HVAC Simulation Guidebook Volume I

HVAC Simulation Guidebook Volume I
Advanced
HVAC Simulation Guidebook
Volume
VolumeIII
Second
Edition
The High
Performance
July
2012Process
Building
Table of Contents
Part 1: Underfloor Air Distribution and Thermal Displacement
Ventilation
Introduction to Part 1................................................................................................................................1
What are Underfloor Air Distribution and Thermal Displacement Ventilation?.........1
What differentiates UFAD and TDV systems from overhead
distribution systems? ..............................................................................................................................2
Barriers to Modeling UFAD and TDV in DOE-2 ..........................................................................3
• Calculation of cooling loads isn’t available..........................................................................4
Thermal Displacement Ventilation and Underfloor Air Distribution Modeling............4
Modeling Issues............................................................................................................................................6
• System Selection...............................................................................................................................7
• Supply Air Temperature................................................................................................................8
• Dehumidification................................................................................................................................8
• Air Volume............................................................................................................................................8
• Static Pressure....................................................................................................................................8
• Economizer Controls........................................................................................................................9
• Building Skin Loads.........................................................................................................................9
• Perimeter Systems............................................................................................................................9
Modeling Methodology .........................................................................................................................10
• Modeling TDV and UFAD in EnergyPro Version 5 (or later)....................................12
• Revising the Lighting, Equipment, and Occupant Inputs
Directly in DOE-2.1e......................................................................................................................12
Part 2: Energy Efficient Chillers
Introduction to Part 2.....................................................................................................................15
How Chiller Performance is Specified in DOE-2.................................................................16
• CAPFT - Chiller Capacity as a Function of Temperature...........................................17
• EIRFT - Energy Input Ratio as a Function of Temperature.....................................17
• EIRFPLR - Energy Input Ratio as a Function of Part Load Ratio..........................18
Improved Models for Variable Speed Chillers in DOE-2.2............................................. 21
Data Required for Specifying Chiller Performance Curves in Doe-2........................ 22
Obtaining Data Required for Chiller Performance Curves............................................. 23
Methods for Creating Custom DOE-2 Chiller Models.......................................................24
Methods for Including Chiller Data in DOE-2 Performance Curves........................... 26
• Calculating the CAPFT Curve in DOE-2...............................................................................26
• Calculating the EIRFT Curve in DOE-2................................................................................29
• Calculating the EIRFPLR Curve in DOE-2..........................................................................30
Guidelines for Creating Accurate Custom Chillers............................................................39
Summary.............................................................................................................................................39
Part 3: Advanced Control Sequences
Introduction to Part 3.....................................................................................................................43
Variable Speed Drive Control Sequences..............................................................................44
• Variable Primary Flow Chilled Water Distribution.......................................................44
• Primary/Secondary Chilled Water Distribution..............................................................46
• Cautions for Modeling Variable-Speed Pumps and Variable-Chilled Water ....... Flow in eQUEST .............................................................................................................................51
• Variable Flow Condenser Water System............................................................................52
Condenser Water System Operation...............................................................................................53
• Cooling Tower Cell Control........................................................................................................53
• Cooling Tower Capacity Control..............................................................................................53
• Condenser Water Temperature Reset..................................................................................55
• Modeling Condenser Water Load Reset in eQUEST.....................................................56
Chilled Water Loop Temperature Reset.................................................................................57
• Defining Chilled Water Temperature Reset in eQUEST..............................................57
Hot Water Loop Temperature Reset........................................................................................58
The first edition of this report (2007) was prepared for Pacific Gas and Electric Company by CTG
Energetics, Inc. for the statewide Energy Design Resources program (www.energydesignresources.
com). For this second edition, Lincus Incorporated performed an engineering review and contacted
selected simulation software experts to update passages affected by recent changes in the California
Building Energy Efficiency Standards (Title 24 2008). Editorial coordination and support for the
second edition was provided by GeoPraxis Inc. The original content creator was not actively involved
in this engineering review, and therefore is not responsible for the updates to the affected passages.
This report was funded by California utility customers under the auspices of the California Public
Utilities Commission.
Neither Pacific Gas and Electric Company, Southern California Edison, nor any of its employees and
agents:
1. Makes any written or oral warranty, expressed or implied, regarding this report, including, but not
limited to those concerning merchantability or fitness for a particular purpose.
2. Assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any
information, apparatus, product, process, method, or policy contained herein.
3. Represents that use of the report would not infringe any privately owned rights, including, but not
limited to, patents, trademarks or copyrights.
PART 1: Underfloor Air Distribution and
Thermal Displacement Ventilation
Underfloor air distribution (UFAD) and thermal displacement ventilation (TDV) have
become increasingly common in commercial new construction because they are energyefficient, enhance indoor air quality, and increase flexibility for space reconfiguration.
However, conflicting opinions exist concerning the benefits of UFAD and TDV. This
often leads to inappropriate analysis and unrealistic customer expectations. There are
many different notions regarding the energy efficiency of UFAD and TDV systems, with
some people claiming that these systems save little or no energy, while others suggest that
they can cut HVAC energy usage by fifty percent or more. To help the energy modeler
evaluate the energy benefits of UFAD and TDV, this simulation guidebook identifies the
key characteristics that distinguish UFAD and TDV systems from traditional overhead
systems and presents a logical, engineering-based method for analyzing UFAD and TDV
with DOE-2-based simulation programs.
What are Underfloor Air Distribution and
Thermal Displacement Ventilation?
This simulation guidebook is concerned with methods for analyzing air distribution systems that
deliver cooling and heating air at floor level instead of from the ceiling. An example of such a system is
underfloor air distribution (UFAD), where conditioned air is delivered at a moderate velocity (650
to 800 feet-per-minute) via a 10” to 16” plenum space underneath an access floor system (Figure
1a). Another example is a Thermal Displacement Ventilation (TDV) system that delivers supply air
horizontally at low velocity (50 to 100 feet-per-minute) from wall-mounted diffusers without using an
underfloor plenum (Figure 1b).
part 1: underfloor air distribution and thermal displacement ventilation
1
Figure 1:
Diffuser for Underfloor Air Distribution System1 (1a-left) versus Diffuser for Thermal
Displacement System2 (1b-right)
(a) A typical
underfloor air
distribution system
consists of a raised
access floor, a 10”
to 16” underfloor
plenum, and air
delivery diffusers. (b)
A thermal displacement
ventilation system
delivers low velocity
air at floor level. An
access floor is not
usually employed for
such a system.
While there are differences in the performance characteristics between UFAD and TDV systems, the
modeling methodology described in this simulation guidebook applies to both. Energy modelers must
exercise judgment to adjust the methods to suit either system. Guidelines are provided throughout this
guidebook that can be applied to account for differences between the two systems.
What differentiates UFAD and TDV systems from overhead distribution systems?
Traditional space conditioning systems supply heated or cooled air from diffusers mounted in a suspended ceiling grid. The design assumption made is that supply air completely mixes with the air in the
room, and as a result, all of the air within the conditioned space reaches a homogeneous temperature
(Figure 2).
Designers go to great effort to select diffusers that promote this mixing effect so that cold air does not
“dump” onto the occupants below. In an overhead mixing system, cold supply air mixes with hot air
that accumulates near the ceiling as a result of heat generated
1
2
2
Source: Tate Access Floors
Source: Halton Group
HVAC SIMULATION GUIDEBOOK - 2ND EDITION
by people, lights, and equipment. While an overhead mixing system concept can provide good occupant comfort, it wastes energy by providing comfortable conditions from the floor all the way to the
ceiling. It would be more efficient to limit the distribution of heated or cooled air only to the lower
volume (for example, up to seven feet above the floor) of the room where the occupants are located.
Overhead Air Delivery Provides Homogeneous Temperature Distribution3
Figure 2:
Traditional overhead
air distribution
systems are designed
to completely mix
supply air with
room air. The goal
is to provide a
uniform temperature
distribution from
Thermal Displacement Ventilation Encourages Stratification4
floor to ceiling.
Figure 3:
By delivering cool
air at floor level and
drawing warmer air
from the ceiling, TDV
encourages thermal
stratification.
Barriers to Modeling UFAD and TDV in DOE-2
Uniform temperatures are assumed throughout the entire conditioned space
Most simulation programs based on DOE-2.1e or DOE-2.2 (such as eQUEST and EnergyPro) determine space cooling loads as a summation of all heat losses and heat gains within a space, without regard
to how the loads are influenced by airflow patterns and the buoyancy of warm air. Stated another way,
most simulation programs are not aware that hot air rises, and therefore assume a uniform temperature
throughout the conditioned volume.
3
4
Source: CTG Energetics, Inc.
Source: CTG Energetics, Inc.
part 1: underfloor air distribution and thermal displacement ventilation
3
For example, consider the following internal load calculation:5
People (sensible + latent)
30 occupants x 450 Btu/hr-person 13,500 Btu/hr
Lights
900 SF x 0.6 W/SF x 3.413 Btu/hr-W
1,843 Btu/hr
Equipment
900 SF x 1.0 W/SF x 3.413 Btu/hr-W
TOTAL
3,072 Btu/hr
18,415 Btu/hr
Most simulation programs calculate the required cooling capacity to meet these internal loads as the
sum of the loads. The fact that hot air rises (producing warmer temperatures near the ceiling and
cooler temperatures near the floor) is not accounted for. For overhead mixing-type air distribution this
approach is satisfactory because overhead diffusers are selected and placed to promote mixing of supply
air and room air. The flow of supply air from the ceiling pushes the hot air near the ceiling down to
the level of occupants.
Calculation of cooling loads isn’t available
When conditioned air is delivered at floor level at low velocity, it does not significantly mix with the
The effect of thermal
hot ceiling air. Accordingly, floor-supplied air is not as disruptive to thermal stratification as overhead
stratification is that
delivery (Figure 3). This is advantageous because the hot ceiling air can be drawn directly into the
cooling loads are
return air system and exhausted from the space instead of neutralized by mixing with cold air. The
reduced relative to
those of overhead
delivery systems,
effect of thermal stratification is that cooling loads are reduced relative to those of overhead delivery
systems, but most simulation programs do not reflect this change. Referring to the previous load cal-
but most simulation
culation example, a UFAD system would reduce the cooling load resulting from internal heat gains by
programs do not
nearly 50 percent. More complex and time-consuming analysis methods, such as computational fluid
reflect this change.
dynamics (CFD), must be employed if one wishes to calculate cooling loads that account for thermal
stratification. In most cases it is not practical to perform CFD analysis, and such analysis cannot be
submitted to show Title 24 compliance.
Thermal Displacement Ventilation and Underfloor Air
Distribution Modeling
The strategy for modeling TDV and UFAD systems is to move a portion of the heat gain from people,
lights and equipment from the conditioned space to an unconditioned plenum space. While projectspecific information about how much of each internal load should be apportioned to the plenum is
highly desirable, such data is infrequently available. Table 1 provides reasonable estimates for both
UFAD and TDV systems. Figure 4 and Figure 5
5
CA Energy Commission, 2008. Non-Residential Alternative Calculation Manual (ACM) Approval
Method, CEC-400-2008-003-CMF, Table N2-5 (default occupancy type)
4
HVAC SIMULATION GUIDEBOOK - 2ND EDITION
show how lighting and receptacle loads would be redistributed in a typical underfloor air system. A
similar approach would be employed for occupant heat gain.
Internal Load Distribution Values for Typical Underfloor Air and Thermal Displacement
Ventilation Systems6
Table 1
Room Cross Section with All Loads Modeled in Space
Figure 4:
Cross-section view of
typical room showing
conditioned and
unconditioned spaces,
with typical lighting
power density (LPD)
and equipment power
density (EPD). The
total internal heat
gain from these
sources is 1.95 W/ft2.
6
These factors are meant to be illustrative only. Please see your compliance software documentation
or the Modeling Methodology section on page 10 for more detailed information.
part 1: underfloor air distribution and thermal displacement ventilation
5
Room Cross Section with Fraction of Loads Reallocated to Plenum7
Figure 5:
To simulate the
reduced heat gain
from lighting and
receptacle loads,
much of the load is
reassigned to the
unconditioned return
air plenum. The effect
is that cooling loads
in the conditioned
space are reduced
while the return
air temperature is
increased. The total
gain from lights
and receptacles
is unchanged;
however, it has
been apportioned
differently between
conditioned and
unconditioned space.
