That 40kJ/L is for the entire system including the cyronics. Get rid of the cyronics and the system gets far far more energy dense.

Yes, that's quite likely how they come up with this number. But is it enough? What's the most energy dense configuration that can be made with room temp SC and does it compete with lithium batteries?

I just took a look at the source paper [0]

""" Although the attainable magnetic flux density limits the energy per unit volume given by Equation (1) ( B2 /2μ o), the real limit of the energy stored in a SMES is mechanical. [...] The relation defines the minimum mass of the mechanical structure in pure tension to support the radial electromagnetic forces. Force-balanced coils [5] minimize the working stress and thus the mass of the structure. """

So it looks like they 1) don't look at cryo and 2) the limiting factor is the stress due to EM fields.

[0] https://snf.ieeecsc.org/sites/ieeecsc.org/files/CR5_Final3_0...

Ah, ok. It seems that this SMES thing is not a solution at all for dense energy storage.

Do you have a source for this?