Humans are dependent on energy. To maintain our highly ordered lives, a great amount of energy must be expended. Ultimately, this energy comes almost entirely from the sun (with the exception of geothermal, tidal, and a few other minor sources). Fossil fuels gained their energy from the plants and animals from which they were made. Wind power takes advantage of the temperature gradients caused by the sun’s heat. This list of sun-derived energy sources goes on and on.
However the reserves of some of these energy sources do not go on and on. It is estimated that oil reserves will be depleted in less than fifty years and coal will only last a few hundred. This means we seriously need to consider alternatives that do not rely on or produce long term effects. Hydroelectric power is one such example. However, over 50% all of the available waterways in the U.S. are currently utilized while hydroelectric sources only supply 7% of the power in the U.S.
So what about solar cells? What exactly do they do and how realistic are they as a possibility of power? The answers to these questions are complicated, but the general situation can be easily understood without any complex explanation.
Over 1000 watts falls on a square meter of ground if the sky is clear and the sun is directly overhead. If all of this light could be converted to usable energy, just over 1 horsepower would be produced. However, these two sentences neglect the most serious limitations of solar power. Lighting conditions at a location at any given time are far from ideal. The sky can be overcast and the sun can be at much lower angles in the sky. These situations can easily reduce the light intensity by a factor of 2 or 3. The next stumbling block is solar energy conversion efficiency. The most efficient solar cells ever made convert around 30% of light energy into usable electrical energy. These are only made in small sizes at great costs in laboratories. Cells that can be produced in large quantities are of much lower efficiencies, usually around 15%.
It may seem that these efficiencies limit the potential of solar cells for power production. However, the efficiencies are actually quite high when compared with other methods of solar energy conversion. Plants, which are a common fuel source, convert sunlight to chemical energy through photosynthesis with an efficiency of about 1-3%. The conversion of this chemical energy to usable energy (via burning) is only around 30% (due to carnot limitations). The combined efficiency is much lower than that of solar power, which means a much greater are would be required to grow all of our fuel than would be required by a photovoltaic collector with the same output. If all of the energy consumed in the U.S. was supplied by solar collection systems with roughly 15% efficiency, about 2% of the land area would be used.
So if we could get all of our energy by covering 2% of the country with solar collectors, why aren’t we doing it? Because it would be a lot more expensive than power from fossil fuels. But, the situation is changing. Advances in technology are being made that lower the cost solar collection systems. One such advance is the development of high efficiency cells that can utilize light concentrated by mirrors. These cells have efficiencies between 20% and 25% and can operate under light that is around 100 times as intense as normal sunlight. In addition the cells must be cooled, and the heat extracted can be used for other purposes. The combination of these advances in technology and increasing oil prices may make solar power a cost-effective alternative in the future.
Whether or not solar collection systems become our prime power source in the future, there will always be demands for the longevity and simplicity associated with solar systems. Even now they are employed to serve in a range of applications. Satellites, remote power systems, roadside telephones, and solar glides are just a few examples. In order to maintain and expand solar systems in the future, knowledge about their design and functionality must be spread and advanced.