System and component modelling and optimisation for an efficient 10 kWe low-temperature organic Rankine cycle utilising a radial inflow expander (original) (raw)
Related papers
Energies, 2018
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 stage.
Energies, 2016
A multi-variable optimization program has been developed to investigate the performance of Organic Rankine Cycles (ORCs) for low temperature heat recovery applications using both turbine and twin-screw expanders when account is taken of performance variation due to changes in ambient conditions. The cycle simulation contains thermodynamic models of both types of expander. In the case of the twin-screw machine, the methods used to match the operation of the expander to the requirements of the cycle are described. The performance of turbine expanders in a superheated ORC has been modelled using correlations derived from operational data for single stage reaction turbines to predict the turbine efficiency at "off-design" conditions. Several turbine configurations have been considered including variable nozzle area and variable speed. The capability of the cycle model has been demonstrated for the case of heat recovery from a steady source of pressurized hot water at 120˝C. The system parameters are optimised for a typical operating condition, which determines the required size of heat exchangers and the expander characteristics. Performance at off-design conditions can then be optimized within these constraints. This allows a rigorous investigation of the effect of air temperature variation on the system performance, and the seasonal variation in net power output for the turbine and twin-screw ORC systems. A case study is presented for a low temperature heat recovery application with system electrical power output of around 100 kWe at design conditions. The results indicate that similar overall performance can be achieved for ORC systems using either type of expander.
Expander design procedures and selection criterion for small rated organic rankine cycle systems
Energy Science & Engineering, 2020
The expander selection and the optimization of its performance are fundamental in low-temperature heat recovery systems. The parameters that must be considered are high efficiency isentropic, pressure ratio, power, lubrication requirements, complexity, rotation speed, dynamic balancing, reliability, and cost. In these systems, the turboexpanders are not always the optimal solution. For these reasons, taking advantage of all the research in the field of dynamic and volumetric machines, a design procedure for all expander types has been presented. 1.1 | Inward-flow radial turbine The radial flow turbines (IFR) are used in turbochargers for commercial engines and fire pumps. They are very compact
Conventional Organic Rankine Cycles (ORCs) using ambient air as their coolant cannot fully utilize the greater temperature differential available to them during the colder months. However, changing the working fluid composition so its boiling temperature matches the ambient temperature as it changes has been shown to have potential to increase year-round electricity generation. Previous research has assumed that the cycle pressure ratio is able to vary without a major loss in the isentropic efficiency of the turbine. This paper investigates if small scale ORC systems that normally use positive-displacement expanders with fixed expansion ratios could also benefit from this new concept. A numerical model was firstly established, based on which a comprehensive analysis was then conducted. The results showed that it can be applied to systems with positive-displacement expanders and improve their year-round electricity generation. However, such an improvement is less than that of the systems using turbine expanders with variable expansion ratios. Furthermore, such an improvement relies on heat recovery via the recuperator. This is because expanders with a fixed expansion ratio have a relatively constant pressure ratio between their inlet and outlet. The increase of pressure ratio between the evaporator and condenser by tuning the condensing temperature to match colder ambient condition in winter cannot be utilised by such expanders. However, with the recuperator in place, the higher discharging temperature of the expander could increase the heat recovery and consequently reduce the heat input at the evaporator, increasing the thermal efficiency and the specific power. The higher the amount of heat energy transferred in the recuperator, the higher the efficiency improvement.
IEEE Access, 2019
In an organic Rankine cycle (ORC), the study of the cycle efficiencies and the turbine is essential to know the performance in the generation of electrical energy. The proper selection of a working fluid is relevant, because it must be environmentally friendly and compatible with the ORC plant. This article presents an experimental study for the analysis of the cycle and thermal efficiencies on a 1-kW ORC system and the isentropic efficiency of its scroll expander. The test was performed on a 1-kW ORC with scroll expander system with R245fa as the working fluid. Furtheremore, it was considered a working temperature below 100 • C, which is used in waste heat recovery systems, to determine the performance of the ORC. The enthalpy is estimated with the Coolprop software. For estimating the cycle and thermal efficiency, the net power and the thermal power, which are supplied to evaporate the working fluid, were considered. The isentropic efficiency of the expander was calculated by the scroll mechanical work and the hydraulic work at the scroll expander. The results show that the mean isentropic efficiency of the fluid in the prototype test for ORC in the generation of 1000 W was about 60%, a promising value for the generation of electrical energy using the residual heat from industry.
