Looks like a "cool but useless" idea to me. The spring is not being used to 'really' store energy... So, you have a crappy (low capacity) rechargeable battery

Looks like a "cool but useless" idea to me. The spring is not being used to 'really' store energy... So, you have a crappy (low capacity) rechargeable battery, that is paired with a crappy mechanical charger. You would be much better served to have good rechargeable batteries, that you put in a good charger (which could be mechanical, if that's what you want).