Net Power Efficiency Research Articles

Exergoeconomic investigation and multi-objective optimization of different ORC configurations for waste heat recovery: A comparative study

This paper compares four different architectures of organic Rankine cycle (ORC) namely the Basic ORC, Recuperative ORC, Regenerative ORC, and Recuperative-Regenerative ORC (RR-ORC) from the exergoeconomy viewpoint. First, a parametric analysis is performed to understand the effect of evaporator temperature, condenser temperature, and pinch point temperature difference on the performance of each system configuration. Next, a multi-objective optimization is performed to optimize the performance of the four configurations using Pareto Envelope-based Selection Algorithm-II (PESA-II) with exergy efficiency and the system cost rate as the objective functions. Pareto fronts, representing the set of optimal solutions, are obtained for each configuration, and multi-criteria decision analysis is done using the Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) to select the best optimal solution. Next, the exergetic and exergoeconomic performance of the four ORC configurations is evaluated at their corresponding optimal conditions in order to provide a comparative assessment. It was found that, under optimal conditions, the RR-ORC performs superiorly to the other three configurations. Compared to the Basic ORC, the net power and exergy efficiency of the RR-ORC are 16.19% and 15.33% higher, while the system cost rate is 1.68 % low. After, the RR-ORC, the Regenerative, and the Recuperative ORCs are the second and the third-best configurations. It was also found that in all four configurations, the cost rate of exergy loss accounts for approximately 60 % of the total system cost rate.

Optimization of the boiler pressure and working fluid in a binary organic Rankine cycle for different heat sources

ORC is a common power generation system that is driven by different heat source temperatures. Boiler pressure and working fluid are effective factors on the cycle performance. The use of mixtures can improve performance. Selecting a proportionate mixture is a great challenge. In this paper, the waste heat recovered from an industrial complex was utilized to generate power by an ORC. A large number of binary mixtures were considered as the working fluid. Based on the optimization algorithms such as Genetic Algorithm and Particle Swarm Optimization, the effects of various components and their concentrations in the mixture and boiler pressure were simultaneously optimized. The optimal conditions including components of the binary mixture, mixture concentration, and boiler pressure were presented for different heat source temperatures in the range of 80–190 °C. R32-R290 with a concentration of 0.69–0.31 at the pressure of 37.06 bar had the highest net power and exergy efficiency for heat source temperature of 80 °C. For temperatures of 100 °C and 130 °C, R290-R143a and R290-R152a with concentrations of 0.2–0.8 and 0.57–0.43 were the most appropriate mixtures at their optimal pressures, respectively. The optimum binary mixtures for heat source temperatures of 160 °C and 190 °C were R600-R21 (0.31–0.69) and R600-R245fa (0.33–0.67), respectively.

Performance analysis of ORC low temperature waste heat power generation system

For 90~180°C low temperature waste heat resources, three different types of organic working fluids, benzene, isopentane and R245fa are selected from the perspectives of safety, environmental protection and physical properties of working fluid. The mathematical models of the system are established through MATLAB and REFPROP and the influence of the evaporation temperature on the performance of the ORC low temperature waste heat power generation system is studied. The results show that the thermal efficiency and net output power of the system increase with the increase of evaporation temperature; the evaporation pressure of CFC-type working fluids is relatively low; the net output power and thermal efficiency of HFC and HC-type working fluids are generally relatively low. The evaporator has the largest irreversible loss in the system, followed by the condenser and expander while the working fluid pump has the smallest irreversible loss.

Experimental analysis of R134a working fluid on Organic Rankine Cycle (ORC) systems with scroll-expander

Waste Heat Recovery System (WHRS) technology, e.g., Organic Rankine Cycle (ORC) has been explored as a solution to the increase in geothermal potential, possibly applied in binary or combined form, as a single-flash plant. This approach utilizes waste brine obtained from the separator. The purpose of this study, therefore, was to determine the performance of R134a refrigerant on ORC systems, using scroll-expander. The experiment was conducted with motor different frequency, at 7.5 Hz, 10 Hz, 12.5 Hz, 15 Hz, 17.5 Hz, and 20 Hz, evaporator temperature regulation of 95 °C, condenser at 10 °C, and an initial working fluid pressure of 5 bar. The results showed modifications in the ORC system performance with scroll-expander, alongside the motor frequency, which influenced the net power and energy efficiency. In addition, the maximum speed produced by the expander was 505.8 rpm, while the highest for energy efficiency achieved from the system was 3.17%, at a motor frequency of 7.5 Hz, with a resulting net power of 584.5 Watt.

