Subcritical Organic Rankine Cycle Research Articles

Exergy efficiency improvement by compression heat recovery for an integrated natural gas liquefaction-CO2 capture-NGL recovery process

For liquefied natual gas production, cryogenic distillation offers lower thermal energy consumption and a more compact footprint for carbon capture unit than the widely used chemical absorption. However, the reduction in thermal energy consumption comes at the cost of increased power consumption. To reduce the power consumption of cryogenic distillation based natural gas lqiuefaction plants, an integrated process for liquefied natual gas production, cryogenic CO2 capture and natural gas liquids recovery is proposed, which is called the base process. This base process is further integrated with a heat recovery unit (Organic Rankine cycle or Kalina cycle) to investigate the optimal utilization of compression heat in the system. The proposed processes are modeled in Aspen HYSYS and optimized using a genetic algorithm installed in Matlab. The results show that specific power consumption of the base case is 0.385 kWh/Nm3 NG. Due to the lack of compression heat recovery, significant exergy loss occurs at the water coolers, accounting for approximately 37 % of the total system exergy destruction. Among the evaluated methods, the supercritical Organic Rankine cycle, using R1234ze as the working fluid, recovers approximately 60 % of the exergy from compression heat with an 8.7 % improvement in system exergy efficiency. The subcritical Organic Rankine cycle, using R600a as the working fluid, achieves a slightly lower exergy efficiency improvement of 7.1 %. The Kalina cycle, while less efficient, still improves the system’s exergy efficiency by 6.3 %. Economic analysis reveals that the total annualized cost of the base process is $8,441,416, with a levelized cost of $0.1411/kg. The subcritical Organic Rankine Cycle provides the most cost-effective solution, with the lowest total annualized cost of $8,305,154 and a levelized cost of $0.1402/kg. The Kalina cycle ranks second in cost-effectiveness, with a total annualized cost of $8,345,914 and a levelized cost of $0.1405/kg NG. Although the supercritical ORC requires the highest capital investment, it proves more cost-effective than the base process due to its reduced annual operating costs.

Review on the development of solar geothermal complementary power generation system

Nowadays, due to the burning of fossil energy has many negative impacts on the environment, people are looking for new sustainable energy sources that can replace fossil energy, and among them, geothermal energy and solar energy are more prominent, but the two energy sources alone have certain shortcomings, and since complementary solar geothermal energy systems compensate for some of the shortcomings of the two energy sources, much research has been done in this field. This essay summarizes the conclusion from those studies and identify the shortcomings. The largest losses in the system are always in the solar power system. The component with the highest losses may be the evaporator or the trough solar collectors with over 50%. In a solar-geothermal complementary power system, the two energy sources promote each other and the system is more efficient in the summer than in the winter. In a solar-geothermal Organic Rankine Cycle power generation system, Supercritical Organic Rankine Cycle can be a better option than subcritical Organic Rankine Cycle for solar geothermal hybrid power production and cost control. And hybrid power plants benefits will increase over time

Recommended operating conditions and performance evaluation of commonly used hydrofluoroolefin (HFO) and hydrochlorofluoroolefin (HCFO) refrigerants in organic Rankine cycle

The application of the first three generations of organic refrigerants, namely, CFCs, HCFCs, and HFCs, in an organic Rankine cycle has been widely investigated. As environmentally friendly successors to the above three generations of refrigerants, hydrofluoroolefins (HFOs) and hydrochlorofluoroolefins (HCFOs) have also attracted increasing research interest for their application in the organic Rankine cycle (ORC). Based on molecular complexity, characteristic points on saturation liquid and vapor curve and area of characteristic regions in temperature-entropy (T-s) diagram, eight common hydrofluoroolefins (HFOs) and hydrochlorofluoroolefins (HCFOs) are classified, analyzed, and evaluated. In addition, the type of heat source and specific operating conditions in which these eight hydrofluoroolefins (HFOs) and hydrochlorofluoroolefins (HCFOs) are most effective are recommended. The results show that molecular complexity of hydrofluoroolefins (HFOs) and hydrochlorofluoroolefins (HCFOs) mainly depends on the length of the C chain, and then on the number and location of Cl and F atoms. HFO-1243zf has the best comprehensive evaluation indicator, and it is noteworthy as a recommended working fluid for subcritical ORC systems. The result obtained in this paper may provide a reference for the design and actual operation of Organic Rankine Cycle system using HFO and HCFO as working fluids.

