Wind power and energy storage technologies State of the art

138 Wind power and energy storage technologies State of the art Wind power and energy storage technologies State of the art Raúl Sarrias 1, Luis M. Fernández 1, Carlos A. García 1, and Francisco Jurado 2 1 Department of Electrical Engineering, University of Cadiz, EPS Algeciras, Avda. Ramón Puyol, s/n. 11202 Algeciras (Cádiz), Spain e-mail: raul.sarriasmena@alum.uca.es, luis.fernandez@uca.es, carlosandres.garcia@uca.es 2 Department of Electrical Engineering, University of Jaen, EPS Linares, C/ Alfonso X, nº 28. 23700 Linares (Jaén), Spain e-mail: fjurado@ujaen.es Abstract. Renewable resources rise as the principal alternative to traditional fossil fuels based energy generation methods, and among them, wind power stands out. Natural resources supply a cleaner energy, although it is necessary to solve the problems they cause on the electricity grid. The inherent variability and unpredictability of wind affect the stability of the power system. Energy storage systems appear to overcome some of these difficulties, since they allow the decoupling between production and demand. Many investigations are being carried out in the field of storage systems. This work will do a review of the most important devices, describing their main characteristics and comparing their ability to operate under different conditions. Key words: Energy storage, wind power, battery, supercapacitor, flywheel, superconducting magnetic energy storage, hydrogen energy. 1. Introduction. In an attempt to find less environmentally hazardous resources, renewable energies are supported by authorities and organisations in most of the industrialised countries. Energy storage appears to overcome some of the problems presented by these means of generation. The possibility of keeping energy stored, and releasing it as electricity subsequently, solves the drawbacks of wind power generation, such as random variability, or low accuracy wind forecasts. In this study, several technologies are analysed. Electrochemical batteries, redox flow batteries, sodium sulphur (NaS) batteries, flywheel, supercapacitor, superconducting magnetic energy storage systems and the hydrogen technology are described and compared in order to state whether one or another is preferable under certain working requirements, indicating the conclusions drawn from the research in the last section. 2. Storage technologies. The most remarkable energy storage technologies currently in use are analysed in this section. 2.1. Electrochemical Batteries. Attending to their physical structure, electrochemical accumulators are comprised of an arrangement of cells, which are formed basically by electrodes, electrolyte and electrical insulator. Energy is stored in a chemical form when supplying energy to the battery. When necessary, this energy is released as

Wind power and energy storage technologies State of the art 139 electricity by connecting a load to the electrodes. Therefore, reversible chemical reactions take place, charging and discharging the battery respectively, by electrons transference. Among the vast variety of electrochemical batteries, lead-acid and lithium-ion stand out. Lead dioxide and sponge lead are used as positive and negative electrode respectively in lead-acid batteries. Both electrodes are immersed in a sulphuric acid solution which works as electrolyte, while a micro-porous material provides electrical insulation [1]. With large appliance in the field of portable devices, the Li-ion type is currently the most remarkable among the lithium-based batteries. Li-ion batteries consist of a positive electrode made up of a lithium-metal oxide and graphitic carbon as negative electrode, using lithium salts for the electrolyte. 2.2. Redox Flow Batteries. Its working technique differs from that of the electrochemical batteries. Redox flow batteries are also known as simply flow batteries. The system consists of two tanks where the electrolytes are stored separately. When necessary, both electrolytes are pumped to a cell, where the electrodes and an ionselective membrane allow the ion exchange between the electrolytes. One of the most outstanding features of this technology is the ability to decouple the energy and power capacities, since energy supply depends on the volume of the reservoir tanks, whereas the power rates are limited by the ions transfer speed through the selective membrane. 2.3. Sodium Sulphur Batteries (NaS). Sodium sulphur batteries present a solid beta alumina ceramic electrolyte, which is surrounded by molten sulphur in its external surface to form the positive electrode. In the internal surface, the electrolyte contains molten sodium as the negative electrode. A high temperature, from 300 to 350ºC, is needed in order to maintain sulphur and sodium in liquid state. Therefore, to start the reaction heat must be provided; however, once it is running, the heat produced by continuous charging and discharging cycles is enough to maintain temperature in the required range [2], thus avoiding the need for an external heat supply. 2.4. Flywheel. The flywheel system stores energy in the kinetic form in a rotating mass (Fig. 1). The quantity of stored energy is a function of the spinning velocity of the rotor and its moment of inertia. Attending to the former, these devices can be classified into low or high speed flywheels. Low-speed rotors, made mainly of steel and metal alloys, are much heavier than the high-speed ones, where composite materials are used. In order to reduce internal losses and self-discharge rates, the rotating mass is kept in a vacuum enclosure, what reduces air friction. With the same aim, magnetic bearings are utilised, hence avoiding shaft friction and the subsequent need for cooling the system. Fig. 1 Flywheel Scheme [2]

