Small-scale Organic Rankine Cycle Research Articles

Combined cold, heat and power system, based on an organic Rankine cycle, using biomass as renewable heat source for energy saving and emissions reduction in a supermarket

Supermarkets are recognized worldwide as great energy consumers, especially in developed countries. Typically, energy consumptions are due to cold, heat and electricity requirements. So a Combined Cold, Heat and Power (CCHP) system (also called trigeneration system) can be used to reduce energy demands and, hence, greenhouse gas emissions to the atmosphere. In this way, this work proposes a small-scale Organic Rankine Cycle (ORC) CCHP application in a supermarket. The ORC has been designed to be used with a biomass boiler, and to produce electricity and useful heat at two different temperature levels related to an absorption chiller and supermarket heating demand.With this in mind, this work conducts the experimental characterization of the CCHP system in a test bench. Thereby, the experimental results obtained are used to predict the expected performance of the system under the supermarket conditions, as well as to quantify the energy, environmental and economic benefits of the application.

Working fluid selection for a small-scale organic Rankine cycle recovering engine waste heat

This paper reports the design and evaluation of a 1 kWe Organic Rankine cycle using different working fluids for engine coolant and exhaust recovery from a 6.5 kW small ICE. Six working fluids have been selected to evaluate and compare the performance of the ORC system. The net power output, thermal efficiency, rotational speed of the scroll expander and condenser load of the ORC system have been studied. Results indicated R134a and R125a have better overall performance than other candidates when the designed inlet temperature of the expander is higher than 150 °C. The highest net power and thermal efficiency are respectively 1.2 kW and 13% when R125a is used as the working fluid. R600 and R245fa are desirable to be used when the optimal rotational speed of the scroll expander is about 3000 RPM. The proposed ORC engine coolant and exhaust waste heat recovery system has the advantages of simple system layout, low dumped heat load of condenser, high power output.

Experimental investigation of four volumetric expanders

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 and two scroll expanders. First, based on a literature review, a comparison between these three technologies of volumetric expansion machines is performed. Afterward, four displacement expanders (~2kW) are tested in a small-scale ORC unit with fluid R245fa: a variable speed swashplate piston prototype, an oil-free twin-screw, a modified lubricated hermetic scroll compressor operating at a constant speed and a variable speed modified scroll compressor. The maximum effective isentropic efficiencies measured are 53% for the piston expander and the screw expander, 76% for the variable-speed scroll and 81 % for the constant-speed scroll machine. However, these performance do not reflect the highest efficiencies achievable by each expander: the test-rig has experimental limitations in terms of mass flow rate and pressure drop (among others) which restrict 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-rig. When selecting an expander, other considerations than the efficiency have to be taken into account such as the flexibility, the operating conditions, the costs and the components compactness. Based on experimental results and practical considerations, some guidelines are drawn to help the reader to properly select a volumetric expander.

Experimental study on free piston linear generator (FPLG) used for waste heat recovery of vehicle engine

In this study, a novel free piston expander coupling with a linear generator prototype is presented to recover exhaust waste heat efficiently from vehicle engine. The free piston linear generator (FPLG) can be used in a small-scale organic Rankine cycle (ORC) system and can directly convert the thermodynamic energy of working fluid into electricity. The effects of intake pressure, operation frequency as well as external load resistance on operation characteristics and output performance are discussed. The results show that the piston displacement profile is similar to a sinusoidal wave. The piston amplitude increases significantly with the increase of the intake pressure. It is observed that the increase in the operation frequency brings about the decrease in both piston amplitude and motion symmetry. In addition, the peak piston velocity and power output show nearly linear relation with the intake pressure, which indicates that the power output is sensitive to the piston velocity. When the operation frequency is 1.0Hz and the external load resistance is 20Ω, the maximum peak velocity is about 0.69m/s and the highest power output 96W is obtained. The conversion efficiency of the FPLG increases with the increase of the intake pressure. But the conversion efficiency increases slightly when the intake pressure is higher than 2.2bar. When the operation frequency is 2.0Hz, energy conversion efficiency of the FPLG can reach up to 45.82% with an intake pressure of 2.6bar.

