A recent report on thermal energy storage describes how supercritical CO2 can be used to increase the efficiency of thermal energy storage solutions. Why is thermal energy storage useful for use with mains electricity, what is supercritical CO2, and how can it be used in thermal storage solutions?

Why thermal energy is good for energy storage?

As the concerns for climate change continue to increase, so does the desire to reduce dependency on non-renewable energy. While energy sources such as hydropower and geothermal allow for on-demand power and easy energy storage, they are only plausible in areas that have rivers and geothermal activity.

Solar and wind are the next two sources of renewable energy that can be placed anywhere with sunlight and wind, which is extremely common. This is why these two energy sources are often the most sought after. However, photovoltaic solar panels and windmills cannot store the energy they produce, creating challenges when meeting energy demand during night and non-windy days.

However, solar thermal energy is one potential solution that instead utilises mirrors to gather sunlight and shine it onto a tower that uses the heat to melt salt. This molten salt is then pumped into a heat exchanger that creates steam, turning a turbine to generate electricity. One major advantage of molten salt solar plants is that the salt remains molten well into the night after the sun has gone, and this can be used to continue generating power when needed.

However, the need to turn water into steam to drive turbines presents challenges, but the use of supercritical CO2 could change this.

What is supercritical CO2?

Supercritical CO2 can be considered a “fourth” state of matter that looks like a fluid but behaves like a gas. When any material is in a supercritical state, it has the properties of a gas in that it can diffuse and move around and yet has the density of a liquid while also having the ability to dissolve compounds (assuming it’s a solvent).

Making a fluid supercritical is extremely difficult; the fluid must be under sufficient pressure that prevents the fluid from turning into a gas, the temperature must be high enough to prevent the fluid from turning into a solid, and the pressure must be low enough to prevent the fluid from turning back into a solid.

How could supercritical fluids help with thermal energy storage?

The ability for supercritical fluids to have near-zero viscosity allows them to be easily pumped with little resistance, while their density gives them superb thermal mass. Combining these two allows for supercritical CO2 to be an ideal medium for use in power cycle technology where heat from a source is used to turn a turbine. But how does this affect thermal energy storage solutions?

One of the biggest advantages of supercritical fluids in thermal energy storage is that supercritical fluids are extremely sensitive to small temperature changes; a slight increase in temperature results in a large increase in pressure. This increased thermal energy transfer efficiency essentially means that less energy is wasted on raising the temperature of a fluid into a gas. Thus, a supercritical fluid can better bridge a thermal storage solution to the power generation system with fewer losses compared to steam-driven systems.

In fact, this was recently demonstrated by the Southwest Research Institute of Texas, which demonstrated a supercritical CO2 power transfer system for use with concentrated solar power plants. The system improved energy efficiency by 10% and reduced the overall footprint of turbomachinery (i.e., turbines) by 20 times.

“sCO2 power cycle technology is a fraction of the size of conventional turbomachinery, offering improved performance for numerous applications. The successful MW-scale demonstration of sCO2 technology at full-cycle conditions is an exciting milestone,” 

Dr Tim Allison: Director of SwRI’s Machinery Department.

Supercritical CO2 offers the best of both gasses and fluids, which could be the key to future thermal energy storage solutions.