Drag reduction within radial turbine rotor passages using riblets

In this paper, reducing the friction losses in a radial inflow turbine rotor surface by adding engineered features (riblets) is explored. Initially, computational fluid dynamics analysis was used to study the operating mechanism of riblets and to test their ability to reduce drag within the rotor passage when running the turbine at the design point. Thereafter, riblets with different heights and spacing have been implemented at the rotor hub to study the effect of riblets geometry and arrangement on the drag reduction, which leads to determine the riblet geometry where the maximum benefit on turbine performance can be achieved. The effect of riblets on boundary layer development and on the secondary flow generation within the rotor passage has been examined. It was found that the introduction of riblets could reduce the wall shear stress at the hub surface, and on the other hand, they contribute to increasing the stream-wise vorticity within the rotor passage. The maximum wall shear reduction was achieved with riblet with relative height hrel = 2.5% equivalent to 19.3 wall units, while the maximum performance happens when using riblets with hrel = 1.5% equivalent to 11.8 wall units as the later contributes less in secondary flow generation within the passage. For riblets with height more than 19.3 wall units, the overall effect is negative, as they cause an increase in drag and give rise to secondary flow leading to lower turbine performance

A new method to identify the optimal temperature of latent-heat thermal-energy storage systems for power generation from waste heat

The integration of thermal-energy storage (TES) within waste-heat recovery power generation systems has the potential to improve energy-efficiency in many industrial processes with variable and/or intermittent waste-heat streams. The first objective of this paper is to present a novel model of these systems that can be used at an early design stage to provide fast and accurate estimates of performance. More specifically, the method can identify the optimal temperature of latent-heat TES systems for waste-heat recovery applications based only on the known heat-source and heat-sink conditions (i.e., temperature, mass-flow rate and specificheat capacity), and can assess both single-stage and cascaded systems. The model has been validated against optimal organic Rankine cycle systems identified from a thermodynamic cycle optimisation. The second objective is to identify the characteristics of optimal systems for different heat-source profiles. The results indicate that, for a given application, there exists an optimal temperature for the latent-heat TES system that depends primarily on the relative size of the heat sink. Moreover, it is found that, for a heat engine operating with TES, the power rating ranges between 25% and 60% of the corresponding power rating for an optimal heat engine, operating without TES, that adapts instantaneously to heat-source fluctuations, whilst the total energy production is reduced by between 45% and 85% respectively. Finally, a small deviation is observed between the results obtained for the different heat sources considered, which suggests that these findings can be extrapolated to other heat sources not considered within this study

Challenges in the Development of Micro Gas Turbines for Concentrated Solar Power Systems

Parabolic solar dish systems have gained more interest recently as a reliable way for harnessing the solar power in form of electricity. Micro gas turbines can be usedas engines in such system to convert the heat available from the solar collector o electricity. In this paper the technical challenges related to using micro gas turbines for utilising concentrated solar power will be addressed based on the experience gained from the EU funded project OMSoP (Optimised Microturbine Solar Power system) which aims todevelop and demonstrate a micro gas turbine coupled to a parabolic dish for the power range of 5–10 kW. The technical challenges related to the turbomachinery design, rotordynamics and dynamic stability, control system, power electronics and thermal storage will be briefly reviewed. Techno economic considerations of the system will also be discussed

Towards virtual testing of compression systems in gas turbine engines

Current trends in the computational fluid dynamics (CFD) analysis of gas turbine engines are in the direction of the so called “virtual testing”. Although this term is used nowadays loosely in the context of this application, the ultimate objective of virtual tests is to replace partly or fully rig and engine tests during the design and certification of engines. In the past few decades, significant developments have been achieved in the discretisation methods and the associated CFD algorithms. Combined with the rapid developments in hardware in both speed and memory which are becoming increasingly available at affordable prices, the simulation of full engine or rig tests are increasingly becoming a reality. This paper describes a method by which virtual tests can be conducted on a low pressure compression system of a gas turbine engine using smart boundary conditions and allowing the sweep along a speed characteristic or sweep along a working line during the mapping of the compressor characteristic in a similar fashion to a typical rig test. The low pressure compression system is equipped with a variable downstream nozzle and the rotational speed is allowed to vary during the computations. The simulations are validated using NASA rotor 67 experimental data against which good agreement was obtained

Simultaneous Cycle Optimization and Fluid Selection for ORC Systems Accounting for the Effect of the Operating Conditions on Turbine Efficiency

