What is a supercritical carbon dioxide (sCO2) and why do we use it?

Carbon dioxide usually behaves as a gas at standard temperature and pressure (STP), or as a solid called “dry ice” when cooled and pressurised sufficiently. If the temperature and pressure are both increased from STP to be at or above the critical point (31°C and 73,8 bar), it adopts properties midway between a gas and a liquid (see Figure 1). It expands in its container like a gas but with a density like that of a liquid. Such conditions can be profitable to increase the power conversion performance.

Its high density comparing to other working fluids (sCO2 is nearly twice as dense as steam) makes it more energy dense. Moreover, it is benign (non-explosive, non-flammable, non-corrosive and non-toxic), thermally stable, and relatively inexpensive. 

These qualities make the sCO2 a prime candidate for power cycles.

  • Its density leads to a reduction of compression work and efficiency increase (see Figure 2)
  • This allows the size of all the system components to be considerably reduced without losing performance (size of a sCO2 turbine can be up to 1/10 of a steam turbine – see Figure 3)

Therefore, sCO2 gas turbines are highly efficient, cost-effective and flexible, with minimum environmental and space impact

It is also relevant to mention that the sCO2 will be contained inside a closed-loop Brayton cycle and therefore, there is no need for an operator. As a result, this system is relatively independent and is very well suited for a remote operation.


Within the project, the following power block components are developed and manufactured:

Waste Heat Recovery Unit (WHRU)

The flue gas from the CEMEX demonstration plant currently passes through cooling towers and filtration units. In the CO2OLHEAT project, this gas will pass through the WHRU, where the heat will be exchanged with a Heat Transfer Fluid (HTF). The WHRU will be designed in a modular and easy-to-install way to facilitate the integration in the power plant process, so that 10 MW heat is supplied to the sCO2 system, 24/7.

The WHRU is positioned at an extraction point with high flue gas temperature, which is upstream of the filtration unit. Consequently, the WHRU is subjected to dust-laden and corrosive gas. The selection of corrosion-resistant alloys for the WHRU, and a clogging-resistant design will be handled by testing and virtual prototyping.

Additional design constraints emerge from the requirement for conveniently coupling of the industrial WH source with the skid integrating the sCO2 turbomachinery so that the WH can be efficiently valorised.

(The above picture depicts a Bosal WHRU, 300 kW)

sCO2 Power Turbine

The Siemens Power turbine is a single-flow, double-shell design with inner and outer casing. The outer casing is of the barrel type with a circumferential split which allows rotational-symmetrical design without local material build-up even for high temperatures and pressures thus avoiding unsymmetrical deformation and thermal loading. The high energy density of sCO2 and its special properties result in comparatively compact turbomachines whose operating characteristics in terms of aero- and rotor-dynamics have to be evaluated and new design approaches including control and protection concepts must be developed.

Special attention must be paid to the aerodynamic optimization of the inlet and exhaust as well as the blade path since the high density of the working medium CO2 is associated with high losses at high velocities. The drum blading comprises reaction stages whereby the exact degree of reaction is optimized individually for each individual stage. The significantly higher demands on seals, both within the turbine and for sealing against the environment, requires new sealing concepts matching the requirements on performance and operational safety.

sCO2 Turboexpander

Baker Hughes is designing, manufacturing, and testing a sCO2 compressor coupled with a sCO2 turbine that act as compressor train driver. Experience and results from sCO2Flex project will be capitalised to design the CO2OLHEAT turbomachines.

Compressor suction operating condition is close to sCO2 critical point; for this reason, special attention shall be taken in aerodynamic design of suction plenum and first compressor impeller to cope with the sCO2 thermodynamic properties. To optimize compressor’s efficiency and operability an IGV system of the compressor will be provided.

sCO2 turbine suction temperature and pressure are above state of the art and as such challenging. A non-conventional configuration shall be designed to allow for the DGS installation and cooling. DGS can provide a benefit in terms of turbine efficiency optimization. Therefore, this design would represent a significant technological step and the resulting demonstration device has the potential to become a reference for sCO2 power system for WHR.

sCO2 Recuperator Heat Exchanger

Heatric is designing and manufacturing the Recuperator Heat Exchanger to be part of the CO2OLHEAT Power Block. In addition Heatric will design and manufacture a test mock-up representative of the Recuperator Heat Exchanger to be tested at Brunel University as part of improving the operation and control of the CO2OLHEAT Power Block. Heatric past experience working on other major sCO2 projects will be used to support the optimisation and integration of the Recuperator Heat Exchanger for the CO2OLHEAT Power Block.

