Rankine Cycle Engine Research Articles

Thermal efficiency and boundary analysis of compression ignition internal combustion Rankine cycle engine

The compression ignition internal combustion Rankine cycle (CI-ICRC) is a novel combustion cycle that integrates oxy-fuel combustion, direct water injection (DWI), and waste heat recovery into diesel cycle to achieve ultra-high thermal efficiency and zero emissions. NOX formation is eliminated by utilizing oxygen instead of air, and water is injected into the cylinder to improve thermal efficiency. To achieve over 50% indicated thermal efficiency in the CI-ICRC engine, a thermodynamic model of the CI-ICRC engine is established in this paper to explore the potential thermal efficiency and boundary under different DWI strategies. Theoretical results demonstrate that the thermal efficiency of CI-ICRC engine is mostly affected by DWI mass and temperature, the thermal efficiency is dramatically enhanced as DWI mass and temperature increase. Under a water-to-fuel ratio of 20 and DWI temperature of 423K, the calculated thermal efficiency reaches 76.8%. Moreover, exhaust waste heat is mainly recovered through DWI, which increases the working fluid within the combustion chamber, thereby increasing the expansion work and improving cyclic thermal efficiency. Additionally, specific heat ratio of the mixture within the cylinder can be enhanced by 0.04 through DWI, improving the thermal efficiency by 9% at an in-cylinder temperature of 800 K.

Theoretical study of a downdraft gasifier, microturbine, and organic Rankine cycle fuelled by date molasses waste for distributed generation applications in Iraq

AbstractThe main objective of this work is the modeling of a combined heat and power (CHP) system that transforms waste from manufacturing date molasses in the Babylon province in central Iraq into electrical and thermal energy. A compact CHP facility that uses date waste (pomace and pits) as a feedstock is the subject of theoretical modeling and simulation research. The planned power plant consists of a downdraft gasifier, a microturbine (MT), and an organic Rankine cycle engine (ORC) as a bottom unit to maximize electric generation by recovering the MT exhaust gas heat and achieving a maximum electric output. The net electric production is 361.68 kW (280 kW from the MT unit and 81.68 kW from the ORC unit). To determine the best‐working fluid for the ORC system as a whole, three working fluids, including isopentane, benzene, and R113, were investigated. The temperature difference between the heat source and the fluid's boiling point is a key factor in choosing a more appropriate ORC working fluid. Isopentane is the most suitable working fluid for CHP applications, according to the findings of the simulation. The electric efficiency achieved is 30%, and the overall efficiency of the CHP plant was 56.87% from the gasification of date pits. The simulation results showed a satisfactory producer gas lower calorific value of 3.92 MJ k‐g‐1 for date pomace and 4.65 MJ k‐g‐1 for date pits, and cold gasification efficiencies of 73% and 76% for pomace and pits, respectively. © 2023 Society of Industrial Chemistry and John Wiley & Sons Ltd.

Flame Characteristics of Gasoline and a Water-in-Gasoline Mixture under a Controllable High-Temperature Thermal Atmosphere.

This study explores the flame characteristics of gasoline and a water-in-gasoline mixture and provides some optimization suggestions for the control strategy of water-in-gasoline mixture injection in the internal combustion Rankine cycle engines. The influences of co-flow temperature, injection pressure condition, and water content on the flame characteristics were investigated under a controllable high-temperature thermal atmosphere, including flame pattern, liftoff height, brightness, and local peak temperature distribution. Results obtained show that the water content has the most significant effect on the flame characteristics. The flame pattern gradually changes from slender to lumpy as the water content rises, while the high-temperature zone increases and becomes uniform. There is an appropriate water content range of 10-20% in which the flame characteristics are generally better. Furthermore, as the water content increases from 20 to 30%, the flame characteristics deteriorate, with a sharp decrease in the maximum flame temperature and brightness.

