Integrated Electrochemical-Thermal System for High Efficiency and Low Cost Hydrogen Compression

Abstract

Three approaches to transport and delivery hydrogen at large scale have been identified by the US Department of Energy [1]. Each of the approaches requires the presence of high-pressure hydrogen compression 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, especially in terms of reliability, efficiency and investment and lifetime costs. Valid alternative processes are represented by electrochemical compression (EC) systems and thermal compression systems, exploiting the properties of suitable hydrogen absorption materials. A 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. Therefore, providing high temperature thermal power during the desorption process, the hydrogen pressure can be increased without the use of electricity. The paper presents the results obtained from a project funded by DOE, involving Greenway Energy, Savannah River National Laboratory and Sustainable Innovation. An electrochemical system, using Nafion-117® membrane, was successfully tested at high operating temperatures (≈ 150 °C) and pressures up to 100 bar, representing the baseline configuration for the first stage of the compression system. The second stage of the compressor, based on metal hydride systems, was tested using a commercially available Ti1.1CrMn material as the baseline alloy, achieving pressures on the order of 500-600 bar at temperatures on the order of 150 °C. A novel heat transfer system was also identified and designed to recover the available waste heat from the electrochemical system and drive the thermal compression process in the MH system. Overall system efficiency and costs were also assessed, with the results presented and discussed.