Small-scale Organic Rankine Cycle Research Articles

Thermodynamic and economic analyses of a hybrid waste-driven CHP–ORC plant with exhaust heat recovery

A smart hybrid power plant comprising a waste-fired CHP (combined heat and power) plant accompanied by a small-scale organic Rankine cycle was recently designed and analyzed thermodynamically. The objective of this hybridization is to maximize the share of electricity production of waste-CHP plants rather than a higher heat production rate in a cost-effective manner. In this work, utilization of the exergy of the hot flue gas of the waste-fired CHP unit in order to increase the potential of the organic Rankine cycle for maximizing the net power output of the hybrid cycle is proposed and techno-economically analyzed. In addition, the effect of using alternative environmentally-friendly organic working fluids on the performance of the system was investigated. The results of the study show that by the flue gas potential utilization of the same CHP unit, the size of the organic Rankine cycle may increase significantly, leading to a larger power output of the plant. As such, the net exergy and energy efficiency values of the combined plant in various operational conditions are improved compared to its primary configuration by, respectively, about 10% and 20%, when both of the main CHP cycle and the organic Rankine cycle are working at nominal load. In addition, this exergy utilization is beneficial economically as well, decreasing the payback period of the parallelization project by about 10% (from 7.4 years to 6.7 years). The alternative organic working fluid does not make a significant change in the technical and economic performance indices of the system.

Experimental study and artificial neural network based prediction of a free piston expander-linear generator for small scale organic Rankine cycle

In this paper, a novel free piston expander-linear generator (FPE-LG) prototype has been developed for small scale organic Rankine cycle system. The effects of three key operating parameters, including intake pressure, operation frequency and external load resistance on piston dynamics, output characteristics of the linear generator, and system energy conversion efficiency are investigated. An artificial neural network (ANN) based prediction model is established after evaluating different learning rates, hidden layer neural numbers and train functions. The ANN model is also validated and tested using the experimental data with consideration of mean squared error and correlation coefficient. Finally, combined the genetic algorithm with the ANN model, a parametric optimization and performance prediction for maximum power output of the linear generator is conducted. The results show that the free piston assembly operates stably with good consistency. Higher intake pressure and external load resistance are beneficial for improving the piston dynamics and output characteristics of the linear generator while the optimal operation frequency corresponding to the maximum peak power output is more dependent on the coordinated variation of the operating parameters. The maximum system energy conversion efficiency can reach up to 28.81% with the intake pressure of 0.2 MPa, operation frequency of 1.5 Hz and external load resistance of 5 Ω. The proposed ANN model shows a strong learning ability and generalization performance. The correlation coefficients between the ANN predictions and experimental data obtained from the validation and test processes are all close to 1. The optimized peak power output can reach up to 100.47 W based on the proposed ANN model. The ANN based method can provide a useful guidance for the performance prediction and coordinated optimization with the least deviation and high accuracy.

Experimental investigation on the effect of working fluid charge in a small-scale Organic Rankine Cycle under off-design conditions

Abstract Off-design performance evaluation is one of the major topics of ORC-related investigations. However, there is a lack of adequate research on the influence of working fluid charge on system off-design performances. This paper presents the experimental results of a 3 kWe ORC prototype. Effect of three different working fluid charge on system key operating parameters have been investigated. The performance of system with different charge levels were measured and compared. Moreover, the pump cavitation and system performance fluctuation were discussed. The results indicated that overcharging working fluid will slightly improve the output power of the expander and results in sharp decline of vapor superheat by high shaft torque. The highest power output of the expander was obtained by 40% pump stroke, and the calculated and measured values were 2.79 kWe, and 2.28 kWe, respectively. Furthermore, the isentropic efficiency of the pump can be also improved by overcharging working fluid. However, such improvement was more obvious with smaller pump stroke. Meanwhile, undercharging working fluid will lead to insufficient liquid level in condenser, and pump cavitation has been detected. Therefore, plenty amount of working fluid charge is necessary to maintain a certain liquid level in the condenser and to avoid pump cavitation.

