APPENDIX 7

- A. MODIFICATIONS TO SIMULATIONS FOR ENERGY USE

CALCULATIONS

TABLE OF CO NTENTS

7- A.1 STATISTICAL PROCESS FOR ASSIGNING VENTILATION RATES

EMISSIONS

.......................................................................................................... 7-A-1

7- A.2 ECONOMIZER USAGE AND PERFORMANCE

............................................... 7-A-4

7- A.3 BLAST PACKAGED DX COOLING EQUIPMENT MODEL

........................... 7-A-5

7-A.3.1 DX Condensing Unit.............................................................................................. 7-A-5

7-A.3.1.1 Available Capacity

............................................................................... 7-A-6

7-A.3.1.2 Full Load Power Consumption ............................................................ 7-A-7

7-A.3.1.3 System Power Consumption ................................................................ 7-A-8

7- A.4 DEVELOPMENT OF PART

- LOAD PERFORMANCE CURVE

....................... 7-A-9

7-A.4.1 Degradation of Compressor Performan ce

.............................................................. 7-A-9

7-A.4.2 Minimum Saturated Condensing Temperature .................................................... 7-A-10

LIST OF TABLES

Table 7 -A.1.1 Statistical Summary of Minimum Outdoor Air in BASE Study Buildings .. 7-A-3

Table 7 -A.1.2 Minimum Outdoor Ventilation Rate for 1,033 Buildings ............................. 7-A-3

Table 7 -A.4.1 Coefficients for BLAST Fraction of Full Load Curve.................................. 7-A-9

LIST OF FIGURE S

Figure 7 -A.1.1 Annual Consumption Loss for Lower Temperature Changes in DICE,
FUND, and PAGEFrequency Distribution of Minimum Outdoor Air Flow for the BASE Study Office Buildings and a Log Normal Distribution (n = 5,000) ................................................................................ 7-A-2 Figure 7 -A.1.2 Frequency Distribution of Outdoor Ventilation Rates for Each Building Type .............................................................................................................. 7-A-4 Figure 7 -A.4.1 Ca lculated Condenser Efficiency Degradation Factor Used in Current Analysis and Comparison with other Data Sources .................................... 7-A-10

7-A-iAPPENDIX 7

- A. MODIFICATIONS TO SIMULATIONS FOR ENERGY USE

CALCULATIONS

As described in chapter 7, the U.S. Department of Energy (DOE) performed simulations of cooling loads for 1,033 commercial buildings in order to calculate the energy use of commercial unitary air conditioners (CUAC) operating under a range of ambient conditions. The simulation runs provided hourly building cooling loads and fan operating hours. The Department used the Building Loads and System Thermodynamics (BLAST) program to calculate the building air-conditioning loads and fan operating hours. 1 The methods and parameters for the simulations were based on those described in the Technical Support Document (TSD) for the Advance Notice of Proposed Rulemaking (ANOPR) 2 for CUAC that DOE published in 2004. Some of the methods and parameters were modified and updated to better captur e the on

- site performance of CUAC when operating. Listed below are the parameters and methods that were modified for the current simulation of CUAC operation.

1) Statistical process for assigning ventilation rates 2) Economizer usage and performance 3) BLAST Packa ged DX cooling equipment model

a. DX condensing unit

i. Available capacity

ii. Full load power consumption

iii. System power consumption 4) Development of part- load performance curve

a. Degradation of compressor performance

b. Minimum saturated condensing temperature

Each area of modification is discussed further below.

7- A.1 STATISTICAL PROCESS FOR ASSIGNING VENTILATION RATES

EMISSIONS

In the present analysis DOE assigned minimum outdoor ventilation flow rates for each of the 1,033 buildings in the set of buildings simulated based partly on the U.S. Environmental Protection Agency's Building Assessment Survey and Evaluation (BASE) study performed by the National Institute for Standards and Testing (NIST). 3 The field data for BASE were collected from many randomly selected General Services Administration (GSA) office buildings throughout the United States, where a standardized protocol was used to measure ventilation rates. NIST excluded from the data set buildings known to have indoor air quality problems. The Department found the average ventilation rates in those office buildings to be approximately

two -thirds of the ventilation rates derived from ASHRAE Standard 62-1999, but with significant variation among the buildings NIS T studied. For this analysis, the ventilation rate was modified for some building types, and a Monte Carlo distribution of multipliers was applied based on data from the NIST BASE study to establish a minimum outdoor air ventilation rate for each of the 1,033 buildings analyzed.

