water. Higher heat capacities add to the energy penalty in the solvent regeneration step. About two thirds of CCS cost is attributed to capture, making it the limit to CCS deployment. Optimizing capture would significantly increase CCS feasibility since the transport and storage steps of CCS are rather mature. An alternate method is chemical looping combustion (CLC). Looping uses a metal oxide as a solid oxygen carrier. Metal oxide particles react with a solid, liquid or gaseous fuel in a fluidized bed combustor, producing solid metal particles and a mixture of CO2 and water vapor. The water vapor is condensed, leaving pure CO2 , which can then be sequestered. The solid metal particles are circulated to another fluidized bed where they react with air, producing heat and regenerating metal oxide particles for return to the combustor. A variant of chemical looping is calcium looping, which uses the alternating carbonation and then calcination of a calcium oxide based carrier. A 2019 study found CCS plants to be less effective than renewable electricity. The electrical energy returned on energy invested (EROEI) ratios of both production methods were estimated, accounting for their operational and infrastructural energy costs. Renewable electricity production included solar and wind with sufficient energy storage, plus dispatchable electricity production. Thus, rapid expansion of scalable renewable electricity and storage would be preferable over fossil-fuel with CCS. The study did not consider whether both options could be pursued in parallel. In 2021 High Hopes proposed using high-altitude balloons to capture CO2 cryogenically, using hydrogen to lower the already low-temperature atmosphere sufficiently to produce dry ice that is returned to earth for sequestration. In sorption enhanced water gas shift (SEWGS) technology a pre-combustion carbon capture process, based on solid adsorption, is combined with the water gas shift reaction (WGS) in order to produce a high pressure hydrogen stream. The CO2 stream produced can be stored or used for other industrial processes. Compression After the has been captured, it is usually compressed into a supercritical fluid. The is compressed so that it can be more easily transported. Compression is done at the capture site. This process requires its own energy source. Like the capture stage, compression is achieved by increasing the parasitic load. Compression of is an energy intensive procedure that involves multi-stage complex compressors and a power-generated cooling process. Transport Large volumes of highly pressurized are transported via pipelines. For example, approximately 5,800