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Impact of typical weather year selection approaches on energy analysis of buildings.

INTRODUCTION

Detailed building energy simulation tools such as DOE-2 (LBL 1981) and EnergyPlus (Crawley et al. 2000) are commonly used to design sustainable buildings. These tools require hourly typical year weather files in order to estimate building energy use and building indoor comfort. Several procedures do exist to develop typical weather data using a single year of hourly data that are selected to represent the range of weather patterns that can be found in a multi-year data set (Keeble 1990).

Several approaches have been utilized to develop and format a typical weather year for building energy analysis including the ASHRAE Test Reference Year or TRY (ASHRAE 1976), Typical Meteorological Year or TMY (Hall et al. 1978), the Weather Year for Energy Calculations (Crow 1981), TMY2 (Marion and Urban 1995), ASHRAE International Weather for Energy Calculations or IWEC (Thevenard and Brunger 2002), and more recently TMY3 (Wilcox and Marion 2008). Other selection approaches have been proposed (Hui 1996).

Limited analyses have been reported to assess the impact of the selection criteria for generating the typical weather year on predicting the performance of building energy systems (Arigirou et al. 1999 and Massie and Kreider 2001). In particular, Argiriou et al. (1999) tested several different TMY weather files generation procedures for Athens with 20 years (1977 to 1996) measured weather data. They considered several configurations of weighting factors and four methods to generate typical weather year including: the TMY method (Hall et al. 1978), a Danish method (Lund and Eidorff 1980), Festa-Ratto method (Festa and Ratto 1993), and 20-year average meteorological year. They developed weather data evaluation system based on building and solar systems. Specifically, they utilized a simple solar water heating system, a building, a photovoltaic system, and a large scale solar heating system with inter-seasonal storage, and PV system. TRNSYS is used in the evaluation analysis (Anon 2000). A modified Festa-Ratto method was found to provide the best data set for Athens. Massie and Kreider (2001) estimated the discrepancies between TMY and TMY2s in predicting the performance of a photovoltaic system and a wind turbine.

In this paper, a series of sensitivity analyses is presented to assess the impact of the typical weather selection criteria on building energy analysis results. In particular, the impacts on annual energy use predictions of weighting factors associated with various weather variables and of the length of historical data used in the selection procedure are evaluated throughout the paper. The analysis is carried out for 10 U.S. sites for which measured weather data for at least 30 years are reported. First, a brief description of the prototypical office building used in the simulation analysis is provided. Then, the results of the simulation analyses are presented and discussed.

BUILDING MODEL DESCRIPTION

For this analysis, a prototypical office building was modeled using a whole-building hourly simulation tool (LBL 1981). The prototypical building model consists of 3-story office building with a gross floor area of 48,000 [ft.sup.2] (4461 [m.sup.2]) as illustrated in Figure 1. A power density of 0.8 W/[ft.sup.2] (8.7 W/[m.sup.2]) is assumed for lighting systems equipped with electronic ballasts and daylight control sensors. Daylight control coverage area is 54% covering all the perimeter offices. Office equipment power density is assumed to be 1.0 W/[ft.sup.2] (10.8 W/[m.sup.2]) for computers, laser printers, photocopiers, and facsimile machines. The building envelope is assumed to include 40% fenestration-to-wall ratio with glazing varying by climate to meet ASHRAE Standard 90.1 (ASHRAE 2004). The outside air ventilation rate is set to be 20 CFM/person (9.5 L/s per person). The HVAC system for the building consists of a variable air volume (VAV) system with hot water reheat coils and dry-bulb outside air economizer. The central plant includes 0.55 kW per ton centrifugal chillers and a 90% efficiency gas fired boiler. Table 1 provides a summary of the basic features of the prototypical office building. Table 2 lists the 10 U.S. sites used to carry out the analysis presented in this paper.

[FIGURE 1 OMITTED]
Table 1. Summary of the Basic Features of the Prototypical Office
Building Model

      Architectural              Mechanical

Floor Area   16,000 sf      Design      75[degrees]F/72[degrees]F
             (1487          Temp.       [Cooling/Heating]
             [m.sup.2])                 (24[degrees]C/22[degrees]C)

Gross Area   48000 sf       Thermo.     76[degrees]F/82[degrees]F
             (4461          Set         (24.5[degrees]C/28[degrees]C)
             [m.sup.2])                 [Cooling]

                                        60[degrees]F/64[degrees]F
                                        (15.5[degrees]C/18[degrees]C)
                                        [Heating]

Peri. Depth  20 ft (6.1 m)  HVAC        VAV+Reheat
             [57% of floor
             area]

Wall         Wood, metal    Fans        Variable Speed Drives (VSDs)
             frame, R-19
             batt. Overall
             R-10.5

Roof         Built-up       OA          20 cfm/person (9.5 L/s per
             roof, metal                person) Economizer control
             frame, R-18
             insulation,
             Overall R-22

WWR          40% (floor to  Chiller     0.55 kW/ton
             ceiling)

Glazing      Variable with  Boiler      90%
             climate

                            Daylight    Perimeter zones
                            Control

Shading      3 ft (0.9 m)               50fc design level
             overhang on
             S, E, and W

LPD/ EPD     0.8 /1.0                   Dimming control
             W/sf

             (8.7/10.8
             W/[m.sup.2])

Table 2. Selected 10 U.S. Sites

 USAF        Stations         STATE    LAT     LON    ELEV   Climate
                                                       (m)    Zone
                                                            (ASHRAE
                                                            Std 90.1)

725180  ALBANY COUNTY ARPT      NY   42.45N  073.48W    89     5A

722190  ATLANTA INTL ARPT       GA   33.39N  084.25W   315     3A

911820  HONOLULU INTL ARPT      HI   21.21N  157.56W     5     1A

723860  LAS VEGAS/MCCARRAN      NV   36.05N  115.10W   664     5B

726410  MADISON/DANE RGNL       WI   43.08N  089.20W   264     6A

722020  MIAMI INTL AIRPORT      FL   25.49N  080.17W     4     2A

725500  OMAHA/EPPLEY FIELD      NE   41.18N  095.54W   299     5A

722780  PHOENIX/SKY HARBOR      AZ   33.26N  112.01W   337     3B

727930  SEATTLE-TACOMA INTL     WA   47.27N  122.18W   137     5B

722740  TUCSON INTL AIR PORT    AZ   32.07N  110.56W   779     3B


IMPACT OF TYPICAL WEATHER YEAR

Using the simulation building model, annual total energy consumption and peak electrical demand are estimated for various annual typical weather data sets and are compared to those mean values obtained from 30 years of simulation data. The typical weather year data sets considered in this analysis include:

* IWEC, generated data sets for the 10 U.S. sites using the basic IWEC selection procedure (Thevenard and Brunger 2002),

* TMY, generated data sets for the 10 U.S. sites using the TMY selection procedure outlined by Hall et al. (1978),

* TMY2n, generated data sets for the 10 U.S. sites using the TMY2 selection procedure outlined by Marion and Urban (1995),

* TMY2r, existing data sets for the 10 U.S. sites published by NREL (Marion and Urban 1995), and

* EqWt, generated data sets for the 10 U.S. sites using a selection procedure based on equal weighting factors as described in Donghyun et al. (2008).

