Micro-scale Organic Rankine Cycle 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.

Experimental investigation of a revolving vane expander in a micro-scale organic Rankine cycle system for low-grade waste heat recovery

Organic Rankine cycles (ORCs) are a reliable solution for power generation from low-temperature waste heat. While the ORC is a mature technology in medium to large scale applications, several challenges remain to be solved for its widespread adoption in micro-scale, including the development of a low-cost, reliable, and efficient expander. In this study, a modified revolving vane expander (M-RVE) prototype is experimentally tested in a micro-scale ORC system for the first time. The M-RVE is tested in the range of suction pressures of up to 13.0 bar(a), suction temperatures of up to 63 °C, and shaft speeds of up to 1850 rpm. The M-RVE demonstrated filling factors of 1.2–2.7, an isentropic efficiency of up to 42.5%, and shaft power of up to 134 W. The significant impacts of lubrication on the performance of the M-RVE are discussed. A comparable performance with existing expanders in the literature is achieved, especially at pressure ratios close to the expander built-in volume ratio. The results of this study provide better guidance for further use of the M-RVE in micro-scale ORC systems and provide a benchmark for future improvements of the M-RVE prototype.

Development of a smart control unit for small-scale concentrated solar combined heat and power systems for residential applications

Solar energy has a significant potential for future power generation but its intermittent and variable nature results in fluctuations of the operational performance of solar power plants. Despite thermal energy storage (TES) systems improving the flexibility and the sustainability of the performance of Concentrated Solar Power (CSP) plants, smart management is required to deal with the complex dynamic variations in the behaviour and interaction of the different plant's subsystems. In this paper, the design, manufacture, and validation of a smart control unit with extended capabilities for a small-scale CSP combined heat and power (CHP) system are described. More precisely, the control unit has been designed to control and optimise the operation of a micro-scale Organic Rankine Cycle (ORC) unit coupled with Linear Fresnel Reflectors solar field and an advanced latent heat thermal energy storage tank which were developed by a consortium of universities and companies in the framework of the EU-funded project “Innova Microsolar”.In parallel to the designing and building the smart control unit, an advanced simulator has been developed in Matlab/Simulink® to investigate the performance of the plant for a wide range of varying ambient and operating conditions. The simulation framework has been connected to the real control unit according to a hardware-in-the-loop (HiL) approach to optimise the control logic of the integrated plant to overcome potential technical and reliability issues during the commissioning of the plant. The developed hardware and the proposed scientific approach can be extended to a wide range of complex solar energy systems equipped with TES and to be integrated into the built environment.

Assumption-free modeling of a micro-scale organic Rankine cycle system based on a mass-sensitive method

Organic Rankine cycle (ORC) systems are one of the most suitable technologies to produce electricity from low-temperature sources. In this paper, the main components of a non-regenerative, micro-scale ORC unit are modeled using the experimental results. These components are then used as functions in the system-level solver developed in MATLAB© to predict the performances of the system at off-design conditions. The proposed system solver is based on a novel approach, in which no assumptions are made about the system’s state, and only the components’ specifications and the real system boundaries that an operator encounters are adopted as inputs. To this end, the conservation of mass is considered in addition to the conservation of energy in the modeling of the system.Using the assumption-free model, the performances of the ORC system are mapped in the range of the experimental data considering the pump and the expander speeds as variables. The results show that the optimum system net electric performance is achieved at the pump and the expander speeds of 400 rpm and 900 rpm approximately. However, the pump is prone to the risk of cavitation due to low subcooling at the condenser outlet at this condition. Moreover, zero superheating is calculated at the expander suction that is not recommended for its operation.Hence, the developed assumption-free, object-oriented, mass-sensitive model has led to the full understanding of the system limitations and losses in the case of waste heat recovery applications. The proposed approach could be extended also to other ORC systems thus mapping their performances at off-design conditions without making artificial assumptions.

