Hydrogen Production: Surge Storage and Transport Filling Cycle Considerations

Hydrogen storage and filling for transport are as important to the success of commercial hydrogen production systems as the method or feedstock source itself. Consequently, understanding hydrogen storage and loadout functional needs will improve the overall engineering design of your hydrogen production project.

Storage and Logistics

Why is storage a key design aspect for commercial production? Without buffer capacity between production and loadout, the two are hard-piped with the same instantaneous rate. This means you can only load as fast as production allows, and production cannot outpace the fill rate. A bottleneck is inherently built in. By including surge storage volume between production and loadout, the two can operate at separate, independent rates (at least for a while, depending on storage sizing).

In order to select the proper buffer/surge storage—and avoid one operation restricting the other—one should understand the project’s production and loadout logistics. In the simplest case, production runs at a constant rate while loadout will vary across fill cycles and be intermittent between vessel changes. The setup of a production process is important as well. For example, it may include more than one process train operating in parallel, or possibly multiple final compressor stages with differing final pressure targets. Transient process conditions, such as startup/shutdown or process upsets, need to be accounted for.

Storage and Transport Methods

The first major category is the physical containment of hydrogen. One of the most common methods is the use of  compressed gas pressure vessels, which range from 3,000 psi to 10,000 psi and are fabricated in various sizes and materials. Another method is underground storage in large subterranean expanses, such as salt caverns or depleted gas reservoirs and aquifers. The third physical storage method is liquid hydrogen. Hydrogen becomes liquid at -253°C atmospheric pressure, and is contained just below this temperature in specialized insulated vessels. 

The second storage category is chemical carriers, where hydrogen is chemically bound to a material and then contained or transported. Metal hydrides, for example, are solid media that bind hydrogen using moderate heat and pressure. There are also Liquid Organic Hydrogen Carriers (LOHC) and the “power fuels” of ammonia and methanol, so named because the “carrier” itself can function as a fuel as well.

How do these methods compare? Of the physical containment methods, compressed gas vessels are the least expensive and already in service across the globe. The density, however, is relatively low, and they have significant hazard potential with flammable gas at very high pressures. Underground storage is good for large volumes and long-term storage, but is limited by geography. Additionally, they are not conducive to high turnover cycles, inherent in surge capacity between production and export operations. Liquid hydrogen has significantly higher energy costs than compressed gas, and presents similar high hazard potential. But its density is much higher, meaning a given quantity of hydrogen requires smaller or fewer vessels to contain/transport it.

All chemical carriers have relatively high densities and slow-release rates, making them generally unsuitable for surge storage in production applications. However, since the hydrogen is present in a stable solid form that can be kept at ambient conditions, metal hydrides have the lowest hazard potential. Technical progress has been made with these materials recently, and it may not be long before commercially viable options for production applications become available.

When the service points are close to the production/loadout location (within 100 miles), utilizing compressed gas via tube trailers is the method of choice. Liquid hydrogen transport requires fewer truck loads for the same quantity of hydrogen, and as the distance increases, the additional fuel savings begin to offset the higher energy costs to produce the liquid. For long distances, the lowest energy method is pipeline transport, but it has the highest relative capital cost and significant regulatory considerations.  The most practical method for overseas transport is liquid form, either as liquid hydrogen or a power fuel (typically ammonia).

Loadout and Filling Cycles

As hydrogen fills the tube trailer, the container’s pressure and temperature increases; although generally similar in nature, the specific relation of temperature, time, and fill rate is unique for every application (e.g., vessel configuration, material of construction, physical fill parameters, set of operating conditions like temperature, pressure, etc.). Determining the applicable fill profile is not simple or straightforward. Generally, computational flow dynamics (CFD) or other modeling tools are used to characterize a profile in a specific application, but it is important to understand what the profile looks like.

All compressed gas vessels used for transport are made of composite fiber wrap, which minimizes weight and maximizes pressure limits. The temperature threshold at which the pressure integrity may begin to degrade (~85°C) is substantially lower than heavier materials like steel. Therefore, it becomes critical to manage the vessel temperature during the fill. One approach is to lower the inlet hydrogen temperature (e.g., precooling) before filling the trailer. Once the temperature begins to approach the threshold, the only option is to slow the fill rate, effectively increasing the overall fill cycle time. In order to minimize the fill rate modulation required, other approaches are more desirable.

One such strategy is to use a cascade fill method. Quantities of hydrogen are staged into multiple surge groups, each filled to a different incremental pressure: initial, intermediate, and final fill. During the initial sub-cycle, the empty tube trailer is filled by the first bank, or lowest pressure. As the trailer pressure approaches that of P1, the system switches and continues filling the container until it is nearly at P2 pressure. The system then switches to the third bank and finishes the fill cycle with a final trailer pressure at P3, the target export pressure.

By incrementally stepping up fill pressures in sub-cycles, the necessary amount of throttling modulation may be decreased in each group. This results in a greater average/net fill rate and shorter fill cycle, while still remaining safely below the temperature threshold. Additionally, there may be some reduction in compression costs. The specific improvements in net fill rate and compression costs depend on the specific application, and require modeling for the various sub cycles. However, when these have been determined, the accuracy of design logistics greatly improves.

In conclusion, a multitude of variables can affect the success of your hydrogen production project. Make sure these are considered early on in the design process by characterizing the expected outcomes before executing the detailed engineering design.

Article originally published on the POWER Engineers website.