Solar Electricity

Early Discovery and Innovation

Portrait of physicist Alexandre Edmond Becquer...

In 1839 French physicist Alexandre-Edmond Becquerel discovered that an electrical charge resulted from the absorption of sunlight by a conductive material. American inventor Charles Fritts used Becquerel’s discovery to create the first solar cell in 1883. Charles Fritts discovery lead five years later to the first patent in the US for solar cells that was issued to Edward Weston in 1888 (US389124 and US389125). Albert Einstein explained the photoelectric effect in 1905 for which he received the 1921 Nobel Prize in Physics. Thirty-four years later Bell Labs scientists D.M. Chapin, C.S. Fuller and G.L. Pearson invented the first generation of modern solar cells that was described in their Journal of Applied Physics 25 (5): 676–677 (May 1954) article:

“A New Silicon p-n Junction Photocell for Converting Solar Radiation into Electrical Power”

Fabrication of materials with electrical properties focused attention on germanium and silicon as semiconductors of electrical charges. Germanium provides higher electrical efficiency than silicon but is not as efficient as silicon at higher temperatures and a costly material to manufacture. Silicon is more abundant in nature and could be manufactured in higher quantities at significantly lower unit cost.

While silicon ruled terrestrial solar power applications germanium remained the preferred material for solar cells used to power satellites and other spacecraft. The high cost of germanium was balanced by more efficient solar cells that reduced the solar array size and associated weight onboard spacecraft. Similar environmental and financial constraints did not apply to land based solar power systems that were best suited to less-efficient but significantly lower-cost silicon solar cells.

Solar cell production quantities and encapsulation quality increased with improved manufacturing methods that resulted in longer life and more reliable solar modules at lower unit costs. The cost of poly-silicon used to fabricate polysilicon solar modules declined from US$170 per kilogram (2.2 lbs.) in December 2008 to under US$35 per kilogram in November 2011.

Thin Film Solar Modules

A solar module is a collection of solar cells encapsulated in a rigid rectangular vacuum-sealed glass package. Thin Film solar cells are an alternative to silicon based solar cells. Layers of photo-sensitive chemicals are deposited onto a thin film substrate that is encapsulated into a solar module. Thin Film (TF) solar modules have not as yet (1Q2012) achieved competitive kilowatt hour (cost/kWh) production efficiency compared to mono-crystalline silicon, multicrystalline or polysilicon (poly-Si) solar modules.

Electricity from Photons

Sunlight carries photons that are absorbed by solar cells made of semi-conductor material. The top and bottom of a silicon solar cell is doped with a chemical of different opposing atomic structure and electrical charge. The top thinner layer of an n-type solar cell has a dominant negative charge and a dominant positive charge on the bottom thicker layer.

A conductive dye is printed in a finger-like pattern on the sun-facing negative charge side of the solar cell to increase reception and absorption of photons. Pair of conductors called busbars carries the current from the solar cell’s finger-like photon receptors to serially adjacent solar cells.

Photons from sunlight absorbed into solar cells dislodge atomic particles from normal state. An invisible band gap separates the top and bottom of a solar cell that forms a resistive barrier to reconnection of electrons with attractive charge holes. Disoriented negative electrons find resistance reconnecting with positive holes. Dislodged electrons scurrying around in search of available holes to return to normal state generate an electrical current at the voltage rating of the solar cell. The time that electrons spend in an excited state in search of holes extends the duration of electrical current in the solar cell. Metal busbars bridge negative and positive sides of adjacent solar cells in series. As current passes between solar cells it carries voltage from each solar cell. Solar cell current multiplied by the sum of voltage from each solar cell in a solar module results in electricity measured in direct current (DC) watthours (Wh). An inverter converts the DC Wh to alternating current (AC) to power AC loads or permit transmission on the electrical grid.

Solar Modules, Arrays and Balance of Systems

The Mueller Austin solar array at 30.3065° -97...

A solar module encapsulates 60 or 72 solar cells, usually in six columns of 10 or 12 solar cells. The sum of each cell’s voltage carried by the current across the solar cells determines the electrical power or watthours (Wh) that the solar module can produce. Solar modules are assembled into solar arrays that make-up the energy receivers and conductors of direct current (DC) electricity.

An inverter is an electrical conversion and power management device that is an integral component of all grid-tied solar power systems. The inverter converts direct current (DC) electricity to alternating current (AC) electricity standards required for transmission to the AC electrical grid. The inverter’s durability, capacity and efficiency to optimize and manage DC to AC electricity power transmission is an important determinant to investment pay-back and rate of return of the solar power system.

The inverter, racking, wiring, junction/disconnect boxes and wiring/grounding chases are referred to as the Balance of Systems (BOS). Electrical grounding devices/wiring and racking systems protect and support the solar array on rooftops or on ground-mounted projects. Wiring delivers the electrical power generated from the solar array to the inverter and to the electrical grid. Junction boxes are conduits that consolidate wiring from modules to the inverter. Disconnects are devices that shutdown DC and AC transmission of electricity within the solar power system for maintenance or emergency purposes.

Solar power system generation is increased by racking that positions solar arrays on a slope or angle to receive more direct sunshine based on the latitude of the project site. Sun trackers provide additional optimization using single or dual-axial positioning to adjust the solar arrays tilt angle and orientation to receive direct sunlight throughout the day.

Peak Sun Hours

The pay-back and investment return from a solar power system located at a particular site requires due diligent assessment of several parameters. Sunlight rises at dawn in the east and declines in the west at dusk as the earth spins before the sun and daylight turns to darkness. Solar power generation occurs only when there is direct and indirect or diffusive sunlight. A high density of photons delivered over a long duration of time results in higher peak sun hours that offers greater solar power generation opportunity. Seasonal changes impact on the duration of peak sun hours as the solar window expands and shrinks between the summer (June 21st) and winter (December 21st) solstices.

Solar power generation systems have a long history in supplying space satellite power requirements. Early solar power systems continue in service after 30 years. Solar module manufacturers offer 25 year limited warranties. Selection of quality solar power system components and contractors is a critical success factor to ensure reliable optimal electricity generation over several decades.

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