140 Wind power and energy storage technologies State of the art A reversible motor/generator couples the rotating system and the electricity grid. When storing energy, the reversible device works as a motor to accelerate the rotor. On the other hand, it generates electricity decelerating the rotor when discharging. As seen in Fig. 1, the rotor and the motor/generator are coupled to the same shaft. 2.5. Supercapacitor. A supercapacitor stores energy in the electric field created between two electrodes, which are separated by a thin liquid electrolyte layer of only a few Å. Hence, supercapacitors follow the same principles as common capacitors. Nevertheless, specific electrode structures and materials allow high rates of capacitance per unit volume. They present a longer lifespan than batteries when operating under frequent charge/discharge cycles, due to the fact that no chemical reactions are needed to store or release energy. Among the different types, it is the Electrochemical Double Layer Capacitor (EDLC) that stands out. 2.6. Superconducting Magnetic Energy Storage (SMES). Huge magnetic fields store energy that can be released as electricity using the SMES system. Complex processes and equipment are needed to create and maintain magnetic fields that can range up to several Tesla. The device is based on the principles of electromagnetism, using a direct current flow through a superconducting coil to generate magnetic fields. A cooling system is needed for the superconducting material to be kept at about -270ºC. Liquid helium in a vacuum enclosure is used to reach this exigency. In this operation, the cooling system consumes considerable amounts of energy, what increases the running costs of the whole system, thus reducing its economic competitiveness against other technologies. 2.7. Hydrogen Technology. Three main stages can be distinguished. These are hydrogen production, hydrogen storage and electricity generation. These stages require specific equipment and can be developed in different locations. An electrolyzer splits hydrogen and oxygen from water when supplying electric energy. As a consequence, oxygen is released to the atmosphere and hydrogen is stored, which is the second stage of the process. Currently, four storage technologies are available [2]: Hydrogen pressurisation and adsorption in metal hydrides are more developed, while liquefaction and adsorption on carbon nanofibres are still under research. In a final stage, stored hydrogen reacts with oxygen in a fuel cell, generating electricity via inverse electrolysis. As a result, water is wasted without hazardous effects on the environment. Fuel cells consist of a two electrodes-electrolyte structure. Depending on the materials utilised, different types exist. The hydrogen technology is expected to be largely used in massive wind power generation. In Fig. 2, a combined wind-hydrogen scheme is shown. Fig. 2 Combined Wind-Hydrogen Scheme [3]

Wind power and energy storage technologies State of the art 141 As seen in Fig. 2, a combined wind-hydrogen system can operate either connected to grid or isolated. Moreover, hydrogen can be stored and consumed in a fuel cell, completing the whole energetic cycle in the same wind farm; or otherwise, transported to a different location and used in an external system. 3. Working requirements. In order to choose the most suitable storage system for a certain application, it is necessary to clarify the requirements needed for the devices to have an appropriate performance. As a consequence, the following classification settles the basic working features for large-scale energy storage devices: Power Quality: By this application, a proper quality in the electricity supplied to the consumers is ensured. Stored energy is released to the grid for only a few seconds or less. Due to the fast response, short-term fluctuations produced in wind power generation can be easily reduced when using storage for this application. Bridging Power: The stored energy is used as an emergency buffer able to release energy ranging from seconds to a few minutes when switching between different energy sources. Energy Management: The basic idea is to store energy when generation exceeds consumption, being available subsequently when required. Energy is released for a longer period of time, typically ranging from several minutes to hours. This technique allows decoupling the stages of energy generation and consumption, which is also known as load levelling. Therefore, a profit can be made by storing energy during off-peak hours, when the cost of electricity is lower, and then releasing during peak hours, obtaining economic surpluses in the process. This alternative will solve some of the complications caused by the long-term variability of wind, being especially useful for isolated wind generation, and enhancing the advantages of wind power connected to network as well. In Table I [4] is shown the importance of a few parameters, in order for the devices to be used in a power or energy application. POWER ENERGY Large storage capacity - + Large power capacity - + Power gradient ++ - Calendar lifetime + + Access time ++ - Low self-discharging - + ++ very important + important - less important Table I. Energy and Power Requirements [4] As seen, for energy usages it is essential for the device to have a large storage capacity. As well, low self-discharging rates and long lifetime are desirable. On the other hand, a short access time and a high power gradient are crucial if the device is expected to have a proper performance under power quality requirements. 4. Comparison of the technologies. It is necessary to compare the performance of the storage systems. Attending to their behaviour, one device or another will be more suitable for each use. In this section the main characteristics of the systems will be analysed in detail, in order to state the application that better adapts to their possibilities.

