ASHRAE Journal - October 2022 - 32

COLUMN ENGINEER'S NOTEBOOK
TMY database was an effort to improve upon the Test
Reference Year (TRY) database, which was one of the
earliest building energy simulation databases.II
National Renewable Energy Laboratory (NREL) produced
the TMY 2 and TMY 3 data sets by updating the
Sandia data using the National Solar Radiation Database
(NSRDB). Recently, gridded TMY data has become available
that is based on satellite readings taken on a 4 km
by 4 km (13,123 ft) grid from 1998-2017.
TMY2 vs. TMY3
The TMY3 data sets are hourly values of solar radiation
and meteorological information for a one-year period
based on records from 1991-2005. TMY2's are similar, but
for the years 1961-1990.**, ††
The TMY3 data set contains
data for 1,020 locations vs. 239 for the TMY2 data set.
Gridded TMY Data
Gridded data is amazing, at least to me. You can click
on any point in the colored area on the National Solar
Radiation Database (https://nsrdb.nrel.gov/) and once
you set up your data query, obtain a data set for the
4 km (13,123 ft) square that your selected point falls
into. A similar website developed by the European
Commission, Joint Research Centre (Photovoltaic
Geographical Information System) is here: https://
tinyurl.com/EuropeanPVData.
The data is derived from readings taken by satellites,‡‡
which (I think) is the reason for a hard line with no data
north of the arctic circle; the satellite viewing angle is
skewed enough so that above that latitude the data is
unacceptable for analysis purposes.
CWEC Data
CWEC stands for Canadian Weather Year for Energy
Calculation and includes 492 data sets created from
30 years of CWEEDS (Canadian Weather Energy and
Engineering Datasets) data.§§
The data may not be totally
in compliance with the TMY standard (more on that in a
minute) but has been coordinated with ASHRAE.
IWEC Data
The ASHRAE IWEC 2 database contains weather files for
3,012 locations outside the United States and Canada.###
The International Weather for Energy Calculations files
are derived from Integrated Surface Hourly (or similar)
weather data originally archived at the National Climatic
Data Center based on periods of record for at least 12
years up to 25 years. This data also may not be fully compliant
with the official TMY standard.
A Closer Look at TMY Data
If you look at the actual data sets, they all have different
numbers of variables in different columns. So, while
conceptually similar, they are not identical regarding
format or the details of the normalization algorithms.
The ISO Standard Behind TMY Data
An ISO standard defines how one goes about putting
a TMY data set together.II II
There is a very significant
aspect of this standard that I feel one needs to be aware
of. To quote the TMY3 user's manual(emphasis is mine):
Because the TMY algorithm assigns priority to the solar
radiation elements, the selected months may or may not be typical
for other elements. Cloud cover, which correlates well with
solar radiation, is probably reasonably typical. Other elements are
not related to the elements used for selection; consequently, their
values may not be typical.
Maximum and minimum temperature, dewpoint and
wind data are given the least weight and solar data is
heavily weighed. The means are daily means calculated
from the hourly values for each month and year in the
data set. A statistical analysis technique is used to assess
all the months in the data set to determine the month
IICrawley, D. 1998. " Which weather data should you use for energy simulations of commercial buildings. " ASHRAE Transactions
104(2):498-515.
**Source: " Users Manual for TMY3 Data Sets " from NREL, a great resource for understanding TMY data: https://tinyurl.com/
NRELTMY3Manual
††Downloads here from the National Solar Radiation Database's Archived Data: https://tinyurl.com/TMYDataArchive
‡‡This paper, " Assembling Typical Meteorological Year Data Sets for Building Energy Performance Using Reanalysis and Satellite-Based
Data, " describes how this works: https://tinyurl.com/TMYFromSatellites
§§CWEC Engineering Climate Data Sets downloads here: https://tinyurl.com/CWECData
###Download the International Weather for Energy Calculations files here for a fee: https://tinyurl.com/IWECData
II II ISO 15927-4-Hygrothermal Performance of Buildings-Calculation and Presentation of Climatic Data, Part 4: Hourly Data for Assessing
the Annual Energy Use for Heating and Cooling
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ASHRAE JOURNAL ashrae.o rg O CTO B E R 2022

https://nsrdb.nrel.gov/ http://www.tinyurl.com/EuropeanPVData http://www.tinyurl.com/EuropeanPVData https://www.tinyurl.com/nreltmy3manual https://www.tinyurl.com/nreltmy3manual https://www.tinyurl.com/TMYDataArchive https://www.tinyurl.com/TMYFromSatellites https://www.tinyurl.com/CWECData https://www.tinyurl.com/IWECData https://www.ashrae.org/

ASHRAE Journal - October 2022

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