IEEE Future Directions
Related Articles
Energy comes in different types, electrical, chemical, thermal, nuclear, movement… Each type has its own advantages related to transport, storage and usage.
As an example, storing electrical energy is trickier than storing chemical energy. The density we can achieve with storage technologies is way lower for electrical than for chemical energy, with nuclear energy providing the highest density (following values are expressed in Wh per volume – 1 cubic decimeter, i.e.1 liter):
- Uranium: 427,734,231,076
- Jet Fuel (kerosene): 10,389
- Diesel: 9,944
- Gasoline: 9,500
- Fat (our body fat!): 9,440
- Liquified Natural Gas: 6,167
- Hydrogen (at 700Bar): 1,555
- Rechargeable Lithium Ion: 230-731
- Supercapacitor: 14-17
- Capacitor: 0.0027-0.277
It is not surprising that airplanes use jet fuel rather than gasoline. Its energy density is 10% better than gasoline and that means less weight to transport and less space, both crucial to aviation. Solar Impulse accomplished an amazing feat by flying a plane all around the Earth powered by solar energy only. However, that plane was able to carry only the pilot, was built with advanced materials -very very light-, and took months to circle the Earth (it stayed on the air for over 23 days to fly the 40,000km around the planet).
Now, you might say that the beginning of aviation was a tiny aircraft barely able to carry its pilot for a few hundred metres and now, 100 year later, we have the A380, weighting 560 tons (at take off) and able to carry 555 passengers (notice that this turns out to be about one ton per passenger, the Solar Impulse weights 1,600Kg and carry one person…). Couldn’t we imagine similar progresses leading to solar power in commercial aviation in a hundred year time (or sooner, like in 2050)? Experience from the past should make me, us, cautious to state that this will not happen. Yet, even the people behind Solar Impulse stated that what they wanted to do was to demonstrate the potential of solar power to apply it in several areas, like cities, but not in aviation.
Energy density is a physical barrier, not a technological one, although technology helps in making the most out of the theoretical limits of energy density in a given material.
Of course, what one could do is to use solar power to create solar fuel, that is use solar power to separate hydrogen and oxygen from water and use this as fuel (we are using this in some rockets). But forget about scaling up a Solar Impulse to create a A380 equivalent (the wing span of Solar Impulse was bigger than the wing span of the A380 to harvest the Sun light energy sufficient to carry one person…).
The other crucial factor is continuous availability of energy. Current electrical grids are a marvel of electrical engineering managing to deliver exactly the same amount of power that users need. This requires a precise balancing between production and consumption. Some production sources are more flexible than others: as an example, hydroelectric power can be used to distribute power in the grid as well as to pump water upstream with electric pumps when the energy produced exceeds demand. Electricity produced by coal plants, on the other hand, is more difficult to manage, since it takes time to decrease/increase the output of a plant, a similar situation applies to nuclear plants.
In all these cases the problem is to meet the changing needs of demand.
For renewable energies, in addition to balance the demand there is the issue of managing the variability of production. Solar energy goes to zero (in practice) during the night and it decreases significantly in cloudy days, wind may stop blowing at any time, regardless of the need of power by the users, tides alternate daily and their periodicity is not necessarily in synch with power demand…
This brings to the fore the problem of energy storage and its density, that I discussed above. Storage is a much crucial issue when we are sourcing energy from renewable sources, because of the (basic) impossibility of controlling their output. One has to harvest as much energy when it is available, store it, and deliver to user when it is demanded.
Tackling this issue is a major hurdle and no effective centralised solution (for energy storage) is available (economically affordable) today. In fact, the growing power production based on renewable sources requires a balancing that is made possible today by tweaking the fossil/hydroelectric production in the grid. When renewable sources are feeding power in the electrical grid, the power utilities trim down the other production sources and when renewable decrease their output the demand is met by the other production sources. Very little energy storage is involved.
However, by 2050, storage may take a swing and this can lead to a significantly different power management.
