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Friday, June 4, 2010 | |

There are many renewable energy sources in the world such as hydrogen, solar, wind, biomass, hydropower, and geothermal energy. Many people think that hydrogen will be the most important fuel of the future because it meets so many requirements of a good energy system. Experts agree the ideal energy system should include the characteristics listed below:
  • should rely on domestic energy sources.
  • should be able to utilize a variety of energy sources.
  • should produce few harmful pollutants and greenhouse gas emissions.
  • should be energy efficient (high energy output from the energy input).
  • should be accessible (easy to find, produce or harness).
  • should result in stable energy prices.

Hydrogen is a versatile energy carrier that can be used to power nearly every end-use energy need. The fuel cell — an energy conversion device that can efficiently capture and use the power of hydrogen — is the key to making it happen.
  • Stationary fuel cells can be used for backup power, power for remote locations, distributed power generation, and cogeneration (in which excess heat released during electricity generation is used for other applications).
  • Fuel cells can power almost any portable application that typically uses batteries, from hand-held devices to portable generators.
  • Fuel cells can also power our transportation, including personal vehicles, trucks, buses, and marine vessels, as well as provide auxiliary power to traditional transportation technologies. Hydrogen can play a particularly important role in the future by replacing the imported petroleum we currently use in our cars and trucks.

What Is A Fuel Cell

A fuel cell is a device that produces a chemical reaction between substances, generating an electric current in the process. It is an electrochemical energy conversion device. Everyone uses another electrochemical energy conversion device, i.e. a battery. A battery contains substances that produce an electric current as they react. When all of the substances have reacted, the battery is dead; it must be replaced or recharged.

With a fuel cell, the substances (in this case, hydrogen and oxygen) are stored outside of the device. As long as there is a supply of hydrogen and oxygen, the fuel cell can continue to generate an electric current, which can be used to power motors, lights, and other electrical appliances.

Why Fuel Cells?

Fuel cells directly convert the chemical energy in hydrogen to electricity, with pure water and potentially useful heat as the only byproducts.
  • Hydrogen-powered fuel cells are not only pollution-free, but also can have two to three times the efficiency of traditional combustion technologies.
  • A conventional combustion-based power plant typically generates electricity at efficiencies of 33 to 35 percent, while fuel cell systems can generate electricity at efficiencies up to 60 percent (and even higher with cogeneration).
  • The gasoline engine in a conventional car is less than 20% efficient in converting the chemical energy in gasoline into power that moves the vehicle, under normal driving conditions. Hydrogen fuel cell vehicles,which use electric motors, are much more energy efficient and use 40-60 percent of the fuel’s energy — corresponding to more than a 50% reduction in fuel consumption, compared to a conventional vehicle with a gasoline internal combustion engine. 
In addition, fuel cells operate quietly, have fewer moving parts, and are well suited to a variety of applications.

How Do Fuel Cells Work?

A single fuel cell consists of an electrolyte sandwiched between two electrodes, an anode and a cathode. Bipolar plates on either side of the cell help distribute gases and serve as current collectors.

There are many types of fuel cells, but the most important technology for transportation applications is the polymer electrolyte (or proton exchange) membrane or PEM cell. A PEM fuel cell converts hydrogen and oxygen into water, producing an electric current during the process.

In a Polymer Electrolyte Membrane (PEM) fuel cell, which is widely regarded as the most promising for light-duty transportation, hydrogen gas flows through channels to the anode, where a catalyst causes the hydrogen molecules to separate into protons and electrons.  The anode is the negative side of the fuel cell. The anode has channels to disperse the hydrogen gas over the surface of the catalyst. Hydrogen gas under pressure enters the fuel cell on the anode side and reacts with the catalyst.

The polymer electrolyte membrane (PEM) is a specially treated material that conducts positive ions (protons), but blocks electrons from flowing through the membrane.  While the protons are conducted through the membrane to the other side of the cell, the stream of negatively-charged electrons follows an external circuit to the cathode. This flow of electrons is electricity that can be used to do work, such as power a motor.

On the other side of the cell, oxygen gas, typically drawn from the outside air, flows through channels to the cathode.  The cathode is the positive side of the fuel cell. It has channels to distribute oxygen gas to the surface of the catalyst. The oxygen reacts with the catalyst and splits into two oxygen atoms. Each oxygen atom picks up two electrons from the external circuit to form an oxygen ion (that combines with two hydrogen ions (2H+) to form a water molecule (H2O). This union is an exothermic reaction, generating heat that can be used outside the fuel cell.