A geospatial analysis approach for the operational assessment of solar ORC systems. Case study: Performance evaluation of a two-stage solar ORC engine in Greece

Renewable energy potential depends on many variables that vary both in space and time. Therefore, geospatial analysis approaches may effectively support proper planning and technoeconomic analysis. In this study, a simulation model of a two-stage Organic Rankine Cycle (ORC) engine combined with evacuated tube solar collectors was developed and applied throughout Greece. The simulation model uses daily meteorological data for a thirty-four years reference period. These data are downscaled in hourly time step and used to predict the system's performance in spatially distributed form based on experimental data. The obtained results are statistically analysed and a set of ranking maps presenting various aspects of the integrated system performance is created. The results indicate that there is a profound spatial variability. The interannual average power production ranges spatially between 17.4 and 27.7 kWh/d, the total efficiency between 0.022 and 0.028 and the corresponding energy cost between 0.41 and 0.7 €/kWh. Finally, the effect of climate variability on energy production performance and the presence of trends are investigated. The proposed approach was proven to be an effective tool for the assessment of the feasibility and potential of a solar heat-to-power engine for Greece and to be expanded in other regions as well.

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.

A Control-Oriented ANFIS Model of Evaporator in a 1-kWe Organic Rankine Cycle Prototype

This paper presents a control-oriented neuro-fuzzy model of brazed-plate evaporators for use in organic Rankine cycle (ORC) engines for waste heat recovery from exhaust-gas streams of diesel engines, amongst other applications. Careful modelling of the evaporator is both crucial to assess the dynamic performance of the ORC system and challenging due to the high nonlinearity of its governing equations. The proposed adaptive neuro-fuzzy inference system (ANFIS) model consists of two separate neuro-fuzzy sub-models for predicting the evaporator output temperature and evaporating pressure. Experimental data are collected from a 1-kWe ORC prototype to train, and verify the accuracy of the ANFIS model, which benefits from the feed-forward output calculation and backpropagation capability of the neural network, while keeping the interpretability of fuzzy systems. The effect of training the models using gradient-descent least-square estimate (GD-LSE) and particle swarm optimisation (PSO) techniques is investigated, and the performance of both techniques are compared in terms of RMSEs and correlation coefficients. The simulation results indicate strong learning ability and high generalisation performance for both. Training the ANFIS models using the PSO algorithm improved the obtained test data RMSE values by 29% for the evaporator outlet temperature and by 18% for the evaporator outlet pressure. The accuracy and speed of the model illustrate its potential for real-time control purposes.

Chemical kinetic analysis of in-cylinder ion current generation under direct water injection within internal combustion Rankine cycle engine

Direct water injection provides feasible solution for combustion optimization and efficiency enhancement within internal combustion Rankine cycle engine, while the feedback signal of close-loop direct water injection control is still absent. Ion current detection monitors in-cylinder electron variation which shows potential in revealing direct water injection process. For better understanding of unprecedented augment of ion current signal under direct water injection within internal combustion Rankine cycle engine, a chemical kinetic model is established to calculate the effect of intake oxygen fraction, fuel quantity, initial temperature, and residual water vapor on in-cylinder electron formation based on GRI Mech 3.0 and ion current skeleton mechanism. The simulation results indicate direct water injection process show significant impact on in-cylinder electron formation through chemical interactions between H2O and other intermediate species including HO2, O2, CH3, and H, these reactions provides additional OH radical for propane oxidation facilitation, which result in large portion of CH radical formation and therefore, lead to higher in-cylinder electron generation. The initial temperature plays a vital role in determining whether residual water vapor show positive or negative effect by in-cylinder temperature co-ordination of direct water injection. Results of this work can be used to explain phenomenon related to direct water injection and ion current signal variation under both internal combustion Rankine cycle or traditional petrol engine.

Investigation of phosphoric acid fuel cell, linear Fresnel solar reflector and Organic Rankine Cycle polygeneration energy system in different climatic conditions

It has been proven that the future of energy demands for human society is related to clean energy sources such as solar energy. On the other hand, fuel cell technology converts the chemical energy of a fuel into electrical energy. Meanwhile, the polygeneration system based on (semi) renewable energy can be a viable alternative to conventional fuel-based systems. However, examining their performance for widespread implementation requires further and powerful studies. The aim of the present work is to evaluate the performance of the polygeneration energy system to produce electricity, heat, hydrogen and cool. The novel hybrid power system consists of the 10 kWel phosphoric acid fuel cell, linear Fresnel solar reflector (LFR), Organic Rankine Cycle (ORC) and Stirling engine (SE). In the novel energy conversion process proposed, the fuel cell generates electricity and heat, and its waste heat is used to generate additional electricity in the SE. The fuel and oxidant required by the fuel cell are supplied by an electrolyzer that is powered by an ORC system. A solar thermal collector provides the duty required by the evaporator of ORC system. The effect of key parameters such as current density, solar radiation and flow rate of hydrogen on the system energy and exergy performance is also investigated. To evaluate the performance of the LFR system the climatic conditions of Tehran (in Iran) and Beijing (in China) are considered. Results revealed that, the SE and ORC system separately generates 1.48 and 26.54 kW of electricity, respectively. Furthermore, the electrolyzer consumes 23.36 kW of electric power and 3.64 kW of cooling is produced by the chiller. In addition, the energy efficiency and total exergy destruction of the proposed system are 71.32 % and 57.94 kW, respectively.

