The village of Mapledurham is adjacent to that of Caversham where I live, near Reading in the English south east, and is a pleasant 4 mile walk from here tracing the banks of the River Thames. Mapledurham House is a beautiful Grade 1 listed manor house, set on the Mapledurham Estate which holds the last commercial working water mill on the river. The estate has belonged to the Blount family since 1492. Later, the Blount sisters Teresa and Martha became friends of the poet Alexander Pope to whom he sent one of his most famous poems, "The Rape of the Lock".
There has been a turbine at Mapledurham since the 1920s which provided electricity to the manor house for many years but is no longer operational. This, however, has been replaced by a brand new turbine based on an Archimedes screw design and at full-power generates almost 100 kW. Of this, some 3 kW are drawn-off to run and control the installation. The output is not constant of course and the device is automatically turned-off when the lock is used and the water level falls, then it ramps back up to 85 - 95% of full capacity. Over a year it is thought that some 500,000 kWh of electricity will be produced at a value of £100,000 which will be sold-on to the energy company Npower.
I have been told that over a 20 year period, the project should generate around £1 million profit, which will be used to restore the listed mill buildings, and thus it appears as a sound business investment. The installation capital, provided by the Mapledurham estate is around £650,000, including interest. The electricity will be bought by the chain Marks and Spencer, who are keen to increase their use of green energy. The Mapledurham Estate is also investigating the use of cow manure to generate biogas, since they have a dairy herd, which also supplies Marks and Spencer.
The turbine itself weighs 24.7 tonnes and has a diameter of 3.5 metres. Its dimensions are such that an eight foot railway sleeper should be able to pass through it without causing damage, and obviously anything up to that size coming from further upstream. The length of the turbine has been truncated so that it fits within the limited space available to it at the mill, which has marginally reduced its maximum power output from around 120 kW.
From an ecological perspective, the choice of the design means that fish too will be able to swim through the turbine without coming to harm. Further down river on the weir at Caversham Lock is a fish-ladder to allow fish similar safe passage. The banks of the "pond" into which the water flows through the turbine will need to be reinforced, because of the phenomenon of resonance. When the turbine is operating at high capacity there is little problem, but when that drops below about 30%, and particularly down to around 10%, a wake is created that washes-away the banks of the pond. These are being reinforced with a steel barrier, and it is intended to place boulders into the pond partly for aesthetic reasons and I imagine that also the wake will be broken-up by flowing around them.
There are two turbines to be installed below Windsor Castle, when they are brought over from The Netherlands, and there is a smaller 18.5 kW hydroelectric generator which provides power for the Mill Theatre at Sonning, some four miles further down river from Caversham, with any excess electricity going onto the national grid.
It seems sensible to extract far more power from the rivers in this way, appearing as a "green" source. However, the installation of such turbines will in many locations necessitate considerable engineering and the adaptation, e.g. of weirs to place them. Effects on river-flow and local ecology must also be considered, but this is a good example of using a local advantage, e.g. a river to provide energy at the local level. In order for Transition Towns to emerge fully as resilient local communities that generate much of their own energy, such developments should be encouraged.
That noted, the turbine at Mapledurham is expected to run for 10 years before maintenance is required, but then the "black box" that is the generator needs to be serviced or replaced. The mechanical parts of the turbine, e.g. bearings, are expected to last for 20 years, but keeping the installation running will probably still depend on parts brought in from elsewhere and by some means of transportation, which currently would be powered by liquid fuel. It is debatable what forms of transport will be available to us in the decades to come and hence the viability of technology that cannot be produced and serviced by local hands.
A video of the turbine in operation may be viewed on Vimeo