Small-scale Organic Rankine Cycle Research Articles

Experimental results of a small-scale organic Rankine cycle: Steady state identification and application to off-design model validation

This paper presents experimental results from a prototype of an 11 kWe organic Rankine cycle (ORC). Special attention has been paid to detecting reliable steady state data points. This has been achieved by application of a novel steady state detection algorithm. This method also allows evaluating the performance of the sensors in relation to remaining steady state fluctuations. The reliability of the experimental data is evaluated by investigating the heat balances over the heat exchangers and through error propagation of the measurement uncertainties. In addition, the experimental data from the setup is used for validating an off-design ORC model. The proposed simulation model solely requires the pump and expander speed as inputs besides the given heat source and sink conditions while subcooling is assumed to be constant. The important dependent model outputs are the evaporation pressure, the condensation pressure and the working fluid mass flow rate. All three predicted outputs show a maximum deviation of less than ±1% from the measured values. The modelled net power output deviates less than ±2% from the measured values. In general, this is a satisfactory result that gives confidence in using these models in future off-design simulations.

Statistical method for the determination of the minichannel heat exchanger’s thermal characteristics

The paper presents a new statistical method for the determination of the experimental thermal characteristics of mini- and microchannel heat exchangers. These exchangers are suitable for small scale ORC (Organic Rankine Cycle) installations, CHP (Combined Heat and Power) generation or other applications like electronic equipment. It is not possible in practice to measure the temperature inside mini- and microchannels and only statistical methods may be used to determine the exchanger’s characteristics. There are many such methods which are the modifications of the Wilson plot method. However, the presented method has some advantages. Both fluid flows are treated symmetrically and flow rates and temperature values may be changed simultaneously. In many other methods it is necessary to stabilize one fluid flow while changing the second one. The correlations for Nusselt number may easily be chosen without changing the method itself. The linear coefficients are calculated analytically, but in the case of additional nonlinear coefficients, the numerical method may easily be used. The presented statistical method was verified based on the experimental data of the prototype minichannel heat exchanger.

Experimental characterization and comparison of an axial and a cantilever micro-turbine for small-scale Organic Rankine Cycle

Electricity generation from waste heat by means of Organic Rankine Cycle is a promising method to increase the efficiency of industrial processes. However, due to special boundary conditions, such systems have to be robust, efficient in full load as well as in part load and scalable down to the power range of less than 15 kW. This study presents experimental results on the behaviour of two small-scale turbines, an axial impulse turbine and a radial cantilever turbine with a maximum power of about 12 kW. The turbine characteristics (isentropic efficiency and swallowing capacity) are given with respect to pressure ratio (ranging from 12 to 24) and rotational speed (ranging from 18,000 to 30,000 rotations per minute). For the axial turbine, a maximum isentropic efficiency of 73.4% has been reached in the ORC test bed. The maximum isentropic efficiency of the cantilever turbine is even higher and reaches 76.8%. The values are compared to volumetric expanders and it is shown that micro-turbines can compete even in the power range addressed in this study. As the turbine characteristics are given for full load and part load conditions, they can be implemented in simulation tools to allow for a more realistic calculation of ORCs with fluctuating heat sources.

A study of clearance height on the performance of single-screw expanders in small-scale organic Rankine cycles

A single-screw expander (SSE) is a type of volumetric expander that can be applied to low-grade heat and waste heat recovery utilizing the organic Rankine cycle system. In this paper, a new thermodynamic modeling of SSEs considering the friction power consumption is carried out to examine the performance. The mathematical model was verified by experimental results, and the calculation results have a good agreement with the experimental results. The clearance height has great influence on the expander performance. The influence of clearance height on the leakage flow rate, friction power consumption, volumetric and isentropic efficiency were investigated. Results show that path L8, L7 and L2+4 are the three main leakage paths, and path L8, L7 and L9 are the three main friction power consumption. Compared with the meshing clearance height, the fitting clearance has a more significant effect on the performance of SSEs. Setting a higher fitting clearance and meshing clearance is an effective measure to reduce friction power consumption and prevent gaterotor wear, respectively. It would be favorable if the fitting and meshing clearance height is no less than 0.04 mm and 0.02 mm, respectively.

A Generalised Assessment of Working Fluids and Radial Turbines for Non-Recuperated Subcritical Organic Rankine Cycles

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 ( T cr = 0.830 T hi + 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.

