Geothermal Organic Rankine Cycle Research Articles

Techno-economic analysis and optimization of a binary geothermal combined district heat and power plant for Puga Valley, India

This article investigates the energy, exergy, and economic performance of a Puga Valley-specific binary geothermal combined district heat and power plant, taking into account geotechnical data and related cost functions. The power plant uses an organic Rankine cycle (ORC) with R123 as the working fluid, utilizing waste heat for district heating to enhance efficiency. Variations in operating and design parameters such as geothermal fluid flow rate, temperature, and ORC temperature of the evaporator and condenser are found to have significant effects on the overall performance of the system and cost rate. A multi-objective grey wolf optimization technique based on artificial neural networks has been used to determine the ideal values of the above-mentioned parameters. The optimization study revealed a Pareto optimal curve with overall exergy efficiency and total cost rate as objective functions from which the optimal point was identified using the technique for order preference by similarity to the ideal solution (TOPSIS) method. While operating the proposed CHP plant at TOPSIS optimal point, the plant delivers a net electrical output of 1.08 MW alongside 4.19 MW of thermal energy, resulting in a total cost rate of 33.84 USD/h, with energy and exergy efficiencies peaking at 32.26 % and 36.8 %, respectively. The exergy analysis at the optimal point also revealed a notable reduction of 550 kW in the total exergy destruction rate compared with the base-case scenario. Furthermore, the design requirements of a 5 MW net power output CHP plant for effective recovery of the Puga Valley geothermal resource are discussed.

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.

Comparative analysis of geothermal binary ORC systems: performance and environmental considerations for CO2 andwater as geofluids

This study considers the process simulation of geothermal binary Organic Rankine Cycle (ORC) systems which utilizes CO2 and water as geofluids for electricity generation. The simulation was performed using Hysys v11 software by using Peng Robinson’s fluid property package. Two dry working fluids including isopentane and n-pentane, were used. The effects of geofluid temperature and working fluid mass flowrate on power generation, as well as the maximum pressure of working fluids were evaluated. The result showed that power generation increases with higher geofluid temperature due to enhanced heat transfer. Isopentane outperformed n-pentane, attributed to its superior thermodynamic properties. CO2 showed better performance as geofluid than water highlighting its superiority, observed in the increased power generation. The unique characteristics of CO2 enable efficient heat transfer at lower temperatures, making it an environmentally friendly and effective choice. Contrarily, the use of water as a geofluid poses some implications for local ecosystems and water resources. From an environmental perspective, CO2 shows greater potential for reduced environmental impact, which aligns with the transition to cleaner energy sources. However, the economic considerations suggest a trade-off, as CO2 projects may entail higher upfront costs compared to water-based systems. Regulatory factors and economic feasibility, therefore, play a crucial role in the choice of geofluid for geothermal power generation.

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.

Investigation of the geothermal flash cycle developed by a thermoelectric generator and PEMFC: 3E analysis and optimization

Investigations have revealed that geothermal organic Rankine cycles (ORCs) have a low efficiency and high exergy destruction rate, especially in the heat exchangers network, which drastically reduces the whole system's performance. To tackle this problem, in the present research, the effects of employing thermoelectric generators (TEG) and proton exchange membrane fuel cell (PEMFC) on the performance of a geothermal ORC will be discussed from thermodynamic and economic points of view. In this study, three geothermal flash cycles are investigated. The first system, GFC, is a geothermal flash cycle. The second system is the GFC boosted by a TEG thermoelectric generator, which is applied instead of the condenser (GFTC). The third system is GFTC, developed using a PEMFC (GFTPC) before the evaporator. These systems are compared from energy, exergy, and economic perspectives. Parametric analysis is conducted for the system with the best performance and the effects of the evaporator temperature, evaporator quality, flash temperature, and PEMFC temperature on the system performance. Based on the results, the net power outputs of the GFTC and GFTPC are 10.21 % and 40.49 % higher than those of the GFC, respectively. GFTC and GFTPC have higher energy and exergy efficiencies and lower LCOE and payback period than GFC. In the GFTPC, a 32.86 % increase in the net power output, 20.81 % enhancement in the energy efficiency, and 0.22 % increase in the exergy efficiency are observed with an increase in the PEMFC temperature from 75 to 115 °C. Moreover, The PEMFC temperature increment reduces the LCOE and payback period of the GFTPC system. Also, the multi-objective optimization for GFTPC design with three decision variables has been done.

