Efficiency Of Combined System Research Articles

Thermodynamic analysis of a solar refrigeration system based on combined supercritical CO<SUB align="right">2 power and cascaded refrigeration cycle

This communication proposed a solar driven system based on supercritical CO2 (sCO2) power cycle integrated with cascaded refrigeration cycle (CRC) to refrigerate a thermal load of below than -40°C. The impact of varying solar irradiance (DNI), type of solar heat transfer fluid (SHTF), and the working fluid of CRC on refrigeration capacity and exergy efficiency of combined system are investigated. Results indicate helium as the most efficient SHTF with air to follow. It is shown that maximum refrigeration is produced when operating with helium as SHTF and propylene as the refrigerant for CRC. From exergetic point of view, propylene provides higher exergy efficiency; CO2 is the one with lower, while N2O presents intermediate results. The exergy efficiency of the system for the propylene, N2O and CO2 is determined as 9.64%, 8.73%, and 8.47%, respectively.

A Comparative Energy and Exergy Analysis Between Organic Rankine Cycle and Kalina Cycle System 11 for the Waste Heat Recovery of a PEM Fuel Cell Power Station

Abstract In this study, the performance analysis of waste heat recovery systems in a power generation system consisting of 13000 Proton Exchange Membrane Fuel Cells (PEMFC) in a stack has been investigated. Organic Rankine Cycle (ORC) and Kalina Cycle System 11 (KCS11) as bottoming cycles to convert generated waste heat of stack into electricity were compared with each other in a defined hybrid system. The improvement of system with an exact energy and exergy analysis after utilizing the waste heat in the hybrid system has been analyzed. Results show that the energy efficiency of combined system using Organic Rankine Cycle and Kalina Cycle System 11 increase by about 5% and 1.75% respectively. In addition, exergy analysis results indicate that exergy efficiency of combined system using Organic Rankine Cycle and Kalina Cycle System 11 increases by about 4% and 1.5% respectively. The total exergy destruction rate obtained for the hybrid power systems is 235.5 kW when ORC is used and 329.7 kW when KCS11 is used respectively. Results show that in presented systems ORC has higher energy and exergy efficiencies than KCS11 but different used working fluids and equipment of systems must also be considered from an economical point of view.

Techno-economic evaluation of a combined biomass gasification-solid oxide fuel cell system for ethanol production via syngas fermentation

Bioethanol is regarded as a high-quality and clean liquid fuel with a renewable production process through bioconversion and chemical synthesis technology. Syngas fermentation route has received increasing attention due to mild reaction conditions and the elimination of expensive pretreatment and enzyme. To improve the economy and energy efficiency of the bioethanol production, a novel combined biomass gasification-syngas fermentation-solid oxide fuel cell system is proposed in this work. The techno-economic assessment is performed by the parametric analysis of the gasification and fermentation process. The sensitivity analysis shows that the dual fluidized bed with woodchips has the highest CO and H2 content, and the ethanol yield can be enhanced by improving the substrate conversion rate. A tentative economic assessment is performed with a total production cost of 430.1 €/t ethanol, revealing the economy of the hybrid gasification-syngas fermentation process. Besides, the problem of syngas distribution and fermenter exhaust recycling is further explored by introducing the SOFC system to realize the cascade utilization of energy. The trade-off between the combined system efficiency, total annual cost, and product price is conducted by multi-objective optimization with the technique for order of preference by similarity to ideal solution (TOPSIS) method. The optimized system can reach the system efficiency of 55%, the minimum total annual cost of 19.9 M€, and the total product price of 64.5 M€. The proposed system could demonstrate the feasibility of bioethanol production via biomass syngas fermentation with a high-efficiency and cost-competitive pathway.