7-A-1For this analysis, the variable of interest is the volume of outdoor air, reported in cubic feet per minute per square foot (cfm/ft 2 ), under minimum ventilation conditions. Those data were available for 148 of the BASE buildings. The data are summariz ed in Table 7- A.1 .1 and shown graphically in Figure 7-A.1.1 . Also shown in Figure 7- A.1 .1 is a log normal frequency distribution having the same mean and standard deviation as the BASE buildings, but using a sample size of 5,000 buildings. Based on preliminary examination of the data, t he Department assumes that the minimum outdoor air volume follows a log normal distribution.

Figure 7- A.1 .1 Annual Consumption Loss for Lower Temperature Changes in DICE, FUND, and PAGE Frequency Distributi on of Minimum Outdoor Air Flow for the BASE Study Office Buildings and a Log Normal Distribution (n = 5,000)

7-A-2Table 7- A.1 .1 Statistical Summary of Minimum Outdoor Air in BASE Study Buildings

Statistical Parameter Outdoor Air (cfm/ft 2 )

Mean 0.097

Standard Deviation 0.078

Minimum 0

10 th Percentile 0.036

25 th Percentile 0.046

Median 0.09

75 th Percentile 0.125

90 th Percentile 0.17

Maximum 0.608

For each building type in the 1,033-building set, DOE used a log normal random number generator to generate a minimum outdoor ventilation rate. The random number generator required as input two variables—the population mean and the standard deviation. For each building type, the mean ventilation rate from the ANOPR TSD was adjusted by a factor of 0.643 to provide the statistical mean used in the log normal random number generator. This adjustment represents the ratio of the median ventilation rate for office buildings in the BASE study (0.09)

to the ventilation rate for office building s used in the ANOPR TSD (0.14). For all but the retail building type, the mean given in the ANOPR TSD is the value used in the ANOPR analysis. The mean retail ventilation rate varied depending on the number of stories in the building. For this analysis the mean value for retail buildings from the ANOPR TSD (0.283 cfm/ft 2 ) was adjusted by a factor of 0.643 to provide the statistical mean input to the ventilation rate.

The Department used the standard deviation for the minimum ventilation r ate in the BASE study (0.078) for all building types in the 1,033-building set as the standard deviation in the log normal distribution. Table 7- A.1 .2 summarizes the minimum ventilation rates calculated based on this statistical approach and then used in the analysis. Figure 7- A.1 .2 shows the

statistical distribution of ventilation rates for each building type in the 1,033-building set.

Table 7- A.1 .2 Minimum Outdoor Ventilation Rate for 1 ,033 Buildings

Building

Type Number of Buildings Outdoor Ventilation Rate (cfm/ft 2 )

Average Maximum Minimum Assembly 75 0.59 0.82 0.41

Education 99 0.27 0.44 0.15

Food

Service 57 0.38 0.58 0.26

Office 357 0.09 0.51 0.01

Retail 408 0.19 0.55 0.04

Warehouse 37 0.04 0.29 0

7-A-3Figure 7- A.1 .2 Frequency Distribution of Outdoor Ventilation Rates for Each Building Type

7- A.2 ECONOMIZER USAGE AND PERFORMANCE

The data set from DOE’s 1995 Commercial Building Energy Consumption Survey (CBECS 95), described in detail in chapter 7, has a single variable ( ECN 6

) that indicates whether the building in question has an economizer to provide “free cooling.” The survey provides no indication of the control or operation status of the economizer, or whether it is used on all the building's CUAC systems. The Department modified the assumption regarding economizer

usage. Field studies indicate that economizers experience a wide range of failure modes and
deviations from proper operation, which are difficult to quantify. 4

, 5

, 6

, 7 The studies, however, indicate that at least a third of installed economizers do not function properly and that

economizer controls often are disc onnected from the HVAC system. Given the available data, the Department changed the modeling by assigning to each building in CBECS95 that reported

economizer use a 30

- percent probability of having a non

- operational economizer. Non

operational means that D OE assumed a fixed ventilation rate for all operating hours of the year.