Thermodynamic evaluation and modelling of an auto-thermal hybrid system of chemical looping combustion and air separation for power generation coupling with CO2 cycles

Chemical looping technologies have been demonstrated to have favorable economy for CO2 separation and high exergy efficiency. In order to realize the energy cascade utilization, a hybrid system combing autothermal coal-fueled chemical looping combustion (CLC), chemical looping air separation (CLAS), supercritical CO2 Brayton cycle and transcritical CO2 cycle is constructed for high-efficiency power production and CO2 separation. The proposed hybrid system is first modeled and simulated under fundamental condition to investigate the basic performance and exergy efficiency of each component. Then the thermodynamic evaluations of the system performance are conducted under different conditions with five variables. Finally, using these simulation results as training samples, a neural network model (NNM) of the system is constructed to predict the power output under variable working parameters. As the energy is used in different subsystems according to its grade, the net power efficiency of this hybrid system can reach to a high value of 64.59%. The established NNM is accurate for power prediction under random conditions with correlation coefficient of 0.9931.

Techno-economic and environmental assessment of staged oxy-co-firing of biomass-derived syngas and natural gas

The utilisation of biomass is regarded as a promising technology as a negative CO2 resource and is widely under investigation. Although being a developing technology predominantly used as the axillary fuel, the interest in biomass gasification has increased since the last decade due to increased attention in sustainable bioenergy. Therefore, in this study, a novel process of biomass-derived syngas and natural gas oxy-co-firing is being investigated. Aspen Plus® is employed to develop the staged oxy-co-firing of biomass-based syngas and the natural gas cycle. The results revealed that the net power efficiencies are in the range of 42.6% and 45.5%. Furthermore, the economic assessment reveals that the LCOE increases with the increment of biomass flowrate with the highest cost of €79.6/MWh for 7 kg/s biomass mass flow rate. The effect of different biomass prices on LCOE is also investigated. It proved that the implementation of biomass with the price of €80/tdry results in the competitive LCOE to the state-of-the-art power cycles without carbon capture system (€62.9/MWh). On the other hand, the environmental analysis reveals that in order to have a negative CO2 emission power cycle, the minimum biomass flowrate should be at 7 kg/s which results in specific negative CO2 emission and total life cycle of CO2 emission of 195.5 and 168.5 kg CO2/MWh, respectively.

Using a Partially Evaporating Cycle to Improve the Volume Ratio Problem of the Trilateral Flash Cycle for Low-Grade Heat Recovery.

This study examined the trilateral flash cycle characteristics (TFC) and partially evaporating cycle (PEC) using a low-grade heat source at 80 °C. The evaporation temperature and mass flow rate of the working fluids and the expander inlet’s quality were optimized through pinch point observation. This can help advance methods in determining the best design points and their operating conditions. The results indicated the partially evaporating cycle could solve the high-volume ratio problem without sacrificing the net power and thermal efficiency performance. When the system operation’s saturation temperature decreased by 10 °C, the net power, thermal efficiency, and volume ratio of the trilateral flash cycle system decreased by approximately 20%. Conversely, with the same operational conditions, the net power and thermal efficiency of the partially evaporating cycle system decreased by only approximately 3%; however, the volume ratio decreased by more than 50%. When the system operating temperature was under 63 °C, each fluid’s volume ratio could decrease to approximately 5. The problem of high excessive expansion would be solved from the features of the partially evaporating cycle, and it will keep the ideal power generation efficiency and improve expander manufacturing.

