The world's entire annual energy need could be met if there was a way to capture just one hour's worth of the solar energy continuously beamed at Earth. This factoid is the underlying rationale for targeting solar energy, which is free, effectively inexhaustible, clean and silent.
But there are major technical issues impeding solar energy utilisation. Radiation is intermittent and weather-dependent. Batteries are required at night. Solar technologies and storage systems are expensive and require a lot of surface area. Converting solar energy to usable electricity that may be transmitted via a power grid is tricky.
The most popular method of generating solar power is via photovoltaic (PV) cells. Semiconductors develop an electrical current upon being exposed to sunlight - this photovoltaic effect has been well-known since the 19th century.
Panels with semiconductor cells convert solar radiation into electricity and about one per cent of global electricity is generated via PV solar. The primary panel material is silicon. The costs have dropped considerably with current panel prices about 65 per cent lower than 2010 prices. PV generates direct current, which is converted to alternating current, using inverters, if the power has to be wheeled onto a grid. Inverter costs have fallen and off-grid PV is also popular.
It is thanks to various forms of policy support (tax breaks, high tariffs for solar, no wheeling charges,and so on) that solar PV has become more or less competitive with conventional power. Under ideal weather conditions, in places such as Germany, solar can be cheaper than conventional power.
Solar energy capacity has grown at 20 per cent a year over the past 15 years, thanks to falling prices and efficiency gains. Japan, Germany, the US and China are major markets. Given tax incentives, solar power tends to hit break-even somewhere between five to 10 years in these markets.
India also holds vast solar potential. The country receives 3,000 hours of annual sunshine on average (over eight hours a day). India is rapidly building capacity, thanks to the central Jawaharlal Nehru National Solar Mission (JNNSM) and the solar policies of many states.
The first JNNSM target is an installed solar capacity of 20,000 megawatts (Mw) by 2020, which would then increase to 100,000 Mw by 2030 and to 200,000 Mw by 2050. The implication is that India's solar capacity could grow at a CAGR of 40 per cent over the next five to six years. Recent solar auction tariffs in India come in at about Rs 6.5/ kilowatt-hour, which is higher than coal-based thermal auction rates (around Rs 5.5), but lower than the cost of diesel-generated power.
Small-scale solar PV on rooftops are already common. In a power-deficient country like India, individuals compare the cost of solar panels to that of diesel gensets. Solar is still expensive in terms of initial capital expenses. But if excess solar power can be wheeled onto the grid, the proposition becomes very attractive.
Some interesting variations have developed around the efficient deployment of solar PV. For example, thin-film solar PV can be directly integrated into roofs at construction. Smart trackers keep panels oriented toward direct sunlight and increase conversion efficiency.
Concentration is another very useful technology. Using lenses and mirrors, sunlight can be concentrated to generate much higher energy. Schoolchildren use the same principle to set fire to paper by using a magnifying glass to focus sunlight. Archimedes was said to have designed a giant glass to set fire to ships.
Concentration is often used for solar thermal power, and less commonly for PV. Solar energy is concentrated to heat water and turn it into steam, which can drive a turbine to generate power. One of the technical problems for using PV concentration is that it causes heat of an order that can destroy or rapidly degrade silicon PV cells. Solar thermal can also be used for water-heating systems without producing electricity.
Solar technologies do require a lot of space. This is one limitation. The state of Gujarat found an innovative answer by putting PV arrays up on canals and waterways. Another issue is that the mining of materials such as silicon and rare earth metals and the manufacture of solar equipment can involve high environmental impact.
Panels cut off sunlight, which means visibility issues. The Michigan State University, US, has just demonstrated transparent solar concentrators, which will not block vision. But it will take a while before transparent panels are available for commercial deployment.
Concentrated solar power technologies can cause other environmental issues. It can kill birds in vast numbers, literally frying them if they fly into a concentrated beam of light. A major project in America's Mojave Desert has highlighted this problem. The BrightSource plant at Ivanpah, California cost $2.2 billion to build. It sprawls across 1,600 hectares and uses 350,000 large mirrors to focus sunlight onto three boiler towers, which together generate a total of about 392 Mw. The beams are deadly for birds, and according to one estimate, about 28,000 birds are killed every year at this plant.
In India as well, environmental concerns have raised their head and there may be roadblocks in plans to build ultra-large solar plants with capacities of 500 Mw-plus, The Rajasthan government may cancel a 4,000-Mw solar ultra megapower project, which was to be located near Sambhar Lake. The 23,000-acre area is a prime site for migratory birds. (It has been alleged that politics also plays a part in the decision to cancel.)
Nevertheless, India is looking to multiply solar capacity by eight to 10 times by 2020. Given the fall in solar equipment costs, a lot of small-scale capacity will develop. As solar panels become ubiquitous, they would change architectural and design norms over the next five to 10 years.