Jean,
Happy holidays!
I'm sure the satellite imagery is used to get some form of cloud cover, but that's become strictly an intermediate product in deriving the global horizontal and direct normal radiation, which is what's of the most interest and relevance to the end users. In fact, all the services that I've accessed except one provide only the radiation values. NREL's NSRDB (National Solar Radiation Data Base) does provide something called "Cloud Type" which I've always include in my downloads just out of curiosity, but I have little idea what it is or how it might be of use. I believe it's a classification of the cloud cover, NOT the amount, but for starters I don't have the coding, but then I haven't bothered to check since the NSRDB already gives you the radiation.
There are also differences in how various places have approached this technique. NREL uses what they call a "Physical Model" that I believe utilizes the excellent work by Chris Gueymard in his REST2 Clear Sky Model, which is also physically based. The SUNY Model developed by Richard Perez, a pioneer in this field, and which has been used in several commercial services, takes a more empirical approach. On the other hand, Prof. Takenaka in Japan uses a neural network but I'm not clear what he's training it on although the pictures are very nice (see http://www.amaterass.org/current/current.dwn.sw.flx.sfc.msm.bin.gif ).
Joe
Joe Huang White Box Technologies, Inc. 346 Rheem Blvd., Suite 205A Moraga CA 94556 yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx http://weather.whiteboxtechnologies.com for simulation-ready weather data (o) (925)388-0265 (c) (510)928-2683 "building energy simulations at your fingertips"
Hi Joe,Which parameters are derived from satalite imaging?-Cloud cover-Radiation-...?Jean
Am 03.01.2018 4:16 vorm. schrieb "Joe Huang YJHuang@xxxxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx>:
Nick,
This is an interesting topic that has evolved in an unexpected way. Now that NREL has mastered the technology of satellite-derived solar radiation, they have regarded weather station data as an impediment, since there's no way to get such measured data to match the 5-kilometer grid of the satellite-derived solar. Therefore, they have abandoned the weather station data (which was was used in all the TMYs to date) and gone instead to Reanalysis Data from NOAA's MERRA, which is running a climate forecasting model in retrospective mode. I don't have time now to discuss reanalysis, except to say that from what I've seen the results are decidedly "iffy". Almost two years ago, I managed to get a Work Statement through ASHRAE for someone to take a good look at reanalysis data.
So, what does this mean for your question? NREL's National Solar Radiation Data Base (NSRDB) can now get you the hourly time series or a TMY for more than a million grid points over the US, all with satellite-derived solar radiation but MERRA results for the other climate parameters. I've told NREL several years ago that for the building simulation industry it would be much better to merge the satellite-derived solar with actual station data that give good accurate coverage of urban areas where buildings are located. NREL's response was that they'd be happy to do this, but someone has to pay them for the work.
In response to your hypothesis below, I think you're being too disparaging of the previous modeling efforts while raising too high your expectations of satellite solar. It's not that the previous models failed to account for local climate conditions, but that they lacked good data to drive them. For example, all models included terms for cloud cover and clearness, or for the more detailed physical models arcane parameters like aerosol optical depth, preciptable moisture, etc., but how available are the input data and how reliable are they? The advantages of satellite-derived solar are that they provide a comprehensive and objective view of the cloud conditions, which combined with satellite measurements of the atmospheric conditions and improved modeling, results in accuracies that previous modeling efforts can not attain. As far as discerning localized effects of smog and dust in urban areas, that would still depend on whether there's sufficient monitoring at that spatial and time scale to detect the differences. What I mean is that it's one thing to observe that in general urban locations have more smog and particulates than rural locations, but it's something else to quantify the resultant differences in solar radiation over time and distance.
I'd like to take the opportunity here to step back and comment on the status of weather data for the building energy community, My interactions with NREL has brought the realization that we have been piggy-backing on the efforts of others outside our community for our weather data. I don't intend to pick on the NREL Solar Program, several of whom I consider friends and colleagues, but their target client is the solar power industry. Since solar power arrays can be installed anywhere, preferably in rural uninhabited locations, it makes sense to go to satellite-derived solar. It's also clear that to serve that industry, NREL would focus its efforts on getting the best solar values, while all the other climatic parameters, like temperature, humidity, wind speed, etc., are secondary, which may be why getting them from MERRA is a satisfactory choice. The focus on solar is also evident in the weighting used by NREL to develop the TMYs, with 50% weight placed on the 2 solar and 50% on the remaining 8 non-solar parameters.
For the building energy community, or priorities are somewhat different. Since 99% of buildings are located in urban locations, we should focus much more on climate in urban areas. Luckily, that's also where the great majority of existing weather are located, which is why I'm resistant to throwing out measured weather data and replacing them with synthetic data, no matter how much they've been "seeded" with real data. As for the weighting of climate parameters in selecting the typical months, why not use building energy simulations and weight them by the distribution of heating and cooling loads?
Joe
Joe Huang White Box Technologies, Inc. 346 Rheem Blvd., Suite 205A Moraga CA 94556 yjhuang@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"On 1/2/2018 9:30 AM, Nicholas Caton wrote:
Hi Joe,
From your perspective, are NREL or any other government/professional bodies making moves/indications to update/refresh the current TMY3 sets to utilize satellite-derived solar radiation? Seems like a no-brainer for our industry, but is there a counter-argument? It seems likely, but has there been rigorous comparisons of satellite-derived solar radiation against measured values and/or our “present-day” solar models used to derive solar radiation information for building energy simulation?
Also, I’m trying to understand and correctly characterize the impact of this development in simple terms my brain can follow. Is it fair to say:
The solar models used in developing weather files for building energy simulations to-date in our industry (including all/most industry-standard TMY weather sets), because they have been using solar radiation derived from (evolving) solar models, have not accounted for the likes of local climate cloud cover / smog / dust? Seattle (~47°N) has perhaps been seeing as much sunlight through the winter as Paris (~48°N)?
Does satellite-derived solar radiation address some or all of these local climate issues (cloud cover, smog, dust) affecting direct/indirect solar radiation?
Thanks sincerely for all your teaching Joe,
~Nick
Nick Caton, P.E., BEMP
Senior Energy Engineer
Regional Energy Engineering ManagerEnergy and Sustainability Services
Schneider ElectricD 913.564.6361
M 785..410.3317
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E nicholas.caton@schneider-electric.com 15200 Santa Fe Trail Drive
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Lenexa, KS 66219
United States
From: Equest-users [mailto:equest-users-bounces@
lists.onebuilding.org ] On Behalf Of Joe Huang via Equest-users
Sent: Thursday, December 14, 2017 8:13 PM
To: BLDG-SIM <bldg-sim@lists.onebuilding.org> ; EnergyPlus_Support <EnergyPlus_Support@yahoogroups.com> ; equest-users@lists.onebuilding.org
Subject: [Equest-users] White Box Technologies brings simulation weather data to the satellite age
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 HuangWhite Box Technologies, Inc.346 Rheem Blvd., Suite 205AMoraga CA 94556yjhuang@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"
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