Written by: Kristjan Eljand | Technology Scout
Intro
Energy storage will play a major role in the near future. In this article, I’ll introduce the diverse world of energy storage technologies ranging from Li-Ion batteries to gravitational storage and try to give the intuition of what could be the valid use-cases for these technologies.
The near-term future of electricity will be about renewable energy (mainly wind and solar), near-zero-emission households and electric vehicles. These trends will inevitably lead to a sharp increase in the demand for energy storage:
- One of the main challenges of the world built on renewables is the fluctuation of their output. In other words, we can generate electricity from the wind only if the wind blows and we need the sun to generate solar energy. The solution is the energy storage-saving part of the generated electricity for low-wind/no-sun periods.
- Electric vehicles use batteries to carry all of the necessary energy — the capacity of those batteries needs to match the energy that is currently produced by burning gasoline and diesel.
Our day-to-day experience with energy storage is mainly related to smart-phones and other small electrical devices that use Lithium-ion batteries as their energy carries. Thus, Li-ion is becoming a synonym of energy storage, but the actual environment is much wider and developing rapidly. In this article, we are able to touch only part of this world, but I hope you’ll find it exciting.
Introduction to selected technologies
Pumped Hydro Energy storage
Pumped hydro is a technology that stores energy by pumping water from a lower to a higher reservoir and then releasing it back through the connection, passing through a turbine(s), which generates electricity. The capital components of a conventional pumped hydro facility include two water reservoirs, a waterway to connect them, and a power station that includes a pump and turbine.
Pumped hydro is considered to be the most mature energy storage technology. A majority of the projects operational today originate from the 1970s and 1980s and the concept originated long before that time. Today, pumped hydro provides more than 98% of all installed capacity of energy storage (DOE 2018a).
It is a suitable technology for very large long-term storage projects (100MW+). The initial investment cost is relatively high but once implemented it can run for more than 15 thousand storage cycles. Thus, the long-term project cost should be between 106–200 $/kWh (energy.gov, 2018).
Other Gravitational Energy Storage options
Pumped hydro is not the only technique that seeks to store energy by taking something to higher altitudes. In recent years, people have been very creative when it comes to gravitational energy storage.
For example, ARES has designed a gravitational storage system where heavy concrete blocks are carried to higher altitudes on rails. When the carrier drives downhill with the block, energy is released and transformed into electricity. ARES says that this kind of system can be scaled from 100–3000 MW.
Another start-up, Energy Vault is using a crane-based system to lift composite bricks. Again, energy is stored in the elevation gain. The system could be scaled from 4–8 MW with continuous power discharge for 8–16 hours. It’s also noteworthy that the company recently closed a funding round with $110 million from Softbank.
Both ARES and Energy Vault are just getting started and it is unclear what kind of conditions make a good use case for them.
Compressed Air Energy Storage
Compressed Air Energy Storage (CAES) system is based on using electricity to compress air and store it in underground caverns. The air is released when needed and passed through a turbine to generate electricity. Even though the compressors and gas turbines used are considered to be a mature technology, only a few CAES projects have been implemented globally.
Similarly to pumped hydro, CAES could be a valid option for large long-term storage projects. Due to smaller initial investment costs and comparable lifetime (more than 10 thousand storage cycles), it can potentially be even cheaper than pumped hydro (94–229$/kWh) (energy.gov, 2018).
Flywheel
Flywheel energy storage (FES) works by accelerating a rotor (a spinning wheel or flywheel) to very high speed and maintaining the energy in the system as rotational energy. The basics of this technology are easy to understand when you imagine spinning yourself while holding a heavy bag — when you release the bag, it’ll fly away because you gave it the energy.
Compared to other ways to store electricity, flywheel systems have a long lifetime, large maximum power output and the system can be charged very fast (in less than 15 minutes). Flywheel efficiency can be as high as 90% but it comes with high cost (11 520 $/kWh) (energy.gov, 2018). Thus, the flywheel could be a valid energy storage option for applications that need very high power for a short period.
Li-Ion Batteries
Li-Ion batteries are the main energy carriers for our smartphones and currently available Electric Vehicles. Rechargeable batteries make use of chemical reactions to store and generate electricity but the exact electrochemistry is out of the scope of this article.
Li-Ion technology is popular due to high energy density (it can store a lot of energy for every gram) and low self-discharge (battery keeps its energy level when not used). Contrary to pumped hydro and compressed air, Li-ion batteries can also be used in small projects. The initial investment cost per kW is comparable to pumped hydro but due to much lower lifetime (ca 4–5 thousand storage cycles) its lifetime storage cost is approximately 393–581$/kWh (energy.gov, 2018) making it ca 3–4x more expensive than pumped hydro.
That said, one of the key aspects of battery technologies is that they are becoming cheaper and cheaper. Between 2010 and 2017, battery prices decreased by 80% and it is expected that the trend continues (EASE 2016). NB: this cost decrease is very important because it will open the possibilities for much cheaper electric vehicles in the upcoming decade.
Lead-acid batteries
The lead-acid battery is another very popular battery technology. It is relatively cheap and can supply high surge currents (high energy power). These features make them attractive for use in motor vehicles (a regular car battery is a lead-acid battery).
Lead-acid can be potentially used in similar conditions as Li-Ion but due to even smaller lifetime (ca 900 storage cycles), its long-term storage cost can be expected to be higher.
Flow batteries
Flow batteries are also using electrochemistry to generate electricity but the logic is very different than other battery systems (links for those who want to dive deeper into the differences between Li-Ion and Flow-batteries). These differences allow flow batteries to keep 0% state of charge (traditional Li-Ion battery can’t be fully discharged) and unparalleled cycle lives (10 thousand cycles compared to 3–5 thousand cycles of Li-ion batteries). That said, the technology is not yet mature enough to compete with traditional rechargeables.
Summary
The global energy storage is clearly dominated by pumped hydro that accounts for more than 98% of all installations. The largest part of the rest is taken by the Li-ion and its relevance can be expected to grow in the upcoming years.
Key findings:
- In small scale storage projects, Li-Ion batteries offer the best option in terms of cost, performance, cycle life and technological maturity;
- In large and long-term projects, pumped hydro and compressed air give the lowest cost in $/kWh. Pumped hydro is a more mature technology and has better efficiency.
- Redox flow batteries could become a serious contender in many projects, but the overall cost, performance and technological readiness need large improvements;
- Flywheels and ultracapacitors (not covered in this article) are valid only for applications that need very high power for only a short period;
- In general, the price of different battery energy storage systems (BESS’s) is expected to decrease substantially while the substantial reduction in costs of pumped hydro and compressed air is unlikely.
References
Overview of Energy storage tech in Wikipedia: https://en.wikipedia.org/wiki/Energy_storage;
LAZARD’s analysis of the Levelized cost of storage: https://www.lazard.com/media/450774/lazards-levelized-cost-of-storage-version-40-vfinal.pdf
Energy Storage Technology and Cost Characterization Report: https://www.energy.gov/sites/prod/files/2019/07/f65/Storage%20Cost%20and%20Performance%20Characterization%20Report_Final.pdf
