Search

This paper presents a theoretical model on the payback period of a subcritical ORC (organic Rankine cycle) system for recovering low-grade waste heat of flue gas. Based on the minimum payback period principle, a detailed internal parameter optimization is carried out. Furthermore, the influences of external parameters on the payback period are analyzed and a new criterion of screening working fluids is proposed. Results show that the payback period of the ORC system decreases before increasing with the increase of evaporation temperature, condensation temperature, and the pinch point temperature differences in the evaporator and condenser. The minimum payback period of the ORC system decreases monotonously with the increase of the inlet temperature, mass flow rate of the flue gas, and the electricity price. When the minimum payback period and the net power output are used as indicators of selecting working fluids of the ORC system, this work identifies a specific working fluid for a given inlet flue gas temperature. We demonstrate this process by showing that R236a, R245fa, and R113 should be selected as the working fluids when the inlet temperatures of the flue gas are around 150 °C, 200 °C and 250 °C, respectively.

In aim to reduce the greenhouse-gas emissions and improve the low-grade heat efficiency, a modified CCHP (combined cooling, heating, and power) system is proposed using supercritical CO2. This cycle combines a Brayton cycle and a transcritical ejector refrigeration cycle by adding an extraction turbine. A mathematical model is developed to simulate the modified CCHP system. Parametric analysis and exergy analysis are conducted to investigate the effects of key thermodynamic parameters on the performance and exergy destruction. Due to the difficulties in the thermal efficiency evaluation for CCHP system, a more practical performance metric is introduced in order to quantify system performance. The results indicate that both higher extraction rate and extraction pressure are helpful to gain more refrigeration. For the conditions considered, the exergy efficiency of the modified CCHP with the extraction turbine is higher than that of the CCHP with the no-extraction turbine from 10.4% to 22.5%. Furthermore, there is a large increase in the turbine power output and the exergy efficiency with increased turbine inlet temperature. It reveals that a rise of heat source quality benefits the system performance.

A multi-objective function F (X) by the incorporation of single-objective functions, i.e., net power output W net , exergy drop of the exhaust gas from inlet to outlet Δ E g , total exergy destruction rate I and system total cost C 2013 , has been put forward as the subcritical organic Rankine cycle (ORC) performance indicator and solved with the method of linear weighted evaluation function. The optimization of the evaporation temperature T e and condensation temperature T c of subcritical ORC has been carried out by the use of pure working fluids R600a, R245fa, R601a, R601, R123, and non-azeotropic mixed working fluids R600a/R601a, R245fa/R601a, R245fa/R601, R600a/R245fa. The results reveal that there exist optimal evaporation temperature T e,opt and condensation temperature T c,opt minimizing the multi-objective function F (X); while T e,opt and T c,opt can not exactly exist when choosing single-objective functions, i.e., W net , I , C 2013 and Δ E g as the performance indicators. Although T e,opt and T c,opt vary with different working fluids, the ORC performance of mixed working fluids is not always better than that of pure working fluids. The multi-objective optimization of ORC shows superiority to the single-objective optimization, owning to its pursuit of the best comprehensive performance.

The performance of the ORC (organic Rankine cycle) systems using zeotropic mixtures as working fluids for recovering waste heat of flue gas from industrial boiler is examined on the basis of thermodynamics and thermo-economics under different operating conditions. In order to explore the potential of the mixtures as the working fluids in the ORC, the effects of various mixtures with different components and composition proportions on the system performance have been analyzed. The results show that the compositions of the mixtures have an important effect on the ORC system performance, which is associated with the temperature glide during the phase change of mixtures. From the point of thermodynamics, the performance of the ORC system is not always improved by employing the mixtures as the working fluids. The merit of the mixtures is related to the restrictive conditions of the ORC, different operating conditions results in different conclusions. At a fixed pinch point temperature difference, the small mean heat transfer temperature difference in heat exchangers will lead to a larger heat transfer area and the larger total cost of the ORC system. Compared with the ORC with pure working fluids, the ORC with the mixtures presents a poor economical performance.

