The United States Department of Energy (DOE) has identified three scenarios to transport and delivery hydrogen at large scale [1]. Each of the approaches requires the presence of high pressure hydrogen systems. Currently DOE set its fueling station pressure targets at approximately 875 bar with flow rates up to 100 kg/h [1]. Among the other targets for hydrogen compression systems, DOE identified the uninstalled cost target for the FY 2020 at 275,000 $, the energy requirement at 1.6 kWh/kg, availability equal to 85% and annual maintenance cost equal to 4% of the uninstalled cost[1] [1]. The current mechanical compressors cannot achieve the DOE targets and they have several additional drawbacks working at the specified operating conditions. Valid alternative processes are represented by electrochemical compression (EC) systems and thermal compression systems, exploiting the properties of suitable hydrogen absorption materials. Here, an alternative two-stage hybrid compressor system is proposed, with a first stage (lower pressure) EC, coupled in series with a second stage (higher pressure) metal hydride (MH) thermal compression system. The EC operates at compression ratios on the order of 10-20, reaching pressures of about 100-200 bar. Molecular hydrogen is oxidized at the anode producing protons and electrons. They are driven through the proton exchange membrane and combine with electrons at the cathode to deliver high pressure hydrogen. The outlet pressure is maintained at relatively low values (≤ 100-200 bar) in order to avoid hydrogen back diffusion. The second stage of the system, operating at higher pressures, is comprised of a thermal compression system, based on MH materials. Such compounds absorb hydrogen in an exothermic chemical reaction and release the absorbed hydrogen in the reverse endothermic chemical reaction. Their equilibrium pressure is a direct exponential function of the operating temperature, determined by the Van’t Hoff equation. Therefore, providing high temperature thermal power during the desorption process, the hydrogen pressure can be increased without the use of electricity. The paper presents recent results obtained from the project, funded by the DOE-FCTO, involving Greenway Energy, Savannah River National Laboratory and Sustainable Innovation. A techno-economic model of the hybrid system was developed and adopted to identify the initial baseline system configuration. Several MEA concepts have been examined for the EC, to find the optimized solution to be coupled with the MH system. The baseline system is currently based on Nafion^(R) 117 membranes, operating at temperatures of 150 ^oC and pressures up to 100 bar. Results show suitable voltage and current density values of the EC, reaching adequate technical performance of the EC system and ability to be paired with the MH compressor. Long term tests of the EC system are planned to be run, to assess the performance time degradation of the Nafion^(R) membrane at the operating conditions of interest. Specific techno-economic analyses have also been carried out for the MH system, identifying two existing MH material formulations, based on Ti hydrides, as the baseline candidates for the current application. A new finned tube configuration was selected to realize an effective heat exchange between the heat transfer fluid (i.e. water at pressures of 7-7.5 bar) and the MH material. Detailed transport models were developed to simulate the performance of the MH configuration. The adopted solution demonstrated the ability to transfer the heating and cooling (sensible and latent) power required to desorb and absorb hydrogen in the MH system in about 10 minutes for half a cycle. A matching operating point between the EC system and the MH system was also identified to recover the waste heat available from the EC system into the MH system at about 130-150 ^oC, without any additional external heating power. The system, integrating an electrochemical compressor with a pure thermal compressor, demonstrated the potential to closely approach the DOE 2020 technical targets for high pressure compression.

References

[1] US DOE Fuel Cell Technologies Office Multi-Year Research, Development and Demonstration Plan - 2015 Delivery Section, 3.2 Hydrogen Delivery

[1] The targets are for: inlet pressure of 100 bar, hydrogen flow rate of 100 kg/h [1]