Tidal energy generates electricity by harnessing the predictable movement of ocean tides. This process involves large structures such as barrages, tidal stream turbines, or constructed lagoons. While tidal energy does not produce the traditional air emissions or chemical waste associated with combustion, its infrastructure causes significant, localized changes to the marine environment. The environmental challenge for tidal power is ecological disruption, not traditional pollution.
Addressing Emissions and Chemical Waste
Generating electricity from tidal energy is a purely mechanical process relying on the kinetic or potential energy of moving water. During the operational phase, tidal power plants release no greenhouse gases, such as carbon dioxide, nitrogen oxides, or sulfur oxides, into the atmosphere. This absence of combustion byproducts contrasts sharply with continuous emissions from fossil fuel power stations. The life-cycle carbon footprint of tidal energy, including manufacturing and installation, is estimated to be very low, often under 50 grams of CO2 equivalent per kilowatt-hour.
The process also generates negligible liquid or chemical waste streams during normal operation. Unlike nuclear or coal plants, there is no need for large-scale water treatment or ash disposal. Minor waste, such as spent lubricants from the turbines, is typically contained and managed during routine maintenance. This makes the day-to-day running of a tidal facility free of conventional air and water pollutants.
Alteration of Water Flow and Sedimentation
The large structures used in tidal energy systems fundamentally change the physical dynamics of the water they occupy. Tidal barrages, which are dams across an estuary or bay, create the most pronounced physical impact. These barriers block the natural tidal cycle, leading to significantly altered water levels and flow rates on the basin side.
The restricted flow reduces the natural scouring action of the tides, resulting in increased siltation and sediment build-up behind the barrier. This build-up can lead to the loss of intertidal mudflats, which are biologically rich habitats. Changing the basin’s flushing rate also affects water quality parameters like salinity and dissolved oxygen. Reduced mixing in some impoundments can lead to thermal stratification and the development of low-oxygen, or anaerobic, conditions near the seabed.
Tidal stream devices are underwater turbines placed in fast-flowing channels and have a smaller physical footprint. By extracting kinetic energy, these devices slightly slow the water flow, altering local sediment transport dynamics. This energy extraction can cause localized scour, or erosion, immediately surrounding the turbine foundations due to increased turbulence. The presence of large arrays of these turbines could potentially change the regional distribution of sediment and the stability of the seabed.
Impact on Marine Wildlife and Habitats
The most significant concerns regarding tidal energy relate to the direct and indirect consequences for marine ecosystems. Large structures like barrages act as physical barriers, severing migratory routes that fish, such as salmon and eel, and marine mammals use for breeding and feeding. This interruption can severely impact the reproductive success and population health of these species. Tidal lagoons, which are partially enclosed structures, also permanently modify the intertidal zone, destroying existing habitat for shoreline organisms.
Direct mortality is a risk, particularly for fish and diving seabirds that pass through the spinning blades of turbines. While new turbine designs incorporate slower rotation speeds and mechanisms to deter marine life, the collision risk remains a complex, site-specific challenge. Studies suggest that marine mammals, like harbor seals and porpoises, may be able to detect and avoid operational turbines, but this behavior is not guaranteed across all species.
The operation of tidal devices also introduces noise pollution into the underwater environment. Both the installation phase, which involves activities like pile driving, and the continuous operation of the turbine machinery generate acoustic energy. This underwater noise can interfere with the communication, navigation, and foraging behaviors of acoustically sensitive species, including whales and dolphins. The noise may also create a persistent acoustic barrier, functionally excluding marine life from otherwise suitable habitats.
Environmental Footprint Compared to Fossil Fuels
When evaluating the total environmental footprint, the localized ecological harm caused by tidal energy must be weighed against the widespread, global damage from fossil fuels. Fossil fuel combustion releases massive quantities of greenhouse gases that drive climate change, ocean acidification, and sea-level rise, representing a systemic threat to all ecosystems. It also produces air pollutants that cause acid rain and severe human health issues.
Tidal power offers a reliable, large-scale, and long-term source of electricity with minimal operational atmospheric emissions, despite its severe localized impacts on marine environments. Its environmental consequences are spatially concentrated and can potentially be mitigated through careful site selection and the use of modern, fish-friendly turbine technology. The transition to tidal power replaces a globally damaging energy source with one whose impacts are contained to a specific marine area. Therefore, from a global climate perspective, tidal energy represents a significant environmental improvement, provided that robust mitigation strategies are in place to protect the affected local ecology.