How Much Heat Energy Can Be Harvested from Sunlight Per Square Meter in the UK?

[physea]

hello!
I find a lot of information about the energy of sunlight per square meter in the UK region. (https://photonenergy.co.uk/new-build-solar-pv/solar-map-of-the-uk)

However, these kWh/m2 must be in the form of light energy or perhaps electricity (after adjusting with an efficiency coefficient).

What I would like to know if the heat energy we can harvest from a m2 in the UK or on Earth. The heating capability of sunlight, I assume is partly due to the infrared part of its radiation as well as ...?? I am not sure.

In other words, does anyone know, how much heat energy I can harvest from sunlight per m2?

thanks!

 

[DaveC426913]

I'm not sure I understand.

About 1000W of sunlight falls on an ideal square metre.
How is that not what you are looking for?

 

[physea]

I'm not sure I understand.

About 1000W of sunlight falls on an ideal square metre.
How is that not what you are looking for?

OK, but what energy is that 1kW? Electromagnetic energy? Electrical energy?
How much of that can be converted to heat energy? 100%?
If I throw that light to a box with water, it will heat it as an electric heater of 1kW?

 

[davenn]

OK, but what energy is that 1kW? Electromagnetic energy?

Electromagnetic which includes everything from radio frequencies up through infrared through visible into ultraviolet. But they are primarily referring to IR-Vis-UV. Extreme UV and higher get blocked by the atmosphere ... fortunately

The figure for solar energy density when the surface is perpendicular to the incoming light is approx. 1350W/m²
This falls off substantially as the angle becomes much less than 90 degrees
The avg energy density for the whole Earth is around 163W/m²Dave

https://en.wikipedia.org/wiki/Sunlight

 

[physea]

I have already read that Wikipedia article, but I would like more precise info.

If we assume 1kW/m2, which of this energy is heat or can be converted efficiently to heat? (apart from using an expensive photovoltaic)

thanks!

 

[phinds]

https://www.eia.gov/energyexplained/index.cfm?page=solar_thermal_power_plants

 

[jim hardy]

Paint a piece of metal flat black and lay it in the sun. It'll quickly get too hot to handle.

If we assume 1kW/m2, which of this energy is heat or can be converted efficiently to heat? (apart from using an expensive photovoltaic)

Like any heat engine that depends on what is temperature difference between heat source(your panel) and heat sink (where you're sending that heat to).

And then there's re-radiation. Solar collectors are limited to about 400F because above that temperature they radiate away( as long wave infrared) just about all the energy they collected from sun's mostly visible spectrum. Blackened Nickel coating is a work-around to that problem.

You might read up on black body radiation and emissivity

Here's a practical application
https://www.motherearthnews.com/renewable-energy/solar-air-heater-zmaz06djzraw

 

[rootone]

Hmm, how about flat black metal plates with a thin layer of water being circulated above them?
Seems to be impossibly simple, someone must have tried that.

 

[jim hardy]

Hmm, how about flat black metal plates with a thin layer of water being circulated above them?
Seems to be impossibly simple, someone must have tried that.

Prior to WW2 most houses in Miami had solar water heaters.
They were a flat copper sheet with serpentine copper pipes soldered on. The panel was mounted in a box , usually Redwood or Cypress, and set on the roof facing south. A tank was mounted nearby and water circulated by gravity, google "thermosiphon" .
Simple enough that a home handyman can build one and they work great in that climate - latitude25 N. .

Not different in principle from what you proposed.

In the 1970's U of Florida did a study on solar water heating technology and concluded that for residential application the pre-WW2 low tech approach is more cost effective than hightech evacuated or focusing collector systems.

On a larger scale,
here's a hybrid power plant. Solar collectors heat feedwater going to the boiler of an otherwise ordinary power plant .

closer look at a collector

Heat transfer fluid is heated in the tube at the focus of the mirror and pumped over to the power plant. There it preheats feedwater for the boiler. That's energy they didn't have to buy from the gas or coal company.
And, the plant can still make power at night and on cloudy days.

>>>>>>>>>>>>> Boring Anecdote Alert, # ∞-1 <<<<<<<<<<<<<<<
You've got to be careful around those things.
The guys once had a mirror pointed down to do some work on it. When the sun came up it set fire to their truck - not thinking ahead they'd parked it in front of the mirror.

old jim

 

[russ_watters]

How much of that can be converted to heat energy? 100%?
If I throw that light to a box with water, it will heat it as an electric heater of 1kW?

Yes, 100% of sunlight eventually becomes thermal energy.

 

[davenn]

If we assume 1kW/m2, which of this energy is heat

what did I say in my post ?

Electromagnetic which includes everything from radio frequencies up through infrared through visible into ultraviolet.

it is ALL electromagnetic radiation

 

[physea]

https://www.eia.gov/energyexplained/index.cfm?page=solar_thermal_power_plants

Damn that was what I had in mind.

