Sunlight

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Understanding sunlight is very important to harvest the energy from it. There are many materials that simply do not absorb it. Some materials absorb small portions of it, reflect most of it and over heat. That may seem counter intuitive. Other materials may absorb most of it and never get hot, which is also counter-intuitive.

Sunlight is electromagnetic radiation just like microwaves, radio waves, x rays, uv light and radiant heat from a radiator. The radiation that we recognize as radiant heat from a radiator has very long wavelengths. Sunlight has much shorter wavelengths. Sunlight energy is typically transformed from a short wave energy to a long wave energy by Earth objects and materials. This transformation takes place when an object absorbs the sunlight, heats up and then tries to throw off that heat. Since it doesn't get as hot as the sun, the wavelengths of energy that is throwing off are lower energy or longer wavelength. The heat any object throws off is related to the temperature of it. If its red hot then the wavelengths of energy it is throwing off are in the visible portion of the spectrum. If it' not that hot it just means the energy it is throwing off is not high enough frequency to be in that range.

Approximately 49% of the incoming sunlight is in the infrared radiation (IR) portion of the spectrum. This is not visible to our eyes. You need a special camera to be able to see its effects. This energy is short wavelength and therefore very high energy. Approximately 43% of sunlight is visible light. Visible light is also very high energy. We can see the effects of it as some of the sunlight is absorbed by all objects and some is reflected. You cannot see the absorbed part but you can see the reflected part. Many plants absorb the red part of the spectrum and reflect back the green so they appear green. About 7% of the incoming sunlight is in the UV range. What makes it down to Earth typically doesn't pass through glass. The remaining 1% is x-rays, gamma rays and radio waves. See the graph below.

As the solar radiation moves towards the Earth's surface, most of the UV is absorbed by the ozone layer and some of the IR is absorbed by certain atmospheric gases and clouds. The remaining radiation is mostly visible, which represents most of the energy striking the Earth's surface. You can see the effects of the visible portion and feel the effects of the infrared portion. Both are being absorbed by objects, transformed from high to low energy and then emitted as heat. There is no obvious way to tell the difference of the source of the heat in direct sunlight without special tools and cameras.

Electromagnetic radiation can travel through a vacuum. It doesn't need air or material to transfer it from one place to another. Radiation coming off a wood stove or radiator is the same. You don't have to be in contact with it to feel the heat. If you got rid of the air, it wouldn't change anything. If that's the case, then it is the case for all objects giving off heat no matter what temperature they are. Solar radiation travels through space before striking the Earth. The short IR and visible light portion of the spectrum travel through glass almost as if it isn't even there. To determine how much passes through glass, refer to the solar heat gain coefficient (SHGC) and the Visible Transmission (VT) coefficients of the glass you are considering from the manufacturers specifications. This information will be with the performance data for a given window. The SHGC tells you how much of the Infrared (IR) portion of the incoming sunlight is passing through the glass and the VT tells you how much of the visible portion of the spectrum is passing through.

All radiation energy is either absorbed, reflected or transmitted through the object that it strikes. How much of it is allocated to each of those possibilities depends on the object. If its reflected, all radiation travels in straight lines. It will be reflected off at the same angle that it strikes. Energy is emitted in the form of radiation in all directions from an object. It is not like a laser.

If the energy is absorbed, the material absorbing the energy will heat up and its temperature will increase. Most objects try to maintain some sort of thermal equilibrium with their surroundings. If they are hotter than their surroundings they will try to give up heat and cool off. It they are colder, like an ice cube on a summer day, they will absorb heat. Most objects give off their heat energy in the low energy long wavelength portion of the electro magnetic spectrum. This is also true for wood stoves and radiators unless they are glowing in the visible portion of the electromagnetic spectrum. Energy in this longwave portion of the spectrum does not pass through glass. Glass is the same exact material that lets short wave radiation pass right through it but blocks long wave radiation. They are both radiation, just different wavelengths. Glass is transparent to short wave radiation in certain parts of the spectrum and opaque to long wave. As a material it has high transmission to short wave and high reflectanceto long wave radiation. In order for electromagnetic radiation in the long wave portion of the spectrum to get to the other side of the glass it has to heat the glass up and then let the glass try to reach some equilibrium with its surroundings . The wavelength of energy it absorbed is going to be different than the wavelength of energy it emits, otherwise the energy will just pass right through it. It has to absorb energy in certain wavelengths and heat up. When it achieves a certain temperature then it can start giving off heat energy which will then correspond to the temperature it is at. Since the temperature is different between the incoming and the outgoing so is the wavelength. Every material absorbs energy at specific wavelengths and gives off energy at specific wavelengths. They may or may not be the same wavelength. The wavelengths, either absorbed or emitted are directly related to the temperature that corresponds with that wavelength. Every material has a set of rules for each and every wavelength. A particular wavelength is either absorbed, transmitted or reflected.

