Riversimple FCEV weight investigation

Prologue

[Updated 02/03/2019 with new information]

It was my opinion that FCEV technology missed the boat (for personal transportation), if it had been where it is now 15-20 years ago I would've been very excited, but advances in Li-ion battery technology has made it a non-starter.

Having had some interesting debates on twitter about this and with the Riversimple Rasa nearly here I thought I'd try to figure out what would happen if Riversimple ditched the FCEV for the Rasa and switched to Li-ion.

I'll try to document my sources where I remember.

The specifications for the Rasa were taken from here: arstechnica.com

Specifications for supercapacitors and Li-ion batteries were taken from wikipedia.org

Li-ion cars are too heavy and inefficient

But are they? 
Riversimple haven't released details of the specific components they have used so I have to investigate and guess a little.
  • The Rasa has 1.5kg of hydrogen @ 33kwh/kg giving total energy of 50kwh
  • The Rasa uses the Hydrogenics HD 8-500 fuel cell. It weighs 49kg and has a peak efficiency of 51% giving an effective total usable energy of 25.5kwh
  • From luxfercylinders.com I determine the weight of the fuel + tank is about 38kg
  • The Rasa uses 120 JSR Micro 3300F supercapacitors which are stated as 1.9Mj or about 0.5kwh, possibly 10 banks of these, with a total weight of 64kg.
  • Excluding all other parts the powertrain for the Rasa is therefore about 151 kg
  • The Tesla Model 3 gets 168 wh/kg or 156.25 wh/kg usable.
  • Therefore I determine a li-ion battery with capacity 25.5 kwh weighs 163.2 kg
The Rasa weighs 580kg, therefore replacing the H2 fuel cell architecture with a the M3 architecture would increase the weight of the vehicle by 12.2kg or 2.1%.

The point

It is important to note that the above is about technology available right now and is only to highlight my point which is that BEV vehicles are not in themselves significantly heavier than FCEV vehicles. I'm not arguing that the Rasa should switch to batteries (although Riversimple says they are in favour of both so why not do a battery version too?), but rather that if you're only concerned with weight either technology is pretty similar.

There are a number of reasons why RS may have made the design decisions they did and I still believe that FCEV vehicles will have niche applications, at least in the short/medium term. I'm not sure that Rasa has a good future due to the way the market has developed but I'm still wishing the company well and hope they succeed.


The future


Right now Solid Energy Systems are making and selling batteries with between 400 and 500 wh/kg and claim to be scaled up enough to be making them for cars by 2020. Given a wh of 450 would give a battery weight (cells only) of about 64 kg and my estimate of about 85kg at the pack level. A Rasa powered by these batteries could therefore weigh 66 kg less than its current H2 drive-train.

Whats interesting about the above is that it really demonstrates the pace of development of battery technology and its showing no sign of slowing anytime soon. Having said that FC tech could also improve. There is a lot of headroom in efficiency for a start, if they could get 75% that would make a big difference to these numbers, although I haven't read anything suggesting that is happening in the immediate future.

Summary

In short, whilst I applaud Riversimple for their circular economy business model, and green aspirations, the argument that we need FCEV for weight reasons is entirely nonsense.



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