To be consistent with the published TMY2r weather files, meteorological data sets of 30 years spanning from 1961 to 1990 were used in generating IWEC, TMY, TMY2n, and EqWt weather files (Donghyun et al. 2008).

Table 3 summarizes the results of the simulation analysis and provides the annual energy use differences in terms of heating, cooling, and total using the simulation results obtained using the 30-year average data set as a reference. Generally, the largest differences (over 5%) are found for heating energy use and for warm climates such as Phoenix and Las Vegas. For all sites and weather data sets, cooling energy differences are generally small ranging from -3 to 1%. In terms of total building energy use, IWEC and TMY2n data sets provide the least differences and EqWt and TMY2r data sets exhibit the largest differences. However, these differences are not significant and are generally below 2%.
Table 3. Annual Total Building Energy Use Differences Between Typical
Weather Data and 30-Year Average

                                 IWEC            EqWt           TMY

Albany    Elec (kWh)       296369   -0.5%  297680   0.2%  296918   0.0%
          Gas (Mbtu/GJ)     736.4   -0.2%   735.7   1.3%   735.9   1.3%
          Total (MBtu/GJ)  1747.9    1.4%  1751.7  -0.7%  1749.3  -0.5%

Atlanta   Elec (kWh)       314559   -0.5%  315552  -0.2%  310582  -1.7%
          Gas (Mbtu/GJ)     243.4    6.4%     222  -2.9%   253.5  10.9%
          Total (MBtu/GJ)  1317.0    0.7%  1299.0  -0.6%  1313.5   0.5%

Honolulu  Elec (kWh)       379932    0.0%  378877  -0.3%  368211  -3.1%
          Gas (Mbtu/GJ)      46.9    1.9%    46.9   1.9%    46.5   1.0%
          Total (MBtu/GJ)  1343.6    0.0%  1340.0  -0.2%  1303.2  -3.0%

Las       Elec (kWh)       333272   -0.1%  332231  -0.4%  333191  -0.1%
Vegas     Gas (Mbtu/GJ)     123.4   -3.3%   124.5  -2.5%   128.7   0.8%
          Total (MBtu/GJ)  1260.9   -0.4%  1258.4  -0.6%  1265.9   0.0%

Madison   Elec (kWh)       302346    0.0%  301992  -0.1%  302267   0.0%
          Gas (Mbtu/GJ)     793.2   -2.3%   790.1  -2.7%   817.9   0.7%
          Total (MBtu/GJ)  1825.1   -1.0%  1820.8  -1.2%  1849.5   0.3%

Miami     Elec (kWh)       377545   -0.5%  377545  -0.5%  378260  -0.3%
          Gas (Mbtu/GJ)        49    1.4%      49   1.4%      49   1.4%
          Total (MBtu/GJ)  1337.6   -0.4%  1337.6  -0.4%  1340.0  -0.2%

Omaha     Elec (kWh)       311337    0.2%  309205  -0.5%  310159  -0.2%
          Gas (Mbtu/GJ)     655.8    1.9%   631.9  -1.9%   627.8  -2.5%
          Total (MBtu/GJ)  1718.4    0.8%  1687.2  -1.0%  1686.4  -1.1%

Phoenix   Elec (kWh)       358285    0.7%  357580   0.5%  355185  -0.2%
          Gas (Mbtu/GJ)      70.4  -11.5%    75.7  -4.8%    79.2  -0.4%
          Total (MBtu/GJ)  1293.2    0.0%  1296.1   0.2%  1291.4  -0.2%

Seattle   Elec (kWh)       268311   -0.7%  269452  -0.3%  270812   0.2%
          Gas (Mbtu/GJ)     400.5   -0.2%   381.9  -4.8%   400.6  -0.2%
          Total (MBtu/GJ)  1316.2   -0.5%  1301.5  -1.7%  1324.9   0.1%

Tucson    Elec (kWh)       333749   -0.2%  333126  -0.4%  335585   0.3%
          Gas (Mbtu/GJ)      87.5   -6.4%    91.7  -2.0%    89.1  -4.7%
          Total (MBtu/GJ)  1226.6   -0.7%  1228.7  -0.5%  1234.5  -0.1%

                                 TMY2n           TMY2r

Albany     Elec (kWh)       297164   0.1%  297259   -0.3%
           Gas (Mbtu/GJ)     735.4   1.2%   730.6    0.1%
           Total (MBtu/GJ)  1749.6  -0.6%  1745.1    0.6%

Atlanta    Elec (kWh)       314401  -0.5%  315018   -0.3%
           Gas (Mbtu/GJ)     232.2   1.5%   228.6    0.0%
           Total (MBtu/GJ)  1305.3  -0.2%  1303.8   -0.3%

Honolulu   Elec (kWh)       380320   0.1%  377504   -0.7%
           Gas (Mbtu/GJ)      46.9   1.9%    46.2    0.4%
           Total (MBtu/GJ)  1344.9   0.1%  1334.6   -0.6%

Las Vegas  Elec (kWh)       331743  -0.6%  333071   -0.2%
           Gas (Mbtu/GJ)     123.5  -3.3%   114.4  -10.4%
           Total (MBtu/GJ)  1255.7  -0.8%  1251.2   -1.2%

Madison    Elec (kWh)       301990  -0.1%  300074   -0.7%
           Gas (Mbtu/GJ)     794.8  -2.1%   779.3   -4.0%
           Total (MBtu/GJ)  1825.5  -1.0%  1803.5   -2.2%

Miami      Elec (kWh)       377428  -0.5%  379842    0.1%
           Gas (Mbtu/GJ)        49   1.4%    47.8   -1.1%
           Total (MBtu/GJ)  1337.2  -0.4%  1344.2    0.1%

Omaha      Elec (kWh)       309008  -0.6%  310159   -0.2%
           Gas (Mbtu/GJ)     634.9  -1.4%   627.8   -2.5%
           Total (MBtu/GJ)  1689.5  -0.9%  1686.4   -1.1%

Phoenix    Elec (kWh)       358034   0.6%  355185   -0.2%
           Gas (Mbtu/GJ)      72.1  -9.3%    79.2   -0.4%
           Total (MBtu/GJ)  1294.1   0.0%  1291.4   -0.2%

Seattle    Elec (kWh)       269322  -0.3%  270812    0.2%
           Gas (Mbtu/GJ)     385.6  -3.9%   400.6   -0.2%
           Total (MBtu/GJ)  1304.8  -1.4%  1324.9    0.1%

Tucson     Elec (kWh)       332106  -0.7%  335585    0.3%
           Gas (Mbtu/GJ)      94.1   0.6%    89.1   -4.7%
           Total (MBtu/GJ)  1227.6  -0.6%  1234.5   -0.1%


Table 4 presents the differences in electrical annual peak demand values obtained for various typical weather data using the 30-year averages as references. For all sites and weather data sets, peak demand differences are below 5%. The averages of the differences for all 10 cities range from 1.6% (IWEC) to 2.4% (TMY). The use of TMY2n and TMY2r leads to respectively 1.84% and 2.17% average differences for the annual electrical peak demand for the prototypical office building considered in this analysis. Figures 2 and 3 provide scatter diagrams for respectively the monthly electrical peak demand and total building energy use obtained from the energy analysis of the prototypical office building using TMY2n and TMYr against the same results generated using 30-year average data.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]
Table 4. Annual Electrical Peak Demand Differences Between Typical
Weather Data and 30-Year Average

