At a large number of hydropower plants there is enforced restrictions on biological flow, i.e. the dam must release a certain amount of water to the original river. This amount of water is seldom utilised for power purposes and represent a loss. Some exceptions is known, the Leiro power station in Eidfjord municipality in Hordaland county (Norway) utilises the water released from the Sysen dam in order to maintain the minimum water flow at Vøringsfossen – one of Norway´s main tourist attractions. A minimum of 12 m3/sec of water is released from the dam in the period 1 June until 15 September. The plant was set in operation during the summer 2011, have an installed power of 5.0 MW and an estimated production of 8.0 GWh. The Sysen dam has a difference between (HRL) and (LRL) of 66 m. The Dam is 1160 meter wide and 81 meter high. A production of 8.0 GWh, is a considerable gain, that otherwise would have been lost.
The large difference in head due to the large difference between Highest Regulated water level (HRL) and the Lowest Regulated water level (LRL) is a main concern for any turbine designer, as this will effect both the turbine efficiency and the cavitation performance of the turbine. It is the relative change of head, and not the absolute change in meters that is of importance in this context. The higher specific speed the turbine have, the larger effect, i.e. Propeller type of turbines suffer more from head variations than Francis type of turbines, deductible directly from the respective efficiency hill diagrams.
To some extent, one can overcome this problem by introducing variable speed operation. Although, permanent magnet generators are of limited availability, generally are more expensive than normal synchronous generators and need power electronics in order to maintain the correct grid frequency, in many cases this still will be a profitable solution. Alternatively, a double feed generator can be used, although this limit the speed variation possible to achieve. The main concern will be the distance to a connecting grid, as long connecting cables can kill any feasible project. A was project proposed for a dam with biological flow in the South of Norway, where the Head variation was 4 m, varying from 12.8 m to16.8 m, i.e. a change of approximately 23 %. In addition, the biological flow restriction was set to 2.0 m3/sec for 7 months of the year, while it for the remaining 5 months was 3.0 m3/sec. The project had an estimated production of approximately 2.4 GWh, and would have a payback time of 10 years based on an assumption of future electricity prizes. The project would have given a positive contribution to cash flow from the first year of operation. The use of a variable speed propeller turbine was essential to the project. A synchronous speed turbine would not be suitable under such operating conditions. As the outlet from the power station was at the same point as the release of biological flow, no violation of the biological flow restrictions was foreseen. The installation would have given a good documentation of the released biological flow, something that was not possible by releasing flow through the floodgates. Unfortunately, the power company made other priorities.
There are many existing hydropower projects around the world where a similar project, utilising the biological flow released from dams, can be introduced. Giving the operator increased profit and give the licensing authority’s better control with how the biological flow restrictions is practised.