A vehicle can be propelled by hydrogen fuel in a number of ways. Hydrogen can be used as a rocket fuel although this is more applicable for the space industry rather than automotive.
Perhaps the most obvious automotive use is to combust hydrogen in a spark ignition engine. Relatively few modifications are required to the actual engine and in many cases, the control system allows switching between burning gasoline and burning hydrogen. A dual-fuel system such as this decreases any anxiety regarding the ability to refuel with hydrogen until there is a hydrogen infrastructure in place. In addition to an either/or state, it is also possible to co-burn hydrogen and gasoline with the benefit of lowering harmful emissions, fuel consumption and tailpipe CO2.
A fuel cell is an electrochemical conversion device which is designed to produce electricity from a fuel source such as hydrogen, hydrocarbon fuels or alcohols. The first rudimentary fuel cell was developed in 1839 however, their commercial usage didn’t take off until the NASA space programmes from the late 1950s onwards.
In a similar way to batteries, fuel ‘cells’ need to be electrically strung together to increase the voltage of the whole device. Although different cell chemistries deliver slightly different voltages, let’s say a lithium ion cell has a typical voltage of 3.2 V so a number of them need to be electrically connected to make a battery ‘pack’. Similarly, an individual fuel cell delivers around 1.4 V so a number of them need to be connected together and this then is referred to as a fuel cell ‘stack’ although commonly the whole device is known as a fuel cell.
There are many types of fuel cells using a number of different fuels but the proton exchange membrane (PEM) fuel cell using gaseous hydrogen is regarded as the best long term solution for fuel cell technology. The chemical reaction of hydrogen and oxygen in the fuel cell generates electrical power and water. The gaseous hydrogen can be supplied from hydrogen storage tanks or from an on-board reformer that produces the hydrogen from other fuels such as methanol.
The electricity from the fuel cell is used to drive an electric powertrain in the vehicle, however, the fuel cell power must be supplemented by some other form of electrical energy as a fuel cell takes some time to ‘warm up’. In addition to this, a fuel cell is good at operating continuously but not so good operating in a transient manner. The transient requirement for electrical energy (for vehicle acceleration) can be supplied by a battery pack with the fuel cell used very effectively as a range extender.
The benefit of using a fuel cell compared with a combustion engine is that at best, the thermal efficiency of an engine is around forty percent whereas a fuel cell is better at up to sixty percent and is continually improving.
There are a number of technologies applicable to on-vehicle storage. The reason for the development of these technologies is that hydrogen exhibits poor energy density per unit volume compared with fuels such as gasoline or diesel (liquid hydrogen has 4 times less energy density than gasoline). This means that a large onboard tank would be needed to store enough hydrogen for a decent driving range. It does however show very good energy density by weight, nearly three times the energy density of gasoline – making it attractive to be used as a clean fuel.
The main technologies for automotive hydrogen storage are compressed or cryogenic, or a mixture of the two however, there is considerable research being conducted into chemical storage.
Two compression levels have emerged in the automotive industry as standard: 350 bar and 700 bar and tanks are available as off-the-shelf production items. The hydrogen remains in a gaseous state but the tank sizes are manageable to package into the vehicle and there will be enough hydrogen for an acceptable driving range. An example of a 350 bar storage system can be seen on the Fuel Cell London Taxi.
An exercise was conducted to identify a suitable tank and a number of options were considered, including 700 bar solutions and multiple tank arrangements. Factors such as cost, size and availability were all evaluated and the solution chosen was to package a 350 bar tank in the engine bay of the vehicle.
The construction of the tank is a seamless aluminium inner tank 10 mm thick, which is overwrapped with carbon fibre filament wound around the tank to a thickness of 25 mm. The construction serves a number of purposes: firstly, to minimise seepage or permeation of the gaseous hydrogen, the molecules are small enough to allow this, however, it can be minimised with specialised tank construction; Secondly, the structure allows the tank to pass certification, including abuse tests such as hydrostatic burst and drop test.
Currently, cryogenic storage systems only exist as demonstration units in a relatively small number of vehicles although industrially, this method of hydrogen storage has been around some time and is well understood.
As the boiling point of hydrogen is so very low, to maintain hydrogen as a liquid, a temperature no greater than -253 °C needs to be provided. Thus, a vehicle using such a system would be refuelled by liquid hydrogen. The tanks that exist at the moment use vacuum as the main insulating medium. Calculations can show that the energy per unit volume of liquid hydrogen is much greater than compressed (even at 700 bar).
However, the downside is that if the vehicle is not used, the tank and contents will start to warm inducing the hydrogen to boil off.
As a purely cryogenic tank is not generally pressurised to a high level, the system would need to vent off the hydrogen to the atmosphere. This may create problems if the vehicle is in a confined space and what it means to the user is that over a period of a few weeks, the hydrogen would be lost.
Research studies have produced prototype tanks that combine cryogenics and compression giving the benefit of storing more hydrogen on board than either of the single systems. These combined systems are far from production but they do help to achieve long term targets for on-board hydrogen storage. A study conducted as part of the US Department Of Energy Hydrogen Programme concludes that a cryo-compressed system has approximately twice the volumetric efficiency of 350 bar systems and has a 40 percent higher volumetric efficiency than 700 bar systems. However these advantages come at the cost of increased off-board energy consumption due to liquefaction energy requirements.
There is significant research being conducted in using various hydrides to store and release hydrogen. Hydrides are compounds of metals and hydrogen that can be ‘charged’ by pressurised gaseous hydrogen and can release the stored hydrogen by heating the hydride. Some methods are not appropriate for on-board storage but are being developed for industrial storage. Hydrides can be liquids, slurries or solids.
Aside from the gravimetric and volumetric characteristics laid out in the chart there are other characteristics of hydrides that need to be considered such as cycle life, refuelling time, kinetics and thermodynamics. Cycle life and refuelling time are self-explanatory but kinetics refers to the rate of hydrogen transfer and thermodynamics refers to operating temperature and the temperature requirements for hydrogen transfer.
All technology roadmaps are looking towards a hydrogen economy and the technology development to make this happen. Fuel cell technology is becoming commercialised to the point that OEMs are already manufacturing fuel cell electric vehicles in large fleet demonstration numbers and from around 2015 onwards, there will be products available for consumers to buy. As with all new technologies, the cost will still be a premium for some time to come and to a certain extent the acceptability of fuel cell electric vehicles will depend on the development of a hydrogen infrastructure. The use of hydrogen in combustion engines could turn out to be an interesting and acceptable stepping stone to a future full hydrogen economy..
Writer: Phil Barker, Chief Engineer H&EV, Lotus Engineering