11.4.5.5: Hydrogen as a “green” fuel suitable for sea- sonal storage

There is an article in a recent issue of the pv magazine titled “Hydrogen is the first viable option for seasonal storage”. In fact, note that all methods which have been discussed in this Chapter so far make it possible to store an amount of sufficient for a day or a few days at most in the case of normal demand. A lithium battery in which one could accumulate a supply “ for the entire winter” would have to be of truly gargantuan size!

Hydrogen actually offers the first feasible method of storing “ green energy ”, which can be maintained for a long time without loss, and when it needs to be used, it will meet the demand for months.

There is no need to create any fancy new technology, because almost all necessary ”puzzle pieces” already exist. What are we talking about? Well, about this old-fashioned method of storing compressed hydrogen. But not in tanks made of steel or carbon fiber composite large enough could not be built in any way.

Let us recall, however, what the method of energy storage using compressed air, presented earlier in the Section in this chapter, taught us. Namely, that compressed gases can be stored in underground caverns excavated in natural salt deposit. It’s not a great problem to make such a cavern because no other tools ade needed than water – skilfully used, it will flush out salt, leaving a cavern of the desired size ans shape.

In Utah, in the city of Delta region, a huge salt dome exists deep underground. Its top is at a depth of about 750 meters below ground level, its thickness is about 1500 m, and it is about 5 km across. Currentrly, several companies are interested in the possibility of creating caverns in this salt deposit for various purposes.

As far as hydrogen storage is concerned, plans have been laid out by Mitsubichi Hitachi Power Systems America to excavate kn the Delta salt dome a storage cavern that could fit the entire Empire State Building. So let’s make a quick estimate. This whole skyscraper has a volume of 37,000,000 cubic feet (approx. 1,050,000 m³). Suppose we fill this cavern with hydrogen at 100 bar (10 MPa). At this pressure, one cubic meter holds 7.3 kg of hydrogen, i.e. the mass of all hydrogen in the cavern will amount to about 7.5 million kg. The heat of combustion of hydrogen is 141.6 MJ / kg, multiplying these two numbers we get 1.062,000 GJ, or 295 GWh.

As we have estimated earlier at the beginning of this chapter, 300 GWh is the amount of energy that California needs in the evening and nighttime hours to completely free itself from the dependence on power generation from fossil fuels. Thus, if just one such cavern were used for storing hydrogen from electrolyzers energized by PV cells during the sunshine hours, and then this hydrogen were used for generating power after the sunset, the goal California has set for itself could be achieved.

It is estimated that in the Delta city region alone up to one hundred such caverns could be built. And similar storage reservoirs can be created in many other places where there are salt deposits. What’s more, not only salt caverns are suitable for storing hydrogen. Natural gas reservoirs from which gas has already been extracted are also suitable. Since such formation were able to hold methane for thousands or millions of years, one can now inject hydrogen into them without much fear that it would escape. And the storage capacity of such depleted gas fields can be enormous! In a recent British Web document (see Page 9) it is estimated that in one depleted gas field off the coasts of the British Islands the energy content of the hydrogen stored would be not several hundred GWh (as in the example discussed earlier), but over ten thousand GWh! In view of the above, we can be confident that there are storage methods capable of providing necessary energy not only for a day or a week, but even over long winter months!

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