Energy Density of Batteries

Ryuk1990

Hi all. I was wondering, are lithium-ion batteries the best type of batteries for electric vehicles?

I also remember watching a video of Elon Musk (from Tesla Motors and SpaceX) claiming that lithium-ion batteries are increasing their energy density by 8 to 9 percent every year. I googled this number and couldn't find any info on it.

That seems unbelievably high. If lithium-ion batteries are becoming that much better so quickly, it seems like we could have an electric 747 flying in the sky sometime in the next 50 to 100 years.

Simon Bridge

Have you seen:
http://www.allaboutbatteries.com/Battery-Energy.html
http://en.wikipedia.org/wiki/Energy_density

But you want to know how lithium-ion batteries have been or are improving so you can make projections. There is a lot of speculation about this - you don't want to google exact figures, instead try terms like "moores law for batteries" or "imrovement rate for battery energy density".
http://www.deloitte.com/view/en_GX/g...56f00aRCRD.htm
... "a 5% increase in energy density is a good year"

Current energy-densities are 0.9-2.3MJ/L
Energy density for jet fuel (BP AvGas) is about 45MJ/L
Projecting a 9% increase in energy density per year, it would take a bit under 50 years to get the same density from the battery as achieved with jet fuel. At 5%, it's more like 80 years (of good years).

There are other considerations - i.e. the total weight of the envisaged electric jetliner does not decrease as energy is consumed, affecting range, and the batteries are denser (masswise) than jet fuel, also affecting range.

Note: it is unlikely that the improvement rate will be steady or that it can continue upwards forever.

Ryuk1990

Thank you Simon.

With lithium being the 3rd lightest element in the universe, lithium-ion battery technology is one of the best things we have available on the market.

However, lithium-ion batteries only increase 2 to 3 percent in energy density in an average year. Eventually they will reach their peak energy density. If we ever want all modes of transportation to go electric and be practical, I can't see lithium-ion batteries being in future transportation.

What research is being done to create a new battery that would have a significantly higher energy density than lithium-ion batteries?

nsaspook

http://www.anl.gov/photos/joint-cent...research-jcesr
http://energy.gov/articles/building-...icles-and-grid

Our (US) tax dollars are being spent on research.

Simon Bridge

Or it may be a moot point ... as oil prices rise, less energy-dense approaches may become economically attractive. We would just have to accept the limitations for the same reason we used to accept steam and horse limitations.

mheslep

That comparison needs a severe adjustment for energy that is actually useful.

Combustion for mechanical work inevitably throws away much, usually most, of the chemical energy content as waste heat. That fact has further consequences for any system built around a combustion engine by way of snowballing peripheral systems: an exhaust gasses system, a heat removal system for the engine and heat insulation for nearby components (or passengers). Then add an air capture, filtering and mixing system. Do the same for the fuel. On and on.

Jet engines are probably the best implementation of combustion engines in existence, but they are not immune to the problems of heat cycles. Batteries and electric motors are immune.

Simon Bridge

Well the devil is in the details and it is a bad idea to draw general conclusions from a single metric of comparison anyway. Post #1 was in terms of energy densities though so the comparison is valid in that spirit.

To work the example more fairly we'd have to look at how we'd use electricity to power an airliner and compare with the efficiency of a comparable jet airliner... it probably would not be fair to build an electric jet engine for comparison - so allow any method. (After all, the challenge is to figure roughly when electric powered airliners could do as well as a current jetliner.)

With the best of today's batteries, the jet engine can throw away 95% of the stored energy and an equivalent battery powered airliner would need to be 100% efficient to match it.

So modify the future projection to include a statement of the needed efficiency.
However - the projected increase of 9% pa is such a hand-wavy number anyway ...

If you just mean to point out that energy efficiencies and engines are not simple, point taken.
http://en.wikipedia.org/wiki/Jet_eng...rgy_efficiency

I would be interested in how you'd go about the same calculation?

