January 2020 Issue
In much of the discussion of renewables as part of the energy mix, the argument often focuses on the unreliable or intermittent nature of these different sources of energy. While at one stage it was true that renewables would not work as firm capacity, the landscape continues to change. With an increasing preponderance of different forms of renewables, such as wind, solar and more recently, tidal and biomass, there is less reliance on any one form of generating electricity or of sun or wind in one place only so the likelihood of everything not generating at the same time is reduced. However, there could still be gaps where the supply is less than demand, which means we still need to consider storage to secure renewables as firm capacity.
The conventional wisdom is that the cost of renewable storage is too high for it to be economically viable, and while that was previously the case, it will not be true going forward.
Renewables plus storage
The cost of solar has dropped by 99 percent over the last four decades, according to MIT, with a trend to lower cost expected to continue. According to the International Energy Agency, the cost of solar power is expected to decline a further 15 to 35 percent by 2024. The cost of wind has also dropped by 66 percent since 2009, according to the Department of Energy (DOE) National Renewable Energy Laboratory (NREL). The combined levelised cost of electricity (LCOE) or average cost of electricity over the lifetime of the plant, that is generated using a mix of renewables and storage, has come down as a result of such decreasing costs. Additionally, the cost of lithium ion batteries has plunged. According to BloombergNEF, the volume weighted average battery pack fell 85 percent from 2010-18, reaching an average of $176/kWh. As increasing the amount of renewables is expected to lead to significant periods of supply when market prices will be very low, with reducing storage costs and renewable generation costs it is becoming increasingly economically viable to have a mix of renewables and storage.
Consumer sources of battery storage
Electric cars and power walls in homes or offices are potential additional sources of power at peak times. When electric cars are not in use, the battery can act as a source of power when needed. Exactly what is possible with electric cars depends on how the owner wants to use the car, but this option can deliver a degree of smoothing out of power, particularly in the evening. Power walls work in a similar way, charging at a time when power is cheap, such as the middle of the night, then discharging at times of high electricity prices, such as when demand is high, so again, it could help to smooth supply and demand.
Batteries alone cannot carry the storage load
Storage does not start and stop with lithium batteries, however. It is not a perfect technology. First, it takes significant energy to produce them. Second, the performance of lithium ion batteries deteriorates over time, so the amount of charge that can be stored reduces and the time to charge increases in relative terms, making them less attractive for energy storage. Third, at present the batteries are still used, if not in their primary application then in a second one. However, continuing to use lithium batteries may lead to a future surplus of unusable batteries.
Beyond battery storage
There are a number of technologies in development which may, at some stage, overtake lithium or be used alongside it for different purposes.
Gravity
There are various forms of gravity storage, but one example consists of a large chunk of rock, typically cut away from the surrounding rock, which can move up and down as a piston. Water is then pumped in from underneath at pressure using electric pumps, which pushes the rock up, then when the time comes to use the power, the flow is reversed and the weight of the rock pushes the water out and that turns a turbine to generate power. The advantage of this technology is the ability to store large amounts of energy in a small space. For example, a rock 100-250m in diameter is typically able to give up to 8 Gwh of storage for six to 14 hours, without major safety risks and for a reasonable cost. Gravity storage is an equivalent of pumped storage which has been in use for many years and can be constructed across a large part of most countries. It is also likely to be the most sustainable option as it requires on-site cutting and construction, but no chemicals or mining. The resultant mechanism is said to have a life of 60 years. However, it is only around 80 percent efficient in energy conversion to and from storage, so that must be factored into the economics.
Compressed air
Compressed air operates on a similar principle. Pump in the air under pressure to an underground cavern, then release the compressed air, again through a turbine. Air is argued to be lower maintenance and has lower cycle costs compared to others. So far, most compressed air storage projects have been large scale but in principle it is possible to have smaller scale versions using compressed air tanks. This is seen as more energy efficient than lithium.
Vanadium oxide
Another alternative is vanadium oxide, or similar electrolytes, where electricity produces two liquids stored separately, which are then combined again to produce electricity. It dates originally from the 1930s but is now coming back to the fore. The advantage of this technology is it is very scaleable in tanks for large-scale, long-term storage.
Hydrogen
The production of hydrogen is old technology, but it has not normally been considered for storage purposes, save on certain island systems, because of the cost of producing and storing the hydrogen. However, like other technologies, the costs are coming down. Hydrogen could become a global energy transfer system, possibly by converting it into and from ammonia to make it easier to transport. It is already used to take wind energy and convert it to energy for use in ships and buses in certain islands where it already compares favourable on cost and reliability. As costs fall and alternative transport technologies develop, it could have far wider applications in the future.
Clearly, there are several alternatives which can help to fill the role of storage, each with different advantages and disadvantages, but together meaning that delivering large-scale, firm, dependable capacity in a mostly renewable energy system is becoming increasingly feasible.
Colin Johnson is a vice president at Charles River Associates. He can be contacted on +44 (0)20 7959 1548 or by email: colinjohnson@crai.com.
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Colin Johnson
Charles River Associates