Compare Energy Use in Variable
Refrigerant Flow Heat Pumps Field
Demonstration and Computer Model
Chandan Sharma
Richard Raustad
Member ASHRAE
Member ASHRAE
ABSTRACT
Variable Refrigerant Flow (VRF) heat pumps are often regarded as energy efficient air-conditioning systems which offer electricity savings
as well as reduction in peak electric demand while providing improved individual zone setpoint control. One of the key advantages of VRF
systems is minimal duct losses which provide significant reduction in energy use and duct space. However, there is limited data available to show
their actual performance in the field. Since VRF systems are increasingly gaining market share in the US, it is highly desirable to have more
actual field performance data of these systems. An effort was made in this direction to monitor VRF system performance over an extended period
of time in a US national lab test facility.
Due to increasing demand by the energy modeling community, an empirical model to simulate VRF systems was implemented in the
building simulation program EnergyPlus. This paper presents the comparison of energy consumption as measured in the national lab and as
predicted by the program. For increased accuracy in the comparison, a customized weather file was created by using measured outdoor temperature
and relative humidity at the test facility. Other inputs to the model included building construction, VRF system model based on lab measured
performance, occupancy of the building, lighting/plug loads, and thermostat set-points etc. Infiltration model inputs were adjusted in the beginning
to tune the computer model and then subsequent field measurements were compared to the simulation results.
Differences between the computer model results and actual field measurements are discussed. The computer generated VRF performance
closely resembled the field measurements.
INTRODUCTION
Due to increasing emphasis on energy efficient operation of the buildings, various HVAC technologies are emerging
in the market. Variable refrigerant flow (VRF) system is one such technology which is an enhanced version of ductless
multi split air-conditioning system. Some of the key advantages offered by VRF systems are simultaneous heating and
cooling operation in heat recovery mode, minimize duct space, and improved zone setpoint control etc. VRF system
technology, benefits and market acceptance issues are discussed in [1]. While the VRF systems were increasingly gaining
market share in the US, a dynamic simulation tool to simulate their performance was missing. Several simplified VRF
models were developed initially [2-4]. Due to increasing demand by the energy modeling community a more detailed VRF
heat pump model was implemented in EnergyPlus [5]. This current paper focuses on validating this detailed VRF model
with field measured data. A customized weather file was created to facilitate fair comparison between measured and
simulated data. Also, building construction, lighting/plug loads of space, and thermostat set-points etc were fed to the
Chandan Sharma is Associate Research Engineer at the Florida Solar Energy Center / University of Central Florida. Cocoa, FL. Richard
Raustad is Senior Research Engineer at the Florida Solar Energy Center / University of Central Florida. Cocoa, FL.
model for increased accuracy. The results of the comparison of simulation outputs and measured data are discussed.
VRF SYSTEM
The specifications of the installed VRF system are shown in Table 1. The table contains system parameters as
obtained from the manufacturer’s catalog data and also as measured in the lab. Measured system parameters are shown in
parenthesis. For the simulation study of the installed VRF system, lab measured parameters are used.
Table 1. VRF Heat Pump System Specification at EPRI Test Facility
System Parameter
Nominal Cooling Capacity
Nominal Cooling Power Input
Cooling COP
Nominal Heating Capacity
Nominal Heating Power Input
Heating COP
Minimum Outdoor Temperature in Cooling Mode
Maximum Outdoor Temperature in Cooling Mode
Minimum Outdoor Temperature in Heating Mode
Maximum Outdoor Temperature in Heating Mode
Terminal Unit Rated Total Cooling Capacity
Terminal Unit cooling SHR
Terminal Unit Rated Total Heating Capacity
Terminal Unit Rated Air Flow rate (Cooling/Heating)
Description
21.1 (18.47) kW [72 (63) kBTU/hr]
5.55 (6.71) kW [18.9 (22.9) kBTU/hr]
3.8 (2.75) W/W [13 (9.4) BTU/W-hr]
23.4 ( 25.39) kW [79.8 (86.6) kBTU/hr]
6.04 (6.47) kW [20.6 (22) kBTU/hr]
3.87 (3.92) W/W [13.2 (13.4) BTU/W-hr]
-5 °C [23 °F]
43 °C [109.4 °F]
-21 °C [-5.8 °F]
35 °C [95 °F]
6000.00 W [20.47 kBTU/hr]
0.79866
6782.00 W [23.13 kBTU/hr]
0.333 (m3/s) [705 CFM]
BUILDING AND TEST CONDITIONS
Geometry of the building is shown in Figure 1 below. The building consists of office space, workout room, cubicles,
lab and warehouse. VRF system indoor units are installed in the lab and warehouse part of the building. The remaining
portions of the buildings were conditioned by separate HVAC systems. The lab is divided into zones 1 and 2 based on
terminal units 1 and 2 and warehouse is divided into zones 3 and 4 based on terminal unit 3 and 4. Zones 1 through 4 are
separated by the remaining space of the building through adiabatic walls.
