Low-temperature Organic Rankine Cycle Research Articles

Introducing a novel hybrid system for cogeneration of liquefied natural gas and hot water using ejector-compression cascade refrigeration system (energy, exergy, pinch and sensitivity analyses)

• A novel integrated structure for natural gas liquefaction was developed. • Integrated cascade ejector refrigeration & low-temperature organic Rankine cycles. • This hybrid system generated 38.39 kg/s LNG as main product & 575.5 kg/s hot water. • The COP of the two stages cascade ejector refrigeration system was 0.8635. • Total exergy efficiency & specific power of the cycle were 36.4% & 0.386 kWh/kg LNG. Multi-component refrigerant compression refrigeration cycles have a high coefficient of performance and difficult controllability. In these systems, it is not easily possible to keep the ratio of stream compositions constant in the event of a leak. Also, due to high power consumption in compression refrigeration cycles, the use of compression-ejector refrigeration systems can reduce energy consumption. In this paper, a novel integrated system for cogeneration of liquefied natural gas (LNG) and hot water using two stages ejector refrigeration system (ERC) and low-temperature organic Rankine cycle (ORC) is developed. The feasibility of using ejector refrigeration systems in the novel integrated structure as the compression refrigeration cycle alternative is investigated. The novel integrated structure produces 38.39 kg/s LNG as the main product and 575.5 kg/s hot water as a byproduct. In the novel integrated structure design, due to the elimination of some equipment in the compression refrigeration systems, high energy consumption is reduced in these units. HYSYS software and MATLAB programming are used to simulate the hybrid system. The specific power consumption and exergy efficiency of the present system are 0.3868 kWh/kg LNG and 36.42%, respectively. The results of exergy analysis illustrate that the most exergy destruction belongs to the heat exchangers, which alone accounts for 60.19% of the total destruction. The heat exchanger network related to each of the multi-stream heat exchangers used in the system is extracted through the pinch method. The sensitivity assessment illustrates that the specific power consumption decreases up to 0.3809 kWh/kg LNG and produced LNG mass flow rate increases up to 39.81 kg/h, respectively when the mole fraction of methane in natural gas increases from 83% to 90%.

Assessment of the performance of a low-temperature Organic Rankine Cycle engine coupled with a concentrating PV-Thermal system

The operation of an integrated CPVT/ORC unit is investigated to identify the possibility of combining efficiently an ORC engine with a CPVT collectors’ field. The combined system is designed to operate so that the heat provided by the CPV field is at a temperature level in the range of 70–90 °C. The performance of the ORC engine is assessed towards comprehending its behavior under varying thermal loads and, as a result, maximizing the overall system electricity production. The analysis reveals that the electricity production from the ORC engine may be in the order of approximately 17 % of the total output of the combined system and that the implementation of the examined technological solution could increase the solar energy conversion efficiency of the CPV field, provided that the ORC engine operates at its maximum thermal efficiency. Finally, the preliminary economic analysis highlighted the potential of the integrated unit, which could be competitive even for small-scale systems.

Energy, exergy and pinch analyses of a novel energy storage structure using post-combustion CO2 separation unit, dual pressure Linde-Hampson liquefaction system, two-stage organic Rankine cycle and geothermal energy

The cost of electricity production and its purchase price markedly differ at different hours of the day; therefore, the idea of storing electricity during off-peak hours (low price) and its use during peak hours (high price) is discussed. In this study, a novel integrated structure for separating CO2 from the output smoke of a power plant using a chemical absorption unit via the post-combustion and CO2 liquefaction method by the dual pressure Linde-Hampson liquefaction system during off-peak hours is developed. The low-temperature organic Rankine cycle and the gas turbine power production unit are used to produce power during peak hours. The post-combustion CO2 separation unit produces 9307 kmol/h CO2 from the power plant's output smoke by absorbing 249.6 MW heat flow from the geothermal energy and 24.42 MW power. At hours when the energy demand is low, the produced CO2 is converted into condensed liquid form by receiving 18.31 MW power. When the demand for electric energy consumption is increased, this integrated structure produces 11.61 MW power. The multi-functional LCES system round trip and baseline LCES system exergy efficiencies are 36.75% and 58.48%, respectively. The exergy analysis results illustrate that the highest exergy destruction contribution in the equipment belongs to the fractionating columns (48.52%), heat exchangers (43.62%), and compressors (2.86%). The heat exchanger network related to the HE12, HE15, HE16, and HE18 multi-stream heat exchangers are extracted by pinch analysis. The sensitivity analysis illustrates that the exergy efficiency of the power generation cycle, baseline LCES plant, and multi-functional LCES structure increase up to 33.19%, 60.87%, and 21.14%, respectively when the pumped pressure of liquid carbon dioxide at peak consumption increases from 80 bar to 120 bar.

