Organic Rankine Cycle Turbogenerators Research Articles

Organic Rankine cycle turbogenerator cooling – Optimization of the generator water jacket heat exchange surface

Discharge of excess heat from the micro-turbine electric generator is an important issue for the micro-turbine operation and should be considered during the design process. The manuscript is intended to present a new tool for rapid shape optimization of the heat exchanging surface of the ORC (Organic Rankine Cycle) micro-turbine electric generator. The optimization of the water jacket geometry of a 10 kWe ORC micro-turbine generator is performed by making use of a modified, with respect to the models presented in the literature, 0D heat transfer model of the finned heat exchange surface of the cooling water jacket, and a genetic algorithm. As a result of optimization, an apparent reduction in fin size and increased fin density at the finned wall surface is achieved, yielding a significant increase of the heat transfer coefficient and a temperature decrease of the electric generator stator. The temperature at the shell surface tangential to the electric generator stator was found to decrease by over 6°C. A considerable reduction of temperature gradients around the water jacket wall is also reported.The results of optimization from the 0D model are verified by numerical simulation using a full 3D CFD model of fluid flow and conjugate heat transfer (CHT) in Ansys CFX for the original and optimized geometries. The CFD CHT model was first validated using experiment-based correlations for heat transfer in liquid flow over a circular cylinder pipe for a wide range of Reynolds numbers.The obtained temperature decrease in the optimized ORC electric generator stator for the nominal operating point of the ORC generator is found similar for both 0D and 3D models. The temperatures calculated in the 0D model at the shell surface tangential to the electric generator stator deviate from respective CFD values by 0.5–1.5°C. Another operational case beyond the nominal operating point has been analysed making use of the 3D CFD method as well, showing that the optimized geometry is characterized by an improved heat exchange performance not only for the nominal operating point.

Experimental Investigation of a Small-Scale Organic Rankine Cycle Turbo-Generator Supported on Gas-Lubricated Bearings

Abstract Waste heat recovery (WHR) is expected to contribute to reducing CO2 emissions from trucks. Organic Rankine cycle (ORC) systems show the highest potential for this application, but still lack efficient small-scale expansion devices, in practice. A novel turbogenerator (TG) supported on gas-lubricated bearings is presented in this paper. The device combines a single-stage radial-inflow turbine and a permanent-magnet machine in a single rotating part supported on aerodynamic bearings, lubricated with the working fluid (R245fa). The oil-free expander was tested within a dedicated ORC test setup. It was driven up to its nominal speed of 100 krpm, generated up to 2.3 kW of electrical power, and reached a peak overall efficiency of 67%. Although the prototype was not actively cooled, the mechanical losses of the rotor shaft and the iron loss of the electrical machine reached their nominal levels. Only the copper loss was at a part-load level. The electromechanical efficiency of the TG reached 91% and is expected to increase while testing the device at higher load. This proof of concept confirms the high-speed and low-loss potential of gas-lubricated bearings for small-scale dynamic expanders.

Toward the Integrated Design of Organic Rankine Cycle Power Plants: A Method for the Simultaneous Optimization of Working Fluid, Thermodynamic Cycle, and Turbine

Abstract The conventional design of organic Rankine cycle (ORC) power systems starts with the selection of the working fluid and the subsequent optimization of the corresponding thermodynamic cycle. More recently, systematic methods have been proposed integrating the selection of the working fluid into the optimization of the thermodynamic cycle. However, in both cases, the turbine is designed subsequently. This procedure can lead to a suboptimal design, especially in the case of mini- and small-scale ORC systems, since the preselected combination of working fluid and operating conditions may lead to infeasible turbine designs. The resulting iterative design procedure may end in conservative solutions after multiple trial-and-error attempts due to the strong interdependence of the many design variables and constraints involved. In this work, we therefore present a new design and optimization method integrating working fluid selection, thermodynamic cycle design, and preliminary turbine design. To this purpose, our recent 1-stage continuous-molecular targeting (CoMT)-computer-aided molecular design (CAMD) method for the integrated design of the ORC process and working fluid is expanded by a turbine meanline design procedure. Thereby, the search space of the optimization is bounded to regions where the design of the turbine is feasible. The resulting method has been tested for the design of a small-scale high-temperature ORC unit adopting a radial-inflow turbo-expander. The results confirm the potential of the proposed method over the conventional iterative design practice for the design of small-scale ORC turbogenerators.

