Using Motors as Generators at Small Scale Hydroelectricity Plants

One of Wales’ most beautiful tourist destinations is now also home to a sensitively designed “green” renewable energy power generation project. The flow of the Afon Lliw, one of the rivers feeding into Lake Bala, is being used at the hamlet of Dolhendre to generate some 600MWh of hydroelectric energy per year, using no fuel whatsoever.

The installation was built for and will be run by DHJC Ltd, a joint venture formed specifically for this project between Derwent Hydroelectric Power Ltd (DHPL) and investor John Cartmell. DHPL specialises in small hydro plant construction and operation, and regularly uses Exico motors in its projects as generators.

In principle generators are the exact opposite of motors but, significantly, they are very similar in construction. DHPL designs its small hydro schemes using electric motors that act as generators. Water jets are turned onto the water turbine with which the Exico motor is directly coupled. Accelerating slowly, the synchronous speed is reached after about a minute and the turbine-generator is automatically locked onto the grid.

The output is governed by the water available in the river at any time (the scheme is known as “run-of-river” as there is no storage of water at the intake) and the maximum output is limited by the capability of the turbine – the rated power of the motor is usually specified to be somewhat greater.

On emergency shutdown the electrical load is instantly removed from the generator so the turbine-generator set is designed to run for short periods at overspeeds of about twice normal speed.

One of the key stages of designing small scale hydroelectric generator systems is identifying the best turbine design; this needs to be a bespoke design based on the gross head of water and the flow volume (which may vary throughout the year). With the turbine fully specified and built, the rest of the generation system can often be based on standard industrial parts, such as motors and couplings.

The Dolhendre installation uses two Exico motors, one of nominal rating 125kW and one of 55kW, both driven by “Turgo” turbines attached to their ‘output’ shafts, which draw power from the flow of the Afon Lliw.   The turbines limit the maximum power of this station to about 140kW in total.

Collectively the motors generate some 600MWh per year, most of which is fed directly into the National Grid although about 20MWh is used by the caravan park upon whose ground the turbine house stands.
One generator was installed and came on stream in July 2015, the other one five months later, just days after Christmas.

Lucie Hodkova of Exico explains that it is not unusual for her to supply motors to unique applications: “At Exico we have a wide range of motors, both standard and specialised. Obviously a lot of our work is supplying motors to fairly conventional applications like conveyors and mixers. However, a good proportion of our larger and more unusual motors are sourced for one-off type installations.

“The Dolhendre project is just such an application. Firstly it is way off our normal industrial stomping ground, simply locating the site was an adventure in itself! Then delivering and installing the motors in such a remote location was a bit of a job. Finally, to top it all they were set up as generators.”

The Dolhendre project also involved the construction of a turbine house, which is partially buried in the solid rock so that it is inconspicuous in the landscape and the laying and burying of a long inflow pipe from a weir that DHJC had built some way upstream Finally, a screened water outlet returns the water to the River Lliw after it has been through the turbine.

Planning for the project took five or more years of negotiation, dealing with three separate landowners, making access arrangements, getting a water abstraction licence and planning permission, and even felling some 100 good sized forestry trees in advance of their normal harvesting date. On top of all this there were eco-surveys for riverlife, birds, large wildlife such as deer, reptiles, small mammals and flora.

Overall it is calculated that the return on investment for the project will be over 10% in a good year for rainfall so full pay back will take up to ten years.