Development, experimental testing and techno-economic assessment of a fully automated marine organic rankine cycle prototype for jacket cooling water heat recovery

The final development stages and continuous operation testing of a micro-scale Organic Rankine Cycle (ORC) prototype running with R134a are presented. The prototype was designed to produce electricity by recovering heat from the hot jacket cooling water of a marine diesel engine at a temperature of 85 °C, using seawater as its heat sink. The study includes an analysis of the safety, dimensional, logistical and classification considerations that had to be taken into account considering the installation of the prototype in the engine room of a real newly built cruise ship. Furthermore, a special focus is given on the control system and the operation strategy of the prototype, which were developed to ensure its stand-alone, fully automated operation. The 24-h and 48-h continuous operation tests showcased the prototype’s capability of stable operation and robust response to load changes. Finally, the study includes a techno-economic assessment study of an upscaled prototype version, which highlighted the strong influence of the seawater temperature on its economic performance. In particular, in the base case scenario, the depreciated payback period was found equal to 11.54 and 10.31 years in the Mediterranean and the North Sea, respectively.

Experimental modeling of a lubricated, open drive scroll expander for micro-scale organic Rankine cycle systems

Abstract Scroll compressors converted into expanders represent a good choice in micro-to-small-scale organic Rankine cycle (ORC) applications. Several studies have tested and modeled scroll expanders in ORC systems in the last decade to assess their performance and reliability in off-design conditions. In this work, SANDEN TRS090F scroll compressor is converted into an expander and tested in a micro-scale ORC system using R134a for power generation from low-grade heat. The experimental data are used to modify the available semi-empirical model in the literature considering a polytropic expansion, a more detailed suction pressure drop model, and a variable loss power correlated to the expander pressure ratio. In addition, the two known expander geometrical parameters, the built-in volume ratio, and the swept volume are considered as fixed inputs to the model instead of being determined as part of the model results. In general, the results of the analysis show that the proposed model can predict the expander's overall performance with good accuracy in different operating conditions. The maximum deviation between the model results and most of the measurements is 5% for the mass flow rate and the shaft power, 15% for the overall isentropic efficiency, and 3 K for the discharge temperature. Then, the impact of the different losses is presented, and finally, the validated model is used to generate the performance maps of the studied expander at different working conditions.

Component-Oriented Modeling of a Micro-Scale Organic Rankine Cycle System for Waste Heat Recovery Applications

Organic Rankine cycle (ORC) systems are some of the most suitable technologies to produce electricity from low-temperature waste heat. In this study, a non-regenerative, micro-scale ORC system was tested in off-design conditions using R134a as the working fluid. The experimental data were then used to tune the semi-empirical models of the main components of the system. Eventually, the models were used in a component-oriented system solver to map the system electric performance at varying operating conditions. The analysis highlighted the non-negligible impact of the plunger pump on the system performance Indeed, the experimental results showed that the low pump efficiency in the investigated operating range can lead to negative net electric power in some working conditions. For most data points, the expander and the pump isentropic efficiencies are found in the approximate ranges of 35% to 55% and 17% to 34%, respectively. Furthermore, the maximum net electric power was about 200 W with a net electric efficiency of about 1.2%, thus also stressing the importance of a proper selection of the pump for waste heat recovery applications.

Impact of the expander lubricant oil on the performance of the plate heat exchangers and the scroll expander in a micro-scale organic Rankine cycle system

Organic Rankine Cycles (ORCs) are recognized as suitable systems to convert the thermal energy from low-grade heat sources to electricity. However, their performance and reliability are subjected to several deficiencies especially at micro-to-small scales. In this work, the impact of the expander lubricant oil on the performance of the heat exchangers and the scroll expander of a non-regenerative, micro-scale ORC unit is investigated. In particular, the oil circulation rate (OCR) is theoretically assessed for each experimental data set based on the thermal balance between the hot and cold streams of the condenser. Then, the performance of the other components is assessed using the lubricant-R134a mixture properties, assuming the same OCR.Results have shown that the presence of the lubricant oil leads to 5–15% capacity loss of the evaporator and the condenser of the studied ORC system. The calculated mass charge of the evaporator can also be underestimated up to 6.5% approximately if the oil is neglected. In addition, neglecting the lubricant oil may lead to over-estimation of the expander mechanical efficiency and under-estimation of its volumetric efficiency up to about 50% in very low shaft speeds. Hence, despite it is usually neglected in the literature, the results of the present analysis show that the impact of expander oil is relevant in micro-scale ORC systems.

