Figure 1: Pressure reducer drinking water system – Tunisia

What 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…

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