Techno-economic assessment of calcium and magnesium-based sorbents for post-combustion CO2 capture applied in fossil-fueled power plants

This work evaluates the most relevant techno-economic performances of calcium and magnesium-based sorbents used in high temperature solid looping cycles for post-combustion CO2 capture applied in fossil power generation. The evaluated power plants generate 1000 MW net electricity with 90% CO2 capture rate. As fuels, natural gas was assessed for combined cycle power plants since coal and lignite were assessed for super-critical power plants. As comparison benchmark systems, the correspondent plants with non-capture feature and with carbon capture using reactive gas–liquid absorption were also assessed. As the results show, both calcium and magnesium carbonate looping concepts applied in solid fossil fuel super-critical plants induce lower carbon capture energy penalty compared to the reactive gas–liquid absorption by about 1–2.2 net efficiency percentage points. The Calcium Looping (CaL) cycle shows higher net power efficiency than the Magnesium Looping (MgL) cycle by about 1–1.4 net percentage points. In case of natural gas combined cycle plants, the chemical absorption system has higher net power efficiency than CaL and MgL systems by about 4–6 percentage points. The key economic indicators (e.g., electricity production cost, CO2 capture costs etc.) show that both evaluated carbonate looping systems perform better than amine-based chemical scrubbing.

Integration and optimization for a PEMFC and PSA oxygen production combined system

In this research, a new type of high-temperature proton exchange membrane fuel cell and pressure swing adsorption (PEMFC-PSA) oxygen generation integrated system is proposed. The thermodynamic model of the integrated system of PEMFC-PSA oxygen production was established. The effects of oxygen concentration, temperature, oxygen flow rate and pressure on the efficiency and net power of the PEMFC-PSA integrated system are studied. The results show that as the oxygen concentration increases, the net power of the new integrated system increases to 85% of the oxygen concentration, which corresponds to the maximum net power value of 101 W, and then decreases. Therefore, the oxygen concentration fed to the system is 85% to produce the maximum net power, and the system should operate under air conditions for maximum efficiency. With the increased operating temperature, pressure and oxygen inlet speed of the system, the maximum net power and maximum efficiency of the new integrated system are achieved.

Modelling and optimization of a flexible hydrogen-fueled pressurized PEMFC power plant for grid balancing purposes

In a scenario characterized by an increasing penetration of non-dispatchable renewable energy sources and the need of fast-ramping grid-balancing power plants, the EU project GRASSHOPPER aims to setup and demonstrate a highly flexible PEMFC Power Plant, hydrogen fueled and scalable to MW-size, designed to provide grid support.In this work, different layouts proposed for the innovative MW-scale plant are simulated to optimize design and off-design operation. The simulation model details the main BoP components performances and includes a customized PEMFC model, validated through dedicated experiments.The system may operate at atmospheric or mild pressurized conditions: pressurization to 0.7 barg allows significantly higher net system efficiency, despite the increasing BoP consumptions. The additional energy recovery from the cathode exhaust with an expander gives higher net power and net efficiency, adding up to 2%pt and reaching values between 47%LHV and 55%LHV for currents between 100% and 20% of the nominal value.

Blocked serpentine flow field with enhanced species transport and improved flow distribution for vanadium redox flow battery

A blocked serpentine flow field is proposed for vanadium redox flow battery (VRFB) in order to improve electrolyte penetration and convection in the porous electrode such that the overall battery performance can be boosted. A three-dimensional mathematical model with considering non-uniform electrode structure under compression and electrode intrusion into channels was applied for simulating and comparing the VRFB performance with conventional serpentine flow field and blocked serpentine flow field. It was indicated that for blocked serpentine flow field, with increasing the block height from 0 mm to 1.5 mm, the discharge voltage of the battery is obviously boosted due to the enhanced species transport and improved flow distribution in the porous electrode. Also, it was shown that the overall battery performance in terms of net discharge power and system efficiency reaches maximum for the blocked serpentine flow field with 1.4 mm block height among different flow field designs, which is attributed to the tradeoff between the effects of increased pump loss and the improved charge-discharge performance. This study can provide beneficial advice for designing the suitable flow field for VRFB with improved mass transport performance.

