Scientists have known of the photovoltaic effect for more than 150 years. Photovoltaic power generation was not considered practical until the arrival of the space program. Early satellites needed a source of electrical power and any solution was expensive. The development of solar cells for this purpose led to their eventual use in other applications.
DISCOVERY AND DEVELOPMENT OF PHOTOVOLTAIC POWER
The photovoltaic effect has been known since 1839, but cell efficiencies remained around 1% until the 1950s when U. S. researchers were essentially given a blank check to develop a means of generating electricity onboard space vehicles. Bell Laboratories quickly achieved 11% efficiency, and in 1958, the Vanguard satellite employed the first practical photovoltaic generator producing a modest one watt.
In the 1960s, the space program continued to demand improved photovoltaic power generation technology. Scientists needed to get as much electrical power as possible from photovoltaic collectors, and cost was of secondary importance . Without this tremendous development effort, photovoltaic power would be of little use today.
Early photovoltaic development
YEAR | DEVELOPMENT |
1839 | Antoine-César Becquerel, a French physicist, discovered the photovoltaic effect. In his experiments he found that voltage was produced when a solid electrode in an electrolyte solution was exposed to light. |
1877 | W.G. Adams and R.E. Day observed the photovoltaic effect in solid selenium. They built the first selenium cell and published “The action of light on selenium,” in Proceedings of the Royal Society. |
1883 | Charles Fritz built what many consider to be the first true photovoltaic cell. He coated the semiconductor selenium with an extremely thin layer of gold. His photovoltaic cell had an efficiency of less than 1%. |
1904 | Albert Einstein published a paper on the photoelectric effect . |
1927 | A new type of photovoltaic cell was developed using copper and the semiconductor copper oxide. This device also had an efficiency of less than 1%. Both the selenium and copper oxide devices were used in applications such as light meters for photography. |
1941 | Russell Ohl developed the silicon photovoltaic cell. Further refinement of the silicon photovoltaic cell enabled researchers to obtain 6% efficiency in direct sunlight in 1954 . |
1954 | Bell Laboratories obtained 4% efficiency in a silicon photovoltaic cell. They soon achieved 6% and then 11%. |
1958 | PV cells were first used in space on board the Vanguard satellite. |
A typical photovoltaic cell consists of semiconductor material (usually silicon) having a pn junction as shown in figure
Sunlight striking the cell raises the energy level of electrons and frees them
from their atomic shells. The electric field at the pn junction drives the electrons into the n region while positive charges are driven to the p region. A metal grid on the surface of the cell collects the electrons while a metal back-plate collects the positive charges.
POWER OUTPUT AND EFFICIENCY RATINGS
The figures given for power output and efficiency of photovoltaic cells, modules, and systems can be misleading. It is important to understand what these figures mean and how they relate to the power available from installed photovoltaic generating systems.
Power Ratings
Photovoltaic power generation systems are rated in peak kilowatts (kWp). This is the amount of electrical power that a new, clean system is expected to deliver when the sun is directly overhead on a clear day. We can safely assume that the actual output will never quite reach this value.
System output will be compromised by the angle of the sun, atmospheric conditions, dust on the collectors, and deterioration of the components. When comparing photovoltaic systems to conventional power generation systems, one should bear in mind that the PV systems are only productive during the daytime. Therefore, a 100 kW photovoltaic system can produce only a fraction of the daily output of a conventional 100 kW generator.
Efficiency Ratings
The efficiency of a photovoltaic system is the percentage of sunlight energy converted to electrical energy. The efficiency figures most often reported are laboratory results using small cells. A small cell has a lower internal resistance and will yield a higher efficiency than the larger cells used in practical applications. Additionally, photovoltaic modules are made up of numerous cells connected in series to deliver a usable voltage. Due to the internal resistance of each cell, the total resistance increases and the efficiency drop to about 70% of the single-cell value. Efficiency is higher at lower temperatures. Temperatures used in laboratory measurements may be lower than those in a practical installation.
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