Maximum Thermodynamic Efficiency Research Articles

Optimal Values of Power Equipment Parameters when Degassing of Solid Municipal Waste Polygons

The possibility of using the energy values of landfill gas to generate electrical and thermal energy based on a contact cogeneration gas turbine installation has been studied. The optimal values degree of increase pressure in the cycle are determined, which make it possible to determine the maximum specific electrical power and thermodynamic efficiency of a cogeneration gas turbine installation with steam injection, in which landfill gas from solid waste landfills is used as an energy carrier. It is concluded that using the proposed engineering solution, landfill gas from municipal solid waste can be usefully implemented in a cogeneration contact gas turbine installation, which will save the country's national wealth and at the same time solve pressing problems of modern cities associated with the disposal of municipal solid waste.

INVESTIGATION OF THE EFFECT OF OPERATING PARAMETERS ON NERNST VOLTAGE IN HYDROGEN-OXYGEN FUEL CELLS

In hydrogen-oxygen fuel cells, operating parameters have an influence on the maximum expected open circuit (Nernst) voltage. Even though fuel cells have been the subject of many research, none of them have theoretically investigated the impact of various operating parameters, particularly concerning Nernst voltage and maximum thermodynamic efficiency. In this study, a computer program was developed to theoretically determine the effect of various operating parameters on the Nernst voltage in hydrogen-oxygen fuel cells. This computer program was developed in MATLAB to mathematically examine the effects of hydrogen and oxygen mole ratios, anode and cathode pressures, and operating temperatures on the maximum expected open circuit voltage. When calculating Nernst voltages and maximum thermodynamic efficiency for fuel cell reactions containing water as a by-product, the effects of higher heating value (HHV) and lower heating value (LHV) are also considered in the solutions. As a result, it was also concluded that temperature increase reduces the fuel cell Nernst voltage and maximum thermodynamic efficiency. Therefore, it was observed from the figures that the best conditions for the Nernst voltage occur when HHV is assumed, the temperature is 353 K, the mole ratios of hydrogen and oxygen are 1.0, the anode and cathode pressures are 5 atm and 6 atm, respectively. In terms of thermodynamic efficiency, it was determined that there was a maximum increase of 92.2% in the LHV assumption compared to the HHV assumption at the temperature of 1000 K, provided that other operating parameters were kept constant.

Quantum Interference Enhancement of the Spin-Dependent Thermoelectric Response.

We investigate the influence of quantum interference (QI) and broken spin-symmetry on the thermoelectric response of node-possessing junctions, finding a dramatic enhancement of the spin-thermopower (Ss), figure-of-merit (ZsT), and maximum thermodynamic efficiency (ηsmax) caused by destructive QI. Using many-body and single-particle methods, we calculate the response of 1,3-benzenedithiol and cross-conjugated molecule-based junctions subject to an applied magnetic field, finding nearly universal behavior over a range of junction parameters with Ss, ZsT, and reaching peak values of , 1.51, and 28% of Carnot efficiency, respectively. We also find that the quantum-enhanced spin-response is spectrally broad, and the field required to achieve peak efficiency scales with temperature. The influence of off-resonant thermal channels (e.g., phonon heat transport) on this effect is also investigated.

(Invited) Atmospheric Plasmas Synthesized Nanocrystals with Quantum Confinement and Quantum Hybrids in Photovoltaics

