Solar Photovoltaic

What is photovoltaic energy?

‘Photovoltaic’ is made up of two words: ‘photo’, meaning light, and ‘voltaic’, meaning electricity. Photovoltaic technology, the term used to describe the hardware that converts solar energy into usable power, generates electricity from light.

PV Technology

At the heart of photovoltaic (PV) technology is a semi-conductor material which can be adapted to release electrons, the negatively charged particles that form the basis of electricity.

PV cells are generally made either from crystalline silicon, sliced from ingots or castings or from grown ribbons, or thin film, deposited in thin layers on a low-cost backing.

The most common semi-conductor material used in photovoltaic cells is silicon, an element most commonly found in sand. There is no limitation to its availability as a raw material; silicon is the second most abundant material in the earth’s mass. To date, most cell production has made use of silicon.

All PV cells have two layers of semi-conductors, one positively charged and one negatively charged. When light shines on the semi-conductor, the electric field across the junction between these two layers causes electricity to flow, generating DC (direct current). The greater the intensity of the light, the greater the flow of electricity.

A photovoltaic system therefore does not need bright sunlight in order to operate. It can also generate electricity on cloudy days. Due to the reflection of sunlight, days with slight cloud can even result in higher energy yields than days with a completely cloudless sky. Generating energy through solar PV is quite different from how a solar thermal system works, where the sun’s rays are used to generate heat, usually for hot water in a house, swimming pool etc.

Crystalline Silicon

Crystalline silicon is the mainstay of most PV modules, representing approximately 90% of the current market. Although in some technical parameters it is not the ideal material for solar cells, it has the benefit of being widely available, well understood and uses the same technology developed for the electronics industry. Efficiency generally ranges from 12-17%, although efficiencies of more than 20% have been obtained with silicon cells already in mass production. This means that 20% of the incoming insulation can be transferred into electricity.

As well as the efficiency of the solar cells, their thickness is also an important factor. Wafers - very thin slices of silicon - are the basis for crystalline solar cells. Thinner wafers mean less silicon needed per solar cell and therefore lower cost. The average thickness of wafers has been reduced from 0.32 mm in 2003 to 0.17 mm in 2008. Over the same period, the average efficiency has increased from 14% to 16%. By 2010, the aim is to reduce wafer thickness to 0.15 mm whilst increasing efficiency to an average of 16.5%.

Thin Film

Thin film modules are constructed by depositing extremely thin layers of photosensitive materials onto a low-cost backing such as glass, stainless steel or plastic. Thin film manufacturing processes result in lower production costs compared to the more material-intensive crystalline technology, a price advantage which is currently counterbalanced by substantially lower efficiency rates (from 5% to 13%).

Concentrated PV

Some solar cells are designed to operate with concentrated sunlight. These cells are built into concentrating collectors that use a lens to focus the sunlight onto the cells. The idea is to use very little of the expensive semi-conducting PV material while collecting as much sunlight as possible. Efficiencies are in the range of 20 to 30%.

Flexible Cells

Based on a similar production process to thin film cells, when the active material is deposited in a thin plastic, the cell can be flexible. This opens the range of applications, especially for building integration (roofs- tiles) and end-consumer applications.