The thermo-physical properties of the construction materials of the building are shown in Table-2 below.
Table 2. Thermo-Physical Properties of the Construction Material of the Building
Roof
Wall
Floor
Layers (outer to inner)
Thickness(m)
[ft]
Conductivity
(W/m.K)
[BTU/hr/ft/F]
Density
(kg/m3)
[lb/ft3]
Specificheat(J/kg.K)
[BTU/lb/F]
Roof membrane
Metal decking
Roof indulation
0.0095[0.031]
0.0015[0.005]
0.30 [0.984]
0.0016 [0.0009]
45.006 [26]
0.090 [0.05]
1121.29 [70]
7680.00[480]
265.00 [16.5]
1460.00 [0.35]
418.40 [0.1]
836.80 [0.2]
Filled concrete blocks #1
Filled concrete blocks #2
Filled concrete blocks #3
0.079 [0.26]
0.127 [0.42]
0.089 [0.29]
1.378 [0.8]
1.065 [0.615]
1.437 [0.83]
1731.45[108]
1031.57 [64]
1647.86[103]
838 [0.2]
419 [0.1]
838 [0.2]
High weight concrete
0.1016 [0.33]
1.31 [0.75]
2243.0 [140]
837.0 [0.2]
Figure 1
Multizone building for the VRF field validation.
EPRI Field Data
EPRI has monitored the following variable at the test site.
1)
Outdoor temperature and relative humidity
2)
Return air temperature and relative humidity at indoor units
3)
Supply air temperature and relative humidity at indoor units
4)
Indoor unit power and energy consumption
5)
Outdoor unit (compressor and condenser fan) power and energy consumption
Internal Gain
Table 3. Internal Gain
Zones
Floor Area (m2)
[ft2]
No. Of People
Lighting
(W/m2)
[BTU/hr/ft2]
Zone 1
Zone 2
Zone 3
Zone 4
91.7 [987]
81.3 [875]
97.6 [1050]
97.6 [1050]
1
1
2
2
8.6 [2.72]
8.6 [2.72]
3.8 [1.2]
3.8 [1.2]
METHODOLOGY
Electric
Equipment
(W/m2)
[BTU/hr/ft2]
17.16 [5.44]
17.16 [5.44]
25 [7.92]
55 [17.43]
Developing model:
Performance curve of the VRF system installed at EPRI test facility were derived using manufacturer’s catalog data.
These performance curves along with lab measured rated conditions were used to model VRF system in the modeling
program. Using the construction drawings of the building, EPRI test site model was created in the modeling program. VRF
system installed at EPRI had four terminal units serving two different zones. The building model in the model was divided
into four fictitious zones each served by one terminal unit. Other required input parameters such as occupancy of the
building, lighting/plug loads, and thermostat set-points were input to the model. In the absence of measured solar
irradiance, and wind speed/direction data, TMY data at Knoxville, TN is used in the simulation weather file. A custom
weather file was then created using lab measured outdoor dry-bulb temperature, relative humidity and TMY data. This was
done to facilitate better approximation of outdoor environments of the building in the simulation model. In the absence of
measured wind speed and direction data, infiltration at the EPRI test site was taken from modeling program’s reference file.
The initial infiltration value was adjusted once to tune the computer model and then subsequent field measurements were
compared with the simulation results.
Simulation and Validation
After the model was developed, detailed simulations were run and the outputs were compared against field data.
Measured total indoor and outdoor unit energy was compared with total predicted energy (VRF heat pump cooling electric
consumption rate + Terminal unit fan energy + Terminal unit parasitic cooling electric consumption rate). Predicted VRF
heat pump cooling electric consumption rate includes electricity used by the compressor, crankcase heater, and the
condenser fan. Predicted parasitic cooling electric consumption includes electricity used by the zone terminal unit’s
transformers, controls, or other electricity consuming devices. As discussed in the Results section below, the measured and
predicted energy is found to be in good agreements.
RESULTS
Figure 2 illustrates the total energy consumption comparison between measured and predicted data. Starting from
15th September 2012 onwards, units1 and 2 were turned off during field testing. The figure also shows average daily zone
return air temperature of zones 3 and 4 and average daily outdoor temperature. From Figure 2 it is clear that zone
temperatures are mostly maintained at set-point temperature of 72 F (22.22 C) throughout the testing period. Also, the total
energy consumption profile for both measured and predicted data follows daily average outdoor temperature profile.
Figure 2
Daily energy (field data and simulation output).
The variation of daily energy consumption with respect to temperature difference between zone return air and
outdoor air is shown in Figure 3 below. As the temperature difference decreases, the energy consumption also decreases up
to a point and then increases. Both measured and predicted data follow similar profile though predicted data diverges from
measured data at very low ambient temperature.
Figure 3 also shows daily energy comparison relation between measured and predicted data. As can be seen from the
graph, measured and predicted data are mostly in good agreement (±25%) with some discrepancies at low energy
consumption. This discrepancy can be attributed to divergence of measured and predicted energy in the heating season.
