Author: Christian Murphy
Posting Date: July 30, 2003

When interest rises in how we will continue to supply the energy we use, the popular media engages in discussions of these partially technical matters. As a result, descriptive terms are often not well defined and used loosely. One distinction of importance that is often glossed over is the difference between energy carriers and energy sources.

An energy carrier is a way of carrying or transmitting energy. Electricity is an example as it transmits energy over hundreds of kilometers. Generally, energy is not naturally available as electricity in a form that is accessible to us.¹ This means that electricity is not an energy source. An energy source is where the energy actually comes from; so the coal that is burned to produce electricity is the energy source. It is the naturally occurring form of energy that has not been processed by humans.² To create an energy carrier from an energy source a conversion process must occur.

Some confusion may occur as energy sources are moved around and used directly as are energy carriers. Natural gas is an energy source that is piped to many homes in the U.S. for use. Being an energy source does not eliminate the possibility of transportation and use, but there are some sources that are converted to a more usable form before performing work. Oil is converted into gasoline (among several options) before being used in cars. It is, however, possible to burn crude oil in some electricity generation plants without first processing it.

Thermal energy (heat), like electricity, is both an energy carrier and an energy source. Heat sources are geothermal energy and solar thermal energy. Thermal energy is generally not thought of as an energy carrier as it is used as a carrier in limited situations like for transporting space cooling or heating substances over short distances.

As fuels are just about anything that burns, fuel spans both energy sources and energy carriers. Coal is considered a fuel but is clearly an energy source while gasoline is also fuel but is an energy carrier. All fuels originate with an energy source.

The table below lists all the readily available energy sources. It also shows any associated energy carriers that come predominantly from that energy source.

General Energy Carriers

While the energy carriers in the table above come primarily from a specific energy source, there are a couple of carriers that are quite general and can be produced from any energy source.The two general pure energy carriers of current interest are: hydrogen and electricity.

Electricity is the only exception to the source/carrier separation. It is a pure carrier except in the static form that exists in our atmosphere as mentioned in the footnote from above. We cannot currently effectively extract this resource, so for this discussion electricity falls into the pure carrier category. Electricity is highly used in the U.S. as an energy carrier as it transports well over long distances. We also have a significant investment in infrastructure to facilitate its use, and it is consumed directly in many of the devices we use everyday.

Hydrogen is new kid on the block and its technologies are still being developed but it shows great potential, with one caveat. Hydrogen is espoused to be the solution to our energy problems and while hydrogen shows great promise as an energy carrier and energy storage medium, it is not an energy source and cannot be presented as such. The process that creates hydrogen is of critical importance. An economy run with hydrogen created through the conversion of coal or petroleum will experience many of the same difficulties as our current economy with energy insecurity and pollution. We can also create a worse problem because energy will be lost in the conversion of the energy source (coal) to the energy carrier (hydrogen). The promise of hydrogen is that it can be generated by renewable technologies such as wind, solar and biomass and has the advantage over electricity that it can be readily stored.

Our energy carriers come from energy sources and The energy sources of the table above are all the only places we can get significant energy from. The good news is there is plenty of energy there to fulfill our needs. We merely need to insure that existing resources be converted into a form that is usable for us with a minimum of impact and cost.

Footnotes

(1) There is energy stored in the atmosphere as static electricity that is sometimes expressed as lightning strikes, but we have yet to figure out how to use that as an energy source. Also note that the small amount of electricity that can be generated from citrus fruits is stored as chemical energy (as in a battery) and is not stored in the fruit as electricity. It is possible for us to store electricity in devices called capacitors and inductors but currently it can only be done cost effectively with relatively small amounts of energy.

(2) Processed here refers to a conversion process and does not include the resource extraction like mining of the coal or drilling for petroleum. Processed does include the conversion of petroleum to gasoline.