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Looking to the Future: Energy Principles and Technologies
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These days, most of our electricity is generated by generators spun by turbines in which spinning magnets exert an electromotive force on tightly wound copper wires. But remember, energy cannot be created or destroyed; the energy to turn the magnet must come from somewhere. Typically, the energy to turn the magnet comes from fossil fuels, nuclear power, or falling water in hydroelectric dams. But there are other ways to generate electricity.Wind turbines put a new spin on an old technology.Windmills which convert wind energy into mechanical energy have been around for millennia and were commonly used for grinding grains or pumping water. So it was a small leap to adapt windmills for generating electricity. Wind turbines are now made of high tech materials in sleek aerodynamic shapes and come in a wide variety of sizes. The smallest are less than a meter (~three feet) in diameter and supply a small amount of electricity to boats and rural homes. The largest are as much as 100 meters (~300+ feet) in diameter which can supply enough electricity to power a small town. They are widely used in Europe where a few large wind farms produce electricity for the power grid and many homes and businesses harness the wind in smaller applications.
Modern wind turbines have peak efficiencies of about 30%. This may not seem like much, given that the efficiency of the canal power plant is between 35 and 40%. But consider that wind power is infinitely and freely available, requires no transportation, and produces no pollutants in the process of generating electricity. In the end, wind turbines often produce electricity at or below the cost per kilowatt-hour of an oil or coal-fired power plant, which makes wind turbines an attractive option for generating electricity in windy places.
Photovoltaic (PV) cells capture the sun’s energy and convert it into electrical energy. The cells are usually made of layers of silicon with chemical additives called dopants that set up an electrical potential in the layers (see diagram). When sunlight hits the panel, some of the electrons in the silicon of the "p-type" layer are excited and move away from their proton partners, leaving a positively charged "hole." The excited negatively-charged electrons move toward the "n-type" layer where a metal contact can conduct the electrical current to a "load" such as a heater or a home appliance. The load is also connected to the metal contact on the back of the PV cell, completing the circuit.Individual PV cells do not generate very much electricity (modern, low-cost cells have efficiencies of about 12%), so normally many cells are connected together to make a single panel, and often two or more panels will be connected together to generate the desired amount of electricity. And since they do not generate any electricity at night, the electricity generated by PV cells is typically stored in a battery which can be tapped for a continuous supply of power.
In the US and Europe, a few big PV installations generate a large amount of electricity which is sold to the grid, but most PV applications are small. Solar power provides electricity to the US Coast Guard’s lights and foghorns and to most of the satellites which circle the earth. Solar panels are particularly useful in developing countries where they provide power to run water pumps and small household appliances in remote rural communities. In homes in Europe and the US, PV panels are commonly used to supplement or replace electricity from the power grid. Tiny PV cells can power calculators, watches, some cell phones, and even laptop computers.
In years past, PV cells were too inefficient and expensive to justify using them in all but the most remote locations, namely in space and at sea. But recent advances in manufacturing and design have lowered the price and increased the efficiency of high quality PV cells to 20% or more. Promising new technologies link PV cells to hydrogen fuel cells.
Hydrogen Fuel Cells, which were first used in space, generate electricity using energy released when hydrogen reacts with oxygen to form water. The hydrogen for this reaction is produced by extracting it from a high-energy molecule and can come from any of a number of sources, including natural gas, methanol, gasoline, or even water. Photovoltaic cells can provide enough electricity to split water molecules apart into hydrogen and oxygen, a process called electrolysis (-lysis means "splitting"). Once the hydrogen is generated from any of these sources, it can be fed directly to the fuel cell or it can be stored for later use.
Remember that the element hydrogen is simply one electron hovering around a nucleus containing one proton. When hydrogen enters the fuel cell it flows through a porous positively-charged electrode which separates the single electron from the nucleus of the atom. The remaining proton, a hydrogen ion (H+), moves through an electrolyte (a conducting solution or gel), pulled by the charges of the electrodes. Finally, it passes through a negatively charged electrode, where it encounters oxygen and meets incoming electrons from the circuit. The hydrogen and oxygen react to form water which is released as steam from the fuel cell. This process generates an electrical current in the electrodes, and we can store that energy in a standard battery or use it immediately to do work.
It is important to note that in fuel cells, hydrogen is not in itself a source of energy, but serves as an efficient way to store energy. Remember that producing the hydrogen gas requires an energy source such as PV, natural gas, methanol, or gasoline, and most of that energy is stored as potential energy in the relatively unstable hydrogen gas. Most of this stored energy is released when the hydrogen encounters oxygen and reacts to form water. The fuel cell is simply a device to control the reaction and convert the energy from the reaction directly into electrical current.
Fuel cells are extremely efficient compared to other methods of generating electricity; they commonly convert 40 to 50% of the energy used to generate hydrogen into electricity and the fuel cell’s waste heat can be used to heat buildings, adding to the unit’s efficiency. Some large experimental fuel cells have efficiencies of up to 60% for electrical generation alone. Operating a fuel cell can be entirely non-polluting if PV panels are used as an energy source, and they can make much more efficient use of fossil fuels than conventional combustion technologies, reducing the amount of pollutants generated in producing the same amount of electricity. Because they are clean, quiet, efficient, can use a wide variety of energy sources, and serve a wide variety of functions, fuel cells are likely to play a major role in energy generation in the near future. They are already used in vehicles, homes, and industry. In the near future, fuel cells will power cellular phones, computers, and affordable automobiles.