Waste Heat Conversion to Power via Organic Rankine Cycle

several studies have been carried out on how to recover waste heat using Organic Rankine cycle. Each study applied different working conditions, power scale and cycle configuration, hence an assessment of working fluids is given for specific case. As a result, no single working fluid have been identified that would meet an entire heat source temperature levels. This motivated us to examine various substances for use as working fluids for subcritical ORC systems operating in the temperature range (543 - 633K). In this study, with a given finite thermal source, performance of working fluid candidates are evaluated and assessed using four criteria: thermal efficiency, exergy efficiency, total exergy destruction and net power output. Results of the study indicate that from a performance point of view, the linear alkanes such as nonane seem to be a suitable fluid in the temperature range (543-573K). Beyond aforementioned temperature range it appears that aromatic hydrocarbons such as toluene, ethylbenzene, P-xylene, O-xylene, propylbenzene preformed better. Under this condition, they represent comparably similar proposed screening criteria parameters and comparably high evaporating pressure, with only occasional slight differences between them.

Thermodynamic and economic analysis of organic Rankine cycle combined with flash cycle and ejector

Increasing the net output power of the Organic Rankine Cycle (ORC) can increase the possibility of developing low-medium temperature heat sources. It is equally important to effectively control the system's economy. So, subcritical double-pressure ORC combined with flash cycle and ejector (S-EFDPORC) and transcritical ORC combined with flash cycle and ejector (T-EFDPORC) are proposed in present work. The thermodynamic and economic analyses on five proposed systems within the heat source temperature range of 100 °C–200 °C are conducted. The net output power and levelized energy cost (LEC) are treated as optimization objectives in thermodynamic and economic performance comparisons, respectively. The non-dominated sorting and three multi-objective optimization methods (LINMAP, TOPSIS, and Shannon Entropy) are used to perform the multi-objective optimization. The results show that S-EFDPORC and T-EFDPORC can both effectively improve the net output power compared with subcritical double-pressure ORC (S-DPORC) and transcritical double-pressure ORC (T-DPORC), while subcritical ORC combined with flash cycle and ejector (S-EFORC) can only improve the net output power within a certain temperature range. When the optimization objective is the net output power, compared with S-DPORC and T-DPORC, the growth rate of net output power in S-EFORC is more than 20% when the temperature is below 160 °C. When the temperature of the heat source is below 160 °C or above 180 °C, the growth rates of net output power of S-EFDPORC and T-EFDPORC increase more than 25%. When LEC is treated as optimization objective, compared with S-DPORC and T-DPORC, S-EFORC reduces LEC by more than 10%. S-EFDPORC and T-EFDPORC increase the LEC of the system, and the maximum growth rates of LEC are 30% and 60% respectively, resulting in worse economic performance. With three multi-objective optimization methods, both S-EFDPORC and T-EFDPORC can increase net output power, but their economy will deteriorate at most heat source temperatures. Although S-EFORC increase the net output power when the heat source temperature is below than 177 °C, LEC reduces throughout the heat source temperature range.

Highlighting the effect of the lower operating limit of the condenser on ORC working fluids selection

In this paper, the study of the effect of the lower operating limit of the condenser on ORC working fluids selection was studied. A sample of 56 fluids constituting the starting universe was extracted in EES software. Two scenarios were studied depending on the definition of the condensing limit. The results shown that the starting universe is partitioned into two quite defined and distinct sub-universes for which two fluids taken separately in each sub-universe cannot be compared due to a condensing limit constraint. Furthermore, it was found that the size of each sub-universe depends not only on the operating condition of the condenser but also on the condensing limit constraint. By extending the study to a subcritical organic Rankine cycle with an internal heat exchanger, it was found that good performance can be obtained by defining the condensing limit from pressure compared to that obtained using temperature. Therefore, it was established that the lower operating limit of the condenser is an additional criterion to be taken into account when performing a working fluids selection for an ORC system in order to conduct a real comparative study. The work could shed some lights on designing of ORC.

