The energy density of solar radiation at Earth’s distance from the Sun (150,000,000 kilometers) averages 1361 joules per second per square meter. This is the value of what we call the “solar constant”, although though there is some variation due to Earth’s elliptical orbit.
Consider that the Sun is radiating out energy in every direction, and that we are so far away from it. And yet, even at this distance we exist in 1.36 kilowatts of power per square meter.
Some of this energy is deflected and absorbed in Earths’ atmosphere, but on a clear-sky day (what we call a “sunny” day) we are receiving significant energy from the moving star we are following in a spiral path, our Sun.
One of the reasons we don’t understand HOW much energy there is is because we see with it, and consider it insubstantial. About 42% of solar energy is visible light. Light IS energy.
On a sunny day around sea level we receive around 1000 joules of energy per second (1000 watts of power, 1 kW) per square meter perpendicular to the Sun. [This is equal to 833 watts per square yard. For interest, 746 watts = 1 horsepower…so, a little over one horsepower per square yard. A horse’s energy! Think of the thundering horses of the Sun!
SAFETY NOTE: Because this IS a significant amount of energy, it really isn’t an ideal subject for young children. Concentrating or reflecting sunlight can cause burns or eye damage. Get into fresnel lenses, and you can be up at 850 degrees C, enough to melt aluminum. Reflect sunlight into eyes, and, just as would looking into the Sun directly, eyes can be damaged. Its problematic even with magnifying glasses and reflective solar cookers – sunlight is energy, and requires careful handling. I usually handed out u.v. protective sunglasses to kids at fairs; but safety must be addressed before heading off to concentrate solar energy.
A hands-on elementary school exercise that safely explores the concept of solar heating is the lesson Smooth Black Stone at the Lessons section of this site.
Principles of Solar Cooker Design: This is the best I’ve found for explaining solar energy and heat principles simply and clearly. There is really nothing like building a solar cooker or two to get hands-on experience with solar energy use, building techniques, and cooking…all rather useful life skills!
If you’re a fan of Thermodynamics try here. [Also from Solar Cookers International, but at an advanced level.]
ENERGY ARRIVES AND DEPARTS
Our planet is in a peculiar situation regarding heat. The interior of our planet is molten rock, kept heated by the slow energy-releasing break-down of large radioactive elements. This heat slowly escapes through the Earth’s crust. However, this by itself would not be enough to keep the surface of our planet warm. For warmth and light, and the active formation of life by plants, we require the energy received from the Sun.
Even so, without heat-trapping gases in our atmosphere, the Earth’s overall temperature would be about minus 18 degrees Celcius, rather than the 14 degrees Celcius which is the Earth’s average temperature today.
The reason for this heat-trapping is that the absorption spectra of molecules is quite specific, and they are only able to accept energy at certain absorption bands, wavelengths on the electro-magnetic spectrum. Absorption of energy can only take place when the energy of the radiation exactly matches the difference between electron energy levels. Fortunately for us, carbon dioxide is such a gas, allowing light energy to pass through, but absorbing some of the lower frequency infra-red beamed back into Space at night from our warmed surface.
Earth’s surface emits heat radiation at a lower temperature than the Sun, so the wavelengths are lower. Turns out that at around the 1500 nanometer band, carbon dioxide can absorb energy. We emit more energy at that wavelength (because we are cooler) than the Sun’s incoming radiation has in it – so the heat-trapping occurs on the way out of our system at night…simply, we radiate into that zone of the electromagnetic spectrum CO2 finds useful – it can accept energy of that wavelength, of that wave-packet size.
HEAT ABSORPTION AND TRANSFER
From our level of reality, there are classically three ways in which we consider heat energy to be spread: by radiation, by conduction, and by convection.
The color and reflectivity of a material help us to determine the wavelengths and amount of solar energy radiation an object will absorb (see also Solar Optics).
Darker objects absorb more energy than light objects, and reflective objects deflect energy.
Generally one speaks about the absorption or emissivity of an object. If that object is our planet, the word “albedo” is used. High albedo, high reflection; low albedo, absorption.
CONDUCTION is the way heat travels through a solid, by the vibration of molecules exciting other molecules around them.
