Turning heat into useful energy is difficult, but advancements in the field of thermal technology have made it more possible than ever before.

This means that less fuel needs to be consumed to produce the same amount of useful energy.

If we want useful energy forever, we need a road that is downhill both ways.

This prevents the body from oxidizing food to produce useful energy.

Thus, a battery using this isotope might continue to supply useful energy for at least half that time.

In other words the amount of useful energy is always less after a change.

There is little danger over the next few million years that we shall exhaust the store of useful energy.

In Europe only 42% of the energy produced is converted into useful energy.

However, it will never be practicable to convert more than a small percentage of this into useful energy.

But he added that it might be "many years" before the process produces useful energy.