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.

Design, Integration, and Control of Organic Rankine Cycles with Thermal Energy Storage and Two-Phase Expansion System Utilizing Intermittent and Fluctuating Heat Sources—A Review

In order to lessen reliance on fossil fuels, a rise in interest in the utilization of fluctuating and intermittent heat sources derived from renewable energy (such as solar thermal, ocean thermal, and geothermal) and waste heat has been observed. These heat sources could be used to generate electricity at relatively low and medium temperatures, for example, through the organic Rankine cycle (ORC). In some case studies, various approaches have been developed to deal with and design ORCs in the desired operating condition utilizing suitable working fluids. This article aims to review some designs and integrated systems of ORC with thermal energy storage (TES) and a two-phase expansion system focusing on the utilization of medium- and low-temperature heat sources in which some subcritical ORCs are presented. Moreover, several possible control systems (both conventional and advanced ones) of ORC with TES and a two-phase expansion system are reported and compared. At the end of this article, the possible future developments of design and control systems are discussed to describe advanced ORC for utilizing low-grade heat sources. This study aims to provide researchers and engineers with an insight into the challenges involved in this process, making industrialization of ORC technology more extensive, in particular when combined with TES and a two-phase expansion system.

Study on performance evaluation framework and design/ selection guidelines of working fluids for subcritical organic Rankine cycle from molecular structure perspective

Organic Rankine cycle (ORC) is a promising power cycle to convert low-grade thermal energy. The quick and easy evaluation of the potential working fluids lacking thermophysical property data is important for the working fluid selection of ORC. This paper proposes a systematic framework of working fluid evaluation from the molecular structure perspective, which couples the quantitative structure property relationship (QSPR) and universal equations of state. The thermodynamic performance of ORC is first correlated with four key thermophysical properties based on universal equations of state. Then, these thermophysical properties are obtained from the corresponding QSPR models based on the molecular structure of working fluids. The influence factors of ORC performance at a micro level are investigated using the QSPR models. Moreover, the effects of molecular characteristics on thermal efficiency are analyzed. The criteria for working fluid design/selection are recommended aiming at the improvement of thermal efficiency. The proposed framework shows sufficient accuracy with the relative errors of −0.41 - 15.85% and mean absolute error of 8.29% compared with the results based on REFPROP. It is found that the cyclization of carbon chain is an ideal method for the working fluid design based on alkanes and alkenes. This work can provide new insights for the evaluation, selection, and design of working fluids on the molecular structure level.

Comparative Thermodynamic Optimization of Organic Rankine Cycle Configurations for Geothermal Power Generation from an Abandoned Oil Well

This study was aimed at quantifying the net electrical power producible from an abandoned oil well in Nigeria using different organic Rankine cycle (ORC) configurations and working fluids. The geological features of a typical Nigerian oil well were employed in the study and a borehole heat exchanger was used for simulating the thermodynamic parameters of the heat source. Specifically, a subcritical ORC without a recuperator (SBC), a subcritical ORC with a recuperator (SBC-R), a supercritical ORC without a recuperator (SPC), and a supercritical ORC with a recuperator (SPC-R) were analyzed, using R115, R236fa, and R1234yf as working fluids. Results showed that between 272 kW and 875 kW of electrical power could be produced from the abandoned oil well using the most basic ORC configuration (SBC). Furthermore, it was obtained that the introduction of a recuperator would increase the ORC net power by about 13% for R236fa, 33% for R1234yf, and 107% for R115. Similarly, a switch from a subcritical ORC to a supercritical ORC configuration would increase net power for all the working fluids. Specifically, an increase in net power was estimated at 3.6% for R236fa, 46% for R1234yf, and 152% for R115 regarding a switch from the SBC to the SPC. Moreover, decreasing the condensation pressure of the ORC plants was observed to improve net power in all cases.

