Pick up a research paper on battery technology, fuel cells, energy storage technologies or any of the advanced materials science used in these fields, and you will likely find somewhere in the introductory paragraphs a throwaway line about its application to the storage of renewable energy. Energy storage makes sense for enabling a transition away from fossil fuels to more intermittent sources like wind and solar, and the storage problem presents a meaningful challenge for chemists and materials scientists Or does it?
Several recent analyses of the inputs to our energy systems indicate that, against expectations, energy storage cannot solve the problem of intermittency of wind or solar power. Not for reasons of technical performance, cost, or storage capacity, but for something more intractable: there is not enough surplus energy left over after construction of the generators and the storage system to power our present civilization.
The problem is analysed in an important paper by Weibach et al.1 in terms of energy returned on energy invested, or EROEI the ratio of the energy produced over the life of a power plant to the energy that was required to build it. It takes energy to make a power plant to manufacture its components, mine the fuel, and so on. The power plant needs to make at least this much energy to break even. A break-even powerplant has an EROEI of 1. But such a plant would pointless, as there is no energy surplus to do the useful things we use energy for.
There is a minimum EROEI, greater than 1, that is required for an energy source to be able to run society. An energy system must produce a surplus large enough to sustain things like food production, hospitals, and universities to train the engineers to build the plant, transport, construction, and all the elements of the civilization in which it is embedded.
For countries like the US and Germany, Weibach et al. estimate this minimum viable EROEI to be about 7. An energy source with lower EROEI cannot sustain a society at those levels of complexity, structured along similar lines. If we are to transform our energy system, in particular to one without climate impacts, we need to pay close attention to the EROEI of the end result.
The EROEI values for various electrical power plants are summarized in the figure. The fossil fuel power sources were most accustomed to have a high EROEI of about 30, well above the minimum requirement. Wind power at 16, and concentrating solar power (CSP, or solar thermal power) at 19, are lower, but the energy surplus is still sufficient, in principle, to sustain a developed industrial society. Biomass, and solar photovoltaic (at least in Germany), however, cannot. With an EROEI of only 3.9 and 3.5 respectively, these power sources cannot support with their energy alone both their own fabrication and the societal services we use energy for in a first world country.
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14 August 2017
15 August 2017