Modeling Issues
There are a number of issues that must be considered when modeling thermal displacement or underfloor air distribution systems. These issues include:
•
System Selection
•
Supply Air Temperature
•
Dehumidification
•
Air Volume
•
Static Pressure
•
Economizer Controls
•
Building Skin Loads
•
Perimeter Systems
7
6
Illustrations: CTG Energetics, Inc.
HVAC SIMULATION GUIDEBOOK - 2ND EDITION
System Selection
The energy modeler must take care to choose the appropriate HVAC system type from those available
within the simulation program. The exact system choice should reflect whether the system operates
in constant or variable-volume fashion, the source of heating and cooling, and the way that outside air
ventilation is managed. The following are examples of potential system selections:
Example #1: Access Floor System with Manually Adjustable Diffusers. This sort of system, which
may include a large number of round, manually adjustable “swirl” diffusers (approximately one
diffuser per 75 to 100 ft2 of conditioned area), has become increasingly common in office buildings. Because occupants have some control over the airflow in their workspace - but cannot
completely shut off the air supply - the system operates essentially as a variable-air-volume system
with a high minimum airflow rate. Such systems are most commonly employed as part of a
chilled water cooling system. In the DOE-2 simulation environment, such a system could be
modeled using system type VAVS (variable-air-volume, with chilled water cooling). The minimum airflow rate (MIN-CFM-RATIO) would be set high to reflect the diversity of loads in the
conditioned space and also the limited turndown offered to occupants. It is common for turn-
The energy modeler
must take care
to choose the
appropriate HVAC
down ratios to be 70 to 80 percent of full flow, though modeling assumptions should be verified
system type from
with the HVAC engineer. In many cases, a 100% outside-air-economizer cycle will be employed
those available
and the specific program inputs should reflect the fact that the cooling requirements can be met
using warmer air than with overhead systems (i.e. 64ºF to 67ºF).
within the simulation
program.
Example #2: Access Floor System with Thermostatically Controlled VAV Zone Terminals. This
system is similar in some respects to Example #1, but the large number of “swirl” diffusers is
replaced with a reduced number of thermostatically-controlled VAV terminals located in the
underfloor plenum. Zoning for such systems is often comparable to overhead systems in terms
of the average area per zone. Such systems usually offer higher turndown than the manual “swirl”
diffusers and are automatically controlled based on space temperature. As a result, the minimum
airflow ratio (MIN-CFM-RATIO) will frequently be lower with this air distribution strategy.
Reviewing the zone schedule prepared by the mechanical engineer should provide information
about the minimum airflow for each VAV terminal. Without such data, it is reasonable to assume
a minimum airflow ratio of 50 percent until more detailed information is available.
Example #3: Thermal Displacement System with Constant Volume Delivery. This system design is
most frequently used in classrooms or other assembly areas. A common configuration for thermal displacement systems consists of four-pipe fan coil units for each zone (or classroom), with a
central air-handling unit distributing outside air to each unit. The fan coil units deliver constant
volume supply air horizontally at low velocities from wall-mounted diffusers. The system can be
modeled in DOE-2 using system type FPFC (four-pipe fan coil). The energy model system inputs
should reflect the supply air temperature and volume design conditions associated with TDV, and
fan energy inputs for each fan coil should account for the contributions from the central outside
air supply unit.
part 1: underfloor air distribution and thermal displacement ventilation
7
Supply Air Temperature
Since TDV and UFAD generally introduce conditioned air in close proximity to the building occupants, the air is delivered at a temperature only slightly (5ºF to 10ºF) below space temperature set
points. This corresponds to a 64ºF to 67ºF supply air temperature set point (MIN-SUPPLY-T or
COOL-SET-T) as opposed to a 55ºF set point for traditional mixing systems.
Dehumidification
Due to the elevated supply air temperature associated with TDV and UFAD, the mechanical designer
must give close attention to humidity control for these systems. With the exception of cool, dry
climates, cooling coils provide inadequate removal of latent load when cooling to only 64ºF or 67ºF.
Consequently, most TDV and UFAD designs need to implement supplemental humidity control
features to avoid the decreased comfort and indoor air quality associated with high space humidity
conditions. In a common TDV or UFAD humidity control scheme, the chilled water coil cools a
mixture of outside air and return air down to 55ºF, and this conditioned air is then mixed with the
remainder of the return air to increase the temperature back up to the supply air temperature set
point. Although the limitations of DOE-2 prevent the accurate modeling of this humidity control
scheme, energy modelers should keep in mind that this form of humidity control will achieve less
energy savings than projected by a DOE-2 model with a high supply-air temperature set point.
Air Volume
Design supply airflow calculations for the space must account for both the elevated supply air temperature and the redistribution of a portion of the occupant, plug and lighting loads from the space
to the return air. Ignoring the high supply-air temperature for TDV and UFAD systems will result
With the exception of
cool, dry climates,
cooling coils provide
inadequate removal
of latent load when
cooling to only 64ºF
or 67ºF.
in an underestimation of supply air volume, and neglecting to redistribute a portion of the space
loads to the plenum will result in supply air-flow rates that are up to two times greater than the
amount required to condition the space. Typically, supply air flows for a true TDV system exceed
those of a corresponding overhead mixing system by only five to twenty percent.8 Supply air flow
rates for UFAD systems range from twenty-five percent less to fifteen percent more than traditional
overhead systems.9
Static Pressure
In most UFAD systems, the underfloor plenum serves as the primary source of air distribution.
Consequently, UFAD systems generally use far less ductwork than corresponding overhead systems,
resulting in reduced static pressure at the supply fans when compared against standard overhead systems. However, due to the wide variance in UFAD design, energy analysts should confirm estimated
values for static pressure with the mechanical designer prior to modeling savings associated with
reduced fan static pressure. The fan energy savings linked to lower fan static pressures will generally
not be reflected in Title-24, since the standard case changes with the proposed case for inputs related
to fan power. However, as per 2008 Title 24 ACM, title 24 directs compliance software to accurately
simulate potentially lower central fan static pressure requirements.
8
“Underfloor Air Distribution and Access Floors.” Energy Design Resources Design Brief.
9
Webster, Tom, Bauman, Fred, and Reese, Jim. “Underfloor Air Distribution: Thermal
Stratification.” ASHRAE Journal. May 2002. Vol. 44, No. 5, Pg. 34.
8
HVAC SIMULATION GUIDEBOOK - 2ND EDITION
Economizer Controls
TDV and UFAD systems can often take advantage of increased hours of economizer operation due
to the higher temperature of air delivered to the space. In most California climate zones, raising
the supply air temperature from 55ºF to 65ºF can extend economizer operation by 2,000-2,500
hours per year.10 However, the humidity control requirements in many of these climates will limit
the hours of additional economizer operation, resulting in reduced free cooling benefits. In climate
zones that require additional dehumidification, the economizer operation must be integrated with the
humidity control to maintain proper humidity conditions. This requires differential enthalpy-based
In most California
economizer operation to ensure that the humidity of the outside air remains lower than that of the
climate zones,
return air. In DOE-2, differential enthalpy control is modeled using the ENTHALPY keyword for
raising the supply air
OA-CONTROL at the system level.
temperature from 55ºF
to 65ºF can extend
economizer operation
Building Skin Loads
If return grilles are located directly above the windows in perimeter spaces served by UFAD or TDV
by 2,000-2,500 hours
per year.
systems, a significant portion of the convective cooling load associated with the building skin can be
funneled directly into the return air plenum.11 A precise energy model for UFAD and TDV systems
can account for the energy savings associated with this phenomenon by reapportioning some of the
glazing and exterior walls in the occupied space to the adjacent plenum. However, unless lightingrelated gains are properly apportioned, this methodology may result in the loss of legitimate automated daylighting control savings in DOE-2-based programs.
Perimeter Systems
Perimeter system approaches vary widely for both UFAD and TDV systems. In some cases, perimeter underfloor air plenums for UFAD systems are separated from interior underfloor air plenums with
dividers; in another approach, underfloor ductwork provides perimeter spaces with a separate source
of supply air, and sometimes perimeter spaces are entirely served by overhead systems. Baseboard
heating can also be provided as the primary heating source for perimeter zones served by TDV or
UFAD systems. The Underfloor Air Distribution Design Guide (ASHRAE, 2003) provides a good
overview of the range of perimeter system designs commonly applied in conjunction with underfloor
air distribution.11 Energy analysts should use their judgment to select the type of space heating and
zone terminal units in DOE-2 that most closely represent the perimeter system design for their project. Modelers should also anticipate the effect factors such as supply plenum temperature decay and
return air temperature decrease may have on the otherwise normative assumptions outlined above. In
addition, a simplified cooling load design tool and a growing body of UFAD research and case studies are now available.12
10
“The Case for TDV in California Schools.” California Energy Commission’s Public Interest Energy Research (PIER) Program.
http://www.archenergy.com/ieq-k12/thermal_displacement/thermal_displace_background.htm
11
Bauman, Fred S. and Daly, Alan. Underfloor Air Distribution (UFAD) Design Guide. Atlanta: ASHRAE, 2003.
12
Schiavon, S., F. Bauman, K.H. Lee, and T. Webster, 2010. Development of a simplified cooling load design tool for underfloor air
distribution systems. Final Report to CEC PIER Program (500-06-049).
Webster, T., F. Bauman, D. Dickerhoff and Y. Lee. 2008. Case Study of Environmental Protection Agency (EPA) Region 8
Headquarters Building.
Bauman, F., T. Webster, and H. Jin, 2006. Design Guidelines for Underfloor Plenums. HPAC Engineering (July 2006).
http://www.cbe.berkeley.edu/research/publications.htm#ufad
part 1: underfloor air distribution and thermal displacement ventilation
9
Modeling Methodology
For many years, energy modelers have been able to apply the basic TDV and UFAD modeling strategies described above by editing the BDL input file in native DOE-2. Alternative Calculation Methods
(ACMs) specified in the 2008 Title 24 standards13 now permit software developers to offer an approved
approach to modeling Underfloor Air Distribution (UFAD). With this method, users are able to
assign a percentage of the occupant, lighting and plug loads to the return air plenum. However, not all
approved software providers have fully implemented this optional capability.
The 2008 Title 24 ACM places certain requirements on compliance software used in modeling of
UFAD.
The Compliance Software shall semi-automatically include provisions to accurately simulate the following factors:
1.
Heat transfer to the underfloor supply plenum and its effect on net room load and thus airflow
requirements (see below)
Common
Simulation
Software
2.
Effect on airflow requirements due to room supply and return temperatures greater than conventional overhead systems.
This guidebook uses
research generated
from the following
3.
energy use) of the combination of higher room supply temperatures and heat gain to the supply
energy simulation
plenum.
software packages:
EnergyPro v. 3.142,
Effect on AHU leaving temperature and thus economizer performance (and its impact on cooling
4.
Potentially lower central fan static pressure requirements
DOE-2.1e release 134.
5.
Effect on total building fan energy due to variable speed fan coils for cooling
Keep in mind that this
6.
Realistically simulate typical UFAD system types.
eQUEST v. 3.44 with
DOE2.2 release 42k6, and
software is constantly
updated. Review the
documentation of
later releases for any
changes to software
inputs or keywords
The Compliance Software shall use the following guidance to accurately simulate realistic energy performance of UFAD systems:
Reduce zone load to simulate heat transfer to the supply plenum (for ACMs that do not explicitly
that might impact the
model supply plenums) – zone heat gain is reduced by applying a Room Cooling Load Ratio (RCLR)
modeling methodology
to the people, lighting and equipment loads. The Compliance Software shall use an RCLR of 0.6,
discussed in this
meaning that 60 percent of the heat gain shall remain in the space and 40 percent shall be assumed to
simulation guidebook.
transfer into the underfloor supply plenum.
Split the remaining space load determined above between room and return plenum to simulate room
air stratification. The Compliance Software shall automatically assign the following factors to each of
occupant, lighting and equipment heat gains: 85 percent to space and 15 percent to return plenum.
13
Nonresidential Alternative Calculation Method (ACM) Approval Manual - 2008 Building Energy
Efficiency Standards. California Energy Commission 2008 Publication # CEC-400-2008-003-CMF
10
HVAC SIMULATION GUIDEBOOK - 2ND EDITION
The diffuser discharge temperature (i.e., supply to the zone) shall be assumed to be 65°F. The required
supply air temperature from the air handler shall be calculated using the Room Cooling Load Ratio
definition above.
