Increase of Hydropower Efficiency Using the Ejection Effect with Lateral Conduit

The aim of the present research work is to assess the ejection effect in a low-head hydropower plant with lateral conduits. The study includes six geometrical variants, under submerged downstream condition using 1:70 scale models tests. The model test results are compared to theoretical equations from the scientific literature, and new equations are proposed to assess ejection effects.

The physical models were implanted in a 32 m long rectangular channel. All variants consist of three vertical Kaplan turbines, four lateral bottom conduits, an apron and two lateral piers towards the main channel.

Steady water discharges through the turbines and lateral conduits, as well as flow elevations upstream and downstream the plant, were calibrated by means of 82 model tests under submerged downstream conditions.

Pressure heights at the bottom of the turbine draft tube outlets were measured using 10 piezometers to assess the total ejection effect (flow in the lateral bottom conduits is present) and the gain of head due to the turbines (no-flow in the lateral conduits). The effective ejection assessed is the difference between the two of them.

Theoretical models of Slisskii (1953) and Krei (1920) produced effective ejection results for the initial geometrical configuration of the plant, adjusting the measured values of the gain of head through an expression of Slisskii (1953). Equations of Bernoulli and Conservation of Momentum were applied in this study when the geometrical variants of the plant became too complex.

For each alternative, correction factors relating the rate between observed and calculated values of the effective ejection as a function of the rate of the turbine and total discharges were obtained. All the applied theoretical models were satisfactorily calibrated, with absolute errors varying from 2.3 to 5.8 %.

The calibrated equations produced remarkably consistent results for the effective ejection assessment from 360 hypothetical flow scenarios. The best scenario was identified with values of the maximum ejection effect ranging from 0.69 to 1.20 m.

Results showed that the effective ejection is more sensitive to geometrical variations of the plant for a shallower submerged condition of the structure. Such differences in between scenarios are somehow reduced for deeper submergences.