Active solar power technologies were first developed during the 1950s. At that time, solar power was advanced due to its use by NASA and the space industry. The energy crisis in the 1970s brought the idea of using solar power to provide electricity for individuals and industries on Earth to the forefront, particularly when then-President Jimmy Carter installed solar panels on the White House in Washington, DC.
Solar technologies are characterized as either passive or active. Active technologies use photovoltaic panels or solar thermal collectors to convert solar energy to electrical energy.
Passive solar techniques include orienting a building to the sun, selecting materials with thermal properties and designing buildings that take advantage of natural properties of insolation.
The photoelectric effect is a property of certain materials that allows photons of sunlight to be absorbed and then released as free electrons. Capture of these free electrons results in an electric current. Through photovoltaics, sunlight is converted into electricity.
Solar power can be actively produced by heat engines or photovoltaics. Some applications for solar technologies include heating and cooling, potable water through distillation and disinfection, hot water, thermal energy for cooking and process heat for industrial purposes. Solar energy can be scaled to individual use (such as solar cookers or rooftop panels used to heat a home), to communities, or to industry in the deployment of solar concentrating power arrays.
Concentrating Solar Power (CSP) systems use lenses or mirrors and tracking to focus a large area of sunlight into a small beam. The concentrated light is used as an energy source for solar power plants and can provide what is called “process heat” for commerce and industry.
The most frequently deployed CSP technologies are solar trough, parabolic dish and solar power tower. In all CSPs a working fluid is heated by the concentrated beam and is then used to generate or store power.
A solar trough is an array of parabolic reflectors that concentrate light onto a receiver. The reflector is made to track the Sun. Trough systems provide the best land-use and are the most widely deployed and cost-effective CSP technology.
In a parabolic dish system a single parabolic reflector concentrates light onto a receiver. These systems track the Sun along two axes and have the highest efficiency among CSP technologies.
An array of tracking reflectors is used to concentrate light onto a central receiver in solar power tower technologies. Power towers offer higher efficiency and better energy storage capability than trough systems.
An impediment to widespread deployment of solar power is that solar energy is not available at night. We are accustomed to a continuous power supply and therefore storage is an important issue for solar energy deployment.
To solve this problem, thermal mass systems are being explored as an option to store solar energy as heat to account for daily or seasonal fluctuations. These systems use materials with high specific heat capacities such as water, earth and stone. Other materials such as paraffin wax and Glauber’s salt are also good thermal storage media, as are molten salts. These media are low-cost, have high specific heat capacity that can deliver heat at useable temperatures.
A barrier to the widespread deployment of solar energy technologies has been the cost of solar collectors that use expensive silicon crystals. Solar power via silicon technology is currently about five times the cost of obtaining electricity through coal. This technology is expensive because large silicon crystals are hard to grow.
Another problem with silicon cells is that they only have a power efficiency of about 22-23 %. Solar cells typically added to homes are about 15-18% efficient. The most efficient cells are those on satellites, which approach 50%. Emerging solar electricity technologies using smaller, cheaper crystals, such as copper-indium-gallium-selenide, that can be shaped into flexible films may be the solution to this problem.
Deriving energy from solar power is 85 times more efficient than growing corn for use as ethanol. On a single plot of land, enough ethanol can be produced to drive a car 30,000 miles per year. If the same acreage was covered with solar cells, the car could be driven 2,500,000 miles per year.
Manufacturing solar cells requires heavy metals such as lead, mercury and cadmium and produces some greenhouse gases. These heavy metals are toxic to our environment. However, researchers found that by switching from fossil fuel plants to solar power, air pollution would still be cut by about 90%. Even comparing direct emissions from production of cadmium telluride solar cells with coal power plants, toxic emissions would up 300 times lower.
Most of the toxic emissions from making solar cells come from fossil fuel-burning power plants, which provide the electricity needed for manufacture. If solar cell manufacturing factories used solar power to produce their own electricity, they could reduce toxic emissions dramatically.
An emerging experimental technology includes the development of solar cells that make fuel instead of electricity, much in the way that a plant can capture solar energy and change the chemical bonds to provide fuel (food) for itself. The implications for such a technology, were it to be developed and deployed, are that we could deal with global warming problems by pulling CO2 out of the air and using fuel-producing solar cells to “photosynthesize” our energy.
Another developing technology is the use of solar chimneys and ponds. A solar updraft tower (also called a solar chimney) is basically a large greenhouse. Sunlight shining on the greenhouse causes the air inside to heat and expand. The expanding air flows toward a central tower, where a turbine converts the air flow into electricity.
A solar pond is a pool of salt water 1′”2 m deep that collects and stores solar energy. Salts in the pond’s water create a density gradient that hampers convection. Layers of water successively increase from a weak salt solution at the top to a high salt solution at the bottom. Solar ponds produce temperatures of 90 C (194 F) in their bottom layer.
Growth in solar power technologies has averaged 40% per year since 2000. Solar water heating has increased on average 20% per year since 1999. Solar water heating is the most widely deployed solar technology.
Government incentives now make it attractive to install solar energy technologies in return for a power buy-back agreement. It is estimated that 50-90% of commercial installations in the past three years are taking advantage of these incentives. In Canada, incentives allow homeowners with solar panel installations to draw power from the grid for about 20¢/kWh. They may sell any excess that they produce back to the grid for about 41¢/kWh.
Clearly, the solar power field is ripe for development and deployment. Continuing development seeks to address the few disadvantages to solar power technologies and to help make the transition to renewable energy sources a reality for our country and our world.
