Different Types of Storage for the Electrical Grid
In a previous post, we discussed why we need storage and how the electrical grid and renewable energy works. Here, we will explain the multi-faceted topic that is grid-scale storage. Most people are familiar with how energy storage works in the form of lithium ion batteries in our cell phones, toys, electronics, and cars. However, grid-scale electricity storage is a bit different and offers different benefits.
Just as in a home, where calculator batteries and car batteries have different sizes and are used for different purposes, a grid also needs different kinds of storage (usually with power capacities of one to several hundred MW). You wouldn’t tote around a car battery to charge your iPhone because it’s too big. The electrical grid also needs different sized batteries to serve different purposes.
Since electricity cannot be stored as electricity itself, the energy from an electric field can only be stored if it is converted to another form of energy, and from there, converted back into electrical energy. The mediums for possible energy storage are varied, and include:
- Mechanical energy, such as the rotational kinetic energy of a flywheel, the gravitational potential energy of pumped hydro, or thermodynamic free energy of compressed air energy storage
- Electrochemical energy created through chemical reactions in batteries
- Chemical energy such as the creation of hydrogen through the electrolysis of water
At grid scale, the following technologies serve the different storage segments:
Small storage capacity, fast response
Fast response assets can support frequency regulation on the grid for seconds to minutes. Frequency regulation is standby power or storage for short-term (measured in seconds) grid frequency imbalances to maintain 60hz. An example of this storage asset is a flywheel.
Medium-sized storage capacity
Medium-size assets can time-shift several MWh of generation from one part of the day to the next, and it includes assets like Li-ion battery packs.
Large-sized storage capacity
Large, long-term storage assets can shift days, weeks or months of surplus to another time when there is an energy deficit. This is referred to as long-duration energy storage, or LDES for short, and it’s the area we will focus the most on.
Grid-scale electricity storage resources can be categorized by several key characteristics:
- Speed of dispatch (how fast the resource can come online and deliver power)
- Power capacity (the maximum time-rate of energy output), usually measured in MW
- Power duration (the length of time a given power capacity lasts), usually measured in hours
- Total energy storage capability, usually measured in MWh and is the product of capacity x duration
Creating long duration energy storage economically has been challenging because of the relationship between power capacity (MW) and energy storage (MW x hours). As shown above, if each green block represented a MWh of storage in a battery, you would have a linear increase in marginal cost for adding each MWh of storage. Therefore, building energy storage resources that can consume and/or dispatch many hundreds of MW for several hours is often cost-prohibitive since there are rarely economies of scale in expanding either the MW or hours.
Pumped hydro, which accounts for 96% of global long duration energy storage today, can achieve economies of scale; however, new pumped hydro facilities are unlikely to be developed in the U.S. in the future due to the lack of land availability.
Therefore, we are addressing this need with advanced compressed air energy storage technology (ACAES), which has a 220 times smaller footprint than pumped hydro. In addition, ACAES uses existing geography so we don’t have to disturb the land or build massive containers, and it uses free and abundant air.
ACAES can scale storage duration and maintain the same power capacity at zero incremental cost due to the nature of certain geologic formations that could serve as the storage medium.
When we look at the cost / MWh, scalability, environmental impact, the lifecycle and longevity, and the commercial and market readiness, TerraStor’s ACAES solution outperforms other storage assets.