Bachelor thesis 15HE credits Electrical Power Engineering 2014-05-25 Lastområdesutvidgning med aktiv flödeskontroll i Francisturbiner Expanding operation ranges using active flow control in Francis turbines Sebastian Adolfsson Serial No. EL1405 Department of Applied Physics and Electronics, Umeå University Acknowledgements I want to thank all personnel at Jämtkraft that has been very helpful and provided with experiences and information about their facilities. Also I want to thank my handlers Michel Cervantes at LTU1, Talib Morad at UMU2 and Peter Palmqvist at Jämtkraft for the aid they have provided with. I want to thank Knut Magne Olsen, technical chief at Troms Kraft who has been very helpful and given a well guided tour at Skibotn power plant. I also thank Torbjørn Kristian Nielsen for the study visit at NTNU3. For technical information I want to show special gratitude towards Håkon Francke at FDB4 that has shared and allowed presentation of his test results and extensive knowledge about the water injection technique. I thank Patrick March at HPP5 Inc for allowing presentation of his graphical findings regarding aeration techniques and efficiency. For allowing publication of research material I thank Smith Brennan T, Sabin Mathew, Jo Magnus Solberg, Patrick March and Håkon Francke. I also thank Henrik Lindsjö at Andritz Hydro and Urban Andersson at Alstom for discussions and practical perspectives on ae ration techniques. For economic information I show large gratitude towards Morten Kjeldsen at FDB for making an economic estimate of installing the peripheral water injection system at the case study. 1 Luleå University of Technology 2 Umeå University 3 Norwegian University of Science and Technology 4 Flow Design Bureau 5 Hydro Performance Processes ~ ii ~ Sammanfattning Denna rapport innehåller en undersökning av fluidinjiceringstekniker för att reducera skadliga flödeseffekter i sugröret hos Francisturbiner som arbetar utanför optimal last. Det finns ett fokus på implementerbarhet hos Jämtkrafts anläggningar och två vattenkraftverk har undersökts som är placerade i serie med varandra vid namnen Lövhöjden och Ålviken. Det enda lönsamma scenariot som upptäcktes med en viss grad av säkerhet var en ökning av lastområdet uppåt för att tillåta överlast. Upptäckter visade att både luft och vatten kan införas på olika platser för att förbättra hydraulisk verkningsgrad kring turbinens delar såväl som att reducera tryckpulsationer i skadliga lastområden. Investeringar i sådana system har visat sig vara användbara och lönsamma på flera platser med dåligt anpassade manövreringsvillkor. Men på grund av förluster i effektivitet då injektionssystem körs visar det sig olönsamt i situationer där de inte förbättrar lastområdet på ett sådant vis att det resulterar i ökad årlig eller topp-produktion. ~ iii ~ Abstract This report contains an investigation of fluid injection techniques used in the purpose of reducing deleterious flow effects occurring in the draft tube of Francis turbines when operating outside nominal load. There is a focus on implement ability at Jämtkrafts hydroelectric power plants and two power plants were investigated, located in series with each other named Lövhöjden and Ålviken. The only profitable scenario found with some degree of certainty was an increase in the operating range upwards to allow overload operation. Findings show that both air and water can be introduced in various locations to improve hydraulic efficiency around the turbine parts as well as reduce pressure pulsations in harmful operating regions. Investments in such systems have proven useful and profitable at several facilities with poorly adapted operating conditions. But due to losses in efficiency when operating injection systems, it turns out unprofitable in situations where it does not improve the operating range in a way that is resulting in increased annual or peak production. . ~ iv ~ Table of contents Acknowledgements ................................................................................................................................. ii Sammanfattning...................................................................................................................................... iii Abstract .................................................................................................................................................. iv 1 Introduction ..................................................................................................................................... 1 2 Aim .................................................................................................................................................. 2 3 Method and sources ......................................................................................................................... 2 3.1 Air injection ............................................................................................................................. 2 3.2 Water injection ........................................................................................................................ 2 4 Structure .......................................................................................................................................... 2 5 Theory ............................................................................................................................................. 2 5.1 Francis turbine ......................................................................................................................... 2 5.2 General flow characteristics .................................................................................................... 4 5.3 Deleterious flow effects ........................................................................................................... 6 5.3.1 Vortex breakdown ........................................................................................................... 7 5.3.2 Vortex rope ...................................................................................................................... 