Comparative Analysis of Small-Scale Organic Rankine Cycle Systems for Solar Energy Utilisation

Small-scale organic Rankine cycle (ORC) systems driven by solar energy are compared in this paper, which aims to explore the potential of power generation for domestic utilisation. A solar thermal collector was used as the heat source for a hot water storage tank. Thermal performance was then evaluated in terms of both the conventional ORC and an ORC using thermal driven pump (TDP). It is established that the solar ORC using TDP has a superior performance to the conventional ORC under most working conditions. Results demonstrate that power output of the ORC using TDP ranges from 72 W to 82 W with the increase of evaporating temperature, which shows an improvement of up to 3.3% at a 100 °C evaporating temperature when compared with the power output of the conventional ORC. Energy and exergy efficiencies of the ORC using TDP increase from 11.3% to 12.6% and from 45.8% to 51.3% when the evaporating temperature increases from 75 °C to 100 °C. The efficiency of the ORC using TDP is improved by up to 3.27%. Additionally, the exergy destruction using TDP can be reduced in the evaporator and condenser. The highest exergy efficiency in the evaporator is 96.9%, an improvement of 62% in comparison with that of the conventional ORC, i.e., 59.9%. Thus, the small-scale solar ORC system using TDP is more promising for household application.

Low temperature and power solar energy conversion for domestic use

This article summarizes the design, the manufacturing and the testing of a small scale Organic Rankine Cycle using Concentrated Solar Power as the heat source. First, the choice of the nominal point of the cycle is detailed, including the operating environment of the ORC (characteristics of the solar field and ambient temperature) and the constraints of operation. The paper presents the design of a radial turbine with a power of 3 kW and the numerical simulations of flow in the fluidic part of the turbine (nozzle, wheel etc). It also deals with system engineering, the calculation and the selection methodology of main components such as exchangers, pumps, and sensors.In the second part, the results of the laboratory tests is presented. The experiment was performed using as hot source, a thermal oil boiler. The analysis of the results shows isentropic efficiency around 40% on a rather large range of pressure drop, and a cycle efficiency of 5 % which, is encouraging in view of the first test conditions.

Performance analysis of a single-piston free piston expander-linear generator with intake timing control strategy based on piston displacement

A single-piston free piston expander-linear generator (FPE-LG) prototype for a small-scale Organic Rankine Cycle system is presented. The steady-state operation of the prototype is successfully achieved, which indicates the feasibility of single-piston FPE-LG with intake timing control strategy based on piston displacement. The effects of intake duration, intake pressure, theoretical stroke, and external load resistance (R) on operational performance and energy conversion efficiency are investigated. Results show that the piston is running at a relatively high velocity when it approaches the predetermined special displacement point (X1 or X2). The peak voltage, current, and power output increase linearly as the intake pressure and duration increase. The theoretical stroke exerts a minimal effect on peak velocity and peak voltage output but considerably affects the operating frequency. The peak power output and work-electric conversion efficiency (ηw-e) increase first and then decrease with increasing external load resistance. The peak output power and ηw-e reach the maximum value of 32.5 W and 30.8%, respectively, with an optimal R = 80 Ω. As the intake duration increases, the indicated efficiency increases gradually, whereas ηw-e shows an increasing and then decreasing trend. The optimal intake duration for reaching the maximum ηw-e is variable under different operation conditions.

