1. Quick Summary
Tidal energy exploits the predictable rise and fall, or the flow, of water driven by gravitational interaction with the Moon and Sun. Wave energy extracts energy from wind-generated surface waves.
Both are attractive because they are predictable and dense: water is far heavier than air, so a modest flow carries substantial power.
Both are difficult because the marine environment combines corrosion, biofouling, storm loading and difficult access in a way no land-based technology faces.
2. What It Means
Tidal range schemes impound water behind a barrage and release it through turbines, like a low-head hydro installation. Tidal stream devices place turbines in fast-flowing channels, closer in concept to underwater wind turbines.
Power in a flow rises with the cube of speed. A site with double the current speed offers about eight times the power, which is why site selection dominates everything else.
Wave devices convert the motion of the surface into mechanical or pneumatic motion. Designs differ widely, reflecting that the resource is irregular in both height and period.
Capacity factor is high and predictable for tides. Unlike wind and solar, output can be forecast years ahead, which is a distinct grid advantage.
3. Why It Happens
Extreme loads dominate design. A device sized for average conditions will be destroyed by a once-in-decades storm, so structure is sized for the extreme and spends most of its life under-used.
Corrosion and biofouling are continuous. Salt water attacks metals and concrete, and organisms colonise surfaces, changing hydrodynamic performance and accelerating wear.
Maintenance access is expensive. Working underwater or in strong currents requires vessels and weather windows, so a small failure can cost more to fix than the component.
The resource is site-specific. Only a limited number of locations combine high flow or good wave climate with reasonable grid connection and acceptable environmental impact.
Environmental effects are local and hard to generalise. Changing flow and sediment transport affects estuary ecology, which is why each proposal is assessed on its own terms.
4. Real Examples
Tidal barrages have operated for decades at a small number of sites, showing the technology works while also demonstrating high capital cost and ecological trade-offs.
Tidal stream turbines in narrow channels with strong currents have run for extended periods, and the main remaining challenge is cost per unit of energy rather than feasibility.
Wave energy has produced many distinct device concepts, and the diversity itself is a signal that no single approach has yet proven dominant.
Hybrid projects that co-locate with offshore wind share grid connection and vessels, reducing the cost of both.
5. How It Affects Us
Predictability is the strongest grid argument: tidal generation can be scheduled, unlike wind or solar.
Coastal communities and fisheries are the most directly affected stakeholders, and their concerns often determine whether a project proceeds.
Cost remains well above mature renewables, so deployment depends on support mechanisms or on niche applications such as remote islands.
Supply chains overlap with offshore wind and marine engineering, which lowers the barrier for countries with existing maritime industry.
6. Key Takeaways
- Tides are predictable; waves are energetic but irregular. Both are far denser than wind.
- The engineering challenge is surviving the marine environment, not generating electricity.
- Site quality dominates economics, because power scales with the cube of flow speed.
- Environmental assessment is inherently local and cannot be settled by global averages.