 Solar Constant
The solar constant is the average rate of solar energy arriving
at the outer edge of the earth's atmosphere, before any losses.
The generally-accepted value is 429.2 BTU per hour per square
foot.
The actual rate of radiation varies about 3 percent either
way from the average. If all of this energy could be collected
and used, it would take about three hours to collect all the
energy used on earth for a full year!
Insolation
Insolation is the rate of solar energy arriving on a specific
flat surface perpendicular to the line of the sun. At sea level,
the least possible loss is 29 to 30 percent. The maximum possible
insolation is therefore about 70 to 71 percent of the solar constant.,
or about 320 BTU per hour per square foot.
No solar collector, regardless of shape or design can deliver
more than this maximum possible value, without energy input from
some other source.
Efficiency
Efficiency of any energy-consuming device or system is the
ratio of output divided by input, and can never be more than
1.0 or 100 percent.
Solar Fraction
The solar fraction is the ratio of solar energy used divided
by total energy used in the same application. It cannot possibly
be more than 1.0 (or 100%). Note that solar fraction is distinctly
different from efficiency.
Wavelength
Conversion
Solar radiation, mostly in the form of visible light, is contained
in a "channel" of short wavelengths. When it is absorbed,
it becomes thermal energy and has been converted to wavelengths
about ten times longer, in a new "channel".
Greenhouse
Effect
Many transparent materials will pass light freely, but will
not freely pass the longer wavelength "channel" of
thermal (heat) energy. Greenhouses and many solar collectors
use this effect by applying glass or plastic covers to prevent
re-radiation of the thermal energy.
Black Body
A "black body" is any material capable of absorbing
radiant energy, and therefore also is capable of re-radiating
the energy. A "perfect" black body absorbs and re-radiates
100% of the radiant energy striking it. "good" black
bodies are used in solar collectors and they absorb and re-radiate
(if not cooled) 90 to 96 percent of radiant energy arriving
Selective
Surface
Certain special coatings can be used in solar collectors to
reduce the re-radiation ability without appreciably reducing
energy-absorption ability. The only such "selective"
surface now well-proven and in common use is a special black
chrome electroplate.
Absorber
In a solar heating collector, the absorber is that portion
of the collector which receives the radiant energy from the sun
and converts it to heat at longer wavelengths. It is usually
a flat black surface with high absorbtance, i.e. a black body.
Collector
A solar collector is the entire assembly, including at least
the absorber and heat exchanger, and any insulation, glazing,
plumbing and enclosure.
Flat Plate
Collector
The flat-plate solar collector is one of many possible types
of solar collectors. It is the most efficient type of collector
for use with temperatures between the freezing and boiling points
of water and up to about 350 degrees F. when used with air as
the working medium. Flat plate collectors are normally used with
the flat surface facing south and tilted to an angle appropriate
to the intended use.
Tracking Collector
A tracking collector is any type of collector installed to
move and follow the sun, and may include flat plate collectors.
Concentrating
Collector
Concentrating Collectors use a specially-shaped reflecting
surface to concentrate radiation in an area smaller than the
reflector, thus producing a higher temperature. Concentrating
collectors must track the sun for full effectiveness, and can
not collect more solar energy than the same area flat
plate collector.
Diffuse Radiation
Diffuse radiation is light energy arriving by reflection or
scattering from some direction other than directly from the sun.
Diffuse radiation is accepted by flat plate collectors but not
by concentrating collectors. Therefore flat plate collectors
will produce on cloudy days while concentrating collectors will
not.
Conduction
One of the three ways in which heat is transferred or lost.
Conduction transfer or loss occurs due to the temperature difference
between two surfaces of the same material, and the heat transfer
is directly through the material.
Convection
The second of three forms of heat transfer. In this form of
transfer, liquid or gas, such as air is heated, then moves away
from the source of heat, being replaced by cooler material which
repeats the process of carrying away heat.
In natural convection, the heated fluid becomes lighter due
to expansion and rises away from the source, being replaced by
cooler, heavier fluid.
In forced convection, the fluid is driven by some outside
force, such as a fan, pump, or wind. It is heated by contact
with the source of heat and carries the heat away as it is moved
by the outside force.
Radiation
The third method of heat transfer or loss. Radiation occurs
by transfer of energy through empty space. The amount of heat
transferred by radiation is proportional to the difference between
the fourth powers of the absolute temperatures of the
radiating surface and the radiation receiver surface. When solar
collectors are operated at higher temperatures, the radiation
losses increase very rapidly with temperature and are the largest
losses responsible for loss of efficiency. At lower temperatures,
radiation losses are small and conduction and convection losses
are the main sources of loss. Both convection and conduction
losses are proportional to temperature differences.
Therm
A therm (or thermal unit) is a term commonly used to express
the heat equivalent of a quantity of natural gas for billing
purposes. A therm is equal to 100,000 BTU, or 29.3 KWH
Temperature
vs. Energy
Knowledge of temperature is necessary to knowledge of thermal
energy, but is not enough. Both the temperature and quantity
of material containing the energy must be known. We could not
reasonably expect a match flame at 2,000 degrees to be able to
heat a swimming pool, but solar collectors at 90 degrees will
do it readily if we use enough of them.
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