                       IWEC    EqWt    TMY    TMY2n   TMY2r

Albany     Peak (kW)  135.9   136.9   135.9   135.9   135.4
           Diff. %      0.3%    1.0%    0.3%    0.3%   -0.1%

Atlanta    Peak (kW)  154.5   154.9   151.8   154.5   147.8
           Diff. %      3.5%    3.7%    1.7%    3.5%   -1.0%

Honolulu   Peak (kW)  150.5   150.5   146.7   151.8   147.2
           Diff. %      0.5%    0.5%   -2.0%    1.4%   -1.7%

Las Vegas  Peak (kW)  165.5   165.4   162.3   165.5   168.1
           Diff. %      1.6%    1.5%   -0.4%    1.6%    3.2%

Madison    Peak (kW)  144.5   144.3   147.1   139.4   145.3
           Diff. %      2.8%    2.7%    4.6%   -0.8%    3.4%

Miami      Peak (kW)  164.7   164.7   156.0   164.7   159.3
           Diff. %      3.1%    3.1%   -2.4%    3.1%   -0.3%

Omaha      Peak (kW)  149.8   150.5   161.4   150.4   161.4
           Diff. %     -1.6%   -1.1%    6.1%   -1.2%    6.1%

Phoenix    Peak (kW)  171.1   171.1   169.8   171.1   169.8
           Diff. %     -0.6%   -0.6%   -1.3%   -0.6%   -1.3%

Seattle    Peak (kW)  117.2   115.0   121.3   121.7   121.3
           Diff. %      1.1%   -0.8%    4.6%    4.9%    4.6%

Tucson     Peak (kW)  158.3   154.9   156.4   154.9   156.4
           Diff. %      1.2%   -1.0%    0.0%   -1.0%    0.0%


Figure 4 illustrates the scatter diagrams of monthly cooling and heating energy end-uses obtained using TMY2n and TMY2r weather data plotted against monthly means calculated for the simulation results generated for the 30-year period (1961 to 1990) for all the 10 sites. Table 5 summarizes the statistical results of the comparative analysis between TMY2n and TMY2r and the mean values obtained for the 30-year for both heating and cooling energy end-uses.

[FIGURE 4 OMITTED]
Table 5. Mean and Standard Deviation of the Monthly Values for Monthly
Cooling and Heating Energy End-Uses Obtained for the 30-Year Average,
TMY2n, and TMY2r

                     Cooling Energy (kWh)   Heating Energy [MBtu (GJ)]

              Mean                 27329.7           Mean       26.7

30 Avg.  Standard Deviation         4986.3  Standard Deviation  42.1

         Confidence Level (95.0%)    901.3  Confidence Level     7.6
                                            (95.0%)

         Mean                      27262.6  Mean                26.4

TMY2n    Standard Deviation         4986.1  Standard Deviation  42.2

         Confidence Level (95.0%)    901.3  Confidence Level     7.6
                                            (95.0%)

         Mean                      27287.6  Mean                26.2

TMY2r    Standard Deviation         4922.9  Standard Deviation  42.2

         Confidence Level (95.0%)    889.9  Confidence Level     7.5
                                            (95.0%)


Both TMY2n and TMY2r annual weather data sets provide energy use predictions that are in good agreement with the mean values of the simulation results compiled for 30 years. Generally, the results obtained using TMY2n weather are statistically slightly closer to the 30-year mean predictions for both natural gas and electricity annual energy use than those generated with TMY2r. The mean of the differences associated with TMY2n and TMY2r are only 0.15% and 0.25%, respectively, while the standard deviation of the differences is only 0% and 1.3%, respectively.

IMPACT OF WEIGHTING FACTORS

Using the simulation building model, annual total energy consumption are estimated for various TMY2 weather data sets obtained using different configurations weighting factors and are compared to those mean values obtained from 30 years weather data. Table 6 lists the weighting factors associated with all weather parameters (Donghyun et al. 2008). Dew point temperature (DPT) and wind speed (WSP) weighting factors are set to be constant (10%) due to their relatively small effect on building energy use. A 6-digit code is assigned to each weighting factor configuration. The first 2-digit set is the weight factor (in fraction multiplied by 10) associated with dry-bulb temperature (DBT), the second 2-digit set is the weighting factor associated with the global horizontal solar irradiation (GHI), and the third 2-digit set is the weighting factor associated with the direct normal solar irradiation (DNI). For example, the code 040400 refers to the case where a weight factor of 0.4 is assigned for DBT, 0.4 for GHI, and 0.0 for DNI.
Table 6. Weighting Factors Used for the Sensitivity Analysis

Weights  DBT  DPT  WSP  GHI  DNI

080000   0.8  0.1  0.1   0    0

060200   0.6  0.1  0.1  0.2   0

040400   0.4  0.1  0.1  0.4   0

020600   0.2  0.1  0.1  0.6   0

000800    0   0.1  0.1  0.8   0

060002   0.6  0.1  0.1   0   0.2

040202   0.4  0.1  0.1  0.2  0.2

020402   0.2  0.1  0.1  0.4  0.2

000602    0   0.1  0.1  0.6  0.2

040004   0.4  0.1  0.1   0   0.4

020204   0.2  0.1  0.1  0.2  0.4

000404    0   0.1  0.1  0.4  0.4

020006   0.2  0.1  0.1   0   0.6

000206    0   0.1  0.1  0.2  0.6

000008    0   0.1  0.1   0   0.8


Table 7 summarizes the results of the simulation analysis with various weighting factors and provides the annual energy use differences in terms of heating, cooling, and total using the simulation results obtained using the 30-year average data set as references. Generally, the largest differences (over 5%) are found for heating energy use and for warm climates such as Atlanta, Phoenix and Las Vegas. For all sites and weather data sets, cooling energy differences are generally small ranging from 3.1% to -1.5%.
Table 7. Annual Energy Use Predictions Using TMY2 with Different
Weighting Factors Compared Against Mean Annual Energy Use Obtained with
Multi-Year Simulations

                                         Albany             Atlanta

Number of Yr       Energy       Energy Use  Diff %  Energy Use  Diff %

30yr Avg of   Elec(MWh)             297       --        316       --
Multi Year    Gas(MBtu [GJ])        726       --        229       --
Simulation    Total(MBtu [GJ])     1740       --       1307       --

080000        Elec(MWh)             298      0.3%       314     -0.5%
              Gas(MBtu [GJ])        734      1.1%       239      4.3%
              Total(MBtu [GJ])     1750      0.6%      1312      0.3%

060200        Elec(MWh)             298      0.4%       314     -0.7%
              Gas(MBtu [GJ])        730      0.5%       237      3.8%
              Total(MBtu [GJ])     1747      0.4%      1309      0.1%

060002        Elec(MWh)             298      0.3%       315     -0.4%
              Gas(MBtu [GJ])        734      1.0%       241      5.4%
              Total(MBtu [GJ])     1751      0.6%      1316      0.6%

040400        Elec(MWh)             296     -0.4%       314     -0.6%
              Gas(MBtu [GJ])        737      1.4%       232      1.5%
              Total(MBtu [GJ])     1747      0.4%      1304     -0.3%