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.

Investigation on the use of a novel regenerative flow turbine in a micro-scale Organic Rankine Cycle unit

Reliable and low-cost expanders are fundamental for the competitiveness of small-scale Organic Rankine Cycle (ORC) plants using low-temperature heat sources. Regenerative flow turbines (RFTs) can be considered a low-cost and viable alternative expander, yet their performance needs to be fully investigated. Therefore, the use of an RFT in a micro-scale ORC test bench is investigated in this work through a modelling study. Specifically, three-dimensional CFD simulations are carried out to assess the performance of the considered expander with varying operating conditions and a numerical model of a non-regenerative, small-scale ORC system is developed to investigate its potential in waste heat recovery (WHR) applications.Using R245fa as the working fluid, the CFD analysis shows that the expander achieves a maximum total-to-static isentropic efficiency of about 44% in the investigated operating range. The small-scale ORC system has a net output power in the range 100–600 W and a net cycle efficiency of 1–2.3%. Moreover, a comparison with two scroll expanders having different built-in volume ratios shows that the RFT operates with higher isentropic efficiencies in low mass flow rates and pressure ratios thus highlighting its suitability for low-temperature WHR applications, especially when considerable fluctuations of the heat source are expected.

Thermo-economic Review of Micro-scale Organic Rankine Cycle Integrated into Vehicle Engines for Waste Heat Recovery

Thermo-economic Review of Micro-scale Organic Rankine Cycle Integrated into Vehicle Engines for Waste Heat Recovery

Experimental study of micro-scale organic Rankine cycle system based on scroll expander

The organic Rankine cycle (ORC) is a promising technology to recover low grade waste heat. In this study, the experimental research is conducted to investigate the dynamic response performance of a micro-scale ORC system driven by low-grade heat sources at different loads. A scroll compressor is modified as the expansion machine in the ORC module and its isentropic efficiency under variable loads conditions is tested in the experiments. This work is investigated to assess the steady-state and dynamic performance of micro-scale ORC system at different loads conditions. The maximum electricity production and isentropic efficiency of the expander are 120 W and 43%, respectively, under the steady state experiments. On the other hand, in the dynamic experiment, the micro-scale ORC system has balance ability for disturbance caused by the load changes. If the disturbance is too large, the ORC system is unstable and could not be restored to a stable state by simple load reduction operation. The outcomes of experiment can establish foundation to the optimal system controls and give more information of optimal system design for the micro-scale ORC system.

Experimental study of a low-temperature micro-scale organic Rankine cycle system with the multi-stage radial-flow turbine for domestic applications

The paper presents an experimental evaluation of the efficiency of a small-scale low-temperature organic Rankine cycle (ORC) as an mCHP (micro Combined Heat and Power) system. Ultimately, the cogeneration system is dedicated to individual households and should be able to supply them with electricity and heat. A high-speed four-stage radial-flow microturbine with a nominal power of 2.5 kWe was used as an expansion machine. The low-boiling medium under the trade name of HFE7100 was used as a working medium in the system. Experimental tests of the ORC system were performed with various loads and under various operating conditions. The main purpose of the work was to achieve the maximum power of the microturbine at maximum possible efficiency of the ORC system. For this purpose, the supply system of the microturbine was improved by using an additional hole through which the blade system is fed. After the improvement, the microturbine produced about 3.9% more electric energy; this was due to the increase in the rotational speed (by approx. 7.2%) as compared to earlier measurement results. The maximum electric power generated by the microturbine was 2.12 kWe and was achieved at a rotational speed of about 22,440 rpm. The maximum thermal efficiency of the ORC was about 6.5%. The cycle had the maximum isentropic efficiency of the microturbine at 71% and it was lower by 1% than the value obtained using a numerical model. The improvement of the microturbine’s supply system enhanced the exergetic efficiency of the ORC system and its maximum value was about 21.5%.

