Solar cells and electricity generation from them at present is no more a new concept to most of the people. Solar electricity is being clean (pollution free), silent, limitless and free will play a great role in the times to come in the present energy driven civilization. Moreover, photovoltaic is now a proven technology which is inherently safe as opposed to other dangerous electricity generating technologies. Although practical solar cells have only been available since the mid 1950s, scientific investigation of the photovoltaic effect started in 1839 and the effect was first observed in a solid material (selenium) in 1877. Today's commercially available silicon solar cells are reported to have efficiencies exceeding 15% of the sunlight falling on to them into electricity, at a fraction of the price of thirty years ago. There are now a variety of methods for the practical production of silicon solar cells (amorphous, single crystal, polycrystalline, thick film, ribbon, sliver, etc), as well as solar cells made from other materials (copper indium diselenide, cadmium telluride, etc).
Types of solar cells:
The most common form of solar cells are based on the photovoltaic (PV) effect in which light falling on a two layer semi-conductor device produces a photo-voltage or potential difference between the layers. This voltage is capable of driving a current through an external circuit and thereby producing useful work. The most common configuration of this device, the first generation photovoltaic, consists of a large-area, single layer p-n junction diode, which is capable of generating usable electrical energy from light sources with the wavelengths of solar light. These cells are typically made using silicon. However, successive generations of photovoltaic cells are currently being developed that may improve the photo-conversion efficiency for future photovoltaic. The second generation of photovoltaic materials is based on multiple layers of p-n junction diodes. Each layer is designed to absorb a successively longer wavelength of light (lower energy), thus absorbing more of the solar spectrum and increasing the amount of electrical energy produced. The third generation of photovoltaic is very different from the other two, and is broadly defined as a semiconductor device which does not rely on a traditional p-n junction to separate photo-generated charge carriers. These new devices include dye sensitized cells, organic polymer cells, and quantum dot solar cells.
Efficiency of solar cells:
The single crystal silicon solar cells can currently convert only up to 24% of the solar energy into electricity, because the radiation in the infrared region of the electromagnetic spectrum does not have enough energy to separate the positive and negative charges in the material. Polycrystalline silicon solar cells have an efficiency of less than 20% at present and amorphous silicon cells, are currently about 10% efficient, due to higher internal energy losses than single crystal silicon. However PV cells made of multi-junction Gallium Arsenide and other similar alloys have achieved efficiencies as high as 30%. Other thin film cells besides amorphous silicon such as Cadmium Telluride and Copper Indium Diselenide have achieved research efficiencies of 16% and almost 18% respectively. Using the commercially available solar cells (as of 2005) and system technology leads to system efficiencies between 5 and 15%. As of 2005, electricity generation will cost ranged from ~ Rs.25/kWh down to ~ Rs.12/kWh in regions of high solar irradiation which comparable with the prevailing retail electric pricing (as of 2005) worldwide. There is a common myth that solar cells never produce more energy than it takes to make them. While the expected working lifetime is around 40 years, the energy payback time of a solar panel is anywhere from 1 to 30 years (usually under five) depending on the type and where it is used. This means solar cells can be net energy producers and can "reproduce" themselves (from just over once to more than 30 times) over their lifetime.
Applications of solar cells:
The market for solar cells at presently is growing at over 30% per year, and the cost of panels is declining continuously in real terms, due to both new technologies and mass production. The need and possible applications for solar electricity is growing as follows:
- the need for low maintenance, long lasting sources of electricity, suitable for places remote from both the main electricity grid and from people; eg satellites, remote site water pumping, outback telecommunications stations and lighthouses
- the need for cost effective power supplies for people remote from the main electricity grid; eg Indigenous and non-Indigenous isolated settlements, outback sheep and cattle stations, tourism and travellers
- the need for non-polluting and silent sources of electricity; eg tourist sites, caravans and campers
- the need for a convenient and flexible source of small amounts of power; eg calculators, watches, light meters and cameras
- emergency power systems
- vaccine and blood storage refrigerators for remote areas
- aeration systems for ponds
- power supplies for satellites and space vehicles
- portable power supplies for camping and fishing
- building integrated photovoltaic cells
- cathodic protection systems
- electric fences
- remote lighting systems
- telecommunications and remote monitoring systems
- solar powered water pumping
- water treatment systems
There are confident predictions from leading solar cell manufacturers that the price of solar electricity will be competitive with mains electricity within a few years. These predictions generally refer to power at the panel, and do not take into account the various other system costs. Because, the prices of the balance of systems components are not declining as rapidly as the cost of panels, so the total system costs will decline more slowly. This factor is encouraging research into appliances that can be used directly from the panels, and do not need to rely on inverters and battery storage.