Solar, Wind and Tidal Powered Cell.

Made by Abhishek Dey Das and Aveek Mukherjee for Kendriya Vidyalaya Science Exhibition.

Introduction

Today’s world is useless without energy. The trains, planes, computers and other modern gadgets and devices require electricity, the concept of  which has been prevalent since 150 BC when the Hero Reaction was performed. Our cars, buses and modern means of transport all require petroleum to run. Most of these are run on conventional sources of energy because it is cheap and easily available. But the de-merits brought in the train of conventional source of power is often overlooked.

So what exactly are the de-merits of using a conventional source of energy? Let’s take a glance……

De-merits of Diesel Power:

1)the cost of fuel is high.

2) comparatively high plant costs per kilowatt.

3) Site should be near sources of fossil fuel,

Health Hazard due to exposure to Total Petroleum Hydrocarbons (TPH):

Health effects from exposure to TPH depend on many factors.  These include the types of chemical compounds in the TPH, how long the exposure lasts, and the amount of the chemicals contacted.  Very little is known about the toxicity of many TPH compounds.  Until more information is available, information about health effects of TPH must be based on specific compounds or petroleum products that have been studied. 

The compounds in different TPH fractions affect the body in different ways.  Some of the TPH compounds, particularly the smaller compounds such as benzene, toluene, and

xylene (which are present in gasoline), can affect the human central nervous system.  If exposures are high enough, death can occur.  Breathing toluene at concentrations greater than 100 parts per million (100 ppm) for more than several hours can cause fatigue, headache, nausea, and drowsiness. When exposure is stopped, the symptoms will go away.  However, if someone is exposed for a long time, permanent damage to the central nervous system can occur.  One TPH compound (n-hexane) can affect the central nervous system in a different way, causing a nerve disorder called "peripheral neuropathy" characterized by numbness in the feet and legs and, in severe cases, paralysis. This has occurred in workers exposed to 500–2,500 ppm of n-hexane in the air.  Swallowing some petroleum products such as gasoline and kerosene causes irritation of the throat and stomach, central nervous system depression, difficulty breathing, and pneumonia from breathing liquid into the lungs.  The compounds in some TPH fractions can also affect the blood, immune system, liver, spleen, kidneys, developing fetus, and lungs.  Certain TPH compounds can be irritating to the skin and eyes.  Other TPH compounds, such as some mineral oils, are not very toxic and are used in foods.

One TPH compound (benzene) has been shown to cause cancer (leukemia) in people.  The International Agency for Research on Cancer (IARC) has determined that benzene is carcinogenic to humans (Group 1 classification).  Some other TPH compounds or petroleum products, such as benzo(a)pyrene and gasoline, are considered to be probably and possibly carcinogenic to humans (IARC Groups 2A and 2B, respectively) based on cancer studies in people and animals.  Most of the other TPH compounds and products are considered not classifiable (Group 3) by IARC. 

Usually coal used to produce electricity could be of any type. Types such as bituminous coal may contain fossil of ancient animals and may be lost while making coal. But that’s not the greatest immediate threat to us. Let’s take a look at the graph below:

The graph shows the average temperature of the northern hemisphere over the past 1000 years. During the first 800 years, the graph shows a gradual decrease, which is thought to have been caused by the way the tilt in the Earth's axis changes in relation to the Sun and the Earth's orbit around the Sun. The last 100 years show a sudden increase in temperature which is attributed by many to greenhouse gases.

Global warming is usually caused by green house gases such as water vapor, carbon dioxide, methane, and then other lesser greenhouse gases (in the decreasing order of their magnitude). Since in a fossil fuel powered form of electricity, coal is burnt to heat water so that it can turn a turbine, there should be no problem in guessing from where all the green house gases come from. Let’s look at the temperature variation curves for the last 140 and 1000 years.

These graphs show a very discernable warming trend, starting in about 1900. It might seem a bit surprising that warming started as early as 1900. How is this possible? The reason is that the increase in carbon dioxide actually began in 1800, following the deforestation of much of Northeastern American and other forested parts of the world. The sharp upswing in emissions during the industrial revolution further added to this, leading to a significantly increased carbon dioxide level even by 1900.