8 5.3.3 Hydro acoustic resonance ................................................................................................ 9 5.4 Countermeasures ................................................................................................................... 10 5.4.1 Air injection ................................................................................................................... 10 5.4.2 Water injection .............................................................................................................. 13 5.4.3 Other countermeasures .................................................................................................. 21 5.5 Case study Lövhöjden power plant ....................................................................................... 24 6 Results ........................................................................................................................................... 26 6.1 Technical summary ............................................................................................................... 26 6.1.1 Air injection ................................................................................................................... 26 6.1.2 Water injection .............................................................................................................. 29 6.2 Economic summary ............................................................................................................... 40 6.2.1 Air injection ................................................................................................................... 41 6.2.2 Water injection .............................................................................................................. 41 6.3 Case study.............................................................................................................................. 42 7 Analysis, discussion and conclusions ............................................................................................ 43 8 Proceeding work/Research/Recommendations ............................................................................. 44 9 Appendix ....................................................................................................................................... 45 ~ v ~ Nomenclature Following physical quantities are mentioned in the report; they are explained individually in the text [ ] [ ] [ ] [ ] [ ] [ ] ̇ [ ] [ ] [ ] [ ] Following abbreviations are used in the report ~ vi ~ 1 Introduction Today hydropower acts as an important part of the Nordic electrical grid due to capabilities of storing fuel and varying production of both active and reactive power. The ability to sell regulating power is becoming increasingly desirable with establishment of the north European power market NORDPOOL together with development in intermittent power production sources. Investments to increase power production in facilities using renewable resources are also eligible to receive electricity certificates in the EU, a factor that increases incentives to invest in hydropower drastically. Most hydropower plants equipped with reaction turbines, especially Francis, are only able to produce power within a limited operational range around BEP6 of the turbine. Undesired operational modes usually exist due to the occurrence of deleterious flow effects in the turbine runner and draft tube when operating at part loads. These effects are mainly pressure surges due the draft tube geometry and residual swirl in the flow entering the draft tube. The residual swirl is due to mismatch in angular momentum provided by the guide vanes and the angular momentum extracted by the turbine, resulting in uneven mass transfer of water to its surrounding structure. Some periodic fluctuations even risk falling inside the resonance frequencies of the water system, leading to so called hydro-acoustic resonance, causing hazardously large vibrational amplitudes. Water flows needed to maintain the rivers in accordance with water-rights judgments or to maintain downstream reservoirs are often so low that they fall outside the acceptable operational range of the turbine. This can lead so scenarios where water is spilled pass the turbine without utilizing it for power production. Also a normal production scheme is to run a facility at max efficiency during peak hours and stop it at lower price periods in order to maximize the extracted profit from water reservoirs, a scenario in which the equipment may suffer many and harmful annual start and stop cycles. Countermeasures can be made to reduce destructive flow effects, resulting in increased plant availability that in turn reduces conflicts of interest in power production planning and reliability viewpoints. In some cases it is possible to increase the annual power production at a hydropower facility by being able to utilize smaller flows then originally designed for. Many countermeasures include physical inserts in the draft tube or turbine, something that risks having a negative impact on the turbine efficiency at BEP. This study is focusing on the investigation of two techniques where fluid is injected in the draft tube flow in order to reduce the pressure surge phenomena’s. Water and air are the working fluids in these systems and even with consideration to their differences they have both been proven to produce positive results and can both be turned off to operate at BEP. Pay-back times of these investments has in the most successful case been a few months, but most facilities are too small to make a noticeable profit or have already solved the operational range problem by installing several turbines in parallel or are using turbines with extended operating range. The goal is to attain knowledge about implement ability and profitable scenarios related to these techniques at hydropower stations owned by the job requestor. Also a presentation of the problems present at the undesired operating modes of the Francis turbine should be made. Information is gathered via research of literacy and company documentation as well as from adepts within the field. Results show a good degree of overall system implement ability and an operating environment with low economic incentives at all enquired facilities. 