IOP Conference Series: Materials Science and Engineering, 2015
Low temperature organic Rankine cycles offer a promising technology for the generation of power from low temperature heat sources. Small-scale systems (~10kW) are of significant interest, however there is a current lack of commercially viable expanders. For a potential expander to be economically viable for small-scale applications it is reasonable to assume that the same expander must have the ability to be implemented within a number of different ORC applications. It is therefore important to design and optimise the cycle considering the component performance, most notably the expander, both at different thermodynamic conditions, and using alternative organic fluids. This paper demonstrates a novel modelling methodology that combines a previously generated turbine performance map with cycle analysis to establish at what heat source conditions optimal system performance can be achieved using an existing turbine design. The results obtained show that the same turbine can be effectively utilised within a number of different ORC applications by changing the working fluid. By selecting suitable working fluids, this turbine can be used to convert pressurised hot water at temperatures between 360K and 400K, and mass flow rates between 0.45kg/s and 2.7kg/s, into useful power with outputs between 1.5kW and 27kW. This is a significant result since it allows the same turbine to be implemented into a variety of applications, improving the economy of scale. This work has also confirmed the suitability of the candidate turbine for a range of low temperature ORC applications.
Energy Conversion and Management, 2016
Most studies on the organic Rankine cycle (ORC) focused on parametric studies and selection working fluids to maximize the performance of organic Rankine cycle but without attention for turbine design features which are crucial to achieving them. The rotational speed, expansion ratio, mass flow rate and turbine size have markedly effect on turbine performance. For this purpose organic Rankine cycle modeling, mean-line design and three-dimensional computational fluid dynamics analysis were integrated for both micro axial and radial-inflow turbines with five organic fluids (R141b, R1234yf, R245fa, n-butane and n-pentane) for realistic low-temperature heat source <100°C like solar and geothermal energy. Three-dimensional simulation is performed using ANSYS R17-CFX where three-dimensional Reynolds-averaged Navier-Stokes equations are solved with k-omega shear stress transport turbulence model. Both configurations of turbines are designed at wide range of mass flow rate (0.1-0.5) kg/s for each working fluid. The results showed that n-pentane has the highest performance at all design conditions where the maximum total-to-total efficiency and power output of radial-inflow turbine are 83.85% and 8.893 kW respectively. The performance of the axial turbine was 83.48% total-to-total efficiency and 8.507 kW power output. The maximum overall size of axial turbine was 64.685 mm compared with 70.97 mm for radial-inflow turbine. R245fa has the lowest overall size for all cases. The organic Rankine cycle thermal efficiency was about 10.60% with radial-inflow turbine and 10.14% with axial turbine. Such results are better than other studies in the literature and highlight the potential of the integrated approach for accurate prediction of the organic Rankine cycle performance based on micro-scale axial and radial-inflow turbines.
Characterising a turbine for application in an organic Rankine cycle
Turbines are critical components of electricity generating low grade WHR&U (waste heat recovery and utilisation) systems, and determining the performance of the turbine is crucial to the successful implementation of these systems. In this paper, an experimental waste heat turbine kit e with the turbine rotor later replaced with an in-house designed and manufactured radial inflow turbine rotor e was sourced, assembled and tested in a designed and built test bench, using air as the working fluid. From the obtained test data, the turbine performance characteristics were determined, and plotted as turbine performance maps. The determined performance characteristics for the turbine working with air were then further used to scale the turbine for a refrigerant-123 application, thus predicting its performance for integration into an organic Rankine cycle low grade WHR&U system.
Energy, 2020
Abstract Reliable and low-cost expanders are fundamental for the competitiveness of small-scale Organic Rankine Cycle (ORC) plants using low-temperature heat sources. Regenerative flow turbines (RFTs) can be considered a low-cost and viable alternative expander, yet their performance needs to be fully investigated. Therefore, the use of an RFT in a micro-scale ORC test bench is investigated in this work through a modelling study. Specifically, three-dimensional CFD simulations are carried out to assess the performance of the considered expander with varying operating conditions and a numerical model of a non-regenerative, small-scale ORC system is developed to investigate its potential in waste heat recovery (WHR) applications. Using R245fa as the working fluid, the CFD analysis shows that the expander achieves a maximum total-to-static isentropic efficiency of about 44% in the investigated operating range. The small-scale ORC system has a net output power in the range 100–600 W and a net cycle efficiency of 1–2.3%. Moreover, a comparison with two scroll expanders having different built-in volume ratios shows that the RFT operates with higher isentropic efficiencies in low mass flow rates and pressure ratios thus highlighting its suitability for low-temperature WHR applications, especially when considerable fluctuations of the heat source are expected.
2019
Organic Rankine Cycles (ORCs) have an interesting potential in small-scale power production using low-temperature heat sources. Despite their unique applications in power production from lowtemperature heat sources, the widespread use of small-scale ORC systems is still challenging especially due to high primary cost resulting in long-term payback periods. The reason goes back to several inherent technical limitations especially related to the available expander machines, which are usually accounted for a significant amount of the total investment cost of such systems. In this paper, a Regenerative Flow Turbine (RFT) adopted in a small-scale ORC prototype is investigated by means of a modelling study. The characteristic curves of the considered turbine have been obtained using CFD analysis to evaluate its performance in an ORC unit working with R245fa. Results of CFD have shown that performance of the RFT has been improved considerably using R245fa and real gas model instead of air ...