Geothermal Organic Rankine Cycle Research Articles

Thermo-Economic Optimization of Organic Rankine Cycle (ORC) Systems for Geothermal Power Generation: A Comparative Study of System Configurations

The suitability of organic Rankine cycle (ORC) technology for the conversion of low- and medium-grade heat sources to useful power has established this as a promising option in geothermal power-generation applications. Despite extensive research in this field, most of which has focused on parametric analyses and thermodynamic performance evaluations, there is still a lack of understanding concerning the comparative performance of different plant configurations from both thermodynamic and economic perspectives. This study seeks to investigate the thermo-economic performance of subcritical and transcritical geothermal ORC power-plants, while considering a range of working fluids and the use of superheating and/or recuperation. A specific case study based on the exploitation of a medium-temperature geothermal heat source (180 °C, 40 kg/s) is conducted. Multi-objective optimization is performed to maximize the power/exergy efficiency (i.e., resource use) and minimize the payback period. The optimization results of different configurations are compared and the influence on system performance of superheating, recuperation, and subcritical vs. transcritical operation are evaluated. The results reveal that superheating is preferable for working fluids with low critical temperatures, but would hinder the performance of fluids whose critical temperature is higher. Recuperation is not attractive under most operating conditions, since the thermal performance improvement and cooling water saving cannot compensate the cost associated with the installation of the additional heat exchanger. Finally, transcritical ORC system are favoured thanks to the better thermal match between the heat source and the working fluid in these configurations. A more generalized geothermal heat source is then considered to explore the optimal configuration over a range of heat sources, which indicates that non-recuperated transcritical-cycle systems with working fluids whose critical temperature is close to the heat-source temperature are generally favourable.

Design and off-design optimization procedure for low-temperature geothermal organic Rankine cycles

In this paper, a two-step optimization methodology for the design and off-design optimization of low-temperature (110–150 °C) geothermal organic Rankine cycles (ORCs) is proposed. For the investigated conditions—which are based on the Belgian situation—we have found that the optimal ORC design is obtained for design parameter values for the environment temperature and for the electricity price which are both higher than the respective yearly-averaged values. However, the net present value is negative (−12.62 MEUR) which indicates that the low-temperature (130 °C) geothermal electric power plant is not economically attractive for the investigated case. Nevertheless, and demonstrated by the results of a detailed sensitivity analysis, a low-temperature geothermal power plant might be economically feasible for geological sites with a higher brine temperature or in a country with a more favorable economic situation; e.g., with higher electricity prices (∼70 EUR/MWh). The novelty of our paper is the development of a thermoeconomic design optimization strategy for low-temperature geothermal ORCs, accounting for the off-design behavior already in the design stage. The generic methodology is valid for low-temperature geothermal ORCs (with MW scale power output) and includes detailed thermodynamic and geometric component models, is based on hourly data rather than monthly-averaged data and accounts for economics.

Transient Simulation of Geothermal Combined Heat and Power Generation for a Resilient Energetic and Economic Evaluation

Geothermal power plants based on the organic Rankine cycle (ORC) are used to convert the thermal power of brine into electricity. The efficiency and profitability of these power plants can be increased by an additional heat supply. The purpose of this study is to evaluate different combined heat and power (CHP) concepts for geothermal applications by thermodynamic and economic considerations. Therefore, a dynamic simulation model of a double-stage ORC is developed to perform annual return simulations. The transient ORC model is validated in a wide range by operational data of an existing power plant in the German Molasse Basin. A district heating system is considered and the corresponding heat load profiles are derived from a real geothermal driven heating network. For CHP, parallel and combined configurations are considered. The validation of the transient model is satisfying with a correlation coefficient of 0.99 between the simulation and real power plant data. The results show that additional heat extraction leads to a higher exergetic efficiency and a higher profitability. The exergetic efficiency and the profitability are increased by up to 7.9% and 16.1%, respectively. The combined concept shows a slightly better performance than the parallel configuration. The efficiency can be increased by up to 1.3%. In economic terms, for CHP the annual return can be increased by at least 2,500,000 €. In principle, the dynamic model shows reliable results for high power gradients. This enables an investigation of geothermal ORC models for the reserve market in future works.