Design and Manufacturing of Micro-Turbomachinery Components with Application of Heat Resistant Plastics

Abstract The article discusses issues associated with the use of modern plastics for the construction of high-speed fluid-flow machines. Currently available plastics exhibit high chemical resistance as well as dimensional and shape stability across a wide temperature range. This allows them to be used for manufacturing components of micro turbomachinery, thereby reducing production time and costs. This article discusses the criteria for the selection of plastics suitable for a particular machine, namely micro turbogenerator operating in the organic Rankine cycle (ORC). In addition to the initial selection of materials based on their chemical and physical properties, strength calculations of selected turbogenerator subassemblies were carried out. The obtained results confirmed that some plastics can replace traditional materials used in the manufacture of ORC turbogenerators. This concerns, in particular, the components of the microturbine blade system. After the manufacture of a trial series of such components, it became apparent that, with appropriately chosen plastics, it is possible to shorten the machining time and reduce production costs, all while maintaining the required dimensional tolerances. The results obtained so far prove that it is possible to use plastics to produce components of modern turbomachines, for instance, parts of high-speed microturbines that have to withstand high operating temperatures.

Combined Turbine and Cycle Optimization for Organic Rankine Cycle Power Systems—Part B: Application on a Case Study

Organic Rankine cycle (ORC) power systems have recently emerged as promising solutions for waste heat recovery in low- and medium-size power plants. Their performance and economic feasibility strongly depend on the expander. The design process and efficiency estimation are particularly challenging due to the peculiar physical properties of the working fluid and the gas-dynamic phenomena occurring in the machine. Unlike steam Rankine and Brayton engines, organic Rankine cycle expanders combine small enthalpy drops with large expansion ratios. These features yield turbine designs with few highly-loaded stages in supersonic flow regimes. Part A of this two-part paper has presented the implementation and validation of the simulation tool TURAX, which provides the optimal preliminary design of single-stage axial-flow turbines. The authors have also presented a sensitivity analysis on the decision variables affecting the turbine design. Part B of this two-part paper presents the first application of a design method where the thermodynamic cycle optimization is combined with calculations of the maximum expander performance using the mean-line design tool described in part A. The high computational cost of the turbine optimization is tackled by building a model which gives the optimal preliminary design of an axial-flow turbine as a function of the cycle conditions. This allows for estimating the optimal expander performance for each operating condition of interest. The test case is the preliminary design of an organic Rankine cycle turbogenerator to increase the overall energy efficiency of an offshore platform. For an increase in expander pressure ratio from 10 to 35, the results indicate up to 10% point reduction in expander performance. This corresponds to a relative reduction in net power output of 8.3% compared to the case when the turbine efficiency is assumed to be 80%. This work also demonstrates that this approach can support the plant designer in the selection of the optimal size of the organic Rankine cycle unit when multiple exhaust gas streams are available.

Part-Load Performance of aWet Indirectly Fired Gas Turbine Integrated with an Organic Rankine Cycle Turbogenerator

Over the last years, much attention has been paid to the development of efficient and low-cost power systems for biomass-to-electricity conversion. This paper aims at investigating the design- and part-load performance of an innovative plant based on a wet indirectly fired gas turbine (WIFGT) fueled by woodchips and an organic Rankine cycle (ORC) turbogenerator. An exergy analysis is performed to identify the sources of inefficiencies, the optimal design variables, and the most suitable working fluid for the organic Rankine process. This step enables to parametrize the part-load model of the plant and to estimate its performance at different power outputs. The novel plant has a nominal power of 250 kW and a thermal efficiency of 43%. The major irreversibilities take place in the burner, recuperator, compressor and in the condenser. Toluene is the optimal working fluid for the organic Rankine engine. The part-load investigation indicates that the plant can operate at high efficiencies over a wide range of power outputs (50%–100%), with a peak thermal efficiency of 45% at around 80% load. While the ORC turbogenerator is responsible for the efficiency drop at low capacities, the off-design performance is governed by the efficiency characteristics of the compressor and turbine serving the gas turbine unit.