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.

Enhancing geothermal ORC power generation with SOFC: A comprehensive parametric study on thermodynamic performance

Geothermal energy is a promising renewable energy source due to its abundant global reserves. However, most geothermal resources are of low grade, severely restricting their power generation capacity and efficiency. In this study, the geothermal organic Rankine cycle power generation system is combined with a solid oxide fuel cell-gas turbine system to improve the overall power supply and heat utilization efficiency. The heat source and cycle temperature are upgraded using high-temperature solid oxide fuel cell waste gas, and the combined system’s efficiency is further improved through cascade heat utilization. Electrochemical-thermodynamic models are developed for the proposed system, and energy-exergy analysis is carried out. The effects of fuel cell current density, operating temperature and geothermal grade on the combined system’s performance are comprehensively analyzed. Compared to the standalone geothermal system, the energy efficiency and output power of the organic Rankine cycle increase by 8.11% and 7.95% respectively in the baseline case. Under the lowest geothermal temperature condition, the cycle output power is improved by 22.64%. The net power of the combined system can vary from 358 kW to 846 kW by adjusting the fuel cell current density from 1000 A m−2 to 8000 A m−2, demonstrating a flexible power supply capability. The optimal solid oxide fuel cell operating temperature is 1023.15 K, resulting in the highest system overall energy efficiency of 14.85%. The presented work provides essential guidelines for improving geothermal power generation performance and designing applicable combined systems.

Investigation of Energy and Exergy of Geothermal Organic Rankine Cycle

In this study, modeling and thermodynamic analysis of the combined double flash geothermal cycle generation was conducted using zeotropic fluid as the working fluid in the Organic Rankine Cycle (ORC). The analysis was performed based on the first and second laws of thermodynamics. Hexane, cyclohexane, isohexane, R245fa, and R236ea exhibit good performance at higher temperatures. In this study, three fluids—hexane, cyclohexane, and isohexane—were used. First, the model results for the pure fluids were compared with those of previous studies. Then, the important parameters of the cycle, including the efficiency of the first law of thermodynamics, the efficiency of the second law of thermodynamics, net productive power, and the amount of exergy destruction caused by changing the mass fraction of the refrigerant for the zeotropic fluids (investigated for the whole cycle and ORC), were obtained and compared.

Thermodynamic Optimization of Advanced Organic Rankine Cycle Configurations for Geothermal Energy Applications

The Organic Rankine Cycle (ORC) is commonly accepted as a viable technology to convert from low to medium temperature geothermal energy into electrical energy. In practice, the reference technology for converting geothermal energy to electricity is the subcritical simple ORC system. Over time, geothermal ORC plants with more complex configurations (architectures) have been developed. In the open literature, a large number of advanced architectures or configurations have been introduced. An analysis of the scientific literature indicates that there is some confusion regarding the terminology of certain advanced ORC system architectures. A new categorization of advanced configurations has been proposed, with a special emphasis on the application of geothermal energy. The basic division of advanced plant configurations is into dual-pressure and dual-stage ORC systems. In this study, the real potential of advanced ORC architectures or configurations to improve performance as compared with the simple ORC configuration was explored. The research was conducted for a wide range of geothermal heat source temperatures (from 120 °C to 180 °C) and working fluids. Net power output improvements as compared with the basic subcritical simple ORC (SORC) configuration were examined. The ability to produce net power with different ORC configurations depends on the magnitude of the geothermal fluid temperature and the type of working fluid. At a lower value of geothermal fluid temperature (120 °C), the most net power of 18.71 (kW/(kg/s)) was realized by the dual-pressure ORC (DP ORC configuration) with working fluid R1234yf, while the double stage serial-parallel ORC configuration with a low-temperature preheater in a high-temperature stage ORC (DS parHTS LTPH ORC) generated 18.51 (kW/(kg/s)) with the working fluid combination R1234yf/R1234yf. At 140 °C, three ORC configurations achieved similar net power values, namely the simple ORC configuration (SORC), the DP ORC configuration, and the DS parHTS LTPH ORC configuration, which generated 31.03 (kW/(kg/s)) with R1234yf, 31.07 (kW/(kg/s)) with R1234ze(E), and 30.96 (kW/(kg/s)) with R1234ze(E)/R1234yf, respectively. At higher values of geothermal fluid temperatures (160 °C and 180 °C) both the SORC and DP ORC configurations produced the highest net power values, namely 48.58 (kW/(kg/s)) with R1234ze(E), 67.23 (kW/(kg/s)) with isobutene for the SORC configuration, and 50.0 (kW/(kg/s)) with isobutane and 69.67 (kW/(kg/s)) with n-butane for the the DP ORC configuration.