Nanocrystals share lot of advantages of organics namely scalable and controlled synthesis, an ability to be processed in solution while additionally retaining the broadband absorption and superior transport properties of traditional photovoltaic semiconductors. Nanocrystal solar cells have the potential to considerably increase the maximum attainable thermodynamic conversion efficiency (> 50%). Nanocrystal solution-processed can be used in solar cell structure not only as an absorber but also as electron and hole transport layer where the HOMO and LUMO levels can be efficiently controlled by size and/or plasma induced surface engineering directly in colloidal solution. Solution-processed and surface engineered nanocrystals with quantum confinement can be then further used to fabricate new class of quantum hybrids when blended for instance with polymers or perovskites and serves as absorbing and/or e-h transporting material. In this presentation, we overview the atmospheric plasma-based approaches to synthesis and surface engineering of nanocrystals with quantum confinement. We will compare surface engineering by fs laser processing in liquid solutions and synthesis of nanocrystals with strong quantum confinement by atmospheric plasmas. Moreover, to understand the thermal stability of nanocrystals observed experimentally, we calculate the cohesive and the formation energies of nanocrystals by means of first-principle calculations. Finally, we overview our recent progress in integration of surface engineered nanocrystal as a quantum hybrids incorporated within perovskites solar cells.

Thermodynamic investigations on internal generator cooling for hermetic expanders in Organic Rankine Cycles

Organic Rankine Cycle (ORC) technology is an effective technical path to realize power generation from low or medium grade energy. Utilizing hermetic expanders with internal generators has become an important development direction of ORC system because the machines are easy to get full tightness. However, due to the high temperature of the gas in expander, it will be very difficult to cool the internal generators in hermetic expanders. This paper introduces three schemes for cooling the internal generators in hermetic expanders, and establishes corresponding thermodynamic models as well as quantitatively analyzes the influences of each cooling scheme on ORC system. The simulation results show that the spray cooling scheme has the least influence on the thermodynamic efficiency of the system whether R245fa or R123 is used as the working fluid, and the maximum thermodynamic efficiency drop of 0.21% caused by cooling the generator is much smaller than the other two schemes. In medium-high temperature conditions, both spray cooling and exhaust-spray cooling schemes can effectively reduce the generator temperature. However, the latter has a much larger impact on the thermodynamic efficiency of the system than the former one. The conclusions can provide a guide for the design of hermetic expanders.

Thermodynamics of Hurricanes Revisited

Hurricanes and tropical storms are heat engines operating between warm tropical oceans and the cold upper troposphere. The purpose of this article is to examine the existing theories for hurricanes and tropical storms, and to discuss their validity. It is argued that contrary to previous claims that hurricanes are Carnot engines, these systems operate at efficiencies considerably below their maximum thermodynamic efficiency. As such, the validity of the current theories of thermodynamics of hurricanes remains questionable, and the phenomenon continues to be a geophysical enigma.

Phase transition in hydrophobic weak polyelectrolyte gel utilized for water desalination

By this article, we continue studying weak polyelectrolyte hydrogels and their application as a desalination agent. We modeled the desalination process as a four-step reversible thermodynamic cycle transferring ions from a low salinity solution to a high salinity one. The cycle implies reversibility at any stage, therefore the method can achieve the maximum thermodynamic efficiency, comparable to the reverse osmosis. As a driving force for ions movement, we use the fact that compression of the gel leads to a decrease in the gel ionization degree, and therefore to a release of ions entrapped in the gel. We considered the gel composed of ionogenic units modified by hydrophobic pendants and showed that this modification significantly increases the number of transferred ions. This increase is caused by a first-order phase transition originated from an interplay between repulsive electrostatic and attractive steric interactions. The transition happens during hydrogel compression. At a certain pressure, the gel collapses abruptly, changing its volume almost to that of a dry state, and releases almost all ions collected inside. Employing the phase transition allows us to model the desalination cycle, which transfers a much larger number of ions and works at rather low pressures <10bar.

Regulation of ATP hydrolysis by the ε subunit, ζ subunit and Mg-ADP in the ATP synthase of Paracoccus denitrificans