The Compliance Software shall allow the use of a higher supply air temperature, as well as the application of supply temperature reset by either demand or outdoor drybulb temperature. Additionally, the
Compliance Software may also optionally accommodate higher chilled water temperatures on systems
that utilized chilled water coils.
The Compliance Software shall make an entry in the special features and remarks section of the PERF-1
report noting the use of an underfloor air distribution system.
DOE Keyword: LIGHTING-W/SQFT
EQUIPMENT-W/SQFT
AREA/PERSON
MIN-SUPPLY-T
CHILL-WTR-T
AHU SAT
Economizer type
PIU W/CFM
AHU design static pressure
Input Type: Default
Tradeoffs: Yes
Modeling Rules for Proposed Design: The Compliance Software shall model all optional underfloor air
distribution system features as input by the user according to the construction documents for the building. Additional supporting calculations can be included to assist the user in determining appropriate
input.
Default: n/a
Modeling Rules for Standard Design (New): The Compliance Software shall model the standard design
according to the requirements of the Required Systems and Plant Capabilities.
Modeling Rules for Standard Design (Existing Unchanged & Altered Existing): Compliance Software
shall model the existing system as it occurs in the existing building. If the permit involves alterations,
Compliance Software shall model the system before alterations.
Although eQUEST 3.64 does not offer UFAD compliance capabilities,14 it does now give users the ability to define their own convective/radiative splits for all categories of internal heat gains in a space. The
default values for these new keywords are the same as the previous hard-coded values, with the exception
of the electric equipment loads. Previously, the program assumed a radiant fraction of 0.70; the default
14
JJH & Associates, 2009. eQUEST/D2Comply ACM Non-Residential Certification Application
(23 December, 2009).
part 1: underfloor air distribution and thermal displacement ventilation
11
for the new keyword is 0.30 to better reflect the large convective gains associated with computers and
peripheral equipment. In addition, various lighting keywords have been expanded to allow these properties of each lighting system to be independently specified.
The following step-by-step modeling methodology may be used with EnergyPro 5 and DOE-2.1e and
assumes that a UFAD system has not yet been implemented.
Modeling TDV and UFAD in EnergyPro Version 5 (or later)
An energy modeler can simulate TDV or UFAD with EnergyPro by:
•
Inputting system inputs directly in EnergyPro;
•
Using the EnergyPro Win/DOE module to generate a DOE-2 input file, and
•
Revising the lighting, equipment, and occupant inputs directly in DOE-2.1e. This step is
explained below.
Revising the Lighting, Equipment, and Occupant Inputs Directly in DOE-2.1e
Step 1. Define at least one but no more than three return air plenums for each system. In cases where
the building design defines a return air plenum, model the plenum as drawn in the plans. If no return
air plenum is defined in the plans, add a plenum with a height of three feet, and an area equal to the
building area served by the system.
Step 2. Select the DOE-2 system type as outlined in the system selection instructions above. Set the
cooling supply air temperature in the cooling tab for each system. Model the economizer type as
designed (often differential enthalpy for TDV systems). For each system, confirm that any variable
speed fans are appropriately defined under the Fans tab.
Step 3. For variable volume systems, select a zonal system from the mechanical tab for each zone, with
minimum air flow set in accordance with the system selection guidelines.
Step 4. From the File menu, select Calc Manager / Options / Win/DOE. Confirm that Delete DOE
files after run is unchecked.
Step 5. To generate the DOE-2 files, select Calc Manager / Calculate.
Step 6. From your EnergyPro Win/DOE directory, open the Title-24 proposed input file titled
[FileName]-Proposed.doe (where filename is the name you entered for the project in EnergyPro).
Step 7. Complete steps 2-5 for Modeling TDV in DOE-2 as outlined above.
12
HVAC SIMULATION GUIDEBOOK - 2ND EDITION
Step 8. Create a text file in your Win/DOE directory using the following syntax:
doe21e “[FileName]-Proposed.doe” [EnergyProWeatherPath]\[WeatherFile]
[FileName] represents the name of your project, [EnergyProWeatherPath] represents the path to the
EnergyPro weather directory, and [WeatherFile] represents the name of the weather file used for your
project. For example, the text file for a project titled Office and located in Sacramento, CA (climate
zone 12) would contain the text:
doe21e “office-Proposed.doe” C:\EP5\Weather\CZ12RV2.WY2
...assuming that the EnergyPro directory was located in C:\EP5.
Step 9. Change the extension of the text file to .bat to create a batch file that can run your project in MS
DOS. (To run the simulation, navigate to the .bat file using either Windows Explorer or My Computer,
and double-click on the .bat file)
Step 10. As an error-checking routine, the energy modeler should confirm that the lighting and equipment energy usage for the original Title-24 proposed case are equal to those shown in the revised Title24 proposed case.
part 1: underfloor air distribution and thermal displacement ventilation
13
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14
HVAC SIMULATION GUIDEBOOK - 2ND EDITION
PART 2: Energy Efficient Chillers
Advances in heat transfer surface technology, digital control, and variable frequency drives have resulted
in chillers that are much more efficient at part load and low lift conditions than those available fifteen
years ago. For example, many chillers equipped with Variable Frequency Drives (VSDs) perform up
to three times better at 30-50% load when chilled water supply temperature is raised and entering
condenser water temperature is lowered. Initially, VSDs were only available on centrifugal chillers but
now some rotary screw and scroll models can use them as well.
COMMON
Simulation
software
This guidebook uses
research generated
To achieve any savings, condenser water temperature must be lowered on centrifugal chillers with
from the following
VSDs. This is due to the fact that these chillers operate with both inlet vanes and VSDs to achieve
energy simulation
both capacity reduction and to keep out of surge. If the entering condenser water temperature is kept
software packages:
high (high chiller lift), the capacity control is entirely with the inlet vanes, and the chiller will be less
efficient than the same chiller without a VSD due to the drive losses.
EnergyPro v. 3.142,
eQUEST v. 3.44 with
DOE2.2 release 42k6, and
DOE-2-based simulation programs have the capability to accurately model the chiller performance if
DOE-2.1e release 134.
the programmer specifies appropriate performance curves. However, this approach is often overlooked
Keep in mind that this
by building simulation programmers, who opt to use default chiller performance curves rather than
software is constantly
develop curves calibrated for the specific chillers under investigation. This significantly limits the
updated. Review the
effectiveness of the energy model as a tool for chiller selection and optimization. By developing chiller
performance curves to match the performance of the specific chillers being modeled, energy modelers
can accurately reflect the product capabilities of each chiller, and avoid the over or underestimation of
savings that commonly occurs with default curves.
documentation of
later releases for any
changes to software
inputs or keywords
that might impact the
modeling methodology
Accordingly, this simulation guidebook addresses the following topics to present strategies for modeling
discussed in this
customized chiller curves in DOE-2-based simulation programs:15
simulation guidebook.
•
Chiller curves used to define chiller performance data in DOE-2;
•
Two methods for developing chiller curves and implementing them into the DOE-2 model,and
15
Always review your software documentation for “New Features” that may extend or update the
introductory methods presented here.
PART 2: Energy Efficient Chillers
15
•
Manufacturer’s data necessary to generate chiller curves.
Equations for Energy Input Ratio as a Function of Part-Load Ratio16
Figure 9
How Chiller Performance is Specified in DOE-2
DOE-2-based simulation programs give users several options for creating calibrated models of chillers. (A handy reference on this subject is provided in the ASHRAE Symposium Paper, “Tools and
Techniques to Calibrate Electric Chiller Component Models.”17) These programs use the following
16
Source: ASHRAE
Hydeman, M; K Gillespie. “Tools and Techniques to Calibrate Electric Chiller Component
Models.” ASHRAE. Atlanta GA. AC-02-9-01. January 2002.
17
16
HVAC SIMULATION GUIDEBOOK - 2ND EDITION
three chiller curves to define the impact of varying chilled water temperature, condenser temperature,
and load on chiller performance and capacity:
•
CAPFT - Capacity as a Function of Temperature. This curve adjusts the available capacity of the
chiller as a function of evaporator and condenser temperatures (or lift).
•
EIRFT - Energy Input Ratio as a Function of Temperature. This curve adjusts the efficiency of
the chiller as a function of evaporator and condenser temperatures (or lift).
•
EIRFPLR - Energy Input Ratio as a Function of Part-Load Ratio. This curve adjusts the efficiency
of the chiller as a function of part-load operation.
The format of these curves is shown in Figure 9. Using Equations (4) to (7), the power under any
conditions of load and temperature can be found from equation 8 (see Figure 9). Each of these curves
is described below:
CAPFT- Chiller Capacity as a Function of Temperature
This curve defines how chiller cooling capacity changes with different refrigerant lift conditions. The
curve can be directly calculated in the DOE-2 program by entering an array of data points that each
contains three numbers: the chilled water supply temperature, the condenser temperature, and the
corresponding value of CAPFT. For example, the data point (44, 85, 1.0) for a water-cooled chiller
translates as, “...when the chiller is producing 44 degree F chilled water and the entering condenser
water temperature is 85 degrees F, the chiller provides 100% of its rated capacity.” This would make
sense to many mechanical engineers because 44 and 85 correspond to the ARI rating conditions
under which chiller capacity and integrated part load value (IPLV) are calculated. All data for the
CAPFT curve assumes the chiller is operating at 100% of motor load.
CAPFT is defined from equation 7 above at the point where the “part-load ratio” (PLR) is unity. This
is shown in equation 9:
(9)
EIRFT- Energy Input Ratio as a Function of Temperature
This curve defines how chiller efficiency changes with different refrigerant lift conditions. The
curve can be directly calculated in the DOE-2 program by entering an array of data points that each
contains three numbers: the chilled water supply temperature, the condenser temperature, and the
EIRFT. For example, the data point (44, 85, 1.0) translates as, “...when the chiller is producing 44
degree F chilled water and the entering condenser water temperature is 85 degrees F, the chiller operates at its nominal full-load efficiency.” All data for the CAPFT curve assumes the chiller is operating
at 100% of motor load.
EIRFT is defined from equation 8 above at the point where the “energy input ratio as a function of
part-load ratio” (EIRFPLR) is unity. This is shown in equation 10:
(10)
PART 2: Energy Efficient Chillers
17
CAPFT Curve18 EIRFT Curve19
Figure 10 (left):
CAPFT- Capacity
as a Function of
Temperature. This
curve adjusts the
available capacity
of the chiller
as a function
of evaporator
and condenser
temperatures (or lift).
EIRFPLR- Energy Input Ratio as a Function of Part Load Ratio
Figure 11 (right):
This curve defines how the efficiency of a particular chiller varies with the amount of cooling that the
EIRFT- Energy Input
chiller is providing. The curve can be directly calculated in the DOE-2 program by entering an array
Ratio as a Function
of Temperature. This
curve adjusts the
efficiency of the
chiller as a function
of data points that each contains two numbers: the part load ratio (PLR) and the EIRFPLR. For
example, the datapoint (1.0, 1.0) translates as, “...when the chiller is at 100% of its load it is at 100%
of its power draw at the current lift conditions.” All data for the EIRFPLR is relative to the current
lift conditions. The PLR is calculated from equation 7 above. It is important to note that this is not
of evaporator
the ratio of the current capacity to the rated capacity of the chiller, but is the ratio of current chiller
and condenser
capacity to available chiller capacity at the current lift conditions (determined by the CAPFT equa-
temperatures (or lift).
tion).
EIRFPLR is defined from equation 8 above at the current lift conditions. This is shown in equation 11:
(11)
Note that the CAPFT and EIRFT curves are defined in terms of a reference condition (Qref and Pref).
The default DOE-2 curves are all normalized to the ARI reference conditions of 44°F chilled water supply temperature and 85°F condenser water supply temperature. For user-defined curves, the reference
can be chosen at any point (though the design condition is typically selected). However, it is critical
that the DOE-2 keywords Size and EIR (energy input ratio) be selected at this reference point.20 Failure
to do this will usually result in incorrect chiller comparisons as well as incorrect peak kW estimates. The
following formulas define Size and EIR.
18
Source: ASHRAE
Source: ASHRAE
20
Winkelmann, F.C., Birdsall, B.E., Buhl, W.F., Ellington, K.L., Erdem, A.E, Hirsch, JJ, Gates, S.
“DOE-2 BDL Summary, Version 2.1E.” November, 1993.
