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Round Trip Efficiency
To justify the use of hydrogen as the fuel of the future it is important to establish exactly how efficient it can be. This achieved by considering the overall efficiency from production to consumption, in this case in hydrogen powered cars. The hydrogen efficiency is characterised by the following equation:
Efficiency = LHVH2 / Energy useful input
The LHVH2 is the lower heating value
of hydrogen (energy produced when hydrogen is burned) 285.84 MJ/ kmol.
Energy useful input is the energy supplied per kmole of hydrogen produced
Onsite hydrogen production:
For onsite hydrogen production to be cost effective components costs need to be reduced. Carbon nickel electrodes could be used over the more expensive metals such as titanium along with simple alkali activator. The current power requirement for such a process is around 4.6kWh / Nm3 of hydrogen produced. The operating temperature is around 80 °C and 30 bar absolute to avoid the difficult initial compression stages after production. The electrolysis process is only one part of the overall hydrogen production as shown below:
Hydrogen On-Site Production Process
The electrolysis unit operates as previously described with a filament heater raising the water temperature from 15 °C to 80 °C and a centrifugal pump raising the pressure from atmospheric to 30 bar. The refuelling station has to supply 400 kg a day or 4451.91 Nm3 of Hydrogen. The Hydrogen and oxygen are produced at high purity 99.5% excluding the need for further processing and are passed to condenser units where any water vapour condenses and is returned to the electrolysis unit. Both the hydrogen and oxygen are then compressed for storage, to 200 and 30 bar absolute respectively. The energy required to compress the oxygen is not relevant to the hydrogen round trip efficiency and so has been discounted from any calculations. However the oxygen produced is considered in the cost analysis of the station. The hydrogen and oxygen are then stored in stainless steel underground storage tanks. A summery of the electrical energy required in the production stages is give below:
Electricity required in electrolysis unit = 76003.69 MJ/day
Electricity required to raise water temperature = 1.05 MJ/day
Electricity required in water pump = 21.79 MJ/day
Electricity required in hydrogen compression = 1189.09 MJ/day
Total energy required in production per day = 77215.62 MJ/day
η Hydrogen production = 285.84*198.61/(21.79 +1.05 +76003.69 + 1189.09) = 73.52 %
This is comparable with η Gasoline
production = 88-91% (Depending if MTBE additive is used as it must be
produced and shipped and mixed with the gasoline.)
Other areas of efficiency loss
Other areas where efficiency is affected are storage due to hydrogen diffusion through the vessel wall, fuel dispensing due to possible leaks and finally the hydrogen vehicle itself where the fuel cell and the car mechanics combine to reduce the overall efficiency:
Other Areas of Efficiency Considerations
Final Round Trip Efficiency
Renewable power sources give a final hydrogen Roundtrip Efficiency of 32.03%.
Using Non renewable power (efficiency <40%) gives a hydrogen roundtrip efficiency of < 13.1 %.
Gasoline Roundtrip Efficiency is between 28.54 - 31.36% depending on the use of MTBE additive.
This is comparable with hydrogen production using renewable energy sources (32.03%) but if conventional energy sources are used then the hydrogen roundtrip efficiency is too low to be considered competitive (13.1%).
Areas of improvement
The first area of improvement lies with the electrolysis unit. The amount of energy currently required to produce 1 Nm3 of hydrogen is 4.6 kWh. However it is theoretically possible to have 100% efficiency, that is 3.5kWh per Nm3 of hydrogen. But this would be very difficult to achieve and predictions are that a value of 3.7 kWh per Nm3 of hydrogen is a more realistic goal. This raises the production efficiency to around 91.03%.
Future Electrolysis Performance
Another area where real progress could be made is in the hydrogen fuel cell. Currently fuel cells are 50% efficient in converting hydrogen to electrical energy, but it assumed that this efficiency could rise to 60%.
If these improvements were applied, the
roundtrip efficiency would then be 48.69% using renewable energy and 19.91%
using non-renewable sources.