• Energy Research

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.

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.

The recovery of excess energy in water supply networks has been a topic of paramount importance in recent literature. In pressurized systems, a pump used in inverse mode (Pump As Turbine, PAT) demonstrated to be a very economical and reliable solution, compared to traditional energy production devices (EPDs). Due to the large variability of flow rate and head drop within water distribution networks, the operation of PATs could be performed by a series-parallel regulation system based on an electronic or a hydraulic principle. Despite the low cost of the PATs and of regulation and control systems, a great barrier to the diffusion of a small hydro power plant in water distribution is represented by the necessity of additional civil works to host the whole plant. Based on laboratory and numerical experiments, the present paper proposes a new low-cost technology, overcoming most of the limitations of the present technologies when low energy is available and high discharge variation occurs. The operating conditions of the plant are properly optimized with reference to the working conditions of a case study. Despite the laboratory prototype having exhibited a significantly low efficiency (i.e., 16%), due to the use of small centrifugal pumps suitable for the analyzed case study, in larger power plants relying on more efficient semi-axial submersed pumps, the energy conversion ratio can increase up to 40%. The results of this research could be useful for network managers and technicians interested in increasing the energy efficiency of the network and in recovering energy in the peripheral branches of the network were a large variability of small flow rates are present.

A new strategy to increase the energy efficiency in a water network exists using turbo pumps, which are systems consisting of a pump and a turbine directly coupled on a same shaft. In a turbo pump, the pump is fed by a turbine that exploits a surplus head in a freshwater network in order to produce energy for one system (wastewater) and reduce the excess pressure in another (drinking water). A pump as turbine (PAT) may be preferred over a classic turbine here due to its lower cost. The result of such a coupling is a PAT–pump turbocharger (P&P). In this research, the theoretical performance of a P&P plant is employed using data from a real water distribution network to exploit the excess pressure of a freshwater stream and to feed a pump conveying wastewater toward a treatment plant. Therefore, the P&P plant is a mixed PAT–pump turbocharger, operating with both fresh and wastewater. A new method to perform a preliminary geometric selection of the machines constituting the P&P plant has been developed. Furthermore, the plant operation has been described by means of a new mathematical model under different boundary conditions. Moreover, the economic viability of the plant has been assessed by comparison with a conventional wastewater pumping system working in ON/OFF mode. Therefore, the net present value (NPV) of the investment has been evaluated in both situations for different time periods. According to the economical comparison, the PAT–pump turbocharger represents the most economically advantageous configuration, at least until the useful life of the plant. Such convenience amounts to 175% up to a time period equal to 20 years.

One of the hardest challenges in irrigation systems is to reduce the use of water resources. The goal “more crop per drop“ has fostered the improvement of hydraulic infrastructure by replacing old open channel networks with pressurized systems (e.g., drip and sprinkler irrigation). This has resulted in reduced water losses, as well as the possibility for farmers to benefit from on-demand water. On the other hand, this improvement in water efficiency has controversially resulted in a large increase in energy consumption, since pressurized systems demand large flows with high pressures. The high energy requirements, along with the investment and operational costs, may compromise the viability of such irrigation systems. Several strategies to save both water and energy resources have, therefore, been proposed in the recent literature. Among these, the exploitation of excess pressure to produce energy by means of energy production devices (EPDs) has been proven to be a very effective strategy. Pumps-as-turbines (PATs) are, by far, preferred over traditional EPDs, such as pico- or micro-turbines, being very cost-effective solutions. In this study, the optimal location of PATs within an irrigation system is investigated with reference to a whole irrigation season. A case study network, the Zújar Canal irrigation district, is considered, for which accurate records of hourly water demands are available due to a diffuse telemetry system over the irrigation area. The aim of the optimization is to search for the best number and location of PATs within the irrigation network in order to maximize the energy production and minimize the investment costs. The hydraulic resolution of the network is coupled with the optimal location problem in one single mathematical model and the optimization procedure is performed via the use of a global optimization solver. The promising results in terms of the hydropower potential prove the effectiveness of the PAT installation within the case study network.

Water systems are usually considered low efficiency systems, due to the large amount of energy that is lost by water leakage and dissipated by pressure reducing valves to control the leakage itself. In water distribution networks, water is often pumped from the source to an elevated tank or reservoir and then supplied to the users. A large energy recovery can be realized by the installation of energy production devices (EPDs) to exploit the excess of pressure that would be dissipated by regulation valves. The feasibility of such a sustainable strategy depends on the potential of energy savings and the amount of energy embedded in water streams, assessed by means of efficiency measures. Alternatively, energy savings can be pursued if the water is directly pumped to the network, bypassing the elevated reservoir. This study focuses on the comparison of two solutions to supply a real network, assessed as a case study. The first solution consists of water pumping to a reservoir, located upstream of the network; the excess of energy is saved by the employment of a pump as turbine (PAT). The second scenario is characterized by a smaller pressure head since a direct variable speed pumping is performed, bypassing the reservoir. The comparison has been carried out in terms of required energy, assessed by means of a new energy index and two literature efficiency indices. Furthermore, differing design conditions have been analyzed by varying the pumping head of both the scenarios, corresponding to different distances and elevation of the water source.

Water distribution network are energy-demanding systems affected by low efficiency. In such systems, the pressure is generally kept under control by regulation valves to reduce the waste of water d...