“. . . all the coal deposits of Pennsylvania would not liberate a thousandth part as much heat as does the surface of the sun in that unit of time,”
- Samuel P. Langley, 1880
The Mechanics of Energy
Energy is the capacity of matter to perform work. Energy exists in multiple forms - mechanical, thermal, chemical, electrical, radiant, and atomic. One form of energy can be converted into any other form of energy if exposed to the appropriate processes. For example, sunlight, a form of radiant energy, is converted into carbohydrates, a form of chemical energy, by plants through a process called photosynthesis. Animals transform chemical energy stored in plants into either kinetic energy (physical movement) or the chemical bonds - a second form of chemical energy - that hold together a living person's body. Otherwise, plants die and over eons of time morph into fossil fuels like oil and natural gas.
Synopsis of Solar Energy
The Sun is about 900,000 miles across and is at least 10 million degrees at its center. The surface of the sun is roughly 6,000°C and its hot gases emit light that has a spectrum ranging from the ultraviolet, through the visible, into the infrared. Photovoltaic or solar cells convert solar power directly into electrical power. Light consists of discrete particle-like packets of energy called photons. Sunlight contains photons with energies that reflect the sun’s surface temperature; in energy units of electron volts. The energy density packed into the photons vary, but the visible region of the light spectrum tends to contain among the highest concentrations of energy that hits the planet.
More energy from sunlight strikes the Earth in one hour than all the energy consumed on the planet in a year. At high noon on a cloudless day, the surface of the Earth receives 1,000 watts of solar power per square meter. Sunlight provides by far the largest of all carbon-neutral energy sources. Heat travels in all directions from the Sun and is the ultimate source of all energy on Earth. This energy is responsible for all sorts of weather events, not only scorching heat waves. For instance, wind occurs when sunlight heats the ground, which heats the air above it, which rises, so that cool air whisks in to take its place.
NASA Map of World Solar Energy Potential
In the past decade, solar energy has attracted significant attention from investors, policymakers and the public generally because it is widely available, geopolitically secure and environmentally sustainable. Indeed, solar energy does not create greenhouse gases as a byproduct of generating electricity. Not surprisingly, it is widely considered among the most compelling solutions available for the world's need for clean, abundant sources of energy. Skeptics need only consider the $7.5 billion solar-energy industry - still growing at a rate of more than 30% every year — to appreciate the growing popularity of solar energy in mainstream electricity markets. Still, in 2001, solar electricity provided less than 0.1% of the world's electricity.
What "Efficiency" Means in the Solar-Energy Sector?
The efficiency of a solar cell is a measure of its ability to convert the energy that falls on it in the form of EM radiation into electrical energy, expressed as a percent. The power rating of a solar cell is expressed in watts, as either as peak watt (Wp), which is a measure of maximum possible performance under ideal conditions, or under more real-life conditions including normal operating cell temperature and AMPM (whole day rather than peak sunshine) standard ratings. The following chart shows solar-efficiencies for several of the leading-edge solar cell technologies.
Solar Cell Efficiency Increase from 1976 to 2004
Solar Water Heating
Energy used for water heating is a significant fraction of the total energy demand in the commercial and residential sectors. In 2004, water heating in the residential sector consumed about 23% of all residential natural gas use, 8% of all residential electricity use, and about 12% of total residential energy expenditures. 3 Nationwide, about 8% of all end-use natural gas is used to heat water in commercial and residential buildings
In the 1890s, solar water heaterswere being used all over the United States. They proved to be a big improvement over wood and coal-burning stoves. Artificial gas made from coal was available too to heat water, but it cost 10 times the price we pay for natural gas today. Many homes used solar water heaters. In 1897, 30 percent of the homes in Pasadena, just east of Los Angeles, were equipped with solar water heaters. As mechanical improvements were made, solar systems were used in Arizona, Florida and many other sunny parts of the United States. By 1920, ten of thousands of solar water heaters had been sold. By then, however, large deposits of oil and natural gas were discovered in the western United States. Gradually, low-cost fossil fuels made their way into the mainstream and replaced most solar water systems.
The oil shocks and energy crises of the 1970s restored the appeal and policy support for solar-water heating in the United States, which expanded significantly in the late 1970s and early 1980s as a result of increasing energy prices and generous tax credits. When federal tax credits ended in 1985 and energy prices fell, the U.S. market for solar-water heating virtually disappeared. Nearly three decades later, the combined influence of public concern about climate change and the anticipation of rising energy prices in the near-term future has sparked a second revival of solar-water heating. In particular, the state of Hawaii - which has the highest electricity prices of any state in the nation - has promoted solar-water heating with a range of government incentives for consumers and producers.
The Future of Solar-Energy
Solar power is in rapid growth mode; new manufacturers and installers of photovoltaic solar cell systems are cropping up everywhere. With present technologies, even assuming continued rapid growth, solar cells are predicted to only supply about 5% of the huge amount of carbon-free energy we will need by 2050.
Most present production of solar power is based on crystalline silicon cells, the first generation technology. The second generation, now starting to be commercialised, is based on thin-film cells and cells made from inexpensive oxide semiconductor materials coated with light sensitive dyes and from photoactive organic polymeric materials. These approaches may yield much lower costs, but at present have significantly lower conversion efficiencies.
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