Selection of Organic Fluid Based on Exergetic Performance of Subcritical Organic Rankine Cycle (ORC) for Warm Regions

The organic Rankine cycle (ORC) exhibits considerable promise in efficiently utilizing low-to-medium-grade heat. Currently, there is a range of organic fluids available in the market, and selecting the appropriate one for a specific application involves considering factors such as the cycle’s thermodynamic performance, plant size, and compatibility with turbomachinery. The objective of our study is to examine the exergetic performance of the ORC with internal heat regeneration. We analyze 12 different organic fluids to evaluate their suitability based on parameters like exergy efficiency and heat exchange area requirements. Additionally, we investigate the need for internal heat regeneration by comparing the overall exergy performance with a simpler ORC configuration. To ensure broad applicability, we consider source temperatures ranging from 150 to 300 °C, which are relevant to industrial waste heat, geothermal sources, and solar energy. For each case, we calculate specific net power output and the UA value (heat exchanger conductance) to gain insights into selecting the appropriate organic fluid for specific source temperatures. Cyclohexane, benzene, isopropyl alcohol, and hexafluorobenzene show poor exergy efficiency due to their high boiling points. Pentane and cyclopentane provides the highest exergy efficiency of 62.2% at source temperature of 300 °C, whereas pentane is found to be the most suitable at source temperatures of 200 and 150 °C with exergy efficiency of 67.7% and 61.7%, respectively. At 200 °C source temperature, RE347mcc achieves 65.9% exergy efficiency. The choice of organic fluid for a given heat source is highly influenced by its critical properties. Moreover, the normal boiling temperature of the organic fluid significantly impacts exergy destruction during the condensation process within the cycle.

Effect of critical temperature and category of zeotropic mixture working fluids on the thermal performance in subcritical ORC

Several thermodynamic properties are crucial in selecting the optimal working fluids and significantly affect the performance of the Organic Rankine cycle (ORC); one of them is the liquid-vapor critical temperature. Four different thermodynamic models were used; they differed by the definition of maximal (TEva) and minimal (TCond) temperatures. Three different scenarios were investigated in all models by picking components from different working fluid classes to prepare zeotropic mixtures. The main focus of our study was on the investigation of the effect of the critical temperature (Tcr) of the mixture and the class of the components (wet/wet, wet/dry, and dry/dry) on thermal efficiency (ηth) and net-work output (Wnet). Results indicate that occasionally, the zeotropic mixtures with higher critical temperatures (but not always) outperform other mixtures under given conditions. Consequently, it cannot be asserted that the higher thermal efficiency and net- work output are monopolized by the highest critical temperature of all mixed working fluids. Selecting a certain mixing ratio and the appropriate model (evaporator and condenser temperatures set as on the bubble and dew points) that matches the ORC application can assist in obtaining the highest thermal efficiency and net-work output.

Continuous decentralized hydrogen production through alkaline water electrolysis powered by an oxygen-enriched air integrated biomass gasification combined cycle

This research work presents an innovative approach for continuous decentralized production of renewable hydrogen from woody biomass. Alkaline water electrolysis (AWE) is used to produce high-purity hydrogen, while the oxygen by-product is mixed with ambient air and used to fire a biomass-fueled downdraft gasifier in order to produce an upgraded producer gas with a lower heating value (LHV) between 7–8 MJ/Nm3. This fuel gas is then subjected to a conditioning stage and eventually fed to a combined cycle consisting of a recuperative gas turbine as topping unit and a regenerative subcritical organic Rankine cycle as bottoming unit, which together allow for a combined electric power generation efficiency close to 40%. Most of the net AC power from the integrated gasification combined cycle (IGCC) is rectified to DC power and ultimately used to power an alkaline electrolyzer, with a minor share allocated to all the required utilities and ancillary equipment, including hydrogen compression to 200 bar. The results from simulation of the hybrid IGCC-AWE plant under steady-state operating conditions in Aspen Plus V.11 indicate an optimal efficiency of 17.6% based on the LHV of hydrogen. Thus, if sized for a biomass consumption of 1 t/h, the proposed plant is capable of providing around 26 kg/h of compressed hydrogen at 200 bar.

