Solar Electricity: A Renewable Energy Source

By Navin Gautam, System Design Engineer, Future Electronics

Renewable energy refers to the sources of energy which are replaced or renewed by natural processes. The sun, our chief source of energy, is a giant nuclear machine. Almost all renewable energy sources are derived in one form or another from the sun.

The potential of energy from the sun for the replacement of energy required by today’s rapidly growing world is very high. The solar energy striking the earth’s surface can be as high as approximately 600 – 1500W/sq meter. However, due to various climatic and geographic conditions, this radiation is not uniform in different parts of the world. When the sun is positioned directly overhead or 90° from the horizon, the incoming radiation strikes the surface of the earth at a right angle and is the most intense. If the sun is at an angle above the horizon, the incoming radiation strikes the earth’s surface at an angle causing the rays to be spread out over a larger surface area and reducing the intensity of the radiation. Apart from the angle of incidence and dust particles in the atmosphere, clouds and terrain play an important   role in the intensity of the solar radiation striking the surface of the earth. The total amount of solar energy falling on one square meter surface area over a day, averaged over a period of time, is referred to as insolation.

The insolation index of a location gives a measure of how much energy on average that surface receives from the sun. Typically, this index ranges from 2.26 to 6.39 kWh/day/m2  in earth surface. Insolation of  2.26 is similar to the solar energy falling in some parts of the UK and is considered to be very low; 6.39 falls in the higher range of insolation that can be received in some places on the African continent. The information about insolation is very important for any kind of solar energy related installation anywhere in the world.

Converting Solar Energy to Electricity

Solar modules are used to convert solar energy to electricity. Silicon solar modules use a state-ofthe art technology perfected by nearly 40 years of manufacturing. However, the energy conversion efficiency of these modules has not been significantly increased over time while other technologies in the world are having exponential growth. The most commercially installed solar modules to date are only 10-16% efficient. However, some researchers have achieved even 40% or higher efficiency, but only in controlled lab conditions. This gives a clear picture that only 10-16% of the solar radiation falling on the earth’s surface is being converted to electric energy. Taking a typical context of North American insolation of 3.5KWh/m2, one square meter of solar panel can collect 350Wh/day to 560Wh/day of energy. If we look over the typical electrical energy requirement of a North American home, it ranges from 14000Wh/day to 50000Wh/day.

Solar Electrical Energy: Common Components

A typical solar electric system comprises one or more solar panel modules, power conversion units, charge controller units and batteries. The battery and the charge controller may or may not be included in a system, depending on the application of the solar electricity.

Solar Panels

Solar panels are simply a P-N junction of a semiconductor, constructed in such way that the maximum junction area is exposed to light. Tempered glass is used to cover the cells so that they are protected from the environment. The textured cell surface and the tempered glass together reduce the reflection of the light. A single P-N junction cell does not produce much voltage. Hence, a solar module consists of several solar cells connected in series to achieve the desired output voltage and power.

The solar panels can be broadly categorized as bulk silicon solar cells and thin film solar cells. The bulk solar cells are made from 180-240 µm thick self supporting silicon wafers. The wafers are processed to make a P-N junction and then connected together to form a solar cell module. Bulk solar cells
are also referred as crystalline solar cells. They can be further categorized into mono-crystalline and poly-crystalline form. Mono-crystalline wafers are cut from cylindrical ingots of silicon crystal; hence, they are expensive, but make highly efficient solar modules. Poly-crystalline silicon cells are made from cast square ingots, making them less expensive to produce than mono-crystalline. However, polycrystalline silicon cells are less efficient.