To store electrical energy in sufficient quantities onboard a vehicle calls for extremely efficient batteries (battery energy density and power).
While close to 500 kWh are stored in a 50-liter gasoline tank, the battery of an average electric vehicle contains 15 kWh, weighs 150 kg and requires over 7 hours for a normal charge. Taking into account the respective engine efficiencies of the thermal engine at 25 % and the electric engine at 90 % the electric/thermal energy ratio at the wheel is more favorable than that of stored energies. However it does not exceed 12 %.
Producing electric energy onboard using a fuel cell avoids storing it and enables a fast fuel charge-up. Present-day demonstration vehicles use a hydrogen fuel cell, the cell, reservoir and battery assembly of which weigh less than 200 kg for a delivered output of 80 kW.
William Grove established the principle for the fuel cell in 1839 but the technology of the day did not enable it to be produced. The NASA reintroduced this technology in 1962 for its space flights. In 1994, Mercedes presented its Necar 1 prototype fitted with a fuel cell produced by the Canadian firm, Ballard. By the end of the 2000s, demonstration vehicles had traveled several million kilometers.
Fuel cell principle
The fuel cell transforms chemical energy into electric energy. For it to function, it has to be constantly supplied with reactants - oxygen and fuel, generally hydrogen, chosen for its reactive capability enabling sufficiently high specific power to be obtained for application to the automobile.
The oxygen and hydrogen react on the electrodes when a catalyst is present, generally platinum, to produce the electric energy at a voltage of 0.7 V and ... water. Like combustion engines, fuel cells generally use the oxygen in the air for this combustion. The operating temperature is in the region of 80°C.
Illustration Shanghai 2004 Report: P.55 fig 3
The hydrogen directed towards the anode is split by oxidization into protons and electrons 2H2-->4H+ + 4 e-
The protons cross the membrane due to the electric field and the electrons move into the external electric circuit.
At the cathode, the protons, electrons and oxygen come together to form water
4 H+ + 4 e- + 02 --> 2 H2O. This reaction also produces heat, hence an output that is not as good as from a battery.
Current demonstrators use permeable polymer membrane cells. In a few years, the weight and space taken up by these devices, for a long while unacceptable, has been divided by ten. The entire assembly (cell, reservoirs and battery) now weighs less than 200 kg and delivers power of around 80 kW. The general space taken up therefore tends to be similar to the motor-tank assembly of an internal combustion vehicle. Moreover, many demonstrators are now standard vehicles that have been modified. In the future, specific architectures will nonetheless have to be designed, as for the Honda FCX Clarity. Daimler is expected to market Mercedes automobiles equipped with fuel cells by 2015.
The storage of hydrogen...
Hydrogen has a very high specific energy density (33 kWh/kg - treble that of gasoline) but its energy density is very low which is a disadvantage for storage onboard a vehicle. It is therefore stored in high-pressure tanks (350 or 700 bar) or in liquid form at very low temperature (-253°C), maintained thanks to a small cryogenic leak, with the evaporation consuming a large quantity of heat.
In the longer term, researchers continue to look at the storage of hydrogen in solid absorbent materials such as metal hydrides, complex hydrides, active carbon, microspheres or nanofibres, etc. Recent research on storing hydrogen in nano-engineered magnesium hydride disks doubles the storage density of a 700 bar tank. However, the limited speed of hydrogen retrieval for rapid acceleration gives the preference to an application for static storage as for example in service station recharging.
… and vehicle autonomy
The autonomy of a vehicle is naturally directly dependent on the storage capacity and output of the fuel cell. What sort of autonomy can a mid-range vehicle consuming 130 Wh/km expect to achieve given that the average output of the hydrogen cell is around 46 % and that of the electric motor 95 %?
The quantity of hydrogen should therefore be sufficient to supply 297 Wh/km. Since the energy density is 33 kWh/kg, the vehicle consumes 0.009 kg H2/km i.e. 0.9 kg H2/100 km.
An autonomy of 500 km, which is what potential buyers require, therefore calls for close to 4 kg of hydrogen onboard.
