Energy storage systems and their optimal application in power distribution networks

Battery energy storage systems, BES (Batteries Energy Storage), use devices where energy is stored in electrochemical form to later generate and supply energy.

Batteries, also sometimes called accumulators or cells, generally consist of two electrodes, anode and cathode (where the oxidation and reduction reactions take place, respectively) and an electrolyte, which is the medium through which the ions circulate.

There are a wide variety of battery technologies for energy storage: lead-acid, lead-acid, sodium, sodium-acid
There are a wide variety of battery technologies for energy storage: lead-acid, sodium-sulfur, nickel-iron, nickel-cadmium, zinc-air, air-iron, lithium-polymer, etc. Due to this diversity, many of their characteristics can vary considerably, but in general they have as advantages high or very high energy densities and require little or no maintenance. Similarly, their main common disadvantages are the management and treatment of their components, reagents and/or products, which are usually toxic, and their reduced life cycle.

The lead-acid battery is the most frequently used electrochemical storage device on the market in terms of energy. It is a well-established technology and the most mature of all battery storage systems, having been discovered in 1859.

The lead acid battery consists of a lead electrode, Pb (anode) and a lead dioxide electrode, PbO2 (cathode), which are separated by an insulating membrane and immersed in an electrolyte of dilute sulfuric acid solution, H2SO4.

During the discharge cycle, both electrodes are transformed into lead (II) sulfate, PbSO4, and return to their initial state during the charge cycle. There are two main types of lead acid batteries: wet or flooded batteries, which is the most common topology used, and sealed or valve-regulated batteries, which are currently under development and research.

The main characteristic of wet batteries is that the electrolyte is in a liquid state and water evaporates, which implies the need for maintenance to replenish it, while sealed batteries are characterized by having a valve that controls the release of the gases produced, reducing maintenance. The latter can consist of gelled or absorbed electrolyte; in fact, another possible classification is based on the state of the electrolyte in the battery: liquid electrolyte, gelled electrolyte and absorbed electrolyte. In absorbed electrolyte batteries, the electrolyte is in a liquid state, but absorbed in a microporous separator that isolates the electrodes and is composed of glass fiber. This method reduces the volume of electrolyte, since only the minimum necessary for the reaction to take place is used. The advantages of this type are no or almost no maintenance, small footprint and very low outgassing.

The main disadvantages are that they are sensitive to variations in ambient temperature and that the highest performance is only achieved when charging is carried out before the battery reaches 50% discharge. Gelled electrolyte batteries contain a silica additive that acts as a desiccant and ensures that the electrolyte remains in a solid state, in the form of a gel. The use of this type is optimal when high depths of discharge and high temperature environments are foreseen, thus extending their life, but they are extremely sensitive to overcharging which can prematurely damage the battery and thus reduce its life. In general, all types of lead acid batteries have a low cost, 250-500 €/kWh [3], a life cycle of 1000-1800 cycles (although it depends largely on the depth of discharge) and a high efficiency, around 80%]. Depending on the operating temperature of the system, battery life can vary from 5 to 15 years. In fact, high operating temperatures, up to 45°C, can improve battery performance in terms of increased capacity, but equally capacity, but still greatly reduce the life time of the system of the system. Thanks to their low daily self-discharge, which is reported to be less than 0.1%, lead acid batteries are suitable for energy storage over long periods of time.

Unfortunately, their relatively short life span and poor performance in the face of ambient temperature variations make these batteries their main disadvantages. Other associated disadvantages are the need for periodic maintenance to replenish water (only in the case of the liquid electrolyte wet battery) and their low specific energy, 180 W/kg and power, 30-50 Wh/kg, due to the high intrinsic density of lead. In addition, they show difficulties in cyclic power delivery due to partial states of charge and overcharging or decrease of their nominal voltage, can generate the creation of lead sulfate crystals – lead sulfate phenomena.

This type of technology is a good and common storage option as a support for regulation and power quality, as an uninterruptible power supply system, UPS, and in some rolling reserve applications. An example of a lead-acid BESS application is the Metlakatla, Alaska, facility (Metlakatla, Alaska) comprised of valve-regulated lead-acid batteries
regulated lead-acid batteries, which has been operating since 1997 as a frequency control, and helps improve grid power quality with a maximum output of 1 MW and 1.4 MWh of energy. The world’s largest installation of this type is located in California, USA, which with a capacity of 40 MWh is capable of operating for 4 hours at a power of 10 MW.

Sodium sulfide, NaS, batteries are a relatively mature technology, with the first research dating back to the 1960s, which now has research dates back to the 1960s, and there are now more than 220 installations worldwide, with a total power of 316 MW and a capacity of almost 2,000 MWh. more than 220 installations worldwide, with a total power of 316 MW and a capacity of almost 2000 MWh. The NaS battery is made up of two liquid electrodes, the positive electrode of sulfur, S, and the negative electrode of sodium, Na, separated by a solid electrolyte ceramic electrolyte of beta-alumina, β-Al2O3.

The beta-alumina electrolyte allows only sodium ions to flow through it, so that sodium can combine with sulfur to form sodium polysulfides, Na2Sx. This flow corresponds to the discharge of the battery, while during charging the reverse process takes place and the sodium polysulfide decomposes into sodium and sulfur ions. Both reactions are carried out at high temperatures, 300-350 ºC, so that the electrochemical reaction can take place and a potential difference of approximately 2 V can be produced of approximately 2 V. A high energy density, 151 kWh/m3 (approximately 3 times higher than lead-acid batteries) and a very high efficiency of 85%, together with a very high durability are its main advantages durability are its main advantages. Other important characteristics of NaS batteries are that they have a life cycle of approximately 2500 cycles, their maintenance is low, their self-discharge is practically null, and 99% of their components are recyclable.