Energy and Exergy Analyses of Geothermal Organic Rankine Cycles Considering the Effect of Brine Reinjection Temperature

The organic Rankine cycles (ORCs) have been used to convert low-enthalpy geothermal brine into power. Parameter optimization and working fluid selection are the main approaches to enhance geothermal ORC performance. This work uses environmentally friendly fluids, including R1224yd(Z), R1233zd(E), R1336mzz(Z), R601 and R601a, as the geothermal ORC working fluids. The evaporation temperatures of the selected fluids were optimized to maximize the cycle net power outputs. The thermodynamic characteristics are investigated with the consideration of the influence of the allowed reinjection temperature (ARIT). Among the selected fluids, R1224yd(Z) has the highest optimal evaporation temperature with the maximum turbine power output for a brine inlet temperature (BIT) higher than 120 °C, especially at a lower allowed reinjection temperature. However, the parasitic power consumption by the pumps in an ORC with R1224yd(Z) is also higher than with the other four fluids. The net power output for ORC with R1336mzz(Z) is slightly more than that with R1224yd(Z). Although the optimal evaporation temperature for a RORC is lower than that for an ORC, the higher preheater inlet temperature leads to a higher geothermal heating exergy efficiency and more power output for a BIT less than 120 °C. The RORC with R1336mzz(Z) produces 2.6% more net power than an ORC for a brine inlet temperature of 100 °C. As the ARIT increases from 70 °C to 75 °C, the plant exergy efficiencies of ORCs are decreased by 6–8% for a geothermal brine inlet temperature of 100 °C.

Design and operation of organic Rankine cycles for varied power load

ABSTRACT The organic Rankine cycles (ORC) always operate under off-design conditions due fluctuations in renewable energy access and actual user demand. Therefore, this study considered geothermal ORC as the research object to obtain the optimal design. The off-design performance was investigated under varied load curves by selecting different design loads. First, the optimal ORC system with a constant-rate load was obtained through minimizing the specific investment cost. For this optimal system, the exergy efficiency initially increased and then decreased under off-design operation conditions. Moreover, the peak efficiency was improved by increasing the design pressure. Second, the load curves of different users were considered, including industry, business, and residents. To achieve the best operational performance of the ORC for these varied load curves, different rate loads and pressures were selected, and then the off-design performance was studied. The results demonstrate that the rate load and maximum demand load are not optimal and that the optimal load ratio (rate load divided by the highest demand load) is 60%. The operating evaporation pressure obtained through the design is 1.9 MPa. For the optimal ORC, compared with a system with a load ratio of 1.0 (rate load is equal to the highest demand load), the total investment cost decreased by approximately 30.7%, and the average exergy efficiency increased by 19.2%.

Simultaneous optimization of design and operation of an air-cooled geothermal ORC under consideration of multiple operating points

We simultaneously optimize both the design and operation of an air-cooled geothermal organic Rankine cycle, maximizing total annual return (TAR), while considering multiple operating scenarios based on different ambient temperatures. In order to accurately capture realistic off-design behavior of the heat exchangers and turbine, as well as for the overall system, we incorporate component models that consider performance variations with both size and operating conditions. We employ a hybrid mechanistic data-driven modeling approach, involving artificial neural networks (ANNs) as surrogate models for accurate fluid properties, as well as for intermediate expressions for which ANNs improve tractability of the optimization problem. We demonstrate the importance of considering multiple operating conditions within design problems and propose a methodology for formulating and solving such problems globally, using our open-source solver MAiNGO.

Optimization of low temperature geothermal organic Rankine power generation system

In this paper, a low-temperature geothermal organic Rankine Cycle (ORC) power generation system model is established. R245fa is used as the circulating working fluid. The calculation program is compiled using the engineering calculation software EES, and the thermodynamic parameters and thermodynamic properties of the main state points of the ORC are theoretically calculated. The effects of narrow point temperature difference, evaporation temperature, geothermal water temperature and condenser end temperature difference on system performance are analyzed. The results show that the narrow point temperature difference has a significant impact on the ORC performance. Considering system performance and economic cost comprehensively, the best narrow point temperature difference and the best condenser end temperature difference are both within the range of 5–7 K.

