Armando Carravetta

Energy Production in Water Distribution Networks: A PAT Design Strategy

Pump operating as turbine (PAT) is an effective source of reducing the equipment cost in small hydropower plants. However, the manufacturers provide poor information on the PAT performance thus representing a limit for its wider diffusion. Additional implementation difficulties arise under variable operating conditions, characteristic of water distribution networks (WDNs). WDNs allow to obtain widespread and globally significant amount of produced energy by exploiting the head drop due to the network pressure control strategy for leak reductions. Thus a design procedure is proposed that couples a parallel hydraulic circuit with an overall plant efficiency criteria for the market pump selection within a WDN. The proposed design method allows to identify the performance curves of the PAT that maximizes the produced energy for an assigned flow and pressure-head distribution pattern. Finally, computational fluid dynamics (CFD) is shown as a suitable alternative for performance curve assessment covering the limited number of experimental data.

PAT Design Strategy for Energy Recovery in Water Distribution Networks by Electrical Regulation

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.

Hydropower Potential in Water Distribution Networks: Pressure Control by PATs

Pressure control is one of the main techniques to control leakages in Water Distribution Networks (WDNs) and to prevent pipe damage, improving the delivery standards of a water supply systems. Pressure reducing stations (PRSs) equipped by either pressure reducing valves or motor driven regulating valves are commonly used to dissipate excess hydraulic head in WDNs. An integrated new technical solution with economic and system flexibility benefits is presented which replaces PRSs with pumps used as turbines (PATs). Optimal PAT performance is obtained by a Variable Operating Strategy (VOS), recently developed for the design of small hydropower plants on the basis of valve time operation, and net return determined by both energy production and savings through minimizing leakage. The literature values of both leakages costs and energy tariffs are used to develop a buisness plan model and evaluate the economic benefit of small hydropower plants equipped with PATs. The study shows that the hydropower installation produces interesting economic benefits, even in presence of small available power, that could encourage the leakage reduction even if water savings are not economically relevant, with consequent environmental benefits.

Cost-Benefit Analysis for Hydropower Production in Water Distribution Networks by a Pump as Turbine

AbstractThe use of microhydroelectric plants in urban pipe networks, based on the combination of a pump as turbine (PAT), two regulating valves, and two pressure meters, is proposed along with simple automation rules. Its economic benefit is tested on a small pipe network, where the network geometry as well as the demand coefficient variation in time and space have been inferred from previously collected data and existing analysis. A similar analysis has been also carried out for different scenarios in which the reduction of pipe installation cost due to a diameter reduction is compared with the increased benefit in energy production. The case study shows that a small increment of the pipe installation cost, with respect to the minimum required by the nodes minimum pressure, can lead to a larger benefit for energy production.

An improved affinity model to enhance variable operating strategy for pumps used as turbines

ABSTRACT The use of pumps operating as turbines (PATs) is attractive to optimize the equipment costs of small hydropower plants. Unfortunately, the lack of information on the performance of PATs restricts the wide use of this technology. If a single characteristic curve is available, the behaviour of a PAT can be predicted by the application of the turbomachinery affinity law. In this paper, the reliability of the affinity law to predict the behaviour of a machine under variable speeds is discussed, and the results of this model are compared with an experimental database which includes the performance curves of five PATs operating at different speeds. The results show that the difference between the theoretical model and the experimental results is significant. Therefore, a new model, based on a relaxation of the affinity equations, has been proposed, in order to minimize the errors between the predicted and measured characteristic curves.

Numerical simulation on pump as turbine: Mesh reliability and performance concerns

A pump used as turbine (PAT) for the transformation of hydraulic energy in electricity is the new trend in the management of water distribution networks. So far few application of such devices are known for the lack of adequate studies on the PAT project and on the interactions between PAT and the water system. Overall efficiency, flexibility under flow rate and pressure variations, response to unsteady state conditions are still open questions. Some of these problems are discussed in the paper by numerical computation.

A ground monitoring system based on TDR tests

A ground monitoring system that exploits electromagnetic field properties of accounting of both physical and geometrical features of the surrounding medium is proposed. The system is based on TDR tests, assisted by a control and processing unit, and aims at detecting starting cavities underground in the soil. The digital signal processing approach deployed for waveform analysis, which plays a fundamental role in detecting and characterizing underground anomalies, is described.

Pressure Coefficient in Dam-Break Flows of Dry Granular Matter

AbstractThe propagation of dry granular flows, such as rock and snow avalanches, can be described by depth-averaged models. Different from classical shallow-water equations, these models take into account the anisotropy of normal stresses inside the flowing pile through using an earth pressure coefficient in the pressure term. A new regularization function for calculating the pressure coefficient in the Savage-Hutter-type models at the early stages of dam-break flows and collapses is proposed. In such circumstances the flow lines are significantly curved with respect to the basal surface and a special treatment of the earth-pressure coefficient is required for obtaining a satisfactory agreement with experimental data. The comparison between numerical simulations and laboratory experimental data shows an apparent improvement in describing the early stages of dam-break waves over rough beds. The comparison with experiments over smooth bed surface exhibits minor evidence of improvement. Nonetheless, in this ...

A two-layer depth-averaged approach to describe the regime stratification in collapses of dry granular columns

The dynamics of dry granular flows is still insufficiently understood. Several depth-averaged approaches, where the flow motion is described through hydrodynamic-like models with suitable resistance laws, have been proposed in the last decades to describe the propagation of avalanches and debris flows. Yet, some important features of the granular flow dynamics cannot be well delivered. For example, it is very challenging to capture the progressive deposition process, observed in collapses and dam-break flows over rough beds, where an upper surface flow is found to coexist with a lower creeping flow. The experimental observations of such flows suggest the existence of a flow regime stratification caused by different momentum transfer mechanisms. In this work, we propose a two-layer depth-averaged model, aiming at describing such a stratification regime inside the flowing granular mass. The model equations are derived for both two-dimensional plane and axi-symmetric flows. Mass and momentum balances of each...

Assessment of Rheological Characteristics of a Natural Bingham-Plastic Mixture in Turbulent Pipe Flow

Estimating rheological parameters of a non-Newtonian fluid is performed with rheometers, but experiments are limited to fine sediments, in the absence of appreciable sedimentation. An approach based on pipe flow measurements may be more flexible and convenient. The aim of this paper is to experimentally verify the latter approach in order to assess the rheology of natural mixtures of heavy materials with high tendency toward sedimentation.