Design and Operational Control Strategy for Optimum Off-Design Performance of an ORC Plant for Low-Grade Waste Heat Recovery

The applicability of organic Rankine cycle (ORC) technology to waste heat recovery (WHR) is currently experiencing growing interest and accelerated technological development. The utilization of low-to-medium grade thermal energy sources, especially in the presence of heat source intermittency in applications where the thermal source is characterized by highly variable thermodynamic conditions, requires a control strategy for off-design operation to achieve optimal ORC power-unit performance. This paper presents a validated comprehensive model for off-design analysis of an ORC power-unit, with R236fa as the working fluid, a gear pump, and a 1.5 kW sliding vane rotary expander (SVRE) for WHR from the exhaust gases of a light-duty internal combustion engine. Model validation is performed using data from an extensive experimental campaign on both the rotary equipment (pump, expander) and the remainder components of the plant, namely the heat recovery vapor generator (HRVH), condenser, reservoirs, and piping. Based on the validated computational platform, the benefits on the ORC plant net power output and efficiency of either a variable permeability expander or of sliding vane rotary pump optimization are assessed. The novelty introduced by this optimization strategy is that the evaluations are conducted by a numerical model, which reproduces the real features of the ORC plant. This approach ensures an analysis of the whole system both from a plant and cycle point of view, catching some real aspects that are otherwise undetectable. These optimization strategies are considered as a baseline ORC plant that suffers low expander efficiency (30%) and a large parasitic pumping power, with a backwork ratio (BWR) of up to 60%. It is found that the benefits on the expander power arising from a lower permeability combined with a lower energy demand by the pump (20% of BWR) for circulation of the working fluid allows a better recovery performance for the ORC plant with respect to the baseline case. Adopting the optimization strategies, the average efficiency and maximum generated power increase from 1.5% to 3.5% and from 400 to 1100 W, respectively. These performances are in accordance with the plant efficiencies found in the experimental works in the literature, which vary between 1.6% and 6.5% for similar applications. Nonetheless, there is still room for improvement regarding a proper design of rotary machines, which can be redesigned considering the indications resulting from the developed optimization analysis.

Thermal performance analysis on steady-state and dynamic response characteristic in solar tower power plant based on supercritical carbon dioxide Brayton cycle

ABSTRACT Supercritical Carbon Dioxide (S-CO2) Brayton cycles and Concentrating Solar Power (CSP) plants are gaining increasing attention due to environment pollution problems and their higher efficiencies. At present, there is a lack of researches on the dynamic thermal analysis of CSP plants based on S-CO2 Brayton cycle, which needs to be resolved urgently. Therefore, a one-dimensional dynamic model of the printed circuit heat exchanger (PCHE) is developed, which is incorporated into transient model of the solar tower power plant. And the Modified Euler or Fully-implicit Runge-Kutta Method is used to complete the dynamic solution. The results indicate that the pressure ratio equaling to 3.5 and the zigzag PCHE regenerator with a diameter of 2 mm will be the best choice for our designed power plant to achieve the balance between larger net output work and power efficiency improvement, which provides the reasonable design parameter for more complicated three-dimensional transient models. By taking the binary nitrate working temperature limitation and common influences of temperature and pressure on S-CO2 thermophysical properties into consideration, as the direct normal irradiance (DNI) decreases, the S-CO2 heat transfer coefficient will increase. When DNI value changes between 780 W/m2 and 970 W/m2, the cold side S-CO2 outlet temperature of PCHE regenerator and S-CO2 Brayton cycle power efficiency will produce 114.65 K and 8% fluctuation in no more than 15 minutes.

