Hamish wrote:
Hi Jaro, Marcel, Tomas,
I was wondering if you could contribute some insight WRT this thread on
the GRASS users' list:
http://thread.gmane.org/gmane.comp.gis.grass.devel/17118
I fear my advice so far is rather bad.
thanks,
Hamish
Hamish,
Your advices were right. I am not sure if anything is still unclear, but I would add some minor clarifications:
Is this essentially this same thing, as it is "integrated" over an
hour when summed for the entire day?
I would guess sum midnight to midnight hourly averages of W/m2 = mode2 ?
Effectively, "per day" means from sunrise to sunset time (these are calculated internally), during night there is no solar radiation. Small amount of available diffuse radiation shortly before the sunrise and after the sunset are neglected. We use "per day" unit for situations where we need to calculate amount of available energy for a certain period of year (e.g. months). If you need radiation within one day you can use mode 1 - irradiance values [W/m^2] to calculate irradiance for a specific time and then multiply with your time interval.
Some differences comparing with measurements may arrise from different time systems. r.sun uses local solar time, while practicaly we use "civic" times with time zones (and also modified with "summer/daylight saving time").
Marcel can send you more information about this. I believe the JRC team has some modification of r.sun that uses civic time.
I was aware of the Joule - Watt relationship, but was not quite sure why r.sun was using W*h instead of Joules. Since a Mega-Joule is a much more readily used unit of enery, I think that I will do the conversion from now on.
This is solely for practical reasons. All calculations are done using watts (e.g. solar constant, irradiance), so expressing radiation in Wh is straightforward and gives a better idea how much energy is available, for example, for solar energy applications (photovoltaics, thermal systems). You can still make a simple conversion if you need joules.
Jaro
Hello,
I looked at the discussion related to the units used in r.sun and can just summarize what were other have already said:
The mode 1 gives values of instantaneous solar power (irradiance) impinging on a surface in "Watts per square meter" (W/m2) or in other words in "joules per second per square meter" (J/m2/s)
The mode 2 integrates solar energy (irradiation) according to user-defined steps that is received by square meter during the whole day (actually from sunrise to sunset). The result is given in Watt-hours per square meter (Wh/m2) -> if multiplied by 3600, the daily total energy received by the surface would be expressed in joules per square meter (J/m2).
We are aware of using Watt-hours (as non-SI unit), but this is because we work in the field of photovoltaic applications (solar electricity), where this unit is still preferred.
Best regards,
Marcel
--
Marcel Suri
European Commission - DG Joint Research Centre
Institute for Environment and Sustainability
Renewable Energies Unit
via E. Fermi 1, TP 450
I-21020 Ispra (VA), Italy
phone: +39 0332 786661, fax: +39 0332 789992
http://re.jrc.ec.europa.eu/pvgis/pv/
******
Interested in solar radiation data? Please help us to understand better your needs, and fill in a questionnaire at:
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Jaro Hofierka wrote:
Hamish wrote:
Hi Jaro, Marcel, Tomas,
I was wondering if you could contribute some insight WRT this thread on
the GRASS users' list:
http://thread.gmane.org/gmane.comp.gis.grass.devel/17118
I fear my advice so far is rather bad.
thanks,
Hamish
On Friday 01 December 2006 02:50, Jaro Hofierka wrote:
Hamish wrote:
> Hi Jaro, Marcel, Tomas,
>
> I was wondering if you could contribute some insight WRT this thread on
> the GRASS users' list:
>
> http://thread.gmane.org/gmane.comp.gis.grass.devel/17118
>
>
> I fear my advice so far is rather bad.
>
> thanks,
> Hamish
Hamish,
Your advices were right. I am not sure if anything is still unclear, but
I would add some minor clarifications:
>> Is this essentially this same thing, as it is "integrated" over an
>> hour when summed for the entire day?
>
> I would guess sum midnight to midnight hourly averages of W/m2 = mode2 ?
Effectively, "per day" means from sunrise to sunset time (these are
calculated internally), during night there is no solar radiation. Small
amount of available diffuse radiation shortly before the sunrise and
after the sunset are neglected. We use "per day" unit for situations
where we need to calculate amount of available energy for a certain
period of year (e.g. months). If you need radiation within one day you
can use mode 1 - irradiance values [W/m^2] to calculate irradiance for a
specific time and then multiply with your time interval.
Some differences comparing with measurements may arrise from different
time systems. r.sun uses local solar time, while practicaly we use
"civic" times with time zones (and also modified with "summer/daylight
saving time").
Marcel can send you more information about this. I believe the JRC team
has some modification of r.sun that uses civic time.
> I was aware of the Joule - Watt relationship, but was not quite sure why
> r.sun was using W*h instead of Joules. Since a Mega-Joule is a much more
> readily used unit of enery, I think that I will do the conversion from
> now on.
This is solely for practical reasons. All calculations are done using
watts (e.g. solar constant, irradiance), so expressing radiation in Wh
is straightforward and gives a better idea how much energy is available,
for example, for solar energy applications (photovoltaics, thermal
systems). You can still make a simple conversion if you need joules.
Jaro
Jaro, others;
Thank you for the quick clarifications. I would be happy to summarize this for
the r.sun manual page- with your revision before posting.
Cheers,
--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341