Experimental investigation of low-temperature organic Rankine cycle using waste heat from gas turbine bearings for different conditions

In power plants, a special type of oil is used for the lubrication and cooling of turbine bearings. The temperature of the cooling oil used for heat extraction from the bearings varies between 78 and 90 °C. Using waste heat from this oil will be a great advantage in terms of heat recovery. The overall thermal efficiency of gas turbine in power plants will also increase using this heat. In the literature, there are studies on organic Rankine cycle systems, but it has been seen that there are no studies on the use of waste heat from gas turbine bearings. In this paper, low-temperature organic Rankine cycle using waste heat from gas turbine bearings for different working conditions was investigated as experimentally. For this purpose, a prototype experiment setup was designed and thermodynamic analyses were carried out experimentally. Experimental organic Rankine cycle system was designed to operate in the temperature range of 78–90 °C using the waste heat in the lubricating oil. R134a was used as the working fluid in the prototype organic Rankine cycle system. The thermal efficiency, turbine expansion ratio and turbine power of the prototype organic Rankine cycle system were determined depending on different waste heat source temperatures and working fluid volumetric flow rates. According to experimental results for prototype organic Rankine cycle system, the highest thermal efficiency, turbine expansion ratio and turbine power are 6.84%, 1.94 and 1.22 kW, respectively, at a waste heat source temperature of 86.5 °C and volumetric flow rate of working fluid is 4.7 L/min.

Modeling of an Organic Rankine Cycle Waste Heat Recovery system for automotive engine applications

The remarkable investments made by manufacturers over the last few decades have contributed to improving the performance of internal combustion engines in every aspect: lower polluting emissions, greater specific power and thermal efficiency. Despite this, on an average, about 40% of the thermal power theoretically available from the combustion of the fuel is still stored in the exhaust gases and therefore dispersed in the environment. In this work the modeling and validation of a waste heat recovery (WHR) plant will be described, combining the engine with a low temperature Organic Rankine Cycle (ORC) system, in order to investigate the feasibility of this system on board of a vehicle, analyzing the quantity of thermal power recovered and made available in the form of electrical power. The ORC plant is modeled using a 0D/1D thermo-fluid dynamic approach. Starting from experimental tests, a map-based model for the piston pump and the scroll expander has been developed. The model has been validated through the use of a vector optimization technique, exploiting a genetic algorithm (MOGA). Subsequently, this system has been coupled to a spark ignition engine for automotive applications, adapting its speed range to comply with the ORC experimental tests. To have an accurate control over the expander inlet temperature, a bypass circuit and two throttles actuated by a PI controller have been implemented. The simulations were performed by considering 18 engine points at maximum load and different rpm. An average thermal efficiency increase of the system of 2.6% was obtained by introducing the recovery plant, and wide improvement chance can be foreseen in the case of ORC full-power use.

Carnot cycle in practice: compensating inefficiencies of ORC expanders through thermal regeneration

