The Southwest Research Institute (SwRI), based in the US state of Texas, has recorded the highest temperature and pressure conditions ever reached in materials testing for supercritical carbon dioxide (sCO2) – a state of carbon dioxide that behaves like both a gas and a liquid.
It is a breakthrough that could advance the potential for more efficient and scalable power generation using sCO2, which is a technology that harnesses the unique properties of CO2 in a supercritical state to improve energy transfer.
While conducting material testing for a high-pressure, high-temperature sCO2 turbine, SwRI achieved conditions of 1,150°C at 300-bar. Before this, the highest recorded pressure and temperature for sCO2 was 800°C at 300-bar. The oxy-fuel turbine was designed to operate at a maximum temperature of 1,150°C.
To achieve these higher temperatures, the research team had to overcome major challenges associated with the extreme conditions needed to keep CO2 in a supercritical state.
“We assessed the performance of different materials and coatings under extreme conditions,” said Dr Florent Bocher of SwRI.
He explained that the team aimed to build a vessel to test materials at the highest temperatures and pressures based on the turbine design.
“The mechanical properties of the vessel materials decline as temperatures rise. This makes it impossible to safely use a traditional high-pressure and high-temperature experimental setup, which uses external heating.”
What is supercritical CO2? Supercritical CO2 is simply carbon dioxide that is pressurised and heated above its critical point. The critical point is a point at which the distinction between liquid and gas can no longer be made and, in this case, the CO2 takes on the properties of both states. This is useful because sCO2 is highly responsive to changes in temperature or pressure. A small increase or decrease in these conditions causes a large change in its density, which is a key factor in efficiently transferring energy. This makes sCO2 ideal for use in power cycles, especially in turbines, because it can produce large amounts of energy with relatively small variations in operating conditions. Other major uses of sCO2 include extracting essential oils from plants, which is an opportunity that is seeing rapid growth through the growth of the legal cannabis industry in many US states.
To achieve the necessary conditions, the team modified an autoclave – a sealed container typically used for sterilising materials under pressure – by installing an induction coil. This kept the vessel’s exterior cool while allowing the interior to reach the required temperatures.
“This new capability is crucial for both current and future research areas and technologies that demand extreme testing conditions,” said Bocher.
The extreme conditions needed to keep CO2 in its supercritical state put immense stress on materials used in the construction and operation of the power plant, particularly in turbines.
High temperatures and high pressures can impact the materials through mechanical stress, and CO2 – although not corrosive when in its normal gaseous or liquid states – can interact with impurities like water and oxygen when supercritical. These contaminants can form carbonic acid, which is highly corrosive.
This technology is likely to prove useful in testing materials for other extreme applications, such as molten salt energy production and storage, as well as hypersonics research.
The $170m Supercritical Transformational Electric Power (STEP) demo pilot plant generated electricity for the first time last year ©SWrI
The $6.4m project, funded by the US Department of Energy, is focused on designing an oxy-fuel turbine for a direct-fired sCO2 power plant. This work serves as a key step in developing the turbine for a planned sCO2 demonstration facility.
In contrast, the $170m, 10-megawatt sCO2 demonstration facility is a separate project underway at SwRI, which will serve as a larger-scale application of this technology.
In addition to SwRI’s demonstration projects, major energy companies and research institutions worldwide are exploring sCO2 systems, including projects like the 50-megawatt Allam Cycle plant in the UK and an initiative led by NET Power in the US that promises to generate power from natural gas with zero emissions and no water usage.
