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Solar Cells

Photovoltaic (PV) cells - photocells

 

Characteristics

 

Solar cells do not utilise chemical reactions to produce electric power. They convert sunlight energy into electric current. They do not store energy.

The cell voltage is proportional to the amount of sunlight falling on the cell but is generally less than 0.5 Volts.

The sun's energy reaching the surface of the earth is roughly 1 kilowatt per square metre.

Solar cell conversion efficiencies have improved in recent years to between 20% and 30% but they still only generate 150 W/m2 in bright sunlight. A reasonable maximum size of a solar array in a typical car would be 3 m2. In the best case, in bright sunlight, this would generate only 0.5 kW (less than 1 bhp) of power which is not enough to drive a road vehicle or even to charge a reasonable sized battery.

Taking into account the hours when there is no sun, or hazy conditions and that the solar cell would often be shielded or tilted at an angle to the sun, the average power generated by the array would be less than 30 W/m2 even in a sunny climate. This makes solar power impractical for driving any other than specialist low power demonstration vehicles.

 

Cells convert sunlight energy into electric current they do not store energy. Sunlight is the “fuel”

 

Typical commercially available PV panels have an efficiency of about 15%, which means that they can deliver about 150 watts of power per square metre.

Polycrystalline cells have the highest efficiency (10-30%), and very long lifetimes because the crystal structure is very stable, but they are very expensive.

Thin film or amorphous silicon cells are the type most commonly found in calculators. Each cell is made from non-crystalline thin films of silicon atoms, and typically have a uniform gray appearance. These cells are cheaper but they tend to have lower efficiencies (5-10%) and shorter lifetimes.

 

A domestic solar power system includes the following components

  • PV panels
  • Batteries: Typically about 12 deep-cycle lead acid batteries
  • Charge controller: To regulate the charging of the batteries
  • Inverter to convert the low voltage DC power from the batteries into AC mains power for use by appliances

 

Advantages

Inexhaustible energy source

PV manufacturers guarantee their products for up to 20 years.

 

Shortcomings

Depends on sun

Typically, the output of any industrial PV module is reduced to 5-20% of its full sun output when it operates under cloudy conditions.

Low cell voltage <0.5 Volts.

Very low conversion efficiency

PV systems convert light directly into electricity using semiconductor technology @ around 30% efficiency
Thermal systems (hot water, pool heaters) produce heat from the sun’s radiation @ +40 % efficiency

Large surface area of cells exposed to the sun needed for high power systems.

High capital cost for high power systems.
Deep discharge batteries are required to store and level the power generated from PV systems.

 

Applications

Suitable for low power applications such as calculators, portable lamps, watches and battery chargers.

Remote telemetry and communications.

Advertising signs

Satellites, the first application to use solar power.

A typical house in the USA sunbelt can be powered with about 200 square feet of solar panels.

 

See more on this page Solar Power Applications

 

Still not cost effective for high power applications.

 

Costs

In the sunny parts of the USA, the cost of larger PV systems (greater than 1 kW) is measured in "levelised" costs per kWh--the costs are spread out over the system lifetime and divided by kWh output. The levelised cost is now about $0.25 to $0.50/kWh. At this price, PV is cost effective for residential customers located farther than a quarter of a mile from the nearest utility line.

 

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History

See also Grätzel Cells

Cell Chemistry Comparison Chart

 

 

 

 

 

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