Effects of reinjection temperature on thermodynamic performance of dual-pressure and single-pressure geothermal ORCs

The geothermal water reinjection temperature should keep at a high level to avoid silica oversaturation but the permissible reinjection temperature varies with geothermal source characteristics. The reinjection temperature directly affects geothermal source and working fluid temperature profiles, and then affects the organic Rankine cycle (ORC) thermodynamic performance. This work optimized the evaporation temperatures for dual/single-pressure ORCs using isopentane as working fluids to maximize the net power outputs and analyzed the effects of the reinjection temperature on ORC thermodynamic performance. The net power output of a dual-pressure ORC decreases with increasing permissible reinjection temperature. The low-temperature geothermal heat can be more efficiently used by a low-pressure evaporation in a dual-pressure ORC than a single-pressure ORC, but the benefit in power output for a dual-pressure ORC over a single-pressure ORC is lower for higher reinjection temperatures. The evaporation exergy destruction rates for dual-pressure ORCs are considerably lower than single-pressure ORCs for higher reinjection temperatures and geothermal water inlet temperatures.

Off-design performance analysis of basic ORC, ORC using zeotropic mixtures and composition-adjustable ORC under optimal control strategy

The geothermal organic Rankine cycle (ORC) power system may frequently operate under off-design conditions due to the variations of geological condition, ambient temperature and actual operation. To comprehensively investigate the off-design performance of ORC using zeotropic mixtures and the effectiveness of composition regulation system on off-design operation. Off-design performances of basic ORC using pure working fluid (BORC), ORC using zeotropic mixtures (MORC) and composition-adjustable ORC (CAORC) are compared. The off-design performance prediction model for each ORC is built based on specifically designed critical components. An optimal control strategy is adopted to maximum the net power output with various geothermal water mass flow rates, geothermal water inlet temperatures and cooling water inlet temperatures. The results imply that variations of control variables and operating parameters are similar for the three ORCs during off-design operation. Compared with BORC, the net power output of MORC and CAORC can be increased by 2.4–5.3%, and the increment is larger when the system efficiency of BORC is lower. Simultaneously, except under extreme conditions where geothermal water inlet temperature is very high and cooling water inlet temperature is quite low, the net power output of CAORC is only a little larger than that of MORC.

Thermo-economic analysis of a novel power generation system integrating a natural gas expansion plant with a geothermal ORC in Tianjin, China

In this paper, a novel power generation system integrating a natural gas expansion plant with a geothermal organic Rankine cycle (ORC) was proposed. The paper begins with an introduction to the thermodynamic and economic models of the system. Secondly, contrastive analyses of three candidate working fluids were made, leading to the selection of R600 as the best fluid for the system, and the effects of gas pipeline parameters fluctuation on the system performance were originally investigated and analyzed based on the actual operating data of a natural gas PRS, located in Xiqing district, Tianjin, China. Finally, a multi-objective optimization for the system using R600 as working fluid was done to find the optimal evaporation temperature based on the TOPSIS decision-making method, and exergy loss analysis was accomplished at the optimal condition. The results indicated that the proposed system had a net profit of 3.97 M$ in the life time and a small payback period around 2.0 years at the optimal evaporation temperature of 45.5 °C where the system had a high thermal efficiency of 89.8% and exergy efficiency of 84.13%. Additionally, the condenser has the greatest potential to reduce the total exergy loss of the system.

Field performance evaluation of geothermal ORC power plants with a focus on radial outflow turbines

The calculation of the performance of an ORC (Organic Rankine Cycle) and its components (in particularly the turbine) is generally not a trivial task because of the lack of a proper instrumentation, the inaccuracy of the available measurements and the uncertainty on fluid thermodynamic properties calculation. These limitations can greatly affect the final computed results especially for complex fluids with a small temperature drop in expansion. A strategy to solve the inconsistency of the measured data set is to verify the energy and mass balance of each component and of the overall plant using also information related to the geometry of specific components, primarily the turbine as well as electrical measurements. After a brief description of the radial outflow turbine and of its main features, two operating geothermal ORC (Organic Rankine Cycle) plants installed by Exergy Spa in Turkey are described. For both plants a methodology for the consistency check of the experimental data set is presented with a description of the installed instrumentation, the test procedure and methods used to calculate the turbine efficiency as well as the overall power cycle performance.

