Organic Rankine Cycle Test Rig Research Articles

Impact of the Lubricant on a Modified Revolving Vane Expander (M-RVE) in an Organic Rankine Cycle System

The expansion device is the critical component of micro-to-small scale organic Rankine cycle (ORC) systems, substantially affecting system efficiency and cost. Low isentropic efficiency and lubrication requirements are the main issues associated with using volumetric expanders in ORC systems. Despite lubrication contributing to reducing internal leakages in an expander, it may compromise the performance of the ORC system by adversely affecting the evaporator’s thermal capacity. This study tests a recently developed and modified revolving vane expander (M-RVE) in a micro-scale ORC test rig by implementing an adjustable oil mass flow rate. The impact of the lubricant oil on the performance of the M-RVE prototype is investigated within a wide range of oil circulation rates (OCR). The results demonstrate a negligible improvement in the filling factor for OCRs higher than 1%. Moreover, the shaft power is not considerably sensitive to OCR, while the calculated isentropic efficiency of the expander improves with OCR. Furthermore, the impact of the lubricant oil on the performance of the evaporator is studied, assuming the exact OCR as the expander and measured temperature and pressure similar to the pure refrigerant for the lubricant-refrigerant mixture in the evaporator. The study shows that the evaporator capacity is penalized with OCR, especially for values higher than 1%. Hence, an OCR of about 1% is a good compromise, and it can be used as a guideline for designing revolving vane expanders for micro-scale ORC systems without a dedicated lubricant oil circuit.

Investigation of an artificial cold source with variable temperature for ORC test rig

Organic Rankine cycle (ORC) power generation system is applied to the cascade heat utilization system. The temperature of cold and heat sources has the significant effect on the performance of ORC system. To study this effect efficiently and easily operated, an artificial cold source with variable temperature in the range of -25–35°C was designed for ORC test rig, and an experimental study was conducted. The experiment investigated the effect of outdoor temperature and compressor frequency on the cold source temperature for a heat source output temperature in the range of 50–120°C. An experimental empirical formula for adjusting the cold source temperature independently was established, modified, and experimentally verified. The results demonstrated that the modified formula with two variables exhibited a quadratic polynomial relationship with the correlation coefficient R2 values in the range of 0.9992–0.9995. The verification experiments demonstrated that the calculated value was within ±0.5°C error range from the experimental value. The empirical formula can realize the accurate cold source temperature for ORC test rig. Eventually, the novel artificial cold source with variable temperature test rig can be used to conduct experimental studies on the operational performance of ORC systems under different temperature conditions from subtropical to temperate regions.

Experimental evaluation of an ORC-CHP architecture based on regenerative preheating for geothermal applications

The Organic Rankine Cycle (ORC) can be applied to generate power from low-temperature heat sources and thus supports a sustainable energy system. This paper contributes to ORC development, particularly for geothermal applications, to increase competitiveness of the technology. To this end, new plant architectures based on regenerative preheating are being investigated. The concept of regenerative preheating is investigated in literature by numerical studies and its thermodynamic and economic performance is predominantly evaluated positively. However, regenerative preheating has not yet been realized in experimental ORC test rigs or commercial products. For this purpose, a novel ORC test rig is designed, constructed and commissioned. For evaluation purposes, the regenerative preheating concept is compared to a standard ORC configuration. As a result, a 9.9% higher net thermal efficiency is achieved with regenerative preheating, while the net power output is equal to the one achieved with the standard configuration. This result is particularly important for combined heat and power (CHP) generation due to the reduced cooling of the heat source. In order to evaluate this concept for CHP applications, a novel ORC-CHP architecture based on regenerative preheating is experimentally compared to three state-of-the-art ORC-CHP concepts. Moreover, three different supply and return temperatures of the district heating system (DHS) are considered. The results reveal that the novel architecture, in combination with low- and medium-temperature DHS, leads to an increased part-load performance and a wider operating range. This enables up to 9.4% increase in annual net electricity production, which accounts for additional revenues of 4.56million € in the case of a typical geothermal project. Thus, it can be concluded that this novel ORC-CHP architecture is beneficial from a technical perspective and also promising in terms of economics.

