Transportation by air and water typically requires a power source that has high energy density. Currently, the high energy demand of aviation and marine travel can only be satisfied by liquid fuel, such as petroleum based products.
It is necessary to find an alternative fuel that can be produced on a large scale which is suitable for aircraft and marine vessels. Biofuel has been mentioned by some as a viable option. However, although it has an acceptable energy density, it would take an enormous amount of land area to provide enough feedstock to produce it. Even a biofuel such as ethanol, which has a high energy density of 30 GJ/ton, would require an enormous amount of farm land. For example, to replace petroleum use for aviation and marine travel in the United States with an equivalent amount of ethanol, it would take a land area larger than Florida! The sheer scale of this means it cannot be done.
Battery-electric power is also unfeasible, but for different reasons. The energy density of batteries is currently far too low for the demands of aircraft and seafaring vessels. For transportation use, battery storage capacity is currently only suitable for light-duty vehicles such as light trucks and automobiles. It is unlikely for now and probably well into the future, that battery technology will progress enough to be able to satisfy the energy requirement of very large machines.
Modern battery packs currently have about 100 times less energy density than petroleum based fuel. They would therefore be too heavy and bulky for the same energy output. For large marine vessels and especially aircraft, the much higher weight and bulk of batteries would be highly impractical. For a commercial passenger jet carrying 200 tons of fuel, an equivalent amount of batteries would weigh 10,000 tons! And this is assuming that electric motors are roughly twice as efficient as jet turbines. This number is even more unimaginable for large marine vessels like ships which might carry thousands of tons of fuel. Size and scale play a key role in determining whether battery-electric power is feasible.
For a light duty vehicle such as a car, you would only be adding several hundred pounds of weight. The large weight difference between battery storage and gas (or diesel) would be offset somewhat since the engine is replaced with a much lighter electric motor. And since electric motors are much more efficient than Internal Combustion Engines, you get much better mileage even though the vehicle is significantly heavier.
Eliminating petroleum dependence for air and marine travel will be among the last technological hurdles we face as we wean ourselves off oil. Fortunately, there is a potential fuel that can be used which provides all the energy requirements for these modes of travel. It is hydrogen.
Hydrogen may be the only long-term solution to powering aircraft and sea faring vessels. It has one of the highest known energy densities, higher than even petroleum. But due to its low mass density at normal ambient conditions (as a gas), it needs to be compressed to a manageable volume before being used (ref: http://gltrs.grc.nasa.gov/reports/2006/TM-2006-214365.pdf).
The main advantage of hydrogen is that it pollutes very little when burned, emitting mainly water vapor. However, a potential issue speculated by some, is that water vapor “exhaust” during high-altitude flight may contribute to the greenhouse effect (global warming). One possible solution is to fly aircraft at lower altitudes, even though fuel efficiency may decrease slightly (ref: http://www.climnet.org/CTAP/techsheets/CTAP11_Aviation.pdf).
Notwithstanding, the use of hydrogen shows serious promise. It can be produced on a large scale from clean, renewable sources, such as wind and solar power. Using electricity (electrolysis) hydrogen can be produced from water with an efficiency of 70%, meaning that 70% of the electrical energy used to produce it is recovered when it is “burned” (ref: http://www.nrel.gov/hydrogen/pdfs/36734.pdf). Note that this 70% also accounts for the energy required to compress it to a manageable volume.
Power generated by wind turbines can be used to produce hydrogen locally, avoiding transmission losses in the process. And due to the very large amount of electricity required it may be best to set up a (stand-alone) dedicated network of wind turbines in remote regions that are located far away from industrial and highly populated areas. This network of turbines would be used solely for hydrogen production. The reason for this is twofold. First, this helps avoid “competition” for energy, as industrial and highly populated areas would need their own uninterrupted power supply. Furthermore, it is more efficient to transmit power over as short a distance as possible, in order to reduce transmission losses. So industrial and populated areas would get their power from wind turbines that are located in closest proximity. While the dedicated network of hydrogen producing turbines would be operating in remote regions. The remote regions can be located in windy areas such as in northern Canada, Alaska, Asia, which have relatively low population density.
