Monday, March 17, 2008

Hydro Power

Instead of using steam to drive generator turbines, a hydro plant uses the force of falling or flowing water.
Hydro power has been used to make electricity in the Midwest since the early 1900s, with many facilities built by the Works Progress Administration in the 1930s.
There are two types of hydroelectric power plants:
  • A high-head plant takes advantage of the force of falling water. Large-scale facilities like the Hoover Dam and Grand Coulee Dam are examples of high-head hydro plants. Dams are built along major rivers to create reservoirs; the utility controls the flow of water through the dam in response to the demand for electricity.
  • A run of the river plant, like those found in the Midwest, relies on the flow of the river to spin the turbines. These plants produce a much smaller amount of electricity.
    The benefits of hydro power are many: no hazardous emissions or solid waste, no fuel costs and it's entirely sustainable. Hydro plants are reliable, low maintenance and provide flood control.

Drawbacks to hydro power :

  • Environmental groups have pointed out the drawbacks to hydroelectric power, especially from large-scale dams and reservoirs. The most dramatic is the impact on wildlife - the reservoirs can alter water temperature and prevent the migration of fish.
  • While "run of the river" hydro plants have a much smaller environmental impact, their use is constrained by the lack of control. The electricity produced at these plants cannot be increased or decreased according to demand, and the flow of the river is dependent on the area's precipitation.
  • Many of the nation's hydroelectric plants are aging, and flooding in recent years has irreparably damaged several in the Midwest. The initial costs of building or replacing a hydro plant are high, and usually not cost-effective in the Midwest, so most utilities are investing in other forms of renewable energy.

Everyday in Nepal.





Everyday at a different, unknown time the electricity throughout all of Nepal simply blacks out. Most of Nepal’s energy (maybe all) comes from hydroelectric power, and during the dry season, October through May, there is neither enough dams nor water to power the country. Everyone knows of this as load shedding, which does exactly that: sheds the load on Nepal’s electrical grid. For a country trying to develop itself, losing power 3-4 hours a day (sometimes more) certainly does not help their industry or productivity in any way. Newspapers talk of many solutions, from nuclear energy to more dams, but all solutions require excessive amounts of money that Nepali’s do not have. Above, Sita cooks in the dark. Obviously, unlit cooking happens everywhere, but in a major metropolitan area, in a middle class home? Yea, weird.
Coupled with an absence of electricity, petrol has become a much sought after commodity. The past weeks have seen indefinite strikes in an area known as the Terai, mainly the southern half of Nepal. These strikes are linked to one of Nepal’s many ethnic groups, the Madeshi, feeling left out of the political process. In fact, they feel so underrepresented in the fledgling political sphere, they want their own country. These strikes, or “Bandahs” in Nepali, are situated along the Nepal-Indian border where a majority of Nepal’s imports, including petrol, cross into the country. As a result, and at least in Kathmandu (I haven’t been or seen it anywhere else), long lines form at the limited number of petrol pumps around the city.
Inextricably linked to the border stand-offs, the supply of kerosene, used for cooking and the heating of people homes, has not escaped the bamboozling turbulence. More long lines, more waiting, and as can always be predicted: the distributor runs out. People walked home with empty jugs, just as they sometimes walk away without fuel in their gas tank. What amazes me is these people simply shrug their shoulders and walk away. (notice they have coca-cola and an ipod, but still empty jugs)
My first 3 days in Kathmandu there was another bandah in the city proper. Tires were burned in the streets, people shouted passionately, and all businesses and traffic were shut down. Unfortunately, I was very green and didn’t have the courage to take any pictures. This bandah was pointed at the government and their announcement to raise fuel prices. After 3 days of screwball shenanigans (the tires smelled terrible, some busses were destroyed with bricks), the government reneged the original price hike, much to the people’s happiness. Following the analysis of these events, I’ve concluded absolutely nothing. Economically speaking, prices for fuel go up. Simple. The people of Nepal are very poor. Complicated. Looking at things socially complicates it even further. No fuel for heat, for cooking, or for day-to-day business, not only exasperates poverty, it creates poverty.
http://www.eyeini.com/eyeini

Friday, March 14, 2008

Hoftun wishes for Nepal




Odd Hoftun, a hydropower engineer from Norway, came to Nepal in 1958 to build Tansen Hospital and later set up Butwal Technical School. Hoftun was back in Kathmandu recently and talked to Nepali Times about lessons learnt in hydropower development in Nepal.