Energy Exploitation of High-Temperature Geothermal Sources in Volcanic Areas—a Possible ORC Application in Phlegraean Fields (Southern Italy)

This work aims to investigate the energy performances of small-scale Organic Rankine Cycles (ORCs) for the exploitation of high temperature geothermal sources in volcanic areas. For this purpose, a thermodynamic model has been developed, and a parametric analysis has been performed that considers subcritical and transcritical configurations, and different organic fluids (isobutane, isopentane, and R245ca). The investigation illustrates the significant effect of the temperature at the entrance of the expander on the ORC behaviour and the rise in system effectiveness when the internal heat exchange (IHE) is adopted. As a possible application, the analysis has focused on the active volcanic area of Phlegraean Fields (Southern Italy) where high temperature geothermal reservoirs are available at shallow depths. The work demonstrates that ORC systems represent a very interesting option for exploiting geothermal sources and increasing the share of energy production from renewables. In particular, the investigation has been performed considering a 1 kg/s geothermal mass flow rate at 230 °C. The comparative analysis highlights that transcritical configurations with IHE guarantee the highest performance. Isopentane is suggested to maximise the ORC electric efficiency (17.7%), while R245ca offers the highest electric power (91.3 kWel). The selected systems are able to fulfil a significant quota of the annual electric load of domestic users in the area.

Experimental study of a small scale organic Rankine cycle waste heat recovery system for a heavy duty diesel engine with focus on the radial inflow turbine expander performance

The purpose of this work is to experimentally evaluate the effect on fuel efficiency of a small scale organic Rankine cycle (ORC) as a waste heat recovery system (WHRS) in a heavy duty diesel engine that operates at steady state conditions. The WHRS consists of two operating loops, namely a thermal oil loop that extracts heat from the engine exhaust gases, and the working fluid loop which is the ORC system. The expansion machine of the ORC system is a radial inflow turbine with a novel back-swept blading that was designed from scratch and manufactured specifically for this WHR application. The engine test conditions include a partial engine load and speed operating point where various operating conditions of the ORC unit were tested and the maximum thermal efficiency of the ORC was defined close to 4.3%. Simultaneously, the maximum generated power was 6.3 kW at 20,000 rpm and pressure ratio of 5.9. The isentropic efficiency reached its peak of 35.2% at 20,000 rpm and 27% at 15,000 rpm. The experimental results were compared with the CFD results using the same off-design conditions, and the results were in good agreement with a maximum deviation of 1.15% in the total efficiency. Last but not least, the engine-WHRS energy balance is also discussed and presented.

A small-scale solar Organic Rankine Cycle power plant in Thailand: Three types of non-concentrating solar collectors

Power generation from solar energy is one of many alternative ways to solve the current energy crisis and environmental problems affecting our world. In this study, a system that utilizes low-temperature heat (under 100 °C) from solar energy to generate electricity by a small-scale Organic Rankine Cycle system is proposed. The system is analyzed using three different capacities of the ORC system with R-245fa (20, 40 and 60 kWe) in combination with solar water heating system (SWHS), using four different models (SORC-I, SORC-II, SORC-III, and SORC-IV). Compound parabolic concentrator (CPC), evacuated-tube and flat-plate collectors were used to generate heat with optical efficiency (FR(τα)e) of 0.72, 0.57, 0.74, overall heat transfer coefficient (FRUL) of 0.97, 0.75, 3.62 W/m2-K, and collector area of 2.16, 2.37, 2.08 m2 per unit, respectively. The maximum power output, the CO2 emission, and the economic analysis in terms of levelized cost of electricity (LCOE) were evaluated by a mathematical model. The weather data from Bangkok, a representative city in the central part of Thailand, was used for simulations. The simulation results shown that, if the number of collectors is the same on all systems, the system can produce the highest power output when combined with the CPC collectors. In terms of the economic analysis, the 60 kWe ORC system has the lowest LCOE value of 0.20 USD/kWh if coupled with 950 units of evacuated-tube collectors without initial investment of the collectors into consideration. In this case, the system can produce the power output of 113.5 MWh/Year, and reduce CO2 emission of 62.2 Ton CO2 eq./Year. If investment cost is taken into account, a LCOE of 0.67 USD/kWh can be achieved by the same system but coupled with 900 units of the same collectors. In this case, the system power output is 110.0 MWh/Year, and CO2 emission are reduced by 60.3 Ton CO2 eq./Year.

