Photovoltaics
Photovoltaic devices convert light directly into electricity. Unlike so many other types of electricity generation, photovoltaics don't need to heat water to steam to turn a turbine to turn a generator.
Photovoltaics (PV) use the Sun's energy, electromagnetic radiation to produce electricity. Radiation is the product of the Sun's process of nuclear fusion. The Sun fuses together hydrogen atoms to form helium and the energy left over in the process is emitted by the Sun in packets of energy called quanta or more commonly called photons. Photons are particles that exhibit the properties of waves such as frequency, wavelength and amplitude. Photons travel through empty space at 186,000 miles per second, the speed of light, arriving at Earth eight minutes after their emission. When the photons reach Earth they immediately begin doing work by exciting the electrons in the various substances they encounter.
In 1886, Heinrich Hertz noticed that shining shorter wavelength light on metal caused the electrons of the metal to be emitted. A phenomenon called the photoelectric effect.
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This was the first example of converting light energy to electric energy.
The flow of electrons (electric charge) or current is what we call electricity. Our modern world is heavily reliant on electricity to do work, such as power our light bulbs, keep our food cold, and operate our televisions and radios. Electricity is considered a secondary energy source. This means that another energy source must be transformed to produce electricity. For the last hundred years we have used the thermal energy from burning fossil fuels to create electricity.
Atoms want to be in equilibrium. They want to have an equal number of protons and electrons resulting in a neutral charge. When an atom is missing an electron an electron from a neighboring atom is attracted to this void and can move into the vacancy. This leaves a vacancy of its own which must be filled by another electron. This constant flow of electrons is called current. What makes the electrons flow is the potential (charge) difference between the two atoms. The atom with the higher energy potential or more electrons will want to give away those electrons to the atom with a lower energy potential or less electrons. This potential difference is called voltage.
Electrons flow differently through different types of materials. The intensity of electron exchange is based on atomic structure. In the center of an atom exists the protons and neutrons. Orbiting the nucleus, at different energy levels are the electrons. The electrons in the outermost level are the electrons available for transfer and are called the valence electrons. At this energy level atoms can have multiple missing electrons, too many electrons, or just the right amount. This factor is the primary cause of electron movement. Ultimately all the atoms want to have just the right amount.
The materials that have just the right amount hold onto their electrons very tightly and don't allow them to flow. These materials are called insulators. Rubber, plastic, and wood are examples of insulators. The handles of many kitchen utensils are made of these materials to prevent the conduction of thermal energy. Other materials that have too many or not enough electrons, allow electrons to flow easily. These materials are called conductors. Metals such as copper, aluminum, and gold are examples of conductors. The cord that you plug into the wall from an electric appliance has a copper interior to allow electrons to flow, covered by a plastic coating to prevent those same electrons from making a circuit with your hand. There are some atoms that have very few missing electrons. Materials made of these atoms do not readily exchange electrons because they can make covalent bonds with neighboring atoms, but can be encouraged to do so. Silicon for instance has four valence electrons. With this arrangement, silicon can bond with four other silicon atoms to form a very ordered crystalline structure. Having all of its valence electrons bonded silicon has no affinity for giving up electrons. This behavior can be changed by doping the silicon with another element such as phosphorus or boron. Phosphorus has five valence electrons leaving one electron available for transfer. Boron has three valence electrons leaving a vacancy to be filled. Adding energy to these materials allows the unbonded electron to rise to the conduction band, which is a higher energy level than the valence level. In the conduction band the electron is free to leave the atom's orbit and to move throughout the material. These materials are called semiconductors. Because the flow of electrons can be controlled in these materials they have been very useful in the development of electronics such as computers. Silicon and germanium are examples of semiconductors. Semiconductors are at the heart of how photovoltaics work.
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Photovoltaics fall into three main categories: Silicon, thin-amorphous, and dyesensitive. Each has its advantages and disadvantages based on materials used, efficiencies, optimal working conditions, and applications.
Silicon cells are the most widely used photovoltaics. They have been in use since the 1950s. This type of photovoltaic (PV) cell is commonly referred to as a crystalline silicon cell. It consists of thin slices of silicon. Silicon, the fourteenth element on the periodic table is considered a semiconductor because it is more conductive than nonmetals and less conductive the metals. Silicon has four valence electrons. Every silicon atom can bond with four other silicon atoms creating a tetrahedral lattice. This lattice is key to it's ability to conduct electricity. Silicon is one of the most abundant elements in nature, however it does not exist on it's own . Silicon can be found in the common mineral silicon dioxide, SiO
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, more commonly known as silica or quartz .