7-A-47- A.3 BLAST PACKAGED DX CO OLING EQUIPMENT MODE L

The simulations for this analysis utilized BLAST’s Packaged DX cooling system model. The BLAST model, which is a mathematical representation of the performance of a direct expansion (DX) cooling system (e.g., a packaged unitary air conditioner), is used to calculate the sensible and latent cooling capacity, condenser energy consumption, and fan power energy consumption of a CUAC system, hour-by-hour, in response to the cooling requirements of a building. The model comprises three components: the DX evaporator coil model, to deal with calculating the cooling coil loads on the system; the DX condensing unit model to determine the energy consumption of the system in response to coil loads; and the supply fan model, to calculate the necessary supply air temperature and fan energy consumption. Detailed information on the DX evaporator and condensing unit models and the calculation of fan energy is found in

the BLAST Technical Reference Manual.

The following sections describe changes DOE made to BLAST model parameters used in the ANOPR TSD. Models for the evaporator coil model, DX condensing unit, and available capacity were either changed or expanded.

7-A.3.1 DX Condensing Unit

F or the ANOPR analysis of building energy and end use loads, the simulations were based on default performance curves incorporat ed in the BLAST software package.

DOE used BLAST’s Packaged DX condensing unit model to determine the required hourly energy input to the condensing unit (the compressor and condenser fans are treated as a single condensing unit) in response to the total cooling load from the DX cooling coil model. The condensing unit model adjusts the condenser's efficiency in response to outdoor air conditions and cooling coil load.

The BLAST formulation for the DX condensing system performance is based on a curve fit of the actual system performance o f a particular piece of equipment to a thermodynamic model of the ideal performance of the system. In addition, the DX condensing unit model has a default value for the overall heat-transfer coefficient of the condensing coil ( referred to as the

condenser UA value , CUA) that is consistent with the BLAST default system capacity. To accurately represent performance of cooling systems at capacities other than the BLAST default capacity, the default value is replaced by a user-specified input value for CUA. Thi s substitution

enables the model to accurately calculate system efficiency under design temperatur e and load conditions that match the user-input nominal cooling efficiency (representative of the performance of the condensing unit under nominal full load design cooling conditions).

Three separate curves are provided in BLAST to simulate the performance of the condensing system. These curves are used to calculate the (1) available capacity (AVAILCAP) for a given hour; (2) the full load power ratio (FPLR), the ratio of the electrical input power for the condenser section to the cooling provided (in consistent units); and (3) the fraction of full load power (FFL), which provides the ratio of the actual condenser power requirements to the

7-A-5full load power requirements for a given hour’s cooling load. The formulation of the performance curves is described below.

7-A.3.1.1 Available Capacity

In BLAST the formulation of the curve for available cooling capacity ( AVAILCAP ) is based on a curve fit of the actual system performa nce to a theoretical model of the condenser

system ( Equation 7- A.3 .1 ).

AVAILCAP = NOMCAP × ( RCAVCD 1 + RCAVCD 2 × ∆T + RCAVCD 3 × ∆T 2 )

Where:

NOMCAP = nominal capacity of the cooling system, and default values for the coefficients are:

RCAVCD 1 = 0.98772

RCAVCD 2 = -0.02288

RCAVCD 3 = 0.00027

ΔT = “equivalent temperature difference” defined by:

∆T = SCT

− DSCT

SCTTR − ( SST

− DSST )

Where:

SCT = saturated condensing temperature (°F),

DSCT = design saturated condensing temperature (°F),

SCTTR = SCT temperature rise coefficient (°F),

SST = saturated suction temperature (°F), and

DSST = design saturated suction temperature (°F).

DSST, DSCT , and SCTTR are given as parameter inputs to the model. The default values for those parameters are:

DSST = 40 (°F)

DSCT = 122 (°F)

SCTTR = 2.63

In order to calculate ΔT, first the SST is estimated by:

SST = DSST + ( 1

− CPLR ) × UTR

Where:

SST = actual saturated suction temperature,

CPLR = cooling part-load ratio, calculated by coil modeling,

DSST = design saturated suction temperature, and

7-A-6UTR = unloader throttling range. a

The default value for UTR , which is a parameter input to the model, is 4 °F.

Next the saturated condensing temperature is estimated as:

SCT = Max � MSCT

LOAD × 1+ DFLPR

CUA

Where:

SCT = actual saturated condenser temperature,

OADB = outdoor dry-bulb temperature,

LOAD = total load on the condensing unit cal culated by coil modeling,

MSCT = minimum saturated condenser level,

DFLPR = design full load power ratio, and

CUA = condenser U- factor

- area product.

