Exergy Analysis of Organic Rankine Cycles with Zeotropic Working Fluids

Waste heat recovery is one of the possible solutions to improve the efficiency of internal combustion engines.Instead of wasting the exhaust stream of an energy conversion system into the environment,its residual energy content can be usefully recovered,for example in Organic Rankine Cycles(ORC).This technology has been largely consolidated in stationary power plants but not yet for mobile applications,such as road transport,due to the limitations in the layout and to the constraints on the size and weight of the ORC system.An ORC system installed on the exhaust line of a bus powered by a natural gas spark ignition engine has been investigated.The thermal power available at engine exhaust has been evaluated by measuring gas temperature and mass flow rate during real driving operation.The waste thermal power has been considered as heat input for the ORC plant simulation.A detailed heat exchanger model has been developed because it is a crucial component for the ORC performance.The exergy analysis of the ORC was performed comparing different working fluids:R601,R1233zd(E)and two zeotropic blends of the two organic pure fluids.The model allowed the evaluation of the ORC produced energy over the driving cycle and the potential benefit on the engine efficiency.

Evaluation of working fluids for organic Rankine cycles using group-contribution methods and second-law-based models

The group-contribution (GC) methods suffer from a limitation concerning to the prediction of process-related indexes, e.g., thermal efficiency. Recently developed analytical models for thermal efficiency of organic Rankine cycles (ORCs) provide a possibility of overcoming the limitation of the GC methods because these models formulate thermal efficiency as functions of key thermal properties. Using these analytical relations together with GC methods, more than 60 organic fluids are screened for medium-low temperature ORCs. The results indicate that the GC methods can estimate thermal properties with acceptable accuracy (mean relative errors are 4.45%-11.50%);the precision, however, is low because the relative errors can vary from less than 0.1% to 45.0%. By contrast, the GC-based estimation of thermal efficiency has better accuracy and precision. The relative errors in thermal efficiency have an arithmetic mean of about 2.9% and fall within the range of 0-24.0%. These findings suggest that the analytical equations provide not only a direct way of estimating thermal efficiency but an accurate and precise approach to evaluating working fluids and guiding computer-aided molecular design of new fluids for ORCs using GC methods.

Thermodynamic Performance Analysis of Geothermal Power Plant Based on Organic Rankine Cycle (ORC) Using Mixture of Pure Working Fluids

The selection of working fluid significantly impacts the geothermal ORC’s Efficiency.Using a mixture as a working fluid is a strategy to improve the output of geothermal ORC.In the current study,modelling and thermodynamic analysis of ORC,using geothermal as a heat source,is carried out at fixed operating conditions.The model is simulated in the Engineering Equation Solver(EES).An environment-friendly mixture of fluids,i.e.,R245fa/R600a,with a suitable mole fraction,is used as the operating fluid.The mixture provided the most convenient results compared to the pure working fluid under fixed operating conditions.The impact of varying the evaporator pressure on the performance parameters,including energy efficiency,exergy efficiency and net power output is investigated.The system provided the optimal performance once the evaporator pressure reached the maximum value.The efficiencies:Energy and Exergy,and Net Power output of the system are 16.62%,64.08%and 2199 kW for the basic cycle and 20.72%,67.76%and 2326 kW respectively for the regenerative cycle.

Life Cycle Assessment Introduced by Using Nanorefrigerant of Organic Rankine Cycle System for Waste Heat Recovery

The use of nanorefrigerants in Organic Rankine Cycle(ORC)units is believed to affect the cycle environment performance,but backed with very few relevant studies.For this purpose,a life cycle assessment(LCA)has been performed for the ORC system using nanorefrigerant,the material and energy input,characteristic indicators and comprehensive index of environmental impact,total energy consumption and energy payback time(BPBT)of the whole life cycle of ORC system using Al_(2)O_(3)/R141b nanorefrigerant were calculated.Total environmental comprehensive indexes reveal that ECER-135 index decrease by 1.5%after adding 0.2%Al_(2)O_(3)nanoparticles to R141b.Based on the contribution analysis and sensitivity analysis,it can be found out ORC system manufacturing is of the most critical stage,where,the ECER-135 index of ORC component production is the greatest,followed by the preparation process of R141b,transportation phase,and that of Al_(2)O_(3)nanoparticles preparation is small.The retirement phase which has good environmental benefits affects the result significantly by recycling important materials.Meanwhile,the main cause and relevant suggestion for improvement were traced respectively.Finally,the environmental impacts of various power generations were compared,and results show that the power route is of obvious advantage.Among the renewable energy,ORC system using Al_(2)O_(3)/R141b nanorefrigerant with minimal environmental impact is only 0.67%of coal-fired power generation.The environmental impact of current work is about 14.34%of other nations’PV results.