Heat Flux Based Optimization of Combined Heat and Power Thermoelectric Heat Exchanger

We analyzed the potential of thermoelectrics for electricity generation in a combined heat and power (CHP) waste heat recovery system. The state-of-the-art organic Rankine cycle CHP system provides hot water and space heating while electricity is also generated with an efficiency of up to 12% at the MW scale. Thermoelectrics, in contrast, will serve smaller and distributed systems. Considering the limited heat flux from the waste heat source, we investigated a counterflow heat exchanger with an integrated thermoelectric module for maximum power, high efficiency, or low cost. Irreversible thermal resistances connected to the thermoelectric legs determine the energy conversion performance. The exit temperatures of fluids through the heat exchanger are important for the system efficiency to match the applications. Based on the analytic model for the thermoelectric integrated subsystem, the design for maximum power output with a given heat flux requires thermoelectric legs 40–70% longer than the case of fixed temperature reservoir boundary conditions. With existing thermoelectric materials, 300–400 W/m2 electrical energy can be generated at a material cost of $3–4 per watt. The prospects of improvements in thermoelectric materials were also studied. While the combined system efficiency is nearly 100%, the balance between the hot and cold flow rates needs to be adjusted for the heat recovery applications.

A concentrated solar spectrum splitting photovoltaic cell-thermoelectric refrigerators combined system: Definition, combined system properties and performance evaluation

Currently, a concentrated solar spectrum splitting photovoltaic cell driven semiconductor thermoelectric refrigerator has no relevant report. In this work, a new style model of the concentrated solar spectrum splitting photovoltaic cell-thermoelectric refrigerators (CSSPV-TERs) combined system is established. On the basis of numerical calculation and theoretical study in detail, the influences of the critical parameters on the property of the CSSPV-TERs combined system are analyzed and evaluated. The total efficiency of the CSSPV-TERs combined system is 23.4%. Meanwhile, the current density J, structural factor c and numbers of thermoelectric devices N has been optimally selected to obtain the best COP. The three critical optimal parameters, N = 7, J = 11.7 A/cm2, and c = 0.000125 cm−1, of the CSSPV-TERs combined system can be obtained, which cause the maximum COP, i.e., COPmax = 0.078. The maximum COP and total efficiency of the CSSPV-TERs combined system are superior to other thermoelectric refrigeration combined systems. The research results and new model not only can provide a valuable reference and theoretical support for the optimal design of the novel solar cell driven thermoelectric refrigeration system, but also improve the conversion efficiency of the combined system by efficiently utilizing full-spectrum solar energy.

Energetic and exergetic analyses of a solar powered combined compression-absorption refrigeration system

This study presents a theoretical framework for the thermodynamic analysis of a combined system which can simultaneously produce cooling for both refrigeration (−5°C) and air conditioning (10°C) for the hot climatic regions in an eco-friendly and carbon free manner. A tower solar collector unit was employed to drive the proposed combined cooling system which consists of a hydrocarbon operated vapour compression refrigeration system that is fed by power from steam turbine shaft, and a waste heat operated single-effect LiBr-H2O operated absorption chiller. A mathematical model using the balances of energy and exergy over the components of the system was developed, and solved using engineering equation solver software. A parametric analysis was conducted to estimate the influence of operating conditions on the system performance. The results show that at baseline operation of the system, for R600a operated VCR, the energetic efficiency of combined system was found to be 73.66% and the exergetic efficiency was 36.43%, while for R290 operated VCR, the energetic and exergetic efficiencies of the combined system were found to be 37.17% and 21.04%, respectively.