The DX condensing unit model has a default value for the overall heat- transfer

coefficient of the condens ing coil (the CUA). BLAST also provides default values for system capacity. The default system capacity for the BLAST model is 563 kBtu/ h; the default CUA is

27.43 kBtu/h-° F. To represent performance of cooling systems accurately at capacities other than the BLAST default capacity, the user specifies an input value to replace the default CUA value.

The user

- specified value enables the model to accurately calculate system efficiency under design temperature and load conditions that match the user-input nom inal cooling efficiency (representative of the performance of the condensing unit under nominal full load design cooling conditions). When analyzing equipment correctly sized for a simulation, the CUA must scale with the nominal equipment capacity in order to maintain the same system performance when modeling equipment of different sizes. DOE maintained the ratio between the two values to preserve the form of the BLAST capacity and full load performance curves for the CUAC analysis.

7-A.3.1.2 Full Load Power Consumpt ion

Full load power consumption is a function purely of the SST and SCT. BLAST first estimates the Carnot efficiency η c of a thermodynamic heat pump operating between the SST and

SCT.

a This equation reflects modifications to the BLAST model to account for the condensing unit capacity under design

conditions. The equation was confirmed at Pacific Northwest National Laboratory and verified by the BLAST support office at the University of Illinois on 10/4/2002 and by the algorithm’s author (Prof. Doug Hittle, Colo rado State University) on 10/16/2002. The modification made to the BLAST model also allow the use of the nominal value of 563 kBtu/h identified in the air conditioner performance data described in BLAST's documentation.

7-A-7η c = SCT

− SST

SCT + 460°F

Next, the actual full load power ratio (FLPR) (the instantaneous full load system efficiency) is calculated based on the DFLPR and an adjustment curve fit that uses the theoretical Carnot efficiency as an input:

FLPR = DFLPR × ( ADJECD 1 + ADJECD 1 η c + ADJECD 1 × η c 2 )

Whe re:

FLPR = full load power ratio

ADJECD 1 = 0.2984

ADJECD 2 = 0.1334

ADJECD 3 = 34.603

The values for ADJECD 1

, ADJECD 2, and ADJECD 3 , which are provided as system defaults, allow for adjusting the nominal full load power ratio to different environmental temperatures.

7-A.3.1.3 System Power Consumption

The system power consumption for a given hour is determined through a three- step process. First, the part load ratio ( PLR ) on the equipment is calculated as:

PLR = LOAD

AVAILCAP

Where LOAD is the coolin g coil load for that hour.

Next, the fraction of full load power ( FFL ) is calculated based on a curve fit of the PLR defined above. A default curve fit is provided by BLAST:

FFL = RPWRCD 1 + RPWRCD 2 × PLR + RPWRCD 3 × PLR 2

Where:

RPWRCD 1 = 0.1456

RPWRCD 2 = 0.9554

RPWRCD 3 = -0.10476.

The basis of the default FFL curve fit, as defined in the BLAST Technical Reference and Help

Manual 1 , derives from the original performance data under part load conditions. Finally, the hour’s condensing unit power consumption ( Power ) is calculated as:

Power = AVAILCAP × FLPR × FFL

7-A-87- A.4 DEVELOPMENT OF PART

- LOAD PERFORMANCE CUR VE

As the BLAST DX model operates at part load, less heat moves to the condenser, which

results in a cooler ave rage condenser temperature for the hour, further modifying system performance through the other system curves. The Department developed a new part

- load performance curve for the BLAST DX condenser unit model, whereby overall performance would better repres ent a multi -compressor system. DOE's use of a two- compressor system was based on the modeling methodology used in the Federal Energy Management Program's (FEMP's) high efficiency rooftop procurement project. 8

The Department used several parameters to define the performance of the BLAST DX unit at other than full-load design conditions. These parameters included a system performance curve to determine the variation in cooling capacity when deviating off of design- temperature conditions, a system performance curve to determine the variation in hourly full-load power consumption (e.g., maximum instantaneous power) at off-design cooling conditions, and a system performance curve to determine actual power consumption at part load.