Nepali Times: You were here in 1990 when the Jana Andolan I took place. Do you get a sense of déjà vu?
Odd Hoftun: The things I have seen in Nepal have overwhelmed me. Kathmandu is overcrowded and growing beyond its limits. I was visiting Butwal, and it is also a changed place. Many of these changes are positive. I am an optimist. I don’t see things going backwards in Nepal. One does wish, however, that certain things would move quicker.

Nepali Times: What lessons has Nepal learnt, or not learnt, about hydropower in the last 17 years?
Odd Hoftun: We have learnt that hydropower development takes time, but also requires a lot of investment. It takes realistic planning, and big projects are simply unrealistic. My philosophy has been to start small and grow over time. Changes in legislation have been very favourable. Some infrastructure has been developed. Opening up to the private sector was unthinkable 20 years ago, but this has now become what many people think of as a solution. The introduction of small hydropower schemes has proved that this it is one way in which hydropower can be developed in Nepal. If you add up all the micro-plants, there is quite a lot of capacity even with them.
There was a time when people used to think only foreign investors and foreign experts could develop hydropower in Nepal. We know that is no longer true because Nepal now has the capability and resources to handle these things. Companies like Himal Hydro, Nepal Hydro and Electric, and Butwal Power Company all started as small companies. Today they have grown and become internationally competitive.
What is still lacking is an understanding from the government on how to work with the private sector. There has to be an element of trust. It seems to me that there’s scope for private investment also in power distribution. Whatever the private sector is doing in hydropower, or in power distribution, there’s an urgent need for a realistic and predictable government framework. It shouldn’t be that one minister says one thing, then another comes in and says something else.

Nepali Times: Can politics be blamed?
Odd Hoftun: They say people get the politicians they deserve, so I wouldn’t blame them. I can’t come here and offer a prescription for how things should be done. I would just like to say that one must be realistic, there must be trust, and there must be consistency over the years. There was a time when I thought the Melamchi water project should become a public-private enterprise. The government was supportive of the idea, but when outside financing institutions got involved the idea was dropped. I was sorry about that because it could have been a multi-purpose regional development model so that not just the city people would benefit.

Nepali Times: Is the Indian market a blessing or a curse?
Odd Hoftun: It can be both. One must look 20 years ahead to make something like that work. The potential is tremendous. In Nepal, big hydropower projects are much talked about, but no one is even thinking about transmission lines across the border or within the country. It is difficult for private parties to run the grid. India is in desperate need of clean energy and Nepal can provide it. That said, just because there is demand in India, doesn’t mean Nepal should rush into big projects. In some places the small hydro concept is ideal. Ultimately, however, these small power plants need to be connected to the grid, which is why they are good for rural development. The big projects take a long time to start, and the rush upsets the local economy. Big projects should and must be undertaken, but that is only possible through export to India. However, there has to be a fair agreement and a very high level of trust between the two nations for something like that to work.

Nepali Times: How would you advise the Norwegian government about investing in Nepali hydropower?
Odd Hoftun: We have hydropower in Norway that has been developed, while in Nepal it is just starting. This is the time for Norway to invest in Nepal. There are many qualified and experienced Nepali people in the legal, financial and technical fields who can take care of Nepali interests. So Nepal would be an ideal partner for cooperation from the Norwegian side.

