The storage of wind energy The greenhouse effect was long seen as a theory of scientists, rather than a substantiated truth. In 2005 Al Gore changed the public opinion with his movie 'an inconvenient truth'i. A contributing factor to the emission of greenhouse gases in the last one hundred and fifty years have been the unremitting use of fossil fuels. Gore convinced governments to take the first step in the search for cleaner, alternative energy sources. The search for these energy sources has led to a lot of new insights and ideas about which energy source should be used after the era of fossil fuels. According to researchers, the most feasible alternative energy source is wind energy. Wind energy is a well-known and reliable energy source already in use. In the Netherlands, wind energy is obtained from windmills, which are put together in wind farms. The Dutch government intends to place a new large wind farm in the north sea. The electricity grid of this new wind farm has to be attached to the national grid. However, wind farms are faced with the unpredictability of wind. During the night, when the electricity consumption is low, an overload of the national grid can occur, because of the hard blowing wind. During the day, when the electricity consumption is high, the energy demand cannot be met, because the wind may not blow hard enough. Therefore, the wind obtained from the wind farm should be stored to meet the demands of the national grid at any time. The storage of wind energy can be enabled by three different flexibility strategies. These flexibility strategies are hydrogen, batteries and pumped-storage hydroelectricity. Hydrogen Hydrogen can be produced from water with hydrolysis. To produce hydrogen energy obtained from the wind farm can be used. The generated hydrogen can then be transformed into energy with a fuel cell or with a modern gas turbine. The use of a gas turbine is considered to be the most efficient method to convert hydrogen into energy. A gas turbine consists of three main components: a compressor, a combustion chamber and a turbine. In the internal combustion engine, chemical energy is added to the gas stream, where hydrogen is mixed with air and inflamed. Thereafter, the temperature and pressure increase, which forces the combusted products into the turbine. The turbine consists of a rotor, which starts spinning due to the constant movement of the gas that goes through the turbine. The spinning rotor generates mechanical energy. Mechanical energy can be put into use for many vehicles. The use of hydrogen in gas turbines is relatively new. Therefore, researchers attempt to improve this process. At this point, the highly flammable character of hydrogen combined with the temperature, which occurs in the gas turbine, constitutes a major problem.ii Hydrogen has to be stored during times of low energy demand and has to be transported to equip energy where needed. For the storage of hydrogen different aspects should be taken into account: technology, efficiency and safety. Hydrogen is not carcinogenic, toxic, radioactive and polluting. The threats of hydrogen are the odourless and colourless features of the substance. Therefore, detectors are required in all places where hydrogen is produced, distributed and stored. Hydrogen is fourteen times lighter than air, which increases the chance to evaporate. For this reason, hydrogen should be stored properly in confined spaces. When hydrogen leaks, the risk of explosion decreases, because the hydrogen quickly mixes with the air. The gas rises approximately twenty meters per second, which is six times faster than air. Hydrogen will explode when it occupies a volume of eighteen to fifty-nine per cent.iii Two different possibilities of hydrogen transportation are transportation through pipelines and via tankers. The transportation of hydrogen through a system of pipelines cannot be seen as a short-term resolution. A new system of pipelines should then be constructed underground, which requires a whole new infrastructure. Moreover, transport via pipelines requires a large amount of hydrogen to be profitable. The main advantage is that the pipelines hardly need maintenance. Transporting hydrogen by tankers can be seen as an short-term resolution. A forty-ton trailer can carry approximately three hundred and fifty kilograms of hydrogen. Furthermore, if the transported hydrogen is liquefied, the same truck can carry approximately three thousand and five hundred kilograms of hydrogen. However, transportation of the same volume of fossil fuel produces more than twenty times the amount of energy compared to hydrogen. Moreover, accidents with tankers can occur, which may cause extremely dangerous situations. Batteries Batteries consist of a series of cells. These series of cells include two electrodes, which are submerged in an electrically conductive agent, an electrolyte. These building blocks make it possible to store chemical energy in batteries. The use of batteries for energy storage on a large scale could be achieved with secondary batteries, because these are rechargeable. Furthermore, for the storage of electricity batteries should be capable to recharge quickly and release energy more than once. A great advantage of the use of batteries is that they can provide energy when needed. A sudden high demand of energy from the national grid can be processed. A disadvantage is that batteries discharge slowly, which make them only appropriate to store energy for a relatively short period of time. One additional disadvantage is that batteries have a decreasing trend of storage capacity with repeated use.iv v Three different batteries could, in theory, be used for the storage of wind energy. In Japan, one of these batteries, sodium sulphur batteries, is already used for the storage of wind energy. Sodium sulphur batteries are used in industries where it is important to maintain the continuity of the electricity accurately. Sodium sulphur batteries consist of cheap materials. The efficiency of these batteries is approximately ninety per cent. However, to initiate the electrochemical process in sodium sulphur batteries the temperature has to be maintained at three hundred degrees. Therefore, the efficiency