Energy Return on Investment (EROI) is a concept used to evaluate the efficiency of energy production. It quantifies the ratio of usable energy delivered from an energy source to the total energy expended to obtain it. Understanding EROI helps in assessing the true viability and sustainability of various energy sources. This metric provides a consistent framework for comparing different energy systems, from fossil fuels to renewables.
Understanding Energy Return on Investment
EROI is calculated as a simple ratio: the amount of energy yielded divided by the amount of energy invested to produce that yield. Think of it like a farmer who invests seeds, water, and labor to grow crops; the EROI would compare the energy content of the harvested crops to the energy put into growing them. If the farmer expends more energy than the crops provide, the endeavor is unsustainable.
The “energy invested” includes all energy inputs across the entire lifecycle of an energy source. This encompasses the energy used for exploration, extraction, processing, transportation, and even the manufacturing of the infrastructure itself, such as drilling rigs or solar panels. Similarly, “energy returned” refers to the usable energy delivered to society. A higher EROI indicates a greater amount of net energy is available for other societal activities.
Significance of EROI
EROI serves as a metric for assessing the true cost and sustainability of energy production, extending beyond mere monetary expenses. It highlights the net energy available to society, which is the actual energy surplus that powers all other economic and social functions.
This concept reveals how much energy is left over after the energy production process is complete. If an energy source has a low EROI, a large portion of its generated energy must be reinvested to produce more energy, leaving less for other societal needs like transportation, manufacturing, or healthcare. For a fuel or energy source to be considered viable, it needs an EROI ratio of at least 3:1. An EROI of around 7:1 is considered the economic break-even point for developed countries to sustain a complex socioeconomic system.
EROI Across Energy Sources
EROI values vary across different energy sources and can change over time as technologies evolve and resources become less accessible. Historically, easily accessible conventional oil had very high EROI values, estimated as high as 100:1 in the early 20th century. As the most readily available reserves have depleted, the EROI for conventional oil has declined, with modern estimates for new wells ranging from 10:1 to 20:1. Offshore drilling and oil sands mining have lower EROI values, with offshore oil ranging from about 7:1 to 15:1 and oil sands from 3:1 to 5:1, due to the increased energy required for extraction and processing. The weighted average EROI of all oil liquids is projected to decrease to a plateau of 6.7 by 2050.
Renewable energy sources present a different range of EROI values. Large hydroelectricity projects have a high EROI, averaging around 110:1 over a 100-year operational period. Wind turbines also show EROI values, with operational wind turbines averaging 19.8:1 based on 2018 data, and newer models like the Vestas V150 reporting an EROI of 31:1. Solar photovoltaic (PV) systems have a lower EROI compared to hydro or wind, with a global average EROIext (extended boundary) of about 1.8:1. While the EROI of renewables has been lower than fossil fuels, continuous technological improvements in areas like solar cell efficiency and wind turbine design are expected to enhance their EROI in the future.
EROI and Future Energy Pathways
Understanding EROI is important for shaping future energy infrastructure and policy decisions. It provides insights into the true energy cost of various options, moving beyond just monetary considerations. As the EROI of easily accessible fossil fuels declines, societies face the challenge of investing more energy to obtain the same amount of net energy. This trend necessitates a careful evaluation of alternative energy sources and their respective EROI values to ensure a sustainable energy transition.
A declining overall EROI for a society could impact economic growth and resource availability, as a larger proportion of gross energy production would be consumed by the energy sector itself. EROI analysis guides the identification of viable alternative energy sources and informs strategic investments in energy infrastructure. It helps in planning for a future where energy systems must deliver sufficient net energy to support ongoing societal needs and development.