Effects of evaporator pinch point temperature difference on thermo-economic performance of geothermal organic Rankine cycle systems

Organic Rankine cycle (ORC) systems are being used to convert medium-low temperature geothermal energy into electricity. However, the ORC system efficiencies need to be increased and the investment costs need to be reduced to further promote this technology. The evaporator pinch point temperature difference (PPTD) is a key parameter affecting the thermodynamic and economic performance. A lower evaporator PPTD leads to higher turbine power output; however, this also increases the heat transfer area and the investment cost. Therefore, this work optimizes the evaporation temperatures to maximize the net power outputs for evaporator PPTDs of 4–15 °C and brine inlet temperatures of 100–150 °C. The heat transfer area per unit power output, the levelized cost of electricity (LCOE) and the dynamic payback period (PBP) at the optimal conditions are also analyzed. ORCs produce 1.7–2.6% more net power with every 1 °C decrease of the evaporator PPTD for brine inlet temperatures higher than 130 °C. The total area per unit power output first decreases to a minimum at an evaporator PPTD of about 7 °C and then increases slightly with increasing evaporator PPTD. The LCOE and the dynamic PBP of a basic ORC reach a minimum at an evaporator PPTD of about 7 °C with the minimum at 5–6 °C for drilling costs higher than 500 $/m.

Construction and preliminary test of a geothermal ORC system using geothermal resource from abandoned oil wells in the Huabei oilfield of China

In this study, a low-temperature geothermal organic Rankine cycle (ORC) system was designed to generate electric power using geothermal resource from abandoned oil wells in the Huabei oilfield of China. Several main sections were constructed including the geothermal water cycle, intermediate water cycle, ORC, cooling water cycle, lubricant oil cycle, bypass and control systems. Using R245fa as the working fluid, a four-stage axial turbine was specially designed and manufactured, to drive a grid-connected alternator. An onsite preliminary test was conducted in the oilfield and part of the experimental data were presented and analyzed. The turbine isentropic efficiency and ORC efficiency were evaluated. Besides, an intermediate cycle efficiency was proposed to show the influence of the heat loss of the intermediate water cycle on the system performance. Experimental results showed that a turbine efficiency of 78.52% and an ORC efficiency of 5.33% could be obtained. The average of intermediate cycle efficiency was 77.98%, indicating that the heat loss of the intermediate water cycle was considerable and should not be ignored. The construction and preliminary test of this ORC system could provide some meaningful references for the researchers and engineers to establish desirable geothermal ORC power plants using abandoned oil wells.

Deterministic Global Optimization of the Design of a Geothermal Organic Rankine Cycle

Herein, a framework for deterministic global optimization of process flowsheets is adapted to the design of an organic Rankine cycle for geothermal power generation. A case study using isobutane as working fluid is considered for the optimal sizing of components and selection of operating conditions at different ambient temperatures. The framework can provide the global optimum in reasonable calculation times within tight tolerances. In contrast, most local solvers applied are found to be inadequate. The CPU times are substantially smaller compared to a state-of-the-art global solver. For the case considered, recuperation can increase net power output but not necessarily economics.

A Retrofit for Geothermal Organic Rankine Cycles based on Concentrated Solar Thermal Systems

In this study, a solar thermal retrofit unit for geothermal ORC power plants is analysed under thermodynamic and technical aspects. For the retrofit, the ORC module is extended by a solar superheater and the additional heat input is realized by parabolic trough collectors. The evaluation of the entire system is based on simulations over the period of one year. Therefore, a case study considering geothermal conditions of the Aydin-Salavatli field in the Büyük Menderes Graben in Turkey is conducted. Quasi-stationary simulations of the system are performed considering off-design behaviour of the power plant components. The calculations are based on hourly climate data. The obtained operational parameter of the ORC and solar-field emphasise the technical feasibility of the selected superheating concept in the context of thermal stability of the working fluid. For the considered solar-field sizes, the annually generated electricity is increased up to 4.5 % compared to the stand-alone geothermal power plant.

Field Performance Evaluation of ORC Geothermal Power Plants Using Radial Outflow Turbines

This paper, after a brief description of the radial outflow turbine and of its main features, discloses the field performances evaluation of two operating geothermal ORC (Organic Rankine Cycle) plants installed by Exergy Spa in Turkey. The work describes the test procedure, the measurements and calculation methods used to obtain the turbine efficiency as well as overall power cycle performance from the set of available experimental data.

