Are Thermal Technologies the Future of Electric Storage?
by Bob Shively, Enerdynamics President and Lead Facilitator
As variable wind and solar renewable generation expands, electricity systems need more and longer-duration storage than conventional batteries can provide. Grids require ways to absorb low-cost electricity when it is abundant and deliver useful energy when it is needed. Current battery solutions provide short-duration storage (2 to 6 hours), but not long-duration storage. They also require substantial capacity to supply sufficient electricity for large thermal loads, such as industrial heating. Thermal energy storage, often called a thermal battery, is poised to become an important addition to storage options.
A thermal battery converts electricity into stored heat for later use. Rather than relying on electrochemical materials used in batteries, it can use relatively common storage media such as crushed rock, brick, graphite, ceramics, molten salts, or thermochemical materials. The stored energy may then later be delivered as hot air, steam, or direct process heat. In the future, technologies may also allow it to be converted back to electricity. A similar process can also be used for those that require cooling rather than heating.
The key question is not whether thermal storage will replace lithium-ion or other electrochemical batteries. It will not. The more useful question is where storing energy as heat creates greater value than storing it in batteries.
Industrial heat is a strong use case, but not the only one, for thermal storage
A key near-term opportunity is direct industrial heat. Many industrial processes need continuous, high-temperature energy for drying, steam generation, chemical processing, calcination, and other operations. Thermal storage provides flexibility for when the facility takes energy from the grid, allowing facilities to purchase extra power when prices are low, avoid purchases when prices are high, and use the energy when needed for the industrial process. It can also allow industrial processes to participate in demand-side management programs and enhance reliability by providing onsite storage of energy available even when the grid is down. Thermal storage can be sized economically for long durations because many storage media such as rocks, bricks, sand, concrete, and salts are potentially less expensive than battery cells. The characteristics of thermal storage allow it to be used where large amounts of electrochemical batteries are not cost-effective.
Another example of thermal storage already in use is residential hot water heating. Utilities have proven that temporarily interrupting heating in water heaters is not noticeable to customers since their hot water tank is providing gallons of stored hot water. By aggregating large numbers of remotely controlled water heaters across a service area, meaningful capacity can be obtained without impacting customers.
Many other examples of thermal storage are being developed and implemented around the world. Existing examples include industrial ice and chilled
water storage, district energy, and concentrated solar power with molten salt storage. Future uses under development include building integrated phase-change materials, cold underground storage, and data center cooling.
A flexible asset for the grid
Thermal storage helps electricity grids operate more efficiently. It can charge thermal storage during the lowest-cost hours when solar or wind output is plentiful and wholesale prices are low or even negative. The stored heat can then serve demand later during system peaks. It can also be used to trim peak demand when grids are under stress. This not only reduces costs but also has the potential to reduce necessary grid infrastructure investments. Instead of curtailing excess renewable output, the system can use it to charge thermal storage. And instead of building generation, transmission, and distribution to meet peak demand, system peaks can be managed through thermal storage. This improves utilization of existing grid assets while reducing curtailment.
Not one technology
Thermal energy storage includes several distinct technology families:
- Sensible-heat storage raises the temperature of a material such as water, rock, concrete, ceramic particles, or molten salt. It is generally the most established form of thermal energy storage.
- Latent-heat storage uses phase-change materials, which absorb or release substantial heat while melting or solidifying. These systems can store more energy in a smaller volume within a defined temperature range. Some latent-heat systems are now in use, but they are not widespread.
- Thermochemical storage uses reversible chemical reactions to store energy. It offers high theoretical energy density and potentially longer storage duration, but it is less mature and faces challenges involving materials durability, cycling, and system complexity.
The best technology depends on the required temperature, charge and discharge rate, duration, available space, industrial process design, and whether the customer needs heat, cooling, or electricity.
What is the future for thermal energy storage?
Thermal technologies are unlikely to be the future of electric storage. But they will likely be an important part of the future energy-storage portfolio. Strongest roles include turning low-cost renewable electricity into dispatchable industrial heat, providing the grid with new sources of flexible demand, and providing long-duration grid storage currently unavailable through battery energy storage systems. For use to become widespread, further development in technology and customer adoption will be required.
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