040202        Elec(MWh)             298      0.3%       313     -0.9%
              Gas(MBtu [GJ])        735      1.2%       232      1.5%
              Total(MBtu [GJ])     1752      0.7%      1301     -0.5%

040004        Elec(MWh)             297      0.0%       313     -0.9%
              Gas(MBtu [GJ])        726     -0.1%       232      1.5%
              Total(MBtu [GJ])     1739      0.0%      1301     -0.5%

020600        Elec(MWh)             297      0.0%       313     -1.1%
              Gas(MBtu [GJ])        736      1.3%       235      2.9%
              Total(MBtu [GJ])     1749      0.5%      1302     -0.4%

020402        Elec(MWh)             297      0.0%       314     -0.7%
              Gas(MBtu [GJ])        736      1.3%       233      1.8%
              Total(MBtu [GJ])     1749      0.5%      1304     -0.2%

020204        Elec(MWh)             297      0.0%       315     -0.4%
              Gas(MBtu [GJ])        721     -0.7%       225     -1.5%
              Total(MBtu [GJ])     1735     -0.3%      1300     -0.6%

020006        Elec(MWh)             297      0.1%       314     -0.5%
              Gas(MBtu [GJ])        726     -0.1%       226     -1.4%
              Total(MBtu [GJ])     1740      0.0%      1298     -0.7%

000800        Elec(MWh)             296     -0.3%       314     -0.7%
              Gas(MBtu [GJ])        739      1.8%       234      2.4%
              Total(MBtu [GJ])     1750      0.5%      1305     -0.2%

000602        Elec(MWh)             296     -0.3%       315     -0.5%
              Gas(MBtu [GJ])        739      1.8%       211     -7.5%
              Total(MBtu [GJ])     1750      0.5%      1285     -1.7%

000404        Elec(MWh)             296     -0.3%       315     -0.5%
              Gas(MBtu [GJ])        739      1.8%       240      4.7%
              Total(MBtu [GJ])     1750      0.6%      1313      0.4%

000206        Elec(MWh)             297      0.1%       313     -0.8%
              Gas(MBtu [GJ])        725     -0.2%       244      6.8%
              Total(MBtu [GJ])     1740      0.0%      1314      0.5%

000008        Elec(MWh)             296     -0.3%       314     -0.6%
              Gas(MBtu [GJ])        736      1.3%       244      6.8%
              Total(MBtu [GJ])     1747      0.4%      1317      0.7%

                                        Honolulu          Las Vegas

Number of Yr  Energy            Energy Use  Diff %  Energy Use  Diff %

30yr Avg of   Elec(MWh)             380       --        334       --
Multi Year    Gas(MBtu [GJ])         46       --        128       --
Simulation    Total(MBtu [GJ])     1343       --       1266       --

080000        Elec(MWh)             379     -0.4%       334      0.2%
              Gas(MBtu [GJ])         46      0.2%       128      0.3%
              Total(MBtu [GJ])     1338     -0.4%      1269      0.2%

060200        Elec(MWh)             379     -0.4%       334      0.1%
              Gas(MBtu [GJ])         47      1.9%       123     -3.3%
              Total(MBtu [GJ])     1339     -0.3%      1263     -0.2%

060002        Elec(MWh)             379     -0.2%       334      0.0%
              Gas(MBtu [GJ])         46     -0.3%       123     -3.7%
              Total(MBtu [GJ])     1340     -0.2%      1261     -0.4%

040400        Elec(MWh)             380      0.0%       333     -0.1%
              Gas(MBtu [GJ])         47      1.9%       123     -3.3%
              Total(MBtu [GJ])     1344      0.0%      1261     -0.4%

040202        Elec(MWh)             380      0.0%       333     -0.2%
              Gas(MBtu [GJ])         46     -0.3%       123     -3.8%
              Total(MBtu [GJ])     1343      0.0%      1259     -0.6%

040004        Elec(MWh)             380      0.0%       332     -0.6%
              Gas(MBtu [GJ])         46     -0.3%       124     -3.0%
              Total(MBtu [GJ])     1342     -0.1%      1256     -0.8%

020600        Elec(MWh)             378     -0.4%       331     -0.7%
              Gas(MBtu [GJ])         47      1.9%       138      7.7%
              Total(MBtu [GJ])     1339     -0.3%      1269      0.2%

020402        Elec(MWh)             379     -0.3%       331     -0.9%
              Gas(MBtu [GJ])         47      1.9%       124     -3.1%
              Total(MBtu [GJ])     1341     -0.2%      1252     -1.1%

020204        Elec(MWh)             380      0.0%       332     -0.6%
              Gas(MBtu [GJ])         46     -0.3%       124     -3.2%
              Total(MBtu [GJ])     1343      0.0%      1255     -0.9%

020006        Elec(MWh)             380     -0.1%       332     -0.6%
              Gas(MBtu [GJ])         46     -0.3%       126     -1.3%
              Total(MBtu [GJ])     1341     -0.1%      1257     -0.7%

000800        Elec(MWh)             380     -0.1%       331     -0.8%
              Gas(MBtu [GJ])         47      1.9%       131      2.5%
              Total(MBtu [GJ])     1343     -0.1%      1260     -0.5%

000602        Elec(MWh)             380     -0.1%       329     -1.5%
              Gas(MBtu [GJ])         47      1.9%       133      4.2%
              Total(MBtu [GJ])     1342     -0.1%      1254     -0.9%

000404        Elec(MWh)             381      0.1%       332     -0.3%
              Gas(MBtu [GJ])         45     -1.4%       129      0.7%
              Total(MBtu [GJ])     1344      0.1%      1263     -0.2%

000206        Elec(MWh)             380     -0.1%       333     -0.2%
              Gas(MBtu [GJ])         45     -1.4%       122     -4.4%
              Total(MBtu [GJ])     1342     -0.1%      1259     -0.6%

000008        Elec(MWh)             381      0.2%       330     -1.0%
              Gas(MBtu [GJ])         46     -0.3%       126     -1.1%
              Total(MBtu [GJ])     1346      0.2%      1254     -1.0%

                                        Madison

Number of Yr       Energy       Energy Use  Diff %

30yr Avg of   Elec(MWh)             302       --
Multi Year    Gas(MBtu [GJ])        812       --
Simulation    Total(MBtu [GJ])     1844       --

080000        Elec(MWh)             302     -0.1%
              Gas(MBtu [GJ])        813      0.1%
              Total(MBtu [GJ])     1844      0.0%

060200        Elec(MWh)             302      0.0%
              Gas(MBtu [GJ])        798     -1.7%
              Total(MBtu [GJ])     1830     -0.7%

060002        Elec(MWh)             302      0.0%
              Gas(MBtu [GJ])        793     -2.4%
              Total(MBtu [GJ])     1824     -1.1%

040400        Elec(MWh)             302      0.1%
              Gas(MBtu [GJ])        804     -1.0%
              Total(MBtu [GJ])     1837     -0.4%

040202        Elec(MWh)             303      0.1%
              Gas(MBtu [GJ])        804     -1.0%
              Total(MBtu [GJ])     1837     -0.4%

040004        Elec(MWh)             304      0.4%
              Gas(MBtu [GJ])        804     -1.0%
              Total(MBtu [GJ])     1841     -0.2%