Small Hydro Power

For a small hydro system to work, all you need to do is simply harness the energy of continual running water with a turbine, and convert this energy into either DC or AC electricity. Small hydro power systems are classified as mini, small, and micro depending on the capacity of the plant. There are already many villages in India and Nepal that have access to small hydro electricity.
Record shows that small hydro power was introduced in India in 1897 during the time of British colonization. Various cases have been reported which say the small hydro power installed during the early 1900s are still in good condition and functioning properly. Only after Independence did India start working extensively on Small Hydro Power under Five-Year plans. With the effort made by MNES (Ministry of Non-conventional Energy Sources) during the period of the Eighth Five-Year plan, Small hydro was upgraded and additional funds allocated for improvement and expansion. The total installed capacity of small hydro projects in India is 144.28 mw while another 241.87 mw is under construction.
In India, the Hilly Hydro Project funded by United Nations Development Programme–Global Environment Facility have shown considerable amount of interest in the sector in 13 Himalayan states. Many demonstration project on the use of small hydro power for lighting, heating, cooking, irrigation, and small industry have been carried out in the 13 states to check deforestation trends. Various social and technical aspects namely, geography, people, end-use, head, flow, and total power generated have been considered. With the lending operations funded primarily by the World Bank and the Global Environment Facility, the Asia Alternative Energy Program (ASTAE) has assisted with the design of mini-hydro components. 15 small hydro schemes representing 68 MW of capacity are under operation in India.
Nepal being a country of rural and isolated committees, the suitability of micro-hydro systems is distinctly visible. The government of Nepal has great plans to increase the mega Hydro Power generation, and has shown almost the same amount of interest in developing small hydro power, but the policy toward small hydro is not very clear yet. Basically, there are foreign-aided projects that support the development of the power sector in Nepal, including the development of mini- and small hydro resources. Recently, some initiatives have been taken by facilitating Kathmandu University with programs for small hydro power research. In India, Alternative Hydro Energy Centre, set up by ministry of Non-conventional Energy Resources in the year 1982, at the University of Roorkie, Roorkie, has been working for the cooperation in small hydro power research and technology transfer, project design, development and consultancy services.
Both in India and Nepal, small hydro power plants has been considered for rural electrification. This has helped to reduce dependence on fuel wood, generate small enterprises and uplift the rural economy. Nepal is second only to Brazil in terms of hydro power. Nepal has numerous rivers, so there is further prospect of small hydro power in Nepal. The availability of water beyond 4000 meters of height has made permanent settlement possible. At such great heights, one of the most economical means to make electricity available is through the use of small hydro plants.
Small hydro has proved to be very advantageous in both India and Nepal. Micro-hydro has been very helpful in saving trees in the rural parts of both the countries. In addition, small hydro has been instrumental in enhancing economic development and living standards especially in remote areas with limited or no electricity both in India and Nepal. Ghandruk Power Plant in small village Ghandruk in the Annapurna region of Nepal, provides electricity to all houses for lighting purposes. Lodges (usualy meant for foreign tourists) and households have electricity for cooking. Since it is cost-effective, time-saving, and labor-saving, lodge-keepers are very positive about it and are ready to maintain it for ever. They have been able to make good profit out of it. There has been an increase in the local incomes in the Annapurna region. Employment opportunities to people were created when village electrification committee was established to oversee the project. Also when Forest Management Committee regulated some laws regarding the protection of forests, local people benefitted with better employment opportunities. Economic structure has changed and is way too different from many of the rural communities of Nepal. Revenue generated from the households and lodges is used for the village development. It has enhanced the interest of local manufacturers in manufacturing devices out of locally available materials.
Micro-Hydro systems are preferable from an environmental point of view as seasonal river flow patterns downstream are not affected and there is no flooding of valleys upstream of the system. Because of the fact that the motion of the river operates the machine, a complex mechanical governor system is not required, which reduces costs and maintenance requirements. The systems can be build locally at low cost, and the simplicity gives rise to better long term reliability. Dry season, and the lack of storage of excess power generated can be hazardous though.
The future of small hydro power both in India and Nepal is bright. India can provide consultancy services in number of areas of small hydro power like planning, investigation and hydro resource survey, engineering designs for construction works, techno-economic analysis of small hydro power, to name a few. With more private sector participation, the chances of improvement are very high. Even without it, it is likely that there will be a shift in the prority to small hydro projects considering the environmental consequences of large hydro projects, the fragile geology and active seismicity of the region, and the high costs associated with the large hydro projects.
(source:http://web.grinnell.edu/courses/ant/S00/ANT154-01/magar/Hydro.html)

