The U-212, aka the Type-212 class. Metal Hydride technology allows it to stay silently submerged for weeks at a time.  

Introduction  

Metal Hydride technology is not one of the many “someday/maybe” energy technologies floating around in academic or commercial science laboratories.   

Metal Hydride technology is real, as real as the modern German U-Boats facing off with Russia in the Baltic Sea.   

The sub can generate up to 240 kW of electricity via power from metal hydride storage. When relying solely on hydrogen power, the U-212 can stay submerged and travel at 10 mph for up to 3 weeks. It does not rely on a snorkel to achieve this feat. When operating in this submerged AIP (air independent propulsion mode), it is, incredibly, even more silent than a nuke sub.  

Now, a bit about the metal hydrides and the science that this technology is based on. Then, some more real-world examples of this not-well-known technology, and some very exciting possibilities.  

Hydrogen, Fuel Cells and Electricity  

A fuel cell is a device that, when presented with hydrogen and oxygen, produces two things: water and electricity. This electricity can be used to power an electric motor that propels our submerged sub silently through the water.  

The chemical equation of this energy transaction is: 2 H2 + O2 → 2 H20 + electricity.  

Oxygen is stored in the form of liquid oxygen (LOX). While it isn’t easy, it’s not impossible. Hydrogen is another story, and that’s where metal hydrides come in, but more about that later. The important thing to keep in mind for U-212 and its variants is that this arrangement allows for the vessel to stay silently submerged for as long as 2 weeks, albeit while traveling at reduced speeds.   

OK, so How do Metal Hydrides Fit In?  

Hydrogen in gaseous form is an accident waiting to happen. A prime example of this hazard was the Hindenburg, a hydrogen-based airship.  

The Hindenburg Airship Hydrogen Disaster.

Some ballpark comparisons:  

• Hydrogen gas stored at atmospheric pressure, contained in a one-meter cubed chamber, amounts to only about 90 grams of the element.   
• Even when compressed as high as is practicable (about 690 times atmospheric pressure), only 40 kg of hydrogen can be contained in the same volume.   
• For liquid hydrogen, the figure increases to 70 Kg/meter3. Both of these modalities require far too much space and present way too great a flammability danger to be usable on any sea-going vessel, much less on a submarine.  
• For metal hydrides, the figure can reach 80 Kg/meter3 or more. Most importantly, metal hydrides are stable over long periods of time and present little flammability risk.  

Of course, gaseous or liquid hydrogen is both far too dangerous to store in a submarine.  

OK, so just What Are Metal Hydrides?  

As described by Fraunhofer^[1], “Hydrogen can be safely stored in a very compact form and at low pressure through a chemical reaction with a hydrogen-absorbing alloy”. The process is illustrated below.   

A schematic Illustration of Metal Hydride Formation. Image source Fraunhofer  

The left side of the diagram is a three-dimensional illustration of a metal, with uncaptured H2 molecules nearby. On the right, we see the same metallic lattice with tiny hydrogen atoms “held captive” in the interstitial spaces between the lattice’s metallic atoms.  

As illustrated by the bidirectional arrows, it is a reversible process – the lattice can capture hydrogen atoms or release them. Essentially, the metallic lattice stores the hydrogen in a very stable manner. Unlike gaseous hydrogen, there is little dangerous, wasteful leaking. And, unlike either liquid or gaseous hydrogen, storage requirements are simple and inexpensive.  

The right side of the image illustrates a cylindrical Metal Hydride storage module. The right side displays a multi-module assembly. Image source: Fraunhofer  

A one-meter-cubed container of metal hydride contains more hydrogen than is possible with gaseous storage, as well as more than with liquid storage.  For that reason, metal hydrides are said to be volumetrically efficient.  However, when considering the mechanism’s weight efficiency, the weight of the metal must be considered along with the weight of the hydrogen. This lowers the gravitational efficiency of metallic hydride storage.   

To supply the same equivalent energy, a metal hydride system would have to be considerably heavier than a modern automotive Lithium Ion Battery (LiB). This would make metal hydrides inappropriate for mobile devices and EVs.   

However, submarines and devices like forklifts specifically need ballast. And a heavier weight is no disadvantage for many energy storage applications.    

Validating Metal Hydride Technology  

The National Renewable Energy Laboratory (NREL) and its collaborators have launched their “High Efficacy Validation of Hydride Mega Tanks at the ARIES Laboratory (HEVHY METAL)”. As described by NREL’s Katherine Hurst^[2],  “The ARIES platform and infrastructure in Colorado aims to help accelerate the deployment of innovative energy technologies related to renewable energy, storage solutions, and interactive loads,” She goes on to say that “By integrating GKN Hydrogen’s technology and collaborating with major utilities like SoCalGas, we are developing solutions to tackle the complexities of modern energy systems.”   