020600        Elec(MWh)             301     -0.3%
              Gas(MBtu [GJ])        791     -2.6%
              Total(MBtu [GJ])     1819     -1.3%

020402        Elec(MWh)             302     -0.1%
              Gas(MBtu [GJ])        789     -2.9%
              Total(MBtu [GJ])     1819     -1.3%

020204        Elec(MWh)             303      0.2%
              Gas(MBtu [GJ])        795     -2.1%
              Total(MBtu [GJ])     1828     -0.9%

020006        Elec(MWh)             303      0.3%
              Gas(MBtu [GJ])        791     -2.6%
              Total(MBtu [GJ])     1826     -1.0%

000800        Elec(MWh)             301     -0.3%
              Gas(MBtu [GJ])        775     -4.5%
              Total(MBtu [GJ])     1804     -2.2%

000602        Elec(MWh)             302     -0.1%
              Gas(MBtu [GJ])        784     -3.5%
              Total(MBtu [GJ])     1814     -1.6%

000404        Elec(MWh)             303      0.2%
              Gas(MBtu [GJ])        772     -5.0%
              Total(MBtu [GJ])     1805     -2.1%

000206        Elec(MWh)             303      0.2%
              Gas(MBtu [GJ])        772     -5.0%
              Total(MBtu [GJ])     1805     -2.1%

000008        Elec(MWh)             303      0.2%
              Gas(MBtu [GJ])        768     -5.4%
              Total(MBtu [GJ])     1802     -2.3%


For the annual total building energy use, the differences in the simulation results obtained from the TMY2 data sets and mean value for the 30-year period are below 3% regardless of weighting configuration. However, these differences can reach 8% for the annual heating energy use when the weighting factor on dry bulb temperature is 0.2 or zero.

Figure 5 illustrates the [R.sup.2] variation of the correlation between monthly total building energy use values predicted using TMY2 and those obtained from averaging 30 years of simulation data as a function of the weighting factors associated with 5 U.S. sites. The [R.sup.2] values are almost equal to 1.0 for all weighting factor configurations except in Atlanta. The lower [R.sup.2] values (i.e., [R.sup.2] > 0.95) are obtained when the weighting factor of DBT is set to zero implying that DBT is a crucial parameter to the TMY weather data selection process.

[FIGURE 5 OMITTED]

IMPACT OF THE NUMBER OF WEATHER YEARS

Using the simulation building model, annual total energy consumption are estimated for various TMY2 weather data sets generated from several periods with varying number of years and are compared to those mean values obtained from 30 years weather data. Table 8 summarizes the results of the simulation analysis and provides the annual energy use differences in terms of electricity, natural gas, and total energy use using the simulation results obtained using the 30-year average data set as references. Generally, the largest differences are found as expected for TMY2 generated from small number of years.
Table 8. Annual Energy Use Predictions Using TMY2 Generated with
Different Number of Years Compared Against Mean Annual Energy Use
Obtained with Multi-Year Simulation

                                         Albany            Atlanta

Number of Yr        Energy      Energy Use  Diff %  Energy Use  Diff %

30yr Avg of   Elec(MWh)             297        --       316        --
Multi Year    Gas(MBtu [GJ])        726        --       229        --
Simulation    Total(MBtu [GJ])     1740        --      1307        --

28yr (63-90)  Elec(MWh)             296      -0.4%      315      -0.3%
              Gas(MBtu [GJ])        748       2.9%      220      -3.7%
              Total(MBtu [GJ])     1757       1.0%     1295      -0.9%

26yr (65-90)  Elec(MWh)             297       0.0%      315      -0.4%
              Gas(MBtu [GJ])        723      -0.5%      211      -7.7%
              Total(MBtu [GJ])     1736      -0.2%     1285      -1.7%

24yr (67-90)  Elec(MWh)             297       0.1%      314      -0.6%
              Gas(MBtu [GJ])        725      -0.2%      218      -4.8%
              Total(MBtu [GJ])     1739       0.0%     1290      -1.3%

22yr (69-90)  Elec(MWh)             297       0.0%      316      -0.1%
              Gas(MBtu [GJ])        727       0.0%      206      -9.7%
              Total(MBtu [GJ])     1740       0.0%     1284      -1.8%

20yr (71-90)  Elec(MWh)             297       0.1%      315      -0.4%
              Gas(MBtu [GJ])        720      -0.9%      210      -8.0%
              Total(MBtu [GJ])     1735      -0.3%     1285      -1.7%

18yr (73-90)  Elec(MWh)             297       0.0%      317       0.2%
              Gas(MBtu [GJ])        720      -0.9%      191     -16.5%
              Total(MBtu [GJ])     1734      -0.3%     1271      -2.8%

16yr (75-90)  Elec(MWh)             297      -0.1%      318       0.7%
              Gas(MBtu [GJ])        703      -3.2%      216      -5.4%
              Total(MBtu [GJ])     1715      -1.4%     1303      -0.3%

14yr (77-90)  Elec(MWh)             297       0.1%      319       0.9%
              Gas(MBtu [GJ])        710      -2.3%      215      -6.1%
              Total(MBtu [GJ])     1724      -0.9%     1303      -0.3%

12yr (79-90)  Elec(MWh)             298       0.3%      318       0.8%
              Gas(MBtu [GJ])        722      -0.5%      202     -11.5%
              Total(MBtu [GJ])     1739      -0.1%     1289      -1.4%

10yr (81-90)  Elec(MWh)             298       0.2%      318       0.5%
              Gas(MBtu [GJ])        712      -2.0%      209      -8.4%
              Total(MBtu [GJ])     1728      -0.7%     1293      -1.1%

08yr (83-90)  Elec(MWh)             299       0.6%      321       1.4%
              Gas(MBtu [GJ])        702      -3.4%      188     -17.6%
              Total(MBtu [GJ])     1722      -1.0%     1282      -1.9%

06yr (85-90)  Elec(MWh)             297       0.1%      320       1.2%
              Gas(MBtu [GJ])        691      -4.8%      201     -12.3%
              Total(MBtu [GJ])     1706      -1.9%     1292      -1.2%

05yr (86-90)  Elec(MWh)             298       0.3%      321       1.6%
              Gas(MBtu [GJ])        682      -6.1%      197     -14.0%
              Total(MBtu [GJ])     1698      -2.4%     1292      -1.2%

04yr (87-90)  Elec(MWh)             299       0.7%      319       1.0%
              Gas(MBtu [GJ])        653     -10.0%      194     -15.2%
              Total(MBtu [GJ])     1674      -3.8%     1284      -1.8%

03yr (88-90)  Elec(MWh)             298       0.4%      322       2.0%
              Gas(MBtu [GJ])        680      -6.4%      188     -17.8%
              Total(MBtu [GJ])     1697      -2.4%     1288      -1.5%

02yr (89-90)  Elec(MWh)             297      -0.1%      326       3.1%
              Gas(MBtu [GJ])        661      -9.0%      182     -20.4%
              Total(MBtu [GJ])     1673      -3.8%     1294      -1.0%