The Organic Rankine Cycle (ORC) is a thermodynamic cycle that can operate with a hot source over a wide range of temperatures, especially with low-grade heat (below 200°C). One of the main limitations for the success of small-scale ORC cycles (few to 100 kWe) is the relatively low isentropic efficiency of the typically used turbomachinery. Low turbine efficiency leads to low ORC cycle performance. To increase the performance of the cycle, the turbine efficiency must be increase, however, this significantly increases the cost of the machinery, making the cycle less profitable. In this work, the performance evaluation of low-temperature ORC cycles (100-150°C) with heat extraction along the expansion process is investigated, in an attempt to overcome this limitation. The studied cycle works in the same way as a conventional ORC, except that during the expansion process, heat is extracted. This heat is re-used later in the cycle, just before the hot source, allowing to reduce its load. The different cycles presented in this paper, using pentane as working fluid, are compared based on their exergetic and energetic efficiencies. The influence of three parameters on the cycle performance is studied: the regeneration ratio, the maximum temperature of the cycle and the turbine isentropic efficiency. In the case of a cycle using pentane with a maximum temperature of 150 °C and an turbine isentropic efficiency of 65%, the energy efficiency increases from 6.2% to 16.3% when going from no regeneration to full regeneration, and the exergy efficiency increases from 21.1 to 45.8%.. Secondly, the influence of the maximum temperature of the cycle is studied. Using pentane as the working fluid, the higher the maximum temperature is, the larger the benefits of heat extraction. However, this temperature cannot exceed the critical temperature of the organic fluid to stay in the case of a subcritical cycle. Finally, considering the turbine isentropic efficiency, it is possible to demonstrate that using a less efficient turbine, for example in small ORC systems, the performance of a cycle with an ideal turbine isentropic efficiency (100%) can be achieved compensating at cycle level the turbine losses with the heat extraction along the expansion process.

Investigation of the Use of Low Temperature Geothermal Organic Rankine Cycle Engine in an Autonomous Polygeneration Microgrid

Low-enthalpy geothermal resources (<150 °C) can be used for electricity generation and are widespread around the world, occurring at shallow depths. At the same time, in many parts of the world, there are existing low-enthalpy geothermal wells that are used for a multitude of applications such as for buildings’ heating and agriculture-related applications. The dominant technology to convert low-grade heat (<150 °C) to electricity is the Organic Rankine Cycle (ORC). The autonomous polygeneration microgrid (APM) concept aims to holistically meet in a sustainable way the needs of an off-grid community in terms of electrical loads, space heating and cooling, potable water production through desalination, and the use of hydrogen as fuel for transportation, in the most cost-effective manner possible. Photovoltaics (PVs) and wind turbines have been investigated extensively, since PVs can be installed practically anywhere in the world and wind turbines in areas with sufficient wind potential. The aim of this paper is to investigate techno-economically the potential of utilizing low-enthalpy geothermal resources in small-scale APMs through an ORC engine to fully satisfy the needs of small settlements. In order to accomplish this task with confidence, a case study for the Greek island of Milos has been developed and a typical settlement has been considered. It is worth mentioning that experimental results from a realized low-power (<10 kWe) ORC engine manufactured to operate at temperatures up to 140 °C are used to add reliability in the calculations. In order to meet the needs of the people, four different APMs based on PVs, wind turbines, and geothermal ORC of different but appropriate configurations were designed and sized through optimization. The optimization process was based on particle swarm optimization (PSO). The comparative examination of the results shows that the use of a low-power, low-temperature ORC engine in an APM is technically feasible; more cost effective than the configurations based on PVs, wind turbines, or combination of both; and has increased environmental sustainability.

Thermodynamic, Economic and Sustainability Analysis of Solar Organic Rankine Cycle System with Zeotropic Working Fluid Mixtures for Micro-Cogeneration in Buildings

Globally there are several viable sources of renewable, low-temperature heat (below 130 °C), particularly solar energy, geothermal energy, and energy generated from industrial wastes. Increased exploitation of these low-temperature options has the definite potential of reducing fossil fuel consumption with its attendant very harmful greenhouse gas emissions. Researchers have universally identified the organic Rankine cycle (ORC) as a practicable and suitable system to generate electrical power from renewable sources based on its beneficial usage of volatile organic fluids as working fluids (WFs). In recent times, researchers have also shown a preference towards deployment of zeotropic mixtures as ORC WFs because of their capacity to improve thermodynamic performance of ORC systems, a feat enabled through the greater matching of the temperature profiles of the WF and the heat source/sink. This paper demonstrates the thermodynamic, economic and sustainability feasibility, and the notable advantages of using zeotropic mixtures as WFs through a simulation study of an ORC system. The study examines first the thermodynamic performance of ORC systems using zeotropic mixtures to generate electricity powered by a low-temperature solar heat source for building applications. A thermodynamic model is developed with a solar-driven ORC system both with and excluding a regenerator. Twelve zeotropic mixtures with varying compositions are evaluated and compared to identify the best combinations of mixtures that can yield high performance and high efficiency in their system cycles. The study also examines the effects of the volume flow ratio, and evaporation and condensation temperature glides on the ORC’s thermodynamic performance. Following a detailed analysis of each mixture, R245fa/propane and butane/propane are selected for parametric study to investigate the influence of operating parameters on the system’s efficiency and sustainability index. For zeotropic mixtures, results disclosed that there is an optimal composition range within which binary mixtures are inclined to perform more efficiently than the component pure fluids. In addition, a substantial enhancement in cycle efficiency can be obtained by a regenerative ORC, with cycle efficiency ranging between 3.1–9.8% and 8.6–17.4% for ORC both without and with regeneration, respectively. Results also revealed that exploiting zeotropic mixtures could enlarge the limitation experienced in selecting WFs for low-temperature solar ORCs. Moreover, a detailed economic with a sensitivity analysis of the solar ORC system was performed to evaluate the cost of the electricity and other economic criteria. The outcome of this investigation should be useful in the thermo-economic feasibility assessments of solar-driven ORC systems using working fluid mixtures to find the optimum operating range for maximum performance and minimum cost.

