I've seen a different version of the space ship/shield absorber type game, and this seems to be very closely modeled after the one I played. However, I liked this one a lot more than the other.
It did seem a bit easy - especially when you realize some ships are just there to provide you energy and are very easy to kill - and I wasn't 100% sure what the way of losing was. But there were definitely tense parts when I ran out of energy and had to scavenge.
Despite the upgrades referring to it, I never saw the shields regenerating. Perhaps they meant something different than what I expected; that was the efficiency of my absorption, maybe?
I only found the machine gun and piercing gun useful. The others were too slow and not purpose specific enough.
Nice adaptation of an existing concept, well done!