Waste heat recovery technologies for offshore platforms

This article aims at finding the most suitable waste heat recovery technology for existing and future offshore facilities. The technologies considered in this work are the steam Rankine cycle, the air bottoming cycle and the organic Rankine cycle.A multi-objective optimization approach is employed to attain optimal designs for each bottoming unit by selecting specific functions tailored to the oil and gas sector, i.e. yearly CO2 emissions, weight and economic revenue. The test case is the gas turbine-based power system serving an offshore platform in the North Sea.Results indicate that the organic Rankine cycle technology presents larger performances compared to steam Rankine cycle units, whereas the implementation of air bottoming cycle modules is not attractive from an economic and environmental perspective compared to the other two technologies.Despite the relatively high cost of the expander and of the primary heat exchanger, organic Rankine cycle turbogenerators appear thus to be the preferred solution to abate CO2 emissions and pollutants on oil and gas facilities.As a practical consequence, this paper provides guidelines for the design of high-efficiency, cost-competitive and low-weight power systems for offshore installations.

Application of Unscented Kalman Filter for Condition Monitoring of an Organic Rankine Cycle Turbogenerator


 
 
 This work relates to a project focusing on energy optimization on offshore facilities. On oil and gas platforms it is common practice to employ gas turbines for power production. So as to increase the system performance and reduce emissions, a bottoming cycle unit can be designed with particular emphasis on compactness and reliability. In such context, organic Rankine cycle turbogenerators are a promising technology. The implementation of an organic Rankine cycle unit is thus considered for the power system of the Drau- gen offshore platform in the northern sea, which is the case study for this project. Considering the plant dynamics, it is of paramount importance to monitor the peak temperatures within the once-through boiler serving the bottoming unit to prevent the decomposition of the working fluid. This paper accordingly aims at applying the unscented Kalman filter to estimate the temperature distribution inside the primary heat exchanger by engaging a detailed and distributed model of the system and available measurements. Simulation results prove the robustness of the unscented Kalman filter with respect to process noise, measurement disturbances and initial conditions.
 
 

Design methodology for flexible energy conversion systems accounting for dynamic performance

This article presents a methodology to help in the definition of the optimal design of power generation systems. The innovative element is the integration of requirements on dynamic performance into the system design procedure. Operational flexibility is an increasingly important specification of power systems for base- and part-load operation. Thus, it is crucial to discard, in an early phase of the design process, plant configurations which feature unacceptable dynamic performance. The test case is the preliminary design of an off-grid power plant serving an off-shore platform where one of the three gas turbines is combined with an organic Rankine cycle turbogenerator to increase the overall energy efficiency. The core of the procedure is a stationary model, capable of performing the on-design thermodynamic cycle calculation, and the design of the components of the system. The results of these simulations are used within the framework of a multi-objective optimization procedure to identify a number of equally optimal system configurations. A dynamic model of each of these systems is automatically parameterized, by inheriting its parameters values from the design model. Dynamic simulations allow then to discriminate among the initial set of solutions, thus providing the designs that also comply with dynamic requirements.

Optimal design of compact organic Rankine cycle units for domestic solar applications

Organic Rankine cycle turbogenerators are a promising technology to transform the solar radiation harvested by solar collectors into electric power. The present work aims at sizing a small-scale organic Rankine cycle unit by tailoring its design for domestic solar applications. Stringent design criteria, i. e., compactness, high performance and safe operation, are targeted by adopting a multi-objective optimization approach modeled with the genetic algorithm. Design-point thermodynamic variables, e. g., evaporating pressure, the working fluid, minimum allowable temperature differences, and the equipment geometry, are the decision variables. Flat plate heat exchangers with herringbone corrugations are selected as heat transfer equipment for the preheater, the evaporator and the condenser. The results unveil the hyperbolic trend binding the net power output to the heat exchanger compactness. Findings also suggest that the evaporator and condenser minimum allowable temperature differences have the largest impact on the system volume and on the cycle performances. Among the fluids considered, the results indicate that R1234yf and R1234ze are the best working fluid candidates. Using flat plate solar collectors (hot water temperature equal to 75 ?C), R1234yf is the optimal solution. The heat exchanger volume ranges between 6.0 and 23.0 dm3, whereas the thermal efficiency is around 4.5%. R1234ze is the best working fluid employing parabolic solar collectors (hot water temperature equal to 120 ?C). In such case the thermal efficiency is around 6.9%, and the heat exchanger volume varies from 6.0 to 18.0 dm3.