The main disadvantage is the need for electric heaters, which are used to reach the initial operating temperature, and their maintenance during standby periods (in operation the temperature is maintained by the Joule effect), and which reduce performance. The properties of NaS batteries make them suitable for power quality applications, emergency power supplies and for stabilizing energy from intermittent renewable sources such as wind or photovoltaic installations.

The largest existing installation with NaS technology is located in Aomori, Japan and with an output and capacity of 34 MW/245 MWh, it supports a wind farm.

Nickel-cadmium batteries, NiCd, have been developed since 1950, making them a highly established technology and converting them, along with batteries. They are mainly composed of two nickel-cadmium electrodes, a separator and an alkaline electrolyte, usually potassium hydroxide, KOH. The positive electrode consists of nickel hydroxide, Ni(OH)2, while the negative electrode is cadmium hydroxide, Cd(OH)2, both are spiral wound and isolated by a porous separating membrane.

Nickel-cadmium batteries are a good alternative to lead-acid batteries, acid batteries. They are more robust, and have a longer life in stationary applications, around 2000-3500 cycles, although it must be taken into account that the life cycle is very variable as it depends directly on the depth of discharge, DoD.

Among the main advantages of Ni-Cd batteries, we can highlight their good power density and high power density and high energy density, 50-75 Wh/kg; as well as high reliability and very low maintenance requirements. In addition, they can inject their rated power into the mains for 2 hours.

Other advantages of Ni-Cd batteries, with respect to Pb-acid batteries, are that they have a longer useful life, are more robust, and are not affected by temperature variations; all this allows them to be used in multiple high-power storage applications.

The main drawback of Ni-Cd batteries is their relatively high cost, around 800 €/kWh, which is due to their expensive manufacturing process and the need for recycling, which has led to their limited success in the market. The obligation to recycle at least 75% of their components, imposed by the European Union since 2003, is motivated by the presence of heavy metals in these batteries, nickel and cadmium, which are highly toxic to humans and the environment.

An example of Ni-Cd battery storage, to cover short periods of time due to lack of power supply from generators
of electricity supply from generators, is the plant installed in the city of Fairbanks, Alaska, in Fairbanks, Alaska, which is capable of providing 27 MW of nominal power for 15 minutes] and 40 MW for 7 minutes. This facility is considered an “electrical island”, being the largest Ni-Cd battery power plant in the world.

The first lithium batteries date back to the 1960s, but they were based on lithium metal and were not rechargeable. From the 1980s, with the incorporation of the graphite anode by Bell Labs, the development of lithium-ion batteries began, but it was not until 1990 that the first lithium-ion batteries, produced by Sony, could be marketed. Since then, the field of research has made possible their development and evolution, achieving great improvements.

Lithium-ion batteries consist of two electrodes, where the anode is made up of graphitic carbon layers and the cathode is a metal oxide such as LiCoO2 or LiMnO2. The electrodes are immersed in an electrolyte generally consisting of a liquid organic substance, usually organic carbonates, in which various lithium salts such as LiClO4 or LiPF6 are dissolved. At the same time, these electrodes are insulated and separated by a porous membrane of polypropylene or polyethylene, which allows the polyethylene that allows the lithium ions to circulate.

During the charging periods, the lithium ions, Li+, flow through the electrolyte from the positive electrode of LiCoO2 to the negative electrode of graphitic carbon; while during the discharge cycles of the battery, the same process is carried out in reverse.

Li-ion batteries are characterized by their high energy density, 170-300 kWh/m3, high specific energy, 75-200 Wh/kg and very high lifetime with up to 10,000 cycles. Their charging and discharging capacity is very fast, reaching 90% of the nominal power in approximately 200 ms. These properties make them an optimal technology to be applied in installations where weight and response time are crucial.

The main disadvantages of lithium-ion batteries are that their life cycle depends directly on the depth of discharge, which means that they are not the most suitable type of battery for applications where total discharge is necessary. On the other hand, and due to their high fragility, they require continuous management and control of both the operating voltage and temperature, as well as their protection circuits. There are also safety and environmental concerns due to the use of flammable organic electrolytes in their constitution.

However, the main disadvantage is still the high cost. Despite intensive research work to reduce manufacturing costs, the cost is still higher than 500 €/kWh, mostly due to the price of the packaging and the overcharge protection of the internal circuits.

There is a wide range of applications for Li-ion batteries, from small low-power applications for mobile devices to medium-power devices such as electric vehicles, and even for high-power installations such as stationary energy storage. In this sense, there is currently an enormous interest in the development and improvement of lithium-ion batteries, and many companies are devoting time and resources to their research, in order to develop innovative materials that improve their performance.

The company Saft is a clear example of the advances in next-generation batteries and intends to install, during 2015, a BESS energy storage with lithium-ion batteries on the island of Kauai, in the archipelago of Hawaii, USA. The system will consist of 8 containers, each containing 4,060 batteries packaged in 290 modules. The eight units together will provide 6 MW and 4.6 MWh. This facility will provide power to stabilize the electrical grid, regulate distribution grid voltage, serve as a backup, provide frequency support during loss of generation, and mitigate intermittent fluctuations that can occur with renewable energy sources.