Low-temperature Organic Rankine Cycle Research Articles

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.

Optimum exergetic performance parameters and thermo-sustainability indicators of low-temperature modified organic Rankine cycles (ORCs)

The optimum exergetic performance parameters and thermo-sustainability indicators (TSIs) of low-temperature Organic Rankine cycles (ORCs) are presented. The study objectives are (i) to determine the exergetic performance, and TSIs of modified ORCs based on different thermodynamic inputs and (ii) compare the optimum exergetic performance parameters and the TSIs of the ORCs. The TSIs considered include, waste exergy ratio (WER), exergy efficiency (EE), exergetic sustainability index (ESI) and environmental effect factor (EEF). Additionally, a component by component exergy analysis was first performed, followed by the application of the TSI models established by modifying the existing TSI model for open cycle thermal plants. The results indicated that the increase in EE across the ORCs was not greater than 0.3%. The exergy destruction gap across the ORCs was between 1.2 and 23%. The ESI was low with the basic ORC while the modified systems had improved ESIs. However, best exergetic performance and TSIs were obtained at optimum conditions of 0.127≤p1≤2.144MPa, 0.17≤p2≤2.5MPa and 416≤t3≤426. The study inferred that system configuration and type of working fluids are paramount in determining the ORC performance. Moreover, the results obtained can further be used to evaluate the functional sustainability limits of working fluids.

Evaluation of Existing Heat Transfer Correlations in Designing Helical Coil Evaporators for Low-Temperature Organic Rankine Cycles via Inverse Design Approach

ABSTRACTFor evaluating the heat transfer correlations from literature in designing helical coil evaporators for low-temperature organic Rankine cycles (ORC), an inverse evaporator design methodology is used. This is done by taking four already performed measurements with different working fluid mass flow rates (0.21–0.23 kg/s) and saturation pressures (1.93–3.05 MPa, reduced pressures of 0.51–0.82) at the helical coil evaporator (66 m long) in an already operational experimental solar/thermal ORC system as real-case references for the evaporator inverse design's boundary conditions. R-404A is considered as the working fluid. The heating water inlet temperature is changed between 353 K and 373 K. The total length is inversely calculated via 15 helical coil two-phase heat transfer correlations for each case. Their end designs are compared with the actual length. Results show that the correlations can be used interchangeably for designing helical coil evaporators for low-temperature ORCs since the heat transfer resistance is dominant on the shell-side. For evaluating the sensitivity of these results, a secondary analysis was made by means of changing the shell-side correlation's accuracy from its initial value of 10% to more accurate 7%. Predictions with several correlations design shorter heat exchangers at reduced pressures less than 0.7.

Thermodynamic and economic analysis of zeotropic mixtures as working fluids in low temperature organic Rankine cycles

Organic Rankine cycle (ORC) is one of the most technically feasible methods to convert low-grade thermal energy into shaft power. An efficient approach to improve the thermodynamic performance is using zeotropic mixtures that enables better temperature match with the heat source and sink. In this article, a thorough assessment of thermodynamic and economic performance is conducted for the low grade ORCs using pure fluid and zeotropic mixtures. This work also examines the effects of heat source temperature and mixture mass fraction on the net power output, heat exchanger size, and cost-effective performance, and discusses the according basis for the selecting pure fluid or zeotropic mixtures. The results show that zeotropic ORC is capable of producing more net power than pure ORC, particularly for heat source with larger temperature difference. However, it needs much more heat exchanger area and therefore causes an unfavorable economic performance. Under the same heat exchanger area, pure cycle has lower pinch point temperature and higher net power output than zeotropic cycle. Therefore, the selection between pure fluid and zeotropic mixtures is also dependent on whether the power demand can be satisfied in pure ORC under the given pinch point condition.

Multi-parameter optimization and fluid selection guidance of a two-stage organic Rankine cycle utilizing LNG cold energy and low grade heat

Two-stage ORC (Organic Rankine Cycle) is a very competitive configuration. However, most of the works have narrowed the studies within a single or just several fluids in spite of the great effect of working fluids on system performance. In this paper, 400 combinations of working fluids are investigated. The evaporation and condensation pressures of both the high and low temperature ORCs are optimized by PSO (Particle Swarm Optimization) with the objective function of maximum exergy efficiency. The results show that the effect of fluid1 on the system exergy efficiency is much greater than that of fluid2. Critical temperature is a key indicator for fluid1 selection and triple point temperature is a crucial indicator for fluid2 selection. The largest exergy destruction occurs in the condenser of cycle II. The proper solution to reduce this exergy destruction is raising the inlet temperature of LNG, rather than lowering the condensation temperature of cycle II.