Thermodynamic investigation of water generating system through HDH desalination and RO powered by organic Rankine cycle

The present study proposed the water generating system through humidification-dehumidification desalination unit and RO powered by organic Rankine cycle to generate both power and drinking water from the waste heat of Naval Ship by using a low-temperature organic Rankine cycle. Waste heat from the naval ship is used in order to run the topping cycle namely the organic Rankine cycle for power generation which operates the RO system for drinking water. A water-heated humidification dehumidification (HDH) desalination unit is also integrated with the organic Rankine cycle system to produce additional freshwater. The mass and energy balance equations are designed to analyze the thermodynamic performance of the cogeneration of the power-water system at the designed conditions. Suitable working fluids for the organic Rankine cycle are chosen on the basis of thermophysical properties i.e. lower boiling point, critical temperature & pressure and global warming potential. Furthermore, the impact of various vital parameters like extraction pressure, vapor extraction ratio, condenser temperature, turbine inlet pressure, etc. on the performance. It was founded that the impact of extraction parameters on the cycle, the net power, total efficiency and water production is 16.74 kW, 42.10% and 75.18 kg h−1 respectively. The extraction ratio variation causes the improvement in the water production and power generation and lower extraction ratio has better results. The RO System used is direct mechanical coupling which uses the power to run the system and produces the water, 2.4L of drinking water is produced using 16.74 kW power, respectively.

Pinch and exergy evaluation of Kalina/Rankine/gas/steam combined power cycles for tri-generation of power, cooling and hot water using liquefied natural gas regasification

Today, due to the increasing trend of energy demand in the world, the use of energy types with the approach of maximizing the efficiency of energy systems is inevitable. In this paper, an integrated power generation system consisting of low-temperature organic Rankine cycle, gas and steam combined power plant and Kalina power generation unit is developed and analyzed. Liquefied natural gas regasification is employed to supply the cooling for the hybrid system. This integrated system generates 158.5 MW power, 9.498 MW cooling, and 46.02 kg/s hot water. The total electrical, thermal, and exergy efficiencies of the integrated system are 48.62%, 55.18% and 67.74%, respectively. System exergy analysis shows that reactors and heat exchangers accounted for the largest share of total exergy destruction, each accounting for 59.91% and 15.76% of total energy destruction, respectively, indicating that these two parts have more than 75% of the destruction occurred. The pinch method was used to extract the heat exchanger network related to the multi-stream heat exchanger of the integrated system. In the parametric analysis, the effect of air/fuel molar ratio (input to the combustion chamber) on system performance has been investigated. One of the most important results is the increase in the total thermal efficiency of the system to 56.15% if the inlet air into the combustion chamber is reduced to 250.0 kg/s. The parametric analysis results also show that in addition to increasing the efficiency of the system due to the decrease in the amount of incoming air, the ratio of power production as the main product to byproducts (hot water and cooling) increases.

