Potential and challenges of the utilization of CO2-mixtures in supercritical power cycles of Concentrated Solar Power plants

The potential of supercritical Carbon Dioxide power cycles to supersede subcritical steam turbine technology in Concentrated Solar Power applications is widely acknowledged. Some differential features of the former are higher efficiency at similar temperatures (in the range from 600 to 750ºC), smaller footprint, higher flexibility and lower cost. Several theoretical and experimental R&D projects are currently working on aspects such as component development (turbomachinery and heat exchangers), system integration into the solar subsystem (receiver and thermal energy storage system), operability, materials… Nevertheless, whilst progress is being made at a very high pace, there is still a great deal of uncertainty regarding how much sCO2 technology will be able to reduce the cost of solar thermal electricity with respect to contemporary CSP technology. This is mostly caused by the sensitivity of cycle performance to ambient temperature, bringing about a large efficiency drop when this temperature exceeds 35ºC. The root cause for this performance drop is the unfeasibility of compression near the critical point, where the very high density of the fluid reduces density and, therefore, compression work. The SCARABEUS project is based on the addition of certain dopants to carbon dioxide in order to yield a working mixture with higher critical pressure and temperature. As a consequence of these modified critical properties of the fluid, compression near the critical point is enabled even at ambient temperatures as high as 40-45ºC. Moreover, at these high temperatures, condensation and compression in liquid state are still possible. The characteristics of the new working fluids have been proved to enable thermal efficiencies higher than 50% for minimum cycle temperatures as high as 60ºC, hence boosting the performance of CSP plants well beyond of the capabilities of systems based on steam turbines. This implies a substantial reduction of the cost of the plant. Nevertheless, whilst the thermal and economic performances are more favourable for CO2-mixtures, new technical challenges must be faced if the technology is to be mature: thermal stability and potential hazards of the dopants, new turbomachinery and heat exchanger designs adapted to the composition of the mixture, phase separation, materials (selection, compatibility and degradation) and others. This paper introduces the main advantages and technical potential of the SCARABEUS technology along with a discussion of the main challenges faced by the consortium in order to demonstrate the technology and beyond.