Article by Mike Hield
IntroductionNormally a report on a talk is done after the event but in the case of the talk on "The Severn Barrage" I thought a preliminary briefing would be of interest. My own interest arises from a career in SWEB as an electrical distribution engineer and my leisure activity as a dinghy sailor and yachtsman.
HistoryMan used water power as long ago as 200 BC for grain milling and water pumping, around 1100 AD for "Fulling" woollen cloth and later for processing metals. From about 1700 mathematicians and engineers started to analyse the workings of the water wheel and came to realise that the weight of water in the wheel was more significant than the impact from the flow. Isaac Newton (1642-1727) established his Second Law of Motion - i.e. Force is equal to rate of change of Momentum. Leonhard Euler (1707-1783) a Swiss mathematician developed his equation of motion for non viscous flow. Daniel Bernoulli (1700-1782) defined three forms of energy in a fluid ie. height, velocity and pressure; these being interchangeable and the total constant. These ideas formed the basis for analysing the performance of turbines, fans and pumps.
Tidal Mills were very rare as they needed to be away from damaging waves and also the relative small size of the mills made them impracticable for large tidal ranges. In 1779 a mill existed on the R.Trym about a third of a mile from its junction with the R.Avon. Hence Sea Mills as the area is known today, but the original name may have been Saye Mills. Saye being a fine serge like cloth which was the product of the mill. Over on the East coast a fine old tide mill is still working at Woodbridge, Suffolk on the R.Deben. In Wales on the upper north-eastern arm of Millford Haven there is a large tidal mill on the Eastern Cleddau at Blackpool east of Haverfordwest. This mill also benefits from the river running down from Mynydd Presseli hills. All these mills are in very well sheltered positions.
The first water turbine, as opposed to an open water wheel, was developed in France in about 1800 by Benoit Fourneyron. The important difference was that all the blades contributed to the energy all the time, the turbine runner ran completely submerged and the power was delivered by a fast rotating vertical shaft.
James Thomson, brother of Lord Kelvin, invented the Vortex turbine and patented it in 1850. It had adjustable guide vanes and the blades were curved. It was a highly effective and efficient turbine.
In Devon from 1906 until 1934 water wheels or turbines were installed at Buckfastleigh, Ivybridge, Holsworthy, Tavistock, Mary Tavey and Morwellham. Scotland benefits from abundant rainfall collected in lakes and rivers at high altitude. In 1896 the British Aluminium Co. Installed five water driven d.c. generators totalling 3750 kw. at the Falls of Foyers and in 1909 11 Pelton wheel generators at Kinlochleven totalling 25725 kw. Aluminium in those early days was almost a precious metal.
In 1932 a proposal was made for a Severn Barrage upstream from Avonmouth. In 1943 A.T.Starr in his text book "Electric Power" wrote
"The Severn Barrage which was estimated to be capable of producing 500,000 h.p. ( 373 MW ) during a 10 hour day with a peak capacity of 1000,000 h.p. In order to overcome the disadvantage of ebb and flow there were to be two reservoirs involving pumping during high level periods. The large cost of the necessary constructional work has prevented as yet the exploitation of the scheme."
In the 1950's the CEGB began to consider the idea of pumped storage whereby water was pumped to a high level at times of low electrical demand to be used to drive generators to meet sudden peak demands on the system at other times. Dinorwig in Wales was opened in 1984 and all the plant is installed inside a mountain and is the largest of its kind with 6 x 340 MW turbine/generators - motors/pumps.
The Rance Barrage in N.W. France near St.Malo was completed in 1967 and comprises 24 10MW bulb type turbines - total 240MW. The turbines are of the axial flow type and generate on both flood and ebb tides.
Time and Tide
Range is important for tidal barrages and is the difference in height between successive High and Low waters.
Tides arise from the gravitational pull of the moon and sun on the major oceans of the world. The biggest tides occur when the sun, moon and earth are on the same axis and are called spring tides and occur about every two weeks. In the intervening week the sun - world - moon axis become at right angles resulting in a reduced gravitational pull on the oceans. These tides are known as neap tides and the range is about 64% of the spring range. In the Bristol Channel spring tides occur in the evening and early morning and neaps at midday and midnight.
Tidal predictions are for average conditions and in practice can be affected by meteorological conditions such as atmospheric pressure and gales.
So why are tides in some locations much higher/lower than the general levels? The main reason is the shore configuration e.g. the coast of South Wales and England together with the shelving sea bottom. The narrowing and shelving would not affect the level if the rate of rise was very slow but for the momentum of the thousands of tons of water entering at speeds up to 4 or 5 knots. Other effects occur in long closed estuaries and channels which have a natural wave resonance period which when coinciding with the tidal period gives a large amplification of the tide.
Together with changes in tidal heights come tidal currents or tidal streams. Off Portishead streams reach 4.8 knots at springs and 2.6 knots at neap tides. Near Lynmouth on the N.Devon coast, streams reach 4 and 1.9 knots, and at this location a marine current turbine of 300 Kw has recently been installed.
For those unfamiliar with the definition of a knot it is a speed of 1 nautical mile per hour - approx. 1.15 mph. or 0.5 metres per second.
With the introduction of Marine Current turbines which are still under development there are now four types of water turbine
Reaction Turbines - Francis and Kaplan
The Reaction concept is not as obvious as that of the impulse concept of the Pelton Wheel. The water approaches a set of curved blades mounted on a shaft and glides over them thereby changing direction and so imparting pressure on the blades due to centrifugal force, i.e. the force experienced by a passenger in a car when turning very fast. The water enters the blades nearly at a tangent and for the highest efficiency leaves the blades radially and at a reduced velocity
The Type number enables various forms of turbine to be classified. The lower the number the higher the speed of rotation.
Pelton wheels are used for high heads and low volumes and most early turbines were of this type. The high speed is well suited to electric generation.
Francis turbines are used for medium heads and are the type usually used for pump storage schemes.
Kaplans can be used for very low heads and large volumes and are the type proposed for the Severn Barrage.
Marine current turbines are under development.