By Laura J. Bottomley, NC State University
Generators are used to convert energy from other forms into electrical energy. They’re how we get the electric current that’s distributed all over the electric grid. Most generators require some kind of force to turn the shaft. Then the turbines can be turned by the steam created from heat. That heat can come from many different sources, usually by burning them.
Sources Used to Generate Electrical Energy
Many early generators used coal, and many still do. Coal is burned to heat water and create steam. That steam turns the turbine, which is connected to the shaft of the generator. The electricity generated is then conveyed over power lines to end-users. In 2019, the US still used coal for about 23% of electricity production, while China used coal for 58% of electricity production.
Natural gas was the largest source of heat energy for making steam in US electricity production in 2019, at 38% of all sources. Nuclear was the third most common source of electrical energy in 2019, all of it nuclear fission.
Fission separates atoms into less complex particles and releases nuclear energy in the process. We use that nuclear energy to convert hot water into steam and to create the mechanical energy that turns the turbine and the generator shaft to create electrical energy.
There’s also hope for nuclear energy by fusion, which forces simple atoms together into larger atoms and also releases energy to create heat to create steam. This is one of the National Academy of Engineering’s Grand Challenges for the 21st century.
The Miracle of Solar Energy
With solar energy, it’s possible to create electrical energy directly, without needing to turn a turbine and generator as an intermediary. This is an example of a photovoltaic (or PV) solar cell made out of silicon. PV cells make use of a physical phenomenon called the photovoltaic effect.
When the photons of light strike certain types of materials, the hold of these materials on their electrons is loosened, and then the electrons can be made to move and become part of an electric current. Most solar cells are still made from silicon, but the materials that make good solar cells are very much under research. Researchers are looking at ways to make cells flexible, robust, and cheap because these cells are very stiff and can break easily.
A cell is combined with many others into solar panels so that the currents can get added up into larger, and more useful, amounts of current. The resulting current can be used to power devices directly, or it can be incorporated as a source of electricity onto the electric grid. This can also be done in different ways. The electricity could be used to drive a motor, which is attached to a generator. Alternatively, solar energy can be used to heat water and drive a turbine.
This article comes directly from content in the video series Electrical Engineering for Everyone. Watch it now, on Wondrium.
AC against DC
So why the need for intermediate steps? Why can’t electricity from solar energy be added directly onto the grid? Well, it’s because the electricity produced by solar cells travels in one direction only. It’s direct current or DC. Our electric grid supplies the alternating current of AC. Electricity generated from solar cells has to be converted from DC to AC.
Individual homes that are off the grid could use DC entirely, but most homes that have solar panels on the roof usually have a device called an inverter that converts DC to AC. Inverters can range from watts to megawatts, and their physical size is decreasing rapidly as we start using new materials like silicon carbide.
Not Efficient Enough, Yet
White light contains different proportions of various light colors, depending on its source. Sunlight brings a nearly even distribution across the visible frequencies. It also has a fair amount of radiation at both the shorter wavelengths of ultraviolet and the longer wavelengths of infrared. The glass that covers most solar cells actually blocks ultraviolet radiation.
Long-wave ultraviolet, sometimes known as “blacklight,” and it’s the least dangerous form of ultraviolet. UV-B is more energetic. But a lot of UV-B is blocked by the Earth’s atmosphere. UV-C is the most dangerous to life, which is why germicidal lamps are made with UV-C. But UV-C is entirely blocked by the Earth’s atmosphere, so solar cells on Earth can ignore it.
Of course, human-made light sources do not come anywhere near sunlight in the energies they offer across different frequencies. Solar cells made of different materials differ in their ability to convert various frequencies of sunlight. In fact, one reason you might prefer a specific solar cell over another is its ability to use a wider range of frequencies of light.
Indeed, one of the ways that solar cells are inefficient is in whether they’re able to convert light to electricity across the entire available spectrum of their light sources. In the year 2020, even the most efficient solar cells were only about 20% efficient. This is another reason that researchers are looking at how to make solar cells from different types of materials, even studying the natural process of photosynthesis used by leaves to convert solar energy to other types of energy.
Common Questions about the Problem of Generating Electrical Energy
Many early generators used coal to function, and some generators still do. This is done by burning the coal, which creates steam. This steam is used to turn the turbine which is connected to the shaft of generators, ultimately resulting in electrical energy being delivered to end-users.
The electricity that solar cells produce in DC, which means it only travels in one direction. On the other hand, our electrical grid supplies use alternative currents. But if someone’s home is off the grid entirely, they could hypothetically use direct current to obtain electrical energy.
The energy of the three forms goes from low to high in this order: Long-wave ultraviolet light or blacklight, UV-B, and UV-C. UV-C is most dangerous to life but is blocked by the Earth’s atmosphere. Most solar cells that produce electrical energy are covered with glass that blocks ultraviolet radiation.