Experimental Investigation on Performance of an Organic Rankine Cycle System Integrated with a Radial Flow Turbine

An experimental method is used to investigate the performance of a small-scale organic Rankine cycle (ORC) system which is integrated with a radial flow turbine, using 90 °C hot water as a heat source. The considered working fluids are R245fa and R123. The relationship between cycle performance and the operation parameters is obtained. With constant condensing pressure (temperature), the outlet temperature of the hot water, the mass flow rate of the hot water and the evaporator heat transfer rate increase with increasing evaporating pressure. Turbine isentropic efficiency decreases and transmission-generation efficiency increases with rising evaporating pressure. In the considered conditions, the maximum specific energy is 1.28 kJ/kg, with optimal fluid of R245fa and an optimal evaporating temperature of 69.2 °C. When the evaporating pressure (temperature) is constant, the outlet temperature of the cooling water increases, and the mass flow rate of the cooling water decreases with increasing condensing pressure. Turbine isentropic efficiency increases and transmission-generation efficiency decreases with the rise of condensing pressure. In the considered conditions, the maximum specific energy is 0.89 kJ/kg, with optimal fluid of R245fa and an optimal condensing temperature of 29.1 °C. Turbine efficiency is impacted by the working fluid type, operation parameters and nozzle type.

Economic investigation of low cost Organic Rankine Cycle incorporated in a combined solar photovoltaic cum thermal plant

Current rise in small scale solar thermal combined electricity generation water heating in Kenya has corresponded with demand growth for power supply in areas poorly served by the national grid. The potential technical approach to this increasing demand encourages the use of PV (Photovoltaic) solar collectors coupled with ORC (Organic Rankine Cycle) system. This paper uses Aspen plus software in modelling ORC system. Thika (00, 370, 41’E) climatic conditions is selected for the present case study to examine performance and cost of the system. The system comprises of PV collectors, oil storage unit, pump, and a small scale ORC engine using an expander. The model evaluates the performance of the system under the change in environmental temperature (Thika town conditions adopted) or varying the mass flow rate of heat transfer fluid (HTF). The results indicate that using Therminol oil 55 as HTF and R245fa (1, 1, 1, 3, 3-pentafluroropropane) as the working fluid and a collector heat source temperature of 163.50C delivered an output power of 15kW with the overall efficiency attained as 8.5%. The proposed system is intended to be potentially suited in Kenyan counties with good solar irradiation and without (or with high cost) access to the national grid electricity supply. Detailed cost analysis of the system was carried out in using Levelized Cost of Electricity (LCOE) order to determine the system viability and payback period and Depreciated Payback Period (DPP). For super heater outlet temperature of 1300C, mass flow rate 3000kg/h, the specific cost of 2kW system is 12, 073 $/kW with a LCOE of 13.5c$/kWh and a payback period of 10 years. While for 15kW system with the LCOE of 12.2 c$/kWh, specific installed cost is about 8, 068 $/kW; the payback period is approximately 6 years.

CFD analysis of variable wall thickness scroll expander integrated into small scale ORC systems

Abstract The studies of constant wall thickness scroll expander have pointed out that geometries with large built-in volume ratios are necessary to achieve high performances in small-sized organic Rankine cycle (ORC) units. The variable wall thickness expander design offers the opportunity of increasing the geometric expansion ratio with the number of scroll turns remaining unchanged to avoid sealing and lubricating issues. In this paper, unsteady and three-dimensional computational fluid dynamics (CFD) simulations of scroll expander using variable wall thicknesses were therefore carried out to investigate the effects of the geometry on the internal flow behaviour. The scroll expander was integrated into an ORC unit fed by R123. The dynamic mesh technology of ANSYS Fluent was applied to generate the deforming mesh in the expander working chambers. The aerodynamic performance analysis yielded how over-expansion phenomena occurred at low pressure ratio while under-expansion phenomena were existing at high pressure ratio which are consistent with the thermodynamic theory of scroll expander. The higher pressure ratio was also contributing to higher temperature drops during the expansion process. Moreover, the occurrence of flank leakages through the radial clearances and its effects on the flow field were pointed out further proving the thermodynamic theory of scroll expander.

