HYDROPOWER LOW HANGING FRUIT: hydropower in existing water infrastructure W hat you see in Figure 1 below does not show a high pressure massage shower but rather a device that ‘destroys’ energy, a so-called pressure reducer. It is, in fact, part of the Tunisian drinking water system representing many of this type of pressure reduction system. EXPLOITING SURPLUS ENERGY IN WATER SUPPLY SYSTEMS In drinking water systems, especially in mountainous or hilly regions, where water intakes or reservoirs are situated at higher altitudes, excess pressure is reduced, meaning energy is ‘destroyed’, through valves in order to prevent pipes from bursting. However, the available energy could instead be used to generate electricity, an approach which has been successfully applied in Switzerland for more than 50 years. Whether this is economically attractive depends on the marginal cost (or feed-in tariffs) in respective countries. A study in 2012 revealed an overall hydropower potential of about 35 GWh per year in the water supply system of SONEDE, the Tunisian water supply utility. To harness this ‘lost’ energy, implementation included on-the-job and classroom training on technical, economic, legal, regulatory and institutional aspects for local staff of the water and electricity utilities SONEDE and STEG. It is considered crucial to include key stakeholders in the water sector as well as energy sector in order to avoid conflicts of interest from the very beginning. How to estimate the potential? The process commences with the collection and analysis of data and information on the water supply system. The location of pressure reducers, pressure and flow data over the course of the day and the year are important input data for the assessment. Both parameters – pressure and flow – are interdependent since higher flow leads to higher friction losses. An initial estimate can be based on maxima and minima of (surplus) pressure and of flow, whereas a more accurate calculation requires the temporal course of both. Figure 2 depicts locations where ideally a turbine can be installed. In all systems a by-pass is required in order to ensure safe drinking water supply at all times, even when the turbine is not operational, for instance during Figure 1: Pressure reducer drinking water system – Tunisia 76 maintenance work. In general, using hydropower in transmission pipelines is technically easier than in distribution pipelines, because flow and pressure in transmission pipelines are less fluctuating. Figure 2 What are your implementation options? In many cases, a cost reduction and other synergies can be achieved when the installation of a turbine is considered during the time when pipes in the drinking water systems are in need of rehabilitation and repairs. This could be during a period when a loss reduction project is planned to reduce leakages. In this instance it would be advisable to replace the old pipe infrastructure with higher pressure class and bigger diameter pipes to facilitate a reduction in pressure losses and thus make optimum use of the available hydropower potential. Another option is to integrate a hydropower system from the very beginning, meaning when a new water supply system is planned and implemented. This allows designing the system for both purposes – water supply and electricity generation – and thus also achieving cost reductions. ESI AFRICA ISSUE 1 2015