Table of Contents
Introduction
Tidal energy uses the regular rise and fall of sea levels and the movement of tidal currents to generate useful power. Unlike wind or solar, tides are driven mainly by the gravitational interaction between Earth, the Moon, and the Sun, which gives tidal energy a special degree of predictability. This chapter focuses on the physical and technical principles that underlie tidal energy, without going into detailed technology types, environmental issues, or wider marine energy topics, which are covered elsewhere.
Origin Of Tides
Tides originate from gravitational forces and the rotation of the Earth. The Moon exerts a gravitational pull on the oceans, creating a bulge of water that we experience as a high tide. On the opposite side of the Earth, inertia creates another bulge. As the Earth rotates, coastlines move through these bulges, giving two high tides and two low tides per day in many locations.
The Sun also exerts gravity, which interacts with the Moon’s pull. When the Sun, Moon, and Earth are roughly aligned, their forces reinforce each other and produce higher high tides and lower low tides. These are called spring tides and occur around full and new moons. When the Sun and Moon are at right angles relative to the Earth, their effects partially cancel and give neap tides, which have a smaller tidal range.
The combination of gravity, Earth’s rotation, ocean basin shapes, and local coastal geography means that tidal patterns vary significantly. Some locations have two roughly equal high tides per day (semidiurnal), others have one dominant high tide (diurnal), and some have a mix (mixed tides). For tidal energy, what matters most is how much the sea level changes and how fast the water flows.
Tidal Range And Tidal Currents
Two related but distinct aspects of tides are central to tidal energy: tidal range and tidal currents.
Tidal range is the vertical difference in water level between high tide and low tide. Some areas have ranges of less than a meter, while others, such as certain estuaries and bays, can exceed 10 meters. High tidal ranges provide the potential to store water at elevated levels and then release it through turbines.
Tidal currents are the horizontal flows of water as the sea level rises and falls. When tides come in, water flows toward the coast or into bays and channels. When tides go out, water flows back to the open sea. In constricted passages such as straits or between islands, these flows can become very strong. Strong tidal currents provide kinetic energy that can be converted to electricity using underwater turbines.
The link between range and currents is simple: a larger tidal range usually drives stronger currents, especially where the coastline and seabed focus the flow through narrow or shallow areas. However, not every large range creates usable currents, and some strong currents occur even where the range is moderate, because of local geography.
Basic Energy Principles For Tidal Power
Tidal energy systems rely on two main physical principles: the potential energy associated with water at different heights and the kinetic energy associated with moving water.
For tidal range systems, the stored potential energy is similar to that of water behind a dam. When there is a height difference between the water inside a basin and the sea outside, the system can release water through turbines and generate power. The theoretical potential energy in a unit area of water due to a height difference $h$ can be approximated by:
$$E_{\text{potential}} \approx \frac{1}{2} \rho g h^2$$
where $\rho$ is the density of seawater and $g$ is the acceleration due to gravity. This energy is usually expressed per unit area of basin.
For tidal current systems, the relevant quantity is kinetic energy in the flowing water. The power available in a tidal current flowing through an area $A$ with speed $v$ is:
$$P_{\text{kinetic}} = \frac{1}{2} \rho A v^3$$
This expression shows that water speed is crucial, because power grows with the cube of velocity. A small increase in current speed leads to a much larger increase in available power.
Key principle: Tidal current power scales with $v^3$. Doubling current speed increases theoretical power by a factor of eight, which is why site selection focuses heavily on the fastest flows.
Not all this theoretical power can be captured. In practice, conversion efficiency and physical limits on how much the flow can be slowed reduce the usable fraction. Nevertheless, these formulas guide the estimation of resource potential and help compare different sites.
Types Of Tidal Energy Concepts At The Principle Level
From a principles perspective, tidal energy concepts fall into two broad categories based on whether they exploit water level differences or moving currents.
Tidal range concepts exploit the potential energy from water at different heights. They use structures that create a controllable difference in water level between a basin and the sea, then run water through turbines while it flows to equalize levels. Basic operating modes can include generating on ebb tide, when water flows from the basin back to the sea, or on flood tide, when water flows into the basin. Some concepts store water to optimize generation timing for electricity demand, but the underlying principle remains the same, conversion of gravitational potential energy to mechanical and then electrical energy.
Tidal current concepts exploit the kinetic energy of horizontal water flow. They place turbines directly in strong tidal streams, similar to underwater wind turbines but operating in a denser fluid. As water flows past the blades, it transfers kinetic energy to the turbine rotor, which turns a generator. These systems rely purely on the natural back and forth motion of the tides and do not create a large artificial height difference. Reversible blades or yaw mechanisms often allow them to operate in both ebb and flood flows, since currents change direction each tidal cycle.
Although technology designs differ widely, all tidal concepts can be traced back to these two principles: exploiting height differences of water levels or exploiting current velocities.
Predictability And Temporal Patterns
A distinctive feature of tidal energy is its high predictability. While day to day wind and solar output depend on weather, tides follow astronomical cycles governed by celestial mechanics. Once a site’s tidal behavior is measured over some time, future tides can be forecast with high accuracy years in advance using well understood tidal harmonics.