Performance comparison of organic Rankine cycle with expansion from superheated zone or two-phase zone based on temperature utilization rate of heat source

The temperature utilization rate of heat source is defined to evaluate the energy utilization rate of organic Rankine cycles (ORCs). Rotary vane pump, plate heat exchangers and scroll expander are adopted to compare the performance of ORCs with expansion from superheated zone or two-phase zone. The results show that temperature utilization rate increases with the increase of mass flow rate of working fluid and the decrease of mass flow rate of heat source. The working conditions are classified into three types of A, B and C according to starting zone of expansion. The temperature utilization rate keeps almost unchanged for types B and C, but the net electric power output efficiency decreases obviously with the increase of working fluid mass flow. The maximum thermal efficiency, net electric power output efficiency and isentropic efficiency of expander are obtained as 6.1%, 3.01% and 83.5%, respectively. The maximum temperature utilization rate can reach 64.4% with the decrease of heat source mass flow. Furthermore, the rotary van pump is suitable for small-scale ORC with stable volume flow rate, relative high efficiency, good sealing condition and long service life. The highest isentropic efficiency and electricity consumption efficiency of pump are 46.1% and 39.8%, respectively.

Simulation of working fluid mass distribution in small-scale Organic Rankine Cycle system under sub-critical conditions

Distribution of working fluid mass will significantly impact off-design performance of the Organic Rankine Cycle. In this paper, both the evaporator and the condenser are divided into three different zones, and the impact of heat transfer correlations and void fraction models selection on the working fluid mass calculation results have been discussed. Afterwards, impact of evaporation and condensation temperatures on working fluid mass distribution in each zone calculated by proposed correlations and models have been evaluated. Four common working fluids which feature different physical properties are taken into account. Results indicate that the selection of heat transfer correlations and void fraction models shows great importance for the calculation of working fluid mass in condenser but rarely affects the calculation results in evaporator. Accordingly, condensation correlation of Shah (1979) and void fraction model of Premoli (1971) is proposed for the discussed system. Furthermore, in condenser the working fluid mass monotonically decreases as the condensation temperature increases, while in evaporator the working fluid mass firstly rises and then falls when the evaporation temperature increases. It can be found that the relevant evaporation temperature at which the largest working fluid mass occurs is related to critical temperatures of the employed working fluids.

Simulation analysis of an innovative micro-solar 2kWe Organic Rankine Cycle plant for residential applications

In this paper a 2kWe small scale Organic Rankine Cycle system coupled with a concentrated solar linear Fresnel lens collector is investigated using a simulation analysis. Coupling small scale solar technologies with ORC system is a well-known solution in low temperature ranges. Efforts exist to achieve higher temperatures and overall efficiencies but at present such systems represent a challenge at small scale. This work is part of the EU funded Innova MicroSolar project about development and optimization of a solar linear Fresnel lens concentrator coupled with 2kWe Organic Rankine Cycle plant and with a PCM thermal storage system for residential applications. In this paper a preliminary assessment of the performance of two of the main components of such system (i.e. solar linear Fresnel lens collector and ORC) is presented. A dynamic analysis of the system is performed by varying the operating conditions. In particular the influence of the system configuration and working temperatures on the ORC overall efficiency is evaluated. The final aim of the work indeed is to assess the best working conditions for such micro-CHP to be integrated in residential buildings.

Experimental study of organic Rankine cycle in the presence of non-condensable gases

Non-condensable gases (NCGs) are inevitable in organic Rankine cycle (ORC) system, and they have adverse impacts. A small-scale ORC test platform using scroll expander and R123 was constructed to investigate the NCGs effect. The expander backpressure (i.e. condenser outlet pressure) and electricity output were examined on different conditions of NCGs mass fraction (xNCG), hot side temperature (Th) and condensation temperature (Tc). Two new parameters, namely reduced coefficient of pressure ratio (RCOPR) and filling ratio of reservoir (FROR), were proposed to reveal the mechanism of ORC performance degradation in the presence of NCGs. The results show that the partial pressure of NCGs (PNCG) in reservoir at work differed from that at static state. Unlike R123, NCGs were blocked by the reservoir and had no access to the pump. The accumulation of NCGs led to unexpected expander backpressure, which could be 0.68 bar higher than the saturation pressure when Th = 140 °C, Tc = 50 °C and xNCG = 1.3%. PNCG generally increased as FROR rose. The FROR changed with Th, Tc and R123 mass flow rate. The relative increment in electricity output of the ORC with xNCG = 1.3% over that with xNCG = 12% was significant, and could reach 114% when Th = 100 °C and Tc = 50 °C.

