History
The development of the solar cell started from the work of the French physicist Mr.Antoine-César Becquerel in 1839. Becquerel discovered the photovoltaic effect when experimenting with a solid electrode in an electrolyte solution; he could observe that voltage developed when light falls on the electrode. After about 50 years later, Charles Fritts could construct his first solar cells using junctions that was formed by coating the semiconductor selenium with an Ultrathin - transparent layer of gold. Fritts's devices were not that much efficient, only transforming less than 1 percent of the absorbed light into electrical energy.
In 1927 one of another metal-semiconductors:- junction solar cell, in this case it is made of copper and semiconductor copper oxide, which had been demonstrated. By 1930s both the selenium cell and copper oxide cell were used in light-sensitive devices, such as photometers which if for using in photography. However, these early solar cells, still had energy-conversion efficiency not more than 1 percent. Luckily this confusion was finally could be overcome with the development of silicon-solar-cell by Mr. Russell Ohl which is in 1941. In 1954, three other great U.S researchers, G.L. Pearson, Daryl Chapin, and Calvin Fuller, convinced a silicon solar cell which was capable of a 6 percent energy-conversion efficiency after using in direct sunlight. By the late 1980's - silicon made cells, and those made of gallium arsenide, with efficiencies of more than 20 percent had been produced. In 1989 a concentrator solar cell- type of device in which sunlight is concentrated onto the cell surface by using lenses, could achieve a great efficiency of 37 percent due to the increasing intensity of the collected energy. Nowadays different solar cells of higher efficiencies and cost are now available.
Thursday, January 7, 2010
How many solar cells would be need in order to provide all of the electricity that my house requires?
If you have been read the HSW article on "How Solar Yard Lights Work", then you can feel how an unbelievable can be produced by a solar cell. The solar panel shown below contains 4 cells, and believe it, each of them can produce 0.45 volts and 100 milliamps, or up to 45 milliwatts. Each cell is measured 2 inches by 0.5 inches. In other words, with these solar cells it can be generated about 45 milliwatts in one square inch (6.45 square cm),how amazing. As the sake of discussion, let's assume that a panel can generate 70 milliwatts per square inch.
What you need to know for calculating how many square inches of solar panel you need for a house.
* How much power the house consumes as an average.
* Where the house is located ( can calculate the mean solar days, average rainfall for a period of time and etc.). This question may be impossible to answer unless you have a specific location selected in mind. So we can assume that on an average day the solar panels can generate their maximum power possible for 5 hours.
A "typical home" in America can be using either electricity or a specific gas to provide heat needed amount heat for the house, the hot water, the clothes dryer and the stove/oven. If you want to power a house with solar electricity, you would use gas appliances instead, because using solar electricity is so expensive. This means that what you would be with solar electricity are the things like the refrigerator, the lights, the TV, the computer, stereo equipment, motors in things like furnace fans, washer and etc. Let's say that all of previously mentioned things is out to 600 watts on average. For the course of 24 hours, you will need about 600 watts * 24 hours = 14,400 watt hours will be definetely needed per day.
From the relevant calculations mentioned above, we should be known that a solar panel can generate up to 70 milliwatts per square inch * 5 hours which is equal to 350 milliwatt hours per day. Therefore you will be actually needed about 41,000 square inches solar cell for the house. That is a solar panel that is measured about 285 square feet (it is about 26 square meters). That is costed around $16,000. Then, because we know that the sun only shines part of the time, not the whole day, you would need to purchase a battery, an inverter and etc., and that definitely often doubles the cost of the installation.
If you want to have a small room air conditioner in your bedroom,what you have to do is double everything.
Because for the reason - solar electricity is so expensive, you would normally prefer at great lengths to reduce your electricity consumption. You may use a portable laptop instead of a desktop computer with a big monitor. You would use fluorescent lights[FLV bulbs] instead of incandescent. You would use a small LCD TV instead of a large color set. You would prefer to a small, extremely efficient refrigerator. By doing above things you might be able to reduce your average power consumption to at-least 100 watts. This would reduce the size of your solar panel and its cost, and this might bring it into the range of possibility.
The most important thing to remember, however is that about 100 watts per an hour purchased from the power grid may only cost about 24 cents a day normally, or in other words $91 a year. That's why you are not able to see more solar houses unless they are in a remote locations. When it costs about $100 a year to purchase power from the grid, it is really hard to justify the matter of spending thousands of dollars on a solar system.
What you need to know for calculating how many square inches of solar panel you need for a house.
* How much power the house consumes as an average.