Cold Energy Utilization in LNG Regasification System Using Organic Rankine Cycle and Trilateral Flash Cycle

"Cold energy" refers to a potential to generate power by utilizing the exergy of cryogenic systems, like Liquefied Natural Gas (LNG), using it as the cold side of a thermodynamic cycle, while the hot side can be even on the ambient temperature. For this purpose, the cryogenic Organic Rankine Cycle (ORC) is one type of promising solution with comprehensive benefits to generate electricity. The performance of this cycle depends on the applied working fluid. This paper focuses on the applicability of some natural working fluids and analyzes their performance upon cold energy utilization in the LNG regasification system. An alternative method, the cryogenic Trilateral Flash Cycle (TFC), is also presented here. The selection of working fluid is a multi-step process; the first step uses thermodynamic criteria, while the second one is addressing environmental and safety issues. It will be shown that in LNG regasification systems, single cryogenic ORC performs higher net output power and net efficiency compared to single cryogenic TFC. Propane as working fluid in the single cryogenic ORC generates the highest net output power and net efficiency. It is demonstrated, that concerning 26 novel LNG terminals, a net power output around 320 MW could be recovered from the cold energy by installing a simple cycle, namely a single-step cryogenic ORC unit using propane as working fluid.

Integration of S-CO2 Brayton cycle and coal-fired boiler: Thermal-hydraulic analysis and design

An integration system for 1000 MW S-CO2 connected-top-bottom power cycle with overlap utilization and coal-fired boiler is developed. The adaptive modification module is introduced to eliminate the non-convergence for S-CO2 superheater thermal calculation in the system iteration. Thermal-hydraulic performance in terms of thermal performance, cycle efficiency, heater configurations, heater areas and final parameter evenness are discussed. Firstly, the strong constraint effects of inlet and outlet parameters on superheater thermal calculation convergence are demonstrated and the adaptive modification module is established. Secondly, based on the detailed thermal-hydraulic analysis, different layouts are compared to identify the flow path way and the heat absorption of S-CO2 flow in different heating processes. The midpoint between the two peaks of heat flux is recommended as the start point for the cold fluid placement in cooling wall arrangement. Finally, a new layout is proposed to obtain the highest overall thermal-hydraulic performance, with a net power efficiency up to 47.57% and a total heater area of 316426.18 m2. The design sequences for S-CO2 coal-fired boiler heaters are recommended as well.

Open feed organic heater pressure analysis on single-stage regenerative organic Rankine cycle performance

Single-stage regenerative organic Rankine cycle (SSRORC) is a system that is used for increasing the simple organic Rankine cycle (ORC) performance. Open feed organic heater (OFOH) addition in the ORC system increase power and efficiency of the system. This paper analyzes the SSRORC performance with a variation of P6/P1 ranges from 1.25 to 3.75 with an increment of 0.25, where P6 is the OFOH pressure at the inlet side and P1 is the pressure at the inlet pump 1, respectively. Hot water was used as the heat source with 100 °C and 100 l/min of temperature and volume flow rate as the initial data. R227ea, R245fa, and R141b were chosen as working fluids for performance analysis. The analysis was performed by calculating the heat input, heat loss, pump and turbine power, net power, and thermal efficiency through energy balance. Exergy input, exergy output, and exergy efficiency were analyzed through exergy balance. The results show that P6/P1 = 2 obtains the highest performance than the other pressure ratios for R227ea, while R245fa and R141b obtain the highest performance at P6/P1 = 2.25. R141b has better performance than the other two fluids with 10.97 % and 11.96 % for thermal and exergy efficiency. The results show that the ratio of OFOH pressure at the inlet side to the pressure at inlet pump 1 (P6/P1) in the middle value obtains the best performance.

An experimental study of cell performance and pressure drop of proton exchange membrane fuel cells with baffled flow channels

The pressure drops and pumping powers in flow channels of proton exchange membrane fuel cells are major standards to evaluate flow channel structures. Baffled flow channels provide higher performance, while the pressure drop and pumping power both aggravate. In this work, the impact of baffle leeward length on power output and pressure drops of proton exchange membrane fuel cells are experimentally studied. The pumping power, net power and power efficiency are calculated as well. Experimental results indicate that pressure drops increase when adding baffles in channels, and longer leeward lengths result in lower pressure drops. Higher cell temperature results in lower pressure drops. The time consuming for stabilizing pressure drop are shortened when using baffles having longer leeward lengths. The net power is increased when using larger baffles, and the highest net powers occur under the temperature of 333K or 343K. In addition, raising the cell temperature can increase the power efficiency, and when the leeward lengths of baffles are decreased, the power efficiency is also decreased. When using baffle without sloping leeward sides, the power efficiency can be decreased to 45.14% under lower cell temperature of 323K.

