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Net radiation - monitor


OsZ
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Hey there,

 

I've been trying to find a way to monitor solar radiation, daily or even hourly, for a project I'm working at the moment.

I did found something like https://en.tutiempo.net/solar-radiation/

But the results... give me some doubts, to say the least.

 

For instance, today:

London: 397 wh/m2

Lisbon: 3075 wh/m2

New York: 854 wh/m2

 

Isn't the wattage to high?

The other references (ex: CERES) I found, graphics, mostly, go from -280w/m2 to 280w/m2 or 0 to 300w/m2 - per month.

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3 hours ago, OsZ said:

Hey there,

 

I've been trying to find a way to monitor solar radiation, daily or even hourly, for a project I'm working at the moment.

I did found something like https://en.tutiempo.net/solar-radiation/

But the results... give me some doubts, to say the least.

 

For instance, today:

London: 397 wh/m2

Lisbon: 3075 wh/m2

New York: 854 wh/m2

 

Isn't the wattage to high?

The other references (ex: CERES) I found, graphics, mostly, go from -280w/m2 to 280w/m2 or 0 to 300w/m2 - per month.

The figures you have quoted are in Watt-hours/m2. The associated plot for London indicates that solar radiation actually varied between 9W/m2 and 75W/m2 during the day.

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26 minutes ago, 3gMike said:

The figures you have quoted are in Watt-hours/m2. The associated plot for London indicates that solar radiation actually varied between 9W/m2 and 75W/m2 during the day.

Yes, I do understand that.

Still, aren't those values too much; at least as a total/daily?

 

I honestly have no idea of what should be a normal average value for net radiation-daily. 🤔

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26 minutes ago, OsZ said:

Yes, I do understand that.

Still, aren't those values too much; at least as a total/daily?

 

I honestly have no idea of what should be a normal average value for net radiation-daily. 🤔

No, in fact the value for London is a quite low. Average daily insolation is 520Wh/day in December, rising to 4740 in July

You may find this website quite useful.. https://globalsolaratlas.info/map?c=11.523088,8.4375,3&s=39.504041,-8.261719&m=site

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2 hours ago, OsZ said:

Yes, I do understand that.

Still, aren't those values too much; at least as a total/daily?

As mentioned by 3gMike above, that definitely doesn't seem too high. Keep in mind that Solar irradiance at the top of the atmosphere is ~1350 kW/m^2, and near the surface on a cloudless day with Sol at the zenith (near the equator) it's ~1000 kW/m^2. Of course it's going to be quite different at the more "extreme" latitudes the cities you list are situated at, but this should give an idea of how much power sunlight packs. Near the equator the average irradiance is generally ~400 W/m^2, which would give a daily insolation of 9600 Wh/m^2 over the course of 24 hours, over three times the value listed for Lisbon there.

What seems to confuse you is the figure you quote from NASA. That figure is:

  1. Net radiation, i.e. a measure of both incoming and outgoing energy; as you can see, roughly half of the world seems to be in the negative there (since it's the winter half of the year in the northern hemisphere, more energy is dissipated than absorbed, and as a consequence it gets colder).
  2. As also mentioned by 3gMike above, the measures of the cities is daily insolation, i.e. the total daily energy received from sunlight per area, measured in watt-hours per meters squared; the measure from NASA for net radiation on the other hand is a measure of average power per area, measured in watts per meters squared. Power, measured in watts, is a measure of how much energy is delivered per unit of time, so when you multiply that by time, as with watt-hours, you get the amount of energy delivered during that time. Another measure of energy is joules, and 1 Wh (watt-hour) is equal to 3600 J (joules).

In other words, when it says e.g. that net radiation is 200 W/m^2, that means the atmosphere is absorbing 200 W of power for every square meter over the given time period, so over 24 hours that would be a net energy gain of 4800 Wh for every square meter of atmosphere. As you can see this is also actually more than the value you listed for Lisbon; but again, the value listed for Lisbon is daily insolation, not net energy, which as you can see from the NASA map is actually typically still negative for Lisbon this time of the year, and it's not until March, around the vernal equinox, that net radiation flips and becomes positive there.

