Commissioned in January 1927, the Motukawa scheme uses 99-metres of an available 122-metre difference in elevation between the Manganui River near Tariki, and the Waitara River near Motukawa, to drive three generating sets, the last of which was added in 1938.
To provide temporary power during construction, a 200 kW hydro plant, the remains of which still exist today, was constructed at the head-works near Tariki.
During 2005, a 200 kW turbine was developed and installed in the Motukawa water race to take advantage of the natural fall in the supply race system. This initiative has increased both the overall installed capacity and the resource use efficiency of the Motukawa scheme.
During 2007, an additional efficiency enhancement was added to the Motukawa scheme. The Mangaotea pumps lift water approximately 4-metres out of the Mangaotea stream into the Ratapiko supply race. This improvement added approximately 750 MWh per annum to the total scheme output.
With a total capacity of 5 MW, the Motukawa scheme now has an average annual output of 22 GWh.
In periods of high rainfall the diversion canal drains an adjacent catchment and Lake Ratapiko rises to spilling level. New gates and race works have been developed to reduce flooding of local farmland.
Although eel and other native species are currently unable to pass through the scheme, a successful trap and transfer programme for both juvenile and adult eels has been implemented to ensure migration of this species is not adversely affected.
TrustPower has also agreed to fund extensive riparian planting in catchment areas, to improve water quality and fish habitat.
In 2001, the Environment Court granted Resource Consents for the Motukawa scheme, thus allowing the scheme to operate (within the conditions of these consents) until 2022.
In keeping with TrustPower’s drive to enhance the efficiency of current installations, a study into turbine upgrading opportunities has been undertaken, using state-of-the-art computational fluid dynamics modelling. Results of the study indicate an 8-10% efficiency gain and as a result the enhancement work has been programmed for completion over the next three years. This upgrade is testimony to the sustainability of this type of plant, as it will also reset the life of the equipment for the foreseeable future, thereby avoiding the use of valuable resources to manufacture completely new machines.