Organic Rankine Cycle Power Plant Research Articles

Comparative study on the organic rankine cycle off-design performance under different zeotropic mixture flow boiling correlations

Zeotropic mixture utilization enhances the performance of the Organic Rankine cycle (ORC) significantly. For geothermal ORC power plants, the zeotropic mixture flow boiling correlation plays a vital role in both evaporator design and system performance evaluation. Based on the screened zeotropic mixture R1224yd(Z)/R1234ze(E) (0.452/0.548), a comparative analysis of 4 flow boiling correlations is performed to assess their impact on both evaporator design and system performance under off-design conditions. The results underscore a substantial impact of these correlations on determining the plate length, with the largest relative difference being 53.8%, consequently resulting in uncertainties of 31.7% and 23.4% for the evaporator exergy loss and cost, respectively. Under off-design working conditions, the relative difference in net power output varies from 16.43% to 7.67%. The exergy efficiency variation is up to 11.82% and the electricity production cost (EPC) uncertainty is up to 11.2% at the working condition with the maximum difference in net output work. This study provides valuable insights for selecting the appropriate zeotropic mixture flow boiling correlation in practical evaporator design and evaluating the off-design performance of ORC systems.

A novel TLFD-GA algorithm for multi-objectives optimization of ORC power plants with the effect of pressure drop considered

This research introduces a novel TLFD-GA algorithm for the three-level criteria optimization of organic Rankine cycle (ORC) power plants. It replaces the non-dominated sorting of NSGA-II with three-level fuzzy decision to simultaneously optimize environmental, thermodynamic, and techno-economic criteria. The algorithm provides direct optimization results without needing additional decision-making methods. Furthermore, the impact of pressure drop in plate heat exchangers is investigated, revealing a proportional relationship between pressure drop and the increment of evaporation pressure. A parametric study on 24 basic and recuperative trans-critical geothermal ORC systems indicates that compensating for pressure drop leads to a 5% decrease in thermodynamic performance, a 20% decrease in techno-economic performance, and a 20–60% increase in heat transfer area per net output power. After TLFD-GA optimization, the saving to investment ratio improves by an average of 4.5% across scenarios. R1270 stands out with a remarkable ratio of 3.13 and 4.32 in medium and high-temperature geothermal reservoirs. Moreover, the proposed TLFD-GA optimization algorithm has demonstrated its superiority in terms of superb convergence and stability when compared to NSGA-II, which suggests its prospective application in the low-carbon and green energy fields.

Comparative study of steam, organic Rankine cycle and supercritical CO2 power plants integrated with residual municipal solid waste gasification for district heating and cooling

Among the different waste-to-energy solutions, gasification is considered a promising option and an alternative to landfilling of residual municipal solid waste (RMSW). Therefore, the potential of RMSW air gasification in combination with three different power cycles for district cooling and heating applications is investigated. The model of a fluidized bed air gasifier developed in Aspen Plus is integrated with the models of steam turbine (ST), organic Rankine cycle (ORC), and supercritical CO2 (sCO2) Brayton cycle power plants for the combined cooling, heating, and power production in district networks.The results of the numerical study show that the ST power plant provides higher electrical power compared to the other systems, while sCO2 exhibits better thermal power and the maxima combined energy conversion efficiency. In between, ORCs prove to be a reliable and flexible solution for varying RMSW compositions and temperature levels of the district network. The size of the district network strongly varies with scenarios and in the best case, more than 1,400 residential buildings can be connected to the trigeneration plant considering 20 ktons/year of RMSW input to the gasifier.

Techno-economic assessment and grid impact of Thermally-Integrated Pumped Thermal Energy Storage (TI-PTES) systems coupled with photovoltaic plants for small-scale applications