A comparative thermodynamic analysis of Kalina and organic Rankine cycles for hot dry rock:a prospect study in the Gonghe Basin

Hot dry rock is a new type of geothermal resource which has a promising application prospect in China.This paper conducted a comparative research on performance evaluation of two eligible bottoming cycles for a hot dry rock power plant in the Gonghe Basin.Based on the given heat production conditions,a Kalina cycle and three organic Rankine cycles were tested respectively with different ammonia-water mixtures of seven ammonia mass fractions and nine ecofriendly working fluids.The results show that the optimal ammonia mass fraction is 82%for the proposed bottoming Kalina cycle in view of maximum net power output.Thermodynamic analysis suggests that wet fluids should be supercritical while dry fluids should be saturated at the inlet of turbine,respectively.The maximum net power output of the organic Rankine cycle with dry fluids expanding from saturated state is higher than that of the other organic Rankine cycle combinations,and is far higher than the maximum net power output in all tested Kalina cycle cases.Under the given heat production conditions of hot dry rock resource in the Gonghe Basin,the saturated organic Rankine cycle with the dry fluid butane as working fluid generates the largest amount of net power.

Energy and exergy recovery from exhaust hot water using organic Rankine cycle and a retrofitted configuration

Exhaust hot water （EHW） is widely used for various industrial processes. However, the excess heat carried by EHW is typically ignored and discharged into the environment, resulting in heat loss and heat pollution. An organic Rankine cycle （ORC） is an attractive technology to recycle heat from low-temperature energy carriers. Herein, ORC was used to recycle the heat carried by EHW. To investigate the energy and exergy recovery effects of EHW, a mathematical model was developed and a parametric study was conducted. The energy efficiency and exergy efficiency of the EHW-driven ORC system were modeled with R245fa, Rl13 and R123 as the working fluids. The results demonstrate that the EHW and evaporation temperatures have significant effects on the energy and exergy efficiencies of the EHW-driven ORC system. Under given EHW conditions, an optimum evaporation temperature exists corresponding to the highest exergy efficiency. To further use the low-temperature EHW, a configuration retrofitted to the ORC by combining with flash evaporation （FE） was conducted. For an EHW at 120 ~C and 0.2 MPa, the maximum exergy efficiency of the FE-ORC system is 45.91% at a flash pressure of 0.088 MPa. The FE-ORC performs better in exergy efficiency than the basic FE and basic EHW-driven ORC.

Simulation and performance analysis of organic Rankine cycle combined heat and power system

To improve the overall thermal efficiency of the organic Rankine cycle（ ORC）, a simulation study was carried out for a combined heat and power（ CHP） system, using the Redlich-Kuang-Soave（ RKS） equation of state. In the system,R245 fa was selected as the working fluid. A scroll expander was modeled with empirical isentropic expansion efficiency.Plate heat exchangers were selected as the evaporator and the condenser, and detailed heat transfer models were programmed for both one-phase and two-phase regions. Simulations were carried out at seven different heat source temperatures（ 80,90, 100, 110, 120, 130, 140 ℃） in combination with eight different heat sink temperatures（ 20, 25, 30, 35, 40, 45, 50,55 ℃）. Results showthat in the ORC without an internal heat exchanger（ IHE）, the optimum cycle efficiencies are in the range of 7. 0% to 7. 3% when the temperature differences between the heat source and heat sink are in the range of 70 to90 ℃. Simulations on CHP reveal that domestic hot water can be produced when the heat sink inlet temperature is higher than40 ℃, and the corresponding exergy efficiency and overall thermal efficiency are 29% to 56% and 87% to 90% higher than those in the non-CHP ORC, respectively. It is found that the IHE has little effect on the improvement of work output and efficiencies for the CHP ORC.