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Before it can be used in solar cells it must first be purified. Monocrystalline solar cells are created by heating the mineral into a molten mixture and growing the silicon from a seed. This popular method is called the Czochralski method.
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This method is time consuming and is energy intensive. Other methods are currently being researched. The multicrystalline or polycrystalline solar cell method also involves melting the silicon but it is cast instead of grown. Thin wafers are cut from the solid silicon.
A crystalline solar cell consists of two silicon layers the top layer is doped with another element such as phosphorus, which has five valence electrons, to make it act like it's electrically negative. Doping can occur in the melting process or a thin layer can be applied after it has cooled. This layer is called the n-type semiconductor. The bottom layer is doped with an element that has only three valence electrons such as boron. This makes the layer act like it's positively charged and it is called the p-type semiconductor. The n-type has extra electrons and the p-type has extra "holes" (empty spots for electrons), but in order for the electrons to jump from one to the other the proper energy is needed. On top of the n-type layer are metal contacts and an antireflective transparent layer for protection. At the bottom of the cell is another contact, usually made of aluminum or molybdenum.
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How a Photovoltaic Cell Works (see figure 1)
Step 1. Each silicon atom has four valence electrons. Everything fits neatly, there is no variation in charge to encourage movement of the electrons. At this point the silicon wafer is acting more like an insulator.
Step 2. In the n-type semiconductor layer phosphorous is added during the doping process. Phosphorous has five valence electrons. This leaves one free electron.
In the p-type semiconductor layer boron is added during the doping process. Boron has three valence electrons leaving a hole or a vacancy. Silicon can make another bond, but there is no electron to make a bond with.
The n-type layer is now has extra electrons and the p-type layer is now short electrons.
This discrepancy is the potential difference that encourages the electrons to flow from the n-type layer to the p-type layer.
Step 3. When the two layers are put together the extra electrons in the n-type layer move to the p-type layer. The n-type layer now has vacancies created around phosphorous and the p-type layer now has extra electrons that boron cannot accept. This creates a barrier, preventing anymore exchange of electrons. This barrier is called the pn junction. Without any other additions of energy the cell would remain in this state.
Step 4. When energy is added to the cell via photons from electromagnetic radiation the energy is absorbed by the electrons causing them to move to a higher energy state and break free. With no other additions the electron would eventually drop back down to a lower energy level and the energy would be emitted.
Step 5. Adding a circuit and a load to the cell allows the electrons to flow around the circuit back to the layer that has the vacancy. As this is successively repeated, electricity is produced.
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Solar cells take advantage of a limitless energy source, but they are only able to use a fraction of the incoming energy. Out of the 100 percent of sunlight that reaches a solar cell 55 percent of it is immediately unusable because it exists in a wavelength outside what the electrons need to move to another energy state. More energy can also be lost through heat.
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The energy conversion efficiency is calculated by dividing the useful energy output by the energy input. Efficiency ratings for solar cells range between 4 to 40 percent depending on the technology.
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This poor performance makes solar energy look less desirable than some other fuels sources. However, when you consider that no natural resources are being depleted by using solar energy and no ecosystems are further being harmed by adding a solar panel to an existing structure, this efficiency rating could be considered quite high. Research and development of new photovoltaic technology is ongoing and efficiency is increasing. The are several other factors that can affect the performance of solar cells, many of which revolve around photon flux. Photon flux is the number of photons per second per unit area. Multiplying photon flux by the specific wavelength of a photon will yield the power density of an area.
Wavelength - The electromagnetic spectrum is made up of various wavelengths of radiation. Each wavelength carries with it a certain potential to do work. This potential is measured in electron-volts. The amount of energy that a wavelength carries is responsible for pushing the electrons across the p-n junction allowing it to flow and generate electricity. Energy is measured in electronvolts (eV). The eV necessary is different for all semiconductor materials and is the reason that they are not 100% energy efficient. Out of all the photons that are directed at a cell many of them are reflected and still others are absorbed as heat and light.
Temperature - PV cells work more efficiently at cooler temperatures. Heat increases resistance which decreases current. The exact temperatures are dependent on the type of material used. So, when planning to install solar cells the ambient temperature must be taken into consideration so that the correct type of PV cell can be chosen. If controlling for temperature is not an option than a system can be designed to cool the cells.