Investigating the performance and cost effects of nanorefrigerants in a low-temperature ORC unit for waste heat recovery

The use of nanorefrigerants in refrigeration and heat pump units increases the cycle performance, with similar effects also applied in organic Rankine cycle (ORC) units, but with very few relevant studies. For this purpose, an existing thermodynamic model for the ORC simulation is extended to account for adding Al2O3 and Cu nanoparticles (NPs) in the refrigerant, introducing their effect in the energy balance and the heat transfer and pressure drop at the different parts of the cycle. Numerical analysis of an ORC with refrigerants R245fa and R1234ze(Z) has been then conducted, with a heat source in the range of 80–120 °C and a NP mass fraction of 0–10% wt., aiming at identifying the performance improvement. The results show that the ORC efficiency increases for higher NP mass fraction and especially with Al2O3. This is mostly attributed to the expansion process with the NPs acting as internal heat sources to the vapour organic fluid, whereas the heat transfer and pressure drop improvements are minor. Finally, the preliminary cost analysis showed that the use of NPs in an ORC brings a payback period of 11.7 years with Al2O3, but with high uncertainty as revealed by the sensitivity analysis.

Multi-objective analysis of an influence of a geothermal water salinity on optimal operating parameters in low-temperature ORC power plant

The effects of geothermal brine salinity on optimal operating parameters of an organic Rankine cycle (ORC) power plant are investigated. For a more comprehensive understanding of the problem, a multi-objective optimization study is conducted aiming at minimizing the total heat transfer area while maximizing the exergy efficiency of the ORC. The optimization problem is solved using Non-dominated Sorting Genetic Algorithm-II. The multi-criteria decision making results show that an increase in the salinity (from 0.00 kgkg−1 to 0.12 kgkg−1) lowers the total heat transfer area and exergy efficiency by 4.39% (from 82.0 m2 to 78.4 m2) and 7.72% (from 28.5% to 26.3%), respectively. An adverse impact of the salinity on the exergy efficiency is dominant resulting in higher system size (reaching 8.00%) for obtaining the same exergy efficiency. For the same reason, a lower exergy efficiency (up to 5.00%) is reported for a system with the predefined heat exchangers size. A sensitivity analysis reveals that the salinity influence on the heat transfer area and exergy efficiency is mainly associated with a decrease in the mass flow rate of the working fluid. Moreover, salinity can increase the exergy flow rate of the brine thereby impacting the exergy efficiency. It is concluded that the salinity of the geothermal water should be considered in designing the ORC for a geothermal power plant because of its significant impact on the performance of the system.

A Novel Exergy Indicator for Maximizing Energy Utilization in Low-Temperature ORC

In the last decade, particular attention has been paid to the organic Rankine cycle (ORC) power plant, a technology that implements a classical steam Rankine cycle using low-boiling fluid of organic origin. Depending on the specific application and the choice of the designer, the ORC can be optimized using one or several criteria. The selected objectives reflect various system performance aspects, such as: thermodynamic, economic, environmental or other. In this study, a novel criterion called exergy utilization index (XUI) is defined and used to maximize the utilization of an energy source in the ORC system. The maximization of the proposed indicator is equivalent to bring the heat carrier outlet temperature to the ambient temperature as close as possible. In the studied case, the XUI is applied along with the total heat transfer area of the system, and the multi-objective optimization is performed in order to determine the optimal operating conditions of the ORC. Moreover, to reveal a relationship between the XUI and important ORC performance indicators, a parametric study is conducted. Based on the results, it has been found that high values of the XUI (~80%) correspond to optimal values of exergy-based indicators such as: exergy efficiency, waste exergy ratio, environmental effect factor or exergetic sustainability index. Furthermore, the values of the XUI = 60%–80% are associated with beneficial economic characteristics reflected in a low payback period (<11.3 years). When considering the ecological aspect, the maximization of XUI has resulted in minimization of exergy waste to the environment. In general, the simple formulation and straightforward meaning make the XUI a particularly useful indicator for the preliminary evaluation and design of the ORC. Furthermore, the comparative analysis with respect to other well-known performance indicators has shown that it has a potential to be successfully applied as the objective function in the optimization of ORC power plants.

