HEAD LOSSES IN SMALL HYDROPOWER PLANT TRASH RACKS (SHP)

Small hydropower plants (SHP) are technical facilities that are part of alternative energy sources [Paish 2002]. They are primarily characterised by low unit power (in Poland below 5 MW) and are often constructed on existing barrages. Electrical current produced by these plants is used to meet local demand. Considering the exploitation of SHPs, it is important to ensure a stable flow through turbines. Aggidis et al. [2010] analysed SHP equipment costs depending on the turbine set. The turbines are protected against damage with trash racks applied for capturing water-borne detritus, such as plant debris carried by water. However, trash racks as solid equipment of SHPs cause head losses, and as a consequence reduce the efficiency of the system. These losses result not only from the spacing of bars, their shape and the technical condition of the inlet chamber, but also from plant debris, its nature, and the quantity of accumulated material that effectively limits the flow. The plant debris captured on trash racks is characterised by diversity in terms of species composition related to the vegetation period and the area where hydraulic facilities are located. Therefore, it is important to maintain trash racks clean by regular removal of the accumulated material. In this context, modernised and newly built power plants are fitted with mechanical cleaners. In older facilities, manual intervention for regular cleaning is required. The present study analyses how the bar shape and the orientation angle of trash racks as well as the accumulated plant debris affect head losses. The results were obtained from laboratory tests. The research examined the impact the inclination angle of trash racks (30 degrees, 60 degrees and 80 degrees) has on head loss values for three different shapes of bars (cylindrical, angled and flat rectangular) and various weight portions of plant debris (0.25, 0.375 and 0.5 kg). The summarised losses were determined by measuring the difference in water levels in front and behind the bars using laboratory facilities. The individual components of losses were determined based on empirical relationships, excluding the losses occurring due to plant debris. Subsequently, the loss resulting from the limited flow was calculated based on the balance of calculated and measured losses.