• Energy Research

Abstract A limiting factor in the use of hydropower plants in water supply networks consists in the cost of the electromechanical production device and the equipment necessary for the plant regulation. Pumps as Turbines (PATs) are electromechanical devices that have already been used in many mini and micro hydropower plants. Due to their low cost and reliability, PATs are an optimal solution for pico-hydropower plants, whose regulation has been widely analysed in literature, for power installations of only a few kW. A new power plant for small residential areas is presented herein. The new low-cost plant is provided with two PATs, that can work as single, series or parallel (SSP) turbines, and three on/off valves, whose synchronous operations regulate the flows and the head loss. The proposed design method is based on the maximization of the effectiveness of the system. Experimental data on a PAT are used to simulate the plant in several hydraulic conditions, with the application of the turbomachinery affinity law. The simple plant operations and the resulting interactions with the network confirm the viability of the proposed scheme. An economic assessment shows the feasibility of the new plant in different hydraulic situations.

In hydropower, the exploitation of small power sources requires the use of small turbines that combine efficiency and economy. Banki-Michell turbines represent a possible choice for their simplicity and for their good efficiency under variable load conditions. Several experimental and numerical tests have already been designed for examining the best geometry and optimal design of cross-flow type machines, but a theoretical framework for a sequential design of the turbine parameters, taking full advantage of recently expanded computational capabilities, is still missing. To this aim, after a review of the available criteria for Banki-Michell parameter design, a novel two-step procedure is described. In the first step, the initial and final blade angles, the outer impeller diameter and the shape of the nozzle are selected using a simple hydrodynamic analysis, based on a very strong simplification of reality. In the second step, the inner diameter, as well as the number of blades and their shape, are selected by testing single options using computational fluid dynamics (CFD) simulations, starting from the suggested literature values. Good efficiency is attained not only for the design discharge, but also for a large range of variability around the design value.

AbstractPressure reducing valves are frequently used within water distribution networks to reduce pressure values. The use of a pump working as a turbine (PAT) to recover the dissipated energy is generally considered the most cost-effective solution, but PATs are not provided with any regulation device. Thus, in presence of variable hydraulic conditions, a regulation system has to be inserted in the power plant to obtain the requested backpressure value. A comparison among the feasible regulation systems for PAT reg- ulation is discussed and the optimal solution is found by means of the power plant effectiveness, combining system capability, flexibility and reliability.

In this study, a new methodology based on performance indices is presented to carry out a detailed energy audit of water systems. Given a water network, the proposed procedure allows for a direct assessment of the critical areas in terms of energy efficiency, as well as for a detailed quantification of the energy benefits resulting from the new management strategies proposed to increase the sustainability of the network. To verify the viability of the methodology, a pressure control strategy is proposed, based on the installation of pressure-reducing valves, containing the excess pressure and thus the water leakage within the system. The operation strategy is carefully designed by the use of a global optimization solver, searching for both the number and location of the devices in order to maximize water savings and minimize the investment cost. The energy benefits resulting from the pressure control are then investigated by the assessment of the performance indices. According to the obtained results, the proposed methodology is a useful tool to assess the vulnerability and inefficiency of a given network, as well as to quantify the benefits resulting from the new management strategies.

AbstractNowadays, the installation of pumps as turbines (PATs) in water supply systems (WSSs) is considered attractive because it is able to effectively combine the pressure regulation with the small-scale hydropower generation. One critical aspect concerns the behaviour of the PAT in the presence of solid particles in the flow which impinge against the inner surface of the device, producing a loss of material termed impact erosion. In this paper, the numerical assessment of PAT erosion is performed, by referring, as a case study, to an existing pressure control station in Southern Italy. The variable operative strategy (VOS) was applied to derive the frequency distribution of flow rates over the PAT operation, under the hypothesis of performing the hydraulic regulation of the hydropower plant. The commercial computational fluid dynamics (CFD) code Ansys Fluent was employed for simulating the liquid–solid flow inside the PAT and then coupled with an in-house code to estimate the erosion. The vulnerability of the PAT to wear was analysed by varying its flow rate, aiming at characterizing the decay of PAT components at a constant rotational speed. Finally, a detailed characterization of the PAT response to the particle impingement at its best efficiency point (BEP) was developed and discussed.

AbstractThe proposed work aims at developing a new set of indices to carry out the energy audit of water systems. The new set consists of both supply and distribution indices and results from a global energy balance applied to a reference network. In this study, the energy audit of two case study networks is presented by the aid of the new proposed indices. Moreover, for each network, different scenarios are considered, differing in the management strategy proposed to increase the energy efficiency of the system. The comparison of the scenarios in terms of indices has allowed for evaluating the most efficient strategy to reduce the pressure exceeding within the network, among different solutions. According to the results, with reference to the case study distribution network, installing both pumps as turbines and pressure reducing valves represent the most efficient strategy, containing the excess pressure and also offering possibility for energy recovery. Moreover, with reference to the water supply system assumed as further case study network, the strategy exhibiting better values in terms of indices is represented by a direct pumping towards the downstream distribution network, instead of pumping towards an upper reservoir and using a valve or a turbine to dissipate the excess pressure. In this work, the proposed indices are demonstrated to be a useful tool for water managers to identify, among different solutions, the most effective intervention to increase the energy efficiency of water systems.

In the management of water distribution networks, large energy savings can be yielded by exploiting the head drop due to the network pressure control strategy, i.e., for leak reductions. Hydropower in small streams is already exploited, but technical solutions combining efficiency and economic convenience are still required. In water distribution networks, an additional design problem comes out from the necessity of ensuring a required head drop under variable operating conditions, i.e., head and discharge variations. Both a hydraulic regulation (HR)—via a series-parallel hydraulic circuit- and an electrical regulation (ER)—via inverter- are feasible solutions. A design procedure for the selection of a production device in a series-parallel hydraulic circuit has been recently proposed. The procedure, named VOS (Variable Operating Strategy), is based on the overall plant efficiency criteria and is applied to a water distribution network where a PAT (pump as a turbine) is used in order to produce energy. In the present paper the VOS design procedure has been extended to the electrical regulation and a comparison between HR and ER efficiency and flexibility within a water distribution network is shown: HR was found more flexible than ER and more efficient. Finally a preliminary economic study has been carried out in order to show the viability of both systems, and a shorter payback period of the electromechanical equipment was found for HR mode.