Basics of Fuel Storage
Energy is stored within fuels in the form of solid, liquid, or gas until it is released by combustion
Energy density is a way of conceptualizing how much energy is stored in fuels, either per mass or per volume
A consideration for storing liquid and gaseous fuels in tanks is reducing the risks of leaks or explosions
Electrolysis is an approach to directly store electrical energy in the form of a fuel such as green hydrogen
Energy density of fuels
Energy density is a useful concept for thinking about the amount of energy stored in fuels. Two ways to conceptualize energy density is the amount of energy per unit of volume and the amount of energy per unit of mass of a fuel. The higher the energy density of a fuel, the greater the amount of energy can be stored in it.
Nuclear fuels have the highest energy density by a considerable margin. Hydrogen comes next, followed by methane. Hydrocarbons like propane and butane are next, followed by gasoline, ethanol, and then coal.
Although hydrogen, methane, and natural gas have the highest energy density per mass, another important consideration is volume. Since these are all gasses, their volume is significantly larger than liquid fuels like gasoline and diesel. Therefore, when considering energy density per unit of volume, gasoline and diesel are much higher than gaseous fuels. In fact, this is why gasoline and diesel are so commonly used.
Some factors that the metric of energy density does not take into account include the mass or volume of the storage units (hydrogen and natural gas often need specialized equipment to either pressurize or cool the tank), the efficiency of the energy conversion, and the purity of the fuel.
The energy of fuels is stored indefinitely without any loss until it is released by combustion, making fuels very useful for energy storage and transportation.
Different fuels can be stored in several different forms: solid (coal), liquid (petroleum, ethanol, diesel, liquified gasses), or gas (natural gas, hydrogen). Each of these states of matter provide challenges and opportunities for storage.
Considerations for storage
Liquid and gaseous fuels stored in tanks can spill and leak. Keeping storage tanks underground protects them from weather, corrosion, and other possible sources of damage. One drawback to storing tanks underground is that it is more difficult to detect small leaks. Gasses are particularly challenging to contain, and pressurized tanks have the risk of exploding, so the material and construction of the storage tank is important.
Source. Environmental Protection Agency
There are several challenges when it comes to storing coal as well. If the coal is oxidized and the pile gets to a high enough temperature, there is a danger of spontaneous combustion. This can be reduced by storing the coal in specific stacks, monitoring the temperature, and protecting it from wind and moisture.
Flows to fuels: electrolysis and green hydrogen
As we move towards an increasingly electrified energy system, and away from fossil fuels, storage will be essential in addressing the challenge of intermittent electricity sources such as solar and wind (electricity must be used immediately when generated). One approach is to store electrical energy in the form of the chemical bonds of fuels such as green hydrogen.
Electrolysis is a process that uses electricity to drive a chemical reaction, such as converting water into hydrogen. Then this hydrogen can be stored until it is time for the energy to be used in a fuel cell or by combustion. The hydrogen is considered green hydrogen if the electricity used for this process comes from renewable or low-carbon sources. Although the round trip efficiency for this process is not very high, hydrogen has a higher storage capacity than batteries, and is therefore an attractive technology for the future.
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Questions for deeper thinking
What are some of the considerations for what makes a good fuel? Why might one type of fuel be better than another for a given application?
One requirement for using fuels is to have an infrastructure that makes their use accessible. As we work to transition away from fossil fuels and to renewable fuels, what are some of the challenges and changes that must be put in place to use new fuels (like green hydrogen)? Consider cars as one example.
What are some of the challenges for storing different types of fuels?
Sources and further reading
Alternative Fuels Data Center: Fuel Properties Comparison
Energy Education: Energy density
Wikipedia: Energy density
Wikipedia: Fuel tank
Wikipedia: Underground storage tank
Midwest Industrial Supply: Keys to Proper Coal Storage
US Energy Information Administration: Independent Statistics and Analysis
Energy Storage: Hydrogen Energy Storage
Page last updated: September 7, 2022