Solar Heat
SOLAR CONSTANT The energy
density of solar radiation at Earth's distance
from the Sun averages 1361 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.
From our level of reality, there are classically three ways in which we consider heat
energy to be shared: by conduction, convection,
and/or 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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