Preparing ourselves for our grid storage forum on April 1, over the past weeks we’ve been talking about utility-scale battery energy storage systems (BESS) and the different storage technologies available today, such as Gravity-Powered Energy Storage Technologies.

On today’s blogpost we’ll be discussing the main technologies at play regarding Thermal Energy Storage (TES) solutions.

Thermal storage has become an increasingly crowded space as the awareness of the need for long duration storage has risen. TES is achieved with widely different technologies which aim to solve long-duration storage by using cheap materials and proven technologies. We can classify these according to their storage medium:

Molten Salt

When energy is collected from any source of energy the electricity drives a heat pump, which converts electrical energy into thermal energy by creating a temperature difference. The heat is then stored in molten salt, while the cold is stored in chilled liquid. When electricity is needed, the temperature difference is converted back to electrical energy by a heat engine and electricity is sent back to the grid, effectively “time shifting” from when its generated to when it is needed the most.

Malta’s core technology stores heat in a molten salt medium for later conversion to electricity at a round-trip efficiency of about 60%, yielding more than 6 hours of storage and offering scalability to almost any storage capacity (by simple including a larger tank and adding more heat transfer fluids). Malta's system is composed of off-the-shelf components common in the oil & gas industry, keeping hardware costs low.

Molten salt is a high temperature fluid that can present significant storage challenges over a 20 year lifetime. While off-the-shelf components may prove to be able to achieve a low CAPEX, system will face high O&M costs, just like it does in Concentrated Solar Power (CSP). On the other hand, there is almost no waste from the process as salt does not need to be replaced and materials used (steel and salt) can be largely reused.

Pintail Power’s patented Liquid Salt Combined Cycle (LSCC) provides low-carbon dispatchable power for utility grids, and facilities. Pintail Power takes an unusual approach to long duration storage, including a conventional combustion turbine in its design, taking inspiration from the success of natural gas combined cycle plants. The idea of combined cycle from natural gas to storage increases the efficiency of the steam turbine compared to competitors that use a single cycle and energy storage density would be increased over other thermal systems.

Figure 1. Pintail Power's Liquid Salt Combined Cycle.

While this may provide additional flexibility and efficiency to its systems, nearly all of the company's competitors are emissions-less, a clear preference as emissions goals become more aggressive and net-zero legislation continues to pass.

Molten Silicon

1414 Degrees uses molten silicon as a storage medium to obtain high temperatures of above 1400°C for high energy storage capacity. The TES system stores energy generated from electricity or gas and supplies both heat and electricity in the proportions required by consumers.

This solution can be used for renewable energy shifting and Grid balancing but is more likely to be competitive if it can provide heat for industrial processes as an additional avenue for cost reduction.

Aluminum

This technology distinguishes itself in its choice of a solid thermal storage medium as compared to molten salt or other fluids that come with their complex systems to manage flow like 1414 Degrees, Pintail Power, or Echogen.

Azelio offers a thermal energy storage system that uses aluminum alloy as storage medium. Stored heat is moved via a heat transfer fluid to a working gas that drives a Sterling engine to generate electricity. The system can also provide heat at 55-65°C for use in industrial processes.

The Sweden-based company's core technology provides up to 13 hours of storage and claims to provide power at costs lower than a diesel generator. Still, high temperature thermal storage medium may introduce O&M complications that will only become obvious throughout the product's 30 year life.

Figure 3 Azelio's containerized and scalable solution.

Alumina Energy is developing a thermal energy storage system for long duration applications. The technology aims to offer storage at lower costs than other thermal storage, kinetic, and gravitational storage technologies with a claimed round trip efficiency of greater than 95%.

Alumina Energy offers the Packed Bed Energy Storage System that uses alumina pebbles as storage medium, heating up to 1500°C, achieving a high energy storage capacity. The storage and heat recovery system is coupled with industry standard turbines for electricity generation. Systems are modular and can be developed from MWh to GWh scale and have a 30 year lifetime.

It is most similar to Azelio in technology, but is far behind in maturity. It provides storage for greater than 4 hours and can ramp to full power in under 20 seconds, enabling use as peak power plants.

Thermochemical Storage - Supercritical CO2 cycle

Echogen offers a CO2-based waste heat recovery system used to convert the heat back into electricity by driving turbines at an expected efficiency of 60%, with more than 8-48 hours of storage capacity.

Echogen's 30-year lifetime system takes the heat from the grid, pass it through the compressor and store the heat. When prices are high or there is lack of electricity that heat is released. It uses supercritical CO2 as a working fluid that claims to yield greater power output at lower costs than standard steam systems. The system operates at less extreme temperatures than other thermal storage solutions, allowing it to use lower cost materials: high temperature (sand or concrete) and low temperature (ice) reservoirs.

Echogen has proven experience in bringing its core technology to market for waste heat recovery, and successfully licensing that technology to Siemens and GE. That said, versus other long duration technologies, their approach appears more complex.

With a similar approach to 1414 Degrees, their solution can be used for Renewable energy shifting and Grid balancing but is a better fit for industries that already have steam systems. For those outside of the thermal-mechanical space and use of steam systems, it might create concerns around complexity and would need qualified personnel for the O&M. Other solutions will work better for cases where a quick response is needed (10-15 minutes) as the system needs to warm up first. Figure 2 Echogen's supercritical CO2 cycle.

As this technology does not need water to operate the cycle it is a good solution for cold climates with freezing temperatures. Also, scaling the storage is not expensive as it would require more water, sand and concrete, all three cheap and abundant materials.

Thermophotovoltaic Storage – Insulated Carbon Blocks

Antora Energy is developing a novel solid state thermal energy storage device for long duration applications. The company's technology aims to discharge stored electricity on the order of days. The product will store excess electricity as heat in insulated carbon blocks that can reach temperatures of 2000°C - the highest temperatures in the thermal storage landscape, yielding a very high energy storage capacity that likely exceeds electrochemical solutions.

That heat is then reconverted back to electricity through a thermophotovoltaic device that operates in the infrared spectrum, capturing the glowing of the energized carbon blocks. Using solid-state thermophotovoltaics for heat conversion and thus replacing conventional steam generators and heat exchangers would lower down costs of O&M. Also, being an enormous industry behind PV, the costs and efficiency of the product would be going down along the years. This containerized solution would be easily scalable and competitive from 500kWh.

Conclusions

TES technologies aim to deliver long duration storage - this is the target where Li-ion batteries are defeated. When scaling Li-ion batteries deployments, the cost increase is linear (cost per kWh), with TES systems this does not happen as the storage medium and materials used are easily accessible and generally cheap. In this way, TES systems would be able to provide long duration storage with greater reliability at a fraction of the cost than electrochemical solutions.

Still, most of these solutions are still on development phase or piloting. It is up to these systems development in the next years if we will see a polarization of energy storage solutions in the next decade or Li-ion will monopolize the energy storage industries from electronics, to large scale utilities.