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Part-Load Performance Characterization and Energy Savings Potential of the RTU Challenge Unit: Daikin Rebel

About: The article was published on 2013-09-30 and is currently open access. It has received 3 citations till now.

Summary (4 min read)

1 INTRODUCTION

  • Daikin’s Rebel was the first rooftop unit system recognized by DOE in May 2012 as meeting the RTU Challenge specifications.
  • Funded by the DOE BTO, Pacific Northwest National Laboratory (PNNL) coordinated the laboratory test of a 10-ton Rebel air-conditioning unit with gas heat.
  • The testing was done at Intertek labs (Cortland, NY) in July, 2012.
  • Then, these curves were used in EnergyPlus simulation programs to model the Rebel unit’s performance.

2 PERFORMANCE CURVE DEVELOPMENT FOR DAIKIN REBEL

  • The Daikin Rebel packaged rooftop system was the first unit to meet DOE’s RTU Challenge specification.
  • These three flow fractions are combined with three outdoor dry-bulb temperatures (65°F, 90°F, and 110°F) and two indoor wet-bulb temperatures (61°F and 67°F), leading to a total of 18 steady-state tests.
  • The average values are then used in development of the performance curves.
  • The Rebel unit modulates the fan speed in response to the space thermal loads.
  • In addition, the compressor modulates its speed to control the discharge air temperature.

2.1 DELIVERED COOLING MODIFIER CURVE AS A FUNCTION OF TEMPERATURE

  • As Equation 1 shows, this curve has three independent variables: wet-bulb temperature of the air entering the cooling coil (𝑇𝑤𝑏,𝑖), dry-bulb temperature of the air entering the cooling coil (𝑇𝑑𝑏,𝑖), and discharge-air dry-bulb temperature (𝑇𝑑𝑏,𝑜).
  • The output of this curve indicates the ratio of the total delivered cooling at the specific operating conditions (𝑇𝑤𝑏,𝑖, 𝑇𝑑𝑏,𝑖, 𝑇𝑑𝑏,𝑜, and 𝑓𝑓 = 1 full flow) and the total maximum cooling capacity at the rated conditions (𝑇𝑤𝑏,𝑖 = 67°𝐹, 𝑇𝑑𝑏,𝑖 = 80°𝐹, 𝑇𝑐,𝑖 = 95°𝐹, and 𝑓𝑓 = 1).
  • The regression equation fits the experimental data very well, with 𝑅2 close to 1 .

2.2 ENERGY INPUT RATIO (EIR) MODIFIER CURVE AS A FUNCTION OF TEMPERATURE

  • This curve has four independent variables: wet-bulb temperature of the air entering the cooling coil (𝑇𝑤𝑏,𝑖), dry-bulb temperature of the air entering the cooling coil (𝑇𝑑𝑏,𝑖), dry-bulb temperature of the air entering the air-cooled condenser coil (𝑇𝑐,𝑖), and discharge air dry-bulb temperature (𝑇𝑑𝑏,𝑜).
  • The regression equation fits the experimental data very well, with 𝑅2 = 0.96 .

2.3 DELIVERED COOLING MODIFIER CURVE AS A FUNCTION OF FLOW FRACTION

  • As Equation 3 shows, a cubic curve is used to indicate how the delivered cooling varies with the air flow fraction (𝑓𝑓).
  • The output of this curve indicates the ratio of the total delivered cooling at the specific air flow fraction and the total cooling capacity at the rated air flow rate (𝑓𝑓 = 1).
  • The regression equation fits the experimental data very well, with 𝑅2 close to 1 .

2.4 ENERGY INPUT RATIO (EIR) MODIFIER CURVE AS A FUNCTION OF FLOW FRACTION

  • A cubic curve is also used to indicate how the EIR varies with the air flow fraction (𝑓𝑓).
  • It is not clear what factors triggered the large deviations for those tests.
  • Table 4 shows the regression coefficients of Equation 4, after removing the above four outliers from tests C1-C18 in Table A-3.
  • The following items need to be noted when using the regressed curves: The Celsius unit is selected because a major motivation of this work is to generate performance curves for EnergyPlus, which uses SI units as default.

