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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:

1. The earth is round so the latitude determines the angle to the sun.
2. The earth rotates on an axis that changes through the seasons - summer solstice, fall equinox, winter solstice, spring equinox. 
3. Time of day varies the altitude angle to the sun, with peak energy coming at "high noon".
4. Length of day determines how many accumulative hours the sun is providing energy.
5. Intensity changes as the earth takes an eccentric path around the sun being closest in March, and furthest in September. 
6. 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 atmosphere.

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:
 

Solar Noon is the time of day that the sun is in the center of point between sunrise and sunset.  Time zones span a long distance about 1000 miles on the average.  While the clock says noon at one end of a time zone, the relative position of the sun is is a little different at the leading edge of the time zone then it is at the trailing edge of the time zone.  Solar Noon is the time of day the sun is at it's highest point. Another way to put it is Solar Noon is the time of day when shadows point to true north; magnetic north is not true north.
Solar Azimuth angle is the actual East-West axis angle the sun from rises and sets in. Solar Azimuth calibrates for the "effective latitude" changes due to the axis shift through the seasons.  Every day through the calendar year there will be slight changes to the Solar Azimuth, with two days a year Spring & Fall Equinox where the Solar Azimuth will be the same.  As the days grow longer the angle of the Solar Azimuth gets wider, and as the days grow shorter the angle of the Solar Azimuth narrows.
Solar Altitude (sometimes referred to as elevation) is the angular height of the sun measured from the Horizon. During the Spring and Fall Equinox the Solar Altitude is equal to the Latitude plus 90°. During the Summer Solstice the Solar Altitude is equal to Latitude plus 113.5°. During the Winter Solstice the Solar Altitude is equal to Latitude plus 66.5°.

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

Solar 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 system.

Tracking systems position the solar concentrator to align the focal path for peak efficiency.  There are two axis's of motion:

1. Seasonal (North-South axis) which moves the solar collector up/down in accordance with the time of year (Azimuth).
2. 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 weather conditions.

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 technology.

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 systems.

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.