A new turbine concept is improving the economic feasibility of very low head (VLH) hydro developments, and lowering fish mortality rates
The development of very low head (VLH) hydro sites, below 3.2m, is often technically feasible but unprofitable. The complex civil structures required to direct the water from the intake to the runner and to recover the kinetic energy contained in the water at the runner exit, are just too expensive.
A study of hundreds of low head sites has shown that there is a direct link between head reduction and a rise in concrete volume required for civil engineering works. The result is that, considering a constant output, if the head drops from 3.2m to 1.4m, the corresponding concrete volume required to build the power plant is multiplied by five, while the runner diameter of the turbine is doubled. Often, therefore, VLH sites prove to be unfeasible from an economic point of view.
To help overcome this problem, the VLH turbine concept has been under development by MJ2 Technologies with assistance from Laval University, Canada. The VLH unit is an integrated generating set (IGS), incorporating; a Kaplan runner with eight adjustable blades; a fixed distributor composed of 18 wicket gates with flat bars inserted in between as trash racks; a permanent magnet generator (PMG) directly coupled to the runner shaft; and an automatic trash rack cleaner mounted on the distributor.
The larger size of the VLH runner means that the water is slowed down through the runner and eliminates the need to have sophisticated civil engineering structures for the inlet to, and exit from, the runner. The IGS is fully submersible, which allows for negligible visual and sound impacts. Slanted in its mounting, the unit can be removed from the sluice passageway by a lifting device system. The unit can be raised for maintenance or to recover the full capacity of the sluice during high flow.
The runner blades are adjustable and self-closing, allowing for automatic upstream water level regulation and eliminating the need for a separate closing gate to shut the unit. The unit can also operate in discharge mode (when not generating electricity) and can operate isolated from the distribution network.
Furthermore, its relatively slow rotational speed (34rpm), large runner diameters (from 3.55m up to 5.5m), very low water velocity (less than 2m/sec), together with other patented technical features, makes the unit a fish-friendly turbine. It allows fish migration downstream through the turbine runner itself, and possibly upstream, too.
The VLH turbine concept was not developed for specific site design requirements. Rather, it was developed as a standardised product range. The established product range is 3.55m-5.6m in runner diameter, and the generator output range is between 100kW-500kW for heads ranging from 1.4m-3.2m and flows of 10-30m3/sec. The turbine generator is double regulated with adjustable blades and variable speed, which allows for operation on sites where the head changes with variation of the river flow.
A complete set of small scale model tests was performed at Laval Hydraulic Machinery Laboratory (LAMH) at Laval University, made according to iec 60193 standards. The objective was to accurately measure the performance of the VLH under a wide range of different heads, flows and incline position angles. Final reports indicate a high turbine efficiency of about 90% under nominal head and flow. It also shows nominal behaviour of the turbine to a 35° of incline, and confirms that the nominal efficiency will be maintained even when the head drops down to one-third of the nominal value, thanks to the variable speed generator that will slow down the generator rotation speed when the net head drops. The yearly production of a VLH unit is, therefore, similar to a good quality double regulated Kaplan.
The VLH turbine is equipped with a permanent magnet generator directly coupled to the turbine shaft. The extremely slow rotational speed of the turbine (a maximum of 41rpm) generates a low frequency power (approximately 12Hz). This power output is then processed by the hydroelectric power generation system (HPGS) that is composed of the power electronics equipment, the power controller and the power station controller and its user interface. This sophisticated electronic equipment will generate energy and provide quality interconnection that meets IEC and IEEE applicable standards.
The prototype VLH turbine is installed in Millau, France. It is a DN 4500 type, 4.5m diameter turbine, using a 6m wide water intake channel. Once connected to the grid in April 2007, the first VLH went through exhaustive commissioning tests.
The results have been in line with expectations from the small scale model tests. Runaway speed at full opening (75rpm) and maximum runaway speed (90rpm) were as calculated. The nominal full output of 438kW was reached at the nominal rotation speed of 37rpm under the nominal net head of 2.5m and flow of 22.5m3/sec. Nevertheless, for administrative reasons, the power plant will operate at a maximum 410kW output delivered to the public network.
After almost one year of service, the VLH turbine has operated throughout different seasons, having been commissioned in spring, operated during summer and undergone severe low water levels. Autumn floods and the north wind, which cause extensive leaf fall, have proved the worth and efficiency of the rotating trash rake cleaner. The VLH has been operating in accordance with, or even somewhat higher, than the estimates provided by economic feasibility studies.
Fish friendly design
Optimisation of the VLH turbine by computational fluid dynamics (CFD) was integrated at the design stage in line with fish friendly criteria based on a survey by Idaho National Engineering and Environmental Laboratory (INEEL) for the US Department of Energy.
The VLH fulfils all but one of the criteria. The results are summarised in Table 1.
The last criterion – blade to discharge ring gap – is not fulfilled but the very low average flow speed of <2m/sec is assumed to allow fish to avoid the periphery of the runner. Under such low water speeds most fish can change direction and avoid high speed water lines.
The fish friendliness of the VLH turbine has been also been tested in-situ at the site where the prototype VLH turbine is installed. The fish passage tests have been conducted in three sessions. The first one took place in the spring of 2007 and has tested the downstream migration of the smolts and a second series of tests at the end of 2007 and early 2008 has tested the downstream migration of silver eels (see panel). These tests have been conducted in Millau, France under the supervision of French officials from the Conseil supérieur de la pêche ONEMA. An official report will be available shortly.
Marc Leclerc, General Manager, MJ2 Technologies SARL, 48 rue de la prise d’eau, 12100 Millau, France. For more information, visit www.vlh-turbine.com
|Going live with eel tests|
The recovery net has a floating platform which enables recovery of the fish as they arrive. The platform, the net and its frame are arranged by means of a crane.