Green Collar Careers Solar Energy
Solar Energy Dynamics
As our sun shines it lays down an average of 16 watts per square
foot across the planet. That's an average not taking into account the variables
like latitude and time of day. Take a central latitude in the
USA, say Kansas City at 38° and the average solar energy per square
foot from 8:00AM to 4:00PM is about 60 Watts per square foot. If
you were thinking about the thermal energy for heating, a square foot
of sunlight in KC would average about 205 BTU's per hour.
Typically one hour of sunlight equals the entire amount of total
energy (including fossil fuel and nuclear) used by mankind over a full
year (8760 hours).
Calculating and predicting the precise amount of
solar energy for a given location is a bit complex and based on these
six factors and shown in the video above:
- The earth is round so the latitude determines the angle to the
- The earth rotates on an axis that changes through the seasons -
summer solstice, fall equinox, winter solstice, spring equinox.
- Time of day varies the altitude angle to the sun, with peak
energy coming at "high noon".
- Length of day determines how many accumulative hours the sun is providing
- Intensity changes as the earth takes an eccentric path around the sun being closest
in March, and furthest in September.
- Local weather patterns vary; solar system efficiency drops
quickly with cloud cover.
Some of the sun's energy is absorbed or reflected by the
atmosphere. We've all heard of the vital ozone layer that
reflects dangerous ultra-violet (UV) rays, and Carbon Dioxide (CO2)
absorbs infra-red (IR) rays very efficiently. While a solar
panel can be tilted to aim directly at the sun, the greater the angle
of the curvature of the earth, the more energy is lost to the
Don't let all this overwhelm you; its more important to
understand the concepts then the memorizing the geometry. There
are several solar energy calculators that simplify the task of
calculating the available energy at a given location. There are
three important factors that will determine the placement of the
solar collection system:
Calculating Solar PV Value
For Solar PV systems the
National Renewable Energy Laboratory (NREL) has an excellent
PV Watts Calculator production for any given location.
There are several other Solar PV Calculators available as well.
Calculating Solar Heating (Thermal) Value
energy also includes heat or thermal energy often associated with
solar water heaters. Thermal energy is expressed in
different units then electricity such as calories, joules and
British Thermal Units (BTU). Most thermal appliances
in America tend to be rated in BTU's so we'll refer to BTU's most
of the time.
A BTU is defined as amount of heat required to
raise the temperature of one pound of liquid water by one degree
from 60° to 61°Fahrenheit at a constant pressure of one
atmosphere. Form an energy equivalence ration, it takes 3.41
Watts to equal 1 BTU.
While there is a direct relationship
between the Watt and BTU, thermal efficiencies are significantly
different from PV efficiencies due to thermal losses and basic
system design. The value provided just from heating water
typically favors a flat collector system that doesn't track the
sun. For those units a simple calculator is all that's
required. One of the most commonly used
Flat Panel Solar Water Heating Calculators is provided by the
state of Texas.
Solar Concentrators and Tracking Systems
Solar Concentrators are an array of mirrors or lenses that focus a
large area of light on a smaller footprint of absorber. The
most common concentrator designs are parabolic mirrors although
there are numerous other system designs. In most
applications this requires an alignment of the sun/lens/solar
collector (focal path) so the system may require a tracking
Tracking systems position the solar concentrator to
align the focal path for peak efficiency. There are two
axis's of motion:
- Seasonal (North-South axis) which moves the solar collector up/down in
accordance with the time of year (Azimuth).
- Daily (East-West axis) which moves the solar collector left/right in
accordance with the time of day.
Tracking systems are usually electrically driven gear motor servo
systems with a specific program written to fit the needs of the
location. One advantage of a tracking system is they can turn
the concentrator to "safe positions" in the event of threatening
There are some experimental concentrator
technologies to take a magnifying lens to focus on solar PV
cells due to the high cost of solar cells. To
date there appears to be little if any cost savings using this
Thermal solar energy is quite different. For
larger scale systems, the use of concentrators and tracking
systems offers a much better option to the conventional flat
panel. In fact there are some very exciting technologies
in the field that give large scale thermal systems a very
optimistic future. The renewed scientific champions of
thermal solar energy have developed a method to calculate solar
energy more application specific to concentrating and tracking
Recent innovations have combined a Stirling engine
design that's nearly two centuries old with a thermal collector
system with exceptional efficiencies and cost per kWh.
Stirling Engine Systems worked with the Department of Energy to
develop and test their proprietary design with a resulting
efficiency of 31% - nearly double the best PV system available.
There are also some systems from Finland that incorporate
Stirling Engines which can be used to generate electricity or
directly pump the cooling cycle of an HVAC system. Where
both of these systems are still in their infancy, they also have
commercial applications are are beginning to sell systems.
One other advantage to Solar Thermal systems is they can typically
store energy at a much lower cost then any current battery technology.