3 BUILDING SIMULATION MODELS

  • Figure 6 shows the axonometric view of the big-box retail store modeled in this work.
  • ASHRAE Standard 90.1-2004 is followed to establish the requirements on building envelope thermal performance.
  • The report (Thornton et al. 2011) can be referred to for more details on envelope construction and internal load profiles.
  • In the EnergyPlus building model, each of the five spaces is regarded as one thermal zone.

3.1 SELECTION OF THE PACKAGED EQUIPMENT

  • In general, RTU Challenge-compliant units such as Rebel are considered by building owners/designers as an option for either new construction or existing building retrofits.
  • Other options include RTUs that just meet the Federal minimum standard or other local code 19 requirements, such as ASHRAE Standard 90.1, or high-end RTUs that are usually available from most manufacturers’ product lines.
  • Three “references” are defined here to make the performance comparison more useful.

3.2 REFERENCE 1

  • The AHRI-rated (American Heating and Refrigeration Institute) efficiency of the baseline or Reference 1 equipment performance is based on ASHRAE Standard 90.1-2004 requirement.
  • Therefore, it is necessary to obtain the cooling performance excluding the supply-fan power.
  • Therefore, it is important to apply a degradation factor to consider the actual field performance for existing RTUs.
  • Table 5 lists the curve coefficients used by the Reference 1 model.
  • DCV is not Rebel’s unique feature distinguishable from conventional RTUs.

3.3 REFERENCE 2

  • Reference 2 represents RTUs in the market that just meet the current (2013) Federal regulations for commercial equipment standards.
  • So, it can be used as the baseline to estimate the potential of energy savings from Rebel units in comparison with new RTUs that meet the minimum efficiency requirements.
  • Except for the rated full-load efficiency, RTUs compatible with Reference 2 are the same as Reference 1 and they have the following features: Current Federal minimum standard requires that a 10-ton packaged air conditioner with gas heat have a minimum EER of 11 at rated conditions.
  • Based on Equation 1 and using the overestimation factor of 1.05, the unit’s COP excluding the supply-fan energy consumption is calculated at 3.62. .
  • The unit has single-stage DX cooling and cycles on and off to meet space cooling loads, as defined for Reference 1. .

3.4 REFERENCE 3

  • Reference 3 represents the latest ASHRAE 90.1-2010 requirements.
  • The low fan speed is used for ventilation and 1st stage DX cooling.
  • The switch from low stage to high stage can be based either the space temperature deviation from the cooling set point or time lag for not reaching the cooling set point.

4 ENERGY MANAGEMENT SYSTEM (EMS) IMPLEMENTATION

  • The traditional controls built into EnergyPlus are not capable of modeling many control options required to simulate the Rebel unit, for example, control of supply-fan speed based on the deviation of the space temperature compared to the space set point.
  • To model the sequence of operation of the Rebel unit requires specialized control.
  • The EMS provides a variety of sensors and actuators much like an actual building automation system.
  • The sequence of operations embedded in the EnergyPlus input files is used to override the traditional EnergyPlus control and to add the desired control functionality into the simulation.

4.1 DESCRIPTION OF RTU’S OPERATIONAL MODE

  • Depending on the space temperatureT , the RTU has four basic operation modes: idle, ventilation, heating, and cooling.
  • The RTU operates in the cooling mode if the space temperature is greater than the cooling set point.
  • Otherwise, the mechanical cooling ends when the space temperature decreases below the cooling set point minus the differential.
  • Once initiated, the 2nd stage cooling continues until the space temperature decreases below the cooling set point minus the differential.

4.2 DEFINITION OF RTU’S OPERATION IN DIFFERENT MODES

  • In the ventilation mode, both heating and cooling are off.
  • In addition, the final adjusted fan speed needs to be checked to make sure it lies between the minimum and maximum fan speed for heating.
  • In the economizing (OA cooling) mode, both heating and mechanical cooling are off.