Performance improvement of a radial organic Rankine cycle turbine by means of automated computational fluid dynamic design

There is a growing interest in organic Rankine cycle turbogenerators because of their ability to efficiently utilize external heat sources at low-to-medium temperature in the small-to-medium power range. High-temperature organic Rankine cycle turbines typically operate at very high pressure ratio and expand the organic working fluid in the dense-vapour thermodynamic region, thus requiring computational fluid dynamics solvers coupled with accurate thermodynamic models for their performance assessment and design. In this article we present a steady-state three-dimensional viscous computational fluid dynamics study of the Tri-O-Gen organic Rankine cycle radial turbine, including the radial nozzle, the rotor and the diffuser. The turbine operates with toluene as the working fluid, whose accurate thermophysical properties are obtained with a look-up table approach. Based on the three-dimensional simulation results, together with a two-dimensional fluid dynamic optimisation procedure documented elsewhere, an improved nozzle geometry is designed, manufactured and experimentally tested. Measurements show it delivers 5 kWe or 4% more net power output, as well as improved off-design performance.

Assessment of Waste Heat Recovery From a Heavy-Duty Truck Engine by Means of an ORC Turbogenerator

This paper documents a feasibility study on a waste heat recovery system for heavy-duty truck engines based on an organic Rankine cycle (ORC) turbogenerator. The study addresses many of the challenges of a mobile automotive application: The system must be simple, efficient, relatively small, lightweight, and the working fluid must satisfy the many technical, environmental, and toxicological requirements typical of the automotive sector. The choice of a siloxane as the working fluid allows for the preliminary design of an efficient radial turbine, whose shaft can be lubricated by the working fluid itself. The system's heat exchangers, though more voluminous than desirable, are within acceptable limits. The simulated ORC system would add approximately 9.6 kW at the design point, corresponding to a truck engine power output of 150 kW at 1500 rpm. Future work will be devoted to further system and components optimization by means of simulations, to the study of dynamic operation and control, and will be followed by the design and construction of a laboratory test bench for mini-ORC systems and components.

Enhancement of the Electrical Efficiency of Commercial Fuel Cell Units by Means of an Organic Rankine Cycle: A Case Study

Among the various fuel cell (FC) systems, molten carbonate fuel cells (MCFC) are nowadays one of the most promising technologies, thanks to the lower specific costs and a very high electrical efficiency (net low heating value (LHV) electric efficiency in the range 45%–50% at MWel scale using natural gas as fuel). Despite this high performance, MCFC rejects to the ambient almost half of the fuel energy at about 350–400 °C. Waste heat can be exploited in a recovery Rankine cycle unit, thereby enhancing the electric efficiency of the overall system. Due to the temperature of the heat source and the relatively small power capacity of MCFC plants (from few hundred kWel to 10 MWel), steam Rankine cycle technology is uneconomical and less efficient compared to that of the organic Rankine cycle (ORC). The objective of this work is to verify the practical feasibility of the integration between a MCFC system (topping unit) and an ORC turbogenerator (bottoming unit). The potential benefits of the combined plant are assessed in relation to a commercial MCFC stack. In order to identify the most suitable working fluids for the ORC system, organic substances are considered and compared. The figure of merit is the maximum net power of the overall system. Finally, the economical benefits of the integration are determined by evaluating the levelized cost of electricity (LCOE) of the combined plant, with respect to the standalone MCFC system. In order to assess the economic viability of the bottoming power unit, two cases are considered. In the first one, the ORC power output is approximately 500 kWel; in the latter, about 1 MWel. Results show that the proposed solution can increase the electrical power output and efficiency of the plant by more than 10%, well exceeding 50% overall electrical efficiency. In addition, the LCOE of the combined power plant is 8% lower than the standalone MCFC system.

