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Hydrogen- an Energy Vector for Tomorrow?

11/30/2010

While hydrogen is plentiful throughout the Universe and on Earth, it is almost always combined with other elements. Before it can be turned into an energy vector, it has to be produced. This requires the use of various energy sources - including renewable ones - and processes. Hydrogen will eventually be used in fuel cells or combustion engines.

A hydrogen fuel cell
© Keblow

Hydrogen- an Element that Needs to be Isolated

Hydrogen is the most abundant chemical element in the universe. It is found in the composition of the sun, the stars, and gas planets. However, it is more rarely found on Earth and in the Earth's atmosphere.

Hydrogen is not a primary energy source because it is almost never found in its natural state on Earth.

Hydrogen is always combined with other elements - such as carbon, with which it forms methane (CH₄). On our planet it is mainly found in water, whose molecule is a mix of two hydrogen atoms and one oxygen atom (H₂O). To obtain pure hydrogen, it must be separated from the chemical elements to which it is bound.

Hydrogen is not a primary energy source because it is almost never found in its natural state (i.e. occurring naturally such as oil or natural gas) on Earth. However, it is an energy vector, meaning it can transfer energy from one place to another. Like electricity- which is not a primary energy source- hydrogen has to be produced before it can transfer its own energy.

Hydrogen's main advantage over electricity is that it is relatively easy to store. Also, once it is produced, hydrogen can generate energy without polluting or emitting CO₂.

Fossil Fuels- the Main Source of Hydrogen

Today, 95% of hydrogen is made from wood and fossil fuels (natural gas and oil).

95% of hydrogen is made from fossil fuels.

• The most common hydrogen manufacturing process is reforming^1-2 (whereby molecules are converted through chemical reactions) of natural gas using overheated steam or steam reforming³.

The principle is as follows: the carbonized atoms (C) in methane (CH₄) are separated from water (H₂O). After two successive reactions, they reform separately, producing dihydrogen (H₂) and carbon dioxide (CO₂).

• Another process is charcoal gasification^4-5- the chemical conversion of wood, made up mainly of carbon (C) and water (H₂O). Burned at a very high temperature inside a reactor (at 1200-1500°C) wood releases gases that then separate and reform to obtain dihydrogen (H₂) and carbon monoxide (CO).

Producing Hydrogen in a Renewable Manner

Like electricity, making hydrogen requires energy. So to obtain ‘clean' hydrogen, 'clean' energy must itself be used. There are several possible solutions.

The hydrogen contained in a liter of water can be used to produce 2 kWh of electricity.

• Gasification concerns the entire solid biomass field^6-7- i.e. the many types of organic matter that can be burnt to release gas. While wood (by way of coal) is the main source used, plant waste such as straw can also be used. If biomass is systematically replanted, its carbon footprint will remain low.

• Water electrolysis^8-9. This technique is not often used. It consists of breaking down water (H₂O) into dioxygen and dihydrogen (H₂) using an electric current. ‘Clean' hydrogen can be obtained using the electricity from a renewable source (wind or solar energy). The hydrogen contained in a liter of water can be used to produce 2 kWh of electricity.

Other processes^10-11 are also being studied, like:

• Photosynthetic microbes- Using modified microbes can produce hydrogen under the influence of sunlight.

• Photoelectrolysis- A photoelectrochemical cell (an electric component that breaks down water under the influence of sunlight) can produce hydrogen and oxygen bubbles when immersed in water.

• Thermochemical decomposition of water- water molecules heated to a high temperature (800-1000°C) break down and release hydrogen. The disadvantage of this method is that nuclear energy is used to heat the water, which requires significant investment and a uranium supply.

From Hydrogen Production to Energy Production

Once the hydrogen has been produced, it is converted into useable energy. This is the role of the following applications:

• Fuel cell¹²

A generator that converts the energy from the fuel (hydrogen in this case) into electricity through an electrochemical reaction. The fuel cell contains two electrodes:

   • One holds the anode (the electrode with positive polarity), which contains hydrogen

   • The other holds the cathode (the electrode with negative polarity), which contains oxygen

   • Between the two there is an electrolyte, a solid or liquid body that can control the passage of electrons. In fuel cells, it is usually a polymer containing platinum.

In the anode, the hydrogen molecules dissociate and ions are released into the electrolyte, while electrons are forced to circulate in an external circuit, creating a continuous electric current.

In the cathode, electrons from the electric circuit combine with ions and oxygen to form water and recoverable heat.

Eventually, with hydrogen on one side and oxygen on the other, the fuel cell produces electricity and heat. The only by-product of combustion is steam.

There are 3 types of fuel cell:

- The proton exchange membrane fuel cell (PEMFC), which works at a temperature of 80°C. It uses a polymer electrolyte and is suitable for use in a hydrogen-powered vehicle¹³.

- Direct methanol fuel cells (DMFC) and direct ethanol fuel cells (DEFC), which make direct use of the hydrogen contained in the molecules of two alcohols (methanol and ethanol). They can be miniaturized, for example for use in portable devices¹⁴ such as telephones and laptops.

- The solid oxide fuel cell (SOFC), which uses a double oxide made up of zircon and yttrium as an electrolyte. Its operating temperature of 800°C means that any fuel containing hydrogen can be used (methane, wood, etc.) through internal reforming processes. This very heavy cell is suitable for stationary applications¹⁵ (e.g. electricity production plant for collective housing).

• The combustion engine

Hydrogen can be used as a fuel in a conventional internal combustion engine. When present in combination with oxygen it causes an explosion that drives a piston which starts the engine. In this case, mechanical energy is being created. The focus for this application is transportation. Hydrogen combustion in a spark ignition engine also produces heat, in the form of steam as well as a small proportion of nitrogen oxide.

Of the two applications, at 59%, the fuel cell has better efficiency (i.e. the rate of energy produced compared to the energy required to produce it) compared to the internal combustion engine at 36%. It is also less noisy.

Hydrogen-based applications are recent.

False. The fuel cell was discovered in 1839¹⁶. In the 19^thcentury, hydrogen was combined with carbon oxide to provide public lighting and gas mains¹⁷. In the second half of the 20^th century, scientific research related to the space programs gave it a new boost. The fuel cell is used on many satellites. Nowadays, transportation, domestic energy (heating, air-conditioning, etc.) and telecom companies are the most interested in this technology¹⁸.