We get more useful energy out of renewables than fossil fuels

EnlargeYaorusheng reader comments 13

It doesn't take a lot of energy to dig up coal or pump oil from the ground. In contrast, most renewable sources of energy involve obtaining and refining resources, sophisticated manufacturing, and installation. So, at first glance, when it comes to the energy used to get more energy—the energy return on investment—fossil fuels seem like a clear winner. That has led some to argue that transitioning to renewables will create an overall drop in net energy production, which nobody is interested in seeing.

A new study by researchers at the UK's University of Leeds, however, suggests that this isn't a concern at all—in most countries, renewables already produce more net energy than the fossil fuels they're displacing. The key to understanding why is that it's much easier to do useful things with electricity than it is with a hunk of coal or a glob of crude oil. Energy efficiency and utility

The basic idea behind the new work is that while it's energetically cheap to extract fossil fuels, the stuff that comes out of the ground isn't ready to be put to use. There are energetic costs to making it into a useful form and transporting it to where it's needed, and then there is lost energy when it's being put to use. That's especially notable for uses like internal combustion engines, where significantly less than half of the energy available in gasoline actually gets converted into motion.

So, the researchers propose an alternate form of the energy return on investment (EROI)—something they call useful-stage EROI. This measures how much energy is needed to put a unit of energy to work in a way that society values—heating a home, moving a car, lighting a room, and so on. This is also a more complicated measure because it depends on how the energy is put to use, which will vary from country to country. So, even though natural gas has the same EROI at extraction, it'll have different useful-stage EROIs in a country that primarily uses gas for heating versus one that's using it for electricity generation since those two activities have different efficiencies. Advertisement

To analyze useful-stage EROIs, the researchers built on a previous publication that calculated what it termed final-stage EROIs, which tracked the energy used to get a unit of energy to where it's ready for use—so, all the energetic costs of extraction, processing, and delivery. This information let them track this statistic for the 50-year period from 1970–2020. Then, they built their own country-level energy use database. Since typical efficiencies of various uses are available, that lets them track the useful-stage EROI in each country they have data for.

The one thing this doesn't include is the energy cost of the infrastructure needed to extract fossil fuels, which, in the case of things like off-shore drilling, can be significant. So, the researchers suggest that they're probably overestimating the useful-stage EROIs for fossil fuels. Inefficiencies

Focusing on utility makes a substantial difference. Using the 2020 data, the final, delivered-to-end-user EROI of fossil fuels is quite good, at approximately 8.5, meaning you get about 8.5 units of energy out for every one you invest. (This is averaged across all fuels and uses.) Once you try to do something useful with it, however, it drops dramatically so that the useful-stage EROI is only about 3.5. Which, to be clear, is bad—you want to be getting as much useful energy as possible for every unit of energy you put into things.

Different fuels have very different profiles, however. Natural gas has the highest useful-stage EROI at 9.5, coal is at 7.2, and oil products are only 2, meaning we only get about twice as much energy out of gasoline as we put into producing and using it. Most of these values have been largely unchanged for the past 50 years except for natural gas, which has seen a dramatic drop in the EROI of getting it ready to use (possibly due to the energy costs of fracking—the trend is most notable in the 1980s), and a smaller drop in useful-stage EROI.

A large contributor to these values is how these fuels are put to use. For example, the useful-stage EROI for natural gas in heating buildings is about 12, meaning it can be used reasonably efficiently. The value for heating with oil products is only about 5. Oil products used in road and rail propulsion are also terrible, being just above 2 for rail travel and under 2 for roads.

Renewable energy, in this analysis, is focused on things like wind and solar, which deliver electrons to the grid (things like renewable production of methane are pretty minor at this point). Those can be used for things like heating, rail and road transit, and other uses performed by fossil fuels. Many of these uses are extremely efficient—things like heat pumps and electric motors are much better at turning energy into utility than their fossil fuel equivalents.