For those of you new to the topic, hydrogen is an alluringly energy-dense fuel, which is why it has been used for space operations since the 1950’s. Getting the financials to work out for Earth-based vehicles, though, is another matter. Currently almost all of the hydrogen produced in the US is generated from fossil natural gas, and it is used in various industrial fields including petroleum refining, metallurgy, fertilizer production, and food processing.
The advent of low cost clean power has opened up a whole new scope of applications, because it enables the production of renewable hydrogen by “splitting” water.
More & Better Clean Power
Until recently there have been two main streams of approach to water splitting. One involves the “bionic leaf” concept, in which a small photoelectrochemical cell is placed directly into water to mimic photosynthesis. The other approach can use different forms of clean power — typically solar but also including wind and tidal energy — to generate electricity for large scale electrolysis.
Just last week we noticed the emergence of a third approach, which combines clean power, high heat and a chemical reaction in a high-efficiency hydrogen production system. Depending on your definition of clean power, that could be either a concentrating solar plant or waste heat from a nuclear power plant.
In the latest development, a research team at Purdue University has come up with a multi-level clean power concept they’re calling “hydricity.” Here’s the rundown from Purdue: “Hydricity uses solar concentrators to focus sunlight, producing high temperatures and superheating water to operate a series of electricity-generating steam turbines and reactors for splitting water into hydrogen and oxygen. The hydrogen would be stored for use overnight to superheat water and run the steam turbines, or it could be used for other applications, producing zero greenhouse-gas emissions.”
By “superheating,” the research team means heat beyond the boiling point, between 1,000 to 1,300 degrees Celsius. This produces the ultra-hot steam needed for high-heat water splitting, and the steam can also be used to run turbines as in a conventional concentrating solar plant.
The Energy Storage Connection
In terms of the financials, the key to the whole system is the use of hydrogen as both a stationary and a transportable energy storage platform, enabling it to replace or supplement existing fossil sources. The system would also enable scalable hydrogen-based electricity production, from central power plants to in-home fuel cells supplied by existing (or new) gas lines.
In contrast to conventional batteries (including advanced lithium-ion batteries), the energy storage component of the “hydricity” concept does not degrade over time as explained by the research team: “The overall sun-to-electricity efficiency of the hydricity process, averaged over a 24-hour cycle, is shown to approach 35 percent, which is nearly the efficiency attained by using the best photovoltaic cells along with batteries. In comparison, our proposed process stores energy thermo-chemically more efficiently than conventional energy-storage systems, the coproduced hydrogen has alternate uses in the transportation-chemical-petrochemical industries, and unlike batteries, the stored energy does not discharge over time and the storage medium does not degrade with repeated uses.”
The full paper is being published at the Proceedings of the National Academy of Sciences and Purdue kindly provided us with an advance copy before it is available online. It provides a full explanation of the advantage of using high heat compared to conventional solar thermal power:
…harnessing solar energy at higher temperature has two promising advantages: (i) thermodynamically, the use of high temperature heat for electricity production increases the electricity generation efficiency, and (ii) heat at temperatures in excess of 1000 K enables thermochemical reactions such as hydrogen production via solar thermal water splitting, on which numerous remarkable theoretical and experimental results have been reported in the 6 literature (21-23). Among solar thermal hydrogen production methods, two-step water splitting cycles based on the reduction and reoxidation of metal oxides, are prominent since they achieve high efficiencies that can make these systems economical and implementable in commercial scale.
According to the team, the hydricity concept can be modified to produce high pressure hydrogen. That’s significant because hydrogen must be compressed in order to be transportable and usable.
It’s A Hydrogen World…
As for uses of hydrogen in the chemical industry, the researchers make this point: “The chemical industry is one of the largest consumers of electricity, which can be supplied from a hydricity process using the strategies listed above. Moreover, hydrogen is an indispensable molecule for chemical industry. It is primarily used for hydrogenation, hydrotreating, and catalyst regeneration, which are common operations in all chemical plants…”
And then there’s the food and fertilizer industries: “The production of all nitrogen and compound fertilizers requires the use of large quantities of hydrogen. Additionally, hydrogen is the major compound for the production of saturated fatty acids from vegetable oils.”
As for the auto industry, fuel cell EVs have a lot of catching up to do with their battery EV cousins, but as the saying goes, don’t look back…