The growing share of Renewable Energy Sources (RES) is rising the amount of curtailed energy to preserve grid security. With the aim of evaluating a complementary storage solution to electric batteries for both new and revamping RES power plants, this study investigates the performance of a Thermally Integrated Pumped Thermal Energy Storage (TI-PTES) system integrated with a Photovoltaic (PV) power plant aimed to enhance the energy self-sufficiency of small-scale users. The assessment is carried out for a case study characterized by a demand of electricity, heating and cooling that vary throughout the year. The studied PTES system is based on the integration of a High Temperature Heat Pump (HTHP), two Thermal Energy Storage (TES) sections and an Organic Rankine Cycle (ORC) power plant. Results show the influence of the main design parameters, such as PV size, HTHP size and TES capacities on the overall system performance, evaluated by means of the energy self-sufficiency, energy self-consumption, round-trip efficiency, Levelized Cost of Storage and the Grid Impact indicator, which quantifies the energy exchanges with the grid (feed-ins and withdrawals) to the overall user demand. The influence of seasonality on the energy performance indicators was studied as well. For the case study considered, the best combination of design parameters for the PV-PTES system is identified with reference to a PV plant whose yearly energy production equals the energy demand and for a 55 kWe High Temperature Heat Pump, a 10 kW ORC and a storage volume of 48 m3 for each of the two TES units. The PTES system is characterized by a LCOS of 0.72 $/kWh, a round-trip efficiency of 28.2 % and its integration offers several advantages over the use of a PV plant without any storage section. In particular, the PTES unit leads to a substantial increase in the self-sufficiency, within the range 1–14 %, and a considerable decrease of the grid impact indicator, within the range 2–28 %.

Structural Design and Analysis of a 100 kW Radial Turbine for an Ocean Thermal Energy Conversion–Organic Rankine Cycle Power Plant

In this paper, a 100 kW radial inflow turbine is designed for an ocean thermal energy conversion (OTEC) power plant based on the organic Rankine cycle (ORC) with ammonia as the working fluid. Based on one-dimensional (1D) and three-dimensional computational fluid dynamics (3D-CFD) modeling, the mechanical structure design, static and modal analyses of the turbine and its components are carried out to investigate its mechanical performance. The results show the stress and strain distribution in the volute, stator and rotor, and their maximum values appear, respectively, at the inlet cutout, the tip of the stator outlet and the connection position between the rotor and the shaft. After optimization, all the stresses in the above components are below the allowable values. The frequencies from the first order to the sixth order of the rotor and whole turbine were obtained through modal analysis without prestress and under prestress. The maximum frequency of the rotor and whole turbine is 707.75 Hz and 40.22 Hz, both of which are far away from the resonance frequency range that can avoid resonance. Therefore, the structure of the designed turbine is safe, feasible and reliable so as to better guide actual production.

Energy and exergy optimization of a combined solar/geothermal organic Rankine cycle power plant

Energy and exergy optimization of a combined solar/geothermal organic Rankine cycle power plant

Advanced monitoring of geothermal Organic Rankine Cycles

Geothermal Organic Rankine Cycle (ORC) power plants have tremendous worldwide potential for the utilization of low- and medium-temperature geothermal resources. However, monitoring of such a system is associated with special challenges for plant operators since the generated electrical power fluctuates strongly, which is caused by mainly two reasons: Firstly, due to the changing condensation conditions caused by air-cooled condensers, and secondly, due to the formation of scaling and fouling. Therefore, the process parameters of a geothermal ORC plant are highly variable, both in terms of diurnal and seasonal fluctuations. Considering the currently applied control software in commercial ORC plants, these fluctuations result in challenges regarding a reliable identification of potential degradation processes within the ORC process.This paper presents a novel analytical and data-based methodology for advanced monitoring of the main components of a two-staged subcritical geothermal ORC system in Southern Germany. These components are the heat exchanger, turbine and air-cooled condenser. For this purpose, operational data from more than three years were preprocessed and evaluated.For the shell-and-tube evaporator, this work applies an equation-based simulation model for the calculation of the thermal resistance due to scaling and fouling. For the turbine and condenser, polynomial functions of various degrees and different objective functions for the numerical computation of regression coefficients are examined. Regression models for three years of operation of the investigated geothermal plant were computed and compared with each other.The results indicate that there are no significant negative changes in the operational performance of most ORC main components during the investigated time period. However, negative operational changes are detected over a three-year period for one turbine of the ORC system. The detailed evaluation of the turbine data reveal a decrease of the normalized isentropic efficiency by 10%, which is significantly higher than the calculated RMSE of the developed correlations, thus indicating a clear performance reduction of the turbine. The developed advanced monitoring methodology allows the operator detailed and accurate assessment of the ORC components conditions, which can not be provided by the software solutions of the commercial ORC manufacturer. Furthermore, the presented methodology can also be applied to other thermal cycle systems with similar components, such as the Kalina Cycle.Finally, a user-friendly software application, developed with the MATLAB® App Designer, is presented. The developed software tool facilitates operators to easily and precisely monitor the operation of the main components of a geothermal ORC plant.