5 ENERGYPLUS SIMULATION RESULTS

  • The EnergyPlus simulation models were run for three locations: Houston, Los Angeles, and Chicago.
  • Both HVAC energy savings and cost savings are presented in this section.

5.1 ENERGY SAVINGS

  • Table 7 shows the energy end uses including cooling energy (kWh), fan energy (kWh), heating energy , total RTU electricity (kWh), and total RTU energy .
  • Figure 8 shows the total RTU energy savings (including both electricity savings and natural gas penalties).
  • Both figures have two parts: the top part shows the percentage of savings and the bottom part shows the absolute savings.
  • Table 7 shows that Rebel’s variable-speed supply fan contributes to a significant amount of HVAC energy savings.
  • Rebel units save about 30% RTU electricity consumption in Houston, 19% in Los Angeles, and 28% in Chicago.

5.2 COST SAVINGS

  • Average blended gas and electricity prices from EIA (2013) are used for the analysis.
  • Based on these prices and the energy simulation results from Table 7, energy costs are calculated as shown in Table 9.
  • The percentage savings of electricity cost is 40%, 42%, and 50%, respectively in the above three locations.
  • Using Reference 3 as the baseline, Rebel units lead to about 27% lower HVAC energy cost in Houston, 18% lower cost in Los Angeles, and 15% lower cost in Chicago.

6 DISCUSSION

  • The building model as presented in Section 3 (Building Simulation Models) has four thermal zones, each served by a packaged RTU.
  • The rated full-load efficiency and normalized part-load performance curves representing a 10-ton cooling capacity are used for all models.
  • Higher savings are expected if performance curves in References 2 and 3 closely match the minimum codes and standards requirements.
  • More cooling savings could be obtained if the efficiency loss from compressor cycling is considered in the reference models.
  • Because the cost of these units was not available and because the costs would be specific to a given installation, no attempt was made to estimate the potential payback periods associated with any of the three reference scenarios.

7 REFERENCES

  • Performance rating of commercial and industrial unitary air-conditioning and heat pump equipment, also known as ANSI/AHRI Standard 340/360-2007.
  • Energy Standard for Buildings Except LowRise Residential Buildings, also known as ASHRAE Standard 90.1-2004.
  • American Society of Heating, Refrigerating and AirConditioning Engineers Inc. Atlanta, GA.
  • PNNL-20405, Pacific Northwest National Laboratory, Richland, WA. 36.

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PNNL-22720
Prepared for the U.S. Department of Energy
under Contract DE-AC05-76RL01830
Part-Load Performance
Characterization and Energy Savings
Potential of the RTU Challenge Unit:
Daikin Rebel
W Wang
S Katipamula
September 2013

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(8/00)

PNNL-22720
Part-Load Performance Characterization and Energy Savings
Potential of the RTU Challenge Unit: Daikin Rebel
W Wang
S Katipamula
September 2013
Prepared for
U.S. Department of Energy
under Contract DE-AC05-76RL01830
Pacific Northwest National Laboratory
Richland, Washington 99352

