# Solar Energy

November 11, 2010, 6:13 pm
 Topics:

This article was created as part of the Student Science Communication Project, by a student from Allegheny College. All SSCP articles have been reviewed by internal EoE editors, and by independent experts on each topic.

## History of Solar Energy

As early as 7th Century B.C. humans were harnessing the sun for solar energy. The Greeks and Romans were the first to use concentrated solar rays to ignite fires, illuminate their buildings, and even as military weapons. As early as the 1st Century, humans were taking advantage of solar energy by building homes with south-facing windows to provide heat. In 1767, Horace de Saussure created the world’s first solar collector to cook food and became the first individual to create a device that actively took advantage of the sun’s rays. In the 19th Century progress was made towards the mechanical conversion of light into energy, with the discovery of the photovoltaic effect. Also discovered during this time period was the photoconductive nature of selenium, that when exposed to light, generates an electric current. This principle is the foundation of today’s solar panel technology. In 1891, the first solar water heater was invented, which is an early predecessor to today’s solar thermal technology. In 1954, Bell Laboratories patented the first solar photovoltaic technology capable of creating enough electricity to power electronic devices. It was only 4-11% efficient. The 1970s witnessed a significant reduction in the price of solar cells, with the cost falling from around $100 per watt to around$20 per watt.

Along with a significant price reduction, the 1970s witnessed the introduction of solar technology into housing and space technology in the United States. In addition, research and development continued with the creation of the Solar Energy Research Institute, which is a branch of the Department of Energy specializing in solar energy. In 1982, Solar One came online near Barstow, California. The power plant was capable of generating 10 megawatts of electricity through the use of solar-thermal, proving that solar power is a capable and viable source of large-scale electrical power generation. Photovoltaic (PV) technology also continued to increase in power and efficiency during this time period. Pacific Gas and Electric introduced a 500-kilowatt PV system in 1993, which showed that PV could be effectively tied into the national electric grid to provide energy. From this expensive large-scale implementation in 1993, PV technology costs decreased so much that in 2001 Home Depot began selling solar power systems in some of their California stores. Over the past 10 years there has been drastic growth in both the PV and solar-thermal manufacturing industries. Scientists believe that the impressive growth seen thus far will continue into the future, providing less-expensive and more widely available solar technologies.

## Solar Energy Technologies

### Photovoltaic

The utilization of the photoelectric effect, which was first discovered in 1839 by Edmund Becquerel, is a technique used to generate electricity from solid materials such as crystalline silicon or selenium. Crystalline silicon was the first material to be successfully used in solar PV devices and is still the most widely used material today. These conductive materials convert sunlight directly into an electric current, which can then be fed into the power-grid.

When light strikes a PV cell, three things happen: the light can pass through, reflect off of, or be absorbed into the cell. The absorbed light is converted into electricity. When light photons strike a PV cell, they cause the electrons within the crystalline silicon (or similar material) to become “excited.” These “excited” electrons contain extra energy and escape from their normal positions around the crystalline silicon atoms. Once they have left their normal positions around the atom, they are now free to travel into the electric power grid to power our homes and businesses. These electrons are able to escape their normal positions around the silicon atoms because there is a built-in electric current within the PV cell. This electric current is the “magic” behind the functionality of the PV cell. The current is created by two types of silicon one of which has an excess of electrons, known as “n- type” silicon. The other type of silicon lacks electrons, and is called “p-type” silicon. Photons hit the “n-type” silicon and excite those electrons, causing them to move to the “p-type” silicon. This creates an electric field as the electrons move between the “n-type” and “p-type” silicon along what is termed the "p/n junction." The electric field then propels the excited electrons onto the negative surface of the “n-type” silicon where they enter into the electric circuit and are therefore accessible to the electric grid.

### Solar Thermal

Solar thermal technology has a wide range of applications, from small-scale home units to large-scale power generation facilities. In its most basic form, solar thermal technology is used to heat water. How that water is utilized depends upon the system that it is in. There are several types of solar collectors that heat water, including flat-plate collectors, evacuated tube collectors, and integral collector-storage systems.

There are two types of flat-plate collectors, one of which is a liquid-plate collector. These systems are dark boxes containing pipes filled with water. As the sun’s radiation strikes the dark box, it heats the water within the pipes that are in the box and a pump circulates this water throughout the entire system. These systems are often used for heating pools. The second type of flat-plate collector is an air flat-plate collector. This system works similarly to the liquid-plate collector, except rather than moving water this system moves air. The thermal energy is transferred to the air and thus creates a heating system.

At the most basic level, evacuated-tube collectors are similar to liquid-plate collectors, except the tubing which carries the water has a vacuum-sealed layer surrounding it. The vacuum prevents conductive and convective heat-loss, thus making this system much more efficient than the typical liquid-plate collector.

Integral collector storage systems (ICS) work by pre-heating water before it enters into a conventional natural gas-powered water heater, thus less energy has to be expended in order to heat the water to the desired temperature.

Solar thermal technology can be used in applications much larger than the average household. These systems work similarly to the small-scale systems in that they use the sun’s energy to heat water. Once this water is heated to an optimal temperature, it is used to drive steam turbines, to create electricity that can be put into the electric grid.

## Future Possibilities

Solar technology has over the years been criticized for some of the limitations inherent within this technology. Typically, the solar units that generate power are immoveable and cannot function when there is no sunlight present. Efforts to remedy these fundamental flaws are currently being devised and implemented. To solve the issue of a solar panel’s inability to function while the sun does not shine, two scientists at MIT, Daniel Nocera and Matthew Kanan, have developed a technique to harness the sun’s energy for the “off hours” by using the electricity generated by the solar panels to spilt water into oxygen and hydrogen. The product of this process, hydrogen, could then be used to power a fuel cell connected to a house or building’s electricity system, thus time shifting the availability of the sun’s energy. If fuel cell technology for cars becomes more cost effective, this technology could also be used to supply the hydrogen needed to power vehicles.

One of the hundreds of companies currently trying to improve solar technology is Suntech Power Holdings Co. Their aim is to design a solar tracking system that can have a solar collecting unit mounted upon it. The success of their system depends on their ability to include a sturdy articulation joint. If the company is successful in achieving their vision, this product could increase the amount of solar energy that the panels can collect throughout the day, allowing for the solar paneling to operate at full output all day (assuming the weather conditions are suitable).

Presently, solar technology is in a state of rapid evolution. As the technology becomes more efficient, there will be increasing opportunities to expand the technology into other applications, in ways currently not possible. For example, in addition to the technology being used to power buildings, the technology might to power cars, both as the main source of energy and as a supplement to standard fuel and power sources. Also, while solar technology is currently used to generate electricity on a large scale, there is tremendous potential to increase that amount, supplanting the growing need to consume fossil fuels in the production of energy.

Glossary