Variable speed operation of centrifugal pumps running as turbines. Experimental investigation

Pumps running as turbines are pointed out as a cost-effective solution for energy recovery in pressurised water supply systems. However, these hydraulic machines feature low efﬁciency under variable discharge operation due to the lack of an inlet ﬂow control component. Variable speed operation is an approach for controlling the discharge at the pump as turbine inlet aiming at increasing the operational efﬁciency. This research work presents the experimental investigation for measuring the variable speed characteristic curves of pumps running as turbines, focusing on the turbine and on the extended operation modes. Three single-stage end-suction closed-impeller centrifugal pumps with different unit speciﬁc speed values are tested. Turbine mode test results show that the discharge-speciﬁc energy operating range is broadened with increasing efﬁciency if the machines are operated with variable speed. Extended operation results show that these hydraulic machines do not feature the instability region near the runaway conditions, the so-called the “s-curve”. Outcomes of this experimental investigation provide the required insights for establishing the design technical speciﬁcations of micro hydropower plants with variable speed pumps running as turbines, aiming at maximizing the energy recovered in pressurised water supply systems.

Hill chart modelling using the Hermite polynomial chaos expansion for the performance prediction of pumps running as turbines

Loïc Andolfatto, François Avellan, João Nuno Gomes Borga Delgado

Pumps running as turbines are suitable hydraulic machines for micro hydropower applications. The selection of the proper pumps to install in a given site still remains a major challenge, as pump manufacturers do not provide the characteristic curves data in the turbine mode. Also, the accurate prediction and modelling of the pumps running as turbines characteristic curves still remain a major difficulty as existing methodologies still lack ac-curacy, especially in the part load and full load operating regions. This paper proposes a new two-step methodology based on the Hermite polynomial chaos expansion for predicting the characteristic curves of pumps running as turbines and modelling their variable speed operation, aiming at improving the prediction accuracy. Firstly, bivariate continuous surrogate functions are established for predicting the turbine mode and the ex-tended operation mode characteristic curves inside a closed interval of unit specific speed values. These surrogate functions are developed by calibrating empirical coefficients based on collected experimental data. Secondly, a hill chart model is determined for describing the variable speed operation of a given pump running as a turbine. This hill chart model allows identifying the discharge and the rotational speed set points for maximising efficiency for a given operating condition. The proposed prediction surrogate functions and the variable speed hill chart model are useful engineering tools for improving the design of pump as turbine hydropower plants and for optimising the pump running as turbine control settings to maximise the produced energy.

2019

Methodology for Risk Assessment of Part Load Resonance in Francis Turbine Power Plant

Philippe Allenbach, François Avellan, Bob Greiveldinger, Christophe Nicolet, Jean-Jacques Simond

At low flow rate operation, Francis turbines feature a cavitating vortex rope in the draft tube resulting from the swirling flow of the runner outlet. The unsteady pressure field related to the precession of the vortex rope induces plane wave propagating in the entire hydraulic system. The frequency of the vortex rope precession being comprised between 0.2 and 0.4 times the turbine rotational speed, there is a risk of resonance between the hydraulic circuit, the synchronous machine and the turbine itself an acting as excitation source. This paper presents a systematic methodology for the assessment of the resonance risk for a given Francis turbine power plant. The test case investigated is a 1GW 4 Francis turbines power plant. The methodology is based on a transient simulation of the dynamic behavior of the whole power plant considering a 1D model of the hydraulic installation, comprising gallery, surge chamber, penstock, Francis turbine but also mechanical masses, synchronous machines, transformer, grid model, speed and voltage regulators. A stochastic excitation having energy uniformly distributed in the frequency range of interest is taken into account in the draft tube. As the vortex rope volume has a strong influence on the natural frequencies of the hydraulic system, the wave speed in the draft tube is considered as a parameter for the investigation. The transient simulation points out the key excitation frequencies and the draft tube wave speed producing resonance between the vortex rope excitation and the circuit and provide a good evaluation of the impact on power quality. The comparison with scale model tests results allows resonance risk assessment in the early stage of project pre-study.

2006

Optimization of low-head hydropower recovery in water supply networks

Irene Almeida Samora

Small scale hydropower is emerging as a decentralized source to satisfy local demand for electricity. The interest in micro-hydropower, which refers to installed power below 100 kW, is increasing since this is a solution with low environmental impact. Water supply systems are one of the main manmade water systems presenting potential for micro-hydropower. Although some applications already exist in adduction lines, the urban distribution networks remain unexplored. Because these are complex systems in which flows and pressure vary constantly, specific technology and installation schemes for energy recovery are lacking. This work accesses the potential for hydropower within water supply networks (WSN) and presents an arrangement of micro-turbines specially conceived for this type of installation. The arrangement is based on a novel inline turbine suitable for pressurized systems and its best location within the networks is studied. The five blade tubular propeller (5BTP), preliminarily designed in the framework of the European Project HYLOW, was further developed and tested in this work. An experimental campaign was conducted with a large range of heads and torque measurements to access its characteristic curves and to obtain hill charts. The relative positioning of the turbine-generator shaft regarding the pipe bend was modified from a downstream position to an upstream position. Efficiencies of around 60% were found. The adequate locations in WSN for micro-hydropower plants are identified using an optimization algorithm which considers both the assessment of the energy production and sizing of the main equipment and works. A concept for the implementation of the 5BTP in the field was developed, consisting of an arrangement with up to four turbines inline within a buried chamber created around an existing pipe. Two objective functions, energy production and economic value respectively, are used. A simulated annealing process is developed to optimize the location of a given number of turbines. This procedure takes into account the hourly variation of flows throughout an average year and its effect on the turbine efficiency. The optimization is achieved by considering the characteristic and efficiency curves of a turbine with different impeller diameters and simulating the annual energy production in a coupled hydraulic model. After a convergence analysis for different restrictions and numbers of installed turbines, the algorithm was applied to analyze the feasibility of the proposed arrangement in two case studies in Switzerland: a sub-grid of the city of Lausanne and the complete WSN of the city of Fribourg. In both cases, the implementation of the proposed energy recovery solution seems to be feasible. A detailed analysis of the cost breakdown revealed that the cost of additional pipe work, which are required in each layout to guarantee a by-pass supply in case of maintenance, may have an important role on the investments. Also, the pressure reduction valves locations, if they exist, are likely to be the optimal solutions. Finally, a methodology to quantify the potential for hydropower based on the excess energy in a WSN is proposed and applied to case studies. It allowed to conclude how much the proposed arrangement can extract from the networksâ energy potential. In addition, an attempt was made to produce an expedite method to estimate the energy produced with one 5BTP based on network parameters and dimensional analysis.

EPFL2016

Optimization of low-head hydropower recovery in water supply networks

Irene Almeida Samora

EPFL2016