Effect of resistive load characteristics on the performance of Organic Rankine cycle (ORC)

The effects of various organic Rankine cycle (ORC) components, such as evaporator, expander and condenser on the performance of the system have been extensively studied. However, only a handful of studies have focused on studying the influence of external load characteristics. A small-scale ORC test rig of 3 kWe was constructed to experimentally investigate the effect of resistive load on the performance of the system. In order to achieve this, the study combined the thermodynamic method with electrical analysis. The results showed that the increase in output power caused a simultaneous increase in rotational speed, torque and inlet pressure of the expander under constant load resistance conditions. However, the fluid mass flow rate only affected the expander's inlet pressure. The decrease in load resistance increased the expander's inlet pressure and torque, and reduced the superheating and rotational speed. The influences of load resistance on the isentropic efficiency and net efficiency were mainly thorough the expander's inlet pressure and superheating. The net efficiency also benefited from the ideal cycle efficiency due to that decreasing load resistance enlarged the enthalpy drop of expander. The effects of load resistance on performance parameters were restricted by the mass flow rate of the fluid.

R1224yd(Z), R1233zd(E) and R1336mzz(Z) as replacements for R245fa: Experimental performance, interaction with lubricants and environmental impact

Achieving the 2050 EU climate targets - an economy with net-zero greenhouse gas emissions - requires a significant increase in the share of renewables in the current energy mix. The Organic Rankine Cycle (ORC) could play an important role in this transformation of today’s energy systems. The ORC is able to convert low-temperature heat sources into electrical power by utilizing refrigerants as working fluids. Many of these refrigerants have a significant global warming potential (GWP) and recent legislations therefore increasingly limit their use. In recent years a new generation of refrigerants has been developed. These refrigerants are characterized by low global warming potentials and are considered as replacements for the old high-GWP refrigerants.In this paper R1224yd(Z), R1233zd(E) and R1336mzz(Z) are investigated as possible replacements for the state of the art high-GWP refrigerant R245fa. These novel fluids have quite similar thermophysical properties and can thus potentially be used as drop-in replacements for R245fa. For the application as drop-in replacement and the correct interpretation of measurement data, the interaction of the refrigerant with the used lubricant is important. Therefore, the interaction of all four refrigerants with a typical lubricant oil is experimentally investigated first. A novel model based on Raoult’s law is introduced and validated with experimental data. The model allows the computation of the liquid and vapor phase composition of refrigerant/oil-mixtures using pressure and temperature measurements. In the next step, the performance of the four investigated refrigerants is evaluated experimentally in an ORC test rig. Therefore, the same experimental program was repeated for every refrigerant in order to ensure a fair comparison. To conclude the evaluation of the refrigerants, a Life Cycle Assessment (LCA) is conducted for a theoretical geothermal power plant operated with the four considered refrigerants.The experimental results show, that R1224yd(Z) as well as R1233zd(E) are very suitable low-GWP alternatives for R245fa and could be used as drop-in replacements. R1336mzz(Z) on the other hand shows significantly lower system efficiencies and power outputs compared to the other refrigerants. The highest power output and system efficiency is however still achieved by the state of the art refrigerant R245fa, but these quantities are only slightly lower for R1233zd(E) and R1224yd(Z) . The LCA based on the experimental results for a hypothetical large-scale geothermal ORC application reveals, that despite the slight performance decrease, the application of low-GWP fluids decreases the CO2-equivalent of the system significantly. The usage of R1233zd(E) could decrease the CO2-equivalent by 67% compared to R245fa.

Numerical and experimental investigation on thermal-hydraulic characteristics of a scroll expander for organic Rankine cycle

A new numerical model of a scroll expander is developed applying the deterministic method to analyze its thermal-hydraulic characteristics for the application to the organic Rankine cycle. Several geometrical models are refined as a subsection function of the orbiting angle and area models regarding suction and tangential gas force are newly developed. The geometrical models match well with the actual values extracted from the 3D computer-aided design model. In addition, mathematical models that reflect the effects of mass leakage and heat transfer are integrated in a closed-loop iteration process. Two critical unknown parameters of overall leakage clearance and wall-to-gas temperature ratio are identified via the coupling of the simulation model and experimental data. Experiments were conducted on a 1 kW scale organic Rankine cycle test rig using R245fa as the working fluid. The identified parameters are expressed as functions of the operating conditions and the simulation model is validated within 5% error range. The best isentropic efficiency appeared to be 62.8% according to the test results of the scroll expander. The simulation model allows the analysis of various aspects of the internal expansion process that could not be examined through empirical or semi-empirical models. Mechanisms regarding under and over-expansion and heat transfer are thoroughly investigated for profound understanding of the scroll expander transient behavior and energy level degradation.

