Can we run out of gravitational (tidal) energy? I read an article on energy forms and sources that made me think. Energy comes from somewhere and is limited in various senses. It's most obvious for fuels: we burn coal and oil and at some point we'll have used up all that we have. 
We use solar power, and at some point in time the sun will run out of fuel and we won't have that source anymore (although we'll be dead before that, but that is not the point of this question).
Then the article mentioned tidal power, mainly as water power plants making use of the fact that the moon rotates around the earth and causes tidal waves / water movements.
How does our use of tidal power affect this "resource" ? What is there to "run out of"? Does our exploitation of tidal "gravitational" power have any effect on how long it will last?
For comparison, our use of oil has a direct effect on how long it will last, but not our use of solar power, since it will be there whether we use it or not. 
Could the tidal power theoretically be there forever, if the moon just keeps moving around the earth? 
As far as I know, the moon is slowly moving away from earth and at some point will leave its orbit. So that would be the end of tidal power in this form. 
Sorry if there are several questions in one, but it all belongs together.
 A: The moon does work on the Earth because the Moon orbits around the Earth at a different speed than the Earth's rotation. So, as time passes, different parts of the Earth feel the maximum strength of the Moon's pull, leading to oscillations in the height of the Earth's surface, which can be exploited for energy extraction.
The Earth has always been extracting energy from the difference between the Moon's orbital period and the Earth's rotational period. Whatever energy humans extract will be tiny compared to natural tidal forces. There are two side effects of the changing tides: (1) the Earth's rotation is slowed down, and (2) the Moon is pushed into a higher orbit further from the Earth. Eventually, the Earth's rotation will slow down so much that one rotation will take the same time as a Moon orbit (this is called tidal locking). At this point, no more energy can be extracted from tidal forces because the Earth's surface is no longer changing with time. This will not happen for a very long time since the length of Earth's rotation is increasing by about 2 milliseconds per century.
A: Ultimately, the short answer is yes.  
The moon is slowly moving away from the Earth, at a rate of 4 centimeters per year.  This is due to it being in a slightly higher orbit than equilibrium.  
At the same time, the Earth, being a satellite of the sun, is having its rotation rate slowed, until it will eventually be tidally locked with the sun.
Both of these effects, however, happen on a timescale that is longer than the age our sun will likely attain.  This means that, although the Earth will one day no longer have tides, from either the sun or the moon, it will not happen before the sun expands into a red giant and consumes the Earth/Moon system.
A: It is all question of scale. If we could harness tidal energy at large scale and more effectively it would slow down the earth faster but at the scales practical now it is not even comparable to topographic changes due to erosion of the continental shelves which brake the tides.
Shifting of tectonic plates and mountain ranges could affect the balance of earth as a perfect sphere and change the pull of moon more.
Inconsistency of density of top layers of earth act like ballast to change the rhythm of tides.
