[Yahya (OP)]

"what about using weight at high heights instead of spring"
As I pointed out in an earlier post some domestic lighting systems used this method of power storage in the 1890,s.

there is some part in this device you are not able to understand  , which make it a new and independent invention that did not exist before , this is discharging spring energy using a clockwork mechanism IN STEPS to solve the problem of a multi-stage clockwork.(I pointed out this before)

I am glad you found a chart listing the energy storage capacity of various substances with steel springs right at the bottom , perhaps if you had found this earlier you would not have embarked on developing a spring powered car

you ignored that I listed in my post a kind of spring which has energy density 2500 times steel spring ( per volume MJ/m3) and 10 times li-ion battery.
https://en.wikipedia.org/wiki/Carbon_nanotube_springs

[syhprum]

I trust you have read the last part of the Wikipedia article entitled "Failure processes that limit energy storage"

[Yahya (OP)]

I read it but I can't see details , when these failure processes occur ? is it related to a maximum stress ? a maximum mass for the spring ? manufacturing ? or bad use ?

[evan_au]

this link shows the reduction in price of carbon nano-tube

There is no doubt that carbon nanotubes have amazing properties - mechanical, electrical, thermal and optical.
At present, we are severely limited in our ability to reliably produce nanotubes of a specific diameter, length, number of concentric tubes and "twist" (which determines whether they are electrical conductors, insulators or semi-conductors).

Producing defect-free nanotubes, and mechanically anchoring the nanotubes are other significant challenges with our present technology. But these will be overcome, over time.

Another characteristic of nanotubes is that they have an enormous surface area for their size. If we could produce repeatable nanotubes, we could use them in electrical capacitors, which could also store a significant amount of energy in a small space: closely-packed columns consisting of two conducting nanotubes, separated by an insulating nanotube would make an amazing capacitor!