Most significantly, the facility’s 1.25-MW proton-exchange membrane electrolysis system is compatible with variable solar energy. It can store up to 500 kg of hydrogen infused into metal hydride. That hydrogen can later be converted to electricity and water via the system’s 1-MW proton-exchange membrane fuel cell.    

The energy equivalent of hydrogen is subject to conditions, but using the Lower Heating Value (LHV) definition, 500 kg of hydrogen can yield 16,650 kilowatt hours of electricity. It is often said that the average US household uses about 30 kWh a day, so by that definition, ARIES’ metal hydride system can store enough energy to power our American household for 555 days.  

A Gigantic Chinese Metal Hydride Project  

China’s Da’an project^[3] is aimed at storing hydrogen generated from electrolyzers within metal hydride systems. Among the goals is providing the hydrogen needed for ammonia production. Worldwide, ammonia is a key commodity in many sectors, with about 16 million tons produced in the US alone. The vast majority of this production comes from the steam reforming of natural gas, which poses some extremely negative environmental costs.  

The Da’an project features 1.5 tons of hydrogen storage in a 31.5 m3 container.  According to Da’an, their metal hydride system can keep hydrogen safely stored for 10 years with no loss.  

Doing some approximate back-of-the-envelope calculations:  

• Energy equivalent of 1.5 tons of hydrogen: 50 MWh
• The Da’an project assumes a Wt% (see glossary) of 6.5. That would be a weight of about 13 tons
• 1,500 gallons of gasoline have an energy equivalent of 50 MWh 
• 1,500 gallons of gasoline weigh about 5 tons  
• The volume in cubic meters of 1500 gallons of gasoline is about 5.5 cubic meters 

The Da’an project authors suggest many interesting applications for this technology. Perhaps the most exciting is a demonstration project involving the transportation of hydrogen, in the form of safe, stable metal hydrides, by sea from Shanghai to Malaysia.   

Wrapping Up  

The much-touted hydrogen green energy revolution hasn’t really gone anywhere up till now because hydrogen in gaseous form is too flammable and too hard to hold. Freezing it takes expensive, troublesome equipment, and even then, the smallest element in the universe finds ways to escape containment.   

Enter metal hydrides. Here, hydrogen atoms are sequestered within the spaces between metallic atoms. It’s relatively easy to shove the hydrogen into the metal, and easy to get it out again for use.  

A “charged“ metal hydride is still at least 90% metal and, as of today, 7% or so hydrogen. Energy density is not as good as a lithium-ion battery, but metal hydrides are safe, easy to store and unlike batteries, they do not lose energy over time.  

Challenges and Opportunities  

There are almost limitless possibilities for this technology.   

All throughout America, there are a number of sites that can host utility-scale solar power farms. All the locals are happy, and the cash is lined up. But there’s one problem – there is no National Will to build the power lines and grids necessary to make use of the power generated.  

It may turn out to be practical to energize metal hydrides right at the solar farm and simply deliver them by truck to wherever hydrogen is needed. The first obvious candidate is the chemical industry, a voracious user of hydrogen. Or, perhaps the chemical plants themselves perform the task on site, using regular, non-renewable energy – it still beats steam reforming of natural gas from an ecological standpoint.  

The ideal full cycle would be:    

• Renewable energy to an electrolyzer 
• Electrolyzer produces hydrogen.  
• The hydrogen is stored as a metal hydride 
• The hydrogen exits the metal hydride and enters a fuel cell 
• The fuel produces electricity.  

There are various estimates of the possible efficiency of this full cycle at the present time, but they hover at about 30%. The greatest challenge is the electrolyzer – at present, in most cases, performance does deteriorate with varying electrical input.   

References     

1. Metal Hydride Technology for Hydrogen Storage, Purification and Compression Applications. Fraunhofer 
2. Heavy Metal Debut: A World-Class Metal Hydride System. The National Renewable Energy Laboratory (NREL)   
3. China’s Solid Metal Hydride – an amazing Hydrogen Storage that is Denser, Longer and Safer. Integral   

Glossary of Terms  

• Air independent propulsion mode (AIP). Any marine propulsion technology that allows a non-nuclear submarine to operate without any access to atmospheric oxygen 
• Weight percentage of a metal hydride storage system (wt%). The percentage of the hydrogen’s mass compared to the total mass of the storage metal plus the hydrogen stored within.