Assessment of geothermal potential of Kumaun Himalaya: A perspective for harnessing green energy

Energy is an essential part of human life, and its demand has exponentially increased after the industrial era, especially in developing countries. India has many geothermal springs (GTSs ∼340) in orogenic and non-orogenic belts, but their potential must be better known and utilized as an efficient green energy resource. Therefore, we attempted for the first time to estimate the reservoir temperature of geothermal springs (n = 17) using dissolved silica geothermometry along the Goriganga, Ramganga, and Kali rivers in the Kumaun Himalaya, Uttarakhand. The average and highest reservoir temperatures were 113 °C and 135 °C at Savildhar, respectively. The average reservoir and fluid circulation depths were ∼1.24 km and 1.27 km, respectively. An inverse relationship between reservoir temperatures and the distance of geothermal fields from the Main Central Thrust evinces structural control over the geothermal fields. Most of the studied GTSs (n = 14) have reservoir temperatures between 100 °C and 150 °C, making them medium enthalpy and suitable for Binary Organic Rankine Cycle power plants. Two springs with reservoir temperatures between 70 °C and 100 °C can be used for industrial, space heating, balneotherapy, and other direct uses. Since geothermal power plants do not require any fuel to produce electricity, this study emphasizes on exploiting the geothermal potential of Himalayan geothermal springs.

Innovations for organic Rankine cycle power systems: Current trends and future perspectives

Since the early 2000s, organic Rankine cycle (ORC) technology has experienced rapid development and market uptake. More than 4.5 GW of total ORC power plant capacity has been installed since then. Due to its flexibility, suitability for small- to medium-scale installations, its applicability to low- to medium-temperature heat sources, and compact design, ORC technology is already considered as state-of-the-art. However, to further increase the technical and economic potential of this technology, significant research is needed to further improve efficiency and other key performance indicators, as well as to address environmental and safety aspects. Therefore, it is important to research new technologies and to foster innovations for improving performance; but it is of similar importance to resolve operational challenges with applied research that focuses closer to the market, and the technology provider and user needs. This vision article outlines the current state-of-the-art and presents current research trends. Parts of this article summarize the research progress reported at the 6th International Seminar on ORC Power Systems (ORC2021) and accompanies the associated special issue published following this event. The article highlights research trends at the concept level, but also at the component and system levels and, therefore, provides a holistic overview for the interested reader regarding the current challenges and potential future research activities in this area. Beyond the variety of cycle concepts in Section 2.1, the different heat sources and applications in Section 2.2, rotating device and turbomachinery (pumps and expansion devices) options in Section 2.3, current R&D trends for working fluids are presented in Section 2.4. This editorial concludes with a more applied perspective on ORC systems, covering process control and a general perspective on the technology in Section 2.5. With an increasing number of ORC plants operating in the field, their integration within a wider energy system is also of increasing importance, and this is addressed in Section 2.6. All prior elements are levers toward the grand challenges in ORC technology: increasing efficiency, reducing investment costs, as well as increasing sustainability, environmental performance, and system compactness.

Speed of Sound Measurements in Dense Siloxane D6 Vapor at Temperatures up to 645 K by Means of a New Prismatic Acoustic Resonator.