In normal temperature and pressure conditions, since the density of the hydrogen is 0.085 kg/m3, the corresponding volume would be 47 m3.
At a pressure of 700 bar, with density reaching 42 kg/m3, the volume required onboard is 500 times lower, i.e. 95 liters.
It is obviously storage in liquid form that wins out, with a density of 71 kg/m3 and an 800 times lower volume of hydrogen, i.e. 56 liters. However, the thickness of the insulated walls of the cryogenic tank must be taken into account.
An alternative to only reducing the volume is, in the latter case, to reduce the volume while increasing autonomy. A tank of 67 useful liters would give an autonomy of 600 km.
Storage in the form of metal or complex hydrides results in volumes of around 30 liters for the 4 kg of hydrogen considered above. Research is still ongoing to improve the process, the speed of release of the hydrogen in particular.
Illustration: Figure 10 p.103 Shanghai 2007 Report
Another solution that enables the storage problem to be circumvented would consist of producing the hydrogen onboard by reforming hydrocarbons. However, this is not the best solution in terms of CO2 emissions as sequestration at the level of a vehicle is ruled out. Thus, although a hydrogen fuel cell does not emit CO2 itself, the overall assessment of the vehicle from well-to-wheel expressed in g CO2/km must integrate the emissions linked to the production of hydrogen. Cf. Electric vehicles and CO2 emissions.
Little problems ...
Before being taken to market, a few “secondary” problems need to be ironed out and two major obstacles overcome.
The little problems include performance at a high ambient temperature and operation when cold (below -10°C). Recent tests at -20°C in North America and Japan seem to have lifted the uncertainty on this point. Moreover, longevity and reliability have not yet reached the level of combustion engines.
Similarly, present-day cells cannot supply immediate high output if needed; supercapacitors or batteries are needed to take over: these solutions are then referred to as "dual hybrid".
... and major obstacles
The first major obstacle to be overcome is cost. The reaction of a cell’s electrodes is triggered by a platinum catalyst. Result: the cost price is still much higher than acceptable for the mass market even if considerable progress has been made in recent years in the "dispersion" of the platinum, reducing the quantity required: several tens of grams are still necessary per vehicle.
Concerning the storage of hydrogen, a tank for hydrogen costs 10 times more than for a conventional tank. The costs targeted in Europe for 2020 are 100 euro/kW at the level of a fuel cell powertrain and 10 euro/kWh for the storage of hydrogen. These costs should be halved by 2030.
The International Energy Agency (IEA) estimates the extra cost of a fuel cell vehicle at around 13,000 dollars at present, when compared to a combustion engine vehicle.
The other obstacle does not reside in the cells themselves: they can only be deployed once the distribution and storage of hydrogen become economically realistic. Public initiatives such as the California Fuel Cell partnership, the Japan Hydrogen & Fuel Cell Program, or the HyWays European Roadmap Project (that will lead to the European Hydrogen Energy Roadmap) are endeavoring to pave the way.
In addition, the production output of hydrogen by electrolysis from the network is appreciably the same as that of a combustion engine:
Water electrolysis 80 % Hydrogen compression 90 % Fuel cell 46 %
Electricity and hydrogen production
Hydrogen and safety
Although the easy dispersion of hydrogen makes it less of a hazard than natural gas, which is widely used for domestic applications, the public will need to be clearly informed to quell fears. In fact, the risk of spontaneous explosion with a concentration in the air of 4 % in a confined space is still present in the back of people’s minds. Numerous simulations of road accidents have nonetheless shown there is less risk of explosion than with the fuels currently used.
Once considered the ultimate solution to road transport problems, notably in the perspective of a "hydrogen society", the fuel cell can still be envisaged in the medium term from a technical standpoint but from an economic standpoint (platinum and hydrogen are still expensive) and for environmental reasons (the production of hydrogen should be taken into account), it is open to question. Its development is likely to depend on the evolution of carbon-free production technology, the storage and distribution of hydrogen as well as the service life of electrodes and the availability of catalysts.