iv
ABSTRACT
In 2011, the U.S. Department of Energy’s Building Technology Office (DOE’s BTO), with help
from the Better Buildings Alliance (BBA) members, developed a specification (RTU Challenge)
for high performance rooftop air-conditioning units with capacity ranges between 10 and 20 tons
(DOE 2013). Daikin’s Rebel was the first rooftop unit system recognized by DOE in May 2012
as meeting the RTU Challenge specifications. This report documents the development of part-
load performance curves and there use with the EnergyPlus simulation tool to estimate the
potential savings from the use of Rebel units compared to other standard options.
The conventional direct-expansion cooling model in EnergyPlus and its performance curve
equations are not appropriate to use to capture the Rebel’s performance; therefore, performance
curves suitable for the Rebel unit were developed. The data for the performance curves were
collected through a series of laboratory tests.
A detailed EnergyPlus model was developed for a prototypical big-box retail store. The model
used the new performance curves along with detailed energy management control code to
estimate the energy consumption of the prototypical big-box retail store in three locations
(Houston, Log Angeles and Chicago). The energy consumption by the big-box store was then
compared to a store that used three different reference units. The first reference unit (Reference
1) represents existing rooftop units (RTUs) in the field, so it can be considered as the baseline to
estimate potential energy savings from other RTU replacement options. The second reference
unit (Reference 2) represents RTUs in the market that just meet the current (2013) Federal
regulations for commercial equipment standards. So, it can be used as the baseline to estimate
the potential for energy savings from Rebel units in comparison with new RTUs that meet the
minimum efficiency requirements. The third reference unit (Reference 3) represents the latest
ASHRAE 90.1-2010 requirements. For RTUs with cooling capacity greater than 11,000 Btu/h,
ASHRAE 90.1-2010 (ASHRAE 2010) requires two-speed fan control or variable-speed fan
control.
The following conclusion can be drawn about the comparison of energy cost for Rebel unit with
the three reference units:
Using Reference 1 as the baseline, Rebel units lead to about 45% lower heating,
ventilation and air conditioning (HVAC) energy cost in Houston and Los Angeles, and
33% lower cost in Chicago. The percentage savings of electricity cost averages around
50% for all three locations.
Using Reference 2 as the baseline, Rebel units lead to about 37% lower HVAC energy
cost in Houston, 40% lower cost in Los Angeles, and 29% lower cost in Chicago. The
percentage savings of electricity cost is 40%, 42%, and 50%, respectively in the above
three locations.
Using Reference 3 as the baseline, Rebel units lead to about 27% lower HVAC energy
cost in Houston, 18% lower cost in Los Angeles, and 15% lower cost in Chicago. The
percentage savings of electricity cost is 30%, 19%, and 28%, respectively in the above
three locations.

v
Based on the simulation results, the Rebel RTU Challenge unit, if widely adopted, could lead to
significant energy, cost and emission reductions. Because the cost of these units was not
available and because the costs would be specific to a given installation, no attempt was made to
estimate the potential payback periods associated with any of the three reference scenarios.
However, if the incremental cost for any of the three reference cases is known, one can easily
estimate a simple payback period.

Citations
More filters
Journal ArticleDOI
TL;DR: In this article, the authors present relatively comprehensive assessment results for three RTU variable-speed retrofit options: 1) retrofits of fixed-speed supply fans with variable speed fans for single-compressor-speed units.

12 citations

ReportDOI
31 Mar 2015
TL;DR: In this paper, the authors compared the seasonal efficiency of a state-of-the-art packaged rooftop unit (RTU) and a standard reference HVAC unit in the field.
Abstract: This report documents the testing of a state-of-art packaged rooftop unit (RTU) (HVAC) and a standard reference unit in the field and compares the seasonal efficiency of the two unit.
References
More filters
ReportDOI
01 Feb 2011
TL;DR: In this paper, the authors developed standard or reference energy models for the most common commercial buildings to serve as starting points for energy efficiency research, which represent fairly realistic buildings and typical construction practices.
Abstract: The U.S. Department of Energy (DOE) Building Technologies Program has set the aggressive goal of producing marketable net-zero energy buildings by 2025. This goal will require collaboration between the DOE laboratories and the building industry. We developed standard or reference energy models for the most common commercial buildings to serve as starting points for energy efficiency research. These models represent fairly realistic buildings and typical construction practices. Fifteen commercial building types and one multifamily residential building were determined by consensus between DOE, the National Renewable Energy Laboratory, Pacific Northwest National Laboratory, and Lawrence Berkeley National Laboratory, and represent approximately two-thirds of the commercial building stock.

871 citations

Frequently Asked Questions (1)
Q1. What are the contributions in "Part-load performance characterization and energy savings potential of the rtu challenge unit:" ?

In this paper, the authors report the development of performance curves for the Rebel unit ; and the simulated energy and cost savings for a prototypical “ big-box ” building at three locations ( Houston, Los Angeles, and Chicago ) compared to the same building that uses conventional RTUs.