THE BASICS OF PHOTOVOLTAICS
THE BASICS OF PHOTOVOLTAICS
“Photovoltaic” refers to the creation of voltage from light, and is often abbreviated as just “PV”. A more common term for the photovoltaic cells is “solar cells”, although the cells work with any kind of light and not just sunlight.
A solar cell is a convertor. It changes energy of light into electrical energy. A cell does not store any energy, so when the source of light (typically the Sun) is removed, there is no electrical current from the cell. If electricity is needed during the night, some form of electrical storage (typically a battery) must be included in the circuit.
What are Solar Cells?
There are many materials that can be used to make solar cells, but the most common is the element silicon (This is not to be confused with “silicone” a synthetic polymer). Silicon is the second most abundant element in the Earth crust, next to oxygen, and silicon and oxygen together make quartz or common sand. It is therefore very abundant, as well as non-toxic and safe. This is the same silicon that is used to make computer ships and some of the processing steps involved in making solar cells are similar to the steps in making computer devices. However, solar cells are much larger than typical individual computer circuits, and they must be much less expensive! A typical solar cell used for terrestrial (Earth-based) applications is 3-6 inches in diameter and cost only a few dollars, whereas a tiny computer circuit device might be only a tenth of an inch in length and width and cost tens or hundreds of dollars.
The conversion process occurs instantly whenever there is light falling on the surface of a cell. And the output for the cell is proportional to the input light. The more light the greater the electrical output. The sunlight acts as a fuel for the conversion process and that fuel is delivered fee everywhere in the world. The solar resource is more uniformly distributed over the Earth’s surface than other renewable resources of energy like wind and hydro. These resources are plentiful in certain specific climates and geographic locations, but may depend on exact details of land contour and elevation.
Key benefits of Solar Electricity
i) Energy Independence
There are many others ways to generate electricity, but what are the specific benefits of solar generated electricity from photovoltaic cells. One of the most attractive benefits is that you will have “energy independence”, the ability to create your own electrical power, independence of fossil fuel supplies or utility connections.
ii) Fuel is already delivered
In a sense, you do need sunlight as the “fuel”, but that is already delivered for free all over the planets surface. Other conventional generation methods require access to a site for fuel deliveries. This may limit the choice of suitable sites so as to have road access, and even then access may be prevented due to poor road conditions, or vehicle problems. The cost of delivering fuel to remote locations can be substantial.
iii) Minimal maintenance
Solar electric systems typically require very minimal maintenance because there are so few moving parts. Contrast this with a diesel-powered system, or even other renewable source such as wind generator or hydro generators, which often have costly repairs or regular maintenance of moving parts. Very complex photovoltaic systems do have more parts and may require some maintenance. But when looking at small power requirements, such as for home lighting or remote telecommunications systems, only occasional battery maintenance is required. It would be a mistake to say that photovoltaic systems required no maintenance, but the absolute amount of time and money required for photovoltaic systems is quite low.
iv) Maximum reliability
This is perhaps the primary advantage of photovoltaic when compared to any other form of electrical power generation. Because there are typically few or no moving parts and the complexity of the systems can be kept low, the ultimate reliability of photovoltaic power systems in the real world is quite high. Environmental effects such as lighting strikes, high winds or blowing sand, humidity and heat, or snow and ice, do not affect the photovoltaic generator. The key to reliability is quality and simplicity. If high quality components are used with the solid-state solar generators, and if the component count and complexity of the system design are kept to a minimum, the chance of any failure occurring is remarkably low.
v) Generate where needed
You can think differently about designing power systems for your loads, and not always have to consider a central generator large enough for all your current demands. You can distribute the generation of power t various sites, such as at each classroom, or each house, rather than always having to install a large generator and string power lines to individual users that might be separated by great distances.
Telecommunications systems designers can look to photovoltaic as a way to perhaps be more selective with the locations of their repeaters. For example, an engineer might be considering covering a certain area with repeaters, and think that he is forced to choose sites that are easily access, so that diesel fuel can be delivered and maintenance can be performed. But the sites may not be the optimum for coverage of the area. Instead, by choosing to use photovoltaic power for his sites, he may now consider more remote, inaccessible sites, and may actually be able to install fewer total repeated but end up giving the same cove that the more numerous accessible sites would give.
Because photovoltaic generators can be as small as a few watts, you can truly consider installing just the amount of power that you need at each site. This flexibility is not available from other forms of generation.
vi) Reduced Vulnerability
Because you can avoid stringing ling power lines for many miles or kilometers from some central generation source, many of the problems with utility power losses can be avoided. Ice storms or vehicle accidents can cause power lines to go down, perhaps tens or hundreds of miles from where the power is actually needed. With a reliable photovoltaic power system at your site, you could still have power, while others around you have none.
And if you have chosen to distribute the generation of power to various load sites at your location, you can insure even more reliability and less vulnerability to each load site. For example, if separate homes have their own lighting system, and one user over discharges their batteries, or damages their system, the other uses will be unaffected.
vii) Easily expanded
Photovoltaic power generators are modular by design. More power can be added to an existing array easily. Old modules can be added to new ones without any penalty. Just enough power can be purchased and installed today to meet your current needs, and as demand grows more modules can be added in later years. This also means that financially it is easy to start with a minimal power system today, and then add to the power as your budget allows later.
How Solar Cells Work
People often say that solar cells work by “magic” because there is nothing moving, the result is instantaneous, and no fuel is apparently needed! The basic process by which solar cells convert sunlight into electricity can seem “magical”, but actually is simple.
Internal Field and Electron Flow
Most typical solar cells are made of the element silicon. When light shines on a solar cell, the energy of the light actually penetrates into the solar cell, and on a random basis, “knocks” negatively charged electrons loose from their silicon atoms. To understand this, we can think of light as being made of billions of energy particles called “photons”. The incoming photons act much like billiard balls, only they are made of pure energy! When they collide with an atom, the whole atom is energized, and an electron is ejected or ionized from the atom.
The freed electron now has extra potential energy, and this is what we call “voltage” or electrical “pressure”. The freed electron has energy that could be used to charge battery or operate an electric motor for example. But the problem is how to get the freed electron out of the solar cell. This is accomplished by creating an internal electro-static field near the front surface of the cell during manufacturing. Other materials besides the basic silicon are “grown” onto the silicon crystal structure. They create an electrical imbalance that results in a one-way electrical “broom” that “sweeps” the freed electrons out of the solar cell and pushes them on to the next cell, or on to the load.
As billions of photons flow into a cell that is exposed to light, billions of electrons are knocked loose and gain extra energy. They flow through the internal electro-static field and out of the cell or modules. This flow of electrical charges with extra potential energy or voltage is what we call “electrical current”.
Cells into Modules
Because typical silicon solar cells produce only about ½ volt, we need to connect cells together to give more useful voltages. When electrical generators are connected together in “series”, or positive to negative, the voltage of each generator adds up.
Usually 30-36 solar cells are connected together in a circuit to give a final voltage of about 15-17 volts, which is enough to charge a 12-volt battery. Charging batteries are the primary use fir photovoltaic modules, so most are designed around doing that job. But manufactures could produce different module designs to better match other loads, for example high voltage motors for water pumps or utility connected systems that often operate at hundreds of volts.
If the voltage or current from one module is not enough to power the load, then modules can also be connected together, just as the cells were. Manufactures usually build modules with convenient junction boxes that allow interconnecting in series or parallel.
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