The power produced by a fuel cell depends on several factors, including the fuel cell type, size, temperature at which it operates, and pressure at which gases are supplied. A single fuel cell produces approximately 1 volt or less — barely enough electricity for even the smallest applications. To increase the amount of electricity generated, individual fuel cells are combined in series to form a stack. (The term “fuel cell” is often used to refer to the entire stack, as well as to the individual cell.) Depending on the application, a fuel cell stack may contain only a few or as many as hundreds of individual cells layered together. This “scalability” makes fuel cells ideal for a wide variety of applications, from laptop computers (50-100 Watts) to homes (1-5kW), vehicles (50-125 kW), and central power generation (1-200 MW or more).

What Is Hydrogen?

Hydrogen is the simplest element known to exist. An atom of hydrogen has oneproton and one electron. It is the lightest element and a gas at normaltemperature and pressure. Hydrogen is also the most abundant gas in the universe and the source of all the energy we receive from the sun. Hydrogen has the highest energy content of any common fuel by weight, but the lowest energy content by volume.

The sun is basically a giant ball of hydrogen and helium gases. In the sun.s core, the process of fusion is continually taking place. During fusion, the protons of four hydrogen atoms combine to form one helium atom with two protons and two neutrons, releasing energy as radiation.

This radiant energy is our most important energy source. It gives us light and heat and makes plants grow. It causes the wind to blow and the rain to fall. It is stored as chemical energy in fossil fuels. Most of the energy we use originally came from the sun.s radiant energy.

Hydrogen as a gas (H2), however, doesn’t exist naturally on earth. It is found only in compound form. Combined with oxygen, it is water (H2O). Combined with carbon, it forms organic compounds such as methane (CH4), coal, and petroleum. It is found in all growing things. biomass. Hydrogen is also an abundant element in the earth.s crust.

How Is Hydrogen Made?

Since hydrogen gas is not found naturally on earth, it must be manufactured. There are many ways to do this. The fact that hydrogen can be produced using so many different domestic resources is an important reason why it is a promising energy carrier. In a hydrogen economy, we will not need to rely on a single resource or technology to meet our energy needs.

Steam Reforming

Industry produces hydrogen by steam reforming, a process in which high-temperature steam separates hydrogen atoms from carbon atoms in methane (CH4), as shown below.

     CH + H2O (steam)  ---> 3H + CO
     CO + H2O (steam) ---> CO2 + H2

Today, most of the hydrogen produced by steam reforming isn.t used as fuel but in industrial processes. Steam reforming is the most cost-effective way to produce hydrogen today and accounts for about 95 percent of the hydrogen produced in the U.S. Because of its limited supply, however, we cannot rely on natural gas to provide hydrogen over the long term. Instead, we will need to produce hydrogen using other resources and technologies, such as those listed below.


One way to make hydrogen is by electrolysis.splitting water into its basic elements hydrogen and oxygen. Electrolysis involves passing an electric current through water (H2O) to separate the water molecules into hydrogen (H2) and oxygen (O2) gases.

The electricity needed for electrolysis can come from a power plant, windmill, photovoltaic cell or any other electricity generator. If the electricity is produced by renewable energy or nuclear power, there is no net increase in greenhouse gases added to the atmosphere. Hydrogen produced by electrolysis is extremely pure, but it is very expensive because of equipment costs and other factors. On the other hand, water is renewable and abundant in many areas.

Technological advances to improve efficiency and reduce costs will make electrolysis a more economical way to produce hydrogen in the future.

Photoelectrochemical Production

Photoelectrolysis uses sunlight to split water into hydrogen and oxygen. A semiconductor absorbs energy from the sun and acts as an electrode to separate the water molecules.

Biomass Gasification

In biomass gasification, wood chips and agricultural wastes are super-heated until they turn into hydrogen and other gases. Biomass can also be used to provide the heat.

Photobiological Production

 Scientists have discovered that some algae and bacteria produce hydrogen under certain conditions, using sunlight as their energy source. Experiments are underway to find ways to induce these microbes to produce hydrogen efficiently.

Coal Gasification With Carbon Sequestration

 In this process, coal is gasified (turned into a gas) with oxygen under high pressure and temperature to produce hydrogen and carbon monoxide (CO). Steam (H2O) is added to the CO to produce hydrogen and carbon dioxide (H2 and CO2). The carbon dioxide is captured and sequestered (stored) to prevent its release into the atmosphere.

Nuclear Thermochemical

In this experimental process, the heat from a controlled nuclear reaction is used to decompose water into hydrogen and oxygen in a series of complex chemical reactions.

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Fuel Cell Fundamentals

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