Nice Graphic Representations of Some Thermochemical Hydrogen Cycles; Tables of Cost & Efficiency.

[NNadir]

Following some references in papers of interest, I came across this nice review article with some nice graphic representations of various thermochemical hydrogen cycles. A few are hybrid cycles, and require the input of electricity, but avoid the high energy cost of pure electrolysis, as electricity is a thermodynamically degraded form of energy. (The efficiencies stated here do not include the thermodynamic efficiency of generating (and in some cases transporting) electricity itself, a very important caveat.)

The paper is this one: Onur Oruc, Ibrahim Dincer, Assessing the potential of thermo-chemical water splitting cycles: A bridge towards clean and sustainable hydrogen generation, Fuel, Volume 286, Part 2, 2021, 119325

There is always a lot of junk thinking around about using hydrogen as a consumer fuel, cars, trucks, boats, etc. As the atmosphere is collapsing now and not in some fantasy "hydrogen economy" future, there is not time to rebuild all of the world's infrastructure to handle this difficult to handle energy storage medium; the volumetric energy density is too low even if the mass density of this form of energy storage is chimerically high. However, hydrogen is a very useful captive intermediate, and, via the hydrogenation of carbon dioxide (or monooxide) can produce fuels that can drop into existing infrastructure either as synthetic petroleum or natural gas via FT synthesis - an idea I personally oppose - or as the wonder fuel DME, which can displace all the world's fossil fuels by dropping into existing infrastructure with relatively minor adjustments. (DME will eliminate several requirements now in place for dangerous natural gas infrastructure, notably the need for refrigerants and liquefaction. The critical temperature of DME is well above the boiling point of water, and DME itself - now a component, among other things of hair spray cans - is a refrigerant. In addition, importantly, DME is not a greenhouse gas; it's atmospheric half-life is 5 days.)

Anyway, some graphics on thermochemical cycles from the paper:



The caption:

Fig. 3. Pure and Hybrid Thermochemical Cycle.

I have a criticism of this particular graphic, which is apparently based on the use of a heat source - in my view the only acceptable source is nuclear energy - that is dedicated to thermochemical hydrogen production. My criticism is this: If the thermochemical cycle is coupled to a heat exchange network, it follows that the system can generate electricity as a side product. This is not shown. This criticism applies specifically to the most studied and most advanced hydrogen cycle, the "SI" (Sulfur Iodine) cycle and it's many variants (only one of which is shown here). The SI cycle is superior (in my view) to most other cycles because it only involves fluid phases, and thus can operate continuously. Continuous processes are always economically and environmentally superior to batch type processes, but I note that some processes involving solid phases can involve heterogenous continuous or nearly continuous production.

The SI cycle:




The caption:

Fig. 4. Sulfur-Iodine Thermochemical Cycle.

The sulfur cycle, similar to the SI cycle, but eliminating iodine by the input of electricity:



The caption:

Fig. 5. Hybrid-Sulfur Thermochemical Cycle.

The iron chloride cycle, involving solid phases. This cycle has been evaluated as a side product to the steel industry:




The caption:

Fig. 6. Iron-Chlorine Thermochemical Cycle.

As a personal note, I am happy to report that my son's first assignment in his Nuclear Engineering graduate program is to familiarize himself with the 3D printing of special steels. This idea seems to me as offering hope of eliminating the need for coal derived coke in steel manufacturing.

The much studied copper chloride (other halides have also been studied in this respect).




The caption:

Fig. 7. Copper-Chlorine Thermochemical Cycle.

The following cycle, with which I was unfamiliar until a few days ago, strikes me as interesting given that the ideal placement of nuclear plants seems to be along the coasts, where they can mitigate one effect of climate change now being observed worldwide, megadroughts by offering desalination as a side product. (I ruminated on an approach to this about this here: The Energy Required to Supply California's Water with Zero Discharge Supercritical Desalination.) Magnesium and chlorine are obviously side products of desalination.




The caption:

Fig. 8. Magnesium-Chloride Thermochemical Cycle.

Speaking of thermochemical hydrogen cycles with which I have been unfamiliar - thousands of such cycles have been proposed - I came across a new one, which requires the lowest temperatures that I've ever seen, around 300°C. It is the uranium bromine cycle, where uranium (oxide/bromide) is a catalyst for water splitting. Of course, even though uranium represents the last best hope to save the world, many people have an irrational fear of the element, driven by oblivious and mostly stupid rhetoric. The uranium bromine cycle is not discussed in the publication now under discussion.

Now two of the tables in the paper. This first is normalized to an ideal value of 10; in this case, the highest value is the best in this table. Note that this paper is source agnostic; it considers the solar thermal case, which in my opinion is a waste of time. Land intensive, fossil fuel dependent, and unreliable solar thermal systems - all of which are garbage - need to be located in deserts, which by definition, lack a supply of water. I am nonetheless amused that the capacity factor is simply not addressed, because the capacity factor of these in flight bird fryers is not worthy of discussion.



The second table gives costs using some known types of nuclear reactors, as well as the fantasy solar/dangerous fossil fuel system.



The "MHR" (Modular Helium Reactor), which offers according to this review the lowest cost hydrogen - in the "percent talk" that anti-nukes use to justify the failure of so called "renewable energy" to address climate change, about 25% the cost of solar/dangerous fossil fuel thermal hydrogen - is a Brayton cycle nuclear reactor that will prove impossible to build in the future, because helium will become an increasingly exotic and rare element in the relatively near future. When released to the atmosphere, helium escapes into space as can be seen by utilizing the Maxwell-Boltzmann distribution to recognize that helium atoms exceed the escape velocity of the Earth. (Small amounts may be available to researchers from the decay of short lived actinides like Cm-242 and others available from used nuclear fuels; all of the geologically sequestered helium on this planet comes from the natural decay of uranium and thorium, but has accumulated over billions of years. Tritium decay, which gives "light" helium, He-3 may be another small source.) However other Brayton cycle reactors are certainly possible, and this is not much of a concern to me at least. Indeed their are huge benefits for using Brayton cycles that do not use helium as a working fluid.

Anyway, in a sane future world, as opposed to the insane present world in which we live, this technology will be utilized to save what can yet be saved and restore what can be restored.

I wish you a pleasant Sunday.

[IbogaProject]

1. Thanks

Have you considered sending a proposal to the right congressmembers, senators and the White House?