IRL: I suspect the dwindling oil supplies will make the decision for us.
We'd just have to put up with whatever limitations there are in whatever ends up being used because the alternative just too expensive.

mheslep

I took a shot it some time ago in the aerospace forum in terms of range versus battery energy density, since range is really what's needed up to a point. I came up with a range equation resulting in the following, for an air frame with a glide ratio of 20:1, electric fan/battery efficiency of 0.8: max range = 800KM times battery specific energy density in MJ/kg

So:
0.72MJ/kg (current Li Ion technology), 576 km
2.4MJ/kg required to cross the Atlantic, 1900 km

Extreme air frames like the Global Flyer's L/D=37 could cross the Atlantic w/ 1.2MJ/kg, i.e. the current capacity of lithium sulfur batteries.

If the above is correct, then I think the more important bottleneck for electric aviation is improvement in the energy density of electric motors (via superconductors) to match jet engines.

Stanley514

There is attempts to create Sodium-Sulfur battery which would be capable to work at room temperature.They hope to use solid conductor of few micron thickness and projected energy density is 300 Wh/kg.If this is not a scum this type of battery may become cheaper and more energy dense than any Li-ion battery.For now sodium-sulfur looks like a best type of rechargeable battery known till now,especially if they will manage to work it under room temperature.

Simon Bridge

Well ... engine efficiency and fuel energy-density are complimentary design parameters.

Hmmm - in your prevous attempt you were factoring in the total weight of the aircraft (did I read that right?) ... to do a comparison, we'd need equivalent figures for aviation-fuel ducted fan aircraft.

There's an additional wrinkle - batteries don't get much lighter as they are used up.
Energy density per kg is important but that's not what I'm thinking of ... iirc nearly 40% of the total mass of a jetliner (747-400) is fuel... so it gets significantly lighter over the journey... increasing range per MJ stored at takeoff compared with batteries - unless the aircraft jettison's batteries as it goes.

Anyhow ... by the sort of calculation you did: how would you respond to the idea that we could have an electric 747 in 50-100 years?

Ryuk1990

One thing to consider about electric aircraft is that they would probably fly at much higher altitudes (at around 80,000 feet) than combustion airliners. At higher altitudes there is less atmospheric density and therefore there is reduced drag on the aircraft. Combustion engines can't run efficiently at those altitudes because of the amount of nitrogen. Electric motors on the other hand can.

mheslep

Range is independent of total mass in that particular parameterization. Range dependent variables: L/D, battery energy density, propulsion efficiency, fraction of total mass taken by battery.

No doubt there are several. And several advantages.
Up to 50% fuel fraction for a 747 - a flying gas can at takeoff.
Yes that's a disadvantage for a battery system. Metal air batteries actually gain mass as they oxidize/discharge. On the other hand a propulsion system that does not require O2 means the ceiling might be extended considerably, thus cutting drag, etc.

The 747 was designed for extreme long range, so I expect it would be the last to undergo such a conversion. First up would be air frames suited for short haul shuttle type flights that log a great deal of miles. These would stand to save the most on fuel/energy costs and enjoy the ability perhaps to operate during hours otherwise prohibited by noise regulations.

Also, I suspect electric aviation would first occur with electric motors in combination with fuel cells not batteries, given Boeing already tested a prototype.

mheslep

Yep. I don't know what the ceiling would be. At some point obviously the wings would stall. Also, there is the potential energy penalty - 1 GJ per 100 tons per km of altitude - only a fraction of which can be recovered on descent. That penalty favors long flights over short.

Another advantage might be the ability to completely stop electric propulsion in descent (and thus energy consumption) unlike a jet engine. I have no idea if reasonable flight profiles in descent could be obtained with no power.

mheslep

Interestingly E-car maker and rocket maker Elon Musk has been thinking about electric jets using batteries, or at least some form of propulsion that does not require or carry oxygen. He suggests the optimum altitude for an electric aircraft would be 80,000 ft, and he's thinking about making them supersonic.
(~@40mins)