A neural network approach to the combined multi-objective optimization of the thermodynamic cycle and the radial inflow turbine for Organic Rankine cycle applications

An optimization model based on the use of Neural Network surrogate models for the multi-objective optimization of small scale Organic Rankine Cycles is presented, which couples the optimal selection of the thermodynamic parameters of the cycle with the main design parameters of In-Flow Radial turbines. The proposed approach proved well suited in the resolution of the highly non-linear constrained optimization problems, typical of the design of energy systems. Indeed the use of a surrogate model allows to adopt gradient based methods that are computationally more efficient and accurate than conventional derivative-free optimization algorithms.The intensive numerical experiments demonstrate that assuming a constant efficiency for the In-Flow Radial turbine leads to an error in the evaluation of the performance of the system of up to 50% and that the optimization approach proposed improves the accuracy of the solution and it reduces the computational time required to reach it by two orders of magnitude. An holistic approach in which the turbine and the thermodynamic cycle are designed simultaneously and the use of multi-objective optimization proved to be essential for the design of Organic Rankine cycles that satisfy both size and performance criteria.

Performance analysis of low speed axial impulse turbine using two type nozzles for small-scale organic Rankine cycle

Expander is one of the core components in organic Rankine cycle (ORC). High rotating speed and limited working range are the key limitations on using turbine expander in small-scale ORC system. In this work, an axial turbine, consisting of 6 nozzles and 36 blades with mean diameter of 160 mm, is designed with partially admitted method to evaluate the feasibility of reducing rotating speed for small mass flow conditions and extending working range with R245fa used as working fluid. The results from 3D simulation show reasonable performance in design and off-design conditions. In design condition, the mass flow rate, turbine power output and turbine efficiency using Laval nozzle are 1.035 kg/s, 17.0 kW and 54.1%, higher than using convergent nozzle. However, the performance using convergent nozzle is better than using Laval nozzle when total mass flow is lower than 0.6 kg/s. Furthermore, reducing the number of nozzles can effectively increase power output of turbine and thermal efficiency of ORC for small mass flow conditions. With the designed turbine using 6 Laval nozzles and rotating at 10,000 RPM, the maximum net power output and thermal efficiency of bottoming ORC are 18.3 kW and 7.8%, respectively.

Machine Learning for the prediction of the dynamic behavior of a small scale ORC system

Dynamic modelling plays a crucial role in the analysis of Organic Rankine Cycle (ORC) systems for waste heat recovery, which deal with a highly unsteady heat source. The efficiency of small scale ORCs (i.e. below 100 kW power output) is low (<10%). Therefore, it is essential to keep the performance of the system as stable as possible. To do so, it is helpful to be able to predict the dynamic behavior of the system, in order to perform a maximization of its performance over the time.In this paper, Feedforward, Recurrent and Long Short Term Memory networks have been compared in the prediction of the dynamics of a 20 kW ORC system. Feedforward Neural Network is the simplest among the architectures developed for machine learning. Recurrent and Long Short Term Memory networks have been proved accurate in the prediction of the performance of dynamic systems. This study demonstrates that the three architectures are capable of predicting the dynamic behavior of the ORC system with a good degree of accuracy. The Long Short Term Memory architecture resulted as the highest performing, in that it correctly predicts the dynamics of the system, showing an error prediction lower than 5% and 10% respectively for what concern the prediction 10 and 60 seconds ahead.

Small scale Organic Rankine Cycle testing for low grade heat recovery by using refrigerants as working fluids

Small scale Organic Rankine Cycle testing for low grade heat recovery by using refrigerants as working fluids

Experimental and numerical analyses of a 5 kWe oil-free open-drive scroll expander for small-scale organic Rankine cycle (ORC) applications