So how many wind turbines would be needed?
As an example, let’s consider the petroleum use of the United States for air and marine travel. The amount of petroleum used is, on an energy basis, equal to about 4500 trillion Btu per year (ref: http://www.eia.doe.gov/oiaf/aeo/supplement/pdf/suptab_35.pdf).
Let’s assume the wind turbines used are rated at 5 MW with a capacity factor of 30%. This means that 0.30×5 MW = 1.5 MW is generated on average due to intermittent wind conditions.
Crunching the numbers, and assuming a 70% hydrogen production efficiency, we would need 143,000 5 MW turbines to supply sufficient hydrogen for U.S. air and marine travel. This is several times larger than current installed capacity worldwide.
On a worldwide scale it is estimated that four times this amount is needed, given that the U.S. consumes about 25% of the world’s petroleum. So, for global production of hydrogen for air and marine travel, we would need 570,000 5 MW wind turbines.
The same methodology can apply to solar energy.
If we were to produce hydrogen using electricity produced from solar energy, a similar strategy would be needed.
Concentrated solar thermal power is likely the best candidate for electricity production using the sun. Photovoltaic solar cells would not be nearly as effective for two reasons. First, they produce electricity at a lower efficiency than solar thermal power, and second, they have a low production rate. The highest efficiency solar cells require the production of high-purity Silicon which must be “grown”. This severely restricts their production rate.
Solar thermal power generation is only available in a few key areas of the world, that experience strong sunlight year round with little cloud cover. Deserts and other marginal land are ideal for solar thermal energy production. Select areas in Africa, Australia, Europe, South America, Middle East and the south western United States provide the ideal environments. Solar thermal systems are perhaps even better suited to stand-alone hydrogen production than wind-power simply because they tend to be located in “hot” and dry areas of the world that are geographically far removed from large populations.
This means that a dedicated system of hydrogen production can be set up in remote desert regions where competition for electricity won’t be a big concern.
Power can be generated using Stirling dishes (ref: http://www.stirlingenergy.com), as they are the most efficient means to produce electricity from solar energy. Power generated by Stirling dishes can be easily scaled up by simply adding more units. And unlike solar cells, these dishes are essentially mechanical devices, which lends itself to high volume production, just like automobiles. Furthermore, they are twice as efficient as solar cells, and are currently cheaper to produce in large volume.
So how many Stirling dishes would be needed?
Each dish outputs 25 kW of electricity. And each dish produces 55,000 – 60,000 kWh of electricity per year (ref: http://www.stirlingenergy.com/technology/default.asp).
Crunching the numbers like before (assuming a 70% hydrogen production efficiency), it would take about 31 to 34 million Stirling dishes to satisfy the energy requirement of air and marine travel in the U.S. This number of dishes would occupy a land area of about 30,000 km2 (ref: http://www.stirlingenergy.com/downloads/30-June-2008-Application-Filed-for-Worlds-Largest-Solar-Energy-Generating-Plant.pdf). This is equivalent to a land area with sides measuring 170 km by 170 km. This is much smaller than the land needed for biofuel production, and you don’t have to use up quality farmland in the process!
Like before, you would multiply the above figures by four, for worldwide production. This amounts to global installations with a combined area of about 120,000 km2, roughly the size of Virginia, using 124 to 136 million Stirling dishes.
Due to the very high volume, it would likely take decades to manufacture the number of Stirling dishes, or wind turbines, needed. But consider that there are roughly 800 million vehicles worldwide, so these numbers are not unreasonable.
Now, the above figures are based on matching the energy contained in the petroleum used, with an equivalent amount of energy from hydrogen. Obviously, a considerable energy reduction can be achieved by improving efficiency in transport, such as with the Sky Sail for ships, or more efficient aircraft engines.
Engines (such as) for ships can be easily adapted to accept hydrogen as fuel. However, for aircraft some research would have to take place, both in turbine engine development and in hydrogen storage capacity.
In the meantime we would continue to use petroleum based fuel for aircraft and marine vessels, slowing weaning ourselves off oil in the decades to come.