Why We Did It

Official Opening

Technical Details

       
In 2007 the Town Mill completed a major project to install a micro hydro-electric system at the mill. The system generates 'green' electricity using the River Lim, our renewable resource. This enables us to make more efficient use of the water that already drives the waterwheel and millstones of this old flour mill that dates from 1340. Some of the electricity generated by the hydro system is used in the Town Mill, whilst the rest is sold into the National Grid.

Making Better Use of Renewable Resources.  In 2004, in response to Government energy policies that encourage electricity generation from renewable resources as a means of reducing national CO₂ emissions, the Town Mill Trustees began to investigate using water from the River Lim to produce 'green' electricity, whilst still retaining the watermill’s  traditional flour-making capability.

Checking Feasibility.A Feasibility Study carried out in 2005 showed that a micro hydro-electric system would be viable and would not affect the waterwheel and flour milling.

Project Sponsorship.  Towards the end of 2005 the Town Mill's Hydro Project Team obtained funding for the project from Clearskies Renewable Energy Grants  (part of the DTI), the EDF Energy Green Fund and the Town Mill Trust.

Selecting Engineers.  The funding from the project sponsors enabled the Mill Trustees in 2006 to appoint HydroGeneration Ltd (now part of Segen.co.uk) as the Consultant Engineers to this turn-key project.

The hydro-electric system was designed to fit into the disused wheel pit on the outside of the north wall of the mill building, which is a listed structure. This design solution avoided any structural alterations to the listed building. The location also provides easy access to the water supply from the leat (mill stream) and its discharge into the existing tailrace, already in use for the waterwheel.  

Water from the leat is collected in a header tank and then fed through a penstock (feed pipe) to drive a water turbine. The water is returned to the river through the draft tube (exhaust). The rotating water turbine is then used to drive an induction generator through a pulley and belt system to generate our 'green' electricity. The system is fully automatic and operates around the clock.

Although the main aim of the project was to generate electricity with a micro hydro-electric system, another important aim was to ensure that the system could be used as an educational tool for mill visitors, to demonstrate the combination of modern technology and a renewable energy water source in the generation of electricity. The new digital technology of the hydro-electric system also provides a valuable and interesting comparison with the existing old technology of the working waterwheel, with its cogs and millstones.

The hydro-electric system was officially opened on 1 March 2007, when children from three local schools - Woodroffe School, St Michael's Primary School and Mrs Ethelston's Primary School - operated two sluices to direct river water into the hydro system before the water turbine was switched on to start producing our 'green' electricity.

Official Opening
1 March 2007

The mill had used its waterpower to produce DC electricity in the 1930s, but had to take out the waterwheel in order to find space  for the hydro system. That DC system was removed in 1948 after the National Grid started operating with AC electricity. Sixty years later, the mill is back in the business of making electricity, whilst still being able to use its replacement Victorian waterwheel to make stone-ground flour. The 'green' electricity the Town Mill produces is estimated to save about 13 tons of CO₂ annually.

Establishing System Viability

Data on River Lim flow rates at the mill indicated an annual average flow rate of 0.24 cubic metres per second. This flow, combined with modern hydro technology and a head of water of 4.03 metres, suggested that a Town Mill micro hydro-electric system could produce about 32,000kW annually, with CO₂ savings of about 13 tons annually.

The 2005 Feasibility Study also concluded that operating  the proposed system would not interfere with the watermill's primary role of flour milling.

Overview.  The disused wheelpit on the outside of the north wall of the mill building was selected as the most functionally suitable site for the hydro-electric system.

The 'hydro' part of the system consists of a water tank, 6ft long, 5ft wide and 3ft deep,  that collects river water from the leat (mill stream) to drive a water turbine via a 20 inch diameter penstock (feed pipe). The water returns to the river via the mill tailrace.

The 'electric' side of the system is a generator, driven through a pulley-belt system by the turbine, and a digital control box that controls both the water turbine speed and the generator output so that it is correctly phased and fed to the Mill and the National Grid.

The Water Turbine.  The heart of the system is a German-built Ossberger cross-flow water turbine, designed specifically for this site, with a maximum power output of 7.07kVA. Its split-flow 300mm rotor, operating at 245rpm (max), is digitally controlled through two actuators to optimise power output whilst maintaining synchronised speed.

The Generator.  The water turbine drives a general purpose Brook Crompton 3-phase induction generator. The 4-pole, 50Hz system operates at up to 1500rpm, is rated to 15kW and is modified to match the digital grid connection unit.

Digital Control.  This is provided by an Excitation and Mains Connection Unit produced by Sustainable Control Systems Ltd.
It uses a water-level  sensor on the system's water tank to control and optimise the speed of the turbine's split rotor through the turbines two actuators.

In addition, its G83-standard digital control system enables automatic synchronization and connection of the mill’s 'green' electricity to the National Grid through an export meter.

The fully automatic system operates unmanned round the clock and has an auto cut-out and re-start facility to cope with any unusual conditions in the hydro system itself or in the National Grid.

Protecting the Components.  The water turbine, the generator and the digital control system, are all located inside the stairwells of the mill, between the Stone Floor and the Meal Floor. This positioning was selected so that the system has some weather protection but can still be seen easily by visitors to the mill.

The system was designed specifically to avoid making changes to the mill building, as it is a listed structure. However, work was required in the chosen location of the old wheelpit area and this involved three major tasks in late 2006:-

Construction of a sump in the tail race for the turbine draft tube (to act as an air lock and increase the differential head).

Installation of steel supports  at the end of the leat and in the tail race to hold the system components in position.

Opening of the leat end-wall to feed the water tank.

Installation of the water tank started in November 2006. Its design incorporates a 0.5inch-spacing metal bar screen to protect the system from river debris and to prevent fish from entering the system.

Next, the water turbine was fitted onto its supporting steelwork, after which the the penstock, the pipe that feeds the water from the water tank to the turbine, was connected. This completed the 'hydro' side of the system.

The induction generator was installed behind the water turbine in February 2007 and the pulley and belt drive connected. The digital control system, final electrical connections and metering systems, were the last system parts to be fitted.

Initial Testing. Testing of the system started on 12 February. Water was fed to the turbine for the first time at 1200 on 14 February 2007, a fitting present for Valentine's Day for the HydroGeneration engineers and the Town Mill Project Team.

System Handover. Full system commissioning and trial running of the turbine continued until the system was formally switched on by children from local schools on 1st March 2007. The project had taken 28 months from concept to completion.

The Town Mill
Mill Lane, Lyme Regis, Dorset DT7 3PU, England
Tel: 01297 443579     e-mail info@townmill.org.uk