OPTIONAL EXTRA CREDIT - Energy Return on Energy Invested

Net Energy and Embodied Energy

We're on a roll, so let's continue with the "budget" theme. In Lesson 1, you learned that energy sources contain a certain amount of energy. In Lesson 1, you also learned that there are 3412 Btu in a kilowatt hour and ~125,000 Btu in a gallon of gasoline. But that only tells part of the story. Almost all energy sources require energy inputs in order to get them to the end user. Let's take that gallon of gas as an example. How was energy used to get that gallon of gas to you and your car? Think about where it came from and how it got there, and what happened in between, then read on.

Gas is a product of petroleum (oil). Most oil comes from the rocks in the ground (tar sands notwithstanding), and this oil is accessed by drilling. Drilling requires energy, as does transporting the oil to a refinery, refining the oil to make gas, and transporting the gas to the gas station. Every step in this process requires energy. The important thing to consider is that if you can determine the amount of energy required to get that gallon of gas to you, and subtract that from the energy you get from the gas, you will have net energy. 

It is important to note that it does not matter which form energy is used in this process (electricity, natural gas, oil, etc.), only how much is used. (Remember that we can calculate total energy used by converting to common units!) This "invisible" energy used prior to the end use is usually referred to as embodied energy.

Embodied energy is used for most energy sources, even renewables. For example, wind turbines and solar panels must be manufactured, their components mined or otherwise processed, then they must be installed using energy, etc. Note that embodied energy can actually be stated for just about anything - remember from Lesson 1 that most food required energy to be planted, grown, shipped, and/or processed. All of this represents embodied energy.

Net energy can be calculated as follows:

• net energy = end use energy - embodied energy

Net energy is a good start, but it can get confusing if you analyze different quantities of energy. This is because the net energy depends significantly on the end use energy amount, but that is not the whole story. Let's look at an example. On average you get about 20 times more energy out of gasoline (end use) than was put in (embodied). In other words, the embodied energy in a gallon of gas is about one-twentieth (1/20) of the end use energy (source: Hall, Lambert, and Balogh, 2014). The net energy for 1 gallon of gas would be (Math alert!):

• Since a gallon of gas is ~125,000 Btu, the average embodied energy is 20 times less:
  • 125,000 Btu/20 = 6,250 Btu.
• The net energy for one gallon of gas, using these assumptions, would be:
  • end use energy - embodied energy = net energy
  • 125,000 Btu - 6,250 Btu = 118,750 Btu.

But what if you get 10 gallons of gas?

• The end use energy is:
  • 125,000 Btu x 10 = 1,250,000 Btu.
• The embodied energy is:
  • 1,250,000 Btu/20 = 62,500 Btu.
• The net energy for 10 gallons of gas would be:
  • 1,250,000 Btu - 62,500 Btu = 1,187,500 Btu.

As you can see, net energy is highly dependent upon the end use energy amount being considered. There is a huge difference between 118,750 Btu and 1,187,500 Btu, but those numbers are actually telling the same story.

Energy Return on Energy Invested (EROI)

You can avoid this confusion by calculating the energy return on energy invested, or EROI. EROI is defined by the Encyclopedia of Earth thusly:

Energy return on investment (EROI) is the ratio of the energy delivered by a process to the energy used directly and indirectly in that process.
Credit: Encyclopedia of Earth

Here is the equation. (Note that "Quantity of energy supplied" is the same as end-use energy and "Quantity of energy used in supply process " is the same as embodied energy.):

$$\text{ EROI }=\frac{\text{ quantity of energy supplied }}{\text{ quantity of energy used in the supply process }}$$

Credit: Encyclopedia of Earth, CC BY-SA 2.5

Because it is a ratio, the end use amount does not matter, because it all balances out in the end. Let's look at the gasoline example from above using this equation:

• EROI of 1 gallon of gas: 125,000 Btu/6,250 Btu = 20
• EROI of 10 gallons of gas: 1,250,000 Btu/62,500 Btu = 20

No matter how much gasoline you analyze, you will come up with the same EROI if the assumptions are the same. As noted above, EROI is relevant to almost every energy source. The higher the EROI, the more energy you get out for every energy unit you put in. Calculating EROI often requires using a lot of assumptions, but since this is an important issue, many attempts have been made to calculate it. The article below indicates some of the complexities and uncertainty in calculating EROI, but the authors are able to arrive at general conclusions since they analyzed a number of peer-reviewed research papers on the topic.

When a Barrel of Oil is Not Really a Barrel of Oil

If you read the article, you should clearly see that not all barrels of oil are created equally. The figures below (from the article) indicate the average EROIs of different energy sources. Note that oil and natural gas are lumped together, because they are often extracted together. The authors are careful to point out that these values should not be taken at "face value," but that they are a good indication of the relative EROIs of different sources.

Figure 2.13: The EROI values of various energy sources, aggregated from various published studies. All else being equal, higher EROI is better.
 

Click for a text description

The EROI values of various energy sources aggregated from various published studies. All else being equal, higher EROI is better.

• Above 75 EROI: Hydroelectric
• Near 50 EROI: Coal
• Near 25 EROI: Oil and Gas (World), Wind
• Approximately 10-12 EROI: Oil Shale, Coal, Nuclear, Geothermal, Solar (PV)
• Near 0 EROI: Tar Sands, Ethanol from Biomass, Diesel from Biomass, Natural Gas

Image Credit: Hall, Lambert, and Balogh, CC BY-NC-ND 3.0

Why is EROI important? One of the main reasons is that EROI is more indicative of the true net energy benefit of various fuels than the end use. It takes about the energy from 1 barrel of oil to extract 20 actual barrels of "traditional" oil (it has an EROI of about 20:1), but the same amount of energy, when used to extract tar sands oil, results in only about 4 actual barrels. In other words, EROI indicates that you get about 5 times the amount of energy from traditional oil than from tar sand oil given the same amount of input. 

A very interesting finding in the Hall, Lambert, and Balogh article is that oil discovery in the U.S. has decreased from 1000:1 in 1919 to only 5:1 in the 2010s, meaning we get 100 times less energy now than 90 years ago! (Essentially, we have extracted most of the "easy to get" oil, and do things like deep sea drilling now.) Getting ethanol from corn (recall from Lesson 1 that this is the U.S.'s primary source of biofuel) can require almost as much energy in as energy you get out, depending on how it is grown and processed.

EROI can help policymakers and others decide which energy source is a more efficient use of energy resources. In the context of this course, it is a particularly important consideration for non-renewable resources, because it indicates the net energy benefit of the sources.

One extremely important final thing to note: EROI only describes energy use. It says nothing about the other important impacts and factors. For example:

• Total energy available is an important consideration - if we can get something really efficiently (high EROI), but there is not a lot of it, then that may not help very much.
• Coal has a relatively high EROI but is the most polluting energy source we use.
• Hydroelectricity has a very high EROI, but if done the wrong way can have negative impacts as well.
• Tar sands, on the other hand, have both a low EROI and a very negative impact on the environment.
• Coal (like oil, natural gas, and nuclear) are non-renewable, and thus limited. (Though we are unlikely to run out any time soon for most of these, as you'll see in a future lesson.)
• All resources are available in limited locations and can be difficult to transport efficiently. Local/native resources may be more logical to use, even if they have a relatively low EROI.

In short, EROI is only one consideration to be made.