Estimating the speed of sound for the dense vapor phase of D6 (dodecamethylcyclohexasiloxane, C12H36O6Si6) is particularly relevant to the study of nonideal compressible fluid dynamics (NICFD), the gas dynamics of fluids whose properties depart significantly from those related by the ideal gas model. If molecular complexity is sufficiently large, dense vapor flows may exhibit so-called nonclassical gasdynamic effects, and D6 is a candidate for experimental studies aimed at proving for the first time the existence of these exotic phenomena. More in general, speed of sound measurements in the dense vapor phase are important for NICFD applications: for example, complex organic compounds are employed as working fluids in organic Rankine cycle power plants and the correct prediction of the dense-vapor speed of sound is of paramount importance for the design of the supersonic turbines equipping these systems. This type of measurements is challenging and thus rare, especially in the case of complex organic molecules, given the high temperature at which they must be performed, close to the temperature at which the molecule thermally decomposes. Therefore, a new prismatic resonator (the OVAR, organic vapor acoustic resonator) has been conceived, designed, and realized. It is suitable for speed of sound measurements in the vapor phase of organic compounds at temperatures up to approximately 670 K and pressures up to about 1.5 MPa. The speed of sound of D6 (97.4% pure) has been measured along eight isotherms between 555 and 645 K and from nearly saturated density to densities close to ideal gas conditions (compressibility factor Z ≈ 0.95). The estimated relative uncertainty of these sound speed measurements is 0.14%. In addition, corresponding density values have been obtained with an estimated uncertainty between 0.2 kg·m-3 and 1.2 kg·m-3.

An improved cavitation model with thermodynamic effect and multiple cavitation regimes

Mechanical feed pumps in organic Rankine cycle (ORC) power plants can suffer from cavitation to lose their normal feeding performance or even damage. Cavitation models for organic fluids in ORC systems are lacking presently. Hence, a new cavitation model with thermodynamic effect was proposed. Surface tension-controlled, inertia-controlled, intermediate and heat transfer-controlled cavitation regimes, and two key elements: vapour bubble growth rate and vapour bubble number density are included in the model. A known air or non-condensable gas concentration in the liquid was employed to determine cavitation nuclei number density. The model was coded in ANSYS CFX as user defined model and validated with cavitating flows of organic fluid R114 in a venturi, liquid nitrogen and liquid hydrogen on a tapered hydrofoil and warm water around a hydrofoil NACA 0015 in cavitation tunnels based on visualised cavity length. Two model constants, temperature depression, and minimal cavitation number were correlated to bulk liquid temperature, Reynolds number, and Jakob number. The temperature and pressure profiles of liquid nitrogen and hydrogen on hydrofoil surface were examined against the experimental data. The model was applied to simulate unsteady cavitating flows of organic fluid R245fa in a diaphragm pump. It was shown that the temperature depression and minimal cavitation number cannot be correlated to bulk liquid temperature, Reynolds number and Jakob number. Two model constants can be correlated fairly to Reynolds number. The model underestimates the thermodynamic effect by 43% for R114, 18.6% for liquid nitrogen and 32.6% for liquid hydrogen based on temperature depression. The predicted temperature and pressure profiles on hydrofoil surface agree with the experimental data for liquid nitrogen. The model can produce an expected curve of mean pump flow rate against net positive suction head available.

Thermodynamic Optimization of Subcritical and Supercritical Organic Rankine Cycle Power Plants for Waste Heat Recovery in Marine Vessels

Abstract This study was aimed at comparing the optimal thermodynamic performance of subcritical and supercritical organic Rankine cycle (ORC) plants for waste heat recovery from ship engines. The technical impacts of adopting a supercritical ORC scheme relative to the usual subcritical one have not been explicitly reported in the literature for heat recovery in ship engines, hence this study. The fluids R245fa, R134a, and R600a were employed for analysis due to their versatility in real systems. The ORC plants were modeled and optimized in matlab using established zero-dimensional models to satisfy the first law mass and energy balances. Results showed that introducing a recuperator would increase ORC performance. For the R600a which exhibited the best performance among the three working fluids, a net power output of 488.3 kW was obtained for the subcritical ORC without a recuperator (SYS A) and 543.7 kW for the one with a recuperator (SYS B). Furthermore, a switch to a supercritical ORC configuration increased the net power by about 29% for R134a and 10% for R600a, and increased the thermal efficiency by about 2.2 percentage points for R134a and 0.5 percentage points for R600a, referencing the supercritical configuration without a recuperator (SYS C) and SYS A.