Wednesday, March 12, 2008

How Power Grids Work?

Electrical power is a little bit like the air you breathe: You don't really think about it until it is missing. Power is just "there," meeting your every need, constantly. It is only during a power failure, when you walk into a dark room and instinctively hit the useless light switch, that you realize how important power is in your daily life. You use it for heating, cooling, cooking, refrigeration, light, sound, computation, entertainment... Without it, life can get somewhat cumbersome.

Power travels from the power plant to your house through an amazing system called the power distribution grid.


Power grid distribution lines can be above or under ground. See more power grid pictures.

The grid is quite public -- if you live in a suburban or rural area, chances are it is right out in the open for all to see. It is so public, in fact, that you probably don't even notice it anymore. Your brain likely ignores all of the power lines because it has seen them so often. In this article, we will look at all of the equipment that brings electrical power to your home. The next time you look at the power grid, you will be able to really see it and understand what is going on!

The Power Plant Electrical power starts at the power plant. In almost all cases, the power plant consists of a spinning electrical generator. Something has to spin that generator -- it might be a water wheel in a hydroelectric dam, a large diesel engine or a gas turbine. But in most cases, the thing spinning the generator is a steam turbine. The steam might be created by burning coal, oil or natural gas. Or the steam may come from a nuclear reactor like this one at the Shearon Harris nuclear power plant near Raleigh, North Carolina:


No matter what it is that spins the generator, commercial electrical generators of any size generate what is called 3-phase AC power. To understand 3-phase AC power, it is helpful to understand single-phase power first.


Photo courtesy U.S. Department of Energy
A breakdown of the major power plants in
the United States, by type

The Power Plant: Alternating Current

Single-phase power is what you have in your house. You generally talk about household electrical service as single-phase, 120-volt AC service. If you use an oscilloscope and look at the power found at a normal wall-plate outlet in your house, what you will find is that the power at the wall plate looks like a sine wave, and that wave oscillates between -170 volts and 170 volts (the peaks are indeed at 170 volts; it is the effective (rms) voltage that is 120 volts). The rate of oscillation for the sine wave is 60 cycles per second. Oscillating power like this is generally referred to as AC, or alternating current. The alternative to AC is DC, or direct current. Batteries produce DC: A steady stream of electrons flows in one direction only, from the negative to the positive terminal of the battery.

AC has at least three advantages over DC in a power distribution grid:

  1. Large electrical generators happen to generate AC naturally, so conversion to DC would involve an extra step.
  2. Transformers must have alternating current to operate, and we will see that the power distribution grid depends on transformers.
  3. It is easy to convert AC to DC but expensive to convert DC to AC, so if you were going to pick one or the other AC would be the better choice.
The power plant, therefore, produces AC. On the next page, you'll learn about the AC power produced at the power plant. Most notably, it is produced in three phases.

The Power Plant: Three-phase Power

The power plant produces three different phases of AC power simultaneously, and the three phases are offset 120 degrees from each other. There are four wires coming out of every power plant: the three phases plus a neutral or ground common to all three. If you were to look at the three phases on a graph, they would look like this relative to ground:


There is nothing magical about three-phase power. It is simply three single phases synchronized and offset by 120 degrees.

Why three phases? Why not one or two or four? In 1-phase and 2-phase power, there are 120 moments per second when a sine wave is crossing zero volts. In 3-phase power, at any given moment one of the three phases is nearing a peak. High-power 3-phase motors (used in industrial applications) and things like 3-phase welding equipment therefore have even power output. Four phases would not significantly improve things but would add a fourth wire, so 3-phase is the natural settling point.