Working fluid charge oriented off-design modeling of a small scale Organic Rankine Cycle system

Organic Rankine Cycle system is one of the most widely used technique for low-grade waste heat recovery. Developing of dynamic Organic Rankine Cycle models played an increasingly important part in system performance prediction. The present paper developed a working fluid charge oriented model for an small scale Organic Rankine Cycle to calculate the theoretical value of working fluid charge level for the system under rated condition. The two heat exchangers are divided into three different zones and related heat transfer correlations are employed to estimate the length variation of each zones. Steady state models have been applied to describe the performance of pump and expander. Afterwards, an overall solution algorithm based on the established model has been proposed in order to exact simulate the system’s off-design performance. Additionally, the impact of different working fluid charge volumes has also been discussed. Simulation results clearly shows the variation trend of different zones in both heat exchangers, as well as the variation trend of system operating parameters under various expander output work. Furthermore, the highest thermal efficiency can be reached 6.37% under rated conditions with a working fluid charge volume of 34.6kg.

New performance maps for selecting suitable small-scale turbine configuration for low-power organic Rankine cycle applications

This paper aims to deliver new performance maps for small-scale organic Rankine cycle (ORC) turbines (<20 kW) by assessing the impact of single and two stage turbine configurations on the ORC’s performance driven by low-temperature (<100 °C) heat source. Small-scale axial, radial-inflow and radial-outflow turbines are designed and compared with their single and two stage configurations with the aim of improving the performance of the ORC’s system by increasing its expansion ratio. Therefore, the preliminary mean-line design is coupled with three-dimensional CFD analysis and ORC modelling for all turbines’ configurations to deliver the performance maps for low-power applications. Due to the complex and 3D nature of flow across the turbine stage, CFD analysis was used to investigate in more detail five candidates of small-scale turbines, in single and two stage configurations with three working fluids (R141b, R245fa and isopentane). ANSYS®17-CFX was used to perform 3D CFD analysis of all turbine configurations. RANS equations for three-dimensional steady state and viscous flow were solved with a k-ω SST turbulence model. The performance maps in terms of turbine isentropic efficiency and power output for each turbine configuration are presented according to the operating conditions in terms of expansion ratio, working fluid mass flow rate, and rotational speed with turbine size.The results revealed that the two-stage axial and radial-outflow turbines’ configurations exhibited a considerably higher turbine performance, with overall isentropic efficiency of 84.642% and 82.9% and power output of 15.798 kW and 14.331 kW respectively, with R245fa as a working fluid. Also, the results exhibited that the maximum ORC thermal efficiency for both two-stage configurations was 13.96% and 12.80% for axial and radial-outflow turbines respectively working with R245fa. These results indicated the potential advantages of a two-stage turbine configuration in a small-scale ORC system for the conversion of a low-temperature heat source into electricity as a useful power.

Performance enhancement of a small-scale organic Rankine cycle radial-inflow turbine through multi-objective optimization algorithm

An effective methodology that encompasses a mean-line design, three-dimensional CFD analysis and optimization and ORC system modelling of the small-scale ORC radial-inflow turbine is presented. Three-dimensional CFD analysis and a multi-objective optimization algorithm were achieved using ANSYS®17 CFX and Design Exploration based on 3D RANS with a k-omega SST turbulence model. The 3D optimization technique combines a design of the experiment, a response surface method and multi-objective method. The optimization of the blade geometry was performed using 20 design points for both nozzle and rotor blades, based on the B-splines’ technique to represent the blade angles and thickness distribution. The number of blades and rotor tip clearance were included as design parameters. The isentropic efficiency and power output were introduced as an optimization objective with two organic working fluids, namely isopentane and R245fa. The results of the optimized geometry with R245fa showed that the turbine's and cycle's thermal efficiencies were higher by 13.95% and 17.38% respectively, compared with a base-line design with a maximum power output of 5.415 kW. Such methodology is proved to be effective as it allows the enhancing of the turbine's and the ORC's system performance throughout to find the optimum blade shape of the turbine stage.