Tidal cycles are not aligned exactly with the 24 hour day. The dominant tidal period is about 12.4 hours for semidiurnal tides. This means that the times of high and low tide shift by about 50 minutes later each day. As a result, tidal energy output follows a repeating pattern, but it drifts relative to human activity and electricity demand schedules.
The power output from tidal systems rises and falls through each cycle. For tidal current systems, power peaks around mid tide when currents are strongest and drops near slack water at high and low tide when currents are minimal. For tidal range systems, power is concentrated during the periods when water is allowed to flow through turbines from higher to lower levels. This creates a regular sequence of generating and non generating periods.
Although this variability can be a challenge for matching supply and demand, the fact that it is highly predictable helps grid operators plan. It also allows combinations of geographically dispersed tidal sites, and combinations with other renewables, to smooth overall variability.
Site Characteristics That Influence Tidal Energy Potential
The suitability of a location for tidal energy depends on several physical characteristics related to coastal and seabed conditions as well as tidal behavior.
Tidal amplitude, or range, is central for tidal range concepts. Locations with very large ranges have more potential energy per unit area. Bays and estuaries that naturally concentrate tidal range, due to resonance or funneling, are of special interest. For tidal current concepts, the focus is on current speed and how consistently high speeds occur through the year.
Coastal geometry plays a major role. Narrow straits, channels between islands, and headlands that force water into constricted pathways can create very strong tidal streams. The cross sectional area of these passages, along with how much water must pass through in each tidal cycle, determines the strength of currents. For tidal range, funnel shaped estuaries or basins that naturally trap and release water enhance energy potential.
Seabed depth and shape influence both resource and technology feasibility. Depth determines whether turbines can be installed and maintained and whether shipping can safely coexist. The slope and hardness of the seabed affect how structures can be anchored and how cables can be laid.
The type of tidal regime also matters. Semidiurnal regimes with two similar high tides per day, diurnal regimes with only one dominant high tide, and mixed regimes, each create different generation profiles. Some regimes have particularly strong spring neap contrasts, which means that output can be much higher in some periods than in others within the lunar month.
Finally, environmental and navigational conditions such as exposure to waves and storms, and proximity to shipping lanes, influence practical deployment, but the underlying energy principle remains the same, the harnessing of predictable tidal motions.
Hydrodynamic And Turbine Interaction Basics
When a tidal energy device is placed in flowing water, it interacts with the hydrodynamics of the site. The key principle is that capturing energy from the flow requires slowing it down to some extent. However, if the flow is slowed too much, less water passes through the area and overall energy capture becomes less efficient.
For tidal current turbines, the physical limit on the fraction of kinetic power that can be extracted by a single device is related to the same theoretical concepts as in wind energy. Although tidal flows interact with the seabed and surface in more complex ways, the general idea remains: an optimal extraction level exists where power is maximized without stopping the flow. Arrays of turbines add further complexity, because each device alters the flow field for its neighbors.
The inflow velocity profile, which varies from seabed to surface, and turbulence levels affect how efficiently turbines can operate and how loads are distributed on their blades and structures. Higher turbulence can increase mixing but also raises mechanical stress. Designers must balance energy capture against survivability and structural integrity, guided by basic fluid dynamics.
For tidal range systems, the interaction is more about modifying the natural tidal wave as it moves into and out of a basin. Structures that hold back water and then release it change local height and phase of tides. From a principle perspective, this is an intentional modification of the natural tidal hydrodynamics to create useful head for power generation, at the cost of altering the timing and magnitude of water level changes.
Comparison With Other Renewable Resources From A Principle Perspective
At the level of basic principles, tidal energy stands between hydropower and wind energy. Like hydropower, it often uses potential energy due to water height differences and benefits from the high density of water compared to air. At the same time, tidal current systems resemble wind turbines operating underwater, using the kinetic energy of moving fluid flows.
A crucial distinction is predictability. The driving forces of wind and sunlight fluctuate with weather and cloud patterns, while tidal forces follow astronomical cycles. This makes tidal output highly forecastable, even if it is variable over daily and monthly scales.
Another distinctive feature is bidirectional flow. Most tidal current systems must operate with currents that reverse direction every half cycle. This has implications for turbine design and control, but the underlying principle is straightforward: the same moving water can be harnessed in both directions because kinetic energy exists whenever the fluid has speed, regardless of its direction.
Finally, tidal energy is geographically constrained by coastal and marine environments with suitable tidal conditions. The basic principles of gravitational forcing and fluid flow apply everywhere, but only some locations concentrate these effects enough to provide a usable energy resource.
Summary Of Core Principles
Tidal energy is built on a few key ideas. Gravitational interactions among Earth, Moon, and Sun create regular, predictable tides. These tides manifest both as vertical water level changes, which provide potential energy, and as horizontal currents, which provide kinetic energy. Tidal range concepts focus on exploiting differences in water height across barriers or basins, while tidal current concepts focus on capturing the energy in flowing tidal streams with underwater turbines.
The amount of energy available depends strongly on tidal range, current speed, and local coastal geometry. Power from currents scales with the cube of flow velocity, so fast tidal streams are especially valuable. Tidal cycles are variable in time but follow patterns that can be forecast with high accuracy. When considered alongside other renewables, tidal energy is unique in its combination of high predictability and dependence on specific coastal and marine conditions, all grounded in well understood physical principles.