of the whole process varies from sixty seven to seventy nine per cent. Sodium sulphur batteries have a significant discharge capacity. They are capable of switching quickly between charging and discharging. A great advantage of sodium sulphur batteries is the flexibility of the batteries dealing with dynamic and unpredictable supplies of energy, such as windmills. The use of lithium-ion batteries could be another option for the storage of wind energy. Lithium-ion batteries already cover a large share of the batteries that are being used in the market for small, portable applications (e.g. in iPods). For the use of wind energy storage, lithium-ion batteries should be produced in a larger size. Advantages of these batteries are a high energy density and an efficiency of approximately a hundred per cent. In addition, lithium-ion batteries have a long battery life. The third option is the storage of wind energy in flow batteries. Flow batteries contain a new technology, which allows energy storage on a large scale. This technology enables the flow batteries to store their electro active material mainly outside the active part of the batteries. This has a positive impact on the energy density and power density of the battery. Moreover, the electrodes of flow batteries do not undergo chemical or physical changes leading to a more sustainable and stable performance. Therefore, the battery life of flow batteries are higher compared with other batteries. In addition, advantages of flow batteries are the high efficiency and little maintenance is required. A major disadvantage is the complex system where these batteries consist of. For this reason, the production of these complex system will be expensive.vi Pumped-storage hydroelectricity Pumped-storage hydroelectricity consists of two connected reservoirs with a height difference. During a period of time when the demand for energy is low, the wind energy will be used to initiate a system that pumps water from the lower reservoir to the upper reservoir. During a period of time when the demand for energy is high, the sluice in the upper reservoir will be opened. Therefore, the water from the upper reservoir will flow to the lower reservoir. The flow of the water can be used to produce energy via a turbine, due to gravitational acceleration.vii Pumped-storage hydroelectricity is mainly applied in countries with natural significant height differences. For this reason, the traditional way of pumped-storage hydroelectricity cannot be implemented in the Netherlands. Two other methods can be used, a pump accumulation central at sea or an underground pump accumulation central. A pump accumulation central at sea consists of a reservoir, which is separated from the sea with a dam. When energy is needed, the sluices will be opened to let the sea water flow into the reservoir. When the energy demand is low, the water in the reservoir will be pumped back into the sea. This method has an efficiency varying from seventy five to eighty per cent. An underground pump accumulation central is similar to a pump accumulation central, except a second reservoir is needed underground. A major disadvantage of these methods is the possible defects a reservoir could have. A leak in one of the reservoirs could occur, which may result into dangerous circumstances for the surrounding towns and cities. Furthermore, building a pump accumulation central at sea or an underground pump accumulation central requires a lot of space and money. The possible defects, the needed space, especially for an underground pump accumulation central, and costs will make social acceptance difficult.viii Conclusion The Dutch government wants to place a wind farm in the north sea to enhance the energy supply. The variation in energy demand enforces the government to store the produced wind energy using a flexibility strategy. Three possible flexibility strategies are hydrogen, batteries and pumped-storage hydroelectricity. Hydrogen can be converted into energy with a fuel cell or a gas turbine. Wind energy can be used to produce hydrogen from water via hydrolysis. However, researchers still have to improve gas turbines for full implementation. The transportation of hydrogen can be carried out through pipelines or via tankers. The use of pipelines would require a new infrastructure. The use of tankers would require a lot of tankers to transport the same amount of energy compared with fossil fuels. Rechargeable batteries could also be used for the storage of wind energy. In Japan, sodium sulphur batteries are already in use for this purpose. Lithium-ion batteries could be another option. The third option is flow batteries. These batteries consist of a high energy density, high power density and a longer battery life compared with other batteries. However, the production costs for flow batteries will be high. Pumped-storage hydroelectricity can be implemented in the Netherlands by two methods, a pump accumulation central at sea and an underground pump accumulation central. Building these centrals will cost a lot of money and require a lot of space. Moreover, possible defects in a reservoir could be dangerous. All possible flexibility strategies could be used for the storage of wind energy. However, some major disadvantages could have an impact on the efficiency, costs, safety and social acceptance of the flexibility strategies. The government should pursue the research into these flexibility strategies, before full implementation. References i Al Gore, Davis Guggenheim, Jan. 2006, An Inconvenient Truth, USA, Participant Productions ii H. Th. J. Reijers, A. de Groot, and P. Lako, ''Evaluatie van waterstof-gebaseerde concepten en systemen'', Apr. 2001 iii Lewis, Bernard, ''Combustion, Flames and Explosions of Gases'', 535, 1961 iv J. Andrews, N. Jelley, ''Energy science, Principles, Technologies, and Impacts'', 2007 v A. Sneijder, ''Daces 2050, database clean energy supply 2050, final report'', Sept. 2001 vi Electricity Storage Association, http://www.electricitystorage.org/, Dec. 2012 vii Utrecht centrum voor energieonderzoek, ''Opslag van elektriciteit: status en toekomst perspectief van Nederland'', Aug. 2006 viii KEMA, ''Energie-eiland: haalbaarheidsstudie fase 1'', Jul. 2007