Third Generation of Working Fluids for Advanced Refrigeration Heating and Power Generation Technologies

Since the 1987 Montreal Protocol, chlorinated refrigerants (CFCs and HCFCs) have been pointed out as responsible for the destruction of the ozone layer. The chemical industry has realized suitable replacement for CFC-12 and for HCFC-22 e.g. HFC-134a, HFC-404A, HFC-410A, HFC-507. This generation of refrigerants developed by the chemical industry can be characterized by the no ozone depleting potential and long atmospheric lifetime resulting in global warming potential. The contribution of the HFCs to the global warming brings up to discussion whether the HFCs should be considered as a transitional substance. Historically the use of natural and ecologically safe refrigerants was a strategy to eliminate environmental problems and avoid uncertainties with synthetic replacement fluids. Since ammonia is toxic, carbon dioxide provide high pressure, and the hydrocarbons are flammable, the general conclusion is often drawn that natural fluids gave safety problems. This paper will describe the possibilities of application as working fluids in low-temperature engineering refrigeration, heat pumping and organic Rankine cycles of the hydrofluoroolefins (HFOs) as third generation of synthetic working fluids.

Zeotropic Mixture Selection for an Organic Rankine Cycle Using a Single Screw Expander

The organic Rankine cycle (ORC) is a popular and promising technology that has been widely studied and adopted in renewable and sustainable energy utilization and low-grade waste heat recovery. The use of zeotropic mixtures in ORC has been attracting more and more attention because of the possibility to match the temperature profile of the heat source by non-isothermal phase change, which reduces the irreversibility in the evaporator and the condenser. The selection of working fluid and expander is strongly interconnected. As a novel expander, a single screw expander was selected and used in this paper for efficient utilization of the wet zeotropic mixtures listed in REFPROP 9.1 in a low-temperature subcritical ORC system. Five indicators, namely net work, thermal efficiency, heat exchange load of condenser, temperature glide in evaporator, and temperature glide in condenser, were used to analyze the performance of an ORC system with wet and isentropic zeotropic mixtures as working fluids. The calculation and analysis results indicate that R441A with an expander outlet temperature of 320 K may be the suitable zeotropic mixture used for both open and close type heat source. R436B may be selected with an expander outlet temperature of 315 K. R432A may be selected with an expander outlet temperature from 295 K to 310 K.

HCFO-1224yd(Z) as HFC-245fa drop-in alternative in low temperature ORC systems: Experimental analysis in a waste heat recovery real facility

The organic Rankine cycle (ORC) has been growing in importance as a technology for producing electricity from low temperature waste and renewable heat sources. In small-scale applications, the most used working fluid has been HFC-245fa, although this is being substituted for low GWP alternatives. A new HCFO working fluid, HCFO-1224yd(Z), has appeared as a possible alternative for HFC-245fa. In this study, this alternative working fluid is analysed and, finally, the working fluid is experimentally tested as drop-in alternative in a commercial ORC system. The main purpose is to determine the feasibility of using HCFO-1224yd(Z) as a drop-in replacement for HFC-245fa in a real facility that was initially designed to operate with HFC-245fa. The results show how the use of HCFO-1224yd(Z) offers power output that ranges from 7.5% to 17.4% lower than that provided by HFC-245fa. Although HFC-245fa offers higher power output, the results show that HCFO-1224yd(Z) offers up to 7.7% higher cycle net efficiency; this is due to the higher input thermal rate required by HFC-245fa. Finally, HCFO-1224yd(Z) has been stated as a suitable alternative to HFC-245fa as a drop-in replacement in a small-scale, low-temperature ORC, which increases its attractiveness for heat source with higher temperatures.

Multi-objective approach for determination of optimal operating parameters in low-temperature ORC power plant

