Morani, Maria Cristina (2021) Newly proposed strategies to increase the energy efficiency of water systems. [Tesi di dottorato]

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One of the main challenges in the water industry consists of the reduction of environmental impacts, as well as the containment of energy use. In this research work, new solutions to achieve a sustainable management of water networks have been developed and organized in three lines of research. The main line of research is based on the optimal location of hydraulic devices within a water distribution network in order to maximize the energy production and water savings, as well as to minimize the investment cost. Firstly, the installation of only Pumps As Turbines (PATs) has been analyzed within a literature synthetic network and a new Mixed Integer Non-Linear Programming (MINLP) model has been developed to perform the optimization. Such an optimization model has been defined by a thorough mathematical formulation in order to deal with the extremely hard technical and computational complexities affecting the optimization procedure. In this research, only deterministic solvers have been employed to search the optima, and a comparison of their performance has been also carried out. Most of the computations have been performed by a global optimization solver, which potentially finds the global optimum in both convex and non-convex problems, but is also used to find good quality local optima in very complex problems, where the achievement of the exact solution may require infinite computational time. Compared to other studies in literature on the same network, the proposed study accounts for crucial hydraulic aspects, such as the phenomenon of flow reversion during the day affecting the installation and the operation of the devices, as well as the need for installing machines generating a power above a minimum fixed value. A comparison with such previous literature works has been carried out in order to highlight the effectiveness of the newly proposed optimization procedure. Moreover, to develop a more realistic and comprehensive mathematical model, the simultaneous installation of PATs and Pressure Reducing Valves (PRVs) has been also modeled by the introduction of new variables and mathematical constraints. Indeed, in presence of large water savings but small energy recovery, a PRV might be a more viable solution than a PAT. Compared to other studies in literature optimizing the only location of PATs within the same synthetic network, the simultaneous installation of valves and turbines, as well as the formulation of new hydraulic constraints, has significantly increased the value of the optimization model. In addition, the optimization has been extended to a real water distribution network serving the Blackstairs region (IE), with the aim of testing the robustness of the model and of the optimization procedure in more complex and larger problems. Indeed, the computational complexity affecting the optimization procedure increases according to the size of the network and the mathematical formulation proposed for the synthetic network might be not suitable for such a more complex case study. Compared to the synthetic network where the pressure reduction up to defined minimum requirements has not compromised the hydraulic operation of the system, in the analyzed real water network the exploitation of the available excess pressure to save both water and energy raises the need for employing also pumping systems to supply the most remote nodes of the network. The installation of pumping systems within the network has been therefore included within the optimization procedure and the outcome has been a new model for a Global Optimization of Hydraulic Devices Location (GOHyDeL), suitable for any water distribution network. Such a new model has been the result of progressive findings and hard attempts to deal with the enormous complexities arising during the computation. In all the performed optimization, the maximized water and energy savings and the minimized installation costs have been assessed according to a cost model used by previous authors in literature, in order to make a more straightforward comparison with such literature works. However, more recent cost models available in literature have been also employed to achieve more reasonable and realistic values of the results. According to the comparison between results obtained by using different cost models, despite the employment of more recent models leading to significantly larger investment costs and, thus, smaller values of NPV, the solutions are quite similar in terms of location of installed devices, and the achieved savings are comparable as well. However, among all the devices, the PRVs have resulted to be more affected by the choice of the cost model, due to the strong dependency of the valve costs on the pipe diameter. On the whole, beyond the large feasibility of the model within the optimal location field, a remarkable value of the proposed research also results from the new formulation of mathematical constraints and variables, which requires less computational effort and could find application also in more general optimization problems. The second line of research defines and compares two alternative strategies to supply a real water distribution network. The first solution consists of an elevated reservoir, which is located upstream of a water distribution network and is supplied from the water source by a pumping system. In this scheme, the excess pressure is not dissipated by a traditional valve, but rather a pump as turbine is installed to contain the pressure, thus water leakage, and also recover energy. The second hydraulic scheme instead consists of a pump supplying the downstream network directly from the source. In this scheme there is not an excess pressure to convert in energy, since the elevated reservoir is bypassed and the flow is pumped to the network with lower head. Such new schemes represent two different strategies to increase the energy efficiency of a supply system, as alternatives to the use of elevated reservoir with dissipation of the excess pressure by means of pressure reducing valves. The two schemes have been properly designed in order to find the devices, in terms of diameter and rotational speed, minimizing the energy requirements, thus maximizing the energy efficiency of the whole system. Given the water network supplying a small village in Ballacolla area (IE), the direct supply of the network has resulted a more efficient strategy than the indirect supply scheme with energy recovery. Moreover, the two schemes have been compared by varying the operating conditions, thus considering different combinations of distance and elevation of the source from the water distribution network. The energy audit of the two schemes has been assessed by new energy efficiency indices and also by literature indices. The comparison has showed that the convenience of a scheme over the other significantly depends on the operating conditions. However, with equal values of pumping head in both the schemes, the indirect scheme with energy recovery is up to 5 % more convenient than the direct pumping scheme, which is instead more efficient if the pumping head could be reduced up to 6 %. In the third line of research a new strategy to save energy in the urban water management is presented. The proposed solution consists of a mixed PAT-pump turbocharger, that is a PAT-equipped turbopump exploiting an excess pressure within the fresh water network to produce energy, which is entirely used to carry a wastewater stream towards a treatment plant. In this system, the excess pressure is converted by the PAT in a mechanical torque, which in turn supplies the pump mounted on the same shaft. Such a plant arises whenever wastewater pumping station and excess pressure point could be co-located, thus in low ground areas where high clean-water pressures occur and sewage networks are equipped with pumping systems due to the need to treat the wastewater. In this application, the water distribution network serving Ballacolla area (IE) has been assumed as case study, since it is suitable for the installation of this kind of plant. A preliminary geometric selection of the devices has been performed by a new selection method based on the maximum daily averaged values of fresh and wastewater discharge. Then, the behavior of the plant has been simulated for several wastewater hydrographs by a new mathematical model. The benefits of the plants have been assessed and compared with a conventional wastewater pumping system working in ON/OFF mode. According to the comparison, the higher Net Present Value (NPV) of the MPP plant proves the advantage of this scheme over the conventional system, at least until the useful life of the plant is reached.

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