Experimental analysis of a micro-scale organic Rankine cycle system retrofitted to operate in grid-connected mode

This work presents the retrofitting of a micro-scale organic Rankine cycle system originally designed for off-grid operation, modified to operate in grid-connected mode. Quantitative and qualitative performance comparisons of the grid-connected and off-grid connected organic Rankine cycle system are made based on experimental data of an organic Rankine cycle test rig. The operating strategies of the two systems are discussed in detail. The organic Rankine cycle test rig has a nominal power output of 1 kW with R245fa as the working fluid, a scroll expander as the expansion machine and steam as the heat source. Initially, the experiments were performed in off-grid operation mode with a self-excited alternating current generator. Subsequently, the organic Rankine cycle test rig was retrofitted for grid-connected mode, replacing the alternating current generator with an induction motor, regenerative variable frequency drive and grid connection. The rig modification was carefully done in such a way that the expander rotational speed was not fixed by the grid frequency, offering an additional control parameter for operational control optimisation. The modified system is able to supply 1.162 kW of gross electric power to the grid while a net electric output of 0.967 kW is measured. The same system is able to generate 1.016 kW (gross) and 0.838 kW (net) electric output in grid-connected mode. The net thermal-to-electric conversion efficiency at peak power generation is 7.36% and 4.66% in grid-connected and off-grid operating mode, respectively.

Experimental assessment of the fluid charge distribution in an organic Rankine cycle (ORC) power system

This paper reports on experiments aiming to understand and to characterize how the working fluid spreads in an organic Rankine cycle (ORC) power system. To this end, a 2-kWe ORC test rig is constructed and tested over a wide range of conditions. Besides standard thermo-hydraulic sensors, the fluid charge distribution is measured on-line by bending load cells and infrared imaging techniques. Following a complete experimental campaign (which includes more than 330 steady-state points and fully characterizes the ORC off-design behaviour), a dual data reconciliation method is applied to the raw measurements to obtain a reference dataset. The results are then analysed to assess the charge distribution mechanisms occurring in the ORC and how it is correlated with the system performance. Among other aspects, this paper demonstrates (i) how the charge inventory is highly correlated to the temperature profiles in the heat exchangers, (ii) how the evaporator dictates the operating conditions of the low-pressure components, and (iii) how the system charge and the liquid receiver size can be selected to maximize the ORC overall performance.

Design and Fabrication of Solar Organic Rankine Cycle Test Rig with Helical Coil Heat Exchangers and working fluid selection strategy

Organic Rankine Cycle (ORC) is a modified Rankine cycle which uses organic working fluid to convert heat into useful energy. ORC has proven its potential in the field of exploiting waste heat generated by process industries. Due to environmental concerns and power demand, researchers now started exploiting renewable energy sources like solar and geothermal energies as heat source for ORC. The research in the field of ORC has revealed that, the energy conversion efficiency of ORC is very low and depends on working fluids used in it. Although variety of organic working fluids are available in market, the Zeotropic working fluids have revealed the better performance in ORC systems due to their temperature glide characteristics in evaporator and condenser but the unknown thermophysical characteristics of these working fluids restricts their use in ORC. This motivates a researcher to explore new possibilities to use these working fluids and improve ORC efficiency. In this regard a systematic test setup is designed and developed to test the potential of Zeotropic working fluids with solar energy as heat source.

Experimental investigation of operation behavior of plate heat exchangers and their influences on organic Rankine cycle performance

As an efficient and reliable heat-to-power conversion technology, the Organic Rankine Cycle (ORC) has been recognized and accepted throughout academia and industry. Much effort has been devoted to the theoretical analysis, numerical simulation, and optimization in past decades. Experimental investigation has recently drawn considerable attention to practical applications. Most of experimental studies focused on the performance test of working fluid, expander, pump, and system. Although heat exchangers are the major contributors of exergy destruction and investment cost, few experimental studies have paid attention to heat exchangers and their effects on the performance of the ORC system. In this study, an ORC test rig with switchable evaporators and condensers is established to investigate the operation behavior of these heat exchangers and the influences of their heat transfer area on the performance of ORC system. An equivalent overall heat transfer coefficient, together with the degree of superheat, degree of subcooling, temperature difference and location at pinch point are used to evaluate operation behavior of the evaporators and condensers. A heat exchanger area utilization indicator is proposed, and applied along with the net power output and thermal efficiency to access the performance of the ORC with different heat exchanger combinations. The operation characteristics of these heat exchangers and the ORC system are experimentally investigated. Experimental results demonstrate that the operation behavior of these heat exchangers and the ORC systems are remarkably influenced by their heat transfer areas and combinations.

