Exploiting small, low-head hydro sites with conventional hydro plant is often found to be only marginally economic; indeed, such sites may go unexploited for this very reason. This paper sets out a significantly less costly means of converting energy from a low-head hydro source, using siphons to convert from water to air pressure. Compared with a conventional turbine solution, the civil engineering required is minimal, and the turbines used are relatively inexpensive. Furthermore, much of the plant can be located on dry land. The paper describes tests carried out both in the laboratory and at a low-head site. The results are compared with theoretical predictions, and different designs are compared. In the lab, void fraction values as high as 0.3 have been achieved; with these values of void fraction, plant efficiencies of 65% or more should be achievable in suitable locations. Whilst they are lower than values for conventional plant, such efficiency values are often tolerable, because the operator has to choose between a siphon plant generating some energy, and no plant at all. The global potential of low-head hydro power that is currently unexploited is estimated at 7000 TWh per year, possibly half of total world energy consumption. The siphon system helps to aerate the river water, which gives a significant environmental benefit. Maintenance of the system is straightforward, because all of it is above water and much of it is located on the river bank. Because the system is much less costly than conventional, it enables low-head sites to be exploited which currently cannot. This contributes to sustainability by converting renewable energy that goes to waste at present.