert the voltage to a temperature based on the type of RTD used. The pressure transducers
operate by outputting a current that is proportional to the measured pressure. A simple multiplying
constant was used to convert the current to pressure (kPa). Once the sample period specified by the
user had elapsed the pressure and temperature measurements were averaged and this result was
recorded to an excel spreadsheet for post processing. The block diagram for this portion of the VI can
be seen in Figure 6 below:
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Figure 6: Pressure and RTD signal acquisition.
The WattNode operates by measuring the energy consumption using current transducers and voltage
taps and outputting a pulse to the data acquisition device each time that a specific amount of energy
consumption was recorded. The LabVIEW program interfaced with the counters built into the USB
Chassis to count the pulses from each WattNode. To utilize these counters in the LabVIEW program it
was necessary to programmatically reserve the pins within the actual c-series module; otherwise, as a
default, LabVIEW would lockout the entire module for singular tasks and not allow multiple channel
samples to be taken at once. For this application the counters require an external clock. As a result one
additional counter was used to generate the clock signal required by the other two counters. Thus, the
resulting system uses three of the four counters available on the chassis. To synchronize the entire
program this clock was also used for the pressure and temperature measurement tasks. A simple
multiplier was used to convert the pulses to energy consumed. The energy consumed was divided by
the sample period to obtain power consumption. At the end of each sampling period the total power
consumed by the fan and the compressor was recorded to the excel spreadsheet. The block diagram for
this portion of the VI can be seen in Figure 7 below:
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Figure 7: Counter signal acquisition.
To test the program a set of data was recorded. The cycle time for each measurement during data
acquisition was set to 300 seconds (or 5 minutes). This was due to the frequency by which the
WattNode would pulse (~0.2 Hz) during each measurement. Issues concerning this low frequency will be
discussed later, but this did result in large sampling times. Anything lower than 300 seconds and the
pulse counts would be less representative of the powers they were measuring. Following the
completion of the data acquisition, the recorded data would be post-processed and the enthalpies were
determined for points 1, 2, and 4. Using the enthalpies, Power draw, and rated efficiency required by
Equation 1, the COP was calculated and organized for interpretation.
RESULTS
The results of real time COP monitoring are presented in figure 8. The data is sampled at 10,000 Hz and
averaged in every 5 minutes. The diagram shows the COP values fluctuate with time, rising from 5.5 to
7.3 at the beginning of 40 minutes, while dropping down until to be stable at 60 minutes and hereafter,
with an average value of 6.03. Detailed monitoring data and calculation is illustrated in appendix C.
Possible reasons for the variation of COP might include,



Transit operation of the refrigeration cycle. Given that the measurement started immediately
after the roof top unit was turned on, hence the refrigeration cycle would take some time to be
stable from transition period to steady state. This is the most possible reason.
Variation of the outdoor weather conditions. The fluctuation of COP might attribute to the
variation of the weather conditions, especially the outdoor air temperature. Since the elapsed
time was short (less than two hours), this contribution might not be significant.
Delayed response of the sensors, especially the delayed RTD temperature reading due to transit
heat transfer process. This is a possible reason given we attached the RTD sensors on the
outside skin of the refrigerant lines.
With consideration of the fact that the data was sampled immediately after the rooftop was
operated, the fluctuation of this measured COP is a consequence of transit refrigeration cycle.
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8.00
7.00
6.00
COP
5.00
4.00
3.00
2.00
1.00
0.00
0
20
40
60
80
100
120
Elapsed Time(min)
Figure 8: COP measurement results.
DISCUSSION
The data acquisition hardware implemented in this project were chosen for robustness. All
National Instruments components far exceeded the requirements for measuring the COP of the
refrigeration cycle. This was done because each item was purchased by the WCEC and although they
were intended for this project, they will be used in other experiments in the near future that may have
equipment specification requirements that greatly exceed those that were necessary for this project.
Unlike the data acquisition equipment, the sensors were chosen specifically for this project. The
pressure transducers were chosen because they feature threaded ports designed specifically to mate
with the valves on the refrigerant tubing. The RTDs were chosen over thermocouples because although
they have a slower response they are more accurate than thermocouples. The response time of the
temperature transducers is not critical because the goal of the project is to take steady state
measurements over time intervals large enough that the difference in response times between
thermocouples and RTDs is inconsequential. The WattNodes were chosen because they simplify the
power measurement by internally calculating and outputting the energy consumption as a function of
the measured current and voltage. The WattNode was the only sensor that was not ideal for this
project. The range of the WattNode is 0 to 270 kW, which far exceeds the 1 kW that compressor under
investigation is capable of. Due to this mismatch, the resolution of the WattNode is very poor over the
effective range. However, while a power meter that more closely matched what is being measured
would be ideal, other power meters that were a closer match are several times more expensive.
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Additionally, the low resolution proved to not be an issue because of the long sample periods that are to
be used throughout the course of this experiment.
The results obtained in the test sample indicate that the COP monitoring system is functioning
properly. The magnitude of the COP is slightly higher than would be expected for normal operating
conditions however, due to the unusual meteorological conditions that the unit was tested in (the end
of winter in California climate zone 12), above average efficiency is to be expected. The fluctuations in
the measured COP are all well within the acceptable range and are most likely due to transient effects in
the refrigeration cycle and the uncontrolled environment (outside) in which the unit was tested.