Coupling effect of evaporation and condensation processes of organic Rankine cycle for geothermal power generation improvement

Organic Rankine cycle(ORC)is widely used for the low grade geothermal power generation.However,a large amount of irreversible loss results in poor technical and economic performance due to its poor matching between the heat source/sink and the working medium in the condenser and the evaporator.The condensing temperature,cooling water temperature difference and pinch point temperature difference are often fixed according to engineering experience.In order to optimize the ORC system comprehensively,the coupling effect of evaporation and condensation process was proposed in this paper.Based on the laws of thermodynamics,the energy analysis,exergy analysis and entropy analysis were adopted to investigate the ORC performance including net output power,thermal efficiency,exergy efficiency,thermal conductivity,irreversible loss,etc.,using geothermal water at a temperature of 120℃as the heat source and isobutane as the working fluid.The results show that there exists a pair of optimal evaporating temperature and condensing temperatures to maximize the system performance.The net power output and the system comprehensive performance achieve their highest values at the same evaporating temperature,but the system comprehensive performance corresponds to a lower condensing temperature than the net power output.

Parametric optimization and performance comparison of organic Rankine cycle with simulated annealing algorithm

Taking the ratio of heat transfer area to net power and heat recovery efficiency into account, a multi-objective mathematical model was developed for organic Rankine cycle （ORC）. Working fluids considered were R123, R134a, R141b, R227ea and R245fa. Under the given conditions, the parameters including evaporating and condensing pressures, working fluid and cooling water velocities were optimized by simulated annealing algorithm. The results show that the optimal evaporating pressure increases with the heat source temperature increasing. Compared with other working fluids, R123 is the best choice for the temperature range of 100--180℃ and R141 b shows better performance when the temperature is higher than 180 ℃. Economic characteristic of system decreases rapidly with the decrease of heat source temperature. ORC system is uneconomical for the heat source temperature lower than 100℃.

Slag-washing water of blast furnace power station with supercritical organic Rankine cycle

Organic Rankine cycle(ORC) power plant operating with supercritical parameters supplied by low temperature slag-washing water(SWW) of blast furnace was investigated.A schematic of such installation was presented with a description of its operation and the algorithm of calculations of a supercritical power plant.Two typical organic fluids with sufficiently low critical parameters were selected as candidate working fluids in the plant to study the efficiency of the system with different organic fluids.An analysis of the influence on the effectiveness of operation of a plant was carried out.With the same temperature of slag-washing water,the specific work in turbine of fluid R143a is 45% higher than that obtained for the fluid R125,however,the specific work in pump of fluid R143a is approximate equal into that one of the fluid R125.

Performance comparison and analysis of a combined power and cooling system based on organic Rankine cycle

A novel power and cooling system combined system which coupled organic Rankine cycle(ORC) with vapor compression refrigeration cycle(VCRC) was proposed. R245 fa and butane were selected as the working fluid for the power and refrigeration cycle, respectively. A performance comparison and analysis for the combined system was presented. The results show that dual-pressure ORC-VCRC system can achieve an increase of 7.1% in thermal efficiency and 6.7% in exergy efficiency than that of basic ORC-VCRC. Intermediate pressure is a key parameter to both net power and exergy efficiency of dual-pressure ORC-VCRC system. Combined system can produce maximum net power and exergy efficiency at 0.85 MPa for intermediate pressure and 2.4 MPa for high pressure, respectively. However, superheated temperature at expander inlet has little impact on the two indicators. It can achieve higher overall COP, net power and exergy efficiency at smaller difference between condensation temperature and evaporation temperature of VCRC.

An approach for IC engine coolant energy recovery based on low-temperature organic Rankine cycle

To promote the fuel utilization efficiency of IC engine, an approach was proposed for IC engine coolant energy recovery based on low-temperature organic Rankine cycle(ORC). The ORC system uses IC engine coolant as heat source, and it is coupled to the IC engine cooling system. After various kinds of organic working media were compared, R124 was selected as the ORC working medium. According to IC engine operating conditions and coolant energy characteristics, the major parameters of ORC system were preliminary designed. Then, the effects of various parameters on cycle performance and recovery potential of coolant energy were analyzed via cycle process calculation. The results indicate that cycle efficiency is mainly influenced by the working pressure of ORC, while the maximum working pressure is limited by IC engine coolant temperature. At the same working pressure, cycle efficiency is hardly affected by both the mass flow rate and temperature of working medium. When the bottom cycle working pressure arrives at the maximum allowable value of 1.6 MPa, the fuel utilization efficiency of IC engine could be improved by 12.1%.All these demonstrate that this low-temperature ORC is a useful energy-saving technology for IC engine.