Study of Corrosion Resistance Properties of Heat Exchanger Metals in Two Different Geothermal Environments

Geothermal fluids harnessed for electricity production are generally corrosive because of their interaction with the underground. To ensure the longevity and sustainability of geothermal Organic Rankine Cycle (ORC) powerplants, the choice of heat exchanger material is essential. The performance of heat exchangers is affected by corrosion and scaling due to the geothermal fluids, causing regular cleaning, part replacement, and in the worst cases, extensive repair work. The properties of geothermal fluids vary between geothermal settings and even within geothermal sites. Differences in exposure conditions require different material selection considerations, where factors such as cost, and material efficiency are important to consider. This work studies in-situ geothermal exposure testing of four metals at two geothermal locations, in different geological settings. Four corrosion-resistant materials were exposed for one month at Reykjanes powerplant in Iceland and four months at Chaunoy oil field in France as material candidates for heat exchangers. The tested alloys were analysed for corrosion with macro- and microscopic techniques using optical and electron microscopes, which give an indication of the different frequencies of repairs and replacement. Inconel 625 showed no effects at Reykjanes and cracks at Chaunoy. The others (316L, 254SMO, and titanium grade 2) showed either corrosion or erosion traces at both sites.

R1224yd(Z), R1233zd(E) and R1336mzz(Z) as replacements for R245fa: Experimental performance, interaction with lubricants and environmental impact

Achieving the 2050 EU climate targets - an economy with net-zero greenhouse gas emissions - requires a significant increase in the share of renewables in the current energy mix. The Organic Rankine Cycle (ORC) could play an important role in this transformation of today’s energy systems. The ORC is able to convert low-temperature heat sources into electrical power by utilizing refrigerants as working fluids. Many of these refrigerants have a significant global warming potential (GWP) and recent legislations therefore increasingly limit their use. In recent years a new generation of refrigerants has been developed. These refrigerants are characterized by low global warming potentials and are considered as replacements for the old high-GWP refrigerants.In this paper R1224yd(Z), R1233zd(E) and R1336mzz(Z) are investigated as possible replacements for the state of the art high-GWP refrigerant R245fa. These novel fluids have quite similar thermophysical properties and can thus potentially be used as drop-in replacements for R245fa. For the application as drop-in replacement and the correct interpretation of measurement data, the interaction of the refrigerant with the used lubricant is important. Therefore, the interaction of all four refrigerants with a typical lubricant oil is experimentally investigated first. A novel model based on Raoult’s law is introduced and validated with experimental data. The model allows the computation of the liquid and vapor phase composition of refrigerant/oil-mixtures using pressure and temperature measurements. In the next step, the performance of the four investigated refrigerants is evaluated experimentally in an ORC test rig. Therefore, the same experimental program was repeated for every refrigerant in order to ensure a fair comparison. To conclude the evaluation of the refrigerants, a Life Cycle Assessment (LCA) is conducted for a theoretical geothermal power plant operated with the four considered refrigerants.The experimental results show, that R1224yd(Z) as well as R1233zd(E) are very suitable low-GWP alternatives for R245fa and could be used as drop-in replacements. R1336mzz(Z) on the other hand shows significantly lower system efficiencies and power outputs compared to the other refrigerants. The highest power output and system efficiency is however still achieved by the state of the art refrigerant R245fa, but these quantities are only slightly lower for R1233zd(E) and R1224yd(Z) . The LCA based on the experimental results for a hypothetical large-scale geothermal ORC application reveals, that despite the slight performance decrease, the application of low-GWP fluids decreases the CO2-equivalent of the system significantly. The usage of R1233zd(E) could decrease the CO2-equivalent by 67% compared to R245fa.

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.

