The 'energy return on investment' metric is proving that renewable energy can be as efficient as fossil fuels, says Rick Gould – what exactly is it?
Rapid growth in energy demand is a major obstacle to the decarbonisation of the global economy, with the International Energy Agency predicting that demand will increase by about 30% during the next 20 years. Another obstacle is the historical perception that fossil fuels are more efficient and economical sources of energy than renewable sources. However, the latest developments in a field known as net energy analysis are challenging this view. Indeed, a metric known as the 'energy return on investment' (EROI) is not only showing that renewable energy is increasingly as efficient as fossil-fuelled sources, but also that we are rapidly heading for a conflict in supply and demand, known as the 'energy cliff'.
Fishing for answers
In the 1970s, Charles Hall, now an ecologist and emeritus professor at the State University of New York, was doing his PhD research on migrating fish. He determined that even though the fish expended energy when migrating to a new food source, they gained five joules of energy for each joule they expended. It was not long before, in the early 1980s, Hall and his PhD student Cutler Cleveland turned their attention to society's energy needs and the EROI of fossil fuels.
In simple terms, an EROI is the ratio of the energy available in a unit of oil, coal and gas, divided by the energy required to extract the fossil fuel. Hall and Cleveland's results found their way onto the front page of the Wall Street Journal. So what did they find?
“The EROIs have been falling for decades,“ explains Hall. “They were originally very high, but as fossil fuels become harder to find, we need more and more energy to extract them.“ Indeed, published EROIs for oil in the earlier part of the 20th century were in the order of 80 to 100, whereas estimates at present average nearer 20. Eventually, there will come a point where the energy required for extraction nears the energy value of the fossil fuel itself – an EROI so low that it is uneconomic and unviable to continue extraction.
Researchers soon took an interest in Hall and Cleveland's work, with papers on EROIs popping up in academic journals worldwide. Early work focused on fossil fuels, but during the past two decades researchers have turned their attention to renewable energy and biofuels.
This has inevitably led to comparisons between the EROIs of fossil fuels and renewables. However, many researchers have found different results for the same energy source. Why is this, and how can researchers and analysts determine truly comparable EROIs?
Apples with apples
Dave Murphy, a former PhD student of Hall's and now an associate professor of environmental studies at St Lawrence University in New York, favours an approach to calculating EROIs based on life-cycle assessment (LCA). “The problem with a lot of the comparisons between fossil fuels and renewable energy is that researchers published EROIs for fossil fuels at point of extraction, whereas those for renewables were all at point of use,“ he explains. The EROI at point of use considers all steps necessary to convert the energy source into useful energy, such as electricity at a plug socket.
A lot of published literature on EROIs typically favoured coal and oil over renewables, with especially low EROIs assigned to solar photovoltaic (PV) systems. “These were not comparing apples with apples,“ emphasises Murphy. He and his co-workers developed a protocol that applied the LCA methods prescribed in ISO 14040 and ISO 14044, and found strikingly different results. “When we looked at the energy costs, for example generating electricity from refined oil, the EROIs fell significantly,“ he says – so much so that renewables looked a lot more favourable.
Murphy and his associates proposed two types of EROI – a standard metric, EROIst, for the raw extracted energy source, and a metric for the source at the point of use, EROIpou. “Estimating the EROIpou, when combined with LCA methodology, provides more meaningful comparisons,“ says Murphy. “At the same time, it is important to define the boundaries and inputs for the calculations, as the same energy sources can have different EROIs depending on where they originate and how they are processed.“
EROIs for solar PV, for example, will increase as the technology improves, but will always be higher in regions where the sun shines for longer and at a higher intensity. “Intermittency of renewable energy also complicates the situation,“ says Hall. “We could compensate by building more capacity and using batteries, but this would reduce the EROI.“
Have researchers found a minimum EROI for sustainable energy generation? “This is a challenging area because of all the variables, but we should ideally aim for EROIs that are upwards of seven,“ says Murphy. Notably, several researchers have found that the gains in net energy do not rise much when an EROI is above 10, but drop rapidly below (Figure 2). There also appears to be a break-even point, where EROIs of two to three would sustain infrastructure but not provide for any economic growth.
“We started calling this 'the energy cliff',“ adds Murphy, referring to the point at which EROIs show unsustainable energy use, even before looking at air quality, resource availability and climate change. Many researchers have found this with biofuels, with numerous scientific papers producing EROIs of less than unity. In other words, the research shows that we are using fossil fuels to subsidise biofuels.
Informed decisions
When applied consistently and systematically, EROIs can inform decisions in energy investments for individual sources of energy. In the UK, Paul Brockway at Leeds University works within a team of energy researchers that is taking this further by developing a methodology to calculate fossil fuel EROIs at a national and global level.
Brockway, an engineer by background, was drawn to EROIs for similar reasons to Murphy and Hall. “It is part of looking at future energy constraints to economic growth. If we are approaching a 'net energy cliff', then it's better to know sooner rather than later,“ he explains. “At the same time, I saw that the EROI field was controversial for those working in it and unknown for the greater majority. I wanted to work in the field to help the process of comparing EROI apples to apples.“
Brockway sees the value in using LCA approaches to calculate EROIs at the point of use, having seen the earlier erroneous comparisons between fossil fuels and renewables. That said, the Leeds team is using input-output models to determine EROIs at a national level. This is analogous to LCA, but a more appropriate approach at a macro-scale. “There are different methods for different purposes,“ he says. “LCA is suitable for individual products or site-based calculations, whereas input-output methods are better for larger, economy-wide scales. Also, improving national-level data on renewables will help economy-wide comparisons, while EROIs will help provide information about net energy for the renewables transition.“
Rick Gould, MIEMA CEnv is a technical advisor at the Environment Agency. He is writing in a personal capacity
Image credit: Shutterstock