Intensity of Light - PV cells work differently under different lighting conditions. Some cells work best under bright lighting conditions and others work better under diffuse lighting conditions. In some cases efficiency can actually drop if light intensity is too high. So although it is typically thought that solar cells can only work in areas with full direct sun like the desert, there are types of solar cells that work better with a little bit of cloud cover.
Angle of Incidence - The output of a PV cell is largely dependent on the amount of light that hits the surface, so by changing the angle at which the Sun's rays hit the cell, the amount of light can be changed. As the angle of the Sun's rays is increased or decreased away from 90 degrees the concentration of energy that an area receives decreases and is instead spread out over a larger area. The closer the angle is to a 90 degree angle the more light it receives and the higher the output.
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Distance - Distance also plays a part in performance of the cell. The closer the cell is to the Sun the higher the performance. As the distance increases between the Sun and the cell the radiation spreads out and less photons hit a particular area. In New England this distance might be a factor worth considering as we are closer to the Sun in the winter time and further from it in the summer.
How Much Energy is Produced?
Energy equals power multiplied by time (E = P x t). Calculating energy starts with understanding electricity and its components. Electricity is the flow of electrons. The components of electricity are voltage (V), current (I), and resistance (R). According to Ohm's law, voltage equals current multiplied by resistance (V = I x R).
Voltage Voltage is the potential energy difference needed for electrons to flow. Increasing potential difference increases electron flow. Voltage is measured in the unit volts (V).
Current The number of electrons that pass a fixed point per second is called current. Current is measured in amperes (amps, A). (Current is represented as "I" in Ohms calculations.)
Resistance - Resistance is a property that slows down the flow of electrons. Resistance can be in the form of the wire used in the circuit or the load in the circuit. Resistance is measured in ohms (Ω). (Resistance is represented as "R" in Ohms calculations.)
The best example of this relationship is illustrated by flowing water from a garden hose. If the water is shut off at the spigot and the hose is set on the ground the remaining water will trickle out. If the back of the hose is picked up while leaving the opening on the ground the water will empty faster. This happens because the potential difference between the two were increased. In the case of the hose the height difference is increasing. This is gravitational energy, but the theory is the same. So, an increase in potential energy difference (voltage) increases the flow (current), for an equivalent resistance.
If two garden hoses are set up, one with a 1-inch diameter opening and one with a 1/2 inch diameter opening, water would flow slower through the smaller opening. The smaller opening is resisting the flow of the water, slowing is down. In the same way, changes in materials in an electric circuit can slow the flow of electrons; diameter or gauge of the wire, type of metal the wire is made of, and temperature of the wire.
The relationship between voltage, current, and resistance is as follows: voltage = current x resistance (V = I x R). In order for current to increase voltage must increase or resistance must decrease. Current can also be determined by rearranging the equation. Current = voltage / resistance (I = V / R). Written this way, it's easy to see the inverse relationship between resistance and current.
Once voltage and current are determined, power can be calculated. Power is defined as the rate at which work is being done. Power = voltage x current (P = V x I). Increasing voltage increases the rate at which work is being done. Using the hose example again, by increasing the height difference (voltage) more water can flow past a point in a given second. If the work to be done was to wash the sand of your feet after a day at the beach, increasing the height of the hose would increase the power allowing you to wash the sand away faster.
Electrical power specifically deals with the rate at which energy is transferred by applying voltage. Power is measured in watts (W). Watts = voltage x current (W = V x I).
Electrical energy is the amount of work being done in a given time. With time included in the discussion, it must also be added to the equation. Energy = power x time (E = P x t). It is customary to measure electrical energy in hours (h) and power equals watts (W), so the unit is power per time or watt-hours (Wh). The equation can be simplified to energy = watt-hours (E = Wh). Because the watthour is a small unit, the electric companies use the kilowatthour (kWh). A kilowatt is 1000 watts.
How much does it cost to play video games for two hours? The typical gaming console uses 125 watts of energy per hour (125 Wh x 2 h = 250 Wh).
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The 250 Wh must be converted into kilowatt hours (250 Wh /1000 = 0.25 kWh). The average cost of electricity per kWh is $0.12. Multiply 0.25 kWh by 0.12 (0.25 kWh x 0.12 = 0.03). The cost to play video games for two hours is three cents.