Generation, of the energy carrier HYDROGEN In context ... - needs

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NyOrka Page 8 12/18/2008Electrolysis, the splitting of pure water with electricity will be described later in moredetail as this is the selected technological pathway in this work package of NEEDS.Electrolysis has been in use for decades, and it is currently subjected to furthertechnological evolution for higher efficiency, minimal environmental effects and use withfluctuating electric input. The modularity of the equipment and the possibility to installonsite production facilities near the market gives higher flexibility for location choicesand evades the need for heavy investment for hydrogen distribution.The wide range of methods for production, storage and use phase of hydrogen is shownin figure 4. The oval at the lower half of the figure indicates which technicalspecifications will be addressed in the report. While making hydrogen with electrolysisfrom water and energy all renewable energy forms that give rise to electricity and or highheat can be considered for the process.According to the International Energy Agency, the year 2000 48% of hydrogen wasderived from Natural gas, 18% from Coal and 30% from oil through gasification and 4%from electrolysis. The total quantity was about 500 billion Normal m3 annuallyThe application methods are as varied as are the options for transportation from thehydrogen source to the users. Figure 1gives an overview of some of the hydrogenproduction and application options that are currently in use. The following criteria willform a frame will be set as boarders for the study: Minimisation of environmental effects,boosting efficiency and minimisation of the cost of handling and distribution as well asbuilt in safety requirements. The scope here is though set around the methods that usewater as the source of hydrogen and renewable energy to run the process. 48% ofhydrogen was derived from Natural gas, 18% from Coal and 30% from oil throughgasification and 4% from electrolysis. The total quantity was about 500 billion Normalm3 annually2.2 ElectrolysisAs mentioned the third family of production methods is electrolysis or splitting of waterwith an electric current. Electricity is lead through water and the water molecules splitinto oxygen at the anode and hydrogen at the cathode. The oxygen can be collected andused for specific purposes such as industry or aquaculture. One example of electrolysisfrom a chemical process but is not included further whereas the hydrogen is not as pureand the amounts are limited hydrogen production: the Lurgi process that produceshydrogen as a side-product from the production of Chlorine and sodium compounds. Thepurity if hydrogen made from water is higher and this is essential for PEM Surelyhydrogen can be collected from chemical industry, yet it may not be enough in the longrun and industries that only produce hydrogen for the energy market will be put near tothe markets while chemical plants are usually kept away from human settlements.Therefore options that can make hydrogen on a large scale must be defined.Three types of industrial electrolysis units are used today. Two involve an aqueoussolution of potassium hydroxide (KOH), which is used because of its high
NyOrka Page 9 12/18/2008conductivity, and are referred to as alkaline electrolyzers. These units can be eitherunipolar or bipolar. The unipolar electrolyser resembles a tank and has electrodesconnected in parallel. A membrane is placed between the cathode and anode, whichseparate the hydrogen and oxygen as the gasses are produced, but allows the transferof ions. The bipolar design resembles a filter press. Electrolysis cells are connected inseries, and hydrogen is produced on one side of the cell, oxygen on the other. Again, amembrane separates the electrodes.The third type of electrolysis unit is a Solid Polymer Electrolyte (SPE) electrolyser.These systems are also referred to as PEM or Proton Exchange Membraneelectrolysers. In this unit the electrolyte is a solid ion conducting membrane asopposed to the aqueous solution in the alkaline electrolyser. The membrane allows theH+ ion to transfer from the anode side of the membrane to the cathode side, where itforms hydrogen. The SPE membrane also serves to separate the hydrogen and oxygengasses, as oxygen is produced at the anode on one side of the membrane and hydrogenis produced on the opposite side of the membrane.Electrolytic units are currently capable of producing the largest amounts of hydrogen, andtoday are in use worldwide mostly the lye based technology, the selected case for theNEEDS . The LCA analysis is based on The PEM electrolysis unit is the newest of thetechnologies and is the cleanest candidate for future hydrogen production. Because thesimilarities with PEM fuel cells and PEM electrolyser it is foreseen that this unit willdevelop into the same unit: A reversible fuel cell! It is used for electrolysis to producehydrogen AND for the electric production from hydrogen. Prototypes of these are2.3 Process description of electrolysisRegardless of the technology, the overall electrolysis reaction is the same:H 2O → ½ O 2+ H 2Figure 2 A schematic overview of the process flow within the three main types of electrolysis
Page 1 and 2: NyOrka Page 1 12/18/2008SIXTH FRAME
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