19
18
HVAC SIMULATION GUIDEBOOK - 2ND EDITION
•
Size - Nominal Rated Output Capacity. This input for nominal chiller capacity, expressed in units of one million Btu’s per hour (Mbtu/hr), is used to normalize the CAPFT
curve.
(12)
Most DOE-2.1e user interfaces such as EnergyPro or VisualDOE allow the user to input nominal
chiller capacity in units of tons.
•
ELEC-INPUT-RATIO, or EIR- Electric Input to Nominal Capacity Ratio. This input is used to define the efficiency of the chiller at the reference conditions. The EIR is calculated as follows:
This data is generally entered in unit of kW/ton in DOE-2.1e- based programs such as (13)
EnergyPro and VisualDOE.
DOE-2.1e Sample Text for Chiller Inputs
INPUT PLANT ..
CHWPlnt = PLANT-ASSIGNMENT ..
Figure 12
$ *********************************************************************** $
$
General Chiller Inputs
$
$ *********************************************************************** $
$ electric centrifugal chiller #1
CHILLER1 = PLANT-EQUIPMENT
TYPE = OPEN-CENT-CHLR
SIZE = 2.393
$ Selections available are described in
$ DOE-2.1e BDL summary, Page 50 (Nov 1993)
$ SIZE = Capacity * 0.012 MBTU/hr / ton
$ where capacity is expressed in units
$ of tons
INSTALLED-NUMBER = 1
MAX-NUMBER-AVAILABLE = 1
..
$ part load ratio for electric centrifugal chiller #1
PART-LOAD-RATIO
TYPE = OPEN-CENT-CHLR
ELEC-INPUT-RATIO = 0.199 $ EIR = kW/ton * 3413 Btu/kW / 12,000 Btu/ton
$ EIR should be defined using the same
$ conditions for CHWT & CWT as SIZE
MIN-RATIO = .1
PART 2: Energy Efficient Chillers
19
$ *********************************************************************** $
$
End General Chiller Inputs
$
$ *********************************************************************** $
$ CONTINUE PLANT INPUTS BELOW (NOT SHOWN) ...
Energy Pro 3.1 General Chiller Inputs
Figure 13
VisualDOE General Chiller Inputs
Figure 14
The DOE-2.2 inputs used for normalizing the curve fit data are similar to the inputs described above
for DOE-2.1e-based simulation programs; however, the keyword for SIZE has been modified to
CAPACITY, representing the nominal capacity of the chiller. In later versions of eQuest. the reference
point selected should generally correspond to the most common operating condition (which is typically
not at ARI-rated conditions).
20 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
Improved Models for Variable Speed Chillers in DOE-2.2
DOE-2.2 offers an improved model for variable-speed-driven chillers that includes temperature terms
in the EIRFPLR equations. This model was based on research reported in the ASHRAE Symposium
paper, “Development and Testing of a Reformulated Regression Based Electric Chiller Model.”21 The
research reported in this paper demonstrates the accuracy of the DOE-2 model in predicting the performance of all electric chiller types over a wide range of operating conditions. The two main shortcomings of the original DOE-2 (2.1E) model are the part-load efficiency of variable speed driven chillers
over a range of temperatures, and the performance of these models where the condenser water flow
varies through the chiller. DOE-2.2 improved the part load capabilities of the variable speed model,
and eventually keywords were added to allow the simulation of water-cooled, variable-flow condensers
as well. As of DOE-2.2 v47d (eQUEST v3.64) variable-flow algorithms have been further improved to
better model the chiller’s response to a variable-flow condenser.22
The format of the revised EIRFPLR curve for variable speed chillers in DOE-2 is shown in Figure 15.
Improved Equations for Energy Input Ratio as a function of Part-Load Ratio (EIRF-PLR)
(14)
PLR is defined in equation 7 above
dT
=t
cws/oat
-t
chws
Figure 15
(15)
a3, b3, c3, d3, e3, and f3 are regression coefficients
The variable speed chiller EIRFPLR curve can be calculated in DOE-2.2 by entering an array of data
points that each contains three numbers: the part load ratio (PLR), the difference between the entering condenser water temperature and the leaving chilled water temperature (dT), and the EIRFPLR
(defined in Equation 11 above). For example, the data point (1.0, 41, 1.0) translates as, “...when the
chiller is at 100% of its load, and there is a 41-degree temperature difference between the condenser
temperature and the leaving chilled water temperature, the chiller is at 100% of its power draw at the
current lift conditions.”
21
Hydeman, M.; N. Webb; P. Sreedharan; S. Blanc. “Development and Testing of a Reformulated
Regression Based Electric Chiller Model.” ASHRAE, Atlanta GA. HI-02-18-02.2002.
22
Hirsch, J.J. 2009. Volume 6: DOE-2.2 New Features, Off-Rated and Variable-Flow Condensers.
PART 2: Energy Efficient Chillers
21
Data Required for Specifying Chiller Performance Curves in
DOE-2
To develop curves that accurately model chiller operation, the energy modeler needs access to at least
twenty to thirty records of data which fully cover the range of conditions that will be simulated.23 The
required data consists of one subset of full-load data and a second subset of part-load data. The modeler
is cautioned to ensure that the data covers the full range of conditions under which the chiller will be
modeled.
During the simulation, if the chiller is subjected to conditions outside of the range of tuning data, very
unpredictable and inaccurate results can occur.
Full-load data used for defining the CAPFT and EIRFT curves must represent the entire range of
condenser and chilled water supply temperatures that will be evaluated by the energy model. For
water-cooled chillers, condenser temperature is defined as the entering condenser water temperature;
for air-cooled chillers, the condenser temperature is defined as the outside air drybulb temperature. To
generate the full-load curves, there must be at least six full-load data points, with at least two different
values for both chilled water and condenser temperatures, and the data points must include both the
minimum and maximum chilled water and condenser temperature values that will be evaluated by the
model. The information required for each full-load data point includes chiller capacity, input power,
chilled water temperature, and condenser temperature. Although the energy modeler can generate
With the exception of
cool, dry climates,
cooling coils provide
inadequate removal
of latent load when
cooling to only 64ºF
or 67ºF.
full-load curves with as little as six data points, a significantly greater number of distinct full-load data
points (i.e., 10 to 20 points) should be used to avoid skewed or inaccurate results.
Part-load data used for defining the EIRFPLR curve must represent the complete range of chiller
unloading that will be analyzed within the energy model. At least three distinct data points are required
in order to develop the EIRFPLR curve, but a significantly larger number of points (i.e., 6 to 10 points)
should be used to improve the accuracy of the chiller curve. In DOE-2.2, at least six distinct points are
required when defining the EIRFPLR curve for VSDs, and additional data should be included whenever possible. For each data point defined in any EIRFPLR curve, the minimum amount of information
needed from the chiller sales representative includes chiller capacity, input power, and condenser and
evaporator temperatures. Additionally, each part-load data point must have a corresponding full-load
data point with matching evaporator and temperatures.
23
Hydeman, Mark, and Gillespie, Kenneth L. Jr., pp.3.
22 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
Obtaining Data Required for Chiller Performance Curves
Collecting the data necessary to develop these three performance curves is a lot of work, and getting
the data requires a major time commitment from the chiller sales representative. Manufacturer’s sales
personnel have the software needed to calculate each of the data points required. However, developing
a full set of data (twenty to forty points) can take up to an hour per chiller to develop. In general, a
sales representative will provide the detailed data required to generate chiller curves if a potential sale is
likely and the request for information is coming directly from the customer. This means that the customization of chiller curves must often take place late in the construction documents phase, or during
bids for project construction. When requesting data from the manufacturer, there are two important
issues to consider:
•
The manufacturer will typically confuse “full-load” and “rated load” conditions. It is not
uncommon for a manufacturer to provide a spreadsheet full of data where all of the data is at the
same design capacity. It takes less time to do this than to make sure that the unloading mechanism
(e.g., inlet vanes, slide valve, VSD...) is fully open at a given set of temperatures for each set of data.
If the full load capacity does not vary with temperature, the energy modeler is forced to use the
default DOE-2 EIRFT and CAPFT curves, and then develop a custom curve for the EIRFPLR.
•
The manufacturer’s data includes “tolerance” on the capacity and efficiency as allowed by
ARI Standard 550/590. This tolerance is low at full load (~5%) but gets very high at part load.
Experience with “0 tolerance” performance-based chiller bids shows that the manufacturers
typically inflate their efficiency by the full ARI tolerance.
Condenser Water Temperature Assumptions
Figure 16:
This graph shows
the condenser water
temperatures used
when implementing
ARI Standard 550/5901998 test procedures.
PART 2: Energy Efficient Chillers
23
ARI Standard Tolerances as a function of Full Load24
Figure 17:
The ARI Standard
allows this tolerance
to be applied on
capacity, EER, COP kW/
ton, and heat balance.
Methods for Creating Custom DOE-2 Chiller Models
Generally speaking, there are three methods for developing calibrated DOE-2 chiller models. These
are presented in the order from least to most accurate:
Method 1. Scale the capacity (SIZE) and efficiency (EIR) only, and use the default CAPFT, EIRFT
and EIRFPLR curves. Method 1 should be avoided, as it is the least accurate.
Method 2. Scale the capacity (SIZE) and efficiency (EIR) and create a custom EIRFPLR curve. (This
method retains the DOE-2 default CAPFT and EIRFT curves). Care must be taken to adjust the reference capacity and efficiency to the CAPFT and EIRFT curve reference of 44°F chilled water supply and
85°F condenser water supply temperatures. Energy modelers are often forced to use this method where
there is limited performance data available or where the manufacturer has provided “full load” data all
at a single design capacity. Method 2 is also the best that can be done with field-measured data that
cannot be separated into full and part-load bins, and can provide surprisingly accurate results.
A variation of Method 2 is described in the paper, “Tols and Techniques to Calibrate Electric Chiller
Component Models,” but it requires a database of chiller performance curves. However, the data
should be used with caution, as it is easy to select a reference curve that was not calibrated to the
full range of temperatures used to simulate chiller operation (e.g., the energy modeler is in danger of
extrapolating performance beyond the tuning data).
Method 3. Scale the capacity (SIZE) and efficiency (EIR) and create custom CAPFT, EIRFT and
EIRFPLR curves. This is the preferred method for developing calibrated DOE-2 chiller models.
24
Source: Taylor Engineering
24 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
Manufacturer’s Data Request Form
Figure 18:
The data request
form shown provides
a simple means for
requesting chiller
curve input data from
chiller manufacturers. A more substantial manufacturer’s
data request form is
available in Appendix
B-1 of the EDR
Chilled Water Plant
Design Guide (Dec.
2009). http://energydesignresources.
com/resources/
publications/designguidelines/designguidelines-cooltoolschilled-water-plant.
aspx
The CoolTools Chiller
Bid and Performance
Tool (Excel spreadsheet) is also available at http://energydesignresources.
com/resources/
software-tools/cooltools/cooltoolschiller-bid-and-performance-tool.aspx
PART 2: Energy Efficient Chillers
25
Methods for Including Chiller Data in DOE-2 Performance
gCurves
DOE-2 offers two methods for creating chiller curves: the DATA method and the COEFFICIENT
method. In the DATA method, the programmer defines each data point directly in DOE-2. In the
COEFFICIENT method, the programmer calculates the regression coefficients for each curve based on
the available data, and inputs these regression coefficients into the model. VisualDOE and eQUEST
support both methods for defining chiller curves, while the current version of EnergyPro supports only
the COEFFICIENT method. The COEFFICIENT method is much more time-consuming than the
DATA method, yet produces the same results. Therefore, the most time-efficient method for modeling
custom chiller curves for EnergyPro projects may be to generate a DOE-2 input file from EnergyPro,
modify the input file with the custom chiller curves, and run the input file directly in DOE-2.
Calculating the CAPFT Curve in DOE-2
The CAPFT curve requires three pieces of data per point: the CAPFT, the chilled water supply temperature, and the condenser temperature. Each CAPFT point is calculated as follows:
(16)
Where:
= chiller capacity at specified temperature conditions
= reference capacity, which can be selected based on either the design capacity or the ARI-rated capacity of the chiller, but must be equal to the nominal capacity defined for the chiller in DOE-2.