Energy, Exergy and Economic Evaluation Comparison of Small-Scale Single and Dual Pressure Organic Rankine Cycles Integrated with Low-Grade Heat Sources

Low-grade heat sources such as solar thermal, geothermal, exhaust gases and industrial waste heat are suitable alternatives for power generation which can be exploited by means of small-scale Organic Rankine Cycle (ORC). This paper combines thermodynamic optimization and economic analysis to assess the performance of single and dual pressure ORC operating with different organic fluids and targeting small-scale applications. Maximum power output is lower than 45 KW while the temperature of the heat source varies in the range 100–200 °C. The studied working fluids, namely R1234yf, R1234ze(E) and R1234ze(Z), are selected based on environmental, safety and thermal performance criteria. Levelized Cost of Electricity (LCOE) and Specific Investment Cost (SIC) for two operation conditions are presented: maximum power output and maximum thermal efficiency. Results showed that R1234ze(Z) achieves the highest net power output (up to 44 kW) when net power output is optimized. Regenerative ORC achieves the highest performance when thermal efficiency is optimized (up to 18%). Simple ORC is the most cost-effective among the studied cycle configurations, requiring a selling price of energy of 0.3 USD/kWh to obtain a payback period of 8 years. According to SIC results, the working fluid R1234ze(Z) exhibits great potential for simple ORC when compared to conventional R245fa.

Converting a commercial scroll compressor into an expander: experimental and analytical performance evaluation

The development of low cost small scale Organic Rankine Cycle (ORC) has a very interesting potential in generating electricity using low temperature waste heat sources. Moreover, HVAC companies could significantly extend their market if a commercial scroll compressor can be converted into an expander using similar units. Therefore, this work reports experimental test funded by an Italian HVAC company on a scroll compressor modified to work as scroll expander in a non-regenerative cycle and a subcritical fluid regime, aimed at reducing system cost and complexity. The scroll expander has been tested with its fluid R410A in a ORC cycle in order to obtain the isentropic efficiency of the scroll expander (0.5) and the pump (0.4).On the basis of the experimental tests, a model accomplished by means of MATLAB/CoolProp has been set up to evaluate the performance of the ORC group to achieve 10 kWe as target power output. Four operative fluids have been simulated, i.e. R245fa, R134a, R1234yf, R1234ze, fixing 100°C as evaporating temperature and considering the condenser temperature in the range 20-50°C. The results have showed that R245fa is the most promising working fluid since there is a higher expansion ratio within lower pressure values. As a consequence, not only a lower mass flow rate is necessary, but overall a lower pump consumption is needed, reaching greater overall conversion efficiency (about 6.5% with condensing temperature of 20°C) and power. Thus, a commercial heat pump scroll compressor can be effectively converted into an expander. The fluid selection shows that the most common ORC fluid can be used with relative low performance but at low cost and easy management.

Neural networks for small scale ORC optimization

This study concerns a thermodynamic and technical optimization of a small scale Organic Rankine Cycle system for waste heat recovery applications. An Artificial Neural Network (ANN) has been used to develop a thermodynamic model to be used for the maximization of the production of power while keeping the size of the heat exchangers and hence the cost of the plant at its minimum. R1234yf has been selected as the working fluid. The results show that the use of ANN is promising in solving complex nonlinear optimization problems that arise in the field of thermodynamics.

Development of Open-Drive Scroll Expander for an Organic Rankine Cycle (ORC) Engine and First Test Results

A novel open-drive scroll expander has been designed and included in a small-scale organic Rankine cycle (ORC) engine. According to the design, the heat input is 100 kWth with hot water temperature up to 100 °C and organic fluid pressure 40 bar. These conditions correspond to supercritical state of the vapour organic fluid (R-404a) with designed thermal efficiency of 6%, which is higher than similar operation at subcritical conditions. Due to supercritical operation, a new expander was necessary to be developed. An open-drive scroll expander has been designed with optimized scroll geometry, manufactured, and then integrated in an ORC engine with net capacity of 6 kW. The ORC has been installed at the laboratory for performance tests, which include the variation of heat input and hot water temperature. The tests presented here concern the first series of results for hot water temperature up to 80 °C and low expander speed (580 rpm@10 Hz), limited by severe vibrations at higher speeds. At these off-design conditions, the pump global efficiency reached was about 40%, the maximum thermal efficiency was 1.7% with expander isentropic efficiency almost 40% and volumetric efficiency about 30%, resulting to high internal leakages due to the low speed. These performance values are promising compared to part-load operation of similar set-ups, making it clear that the new expander can bring a superior performance compared to standard converted scroll expanders.

