Wednesday, 15 April 2015

Quarry Bank Mill's Hydro Scheme

As a volunteer guide at Quarry Bank Mill in the 'Power' section (waterwheel and steam engines), the new Hydro Scheme at the mill is of interest to me. While the Great Wheel and the historic steam engines at Styal showcase the technical ingenuity of our forefathers, the Hydro Scheme demonstrates that engineering techniques have moved on apace since those pioneering days of the Greggs of Quarry Bank.

As always, please click on any picture for a larger image.

The water turbine and fish pass at Quarry Bank

The Great Wheel at Styal was in use from 1847 to 1904. It produced 100hp and as a high-breast-shot suspension wheel was state of the art at the time. That was replaced by a Gilkes water turbine which produced 200hp. Both operated with a 9m head of water achieved by a 5m weir (including a 1m extension, later removed), and a further 4m by use of a tunnel which allowed the tail water to be returned to the river about 1.5 miles downstream after it had dropped by that amount.

The Kaplan Turbine in the new Hydro Scheme cannot make use of the tail race tunnel, or the weir extension, so has a head of only 4m. It will produce about 220,000 kWh of electricity per year.

The Hydro Scheme and Fish Pass at Styal, adjacent to the weir. The turbine house housing the Kaplan turbine is the square building top right of the Hydro channel.

Water flow in a river varies throughout the year, being more in winter and less in summer. A water turbine, to be efficient, should be sized to run at 100% capacity for about 25 - 30% of the year. The turbine at Styal takes 1.8 cubic metres of water per second and is sized to be at 100% capacity (about 50kW) for 25% of the year, reduced capacity for 60% of the year, and zero output for the remainder.

In times of flood, tail water rises more quickly that head water, so available head is reduced. This can reduce the head to as little as 2m at styal. Optimal head therefore occurs at times of high constant river flow, but not flood or low flow states.

With the 4m head available at Styal and the flow rate patterns of the River Bollin, either an Archimedes Screw or Kaplan turbine could have been specified. The Archimedes Screw would have been 7m long, whereas a Kaplan is much shorter, more efficient, but needs a fish screen to prevent fish being drawn into the turbine (fish can pass safely through an Archimedes Screw, but not a Kaplan). An Archimedes Screw solution was considered at Styal, but rejected on grounds of efficiency and visual impact. The chosen solution was a Kaplan turbine mounted at the angle of the intake pipe, with the alternator mounted on top of the turbine and driven by a flat belt (an inherently efficient configuration).

At the top of the intake is the intake pit with a course screen in front of it to keep out large pieces of debris such as logs. Behind this is the fish screen with 10mm spaces between the bars, which appears as a solid obstacle to a fish and so makes it unlikely that they would approach the screen. The narrow spaces between the bars on this screen make it susceptible to blockage by small debris items, so it is fitted with an automatic cleaner to lift debris clear of the screen's grill and dump it into a gutter above the screen. The screen cleaner operates either on a timed basis or if a reduced water flow through the screen is detected. A submerged debris pump provides a flow of water to wash the removed debris out of the gutter and back into the river downstream of the turbine.

Behind the fish screen is the bell mouth of the intake pipe, fitted with a cut-off gate to prevent water entering the intake if the turbine is stopped for maintenance, or in an emergency situation. The intake pipe is 1.2m diameter and 30m long, leading down to the turbine house. Downstream of the turbine is the tail water draft tube. This is a 4-to-1 diffuser to reduce the pressure of the tail water in the draft tube (check out your Bernoulli fluid dynamic theory!) and so generate suction behind the turbine and make it more efficient. 

Diagram of a typical Kaplan turbine showing variable pitch inlet guide vanes and the variable pitch turbine blades

In a Kaplan turbine the water enters via the inlet and is spun by variable-pitch non-rotating guide vanes. It then flows onto the rotating turbine runner, which has variable pitch blades. The runner takes the spin out of the water so the flow downstream of the turbine is linear. The pitch of the inlet vanes and the turbine blades is altered by a hydraulic control system to maintain the turbine speed at 350rpm. The flat belt and pulley drive to the alternator maintains that at 1000rpm to give an AC power output at 50Hz, to match main grid frequency. The control panel monitors river levels and turbine rpm and adjusts the turbine accordingly. In the event of a power failure or hydraulic system malfunction, the vanes close under spring tension, the turbine blades go to full-fine pitch, and the turbine slows and stops; it is fail safe.

There is no power generator more efficient than a water turbine. They can reach 94% efficiency; the relatively small turbine at Styal is 86% efficient. By contrast, wind turbines are 40% efficient at best, solar panels 60%. The entire system efficiency at Styal ('water to wire') is about 80%.

The fish pass has turbulence-generators to create a turbulent flow with slow water speed at the edges for the fish to swim up. The turbine outlet flow attracts fish, so they swim towards it and thereby discover the fish pass. A sewage works upstream of Styal has a constant output of relatively clean water into the river which helps to maintain a constant river flow for the Styal turbine.

Total cost of the project was £850k, of which the National Trust paid £570k. The remainder was for the fish and eel pass, funded by the Environment Agency. Annual power generated will be about 220,000kWh, attacting a subsidy of £45.35k, and the value to Quarry Bank Mill for power generated by the turbine, so not having to be purchased from the Grid, is £16.3k

At times the Hydro Scheme generates more power than Quarry Bank Mill requires. The National Trust, being a charity, cannot sell this back to the Grid. It only amounts to about £1,500 per year, and use of technology to offload the Mill's use of gas in favour of using this excess electricity instead are being investigated.


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