Effects of working fluid mixtures on the exergetic sustainability of a solar-biomass organic Rankine cycle power plant

This study assessed the effects of working fluid mixtures on the exergetic sustainability of a solar-biomass organic Rankine cycle (ORC) plant. The design features of a real ORC plant operational at Ottana (Italy) were employed for analyses. Although a lot of studies exist in the literature on thermo-economic implications of mixture working fluids in ORC plants, the aspect of sustainability is often left out. The desire to bridge this gap is the key motivation for this study. Zero-dimensional models were implemented in MATLAB for the design of all the hybrid plant units. The ORC design considered as working fluids the existing pure MM, and mixtures 0.1MM/0.9MDM, 0.8MM/0.2MDM, and 0.9MM/0.1MDMselected actively from the REFPROP database. Furthermore, established off-design models were incorporated into the ORC simulations. The exergetic sustainability index (ESI), the environmental effect factor (EEF), the exergetic improvement potential rate (IP), and the exergetic recoverability ratio (RECR) were adopted as sustainability performance indicators. Additionally, the destruction factor (DF) was used to assess the effects of a working fluid mixture on the sustainability performance of the ORC at the component level. Results showed that under design conditions, the mixture 0.1MM/0.9MDM had worse ESI than pure MM by about 1.5%, while the mixtures 0.8MM/0.2MDM and 0.9MM/0.1MDM improved ESI by about 1.6% and 2%, respectively. At off-design conditions, the optimum HTF mass flow rate with the highest ESI was obtained as about 7 kg/s for all the fluids. For the HTF temperature, 260 o C was obtained as the optimum for the mixture 0.8MM/0.2MDM, and 250 o C for all the other fluids. Also, decreasing the heat sink temperature in all fluids increases linearly the system ESI. Moreover, the ORC condenser showed the highest DF of over 70% for all the fluids, and the use of the best mixture (0.9MM/0.1MDM) as the ORC working fluid would increase the condenser DF by about 2 percentage points relative to pure MM. Future studies should seek to derive a set of general criteria that would aid the selection of cycle and working fluid mixture designs in favor of sustainability.

Model and transient Control strategy design of an Organic Rankine Cycle Plant for waste heat recovery of an Internal Combustion Engine

The multi-sources hybrid polygeneration energy systems are of great interest and topicality as they are one of the most promising technologies in the European’s Green Deal panorama, with the aim of serving users with electrical and thermal energy using a single plant powered by one or more energy sources. In the waste heat recovery field Organic Rankine Cycle (ORC) power plants are becoming increasingly popular, especially for exploiting medium and low temperature heat sources as a micro-small scale power plant. However, the development and diffusion of this technology is still limited due to the high costs and consequently prototype development and experimental assessment of performance is very poor, especially for non-stationary systems. In this work the modelling and validation of a micro-scale waste heat recovery (WHR) plant coupled with a control system is presented. An ORC plant has been modelled through a map-based model approach for the piston pump and the scroll expander while the pipes and the heat exchangers through a 1D thermo-fluid dynamic approach. A preliminary comparison was made between some numerical quantities of the modelled plant and the same experimental quantities in 61 different operating conditions, showing an average error of 50.1%. The model has been calibrated using a vector optimization technique: two calibration parameters of the heat exchangers were calibrated with a genetic algorithm (MOGA II) by reducing the error of 5 quantities obtained from the model with the respective experimental quantities in 15 different operating conditions. The remaining 46 operating conditions were used to evaluate the calibrated model, showing an average error of 3%. Furthermore, in order to provide for the use of the system coupled to highly variable heat sources, such as the exhaust gases of an internal combustion engine, a control strategy has been designed to perform two tasks: leading the ORC performance where the efficiency is higher, acting on the pump speed through a map-based control, implemented by a look-up table control, and protecting the organic fluid from damage caused by high working temperatures through a bypass control system with a PI control, depending on the proportional and integral gains. In order to verify the control strategy behaviour at different thermal transient inputs, a set of simulations has been run, showing a robust and stable manner preserving the organic fluid properties and limiting the superheated steam at expander inlet.