And what about this "ground," as mentioned above? The power company essentially uses the earth as one of the wires in the power system. The earth is a pretty good conductor and it is huge, so it makes a good return path for electrons. (Car manufacturers do something similar; they use the metal body of the car as one of the wires in the car's electrical system and attach the negative pole of the battery to the car's body.) "Ground" in the power distribution grid is literally "the ground" that's all around you when you are walking outside. It is the dirt, rocks, groundwater, etc., of the earth.

Transmission Substation

The three-phase power leaves the generator and enters a transmission substation at the power plant. This substation uses large transformers to convert the generator's voltage (which is at the thousands of volts level) up to extremely high voltages for long-distance transmission on the transmission grid.


A typical substation at a power plant

You can see at the back several three-wire towers leaving the substation. Typical voltages for long distance transmission are in the range of 155,000 to 765,000 volts in order to reduce line losses. A typical maximum transmission distance is about 300 miles (483 km). High-voltage transmission lines are quite obvious when you see them. They are normally made of huge steel towers like this:


All power towers like this have three wires for the three phases. Many towers, like the ones shown above, have extra wires running along the tops of the towers. These are ground wires and are there primarily in an attempt to attract lightning.

The Distribution Grid

For power to be useful in a home or business, it comes off the transmission grid and is stepped-down to the distribution grid. This may happen in several phases. The place where the conversion from "transmission" to "distribution" occurs is in a power substation. A power substation typically does two or three things:
  • It has transformers that step transmission voltages (in the tens or hundreds of thousands of volts range) down to distribution voltages (typically less than 10,000 volts).
  • It has a "bus" that can split the distribution power off in multiple directions.
  • It often has circuit breakers and switches so that the substation can be disconnected from the transmission grid or separate distribution lines can be disconnected from the substation when necessary.


A typical small substation

The box in the foreground is a large transformer. To its left (and out of the frame but shown in the next shot) are the incoming power from the transmission grid and a set of switches for the incoming power. Toward the right is a distribution bus plus three voltage regulators.


The transmission lines entering the substation and passing through the switch tower


The switch tower and the main transformer

Now the distribution bus comes into the picture.

Distribution Bus

The power goes from the transformer to the distribution bus:


In this case, the bus distributes power to two separate sets of distribution lines at two different voltages. The smaller transformers attached to the bus are stepping the power down to standard line voltage (usually 7,200 volts) for one set of lines, while power leaves in the other direction at the higher voltage of the main transformer. The power leaves this substation in two sets of three wires, each headed down the road in a different direction:


The wires between these two poles are "guy wires" for support. They carry no current.


The next time you are driving down the road, you can look at the power lines in a completely different light. In the typical scene pictured on the right, the three wires at the top of the poles are the three wires for the 3-phase power. The fourth wire lower on the poles is the ground wire. In some cases there will be additional wires, typically phone or cable TV lines riding on the same poles.

As mentioned above, this particular substation produces two different voltages. The wires at the higher voltage need to be stepped down again, which will often happen at another substation or in small transformers somewhere down the line. For example, you will often see a large green box (perhaps 6 feet/1.8 meters on a side) near the entrance to a subdivision. It is performing the step-down function for the subdivision.

Regulator Bank

You will also find regulator banks located along the line, either underground or in the air. They regulate the voltage on the line to prevent undervoltage and overvoltage conditions.