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.

Assessment of the energy recovery potential of oil sludge through gasification aiming electricity generation

Thermochemical conversion of oil sludge (OS) residues from crude oil refinery was studied as an alternative for treatment and energy recovery before OS final disposal. A simulation model for OS gasification was developed and validated by using AspenPlus™ V 11.0. Three scenarios by considering different gasifying agents were evaluated, aiming the assessment of different gasification parameters and performance indexes such as gasification temperature, cold-gas efficiency, syngas yield, and LHV. To evaluate the potential use of syngas to generate electricity, three different prime movers such as steam Rankine cycle, gas microturbine, and gas-Internal Combustion Engine (ICE) were modeled. A hydrogen-rich syngas from OS gasification with air/steam mixture was reached (38.2 vol%) at ER 0.20. Syngas LHV and syngas yield ranged between 8.6 and 3.0 MJ/Nm3 and 0.6–1.3 Nm3/kg OS, when ER was increased from 0.25 to 0.45, respectively. The highest energy recovery potential (electricity generation) was obtained when using syngas to power a gas-ICE, with a generation index of about 0.47 kWh/kg-OS at ER ratio of 0.25. Therefore, OS thermochemical conversion into syngas and its subsequent use to generate electricity could be a technological alternative to oily residues management inside an oil refinery that could efficiently transform a dangerous residue into electricity.

Assessment of integrated energy systems for the production and use of renewable methanol by water electrolysis and CO2 hydrogenation

The conversion of renewable hydrogen and recycled CO2 to valuable fuels in power-to-liquids (PtL) systems could be a key solution to reduce CO2 emissions. Within this context, methanol is a promising candidate, both as an energy carrier and a chemical feedstock.This paper is focused on renewable methanol production and its use in a PtL integrated system based on a water electrolysis process fed by renewable energy sources (RES), a methanol synthesis and purification section and a high temperature solid oxide fuel cell. In particular, an analysis and comparison of two power-to-methanol integrated systems based on different electrolysis technology, i.e. the commercially mature alkaline electrolysers and the innovative high temperature solid oxide cells, was carried out. A thermal energy storage system based on a phase change material and an organic Rankine cycle engine can also be included, depending on the chosen electrolysis process.Detailed models of the main sections were developed and implemented by using Aspen Plus. A comprehensive analysis through mass and energy balances was carried out to evaluate performance indices of each section and of the overall plant for the two different configurations. A global efficiency of the overall system slightly lower than 0.35 was obtained for both systems.

Recovery and Utilization of Low-Grade Waste Heat in the Oil-Refining Industry Using Heat Engines and Heat Pumps: An International Technoeconomic Comparison

We assess the technoeconomic feasibility of onsite electricity and steam generation from recovered low-grade thermal energy in oil refineries using organic Rankine cycle (ORC) engines and mechanical vapour compression (MVC) heat pumps in various countries. The efficiencies of 34 ORC and 20 MVC current commercial systems are regressed against modified theoretical models. The resulting theoretical relations predict the thermal efficiency of commercial ORC engines within 4–5% and the coefficient of performance (COP) of commercial MVC heat pumps within 10–15%, on average. Using these models, the economic viability of ORC engines and MVC heat pumps is then assessed for 19 refinery streams as a function of heat source and sink temperatures, and the available stream thermal energy, for gas and electricity prices in selected countries. Results show that: (i) conversion to electrical power with ORC engines is, in general, economically feasible for heat-source temperatures >70 °C, however with high sensitivity to energy prices; and (ii) steam generation in MVC heat pumps, even more sensitive to energy prices, is in some cases not economical under any conditions—it is only viable with high gas/low electricity prices, for large heat sources (>2 MW) and higher temperatures (>140 °C). In countries and conditions with positive economics, payback periods down to two years are found for both technologies.

