Leif Lia

Review of Ice Effects on Hydropower Systems

AbstractHydropower is a major power source in cold region countries. It is also the largest renewable energy source offering significant potential for reduction in carbon emissions. In Norway, hydropower accounts for nearly 99% of the total electricity production. To meet winter demand, storage schemes are implemented in tandem with run-of-river schemes to a large extent in cold region hydropower systems. In these systems, ice creates operational constraints during winter that can lead to reductions in power production. The problems occur in the various phases of the ice regime, mainly due to frazil ice, ice runs, and ice jams. Counteracting these ice problems is usually a difficult task that involves expensive measures and possibly lost production. This paper presents a comprehensive review of the effects of freshwater ice on hydropower systems. It has been shown that ice in freshwater systems poses a number of operational constraints on the various components of a hydropower system at a time when energy...

The Effect of Surge Tank Throttling on Governor Stability, Power Control, and Hydraulic Transients in Hydropower Plants

This paper investigates the effect of surge tank throttling on governor stability, power control, and hydraulic transients in hydropower plants. The work is intended to be practical, but includes some new research. The practical contributions include a methodology for a combined evaluation of the effects of installing surge tank throttles in hydropower plants, and a demonstration of the throttle effects through a case study. The research contributions include the evaluation of the throttle effect on power control, and a comparison of the throttle effects on power control for governor systems with speed feedback exclusively versus combined speed and power feedback. Field measurements are used to calibrate a numerical model of the case-study hydropower plant. The results from the case study show that the throttle has an insignificant positive impact on governor stability. Power control is improved when a throttle is installed; the overshoot of produced power and the time until steady-state conditions occur are reduced. The throttle has a significant effect on the hydraulic transients, and increases the water hammer and reduces the mass oscillations in the system.

Hydraulic scale modelling and thermodynamics of mass oscillations in closed surge tanks

ABSTRACT The design and results from a hydraulic scale model of mass oscillations in a hydropower plant with a closed surge tank constructed as an underground rock cavern are presented. The results from the model test of an existing hydropower plant at scale 1:65 are compared with field measurements. The main contributions of this work include (1) an assessment of whether hydraulic models may be applied to evaluate hydropower tunnels with closed surge tanks, (2) a novel approach to scale atmospheric air pressure, and (3) an evaluation of the thermodynamic behaviour in the model and prototype. The hydraulic model is shown to provide an accurate representation of the maximum (first) amplitude, with a relative error of less than 4%. An estimate of the period of the oscillations has a relative error of less than 1%. The model has higher dampening compared with the prototype, resulting in the 20% relative error of the second amplitude. Both the model and prototype reveal approximately adiabatic behaviour of the closed surge tank.

Displacements as failure origin of placed riprap on steep slopes

ABSTRACT This paper presents results from a scale model study related to the stability of dumped and placed riprap on steep slopes of 1:1.5 (vertical: horizontal) exposed to overtopping. The experiments showed that small rearrangements of the stones in placed riprap, quantified as displacements of particular stones, led to a compaction in the lower part of the riprap and to loosening in the upstream part. The riprap became unstable when the maximum displacements exceeded the size of the longest axes of the riprap stones. The experimental data were used to develop a relationship to describe the development of the displacements taking the load-history into account. The obtained results were indirectly confirmed by comparison with findings of two reports which are described in the present paper. Moreover, placing the riprap stones in an interlocking pattern resulted in five times higher critical discharges compared to randomly dumped riprap.

Horizontal trash rack – a new intake concept for efficient backflushing

At many hydropower intakes in rivers, there are extensive problems with debris fouling the trash rack. An alternative to manual or mechanical cleaning is the hydraulic concept of backflushing, in which water removes debris from the trash rack and flushes it back to the river out of a flushing conduit. A new intake concept with the intake gate located upstream of a horizontal trash rack, for the efficient utilization of backflushing, is tested in a physical model. The experimental results provide an opportunity for energy production during the automatic cleaning. The cleaning efficiency is strongly dependent on the flushing capacity. The costs related to the flushing conduit and a slightly increased head loss can be less important for many cases than the advantages from timely and efficient debris removal, reduced costs for human labor and improved safety. Additional possibilities for downstream fish-passage through the bottom outlet call for further studies.

Field and model tests of riprap on steep slopes exposed to overtopping

The comparability of large-scale field tests of dumped and placed riprap with a stone diameter of 0.37 m and corresponding model tests in a scale of 1:6.5 was investigated in terms of stability, packing density and visually observed flow pattern. The tested riprap protections were exposed to overtopping on a slope of 1:1.5 (vertical: horizontal). The results for dumped riprap revealed similarity between the field and model tests based on the critical stone-related Froude number as a measure of the stability, packing density, flow pattern and overtopping depth. The field and model tests with placed riprap showed good agreement in regard to flow pattern and overtopping depth. However, the placed riprap in the model tests was denser packed and more stable than in the field indicating laboratory effects. Placed riprap withstood up to 10 times higher unit discharges than dumped riprap, 6–8 m2s−1 in the field tests.