Wind and solar power are now the cheapest ways of generating electricity in most parts of the world. The costs have become so low that it’s often cheaper to build new renewable power generation than continue operating existing coal power stations. It may seem strange, therefore, that there is not a focused effort to replace all power generation with wind and solar as soon as possible. Why are we still talking about an energy mix that includes other more expensive and polluting technologies?
As engineers, with an understanding of manufacturing, we often assume that the more of something we make, the cheaper it will become. This is almost always true for the actual cost of manufacturing. However, when production depends on scarce resources, the exact opposite happens. The more you produce, the more you drive up the cost of the resources you need to buy. Unfortunately, this may be true for many types of renewable energy. This is one of two fundamental reasons we need a range of different renewable technologies. The other is intermittency of supply.
Some of the scarce resources required for renewable power include:
- Land in suitable locations for wind and solar power generation
- Geological features suitable for pumped-storage hydroelectricity to store intermittent power at a low cost
- Cobalt and other minerals to store power in lithium-ion batteries
- Rare earth magnets to produce efficient generators for wind turbines
- Minerals to produce solar panels
Land for Wind and Solar Power Generation
One of the first considerations when siting new wind and solar power installations is where they will be located. The site must provide reliable wind or sunlight, and the land must be a reasonable cost. Other societal impacts must also be considered. Additionally, the power must be generated sufficiently close to where it will be used so that power transmission doesn’t represent too much additional cost. Many of the best sites have already been used, which means that new renewable power facilities must choose sites where not as much power can be generated, the land is more expensive or electrical transmission costs will be higher. Despite this, the rapidly falling cost of manufacturing and installing the power generation hardware means that the total cost continues to fall. This cannot go on indefinitely. There will come a point when sites either provide very little energy, are in prime real estate or involve huge transmission costs.
The big question is how close are we to using up the economical sites, causing the price of wind power to rise. The good news is that we are a long way away from reaching capacity. In fact, a study carried out last year by the University of Sussex and Aarhus University found that just the onshore wind sites in Europe could power the whole world. Other studies have shown similar results for other regions and solar energy. When offshore wind is considered, there is real abundance of potential sites for power generation. The transition to wind and solar power will not be slowed by any shortage of suitable locations.
Geological Features for Pumped-storage Hydroelectricity
Wind and solar only produce power when the wind is blowing or the sun is shining. Fully transitioning to these power sources would, therefore, require greatly increased storage capacity within our electrical grids. The only current or near-term technology that can economically store the required quantities of energy is pumped-storage hydroelectricity (PSH). Energy is stored by pumping water up to a large reservoir, typically on a mountain. When the energy is needed, it is allowed to flow back down to another reservoir, often 100s of meters below, through a high-pressure pipe that is typically supported by surrounding rock. At the bottom of the pipe the water flows through a turbine at immense pressure to generate electricity.
Although smaller PSH installations are significantly cheaper than equivalent lithium-ion battery banks, the costs of PSH fall dramatically for larger installations. The lifecycle costs are also far lower. Batteries last at best 15 years. While PSH is typically designed to last 100 years, it could last indefinitely with regular maintenance. PSH is an investment that requires a small maintenance cost. Battery storage needs to be completely replaced periodically. It isn’t an investment. It is simply an expense. Despite the long-term economic advantages of PSH, considerable initial capital investment is required and large installations may take over 10 years to come online.
The issue for PSH is finding suitable locations. Two reservoirs must be sited close to one another but with an elevation difference of more than 500m. There must be suitable geological features to contain each reservoir and support the high-pressure pipe connecting them. The mountainous sites with these features are often in unspoiled natural locations. They have ecological and societal value that makes it difficult for a major engineering project or the construction of concrete dams to disturb them. A huge supply of water is also required, which often means depleting local aquifers.
Many countries have sites that are technically able to provide sufficient energy storage for a full transition to wind and solar. However, full-scale adoption is held back by the desire to make storage available rapidly and without major disruption to valued beauty spots and ecological sites.