Qi
To define CAPFT curves using the DATA method in DOE-2.1e, the energy modeler should group
together the Q
corresponding
chilled water temperature, condenser temperature, and CAPFTi for each
ref
point by enclosing these three values into a single set of parentheses. The first point in the curve must
be normalized to 1.0, and must correspond to the conditions at which the CAPACITY and EIR are
specified. A sample DOE-2.1e CAPFT curve is shown in Figure 20.
In DOE-2.2, curves are defined by grouping all the data for a given input parameter into a single array.
For example, in the CAPFT curve, the chilled water leaving temperatures are
26 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
listed in the “INDEPENDENT-1” array, the condenser temperatures are listed in the
“INDEPENDENT-2” array, and the calculated values for CAPFTi are listed in the “DEPENDENT”
array. The sample DOE-2.1e CAPFT curve shown in Figure 20 below would be defined in DOE-2.2
as shown in Figure 21.
eQuest Chiller Curve- DATA Method
Figure 19:
The screen shot
shows how to define
a chiller curve in
eQuest using the Data
Method.
DOE-2.1e CAPFT Curve- DATA Method
$********************************************************$
$
DOE-2.1e CAPFT Curve - DATA Method $
Figure 20
$********************************************************$
$ Insert curve under PLANT-ASSIGNMENT in the PLANT portion of
$ the DOE-2 input file
TYP-CAPPFT = CURVE-FIT
TYPE = BI-QUADRATIC
DATA
$ (CHWSi, CWSi, CAPFTi)
DATA (44,85,1.000) (42,85,0.981)(40,85,0.946) (38,85,0.911)
(42,75,1.035)(40,75,1.035) (38,75,0.989)
(42,65,1.035)(40,65,1.035) (38,65,1.035)
(50,65,1.035) $*****************************************************************$
$END DOE-2.1e CAPFT Curve - DATA Method
$
$*****************************************************************$
PART 2: Energy Efficient Chillers
27
DOE-2.2 CAPFT Curve- DATA Method
$********************************************************$
Figure 21
$
DOE-2.2 CAPFT Curve - DATA Method
$
$********************************************************$
$ Insert curves after the last input for SPACE
“TYP-CAPFT” = CURVE-FIT
TYPE
= BI-QUADRATIC-T
INPUT-TYPE
= DATA
INDEPENDENT-1
=
(44,42,40,38,42,40,38,50,42,40,38)
$Independent-1 defines chilled water leaving temperature
INDEPENDENT-2
=
(85,85,85,85,75,75,75,65,65,65,65)
$Independent-2 defines condenser water entering tempera-
ture
DEPENDENT
=
( 1.000,0.981,0.946,0.911,1.035,1.035,0.989,
1.035,1.035,1.035,1.035)
$Dependent defines calculated values for CAPFTi
..
$*********************************************************$
$
END DOE-2.2 CAPFT Curve - DATA Method
$
$*********************************************************$
In the COEFFICIENT method, the inputs for chilled water supply temperature, entering condenser
water temperature, and CAPFT are folded into an input matrix which can be solved for the six regression coefficients in equation 4 using the least squares linear regression routine.13 The matrix for the
CAPFT curve appears as follows:
(17)
Where:
tchws
= the chilled water supply temperature (°F)
tcws/oat = the condenser water supply temperature (°F) for water-cooled equipment and the outdoor air dry-bulb temperature (°F) for air-cooled equipment
A typical CAPFT curve in DOE-2.1e using the COEFFICIENT method would be defined as shown
in Figure 22.
28 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
DOE-2.1e CAPFT Curve- COEFFICIENTS Method
$**************************************************************$
$ DOE-2.1e CAPFT Curve - COEFFICIENTS Method $
Figure 22
$**************************************************************$
$ Insert curve under PLANT-ASSIGNMENT in the PLANT portion of
$ the DOE-2 input file
TYP-CAPFT = CURVE-FIT
TYPE = BI-QUADRATIC
$ COEF = (a2, b2, c2)
COEF=( -0.38924542, -0.02195141, -0.00027343
0.04974775, -0.00053441, 0.00067295)
..
$*******************************************************************$
$ END DOE-2.1e CAPFT Curve - COEFFICIENTS Method $
$*******************************************************************$
Calculating the EIRFT Curve in DOE-2
The EIRFT curve is similar to the CAPFT curve, but replaces the “CAPFT” term with a term for
“EIRFT”. Similarly to the CAPFT curve, the first point in the EIRFT curve must be normalized to
1.0, and must correspond to the conditions at which the CAPACITY and EIR are specified. When
implemented correctly, this curve should show the best chiller performance at low lift conditions and
the worst performance at high lift conditions. Each EIRFT point is calculated as follows:
(18)
where:
Qi and Qref are defined in equation 16, and
Pi = chiller input power at specified temperature conditions
Pref = reference input power which can be selected based on either the design capacity or the ARI-rated
capacity of the chiller; but must use the same conditions as Qref.
PART 2: Energy Efficient Chillers 29
Using the DATA method, the EIRFT curves can be defined in DOE-2.1e or DOE-2.2 by implementing the same format as that shown for the CAPFT curves in Figures 20 and 21 respectively. In each
case, the EIRFTi terms should replace the CAPFTi terms.
The EIRFT curve can also be defined in DOE-2 using the COEFFICIENT method. The coefficients
should be calculated by folding values for chilled water supply temperature, entering condenser water
temperature, and EIRFT into an input matrix, and solving for the six curve coefficients in equation 5
using the least squares linear regression routine.14
The matrix for the EIRFT curve appears as follows:
(19)
where:
tchws
=
tcws/oat =
the chilled water supply temperature (°F)
the condenser water supply temperature (°F) for water-cooled equipment and the outdoor air dry-bulb temperature (°F) for air-cooled equipment.
The DOE-2.1e formatting for the EIRFT coefficient method is similar to that shown Figure 22.
Calculating the EIRFPLR Curve in DOE-2
For the EIRFPLR curve, the following pieces of information are required for each data point: the
CAPFT at the current evaporator and condenser temperatures; the EIRFT at the current evaporator
and condenser temperatures; the part-load ratio (PLR), and the ratio of power for part-load to power
for full-load (EIRFPLR) at the given lift conditions. As with the EIRFT and CAPFT curves, the first
point defined for this curve should also be normalized to 1.0, and should correspond to the conditions at which the CAPACITY and EIR are specified. The PLR is defined as the ratio of the present
capacity over the full-load capacity at the given lift conditions. For each data point, the chilled water
and condenser temperatures for the part-load point should be equal to the temperatures used for the
corresponding reference full-load point.
(20)
where:
Qi = part-load capacity
Qref = full-load capacity at the same evaporator and condenser temperatures
CAPFT is the capacity as a function of temperature curve evaluated at the current temperature conditions.
30 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
The EIRFPLR for each point is calculated as follows:
(21)
where:
Pi = part-load input power
Pref = full-load input power at the same evaporator and condenser temperature, for the given part-load
capacity
CAPFTi is the capacity as a function of temperature curve evaluated at the current temperature conditions.
EIRFTi is the energy-efficiency ratio as a function of temperature curve evaluated at the current temperature conditions.
Consider a sample point of data with a part-load capacity of 797 tons, power consumption of 355 kW,
entering condenser water temperature of 75ºF, and chilled water supply temperature of 44ºF. The
reference full-load data point, having the same condenser and evaporator temperature, has a capacity
of 1,150 tons power consumption of 698 kW. The ARI-rated full-load capacity is 1,200 tons, with a
power consumption of 708 kW. ARI-rated values were used for defining SIZE and EIR in the DOE-2
input file.
PART 2: Energy Efficient Chillers
31
VisualDOE Chiller Curve Definition- DATA Method
Figure 23:
Chiller Curve defined
in VisualDOE using the
data method, where X
is the PLR, and Z is the
EIRFPLR.
DOE-2.1e EIRFPLR Curve- DATA Method
$********************************************************$
Figure 24
$ DOE-2.1e EIRFPLR Curve - DATA Method
$
$********************************************************$
$ Insert curve under PLANT-ASSIGNMENT in the PLANT portion of
$ the DOE-2 input file
TYP-EIRFPLR = CURVE-FIT
TYPE = QUADRATIC
DATA $ (PLRi,EIRFPLRi)
DATA =
(1.000,1.0000)
$kw/ton = 0.667
(0.843,0.8595)
$kw/ton = 0.637
(0.749,0.7448)
$kw/ton = 0.621
(0.656,0.6549)
$kw/ton = 0.624
(0.562,0.5649)
$kw/ton = 0.628
(0.468,0.5195)
$kw/ton = 0.693
(0.375,0.4396)
$kw/ton = 0.733
(0.281,0.3598)
$kw/ton = 0.800
(0.187,0.3049)
$kw/ton = 1.017
(0.094,0.2549)
$kw/ton = 1.700
..
$*********************************************************$
$
END DOE-2.1e EIRFPLR Curve - DATA Method
$
$*********************************************************$
32 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
The CAPFT evaluated at the current temperature conditions would be calculated as:
The EIRFT evaluated at the current temperature conditions would be calculated as:
The EIRFPLR point would be calculated as:
Using the DATA method, a typical EIRFPLR curve in DOE-2.1e would be defined shown in Figure
24.
The same EIRFPLR curve, representing part-load chiller performance for a chiller without a Variable
Speed Drive (VSD), should be entered in DOE-2.2 as shown in Figure 25.
DOE-2.1e EIRFPLR Curve without VSD- DATA Method
$********************************************************$
$
DOE-2.2 EIRFPLR Curve without VSD - DATA Method $
$********************************************************$
$ Insert curve after the last entry for SPACE
“TYP-EIRFPLR” TYPE
INPUT-TYPE
= CURVE-FIT
= QUADRATIC
Figure 25:
A sample DOE2.2
EIRFPLR curve for a
chiller with a VSD that
is shown in figure 30.
= DATA
INDEPENDENT = (1.000, 0.843, 0.749, 0.656, 0.562,
0.468, 0.375, 0.281, 0.187, 0.094)
$ independent dataset includes all PLRi terms
DEPENDENT
= (1.000, 0.8595, 0.7488, 0.6549, 0.5649
.5195, 0.4396, 0.3598, 0.3049, 0.2549)
$ dependent dataset includes all EIRFPLRi terms
..
$************************************************************$
$
END DOE-2.2 EIRFPLR Curve without VSD - DATA Method $
$************************************************************$
PART 2: Energy Efficient Chillers
33
In the COEFFICIENT method, inputs for PLR and EIRFPLR are folded into an input matrix which
can be solved for the three curve coefficients using the least squares linear regression routine.24 The
format for the EIRFPLR curve is as follows:
Figure 26
`
(22)
Where a1, b1, and c1 are the regression coefficients for the curve.
The matrix for the EIRFPLR curve appears as follows:
(23)
The EIRFPLR curve should be defined in DOE-2.1e as shown in Figure 26.
DOE-2.1e EIRFPLR Curve- Coefficients Method
$********************************************************$
$
DOE-2.1e EIRFPLR Curve - COEFFICIENTS Method
$
$********************************************************$
$ Insert curve under PLANT-ASSIGNMENT in the PLANT portion of
$ the DOE-2 input file
TYP-EIRFPLR = CURVE-FIT
TYPE = QUADRATIC
$ COEF = (a1, b1, c1)
COEF=(0.27715900,-0.02504500,0.73693600)
..
$*********************************************************$
$ END DOE-2.1e EIRFPLR Curve - COEFFICIENTS Method
$
$*********************************************************$
In DOE-2.2, the EIRFPLR curve for VSDs should be modified to include a third value for each point
of data that indicates the differential between the condenser and chilled water supply temperatures.25
The curve type should be selected as “BI-QUADRATIC-RATIO&dt,” where the first independent
dataset lists the values for PLRi, the second independent dataset lists the values for the temperature differential between the condenser and entering chilled water, and the dependent dataset lists the values
for EIRFPLR. The sample code shown in Figure 30 demonstrates the format for defining this curve
in DOE-2.2.
Once the three chiller curves have been defined, they should each be attached to the corresponding
chiller. In DOE-2.1e, this is accomplished by associating each chiller curve with the corresponding
25
More details regarding the definition of this curve, defined as a “BI-QUADRATICRATIO&dT” curve can be found in the “New Features,” Volume 6 manual by James Hirsch (see
footnote 3 from chapter 1).