6 Best Efficiency Point ~ 1 ~ 2 Aim The aim of the project is to make an economic and technical investigation of two fluid injection techniques. The fluids injected will be air and water which will have different effects, system structure, investment costs, planned installation time and technical properties. A case study shall be made at Lövhöjden, an 8,5MW plant with 99m water head, annual production of 24GWh and is owned and operated by Jämtkraft. The goal is to appreciate the technical implement ability and economic environment surrounding the investment of both the water and air injection systems. 3 Method and sources 3.1 Air injection A literacy study and discussions with adepts at Alstom and Andritz Hydro will act as the fundament for the theoretical understanding of the air injection system. H. Lindsjö at Andritz Hydro is consulted to make an economic estimate of the installation costs of an atmospheric air injection system at the case study. 3.2 Water injection The theoretical understanding will be built from literacy research, meetings with adepts at NTNU and FDB. Also a visit is made to Skibotn, a 72MW facility owned and operated by Statkraft, utilizing the water injection system. At FDB H. Francke provides with results from the systems and M. Kjeldsen is consulted for an estimate of the installation costs at the case study. 4 Structure The report starts with a theoretical introduction of turbine equipment, its related problems and a description of countermeasures to these problems as well as a description of a certain case study. Following the theory is a results chapter which consists of technical and economic summaries of the air and water injection systems separately, referring to results from several documented scenarios. The results chapter ends with the case study findings. The report is finished with an analytical discussion about attained results and further research. 5 Theory This section concerns some basic theory surrounding the investigated phenomena’s and techniques. It is presented to give a general understanding of the key factors. 5.1 Francis turbine The Francis turbine is the most commonly used turbine in hydropower plants globally and can be designed to utilize a large range of water heads with high efficiency. Most Francis turbines suffer a common problem related to pressure surges in the draft tube occurring around half load and operating at these loads is therefore avoided. To explain the Francis turbine makeup, a short review of its parts is presented below. They are presented in the same order as water is passing through the turbine. A cut through picture is seen in Figure 5-1 ~ 2 ~ Penstock – Provides pressure potential and flow of water to the spiral casing. Pressure is exerted by the weight of the above water head.  Spiral Casing – Surrounding the runner with spiral geometry of area decreasing with its length, this is to maintain uniform velocity to the stay vanes. This structure is housing the stay and guide vanes that are spatially distributed around the runner in the center.  Stay vanes – Fixed fins that convert pressure potential to kinetic energy and are reducing swirl of the inlet flow by aligning the direction of the flow towards the runner section.  Guide vanes – Movable fins that are used to regulate flow rate and the angle of attack to the runner. These are connected in a circular structure called a wicket gate, which can be moved hydraulically to change their position simultaneously.  Runner – Converts kinetic energy and pressure potential energy in the water to torque transferred by the shaft. Water enters radially and leaves axially exerting both impulse and lift force to the runner blades.  Draft tube – Connects to the runner outlet and converts kinetic energy to pressure potential due to a diffusing geometry meaning an area increasing along its length.  Shaft – Connecting the turbine runner and generator rotor and is transferring the torque extracted by the runner from the water. This part is not characteristic for the Francis turbine but can due to its sometimes hollow structure be used as an air path when injecting air and is due to the nature of this report mentioned here. Figure 5-1 – Francis turbine [1] When water is leaving the turbine operating at BEP and enters the draft tube, its velocity profile should ideally consist only of an axial and no angular component. When the turbine is operated in part or over load modes the swirl generated by the wicket gate in the spiral casing is not matching the angular momentum extracted by the runner, hence leaving a residual rotational velocity component at ~ 3 ~ the runner outlet. Direction of water flow leaving the runner is depicted by the red vector in Figure 5-2. Figure 5-2 – Velocity distribution for water leaving Francis runner at BEP, Part load and Overload The residual swirling velocity is thought to be the cause of severe pressure surges in the draft tube and it is noted that at part loads the swirling flow is rotating in the same direction as the runner and at over load the swirl is rotating in the opposite direction of the runner. Difference in pressure at runner and guide vanes varies with the difference in water head over the turbine. Power plants with large head variation may suffer operational problems due to the difficulties in designing a turbine runner that can handle the different operating regimes sufficiently. 5.2 General flow characteristics A general understanding of the relation between pressure, elevation and velocity in a fluid flow can be achieved by studying a simple one dimensional version of Bernoulli’s principle as presented below is relative velocity in a control region [ ] is gravitational acceleration [ ] is elevation [ ] is pressure in the control region[ ] is density [ ] The equation is expressing continuity of enthalpy in the flow where the effect from changing each variable easily can be mirrored into the others. The inverse relation between pressure and velocity is of most importance when studying flow behavior since it can give a simple understanding of effects such as cavitation and pressure pulsations. ~ 4 ~