Working-fluid selection and thermoeconomic optimisation of a combined cycle cogeneration dual-loop organic Rankine cycle (ORC) system for solid oxide fuel cell (SOFC) waste-heat recovery

A novel combined-cycle system is proposed for the cogeneration of electricity and cooling, in which a dual-loop organic Rankine cycle (ORC) engine is used for waste-heat recovery from a solid oxide fuel cell system equipped with a gas turbine (SOFC-GT). Electricity is generated by the SOFC, its associated gas turbine, the two ORC turbines and a liquefied natural gas (LNG) turbine; the LNG supply to the fuel cell is also used as the heat sink to the ORC engines and as a cooling medium for domestic applications. The performance of the system with 20 different combinations of ORC working fluids is investigated by multi-objective optimisation of its capital cost rate and exergy efficiency, using an integration of a genetic algorithm and a neural network. The combination of R601 (top cycle) and Ethane (bottom cycle) is proposed for the dual-loop ORC system, due to the satisfaction of the optimisation goals, i.e., an optimal trade-off between efficiency and cost. With these working fluids, the overall system achieves an exergy efficiency of 51.6%, a total electrical power generation of 1040 kW, with the ORC waste-heat recovery system supplying 20.7% of this power, and a cooling capacity of 567 kW. In addition, an economic analysis of the proposed SOFC-GT-ORC system shows that the cost of production of an electrical unit amounts to $33.2perMWh, which is 12.9% and 73.9% lower than the levelized cost of electricity of separate SOFC-GT and SOFC systems, respectively. Exergy flow diagrams are used to determine the flow rate of the exergy and the value of exergy destruction in each component. In the waste-heat recovery system, exergy destruction mainly occurs within theheat exchangers, the highest of which is in the LNG cooling unit followed by the LNG vaporiser and the evaporator of the bottom-cycle ORC system, highlighting the importance of these components’ design in maximising the performance of the overall system.

Off-design optimisation of organic Rankine cycle (ORC) engines with different heat exchangers and volumetric expanders in waste heat recovery applications

Organic Rankine cycle (ORC) engines in real applications experience variable heat-source conditions. In this paper, the off-design performance of small- to medium-scale ORC engines recovering heat from stationary internal combustion engines (ICEs) is investigated. Of particular interest are the employment of screw vs. piston expanders, and two heat exchanger (HEX) architectures. Unlike previous studies where the performance of the expander and HEX are assumed fixed during off-design operation, here we consider explicitly their varying and interacting characteristics within the overall system. Nominal sizing results reveal indicated isentropic efficiencies > 80% for twin-screw and > 85% for piston expanders. Following nominal design, the ORC engine operation is optimised for ICE part-load (PL) operation. Although the heat transfer coefficients in the evaporator decrease by up to 30% at PL, the effectiveness in this HEX increases by 20% due to the larger temperature differences across the component. The screw expander efficiency reduces by up to 3% at off-design operation, whilst that of the piston expander increases by up to 16%. Optimised off-design maps indicate that the ORC engine power output reduces to 77% (piston) or 68% (screw) of its full-load value when the ICE operates at 60% PL, and that ORC engines with plate HEXs generate 5–11% more power than those with double-pipe HEX designs. Under variable ICE operation, smaller ORC engines with piston expanders generate more power than larger engines with screw expanders, highlighting the resilient off-design operation of piston machines. The modelling tool developed here can predict ORC performance over a wide operating envelope and provides performance maps that can be used by operators to optimise ORC engine operation in variable conditions and by ORC vendors to inform component design decisions.

Technoeconomic analysis of internal combustion engine – organic Rankine cycle systems for combined heat and power in energy-intensive buildings