Total monthly energy consumption of VRF heat pump system installed at EPRI test facility was compared as shown
below. As the Figure 4 suggests, total monthly energy consumption decreases as the heating season approaches. Also,
predicted and measured total monthly energy in the cooling season are within 3% and for the heating season the differences
increase up to 13% for October, 26% for November and 30% for December. Histogram in Figure 4 shows the distribution
of the percent error between predicted and measured daily energy consumption. The figure shows that 72% errors are
within ±25% and 79% errors are within ±35%.
Figure 3
Daily Energy Comparison (DeltaT = outdoor air temp – zone return air temp).
Figure 4
Total monthly energy comparison (field data and simulation output) and Total energy error distribution.
Statistical Analysis
In order to evaluate consistency and dependency of measured and simulated data, sample correlation coefficient (r) is
determined as follows:
n
(Xs
r
X s )( X m
Xm)
i 1
n
n
(Xs
X s )2
(Xm
X m )2
(1)
Calculated correlation coefficient (0.92) is presented in table 4 and reflects high correlation. The correlation
hypotheses are verified through a t-test with significance level (α) of 5%. The hypotheses of correlation coefficient are
accepted.
i 1
i 1
Table 4. Sample Correlation of Measured and Correlated Data
Item
Sample correlation coefficient (r)
Cv (RMSE)
Sample size
Total Energy
0.92
21%
153
Coefficient of variation of root mean square error Cv (RMSE) between measured and simulated data is:
n
(Xs
X m )2
i 1
Cv ( RMSE )
n
Xs
(2)
Cv (RMSE) is a normalized measure of the variability of root mean square error between measured and simulated total
energy. In this case, Cv (RMSE) is calculated as 21 % which is a reasonable variability between measured and simulated
data.
CONCLUSION
In this study, Variable Refrigerant Flow Heat Pump model was validated against field data measured at EPRI lab
facility. For the comparison of the measured data and simulation outputs, EPRI building model was developed in the
modling program using the construction drawings. Other inputs to the model contained VRF system model based on lab
measured performance, occupancy of the building, lighting/plug loads, and thermostat set-points etc. In the absence of
measured wind speed and direction at the test facility, the infiltration data is taken from the modeling program’s reference
files. This infiltration data were adjusted once to tune the computer model and then subsequent field measurements were
compared to the simulation results. For increased accuracy in the comparison, a customized weather file was created using
measured outdoor temperature and relative humidity data. The findings of the measured and simulation outputs can be
summarized as follows:
1) About 72% of all the simulated total energy fall within ±25% of the measured data of total energy and about 79%
of all simulated total energy fall within ±35% of the measured total energy.
2) Sample correlation coefficient (r) between measured and simulated energy is around 0.92 which reflects high
correlation. Coefficient of correlation is verified and the hypotheses accepted through t-test.
3) Variability of normalized room mean square error, Cv (RMSE), is around 21% which shows measured and
simulated data have small variability.
Due to inadequacy of some of the important input parameters for the simulation such as internal loads, infiltration
etc., it is highly recommended that further field tests of VRF heat pumps should be conducted. Future field tests should
focus on to obtain information regarding internal loads of the test facility, and wind speed and direction to have better
approximation of infiltration etc. These informations are expected to increase the accuracy of the simulation.
ACKNOWLEDGMENTS
Author would like to thank Dr. Bereket Nigusse from Florida Solar Energy Center and Harshal Upadhye and Ron
Domitrovic from Electrical Power Research Institiute for their kind assistance.
NOMENCLATURE
r = Sample correlation coefficient
Cv(RMSE)= Coefficient of variation of the root-mean-square error
n = Sample size
Subscripts
s
m
i
=
=
=
simulation variable
measured variable
index
REFERENCES
Goetzler, W. 2007. Variable Refrigerant Flow Systems. ASHRAE Journal, pp. 24.
Zhou, Y.P, Wu, J.Y., Wang, R. Z., and Shiochi, S. 2007, Energy simulation in the variable refrigerant flow air-conditioning
system under cooling conditions, Energy and Buildings, 39 (2007) 212-220
Zhou, Y.P, Wu, J.Y., Wang, R. Z., and Shiochi, S. 2008, Simulation and experimental validation of the variable-refrigerantvolume (VRV) air-conditioning system in EnergyPlus, Energy and Buildings, 40 (2008) 1041-1047
Li, Y., Wu, J., and Shiochi, S. 2009, Modeling and energy simulation of the variable refrigerant flow air conditioning system
with water-cooled condenser under cooling conditions, Energy and Buildings, 41 (2009) 949-957
Raustad, R., 2012, A Variable Refrigerant Flow Heat Pump Computer Model in Energyplus, ASHRAE TRNS-001392011.R5
ACKNOWLEDGMENTS
This material is based upon work supported by the Department of Energy under Award Numbers DE-EE0003848.
Disclaimer: "This report was prepared as an account of work sponsored by an agency of the United States
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