Thursday, March 15, 2012

Better Place: Hydrogen Versus Plug-In Electric Vehicles


  There are still a lot of questions about the future of transportation: whether it will be Electric Cars based on Energy storage solution - Batteries or Hydrogen can power the fuel cell and it will be the most efficient way of transportation in post carbon society. Better Place has its own answers for us today. 

Plug-in battery electric vehicles are far more energy efficient than either hydrogen fuel-cell or hydrogen internal combustion engine vehicles.

Hydrogen is often touted as “the next big thing” in transportation fuels, used either in a fuel-cell-powered electric car, or as fuel for vehicles with an internal combustion engine (“ICE vehicle”). This technical note examines the relative merits of using hydrogen to power our cars in either of these ways, compared with using electricity in battery electric vehicles, looking at the entire supply chain (“well-to-wheel”) for both energy sources. Whilst there can be no doubt that hydrogen cars themselves are clean – their direct emissions are mostly water vapour – it is critical for any comparison to examine the entire energy life cycle. This raises the question: are hydrogen cars the best way to use our limited energy resources and how do they compare with electric cars?

Hydrogen gas does not occur naturally on earth. To use hydrogen as a fuel, it first needs to be separated from other atoms with which it is bound up, and isolated it in its elemental form: H2 (hydrogen gas). There are two main ways to make hydrogen gas: from a fossil fuel, or from water by using electricity. Both methods involve a large inherent efficiency loss.
From fossil fuel: Hydrogen gas can be extracted from natural gas (methane) by mixing it with steam under very high temperature and pressure, leading to the production of H2 and carbon dioxide (CO2). Further processing separates the H2 for storage and distribution. Whilst natural gas is both plentiful and cheap, this method of hydrogen production produces vast amounts of CO2, both as a byproduct from the process itself, and also from the production of the electricity and heat required to drive it. As a result, a hydrogen-based transportation system delivers few environmental benefits if the H2 is formed in this way, and it will not be considered further here. It would make much more sense to put the methane directly into the car rather than turning it into H2 first, but even this is far less efficient than using the gas to produce electricity for a pure electric car.1
From water: Electrolysis – where an electric current is passed through water to produce H2 and O2 – is a more environmentally friendly method of hydrogen production. However, since this is the reverse of the combustion reaction, it uses a significant amount of energy to drive the process. The efficiency claims for hydrogen produced in this way are in the range of 50-80%,2 so a massive amount of energy is lost in order to produce the hydrogen from electricity.
Hydrogen is a dangerous and difficult substance to handle, and therefore costly to safely store and distribute. In principle it would be possible to produce hydrogen gas locally, at filling stations or even at homes, in the latter case even perhaps using solar power. However, the cost and safety considerations would be considerable, and so for the purpose of this document we will presume that the hydrogen is produced at central facilities optimised for economical operation and safety.
Distribution: Building a network of underground pipelines for distribution of hydrogen to service stations would be extremely expensive, and would likely also pose a grave and unacceptable safety risk given the explosive nature of the gas, and its tendency to leak through many materials. The only alternative to pipelines would be to distribute the fuel by truck, but because of the low volumetric energy density of compressed H2 and the heavy weight of the steel pressure tanks, it would take more than 20 tanker trucks to distribute the same amount of energy that can be distributed by a single petrol tanker. Hydrogen is easier to transport in large quantities if it’s liquefied, but this requires further large amounts of energy to cool it below -250°C under pressure.
Storage: Wherever it is produced, hydrogen gas must be compressed and liquefied for storage in a vehicle’s specially designed high-strength fuel tank. Once there, it must be used quite quickly, as it otherwise boils off over time.
Safety: There are many issues surrounding the storage and transport of hydrogen in a vehicle. With a gravimetric density 14 times lower than air, H2 has to be compressed to extremes to provide a driver with reasonable range. There is only one hydrogen-fuelled car that has made it past the concept stage: Honda’s FCX Clarity. The pressure inside its tank when fully fuelled is 5000 psi,3 which is 350 times atmospheric pressure. This pressure requires a tank with very thick walls to contain it, which in turn adds considerable weight and bulk to the vehicle (and further reduces its efficiency). The Clarity needs a 173 litre tank (compared to 50 litres in a similarly-sized ICE vehicle) to contain 4.1 kg of H2 that delivers a range of 300 km.
Considering all the inefficiencies of generating, transporting and distributing hydrogen, and comparing them with generating and distributing electricity, how do the “well-to-wheel” efficiencies compare? Ulf Bossel, director of the European Fuel Cell Forum, has published just such a comparison.4 He found that “the power-plant-to-wheel efficiency of a fuel cell vehicle operated on compressed gaseous hydrogen [produced by electrolysis] will be in the vicinity of 22%”, and that “using liquefied hydrogen does not improve the situation… the power-plant-to wheel efficiency of a fuel cell vehicle operated on liquid hydrogen will be in the vicinity of 17%”. In comparison, he finds that electric cars are a much more attractive proposition: “with these numbers, the power-plant-to-wheel efficiency of an electric car with regenerative braking becomes 66%”. This means that a driver could travel three times as far in an electric car as they could in a hydrogen-powered car using the same amount of electricity. Hydrogen-fuelled ICE vehicles are even less efficient than hydrogen fuel cell vehicles,5 and thus provide even poorer overall efficiency again: around 14% and 11% for compressed and liquefied hydrogen respectively.

The distance driven by a vehicle is proportional to the mechanical energy available. Even for the most favourable comparison, being against a hydrogen fuel-cell car, the electric vehicle can drive three times further per kWh of electricity consumed. Compared with a H2-fuelled internal combustion vehicle, the electric car can drive around five times further (see graph below). The fundamental problem of using hydrogen as fuel is that the process uses electricity to produce H2, then more energy to compress and transport it, and more energy again to convert the H2 back into electricity that is finally used to drive the same electric motor that is found in a battery-powered electric car. That is in part why, when concluding his paper to the IEEE entitled “Does a hydrogen economy make sense?”, Bossel answered with one word: “Never.”6
hydrogen note chart

1. See Better Place Australia technical note: “Driving further on natural gas”.
2. Werner Zittel; Reinhold Wurster; "Chapter 3: Production of Hydrogen. Part 4: Production from electricity by means of electrolysis". HyWeb: Knowledge - Hydrogen in
the Energy Sector. Ludwig-Bölkow-Systemtechnik GmbH. .
4. Ulf Bossel, “E fficiency of hydrogen fuel cell, diesel-SOFC-hybrid and battery electric vehicles”, Discussion paper E04, European Fuel Cell Forum, 2003.
5. Henning Lohse-Busch, Thomas Wallner & Neeraj Shidore (2007), “E fficiency-Optimized Operating Strategy of a Supercharged Hydrogen-Powered Four-Cylinder Engine
for Hybrid Environments”, SAE International JSAE 20077209.
6. Ulf Bossel, “Does a Hydrogen Economy Make Sense?” Proceedings of the IEEE, Vol 94, No 10, pp 1826-1837

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