                                       Honolulu           Las Vegas

Number of Yr  Energy            Energy Use  Diff %  Energy Use  Diff %

30yr Avg of   Elec(MWh)             380       --        334       --
Multi Year    Gas(MBtu [GJ])         46       --        128       --
Simulation    Total(MBtu [GJ])     1343       --       1266       --

28yr (63-90)  Elec(MWh)             380      0.1%       332     -0.5%
              Gas(MBtu [GJ])         47      1.9%       130      1.7%
              Total(MBtu [GJ])     1345      0.2%      1262     -0.3%

26yr (65-90)  Elec(MWh)             379     -0.2%       332     -0.3%
              Gas(MBtu [GJ])         47      1.9%       128      0.4%
              Total(MBtu [GJ])     1342     -0.1%      1263     -0.3%

24yr (67-90)  Elec(MWh)             381      0.4%       335      0.3%
              Gas(MBtu [GJ])         45     -1.4%       125     -2.2%
              Total(MBtu [GJ])     1347      0.3%      1267      0.1%

22yr (69-90)  Elec(MWh)             380     -0.1%       334      0.0%
              Gas(MBtu [GJ])         45     -1.4%       128      0.4%
              Total(MBtu [GJ])     1341     -0.1%      1267      0.0%

20yr (71-90)  Elec(MWh)             379     -0.4%       333     -0.1%
              Gas(MBtu [GJ])         47      1.9%       130      1.5%
              Total(MBtu [GJ])     1339     -0.3%      1267      0.1%

18yr (73-90)  Elec(MWh)             381      0.3%       335      0.6%
              Gas(MBtu [GJ])         47      1.9%       121     -5.6%
              Total(MBtu [GJ])     1347      0.3%      1265     -0.1%

16yr (75-90)  Elec(MWh)             381      0.3%       336      0.8%
              Gas(MBtu [GJ])         47      1.9%       121     -5.6%
              Total(MBtu [GJ])     1348      0.4%      1268      0.2%

14yr (77-90)  Elec(MWh)             385      1.3%       338      1.4%
              Gas(MBtu [GJ])         45     -1.8%       121     -5.6%
              Total(MBtu [GJ])     1359      1.1%      1275      0.7%

12yr (79-90)  Elec(MWh)             384      1.2%       336      0.7%
              Gas(MBtu [GJ])         46     -0.3%       120     -5.8%
              Total(MBtu [GJ])     1358      1.1%      1267      0.1%

10yr (81-90)  Elec(MWh)             384      0.9%       333     -0.3%
              Gas(MBtu [GJ])         45     -1.8%       127     -0.8%
              Total(MBtu [GJ])     1354      0.8%      1262     -0.4%

08yr (83-90)  Elec(MWh)             384      1.0%       334      0.0%
              Gas(MBtu [GJ])         46      0.6%       125     -2.1%
              Total(MBtu [GJ])     1356      0.9%      1263     -0.2%

06yr (85-90)  Elec(MWh)             385      1.4%       339      1.5%
              Gas(MBtu [GJ])         46      0.6%       129      0.7%
              Total(MBtu [GJ])     1361      1.3%      1284      1.4%

05yr (86-90)  Elec(MWh)             387      1.8%       335      0.5%
              Gas(MBtu [GJ])         46     -0.5%       123     -3.5%
              Total(MBtu [GJ])     1367      1.8%      1268      0.1%

04yr (87-90)  Elec(MWh)             383      0.9%       341      2.1%
              Gas(MBtu [GJ])         45     -1.8%       127     -0.3%
              Total(MBtu [GJ])     1354      0.8%      1290      1.9%

03yr (88-90)  Elec(MWh)             384      1.0%       339      1.7%
              Gas(MBtu [GJ])         45     -1.8%       139      8.7%
              Total(MBtu [GJ])     1356      0.9%      1297      2.4%

02yr (89-90)  Elec(MWh)             378     -0.4%       339      1.7%
              Gas(MBtu [GJ])         46     -0.3%       140      9.6%
              Total(MBtu [GJ])     1338     -0.4%      1298      2.5%

                                        Madison              Miami

Number of Yr       Energy       Energy Use  Diff %  Energy Use  Diff %

30yr Avg of   Elec(MWh)             302        --       379       --
Multi Year    Gas(MBtu [GJ])        812        --        48       --
Simulation    Total(MBtu [GJ])     1844        --      1343       --

28yr (63-90)  Elec(MWh)             301      -0.3%      377     -0.6%
              Gas(MBtu [GJ])        793      -2.3%       48     -0.5%
              Total(MBtu [GJ])     1821      -1.2%     1335     -0.6%

26yr (65-90)  Elec(MWh)             302       0.0%      378     -0.4%
              Gas(MBtu [GJ])        818       0.7%       48     -1.5%
              Total(MBtu [GJ])     1849       0.3%     1337     -0.4%

24yr (67-90)  Elec(MWh)             303       0.2%      379     -0.2%
              Gas(MBtu [GJ])        811      -0.2%       48     -1.5%
              Total(MBtu [GJ])     1844       0.0%     1340     -0.2%

22yr (69-90)  Elec(MWh)             303       0.2%      378     -0.4%
              Gas(MBtu [GJ])        831       2.4%       48     -1.5%
              Total(MBtu [GJ])     1865       1.1%     1337     -0.4%

20yr (71-90)  Elec(MWh)             303       0.3%      381      0.4%
              Gas(MBtu [GJ])        840       3.5%       46     -4.0%
              Total(MBtu [GJ])     1875       1.7%     1346      0.3%

18yr (73-90)  Elec(MWh)             303       0.3%      380      0.2%
              Gas(MBtu [GJ])        833       2.6%       46     -4.0%
              Total(MBtu [GJ])     1868       1.3%     1344      0.1%

16yr (75-90)  Elec(MWh)             304       0.5%      377     -0.6%
              Gas(MBtu [GJ])        868       6.9%       49      0.6%
              Total(MBtu [GJ])     1905       3.3%     1335     -0.6%

14yr (77-90)  Elec(MWh)             304       0.6%      377     -0.7%
              Gas(MBtu [GJ])        875       7.7%       48     -0.3%
              Total(MBtu [GJ])     1912       3.7%     1334     -0.6%

12yr (79-90)  Elec(MWh)             303       0.3%      381      0.6%
              Gas(MBtu [GJ])        829       2.1%       47     -3.0%
              Total(MBtu [GJ])     1864       1.1%     1349      0.4%

10yr (81-90)  Elec(MWh)             304       0.6%      381      0.6%
              Gas(MBtu [GJ])        793      -2.4%       47     -3.0%
              Total(MBtu [GJ])     1830      -0.7%     1349      0.4%

08yr (83-90)  Elec(MWh)             304       0.7%      378     -0.2%
              Gas(MBtu [GJ])        774      -4.8%       47     -3.0%
              Total(MBtu [GJ])     1812      -1.7%     1339     -0.3%

06yr (85-90)  Elec(MWh)             304       0.4%      381      0.5%
              Gas(MBtu [GJ])        755      -7.1%       47     -3.6%
              Total(MBtu [GJ])     1791      -2.9%     1348      0.4%

05yr (86-90)  Elec(MWh)             302       0.1%      384      1.2%
              Gas(MBtu [GJ])        704     -13.4%       47     -2.5%
              Total(MBtu [GJ])     1736      -5.9%     1358      1.1%