In this paper, a novel power/refrigerating combined-system driven by the low-grade waste heat of the flue gas was introduced. It coupled a transcritical organic Rankine cycle with a vapor compression refrigeration cycle. In order to understand basic characteristics of this novel combined-system, a detailed performance analysis was performed. Results show that the turbine inlet pressure, flue gas inlet temperature and condensation temperature are three important parameters influencing system performance. During the increase of the turbine inlet pressure, the refrigeration capacity and the exergy efficiency exhibit the maximum values. However, with the increase of the flue gas inlet temperature and the decrease of the condensation temperature, both the refrigeration capacity and the exergy efficiency corresponding to the optimal turbine inlet pressure increase. The use of regenerator can improve the system performance and reduce the optimal turbine inlet pressure. Under partial refrigeration load, there is a linear relationship between the electricity output and the refrigeration capacity at a fixed work output from the turbine. The total irreversible loss decreases and the exergy efficiency increases with the decrease of the power ratio. Finally, after screening and comparing various potential working fluids, R134a is recommended as the working fluid of this novel combined-system.

This paper presents an economical evaluation and parametric optimization on subcritical ORC (organic Rankine cycle) system for recovering the low-temperature waste heat of flue gas which is released from industrial boilers. First, the effects of the PPTD (pinch point temperature difference) in evaporator and condenser, and evaporating temperature on the system performance are examined with the EPC (electricity production cost) as the evaluation criterion. The optimal evaporating temperature, PPTDs in evaporator and condenser, as well as condensing temperature corresponding to the minimum EPC are provided simultaneously for the subcritical ORC system under different heat source temperatures. Then, the optimal parameter values are compared for nine potential organic working fluids with the critical point temperature from 124 °C to 235 °C. Results show that there exists a possible relationship between the critical temperature of working fluids and the economical performance of the system. Furthermore, it is suggested that the working fluids including R123, n-pentane, R11 and R141b, whose critical temperature range from 180 °C to 210 °C, are preferable for recovering the low-temperature waste heat of flue gas.

The low-grade heat sources coupled by ORC (organic Rankine cycle) are categorized into two groups. For the first one, the inlet temperature and the mass flow rate are known, and the working mass of the heat source is directly discharged after being used. For the second, the heat release is specific and the working mass of the heat source is usually recycled after releasing heat. The theoretical formulas of net power output and thermal efficiency for subcritical ORC coupling with the two kinds of heat source are proposed to elucidate the selection criteria of working fluids. The new mathematical relation of working fluid selection is given out. The selection of working fluids for subcritical ORC should couple with the types of low-grade heat sources. For the first heat source, both the theoretical analysis and numerical simulation results show that the working fluids with high liquid specific heat and low latent heat of evaporation should be selected as the working fluids. In contrast, the working fluids with low liquid specific heat and the high latent heat of evaporation are better for the second heat source.

Two modified systems based on a KCS (Kalina cycle system) 11 with a two-phase expander to substitute a throttle valve are proposed. The two-phase expander is located between the regenerator and the absorber in the B-modified cycle and between the separator and the regenerator in the C-modified cycle. A thermodynamic performance analysis of both the original KCS 11 and the modified systems is carried out. The optimization of two key parameters (the concentration of working fluid and the temperature of cooling water) is also conducted. It is shown that the two modified cycles have different performance under the investigated conditions. Results also indicate that the C-modified cycle can obtain better thermodynamic effect than the B-modified cycle. The temperature of cooling water plays an important role in improving the system performance. When the cooling water temperature drops from 303 K to 278 K, the C-modified cycle thermal efficiency can be improved by 27%.

The LCA (life-cycle assessment) was applied to evaluate EI (the environmental impact) of ORCPW (organic Rankine cycle power-plant for waste-heat-recovery) in this paper. The model of LCA on the ORCPW was established. The life-cycle of ORCPW was divided into construction, operation and decommissioning phases. The inventory of environmental emissions was listed for the ORCPW with 7 different working fluids. The GWP (global warming potential), AP (acidification potential), EP (eutrophication potential), HTP (human toxicity potential), SWP (solid waste potential) and SAP (soot and dust potential) were investigated. Some EIs of ORCPW were compared with the EIs of other power generation modes. The results show that the construction phase of ORCPW contributes mostly to the GWP and EP. GWP is the most serious EI followed by HTP among all the environmental impacts. The average pay back times of greenhouse gas discharged from ORCPW is calculated on the basis of five other power generation modes. For 7 different working fluids, it is 3–5 years for CO2, about one year for CH4 and 3–6 years for NOx. But CO cannot be paid back during the life-cycle of ORCPW according to the average pay back time.

