Hydraulic air pumps for low-head hydropower

Hydropower is a proven renewable energy resource and future expansion potential exists in smaller-scale, low-head sites. A novel approach to low-head hydropower at run-of-river and tidal estuary sites is to include an intermediate air transmission stage. Water is made to flow through a siphon, rather than a conventional water turbine, and at the top of the siphon the pressure is sub-atmospheric and air is entrained into the water. The siphon forms a novel, hydraulically powered vacuum pump or 'hydraulic air pump' (HAP). Air is pumped by the HAP through a separate air turbine and generator. This arrangement offers dramatic increases in turbine-generator speed and allows better control and matching of components and lifecycle cost reductions due to reduced maintenance costs and the use of smaller rotating machines. This study builds on previous work on such systems by showing why the pumping process can be treated as isothermal. Also, initial test results with a small siphon are presented and compared to existing models. These show a discrepancy between predicted and measured pressure ratios and therefore an overprediction of efficiency and power output using simple mathematical models.