It is with joy and some trepidation to report that White Box Technologies (WBT) is
updating all its historical weather files with satellite-derived solar radiation. Joy
because this overcomes what has been the most significant question mark with weather
files; trepidation because of the amount of work needed to carry out and maintain this
effort. To show that this is more than marketing hype, I need to give a rather long
explanation about this development.
The bane of weather data over the past three decades has been the solar radiation (global
horizontal and direct normal) which are not measured parameters, but derived using various
solar and sky models. All the familiar "typical year" sets, i.e., TMY, WYEC, IWEC, etc.,
let alone the historical weather files, have modeled solar radiation. Although a lot of
work has gone into such models (see M. Iqbal,"An Introduction to Solar Radiation",
Academic Press, 1983), there remain an almost intractible problem of the lack of good
measured solar to tune any of these models. For example, in the ASHRAE IWEC2 weather
files, my team was able to find one or two years' measured data for less than 50 locations,
from which were derived 28 sets of regression coefficients then used for all 3,012 IWEC2
locations.
For the past decade and a half, researchers around the world have been working to derive
solar radiation from weather satellite imagery, driven largely by the needs of the solar
power industry for the siting of solar power plants and getting "bankable" solar estimates
for their arrays. Our little building energy simulation sector can of course benefit by
hanging on the coattails of the solar power industry, but the downside has been to be
totally priced out, since the commercial cost for one year's solar data for one location
(grid cell) typically runs around $1,000.
A welcome development over the last five years is that various government offices or
affiliated consortia are now beginning to also providing access to satellite-derived solar
radiation at minimal or more acceptable costs under various conditions. Over the past
three years, WBT has obtained access to such data and permission for its use in WBT
weather files.
WBT is now either replacing the solar radiation on its historical weather files, or using
satellite-derived radiation to develop custom solar coefficients for each location to
extend the satellite-derived solar to time periods outside the available time window. With
the exception of polar locations above or below 60/66 degrees, island nations in the
Pacific and Indian Ocean, and a few unfortunate "blind spots", the entire land mass is
being covered with at least 10 years up to 18 years of hourly solar records.
Starting in 2018, WBT historical weather files in the following areas will all have
satellite-derived solar radiation for the following time periods: Europe, Africa, South
America east of 66 West, i.e., Brazil and Uruguay (2004 to date), Australia (1999 to
date), and East Asia (2007 to date, access pending). WBT historical weather files in the
following areas will have satellite-derived solar radiation for the indicated time periods
- North America and Central/South American down to 20 South (1998-2015), South Asia
(2000-2014), with modeled solar radiation from 2016 on that has been individually tuned to
the past satellite-derived solar.
Another benefit to the satellite-derived solar is to increases the number of available
weather stations, which in many places has been limited by the lack of cloud cover data
needed to model the solar radiation.? For reasons that are not immediately identifiable,
several English-speaking Commonwealth countries has seen a marked drop in the number of
available stations due to the decreases in the reporting of cloud cover (see plot, ZAF =
South Africa). For example, the number of stations in the UK has dropped by almost 2/3s
between 2001 and 2017 (174 to 64), but with satellite-derived solar, it will go back up to
over 180, while in Australia and South Africa the comparable numbers are from 175 to well
over 500, and from 37 to over 100, respectively.
If interested, customers who have purchased a historical weather files from WBT over the
past five years can get an updated weather file at no cost. Lastly, although it will take
at least a month to update all 10,000 2017 files, it's very quick to do for any specific
location or even 50 or so locations. Therefore, if you have an urgent request please
e-mail me and I will put that at the beginning of the queue for that day.
--
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"
