Hydrogen and Clean Fuels Basics

Hydrogen as a Fuel

As a fuel, hydrogen has a mixture of promising and daunting properties. Hydrogen’s main attractions as a fuel are that it releases lots of energy when burned, and when burned with oxygen, the only product is water.  Fuel cells can use hydrogen to produce electricity through an electrochemical reaction, rather than burning, but again the only chemical byproduct is water. Since greenhouse gases other than water are not formed from use of hydrogen as a fuel, it has great promise in reducing greenhouse gas emissions.  Hydrogen is not found on Earth in large quantities and therefore is not a primary source of energy.  However, hydrogen can be produced from a variety of sources. Of course, it takes some energy to convert other fuel forms to hydrogen and the production cost is an economic issue. For many years, scientists have made hydrogen for study by passing an electric current through water to separate its constituents, a process called electrolysis. It is made industrially by reacting steam with natural gas at high temperatures, a process called steam methane reforming (SMR).

Pressurized hydrogen storage tank by Quantum

Hydrogen has a high level of energy per unit mass; one kilogram of hydrogen when burned produces about the same amount of energy as a gallon of gasoline. However, gaseous hydrogen has a very low density, which means that storing it takes up a lot of space and its energy density per unit volume is very low. Low energy density is one of the main barriers to using hydrogen as a transportation fuel.  As an example, even when compressed to a very high pressure of 10,000 pounds per square inch (psi), it takes more than 6 gallons of hydrogen to equal the energy in 1 gallon of gasoline (see illustration at right).  This low density makes hydrogen storage systems heavy, bulky, and expensive.  Liquid hydrogen is more dense but only exists at very low temperatures close to Absolute Zero (- 459 °F) and must be stored in expensive cryogenic containers. DOE-sponsored research is developing alternate storage methods such as hydrides, compounds of hydrogen with elements such as lithium and aluminum, which have higher energy densities than pure hydrogen and can release the hydrogen as needed for fuel.  Another barrier to widespread use of hydrogen is the difficulty in transporting hydrogen from central production facilities to places where hydrogen can be used to refuel vehicles. Existing pipelines for natural gas or oil cannot be used to transport hydrogen without modifications, and truck transport is too expensive to be widely used. 

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