* Where the house is located ( can calculate the mean solar days, average rainfall for a period of time and etc.). This question may be impossible to answer unless you have a specific location selected in mind. So we can assume that on an average day the solar panels can generate their maximum power possible for 5 hours.
A "typical home" in America can be using either electricity or a specific gas to provide heat needed amount heat for the house, the hot water, the clothes dryer and the stove/oven. If you want to power a house with solar electricity, you would use gas appliances instead, because using solar electricity is so expensive. This means that what you would be with solar electricity are the things like the refrigerator, the lights, the TV, the computer, stereo equipment, motors in things like furnace fans, washer and etc. Let's say that all of previously mentioned things is out to 600 watts on average. For the course of 24 hours, you will need about 600 watts * 24 hours = 14,400 watt hours will be definetely needed per day.
From the relevant calculations mentioned above, we should be known that a solar panel can generate up to 70 milliwatts per square inch * 5 hours which is equal to 350 milliwatt hours per day. Therefore you will be actually needed about 41,000 square inches solar cell for the house. That is a solar panel that is measured about 285 square feet (it is about 26 square meters). That is costed around $16,000. Then, because we know that the sun only shines part of the time, not the whole day, you would need to purchase a battery, an inverter and etc., and that definitely often doubles the cost of the installation.
If you want to have a small room air conditioner in your bedroom,what you have to do is double everything.
Because for the reason - solar electricity is so expensive, you would normally prefer at great lengths to reduce your electricity consumption. You may use a portable laptop instead of a desktop computer with a big monitor. You would use fluorescent lights[FLV bulbs] instead of incandescent. You would use a small LCD TV instead of a large color set. You would prefer to a small, extremely efficient refrigerator. By doing above things you might be able to reduce your average power consumption to at-least 100 watts. This would reduce the size of your solar panel and its cost, and this might bring it into the range of possibility.
The most important thing to remember, however is that about 100 watts per an hour purchased from the power grid may only cost about 24 cents a day normally, or in other words $91 a year. That's why you are not able to see more solar houses unless they are in a remote locations. When it costs about $100 a year to purchase power from the grid, it is really hard to justify the matter of spending thousands of dollars on a solar system.
If photovoltaics are such a wonderful source of free energy, then why doesn't the whole world run on solar power?
Solar-power Costs
Some people have a flawed concept of solar energy. While it's true that sunlight is free, the electricity generated by PV systems is not. As you can see from our discussion of a household PV system, quite a bit of hardware is needed. Currently, an installed PV system will cost somewhere around $9 per peak Watt. To give you an idea of how much a house system would cost, let's consider the Solar House -- a model residential home in Raleigh, North Carolina, with a PV system set up by the North Carolina Solar Center to demonstrate the technology. It's a fairly small home, and it is estimated that its 3.6-kW PV system covers about half of the total electricity needs (this system doesn't use batteries -- it's connected to the grid). Even so, at $9 per Watt, this installed system would cost you around $32,000.
That's why PV is usually used in remote areas, far from a conventional source of electricity. Right now, it simply can't compete with the utilities. Costs are coming down as research is being done, however. Researchers are confident that PV will one day be cost effective in urban areas as well as remote ones. Part of the problem is that manufacturing needs to be done on a large scale to reduce costs as much as possible. That kind of demand for PV, however, won't exist until prices fall to competitive levels. It's a Catch-22 situation. Even so, demand and module efficiencies are constantly rising, prices are falling, and the world is becoming increasingly aware of environmental concerns associated with conventional power sources, making photovoltaics a technology with a bright future.
Some people have a flawed concept of solar energy. While it's true that sunlight is free, the electricity generated by PV systems is not. As you can see from our discussion of a household PV system, quite a bit of hardware is needed. Currently, an installed PV system will cost somewhere around $9 per peak Watt. To give you an idea of how much a house system would cost, let's consider the Solar House -- a model residential home in Raleigh, North Carolina, with a PV system set up by the North Carolina Solar Center to demonstrate the technology. It's a fairly small home, and it is estimated that its 3.6-kW PV system covers about half of the total electricity needs (this system doesn't use batteries -- it's connected to the grid). Even so, at $9 per Watt, this installed system would cost you around $32,000.
That's why PV is usually used in remote areas, far from a conventional source of electricity. Right now, it simply can't compete with the utilities. Costs are coming down as research is being done, however. Researchers are confident that PV will one day be cost effective in urban areas as well as remote ones. Part of the problem is that manufacturing needs to be done on a large scale to reduce costs as much as possible. That kind of demand for PV, however, won't exist until prices fall to competitive levels. It's a Catch-22 situation. Even so, demand and module efficiencies are constantly rising, prices are falling, and the world is becoming increasingly aware of environmental concerns associated with conventional power sources, making photovoltaics a technology with a bright future.