Design and Parametric Study of an Organic Rankine Cycle using a Scroll Expander for Engine Waste Heat Recovery

This paper reports the design and investigation of a small scale Organic Rankine cycle using a targeted scroll expander to recover both coolant and exhaust energy from a diesel fuelled internal combustion engine. The designed system could potentially be used to recover both the heat source energy into electricity using simple control of the mass flow rate of the working fluid. Results indicated that under the optimal conditions this ORC system can generate 1 kW power under the full load operational condition of a 6.5 kW engine. Moreover, the corresponding rotation speed of the scroll expander is about 4000 rpm, which means the expander could be adapted with conventional generator to produce electricity at relatively reasonable cost.

Performance Evaluation and Comparison of Experimental Organic Rankine Cycle Prototypes from Published Data

Abstract The organic Rankine cycle (ORC) is a technology commonly used to convert low-grade heat (80-300 °C) into electricity, for a wide range of power (kW to MW). This technology is suitable for geothermal sources, thermal solar, biomass and waste heat recovery (WHR) from industry or engines. ORC has shown a renewed interest over the last decades thanks to its flexibility and easy maintenance. Working fluid and cycle architecture have been deeply investigated from a theoretical standpoint. Recently, there have been many experimental realization of small scale ORC with a particular investigation of the expander. Database of a hundred ORC experimental prototypes has been built from scientific literature, in order to perform a statistical analysis of ORCs. This database includes both qualitative and quantitative data on ORC facilities such as: operating conditions, performances achieved for components and complete cycle, components type and size used in the set-up, running environment of the ORC. Data compilation showed a wide variety of criteria and definition to evaluate ORC and components performances, making objective comparison complex. A distinction and classification of main criteria used in the ORC literature are presented. Then, a reflection of most suitable criteria for ORC performance evaluation is proposed, with an energy and exergy view, depending on the heat source. Finally, a preliminary database analysis is presented. Reported ORC gross electric efficiency is up to 10% and expander efficiency up to 70-80%.

Experimental Study on a Small-scale R245fa Organic Rankine Cycle System for Low-grade Thermal Energy Recovery

This work conducted an experimental investigation of a small-scale organic Rankine cycle (ORC) system at designed operating and control conditions for low-grade thermal energy recovery application. In the ORC system, R245fa was selected as working fluid while a turboexpander (turbine) with a high speed and permanent magnet synchronous electricity generator was installed to produce electric power and two-plate type heat exchangers were designed as an evaporator and condenser. The effects of condenser cooling water temperatures and R245fa superheat at the turbine inlet on the system performance were measured and analyzed. Practically, to ensure safe operation of the ORC expander, the R245fa superheat at the expander inlet is controlled to remain constant. The experimental results showed that at constant heat source parameters (temperature and flow rate), the turboexpander power output and cycle efficiency increased with lower cooling water temperatures. Under the specified test condition ranges, the maximum turboexpander power generation could achieve 5.405 kW when the cooling water temperature and the pressure ratio were set at 23.0 °C and 7.3 respectively. On the other hand, at certain cooling water temperatures, the superheat at the expander inlet exerted a negative impact on the turboexpander and system performances when the evaporating pressure was kept constant. Ultimately, the superheat was found to be an important control parameter to ensure efficient and safe system operation.

