Before the OED conclusion, two conditions for the revision of concession for Folla-Vindølareguleringen

07.05.2019 09:50

Statkraft gives reasons to keep conditions and maneuvering from the concession decided 56 years ago. According to Statkraft the most important are:

       1.  The regulation is flood damping

2.   Adjustability for the market.

3.   To let minimum water makes point 1 and 2 difficult.

Let us start from the beginning. The equation for effect is:


Where V is the volume of water in time through the turbine in m³/s, D is the density of water 1000 kg/m³, G is the gravitational acceleration 9,81 m/s², H is the water column for the plant, and K is the total efficiency of the turbine system as a coefficient.

With the effect of 130 MW from Trollheim kraftverk the following data are given:

V=38,5 m³/s.

The mean water column is H=377m (the regulated water surface is situated 420-375 meters above sea level at Follsjö according to the concession). Mean level of turbine is 20, 5 meters above sea level.

K=0,91 is the efficiency coefficient 

In general the volume velocity V=41 m³/s is associated with a max water column of H=400 m and V=36 m³/s with H=355 m, the plant gives about 147 MW to 114 MW (the lowest effect E should be lower because K should be lower but it is less important). The water way from Follsjö to Trollheim kraftverk is constant and the plant has in general a linear relation between volume speed and water column. You can adjust the water column H by adjusting the height of the water surface in Follsjö and the maximum water speed will follow accordingly.

We have not found use of data given by Statkraft and are using our own.

An example: In Figure 1 we can see that the plant is effect driven (max haft of the time and the rest hitting the minimum total water of 15 m³/s outside the plant) in May through December and driven consistently for maximum in January thought April. 2/3 of the available effect gives production over the year in Trollheim kraftverk (because of problems, available water, etc.). Statkraft gives the yearly mean effect production to 805 GWh, or 2898 TW. It gives a produced effect of 138 MW. This estimation makes us believe that Statkraft maneuvers according to our data rather than to the ones supplied.

Statkrafts arguments are:

      1.      The regulation is flood damping.

a.       If the dam is full there is no damping. On the contrary, due to that the regulation dries out the mountain sides and valley floor the flood risk is increased.

b.       If the dam has room for water the regulation damps a flood until the dam is full.

Figure 1. The water surface height above sea level in Follsjö. The figure is supplied by Statkraft to NVE with an application for concession of a second turbine in Trollheim kraftverk 2016 but was withdrawn. NVE is using data from this application in the revision of conditions for Folla-Vindølareguleringen.


Figure 2. Total efficiency in % from Statkrafts application of a second turbine in Trollheim kraftverk 1998. The application was withdrawn.


In Figure 1 we can see that the height of the water surface of Follsjö is over 415 meter above sea level 2/3 of the year, it does not give any pronounced flood damping. If we turn back to the equation for effect E we can see why Statkraft prioritize a large degree of filling in Follsjö, it gives a large water column H. You can not have both, what you win in effect E you lose in flood damping and vice versa. Our understanding is that Statkraft neglects flood damping for large effect. The limit is the amount of available water.


2.         The market requirers adjustability

The efficiency k can also affect the effect E. According to information the turbine has fixed guide vanes. The water hits the turbine wheel with a similar angle at different speeds and the efficiency is large in a small band (see Figure 2). Also, the plant has trouble with cavitation at low water speeds. It is very outdated to have fixed vanes, a fixed guide vane ring. In essence there is no adjustability. We assume that a wheel with an efficiency curve to the right of the one in Figure 2 is installed. We see that the efficiency K varies about 1 % and that the water column H can vary with 12 %.


3.       To let minimum water makes it difficult for point 1 and 2.

Minimum water is obvious with today’s vision on biological diversity, without it the river will end up as a power ditch. Most calculations in genetics from hatcheries and biological diversity in rivers conclude that assessments become critical after 50 years of regulation. One example of misunderstanding is from side 11 in Statkrafts comments on the conditions suggested by NVE, January 2019 unsigned:


‘En av utfordringene i Surna er at minstevannføringen er betydelig større enn naturlig vannføring i tørre perioder. Uten reguleringen ville vannføringen i Surna vært lavere enn 15 m³/s nesten 20 % av tiden. Sommeren 2018 ville vannføringen i Surna ved Skjærmo uten reguleringer på det laveste vært 5-7 m³/s. Som følge av reguleringene var ikke vannføringen lavere enn 16-18 m³/s.’


Does Statkraft understand why NVE suggests minimum water as a condition, the most important of the revision? Minimum water is required because it’s bottom surface shall replace biological surface lost in disappearing voids between rocks and gravel, due to the rivers lesser transport capability when it is regulated. Fish juveniles also hide naturally in the voids when there is low water in the river. Minimum water makes stranding of fish less pronounced. The court judged the minimum total water outside the plant to be 15 m³/s, that is 33 % of the mean volume water of 45 m³/s. It coincides well with the Montana Method which gives a minimum of 30 % as a mean over the year. For further reading we refer to NVE:s conditions and final report midsummer 2018.


Two conditions for the revision of Folla-Vindølareguleringen

In addition to NVE:s conditions we suggest to OED that Statkraft should install a movable vain ring, movable guide vanes. It will give an adjustability in Trollheim kraftverk that both the market and Surnas biological diversity will prosper from, in practice there is now no adjustability. Also, OED can specify a lower flood risk by limiting the degree of filling of the dams before a heavy rain. OED can through the revision make Trollheim kraftverk a modern plant and not compensate for the worst consortia treatment in Norwegian history.