Power plant with organic Rankine cycle (ORC) is currently the most perspective technology for electricity production when low-temperature source of energy is to be used. Because of the ‘narrow’ temperature range (ca 70 ÷ 200 K) between upper and bottoming energy sources of the ORC, it is characterized by a low thermal efficiency. Thus, a lot of research and scientific effort have gone into developing new calculation techniques to optimize the work of such a system and the selection of a working fluid. In most of the recent studies a multi-criteria approach was applied and a single optimal operating point of the ORC was indicated as the final result of the optimization. The primary goal of this paper is to present an alternative approach. The general idea is to determine and indicate a set of the optimal operating parameters for which none of the considered criteria dominates significantly. In order to meet these conditions, multi-objective optimization using weight function G and genetic algorithm (GA) is conducted. The outcomes of the proposed model converge with Pareto set findings obtained using commonly known Non-dominated Sorting Genetic Algorithm-II (NSGA-II) method. The analysis is carried out for three sampled working fluids of the ORC: R1234yf, R1234ze and R245fa. The final results show that R1234yf is the optimal choice, since, in case of that fluid, the lowest values of the function G are observed for all weight distribution cases. Specifically, when the economic criterion is dominant (w1 > w2 case), the function G for R1234yf, R1234ze and R245fa is equal to: 0.1069, 0.1270 and 0.1460, while for the environmental criterion predominance (w2 > w1 case), the following values of G are obtained: 0.0985, 0.1260 and 0.1474. Simultaneously, the component criteria of the function G, i.e. the APR parameter (economic criterion) and SI index (environmental criterion) are minimized as well for all considered cases. In accordance with the fluid order given above, the optimized values of the APR parameter for w1 > w2 case are as follows: 0.61 m2 kW−1, 0.68 m2 kW−1 and 0.68 m2 kW−1, while, for w2 > w1 relation, the SI index is equal to: 0.48, 0.49 and 0.48. Furthermore, using the optimized values of the decision variables (pinch point temperature difference, evaporation temperature and condensation temperature), the novel 3-D plots with so-called sustainable surface are developed in the study to reveal the potential optimal ranges of important ORC parameters, such as: thermal efficiency, exergy efficiency or net power output. In general, the method should be a useful tool in the design process of the ORC systems, since it allows for the straightforward indication of several optimal operating points with respect to the adopted criteria. In other words, the designer has a wider choice and can opt for the parameters values that favor the selected objective.

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%.

Numerical Investigation on the Performance of a Regenerative Flow Turbine for Small-Scale Organic Rankine Cycle Systems

In this study, the performance characteristics of a regenerative flow turbine (RFT) prototype have been investigated by means of a computational fluid dynamics (CFD) study. The prototype has been initially designed to be used in gas pipelines replacing expansion valves but, because of the intrinsic characteristics of this kind of expander, its use can be extended to other applications like the expansion process in small-scale organic Rankine cycle (ORC) plants. In the first part of this work, the numerical results of the CFD analysis have been validated with the experimental data reported in literature for the same turbine prototype. After the validation of the model, a detailed study has been carried out in order to evaluate specific features of the turbine, focusing the attention on the typical operating conditions of small-scale low-temperature ORC systems. Results have shown that the considered RFT prototype operates with higher isentropic efficiencies (about 32% at 6000 rpm) at lower mass flow rates, while the power output is penalized compared to other operating points. The numerical analysis has also pointed out the high impact of the losses in the leakage flow in the gap between the blade tips and the stripper walls. Therefore, the CFD analysis carried out has provided a thoughtful understanding of the performance of the expander at varying operating conditions and useful insights for the future redesign of this kind of machine for the application in small-scale ORCs.

Fluid⁻Structure Interaction for Biomimetic Design of an Innovative Lightweight Turboexpander.

Inspired by bird feather structures that enable the resistance of powerful aerodynamic forces in addition to their lower weight to provide stable flight, a biomimetic composite turbine blade was proposed for a low-temperature organic Rankine cycle (ORC) turboexpander that is capable of producing lower weight expanders than that of stainless steel expanders, in addition to reduce its manufacturing cost, and hence it may contribute in spreading ORC across nonconventional power systems. For that purpose, the fluid–structure interaction (FSI) was numerically investigated for a composite turbine blade with bird-inspired fiber orientations. The aerodynamic forces were evaluated by computational fluid dynamics (CFD) using the commercial package ANSYS-CFX (version 16.0) and then these aerodynamic forces were transferred to the solid model of the proposed blade. The structural integrity of the bird-mimetic composite blade was investigated by performing finite element analysis (FEA) of composite materials with different fiber orientations using ANSYS Composite PrepPost (ACP). Furthermore, the obtained mechanical performance of the composite turbine blades was compared with that of the stainless steel turbine blades. The obtained results indicated that fiber orientation has a greater effect on the deformation of the rotor blades and the minimum value can be achieved by the same barb angle inspired from the flight feather. In addition to a significant effect in the weight reduction of 80% was obtained by using composite rotor blades instead of stainless steel rotor blades.