ok so this is maybe a bit more elaborate than a typical shower thought.

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to generate useful energy, you need a hot and a cold spot, between which there is a heat transfer, you can translate a part of that heat flow into useful (mechanical) work.

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In the solar system, the sun is the hot spot while the universe around it (outer space) is the cold spot. Energy continuously flows outward from the sun into the vastness of empty space. This is a temperature gradient.

Again, the law of thermodynamics applies: We can only utilize the sunlight because there is a temperature gradient. The sun is hotter than the vacuum. Therefore there is a heat flow, which can be utilized.

If everything in the universe was the same temperature, solar panels wouldn’t be able to produce electric power. They have to be colder than the light source, otherwise they don’t work.

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Now, if you replace the sun with a black hole, that’s a very cold object. A black hole has a temperature of approximately zero, because it basically doesn’t radiate out any energy. Remember that it’s black because it doesn’t even let any light out. Some argue that there is hawking radiation but it’s minuscule and can be ignored.

(excuse my very crappy editing)

Anyways, black holes don’t radiate, so their temperature is very cold. On the other hand, vacuum is not completely cold, but has a positive temperature: the so called cosmic microwave background temperature. Which gives it a temperature of roughly 3 K above absolute zero (for comparison, room temperature is 300 K). So it is hotter than a black hole.

You see where this is going? We have a temperature gradient, so there is a net heat flow inwards into the black hole. So there is a heat flow, and a carnot engine (as linked above) is able to generate useful mechanical work out of it; solar panels can produce electric power from it. We could power a civilization that way (neglecting concerns of practical feasibility, such as economics and human skill).

Just to inspire you.