04yr (87-90)  Elec(MWh)             304       0.5%      383      1.0%
              Gas(MBtu [GJ])        715     -11.9%       47     -3.6%
              Total(MBtu [GJ])     1752      -5.0%     1355      0.9%

03yr (88-90)  Elec(MWh)             303       0.2%      387      2.0%
              Gas(MBtu [GJ])        746      -8.2%       46     -4.4%
              Total(MBtu [GJ])     1780      -3.5%     1367      1.8%

02yr (89-90)  Elec(MWh)             303       0.3%      391      3.2%
              Gas(MBtu [GJ])        804      -1.0%       46     -4.2%
              Total(MBtu [GJ])     1838      -0.3%     1382      2.9%

                                         Omaha             Phoenix

Number of Yr       Energy       Energy Use  Diff %  Energy Use  Diff %

30yr Avg of   Elec(MWh)             311        --       356         --
Multi Year    Gas(MBtu [GJ])        644        --        80         --
Simulation    Total(MBtu [GJ])     1704        --      1294         --

28yr (63-90)  Elec(MWh)             309      -0.6%      359       0.8%
              Gas(MBtu [GJ])        635      -1.4%       77      -3.0%
              Total(MBtu [GJ])     1690      -0.9%     1301       0.6%

26yr (65-90)  Elec(MWh)             310      -0.3%      357       0.4%
              Gas(MBtu [GJ])        632      -1.9%       84       6.2%
              Total(MBtu [GJ])     1689      -0.9%     1304       0.8%

24yr (67-90)  Elec(MWh)             308      -0.8%      360       1.1%
              Gas(MBtu [GJ])        643      -0.2%       77      -3.0%
              Total(MBtu [GJ])     1695      -0.6%     1305       0.9%

22yr (69-90)  Elec(MWh)             309      -0.7%      360       1.1%
              Gas(MBtu [GJ])        632      -1.9%       77      -3.0%
              Total(MBtu [GJ])     1685      -1.1%     1305       0.9%

20yr (71-90)  Elec(MWh)             310      -0.4%      360       1.2%
              Gas(MBtu [GJ])        643      -0.1%       77      -3.0%
              Total(MBtu [GJ])     1699      -0.3%     1306       1.0%

18yr (73-90)  Elec(MWh)             310      -0.3%      361       1.5%
              Gas(MBtu [GJ])        647       0.5%       77      -3.3%
              Total(MBtu [GJ])     1705       0.0%     1309       1.2%

16yr (75-90)  Elec(MWh)             310      -0.2%      360       1.1%
              Gas(MBtu [GJ])        656       1.9%       78      -2.0%
              Total(MBtu [GJ])     1714       0.6%     1306       0.9%

14yr (77-90)  Elec(MWh)             310      -0.3%      361       1.5%
              Gas(MBtu [GJ])        673       4.6%       75      -6.2%
              Total(MBtu [GJ])     1731       1.6%     1307       1.0%

12yr (79-90)  Elec(MWh)             308      -0.9%      363       2.2%
              Gas(MBtu [GJ])        677       5.2%       73      -8.6%
              Total(MBtu [GJ])     1728       1.4%     1313       1.5%

10yr (81-90)  Elec(MWh)             307      -1.1%      366       2.9%
              Gas(MBtu [GJ])        652       1.2%       73      -8.6%
              Total(MBtu [GJ])     1701      -0.2%     1322       2.2%

08yr (83-90)  Elec(MWh)             311       0.1%      363       2.2%
              Gas(MBtu [GJ])        650       1.0%       73      -8.6%
              Total(MBtu [GJ])     1712       0.4%     1313       1.5%

06yr (85-90)  Elec(MWh)             311       0.0%      363       2.1%
              Gas(MBtu [GJ])        640      -0.5%       69     -13.7%
              Total(MBtu [GJ])     1701      -0.2%     1309       1.2%

05yr (86-90)  Elec(MWh)             311       0.0%      365       2.6%
              Gas(MBtu [GJ])        573     -11.0%       73      -8.7%
              Total(MBtu [GJ])     1634      -4.1%     1318       1.9%

04yr (87-90)  Elec(MWh)             314       0.9%      367       3.1%
              Gas(MBtu [GJ])        593      -7.9%       73      -8.6%
              Total(MBtu [GJ])     1663      -2.4%     1324       2.4%

03yr (88-90)  Elec(MWh)             309      -0.4%      366       2.9%
              Gas(MBtu [GJ])        579     -10.0%       72      -9.7%
              Total(MBtu [GJ])     1635      -4.1%     1322       2.1%

02yr (89-90)  Elec(MWh)             311       0.0%      367       3.2%
              Gas(MBtu [GJ])        646       0.4%       74      -6.8%
              Total(MBtu [GJ])     1707       0.1%     1328       2.6%

                                       Seattle              Tucson

Number of Yr       Energy       Energy Use  Diff %  Energy Use  Diff %

30yr Avg of   Elec(MWh)             270       --        335       --
Multi Year    Gas(MBtu [GJ])        401       --         94       --
Simulation    Total(MBtu [GJ])     1323       --       1235       --

28yr (63-90)  Elec(MWh)             269     -0.3%       333     -0.3%
              Gas(MBtu [GJ])        386     -3.9%        90     -4.2%
              Total(MBtu [GJ])     1305     -1.4%      1228     -0.6%

26yr (65-90)  Elec(MWh)             269     -0.4%       333     -0.3%
              Gas(MBtu [GJ])        382     -4.8%        90     -4.2%
              Total(MBtu [GJ])     1301     -1.7%      1228     -0.6%

24yr (67-90)  Elec(MWh)             269     -0.4%       334     -0.3%
              Gas(MBtu [GJ])        388     -3.3%        90     -4.2%
              Total(MBtu [GJ])     1307     -1.3%      1228     -0.6%

22yr (69-90)  Elec(MWh)             269     -0.3%       334     -0.3%
              Gas(MBtu [GJ])        386     -3.8%        92     -2.1%
              Total(MBtu [GJ])     1305     -1.4%      1230     -0.4%

20yr (71-90)  Elec(MWh)             270     -0.2%       334      0.0%
              Gas(MBtu [GJ])        390     -2.9%        89     -4.7%
              Total(MBtu [GJ])     1310     -1.1%      1231     -0.4%

18yr (73-90)  Elec(MWh)             269     -0.5%       334      0.0%
              Gas(MBtu [GJ])        386     -3.8%        89     -4.7%
              Total(MBtu [GJ])     1303     -1.5%      1231     -0.4%

16yr (75-90)  Elec(MWh)             269     -0.4%       336      0.5%
              Gas(MBtu [GJ])        382     -4.9%        89     -4.7%
              Total(MBtu [GJ])     1300     -1.8%      1237      0.1%

14yr (77-90)  Elec(MWh)             270     -0.1%       338      0.9%
              Gas(MBtu [GJ])        381     -5.0%        95      1.8%
              Total(MBtu [GJ])     1303     -1.6%      1247      1.0%

12yr (79-90)  Elec(MWh)             269     -0.3%       339      1.2%
              Gas(MBtu [GJ])        396     -1.2%        94      0.1%
              Total(MBtu [GJ])     1315     -0.6%      1250      1.2%