A review on geothermal Organic Rankine cycles: modeling and optimization

Employing renewable energy sources for power generation have been developed in recent years due to their several benefits including low emission of greenhouse gases and their inexhaustible nature. Among the applicable renewable energy sources for electricity generation, geothermal energy is one of the most appropriate ones owing to its independent availability from the weather condition. Due to the acceptable performance of Organic Rankine cycles (ORCs) in generating power from low- or medium-temperature heat sources, these cycles are appropriate choices for utilization in geothermal power plants. There are several parameters that influence the rate of power production and the efficiency of geothermal-based ORCs. The impacts of influential parameters have been investigated in several studies by modeling and optimization of these cycles. This article provides a comprehensive review of the previous studies focused on modeling and optimization of the ORCs with different configurations and operating conditions, which would be useful for the researchers working in this field of science. Conclusions of the reviewed studies reveal that geothermal-based ORCs performance is significantly influenced by the applied operating fluid in the cycle, working condition in addition to configuration and architecture of the applied cycle. Moreover, studies that concentrated on the optimization of geothermal-based ORCs are also reviewed and their key conclusions are reflected as well. In the majority of the studies in this field, technical and economic objectives have been considered for optimization of the cycles. Depending on the characteristics of the case studies, the efficiency and cost of the generated power in optimized geothermal-based ORCs vary greatly. Finally, several recommendations are proposed for future studies in this field.

Robust optimization of an organic Rankine cycle for geothermal application

A robust design optimization (RDO) methodology for Organic Rankine Cycles (ORC) is presented, allowing to ensure an improved, stable performance over a large range of operating conditions. In contrast with classical ORC design methods, whereby all modeling hypotheses and operating conditions are considered as perfectly known, i.e. deterministic, the RDO approach allows to account for the manifold sources of uncertainty affecting the system. For geothermal ORC, the latter are related on one hand with the ill-known properties of the geothermal source and, on the other, with intrinsically random parameters, such as the condensation temperature. The proposed RDO approach selects values of the design parameters that maximize the expected (average) performance while minimizing its variance under uncertain nominal operating conditions. The optimal design delivered by the proposed strategy outperforms the one derived from the standard deterministic approach: specifically, the expected power output is increased by 1.5%, while its standard deviation is reduced by 8.5% and the surface of the heat exchangers by 34%. • Geothermal Organic Rankine cycle performance is subject to many uncertainties. • A robust optimization strategy is devised for cycle design under uncertainty. • The design maximizes average performance while minimizing the variance. • Nested Kriging surrogates are used to efficiently speed-up the design loop. • Robust optimum has +1.5% power output and smaller variance wrt standard design.

Deterministic global superstructure-based optimization of an organic Rankine cycle

Organic Rankine cycles (ORCs) offer a high structural design flexibility. The best process structure can be identified via the optimization of a superstructure, which considers design alternatives simultaneously. In this contribution, we apply deterministic global optimization to a geothermal ORC superstructure, thus guaranteeing to find the best solution. We implement a hybrid mechanistic data-driven model, employing artificial neural networks as thermodynamic surrogate models. This approach is beneficial as we optimize the problem in a reduced space using the optimization solver MAiNGO. We further introduce redundant constraints that are only considered for the lower-bounding problem of the branch-and-bound algorithm. We perform two separate optimizations, one maximizing power output and one minimizing levelized cost of electricity. The optimal solutions of both objectives differ from each other, but both have three pressure levels. Global optimization is necessary as there exist suboptimal local solutions for both flowsheet configuration and design with fixed configurations.

Evaluation of Using Gas Turbine to Increase Efficiency of the Organic Rankine Cycle (ORC)

Power conversion systems based on the Organic Rankine Cycle (ORC) have been identified as a potential technology especially in converting low-grade renewable sources or waste heat. However, it is necessary to improve efficiency of ORC systems. This paper focuses on use of low geothermal resources (for temperature range of 80–128 °C and mass flow 100 kg/s) by using modified ORC. A modification of conventional binary power plant is conducted by combining gas turbines to increase quality of steam from a geothermal well. An analysis has been conducted for three different working fluids: R245fa, R1233zd(E) and R600. The paper discusses the impact of parameter changes not only on system efficiency but on other performance indicators. The results were compared with a conventional geothermal Organic Rankine Cycle (ORC). Increasing of geothermal steam quality by supplying exhaust gas from a gas turbine to the installation has a positive effect on the system efficiency and power. The highest efficiency of the modified ORC system has been obtained for R1233zd(E) as a working fluid and it reaches values from 12.21% to 19.20% (depending on the temperature of the geothermal brine). In comparison, an ORC system without gas turbine support reaches values from 9.43% to 17.54%.