Water distribution networks (WDN) are present in all developed cities. Their aim is to deliver drinking water to citizens. New technologies, however, could also allow them to function as renewable energy sources.
What Is a Water Distribution Networks? Why Are They a Source of Energy?
A water distribution networks is a net composed of nodes, where water is injected or collected, and pipes where water flows, which link the nodes. Maximum and minimum thresholds on water pressure and flow velocity must be respected in a WDN in order to avoid technical and engineering complications along pipes and at the nodes. This means that, in certain points, where pressure is higher than the maximum threshold, the excess pressure has to be dissipated. Overpressure is dissipated by installing valves which generate power losses in the water flow (so-called head losses), reducing water pressure. Valves mainly act by reducing locally the cross section of the pipe, generating an obstacle to water flow. Water encounters resistance flowing across this obstacle, losing energy and pressure. Head losses represent an amount of water energy which is lost and wasted.
Recovering Excess Water Energy and Pressure
To avoid dissipating water power, pressure reduction can be generated by installing hydraulic turbines. The excess water pressure is converted into mechanical and electrical power by the turbine. The effect is the same as installing valves—a reduction in water pressure—but now the original surplus of pressure is converted into electricity.
Considering typical turbine efficiency, only 20-40% of the surplus water pressure is lost, and 60-80% is converted into electricity. Typical turbines are bulb and Pelton turbines, or pumps operating as turbines (PAT).
Bulb turbines are hydraulic machines which rotate around a rotational axis parallel to the flow direction. They are composed of a certain number of blades installed around the axis. PAT are geometrically similar to bulb turbines. They are conceived to work as pumps (hence with the aim of lifting water, giving energy to water instead of taking energy from water). If they rotate in the opposite direction, PAT can work as turbines, extracting energy from water.
Bulb turbines and PAT are installed inside pipes. Pelton turbines are installed outside of the pipe. Water is squirted into the turbine blades installed along the turbine external circumference. After that the water has been used by the Pelton turbine, water is reintroduced into the pipe with less energy and pressure. These are the most used turbines in WDN because hydraulic conditions in aqueducts involve high pressures and low flow rates (generally tens/hundreds liters per second).
As we previously reported, water wheels in irrigation systems can work in a somewhat similar way. There, water is used both for irrigation purposes and for energy production.
All of these options should be seen in the wider context of smart cities. As cities look for ways to generate power and carefully manage their water, the integration of electricity production into water pressure management represents a mostly untapped opportunity.
