Key Points You Need to Know When Talking About Energy: Energy Density

One of the main uses of hydrocarbon fuels is transportation.  If we want to eliminate the burning of these fuels as a way of generating energy, we can't just replace hydrocarbon burning power plants with other power plants; we would also need to replace hydrocarbon burning vehicles with other vehicles.

A key reason this is more difficult than it might seem is energy density.

Energy Density

Energy density is, simply, the amount of energy that can be extracted from a given unit of material.  You can talk about either energy density per volume or energy density per weight, but energy density per weight is more frequently used.

Energy density can also be reported in different types of energy.  Batteries are described in terms of Watt-hours per kilogram, typically, whereas things like gasoline are described in terms of Joules per kilogram.  Both are units of energy and are directly convertible (1 Watt hour = 3600 Joules), but Watts are used for electricity and Joules are usually used for heat.

Some reference energy densities

Batteries

The most energy dense batteries out there are Lithium Ion batteries, which range from 100 to 265 Wh/kg.  Larger batteries tend to be less energy dense, as the larger the battery array is, the more weight is taken up by non-energy storing components like compartmentalization and wiring.  Batteries in current Tesla cars are about 165 Wh/kg.  This works out to be about 0.6 Megajoules per kilogram (MJ/kg).

Hydrocarbon (aka "fossil fuels")

Hydrocarbons of all kinds, whether derived from fossil fuels or not, are highly efficient at storing energy.  The range for hydrocarbons goes from the least dense, which is wood at 16 MJ/kg, to natural gas which is all the way at 55 MJ/kg.  Standard gasoline is pretty high at about 46 MJ/kg.

Nuclear

Nuclear fuels are not in the same league in terms of raw energy density.  Uranium-235 has an energy density of 3,900,000 MJ/kg.

These values immediately prompt certain questions:

-How are electric cars able to be even a little bit competitive with gas-powered cars?

Gasoline is about 77 times as energy dense as Li-Ion batteries.  How is it that electric cars are even able to compete with that kind of electrical density?  The glib answer is, "they're not, really".  And I think it is true that electric cars are still only barely commercially viable, if not for the "green cred" that they offer.  It is still rarely the case that if someone needs to buy a car and considers purely normal economic considerations that the rational choice is an all electric car.  But, electric cars have been getting better and better and the choice is not quite so lop-sided as it once was.  Here are some reasons the energy density issue is not as fatal for electric cars, at least of the ordinary passenger variety:

1. The high energy density for gasoline is for *heat* energy.  This energy must be converted to motion by the internal combustion engine (ICE), and that process is constrained by thermodynamic efficiency limitations.  In particular, it is theoretically impossible for the ordinary ICE to get more than about 37% efficiency in converting the heat energy of exploding gas to motion.  This is a hard-and-fast limit imposed by physics (Carnot Theorem).  In *practice* the efficiency is lower still, so the actual energy extracted for useful motion is about 20% or so for the average car.
   
   In contrast, electric motors are one of the most efficient devices we have for converting energy to motion, and Tesla's operate at about 90% energy efficiency.  When you factor in the difference in energy conversion, the advantage of the gasoline powered ICE drops to just 17 times as energy dense as Tesla's Li-Ion batteries.
2. There is an additional crucial difference in how much weight of machinery is required to convert gasoline to motion.  Gasoline powered cars must carry around with them a massive steel engine in order to convert gas to motion, a fairly bulky fuel injection system, and a bulky exhaust / noise muffling system.  Furthermore, because the power produced by an ICE is fairly low compared with the speed people want to drive them, a massive steel transmission system is also required in order to deliver the power at a wide range of speed without stalling out.  All of these things together make up a large proportion of the total weight of the vehicle.
   
   In contrast, electric vehicles are mechanically much simpler than ICEs.  Tesla's system, for example, has a total of 17 moving parts, compared with about 200 for a normal ICE.  Electric motors are also naturally very "torque-y", able to generate large ranges of speeds without the need for a transmission with complex gearing.  The motors are also tiny compared to an ICE, and there is no need for fuel injection or coolant or exhaust.
   
   Therefore, the batteries in an electric vehicle are not just replacing the gas tank; they are effectively also replacing most of the internal machinery that is under the hood of a car.  This considerably boosts the effective energy density of an electric vehicle compared to a gas powered car.  On a typical gas powered car, the combined weight of the engine and transmission is close to 10x the weight of a full gas tank.  This therefore drops the energy density advantage of an ICE care to merely 1.7 times that of a Li-Ion powered car.

This is why it is possible to have electric cars with comparable ranges to gas vehicles nowadays.  It's still a bit of a stretch, but it is possible.

However, there is an important point to realize here!  The fact that electric cars are able to play in the same ballpark as gas powered cars here is very much due to the relative weight of the fuel tank to the engine and transmission components.  This is a factor that is specific to the size and design of the typical passenger sedan.  Therefore this relatively even playing ground does not necessarily apply to other vehicle types.  Airplanes, in particular, are much more sensitive to the amount of fuel they carry--hence electricity powered airplanes are nowhere on the horizon of being practical.  I know less about railroads, but given that the weight of the engine on a train is insignificant compared to the loads that are hauled, I doubt that electrically driven trains are viable either.  Likewise I have a lot of doubt about electrically powered water freight.

Bottom line: complete electrification of transportation is probably not feasible in the near future, even if we're able to transition all personal passenger cars to electric.

-How are non-nuclear power plants able to compete with nuclear power plants, given the absurd energy density of Uranium?

The answer to this is more complex and I will get into this later when I talk about the economics of nuclear power.  For now, the simple answer is that non-nuclear power plants can not compete with nuclear--in terms of fuel cost (well, aside from solar and wind in which the "fuel" is free).  Uranium is very costly to mine, but in actuality this cost is only a negligible part of the total cost of operating a nuclear power plant.  It turns out that the majority of the cost of running a nuclear power plant is the interest that you pay on the loans to build the plant.  More on this later, though.