The most challenging obstacle encountered in the execution of this experiment was the
implementation of the counters in the LabVIEW program for data acquisition. Rather than singular task
sampling, this experiment involves multi-task data acquisition, including simultaneous measurement of
pressure, temperature and power. Additionally, analog signals (temperature and pressure) and digital
pulse signals (reading from Watt Nodes) were sampled at the same time, causing a problem for
synchronous sampling. These complications impeded our progress for several days. In addition, the
LabVIEW program would, by default, lockout the entire module for singular tasks and not allow multiple
channel samples to be taken at once. Therefore, to utilize these counters in the LabVIEW program it was
necessary to programmatically reserve the pins within the actual c-series module. In addition to the
counters used to count each WattNode, another counter was required to generate an “external” clock
for the other two counters. Using this clock source for the WattNode counters caused the counters to
be out of synch with the other measurement tasks in the program. To solve this problem we used the
clock signal generated by the third counter to clock all the other measurements in the program.
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COLOPHON
The results obtained in the experimental test of the data acquisition software and hardware
demonstrates the successful execution of this team’s project. All instrumentation is installed and
operating correctly and the developed LabVIEW program provides an efficient and intuitive user
interface for the long term monitoring of the refrigeration cycle.
When considering the course objective outlined for MAE 276, the execution of this project served as a
very decent platform for their achievement. The entire data acquisition system had been built and
constructed by the project team members. This included the installation of the LabVIEW software and
hardware, measurement devices, and signaling wires for communication. This also included the
culmination of literature review and theory for the project to return quality data in a useable manner. In
addition to the installation and research activities associated with this project, it was also necessary for
the research team to program and troubleshoot those programs. This all served to reinforce the handson and technical understanding as outlined by the course objectives. Through this project, its team
members successfully carried out a project from theoretical conception to physical execution while
solving the inevitable interfacing problems along the way. Few other assignments could have reinforced
the course objectives more effectively.
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Appendix A:
Front panel of designed LabVIEW program
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Appendix B:
Back panel of designed LabVIEW program
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Appendix C:
Worksheet for COP calculation
Measured Data
Calculated Data
t
Wcomp
Wfan
P1=P2
P4
T1 ( C)
T2
T4 ( C)
η
h1
h2
h4
COP
(min)
(kW)
(kW)
(Mpa)
(Mpa)
( °C)
( °C)
( °C)
(Const.)
(kJ/kg)
(kJ/kg)
(kJ/kg)
/
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
0.63
0.63
0.63
0.63
0.63
0.63
0.63
0.63
0.63
0.63
0.63
0.61
0.61
0.61
0.60
0.60
0.61
0.61
0.61
0.61
0.62
0.61
0.61
0.16
0.18
0.16
0.16
0.18
0.16
0.16
0.18
0.16
0.16
0.18
0.16
0.16
0.16
0.16
0.16
0.16
0.16
0.16
0.17
0.17
0.17
0.17
1169.2
1153.0
1200.4
1232.0
1233.6
1197.4
1161.6
1168.3
1163.1
1174.4
1159.1
1171.6
1188.0
1177.4
1193.9
1164.5
1150.2
1182.1
1140.0
1126.8
1135.1
1135.9
1149.8
240.5
197.1
176.1
156.5
152.1
150.7
142.7
152.6
158.7
160.6
159.7
158.3
160.0
156.8
138.0
143.5
142.3
138.6
140.6
140.5
141.5
131.0
130.2
43.0
42.2
43.2
43.5
44.7
45.4
45.4
46.0
45.7
46.9
46.4
49.4
49.9
50.6
51.0
50.5
50.5
51.0
50.3
49.9
50.0
50.1
50.4
20.3
19.4
20.3
20.6
20.7
20.2
19.5
19.4
19.6
19.6
19.5
19.8
19.9
19.7
19.9
19.2
19.0
19.6
18.8
18.4
18.6
18.6
19.0
4.6
3.6
3.5
6.0
7.4
8.3
9.4
10.3
11.1
11.4
11.5
11.3
11.4
11.3
10.9
10.8
11.1
10.8
10.9
10.5
10.6
10.6
10.4
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
398.64
398.06
398.31
398.06
399.28
400.76
401.35
401.85
401.62
402.71
402.43
405.43
405.71
406.64
406.88
406.73
407.04
407.08
406.96
406.69
406.74
406.77
406.91
229.21
227.83
229.20
229.59
229.80
228.96
228.03
227.89
228.18
228.10
228.03
228.35
228.62
228.29
228.54
227.57
227.18
228.06
226.93
226.37
226.70
226.64
227.21
378.21
378.14
378.46
381.04
382.28
383.10
384.18
384.80
385.39
385.65
385.74
385.55
385.60
385.58
385.55
385.42
385.64
385.52
385.53
385.23
385.26
385.45
385.32
5.49
5.58
5.66
6.70
6.63
6.57
6.85
6.80
7.29
6.95
6.98
5.92
5.84
5.60
5.49
5.53
5.54
5.46
5.54
5.53
5.53
5.56
5.47
Note: The table symbols is consistent with equation 1.
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