Visible light which corresponds to very high temperatures is absorbed by Earth objects, converted into lower temperature heat energy and then re-radiated at the lower temperature. It gets transformed from a high energy wave to a lower energy wave. Its the same amount of energy. Energy is neither lost or gained. A short term exposure to high impact sunlight can mean long term trying to get rid of the energy that was absorbed. The wavelengths coming from the sun are a reflection of the temperature of the sun. The wavelengths coming off of heated materials as heat are a reflection of the temperature of that material.

Consider an electric stove top burner. It can be quite hot and still black. If you put your hand near it, you can feel the radiant energy coming off of it. As you add energy in the form of resistance using electricity, the element gets hotter. Eventually it will get so hot that it will glow red. This is a reflection of the fact that it has achieved a certain temperature. Its temperature is the same as the red portion of the visible spectrum. That is in the visible range so its very high energy and short wavelength. At that temperature it gives off heat relative to that temperature, in the red portion of the visible spectrum. If it continues to absorb energy and get hotter, it may glow orange, then yellow and so forth. Gas stoves are even hotter than electric burners. The flames are usually blue. When you check these 2 colors on a spectrum scale you will see the blue color is short wavelength and much higher energy.

Some materials can absorb a tremendous amount of heat, heat up and not give off any heat until they have achieved a certain temperature. At that temperature, they suddenly start radiating all the heat energy contained within them. There are other materials that can absorb a tremendous amount of heat energy and not change their temperature at all. If you put enough heat energy into water such that you raise the temperature of the water to boiling temperature, that doesn't mean the water is going to boil. Once you achieve that temperature the water can absorb a lot more heat energy without changing its temperature at all, then eventually it will boil. Once it starts boiling, as long as you keep adding the right amount of energy, it will keep boiling. At that point it will also be at equilibrium. No matter how much heat you add within reason, it will keep boiling but its temperature will not change one degree. In the case of water, when you achieve boiling temperature, the water can lose the energy it gains by evaporating off water molecules in the form of water vapor or a gas. By analogy, all materials have a specific behavior pattern with respect to heat energy. Some materials can absorb energy without changing their temperature at all until they reach a certain point. Just as water boils exactly at 212 Degrees F, each material absorbing energy will heat up to a certain point and at that point have some strategy to maintain an equilibrium temperature. Usually that means throwing off the heat in the form of long wave radiation instead of evaporating itself into the air.

The visible portion of the electromagnetic spectrum is between 400 nanometers (nm) and 700 nm. The infrared portion of the spectrum ranges from near IR or short wave IR at 1000 nm which is 1 micrometer to the Far infrared or long wave infrared at 1000 micrometers which is 1 mm (millimeter). Within that range, Near IR (infrared) is about 1000 nm, Thermal IR is about 100 micrometers and Far IR is about 1000 micrometers.

The infrared portion of the spectrum is divided into 3 parts. Infrared A is radiation betwen 700 nm and 1,400 nm. Infrared B is between 1,400 nm and 3,000 nm and Infrared C is between 3,000 nm and 1 mm.

As objects on the Earth are heated by the energy from sunlight, they emit radiation in the long wave portion of the IR spectrum. These temperatures are more on the order of room temperature.

From http://faculty.icc.edu/easc111lab/labs/labi/prelab_i.html

The two primary gases that absorb long-wave radiation in the lower atmosphere are water vapor and carbon dioxide. Methane, ozone, and chlorofluorocarbons can also absorb some of the long-wave radiation.

When you consider harvesting and storing sunlight energy you must pay careful attention to your materials. Each material has physical characteristics that define its behavior in the presence of sunlight. The sunlight can be absorbed, reflected or transmitted but even that is wavelength specific. It may absorb certain wavelengths, let others pass right through it and reflect the rest off. With respect to storing energy, each material has certain phyisical characteristics that will dictate how well it stores energy, if at all. Each material has a defined set of physical characteristics that will dictate how much it will heat up and how and when it throws that energy off. This is also wavelength specific.

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