7-A.4.1 Degradation of Compressor Performance

The D epartment used a degradation factor for compressor performance to represent losses during cycling. The factor provides for a 25-percent degradation of the compressor's coefficient of performance (COP) at a compressor load of 0 percent of capacity, decreasi ng to a 0-percent degradation at 100- percent capacity. The Department based this factor on the degradation factors used in the FEMP cost estimator for unitary equipment. 10

DOE developed new coefficients using a successive approximation method for the quadratic fraction of the full load (FFL) curve in BLAST. This method provides a performance degradation that approximates the trends shown for the idealized multi-compressor model for a range of outdoor air temperatures. Table 7- A.4 .1 shows the original values for the BLAST FFL curve and the modified values DOE developed for the current analysis.

Table 7- A.4 .1 Coefficients for BLAST Fraction of Full Load Curve

Source RPWRCD(1) RPWRCD(2) RPWRCD(3)

BLAST Default FFL

Coefficients 0.1456 0.9554 -0.10476

UAC FFL Coefficients

Used for Current Analysis 0.0925 1.2396 -0.3432

Figure 7- A.4 -1 shows the idealized two

- compressor curve used in BLAST, in which the efficiency degradation factor, which is the ratio of the degraded system COP to the full load COP, is shown as a function of the system part load ratio (PLR) for an idealized, two-compressor model. The idealized cycling behavior in this model overlooked some factors, such as minimum compressor run times. Those other factors would result in a somewhat higher efficiency of system operation at system load points of about 0 percent and 50 percent. The idealized curve in

7-A-9Figure 7- A.4 -1 represents a system operating with an average condenser efficiency degradation

factor of greater than 97 percent when the system load exceeds 40 percent.

Figure 7- A.4 -1 shows the new BLAST performance degradation curve assuming a minimum saturated condensing temperature (MSCT) of 80°F and a load profile that varies linearly from 0 percent system load at 70 °F, to 100 percent system load at 95 °F. The modified FFL curve provides a curve for condenser efficiency degradation having somewhat higher performance than the idealized model, especially at PLR values above 20 percent, although performance is sli ghtly lower at PLR values below 20 percent.

Figure 7- A.4 .1 Calculated Condenser Efficiency Degradation Factor Used in Current Analysis and Comparison with other Data Sources

7-A.4.2 Minimum Saturated Condensing Temperature

The BLAST default value for the minimum saturated condensing temperature (MSCT) allowed the refrigerant in a CUAC to reach 100 °F. DOE lowered the user -input parameter representing the allowed MSCT to the minimum condensing temperature of 80 °F reported in the compressor performance literature.

7-A- 10R EFERENCES

1. BLAST Support Office, Volume 1: Building Loads and System Thermodynamics (BLAST) User Manual , 1991. University of Illinois at Urbana-Champaign.

2. U.S. Department of Energy, Technical Support Document: Energy Efficiency Program for Commercial and Industrial Equipment: Commercial Unitary Air Conditioners and Heat Pump , Office of Energy Efficiency and Renewable Energy, Editor. 2004: Washington, D.C. http://www.regulations.gov/#!documentDetail;D=EERE-2006- STD

0103-0078

3. Persily, A. and J. Gorfain, Analysis of Ventilation Data from the U.S. Environmental

Protecti on Agency Building Assessment Survey and Evaluation (BASE) Study , 2004. National Institute for Standards and Testing.

4. Ecotope, Enhanced Operations & Maintenance Procedures for Small Packaged Rooftop HVAC Systems –Protocol Development, Field Review, and Measure Assessment —Final Report , 2002. Eugene Water and Electric Board.

5. Cowan, A., A Review of Recent Commercial Roof Top Unit Field Studies in The Pacific Northwest and California , 2004. Prepared by New Buildings Institute for Northwest Power and Conservation Council and Regional Technical Forum.

< http://rtf.nwcouncil.org//NWPCC_SmallHVAC_Report%28R3%29Final.pdf >

6. California Energy Commission, Integrated Energy Syste ms: Productivity & Building

Science , 2003. Prepared by New Buildings Institute for the Public Interest Energy Research Program. < http://www.energy.ca.gov/pier/project_reports/500-03-082.html >

7. Portland Energy Conservation Inc, Retrocommissioning Report– Facility A, Clearlake,

CA —Final Report , 2001. Prepared by PECI, Portland, OR, in conjunction with the Institute for Market Transformation (IMT) and Pacific Gas and Electric

< http://www.imt.org/resources/detail/retrocommissioning -report-long- term

- care

- facility

a >

8. Federal Energy Management Program, FEMP High-efficiency Rooftop Air -conditioner Procurement Project

. < http://www.pnl.gov/uac/costestimator/main.stm >

7-A- 11