A theoretical expression on performances of transcritical power cycle with the pseudocritical temperature prediction of working fluid

As a representative energy conversion technology of medium and low grade heat source, organic Rankine cycle (ORC) has attracted much attention. Compared with the cycle performance calculation from REFPROP, a theoretical expression can conveniently and quickly predict the cycle performance. Although there exist various theoretical equations to predict the performances of subcritical ORC, few expressions have been proposed for the transcritical cycle. Thus, in this study, based on the pseudocritical temperature of working fluid, a theoretical equation is proposed to predict the performance of the transcritical power cycle. For the required pseudocritical temperature, an artificial neural network (ANN) model is developed to obtain the reduced value (Tpr) from the reduced pressure (Pr), molar mass (M) and critical compressibility factor (Zc), based on the generated 30,670 data of 93 working fluids. From the obtained results, it can be concluded that the developed ANN has the absolute average relative deviation (AARD) 0.44% for the prediction of pseudocritical temperature. For the cycle performance prediction of the proposed expression, when the cycle condition is closer to the critical region of working fluids, the prediction accuracy is lower. Compared with the dry and wet fluids, the isentropic fluid generally has lower deviations of thermal efficiency and net work. Furthermore, for any working fluid, deviations of cycle performances are lower than 10% in the typical ranges of cycle conditions. Meanwhile, the average AARDs of cycle efficiency and net work are 4.93% and 6.36%, respectively. Based on the developed theoretical expression, cycle performance of transcritical cycle can be accurately and quickly predicted for any working fluid.

OPTIMAL DESIGN OF ORGANIC RANKINE CYCLE POWER PLANTS FOR EFFICIENT UTILIZATION of BIOMASS ENERGY IN NIGERIA

This study investigated the optimal design choice among four organic Rankine cycle (ORC) configurations for efficient utilization of solid biomass energy in Nigeria. Although vast opportunities exist for large-scale biomass power plants in the country, there has been little or no practical implementation yet, due to the limitation of technical know-how regarding thermodynamic conversion technologies. To bridge this gap, a thermodynamic optimization technique was applied in this study to the ORC. Specifically, the subcritical ORC (SUBORC), the regenerative subcritical ORC (SUBORC-REGEN), the supercritical ORC (SUPERORC), and the regenerative supercritical ORC (SUPERORC-REGEN) configurations were compared using established zero-dimensional optimization models implemented in MATLAB. Results showed that the SUPERORC-REGEN would be the most preferred choice amongst the options compared. Specifically, a palm kernel expeller (PKE) biomass fuel considered could yield about 1.98 MW of power at a thermal efficiency of about 28%. Additionally, it was obtained that the supercritical ORC would always outperform the subcritical types technically, with or without a regenerator. For the regenerative configurations, results showed that the supercritical ORC would generate 113 kW and 429 kW more net power than the subcritical ORC, respectively for n-pentane and n-butane working fluids. Similarly, the study reiterated that adopting a regenerative configuration would improve ORC performance. For instance, the SUPERORC-REGEN yielded 63% and 73% more power than the SUPERORC, respectively for n-pentane and n-butane working fluids. The practical economic implications of the different ORC configurations should be examined in future studies, alongside the investigation of exergy-based optimization potentials on component basis.