Dynamic Modeling of Organic Rankine Cycle Power Systems

New promising applications of organic Rankine cycle (ORC) technology, e.g., concentrated solar power, automotive heat recovery and off-grid distributed electricity generation, demand for more dynamic operation of ORC systems. Accurate physically-based dynamic modeling plays an important role in the development of such systems, both during the preliminary design as an aid for configuration and equipment selection, and for control design and optimization purposes. A software library of modular reusable dynamic models of ORC components has been developed in the MODELICA language and is documented in the paper. The model of an exemplary ORC system, namely the 150 kWe Tri-O-Gen ORC turbogenerator is validated using few carefully conceived experiments. The simulations are able to reproduce steady-state and dynamic measurements of key variables, both in nominal and in off-design operating conditions. The validation of the library opens doors to control-related studies, and to the development of more challenging dynamic applications of ORC power plants.

Computational Study of a High-Expansion Ratio Radial Organic Rankine Cycle Turbine Stator

There is a growing interest in organic Rankine cycle (ORC) turbogenerators because they are suitable as sustainable energy converters. ORC turbogenerators can efficiently utilize external heat sources at low to medium temperature in the small to medium power range. ORC turbines typically operate at very high pressure ratio and expand the organic working fluid in the dense-gas thermodynamic region, thus requiring computational fluid dynamics (CFD) solvers coupled with accurate thermodynamic models for their performance assessment and design. This article presents a comparative numerical study on the simulated flow field generated by a stator nozzle of an existing high-expansion ratio radial ORC turbine with toluene as working fluid. The analysis covers the influence on the simulated flow fields of the real-gas flow solvers: FLUENT, FINFLO, and ZFLOW, of two turbulence models and of two accurate thermodynamic models of the fluid. The results show that FLUENT is by far the most dissipative flow solver, resulting in large differences in all flow quantities and appreciably lower predictions of the isentropic nozzle efficiency. If the combination of the k−ω turbulence model and FINFLO solver is adopted, a shock-induced separation bubble appears in the calculated results. The bubble affects, in particular, the variation in the flow velocity and angle along the stator outlet. The accurate thermodynamic models by Lemmon and Span (2006, “Short Fundamental Equations of State for 20 Industrial Fluids,” J. Chem. Eng. Data, 51(3), pp. 785–850) and Goodwin (1989, “Toluene Thermophysical Properties From 178 to 800 K at Pressures to 1000 Bar,” J. Phys. Chem. Ref. Data, 18(4), pp. 1565–1636) lead to small differences in the flow field, especially if compared with the large deviations that would be present if the flow were simulated based on the ideal gas law. However, the older and less accurate thermodynamic model by Goodwin does differ significantly from the more accurate Lemmon–Span thermodynamic model in its prediction of the specific enthalpy difference, which leads to a considerably different value for the specific work and stator isentropic efficiency. The above differences point to a need for experimental validation of flow solvers in real-gas conditions, if CFD tools are to be applied for performance improvements of high-expansion ratio turbines operating partly in the real-gas regime.

Real-Gas Effects in Organic Rankine Cycle Turbine Nozzles

Organic Rankine cycle turbogenerators are a viable option as stationary energy converters for external heat sources, in the low power range (from a few kW up to a few MW). The fluid-dynamic design of organic Rankine cycle turbines can benefit from computational fluid dynamics tools which are capable of properly taking into account real-gas effects occurring in the turbine, which typically expands in the nonideal-gas thermodynamic region. In addition, the potential efficiency increase offered by supercritical organic Rankine cycles, which entails even stronger real-gas effects, has not yet been exploited in current practice. In this paper, real-gas effects occurring in subcritical and supercritical organic Rankine cycle nozzles have been investigated. Two-dimensional Euler simulations of an existing axial organic Rankine cycle stator nozzle are carried out using a computational fluid dynamics code, which is linked to an accurate thermodynamic model for the working fluid (octamethyltrisiloxane C 8 H 28 O 2 Si 3 ). The cases analyzed include the expansions starting from actual subcritical conditions, that is, the design point and part-load operation, and three expansions starting from supercritical conditions. Results of the simulations of the existing nozzle for current operating conditions can be used to refine its design. Moreover, the simulations of the nozzle expansions starting from supercritical conditions show that a nozzle geometry with a much higher exit-to-throat area ratio is required to obtain an efficient expansion. Other peculiar characteristics of supercritical expansions such as low sound speed and velocity, high density, and mass flow rate, are discussed.