34 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
eQuest Chiller Curve Definition
Figure 27:
Define EIRFPLR curve
for variable speed
chiller as”BiQuadratic in Ratio
& dT” in eQuest.
chiller type. For example, for an open centrifugal chiller, selected as OPEN-CENT-CHLR, the curves
would be attached under EQUIPMENT-QUAD using the keywords OPEN-CENT-CAP-FT for the
CAPFT curve, OPEN-CENT-EIR-FT for the EIRFT curve, and OPEN-CENT-EIR-FPLR for the
EIRFPLR curve (see Figure 31).
Energy Pro Chiller Curve Definition- COEFFICIENTS Method
Figure 28:
Chiller curve defined
in Energy Pro using
the coefficients
method.
PART 2: Energy Efficient Chillers
35
eQuest Chiller Curve Definition- DATA Method
Figure 29:
Insert points for
EIRF-PLR curve for
Variable Speed Chiller
in eQuest.
36 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
DOE-2.2 EIRFPLR Curve for Chillers with Variable Speed Drives- DATA Method
$**************************************************************$
$
DOE-2.2 EIRFPLR Curve - DATA Method
Figure 30
$
$ (TO BE USED FOR CHILLERS WITH VARIABLE SPEED DRIVES)
$
$**************************************************************$
$ Insert curves after the last input for SPACE
“VSD-EIRFPLR” TYPE
INPUT-TYPE
= CURVE-FIT
= BI-QUADRATIC-RATIO&dt
= DATA
INDEPENDENT-1 = (1.000,0.900,0.751,0.630,0.540
0.450,0.360,0.270,0.180,0.141)
$ independent dataset # 1 includes all PLRi terms
INDEPENDENT-2 = (41.000,37.000,33.000,29.000,25.000
21.000,21.000,21.000,21.000,21.000)
$ independent dataset # 2 includes all dtiterms
DEPENDENT =
(1.000,0.828,0.673,0.542,0.440
0.367,0.330,0.259,0.203,0.184)
$ dependent dataset includes all EIRFPLRi terms
..
$*************************************************************$
$ END DOE-2.2 EIRFPLR Curve - DATA Method for VSD Chillers $
$*************************************************************$
PART 2: Energy Efficient Chillers
37
DOE-2.1e Sample Code showing how to Attach Chiller Curve to a Chiller
Figure 31
$***************************************************$
$
DOE-2.1e Attach Chiller Curves to Chiller
$
$***************************************************$
$ TYP-CAPFT curve, TYP-EIRFT curve, and TYP-EIRFPLR curve
$ are defined under PLANT-ASSIGNMENT as shown in
$ Figures 20, 22, 24, and 26
OPEN-CENT-CAP-FT
TYP-CAPFT
$Attaches TYP-CAPFT curve to open centrifugal chiller
OPEN-CENT-EIR-FT
TYP-EIRFT
$Attaches TYP-EIRFT curve to open centrifugal chiller
OPEN-CENT-EIR-FPLR
TYP-EIRFPLR
$Attaches TYP-EIRFT curve to open centrifugal chiller
..
$*******************************************************$
$ END DOE-2.1e Attach Chiller Curves to Chiller $
$*******************************************************$
In DOE-2.2, chiller curves are attached to the chiller at the position of the code where the chiller is
defined. The format for attaching chiller curves in DOE-2.2 is shown in Figure 32.
DOE-2.2 Sample Code showing how to Attach a Chiller Curve to a Chiller
$**************************************************$
Figure 32
$ DOE-2.2 Attach Chiller Curves to Chiller $
$**************************************************$
“Typical Chiller” TYPE
CAPACITY
= CHILLER
= ELEC-OPEN-CENT
= 3.66
ELEC-INPUT-RATIO = 0.1726
CAPACITY-FT
= “Typ-CAPFT”
$ TYP-CAPFT curve attached to chiller
= “TYP-EIRFT”
$ TYP-EIRFT curve attached to chiller
EIR-FT
EIR-FPLR = “TYP-EIRFPLR” $ TYP-EIRFPLR curve attached to chiller
CHW-LOOP
= “Chilled Water Loop”
CHW-PUMP
= “Primary CHW Loop Pump”
CONDENSER-TYPE = WATER-COOLED
CW-LOOP
= “Condenser Water Loop”
..
$*******************************************************$
$ END DOE-2.2 Attach Chiller Curves to Chiller $
$*******************************************************$
38 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
Guidelines for Creating Accurate Custom Chillers
Due to the complexity involved in creating custom chiller curves, energy modelers have
been known to improperly develop them and end up with results that are less accurate
than those available using default curves. To ensure the greatest accuracy when modeling custom chiller
curves, follow these important guidelines:
•
Get accurate data from the chiller manufacturer. Make sure that the “full load” data is truly at
an operating point with the chiller fully loaded.
•
Derate the data based on the ARI tolerance curve. Do not derate the capacity, but use the full
tolerance to derate the stated power at each point of operation.
•
Make sure that the data points used for the curves extends over the full range of simulated
operation. If you extrapolate performance beyond the data points, you will get useless results.
•
Use sufficient amounts of data. Results will increase in accuracy as the number of data points is
increased, provided that the data points cover the full range of conditions that will be simulated
in the energy analysis program.
•
Pay attention to the order of data input. Whether using the COEFFICIENT method or the
DATA method to define chiller curves, the order of the data input must be entered as outlined
in this simulation guidebook. For example, in the EIRFPLR curve, information for PLR should
be listed first in the data pair, and EIRFPLR should be listed second. If the order of the data is
reversed, then the energy efficiency of the chiller at various part-load conditions will be calculated
incorrectly, and therefore, overall chiller energy use will be inaccurate.
Summary
Using the chiller modeling methodology described in this guidebook, energy modelers can assist the
design team in developing chiller plants with tremendous potential savings. When energy modelers
combine the advanced control sequences modeling strategies discussed in the third section of this simulation guidebook with these strategies for modeling chiller curves, the energy model can become a particularly effective tool for optimizing chilled water plant performance. For example, an article entitled
“Commissioning Tools & Techniques Used in a Large Chilled Water Plant Optimization Project,”26
reported a 6.5 GWH/year reduction in plant energy through chiller replacement and controls in a
17,000 ton chiller plant. Careful development of calibrated energy models for this plant enabled the
team to analyze the various design options, and achieve the maximum possible energy and cost savings
for the plant. These savings were verified by post-retrofit monitoring.
26
Hydeman, M.; S. Tylor; C. Speck; and K. Gillespie. “Commissioning Tools & Techniques Used
in a Large Chilled Water Plant Optimization Project.” Proceedings of the 7th National Conference
on Building Commissioning. PECI, Portland Oregon. May 1999.
PART 2: Energy Efficient Chillers
39
Steps for Creating Custom Chiller Curves in DOE-2-Based Programs
1.
Procure 20-40 points of data from chiller manufacturers, where each point includes condenser
temperature, chilled water supply temperature, chiller capacity, input power, and percent loading. Data should include both part-load and full-load data over the full range of condenser and
chilled water temperatures for which the chiller will operate.
2.
Select the reference point by which the chiller curves will be normalized. For DOE-2.1e, the
reference point should be selected at ARI-rated conditions. For recent versions of DOE-2.2, the
reference point should correspond to the most common full-load operating conditions.
3.
Generate CAP-FT Curve
a.
For each set of full-load data, calculate CAPFTi, defined in equation 16 above.
b.
DATA Method: Enter each set of full load data points for the CAPFT curve into a DOE-2
based simulation program, where each point includes a term for chilled water temperature,
condenser temperature, and CAPFTi. Confirm that the CAPFTi for the first point is normalized to 1.0 based on the reference point identified in Step # 2 above. Figure 20 defines
the format for entering CAPFT curves in DOE-2.1e, and Figure 21 indicates the format
for DOE2.2 CAPFT curves.
c.
COEFFICIENTS Method: Input each set of full load data points including chilled water
temperature, condenser temperature, and CAPFTi into the matrix shown in equation 17,
and solve for the six regression coefficients. Define the curve coefficients in DOE-2.1e
using the format shown in Figure 22.
4.
Generate EIR-FT Curve
a.
For each set of full-load data, calculate EIRFTi, defined in equation 18 above.
b.
DATA Method: Enter each set of full load data points for the EIRFT curve into a DOE-2
based simulation program, where each point includes a term for chilled water temperature,
condenser temperature, and EIRFTi. Confirm that the EIRFTi for the first point is normalized to 1.0 based on the reference point identified in Step # 2 above. Figure 20 defines
the format for entering EIRFT curves in DOE-2.1e, and Figure 21 indicates the format
for DOE2.2 EIRFT curves (where in each case the term for EIRFTi replaces the term for
CAPFTi).
c.
COEFFICIENTS Method: Input each set of full load data points including chilled water
temperature, condenser temperature, and CAPFTi into the matrix shown in equation 19,
and solve for the six regression coefficients. Define the curve coefficients in DOE-2.1e
using the format shown in Figure 22.
40 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
5.
Generate EIR-FPLR Curve
a.
For each set of part-load data, calculate the PLRi and EIRFPLRi as defined in equations 20
and 21 respectively. Note that you will need to calculate the CAPFTi and EIRFTi at each
point in order to calculate the PLRi and EIRFPLRi. If you are defining a curve for a variable speed chiller in DOE2.2, you should also calculate dT as defined in equation 15.
b.
DATA Method: Enter each set of full load data points for the EIRFPLR curve into
a DOE-2 based simulation program, where each point includes a term for PLRi and
EIRFPLRi. Confirm that the PLRi and EIRFPLRi for the first point are normalized to
1.0 based on the reference point identified in Step # 2 above. For curves defined in DOE2.1e, use the format shown in Figure 24. For chillers without variable speed drives, defined
in DOE-2.1e, use the format shown in Figure 25. For chillers with variable speed drives
defined in DOE2.2, include terms for PLRi, dT, and EIRFPLRi as demonstrated in Figure
30.
c.
COEFFICIENT method: Input the data for PLRi and EIRFPLRi into the matrix shown in
equation 23, and solve for the three regression coefficients. Enter the curve coefficients in
DOE-2.1e using the format shown in Figure 26.
6.
Attach the custom chiller curves to the appropriate chiller using Figure 31 for DOE-2.1e, and
Figure 32 for DOE2.2.
PART 2: Energy Efficient Chillers
41
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42 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
PART 3: Advanced Control Sequences
The recent widespread use of digital controls in building construction has greatly expanded the opportunities for optimizing building efficiency. Using digital controls provides more accurate sensing of
data and enhances flexibility for modifying control logic. However, relatively few buildings that use
digital control technologies attain their full potential for cost-effectively minimizing energy demand and
consumption. Common problems that prevent the use of efficient digital control technologies include:
•
Misinformation regarding the risks and benefits of the technology;
•
An inadequate understanding of the energy and cost benefits associated with these •
COMMON
Simulation
software
This guidebook uses
research generated
strategies, and
from the following
Complete ignorance regarding the availability of such strategies.
energy simulation
software packages:
Energy models that accurately demonstrate the operating cost benefits of these technologies can present decision makers with compelling reasons for implementing the strategies into the project design.
Accordingly, this simulation guidebook highlights the following digital control strategies and sequences
EnergyPro v. 3.142,
eQUEST v. 3.44 with
DOE2.2 release 42k6, and
DOE-2.1e release 134.
that may improve efficient operation of water-side systems. The guidebook also provides a guide for
Keep in mind that this
modeling each technology in EnergyPro, native DOE-2.1e, and eQUEST:
software is constantly
updated. Review the
documentation of
•
Variable primary flow chilled water distribution (page 44)
•
Primary/secondary chilled water distribution (page 46)
•
Variable flow condenser water system (page 52)
•
Cooling tower cell control (page 53)
•
Cooling tower capacity control (page 53)
modeling methodology
•
Condenser water temperature reset (page 55)
discussed in this
•
Chilled water temperature reset (page 57)
simulation guidebook.
