Volatility is an inherent characteristic of most renewable energy sources. This is especially applicable for wind and photovoltaic power generation. Historic data shows that in Western and Middle Europe we experience so-called dark doldrums of about two weeks length almost every year. On the other hand, on windy days, especially when combined with sunshine, there is a massive overproduction of electricity which leads to power plant shutdown and energy export at sometimes negative prices. Therefore, it is obvious that an energy storage system which can make use of the overproduction and use it for “bad times” is an essential keystone when building a defossilized sector coupled energy infrastructure.
When looking at the different scenarios for reaching the European Union’s CO2 reduction goals up to 2050, most of them determine an annual hydrogen demand between 30 to 70 million tons per year. Forecasts for Germany range between 10 to 15 million tons per year. It is quite safe to say that a storage system should offer a 60-day reserve for the average demand. In this case that would be 5 to 12 million tons for the European Union.
Fortunately, Europe is blessed with many usable salt deposits and many salt caverns are nowadays used for storing natural gas or oil.
Some hydrogen salt cavern storage sites are already for decades in use in United Kingdom and in the USA, both supplying adjacent chemical and petrochemical facilities.
A typical salt cavern is the result of brine production by flushing water through the underground rock salt deposit. Most caverns in Northern Germany or Poland are located in a depth of 500 – 2,500 m with diameters of 50 to 100 m and a height of 100 to 500 m. Typical maximum allowable filling pressures are 150 to 200 bar. In order for the cavern to remain stable, not all of the filling can be used as working gas. About 1/3 must remain in the cavern as buffer gas, and the remaining 2/3 can be used as working gas for storage operations.
Despite the unique cushion gas requirement, the storage capacity of a cavern is tremendous. An average cavern with 60 m diameter, 300 m height and a filling pressure of 175 bar contains 100 million Nm3 of working gas. In case of hydrogen that is the equivalent of 300 GWh which can be used for heating, steel production, mobility or reconversion to electricity. For the latter with currently available fuel cell technology about 60 % of the energy can be recovered for electricity. This is absolutely on par with the best available gas fired powerplants but has the advantage that no carbon capturing is required. A typical underground gas storage consists of several caverns, called Epe and is located in north-western Germany nowadays can store up to 4 billion Nm3 of natural gas.
When we add up the numbers, we can see that Europe, depending on the abovementioned scenarios, would need around 15 to 33 Epe-equivalents or 2 to 5 times the current German underground storage capacity. As not all the existing sites can be converted to hydrogen usage and keeping in mind the flushing time of 2 to 5 years for a cavern it is obvious that expansion activities must start urgently.
Filling and operating such an underground storage are characterized by variable suction and discharge pressures and especially highly volatile volume flow demands resulting from the volatile behavior of the electricity generation by wind and photovoltaics. Fortunately, NEUMAN & ESSER already has ample experience in this field, combining different control means like variable speed drive and reverse flow control to fulfill the demand in the most energy conserving way. One major difference to natural gas storage is, that they can use turbo compressors for the base load and piston compressors for the peak loads or very small loads. As turbo compressors are unable to efficiently compress hydrogen to these pressures, piston compressors take over also the base load task for hydrogen storage sites. Again, their efficiency is remarkable. When compressing from 60 bar pipeline pressure to 200 bar storage pressure such a baseload compressor consumes about 15 MW with a volume flow of 300,000 Nm3/h or 1.6 % of the energy contained in the hydrogen. At 60 to 120 bar, in the middle of the filling phase, only 0.9 % are required.
