Photovoltaic (PV) cells utilize a special semi conductor material that silently and directly converts solar energy into electricity at the atomic level without using complex machinery usually associated with electrical generation. This is possible because of a material property known as the photoelectric effect, which allows the material to absorb photons of light and release electrons. These free electrons can then be captured resulting in a electrical current that can be used as electricity. Because the resulting electrical current is Direct Current (DC), an inverter must be used to convert it into Alternating Current (AC) before it can be used.  Currently, most photovoltaic cells are manufactured from silicon although other exotic material such as gallium arsenide are becoming more and more common.  Following are the three most common types of photovoltaic systems.

Mono-crystalline

The majority of photovoltaic systems available on the market today are single-crystal silicon based.  These are usually a uniform blue or black and are manufactured by melting highly purified silicon and crystallizing it into ingots which are sliced into thin wafers to make individual cells.  These cells are backed with a metal back-plane to provide support and a electrical contact on the bottom of the cell.  The top of the cell is covered with a thin metallic mesh to allow sunlight through while also providing another electrical contact.  Solar radiation from the sun contains small particles referred to as photons.  As these photons move into a cell and strike electrons, they dislodge the electrons and create empty spaces.  The dislodged electrons move toward the top layer of the cell and into the metallic mesh as the photons continue to dislodge more electrons.  If an electrical circuit is completed from the top mesh and the back-plane, the electrons will flow through the circuit creating a current.  As more cells are connected to the array the current will continue to increase while the voltage remains relatively constant.

 

Polycrystalline

Polycrystalline photovoltaic cells are very similar to mono-crystalline cells in the way they are manufactured and function.  The main difference is that polycrystalline cells are made from a lower quality silicon resulting in reduced efficiency.  This also reduces the manufacturing cost which is the main benefit over mono-crystalline cells.

 

Amorphous Silicon

Amorphous silicon photovoltaic cells, unlike mono and polycrystalline cells, have no distinct crystal structure.  Instead amorphous silicon cells (thin film silicon) are made from depositing thin layers of vaporized silicon in a vacuum onto a support structure (glass, metal or plastic).  Because some light passes through the top layers of the cell, multiple layers are deposited increasing the total power output of the cell.  Despite this, the efficiency of thin film silicon cells is still around half that of comparable mono and polycrystalline cells.

Solar thermal technologies use solar radiation to provide heat for a wide range of applications including space heating, pool heating, domestic water heating, and even power generation.  Solar thermal systems are generally more economically feasible then photovoltaic systems despite recent advances in PV technology. Solar thermal collectors are about five times as efficient as currently available photovoltaic panels, yet they cost about one-tenth as much.  Solar collectors generally fall into one of two categories; concentrating or non-concentrating.  In the non-concentrating type the collector area is the same as the absorber area while in a concentrating type the collector area is separate from the smaller absorber area.

Non-Concentrating Collectors

Non-Concentrating collectors are often used in smaller-scale production situations due to their low cost and ease of maintenance.  These systems integrate both the collector and the absorber into the same component.  This eliminates the need for solar tracking systems although they may still be used to increase the efficiency.

Flat-Plate

Flat-plate collectors are the most common type of non-concentrating collector.  Flat-plate collectors consists of a absorber plate made from a thin sheet of thermally stable polymers, copper, aluminum or steel coated with a matte black finish.  The absorber sheet is commonly backed with a grid or coil of tubing that the heat transfer fluid passes through.  The bottom of the panel is plated with a insulating material while the top is covered with a translucent material to reduce heat losses.  As solar radiation passes through the translucent material and strikes the absorber, it heats up.  This heat is conducted into the tubing and the heat transfer fluid then transfers the heat from the absorber to a storage tank.

 

Evacuated Tube

Evacuated tube collectors are the most efficient type of non-concentrating collector on the market.  They are made using concentric strengthened borosilicate glass tubes.  The outer tube is translucent, allowing solar radiation to pass through unrestricted while the inner tube is treated with a special optical coating resulting in energy absorption without reflection.  The gap between the inner and outer tubes is evacuated creating a vacuum significantly reducing heat loss due convection and conduction thus increasing efficiency.  A heat pipe consisting of a copper tube filled with a proprietary liquid that boils under very low pressure and temperature situations is inserted inside the inner glass tube.  As the the liquid absorbs heat it vaporizes and rises to the top of the heat pipe.  The heat is transferred to a common manifold through which a heat transfer fluid flows removing the heat to be used or stored.  Once the heat has been transferred the liquid condenses and gravity returns it to the base of the heat pipe where the process continually repeats. 