An innovative small-scale two-stage axial turbine for low-temperature organic Rankine cycle

High expander efficiency is required to achieve best performance for small-scale organic Rankine cycle (ORC) systems driven by low-temperature (<100°C) heat sources. In this paper a small-scale two-stage axial turbine is modelled and compared with a single-stage axial turbine, with the aim of enhancing the ORC performance by increasing its pressure ratio. The preliminary mean-line design approach is coupled with three-dimensional CFD modelling and ORC cycle analysis was used to assess the impact of two-stage axial turbine on the ORC cycle performance. Three-dimensional CFD analysis of the single and two-stage axial turbines was performed using ANSYS®17-CFX software. The RANS equations with a k-ω SST turbulence model were solved for three-dimensional viscous steady state flow. The real gas thermodynamic properties of three organic working fluids (n-pentane, R141b, R245fa) are used in modelling the flow with both turbine configurations. Results revealed that the two-stage axial turbine configuration exhibited a substantially higher turbine performance, with overall isentropic efficiency of 83.94% and power output of 16.037kW, compared to 78.30% and 11.06kW from the single-stage configuration, with n-pentane as working fluid and mean diameter of 64mm for the two-stage configuration. Also, results showed that the maximum ORC thermal efficiency was 14.19% compared with 10.5% for single-stage configuration using n-pentane as the working fluid. These results highlight the potential of using two-stage axial turbine in a small-scale ORC’s system for the conversion of low-temperature heat sources into electricity.

Numerical Analysis of an Organic Rankine Cycle with Adjustable Working Fluid Composition, a Volumetric Expander and a Recuperator

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.

Investigation on a small-scale pumpless Organic Rankine Cycle (ORC) system driven by the low temperature heat source

A small-scale pumpless Organic Rankine Cycle (ORC) system driven by the low temperature heat source is established to investigate the overall performance. Hot water temperature from 75°C to 95°C is adopted for performance analysis whereas the environmental temperature is about 25°C. Refrigerant R245fa is selected as working fluid, and scroll expander is employed for power generation. One worth noting fact is that pumpless ORC system shows great potential for low temperature heat recovery. Experimental results indicate that the maximum power output is 232W, which is obtained at 95°C hot water inlet temperature. Correspondingly, the average power output is 204W which is lasted for 6.6min, revealing the high stability for power generation. For different hot water inlet temperature, the highest energy and exergy efficiency of the system are 2.4% and 13.7%. Besides, performance of novel pumpless ORC system is compared with that of our previous prototype. It shows a remarkable improvement in terms of power output, energy efficiency and exergy efficiency. Power generation process is able to keep constant for 95% of the cycle time. As a result, pumpless ORC may become an alternative solution to low grade heat utilization when compared with conventional small-scale ORC.

Small Scale Organic Rankine Cycle (ORC): A Techno-Economic Review

The Organic Rankine Cycle (ORC) is widely considered as a promising technology to produce electrical power output from low-grade thermal sources. In the last decade, several power plants have been installed worldwide in the MW range. However, despite its market potential, the commercialization of ORC power plants in the kW range did not reach a high level of maturity, for several reasons. Firstly, the specific price is still too high to offer an attractive payback period, and secondly, potential costumers for small-scale ORCs are typically SMEs (Small-Medium Enterprises), generally less aware of the potential savings this technology could lead to. When it comes to small-scale plants, additional design issues arise that still limit the widespread availability of the technology. This review paper presents the state of the art of the technology, from a technical and economic perspective. Working fluid selection and expander design are illustrated in detail, as they represent the bottleneck of the ORC technology for small-scale power production. In addition, a European market analysis is presented, which constitutes a useful instrument to understand the future evolution of the technology.

Supersonic flow of non-ideal fluids in nozzles: An application of similitude theory and lessons for ORC turbine design and flexible use considering system performance

