# Heating capacity of sunlight

1. Feb 10, 2018

### physea

hello!
I find alot 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!

2. Feb 10, 2018

### 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?

3. Feb 10, 2018

### physea

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?

4. Feb 10, 2018

### davenn

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/m2

Dave

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

Last edited: Feb 10, 2018
5. Feb 10, 2018

### 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!

6. Feb 10, 2018

### phinds

7. Feb 10, 2018

### jim hardy

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

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.

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

8. Feb 10, 2018

### 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.

9. Feb 10, 2018

### jim hardy

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

10. Feb 10, 2018

### Staff: Mentor

Yes, 100% of sunlight eventually becomes thermal energy.

11. Feb 10, 2018

### davenn

what did I say in my post ?

12. Feb 11, 2018

### physea

13. Feb 13, 2018

### Tunalover

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.

14. Feb 13, 2018

### Tom Kunich

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.

15. Feb 13, 2018

### Staff: Mentor

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%.

16. Feb 13, 2018

### Tunalover

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.

17. Feb 13, 2018

### Staff: Mentor

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.

18. Feb 13, 2018

### Staff: Mentor

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

19. Feb 13, 2018

### haruspex

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.

20. Feb 13, 2018

### jim hardy

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

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...

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.

21. Feb 13, 2018

### Staff: Mentor

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

22. Feb 13, 2018

### 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

23. Feb 13, 2018

### Stavros Kiri

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.]

Last edited: Feb 13, 2018
24. Feb 17, 2018

### 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

25. Feb 17, 2018

### Stavros Kiri

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).