Thus, we see that Global Warming is not something far off in the future - in fact it predates almost every living human being today.

Let’s take a look at another graph showing the in pour of CO2 (carbon di oxide) and one of the primary factors of which is burning of coal to produce electricity.

The solution to all the above problems is of course a non-conventional source of electricity. Our country has been blessed by many sources of non-conventional energy, which if properly trapped can work wonders. To name a few, there are solar energy, tidal energy, bio-gas energy, wind energy and others. Efforts have already been made and is even being made on a daily basis to utilize these to the fullest. But no single conventional energy can provide or supply electricity continuously twenty-four hours a day because there might be not enough wind to rotate the blades of a wind-mill or no solar energy at night. So, the most prudent possibility is then to go back to the conventional fossil fuels for electricity.

Our project deals with making electricity from non-conventional sources and supplying the electricity continuously for twenty-four hours. Electricity generated from a single source will never be able to do this, so the next possibility is to combine different sources. That will allow supply of power for the whole day and we would be looking towards a greener, better and pollution free world.

SWT includes Solar, Wind and Tidal Powered Cells all combined together.

Solar Energy

Converting Photons to Electrons

The solar cells that you see on calculators and satellites are photovoltaic cells or modules (modules are simply a group of cells electrically connected and packaged in one frame). Photovoltaics, as the word implies (photo = light, voltaic = electricity), convert sunlight directly into electricity. Once used almost exclusively in space, Photovoltaics are used more and more in less exotic ways. They could even power your house. How do these devices work?

Photovoltaic (PV) cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. PV cells also all have one or more electric fields that act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

That's the basic process, but there's really much more to it. Let's take a deeper look into one example of a PV cell: the single crystal silicon cell.

Silicon in Solar Cells

Silicon has some special chemical properties, especially in its crystalline form. An atom of silicon has 14 electrons, arranged in three different shells. The first two shells, those closest to the center, are completely full. The outer shell, however, is only half full, having only four electrons. A silicon atom will always look for ways to fill up its last shell (which would like to have eight electrons). To do this, it will share electrons with four of its neighbor silicon atoms. It's like every atom holds hands with its neighbors, except that in this case, each atom has four hands joined to four neighbors. That's what forms the crystalline structure, and that structure turns out to be important to this type of PV cell.

We've now described pure, crystalline silicon. Pure silicon is a poor conductor of electricity because none of its electrons are free to move about, as electrons are in good conductors such as copper. Instead, the electrons are all locked in the crystalline structure. The silicon in a solar cell is modified slightly so that it will work as a solar cell.

Our cell has silicon with impurities -- other atoms mixed in with the silicon atoms, changing the way things work a bit. We usually think of impurities as something undesirable, but in our case, our cell wouldn't work without them. These impurities are actually put there on purpose. Consider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms. Phosphorous has five electrons in its outer shell, not four. It still bonds with its silicon neighbor atoms, but in a sense, the phosphorous has one electron that doesn't have anyone to hold hands with. It doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place.

When energy is added to pure silicon, for example in the form of heat, it can cause a few electrons to break free of their bonds and leave their atoms. A hole is left behind in each case. These electrons then wander randomly around the crystalline lattice looking for another hole to fall into.

These electrons are called free carriers, and can carry electrical current. There are so few of them in pure silicon, however, that they aren't very useful. Our impure silicon with phosphorous atoms mixed in is a different story. It turns out that it takes a lot less energy to knock loose one of our "extra" phosphorous electrons because they aren't tied up in a bond -- their neighbors aren't holding them back. As a result, most of these electrons do break free, and we have a lot more free carriers than we would have in pure silicon. The process of adding impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon is called N-type ("n" for negative) because of the prevalence of free electrons. N-type doped silicon is a much better conductor than pure silicon is.

Actually, only part of our cell is N-type. The other part is doped with boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type silicon ("p" for positive) has free holes. Holes really are just the absence of electrons, so they carry the opposite (positive) charge. They move around just like electrons do.