CO2OLHEAT will use a recuperative sCO2 cycle which, to achieve a target plant efficiency, requires the Recuperator Heat Exchanger to have a large surface area. Given the relatively high temperature, pressure and temperature span (temperature difference between inlet and outlet of the exchanger) combination required, conventional heat exchangers like Shell & Tubes may lead to a large Power Block and unfit for integration inside an industrial plant. In this case, compact exchangers such as Heatric’s PCHE (Printed Circuit Heat Exchanger) are required. Heatric, as part of the supply of the Recuperator Heat Exchanger, will work with the other component suppliers and the system integrator to optimise the Power Block Skid package.

While there are many sCO2 power cycles being tested at various scales, there is no reference as to the optimisation of such cycle for energy intensive industries. There is a need to further understand the behaviour of state of the art components in such markets. This project brings the opportunity to develop, optimise and demonstrate a full size sCO2 Waste Heat Recovery system by combining the experience of several leaders in this cycle and represent a significant technological step towards the validation and deployment of sCO2 power system for WHR for industries such as Cement, Glass, Refineries and many other Energy Intensive Industries.

Primary Heat Exchanger (PHE)

The PHE is transfers heat from the Heat Transfer Fluid, and heats the sCO2 to a temperature as close as feasible to the flue gas temperature. The cooled HTF exits the PHE, and flows in a closed circuit to the WHRU, for reheating.

The cooler is designed as a tubular heat exchanging core, consisting of an array of pressure tubes, containing the sCO2 flow. The Heat Transfer Fluid flows around and between the pressurised sCO2 tubes, thereby heating the sCO2 flow. The tubular array sits in a shell, containing the hot, pressurised HTF.

The design of the PHE is derived from Bosal’s line of heaters for pressurised fluids.

(The above picture depicts a detail of a Bosal heater of 0,5 MW, showing the exit of a tubular array, and the hot (red) and cooled (blue) fluid paths)


The Cooler removes the residual heat of the sCO2, after power conversion. The cooling system has a low environmental footprint, minimising the usage of water and other resources. The heat rejection is guaranteed 24/7, for all climates, as will be validated for the replicator sites. 

(The above picture depicts a detail of a Bosal cooler, 2,5 MW)


Modular Approach – Agile design applied to installation

The CO2OLHEAT project aims to propose a cost saving solution to existing plants: this adds a number of severe constraints to the installation and commissioning phase, as the interference and turndown of existing plant has to be minimized not to jeopardize the economic benefits of the CO2OLHEAT installation.

The Partners choose a Modular approach to be applied to the design philosophy of the plant: this will be achieved by the power block design & assembly on a number of separate SimeROM skid structures to be linked together on the field.

The Modular approach, coming from the offshore and naval Industry, solve smartly the constraints which arise when installing a new plant inside an existing facility:

  • Minimizes the turndown of existing business (allowing large prefabricated activities and pre-testing in the workshop, limiting the installation and the plant start-up times)
  • Is replicable (the same small modules can be installed in other sites, with a slight adjustment of the interconnecting system only)
  • Is flexible
  • Requires smaller crews on site (reducing potential interferences and HSE impacts)
  • Is Agile (each module can be independently designed, manufactured and tested, allowing better focus on single issues)

Grid Oriented Controller

Control algorithms will manage the CO2OLHEAT integrated system, with focus on flexibility enhancement and power grid interoperability. The control algorithm will be developed following the experience of Brunel University of London in control strategy definition of advanced and flexible power cycle as well as expertise of MAS in control narrative implementation. The control strategy will be developed, targeting flexibility enhancement of the whole system. This will be done by performing dynamic simulations (especially looking at off-design conditions) and control software verification before power block installation in Prachovice demosite, while targeting the following objectives:

  • Enhancement of proper models to CO2OLHEAT dynamic modelling: capture and predict the main dynamic phenomena of the demosite plants. The dynamic models will de-risk the control development
  • CO2OLHEAT control strategies definition: in order to contribute to the grid stability, the local grid code requirements will be evaluated in order to include them in the control strategy to take into consideration the power grid interoperability towards promotion of CO2OLHEAT WH2P plants as ancillary service providers
  • Development and validation of advanced predictive controls: to enhance the flexibility and power grid operability, the different CO2OLHEAT components need to be properly integrated and controlled (e.g. the by-pass duct that conveys hot gases at 400°C to the WHRU). The model based predictive control will be validated with Prachovice data and target a multi-hierarchical approach able to consider the systems constraints and interact with the grid request.

The Grid oriented controller targets higher plant efficiency in part load (+20%) as well as quicker transients in lower WH temperature moments (+25%).

Flag of Europe This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement N° 101022831