 

[Tunalover]

hello!
I find a lot of information about the energy of sunlight per square meter in the UK region. (https://photonenergy.co.uk/new-build-solar-pv/solar-map-of-the-uk)

However, these kWh/m2 must be in the form of light energy or perhaps electricity (after adjusting with an efficiency coefficient).

What I would like to know if the heat energy we can harvest from a m2 in the UK or on Earth. The heating capability of sunlight, I assume is partly due to the infrared part of its radiation as well as ...?? I am not sure.

In other words, does anyone know, how much heat energy I can harvest from sunlight per m2?

thanks!

The greatest solar flux I've found is from the US Department of Defense MIL-STD-810G Method 505.3 for the Hot-Dry environment. This provides a diurnal cycle (solar irradiation and ambient air temp on the same graph) with a total solar irradiation that peaks at 1120W/m². The components of this total solar irradiance are the UVA, UVB, IR, and visible bands. So, in theory, in that Hot-Dry environment the most you can ever harvest is 1120W/m². You can download MIL-STD-810G with Notice 1 incorporated at http://quicksearch.dla.mil/qsSearch.aspx. The file is big so it may take a while to download.

 

[Tom Kunich]

Yes, 100% of sunlight eventually becomes thermal energy.

Not really. In terms of the Stefan-Boltzmann Law the Earth is a greybody hence reflects a great deal of the energy delivered. So only about 70% of the solar radiation is converted to heat.

 

[russ_watters]

Not really. In terms of the Stefan-Boltzmann Law the Earth is a greybody hence reflects a great deal of the energy delivered. So only about 70% of the solar radiation is converted to heat.

Good point; it is worth separating the general answer for Earth from the answer useful to the OP. The general (average) answer for Earth is 70%, but for individual applications depends on what it hits. The OP gets to pick, so the answer for the OP is essentially up to 100%.

 

[Tunalover]

Good point; it is worth separating the general answer for Earth from the answer useful to the OP. The general (average) answer for Earth is 70%, but for individual applications depends on what it hits. The OP gets to pick, so the answer for the OP is essentially up to 100%.

Man made surfaces only absorb part of the incident irradiance. Some is reflected. How much all depends on the solar absorptance of the particular surface.

 

[russ_watters]

A solar cell is not a heat engine.

True, but the OP and @jim hardy were referring to heat collectors. Though I doubt thermodynamic considerations get in the way much of the book value until the working fluid gets really hot. It's an interesting and complex problem though.

 

[russ_watters]

Man made surfaces only absorb part of the incident irradiance. Some is reflected. How much all depends on the solar absorptance of the particular surface.

Agreed. So if the OP picks asphalt, for example, the answer is about 96%.

 

[haruspex]

Agreed. So if the OP picks asphalt, for example, the answer is about 96%.

Yes, except that as well as albedo there is the issue of reradiation. The hot asphalt will radiate well as a black body.
The ideal would be a filter permitting the bulk of the sun's power that has made it through the atmosphere, covering an absorber which reradiates at a lower frequency, blocked by the filter. In short, a perfect greenhouse.

 

[jim hardy]

A solar cell is not a heat engine. It converts mostly the center section of the visible wavelengths to energy. It will convert approximately 20-26% or so of the energy falling upon

Yes,, as Russ observed i was thinking about my favorite, old fashioned flat plate residential water heater.

The mechanism by which you get the cooled water back into the high pressure system isn't simple and I'd like to see a report of how much of the original energy is lost in this cycle alone.

There's a detailed report by Bechtel with analysis of that FPL hybrid plant. Will see if i can find it again. The fluid in the collectors is not water but some exotic heat transfer fluid that makes not much pressure at 400F. It is circulated over to the steam plant where it heats feedwater via a heat exchanger...

The tubes that absorb energy would have to be probably black anodized aluminum tubing

I don't know what they are in that plant but can probably find out. A good friend used to work there and i'll inquire.. I do know that Blackened Nickel is a desirable coating. It absorbs visible like a black body but to IR it has very low emissivity , so counteracts the re-radiation effect.

 

[russ_watters]

Several off-topic posts deleted. Please keep the thread focused on the OP's question. Thanks.

 

[jim hardy]

Spectrally selective coatings help solar collectors operate at higher temperature.

Plenty of papers out there..

https://www.sciencedirect.com/science/article/pii/S0040609001010513

The coatings have high spectral selectivity, with solar absorption ranging from 0.88 to 0.94 and thermal emissivity ranging from 0.15 to 0.04, depending on the coating materials and sputtering conditions.

 

[Stavros Kiri]

In other words, does anyone know, how much heat energy I can harvest from sunlight per m2?

What if you use magnifying lenses? (e.g. you can set things on fire, for one thing, etc. ...)
[In other words you can increase the Power [/Energy] Density, so there is no unique answer to your question, etc.]