A typical regulator bank

Up toward the top are three switches that allow this regulator bank to be disconnected for maintenance when necessary:


At this point, we have typical line voltage at something like 7,200 volts running through the neighborhood on three wires (with a fourth ground wire lower on the pole):


Taps

A house needs only one of the three phases, so typically you will see three wires running down a main road, and taps for one or two of the phases running off on side streets. Pictured below is a 3-phase to 2-phase tap, with the two phases running off to the right:


Here is a 2-phase to 1-phase tap, with the single phase running out to the right:


--->

At the House

And finally we are down to the wire that brings power to your house! Past a typical house runs a set of poles with one phase of power (at 7,200 volts) and a ground wire (although sometimes there will be two or three phases on the pole, depending on where the house is located in the distribution grid). At each house, there is a transformer drum attached to the pole, like this:


In many suburban neighborhoods, the distribution lines are underground and there are green transformer boxes at every house or two. Here is some detail on what is going on at the pole:


The transformer's job is to reduce the 7,200 volts down to the 240 volts that makes up normal household electrical service. Let's look at this pole one more time, from the bottom, to see what is going on:


There are two things to notice in this picture:

  • There is a bare wire running down the pole.
    This is a grounding wire. Every utility pole on the planet has one. If you ever watch the power company install a new pole, you will see that the end of that bare wire is stapled in a coil to the base of the pole and therefore is in direct contact with the earth, running 6 to 10 feet (1.8 to 3 m) underground. It is a good, solid ground connection. If you examine a pole carefully, you will see that the ground wire running between poles (and often the guy wires) are attached to this direct connection to ground.

  • There are two wires running out of the transformer and three wires running to the house.
    The two from the transformer are insulated, and the third one is bare. The bare wire is the ground wire. The two insulated wires each carry 120 volts, but they are 180 degrees out of phase so the difference between them is 240 volts. This arrangement allows a homeowner to use both 120-volt and 240-volt appliances. The transformer is wired in this sort of configuration:


The 240 volts enters your house through a typical watt-hour meter like this one:


The meter lets the power company charge you for putting up all of these wires.

Safety Devices: Fuses

Fuses and circuit breakers are safety devices. Let's say that you did not have fuses or circuit breakers in your house and something "went wrong." What could possibly go wrong? Here are some examples:
  • A fan motor burns out a bearing, seizes, overheats and melts, causing a direct connection between power and ground.
  • A wire comes loose in a lamp and directly connects power to ground.
  • A mouse chews through the insulation in a wire and directly connects power to ground.
  • Someone accidentally vacuums up a lamp wire with the vacuum cleaner, cutting it in the process and directly connecting power to ground.
  • A person is hanging a picture in the living room and the nail used for said picture happens to puncture a power line in the wall, directly connecting power to ground.


When a 120-volt power line connects directly to ground, its goal in life is to pump as much electricity as possible through the connection. Either the device or the wire in the wall will burst into flames in such a situation. (The wire in the wall will get hot like the element in an electric oven gets hot, which is to say very hot!). A fuse is a simple device designed to overheat and burn out extremely rapidly in such a situation. In a fuse, a thin piece of foil or wire quickly vaporizes when an overload of current runs through it. This kills the power to the wire immediately, protecting it from overheating. Fuses must be replaced each time they burn out. A circuit breaker uses the heat from an overload to trip a switch, and circuit breakers are therefore resettable.

The power then enters the home through a typical circuit breaker panel like the one above.

Safety Devices: Circuit Breakers


Inside the circuit breaker panel (right) you can see the two primary wires from the transformer entering the main circuit breaker at the top. The main breaker lets you cut power to the entire panel when necessary. Within this overall setup, all of the wires for the different outlets and lights in the house each have a separate circuit breaker or fuse:


If the circuit breaker is on, then power flows through the wire in the wall and makes its way eventually to its final destination, the outlet.


What an unbelievable story! It took all of that equipment to get power from the power plant to the light in your bedroom.