Modeling and simulation of a shell-and-tube heat exchanger for Organic Rankine Cycle systems with double-segmental baffles by adapting the Bell-Delaware method

In respect of an efficient design of Organic Rankine Cycle (ORC) power plants, the heat exchange equipment plays an important role. Shell-and-tube heat exchangers are widely used in such energy conversion systems. The TEMA (Tubular Exchanger Manufacturers Association) E shell type with single-segmental baffles is a relatively simple and the most common design. For this configuration the shell-side heat transfer coefficient is in general determined by the Bell-Delaware method. However, in case of applications with low mean temperature differences like geothermal ORC systems, more advanced flow configurations are applied. In this work a TEMA NFN shell-and-tube heat exchanger with double-segmental baffles is analyzed for single-phase flow. For this design, the standard version of the Bell-Delaware method is not applicable. In this context, the Taborek version of the Bell-Delaware method is adapted to the specific heat exchanger design by adjusting the calculation methods for the main geometrical parameters like cross flow area, leakage areas and effective number of tubes in the overlapping region. The entire heat exchanger is simulated in DYMOLA 2015 FD01. Thereafter, the obtained temperature profiles are validated by real power plant data and deviate from operational data by up to −1.14%. Subsequently, a novel correlation for an enhancement factor Jx is developed. Due to the application to other shell-and-tube heat exchangers with double-segmental baffles, high accuracy is achieved. In addition, a heat exchanger baffle section is simulated in ANSYS FLUENT 15.0 in order to validate locally the novel adaption of the Taborek version of the Bell-Delaware method (main approach) by CFD simulations. The obtained heat transfer coefficients show a sufficient agreement.

Techno-economic analysis of a solar thermal retrofit for an air-cooled geothermal Organic Rankine Cycle power plant

High ambient temperatures are often coupled with high levels of solar irradiation and lead to a significant reduction of the power output of air-cooled geothermal Organic Rankine Cycle (ORC) power plants. Consequently, hybridisation based on solar thermal power is promising. In this work, a solar thermal retrofit based on superheating of the ORC working fluid is analysed under technical and economic criteria considering typical conditions in Turkey. The conducted off-design simulations prove that the isentropic efficiency of the turbine has a major impact on the performance of the entire system. The solar field size and the corresponding degree of superheating is limited in the respect to the built-in components. Therefore, the thermal stability of the examined ORC working fluid is sufficient and a low-temperature parabolic trough field with water as heat transfer fluid can be realized. For the retrofit, detailed simulations over the period of one year show up to 7.8% more electricity, a solar-to-electric efficiency of 10% and a significant power gain during summer. Besides, an increased reinjection temperature could lead to an improved management of the geothermal reservoir. Economically, the medium sized solar field is related to the lowest levelised costs of electricity (LCOE) with 145 $/MWh.

Dynamic Simulation of an Organic Rankine Cycle—Detailed Model of a Kettle Boiler

Organic Rankine Cycles (ORCs) are nowadays a valuable technology to produce electricity from low and medium temperature heat sources, e.g., in geothermal, biomass and waste heat recovery applications. Dynamic simulations can help improve the flexibility and operation of such plants, and guarantee a better economic performance. In this work, a dynamic model for a multi-pass kettle evaporator of a geothermal ORC power plant has been developed and its dynamics have been validated against measured data. The model combines the finite volume approach on the tube side and a two-volume cavity on the shell side. To validate the dynamic model, a positive and a negative step function in heat source flow rate is applied. The simulation model performed well in both cases. The liquid level appeared the most challenging quantity to simulate. A better agreement in temperature was achieved by increasing the volume flow rate of the geothermal brine by 2% over the entire simulation. Measurement errors, discrepancies in working fluid and thermal brine properties and uncertainties in heat transfer correlations can account for this. In the future, the entire geothermal power plant will be simulated, and suggestions to improve its dynamics and control by means of simulations will be provided.

Thermo-economic comparison of air-cooled and cooling tower based Organic Rankine Cycle (ORC) with R245fa and R1233zde as candidate working fluids for different geographical climate conditions

This article compares the part-load operation of air cooled and cooling tower based low-medium temperature geothermal Organic Rankine Cycle (ORC) systems installed at different geographical locations. Working fluid R245fa was compared with a newer competitor R1233zde for thermo-economic performance, environment-friendly and efficient system integration. Monthly averaged, weather data is used to simulate ambient conditions of Ulsan, London, Vegas and Kuala Lumpur. Mathematical models for condenser part load operation were formulated for both air cooled and mechanical draft wet cooling tower based systems. Numerical study and experimental validation was performed for the condenser when wet cooling tower based system was investigated. The ORC system design was optimized for maximum power output to grid and operational control optimization was performed on the heat sink to achieve maximum power output at different ambient or off-design conditions. Economic analysis was performed by comparing the capital investment/kW and levelized cost of electricity (LCOE) over the lifetime of the system. Based on the economic analysis, the results reveal that R1233zde has potential to replace R245fa working fluid when the source temperature is higher (around 145 °C). Cooling tower based system are preferable for hot dry regions while air-cooled systems can be implemented with R1233zde for Ulsan and London.

