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65 ionising solvent such as water). The anode oxidises the hydrogen fuel using a catalyst, often a platinum powder, by removing an electron (oxidation) to leave a positively charged hydrogen ion. The hydrogen ions are drawn through the electrolyte after the reaction. At the same time, electrons are drawn from the anode to the cathode through an external circuit, producing direct current. At the cathode, hydrogen ions, electrons, and oxygen react to form water The advantages of fuel cells are that they are far more efficient than fossil fuels and have a higher energy density. They also tend to be modular units, making for easy installation, they are considered to be zero-emissions devices, and they produce very little vibration that could affect other parts of an unmanned system such as a camera or video unit. On the downside, however, they can prove relatively expensive and complex compared with battery or solar power alternatives, and require specialist design and assembly to make them integral to the unmanned system; they can need specialist maintenance procedures too. In addition, the risk of the system becoming contaminated means they need filters and cleaners to ensure that the materials used are of the best quality, which have to be accounted for in their cost. Hydrogen is not naturally abundant so it must be obtained through water electrolysis or by reforming hydrocarbons, defining it as an energy carrier rather than an energy source. There is also no distribution infrastructure yet for hydrogen for fuel cell use, and issues with hydrogen handling mean special safety precautions must be adopted when dealing with it. Methanol toxicity (where CH 3 is the fuel) and flammability mean that storage and transport will always be a key aspect of the overall support network. Fuel cell integration into the unmanned vehicle, however, is key. The issues that need to be addressed are those that designers wrestle with on a daily basis – where is the energy to be produced, how is it to be stored and delivered to the power unit, and how is the power system going to be contained within the vehicle? What is the impact to the vehicle in terms of weight or cost? Gaseous hydrogen storage The notion of gaseous hydrogen storage as a design feature to fuel UAVs recalls the designs of the first airships, from the Zeppelins of World War I to the Hindenburg and the British R101 in the 1920s and ’30s. Their time ended after a number of disasters, but recently there have been steps to mimic the concept, in such diverse applications as passenger carriage right down to the smallest of UAVs. The challenges with dealing with hydrogen, particularly in its gaseous form, are twofold – its flammability and the volume needed to provide meaningful fuel energy. In a properly sealed system, there should be no problems with fire risks, but the issue of gaseous volume is a tricky one. With the extra structure needed to encompass the fuel in this state, there is the possibility that the design compromises needed for flight cannot be achieved. This is owing to the balance needed between providing enough space for the gaseous fuel and keeping the weight of that structure to the absolute minimum so that any advantages in using this method are not marginalised. The advantage with storing gaseous hydrogen within the structural framework of the UAV, instead of in specific fuel tanks, is that the more traditional ‘silo-based’ design philosophies, such as aerodynamics, structures and mass coexist in a less distinct way. This allows an overall more efficient system to be designed that makes best use of the advantages of gaseous fuel cell technology. Where this concept has been Fuel cells | Focus The fuel cell-powered Hycopter stores its hydrogen in its frame structure rather than specific fuel tanks. The craft will be available in early 2016 (Courtesy of Horizon Energy Systems) Unmanned Systems Technology | Dec 2015/Jan 2016 Fuel cells are far more efficient than fossil fuels and have a higher energy density. They also tend to be modular units, making for easy installation