This paper presented the analysis on the influence of the pinch point temperature difference (PPTD) and the evaporation temperature on the performance of organic Rankine cycle (ORC) in recovering the low temperature waste heat of the flue gas. Both the net power output and the heat transfer area of the evaporator and condenser were evaluated for dry and isentropic working fluids. When the heat and cold source conditions were given, the maximum net power output and the heat transfer area were obtained. The results show that some organic working fluids cannot reach the maximum net power output to avoid the low temperature corrosion. With the increase of the PPTD of the evaporator at a given total temperature difference, the total heat transfer area decreases first and then increases, while the corresponding cost-effective performance (ratio of the net power output to total heat transfer area) displays almost the opposite variation tendency. The PPTD of the evaporator for the optimization cost-effective performance is approximately the same for different organic working fluids. Meanwhile, the isentropic working fluids show better cost-effective performance than dry working fluids.

Organic Rankine cycle (ORC) is a promising technology for converting low-grade waste heat into the electricity. This paper carries out an exergoeconomic analysis and performance optimization of a condenser for a binary mixture of vapors in the ORC system. In this model, the exergy loss due to heat transfer between a binary mixture of vapors and the cold fluid is taken into account and other exergy losses such as pressure drop and flow imbalance are ignored. The objective function is defined as the annual total cost per unit heat transfer rate considering the capital cost and the exergy loss cost. The variation laws of the annual total cost with the number of the transfer unit and the temperature ratios are exhibited. The optimal values of the number of the transfer unit and the heat capacity ratio which minimize the annual total cost have been obtained. The effects of related technical and economical parameters on the optimal performances have also been discussed.

A theoretical formula is proposed to calculate the OET (optimal evaporation temperature) of subcritical ORC (organic Rankine cycle) based on thermodynamic theory when the net power output is selected as the objective function. The OETs of 22 working fluids including wet, isentropic and dry fluids are determined under the given conditions. In order to compare the accuracy of these results, the quadratic approximation method in EES (Engineering Equation Solver) is used to optimize the net power output and the OETs are obtained by numerical simulation. The results show that the OETs calculated by the theoretical formula are consistent with the numerical simulation results. In addition, the average computational accuracy of OETs from the theoretical formula is higher than that from the simplified formula recommended by the related literature. The larger net power output will be produced when the critical temperature of working fluid approaches to the temperature of the waste heat source. According to the maximum net power output, suitable working pressure, total heat transfer capacity and expander SP (size parameter), R114, R245fa, R123, R601a, n-pentane, R141b and R113 are suited as working fluids for subcritical ORC under the given conditions in this paper.

In this paper, exergy transfer effectiveness is defined to describe the performance of heat exchangers operating above/below the surrounding temperature with/without finite pressure drop. It is discussed systemically that the effects of heat transfer units number, the ratio of the heat capacity of cold fluids to that of hot fluids and flow patterns on exergy transfer effectiveness of heat exchangers. Furthermore, the results of exergy transfer effectiveness with a finite pressure drop are compared with those without pressure drop when different objective media, such as ideal gas and incompressible liquid, etc. are applied. The detailed comparisons of the exergy transfer effectiveness with heat transfer effectiveness are also performed for the parallel flow, counter flow and cross flow heat exchangers operating above/below the surrounding temperature.

The exergy transfer characteristics of fluid flow and heat transfer inside a circular duct under fully developed laminar and turbulent forced convection are presented. Temperature is kept constant at the duct wall. The exergy transfer Nusselt number is put forward and the analytical expressions for exergy transfer Nusselt number are obtained as functions of heat transfer Nusselt number, Reynolds number, Prandtl number, etc. The variations of the local and mean convective exergy transfer coefficient, non-dimensional exergy flux, exergy transfer rate, etc. with operating parameters are presented graphically. By reference to a smooth duct and taking air as working fluid, a numerical analysis of the influence of the Reynolds number and non-dimensional cross-sectional position on exergy transfer characteristics has been conducted. The results show that the process parameters and configuration in the fluid flow and heat transfer inside a duct should be properly selected so that the forced convection process could have the best exergy utilization. In addition, the results corresponding to the exergy transfer and energy transfer are compared.