A Brief Introduction To Solar Photovoltaics (PV)

The direct conversion of light into electricity without any moving parts is called the "photovoltaic effect" and was discovered in 1839 by the French scientist Edmond Bequerel. The first solar cells, made of selenium, were used as photographic light meters. P.V. may have caught on then but for the invention of the "infernal" combustion engine and the discovery of oil. Instead, no serious application for P.V. was developed until the advent of the space industry when scientists could not figure a way of getting their wires to reach into outer space! In the early 1950’s, workers at the Bell Laboratories in the USA discovered an improved efficiency silicon solar cell. This opened the way to their use on space satellites throughout the 1960’s. With the oil crisis in 1973-74 serious US government, funding brought major cost reductions opening up new markets.

Many technologies have been developed to tap the power of the sun, of these the simplest and most elegant is surely, solar photovoltaics. Based on solid-state physics and a spin-off of the semiconductor industry, it is simple, dependable, with no moving parts. Above all, the technology is environmentally friendly, silent, and emission free.

What Are Photovoltaics?

Photons to Electricity

Light can be considered to consist of a stream of tiny particles of energy called photons. When photons from light of a suitable wavelength hit a photovoltaic cell, usually made of silicon, they can transfer their energy to some of the electrons in the material so "promoting" them to a higher energy level. Normally these electrons help hold the material together by forming so called "valence bonds" with adjoining atoms and cannot move. In their "excited state", however, the electrons become free to conduct electric current by moving through the material. With one side of the cell being positive (p) and the other negative (n) and with the respective connections made to each, a circuit is formed and the cells generate electricity.

These cells are connected in a similar way to which batteries fit into a torch, that is, positive to negative, and configured to provide the appropriate power desired. The cells are typically mounted in an aluminium frame with a glass cover.

Which kind of panel?

Monocrystaline. These are the most efficient panels available producing the most electricity per square inch. Using highly skilled operators they are manufactured from thin wafers (.25 mm) sliced from a single high-grade crystalline ingot mostly using a process called the 'Czochralski Process'. Until recently almost all such cells were fabricated from extremely pure ‘electronic grade’ polycrystaline silicon, however ‘solar-grade’ silicon is being introduced to the process. Where efficiency and durability are required such as in rugged or extreme places like the North Sea, Monocrystaline can be relied on. They will not, however, work if partly shaded and efficiency drops considerably in cloudy weather.

Polycrystaline - These take up a larger surface area for the amount of power produced. The manufacturing process is less labour and energy intensive which is reflected in the price. In effect, they are much like the previous panel but less efficient.

Amorphous - Also known as 'Thin Film'. The manufacture of this type of module is fully automated and can be used in conjunction with flexible materials such as hard wearing industrial fabrics or plastic to produce flexible panels. The raw ingredients are relatively inexpensive in turn, rigid panels require a greater surface area per Watt than either Monocrystaline or Polycrystaline. In a price per Watt comparison Amorphous panels represent good value and can, for example, be used on new buildings, the price of the panels being off-set against the price of roofing materials.

Thin Film Triple Junction - Thin film Triple Junction P.V. works efficiently in cloudy weather. It is made in three separate layers one on top of the other, called multi-junctions. Each layer is tailored to a different portion of the spectrum. The wide band junction on top absorbs the higher energy photons at the red end of the spectrum, sunlight bright enough to cast a shadow. Following this junction come the narrower bands forming junctions designed to absorb a portion of the lower light frequencies nearer the green and blue portions of the spectrum, therefore utilising the entire spectrum to produce electricity. This means that Triple Layer modules are suited to the cloudier weather conditions we experience in the UK, producing up to 30% more power than equivalently rated panels

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