Numerical predicting the dynamic behavior of heat exchangers for a small-scale Organic Rankine Cycle

Dynamic modeling is a hot topic for investigations of Organic Rankine Cycle (ORC). In this paper, Moving boundary model and discretized model were used to describe the dynamic behavior of heat exchangers in a small-scale ORC using R123 as working fluid. Overall system model is assembled using dynamic models of heat exchangers combing static models of the scroll expander and the pump. The comparison made between the established models and experimental results shows that both the models can reveal the real system performance sufficiently. Moreover, the moving boundary model is numerically faster than the discretized model, and therefore more suitable for engineering applications.

Experimental Investigation of the Operating Point of a 1-kW ORC System

The organic Rankine cycle (ORC) is a promising technology for the conversion of waste heat from industrial processes as well as heat from renewable sources. Many efforts have been channeled towards maximizing the thermodynamic potential of ORC systems through the selection of working fluids and the optimal choice of operating parameters with the aim of improving overall system designs, and the selection and further development of key components. Nevertheless, experimental work has typically lagged behind modelling efforts. In this paper, we present results from tests on a small-scale (1 kWel) ORC engine consisting of a rotary-vane pump, a brazed-plate evaporator and a brazed-plate condenser, a scroll expander with a built-in volume ratio of 3.5, and using R245fa as the working fluid. An electric oil-heater acted as the heat source, providing hot oil at temperatures in the range 120-140 °C. The frequency of the expander was not imposed by an inverter or the electricity grid but depended directly on the attached generator load; both the electrical load on the generator and the pump rotational speed were varied in order to investigate the performance of the system. Based on the generated data, this paper explores the relationship between the operating conditions of the ORC engine and changes in the heat-source temperature, pump and expander speeds leading to working fluid flow rates between 0.0088 kg/s and 0.0337 kg/s, from which performance maps are derived. The experimental data is, in turn, used to assess the performance of both the individual components and of the system, with the help of an exergy analysis. In particular, the exergy analysis indicates that the expander accounts for the second highest loss in the system. Analysis of the results suggests that increased heat-source temperatures, working-fluid flow rates, higher pressure ratios and larger generator loads improve the overall cycle efficiency. Specifically, a 46% increase in pressure ratio from 2.4 to 4.4 allowed a 3-fold electrical power output increase from 180 W to 550 W, and an increase in the thermal efficiency of the ORC engine from 1 to 4%. Beyond reporting on important lessons learned in improving the performance of the system under consideration, comparisons will be shown for making proper choices with respect to the interplay between heat-source temperature, generator load, and pump speed in an ORC system.

Numerical CFD simulations on a small-scale ORC expander using a customized grid generation methodology

Positive displacement machines are the most suitable devices for small-scale waste heat to power conversion units based on an Organic Rankine Cycle (ORC) due to their capabilities of handling small mass flow rates and high pressure ratios. Among the technologies, sliding vane machines provide unique features such as low operating revolution speed, geometrical flexibility and uncomplicated manufacturing. Nonetheless, research and product development in this field have been constrained by the lack of interfaces between deforming and moving fluid domains that characterize sliding vane devices and the design tools at the state of the art. This research work tackles this challenge and presents the development of an analytical grid generation for sliding vane machines that is based on user defined nodal displacement. Through this approach, the numerical methodology discretizes the fluid domains enclosed between the cells and ensures conservation of intrinsic quantities by maintaining the cell connectivity and structure. Transient 3D single phase simulations on a small scale ORC expander were further set up in the ANSYS CFX solver and provided insights on the main flow field as well as in the leakage paths between rotor blade tips and casing. The numerical results were eventually validated with reference to an experimental dataset related to a waste heat to power conversion application in compressed air systems where the sliding vane ORC expander worked with R236fa, at a pressure ratio of 2.65 and at 1551 RPM.

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.