Authors: Russell Muren and Kevin Davis
Take a look at state-of-the art, distributed grid storage technologies and, at first, you may be overwhelmed by what appears to be a variety of approaches. However, upon closer review, you could easily label each as one of the following: conventional battery, flow battery, compressed air system, or flywheel. At Rebound we simply refer to these technologies as “batteries” and believe they aren’t the energy storage solution our grid demands.
To us, a “battery” is any technology using a reversible process that is A) driven in one direction by grid energy during times of low value and B) allowed to proceed in the opposite direction, returning energy to the grid during times of high value. Whether you are driving a reversible chemical reaction, compressing and decompressing air, or spinning up and down a hunk of carbon fiber, the principle is the same. Electricity comes off the grid and into a storage material, which later releases slightly less electricity when needed. It’s the same approach across the board. And, more importantly, each system is improved through a very similar, yet ineffective, incremental R&D approach. Lets look at an example:
In an Adiabatic Compressed Air Energy Storage (ACAES) system, electricity is removed from the grid and used to compress air. The heat generated during compression is removed and stored in a separate storage material while the cooled compressed air is stored in a tank. During discharge, the system operates in reverse with the cool air reheated by the storage material and decompressed via a turbine to generate electricity. This particular technology is improved via three methods:
- Storage vessel development to store the air at higher pressures.
- Thermal storage system development to store the heat of compression at higher temperatures.
- Heat transfer development to remove the heat of compression more efficiently.
Each method above is an incremental R&D approach that potentially improves the system, but that fails to change the underlying rigid concept of ACAES. It’s this rigidity that forces engineers to move towards improvements focused on extreme temperatures, pressures, and chemistries. For example, to increase the storage efficiency associated with the heat-of-compression, new ACAES systems must tolerate storage temperatures above 600°C. Thermal-electric conversion efficiencies at this temperature do approach an attractive 40%, but balance of system components which were once carbon steel, plastics, or aluminum, are now Duplex steels, Nickel-based super alloys, or ceramic-coated materials. This transition triggers a 20-fold, system component cost increase that dwarfs the marginal, one-fold increase in thermal efficiency.
This incremental R&D approach might be appropriate for mature technologies staying ahead of the competition through 5-10% annual improvements, but it won’t cut it for long-term grid storage solutions. None of todays “battery-based” storage technologies are even close to cost competitive. The industry needs an 80% cost reduction in energy storage technology and it needs it now.
At Rebound, we reject the “battery” concept because the key to cost effective grid storage is not an incremental improvement over any of today’s concepts. We must move past this singularly focused technological idea to smarter, systems-based grid storage approaches that enable dramatic cost reductions. The Rebound concept is representative of the technological shift the grid needs to move forward with low cost, reliable energy storage.
Rebound’s technology relies on two, thermally bound processes, not a single reversible process like a “battery.” One process takes low value energy off the grid at night and another uses that energy to enhance the conversion of solar energy into electricity during high value periods. The system operates between -100 and 100°C at ambient pressures using a water-based energy storage material without any novel moving parts. Through this approach Rebound can achieve storage performance exceeding that of conventional “battery” technologies, and at a fraction of the cost.
Rebound believes our system can reach sub-$100/kWh storage costs with current, off-the-shelf technology. Even greater cost reductions can be realized through components re-tooled to our specific operating conditions. Now, do other “battery-based” storage companies say they can get to sub $100/kWh? Of course. Will they get there? No. There is just no realistic and technically defensible path that gets the outdated “battery” concept the 80% cost reductions demanded by industry. The only choice is to move to smarter, systems-based approaches.