Solar electricity generation represents a clean alternative to electricity from fossil fuels, with no air and water pollution, no global warming pollution, no risks of electricity price spikes, and no threats to our public health.Solar energy can also heat water, cool and heat homes, and provide free, natural lighting. And once a system is in place to convert the solar resource into useful energy, the fuel is free.
Just 18 days of sunshine on Earth contains the same amount of energy as is stored in all of the planet's reserves of coal, oil, and natural gas. Outside the atmosphere, the sun's energy contains about 1,300 watts per square meter. Once it reaches the atmosphere, about one-third of this light is reflected back into space, while the rest continues toward Earth’s surface. Averaged over the entire surface of the planet, a square meter collects 4.2 kilowatt-hours of energy every day, or the approximate energy equivalent of nearly a barrel of oil per year. Deserts, with very dry air and little cloud cover, receive the most sun—more than 6 kilowatt-hours per day per square meter on average over the course of the year.
India is endowed with vast solar energy potential. About 5,000 trillion kWh per year energy is incident over India's land area with most parts receiving 4-7 kWh per sq. m per day. Hence both technology routes for conversion of solar radiation into heat and electricity, namely, solar thermal and solar photovoltaics, can effectively be harnessed providing huge scalability for solar in India. Solar also provides the ability to generate power on a distributed basis and enables rapid capacity addition with short lead times. Off-grid decentralized and low-temperature applications will be advantageous from a rural electrification perspective and meeting other energy needs for power and heating and cooling in both rural and urban areas.
From an energy security perspective, solar is the most secure of all sources, since it is abundantly available. Theoretically, a small fraction of the total incident solar energy (if captured effectively) can meet the entire country's power requirements. It is also clear that given the large proportion of poor and energy un-served population in the country, every effort needs to be made to exploit the relatively abundant sources of energy available to the country. While, today, domestic coal based power generation is the cheapest electricity source, future scenarios suggest that this could well change.
The most important components of a PV cell are two layers of semiconductor material commonly composed of silicon crystals. On its own, crystallized silicon is not a very good conductor of electricity, but when impurities are intentionally added—a process called doping—the stage is set for creating an electric current.
The bottom layer of the PV cell is usually doped with boron, which bonds with the silicon to facilitate a positive charge (P), while the top layer is doped with phosphorus, which bonds with the silicon to facilitate a negative charge (N).
The surface between the resulting "p-type" and "n-type" semiconductors is called the P-N junction (see diagram below). Electron movement at this surface produces an electric field that allows electrons to flow only from the p-type layer to the n-type layer.
When sunlight enters the cell, its energy knocks electrons loose in both layers. Because of the opposite charges of the layers, the electrons want to flow from the n-type layer to the p-type layer. But the electric field at the P-N junction prevents this from happening.
The presence of an external circuit, however, provides the necessary path for electrons in the n-type layer to travel to the p-type layer. The electrons flowing through this circuit—typically thin wires running along the top of the n-type layer—provide the cell's owner with a supply of electricity.
Most PV systems are based on individual square cells a few inches on a side. Alone, each cell generates very little power (a few watts), so they are grouped together as modules or panels. The panels are then either used as separate units or grouped into larger arrays.
There are three basic types of solar cells:
Historically, most PV panels were used for off-grid purposes, powering homes in remote locations, cell phone towers, road signs, and water pumps. In recent years, however, solar power has experienced remarkable growth in the United States and other countries for applications where the power feeds into the electricity grid. Such grid-connected PV applications now account for more than 99 percent of the global solar market .
[1, 2, 3] Fraunhofer Institute. 2015. Photovoltaics report.
 International Energy Agency (IEA). 2014. Technology roadmap: Concentrating solar power. Paris, France.
[6, 7] Bird, L., J. McLaren, J. Heeter, C. Linvill, J. Shenot, R. Sedano, and J. Migden-Ostrander. 2013. Regulatory considerations associated with the expanded adoption of distributed solar. Golden, CO: National Renewable Energy Laboratory.