Solving Solar-power Issues
Certainly, no one would accept only having electricity during the day, and then only on clear days, if they have a choice. We need energy storage -- batteries. Unfortunately, batteries add a lot of cost and maintenance to the PV system. Currently, however, it's a necessity if you want to be completely independent. One way around the problem is to connect your house to the utility grid, buying power when you need it and selling to them when you produce more than you need. This way, the utility acts as a practically infinite storage system. The utility has to agree, of course, and in most cases will buy power from you at a much lower price than their own selling price. You will also need special equipment to make sure that the power you sell to your utility is synchronous with theirs -- that it shares the same sinusoidal waveform and frequency. Safety is an issue as well. The utility has to make sure that if there's a power outage in your neighborhood, your PV system won't try to feed electricity into lines that a lineman may think is dead. This is called islanding.
If you decide to use batteries, keep in mind that they will have to be maintained, and then replaced after a certain number of years. The PV modules should last 20 years or more, but batteries just don't have that kind of useful life. Batteries in PV systems can also be very dangerous because of the energy they store and the acidic electrolytes they contain, so you'll need a well-ventilated, non-metallic enclosure for them.
Although several different kinds of batteries are commonly used, the one characteristic they should all have in common is that they are deep-cycle batteries. Unlike your car battery, which is a shallow-cycle battery, deep-cycle batteries can discharge more of their stored energy while still maintaining long life. Car batteries discharge a large current for a very short time -- to start your car -- and are then immediately recharged as you drive. PV batteries generally have to discharge a smaller current for a longer period (such as all night), while being charged during the day.
The most commonly used deep-cycle batteries are lead-acid batteries (both sealed and vented) and nickel-cadmium batteries. Nickel-cadmium batteries are more expensive, but last longer and can be discharged more completely without harm. Even deep-cycle lead-acid batteries can't be discharged 100 percent without seriously shortening battery life, and generally, PV systems are designed to discharge lead-acid batteries no more than 40 percent or 50 percent.
Also, the use of batteries requires the installation of another component called a charge controller. Batteries last a lot longer if care is taken so that they aren't overcharged or drained too much. That's what a charge controller does. Once the batteries are fully charged, the charge controller doesn't let current from the PV modules continue to flow into them. Similarly, once the batteries have been drained to a certain predetermined level, controlled by measuring battery voltage, many charge controllers will not allow more current to be drained from the batteries until they have been recharged. The use of a charge controller is essential for long battery life.
The other problem besides energy storage is that the electricity generated by your PV modules, and extracted from your batteries if you choose to use them, is not in the form that's used by the electrical appliances in your house. The electricity generated by a solar system is direct current, while the electricity supplied by your utility (and the kind that every appliance in your house uses) is alternating current. You will need an inverter, a device that converts DC to AC. Most large inverters will also allow you to automatically control how your system works. Some PV modules, called AC modules, actually have an inverter already built into each module, eliminating the need for a large, central inverter, and simplifying wiring issues.
Throw in the mounting hardware, wiring, junction boxes, grounding equipment, overcurrent protection, DC and AC disconnects and other accessories and you have yourself a system. Electrical codes must be followed (there's a section in the National Electrical Code just for PV), and it's highly recommended that the installation be done by a licensed electrician who has experience with PV systems. Once installed, a PV system requires very little maintenance (especially if no batteries are used), and will provide electricity cleanly and quietly for 20 years or more.
If you decide to use batteries, keep in mind that they will have to be maintained, and then replaced after a certain number of years. The PV modules should last 20 years or more, but batteries just don't have that kind of useful life. Batteries in PV systems can also be very dangerous because of the energy they store and the acidic electrolytes they contain, so you'll need a well-ventilated, non-metallic enclosure for them.
Although several different kinds of batteries are commonly used, the one characteristic they should all have in common is that they are deep-cycle batteries. Unlike your car battery, which is a shallow-cycle battery, deep-cycle batteries can discharge more of their stored energy while still maintaining long life. Car batteries discharge a large current for a very short time -- to start your car -- and are then immediately recharged as you drive. PV batteries generally have to discharge a smaller current for a longer period (such as all night), while being charged during the day.
The most commonly used deep-cycle batteries are lead-acid batteries (both sealed and vented) and nickel-cadmium batteries. Nickel-cadmium batteries are more expensive, but last longer and can be discharged more completely without harm. Even deep-cycle lead-acid batteries can't be discharged 100 percent without seriously shortening battery life, and generally, PV systems are designed to discharge lead-acid batteries no more than 40 percent or 50 percent.