 

[olivermsun]

magnifying lenses could help, but you need to collect the incoming sunlight from a larger area, so there's no real free lunch here

 

[Stavros Kiri]

magnifying lenses could help, but you need to collect the incoming sunlight from a larger area, so there's no real free lunch here

True for the total energy, but the way I understand it he's more interested in Power[/Energy] Density. In the case of focused sunlight, just like with a laser, that clearly changes and you can e.g. boil water (instead of just warming it up) or potentially you can charge e.g. your cell phone's battery a lot faster etc. , but, yes, it happens in a smaller area, so there's obviously no violation of energy principles here.
But here is the deal: no one prohibits you (except for geometrical constraints only) to put more and more lenses (theoretically to each point ...) and increase that way the harvested energy/power density (while the total energy stays the same, I agree).

 

[olivermsun]

True for the total energy, but the way I understand it he's more interested in Power[/Energy] Density. In the case of focused sunlight, just like with a laser, that clearly changes and you can e.g. boil water (instead of just warming it up) or potentially you can charge e.g. your cell phone's battery a lot faster etc. , but, yes, it happens in a smaller area, so there's obviously no violation of energy principles here.

That's just the thing. You could boil water but only a little water, or you could warm up a bunch more water. What are you going to do with it, run it to a heat pump? If so then how much does a tiny capful of boiling water buy you? Similarly charging your cell phone battery. You have a giant lens and a tiny solar panel or a giant solar panel... is there that much difference in the energy collected?

But here is the deal: no one prohibits you (except for geometrical constraints only) to put more and more lenses (theoretically to each point ...) and increase that way the harvested energy/power density (while the total energy stays the same, I agree).

Lenses cost money and space. At what point do they justify their existence relative to a larger collector?

 

[Stavros Kiri]

Lenses cost money and space. At what point do they justify their existence relative to a larger collector?

Less space ... (and they run free)

 

[olivermsun]

Less space ... (and they run free)

You need a lens in front of a collector. How is that less space? And how quickly does the initial investment justify itself compared to simply having a full-area collector?

 

[Stavros Kiri]

You need a lens in front of a collector. How is that less space? And how quickly does the initial investment justify itself compared to simply having a full-area collector?

Depends on the construction and what you're looking for. Even one small lens can set something on fire or boil little water, while a large collector can't (?). It increases the power/energy density. And you can put many of them.

 

[johnbbahm]

Depends on the construction and what you're looking for. Even one small lens can set something on fire or boil little water, while a large collector can't (?). It increases the power/energy density. And you can put many of them.

The energy density is still the same. If the magnifying lens focuses the 1000 watts per square meter down to .01 square meters,
the energy in the .01 square meter is the same, just spread over a smaller area. (You also loose a little from the lens).

 

[jim hardy]

The energy density is still the same. If the magnifying lens focuses the 1000 watts per square meter down to .01 square meters,
the energy in the .01 square meter is the same, just spread over a smaller area. (You also loose a little from the lens).

And as your target gets hotter it re-radiates away some of that energy it has collected.
Radiation goes up as 4th power of temperature.

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/stefan.html

 

[Stavros Kiri]

The energy density is still the same. If the magnifying lens focuses the 1000 watts per square meter down to .01 square meters,
the energy in the .01 square meter is the same, just spread over a smaller area. (You also loose a little from the lens).

By 'energy/power density' here I meant "energy (or power) per unit area".
Since

the energy in the .01 square meter is the same, just spread over a smaller area

the energy density clearly changes (increases).

 

[Rive]

In other words, does anyone know, how much heat energy I can harvest from sunlight per m2?

There are too many variations in this to give a simple answer. There are plenty of 'case study' available for SDHW systems: it worth a shot to check some and pick one with close parameters to your interest.

Be careful, since the methodology is a bit messy and it does matter if it is about solar efficiency or (real life) energy saving.

Also, you can easily find some online systems by giving a google to 'solar hot water system live data'.

 

[bob012345]

Not really. In terms of the Stefan-Boltzmann Law the Earth is a greybody hence reflects a great deal of the energy delivered. So only about 70% of the solar radiation is converted to heat.

30% of the incident radiation is reflected by the earth. That's true but not related to the question here. Of the sunlight falling on a square meter at the surface, 100% can be absorbed. That's would also be true in space. Then, some may be re-radiated according to the SB law but that's a different phenomenon dependent on the temperature. A black body is not a heat engine with a Carnot efficiency. Energy can be converted to heat at 100% efficiency.

 

[bob012345]

And as your target gets hotter it re-radiates away some of that energy it has collected.
Radiation goes up as 4th power of temperature.

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/stefan.html

That's a true statement but not a fundamental limit to collecting heat. Systems are designed to retain absorbed heat all the time. They are not doomed to radiate it all away when they get hot. The question is one of engineering not fundamental physics. The engineering answer is one can collect a very high percent of sunlight as heat, in the 90% range. It has been as high as 99% of available energy converted and stored as heat in sophisticated solar facilities. Converting that heat energy to electricity is where the losses come.