Dynamic modelling and validation of a commercial scale geothermal organic rankine cycle power plant

The organic rankine cycle (ORC) is a heat recovery technology used for renewable energy generation such as the large-scale production of electricity from geothermal resources. In this paper a dynamic model of a commercial-scale geothermal ORC is developed in process simulation software and validated against plant data. The difference in power output between the model and plant was found to be 0.24% with a standard deviation of 1.40% of the average power output. The results from model validation are used to suggest improvements to the model, which is intended to be used for further investigation of optimisation and process control.

Performance analysis of an organic Rankine cycle for a reverse osmosis desalination system using zeotropic mixtures

The use of low-grade thermal energy to power desalination processes by coupling an organic Rankine cycle (ORC) with seawater reverse osmosis (RO) is a promising technology to reduce the cost and environmental impact associated with the use of fossil fuel sources. A low-enthalpy geothermal ORC for a RO desalination system with zeotropic mixtures is proposed. Zeotropic mixtures can improve the thermodynamic performance of ORC systems owing to their excellent temperature glide characteristics during evaporation and condensing processes. A case study with butane/pentane (R600/R601) and butane/isopentane (R600/R601a) is investigated, aiming to analyze the effect of seawater temperature increase on the cycle performance. In the temperature range investigated, the power profit first increases rapidly then decreases. For the mixture R600/R601 at a mole fraction of 0.9/0.1, the maximum power profit value of 29.3kW occurred with the temperature rise of 26K, and for the mixture R600/R601a at a mole fraction of 0.9/0.1, the maximum power profit value of 30.9kW occurred with the temperature rise of 27K. The results show that, it is necessary to consider the effects of seawater temperature increase in both the ORC and RO simultaneously to design the ORC-RO system.

Performance study of a twin-screw expander used in a geothermal organic Rankine cycle power generator

The ORC (organic Rankine cycle) system is an effective technology to generate electricity from low temperature heat sources. The twin-screw expander is a key component that is commonly used in the small-to-medium capacity ORC system to convert thermal energy into work. In this paper, the performance of a twin-screw expander is theoretically and experimentally studied. A mathematical model is developed and subsequently validated using experimental data. The effect of several important factors including expander speed, suction pressure and inlet superheat on the expander performance is investigated. Results indicate that the expander speed and suction pressure have large influences on the expander performance, while the inlet superheat has relatively small effect. The isentropic efficiency of the expander decreases from 0.88 to 0.6 and the expander volumetric efficiency decreases from 0.88 to 0.7 as the expander rotational speed increases from 1250 to 6000 rpm. The results further show that the expander volumetric efficiency decreases from 0.91 to 0.85 as the expander suction pressure increases from 0.33 to 0.47 MPa. Furthermore, the energy conversion efficiency of the studied ORC system using the twin-screw expander is as high as 7.5% under the site conditions.

Thermo-economic analysis of a hybrid solar-binary geothermal power plant

An integrated model is developed for a hybrid solar-binary geothermal system. The system consists of an existing ORC (organic Rankine cycle) utilizing a low-temperature geothermal brine and a solar trough system. The hybrid system is based on the new parallel configuration developed by Ghasemi et al. (2014) which performs better than combined individual systems. The operation of the hybrid system is optimized maximizing the net power output and evaluated for representative days of year 2013. The design of the solar trough system is based on the objective of maximizing net power of the hybrid system at nominal conditions. This design results in a 7% solar share of net power output. The optimized constant-flow and variable-flow solar configurations lead to 5.5% and 6.3% boost in the net power output compared to the optimized geothermal ORC, respectively. The LEC (levelized electricity cost) is used as an economic performance metric to analyze the hybrid system. The analysis shows that LEC can be decreased by 2% for the hybrid compared to a stand-alone geothermal system. An optimization of the stand-alone geothermal ORC results in 8% reduction in LCE suggesting as a better option compared to the hybridization of the geothermal system at this time.