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4 hours ago, OsZ said:

Hey there,

 

I've been trying to find a way to monitor solar radiation, daily or even hourly, for a project I'm working at the moment.

I did found something like https://en.tutiempo.net/solar-radiation/

But the results... give me some doubts, to say the least.

 

For instance, today:

London: 397 wh/m2

Lisbon: 3075 wh/m2

New York: 854 wh/m2

 

Isn't the wattage to high?

The other references (ex: CERES) I found, graphics, mostly, go from -280w/m2 to 280w/m2 or 0 to 300w/m2 - per month.

Howdy!

In looking at your post, I see a couple of issues with units. 
 

If you are looking at/measuring solar radiation at the surface of the earth, you can look at it "instantaneously" at any given time- and the units will be in W/m^2. This is an "instantaneous" value. It is for a single moment in time.

It like you have a 100W light bulb. It puts out 100 W.

When you look at the total for a day or a month, your units will be W*h/m^2 - watt hours per day (or month) 

Watts is for a moment in time Watt-hours is the accumulation over time.

 I hope that makes sense. 
 

Just outside the atmosphere of earth, solar radiation is about 1.4 kW/m^2.

Of that, only about 56% can make it through the atmosphere. This is further reduced by atmospheric conditions which can block from 10-100% (thick clouds and fog). You should also consider the angle of the suns rays. The 1.4 kW/m^2 is the max, with the suns rays perpendicular to the trajectory of the suns rays

 

 

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Thanks a lot, everyone.

My intention is to apply those values to the Penman-Monteith equation; so the data needed is precisely the solar radiation that reaches the surface of the Earth.

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2 hours ago, OsZ said:

Thanks a lot, everyone.

My intention is to apply those values to the Penman-Monteith equation; so the data needed is precisely the solar radiation that reaches the surface of the Earth.

Interesting. Using the first link you provided, to find the average irradiance for that time period you simply have to divide by 24 hours. For London today it gives 857 Wh/m^2 for the day, so divided by 24 hours that gives ~35.7 W/m^2 on average. Note that Global Solar Atlas seems to give give the average daily values over the course of a year rather than recent values, since it's primarily geared towards estimates for people interested in Solar power (photovoltaics), and thus would not be a good estimate of a recent day in e.g. London. From some quick reading I believe the equation you're attempting to calculate is typically for shorter periods of time, e.g. ten-day periods or months, meaning that you'd need more accurate data for that purpose. I'm not sure you could use the link you provided for that either, since it doesn't seem to have historical data, and it's unclear whether the given values are actually reflected in measurements or whether they're simply what was forecasted.

If you want a decent monthly measurement, I would try this: https://re.jrc.ec.europa.eu/pvg_tools/en/

There you choose the place on the map, select "MONTHLY DATA" in the menu to the right of the map, leave the default dataset, choose e.g. 2010-2020 for a good overview, and select "Global horizontal irradiation", and then "Visualize results"; if successful, you should have something that looks like this (for London in my case, and hovering over January 2020):

Screenshot-39.png

Note that this is not "irradiance" (the power, which is what you're after), but just as in the case of the site you provided it's "irradiation" (similar term, but this means the same as "insolation", i.e. the total amount of energy delivered over a given time period, that period in this case being over the course of a month). You'd then take this value for the irradiation that month, 24.64 kWh/m^2, and divide it by the number of hours in that month; for January, that would be 31 * 24 hours, i.e. 744 hours. (24.64 kWh/m^2) / (744 h) = ~33.1 W/m^2. As you can see, that's actually rather close to the estimate based on the daily forecast from your site, but that might be coincidental, because using the forecast value for tomorrow you'd get almost double that. This estimate will be more accurate for January in general. To get an even better estimate for January, average the values from January each year. Note that you're still not getting the actual recent values, which could differ from these averages (the irradiation for January varies between ~23-30 kWh/m^2 in these years), so if that is critical you're going to have to find a proper live/real-time dataset. From searching a bit for that it seems like people providing such datasets are not doing it for free, and I couldn't find a single free real-time dataset. If anyone know where to find something like this feel free to share.

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