Analysis of a combined proton exchange membrane fuel cell and organic Rankine cycle system for waste heat recovery

ABSTRACT Proton exchange membrane fuel cell (PEMFC) has become a promising alternative power source due to its quiet operation, low emission, and high theoretical energy conversion efficiency. However, a relatively large fraction of the energy (around 40 ~ 50%) is dissipated into the ambient environment by the cooling system, leading to a waste of thermal energy. The present study aims to improve the practical PEMFC performance by harvesting the thermal energy from the cooling system utilizing an organic Rankine cycle (ORC) system. The R134a and R245fa are selected as the working liquids for ORC to harvest energy from the PEMFC. A combined PEMFC and ORC model is established based on the first and second law thermodynamic analysis. It is found that the cycle efficiency (ηorc ) of R134a (about 11.33%) is higher than that of R245fa (about 9.16%). The combined system efficiency (ηsys ) for R134a can be increased by approximately 5.84%, compared with that of the fuel cell stack alone. For R134a, the effect of changes in the operating temperature and current density of the PEMFC stack on its cycle efficiency (ηorc ) is investigated. The results indicate that as the working temperature of the PEMFC stack increases, both the ORC and combined system efficiencies (ηorc and ηsys ) increase, while with the increase of the current density of the PEMFC, the ORC cycle efficiency (ηorc ) remains almost constant, and the combined system efficiency (ηsys ) decreases. The exergy efficiency is found to have the same variation trends with the energy efficiency.

Investigations on SOFC-HAT-sCO2 based combined power and heating cycle

Paper pertains to the investigations on combined solid oxide fuel cell (SOFC), humid air turbine (HAT), and supercritical carbon dioxide (sCO2) cycle for power and heating. The energy available in the exhaust of SOFC is utilized in the HAT and in the sCO2 system for harnessing waste heat. Thermodynamic modeling of the combined system is done based on the first law of thermodynamics for analyzing the power output and heat available in the heat exchanger of the sCO2 cycle for process heating. Results have been obtained for parametric analysis of the combined system. Here, combined system efficiency attains the maximum value of 64.81% at considered operating conditions which is more than that of SOFC, HAT, or sCO2 cycle operating in isolation. The maximum power output is 1830.128 kJ/kg of air while the heat available for heating is found to be 536.86 kJ/kg of air. The results presented in this paper will help in understanding the advantages of the operation of the SOFC-HAT-sCO2 combined power and heating cycle.

Steady state simulation on combination of glazed and unglazed flat plate solar collectors for air heating

Unglazed flat plate solar collectors usually have lower costs and can have an advantage in situations where heat loss is low. In this study a steady state simulation of the combination of glazed and unglazed flat plate solar collectors for air heating was performed for various conditions of heat transfer coefficient (3, 12 and 21 W/m2-°C), inlet temperature (30, 45, 60 and 75°C) and solar irradiation (350, 600 and 850 W/m2). The efficiency of combined system of the glazed and unglazed solar collector area was better compared to the system which consists entirely of glazed at a low coefficient of heat transfer (3 W/m2-°C), except for high inlet temperature (more than 60°C) and low solar irradiation (350 W/m2-°C). For the combination of 6m2 unglazed and 9m2 glazed solar collector, the higher percentage of flow rate through unglazed collector slightly increases efficiency except at high inlet temperatures (> 60°C) for high heat transfer coefficient (12 and 21 W/m2-°C) and at low solar irradiation (350 W/m2). For air heating operations which are maintained in equilibrium conditions the temperatures of the air produced were slightly higher than that of the collector inlet.

Harvesting solar thermal energy with a micro-gap thermionic-thermoelectric hybrid energy converter: Model development, energy exchange analysis, and performance optimization

We present a comprehensive analysis of a dual, micro-gap thermionic-thermoelectric hybrid energy converter by developing a detailed theoretical model of the system. The space-charge and near-field effects in thermionic conversion and the temperature-dependent effects in thermoelectric conversion are considered while studying the energy flow through the cascade system. The temperatures and energy exchange channels in the different parts of the system are quantified using a self-consistent iterative algorithm considering the energy balance condition. The model is deployed to simulate the hybrid system performance when energized by a constant heat flux source which could, for example, be concentrated solar irradiance. The effect of solar radiation intensity on the combined system efficiency, and the dependence of the thermoelectric generator performance on the waste heat released from the thermionic top stage, are illustrated with several examples. Based on these analyses, a performance optimization of the hybrid system is carried out. The findings presented in this work offer useful insights into the hybrid generator operation. Moreover, the model developed can be used as a design tool for the practical implementation of such a hybrid system, or for extracting material- and device-related parameters from experimental data.