An experimentally validated, energy focused, optimal control strategy for an Organic Rankine Cycle waste heat recovery system

This paper presents a Nonlinear Model Predictive Controller (NMPC) designed to provide optimal control input for maximum turbine power generation in an Organic Rankine Cycle (ORC) Waste Heat Recovery (WHR) system. While the literature is rich in ORC-WHR system modeling and control approaches in simulation environments, the fundamental dynamic analysis, system aging, thermal inertia, and experimental implementation of power optimization based optimal ORC-WHR control are still lacking. These factors are key to fully understanding and controlling the dynamic behavior of the system and are the main focus of this study. In contrast to prior literature, this work experimentally evaluates the nonlinear dynamics of the ORC system to comprehensively understand the controller design requirements. A power optimization-based Nonlinear Model Predictive Controller (NMPC) is derived utilizing an Extended Kalman Filter (EKF) as a state estimator. Simulation results indicate that optimal turbine power generation is obtained with minimal working fluid superheat for the system under study. Consequently, a superheat-tracking controller is designed, and the performance of the controller is simulated over step inputs. The designed controller is then experimentally validated on an ORC test rig with a 13L Heavy Duty Diesel Engine (HDDE).During experimental evaluation of the controller, it was discovered that the control-oriented model is susceptible to system aging effects and therefore, the model was calibrated online to match the behavior of the aged system. Moreover, evaporator thermal inertia was found to play a vital role attenuating the fluctuating frequency components of the exhaust conditions. The tuned controller provided satisfactory control response for transient engine conditions and maintained the working fluid temperature within acceptable limits.

Experimental analysis of a small-scale scroll expander for low-temperature waste heat recovery in Organic Rankine Cycle

In this work a small-scale non-regenerative Organic Rankine Cycle test rig has been developed to experimentally assess the performance of the scroll compressor SANDEN TRS090 converted into an expander.The performance of the expander has been evaluated using R245fa as working fluid in terms of overall efficiency and filling factor under variable flow rates, expansion ratios, rotational speeds and superheating temperatures. A peak isentropic efficiency of 45% has been obtained for an expansion ratio of 1.9–2.1 whilst the rotational speed has a minor effect. The highest values of expander overall efficiency have been achieved for a filling factor ranging from 2.5 to 3 which correspond to rotational speeds higher than 1000 rpm. During the tests the expander has achieved a mechanical power output of about 650 W for an expansion ratio in the range 2–2.2 whilst the peak values of the mechanical efficiency of the plant were >3% (self-consumption of the pump and auxiliaries excluded) for an expansion ratio of 2.2–2.5.Eventually, it was proved that the machine has no advantages in terms of overall efficiency when the fluid is superheated but the whole ORC system has a higher performance with a superheating grade in the range 10–30 K.

Experimental evaluation of nucleate pool boiling heat transfer correlations for R245fa and R1233zd(E) in ORC applications

In binary geothermal power plants based on the Organic Rankine Cycle (ORC) typically shell-and-tube heat exchangers are used as evaporators. In the shell-side, nucleate boiling of the working fluid takes place on the external surfaces of tubes. For the replacement of fluids with high global warming potential (GWP) or selection of efficient working fluids, a comprehensive evaluation has to be performed. Therefore, the knowledge about the nucleate pool boiling heat transfer coefficient (HTC) in combination with the electrical power output is necessary. In this study, the focus is led on the experimental evaluation of nucleate pool boiling heat transfer correlations for R245fa and its possible replacement R1233zd(E) in ORC applications. The nucleate boiling HTC on a horizontal tube and the electrical power generation of a 1 kW scroll expander are simultaneously measured with an ORC test rig for both fluids. The thermal input is provided by an electrically heated preheater and evaporator. Nucleate boiling takes place on a plain copper tube with an outer diameter of 32 mm and a heated length of 822 mm. The surface temperature of the copper tube is determined by thermocouples within the tube in consideration of thermal conduction. The obtained measurement results, regarding heat transfer characteristics as well as power output, show that the working fluid R245fa performs better at equal saturation temperatures due to the higher density and saturation pressure, and the lower viscosity. The HTC for R245fa is up to 43.2% higher in comparison to R1233zd(E). The experimental HTC are compared to selected nucleate pool boiling HTC correlations. The evaluation reveals that correlations according to Cooper and Gorenflo et al. show the slightest mean absolute deviations between 4.75% and 15.65% for both 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.

