Solar Heat

Solar Power—Radiation from the sun received on Earth

Here is some basic information for those interested in converting solar radiation into power. At the average earth-sun distance of 92.9 million miles the solar radiation intensity is:

435 BTU/(ft2-hr) or 1370 watts/meter2 

After going through the atmosphere and reaching the ground the values drop to:

340 BTU/(ft2-hr) or 1070 W/m2. 

The above value is what you have to work with at noon on a clear day at the equator. As you depart from those conditions the values decrease. A good ballpark value to use for 40 degrees North latitude appears to be around:

300 BTU/(ft2-hr) or 944 W/m2. 

Just so you know there is more to learn about solar radiation, there is the total radiation which is the sum of direct normal radiation (sunlight), diffuse or sky radiation (when you stand in the shade it isn’t dark and your solar calculator still works), and reflected solar radiation (you probably noticed it’s warmer in the sun by the south side of a building).

The above numbers will get you started if you know what to do with them. I’ll talk about heating things up with solar power on a future post so you can figure how much heat you need to change the temperature of things like air, water, or a piece of metal.  

By the way, typical solar cells have about 10% conversion efficiency so you get roughly 94 W/m2 out of them. State-of-the-art solar cells are approaching 15-20% efficiency.