Performance analysis and parametric optimization of supercritical carbon dioxide (S-CO2) cycle with bottoming Organic Rankine Cycle (ORC)

Supercritical carbon dioxide (S-CO2) cycle is proven to be one promising alternative to provide high efficiency and has been developed for a wide range of energy conversion applications. Thermal efficiency of the S-CO2 cycle can be further improved by incorporating an appropriate bottoming cycle utilizing the residual heat. In this paper, an Organic Rankine Cycle (ORC) is added to the S-CO2 cycle for heat recovery. Different recuperative ratios of the topping S-CO2 cycle are considered and the influence of heat source initial temperature and total heat load on the bottoming ORC is evaluated. Two configurations of the S-CO2-ORC combined cycle system are presented, one without a pre-cooler and the other still with a pre-cooler, corresponding to total and partial residual heat recovery respectively. Though the entire residual heat recovery by the bottoming cycle could definitely increase the system thermal efficiency, the low ORC evaporation temperature and mediocre ORC performance leads to a limited improvement. While in the combined cycle system with a pre-cooler, higher ORC evaporation temperature could be attained and it has a remarkable effect on the ORC performance, even though part of the topping cycle residual heat is discharged to the ambient. The simulation results reveal that the S-CO2-ORC combined cycle system performance could be significantly improved through this parametric optimization. The recompression S-CO2 cycle with bottoming ORC is then analyzed and thermal performance is improved based on the previous optimization results. The bottoming ORC could effectively recover the residual heat of the topping S-CO2 cycle and increase the system thermal efficiency, thus it can be considered and applied in similar practical cases.

Thermo-Economic Multiobjective Optimization of a LOW Temperature Organic Rankine Cycle for Energy Recovery

In last decades the scarcity of combustible oil around the world has promoted the quest of new processes to produce power from alternative energy sources. The conventional Rankine cycle with water as working fluid, it is a good choice to convert heat into electricity but inefficient in temperatures below 350 C. Using an organic compound as working fluid the Rankine cycle (Called Organic Rankine Cycle) improves the cycle efficiency and can reduce the economy of the process for energy recovery from low temperature sources. In this work we proposed a set of organic compounds and a scenario of process to obtain simultaneously both optimum values of compositions of the mixture (used as working fluid) and operating conditions of the cycle for a given low temperature source. As design goals we set the minimization of total annual cost and the maximization of thermal efficiency. Because these two goals are in conflict the design problem was posed as a non-linear multi-objective optimization problem with thermodyn...

Potential of Low Temperature Organic Rankine Cycle with Zeotropic Mixtures as Working Fluid

Abstract Organic Rankine cycle (ORC) is one of the most technically feasible methods to convert low-grade thermal energy to electricity. Using zeotropic mixtures as working fluid can provide better temperature matches with the heat source and heat sink, so it is regarded as an efficient approach to improve ORC thermodynamic performance. This paper comparatively investigates the performance of low temperature ORC with pure fluids and zeotropic mixtures. We develop a numerical model considering the size of the heat exchangers to predict both the first law thermal efficiency and cost-effective performance. The results indicate that using mixtures does not gain beneficial effect when the prediction program is improved by taking heat transfer model into account.

An innovative small-scale two-stage axial turbine for low-temperature organic Rankine cycle

High expander efficiency is required to achieve best performance for small-scale organic Rankine cycle (ORC) systems driven by low-temperature (<100°C) heat sources. In this paper a small-scale two-stage axial turbine is modelled and compared with a single-stage axial turbine, with the aim of enhancing the ORC performance by increasing its pressure ratio. The preliminary mean-line design approach is coupled with three-dimensional CFD modelling and ORC cycle analysis was used to assess the impact of two-stage axial turbine on the ORC cycle performance. Three-dimensional CFD analysis of the single and two-stage axial turbines was performed using ANSYS®17-CFX software. The RANS equations with a k-ω SST turbulence model were solved for three-dimensional viscous steady state flow. The real gas thermodynamic properties of three organic working fluids (n-pentane, R141b, R245fa) are used in modelling the flow with both turbine configurations. Results revealed that the two-stage axial turbine configuration exhibited a substantially higher turbine performance, with overall isentropic efficiency of 83.94% and power output of 16.037kW, compared to 78.30% and 11.06kW from the single-stage configuration, with n-pentane as working fluid and mean diameter of 64mm for the two-stage configuration. Also, results showed that the maximum ORC thermal efficiency was 14.19% compared with 10.5% for single-stage configuration using n-pentane as the working fluid. These results highlight the potential of using two-stage axial turbine in a small-scale ORC’s system for the conversion of low-temperature heat sources into electricity.