Comparative design-point and yearly advanced exergoenvironmental analyses of a solar-biomass organic Rankine cycle power plant

ABSTRACT This study compared the results of advanced exergoenvironmental analysis at the design point and over a year of operation of a solar-biomass organic Rankine cycle (ORC) power plant based on a real 630 kW ORC plant operational at Ottana (Italy). Although the advanced exergoenvironmental method is now popular in the literature for energy system analysis, it is commonly applied at a single design point. It is, however, not clear yet if such design-point analysis would be acceptably reliable in transient energy systems that depend on varying weather conditions such as solar, a research gap this study attempted to fill. Total impact rates of 7.02 Pts/h were obtained in the hybrid plant at the design point out of which about 4.39 Pts/h is avoidable; 3.50 Pts/h (about 80%) due to the internal operations of each of the individual plant components (endogenous) and 0.89 Pts/h (about 20%) due to interconnections among the system components (exogenous). Also, total yearly impact rates of 91.14 kPts/year were obtained out of which about 69.93 kPts/year is avoidable; 51.13 kPts/year (about 73%) endogenous and 18.80 kPts/year (about 27%) exogenous. While about 63% of the total hybrid plant’s impact rates are obtained avoidable based on the design-point advanced exergoenvironmental analysis, about 76% is obtained in the yearly analysis. These results imply that applying the advanced exergoenvironmental method to a transient energy system over a year of plant operation would offer significantly different insights relative to the analysis at the design point. Going forward, results of the advanced exergoenvironmental method should be interpreted with caution when applied at single design points, especially for energy systems that rely heavily on intermittent weather parameters for their nominal operation.

Annualized exergoenvironmental comparison of solar-only and hybrid solar-biomass heat interactions with an organic Rankine cycle power plant

This study is aimed at comparing the environmental performance of a solar-only and hybrid solar-biomass organic Rankine cycle (ORC) plant using the exergoenvironmental method. The system studied adopts the design features of a real ORC plant currently running at Ottana, Italy, rated nominally at about 630 kW. Established procedures of the exergoenvironmental methods were applied, which integrate the principles of exergy and life cycle analyses. The method quantifies the yearly environmental impact rates of each of the plant components. The eco indicator-99 impact assessment method was adopted to quantify impact rates. Results showed that implementing the biomass hybridization scheme would improve the annualized exergetic efficiency of the existing solar ORC plant by about 3 percentage points, from about 7% with solar-only to about 10% with hybrid solar-biomass heat sources, although a small increase in the relative irreversibility rate is observed (from 49% to 51%). It would as well improve the capacity factor of the plant. However, the environmental impacts would be impacted negatively. Particularly, the specific exergoenvironmental impact rates were obtained as 27.4 Pts/MWh for the hybrid solar-biomass ORC plant, as against 20.3 Pts/MWh obtained for the solar-only plant, implying that the hybridization strategy would increase impact rate by about 35%. Similarly, it was obtained that biomass hybridization would increase the overall exergoenvironmental impact rates of the ORC plant by about 92,000 Pts/year, due majorly to increased exergy destruction in the plant components and the polluting emissions of the combustor.

Impacts of biomass hybridization on exergetic sustainability of a solar organic Rankine cycle power plant

Due to the low thermo-economic performance of solar ORC plants, several studies are ongoing to investigate the potential merits/demerits of biomass hybridization as a retrofit. Although many of such studies have examined the technical and economic viabilities of various hybrid solar-biomass concepts, due attention has not been directed to the area of their sustainability based on the Second Law of Thermodynamics. This study was aimed at investigating the potential effects of biomass hybridization on the exergetic sustainability of an organic Rankine cycle (ORC) power plant. Design data of an existing solar ORC plant currently operational at Ottana (Italy) were employed for analysis. The referenced ORC plant is rated at about 630 kWe, powered at the moment solely by a concentrated solar power (CSP) field, which integrates linear Fresnel collectors with two-tank thermal energy storage (TES) device. Five different exergetic sustainability indicators were employed in this study to assess the aforementioned ORC plant when fed by hybrid solar-biomass energy resources. Results showed the hybrid plant's destruction factor (DF), exergetic sustainability index (ESI), environmental effect factor (EEF), improvement potential rate (IPR), and recoverability ratio (RECR) of 89.40%, 0.119, 8.435, 4744.8 kW, and 0.89, respectively. Additionally, a comparison of the solar-only and the hybrid solar-biomass schemes revealed that biomass hybridization would improve the exergetic sustainability of the ORC plant. Nevertheless, the high value of IPR obtained suggests that ample opportunities would yet exist to further enhance sustainability indices of the studied ORC plant post biomass hybridization. In sum, biomass hybridization of the solar ORC plant considered in this study would enhance sustainability performance but more opportunities would yet exist to further optimize the system.