142 Wind power and energy storage technologies State of the art In this work the devices are compared in terms of the key factors that make the difference between a power or energy application. For the chosen storage systems, the value of these parameters is shown in Table II. Supercapacitor Storage Capacity [kwh] Available Power [kw] Discharge Time Lifespan [years] Access Time [ms] Selfdischarging Redox Flow NaS Flywheel Electrochemical SMES Fuel Cell 0.5-10 4 500-10 5 up to 2 10 5 1-30 0.1-0.6 10-50 several 10 3 0.5-10 3 10-3 10 4 10-8 10 3 1-10 4 0.9-200 10 3-2 10 5 5-10 4 sec - hours min - days sec - hours sec - min seconds sec - min min - days 5-15 15 15 20 15 20 10 20 1 5-15 2-5 0.5 0.5-5 20 2% - 5% per month None None 20% per hour Table II. Storage technologies characteristics 14% per month Cooling power 3% per day (tank) In the wide group of electrochemical batteries, only lead-acid and Li-ion batteries have been considered, since the rest have not shown an adequate behaviour in industrial usages yet. Lead-acid batteries are currently the most mature storage technology. They have been used with positive results in both energy and power applications. Their massive use is based on the low self-discharge rates they present, as well as their long experience. Nevertheless, their relatively short lifespan limits their use when continuous charge/discharge cycles are required. The Li-ion technology has proved to be an interesting alternative to lead-acid. Li-ion batteries are lighter and present a higher energy-volume rate than lead-acid ones. In addition, their superior efficiency, close to 100%, makes them the most popular choice for portable devices. A more extensive use of this technology is hindered by the remarkably high costs they deal with, reaching values up to 4,000$/kWh, in contrast to a maximum of 1,000$/kWh for lead-acid [5]. Moreover, these batteries may suffer serious damages under certain conditions i.e. deep discharge. This electrical fragility prevents their use as a large-scale storage system. Two new options appear with redox flow and sodium sulphur batteries, which are already in use as an alternative to the traditional large-scale storage systems. Both are still growing technologies and need to improve their performance to reach a higher commercial penetration. Sodium sulphur batteries are able to provide a power pulse in a short instant, or otherwise, an energy supply for a longer period with high efficiency. This feature, together with a short access time, makes them suitable for reducing wind power fluctuations. Their main shortcoming is the need for frequent heat supply to maintain electrolytes molten, thus increasing running costs. Flow batteries present notable energy and power capacity, furthermore, it is essential to highlight their ability to decouple these two features in the design stage. A long discharge time, the lack of self-discharge and total absence of deep discharge damaging, are their major advantages to operate as long duration storages. However, the need for pumping the electrolytes involves considerable running costs that hamper a broader development of this system. Hydrogen technology is currently under intensive research. It is considered to be an adequate energy supply for the near future, since fuel cells are able to provide electricity for a few days time, producing