Study on a high-performance solar thermoelectric system for combined heat and power

In this paper, a high-performance solar thermoelectric system for combined heat and power is developed and investigated. This new system has achieved a higher performance by considering some of the features, including a simplified structure, lower contact thermal resistance, lower convection heat loss, and better heat transfer performance. The mathematical model was built and analyzed. And the models are verified via the experimental prototype test. In addition, the performance of the system under different environmental conditions and operating parameters are evaluated and predicted. The results show that the proposed system can be used for cogeneration of heat water and electric power. And when the solar irradiation is larger than 700W/m2, and water temperature of 13°C, the maximum instantaneous electrical power output was found to be 0.659W at the load resistance 4.2Ω, and the power generation efficiency of the thermoelectric generator (TEG) is 1.956%, which is close to that of a low temperature organic Rankine cycle system. These results indicate the feasibility of the proposed high-performance solar thermoelectric system and give hints for future improvement of the solar thermoelectric system for combined heat and power.

3D Numerical Modeling of Zeotropic Mixtures and Pure Working Fluids in an ORC Turbo-Expander

The present paper provides a numerical study that leads to the proper selection of a working fluid for use in low-temperature organic Rankine cycle (ORC) applications. This selection is not only based on the provision of best efficiency but also to comply with global warming potential (GWP) regulations. For that purpose, different pure organic working fluids, including R245fa, R236fa, R123, R600a, R134a, and R1234yf as well as zeotropic mixture R245fa/R600a, are selected. The investigation is conducted on a single stage radial inflow turbo-expander, which was originally used in the Sundstrand Power Systems T-100 Multipurpose Small Power Unit. The commercial package ANSYS-CFX (version 16.0) was used to perform the numerical study using 3D Reynolds-Averaged Navier–Stokes (RANS) simulations. Peng–Robinson equation of state is adopted in the finite-volume solver ANSYS-CFX to determine the real-gas properties. The obtained results show that, while the use of R134a and R1234yf provides the best efficiency of all the working fluids under investigation, the latter is best selected for its comparatively low global warming effects.

Exergy and energy analysis of a regenerative organic Rankine cycle based on flat plate solar collectors

In current study, a low temperature organic Rankine cycle (ORC) based on flat plate solar collectors with storage tank is considered. Due to low cost applications, water at ambient pressure is used in both the tank and collectors. Also, the cooling is done by water in ambient temperature. Energy and exergy analysis is used to evaluate the performance of the system under various conditions to find the main sources of exergy destruction and the potential to improve them. Some parameters including exergetic efficiency, thermal efficiency, exergy destruction rate, fuel depletion ratio and irreversibility ratio are investigated. Exergy efficiency and exergy destruction ratio are calculated for the overall system according to the second law of thermodynamics based on daily efficiency. Exergy analysis of each sub-system leads to the choice of the optimum physical parameters for minimum local exergy destruction ratios. Four different working fluids are considered including R245fa, R134a, pentane and toluene to evaluate the system. Results show that the solar collector, thermal storage tank and the vapor generator are the main sources of exergy destruction respectively. Also a parametric study shows that there is an optimum daily exergy efficiency based on turbine inlet temperature. Under the same load, pentane has the best performance followed by R245fa, toluene and R134a. The corresponding daily exergy efficiencies are 24.08%, 22.53%, 22.09% and 21.76%.

