Electric vehicle
Electric Vehicle Technology Explained, Second Edition6.4
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| 124 Electric Vehicle Technology Explained, Second Edition 6.4 Energy Efficiency of Reforming When we wish to steer away from fossil fuels the direct electrolysis of water from nuclear or electricity from alternative energy sources becomes more appropriate. Electrical energy generated from sources such as nuclear or solar energy will allow production of hydrogen without the release of carbon dioxide and hence the fuel does not give rise to carbon release and associated global warming at all. Electrolysis is essentially a simple process. An electrical power source is connected to two electrodes typically made from some inert metal such as platinum or stainless steel, which are placed in the water. Ina properly designed cell, hydrogen will appear at the cathode (the negatively charged electrode, where electrons are pumped into the water) and oxygen will appear at the anode (the positively charged electrode). The ideal efficiency is known as the faradaic efficiency. The actual energy efficiency of water electrolysis varies widely, typically between 50 and 80%. 6.5 Hydrogen Storage I – Storage as Hydrogen 6.5.1 Introduction to the Problem The difficulties arise because although hydrogen has one of the highest specific energies (energy per kilogram) – which is why it is the fuel of choice for space missions – its density is very low, and it has one of the lowest energy densities (energy per cubic metre. This means that to get a large mass of hydrogen into a small space very high pressures have to be used. A further problem is that, unlike other gaseous energy carriers, it is very difficult to liquefy. It cannot be simply compressed, in the way that LPG or butane can. It has to be cooled down to about 22 K, and even in liquid form its density is really very low, 71 kg m −3 Although hydrogen can be stored as a compressed gas or a liquid, there are other methods that are being developed. Chemical methods can also be used. These are considered in the next section. The methods of storing hydrogen that will be described in this section are compression in gas cylinders storage as a cryogenic liquid storage in a metal absorber – as a reversible metal hydride; • storage in carbon nanofibres. None of these methods are without considerable problems, and in each situation their advantages and disadvantages will play out differently. However, before considering them in detail we must address the vitally important issue of safety in connection with storing and using hydrogen. 6.5.2 Safety Hydrogen is a unique gaseous element, possessing the lowest molecular weight of any gas. It has the highest thermal conductivity, velocity of sound, mean molecular velocity and the lowest viscosity and density of all gases. Such properties lead hydrogen to have a leak rate through small orifices faster than all other gases. Hydrogen leaks 2.8 times |