Wind power and energy storage technologies State of the art 143 no polluting wastes. However, this technology is not recommended for power applications, since impurity in hydrogen may appear if operating under constant switching charge/discharge cycles. Among its main drawbacks, the low efficiency it presents is remarkable, approaching 40% for the entire system. Besides, high capital costs are required when installing the system. These shortcomings prevent from a wider expansion of this technology. Nonetheless, as stated in [6], fuel cells show acceptable performance for mid-power utilities in isolated areas. With regard to the power applications, there are different options which present better performance than the previously indicated devices. These are mainly the supercapacitors, the SMES system and the flywheels, since they are able to supply high power rates for a short period of a few minutes as maximum. These three devices are especially suitable to curtail the effects of wind power fluctuation, due to their fast response. Supercapacitors possess an exceptional power density up to 10,000 W/kg [2] and energy density, what compensates for their low storage capacity and power availability. They appear as a good option for peak power supply, when sudden power demands occur [2]. However, this technology is still under development and deals with excessively high costs, what sometimes hamper its broader utilisation. With regard to SMES systems, their fast response and notable overall efficiency are remarkable. This device is especially recommended when complete charge/discharge cycles are required on a continuous basis [6], due to the fact that its lifespan is not affected by frequent and deep discharges. In addition, their ability to operate for a few minutes time, allows small-scale energy supply, as well as power quality control with intermittent sources, such us wind power generation. However, cryogenic temperatures are needed in the system. This requires a considerable energy consumption that affects negatively, due to the operational costs it involves. Moreover, the huge magnetic fields produced are sometimes unstable, and demand special infrastructures i.e. underground location. These drawbacks make the SMES be less used than other more developed technologies. Flywheels have shown proper performance in a number of industrial applications, being currently the leading technology in the field of power quality control and power back up for renewable generation. Their principal advantages remain in a short access time, remarkable power rating and long lifetime. Even though capital costs for high-power flywheels are lower than for supercapacitors or SMES systems, a reduction is still needed for them to be able to compete with other more mature technologies. 5. Conclusions. It has been deduced from the analysis that energy storage is already physically feasible and economically affordable in the majority of the cases [6]. Moreover, it is an alternative that will grow in the future as the technology develops. As concluded in many of the references utilised, lead-acid batteries are the chosen device in most of the applications, either under power or energy requirements. This preponderance is based on the large experience acquired with this technology, which allows reduced investment costs as well. For energy management applications, an attempt to substitute lead-acid with other devices is being done. In this respect, the hydrogen technology, and redox flow and sodium sulphur batteries are better positioned. Nevertheless, further R&D work must be done for all these options. NaS and flow batteries need to reduce running costs by improving operation techniques. In the case of hydrogen, it is

144 Wind power and energy storage technologies State of the art necessary to improve in efficiency. Its weak development leads to an expensive system as well. Therefore, a costs reduction is essential for this technology to become commercially affordable. In the field of power applications, flywheels, SMES and supercapacitors will replace lead-acid as soon as they become more economical, since they are more reliable and long-durable technologies compared to the latter. Flywheels present larger experience, showing satisfactory results in industrial applications. On the other hand, supercapacitors perform extraordinarily with frequent and deep charge/discharge cycles. Both these technologies are currently more spread than SMES, which faces considerable difficulties with the use of large magnetic fields. The stability problems produced by magnetism may be solved by improving the power electronics used in these systems. In addition, more economically attractive options must be found for this technology to be utilised broadly. When the storage is designed to work in bridging power applications, a mixture of all the previously indicated characteristics would be the ideal situation. However, since none of the devices presents a high performance in all these aspects, it is necessary to couple two or more systems in order to compensate for each other deficiencies. Therefore, flow batteries and SMES or flywheels are able to operate together. Especially in wind farms, coupled systems optimise electricity generation, due to the number of storage possibilities and the ability to release energy during long periods or short instants. Besides, a more efficient use of wind resources reduces generation costs as well, hence becoming wind power a more economically competitive source. In conclusion, wind power generation penetration will increase as the different storage technologies develop. The costs of electricity production, based on renewable energies, are reduced by energy storage, since the use of these devices, either alone or coupled, optimises the use of the available resources. The systems here described have proved to be the most suitable alternatives for the working features considered. Nevertheless, strong investment in R&D is required. At the same time, further fieldwork is necessary to gain in experience and knowledge of the performance of the storage systems presented in this study under real operating conditions. References. [1] K. C. Divya and J. Østergaard, Battery energy storage technology for power systems An overview, Electric Power System Research 79 (2009), pp. 511-520. [2] I. Hadjipaschalis, A. Poullikkas and V. Efthimiou, Overview of current and future energy storage technologies for electric power applications, Renewable and Sustainable Energy Reviews 13 (2009), pp. 1513-1522. [3] S. A. Sherif, F. Barbir and T. N. Veziroglu, Wind energy and the hydrogen economy - Review of the technology, Solar Energy 78 (2005), pp. 647-660. [4] E. Spahić, G. Balzer, B. Hellmich and W. Münch, Wind energy storages - Possibilities, PowerTech 2007, pp. 615-620. [5] http://www.electricitystorage.org [6] H. Ibrahim, A. Ilinca and J. Perron, Energy storage systems - Characteristics and comparisons, Renewable and Sustainable Energy Reviews 12 (2008), pp. 1221-1250.