Transient heat extraction modeling method for a rectangular type salt gradient solar pond

In this article, a transient modeling method for the heat extraction process in the rectangular solar ponds is presented by using a finite difference technique. Firstly, the thermal behavior of the pond, during the heat extraction operation is modeled. Then the model is verified using experimental data, and the maximum observed percentage error of this method is 2.03%. This method is used to model the heat extraction from a hypothetical pond in Tehran for three months of August, September, and November. The results present the temperature profiles, the extracted heat and the rate of heat extraction. All of these parameters have the highest value in August, and the lowest values are allocated to November. Moreover, the percentages of the extracted heat during the operation are studied. For all the three months, this parameter has a high value in the first 40% of the time needed to full heat extraction. Between the 40% and 70% of the total time, the heat extraction rate reduces drastically, and after the 70% of the total time, it is almost constant at a low value. Also, the modeling results are used in a low-temperature organic Rankine cycle, and it is found that for the designed pond, the pond can be used as the heat source of the cycle only in August and for 7.31h of operation.

Thermodynamic analysis of a low-temperature organic Rankine cycle power plant operating at off-design conditions

This paper deals with an experimental study on a 50-kW Organic Rankine cycle (ORC) power generation plant driven by low-grade heat source. Hot water boiler and solar-thermal system were used as the low-grade heat source providing hot water at temperature ranging from 65 to 95°C. A twin screw compressor has been modified as the expansion machine in the ORC module and its expansion efficiency under variable operating conditions was tested in the experiments. This work was purposed to assess the ORC system and get the performance map at off-design operating conditions in a typical year from the view of the first and the second law of thermodynamics. The maximum electricity production and thermal efficiency were 46.5kW and 6.52% respectively at the optimal operating condition. The highest exergetic efficiency reached 36.3% and the exergy analysis showed that evaporation pressure and condensation pressure were the key parameters to influence the exergy flow and exergetic efficiency. Furthermore, by fitting the actual plant data obtained in different months, an empirical model has been developed to predict the net power output and thermal efficiency with acceptable accuracy. Lastly, as an illustration, the empirical model is used to analyze the performance of the solar-driven ORC system.

Experimental testing of a low-temperature organic Rankine cycle (ORC) engine coupled with concentrating PV/thermal collectors: Laboratory and field tests

A detailed experimental investigation of a small-scale low-temperature organic Rankine cycle (ORC) with R-404A is presented. The tests are first conducted at laboratory conditions for detailed evaluation of the main components at both design and off-design conditions, for variable heat input up to 48 kWth and hot water temperature in the range of 65–100 °C. A scroll compressor in reverse operation is used as expansion machine and a dedicated helical coil heat exchanger is installed, suitable for high-pressure and temperature operation. The ORC pump is a diaphragm pump coupled with an induction motor. The rotational speeds of both the expander and pump are regulated with frequency inverters, in order to have the full control of the engine operation. The ORC has been then connected with concentrating PV/thermal collectors, which produce electricity and heat and provide it to the ORC. These field tests are also presented with the overall focus on the performance of the whole ORC unit and its power contribution to the solar field. The tests have revealed that such low-temperature ORC unit can have adequate efficiency and that its coupling with a solar field is feasible, increasing the power production of the whole system.

Design Sensitivity Analysis of a Plate-Finned Air-Cooled Condenser for Low-Temperature Organic Rankine Cycles

ABSTRACTDue to increasing world energy demand and environmental concerns, sustainable energy production has become crucial. Among sustainable energy sources such as solar, wind, and geothermal, industrial waste heat (heat normally released to the environment) has a big potential. Organic Rankine cycles (ORCs) are promising systems for utilizing low-temperature (100–250°C) waste heat. For an ORC system, the condenser is a key component. An accurate condenser design is important for cycle efficiency and system cost. In the literature, there are in-tube condensation correlations that are used to design condensers. However, they are not necessarily valid for low-temperature ORC conditions and working fluids, and that might lead to inaccurate end designs. This study comprises a summarized literature survey about in-tube condensation correlations. Then an iterative heat exchanger design methodology is proposed that allows performing a design sensitivity analysis on a V-shaped condenser within an input range of geometric parameters and boundary conditions. Nineteen correlations are implemented to calculate rating parameters like pressure drops, total transferred heat, overall heat transfer coefficient, size, cost and degree of subcooling. The deviations at these parameters are represented as the coefficient of variation that indicates the design condition regions where the prediction methods differ or not.

