The EVANS Tidal Power REEF System is a totally new concept in tidal power generation with the prospect of being environmentally benign while still allowing the passage of ships and generating as much or even greater amounts of electricity than the mammoth Cardiff-Weston barrage, often thought to be the only 'Severn Tidal Project'. The REEF system is 'an operating system' and can actually be applied to any location in the Severn estuary.
Unlike other monolithic tidal barrages designed to hold back the full height of the tide (the second highest in the World) the 'REEF' works with only two or three metres of fall (head difference) but 'harvests' the energy over a much longer generation period making it much easier to integrate into the National Grid system and to match the peaks in electricity demand.
Tidal stream turbines (we built the first prototype 18 years ago!) extract far less energy and are very expensive to maintain, which is why it evolved into a 'Tidal Fence', and then into the REEF. With the REEF system, the full range of the tide is harnessed by a large number of simple low-head turbines that make up about half the total cross-sectional area of the channel. It is still massive but more economical in materials than a conventional barrage or the Lagoon.
The twelve-mile route from near Minehead in Somerset to Aberthaw in Wales generates the most power whilst avoiding the bulk of the silt moving up and down the estuary with each tide. The location and small difference in level between the sea and the estuary is critical to shipping, which can pass through opening sections of the REEF on their way up to Avonmouth avoiding expensive delays associated with conventional locks.
The small difference in level also facilitates the safe passage of salmon and other fish species through the special turbines, something that is not possible with conventional turbines with their high pressure drop and sharp edged high-speed runners (rotors). There is nothing to suggest that there should be any more risk (possibly less) from a salmon passing through a turbine than from ascending a natural obstacle of similar height in a river such as a set of rapids, with sharp rocks and high turbulence.
A unique feature of the REEF is the 'Active Tidal Control System' that allows the modification of the water levels during each tidal cycle. The opportunity to swing the generation peaks to match power demand, and the ability to artificially lower the natural levels to reduce flooding, are but two of the possible benefits. A sophisticated computer system will be required to optimize power generation alongside inputs from other renewables, and take metrological and river level information to predict either 'storm surges' or river flooding. Meeting the need to maintain water navigation depths and protect vulnerable wildlife habitats such as salt marshes are also a basic requirements in designing the system.
The 'Tidal Power REEF System' offers more power output, less adverse environmental impacts, lower cost and a genuine benefit to the Nation in terms of ownership, security, employment and a moral legacy for future generations.