The design of optimal organic Rankine cycle (ORC) systems requires the simultaneous identification of the optimal cycle architecture, operating conditions and working fluid, whilst accounting for the effect of these parameters on expander performance. In this paper, a novel method for predicting the design-point efficiency of a radial turbine is developed, which can predict the achievable efficiency based only on the thermodynamic conditions. This model is integrated into an optimization framework in which the working fluid is modeled using the Peng-Robinson equation of state and the fluid parameters (i.e., critical temperature) are simultaneously optimized alongside the cycle conditions. This framework can evaluate recuperated and transcritical cycles, whilst heat-transfer area requirements are estimated based on representative overall heat-transfer coefficients. For a range of heat sources, a single-objective optimization is first completed in which power output is maximized, which is then followed by a multi-objective optimization in which the trade-off between power output and total heat-transfer area is investigated. It is demonstrated that the optimization framework can simultaneously optimize the working fluid and cycle parameters, and identify whether a subcritical or transcritical cycle, with or without a recuperator, is best suited for a particular application, whilst accounting for the effect of these variables on the expander performance. This information is critical to identify optimal cycle configurations and working fluids that result in the best thermodynamic performance, yet exist in the design space in which feasible turbines can be designed. It is found that the optimal critical temperature does not vary significantly between different cycle architectures, and is not affected by whether a single or multi-objective optimization is completed. However, including the expander performance model results in significantly different cycles to optimal thermodynamic cycles

Improving the economy-of-scale of small organic rankine cycle systems through appropriate working fluid selection

Organic Rankine cycles (ORC) are becoming a major research area within the field of sustainable energy systems. However, a major challenge facing the widespread implementation of small and mini-scale ORC systems is the economy-of-scale. To overcome this challenge requires single components that can be manufactured in large volumes and then implemented into a wide variety of different applications where the heat source conditions may vary. The aim of this paper is to investigate whether working fluid selection can improve the current economy-of-scale by enabling the same system components to be used in multiple ORC systems. This is done through coupling analysis and optimisation of the energy process, with a performance map for a small-scale ORC radial turbine. The performance map, obtained using CFD, is adapted to account for additional loss mechanisms not accounted for in the original CFD simulation before being non-dimensionalised using a modified similitude theory developed for subsonic ORC turbines. The updated performance map is then implemented into a thermodynamic model, enabling the construction of a single performance contour that displays the range of heat source conditions that can be accommodated by the existing turbine whilst using a particular working fluid. Constructing this performance map for a range of working fluids, this paper demonstrates that through selecting a suitable working fluid, the same turbine can efficiently utilise heat sources between 360 and 400 K, with mass flow rates ranging between 0.5 and 2.75 kg/s respectively. This corresponds to using the same turbine in ORC applications where the heat available ranges between 50 and 380 kWth, with the resulting net power produced by the ORC system ranging between 2 and 30 kW. Further investigations also suggest that the same pump could also be used; however, the heat exchanger area scales directly with increasing heat input. Overall, this paper demonstrates that through the optimal selection of the working fluid, the same turbomachinery components (i.e. pump and turbine) can be used in multiple ORC systems. This offers an opportunity to improve the current economy-of-scale of small ORC systems, ultimately leading to more economical systems for the utilisation of low temperature sustainable heat sources

System and component modelling and optimisation for an efficient 10 kWe low-temperature organic Rankine cycle utilising a radial inflow expander

Small-scale (10 kWe) organic Rankine cycles for low temperature applications such as heat recovery and solar power present a significant development opportunity but limited prototypes have been developed. This paper aims to address this by describing a system modelling tool which is used to select a working fluid, optimise cycle conditions, and preliminarily size a radial inflow rotor for an experimental test rig. The program is a steady-state sizing and optimisation tool which advances on current models by combining component models and cycle analysis with multi-objective optimisation and turbomachinery design aspects. Sizing and off-design pump and expander models are based on non-dimensional characteristic plots, whilst an additional design program achieves an expander rotor design. A novel objective function couples component and system performance with complexity. Results from an optimisation study indicate that R1234ze is the optimal working fluid for the defined objective function with a predicted net power output of 7.32 kWe, correlating to a cycle efficiency of 7.26%, and evaporator and condenser areas of 1.59 m2 and 2.40 m2, respectively. However, after considering operating pressures and fluid availability, R245fa has been highlighted as the most suitable fluid for a planned experimental radial expander test rig and a preliminary turbine design is proposed

A generalised assessment of working fluids and radial turbines for organic Rankine cycles