Organic Rankine cycles (ORCs) are thermodynamic power cycles designed to generate work from a wide range of heat source conditions. In particular, low-grade waste heat recovery (WHR) (<150 °C) can be effectively exploited with such systems. The efficiency of an ORC is highly dependent on its expander performance. In the low power output range (<10 kW), scroll expanders are cost-effective. Although a number of researchers have investigated the use of scroll compressors as expanders, very little work has been carried out in modeling and investigating the performance of oil-free expanders. In this work, an experimental evaluation of a newly designed open-drive oil-free scroll expander was performed. The expander had a nominal capacity of 5 kW, built-in volume ratio of 3.5, and was integrated into an ORC test-rig with R245fa as working fluid. The experimental data consisted of 75 data points that were used to map the oil-free operation of the scroll expander over five expander rotational speeds (from 800 rpm to 3000 rpm). Two heat sources inlet temperature, i.e. 85 °C and 110 °C, were investigated. The scroll expander achieved a maximum overall isentropic efficiency of 0.58 for the temperature source of 110 °C, for the imposed specific volume ratio of 6.12 at rotational speed of 1600 rpm. For the same heat source, the maximum expander power output was 3.75 kW for an imposed specific volume ratio of 6.55 and rotational speed of 2500 rpm. Besides the experimental work, the performance of the expander was characterized by means of a semi-empirical model to break-down the different loss terms. A well known model available in the literature was extended to account for the major frictional losses in a scroll machine, i.e. bearings, tip-seals and other sources of friction. Additionally, an Artificial Neural Network (ANN) modeling approach was also proposed to achieve higher accuracy in mapping expander performance for use in system simulation. The experimental data and model source codes are provided as supplementary materials.

An appraisal of proportional integral control strategies for small scale waste heat to power conversion units based on Organic Rankine Cycles

Despite the increasing number of Organic Rankine Cycle (ORC) installations at megawatt scale, the waste heat rejected by industrial processes can vary substantially from a few kWh to many megawatt hours. Hence, ORC units with a power output in the range of tens of kilowatts should be developed to tackle the heat recovery and business opportunities that can arise from this market segment. In the current research activity, a dynamic model of a small scale ORC system was developed using a commercial software platform. The unit is equipped with two plate heat exchangers, a centrifugal pump and a radial turbine designed and optimized using an in-house code and a commercial 1D modelling tool. The off-design behaviour of the ORC system has been characterized by varying the inlet conditions of heat source and sink, and the revolution speed of the turbomachines. Moreover, the response to transient thermal inputs at different time scales has also been investigated. Finally, four control strategies have been compared from the performance and energy efficiency perspectives. The results show that the turbine based regulation strategies achieve better control performance while pump based controls are able to regulate the system by maintaining the net power output closer to the design point.

Performance investigation of a free piston expander-linear generator for small scale organic Rankine cycle

This paper presents a free piston expander-linear generator (FPE-LG) for small scale organic Rankine cycle (ORC). Based on FPE-LG experiment configuration, performance investigation and efficiency analysis on FPE-LG are conducted. Experiment results show that intake pressure and external load resistance have great influence on actual stroke, velocity and exhaust temperature. The exhaust pressure is hardly relevant to intake pressure and operation frequency. When FPE-LG operates with low operation frequency, the entropy generation decreases with intake pressure, whereas it increases with intake pressure when the operation frequency is high. The indicated work increases when increasing the intake pressure and external load resistance, whereas it decreases when increasing operation frequency. The indicated efficiency decreases with the intake pressure and external load resistance, and the maximum indicated efficiency can reach up to 81%. The voltage and power output increase when the intake pressure and external load resistance increase, and the maximum voltage and power output can reach 44.4 V and 111.6 W, respectively. High energy conversion efficiency can be achieved in the area of high external load resistance and intake pressure. Generally, the higher power output is, the higher energy conversion efficiency is. The maximum energy conversion efficiency of 73.4% is achieved.

