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

[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.

 

[klimatos]

What I would like to know if the heat energy we can harvest from a m2 in the UK or on Earth.

The answer depends on the precise location, the time of day, the time of year, what year and the multitude of factors that affect the angle of incidence of the radiation. It also depends on the available technology, which changes from year to year. Finally, if it's profit you're interested in, it depends on the cost of your technology compared to the cost of an equal amount of energy from other sources.

There are thousands of scientists, economists, and ordinary business people working on these issues as you read this. Do you have an approach that they haven't thought of?

 

[Stavros Kiri]

What I would like to know if the heat energy we can harvest from a m2 in the UK or on Earth.

Your syntax is not correct or obvious here. What exactly are you asking?

 

[bob012345]

Your syntax is not correct or obvious here. What exactly are you asking?

This was a quote of the original question and its answer but the two were not distinguished.

 

[Stavros Kiri]

This was a quote of the original question and its answer but the two were not distinguished.

Ah, ok. Thanks