Also, the use of batteries requires the installation of another component called a charge controller. Batteries last a lot longer if care is taken so that they aren't overcharged or drained too much. That's what a charge controller does. Once the batteries are fully charged, the charge controller doesn't let current from the PV modules continue to flow into them. Similarly, once the batteries have been drained to a certain predetermined level, controlled by measuring battery voltage, many charge controllers will not allow more current to be drained from the batteries until they have been recharged. The use of a charge controller is essential for long battery life.
The other problem besides energy storage is that the electricity generated by your PV modules, and extracted from your batteries if you choose to use them, is not in the form that's used by the electrical appliances in your house. The electricity generated by a solar system is direct current, while the electricity supplied by your utility (and the kind that every appliance in your house uses) is alternating current. You will need an inverter, a device that converts DC to AC. Most large inverters will also allow you to automatically control how your system works. Some PV modules, called AC modules, actually have an inverter already built into each module, eliminating the need for a large, central inverter, and simplifying wiring issues.
Throw in the mounting hardware, wiring, junction boxes, grounding equipment, overcurrent protection, DC and AC disconnects and other accessories and you have yourself a system. Electrical codes must be followed (there's a section in the National Electrical Code just for PV), and it's highly recommended that the installation be done by a licensed electrician who has experience with PV systems. Once installed, a PV system requires very little maintenance (especially if no batteries are used), and will provide electricity cleanly and quietly for 20 years or more.
Solar-powering a House - Easy
What would you have to do to power your house with solar energy? Although it's not as simple as just slapping some modules on your roof, it's not extremely difficult to do, either.
First of all, not every roof has the correct orientation or angle of inclination to take advantage of the sun's energy. Non-tracking PV systems in the Northern Hemisphere should point toward true south (this is the orientation). They should be inclined at an angle equal to the area's latitude to absorb the maximum amount of energy year-round. A different orientation and/or inclination could be used if you want to maximize energy production for the morning or afternoon, and/or the summer or winter. Of course, the modules should never be shaded by nearby trees or buildings, no matter the time of day or the time of year. In a PV module, even if just one of its 36 cells is shaded, power production will be reduced by more than half.
If you have a house with an unshaded, south-facing roof, you need to decide what size system you need. This is complicated by the facts that your electricity production depends on the weather, which is never completely predictable, and that your electricity demand will also vary. These hurdles are fairly easy to clear. Meteorological data gives average monthly sunlight levels for different geographical areas. This takes into account rainfall and cloudy days, as well as altitude, humidity, and other more subtle factors. You should design for the worst month, so that you'll have enough electricity all year. With that data, and knowing your average household demand (your utility bill conveniently lets you know how much energy you use every month),there are simple methods you can use to determine just how many PV modules you'll need. You'll also need to decide on a system voltage, which you can control by deciding how many modules to wire in series.
First of all, not every roof has the correct orientation or angle of inclination to take advantage of the sun's energy. Non-tracking PV systems in the Northern Hemisphere should point toward true south (this is the orientation). They should be inclined at an angle equal to the area's latitude to absorb the maximum amount of energy year-round. A different orientation and/or inclination could be used if you want to maximize energy production for the morning or afternoon, and/or the summer or winter. Of course, the modules should never be shaded by nearby trees or buildings, no matter the time of day or the time of year. In a PV module, even if just one of its 36 cells is shaded, power production will be reduced by more than half.
If you have a house with an unshaded, south-facing roof, you need to decide what size system you need. This is complicated by the facts that your electricity production depends on the weather, which is never completely predictable, and that your electricity demand will also vary. These hurdles are fairly easy to clear. Meteorological data gives average monthly sunlight levels for different geographical areas. This takes into account rainfall and cloudy days, as well as altitude, humidity, and other more subtle factors. You should design for the worst month, so that you'll have enough electricity all year. With that data, and knowing your average household demand (your utility bill conveniently lets you know how much energy you use every month),there are simple methods you can use to determine just how many PV modules you'll need. You'll also need to decide on a system voltage, which you can control by deciding how many modules to wire in series.
Energy Loss in a Solar Cell
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.
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.
Anatomy of a Solar Cell
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.
A = n type silicon
B = p type silicon
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.
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.
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 anti-reflective coating is applied to the top of the cell to reduce reflection losses to less than 5 percent.
A = n type siliconB = p type silicon
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.
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.
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 anti-reflective 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.
Subscribe to:
Comments (Atom)