Vacuum pumping options for application in solar thermochemical redox cycles – Assessment of mechanical-, jet- and thermochemical pumping systems

Solar powered two step thermochemical redox cycles are often investigated as a pathway to producing renewable fuels, via the splitting of water and carbon dioxide. These processes require a high temperature step for the reduction of the redox material. The reduction thermodynamics also critically depend on the oxygen partial pressure. In generic process assessments, purge gases and mechanical vacuum pumps have been considered for removing the released oxygen. Even though both alternatives are expected to have a significant impact on the overall process efficiency, little effort has been made so far in providing a detailed analysis of the implementation of the related components - especially in the case of vacuum pumps. In this study models are developed for assessing the energy requirements of mechanical vacuum pumps as well as for jet pumps. The models are validated and compared to data provided by manufacturers. In addition, a novel thermochemical pumping concept is introduced and a first thermodynamic assessment is presented. The energy demands of the different pump options are discussed in the context of the targeted application. By comparison with the energy stored in the produced fuel, limitations for the implementation of these systems can be identified. Furthermore, the costs for mechanical and jet pumps have been taken into account for the assessment. While mechanical pumps are energetically favored and could be reasonably applied for pressures down to 150 Pa, jet pumps are the more economic option and can make direct use of waste heat, which should be vastly available in the process at the required temperature level. The concept of thermochemical pumps is energetically very interesting and might shift the limit of realistic operational pressures to lower values, especially when combined with conventional systems.