The aim of this paper is to conduct a generalised assessment of both optimal working fluids and radial turbine designs for small-scale organic Rankine cycle (ORC) systems across a range of heat-source temperatures. The former has been achieved by coupling a thermodynamic model of subcritical, non-recperated cycles with the Peng–Robinson equation of state, and optimising the working-fluid and cycle parameters for heat-source temperatures ranging between 80 °C and 360 °C . The critical temperature of the working fluid is found to be an important parameter governing working-fluid selection. Moreover, a linear correlation between heat-source temperature and the optimal critical temperature that achieves maximum power output has been found for heat-source temperatures below 300 °C ( Tcr=0.830Thi+41.27 ). This correlation has been validated against cycle calculations completed for nine predefined working fluids using both the Peng–Robinson equation of state and using the REFPROP program. Ultimately, this simple correlation can be used to identify working-fluid candidates for a specific heat-source temperature. In the second half of this paper, the effect of the heat-source temperature on the optimal design of a radial-inflow turbine rotor for a 25 kW subcritical ORC system has been studied. As the heat-source temperature increases, the optimal blade-loading coefficient increases, whilst the optimal flow coefficient reduces. Furthermore, passage losses are dominant in turbines intended for low-temperature applications. However, at higher heat-source temperatures, clearance losses become more dominant owing to the reduced blade heights. This information can be used to identify the most direct route to efficiency improvements in these machines. Finally, it is observed that the transition from a conventional converging stator to a converging-diverging stator occurs at heat-source temperatures of approximately 165 °C , whilst radially-fibered turbines seem unsuitable as the heat-source temperature exceeds 250 °C ; these conclusions can be used to inform expander design and selection at an early stag

A preliminary comparison of different turbine architectures for a 100 kW supercritical CO2 Rankine cycle turbine

The aim of this paper is to conduct a preliminary comparison of different turbine architectures for a small-scale 100 kW supercritical CO 2 Rankine cycle. The turbine is required to expand supercritical CO2 from 650°C and 170 bar, down to 50 bar. For such an application, it is not immediately clear which turbine architecture is the most suitable design when considering both aerodynamic and mechanical design constraints. Within this paper, three different turbine architectures are considered, namely radial-inflow, single-stage axial, and two-stage axial turbines. For each architecture, a preliminary design model is constructed which is based on conventional turbomachinery design parameters such as the loading coefficient, flow coefficient and degree of reaction. Using this model, a parametric investigation on the effect of the rotational speed on the required rotor diameter and blade height is conducted and the different turbine architectures are compared. This is completed with the view of establishing the feasible design space for a small-scale supercritical CO2 turbine. For all three architectures, it is found that in order to obtain feasible blade heights it is necessary to maximise the loading coefficient whilst m inimising the flow coefficient, and design the turbine with the minimum allowable diameter. Typically, this results in a turbine design w ith a rotor diameter of 30 mm, a rotor-inlet blade height in the range of 1.74 to 2.47 mm, and a rotational s peed between 150 and 250 kRPM for a single-stage radial or axial turbine, and 75 and 175 kRPM for a two-stage axial turbine. Ultimately, nine candidate turbine designs have been identified, which should be studied further using more advanced 3D CFD and FEA simulations

Development and Validation of a Thermo-Economic Model for Design Optimisation and Off-Design Performance Evaluation of a Pure Solar Microturbine

The aim of this paper is to present a thermo-economic model of a microturbine for solar dish applications, which demonstrates the applicability and accuracy of the model for off-design performance evaluation and techno-economic optimisation purposes. The model is built using an object-oriented programming approach. Each component is represented using a class made of functions that perform a one-dimensional physical design, off-design performance analysis and the component cost evaluation. Compressor, recuperator, receiver and turbine models are presented and validated against experimental data available in literature, and each demonstrated good accuracy for a wide range of operating conditions. A 7-kWe microturbine and solar irradiation data available for Rome between 2004 and 2005 were considered as a case study, and the thermo-economic analysis of the plant was performed to estimate the levelised cost of electricity based on the annual performance of the plant. The overall energy produced by the plant is 10,682 kWh, the capital cost has been estimated to be EUR 27,051 and, consequently, the specific cost of the plant, defined as the ratio between the cost of components and output power in design condition, has been estimated to be around EUR 3980/kWe. Results from the levelised cost of electricity (LCOE) analysis demonstrate a levelised cost of electricity of EUR 22.81/kWh considering a plant lifetime of 25 years. The results of the present case study have been compared with the results from IPSEpro 7 where the same component characteristic maps and operational strategy were considered. This comparison was aimed to verify the component matching procedure adopted for the present model. A plant sizing optimisation was then performed to determine the plant size which minimises the levelised cost of electricity. The design space of the optimisation variable is limited to the values 0.07–0.16 kg/s. Results of the optimisation demonstrate a minimum LCOE of 21.5 [EUR/kWh] for a design point mass flow rate of about 0.11 kg/s. This corresponds to an overall cost of the plant of around EUR 32,600, with a dish diameter of 9.4 m and an annual electricity production of 13,700 [kWh]