Renewable energy consists of solar power and wind power, and is attracting attention as a next-generation energy source due to its large effect in reducing carbon dioxide emissions. However, renewable energy is dependent on natural conditions such as weather. Therefore, the output is unstable and difficult to follow electricity demands, causing restriction of increasing capacity originated from the renewables. In order to solve this problem, it is necessary to store power from renewable energy and absorb and smooth out fluctuations in output.
To achieve this, we have proposed and demonstrated an Advanced Superconducting Power Conditioning System (ASPCS) that combines a superconducting magnetic energy storage device (SMES), a fuel cell generator, and an electrolyzer. ASPCS is a system that splits the fluctuating output power into long-term and short-term components and compensates for a well-controlled power. When there is an oversupply of power generated by renewable energy, the surplus is stored as hydrogen by electrolyzing water with an electrolyzer (EL). Also, when the generated power is insufficient, the hydrogen is supplied to the fuel cell to make up the power-shortage, while high-frequency power fluctuations that cannot be absorbed by fuel cells are leveled by SMES with the short-time charge and discharge ability. This system could be a solution to the enhancing utility grid stability and effective use of renewable energy. SMES that plays the most important rule in ASPCS, is a device that uses superconducting coils to store electric power with extreme high efficiency, due to not converting it into other forms of energy or degrading with the number of cycles.
At present, hydrogen energy is attracting attention as a clean energy that does not emit greenhouse gases when used, and this system also uses hydrogen energy as its central axis. Hydrogen has a boiling point of 20 K at 1 atm and has not been used as a refrigerant due to the danger of wide explosion limits. Besides evaporation during transportation is not negligible. We also have been proposed the effective use of boil-off gas by providing the gas as an energy source for electricity generation from the fuel cells, as well as utilizing the cold heat for cooling Joule heat generated along the transmission lines.
Our research team used MgB2 as a superconducting material to fabricate coils. Because critical temperature is around 39 K, it shows superconducting properties when cooled with liquid hydrogen. This will combine an environmentally friendly power generation system with a liquid hydrogen station for fuel cell vehicles to create an economical, ultra-low-carbon system. In addition, MgB2 can be easily made into a wire following conventional powder-in-tube technology with low cost and the product is light weight.
Liquid hydrogen is flammable and must be operated in a safe manner. For this purpose, we have introduced an indirect liquid hydrogen cooling system based on thermosiphon circulation technology that completely separates the combustible liquid and the power. Cooling by thermosiphon is highly reliable because there is no drive equipment such as a pump. In the current phase of the project, we fabricated a 10 kJ SMES coil system indirectly cooled by liquid hydrogen and demonstrated it both at DC and with varying current at several ramp rate conditions.
In this report, we describe the evaluation of transport properties and losses by changing the current as well as the contact resistance between the coil and terminals. We also use computer simulations with heat balance equations to evaluate and lead to the design of large-capacity future energy storage such as the MJ class, which is comparable to conventional NbTi SMES.
Keywords: superconductivity, MgB2