Supercritical CO2


| PTES system developed by GE, sCO2 is the


working fluid of the heat pump with molten salt thermal storage (up to 480°C). Electric resistive heating is used to heat the molten salt to 565°C. The discharge cycle uses a standard Rankine cycle with water/steam as the working fluid. The cold storage medium is water. MAN/ ABB Electro-Thermal Energy Storage (ETES) utilises ice for cold storage and hot water for hot storage. Both turbomachines operate in sCO2


cycle. Expected roundtrip efficiency (RTE) of pretty much all the PTES systems under development is around 60%.


Molten salt based TES can be combined with an sCO2


power cycle in concentrated solar power (CSP) applications. This is the technology promoted by Heliogen, with turbomachinery developed by Hanwha and tested at Southwest Research Institute. The initial commercial offering is a 5 MWe system with 600°C turbine inlet temperature operating in a split-flow (re-compression Brayton Cycle). Targeted deployment date is early 2024. Another interesting technology for CSP applications is thermochemical energy storage (TCES), which is being investigated by GE’s Global Research Center (under the ARPA-E FOCUS programme) and Echogen (under the ARPA-E DAYS programme). The goal is to combine TCES with TES, which uses the excess solar energy and stores it in molten salt. The excess energy from the grid is used to cool liquid CO2


to solid (dry ice) and release the heat from the molten


Above: Figure 3. STEP turbine skid configuration (the bypass compressor is driven by the turbine)


salt to change solid CO2 to supercritical fluid and


expand it in a turbine. Targeted RTE is 68%. SunShot and STEP


There has been no sustained field experience yet with sCO2


turbines or power plants. The first


attempt to design and operate a larger-than- laboratory scale demo/test was the Advanced Projects Offering Low LCOE Opportunities (APOLLO) project under the aegis of the US DOE’s SunShot initiative. The project was a collaboration between GE and SwRI to develop a high-efficiency sCO2


compression system


(80% compressor efficiency with variable IGVs and OGVs). The compression system in question comprised two compressors, the main compressor and the re-compressor of the split re-compression sCO2


The system was intended as a modular power block for CSP applications. The powertrain (27 000 rpm) also comprises a 14 MW four-stage sCO2


expander for net 10 MW (rated) generator output. Turbine inlet conditions were 715°C and 250 bar with about 85 kg/s sCO2


flow rate. The


pilot plant facility, which, at the time of writing, is under construction on the campus of Southwest Research Institute, San Antonio, TX, USA.


work under the SunShot initiative encompassed six years (2012-2018), achieving considerable success and providing a wealth of lessons learned for next steps, notably, a 10 MWe, $119 million sCO2


The project is being undertaken under the US DOE’s Supercritical CO2


cycle, each rated at 2 MW.


Power (STEP) programme (with the US DOE footing $84 million of the bill). A team led by GTI Energy, SwRI and GE was established to design, construct, commission, and operate the facility. The project goal is to advance the state-of-the- art for the high temperature sCO2


power cycle from TRL 3 to TRL 7.


The STEP turbine skid is shown in Figure 3. The three-stage STEP turbine is designed to incorporate improvements on the SunShot unit, eg, increased casing and rotor life (100 000 hours vs 20 000 hours), shear ring retention rather than bolts, a design for couplings on both shaft ends, and improved aero performance with increased volute flow area (see Figures 4 and 5). Special attention is given to the dry gas seal (DGS) design and thermal management based on the lessons learned from the APOLLO (SunShot) projects.


Above: Figure 4. Comparison of the four-stage SunShot (APOLLO) and three-stage STEP turbines


The test programme is divided into two phases. In Phase I, a simple recuperated variant will be operated to demonstrate 500°C turbine inlet temperature operation with reduced risk. In Phase II, the split-flow (re-compression) cycle with parallel compressors and multiple heat exchangers will be deployed to operate at 715°C turbine inlet temperature operation with maximum efficiency. The announced goal is to demonstrate the pathway to 50% or higher efficiency. Since the heat source of the STEP facility is a natural gas fired heater (similar to a heat recovery steam generator in construction), the efficiency measured in the tests will be significantly lower than 50%.


Above: Figure 5. Full-scale mock-up of the monolithic STEP turbine rotor with the balance piston and three stages. The actual piece is fabricated from a nickel-based alloy


12 | January/February 2023| www.modernpowersystems.com Ongoing research


Achieving 50% or higher net LHV efficiency in a power plant operating in sCO2


Brayton (or Transformational Electric


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