•
Hot water temperature reset (page 58)
later releases for any
changes to software
inputs or keywords
that might impact the
PART 3: Advanced Control Sequences
43
Variable Speed Drive Control Sequences
When controlled correctly, variable speed drives (VSDs) installed on fan, pump, and compressor
motors can significantly limit equipment energy usage by optimizing the part-load performance of
the equipment. In recent years, VSDs have become an increasingly common option for HVAC system equipment due to the improved reliability of VSD technology and more stringent energy code
Following Through
on Control
Strategies
Each control strategy
requirements. However, design teams often only specify equipment with VSDs when this technology
is required to minimally comply with energy code standards. To promote further use of these efficient
technologies, California Title-24 2008 standards prescriptively require variable speed controls for
pumps over 5 hp that serve variable-flow systems.
simulated in the final
building energy model
Using DOE-2 simulation programs to model variable-speed control sequences for waterside systems is
MUST match the equipment
often difficult due to (a) the limited modeling capabilities of DOE-2.1e-based software (i.e. EnergyPro),
data and control schemes
and (b) the vast range of available inputs in DOE-2.2 programs such as eQUEST. This portion of the
defined in the plans and
control sequences for the
building. Unfortunately,
control strategies are
simulation guidebook discusses methods for modeling the following variable-speed control strategies in
EnergyPro, native DOE-2.1e, and eQUEST:
often recommended based
•
Variable Primary Flow Chilled Water Distribution
on initial energy models
•
Primary/Secondary Chilled Water Distribution
and incorporated into the
•
Variable Flow Condenser Water System
final estimated energy
savings for the building,
but are excluded from
final plans or control
sequences. A building
commissioning process
Note that the variable-speed, variable-flow-pumping algorithms in the LBNL/USDOE version of
DOE-2.1e (which is used as the simulation engine for EnergyPro), provide only a rough approximation
of actual variable speed pumping control, whereas later versions of DOE-2.1e, and DOE-2.2 incorporate much more accurate simulation of variable-speed pumping control.
should be used to confirm
that control sequences
Variable Primary Flow Chilled Water Distribution
described in the design
In a variable primary flow chilled water distribution system, VSDs on the primary chilled water
document are used in the
final building design,
and to confirm that
the control sequences
function properly when
the building is completed.
pumps vary the flow through the chillers and out to the chilled water coils based on demand. These
systems generally include a bypass loop to ensure minimum flow through the chiller. When properly implemented, variable primary pumping generally provides the most cost-effective and energyefficient option for chilled water distribution. In EnergyPro, variable primary flow is modeled by
defining the primary chilled water pump data under the secondary chilled water pump tab according
to the following process:
• From the Plant / Chilled Water tab, click each chiller to view the primary pump inputs. Set Flow
Rate per Pump, Pump Multiplier, and Design Power to a value of zero. Close the chiller window.
• Select the Secondary Pumps window by clicking the graphic titled either No Secondary Pumps or
Secondary Pumps. Select the pump type as Variable Speed, and then enter the appropriate Minimum
Flow Per Pump, as well as all the other relevant inputs
44 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
pertaining to the primary chilled water pumps. In DOE-2.1e, energy modelers can simulate variable
primary chilled water flow by setting CCIRC-PUMP-TYPE to VARIABLE-SPEED, and specifying
the minimum electricity consumption as a fraction of full load consumption (CCIRC-MIN-PLR).
Variable Primary Flow Inputs for the
Chiller Window in EnergyPro
Variable Primary Flow Inputs for the
Secondary Chilled Water Pumps in EnergyPro
Figure 33 (left):
To define variable
primary flow in
EnergyPro, set the
values for flow, pump
multiplier, and design
power to 0 in the
chiller window.
Figure 34 (right):
Define variable
primary inputs in the
secondary chilled
water pump window in
EnergyPro.
DOE-2.1e Variable Primary Chilled Water Flow
PLANT-PARAMETERS
$Chilled Water Variable Primary CHW Pumps
CCIRC-PUMP-TYPE = VARIABLE-SPEED
Figure 35
$ defines primary flow as
$ variablespeed; (FIXED-SPEED
$ is the DOE-2default)
$ Minimum electricity
CCIRC-MIN-PLR = 0.300
$ consumption as fraction of
$ full-load consumption
CCIRC-HEAD = 100
CCIRC-IMPELLER-EFF = .7
$ Defines the head pressure
$ of the CHW loop
$ CHW pump impeller
$ efficiency
CCIRC-MOTOR-EFF = .85
$ CHW pump motor efficiency
$ Other Plant Parameters not shown
..
PART 3: Advanced Control Sequences
45
In eQUEST, users can directly model variable primary chilled water flow in the wizard interface using
the following steps:27
•
From the Cooling Primary Equipment screen, select a pump configuration of Single System
Pump(s) Only.28
•
Select chilled water flow as Variable.
•
Select Pump Eff / Control as VSD.
•
If the chilled water loop design specifies a bypass to ensure minimum flow through the chiller,
the loop minimum flow should be specified in the Detailed Data Edit interface under the chilled
water loop inputs.
•
eQUEST automatically defines all chilled water coils in a variable flow loop with 2-way valves.
If the chilled water coils for any system attached to the chilled water loop have 3-way valves
assigned to them, the CHW Valve Type for these systems should be changed to Three-Way in
the Detailed Data Edit interface.
Variable Primary Chilled Water Flow Depicted in eQuest Detailed Edit Mode
Figure 36
Primary/Secondary Chilled Water Distribution
Primary/secondary chilled water distribution systems generally consist of a constant flow loop through
the chillers and a variable flow loop to the chilled water coils. In EnergyPro, this configuration is modeled by (a) entering the constant speed pump data in the chiller window and (b) defining the variable
speed secondary pump data in the secondary pumps window:
•
From the Plant / Chilled Water tab, click each chiller to view the primary pump inputs. Set
Flow Rate Per Pump, Pump Multiplier, and Design Power as specified in the project design.
Close the chiller window.
27
A sample variable primary flow system is shown in the DOE-2.2 Volume 3 Topics Manual released
with the eQUEST program. See Example 7, pp. 310.
28
The selection of “single system pumps only” indicates that the pumps are located at the loop level
only, and no pumps are located at specific chillers. The loop pumps are sized to overcome all pressure
drops associated with the entire system including chillers, piping, and coils. The pump configuration
selection, “individual chiller pumps only,” indicates that each of the chilled water pumps are associated with individual chillers, and no pumps are located on the chilled water loop. The pumps that
serve each chiller are sized to overcome the pressure drop for the chiller they are assigned to plus the
pressure drop across the entire loop.
46 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
•
Select the Secondary Pumps window by clicking the graphic titled either No Secondary Pumps
or Secondary Pumps. Select the Pump Type as Variable Speed, and input the appropriate
minimum flow per pump, as well as all the other relevant inputs pertaining to the secondary
chilled water pumps.
Variable Primary Chilled Water Floor Defined in eQuest Wizard
Figure 37
Although the primary/secondary pumping configuration cannot be directly modeled in DOE-2.1e, the
energy usage of primary/secondary chilled water distribution systems can be approximated by (a) entering data for the variable flow secondary loop under the chilled water pump inputs and (b) inputting
the primary constant flow loop data under the condenser pump inputs.29 The effective head (TWRPUMP-HEAD) for the combined condenser water pump(s) and primary chilled water pump(s) can be
calculated by summing the products of the individual pump head and flow rate, and dividing by the
net system flow rate through the condenser and evaporator (see Figure 38).30
Motor efficiency and impeller efficiency for the pumps should be calculated using the flow-weighted
averages for the condenser water and primary chilled water pumps. This is shown in Figure 39.
In eQUEST, energy modelers can define primary/secondary flow by (a) entering the secondary chilled
water loop inputs in the wizard interface and then (b) completing the primary chilled water loop inputs
in the detailed interface:
•
From the Cooling Primary Equipment wizard screen, select a pump configuration of Single
System Pump(s) Only.
•
Select CHW Loop Flow as Variable and Pump Control for the loop as VSD.
•
When all wizard inputs have been entered, switch to Detailed Edit Mode by clicking Mode /
Detailed Data Edit.
29
This modeling approach assumes that condenser water pumps are constant speed.
“Procedures for Modeling Buildings to MNECB and CBIP.” Version 2.0, November, 2002.
30
PART 3: Advanced Control Sequences
47
Equations for Calculating Effective Head for Combined Condenser Water Pump(s) as an Input
for DOE-2.1e.
Figure 38
(24)
(25)
(26)
Where:
= Net System Flow Rate through the condenser and evaporator (in gpm).
= Flow Rate through the evaporator for each chiller (in gpm)
= Flow Rate through each condenser water pump (in gpm)
= Flow Rate through each primary chilled water pump (in gpm)
= Flow Rate through the condenser for each chiller (in gpm)
(DOE-2 keyword: TWR-PUMP-HEAD) = Effective head for the system (in feet)
= Pump head for each condenser water pump
= Pump head for each CHW pump
= Capacity of each chiller in tons
COMP-TO-TWR-WATER = DOE-2 Keyword to define flow (in gpm/ton)
48 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
Equations for Calculating Motor Efficiency and Impeller Efficiency for Combined Condenser
Water Pump(s) and Primary Chilled Water Pump(s) as an Input for DOE-2.1e
(27)
Figure 39
(28)
Where:
= net motor efficiency (DOE-2 keyword TWR-MOTOR-EFF)
= net impeller efficiency (DOE-2 keyword TWR-IMPELLER-EFF)
= condenser water pump motor efficiency for each pump
= condenser water pump impeller efficiency for each pump
= primary chilled water pump motor efficiency for each pump
= primary chilled water pump impeller efficiency for each pump
DOE-2.1e Primary/Secondary Chilled Water System with Variable Flow to Chilled Water Coils
PLANT-PARAMETERS
$Chilled Water Primary/Secondary CHW Pumps
Figure 40
$ Primary CHW Pumps + CDW Pumps - Constant Speed (Condenser pump includes a weighted
$average including all the primary chilled water pumps and all condenser pumps)
TWR-PUMP-HEAD = 10
$ uses value for effective head described above
TWR-IMPELLER-EFF = 0.77
$ net impeller efficiency
TWR-MOTOR-EFF = 0.9
$ net motor efficiency
$Compression chiller(‘s) condenser and evaporator water flow in units of gpm/ton.
COMP-TO-TWR-WTR = 5.4
$ Secondary CHW Pumps - Variable Speed
CCIRC-PUMP-TYPE = VARIABLE-SPEED
CCIRC-MIN-PLR = 0.300
$ defines secondary flow as variable speed;
$ For VSD Pumps, Minimum electricity
$consumption as fraction of full-load consumption
CCIRC-HEAD = 70
$ Defines the head pressure of the secondary CHW loop
CCIRC-IMPELLER-EFF = .75
$ Secondary CHW pump(s) impeller efficiency
CCIRC-MOTOR-EFF = .90
$ Other Plant Parameters not shown
$ Secondary CHW pump(s) motor efficiency
..
PART 3: Advanced Control Sequences 49
•
Right-click the Project icon in the Waterside HVAC module, and then select Create Pump. Assign a name to the pump, and then select Create from Scratch. Enter the appropriate
data for the primary pump(s) in the resulting pop-up window. Use caution to leave the pump
capacity control as One-Speed Pump.
•
Right-click the Project icon again, and then click Create Circulation Loop. Assign a name
to the loop, and select Create from Scratch. Select Circulation Loop Type as Chilled Water.
Enter the appropriate data for the primary chilled water loop in the resulting pop-up window.
Select Loop Pump as the primary pump(s) just created.
•
Double-click each chiller and change the Loop Assignment for CHW to the primary chiller loop created in the previous step.
•
Double-click the Secondary Loop and set Loop Subtype to Secondary. Select the Primary
Chilled Water Loop as the Attached Primary Loop, and then set Head Setpoint Control to
Fixed or Valve-Reset, depending on the actual controls scheme that will be used in the building.
In the resulting pop-up window, keep Valve-Type as Three Way, and enter any other pertinent
information.
eQuest Detailed Interface for Primary/Secondary Chilled Water Loop
Figure 41:
Final appearance of
primary/secondary
chilled water loop
in eQuest detailed
interface.
To create new pumps, or a new chilled water circulation loop in eQuest, right-click on the Project
icon and select the appropriate icon.
Figure 42
50 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
EQUEST provides an even simpler approach for modeling primary/secondary chilled water distribution
in cases where the primary loop is served by only one chiller:
•
From the Cooling Primary Equipment screen, select a pump configuration of Both System
and Chiller Pumps.
•
Set the CHW Loop Flow as Variable and Pump Control for the loop as VSD.
•
Leave the pump control under the chiller as Single Speed.
Using this primary/secondary configuration, the pump that is attached to the chiller will cycle with
the chiller during low loads. If this is not the way that the primary/secondary pumps will actually be
controlled, the first option for modeling primary/secondary CHW pump configuration should be used.
