At Politecnico di Milano and University of Brescia, the investigation of CO2 blends has been performed both from theoretical and experimental standpoints. Thus, cycle modelling is complemented with thermal stability tests to verify the maximum operating temperature of the fluid and Vapor-Liquid Equilibria (VLE) measurements to predict the thermodynamic properties of the mixture. While thermal stability for pure fluids such as TiCl4 was demonstrated at temperatures up to 650°C, other mixtures exhibited degradation at lower temperatures when they were investigated for utilization in ORC applications (350-400°C).
The previous section has set out the following challenges posed by CSP technologies globally:
I. Higher efficiencies must be achieved to enable a drastic reduction in the size and hence cost of the solar field, which is the main contributor to capital costs.
II. Higher operating temperatures are needed to bring about a drastic reduction of the size and hence cost of the storage system.
III. Operating flexibility is essential, which means being able to keep the two previous features regardless of the boundary conditions of the power plant (ambient conditions and load setting).
IV. Dispatchability is essential and the best way to accomplish it is through thermal energy storage systems which can be implemented easily at a competitive cost.
V. Ultimately, resolving the outlined challenges leads to a lower cost of electricity (LCoE), yielding a technology that becomes cost competitive against photovoltaic systems and even fossil fuel technologies. The target LCoE to achieve this twofold objective is in the range between 50 and 100 €/MWh.
VI. All these objectives are tackled by SCARABEUS, a project conceived to bring about greater strides than would be obtained if steam turbine or combined gas/steam turbine technologies were used. This progress is built upon the following innovations (a discussion about the specific innovations is provided in the next section):
a. Power block efficiency (i): SCARABEUS aims to demonstrate that 50% cycle efficiency is possible with the current state-of-the-art receiver technology. This objective will be achieved by the adoption of sCO2 power cycles with layouts tailored to the specific needs of CSP applications. The approach to cycle selection already used by USE has been recognised as innovative and with great potential in different scientific meetings within the sCO2 community.
b. Power block efficiency (ii): further to the utilization of sCO2 power cycles, SCARABEUS will make use of an innovative feature with regards to the working fluid. The utilization of CO2 blends ensures that the most efficient working cycles can be implemented in locations where ambient temperatures are unusually high (over 40ºC) and, moreover, it enables adapting the composition of the blend dynamically thus allowing the power block to run at its optimum point (peak efficiency) under varying boundary conditions.
c. Reliability: there has been some research with sCO2 cycles using CO2 blends in the past. Nevertheless, these investigations were either theoretical or with limited experiments only. Hence, the feasibility of the concept and the limitations that arise when operating the system in off-design conditions have not been verified to date, let alone considered any aspect of reliability (which is essential to achieving a higher TRL). With respect to previous research, SCARABEUS will validate the feasibility of the concept as a whole and explore different compounds amongst nonhydrocarbon species. Furthermore, SCARABEUS will verify that the proposed blends enable the adoption of very efficient condensation cycles at high ambient temperature and, it will also check the long-term thermal stability of the blends. This key innovation is considered of paramount importance to guarantee the reliability of the technology.
d. Cost of electricity: the aim of SCARABEUS is to reduce the LCoE by 50 €/MWh compared to the best available technology. This will come as a result of the 20% efficiency rise attained in the power block which enables a smaller footprint of both the solar field and thermal energy storage system (twofold cost reduction). The previous review of the state-of-the-art quoted a report by IRENA claiming that the LCoE of CSP is expected to drop down to 80-100 €/MWh by 2025 by improvements to all components including solar field, thermal storage and power block. This means that merging all the mentioned improvements to the solar field and receiver with the SCARABEUS power block (improvement by 20% with respect to sCO2 power block concept), it will be possible to produce fully dispatchable solar electricity at 60-75 €/MWh. Such a scenario will certainly make it possible to move forward towards a 100% renewable energy world.