Thermo-economic analysis and optimization of a novel carbon dioxide based combined cooling and power system

Supercritical carbon dioxide is widely used in power plant and refrigeration system as working fluid. In this paper, a novel combined cooling and power system consists of supercritical carbon dioxide recompression Brayton cycle and transcritical carbon dioxide refrigeration cycle is proposed. In the novel system, the working fluid through the recompressor in supercritical carbon dioxide recompression Brayton cycle is partially replaced by the refrigeration compressor outlet working fluid in transcritical carbon dioxide refrigeration cycle. Mathematical models are established to simulate the combined system under steady-state conditions and the simulated results show that the reactor dominates the exergy destruction followed by the cooler and high temperature recuperator. Effects of five parameters, including turbine discharge pressure, cooler outlet temperature, evaporation temperature, cooling capacity and mass flow fraction are evaluated. The performances of the proposed combined system and the separation system are optimized and then compared from the perspective of thermodynamics. The calculated results show that the combined system’s comparative advantage over separation system will increase with increasing cooling capacity and decreasing evaporation temperature. The exergy efficiency of combined system is up to 2.45% and 5.87% higher than separation system when evaporation temperature is 273.15 K and 253.15 K, respectively. In order to balance the contradiction between the investment and system performance, the multi-objective optimization is conducted with exergy efficiency (to be maximized) and annual cost per heat consumption (to be minimized) as objective functions at different evaporation temperature. Non-dominated sorting generic algorithm-II is employed and Pareto frontier solution for the proposed system is given. Suggestions for engineering practice are provided based on the distributions of some key parameters on Pareto frontier solution.

Thermo-economic analysis of using an organic Rankine cycle for heat recovery from both the cell stack and reformer in a PEMFC for power generation

Distributed power generation is gaining attention as a solution for the transmission loss and site selection in centralized power generation. Polymer-electrolyte membrane fuel cells (PEMFCs) are suitable as a distributed power source for residential areas because of their high efficiency and low environmental impact. This study proposes a combined power generation system for recovering waste heat from both the cell stack and the reformer of a PEMFC by applying an organic Rankine cycle (ORC). The best working fluid with the highest ORC power output (i.e., the highest combined system efficiency) was identified through a parametric study of different working fluids. An economic analysis was also performed for different working fluids, waste heat sources, and types of system operation. The results show that the installation cost of the ORC can be recovered within the fuel cell's lifetime in all design cases. Greater cumulative profit can be generated by maintaining the same power output as the stand-alone PEMFC system for greater efficiency than when increasing the power output to sell surplus power. The results demonstrate that the optimal heat recovery from the PEMFC system is both thermodynamically and economically beneficial.

Thermodynamic studies and maximum power point tracking in thermoelectric generator–thermoelectric cooler combined system

Thermoelectric generator (TEG) operated thermoelectric cooler (TEC) is a highly compatible combination for low-cooling power application. The conventional TEG–TEC combined systems have low operating efficiency and low cooling power because maximum power output from the TEG is not fully utilized. This paper proposes and analyses the combined system with maximum power point tracking technique (MPPT) to maximize the cooling power and overall efficiency. This paper also presents the effect of TEG, TEC source temperature and the effect of heat transfer area in the performance of the combined system. The thermodynamic models of the combined system are developed in MATLAB simulink environment with temperature dependent material properties and analysed for variable operating temperatures. It has been found that, in the irreversible thermodynamic model of the combined system with MPPT, when the hot and cold side of TEG and TEC are kept at a temperature difference of 150K and 10K respectively, the power output of TEG increases from 20.49W to 43.92W, cooling power of TEC increases from 32.66W to 46.51W and the overall combined system efficiency increases from 2.606% to 4.375% respectively when compared with the irreversible combined system without MPPT. The characteristics improvements obtained by this practice in the combined system for the above mentioned operating conditions is also true for other range of operating temperatures. It is also been observed that the external irreversibilities decreases the cooling power and the overall system efficiency of the combined system by 36.49% and by 16.9% respectively.

