Hydropower seems to be an appreciable renewable source. Its use, at low head sites, can be extended in the future. This book, therefore, provides an investigation of this promising resource. The performance of low head Hydraulic Air Compressor is both experimentally and theoretically investigated for various air admission system parameters (air tube length and diameter), downcomer length and hydraulic head. The results are both presented and discussed. The losses in the whole system are classified and investigated.