Cautions for Modeling Variable-Speed Pumps and Variable-Chilled Water Flow in eQUEST
When energy modelers make their chilled water pumping selections within the eQUEST wizard,
eQUEST provides reasonable assumptions regarding valve type and estimated chilled water flow.
However, energy analysts who use Detailed Edit Mode to change flow configurations from constant to
variable flow often make the following errors:
•
CHW valve-types on air handling units are not modified to reflect variable flow. Energy savings
for variable-flow CHW systems hinge on the inclusion of two-way chilled water valves in most,
if not all of the air-handling units. If the valves are left as three-way valves, the chilled water flow
remains constant, and VSDs on the pumps will have no impact on system performance.
•
CHW flow for the comparison system is incorrectly sized using coincident peak flow. When
performing a variable-flow analysis for Title-24 or Savings by Design, energy analysts should (a)
model the base case with variable flow (i.e., two-way CHW valves on the air-handling units) and
constant speed pumps; and (b) model the proposed case with variable-flow and variable-speed
pumps. This ensures that the chilled water flow is sized based on the non-coincident peak flow
rather than the sum of the peak flows through the air-handling units’ chilled water coils.
Primary/Secondary pumping configuration with a single chiller defined in the eQuest wizard
Figure 43
PART 3: Advanced Control Sequences
51
Variable Flow Condenser Water System
Energy Pro has provisions for modeling variable flow condenser water systems. After inserting a new
plant into the model, the user can go make a selection under the hydronic tab of user input window.
The user has the option to make three choices for pump flow control.
1.
One-speed/3-way valve
2.
One-speed/ 2-way valve
3.
Variable-Speed
In eQUEST, variable-condenser water flow is modeled in Detailed Data Edit Mode as follows:31
•
When all wizard data has been finalized, select Mode /Detailed Data Edit.
•
From the Water-Side HVAC module, select the condenser water loop pump(s) from the
condenser water loop.
•
Set Capacity Control as Variable Speed Pump, and then enter the appropriate data for the
minimum speed of the pump.
•
Select each chiller served by the condenser water loop. From the Loop Attachments tab, set
Condenser Water Flow Control as Variable Flow. Select the Minimum Condenser Water
Flow, defined as the “minimum allowable fraction of the design flow through the condenser,” as
specified in the control sequences. This minimum flow should be sufficiently high to maintain
turbulent flow through the condenser.
Variable Condenser Flow defined in eQuest
Figure 44
31
Variable condenser water flow can only be defined in recent versions of eQuest.
52 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
Condenser Water System Operation
Although the variable condenser water flow strategies discussed above will generally improve the
efficient operation of the chilled water plant, this strategy typically affords the greatest benefit when
used with other condenser water system control strategies. The following section of the simulation
guidebook discusses modeling techniques for three additional condenser water system control strategies:
•
Cooling tower cell control
•
Cooling tower capacity control
•
Condenser water temperature reset
Cooling Tower Cell Control
Central plant designs specifying multiple cooling towers or multi-cell cooling towers can capitalize on
the energy savings associated with variable speed tower fans by implementing control sequences to operate the maximum possible number of cells at any given time. In the Max Cells sequence, the control
system enables the greatest number of cells that can operate above their specified minimum flow ratios
at a given time, and spreads the load equally across these cells. Cooling tower applications designed to
provide very low flow will see the greatest energy savings from Max Cells controls.
In EnergyPro, the software hard-codes the Max Cells option into the DOE-2 input file whenever multiple cell cooling towers are defined. In DOE-2.1e, cooling tower cell control can be defined under
plant parameters by changing TWR-CELL-CTRL from MIN-CELLS to MAX-CELLS. The energy
modeler should also check DOE-2 inputs for minimum part load ratios to confirm that they match the
ratios defined in the system control sequences. In eQUEST, multi-cell cooling towers and cooling tower
cell control sequences should be selected from the Detailed Data Edit mode as follows:
•
Double-click the Cooling Tower icon in the Water-Side HVAC module. Enter the total number of cooling tower cells shown in the plans.
•
Set Cell Control as Maximum Cells. Define the cell minimum flow as the smallest ratio of flow
compared against the nominal flow for which each cell can operate.
Cooling Tower Capacity Control
Cooling tower capacity control describes the method used for regulating the exit temperature for the
water leaving the cooling tower. The four options most commonly seen in existing and new buildings
include:32
•
Fluid Bypass: a three-way valve bypasses water around the cooling tower, modulating as needed
to maintain capacity. This is the least efficient control option, since cooling tower fans run
continuously.
32
A full description of each cooling tower fan capacity control keyword is included in EnergyPro,
DOE-2.1e and eQuest documentation.
PART 3: Advanced Control Sequences
53
eQuest Detailed Edit mode input for maximum cell control
Figure 45
•
One-Speed Fan: the fan cycles on and off to maintain the set-point temperature.
•
Two-Speed Fan: the fan cycles between Off, Low-Speed, and High-Speed to regulate the exit water temperature.
•
Variable-Speed Fan: a variable-speed drive controls fan speed, so that the heat rejection capacity
exactly matches the load at the desired set-point. This is the most efficient option, and generally
reduces wear and tear on the fan motor associated with one or two-speed fan operation.
In EnergyPro, the energy modeler can identify fan capacity control as follows:
Cooling Tower Capacity Control Selections in EnergyPro
Figure 46
•
From the Plant / Chilled Water tab, double-click the cooling tower icon. A water-cooled chiller
must be defined on the Chilled Water tab before the cooling tower icon will appear.
•
Click Cooling Tower in the pop-up window, and then select an existing tower to edit, or create
a new cooling tower. Select the capacity control to match the controls shown on the plans.
54
HVAC SIMULATION GUIDEBOOK - 2ND EDITION
In DOE-2.1e, energy modelers can define cooling tower capacity in the PLANT-ASSIGNMENT section as follows:
•
TWR-CAP-CTRL to FLUID-BYPASS, ONE-SPEED-FAN, TWO-SPEED-FAN, or
VARIABLE-SPEED-FAN as defined in the plans.
•
For two-speed fan control, enter the air flow ratio and fan power ratio for the low-speed setting
as defined in equations 29 and 30:
(29)
(30)
•
For variable-speed fan control, use the DOE-2 keyword TWR-MIN-FAN-SPEED to enter the minimum fraction of nominal fan speed at which the fan can operate.
eQuest Detailed Edit Mode Inputs Associated with Variable Speed Fan Control
Figure 47
eQUEST includes fan capacity control selections from the Heat Rejection screen of the wizard. When
wizard selections indicate two-speed or variable speed control, eQUEST generates default assumptions
for minimum fan speed and power. The Detailed Data Edit mode allows the option for modifying
these assumptions in the Water-Side HVAC module:
•
For variable speed fan control, the Min Fan Speed represents the minimum fraction of fan speed
where the fan can operate.
•
Two-speed fan control, Fan Low Flow and Fan Low Elec are defined according to equations
29 and 30, respectively.
Condenser Water Temperature Reset
Chillers operate most efficiently when at low lift conditions where the differential between chilled
water supply temperature and entering condenser water temperature reaches its minimum. Therefore,
reducing condenser water supply temperatures at chiller part-load conditions results in demonstrable
compressor energy savings. The condenser water reset strategies simulated in DOE-2.1e-based programs
are limited to wetbulb temperature reset control, which reduces condenser water set point temperatures
based on outside air wetbulb temperatures. This strategy rarely results in substantial energy savings. In
PART 3: Advanced Control Sequences
55
contrast, a reset control that varies the cooling tower fan speed based on chiller load allows the leaving
condenser water temperature to float with both wetbulb temperature and chiller load, producing sizable chiller energy savings. To demonstrate the energy benefit of condenser water load reset controls
in EnergyPro or other DOE-2.1e-based programs, energy modelers can perform a separate analysis of
these controls, and then integrate their findings into the DOE-2 results during post-processing of the
models. However, the California Energy Commission has not approved this modeling approach for
documenting Title-24 compliance.
Modeling Condenser Water Load Reset in eQUEST
eQUEST lets energy modelers select condenser water load reset and further refine the assumptions for
load reset controls in the Detailed Edit Mode:
•
From the Primary Heat Rejection screen, set Temperature Control as Reset, identify the
Minimum Condenser Water temperature as defined in the control sequences, and then select
the appropriate option for Cooling Tower Fan Capacity control. The system design must
specify Two-Speed or Variable-Speed cooling tower fans for the load reset control strategy to
function properly.
•
When the project design employs condenser water load reset controls in conjunction with
variable-condenser water flow, adjust inputs for minimum reset part-load ratio and maximum
reset speed to reflect the actual control sequences defined in the system. From the Detailed Data
Edit mode, double-click the cooling tower, and then specify the Min Reset PLR as the minimum
part load of the fan at which the minimum condenser water flow occurs. Also define the Max
Reset Speed as the maximum speed of the cooling tower fan during load reset operation.
eQuest Detailed Edit Mode Inputs Associated with Condenser Water Systems
Figure 48:
Enter the information
shown in this graphic
when both condenser
water load reset and
variable condenser
water flow are
implemented into the
project design.
56 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
Chilled Water Loop Temperature Reset
If implemented carefully, chilled water (CHW) temperature reset controls can reduce chiller energy
usage as well as the losses associated with chilled water distribution by increasing the chilled water supply
temperature set-point with decreasing load. The mechanical engineer should carefully examine chiller
constraints when identifying the appropriate CHW reset schedule, since optimum efficiencies for different chillers can vary greatly for any given CHW temperature, CW temperature, and percentage load
conditions. For example, constant speed centrifugal chillers reach maximum efficiencies when loaded
to greater than 80%, while the same chiller model equipped with a VSD performs best at 30-50% load
with a relatively high chilled water temperature. A chilled water temperature control strategy for the
constant speed chiller would have a very low reset range, whereas the variable-speed chiller may have
a reset temperature range of greater than 10ºF. For variable-flow chilled water loops, the design team
must also be careful to optimize the chilled water flow and temperature reset controls to minimize combined chiller and CHW pump energy usage. EnergyPro does not provide a means for modeling chilled
water temperature reset controls directly in the program interface. However, by modifying the input
file generated from EnergyPro, and running the simulation directly in DOE-2.1e, the energy modeler
can simulate the approximate impact of this strategy on building performance. The temperature range
for chilled water reset control is defined in DOE-2.1e using the keyword CHILL-WTR-THROTTLE.
To increase the chilled water temperature with decreasing load, a negative value must be entered for this
keyword. For example, to specify a reset temperature range of 10ºF, the CHILL-WTR-THROTTLE
should be -10ºF. While this modeling strategy loosely represents chilled water reset controls, the DOE-2
simulation results for this strategy are not exact.
Defining Chilled Water Temperature Reset in eQUEST
The eQUEST wizard interface provides a simple method for defining chilled water reset controls from
the Chilled Water System Control and Schedule screen:
•
Set Setpoint as Reset.
•
Enter the appropriate values for CHW Minimum and CHW Maximum temperatures.
•
When the chilled water circulation loop is a variable-flow loop, the loop flow reset should also
be defined as follows:
•
From the Detailed Data Edit mode, double-click the chilled water circulation loop in the
water-side HVAC module.
•
Select the Controls tab from the pop-up window, and then set the appropriate Loop-Flow
Reset. The chilled water reset control sequence should use Variable-Speed Pump controls
to limit the flow to the percentage defined for loop-flow-reset, and then hold the water flow
constant while implementing the CHW reset control strategies.
eQuest Wizard Inputs for CHW Temperature Reset Controls
Figure 49
PART 3: Advanced Control Sequences
57
Hot Water Loop Temperature Reset
Hot Water (HHW) temperature reset controls generally reduce boiler energy usage and distribution
losses by decreasing the hot water supply temperature set-point with decreasing load. Hot water reset
control strategies cannot be modeled directly in DOE-2.1e or EnergyPro. However, in eQUEST, hot
water reset is modeled similarly to chilled water reset; the energy modeler can select Reset Control from
the Hot Water System Control and Schedule screen of the eQUEST wizard. For variable-flow loops,
the energy modeler can enter data in Detailed Data Edit mode to define the loop flow reset ratio for the
pumps, where hot water flow will be held constant as the hot water reset control strategy takes effect.
eQuest Wizard Inputs for HHW Temperature Reset Controls
Figure 50
58 HVAC SIMULATION GUIDEBOOK - 2ND EDITION
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