Development and validation of a single-piston free piston expander-linear generator for a small-scale organic Rankine cycle

To improve the energy efficiency of internal combustion engines, this study develops a single-piston free piston expander-linear generator (FPE-LG) for a small scale organic Rankine cycle (ORC) in a waste heat recovery system. The prototype can operate continuously and stably, thereby indicating that the single-piston FPE-LG has a feasible working principle. The operation characteristics of FPE-LG are revealed, and the effects of the intake valve opening time (IVOT) and the exhaust valve opening time (EVOT) on the output performance are investigated. The experimental results reveal that the piston can run at a relatively high velocity near the dead centers, and peak acceleration is typically achieved when the piston arrives at the operation left/right dead centers (OLDC/ORDC). The piston stroke length is decreased significantly as the operating frequency is increased. The peak power output, peak in-cylinder pressure, peak velocity of the free piston, and utilization ratio of the piston stroke (η) are increased as the external load resistance is increased. In addition, η increases with longer IVOT and can reach up to 67.4% when the IVOT is 40 ms. The exhaust pressure decreases as the EVOT is increased, and the power output can be improved by regulating the IVOT rather than the EVOT.

Experimental and simulation study of a free piston expander–linear generator for small-scale organic Rankine cycle

This paper presents a new free piston expander–linear generator (FPE–LG) for small-scale organic Rankine cycle (ORC) waste heat recovery systems. An FPE–LG experimental test rig is developed, and a simulation model is established with Matlab/Simulink. Motion characteristics and output performance are discussed based on experimental results, and the simulation model is validated. Further investigation of the motion characteristics and output performance of FPE–LG is conducted at a relatively high intake pressure and operating frequency. Results show that intake pressure and operating frequency significantly affect motion characteristics and output performance. The velocity and displacement of the free piston assembly increase with increasing intake pressure, and the peak power output initially increases then decreases with increasing operating frequency. The peak power output is 676 W when the intake pressure, external load resistance, and operating frequency are 10 bar, 9 Ω, and 15 Hz, respectively. Using the simulation model to study FPE–LG operating parameters is convenient, and the model can serve as a reference for the future design and optimization of FPE–LG.

Experimental investigation of a free piston expander-linear generator with different valve timings

In this paper, a new free piston expander-linear generator (FPE-LG) prototype for small-scale organic Rankine cycle (ORC) waste heat recovery system has been developed. Firstly, the in-cylinder pressure, motion characteristics and output performance of the FPE-LG with different valve timings are investigated. Then, the effect of external load resistance on motion characteristics and output performance with cam plate A is analyzed. Finally, the effect of external load resistance on the indicated efficiency with cam plate A is studied. The results show that the intake duration and exhaust duration have significant influence on the motion characteristics and output performance of FPE-LG. Compared with other cam plates, cam plate A (the intake exhaust duration is 60°, and the exhaust duration is 180°) can be considered as the best choice for FPE-LG. The external load resistance not only influences the motion characteristics but also effects the output performance. The displacement and velocity increase with increasing the external load resistance. The peak power output with cam plate A is up to 18.6 W. The indicated efficiency shows a downward tendency when the external load resistance increases, and the maximum indicated efficiency of 66.2% is achieved.

Experimental investigation and optimal performance assessment of four volumetric expanders (scroll, screw, piston and roots) tested in a small-scale organic Rankine cycle system

The aim of this paper is to facilitate the selection of the expander for a small-scale organic Rankine cycle based on an experimental comparison of a piston, a screw, a scroll and a roots expander. First, based on a literature review, a comparison between these four technologies of volumetric expansion machines is performed. Afterward, four displacement expanders [2–4 kW] are tested on two similar small-scale ORC unit with fluid R245fa. The maximum effective isentropic efficiencies measured are 53% for the piston expander and the screw expander, 76% for the variable-speed scroll and 48% for the roots machine. However, these performances do not reflect the highest efficiencies achievable by each expander: the test-rig presents experimental limitations in terms of mass flow rate and pressure drop (among others) that restricts the achievable operating conditions. The calibration of semi-empirical models based on the measurements allows to overcome this issue and to predict the isentropic efficiency in optimal conditions despite the limitations of the test-rigs. Based on experimental results, extrapolated prediction of the semi-empirical model and practical considerations, some guidelines are drawn to help the reader to select properly a volumetric expander.