Parametric Optimization of Organic Rankine Cycle with R245fa/R601a as Working Fluid

In order to select the appropriate working fluids and optimize parameters for medium-temperature geothermally-powered organic Rankine cycle(ORC), R245 fa is mixed with R601 a at geothermal water temperature of 110 ℃. Based on thermodynamics, the characteristics of mixture and its influence on the performance of ORC under different evaporating temperatures and composition proportions are analyzed. Results show that the zeotropic mixture R245fa/R601a(0.4/0.6) has the highest performance. When the evaporating temperature reaches 67 ℃, the outlet temperature of geothermal water is 61 ℃, the net power output is the highest and the thermal efficiency is about 9%.

Dynamic test on waste heat recovery system with organic Rankine cycle

Dynamic performance is important to the controlling and monitoring of the organic Rankine cycle(ORC) system so to avoid the occurrence of unwanted conditions. A small scale waste heat recovery system with organic Rankine cycle was constructed and the dynamic behavior was presented. In the dynamic test, the pump was stopped and then started. In addition, there was a step change of the flue gas volume flow rate and the converter frequency of multistage pump, respectively. The results indicate that the working fluid flow rate has the shortest response time, followed by the expander inlet pressure and the expander inlet temperature.The operation frequency of pump is a key parameter for the ORC system. Due to a step change of pump frequency(39.49-35.24 Hz),the expander efficiency and thermal efficiency drop by 16% and 21% within 2 min, respectively. Besides, the saturated mixture can lead to an increase of the expander rotation speed.

Selection of organic Rankine cycle working fluid based on unit-heat-exchange-area net power

To improve energy conversion efficiency, optimization of the working fluids in organic Rankine cycles(ORCs) was explored in the range of low-temperature heat sources. The concept of unit-heat-exchange-area(UHEA) net power, embodying the cost/performance ratio of an ORC system, was proposed as a new indicator to judge the suitability of ORC working fluids on a given condition. The heat exchange area was computed by an improved evaporator model without fixing the minimum temperature difference between working fluid and hot fluid, and the flow pattern transition during heat exchange was also taken into account. The maximum UHEA net powers obtained show that dry organic fluids are more suitable for ORCs than wet organic fluids to recover low-temperature heat. The organic fluid 1-butene is recommended if the inlet temperature of hot fluid is 353.15-363.15 K or443.15-453.15 K, heptane is more suitable at 373.15-423.15 K, and R245 ca is a good option at 483.15-503.15 K.

BP-PID Control Applied in Evaporator of Organic Rankine Cycle System

According to the problem that the selection of traditional PID control parameters is too complicated in evaporator of Organic Rankine Cycle system(ORC),an evaporator PID controller based on BP neural netw ork optimization is designed. Based on the control theory,the model of ORC evaporator is set up. The BP algorithm is used to control the Kp,Kiand Kdparameters of the evaporator PID controller,so that the evaporator temperature can reach the optimal state quickly and steadily. The M ATLAB softw are is used to simulate the traditional PID controller and the BP neural netw ork PID controller. The experimental results show that the Kp,Kiand Kdparameters of the BP neural netw ork PID controller are 0. 5677,0. 2970,and 0. 1353,respectively.Therefore,the evaporator PID controller based on BP neural netw ork optimization not only satisfies the requirements of the system performance,but also has better control parameters than the traditional PID controller.

