Hydrogen: The 'Do Less with More' Fuel
By Alec Brooks
A favorite vision for hydrogen powered transportation in the future is that hydrogen would be produced by electrolysis with renewable energy. System diagrams often show an electrolyzer next to a graphic of a windmill or solar array to symbolize that the hydrogen will be "green". Such an approach is said to provide zero emissions driving with no greenhouse gas emissions. It is certainly feasible to hook an electrolyzer up to a solar array or windmill – but the question is whether this is the best use of the energy.
The problem with making hydrogen with electricity is the large amount of electricity that is wasted. Fully three quarters of the electricity used to make and compress hydrogen is lost in the process; the electricity coming out of the fuel cell to drive a vehicle's motor is only one quarter of the energy used to make the hydrogen. This begs the question as to whether there might be alternate, less wasteful uses of this energy.
In a recent Los Angeles Times article, writer Dan Neal described his experiences driving a Honda FCX fuel cell vehicle fueled by hydrogen made at Honda's solar hydrogen fueling station. He was struck that a vehicle could be propelled by energy from the sun.
He rhapsodized, "I'm driving pharmaceutical-grade California sunshine: hydrogen generated in an experimental solar-powered station at Honda R&D America's facility in Torrance", and "I'm driving on pure hydrogen, the converted essence of the sun itself."
He went on to explain that Honda's large solar refueling station was designed to generate enough solar energy to produce hydrogen sufficient to drive the FCX about 10,000 miles a year. This works out to about a half a kilogram of hydrogen a day, good for up to 28 miles in the FCX. Honda's published data for the solar refueling station shows that it takes about 64 kiloWatt hours (kWh) of electricity to produce, purify, and compress each kilogram of hydrogen.
So on a daily basis, 32 kWh is consumed to make 1/2 kilogram of hydrogen. Of that 32 kWh, only about 8kWh is provided by the fuel cell system to run the vehicle's drive motor; the other 24kWh is wasted.
Studies comparing hydrogen fuel cell vehicles to other alternatives often have comparative bar graphs showing well-to-tank efficiency and tank-to-wheels efficiency. These measures do not promote a good understanding of the real situation. What really matters is how much energy a vehicle uses per amount of distance traveled -- a function of well-to-tank efficiency and vehicle fuel economy. Vehicle fuel economy is an all-inclusive measure of utilization of energy stored in the vehicle per unit of distance traveled. Tank-to-wheels efficiency is an imprecise concept, and is not a very accurate measure of vehicle energy consumption. Vehicles with very efficient engines can and often do get poor fuel economy.
To illustrate how the 32 kWh consumed every day to make hydrogen could be used more effectively, consider electrical loads that are familiar on a daily basis.
First off, we'd like to provide the same 28 miles of transportation electrically. With a battery electric vehicle or plug-in hybrid vehicle of the same size as the FCX, it will take about 0.30 kWh/mile for battery charging, or 8.4 kWh for 28 miles. This leaves 23.6 kWh (about three quarters) of the energy left over to be used for other purposes. With an overall energy budget of 23.6 kWh, all of the following common household loads can be served using energy efficient appliances:
- Run ten compact fluorescent light bulbs for 5 hours
- Provide the daily energy needs of the refrigerator
- Run the television for 4 hours
- Run the computer for 4 hours
- Run the dishes through the dishwasher, including electrically heating the water
- Run a load of laundry through the washer, including electrically heating the water
- Dry the load of laundry (gas dryer)
- Heat water for four showers
- Run a 3-ton (36000 btu/h) central air conditioner for 5 hours
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