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Converting a Watermill into a Hydroelectric Plant

A mill owner converted the facility's old electromechanical equipment into a 6 kW hydropower plant. The owner installed a Mitsubishi FR-A741 inverter drive with power regeneration capability to convert the electricity produced by the Francis turbine into the frequency required by the main transmission grid. At the same time, variable-speed operation of the turbine is increasing the efficiency of electricity generation at this facility. Since the conversion, the plant has provided a reliable supply of power for the owner's consumption, as well as about 12 MWh of electricity annually that is fed to the grid.

The situation

During the 18th century, a water mill was built in the hamlet of Durrnhof in Bavaria, Germany. The structure was used as a flour mill by a monastic order. In the early 1930s, the current owner converted the facility into a sawmill, replacing the waterwheel with a Francis turbine. The storage pond, with an area of about 1 hectare, and the turbine intake also date from the time of the conversion. The turbine was used to power the saw via a transmission shaft for nearly 30 years, before an electric motor took on this task.

In 1960, as part of a general overhaul, the Francis turbine acquired a new function. Coupled to a direct current generator, it began to produce electricity for private consumption, with a capacity of 2 kW.

Werner Duerr owns the facility and lives with his family in the mill house. Because of the increased capacity that would be available through an upgrade, Duerr decided to modernize the plant. For such an investment to make economic sense, the plant would also need to supply power to the public network. Under the German Renewable Energy Act (EEG), regeneratively produced power from new plants with a capacity up to 500 kW brings a price of €0.1267 (US$0.18) per kWh for a period of 20 years. Duerr calculated that the retrofit will pay for itself after six years.

The decision to opt for inverter technology from Mitsubishi Electric was made after an exhaustive selection process. This included testing another inverter with a regenerative unit. However, this system was not able to provide continuous regeneration, only cyclic feedback. In addition, because of its modular design, that system would have required a lot of space for installation. Without an inverter, the speed range of the turbine with good efficiency is small.

This series FR-A741 frequency inverter with power regeneration increases the efficiency of the water turbine at this small plant and converts the power to the frequency of the main transmission grid.

One important point was the fact that the FR-A741 frequency inverter is particularly suitable for continuous feed-in operation, because the turbine is operated primarily in the lower third of its output range and the control technology of the apparatus best compensates here for the comparatively poor efficiency. Trials performed by Duerr and Eisemann Elektrotechnik of Bamberg using other products showed that these can only feed braking torque back for a short time and are therefore not designed to meet the requirements of power regeneration.

The FR-A741 frequency inverter combines drive technology and a regenerative unit in a single casing. No additional components are required. The braking energy of the motor produced in regenerative operation is fed into the grid with a low level of loss, also generating considerably less heat than a braking resistor. In addition to lower energy costs, the installation space requirement is reduced, as no additional cooling units are required. The integrated AC reactor and the absence of an external braking unit eliminate up to 60% of the wiring, and up to 40% less space is required for installation in the switchgear cabinet, depending on the output capacity.

The inverter is available in an output range of 5.5 to 55 kW and is rated for three-phase operation at input voltages of 323 to 528 V (at 50 or 60 Hz). The FR-A740 series is equipped with a programmable logic controller and advanced functions for speed, torque and position adjustment. The frequency inverter boasts all the performance features of a high-end drive, such as operating time monitoring, network capability, long-life components and simple operation and maintenance. The full output capacity is available without restrictions up to a clock frequency of 14.5 kHz.

Installing the frequency inverter

The central module of the plant, which was converted by Eisemann Elektrotechnik, is a FR-A741 inverter from Mitsubishi Electric. This inverter converts the energy produced by an asynchronous generator to the main grid frequency of 50 Hz. As a generator, the motor has a capacity of 7.5 kW at 1,430 rpm and is connected to the Francis turbine via a belt drive.

The problem many small hydro plants have is the variation in available water supply over time. In partial load operation in particular, this often prevents optimum operation because the efficiency of the turbine and generator — and thus the electrical power output — depends heavily on the flow rate and speed. As in the case of a car, a constant speed delivers optimum efficiency, whereas frequent acceleration and braking reduces efficiency.

For this reason, Duerr manufactured and installed a sensor to continuously record the water level ahead of the turbine inlet and transmits the data to an Alpha XL micro controller supplied by Mitsubishi Electric. Depending on the water level, the sensor sets the frequency of the inverter with the aid of a rotary encoder and also adjusts the position of the turbine blades by means of a DC motor, thus adjusting the supply of water to the turbine. This adjustment maintains the storage pond at a constant level and guarantees uniform operation of the plant. If the water level is high, the inflow opens. The flow rate and speed of the turbine increase and thereby also the feed capacity via the inverter.

Another advantage of the frequency inverter is its efficiency compensation. Due to its design, a Francis turbine has a relatively limited speed range in which it operates with good efficiency. The control technology of the frequency inverter expands the range in which the turbine delivers an adequate amount of energy, meaning the installation can be operated much more efficiently. The feed capacity in full load operation, especially in the winter months (January to March), is now 6 kW — in the partial and low load range during the rest of the year it is about 2 kW.

Thanks to this conversion, operation of the old water turbine is now profitable once again. Duerr's small hydro plant generates up to 18 MWh of environmentally friendly power a year. The family uses about a third of the power produced, with the greater part feeding into the main grid. About 60% of this annual output fed into the grid is generated during the winter months. A four-quadrant meter for the excess infeed measures the amount of energy fed into the supply network.

Conversion of the plant cost about €5,000 (US$7,100) and is expected to have a six-year payback period.

— By Daniel Lehmann, M.Sc., Mar-keting Operations Division, Factory Auto-mation, Mitsubishi Electric Europe B.V., and Werner Duerr, plant owner

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