So where has all this gotten us? The interesting part starts when you put N-type silicon together with P-type silicon. Remember that every PV cell has at least one electric field. Without an electric field, the cell wouldn't work, and this field forms when the N-type and P-type silicon are in contact. Suddenly, the free electrons in the N side, which have been looking all over for holes to fall into, see all the free holes on the P side, and there's a mad rush to fill them in.

Before now, our silicon was all electrically neutral. Our extra electrons were balanced out by the extra protons in the phosphorous. Our missing electrons (holes) were balanced out by the missing protons in the boron. When the holes and electrons mix at the junction between N-type and P-type silicon, however, that neutrality is disrupted. Do all the free electrons fill all the free holes? No. If they did, then the whole arrangement wouldn't be very useful. Right at the junction, however, they do mix and form a barrier, making it harder and harder for electrons on the N side to cross to the P side. Eventually, equilibrium is reached, and we have an electric field separating the two sides.

The effect of the electric field in a PV cell

This electric field acts as a diode, allowing (and even pushing) electrons to flow from the P side to the N side, but not the other way around. It's like a hill -- electrons can easily go down the hill (to the N side), but can't climb it (to the P side).

So we've got an electric field acting as a diode in which electrons can only move in one direction. Let's see what happens when light hits the cell.

When Light Hits the Cell

When light, in the form of photons, hits our solar cell, its energy frees electron-hole pairs.

Each photon with enough energy will normally free exactly one electron, and result in a free hole as well. If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to their original side (the P side) to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage, we have power, which is the product of the two.

How much sunlight energy does our PV cell absorb? Unfortunately, the most that our simple cell could absorb is around 25 percent, and more likely is 15 percent or less. Why so little?

Energy Loss

Why does our solar cell absorb only about 15 percents of the sunlight's energy? Visible light is only part of the electromagnetic spectrum. Electromagnetic radiation is not monochromatic -- it is made up of a range of different wavelengths, and therefore energy levels. (See How Special Relativity Works for a good discussion of the electromagnetic spectrum.)

Light can be separated into different wavelengths, and we can see them in the form of a rainbow. Since the light that hits our cell has photons of a wide range of energies, it turns out that some of them won't have enough energy to form an electron-hole pair. They'll simply pass through the cell as if it were transparent. Still other photons have too much energy. Only a certain amount of energy, measured in electron volts (eV) and defined by our cell material (about 1.1 eV for crystalline silicon), is required to knock an electron loose. We call this the band gap energy of a material. If a photon has more energy than the required amount, then the extra energy is lost (unless a photon has twice the required energy, and can create more than one electron-hole pair, but this effect is not significant). These two effects alone account for the loss of around 70 percent of the radiation energy incident on our cell.

Why can't we choose a material with a really low band gap, so we can use more of the photons? Unfortunately, our band gap also determines the strength (voltage) of our electric field, and if it's too low, then what we make up in extra current (by absorbing more photons), we lose by having a small voltage. Remember that power is voltage times current. The optimal band gap, balancing these two effects, is around 1.4 eV for a cell made from a single material.

We have other losses as well. Our electrons have to flow from one side of the cell to the other through an external circuit. We can cover the bottom with a metal, allowing for good conduction, but if we completely cover the top, then photons can't get through the opaque conductor and we lose all of our current (in some cells, transparent conductors are used on the top surface, but not in all). If we put our contacts only at the sides of our cell, then the electrons have to travel an extremely long distance (for an electron) to reach the contacts. Remember, silicon is a semiconductor -- it's not nearly as good as a metal for transporting current. Its internal resistance (called series resistance) is fairly high, and high resistance means high losses. To minimize these losses, our cell is covered by a metallic contact grid that shortens the distance that electrons have to travel while covering only a small part of the cell surface. Even so, some photons are blocked by the grid, which can't be too small or else its own resistance will be too high.

Finishing the Cell

There are a few more steps left before we can really use our cell. Silicon happens to be a very shiny material, which means that it is very reflective. Photons that are reflected can't be used by the cell. For that reason, an antireflective coating is applied to the top of the cell to reduce reflection losses to less than 5 percent.

The final step is the glass cover plate that protects the cell from the elements. PV modules are made by connecting several cells (usually 36) in series and parallel to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with a glass cover and positive and negative terminals on the back.

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