... you aren't envisioning a giant tunnel in space that the moon would pass through, are you? Like the crude drawing below?

1. We don't have the materials strong enough to attach it to earth with.
2. We don't have enough fuel to launch that into space
3. The turbines would... collide with the surface of the moon to generate electricity?
4. Even if all this worked, each collision / "passing through" would add orbital velocity to the lunar orbit (attached to earth's surface, it would translate some of earth's rotational energy into lunar orbital energy) and thus we'd lose the moon even faster and you'd need to keep the 'turbine' constantly adjusted for the altitude of each encounter.
5. The amount of energy you would generate before the Moon gained escape velocity would only be a small fraction of the energy it would take to launch, build in orbit and maintain said Moon Turbine.

Since you asked about total possible energy, if the moon were treated as if it were on a rail (it's not) and the materials could withstand a collision (they can't) you could potentially harness

$2.096x10^{29} Joules$

If by slowing the rotation of the Earth down to the angular velocity of the moon. Also probably cause a slew of natural (artificial in this case?) disasters.

... you aren't envisioning a giant tunnel in space that the moon would pass through, are you? Like the crude drawing below?

1. We don't have the materials strong enough to attach it to earth with.
2. We don't have enough fuel to launch that into space
3. The turbines would... collide with the surface of the moon to generate electricity?
4. Even if all this worked, each collision / "passing through" would add orbital velocity to the lunar orbit (attached to earth's surface, it would translate some of earth's rotational energy into lunar orbital energy) and thus we'd lose the moon even faster and you'd need to keep the 'turbine' constantly adjusted for the altitude of each encounter.
5. The amount of energy you would generate before the Moon gained escape velocity would only be a small fraction of the energy it would take to launch, build in orbit and maintain said Moon Turbine.

Since you asked about total possible energy, if the moon were treated as if it were on a rail (it's not) and the materials could withstand a collision (they can't) you could potentially harness

$2.096x10^{29} Joules$

by slowing the rotation of the Earth down to the angular velocity of the moon. Also probably cause a slew of natural (artificial in this case?) disasters.

... you aren't envisioning a giant tunnel in space that the moon would pass through, are you? Like the crude drawing below?

1. We don't have the materials strong enough to attach it to earth with.
2. We don't have enough fuel to launch that into space
3. The turbines would... collide with the surface of the moon to generate electricity?
4. Even if all this worked, each collision / "passing through" would add orbital velocity to the lunar orbit (attached to earth's surface, it would translate some of earth's rotational energy into lunar orbital energy) and thus we'd lose the moon even faster and you'd need to keep the 'turbine' constantly adjusted for the altitude of each encounter.
5. The amount of energy you would generate before the Moon gained escape velocity would only be a small fraction of the energy it would take to launch, build in orbit and maintain said Moon Turbine.

... you aren't envisioning a giant tunnel in space that the moon would pass through, are you? Like the crude drawing below?

1. We don't have the materials strong enough to attach it to earth with.
2. We don't have enough fuel to launch that into space
3. The turbines would... collide with the surface of the moon to generate electricity?
4. Even if all this worked, each collision / "passing through" would add orbital velocity to the lunar orbit (attached to earth's surface, it would translate some of earth's rotational energy into lunar orbital energy) and thus we'd lose the moon even faster and you'd need to keep the 'turbine' constantly adjusted for the altitude of each encounter.
5. The amount of energy you would generate before the Moon gained escape velocity would only be a small fraction of the energy it would take to launch, build in orbit and maintain said Moon Turbine.

Since you asked about total possible energy, if the moon were treated as if it were on a rail (it's not) and the materials could withstand a collision (they can't) you could potentially harness

$2.096x10^{29} Joules$

If by slowing the rotation of the Earth down to the angular velocity of the moon. Also probably cause a slew of natural (artificial in this case?) disasters.