A critical overview of solar assisted carbon capture systems: Is solar always the solution?

Mohammad Saghafifar, Samuel Gabra                                                                  International Journal of Greenhouse Gas Control, 2020

This paper focuses on possible integration schemes between solar thermal collectors and carbon capture systems. In its entirety, solar assisted carbon capture systems can be categorized into direct and indirect systems. Key factor in designing solar assisted carbon capture systems
is to match the thermal-grade between the collector and gas separation process.

For post-combustion systems, in particular, amine based systems have been widely considered for integration with solar thermal collectors using either direct or indirect schemes. Comparing direct and indirect integration schemes, results are overwhelmingly in favour of indirect solar integrations. The key merits of indirect integration include its additional flexibility in system operation and the high solar energy utilization factor it offers. For direct systems, solar thermal energy utilization is limited to the reboiler heat duty. Moreover, higher temperature concentrating solar thermal collectors with higher optical efficiencies can be used in indirect systems. These conclusions can be extended to the case of using any other low temperature gas separation technique including other chemical absorption and chemical/physical temperature swing adsorption. Direct integration may only be suitable when high grade heat is required for gas separation, as in calcium looping.

For pre-combustion systems, all the available studies focused on utilizing solar thermal energy for fuel reforming/gasification stage. It is more feasible to supply the H2/CO2 separation energy from other low-grade sources. The advantage of these systems is the inherent ability of storing solar energy chemically in form of the upgraded fuel. In addition, high-grade heat needed for reforming/gasification stage makes solar assisted systems more feasible.

For oxyfuel combustion, both direct and indirect schemes were studied. In direct systems, solar energy is used to derive an endothermic reduction reaction for chemical looping processes. Solar hybrid chemical looping systems have two advantages: first, the inherent carbon dioxide sequestration and second, storing solar thermal energy both chemically and sensibly. For indirect systems, solar energy is utilized within the power cycles whilst air separation systems were used for pure oxygen production. In terms of merits, these systems are similar to indirect systems using low temperature gas separation methods. Possible future research direction for solar assisted carbon capture systems can be classified into solar collectors cost reduction research and innovative integration scheme research. Firstly, the cost of collectors, irrespective of technology, must be reduced. In addition, more research must be carried out on high temperature solar collector integration within high-temperature carbon capture and storage (CCS) systems. For post-combustion systems, ongoing research on solar assisted calcium looping systems is promising. Integrating high-grade solar energy for fuel reforming and gasification in pre-combustion CCS is another potential research route.

An interesting aspect of solar assisted CCS systems is the possibility of its utilization as an energy storage scheme coupled with CO2 capture. In these schemes, extra available solar energy during high insolation periods is employed to further run the endothermic part of the CCS
process and store the product for usage during instances of low solar insolation. The products from these processes can be simply stored.

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