If something hot is touching something less hot, heat energy will be transferred to the cooler item by conduction. Different materials conduct heat at different rates. Metals conduct heat well. Wood, not so much. So we differentiate between conductors and insulators.
When we speak of radiated heat (as from a fire, or radiant heater), we are talking about direct electro-magnetic radiation, just as sunlight is. The packets of wave energy are radiated through air across the room to be absorbed by material – for example, one’s shirt is warmed, one feels warmer, when standing near a fire. The shirt and our bodies are absorbing electromagnetic radiation, and the energy feels “warm” to us.
To experience solar radiation, one need do no more than hold their palms towards the Sun.
When a mass is hot, it radiates its energy to everything around it until everything is at the same temperature. This is entropy – a gradual dispersement of energy to equality everywhere. It’s actually not too surprising, because at it’s edges, all objects radiate energy (according to their temperature) away in every direction.
Buckminster Fuller had it that the Universe exhibited tensegrity, with tension and compression members balanced. In the larger scheme of things, electro-magnetic radiation is radiating away, gravity is pulling in.
Different-temperatured masses of air or water can set up convection currents. The warmed gas or liquid is pushed away from the Earth’s surface as the force of gravity on cooler (and therfore denser) gases or liquids pulls them closer than the less dense warmed gases or liquids. (Water is the great exception here – it is densest at 4 degrees C, and then expands).
We say “heat rises”, and talk about “thermo-siphoning”. Again, the less-dense hotter fluids and/or gases are displaced away from the Earth as cooler, denser liquids and gases are attracted more forcefully by gravity to the Earth.
Convection works well for our planet, with water and wind currents spreading heat from the equatorial regions into the temperate and polar zones. On a smaller scale this principle can be used for the distribution of solar-heated water and air into buildings…though often a fan or pump is employed for this purpose.
TYING IT ALL TOGETHER WITH SOLAR COOKING
Solar radiational energy, after being absorbed by a dark pot in a solar oven, is transmitted by conduction and convection to cook food.
INSULATION, and REFLECTIVE RADIATION BARRIERS
Some materials do not conduct heat well, and are known as thermal insulators. For example, because of the trapped dead air spaces in it, wool does not conduct heat energy nearly as well as a solid metal. So heat energy can be trapped behind wool, which is what is happening when we wear wool clothing. Eventually heat energy will pass through wool, but much more slowly than if the wool insulation wasn’t in place. Insulation doesn’t allow heat energy to travel through it easily; it slows heat transfer down.
What prevents easy heat conduction in wool is all the tiny air spaces. Air doesn’t conduct heat well, and the smallness of the air spaces prevent convection currents from being set up. This same idea is used in fibreglass insulation, blown cellulose, and styrafoam.
Want to keep heat in for a longer while? Use lots of insulation.
It’s not that the insulation is inherently warm, and shares its warmth with the interior object – the coat doesn’t make you hot. It’s that the insulation keeps the heat trapped longer – your own body heat can’t escape quickly. Eventually, if no more energy is added (as heat energy is to your coat by your body metabolizing food), no matter how well one insulates, an object hotter than its surroundings will share its heat with its surroundings, and will eventually wind up at the same temperature. It’s just that insulation slows that dispersion down…a lot.
The same is true for keeping heat out…which is why containers for keeping food cool are insulated. One wonders why they aren’t called out-sulated (but perhaps that’s reserved for the reflective outer coating on the better ones, which bounces heat radiation away). Warmer surroundings attempt to share their heat with the colder food. Got to keep that heat energy out if you want the food to stay cool. Insulation works as well here as it does at keeping heat in…it slows down the transfer of heat energy.
Food coolers and thermoses will keep even more heat energy out (or in) if they are shiny outside. Why?
If you are out camping, try wrapping the food cooler in a wool blanket, and see if the ice lasts longer. Then wrap the whole combination of food cooler and wool blanket in one of those shiny emergency “space blankets” ($2 at camping supply stores). See how much longer again the ice lasts.
While using solar energy to create electricity is about 12 -15 % efficient, direct absorption of solar energy to heat water or air can be up to 75% efficient. Onward to Solar Energy Uses to see solar heat in action!