Thermo-Economic and Heat Transfer Optimization of Working-Fluid Mixtures in a Low-Temperature Organic Rankine Cycle System

In the present paper, we consider the employment of working-fluid mixtures in organic Rankine cycle (ORC) systems with respect to thermodynamic and heat-transfer performance, component sizing and capital costs. The selected working-fluid mixtures promise reduced exergy losses due to their non-isothermal phase-change behaviour, and thus improved cycle efficiencies and power outputs over their respective pure-fluid components. A multi-objective cost-power optimization of a specific low-temperature ORC system (operating with geothermal water at 98 °C) reveals that the use of working-fluid-mixtures does indeed show a thermodynamic improvement over the pure-fluids. At the same time, heat transfer and cost analyses, however, suggest that it also requires larger evaporators, condensers and expanders; thus, the resulting ORC systems are also associated with higher costs. In particular, 50% n-pentane + 50% n-hexane and 60% R-245fa + 40% R-227ea mixtures lead to the thermodynamically optimal cycles, whereas pure n-pentane and pure R-245fa have lower plant costs, both estimated as having ∼14% lower costs per unit power output compared to the thermodynamically optimal mixtures. These conclusions highlight the importance of using system cost minimization as a design objective for ORC plants.

Effects of critical and boiling temperatures on system performance and fluid selection indicator for low temperature organic Rankine cycles

The critical temperature (Tc) and boiling temperature (Tb) of working fluids are important selection criteria for the organic Rankine cycle (ORC) system. In this study, the ratio of Tb and Tc (Tbr) and the vapor expansion ratio (VER) model based on Claussius–Claperyron equation are introduced to compare and explain their effects on the maximum net output power (Wnet,max) and VER of ORC. The investigation of 267 working fluids is done at four heat source temperatures (T5). Maximum vapor enthalpy method is proposed to determine the upper limit of the evaporation temperature, which is the optimization parameter for maximizing the net power output. At low T5 (423.15 and 473.15 K), the obvious relationships between Wnet,max and Tc are independent of Tbr. Therefore, Tc enables to select working fluids with high Wnet,max. However, at high T5 (523.15 and 573.15 K), Tbr is essential to exclude working fluids with optimum Tc (0.89 – 0.90T5) but low Wnet,max. Moreover, at a given Tc, high Tb or Tbr indicates high VER. Consequently, Tb or Tbr is suitable to be used as the second indicator. This paper proposes the optimal combinations of Tc and Tb and the developed composite indicator for selection of working fluids.

Experimental Study of the Influence of Light Intensity on Solar Organic Rankine Cycle Power Generation System

A low-temperature (&lt;120 °C) solar organic Rankine cycle (ORC) power generation experimental facility is designed and built. The influence of light intensity on the system performance is investigated using the experimental facility. The results indicate that the system efficiency can reach 2.2%. The temperature of heat transfer fluid (HTF) decreases linearly with light intensity (I). However, both system efficiency and thermoelectric efficiency first decrease linearly and then drop sharply as I decreases at working fluid flow rates (Vwf) of 200 and 160 L/hr, while they only decrease slightly with I at Vwf of 120 L/hr. The light intensity of the turning point is 824 W/m2 at Vwf of 200 L/hr, which corresponds to an HTF temperature of 75 °C. In addition, it is found that the influence of light intensity on the performance of ORC becomes stronger for higher working fluid flow rate. Moreover, the light intensity and HTF temperature at the turning point increase with working fluid flow rate. The experimental results are of great significance for the design and operation of low-temperature solar ORC power generation system.

Experimental comparison of organic fluids for low temperature ORC (organic Rankine cycle) systems for waste heat recovery applications

This contribution experimentally evaluates and compares the performance of an ORC (organic Rankine cycle) system for stationary bottoming WHR (waste heat recovery) application operating with two different working fluids, SES36 and R245fa. The test rig is a regenerative cycle equipped with a single screw expander modified from a standard compressor characterized by a nominal shaft power of 11 kW. A total of 36 and 43 steady-state points are collected for SES36 and R245fa respectively, over a wide range of operating conditions by changing the expander rotational speed, the pump frequency and the cooling condenser flow rate. The performances of the ORC components are individually evaluated. A maximum expander isentropic efficiency of 60% is reached using SES36 at 3000 rpm, and a value of 52% is reached with R245fa at 3000 rpm. However, for a given pressure ratio the expander output power is higher with R245fa than with SES36. The overall performance of the ORC unit are investigated in terms of first and second law efficiencies and net output power for the two fluids. The results experimentally demonstrate the correlation between the working fluid critical temperature and the ORC unit working characteristics for low temperature waste heat recovery applications. Open experimental data are provided for both fluids.