Model Predictive Control of an Organic Rankine Cycle System

Organic Rankine Cycle (ORC) waste heat recovery systems offer promising engine fuel economy improvements for heavy-duty on-highway trucks. An ORC test rig with parallel evaporators to recover both tailpipe and EGR waste heat from a 13L heavy duty diesel engine was developed and used in this work to demonstrate a novel control strategy based on Model-Predictive Control (MPC). The main control objectives for the ORC system are: (i) regulation of working fluid temperature, (ii) safe turbine operation - away from 2-phase region, and (iii) maximization of waste heat recovery. The MPC uses a built-in moving boundary evaporator model to predict future system response and generate optimal actuator reference commands. Two variants of MPC were considered in this work: an adaptive linear MPC (LMPC) and a nonlinear MPC (NPMC). Compared with the traditionally used PID controller, MPC demonstrates more accurate temperature control and improved disturbance rejection in simulation. Finally, the LMPC and NMPC controllers were implemented on the ORC test rig and showing promising initial test results.

Simultaneous experimental investigation of nucleate boiling heat transfer and power output in ORC using R245fa and R1233zd(E)

In binary geothermal power plants based on the Organic Rankine Cycle (ORC) typically shell-and-tube heat exchangers are used as evaporators. In the shell-side, nucleate boiling of the working fluid takes place on the outer surface of the tubes. For the replacement of fluids with high global warming potential (GWP) or selection of efficient working fluids, a comprehensive evaluation has to be performed. Therefore, the knowledge about the nucleate boiling heat transfer coefficient (HTC) in combination with the electrical power output is necessary. In this study, the focus is led on the investigation of the replacement of R245fa by the low GWP fluid R1233zd(E) in geothermal applications. The nucleate boiling HTC on a horizontal tube and the electrical power of a 1 kW scroll expander are simultaneously measured with an ORC test rig for both fluids. The thermal input is provided by an electrically heated preheater and evaporator. Nucleate boiling takes place on a plain copper tube with an outer diameter of 32 mm and a heated length of 822 mm. The surface temperature of the copper tube is determined by thermocouples within the tube in consideration of thermal conduction. The obtained results, regarding power output as well as heat transfer characteristics, show that the working fluid R245fa performs better at equal saturation temperatures due to the higher density and pressure, and the lower viscosity. The HTC of R245fa is exemplarily up to 43.2 % increased in comparison to R1233zd(E).

Characterizing the performance of a single-screw expander in a small-scale organic Rankine cycle for waste heat recovery

This paper focuses on the experimental and numerical characterizations of a single-screw expander for waste heat recovery organic Rankine cycle (ORC) applications. A down-scaled industrial ORC test-rig has been tested with two different working fluids, R245fa and SES36. The hot source inlet temperature has been set to 125°C and the maximum expander inlet pressure was limited to 1200kPa. A total of 102 steady-state points have been collected by varying the expander pressure ratio between 3 and 9 with rotational speeds in the range from 2000rpm to 3300rpm. The experimental results allowed to assess the relationship between internal built-in volume ratio and imposed expansion ratio at different rotational speeds with respect to shaft and overall isentropic efficiency as well as volumetric performance in terms of filling factor. Results showed that while R245fa allowed approximately a 10% higher power output, the single-screw expander was performing at higher isentropic efficiency with SES36 due to higher pressure ratio achievable under the given working conditions and system limitations which also led to a better matching between ORC system and volumetric expander performance. A semi-empirical model has been developed and calibrated to break down the expander internal losses in the case of R245fa. The model has been exercised to investigate the effect of potential design improvements on the overall performance. The friction losses played a major role in the total loss count followed by suction pressure drops and leakages. As a consequence, the effect of lubrication should be further investigated to reduce leakages and friction. This study demonstrates the potential of single-screw technology as volumetric expander for ORC applications.

An experimental and modelling study of a 1 kW organic Rankine cycle unit with mixture working fluid

The ORC (organic Rankine cycle) technology is appropriate for conversion of low-grade industrial waste heat to electrical power due to its utilization of volatile organic fluids as working fluids. It has been proposed that zeotropic fluid mixtures can improve the ORC performance compared to pure fluids. The purpose of this paper was to demonstrate the feasibility of using a zeotropic mixture as working fluid through an experimental study with a lab-scale ORC (organic Rankine cycle) test rig. In this study, a zeotropic mixture of R245fa and R365mfc (48.5%/51.5% on a mole basis) was examined by focusing on its dynamic behavior in the system until reaching steady state and the performance in a scroll expander, a finned-tube heat extractor, an evaporator and a condenser. The test rig used the exhaust gas from a 30 kW Capstone™ Gas Turbine as its heat source. Computer simulation was conducted at system level with steady state conditions and the results were compared to experimental data.