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Solar Heat
SOLAR CONSTANT
The energy density of
solar radiation at Earth's distance from the Sun averages 1353
joules per second per square metre. Some of this energy is deflected
and absorbed in Earths' atmosphere. On a sunny day at ground level we receive
about 1000 joules of energy per second (1000 watts of power)
per square metre perpendicular to the Sun. [This is equal to 833 watts per
square yard. For interest, 746 watts = 1 horsepower.]
Principles
of Solar Cooker Design
[This is the best I've found for explaining solar energy and heat principles
simply and clearly. There is nothing like trying to build a solar cooker
to get hands-on experience with solar energy, building techniques, and cooking.]
If you're a fan of Thermodynamics
try here.
[Also from Solar Cookers International, but at an advanced level.]
HEAT ARRIVES
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 there. 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.
HEAT CONDUCTION
Although the colour and reflectivity
of a material help us to determine the wavelengths and amount of solar energy
an object will absorb (see Solar Optics),
how well a material conducts heat determines whether the object will
get hot all the way through (or how hot something behind it will get). Conduction
is the way heat travels through an object.
When an object is heated, it will share its heat
energy with everything around it until everything is at the same temperature.
This is entropy - a gradual dispersement of energy to equality everywhere.
There are classically three ways that heat energy can be shared:
conduction, convection, and radiation. If something hot
is touching something less hot, heat energy will be transferred by conduction.
Different materials conduct heat at different
rates. Metals conduct heat well. Materials that slow down heat flow are called
insulators.
CONVECTION
Different-temperatured masses
of air or water can set up convection currents. The warmed gas or liquid is
moved away from the Earth's surface. The force of gravity on cooler (and
therfore more dense) gases or liquids displaces the warmed, less dense gases.
We say "heat rises", and talk about "thermosiphoning". Actually, 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 usually a fan or pump
is employed for this purpose.
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're 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 absorbtion 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!
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