A significant improvement in the economy-of-scale of small-scale organic Rankine cycle (ORC) systems can arise from the appropriate design of components that can be manufactured in large volumes and implemented flexibly into a wide range of systems and potential applications. This, in turn, requires accurate predictions of component performance that can capture variations in the cycle conditions, parameters or changes to the working fluid. In this paper previous work investigating a modified similitude theory used to predict the performance of subsonic ORC turbines is extended to analyse the supersonic flow of organic fluids within 2D converging-diverging nozzles. Two nozzles are developed using a minimum length method of characteristics design model coupled to REFPROP. These are designed for R245fa and Toluene as working fluids with nozzle exit Mach numbers of 1.4 and 1.7 respectively. First, the nozzle performance is confirmed using CFD simulations, and then further CFD simulations are performed to evaluate the performance of the same nozzles over a range of different inlet conditions and with different working fluids. The CFD simulations are compared to predictions made using the original and modified similitude theories, and also to predictions made by conserving the Prandtl-Meyer function for the different operating conditions. The results indicate that whilst the modified similitude model does not accurately predict nozzle performance, conserving the Prandtl-Meyer function allows to predict the nozzle outlet Mach number to within 2% providing there is not a significant change in the polytropic index. Finally, the effect of working fluid replacement on the ORC system is discussed, and preliminary results demonstrate the possibility of matching a particular turbine to a heat source through optimal working fluid selection.

Evaluation of a small-scale waste heat recovery organic Rankine cycle

In recent years, the use of small-scale organic Rankine cycles (ORC) in exhaust gas heat recovery of reciprocating engines has been intensively studied. In this paper, the working fluid selection and experimental results of a small-scale ORC unit utilizing exhaust heat of a diesel engine are presented and discussed. Based on the working fluid selection study, siloxane MDM was evaluated as the most suitable fluid for the experimental system. The experiments were conducted with the aim of studying and analyzing the capability of the ORC process of recovering heat from the diesel engine exhaust. The high pressure MDM vapor was expanded through an expansion valve; thus, no power was extracted from the experimental setup and the main focus was on studying the performance of the process heat exchangers. The system under study was identified to be capable of efficiently recovering the waste heat of the exhaust gases, and the potential of using high molecular weight and high critical temperature fluids as the working fluids in high-temperature, small-scale ORC applications was confirmed. It was concluded that when using siloxane MDM as the working fluid, the requirements for the process sealing to withstand low vacuum conditions as well as the effective removal of non-condensable gases during the operation can be identified as one of the major challenges in achieving the targeted power output from this type of ORC systems.

Novel experimental research on the compression process in organic Rankine cycle (ORC)

Organic Rankine cycle (ORC) is one of the most promising methods for converting medium-low temperature thermal energy into power. Many studies have been performed to optimize the operating conditions, select proper working fluids, design efficient expansion machines, etc. However, as the major power-consuming component in ORC system, the circulating pump is rarely studied. Most of theoretical studies only assume a constant value of isentropic efficiency for the compression process, but this value varies with different studies. A small-scale ORC system is built in the present work to test the performance of a diaphragm pump, and the working fluids of R245fa, R123, R152a and R600a are tested under various conditions. The isentropic efficiencies of diaphragm pump for compressing those working fluids are between 57.22% and 93.51%. The results show that volume flow rate and pressure difference have great influence on isentropic efficiency, and the isentropic efficiency increases with the volume flow rate and pressure difference. Under the condition of same volume flow rate and pressure difference, the isentropic efficiency conforms to the order of ηR245fa>ηR123>ηR600a>ηR152a. The analyses about the variation trend of isentropic efficiency are made from the perspectives of physical properties of working fluid and the driving mode of diaphragm pump, which prove that the hitting velocity of diaphragm to the liquid is the main cause for irreversible enthalpy increment. A novel parameter αV/ρcp is proposed to identify the influence of physical properties of working fluid on the performance of pump. The experimental results show that under same volume flow rate and pressure difference, the isentropic efficiency of pump decreases with the increment of αV/ρcp of different working fluids.

