Abstract
Process and Technology Status – There are three categories of tidal energy technologies. The first category, tidal range technologies use a barrage – a dam or other barrier – to harvest power from the height difference between high and low tide. The power is generated through tidal turbines (most of them come from hydropower design, such as bulb turbines) located in the barrage, and their commercial feasibility has been well established through the operation of plants in France (240 Megawatts (MW)), Canada (20 MW), China (~5 MW) and Russia (0.4 MW) from the 1960s and 1970s. In 2011/2012, South Korea opened the largest and newest tidal barrage (254 MW). New technologies developed for tidal range power generation are tidal ‘lagoons’, tidal ‘reefs’, and tidal ‘fences’, and low-head tidal barrages. The second category, tidal current or tidal stream technologies have had more than 40 new devices introduced between the period 2006-2013. The major differences among the devices are the turbines, which can be based on a vertical or horizontal axis, and in some cases are enclosed (ducted). Full-scale deployment of single turbines have been achieved, and the next step is the demonstration of arrays of turbines (Energy Technologies Institute (ETI)/ UK Energy Research Centre (UKERC), 2014). Up to 2010, the industry was dominated by small entrepreneurial companies, but in the last three years large engineering firms and turbine manufacturers like ABB, Alstom, Andritz Hydro, DCNS, Hyundai Heavy Industries, Kawasaki Heavy Industries, Siemens, and Voith Hydro have entered the market. Furthermore, companies like General Electric (GE) have also shown an interest and are supplying the electrical power systems for some of the prototypes. Also, large utilities like Bord Gáis Energy, Électricité de France (EDF), GDF Suez, and Iberdrola are running demonstration projects. Some tidal current or tidal stream technologies are also used to harvest ocean currents. Compared to tidal currents, ocean currents are unidirectional and generally slower but more continuous. Ocean current technologies are in an early developmental stage, and no full-scale prototype has been tested or demonstrated yet. The final category, hybrid applications are forms of tidal range technologies that have great potential if their design and deployment can be combined with the planning and design of new infrastructure for coastal zones. Project proposals for hybrid applications exist in Canada (British Columbia), China, the Netherlands (Grevelingen), Norway (E39 road project) and the UK (Bristol Channel). Furthermore, there are plans for a hybrid form of tidal range and current power generation called ‘dynamic tidal power’. Again, no full-scale prototype has been tested or demonstrated yet.
Potential – Worldwide, the technically harvestable tidal energy resource from those areas close to the coast, is estimated by several sources at 1 terawatts (TW). The potential for tidal current technologies is larger than for tidal range. Total tidal range deployment in 2012 was around 514 MW, and around 6 MW for tidal current (of which 5 MW is deployed in the UK). Extensive plans exist for tidal barrage projects in India, Korea, the Philippines and Russia adding up to around 115 gigawatts (GW). Deployment projections for tidal current up to 2020 are in the range of 200 MW. An advantage of both tidal range and tidal current energy is that they are relatively predictable with daily, bi-weekly, biannual and even annual cycles over a longer time span of a number of years. Energy can be generated both day and night. Furthermore, tidal range is hardly influenced by weather conditions.
Cost indications – Tidal range power generation is dominated by two large plants in operation, the ‘La Rance’ barrage in France and the ‘Sihwa dam’ in South Korea. The construction costs for ‘La Rance’ were around USD 340 per kilowatt (/kW) (2012 value; commissioned in 1966), whilst the Sihwa barrage was constructed for USD 117/kW in 2011. The latter used an existing dam for the construction of the power generation technology. The construction cost estimates for proposed tidal barrages range between USD 150/kW in Asia to around USD 800/kW in the UK, but are very site specific. Electricity production costs for ‘La Rance’ and ‘Sihwa Dam’ are EUR 0.04 per kilowatt-hour (/kWh) and EUR 0.02/kWh, however these costs are very site specific. Tidal range technologies can be used for coastal projection or water management, which would reduce the upfront costs. On the other hand, additional operational costs may occur due to the control, monitoring and management of the ecological status within the impoundment. Tidal current technologies are still in the demonstration stage, so cost estimates are projected to decrease with deployment. Estimates from across a number of European studies for 2020 for current tidal technologies are between EUR 0.17/kWh and EUR 0.23/kWh, although current demonstration projects suggest the levelised cost of energy (LCOE) to be in the range of EUR 0.25-0.47/kWh. It is important to note that costs should not be considered as a single performance indicator for tidal energy. For example, the costs for both tidal range and tidal stream technologies can fall by up to 40% in cases where they are combined and integrated in the design and construction of existing or new infrastructure.
Barriers and drivers – The greatest barrier to tidal range technology advances are the relatively high upfront costs related to the developments of the dykes or embankments, and the ecological implications of enclosures or impoundments. Moreover due to tidal cycles and turbine efficiency, the load factor of a conventional tidal barrage is around 25%, which leads to high cost of energy. Improvement in turbine efficiency, in particular innovative reversible turbines for ebb and flood generation, should provide a significant increase in energy yield. One important new avenue is the use of tidal range applications to promote ecological improvement. In all these solutions (e.g., in the case of the Sihwa barrage or potentially in case of the Grevelingen lake in the Netherlands), the installation of tidal range technology leads to several important societal benefits besides renewable energy. These include flood defence, improved environmental and ecological water quality, and fisheries and tourism functions. An important new application for tidal range energy under development is one which is focused on harvesting energy from low head tidal differences of less than 2 metres (m). For tidal stream technologies, continued support for demonstration and grid connection of larger scale arrays will be critical. With these experiences, the materials, operation and maintenance costs can be improved. Furthermore, high installation costs of both tidal range and tidal current solutions need to be overcome through capital investments, aiding commercialisation, feed-in tariffs or investment mechanisms in innovation. The simultaneous research and development of new infrastructure of flood defences, coastal restructuring, bridge and road construction, also offer opportunities to advance tidal energy technologies.