For buildings with low heat-to-power demand ratios, the installation of internal combustion engines (ICEs) for the onsite provision of combined heat and power (CHP) results in large amounts of surplus heat. In the UK, such installations risk being ineligible for the CHP Quality Assurance (CHPQA) programme, thereby incurring additional levies. In this work, a technoeconomic optimisation of small-scale organic Rankine cycle (ORC) engines is performed, in which the ORC engines recover heat from the ICE exhaust gases in order to increase the overall efficiency of this combined solution and meet the CHPQA requirements. Two competing system configurations are assessed. In the first, the ORC engine also recovers heat from the CHP-ICE jacket water to generate additional power. In the second, the ORC engine operates at a higher condensing temperature, which prohibits jacket-water heat recovery but allows heat from the condenser to be delivered to the building. When optimised for minimum specific investment cost, the first configuration is initially found to deliver 20% more power (25.8 kW) at design conditions, and a minimum specific investment cost (1600 £/kW) that is 8% lower than the second configuration. However, the first configuration leads to less heat from the CHP-ICE being supplied to the building, increasing the cost of meeting the heat demand. By establishing part-load performance curves for both the CHP-ICE and ORC engines, the economic benefits from realistic operation can be evaluated. The present study goes beyond previous work by testing the configurations against a comprehensive database of real historical electricity and heating demand for thirty energy-intensive buildings at half-hour resolution. The discounted payback period for the second configuration is found to lie between 3.5 and 7.5 years for all of the buildings considered, while the first configuration is seen to recoup capital investment costs for only 23% of the buildings. The broad applicability of the second configuration offers attractive opportunities to increase manufacturing volumes and reduce unit costs. The findings are relevant to a range of buildings with heat-to-power demand ratios from 20% to 100%.

Modeling, Simulation, and Performance Evaluation Analysis of a Parabolic Trough Solar Collector Power Plant Coupled to an Organic Rankine Cycle Engine in North Eastern Greece Using trnsys

Abstract The present work is focused on the development of a simulation model for an existing cogeneration power plant, which utilizes a solar thermal field with parabolic trough solar collectors coupled to an Organic Rankine Cycle engine. The power plant is modeled in the trnsys v.17 software package and its performance has been validated with real operating conditions. The simulated system (concentrated solar power (CSP) field and ORC engine) is the main part of a hybrid power plant located near “Ziloti” village of the Municipality of Xanthi, in northeastern Greece. The construction of the hybrid power plant was funded by the Strategic Co-Funded Project of the European Territorial Cooperation Program Greece–Bulgaria 2007–2013 with the acronym ENERGEIA. The power plant simulated in this paper includes a 234 kWth solar parabolic trough collector (PTC) field, a 5 m3 thermal energy storage tank, and a 5 kWe ORC engine for the production of thermal and electrical energies. The results of the simulations present small deviation in contrast to the real operating data of the CSP power plant coupled with the ORC engine, therefore the simulation model is considered as reliable.

Waste Heat Recovery and Conversion into Electricity: Current Solutions and Assessment

The main energy consumption sectors are the residential, industry and transport. In all of them, a part of the energy consumption is not used and generally rejected as heat in the environment. This is named the waste heat. Firstly, the main way is to optimize the process to reduce the fuel consumption. Then, if there is a residual waste heat, a valorization way is to convert this heat into electricity. Some technologies are developed. The main technology is the Organic Rankine Cycle engine. Then, a new concept, named Turbosol, is based on the quasi-isothermal expansion of a water and oil mixture in a nozzle. Some piston engines are also developed, based on Stirling, Ericsson and Joule cycles. All these technologies are named externally heated engines. Some other research studies concern the thermo-electric effect and the thermo-magnetic effect. In this article, a non-exhaustive list, with description and comments on these technologies is proposed. The aim is to assess the potential of them and identify the current limits. To compare the different technologies in first law efficiency terms is not sufficient. Some new criterions are proposed. The first consideration is to assess the heat rate consumption referred to the heat rate available. To assess the quality of waste heat to power conversion, it is pertinent to evaluate the power output divided by the available heat rate. Then, because of the second law, it is pertinent to evaluate the exergy recovery ratio. These new waste heat criterions are compared to the classical first law efficiency in different cases. Then, the main current issue is to produce enough electrical power output to ensure the profitability. Some thermo-economic considerations are proposed, including the impact of a waste heat taxation.