Theoretical study on a novel ammonia–water cogeneration system with adjustable cooling to power ratios

A novel ammonia–water cogeneration system with adjustable cooling to power ratios is proposed and investigated. In the combined system, a modified Kalina subcycle and an ammonia absorption cooling subcycle are interconnected by mixers, splitters, absorbers and heat exchangers. The proposed system can adjust its cooling to power ratios from the separate mode without splitting/mixing processes in the two subcycles to the combined operation modes which can produce different ratios of cooling and power. Simulation analysis is conducted to investigate the effects of operation parameter on system performance. The results indicate that the combined system efficiency can reach the maximum values of 37.79% as SR1 (split ratio 1) is equal to 1. Compared with the separate system, the combined efficiency and COP values of the proposed system can increase by 6.6% and 100% with the same heat input, respectively. In addition, the cooling to power ratios of the proposed system can be adjusted in the range of 1.8–3.6 under the given operating conditions.

The combined UASB and MBR system to COD and TSS removal and excess sludge reduction for the treatment of high strength wastewater in various operational temperatures

There are many activated sludge plants (ASP) in Iran. Most of them are overloaded and as a result their efficiency is very low. A combined laboratory-scale system (5 L reactors) consisting of an up-flow anaerobic sludge blanket (UASB) and aerobic membrane bioreactor (MBR) was operated at 20 and 30°C with pH between 7.6 and 8.4. The experiment was run to optimize treatment of high-strength enriched municipal wastewater of the Ekbatan treatment plant located in west Tehran. The chemical oxygen demand (COD) of the wastewater was enriched due to the addition of molasses and milk powder. To prevent pH fluctuation of the influent, NaHCO3 and K2HPO4 were added to the wastewater. The excess sludge from the MBR was recirculated into the UASB. The system treated fortified municipal wastewater with a volumetric COD loading rate of 600, 1,200, 1,800, and 2,400 mg/L, and temperatures at 20 and 30°C. The results of the present work indicate an optimum organic loading (7.2–10.8 kg m−3 d−1) with COD removal efficiency (RE) between 74 and 85% in both examined temperatures in UASBR. The total combined system efficiency was, approximately, 98% in COD removal and 100% in total suspended solids removal. Furthermore, the nitrate RE was about 80%. Also, excess sludge reduction was over 90%, and the optimum hydraulic retention time for influent COD concentration of 1,200 mg l−1 is shown to be 4 h. This upgraded system increases the treatment capacity by factor of 5.

Thermionic energy converter : Hernqvist, K. G.; Kanefsky, M.; Norman, F. H., RCA Rev.19(2): 244–258, June 1958. Illus.

We present a comprehensive analysis of a dual, micro-gap thermionic-thermoelectric hybrid energy converter by developing a detailed theoretical model of the system. The space-charge and near-field effects in thermionic conversion and the temperature-dependent effects in thermoelectric conversion are considered while studying the energy flow through the cascade system. The temperatures and energy exchange channels in the different parts of the system are quantified using a self-consistent iterative algorithm considering the energy balance condition. The model is deployed to simulate the hybrid system performance when energized by a constant heat flux source which could, for example, be concentrated solar irradiance. The effect of solar radiation intensity on the combined system efficiency, and the dependence of the thermoelectric generator performance on the waste heat released from the thermionic top stage, are illustrated with several examples. Based on these analyses, a performance optimization of the hybrid system is carried out. The findings presented in this work offer useful insights into the hybrid generator operation. Moreover, the model developed can be used as a design tool for the practical implementation of such a hybrid system, or for extracting material- and device-related parameters from experimental data.