Energy density of Lithium Ion VS Hydrogen

Lithium Ion VS Hydrogen. How do these two energy storage mediums compare? Representing Team Electric we have the Tesla Model S and representing Team Hydrogen we have the Toyota Mirai. 

A Tesla Model S with a 85kWh battery pack contains 7104 cells. Each cell is 65mm tall, have a diameter of 18mm, weighs 45g and contain 0,01258 kWh of energy. Specific energy density, is 0,279kWh/kg and volumetric energy density is 0,760kWh/l. Total weight of the battery cells adds up to 320kg.

However, these numbers are best case scenario. The battery cells need to be contained in a structure, be fitted with a cooling system etc. and all this adds another 220 kg which gives you a total weight of 540kg. Also, some of the battery capacity is reserved in order to increase cycle life and the geometric shape off the battery cells greatly reduce the volumetric energy density. Final specific energy density is 0,155kWh/kg, and volumetric energy density is no higher than 0,597kWh/l.

So how does hydrogen compare to lithium ion?

- Hydrogen specific energy density = 39,44 kWh/kg (141X Tesla 18650 cell)
- Hydrogen volumetric energy density = 2,79 kWh/l (3,67X Tesla 18650 cell)

Seems like a superior victory to team hydrogen, right? Well it`s more complicated than this.

1. You can`t really store hydrogen as liquid so you have to store it as compressed gas instead. When stored at 700 bar the volumetric energy density is lowered to 1.60 kWh/l.
(Internal tank volume is 122,4 liters and the hydrogen capacity is 5kg)

2. With a pressure of 700 bar you need a very strong tank wich adds at a lot of weight and quite a lot volume. The Toyota Mirai hydrogen tank(s) contains 5kg of hydrogen but the tank(s) itself weighs 82,5kg and have an estimated external volume of 150 liter = 1,3kWh/l.

3. The fuel cell weighs 56kg and occupy 37 liters of space. A device called fuel cell boost converter add another 13 liters of space.

4. The 1,6kWh nickel-metal hydrate battery is estimated to weigh 14kg and have a volume of 6 liters.

5. The fuel cell efficiency is around 50%. Available energy: 5kg ⋅ 39,44kWh/kg ⋅ 0,5 = 98,6kWh.
(Fifty percent effiecency is quite optimistic since I have used hydrogens "higher heating value". Lower heating value of hydrogen is 33,33kWh/kg.

6. The fuel cell create a lot of heat which require a big cooling system. This adds weight/volume and increase drag. No data.

MiraiEnergyDensity.png

Final specific energy density is maximum 0,62kWh/kg, and volumetric energy density is no higher than 0,48kWh/l.

Conclusion: Team Hydrogen wins the specific energy trophy (+400%) and Team Electric wins the volumetric trophy (+25%).

To be noted: The hydrogen numbers are quite optimistic since a lot of data is missing!