Possibilities for improving hydrodynamic performance of hydrokinetic turbines

Razvoj tehnologija obnovljivih izvora energije važan je za postizanje ambicioznog cilja Europske unije do 2050. godine, koji podrazumijeva postizanje gospodarstva s nultom neto stopom emisija stakleničkih plinova. Dosadašnji je razvoj uglavnom bio usmjeren na razvoj tehnologija koje koriste intermitentne obnovljive izvore kao što su vjetar i Sunce, dok je kinetička energija sadržana u morima, oceanima i rijekama još uvijek uglavnom neiskorišten potencijal. U literaturi su predloženi različiti koncepti za pretvorbu kinetičke energije vode, a turbine su najčešće korištena tehnologija. Iako je ova tehnologija nadišla početnu fazu testiranja, te su započeti demonstracijski projekti, glavna prepreka za komercijalni uspjeh je relativno niska iskoristivost. Cilj ovog rada je modifikacijom geometrije protočnog dijela kinetičkih vodnih turbina s horizontalnom osi vrtnje ostvariti povećanje hidrauličke iskoristivosti, odnosno koeficijenta snage. Poboljšanja hidrodinamičkih karakteristika se nastoje ostvariti uz istovremeno održavanje jednostavnosti konstrukcije, koja predstavlja jednu od primarnih prednosti ovog načina iskorištavanje hidroenergije. Modifikacijom rotora, u prvom redu oblika lopatica, ostvarena su poboljšanja hidrodinamičkih karakteristika i povećanje koeficijenata snage. Za postizanje visokih koeficijenata snage u širokom radnom području turbine predložen je integrirani pristup konstruiranju lopatica, koji koristi genetske algoritme za optimizaciju hidroprofila i drugih konstrukcijskih parametara rotora, kao što su duljina tetive i kut uvijanja po visini lopatice. Razmotrene su i konstrukcijske izvede rotora s terminezonima, koji se nalaze na vrhovima lopatica i okrenuti su prema podtlačnoj strani lopatica. Njihova je uloga sprečavanje prestrujavanja fluida na vrhu lopatice, koje predstavlja sekundarno strujanje i uzrokuje gubitke na vrhu lopatice. Konstrukcijski parametri terminezona odabrani su na temelju saznanja iz dosadašnjih istraživanja, najviše u području vjetroturbina, a predloženom konstrukcijom ostvareno je povećanje koeficijenta snage za projektni režim rada od 7,2%. Za dodatno povećanje koeficijenta snage, iznad teorijske vrijednosti poznate pod nazivom Betzo-ov limit, koja iznosi 59,3% i vrijedi za rotor u slobodnom toku fluida, razmatraju se različiti statorski elementi. Ideja je da se povećanjem brzine u ravnini rotora i uslijed stvaranja područja niskog tlaka iza turbine poveća i izlazna snaga turbine. Provedene su numeričke simulacije različitih izvedbi turbine u svrhu detaljnije analize strujnih karakteristika pomoću računalne mehanike fluida, te je ustanovljeno maksimalno povećanje koeficijenta snage od 81%. Najviši koeficijenti snage dobiveni su korištenjem statora u obliku difuzora s prirubnicom na izlaznom dijelu. Ustanovljeno je da se korištenjem duljih difuzora općenito postižu viši koeficijenti snage, ali taj je utjecaj značajniji za izvedbe statora bez prirubnice. Za statore s prirubnicom utjecaj povećanja duljine difuzora na povećanje koeficijenta snage postaje zanemariv za duljine veće od promjera rotora. Poboljšane konstrukcijske izvedbe kinetičkih vodnih turbina s horizontalnom osi vrtnje predstavljaju učinkovito, pouzdano ali i ekološki prihvatljivo rješenje za distribuiranu proizvodnju električne energije.Technological advances in renewable energy play an important role in meeting the ambitious goal of carbon neutrality by 2050. Hydrokinetic energy, contained in marine, ocean, and river currents, is still mostly untapped renewable resource that can contribute to the global clean energy transition. Unlike many other renewable energy resources, like solar or wind, energy generation from hydrokinetic energy is quite predictable. Hydrokinetic turbines are particularly applicable to sites where the construction of dams is not technically or economically feasible as well as in isolated areas that are not connected to the main power grid (islanded micro-grids). However, the main barrier to wider commercialization is relatively low efficiency, limited by so-called Betz limit. According to Betz-limit, no turbine can capture more than 59,3 % of energy in open flow, regardless of the turbine design. Most of the available literature deal with large-scaled turbines for tidal applications while the technology for river applications have rarely been reported so far. Different concepts of hydrokinetic turbines presented in the literature are mainly based on the well-developed field of wind turbine design. Depending on the direction of the rotational axis, relative to water flow, hydrokinetic turbines can be classified as vertical axis turbines or horizontal axis turbines. To encourage the expansion of electricity generation using hydrokinetic turbines, especially small and micro-scaled, this work is focused on the investigation of possibilities for improving horizontal axis turbine design. Methods The integrated, optimization-based, design method is proposed to improve rotor performances. The method is based on the Blade Element Momentum Theory (BEM) that combines the blade element theory and the momentum theory