There is no other energy storage system that provides the location flexibility, scalable capacity, and the small surface footprint that this patented hydrogen-based system delivers.
Why is energy storage so critical for renewable energy?
The supply and demand of any electrical grid must be constantly balanced to provide a stable power supply to consumers. Unlike fossil fuels such as coal, which can be stockpiled as energy inventory, solar and wind energy cannot be captured in a container for later use. As these intermittent and variable energy sources increase penetration into the power grid, the need for energy storage will continue to grow and will become a necessity once it exceeds base load power potential. Renewable energy systems are already being taken off line due to over generation during peak production periods.
Why is hydrogen the best option?
Hydrogen is the best option for two reasons: 1) it can be produced in abundance in several ways, including directly by electrolysis that is powered from solar and wind energy, and 2) hydrogen has the highest mass energy density of all other options including two orders of magnitude benefit over battery technology.
Hydrogen solutions have been proposed before, so what makes this solution different?
Current large-scale hydrogen storage systems use manmade salt caverns. There are three major drawbacks associated with this approach: 1) locations are limited to areas with existing suitable salt deposits, 2) fill rates and withdrawal rates have limitations, 3) the net storage capacity of useable hydrogen gas is only about 2/3 of the total amount stored. Surface storage in tanks or vessels is limited to much smaller scale storage capabilities and is not practical for a grid utility scale.
What makes this new system better?
First, the system uses underground storage chambers that are constructed using common practices in mining and the oil and gas industry. A large-diameter blind shaft boring drill in diameters up to 6m and depths up to 600m creates the mining type storage chambers. Oil and gas well type chambers can also be constructed using surface drilling rigs in diameters generally ranging from 30cm up to 60cm to depths of 3000m. These chambers can be lined with hydrogen resistant materials to prevent hydrogen leakage and ensure structural stability in any ground condition. These features allow storage of large volumes of gas at virtually any location, enabling the storage to be positioned wherever is most convenient to the power generation and application.
Second, the chambers are flooded with water prior to filling them with hydrogen using a gravity feed from a surface body of water through a feed pipe to the bottom of the chamber. The accumulating gas in the top of the chamber has to force water out of the bottom of the chamber to make room for additional gas. In order to do this, the gas pressure in the chamber has to overcome the weight of the water in the water pipe. This hydrostatic pressure is used to maintain pressure in the chamber, as it is being filled for storage and emptied for regeneration of electricity. The hydrostatic pressure feature eliminates the need for mechanical compression and the associated costs. The hydrostatic pressure increases at a rate of approximately 10 kPa/m. Pressures of 35 MPa can be achieved with chambers 3,500m deep. The system can also be designed to provide constant hydrogen pressurization by pressuring the water in the chamber rather than relying on hydrostatic pressure. This could be beneficial for systems where the hydrogen is generated under pressure and the requirement is to maintain that pressure in the storage chamber.
Third, the system is fully rechargeable an unlimited number of times. Because of the water filled system, filling and discharge rates are not dramatically restricted by thermodynamic and structural stability concerns, providing an advantage over massive salt cavern storage systems.
Fourth, the use of these manmade chambers provides a system that is fully scalable by varying the size and number of chambers. The table below provides some examples of single chamber storage comparisons. As seen in the table, sufficient storage to provide micro to full utility grid scale power can be provided.
How does the system work?
The diagram at the beginning of the article provides a conceptual illustration of the system. Whenever grid demand falls below the solar or wind farm’s capacity, the excess electricity is diverted to a high-pressure electrolyzer where hydrogen gas is produced. Hydrogen accumulates in the upper portion of the closed chamber and expands in volume as more gas is captured. As gas accumulates in the chamber, the gas pressure forces water back out of the chamber through the fill pipe to the surface reservoir. The gas pressure increase is proportional to the weight of water it displaces, therefore increasing in proportion to its volume. Pressure balance between the gas and water continues at all times in the chamber, controlled by the hydrostatic pressure in the chamber fill pipe.
When additional grid power is needed, the stored hydrogen can be released from the chamber and regenerated into electricity. Allowing gas to escape from the chamber causes the chamber to be refreshed with additional water. Since energy balance in the chamber is preserved at all times, the transfer of water back into the chamber occurs automatically. The energy balance will also control the additional water volume. The water level will automatically reach the proper level based on the amount of hydrogen released.
Power regeneration from the stored hydrogen can be either in the form of fuel cell or gas turbine. Both technologies exist although hydrogen turbines are still under development and fuel cell technologies are constantly improving.
Tom Barczak, Ph.D.,
Solar Wind Storage LLC
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