Novel operation strategy for a gas turbine and high-temperature KCS combined cycle

This paper designs a high-temperature Kalina cycle system (KCS) to recover a gas turbine (GT) waste heat. A novel methodology of predicting off-design performance of GT-KCS combined cycle is presented by introducing geometric parameters of heat exchangers. The inlet guide vane of GT compressor operation strategy is applied in GT. Novel operation strategies named modified sliding pressure operation (MSPO) and novel modified sliding pressure operation (NMSPO) are proposed for KCS. The MSPO uses a fixed KCS turbine inlet temperature and an optimized ammonia concentration at KCS separator inlet under GT part-load conditions, while the NMSPO adopts an optimized ammonia concentration at KCS separator inlet and a fixed temperature difference between KCS turbine inlet and GT exhaust. The results show that the optimal ammonia concentrations at KCS separator inlet under the MSPO and NMSPO decrease with declining GT load. The MSPO has the highest bottoming KCS thermal efficiency among the sliding pressure operation (SPO), MSPO and NMSPO. The NMSPO recovers the most heat from GT exhaust, resulting in the highest KCS net power and GT-KCS thermal efficiency. The NMSPO recommended as the optimal operation strategy could produce 20.72 kW and 25.75 kW more net power than the MSPO and SPO, respectively.

Performance improvement of biomass-fueled closed cycle gas turbine via compressor inlet cooling using absorption refrigeration; thermoeconomic analysis and multi-objective optimization

Compressor inlet air cooling is a practical and applied methodology to enhance the conventional open cycle gas turbine power plants. This methodology in this paper, is proposed to be applied on an innovative biomass-fueled Closed Cycle Gas Turbine (CCGT), in which the exhaust gas waste heat is utilized to run an ammonia-water absorption refrigeration cycle for compressor inlet cooling. Thermoeconomic analysis is presented to investigate the two systems (with and without inlet cooling) and multi-objective optimization is conducted to compare their performances at optimal operating conditions. Levelized Cost of Electricity (LCOE) and exergy efficiency are selected as the two rational objectives. The results indicated that, for all the practical range of operating conditions, compressor inlet cooling significantly improves the system performance in terms of both thermodynamics and economics, despite the additional costs imposed on the overall system by adding the absorption refrigeration cycle. It is found that, under the optimal operating conditions, incorporation of compressor inlet cooling results in an improvement of net power and exergy efficiency by 30.1%, meanwhile the LCOE would be reduced by 22.5% when inlet cooling is employed.

Exergoeconomic and exergoenvironmental analyses and optimization of different ejector based two stage expander-organic flash cycles fuelled by solar energy

In this work, four new architectures of two-stage expander organic flash cycles (EOFCs) based on ejector namely, ejector based EOFC (E-EOFC), ejector based high-pressure evaporation EOFC (EF1-EOFC), ejector based low-pressure evaporation EOFC (EF2-EOFC), and double ejectors based EOFC (DE-EOFC) are proposed and analyzed using exergy, exergoeconomic and exergoenvironmental methods. The thermodynamic, cost and environmental impact indicators of the suggested EOFCs are compared with those of the base EOFC. A performance comparison exhibits that the net power, energy, and exergy efficiencies are improved by 16.83%, 13.05%, and 10.99%, receptively in EF1-EOFC, E-EOFC and DE-EOFC relative to the base EOFC. EF2-EOFC gives the highest reduction in the total cost rate by 7.26% relative to the base EOFC and DE-EOFC with minimum increment in cost rate by 0.66% and a 0.95% improvement in the environmental impact rate relative to the base EOFC is recognized as the appropriate layout from the thermodynamic, economic and environmental impact aspects. In addition, non-dominated sorting genetic algorithm II (NSGA-II) and three decision-making methods are implemented to ascertain the maximum net power and minimum total cost and environmental impact rates of proposed cycles.