Thermodynamic and Techno-economic Analysis of a Triple-pressure Organic Rankine Cycle: Comparison with Dual-pressure and Single-pressure ORCs

Investigation of a triple-pressure organic Rankine cycle(TPORC) using geothermal energy for power generation with the net power output of the TPORC analyzed by varying the evaporation pressures, pinch temperature differences(tpp) and degrees of superheat(tsup) aimed to find the optimum operation conditions of the system. The thermodynamic performance of the TPORC was compared with a dual-pressure organic Rankine cycle(DPORC) and a single-pressure ORC(SPORC) for geofluid temperatures ranging from 100°C to 200°C, with particular reference to the utilization of a hot dry rock(HDR) geothermal resource. Thermodynamic performances of the TPORC system using eight different organic working fluids have also been investigated in terms of the net power outputs. Results show that a higher geofluid mass flow rate can make a considerable contribution to shortening the payback period(PBP) as well as to decreasing the levelized electricity cost(LEC), especially when the geofluid temperature is low. For the temperature range investigated, the order from high to low based on thermodynamic and techno-economic performances is found to be TPORC > DPORC > SPORC. In terms of using geothermal resources within the given temperatures range(100°C–200°C), the TPORC system can be a better choice for geothermal power generation so long as the wellhead geofluid temperature is between 140°C and 180°C.

Thermo-economic Investigation of an Enhanced Geothermal System Organic Rankine Cycle and Combined Heating and Power System

As a potentially viable renewable energy, Enhanced Geothermal Systems(EGSs) extract heat from hot dry rock(HDR) reservoirs to produce electricity and heat, which promotes the progress towards carbon peaking and carbon neutralization. The main challenge for EGSs is to reduce the investment cost. In the present study, thermo-economic investigations of EGS projects are conducted. The effects of geofluid mass flow rate, wellhead temperature and loss rate on the thermo-economic performance of the EGS organic Rankine cycle(ORC) are studied. A performance comparison between EGS-ORC and the EGS combined heating and power system(CHP) is presented. Considering the CO_(2)emission reduction benefits, the influence of carbon emission trading price on the levelized cost of energy(LCOE) is also presented. It is indicated that the geofluid mass flow rate is a critical parameter in dictating the success of a project. Under the assumed typical working conditions, the LCOE of EGS-ORC and EGS-CHP systems are 24.72 and 16.1 cents/k Wh, respectively. Compared with the EGS-ORC system, the LCOE of the EGS-CHP system is reduced by 35%. EGS-CHP systems have the potential to be economically viable in the future. With carbon emission trading prices of 12.76 USD/ton, the LCOE can be reduced by approximately 8.5%.

A thermodynamics comparison of subcritical and transcritical organic Rankine cycle system for power generation

A comparison on subcritical and transcritical organic Rankine cycle（ORC） system with a heat source of 110 ℃ geothermal water was presented. The net power output, thermal and exergy efficiencies and the products of the heat transfer coefficient（U） and the total heat exchange area（A）（UA values） were calculated for parametric optimization. Nine candidate working fluids were investigated and compared. Under the given conditions, transcritical systems have higher net power outputs than subcritical ones. The highest net power output of transcritical systems is 18.63 k W obtained by R218, and that of subcritical systems is 13.57 k W obtained by R600 a. Moreover, with the increase of evaporating pressure, the thermal and exergy efficiencies of transcritical systems increase at first and then decrease, but the efficiencies of subcritical ones increase. As a result, the efficiencies of transcritical systems cannot always outperform those of the subcritical ones. However, the subcritical systems have lower minimum UA values and lower expansion ratios than the transcritical ones at the maximum net power output. In addition, the transcritical cycles have higher expansion ratios than the subcritical ones at their maximum net power output.

Performance Analysis of an Organic Rankine Cycle with a Preheated Ejector

The so-called organic Rankine cycle(ORC)is an effective technology allowing heat recovery from lower temperature sources.In the present study,to improve its thermal efficiency,a preheated ejector using exhaust steam coming from the expander is integrated in the cycle(EPORC).Considering net power output,pump power,and thermal efficiency,the proposed system is compared with the basic ORC.The influence of the ejector ratio(ER)of the preheated ejector on the system performances is also investigated.Results show that the net power output of the EPORC is higher than that of the basic ORC due to the decreasing pump power.Under given working conditions,the average thermal efficiency of EPORC is 29%higher than that of ORC.The ER has a great impact on the performance of EPORC by adjusting the working fluid fed to the pump,leading to significant variations of the pump work Moreover,the ER has a remarkable effect on the working fluid temperature lift(TL)at the evaporator inlet,thus reducing the evaporator heat load.According to the results,the thermal efficiency of EPORC increases by 30%,when the ER increases from 0.05 to 0.4.