Hydrogen Storage using Slurries of Chemical Hydrides

The usual storage technologies considered for hydrogen are compressed hydrogen, liquid hydrogen, metal hydrides, and carbon-based storage systems. Over the past couple of years another method of hydrogen storage and transmission has been under investigation that offers some significant advantages over the usual hydrogen storage technologies.

Thermo Power Corporation has been developing a chemical hydride slurry approach. In this approach, a light metal hydride is used as the hydrogen carrier and storage media. Light metal hydrides such as lithium hydride, magnesium hydride, sodium hydride, and calcium hydride produce hydrogen when they react with water. These materials are typically dry solids at ambient conditions. The oil in the slurry protects the hydride from unintentional contact with moisture in the air and makes the hydride pumpable. At the point of storage and use, a chemical hydride/water reaction is used to produce high-purity hydrogen.

An essential feature of this approach is the recovery and recycle of the spent hydride at centralized processing plants to produce new hydride slurry, resulting in an overall low cost for hydrogen. This chemical hydride slurry system has several benefits:

• it greatly improves the energy transmission and storage characteristics of hydrogen as a fuel,
• it provides a hydrogen storage medium that is stable at normal environmental temperatures and pressures,
• it is pumpable and easily transported,
• it has a high gravimetric and volumetric energy density, with the use of a properly designed reactor it can provide hydrogen at elevated pressures without the use of a compressor,
• it produces the hydrogen carrier efficiently and economically from a low cost carbon source, and since the production of the hydride is a carbo-thermal process performed at a centralized plant, CO2 resulting from the carbo-thermal process for refining lithium is concentrated and amenable to sequestration.

Hydrocarbon Fuel Reformer for Fuel Cell Car

Fuel cell vehicles offer many advantages when compared to internal combustion or battery-powered electric vehicles. Advantages over the internal combustion engine (ICE) include the potential for higher fuel efficiency and lower emissions. The advantages over a battery- powered vehicle include an improved driving range and shorter refueling times.

The fuel efficiency of a fuel cell vehicle is expected to be about twice that for current internal combustion engines and the overall energy consumption (fuel chain and vehicle) is expected to be lower than that of battery-powered vehicles. Emission levels are expected to meet the Super Ultra Low Emission Vehicle Standard, much lower than those from current ICEs.

The ideal fuel for the low-temperature proton exchange membrane (PEM) fuel cells being considered for automotive applications is hydrogen. Currently, the infrastructure for hydrogen refueling is lacking, and hydrogen storage technologies available for onboard storage provide a decreased driving range compared to gasoline and ICE technology.

However, it is apparent that the commercial success of a fuel cell vehicle will be tied to the availability of a refueling infrastructure. In other words, it will be difficult to sell hydrogen-powered fuel cell vehicles without first investing in a hydrogen refueling infrastructure. However, it will be difficult to convince investors to build a hydrogen infrastructure if there are no commercial vehicles to use it.

A solution to this “chicken or the egg” dilemma is to provide an onboard reformer to convert a hydrocarbon fuel into a hydrogen-rich gas for utilization by the fuel cell. This strategy could help introduce fuel cell cars to the marketplace earlier and smooth the transition from internal combustion engine to fuel cell-powered vehicles. Hydrocarbon fuels can use the existing infrastructure for refueling and provide a higher hydrogen density than current hydrogen-storage technologies.

Currently, hydrogen is produced industrially from natural gas using a steam reforming process. A similar process can be used for onboard conversion of natural gas or higher hydrocarbons to hydrogen-rich product gases. However, onboard reforming presents several unique challenges, which include size and weight limitations and the need for rapid startup and the need to be responsive to demand. In addition, since the fuel to be used for onboard reforming is still to be determined, the reformer should be fuel-flexible.

There is some debate about which hydrocarbon fuel is optimal for fuel cell systems. Methanol and ethanol are available commodity chemicals and have numerous advantages as fuel (e.g., water soluble, renewable), and methanol is easy to reform. Gasoline and diesel have advantages over the alcohols, including existing refueling infrastructures and higher energy density. However, they are blends of different kinds of hydrocarbons and are more difficult to reform.

Hydrogen Storage

One of the most important factors in introducing hydrogen as future fuel is transportation and on-vehicle storage of hydrogen. Storing hydrogen that flexibly links its production and user are key factor of the hydrogen fuel utilization. The major contribution to the problem is from low gas density of hydrogen. For example, to store energy equivalent to one gasoline tank, an ambient pressure hydrogen gas tank would be more than 3000-fold the volume of the gasoline tank.

Various storage options have been introduced by many researchers and institutions for last two decades. Here is a brief desription for some of them.

• Compressed Hydrogen. Considering both storage and refueling technologies, probably compressed gas storage is the most promising alternative. High strength steel or other metals are an option from a strength perspective, however, diffusivity of hydrogen through the steel and weight of the steel are major issues for vehicular storage.
  

• Liquid Hydrogen. Storing of hydrogen in liquid form at cryogenic condition is attractive in that it offers low weight and volume per unit energy when compared to compressed hydrogen. But, main issues are hydrogen boil-off, the energy required for liquefaction, and tank cost.

• Metal Hydride. Metal hydrides are specific combinations of metallic alloys, which possess the unique ability to absorb hydrogen and release it later. The life of a metal hybride storage tank is directly related to the purity of the hydrogen it is storing. The alloys act as a sponge, which absorbs hydrogen, but it is also absorbs any impurities together with hydrogen. Thus, the hydrogen released from the tank is highly pure, but the tank’s lifetime and ability to store hydrogen is reduces as the impurities are deposited in the metal pores.

• Carbon Nanotubes. Hydrogen can be adsorbed on a carbon surface. Various forms of carbon with high surface area may be utilized for the storage of hydrogen. Research on this technology has focused on the areas of improving manufacturing techniques and reducing costs as carbon nanotubes move toward commercialization.