Study on Hydrogen Flow and Heat Transfer in Underground Salt Cavern Hydrogen Storage

Hydrogen energy is a green, low-carbon, widely used secondary energy with abundant sources, and is gradually becoming one of the important carriers of energy transformation. The safe and efficient storage of hydrogen energy is particularly important. Underground hydrogen storage technology has received widespread attention due to its large scale and low comprehensive cost. Salt cavern hydrogen storage has good sealing performance, stable structure, and flexible operation, making it the most promising choice for large-scale underground hydrogen storage. According to the thermodynamic characteristics of hydrogen, a heat transfer model of hydrogen flow in salt cavern hydrogen storage is established. Three operating conditions, namely, hydrogen injection, static state, and hydrogen production, are simulated and calculated, and the flow field characteristics of hydrogen under different operating conditions are determined, providing technical support for the research on the design of hydrogen injection and production schemes for salt cavern hydrogen storage.


Introduction to underground salt cavern hydrogen storage
Hydrogen is a widely used, clean, and safe energy carrier that can be used as a power fuel or industrial raw material.With the vigorous development of unstable renewable energy sources such as wind power and photovoltaic, there are inevitably problems such as excess power and difficulty in absorbing it, which pose challenges to the power balance of the grid [1,2].The efficient conversion of renewable energy and large-scale energy storage technology can provide effective ways to solve problems such as wind and solar energy consumption, peak shaving and valley filling, and power balance [3].Among the clean secondary energy sources, hydrogen energy is recognized as the most promising secondary energy source in the 21st century [4].The application of hydrogen energy can help reduce society's dependence on non-renewable energy such as coal and petroleum, promote the transformation of the fuel industry, and reduce carbon emissions in transportation, industrial energy consumption, and building heating processes.Currently, the major developed economies in the world have raised the use of hydrogen energy to a national strategic level, and have formulated long-term research and development plans at the national level.
As an important bridge between hydrogen production and utilization, the importance of hydrogen storage technology cannot be ignored.High pressure gaseous hydrogen storage technology, low temperature liquid hydrogen storage technology, solid hydrogen storage technology, and organic liquid hydrogen storage technology are currently the four main hydrogen storage technologies, with the mainstream method still being high pressure gaseous hydrogen storage [5,6].Underground hydrogen storage technology has received widespread attention due to its large scale and low comprehensive cost.The developed countries in the world, represented by the United States, are conducting technical breakthroughs around underground hydrogen storage technology, which has developed rapidly.At present, the United States, Britain, Germany, Canada, Poland, Netherlands, Denmark and other countries have also formulated hydrogen storage plans in salt caverns [7][8][9].
Salt cavern hydrogen storage refers to the large-scale storage of hydrogen energy in underground salt caverns using the high sealing property of salt caverns, which can be used for short-term or seasonal storage.Compared to surface storage, underground salt cavern storage has the advantages of large reserves, low gas storage cost, and good sealing performance.At the same time, it can also save and optimize surface land resources, making it an ideal place for large-scale hydrogen storage.Hydrogen storage technology is a key technology to promote the development of the hydrogen energy industry [10][11][12].Underground salt cavern hydrogen storage has broad application prospects, but it also faces challenges.Salt cavern hydrogen storage can learn from the construction experience of salt cavern gas storage, but there are differences in the physical properties of hydrogen and natural gas, and the flow characteristics during hydrogen injection and production need to be studied.

Establishment of hydrogen physical properties parameters and models
During the injection and production process of hydrogen storage, the temperature and pressure of hydrogen are constantly changing, so the physical parameters of hydrogen are also constantly changing.During gas injection, the gas is compressed and the temperature increases; During gas production, the gas expands and the temperature decreases.Under operating conditions, a thermodynamic property chart of hydrogen density, constant pressure heat capacity, compressibility factor, thermal conductivity, viscosity, and heat capacity ratio has been established to facilitate the study of the flow characteristics of hydrogen.The thermal property chart is shown in Figure 1.
Where, ρ is the density, t is the time, U is the velocity vector, p is the pressure, μ is the viscosity, I is the identity matrix, C p is the constant pressure heat capacity, Q p is the work done by pressure, W p is the volumetric work, and Q vd is the viscous dissipative work.

Flow characteristics of hydrogen during operation
The flow characteristics of hydrogen during injection and production in salt caverns are obtained by solving the model.Under different conditions, the hydrogen flow law in the underground salt cavern storage chamber is also different, which can be divided into the following three situations: hydrogen injection, standing, and hydrogen production.
Under hydrogen injection conditions, as shown in Figure 2, hydrogen is injected from the wellhead and passes through the wellbore to exchange heat with surrounding rock.Due to the friction between the gases and the friction between the gas and the wellbore, the temperature of the hydrogen in the wellbore increases.When it reaches the cavity, kinetic energy is converted into heat energy, which changes the temperature of the hydrogen in the cavity.The speed of hydrogen in the wellbore is greater than the speed in the cavity.During hydrogen injection, the velocity at the central axis of the cavity is significantly higher than that in the rest of the region.Since the model does not consider the impact of gravity on the results, the hydrogen pressure basically changes uniformly over time.Under static conditions, as shown in Figure 3, there is no forced convection of the hydrogen, but the heat exchange between the hydrogen in the cavity and the surrounding rock results in uneven temperature of the gas in the cavity.The uneven temperature results in uneven density of the hydrogen.Due to the density difference, the hydrogen gas in the cavity undergoes natural convection, resulting in a consistent temperature of the hydrogen in the cavity.Under hydrogen production conditions, as shown in Figure 4, the velocity in the wellbore is higher than the hydrogen velocity in the cavity, and the hydrogen gas flows into the wellbore from the cavity.The size of the wellbore suddenly decreases.Due to the Joule Thomson effect, the temperature of the hydrogen in the cavity decreases rapidly.

Conclusion
The flow characteristics of underground salt cavern hydrogen storage during hydrogen injection, static and production condition were studied in this paper.Research shows that the temperature of hydrogen gas in the wellbore and salt cavity increases during gas injection, and the velocity at the central axis of the cavity is significantly higher than that in the rest of the region.Under static conditions, the temperature of the hydrogen gas in the cavity eventually tends to be uniform.Under hydrogen production conditions, the temperature of the hydrogen gas in the cavity decreases rapidly.In order to ensure the stability of the cavity, the hydrogen production speed cannot be too fast.

Figure 1 .
Figure 1.Hydrogen thermal property chart.After establishing the thermodynamic property chart, a flow and heat transfer model is established.N-S equations are used to describe the flow behavior of the fluid in the model for flow and heat transfer.

Figure 2 .
Figure 2. Flow field and temperature distribution under hydrogen injection condition.

Figure 3 .
Figure 3. Flow field and temperature distribution under static condition.

Figure 4 .
Figure 4. Flow field and temperature distribution under hydrogen production condition.