Experimental investigations into power generation with low grade waste heat and R245fa Organic Rankine Cycles (ORCs)

In this study, experimental research was conducted to investigate the performance of a small-scale Organic Rankine Cycle (ORC) system utilising low grade heat sources to generate electric power at different operating conditions. The experiment setup consisted of typical ORC system components, such as a turboexpander with high speed generator, finned-tube condenser, ORC pump and plate evaporator. R245fa was selected as a working fluid in the experimental system, considering its appropriate thermosphysical properties for the ORC system and low ozone depletion potential (ODP). At constant heat sink (ambient) parameters, extensive experiments were carried out to examine the effects of various important parameters including heat source temperature and working fluid pump speed etc. on system performance. Results showed that at a fixed working fluid speed, the thermal efficiency of the tested ORC system could be improved with an increased heat source temperature. On the other hand, at a constant heat source temperature, the working fluid pump speed could be optimised to maximise system thermal efficiency. Both the heat source temperature and ORC pump speed were found to be important parameters in determining system thermal efficiency and the component operations. The experimental outcomes can instruct future optimal system design and controls.

结构参数对单螺杆膨胀机性能影响的初步实验研究

目前，中小规模有机朗肯循环系统的实用化仍存在许多问题，其中膨胀机性能是阻碍其应用的重要技术瓶颈，对于确定类型的膨胀机，结构参数对其性能影响较大。影响单螺杆膨胀机性能的结构参数主要是内容积比和配合间隙。本文开展了内容积比及间隙对单螺杆膨胀机性能影响的初步研究工作。结果表明：在相同间隙下，大内容积比的膨胀机效率仅为小内容比的50%左右，容积效率比小内容比的膨胀机低20%。对于小内容积比的样机，大配合间隙与小配合间隙相比，除容积效率略有下降外，功率和膨胀机效率基本没有差异；对于大内容积比的样机，大配合间隙与小配合相比，除容积效率略有下降外，功率和膨胀机效率均有一定幅度增加。 Currently, the practical application of medium and small-scale Organic Rankine Cycle system still has many problems. And the performance of expander is an important technical bottleneck which limits its application. For a given type of expander, structural parameters have great influence on its performance. Internal volume ratio and gap are two main structure parameters which impact performance of single screw expanders. This paper carried out a preliminary study on the performance of single screw expanders under the influence of internal volume ratio and gap. The results show that the efficiency of the expander with large internal volume ratio is only about 50% of the one with small internal volume ratio, the volumetric efficiency of the former is lower than that of the latter by 20%. For the prototype of small internal volume ratio, there is no significant difference of power and efficiency of the expander with large fit gap compared with small fit gap, except the volumetric efficiency has a slight downward. For the prototype of large internal volume ratio, power and efficiency of the expander with large fit gap increase a certain extent, but the volumetric efficiency decreases compared with small fit gap.

Optimizing the performance of small-scale organic Rankine cycle that utilizes a single-screw expander

This paper deals with the operation and optimization of a down-scaled industrial organic Rankine cycle (ORC) for low-grade waste heat recovery. The system is a sub-critical regenerative ORC with a nominal power output of 11kW. The ORC unit has been assembled using off-the-shelf components including three identical plate heat exchangers, a liquid receiver, a multi-stage centrifugal pump and a single-screw compressor adapted to operate as an expander. The experimental results are used to evaluate the influence of the expander performance on the behavior of the ORC system at nominal and part-load conditions. The matching between the volumetric expander and the system operating conditions is also analyzed. Results showed that in the case of SES36, both the expander efficiency and system performance were maximized for a pressure ratio between 7 and 9. In the case of R245fa, while the system efficiency achieved values similar to SES36, but the expander maximum isentropic efficiency was 17% lower. Two analyses are carried out to optimize the operation of the ORC unit with R245fa. At first, the insights gained by analyzing the experimental data are used to evaluate the theoretical matching between volumetric expander and the system maximum efficiency in terms of the Second Law of thermodynamics. Secondly, a control-oriented steady-state cycle model based on empirical correlations calibrated on the experimental results is developed. The model is used to implement a feed-forward control strategy based on predetermined steady-state points that allow to optimize the operation of the ORC unit. The frequency of the pump is used as controlled variable. The steady-state experimental data of both working fluids is provided in an electronic annex.