Energy Storage: Batteries Breathing Down the Neck of Hydropower Plants

The method of energy storage depends on whether it is electrical energy, oil and oil derivatives, gas, wind and solar energy, or biodiesel. Each form of energy is stored differently: electrical energy is stored in pumped storage hydropower plants, Ina transports oil from the fields in Moslavina, Slavonia, and Podravina to the Sisak Oil Refinery, and imports to a storage facility in Omišalj, while gas is stored in the underground gas storage Okoli, managed by the state company Underground Gas Storage, and a new storage facility is being built in Grubišna Polje. When it comes to storing biodiesel, the storage time for this energy source is unlimited, but it is important that the interior of the tanks in which it is stored is not coated with paint that reacts upon contact with biodiesel and that they are not corrosive.

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Necessary Balance

Prof. Dr. Hrvoje Pandžić, an associate professor at the Department of High Voltage and Energy of the Faculty of Electrical Engineering and Computing at the University of Zagreb, emphasizes that the profession prefers the terms ‘energy storage’ and ‘energy storage’ when it comes to terminology. He also points out that electrical energy is a transitional form of energy and therefore cannot be stored for later use, but energy from renewable sources can be stored due to increasingly cheaper storage batteries.

– Electrical energy is produced and consumed simultaneously. Therefore, it is necessary to continuously maintain the balance of its production and consumption. Energy storage facilities occasionally behave as consumers (when they are charged) and occasionally as producers (when they are discharged) of electrical energy. Electrical energy is not stored in storage facilities; rather, it is converted into another form of energy, which is subsequently converted back into electrical energy. Of course, this process has its losses, and the efficiency is less than 100 percent – says Prof. Dr. Pandžić.

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He emphasizes that the conventional method of storing electrical energy is pumped storage hydropower plants, such as the Velebit pumped storage hydropower plant. In such hydropower plants, energy is stored in the form of potential energy of water.

Challenges and Limitations

According to Prof. Pandžić, the largest energy storage facilities today are indeed pumped storage hydropower plants. However, the locations for their construction are geographically limited because a river and an elevation with an upper reservoir are necessary. – Given the increasing integration of renewable sources, the need for energy storage in the power system is increasing, and new technologies are being used more and more, primarily batteries (conversion to chemical energy) and flywheels (conversion to kinetic energy). Technological advancement and the drop in battery prices, primarily due to the significant momentum of electric vehicles, make battery storage an increasingly acceptable solution for providing power system regulation services – emphasizes Prof. Pandžić.

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Biodiesel is stored like other oil derivatives in above-ground or underground storage facilities, but under special conditions, emphasizes Ivan Corn, manager of Biodiesel Vukovar, BDV. – The only additional requirement is that the storage facilities are not coated internally with paints that could react upon contact with biodiesel and that they are well maintained, meaning they show no signs of corrosion because metal oxides can negatively affect the oxidative stability of biodiesel and its durability – says Corn. If biodiesel is stored under satisfactory conditions (in storage facilities without corrosion and direct moisture ingress), Corn emphasizes, it can be stored for a longer time (several months), and if the storage facilities are also inerted with nitrogen, then biodiesel has an almost unlimited storage life.

Safe Storage

Crude oil produced in domestic fields is transported by Ina to storage facilities at the Sisak Oil Refinery where it is stored until processing, and imported oil is stored in Omišalj, from where it is transported for processing to the Rijeka Oil Refinery. Oil derivatives are also stored in both of Ina’s refineries from where they are distributed by tank trucks directly to retail outlets or by ship and rail to Ina’s logistics storage terminals in Zagreb, Solin, Osijek, and Ploče. Oil derivatives are then further distributed by tank trucks to end customers. Until 2009, Ina was also engaged in gas storage, but then the state company Plinacro established a new company Underground Gas Storage and purchased a 100 percent business share from Ina. This transferred control over gas storage to the state, and the capacity of the underground gas storage Okoli was increased by 20 percent. This storage is connected to the main gas pipeline and operates through cycles of gas injection and withdrawal.

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Gas is injected from the beginning of April to October, and withdrawn from the beginning of October to April. The gas injection capacity is 160,000 cubic meters per hour. In the gas storage Okoli, at three depleted gas fields located at a depth of 1900 meters, 553 million cubic meters of natural gas can be stored. However, the output capacity, conditioned by the geological characteristics of the gas fields in which the gas is stored, is only 350 million cubic meters of gas per year. To address this issue, an increase in gas storage capacity is planned in three phases: by expanding the existing underground gas storage in Okoli, constructing a peak storage facility in Grubišna Polje, and building a new seasonal underground gas storage. The first phase was achieved by reconstructing Okoli, allowing for a 20 percent increase in gas output capacity.