Capturing the hydrokinetic energy of the Rhone River

In the current economic situation regarding electric generation and conservation of the environment, it is essential to develop sustainable technologies. From this point of view, hydropower is an excellent resource with a consequently high value. The system presented herein aims to optimize the use of this energy while bringing a solution both economic and respectful of the environment. The studied submerged flow current turbines, consist of an assembly of two turbines immersed which converts the kinetic energy of the flow current into electric power without needing any weir structure, contrary to the traditional exploitation of the potential energy in a run-off-river plant. The designer chose Switzerland and more particularly the Rhone River as pilot project site for the first feasibility study in Europe. The study confirms the technical feasibility of the system.

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

Part load flow in radial centrifugal pumps

Olivier Braun

Centrifugal pumps are required to sustain a stable operation of the system they support under all operating conditions. Minor modifications of the surfaces defining the pump's water passage can influence the tendency to unstable system operation significantly. The action of such modifications on the flow are yet not fully understood, leading to costly trial and error approaches in the solution of instability problems. The part-load flow in centrifugal pumps is inherently time-dependent due to the interaction of the rotating impeller with the stationary diffuser (Rotor-Stator Interaction, RSI). Furthermore, adverse pressure gradients in the pump diffuser may cause flow separation, potentially inducing symmetry-breaking non-uniformities, either spatially stationary or rotating and either steady or intermittent. Rotating stall, characterized by the presence of distinct cells of flow separation on the circumference, rotating at a fraction of the impeller revolution rate, has been observed in thermal and hydraulic turbomachines. Due to its complexity, the part-load flow in radial centrifugal pumps is a major challenge for numerical flow simulation methods. The present study investigates the part-load flow in radial centrifugal pumps and pump-turbines by experimental and numerical methods, the latter using a finite volume discretization of the Reynolds-averaged Navier-Stokes (RANS) equation. Physical phenomena of part load flow are evidenced based on three case studies, and the ability of numerical simulation methods to reproduce part-load flow in radial centrifugal pumps qualitatively and quantitatively is assessed. A numerical study of the flow in a high specific speed radial pump-turbine using steady approaches and the hypothesis of angular periodicity between neighboring blade channels evidences the relation of sudden flow topology changes with an increase of viscous losses, impacting on the energy-discharge characteristic, and thus increasing the risk of unstable operation. When the flow rate drops below a critical threshold, the straight through-flow with flow separation zones attached to the guide vanes changes to an asymmetrical flow. Energy is drawn off the mean flow and dissipated in a large vortex-like structure. Besides flow separation in some diffuser channels, time-dependent numerical simulations of the flow in a double suction pump evidence a flow rate imbalance between both impeller sides interacting with asymmetric flow separation in the diffuser. Viscous losses increase substantially as this imbalance occurs, the resulting segment of positive slope in the energy-discharge characteristic is found for a flow rate sensibly different from measurements. Different modes of rotating stall are identified by transient pressure measurements in a low-specific-speed pump-turbine, showing 3 to 5 zones of separated flow, rotating at 0.016 to 0.028 times impeller rotation rate, depending on discharge. For operating conditions where stall with 4 cells is most pronounced, velocity is measured by Laser-Doppler methods at locations of interest. The velocity field is reconstructed with respect to the passage of stall cells by definition of a stall phase obtained from simultaneous transient pressure measurements. Time-dependent numerical simulation predicting rotating stall with 4 cells shows velocity fields that are in reasonable agreement with the measured velocity fields, but occurring at a sensibly higher flow rate than found from experiments. In consideration of the quantitative shortcomings of the numerical simulation, a novel modelling approach is proposed: Replacing the costly 3-dimensional simulation of the major part of the impeller channels by a 1-dimensional model allows a significant economy in computational resources, allowing an improved modeling for the remainder of the domain at constant computational cost. The model is validated with the challenging cases of rotating stall and impeller side flow rate imbalance. The satisfying coherence of the results with the simulation including the entire impeller channels qualifies this approach for numerous turbomachinery applications. It could also provide improved, time-dependent boundary conditions for draft tube vortex rope simulations at reasonable computational cost. Parameter studies modifying deliberately some quantities of mean flow and turbulence at the modeled boundary surfaces can be implemented in the framework of the method.

EPFL2009

THERMOECONOMIC ANALYSIS AND OPTIMISATION OF AIR-BASED BOTTOMING CYCLES FOR WATER-FREE HYBRID SOLAR GAS-TURBINE POWER PLANTS

Raphaël Michel Adrien Marie Sandoz

Among the plethora of alternatives available, concentrated solar power (CSP) appears as one of the most favourable options. The stability and dispatchability of production achievable by the integration of storage and fuel-solar hybridisation are amidst the major advantages of this technology. Nevertheless, conventional CSP plants are based on stream-turbine cycles which consume large amounts of water. In addition to the low thermodynamic efficiency of this type of cycle, the installation of such plants in water-scarce areas is complicated by their reliance on water resources. Thus, the study of new concepts that overcome these drawbacks is necessary for the future of this technology. The availability of high temperature solar receivers for solar tower systems opens the way for the use of gas-turbines in hybrid solar- natural gas configurations. In order to increase the efficiency of the cycle while keeping the water consumption as low as possible, a promising alternative to the recovery of the waste heat in steam-turbines is to use a low-temperature intercooled-recuperated gas-turbine cycle. This work focuses on the analysis and optimisation of the performance of an innovative hybrid solar gas-turbine power plant with an air-based bottoming cycle (ABHSGT). The evaluation considers thermodynamic performance, economic viability and environmental impact as interrelated concerns. With this in mind, detailed steady-state and dynamic models of the power plant have been developed and validated by comparison with existing components. A second model without bottoming cycle has been built for comparison. A multi-objective optimisation using an evolutionary algorithm has then been performed, optimising both capital cost and specific CO2 emissions and resulting in a Pareto-optimal set of possible designs. The analysis of the trade-off curves resulting from the optimisation reveals promising outcomes. The global minimum for the levelised cost of electricity, found at relatively high solar shares, proves the economic potential of the technology. The integration of the bottoming cycle decreases significantly the levelised cost of electricity and the CO2 emissions of the system compared to the reference plant, and higher efficiencies are achieved. The optimal design selected for an in-depth thermoeconomic and environmental analysis exhibits a levelised cost of electricity of 109 [USD/MWhe] for a solar share of 20% and an overall exergetic efficiency 38.5%. The specific CO2 emissions are reduced by 33% compared to those of a simple gas-fired power plant. The water consumption is kept at very low levels compared to other CSP plants, making the system suitable for the deployment in water-scarce areas. In addition, the environmental impact induced by the land use requirements is considerably lower than that of other renewable energy technologies. The sensitivity analysis performed to assess the consequence of changes in varying financial conditions on the levelised cost of electricity and the net present value reveals that the system studied represents a profitable investment in the presence of feed-in tariffs. In the light of the performance obtained in the three aspects considered (thermodynamic, economic and environmental), it can be concluded that the ABHSGT represents a promising alternative to other renewable energy technologies, especially in water-scarce areas.

2012