 

Concentrating Collectors

Concentrating collectors use optical components to reflect light collected over a large area onto the smaller absorber area.  This allows for much higher operating temperatures than non-concentrating collectors increasing efficiency and cost-effectiveness.  However, because light is reflected onto a focal line or point either 1-axis or 2-axis solar tracking must be used to ensure proper alignment and operation.  These types of systems are oftened used for large hot water needs or to generate electricity.

Parabolic Trough

The most common type of commercial concentrating collectors are parabolic trough collectors which consist of large parabolic curved mirrors that collect, reflect and focus light onto a focal line.  A 1-axis solar tracking system is used to track the suns location, rotating the collectors as needed.  A metal absorber tube is located along the entire length of the focal line.  The tube is surrounded in a specially coated evacuated glass tube to reduce conductive, convective and radiative heat losses.  A specialized heat transfer fluid that resists boiling flows through the inner absorber tube picking up heat before it flows through a heat exchanger dumping the heat for use.

 

Parabolic Dish

Parabolic dish collectors are very similar to parabolic trough collectors except instead of using a curved trough to focus light onto a focal line they use a curved dish to focus light onto a focal point.  This requires use of highly accurate 2-axis solar tracking system in order to operate effectively.  Often times these systems are used to focus solar radiation onto a Stirling engine to generate electricity instead of providing heat.

Photon Flux

This is how many photons actually strike the solar panel, it is most commonly affected by the size and orientation of the panels.

• Array orientation - The power output of solar panels is greatly affected by their orientation and tilt angle to the sun. Because the suns position and angle changes in the sky depending on the time of year, solar systems are most efficient if used with a solar tracking mechanism. Static mounted systems can still provide adequate performance if optimized using sun charts to determine the best position and angle.

• Array size - Solar panels are broken into cells which are then connected in parallel. This allows the provided voltage/flow to remain constant no matter the number of cells, this means the power output of a solar system is directly proportional to its area.

Photon Intensity

This is the amount of energy each photon contains, it is most commonly affected by the local climate and the latitudinal position of the panels.

• Latitudinal position - Geographic locations further from the equator experience a seasonal reduction in solar radiation availability. For best performance in these locations the panel angle is often set to the angle of the latitude, however, performance can be improved by adjusting the panel angle on a per season basis or by using a solar tracking system to continuously adjust the panels to the optimum angle.

• Climate - Local climate can significantly affect the power output of solar arrays. During the winter the sun sits lower in the sky decreasing the light intensity and length. Additionally, locations with cloudy, rainy, or snowy conditions for large portions of the year may encounter significant power decreases.

In order determine the area available for a solar system, a tape measure should be used to record the dimension of all areas considered.  Alternatively, satellite images from Google earth can be used although this method is not as accurate as surface slope can not be accounted for.

• Google Maps Area Tool

The solar resource available at the given location must be determined to asses the amount of savings available.  A tool provided by the National Renewable Energy Laboratory may be used for this. 

• PV Watts
• User Guide

Usable Energy (kWh/ft²-yr) - This equation calculates the amount of usable energy available to the photovoltaic solar collectors.

Where,

PC = Production Capacity (kWh/W-yr)

SP = Solar Panel Rating (W/ft²)

n_I = Inverter Efficiency (%)

Solar Panel Output (kW) - This equation calculates the solar panel power output for photovoltaic systems.

Where,

UE = Usable Energy (kWh/ft²-yr)

A = Available Area (ft²)

Monthly Production (MMBtu/mo.) - This equation calculates the monthly production for solar thermal systems.

Where,

S_R = Solar Radiation (Btu/ft²/day)

S_A = Solar Field Size (ft²)

n_T = Overall System Efficiency (%)

CF₁ = Conversion Factor (1,000,000Btu/MMBtu)