An understanding of the principles which give rise to tides
is essential to explain tidal power
Whilst a thorough understanding of the interactions involved is quite complex, the origin of tides can be explained in general terms by investigating the gravitational effects of the Moon and the Sun on the ocean and the effect of centrifugal forces.
The interaction of the Moon and the Earth results in the oceans bulging out towards the Moon, whilst on the opposite side the gravitational effect is partly shielded by the Earth resulting in a slightly smaller interaction and the oceans on that side bulge out away from the Moon, due to centrifugal forces. This is known as the Lunar Tide. This is complicated by the gravitational interaction of the Sun which results in the same effect of bulging towards and away from the Sun on facing and opposing sides of the Earth. This is known as the Solar Tide.
As the Sun and Moon are not in fixed positions in the celestial sphere, but change position with respect to each other, their influence on the tidal range (difference between low and high tide) is also effected. For example, when the Moon and the Sun are in the same plane as the Earth, the tidal range is the superposition of the range due to the lunar and solar tides. This results in the maximum tidal range (spring tides). Alternatively when they are at right angles to each other, lower tidal differences are experience resulting in neap tides.