Research on the concept of a circular economy typically focuses on closing material cycles — the idea being that it avoids the environmental impacts of extracting raw materials and solves the waste problem. However, a team of Swiss researchers believe this approach is too narrow to use as a foundation for a sustainable society because it leaves open the questions of how much and how quickly materials can be cycled and — most importantly — what energy powers these cycles. Largely neglected in such work, they claim, is that such cycles will require large-scale development of renewable energy resources which themselves must be powered in a sustainable manner.
So, Harald Desing and his team from the Swiss Federal Laboratories for Materials Science and Technology (EMPA), St. Gallen, Switzerland, have posed this question: “Is there enough renewable energy available globally to sustainably manage material flows without violating planetary boundaries?”
Writing in the open access journal Energies, they start from the point of view of Earth as a system that only exchanges energy with space. Solar radiation accounts for most of the energy brought into the system and — with small contributions from planetary motion and geothermal energy — the Earth uses this to power subsystems such as oceans, forests and the atmosphere. These, in turn, extract free energy (exergy) from the incoming energy fluxes and convert it to wind, water currents and biomass production.
Whether energy conversions are taking place in the natural Earth system or in a technosphere created by humans, all energy ultimately radiates back into space, the authors note.
The challenge comes as humanity diverts renewable energy fluxes to its own activities and reduces those available to the Earth. If this becomes too large, it will exceed “tipping points,” the authors warn, leading to rapid and irreversible changes in the Earth system — melting ice caps and the resulting acceleration of climate change, for example.
Desing and his co-researchers estimate that Earth needs 99.96% of the energy arriving from space to power its systems, leaving 0.04% for use by mankind. However, they add, this small fraction is still roughly ten times higher than today’s global energy demand.
The researchers used a system of electrical energy equivalents to further study various renewable energy possibilities. This showed some substantial differences in conversion losses depending on whether solar energy, wood or hydropower generates the electricity. In fact, direct solar energy conversion involves fewer conversion steps and fewer energy losses than other renewable options.
However, the scale of solar parks needed to achieve this present its own threat to the Earth’s existing system in terms of disruption to biodiversity, evaporation and, thus, the water cycle, the radiation of heat back to space and much more.
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The land issue also applies to harvesting of what they refer to as chemical energy from agriculture and forestry. This, in turn, competes with food production.
The paper proposes a new method to estimate the global appropriable technical potential (ATP), which considers and respects Earth system boundaries and the human demand for chemical energy.
The new method could be used in global, national and even local scenarios to evaluate questions such as: “Can the current food waste be a significant renewable energy resource?” and “How would an improved conversion technology increase ATP?” Furthermore, with additional data and robust assumptions, the method could answer policy-relevant questions for moving towards a sustainable circular economy such as: “What are priority renewable energy resources for investments?” or “What maximum levels of circularity are achievable with the appropriable renewable energy?”
The authors acknowledge their findings are essentially preliminary and their judgment is subjective and restricted to the selected data sources. “Refined and policy-relevant results could be obtained with more data, more-detailed models and experts’ knowledge in the respective fields of energy options,” they note. All calculation sheets and Matlab code files are available in the supplementary materials section of the paper.
Similarly, adding lifecycle impact assessments for state-of-the-art technologies to the current method could offer a broader perspective.
For now, the EMPA team is exploring what such a pathway from a fossil to a solar society might look like; the solar energy system must not only be large enough to meet global demand but also replace the fossil fuel system quickly enough to avert the climate catastrophe in time, they caution.
Seán Ottewell is Chemical Processing's editor at large. You can email him at [email protected].