Dynamic simulation and experiments of a low-cost small ORC unit for market applications

Organic Rankine Cycle is one of the most efficient, profitable and feasible technology for low-to-medium temperature heat recovery. The boost to the development of small-scale systems that could easily enter the market poses challenges in the design and selection of components, and in the choice of the operating conditions. This work deals with a micro-scale Organic Rankine Cycle (ORC) unit that has been designed to be cheap, efficient and compact, and tested to analyze performance and transient response under large variations of the heat source temperature. A dynamic model is also built and validated against two sets of experimental data: the first one refers to maximum power operation of the system and the second one to partial load operation close to the occurrence of two-phase expansion. The goal is to understand the potential of the system to face market applications, for which best design choices, operational limits and response time to variable heat sources must be identified. The choice of scroll expander, volumetric pump and plate heat exchangers has been made according to the lowest cost-to-efficiency ratio to keep the investment cost below 2500 euro/kW, already in the pre-market phase, in the power range of 3–4 kW. Results of the experiments show a fast time response, with a prompt reaction of the system to variations of the heat source temperature. Simulations demonstrated the ability of the model to capture with good accuracy the system dynamics. In particular, the validation process at maximum power operation (expander power output of 3300 W) shows maximum relative errors of about 4% in the prediction of measurements, whereas, the validation process at partial load operation yields slightly higher errors, namely 5.2% and 6.6% in the prediction of the evaporation pressure and the expander power output, respectively.

Simultaneous experimental comparison of low-GWP refrigerants as drop-in replacements to R245fa for Organic Rankine cycle application: R1234ze(Z), R1233zd(E), and R1336mzz(E)

Micro-scale organic Rankine cycle (ORC) enables to convert low-temperature waste heat into electricity, which is presented as one of combined heat and power (CHP) technologies. The commonly utilized working fluid R245fa will be phased out in the near future because of significant impact to the climate change. In that case, new HFOs (hydrofluoroolefins) refrigerants are suggested as potential alternatives to R245fa because of extremely low GWPs (Global Warming Potential) and zero ODP (Ozone Depletion Potential). This paper experimentally analyzed the applicability of three HFO refrigerants as drop-in replacements to R245fa for micro-scale ORC application including R1234ze(Z), R1233zd(E) and R1336mzz(E). Control options were assigned to the refrigerant mass flow rate and the expander rotational speed. System performance indicators including cycle thermal efficiency and net power output were compared. Besides, the expansion behavior along with the heat transfer performance and pump work were also analyzed. For the whole range of operating conditions, comparing the maximum cycle thermal efficiency, R245fa is 4.6% while R1233zd(E), R1234ze(Z) and R1336mzz(E) are 4.7%, 4.5% and 3.1%, respectively; Comparing the maximum net power output, R245fa generates 11.4% more than R1233zd(E) and 3.1% than R1234ze(Z). Limited range of pressure ratio contributes to the extremely low electricity of R1336mzz(E).

Performance and operation of micro-ORC energy system using geothermal heat source

In the electricity production sector, geothermal energy is considered a reliable energy source because of its independence of seasonal, climatic and geographical conditions. Low-temperature geothermal wells present a huge potential of exploitation, as the development of binary cycles and the technological improvement in drilling make this heat source a competitive solution for electricity generated distribution and self-consumption. The Organic Rankine Cycle (ORC) is currently the best solution to convert heat into electricity using low enthalpy heat sources. The ORC technology is already mature and widespread for medium and large-scale power plants, applying for geothermal, solar, biomass or waste heat recovery exploitation. Micro-scale ORC applications are still not diffused in the market: the system layout, the working fluid selection and the expander architecture can significantly vary depending on the specific realization requirements, thus a standard configuration has not established yet.In this paper, a particular case study of a micro-ORC power system using a geothermal well is presented. The application in analysis is a plug-and-play ORC facility, currently installed and operating in a pool centre. The system layout and the main components are described. The heat source is a geothermal well, which continuously supplies (by pressure difference) liquid water at a temperature lower than 60 °C to a binary Rankine cycle working with R134a. The ORC system is driven by a prototypal radial-piston expander and adopts an external-gear feed pump and a recuperative cycle. It is developed for working continuously, delivering the generated electricity directly into the grid. The facility is provided with temperature, pressure and electric power sensors for monitoring the operation and for a preliminary evaluation of the performance. The global efficiency of expander and feed pump and the ORC net efficiency have been evaluated at the regular working conditions of the geothermal well, showing values equal to, respectively, 53 %, 41 % and 4.4 %.