One interesting solution is to use concrete spheres submerged in the sea close to offshore wind turbines. A study carried out by Goethe University Frankfurt and Saarland University has shown this could deliver economically competitive power. It would, however, be limited to sites with suitable geological features to site both wind turbines and storage spheres in close proximity, with an estimated global capacity of 900 GWh, equivalent to 20 minutes of the worlds average electricity use.
Mineral Resources for Lithium-ion Batteries
There is a growing acceptance that utility-scale lithium-ion battery banks will be required for rapid increases in the use of wind and solar power. PWH capacity cannot be increased rapidly enough and is limited by available locations. Other energy storage technologies just won’t be ready quickly enough, for example, the production of synthetic fuels from solar energy and alternative batteries.
Lithium-ion batteries require cobalt, most of which currently comes for the Democratic Republic of Congo. There are serious ethical issues with these mines, which are closely linked to conflict and child labor. Scaling up battery production for utility storage, as well as increasing use of electric vehicles, will require vastly more cobalt than is currently being produced.
Land-based cobalt reserves are not easily extracted, but there is abundant cobalt in nodules on the ocean floor. For this reason, there is increasing interest in carrying out deep ocean robotic mining to extract these modules. An EU research project has been launched to understand the impact this might have on the ecosystems in the deep ocean.
Extracting the required quantities of cobalt will involve technical challenges and potential ecological damage. In the long term, a scarcity of cobalt should not prevent a barrier to widespread adoption of lithium-ion batteries for utility scale power and automotive electrification. However, the time required for new mines to start producing will present a barrier to rapid deployment.
Wind and Solar Dependence on Critical Metals
Rare earth magnets are currently used to produce efficient generators in wind turbines and electric motors in vehicles. This already consumes a significant amount of the world’s supply of neodymium, dysprosium, praseodymium and terbium. Motors and generators can be constructed without these, using conventional steel and copper windings, but they are less efficient. Superconducting motors and generators offer a possible solution to achieving even higher efficiency and power density without dependence on these rare earth magnets.
All three current photovoltaic technologies are dependent on indium. Silicon cells, which still dominate the market, require silver while the two emerging thin film technologies use a number of critical elements. The leading thin film technology is cadmium telluride (CdTe), which requires a large proportion of tellurium supplies as well as some cadmium. The other main thin film technology, copper indium gallium selenide (CIGS), requires a significant amount of tellurium as well as selenium and gallium.
It is relatively easy to substitute for rare earth magnetic in wind turbines, meaning that availability of critical metals should not have a significant impact on this sector. The major issues are with photovoltaics and energy storage. The amount of photovoltaics are likely to be limited to a small percentage of the energy mix by critical metal availability. A recent report commissioned by the Dutch Ministry of Infrastructure and Water Management shows that achieving the Paris Agreement goals for wind and solar power will require the global production of some metals to grow at least twelvefold by 2050.
Energy storage to mitigate for intermittency in the supply of renewable energy is a major barrier to rapid decarbonisation. Existing technologies will require 10 to 20 years to create pumped-storage hydroelectricity or the cobalt mines required for lithium-ion batteries. Alternative technologies, such as new batteries or synthetic fuels, are likely to take at least that long to be available at scale. For these reasons, it is not realistic to expect battery-electric vehicles to achieve rapid decarbonization. There should be greater emphasis on alternatives, such as active transport, mass transit and electrified highways. We should also be building pumped-storage hydroelectricity with urgency, preserving critical resources for battery use where there are no suitable alternatives. In the long term, hydrogen produced using electrolysis or thermo-chemical reactors might remove storage and transport barriers to renewable energy. However, many technical barriers remain, which is something I will cover in a future article.
It is interesting to note that the world is heavily dependent on China for many of the required critical metals. Renewables may improve energy security, but they do not eliminate strategic interests. As oil loses its strategic importance, we are entering a new age in which geopolitics will be dominated by critical metals. Critical metal dependence for low-carbon technologies is a complex area I will explore in more detail in the second part of this article.