10yr (81-90)  Elec(MWh)             269     -0.3%       337      0.9%
              Gas(MBtu [GJ])        390     -2.8%        95      2.0%
              Total(MBtu [GJ])     1310     -1.0%      1247      0.9%

08yr (83-90)  Elec(MWh)             272      0.5%       339      1.3%
              Gas(MBtu [GJ])        404      0.6%        95      1.9%
              Total(MBtu [GJ])     1330      0.5%      1251      1.3%

06yr (85-90)  Elec(MWh)             273      1.1%       342      2.3%
              Gas(MBtu [GJ])        392     -2.2%        96      2.2%
              Total(MBtu [GJ])     1325      0.1%      1264      2.3%

05yr (86-90)  Elec(MWh)             273      0.9%       342      2.3%
              Gas(MBtu [GJ])        364     -9.4%        93     -0.4%
              Total(MBtu [GJ])     1294     -2.3%      1262      2.1%

04yr (87-90)  Elec(MWh)             272      0.8%       342      2.1%
              Gas(MBtu [GJ])        363     -9.4%        93     -0.2%
              Total(MBtu [GJ])     1293     -2.3%      1259      1.9%

03yr (88-90)  Elec(MWh)             271      0.2%       344      2.7%
              Gas(MBtu [GJ])        406      1.1%        86     -7.7%
              Total(MBtu [GJ])     1330      0.5%      1259      1.9%

02yr (89-90)  Elec(MWh)             274      1.5%       344      2.7%
              Gas(MBtu [GJ])        425      6.0%        89     -5.3%
              Total(MBtu [GJ])     1361      2.9%      1262      2.1%


Figure 6 illustrates the [R.sup.2] variation of the correlation between monthly total building energy use values predicted using TMY2 and those obtained from averaging 30 years of simulation data as a function of the number of years utilized to generate the typical weather year for 10 U.S. sites. For all sites except Atlanta, the correlation [R.sup.2] is above 0.95 when more than 5 years are used to generate TMY weather data. For Atlanta, there are large differences between monthly heating energy use values predicted by TMY2 and mean values for 30 years. At lest 12 years are needed to generate TMY2 in order for the value of [R.sup.2] to exceed 0.90 in Atlanta.

[FIGURE 6 OMITTED]

An analysis of the historical data show that there is an increase in average winter temperatures for Atlanta as well as Phoenix and thus a decrease in annual heating degree over between 1961 and 1990 as shown in Figure 7. Thus, when TMY2 is selected based on only few years (starting from 1961), the heating degree days will be higher than that obtained from either 30-year average, or TMY2 based on 25 or 30 years. Moreover, the results of Figure 7 indicates that the years selected for TMY2 during winter months can affect significantly the heating energy use predicted for an office building in a warm climate with relatively non negligible heating needs such as Atlanta.

[FIGURE 7 OMITTED]

SUMMARY AND CONCLUSIONS

The impact of the selection of typical weather year data on predicting annual building energy use is investigated and compared against 30-year average data using statistical analysis. A comparative analysis based on the average simulation predictions obtained for 30-year period is carried out. Impacts of weighting factors and the numbers of years used to generate typical weather data are investigated for 10 U.S. sites.

The results of the simulation analyses presented in this paper for prototypical office buildings show a maximum 5% difference between the simulation results obtained using any typical weather data sets (TMY, IWEC, and TMY2) and those obtained by averaging the results for 30 years for the 10 U.S. climates considered in this paper. In terms of total building energy use, IWEC and TMY2n data sets provide the least differences against long-term energy use predictions. Moreover, the results indicate that, for the 10 U.S. climates considered in the study, 15 years of recorded data would be sufficient to generate a typical weather year suitable for energy analysis of prototypical office buildings. Additional work is needed to generalize the results for other climates and building types.

REFERENCES

Argiriou A., S. Lykoudis, S. Kontoyiannidis, C.A. Balaras, A. Asimakopoulos, M. Petrakis, and P. Kassomenos. 1999. Comparison of methodologies for TMY generation using 20 years data for Athens, Greece. Solar Energy 66(1):33-45.

Anon. 2000. TRNSYS: Transient system simulation program. Solar Energy Laboratory, University of Wisconsin, Madison, WI.

ASHRAE. 1976. ASHRAE handbook of fundamentals. American Society of Heating Refrigeration and Air-Conditioning Engineers, Inc., Atlanta.

Crawley, D.B., L.K. Lawrie, C.O. Pedersen, and F.C. Winkelmann. 2000. Energy plus: Energy simulation program. ASHRAE Journal 42(4):49-56.

Crow, L.W., 1981, Development of hourly data for weather year for energy calculations (WYEC), including solardata, at 21 stations throughout the US. ASHRAE Transactions 87(1):896-905.

Donghyun, S., M. Krarti, and J. Huang. 2009. Evaluation of typical weather year selection approaches. ASHRAE Transactions 115(2):654-67.

Festa, R. and C.F. Ratto. 1993. Proposal of a numerical procedure to select reference years. Solar Energy 50:9-17.

Hall, I.J., R.R. Prairie, H.E. Anderson, and E.C. Boes. 1978. Generation of a typical meteorological year. In: Proceedings of the 1978 annual meeting of the American Section of ISES, pp. 669-671.

Hui, C.M. 1996. Energy performance of air-conditioned buildings in Hong Kong, PhD Thesis. City University of Hong Kong.

Keeble, E. 1990. Availability of UK climatic data for use in simulation, BEPAC Technical Note 90/1, Building Research Establishment, U.K., October.

LBL. 1981. DOE reference manual version 2.1A. Prepared for the U.S. Department of Energy by Lawrence Berkeley Laboratory, Report LBL-8706 Rev. 2.

Lund, H. and S. Eidorff. 1980. Selection methods for production of test reference years. Appendix D, contract 284-77 ES DK. Final report EUR 7306 EN. Copenhagen: Technical University of Denmark, Thermal Insulation Laboratory.

Marion, W. and K. Urban. 1995. Users manual for TMY2s typical meteorological years. National Renewable Energy Laboratory, Golden, CO.

Massie, D.D. and J.F. Kreider. 2001. Comparison of and discrepancies between TMY and TMY2s predictions for simple photovoltaic and wind energy simulation, ASME Trans, vol. 123.

Thevenard, D.J. and A.P. Brunger. 2002. The development of typical weather years for international locations, Part II, Production, ASHRAE Transaction 108(2):480-486.

Wilcox, S. and W. Marion. 2008. Users manual for TMY3 data sets, NREL Technical Report NREL/TP-581-43156, April, Golden, CO.

This paper is based on findings resulting from ASHRAE Research Project RP-1477.

Donghyun Seo is a graduate student and Moncef Krarti, PhD, PE is a Professor and Associate Chair in the Civil, Environmental, and Architectural Engineering Department at the University of Colorado, Boulder, CO. Joe Huang is president of White Box Technologies, Inc., Moraga, CA, and formerly a staff scientist at Lawrence Berkeley National Laboratory, Berkeley, CA.

Donghyun Seo

Student Member ASHRAE

Yu Joe Huang

Member ASHRAE

Moncef Krarti, PhD, PE

Member ASHRAE
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Author:Seo, Donghyun; Huang, Yu Joe; Krarti, Moncef
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Date:Jan 1, 2010
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