Onboard Hydrogen Storage in Light Vehicles

Kanthi Nagaraj
March 1, 2015

Submitted as coursework for PH240, Stanford University, Fall 2014

Introduction

Onboard Hydrogen Storage continues to be one of the key technical barriers for widespread adoption of Hydrogen fuel cell vehicles. The aim is to be able to mass produce hydrogen storage systems that have comparable performance to traditional fossil fuel powered cars, under varying external conditions.

Requirements of Onboard Storage Systems

In order for hydrogen to achieve wide consumer acceptance, it is important to achieve high volumetric and gravimetric energy density storage of hydrogen on-board vehicles. It is equally important to be able to extract hydrogen and refuel it in the storage system at a quick rate. These metrics were formalized by the FreedomCAR and Fuel partnership between U.S. Council of Automotive Research and U.S. Department of Energy, and set as targets in Hydrogen storage technologies roadmap. [1] These targets were revised in 2009 and the revised targets are reproduced here in Table 1. [2]

Storage Parameter Units 2017 Ultimate
System Gravimetric Capacity: Usable, specific-energy from H2 (net useful energy/max system mass) kWh/kg
(kg H2/kg system)		 1.8 (0.055) 2.5 (0.075)
System Volumetric Capacity: Usable energy density from H2 (net useful energy/max system volume) kWh/L
(kg H2/L system)		 1.3 (0.040 2.3 (0.070)
Storage system cost (& fuel cost) $/kWh net ($/kg H2)
$/gge (gasoline gallon equivalent) at pump		 12 (400) 2-4 8 (266) 2-4
Charging/Discharging Rates
Charging/discharging Rates min 3.3 2.5
System fill time (for 5 kg) kg H2min 1.5 2.0
Minimum full flow rate (g/s)/kW 0.02 0.02
Start time to full flow (20 C) s 5 5
Start time to full flow (-20 C) s 15 15
Transient response 10%-90% and 90% -0% s 0.75 0.75
Table 1: Hydrogen storage systems targets for 2017. [2]

Current Systems

The Hydrogen storage subprogram of the U.S Department of Energy is pursuing the dual strategy with short-term focus on improving compressed hydrogen storage systems and a longer-term focus on developing advanced cold/cryo-compressed and material based Hydrogen storage systems. [3] In 2013, Hydrogen storage subprogram performed an assessment of existing compressed Hydrogen storage systems with respect to 2017 targets. [4] The key findings are captured below in Table 2. [4]

Storage Parameter Units 2017 Ultimate 700 bar 350 bar
System Gravimetric Capacity: Usable, specific-energy from H2 (net useful energy/max system mass) kWh/kg
(kg H2/kg system)		 1.8 (0.055) 2.5 (0.075) 1.5 (0.044) 1.8 (0.054)
System Volumetric Capacity: Usable energy density from H2 (net useful energy/max system volume) kWh/L
(kg H2/L system)		 1.3 (0.040) 2.3 (0.070) 0.8 (0.025) 0.6 (0.018)
Storage system cost $/kWh net
($/kg H2)		 12 (400) 2-4 8 (266) 2-4 17 13
Table 2: Comparison of current compressed hydrogen storage systems with 2017 targets. [4]

In order to understand the state of art in the industry, we should look at the current storage systems in commercial Hydrogen fuel cell vehicles. Many commercial car manufacturers like Honda, Toyota and Hyundai have adopted compressed Hydrogen fuel tanks in their Hydrogen fuel cell vehicles. [5-7] BMW's Hydrogen 7 is an exception to this and uses a liquid Hydrogen fuel tank. [8] It must be noted that these vehicles are in various stages of development and production. Unfortunately, technical information on these storage systems is limited in the public domain.

Conclusion

Clearly, the revised targets for 2017 have not been met yet by Hydrogen storage systems for vehicles. We should note that the current system capabilities are fairly close to meeting the 2017 targets, and given the remaining time frame, it is possible that the targets will be met by 2017.

© Kanthi Nagaraj. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] "Hydrogen Storage Technologies Roadmap," FreedomCAR Fuel Partnership, November 2005.

[2] "Targets for Onboard Hydrogen Storage Systems for Light-Duty Vehicles," FreedomCar Fuel Partnership, September 2009.

[3] "FY 2013 Annual Progress Report," DOE Hydrogen and Fuel Cells Program, U.S. Department of Energy, DOE/GO-102013-4260, December 2013, Section IV.

[4] S. McWhorter and G. Ordaz, "Onboard Type IV Compressed Hydrogen Storage Systems - Current Performance and Cost," U.S. Department of Energy, Record No. 13010, 17 Jul 13.

[5] J. R. Healey, "Test Drive: Honda FCX Clarity is Fuel-Cell Fab", USA Today, 27 Jul 13.

[6] M. Silva, "Hyundai's Fuel Cell Car Drives More Smoothly Than Popular Hybrids," Bloomberg, 14 Feb 13.

[7] K. Chang, "A Road Test of Alternative Fuel Visions," New York Times, 18 Nov 14.

[8] P. Valdes-Dapena, "Coming Soon: Hydrogen Powered BMW," CNN Money, 12 Sep 06.