Organic rankine cycle systems with mixture of pure fluids: On infeasible fluid’s fractions due to the interaction between the mixture glide and the hexs pinchs

The Organic Rankine Cycle (ORC) is a promising technology for power generation from low-grade heat. The selection of working fluids is one of the important key points to improve the performance of an ORC system. Zeotropic mixtures show promising performances as working fluids. In fact, their temperature glide during phase change enables better match between the working fluid and the heat source/sink temperatures. In order to reveal the performance of mixture in ORC system, this paper deals with the thermodynamic model of the subcritical Organic Rankine Cycle (ORC) systems driven by low grade heat source while using zeotropic mixture working fluids with a special consideration to the interaction between phase change glides and the pinch value and their location in both the evaporator and the condenser (HEXs). Zeotropic mixtures of seven pure fluids are evaluated as working fluids for a subcritical ORC system. The mass fraction effects of mixtures on the thermal efficiency are analyzed. For given working conditions (working fluid mass flow, pressure and bubble temperature) the results show that for each considered zeotropic mixture there exist mass fraction ranges that are not consistent with the pinch values constraint in the HEXs and leads to so-called ‘infeasible zones’ with unreal HEXs dimensions. Results shows also that, out of these “infeasible fractions” zone, keeping unchanged the working conditions, the thermal performances of ORC system using zeotropic mixture are always better than the thermal performances of the same systems using the correspondent pure fluids. In addition, out of these highlighted “unfeasible zones” it was found that mixture with high temperature glide improve the thermal efficiency of ORC system.

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.

Investigation of thermal efficiency for subcritical ORC and TFC using super dry working fluids

AbstractEfficiency is a crucial factor for power cycles. Generally, in the same temperature range, the cycle efficiency for a basic organic Rankine cycle (ORC) is higher than a similar trilateral flash cycle (TFC) for almost all working fluids and heat source/heat sink temperature pairs. According to some recent results, by using super dry working fluids, the thermal efficiency of TFC can outperform its ORC counterpart at high temperatures. This study performed the thermodynamics analysis using three dry working fluids with subcritical basic ORC and TFC with zero to partial recuperated heat ratio. The results showed that using recuperative heat exchange effectively contributes to increasing the efficiency of ORC and TFC. However, the rate of efficiency increase in ORC is higher than the TFC, which can be clearly seen, especially at high maximal cycle temperatures. Using super dry working fluids, the recuperator‐free TFC has favorable cycle efficiency at high heat source temperatures. In contrast, by adding heat recovery for both systems, the ORC can outperform the TFC at 0.4, 0.2, and 0.1 recuperated heat effectiveness for Dodecane, Decane, and Nonane, respectively.

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.

Analysis of Organic Rankine Cycle efficiency and vapor generator heat transfer surface in function of the reduced pressure

In the paper presented is analysis of the influence of reduced pressure on efficiency and heat transfer area of vapor generator of Organic Rankine Cycle (ORC) in case of subcritical and supercritical parameters of operation. Compared are two cases of subcritical and supercritical ORC featuring a similar arrangement of heat source supply and heat removal, that is featuring the same temperatures of working fluid before the turbine, and the same condensation temperature in the respective cycles. The analysis is helpful in selection of the appropriate pressure in the vapor generator. In accomplished analyses a selection of wet ORC working fluids are scrutinized for a given range of heat source temperatures with respect to influence on efficiency of thermodynamic cycle and vapor generator area of heat transfer on installation and operation costs to illustrate the issue. Investment cost of a vapor generator in the ORC cycle accounts for a main share of expenditure alongside the cost of the expanding machine. Results of calculations show that from the point of view of cycle efficiency and size of vapor generator the pressures close to critical fluid pressure are usually optimal. Some working fluids feature even an optimal pressure. For the region close to critical point authors elaborated own method for heat transfer coefficients elaboration, which is useful in more exact estimations of heat transfer process in vapor generator. In case of a heat source with a relatively high temperature, it is better to consider a thermodynamic cycle with supercritical parameters even if as a result the vapor generator is slightly larger than for the case of a subcritical cycle. There will always be a more pronounced gain in efficiency compared to the expense induced by the heat transfer surface area of vapor generator.