Download Solar Energy

By Ron Whitelock
Green Energy – Part II:
Solar Energy
In this article, we will take a look
at Solar Energy – what is it; and
how can we use it in our traffic and
transportation projects.
Our Star
As noted in the Part I article on
Green Energy, - it’s hip, it’s cool,
it’s trendy, and it’s green. Solar and
wind power are increasingly becoming topics of conversation as the
world shifts from dirty fuels such as
coal, oil and other fossil fuels, to the
clean and renewable energy promised by the wind and the Sun. The
question is can we take advantage
of this shift and use these resources
to reliably power our projects over
the long run.
The Sun is really, really big! It is the largest object in the solar system, and
it accounts for more than 99% of the total mass in the solar system. It is
difficult to get a true feel for how big it is because nothing in our daily life
comes even close. Imagine, if the Sun were a hallow ball, it would take
about one million Earths to fill it. Now that is big!
Solar Facts
The Sun continuously emits an enormous amount of energy that is dispersed into outer space in all directions. Only a small fraction of this energy
is intercepted by the earth.
The Sun is actually a star, and its mass is made up of 70% hydrogen, 28%
helium, and 2% metals. Its diameter is 1,390,000 kilometers (863,706 miles),
and it is larger than most of the stars in the galaxy.
Our planet Earth is located about 150,000,000 kilometers (93,205,679 miles)
away from the Sun, but the distance is not constant. Since the Earth has an
elliptical orbit around the Sun, it comes close to a distance of 147,000,000
kilometers (91,341,565 miles) from the Sun once a year on the 3rd of January, and it reaches a distance of 152,000,000 kilometers (94,448,421 miles)
on the 4th of July six months later.
Solar Energy
The solar energy reaching the boundary of the earth’s atmosphere is considered to be constant for all practical purposes. Because of the difficulty
in achieving accurate measurements, the exact value is not known with
certainty but is believed to be between 1,353 and 1,395 W/m2 squared
(4439 and 4577 f+2) (approximately 1.4 kW/m2 squared).
Scientists tell us …
• The Earth receives more energy
from the Sun in an hour than is
used in the entire world in one
year
• The area of roof space available
in Australia is enough to provide all of the nation’s electricity, using solar panels
• Weights for weight, advanced
silicon based solar cells generate
the same amount of electricity
over their lifetime as nuclear
fuel rods
• All the components in a solar
panel can be recycled
Page 26
In passing through outer space, which is a vacuum, the different types of
solar energy remain intact and are not modified until the radiation reaches
the top of the earth’s atmosphere. At this point, things start to change.
Not all of the solar radiation received at the periphery of the atmosphere
reaches the surfaces of the earth. This is because the earth’s atmosphere
plays an important role in selectively controlling the passage towards the
earth’s surface of the various components of solar radiation. Near midday on a clear day, about 25 percent of the solar radiation is scattered and
absorbed as it passes through the atmosphere. As a result only about 1.0
kW/m2 of the incident solar radiation reaches the earth’s surface without
being significantly scattered. In late morning and late afternoon, the scattering and absorption is greater.
As the Sun rises, moves across the sky and then sets, it is directly overhead
only for a short period of time. Over the day, the amount of solar energy
reaching the earth varies greatly with the time of day. Scientists provide us
a summary value for this effect – called insolation. The values of solar insoContinued on page 28
IMSA Journal
Green Energy – Part II: Solar Energy . . .
Continued from page 26
lation are commonly expressed in kWh/m2/day. This is the amount
of solar energy that strikes a square meter of the earth’s surface in
a single day. There are several charts available on the internet to
get insolation values for your area for each month of the year. For
example: Kansas City, KS insolation values look like this.
J
F M
A M
J J
A
S O N
D AVE
2.06
2.893.62 4.92 5.58 6.176.21 5.59 4.9 3.49 2.2 1.75 4.11
For design purposes, worst-case is December at 1.75 kWh/m2/day.
Solar Energy Conversion
The first solar cell was constructed by Charles Fritts in the 1880s.
The conversion of solar energy to electricity is called photovoltaic.
The photoelectric is explained in detail in Wikipedia’s” Photoelectric
effect” – for those interested in this level of detail.
This process is also known as active solar vs passive solar which we
get when Sunlight heats the house or the swimming pool. Photovoltaic happens when the Sunlight strikes a photo diode. DC current
is the result. Solar panels typically have an open circuit voltage of
around 20 volts. So if a number of panels are connected in series,
the output voltage can be very dangerous.
The theoretical limit for silicon based solar cells is 29% conversion
efficiency. Currently, solar panels generally available have efficiencies anywhere from 12% to 18%. Use the lower value for design
purposes to allow for panel aging, dirt on the panel etc.
Solar systems are usually designed for the worst case – typically the
month of December – no surprises here!
Let’s test the parameters of running a solar flashing beacon 24/7 in
Kansas City from solar energy.
One Amber Beacon:
Power needed per day:
The insolation value for December:
Solar panel efficiency:
Net power obtained:
Solar panel size :
10w /50% duty cycle = 5w
5w x 24hr = 120 watt-hours/ day
1.75 kWh/m2/day
12%
1750 x .12 = 210 w/ m2/day
120/210 = .57 m2
A 0 .57 square meter panel or next commercially available size up
will power the flashers year round. But what we have not talked
about is the battery and charging system to support the system
during night time or low Sun levels. Battery for these deep cycling
operations is a complete story onto itself.
Sources of Information
There are many ‘solar calculators’ on the internet. One such source
is In My Backyard (IMBY) http://www.nrel.gov/eis/imby/
It is fun to run various scenarios to test your applications for solar
operation.
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I have installed several solar systems in
the 50 – 250 W power range – and they
have performed well in the Toronto area
for several years. So they can work to get
power to areas where utility power is
not practical but a traffic control device
is required.
However, these systems, although simple
in nature, require experience and knowledge to be properly sized for the applications and your area. I strongly suggest that
you consult a reputable supplier to have
your specific needs properly analyzed.
Something to think about!
IMSA Journal