Parametric optimization and comparative study of an organic Rankine cycle power plant for two-phase geothermal sources

For two-phase geothermal sources, Organic Rankine Cycle (ORC) based binary plant is often applied for power production. In this work, a network topology is designed with the open-source Thermal Engineering Systems in Python (TESPy) software to simulate the stationary operation of the ORC plant. With this topology, the performance of six different working fluids are compared. From the thermodynamic perspective, the gross and net power output are optimized respectively. Results show that R600 has the highest gross power output of 17.55 MW, while R245fa has the highest net power output of 12.93 MW. However, the turbine inlet temperatures for these two working fluids need to be designed at the upper theoretical limit. R245ca and R601a require the heat exchange rates of internal heat exchanger to be larger than 1.51 MW and 0.99 MW to satisfy the re-injection temperature limit, which are smaller than the R600 (6.7 MW) and R245fa (6.0 MW) cases. Besides, the working fluid with lower critical state is preferred for a geothermal source with smaller steam fraction to establish a stable ORC plant. The workflow for the ORC design and optimization in this work is generic, and can be further applied to thermo-economic investigation.

Performance analysis and working fluid selection of an Organic Rankine Cycle Power Plant coupled to an Enhanced Geothermal System

This paper presents a coupled Wellbore-Reservoir-Organic Rankine Cycle (ORC) power plant model for sustainable and efficient use of multiple-fractures Enhanced Geothermal System with simulations conducted over 40 years of operation time. A multi-objective optimization of the ORC under off-design conditions was conducted using constrained NSGA-II technique considering specific investment cost, energy and exergy efficiencies as objective functions. Twenty working fluids were considered to select the best one based on the turbine inlet superheated vapor condition, the lowest specific investment cost, the best energy and exergy efficiencies. Optimal operations conditions were determined for each working fluid considering turbine inlet temperature, turbine inlet pressure, condenser temperature, refrigerant mass flow rate, and length, tubes number, inner diameter and outer diameter of evaporator and condenser, isentropic turbine efficiency, isentropic pump efficiency as decision variables. The results show that the working fluids have a significant effect on the reinjection geofluid temperature and slight effect on the production temperature. The best performing working fluid was R1233zd(E) providing an energy efficiency of 19.2–19.32% and the exergy efficiency found is between 56.4% and 58.44% over the 40 years of production in off-design conditions and optimal operation conditions. The energy efficiency and the exergy efficiency decrease with increasing condenser temperature.

A new indicator for minimizing size of an ORC power plant based on heat exchanger and turbine design parameters

• System size index SSI – a novel indicator for minimizing size of the ORC is proposed. • SSI is defined based on heat exchanger area and turbine rotor diameter. • By using heat transfer indicators, the turbine may be oversized by up to 70%. • With the use of turbine indices, the heat exchanger may be oversized by up to 40% . • By applying the SSI index, the ORC components are sized optimally. The increasing interest in mobile applications and modular structures of an organic Rankine cycle (ORC) has resulted in growing importance of the optimal sizing of the ORC installation. Most of the existing works were focused on reducing dimensions of the ORC by applying indicators estimating size of a single type of the ORC component. This study aims to define an indicator which comprehensively evaluates the size of the ORC power plant. The proposed indicator, specified as System Size Index SSI , is formulated based on two key ORC design parameters, namely the total heat transfer area of the heat exchangers and the rotor diameter of the turbine. To examine the versatility of SSI , the analysis was conducted considering different ORC applications scenarios, including modular geothermal power plants and mobile waste heat recovery units. The findings of single-objective optimization procedures have shown that by applying indicators evaluating size of the heat exchangers, the turbine rotor diameter may deviate from its smallest possible dimension by up to 70%. Similarly, by utilizing indices evaluating size of the turbine, the heat exchangers area may differ from its optimal size by up to 40%. Meanwhile, by using the SSI indicator, both the total heat transfer area and turbine rotor diameter did not differ from their smallest possible dimensions by more than 1% for each scenario considered. Finally, based on a bi-objective analysis with SSI and net power output as the criteria, it was concluded that by applying such indicators as APR parameter or performance factor PF , the designer is left with a single design solution, while the adoption of the bi-objective approach gives much more flexibility in finding a compromise between the system size and power output of the installation.