The next time you drive down the road and look at the power lines, or the next time you flip on a light, you'll hopefully have a much better understanding of what is going on. The power distribution grid is truly an incredible system. http://science.howstuffworks.com/power.htm/printable

Tuesday, March 11, 2008

Generating Electricity using

Battery & GeneratorElectricity starts with electrons. In many materials, the electrons are tightly bound to the atoms. Wood, glass, plastic, ceramic, air, cotton ... These are all examples of materials in which electrons stick with their atoms. Because the electrons don't move, these materials cannot conduct electricity very well, if at all. These materials are electrical insulators.
But most metals have electrons that can detach from their atoms and move around. These are called free electrons. Gold, silver, copper, aluminum, iron, etc., all have free electrons. The loose electrons make it easy for electricity to flow through these materials, so they are known as electrical conductors. They conduct electricity. The moving electrons transmit electrical energy from one point to another.
Electricity needs a conductor in order to move. There also has to be something to make the electricity flow from one point to another through the conductor. One way to get electricity flowing is to use a generator.

Using a battery: Using a battery, a fuel cell or a solar cell to produce electricity, there are three things that are always the same:
The source of electricity will have two terminals: a positive terminal and a negative terminal.
  • The source of electricity (whether it is a generator, battery, etc.) will want to push electrons out of its negative terminal at a certain voltage. For example, a AA battery typically wants to push electrons out at 1.5 volts.
  • The electrons will need to flow from the negative terminal to the positive terminal through a copper wire or some other conductor. When there is a path that goes from the negative to the positive terminal, you have a circuit, and electrons can flow through the wire.
  • You can attach a load of any type (a light bulb, a motor, a TV, etc.) in the middle of the circuit. The source of electricity will power the load, and the load will do its thing (create light, spin a shaft, generate moving pictures, etc.).
    Electrical circuits can get quite complex. But at the simplest level, you always have the source of electricity (a battery, etc.), a load (a light bulb, motor, etc.), and two wires to carry electricity between the battery and the load. Electrons move from the source, through the load and back to the source.

Using a generator:A generator is a simple device that moves a magnet near a wire to create a steady flow of electrons.
One simple way to think about a generator is to imagine it acting like a pump pushing water along. Instead of pushing water, however, a generator uses a magnet to push electrons along. This is a slight over-simplification, but it is nonetheless a very useful analogy.
There are two things that a water pump can do with water:

  • A water pump moves a certain number of water molecules.
  • A water pump applies a certain amount of pressure to the water molecules.

In the same way, the magnet in a generator can:

  • push a certain number of electrons along
  • apply a certain amount of "pressure" to the electrons
In an electrical circuit, the number of electrons that are moving is called the amperage or the current, and it is measured in amps. The "pressure" pushing the electrons along is called the voltage and is measured in volts. So you might hear someone say, "If you spin this generator at 1,000 rpm, it can produce 1 amp at 6 volts." One amp is the number of electrons moving (1 amp physically means that 6.24 x 1018 electrons move through a wire every second), and the voltage is the amount of pressure behind those electrons.

AC vs. DC

Batteries, fuel cells and solar cells all produce something called direct current (DC). The positive and negative terminals of a battery are always, respectively, positive and negative. Current always flows in the same direction between those two terminals.
The power that comes from a power plant, on the other hand, is called alternating current (AC). The direction of the current reverses, or alternates, 60 times per second (in the U.S.) or 50 times per second (in Europe, for example). The power that is available at a wall socket in the United States is 120-volt, 60-cycle AC power.
The big advantage that alternating current provides for the power grid is the fact that it is relatively easy to change the voltage of the power, using a device called a transformer. By using very high voltages for transmitting power long distances, power companies can save a lot of money. Here's how that works.
Let's say that you have a power plant that can produce 1 million watts of power. One way to transmit that power would be to send 1 million amps at 1 volt. Another way to transmit it would be to send 1 amp at 1 million volts. Sending 1 amp requires only a thin wire, and not much of the power is lost to heat during transmission. Sending 1 million amps would require a huge wire.
So power companies convert alternating current to very high voltages for transmission (e.g. 1 million volts), then drop it back down to lower voltages for distribution (e.g. 1,000 volts), and finally down to 120 volts inside the house for safety. It is a lot harder to kill someone with 120 volts than with 1 million volts.