Off-design operation of ORC engines with different heat exchanger architectures in waste heat recovery applications

Abstract Organic Rankine cycle (ORC) engines in waste-heat recovery applications experience variable heat-source conditions (i.e. temperature and mass flow-rate variations). Maximising ORC system performance while accounting for off-design operation in response to such variations is of crucial importance for the financial viability and wider adoption of this technology. In this paper, the off-design performance of an ORC engine is investigated in a heat-recovery application from a stationary internal combustion engine (ICE), employing a screw expander and two heat exchanger (HEX) architectures. Unlike previous studies where the ORC expander and HEX performance are assumed fixed during off-design operation, here we consider explicitly the time-varying characteristics of these system components. Nominal system sizing results indicate that the screw expander isentropic efficiency exceeds 80%, and that the heat transfer area requirements of plate HEXs (PHEXs) are 50% lower than those of double-pipe HEXs (DPHEX). Following nominal system design, the ORC engine operation is optimised at conditions relating to part-load (PL) operation of the ICE. Although, the heat transfer coefficients in the evaporator decrease by 30% at PL, the HEX effectiveness increases by up to 20% due to higher temperature differences between the working fluid and the heat source, and the performance of the PHEX appears less sensitive to off-design operation conditions. The optimum PL screw expander efficiency maps reveal only a minor reduction of the expander efficiency (by 2%) relative to the design conditions. Optimum off-design maps indicate that the power output of the ORC engine reduces to 72% of full-load for an ICE at 60% of full-load, and that ORC engines with PHEXs generate slightly more power for the same heat-source conditions. Overall, the tool developed predicts ORC performance over an operating envelope and allows the selection of optimal HEX and expander designs. The findings can be used by ORC plant operators to optimise the ORC engine power output given varying on-site heat-source conditions, and by ORC vendors to inform HEX and expander design decisions.

Off-design optimisation of organic Rankine cycle (ORC) engines with piston expanders for medium-scale combined heat and power applications

Organic Rankine cycle (ORC) engines often operate under variable heat-source conditions, so maximising performance at both nominal and off-design operation is crucial for the wider adoption of this technology. In this work, an off-design optimisation tool is developed and used to predict the impact of varying heat-source conditions on ORC operation. Unlike previous efforts where the performance of ORC engine components is assumed fixed, here we consider explicitly the time-varying operational characteristics of these components. A bottoming ORC system is first optimised for maximum power output when recovering heat from the exhaust gases of an internal-combustion engine (ICE) running at full load. A double-pipe heat exchanger (HEX) model is used for sizing the ORC evaporator and condenser, and a piston-expander model for sizing the expander. The ICE is then run at part-load, thus varying the temperature and mass flow rate of the exhaust gases. The tool predicts the new off-design heat transfer coefficients in the heat exchangers, and the new optimum expander operating points. Results reveal that the ORC engine power output is underestimated by up to 17% when the off-design operational characteristics of these components are not considered. In particular, the piston-expander isentropic efficiency increases at off-design operation by 10–16%, due to the reduced pressure ratio and flow rate in the system, while the evaporator effectiveness improves by up to 15%, due to the higher temperature difference across the HEX and a higher proportion of heat transfer taking place in the two-phase evaporating zone. As the ICE operates further away from its nominal point, the off-design ORC engine power output reduces by a lesser extent than that of the ICE. At an ICE part-load operation of 60% (by electrical power), the optimised ORC engine with fluids such as R1233zd operates at 77% of its nominal capacity. ORC off-design performance maps are generated, for characterising and predicting system performance, which can be used, along with the optimisation tool, by ORC system designers, manufacturers and plant operators to identify optimum performance under real operating conditions.

Effects of Vessel and Water Temperatures on Direct Injection in Internal Combustion Rankine Cycle Engines

This study focused on the effects of vessel and water temperatures on direct injection in internal combustion Rankine cycle engines through experimental and numerical methods. First, a study was carried out with schlieren photography using a high-speed camera for simultaneous liquid–gas diagnoses. Water was directly injected into a constant-volume vessel that provided stable boundaries. We wrote a MATLAB program to calculate spray tip penetration and cone angle from the images. For the further extension of boundary conditions, a numerical model was established and calibrated in AVL-FIRE for the thorough analysis of injection characteristics. Both experimental and numerical results indicated that injection and vessel temperatures have different effects on spray tip penetration. An increase in injected water temperature leads to shorter spray tip penetration, while the spray tip penetration increases with increasing vessel temperature. However, increased injection and vessel temperatures can both decrease the spray cone angle. Moreover, the simulation results also suggested that heat conduction is a main factor in boosting evaporation under top dead center conditions. When the internal energy of water parcels surges, these parcels evaporate immediately. These results are helpful and crucial for internal combustion engines equipped with direct water injection technology.