Which molecule is an energy store
Energy goes to school - energy storage
28 29 06 | ENERGY STORAGE 06 | ENERGY STORAGE 4 4 elements of the chemical energy industry Source: Energiespeicher, Sterner / Stadler (Ed.) CH2O CH2 CH4 H H2O O C6H2O CO2 + H2O CO2 + 2H2O CO2 C BASICS The three decisive elements of the chemical energy industry - and thus also chemical storage - are carbon (C), hydrogen (H) and oxygen (O). The figure shows the three elements and their compounds that are important for the energy industry: ↘ ↘ H2O and CO2 as combustion products and all mixtures of these that lie between these two compounds on the combustion product line. ↘ ↘ CH2O as a representative for biomass and its dried products peat, lignite, hard coal. The highest quality coal is C 6 H2O. It is called anthracite or shiny charcoal. ↘ ↘ Between CH4 and CH2 are gaseous and liquid hydrocarbons such as methane, liquefied gases, gasoline, diesel, and kerosene. ↘ ↘ The energy content and thus the storage potential of a substance increases the closer it is to the CH axis. The closer a substance is to oxygen, i. H. with increasing oxidation, the more the storage potential falls. The most important chemical and energy economic process today is still the combustion of hydrocarbons. In addition to other combustion processes in the transport sector, they increase the proportion of CO2 in the atmosphere. The reduction in global biomass stocks, for example, follows the same principle and also contributes to an increase. The only counterbalance to these processes is photosynthesis, which uses solar energy to convert and bind the relevant carbons back into biomass. However, their turnover is too low to meet the human demand for fossil fuels. An alternative in the chemical energy industry is electrolysis as the production of a chemical energy carrier that is independent of photosynthesis. Hydrogen Hydrogen is the lightest gas and comes first in the periodic table of the elements. The atom has a proton and an electron. Hydrogen is colorless and odorless and only rarely occurs in its pure form in nature. Because it is very reactive, it forms numerous chemical bonds. The earth's crust consists of approx. 50% hydrogen. It is part of water and almost all organic compounds. Bound hydrogen occurs in all living organisms and is therefore the most common element on earth. Hydrogen (H2) and oxygen (O2) result in a detonatable mixture. When it comes into contact with an open fire, the so-called oxyhydrogen reaction occurs. It is a type of combustion (oxidation). Their reaction product is water. Advantages as an energy source: ↘ ↘ No scarcity ↘ ↘ Production with renewable energies possible ↘ ↘ Clean combustion, since almost exclusively water is produced ↘ ↘ Highest gravimetric energy density of gases ↘ ↘ Hardly soluble in water ↘ ↘ High efficiency through "cold" combustion, conversion from hydrogen and oxygen to water in fuel cells possible ↘ ↘ Can be used flexibly in the fields of electricity, heat and transport Disadvantages as an energy source: ↘ ↘ Low volumetric energy density ↘ schwierig Difficult to burn in conventional gas power plants because of the high flame speed ↘ ↘ Since hydrogen is practically only available in bound form - comes and cannot simply be "mined" like other raw materials, its production is energy-intensive - e. B. in water electrolysis with efficiencies of approx. 54 to 84% ↘ ↘ Hydrogen mixes and reacts as the smallest atom very quickly with other elements and can lead to embrittlement of seals and metals in pipes and tanks ↘ ↘ High explosiveness because of a A very wide ignition range of 4 to 77% hydrogen is only used energetically today and to a very limited extent as a fuel in space travel. It is considered to have great potential as a substitute for fossil fuels. Water electrolysis has been known for 200 years and is not a fundamentally new technology. It is understood as the decomposition of water into hydrogen and oxygen with the help of an electric current. STORAGE TECHNOLOGY ELECTROLYSIS There are currently three processes available for water electrolysis, which are technically important, but differ greatly in terms of function, operating conditions and level of development. ↘ ↘ Alkaline electrolysis This has been a tried and tested process on an industrial scale for many years. The cell of an alkaline electrolyzer consists of two half-cells, which are separated by an ion-conducting membrane. Water circulates in them, the conductivity of which is increased by adding potassium hydroxide. Porous electrodes with a large surface are located close to the membrane on both sides. If a voltage is now applied to the electrodes, the water on the cathode side is split into H + and OH -. While the protons that are created react to form H2 molecules, rise and are separated by the electrolyte, the hydroxide ions diffuse through the membrane and react, releasing electrons to water and atomic oxygen. The resulting oxygen molecules are also separated out by the electrolyte. The used water has to be refilled again and again. ↘ ↘ Membrane electrolysis It consists of a proton-conducting membrane that is firmly connected to the electrodes on both sides. Membrane electrolysis is based on fuel cell technology and works the other way round. It is particularly suitable for dynamic operation, even under pressure, but has so far only been tested in smaller systems. ↘ ↘ High-temperature electrolysis Here, water vapor is split instead of liquid water. Therefore, the energy for the phase transition from liquid to gaseous is not to be generated within the electrolysis by electricity, but by external water evaporation z. B. to provide with waste heat from an industrial process. This reduces the electricity requirement by 16%. STORAGE TECHNOLOGY METHANIZATION Electrolytically generated hydrogen can be processed into gaseous or liquid hydrocarbons using CO2. This process is called methanation. In this reaction, carbon monoxide reacts with hydrogen at temperatures of 300 to 700 ° C to form methane and water. This reaction is exothermic, but must be accelerated by a catalyst. CO2 methanation was discovered in France as early as 1902. However, research into energy technology did not begin until much later, as for a long time, due to the cheap resources available, no need for the development of this technology was seen. It was only discussed for the storage of solar energy in the 1970s and 1980s. The first work was carried out on a laboratory scale in Japan in the 1990s. In Germany, CO2 methanation was viewed in the context of fuel cells in the early 2000s. The power-to-gas concept from 2011 finally helped CO2 methanation for energy storage achieve a breakthrough. Today it is an essential part of sustainable energy systems that couple the electricity and gas networks and integrate the heating market and mobility. The existing gas infrastructure is geared towards natural gas (methane). Pure hydrogen can only be integrated into the existing energy infrastructure to a limited extent (5%). When methanated, hydrogen can be stored, transported and used more easily. At the present time, methanation is still not used for the large-scale production of methane, as this can be obtained more cheaply from natural gas. Natural gas consists mainly of methane (CH4). Methane has the following advantages: ↘ ↘ It is relatively inert chemically to other hydrocarbons. ↘ ↘ It has good properties as a fuel. ↘ ↘ It has a high volumetric energy density (factor three compared to hydrogen). ↘ ↘ It can be converted into higher-chain hydrocarbons. ↘ ↘ There is an extensive infrastructure for transport and storage. Disadvantages: ↘ ↘ Additional conversion step and thus a loss of efficiency ↘ ↘ During combustion, CO2 is still released. STORAGE TANK FOR GASES The construction of the gas infrastructure in Germany began as early as the middle of the 19th century, initially as coal gas, later referred to as town gas. It was produced by coal gasification and represented a gas mixture of hydrogen, methane, nitrogen and carbon monoxide. The exact composition varied depending on the gasworks and production process, the type of gas scrubbing and also the coal used. After the transition to natural gas in the 1970s, this process is not yet complete even at the beginning of the 21st century and is gaining new relevance with the energy transition.
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