On this page of our guide to Destroy all Humans you can find information on how to regenerate the shield of your flying saucer. You'll learn how to fix your spacecraft's shield. This option is especially useful when fighting a boss.
Energy absorption works similarly to brain extraction. By using it you can regenerate the saucer's shield and replenish your resources.
Remember that you can only draw energy from the mechanical elements that appear on the map. You can use cars to absorb energy.
This ability is not used to regenerate shield, but it makes it easier to control the flying saucer. By using the Repulse-O-Tron you can repel incoming rockets that are flying in your direction. Usually, reflected rockets hit the opponent who fired them against you.
Unfortunately, the spacecraft's shield does not regenerate automatically. You just have to rely on energy absorption.