to relate blade shape and rotor ability to generate power. The required input parameters are parameters that primarily depend on the chosen location for the turbine installation, such as rotor and hub diameter and tip to speed ratio (). In the proposed design approach BEM method is reformulated in the form of the objective function and nonlinear constraint. The original BEM theory is extended with correction factor to take into account the effect of tip and hub losses. The decision variables in the first objective function are axial and tangential induction factors. Genetic algorithm is firstly used to determine initial chord length and blade twist required as input for hydrofoil shape optimization. Instead of using large number of coordinate points to represent hydrofoil, non-uniform rational B-splines (NURBS) representation is selected to reduce computation cost of hydrofoil shape optimization. The second objective function is formulated to find hydrofoil with high lift coefficient, low drag coefficient and delayed cavitation inception. To formulate single objective function with multiple optimization objectives, weighting factors are used. Data on lift and drag coefficients of optimized hydrofoil are further used to determine optimal chord and blade twist distribution. Rotor design with small attachment at the tip of the blades with the same cross-section shape as blade hydrofoil is also proposed. This addition, called winglet, is designed to reduce the effect of vortices generated due to the pressure difference between the upper and lower blade surfaces and thus increase output power. The potential of overcoming the theoretical efficiency limit for bare turbine rotors in open flow, known as the Betz limit, is also investigated. The idea is to increase the velocity at the rotor plane using diffusers and thus increase power which is proportional to the cube of the water velocity. Numerical simulations were performed using ANSYS Fluent 2020 R2 Academic software to get insight into the hydrodynamics of turbines with different geometries and types of diffusers. Results Numerical simulations results show high power coefficients of bare turbine rotor, designed according to proposed integrated design method. Relatively high values of power coefficients, above 40%, are obtained in wide operation range. The highest power coefficient of bare turbine is 45.4% and it is obtained at tip to speed ratio of 4. With winglet addition at blade tip increase in energy conversion efficiency is more than 7%, which means that power coefficient reaches the value of almost 49 % at design tip to speed ratio. The results indicate that power coefficient increase can be obtained even with very simple stator cross-section design, over a wide operation range. In general, the higher values of diffuser open angle and length increase the energy conversion efficiency. However, it is observed that the impact of length increase becomes weaker when diffuser length reaches the value of rotor diameter, which indicates that optimal diffuser length is around the value of rotor diameter. More considerable power augmentation is obtained with brimmed diffuser. The highest percentage rise of 81% in energy conversion efficiency is obtained with 1.25 length brimmed diffuser with open angle of 15°. However, significant increase in power coefficient was not achieved by using more sophisticated diffuser shapes, like diffusers with foil-shaped cross-sections. Conclusion The simulation results show energy conversion efficiency increase with all the proposed design improvement. The use of optimization algorithms in blade design contribute to the rotor design improvements. The addition of winglets has increased values of power coefficient by reducing the negative effect of vortices at blade tip. However, more considerable power augmentation is obtained with brimmed diffusers. This is due to velocity increase at rotor plane which enable turbine to overcome bare rotor efficiency limitation of 59.3%, Scientific contribution 1) Integrated, optimization based, turbine blade design method is proposed. The method is based on the Blade Element Momentum Theory (BEM) that combines the blade element theory and the momentum theory. The method utilizes genetic algorithm to optimize hydrofoil shape and determine optimal chord length and twist angle distribution along the blade span; 2) numerical simulations results give insight into the influence of different geometries and types of stator elements on the hydrodynamics of horizontal axis hydrokinetic turbines; 3) based on the numerical simulation results, recommendations for design improvements of rotor and stator elements are given; 4) the simulation results can be used in pre-design phase, allowing the designer to select the most effective stator design for specific application; 5) the proposed concepts of micro-turbines can produce electricity by using kinetic energy contained in river streams which make them suitable for distributed energy system; 6) proposed turbine design concepts contribute to more efficient hydropower utilization at locations where conventional hydropower technology cannot provide feasible solutions