Design and Experimental Study of a 5-kW Organic Rankine Cycle with Axial-Flow Turbine for Low-Temperature Heat Recovery

AbstractThe microscale organic Rankine cycle (ORC) system is considered a promising technology to recover thermal energy from widely distributed low-temperature heat sources. By using HFC-R225fa as...

Development of a sliding vane expander in a micro-scale ORC system for utilizing low-grade heat

In power generation unit, the steam Rankine cycle is usually used to generate electricity. At the temperature below 300°C (low-grade heat), the steam Rankine cycle is not efficient to operate. To utilize low-grade heat, Organic Rankine Cycle (ORC) is introduced for producing mechanical work. However, its system performance relies heavily on the design of the expander. This present work aims to propose the development of the sliding vane expander for a micro-scale ORC system. The air motor, which is commercially available, is modified in an attempt to prevent leakage and to consistently use with ORC system. The experimental test rig is constructed to demonstrate performance of the sliding vane expander. It is driven by a 15kW boiler. The condenser is a water cooled plate heat exchanger at which the cooling water temperature is regulated by the chiller. R141b (1, 1-Dichloro-1-fluoroethane), which is a kind of HCFC, is used as the working fluid. The torque of the expander is measured by means of the Prony Brake method, and later used to determine shaft power of the expander. The preliminary test to demonstrate the performance of the expander is implemented. The tested condition is set at boiler and condenser temperature of 90°C and 34°C, respectively. It is found that the sliding vane expander is workable steadily without leakage of working fluid. Also, it provides an acceptable performance. The ORC operated with the sliding vane expander can produce approximately 200W of shaft power while the system efficiency is about 1.5%.

Modelling and testing of a hybrid solar-biomass ORC-based micro-CHP system

SUMMARY Expanders employed recently in organic Rankine cycle (ORC)-based systems suffer from key problems including excessive working fluid leakage, thermal losses, low isentropic efficiency and high cost. The majority of the units available in the market are for medium and large-scale applications (>100 kW) with no commercial micro-scale expanders available and applicable for ORC units for residential and building applications. Moreover, the majority of the studies conducted on ORC expanders employed HFC and HCFC working fluids which have high global warming potential leading to negative environmental impacts. In this study, a micro-scale CHP system based on the ORC technology is theoretically and experimentally investigated to provide the thermal needs and part of the electrical demands for residential applications. An innovative design for a hybrid ORC-based micro-CHP system is proposed using a biomass boiler and a solar concentrator to run the CHP system providing more reliable and clean operation compared to conventional natural gas-driven units. The micro-CHP system employs a new type small-scale scroll expander with a compact design, integrating the generator and the turbine in a single unit. A numerical model was developed using the Engineering Equation Solver (EES) software to simulate the thermodynamic behaviour of the ORC unit predicting the thermal and electrical performance of the overall CHP system. In addition, an experimental setup was built to test the whole ORC–CHP system performance under different conditions, and the effect of various operational parameters on the system performance has been presented using an environmentally friendly HFE7100 working fluid. The maximum electric power generated by the expander was in the range of 500 W at a pressure differential of about 4.5 bars. The attained expander isentropic efficiency was over 80% at its peak operating conditions with no fluid leakage observed. Being mass-produced with low cost in the automotive industry along with the high isentropic efficiency and the leakage-free performance, the proposed compact scroll expander represents a potential candidate to be used in the development of micro-scale ORC–CHP units for building applications. Copyright © 2013 John Wiley & Sons, Ltd.