Abstract
Whether renewable energy sources (RES) will provide sufficient energy surplus to entirely power complex modern societies is under discussion. We contribute to this debate by estimating the current global average energy return on energy invested (EROI) for the five RES technologies with the highest potential of electricity generation from the comprehensive and internally consistent estimations of their material requirements at three distinct energy system boundaries: standard farm-gate (EROIst), final at consumer point-of-use (EROIfinal), and extended (including indirect investments, EROIext). EROIst levels found fall within the respective literature ranges. Expanding the boundaries closer to the system level, we find that only large hydroelectricity would currently have a high EROIext ~ 6.5:1, while the rest of variable RES would be below 3:1: onshore wind (2.9:1), offshore wind (2.3:1), solar Photovoltaic (PV) (1.8:1), and solar Concentrated Solar Power (CSP) (<1:1). These results indicate that, very likely, the global average EROIext levels of variable RES are currently below those of fossil fuel-fired electricity. It remains unknown if technological improvements will be able to compensate for factors, which will become increasingly important as the variable RES scale-up. Hence, without dynamically accounting for the evolution of the EROI of the system, the viability of sustainable energy systems cannot be ensured, especially for modern societies pursuing continuous economic growth.
Highlights
The solution to the coupled problems of climate change, pollution, and the depletion of fossil fuels (FF) needs a rapid transition, in historical terms, from the current fossil-based energy system to one whose main energy sources will be renewable
Accounting for the transmission losses, the energy investments associated to the grids as well as the change from plant to full-system context implies a large gap between the three EROI boundary levels studied, a gap that other authors have found for FFs (e.g., [16,33])
operation and maintenance (O&M) of the grids represent a relevant contribution, increasing around 9% (CSP), 19%, 30% (PV), and 40% the embodied energy of the cradle to gate phase of each technology respectively (EnUNew cap + EnUO&M + EnUDecom + EnUTra following the notation of Equation (4))
Summary
The solution to the coupled problems of climate change, pollution, and the depletion of fossil fuels (FF) needs a rapid transition, in historical terms, from the current fossil-based energy system to one whose main energy sources will be renewable (renewable energy sources, RES). In the ongoing debate about the timing and the features of this transition, environmental, economic, political, and technological issues are pointed out [1,2]. With relation to the latter, the discussion has mainly addressed three interrelated aspects: The requirements and future availability of minerals to build the infrastructures to harness, transform, transport, and store energy [3,4,5,6]. The most popular indicator to measure the ratio of energy surplus with relation to the required energy investments is the energy return on energy invested (EROI), i.e., the ratio of the energy delivered and the energy consumed to deliver that energy in a given time, with very abundant literature focusing both at technology and system level at different geographical scales (see e.g., [3,14,15,16,17,18,19,20,21,22,23])
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