F1FO-ATP synthase is a crucial metabolic enzyme that uses the proton motive force from respiration to regenerate ATP. For maximum thermodynamic efficiency ATP synthesis should be fully reversible, but the enzyme from Paracoccus denitrificans catalyzes ATP hydrolysis at far lower rates than it catalyzes ATP synthesis, an effect often attributed to its unique ζ subunit. Recently, we showed that deleting ζ increases hydrolysis only marginally, indicating that other common inhibitory mechanisms such as inhibition by the C-terminal domain of the ε subunit (ε-CTD) or Mg-ADP may be more important. Here, we created mutants lacking the ε-CTD, and double mutants lacking both the ε-CTD and ζ subunit. No substantial activation of ATP hydrolysis was observed in any of these strains. Instead, hydrolysis in even the double mutant strains could only be activated by oxyanions, the detergent lauryldimethylamine oxide, or a proton motive force, which are all considered to release Mg-ADP inhibition. Our results establish that P. denitrificans ATP synthase is regulated by a combination of the ε and ζ subunits and Mg-ADP inhibition.

Modeling and optimization of an ocean thermal energy conversion system for remote islands electrification

Electrification of remote zones is often characterized by high energy costs as a result of the fossil fuels supply and cost associated to its logistic. This study performs a theoretical model and optimization of an ocean thermal energy conversion (OTEC) system coupled to an organic Rankine cycle (ORC) generator for small scale applications in the San Blas archipelago (Panama). The gross electric power has been previously set at 125 kW for the eleven working fluids selected: ammonia, R152a, R1234yf, R1234ze, R125, R134a, R161, propane, isobutene, RE143a and decafluorobutane. Results show R1234yf gets the maximum thermodynamic and net electric efficiency (3.60% and 2.57%, respectively). Ammonia reaches the maximum net electric power (99.3 kW) and, thus, the lowest pumping losses (20.59% of the gross). Besides, despite decafluorobutane shows slightly lower electric power (98 kW) and efficiencies, this fluid does not present environmental hazardous features. Sensitivity analyses show that all performance parameters of the plant are strongly affected by deep and surface seawater temperature variation. Finally, for a surface seawater temperature of 30 °C and deep 5 °C, the net electric power reached is 94.6 kW for R1234yf, 99.3 kW for ammonia and 98.0 kW for decafluorobutane. The net electric efficiency is 2.57%, 2.53% and 2.42%, and the total area required by the heat exchangers is 890 m2, 940 m2 and 986 m2, respectively.

Theoretical limits of multiple exciton generation and singlet fission tandem devices for solar water splitting.

Photoelectrochemical (PEC) water splitting is one of the most important approaches being investigated for solar fuel generation. In this study, we determine the maximum thermodynamic power conversion efficiencies (PCEs) of PEC water splitting two-bandgap tandem devices that produce multiple carriers per photon absorbed via Multiple Exciton Generation (MEG) or Singlet Fission (SF) and in the presence of solar concentration. Here, we employ a detailed balance thermodynamic analysis to determine the effects of top cell thickness, solar concentration, carrier multiplication, electrode overvoltage (VO), and water absorption on PEC power conversion efficiency for water splitting cells. We have found a maximum PEC power conversion efficiency of 62.9% in cells using two ideal tandem MEG absorbers with bandgaps of 0.3 and 1.2 eV at 1000-suns solar concentration and 0 overvoltage; the maximum PCE for two tandem SF absorbers under the same conditions is nearly the same at 59% with the same values for the absorption thresholds. A very interesting and important result was that, upon thinning the top cell, the range of viable bandgaps for both the top and bottom cells is extended by as much as 0.5-1 eV while still maintaining high maximum conversion efficiency (60-63%). The effects of imposing different solar concentrations from 1X to 1000X and having different tandem configurations of SF and MEG layers were also studied.

Gaseous and particle emissions in low-temperature combustion diesel–HCNG dual-fuel operation with double pilot injection

Alternative fuels and energy vectors are becoming increasingly important in terms of technical, geopolitical, economic, and environmental aspects. In particular, gaseous fuels and vectors, such as fossil or synthetic natural gas (NG) blended with hydrogen, commonly help provide optimal strategies to reduce global and toxic emissions of internal combustion engines, owing to their adaptability, anti-knock capacity, lower toxicity of pollutants, reduced CO2 emissions, and cost-effectiveness. However, diesel engines still represent the reference category among internal combustion engines in terms of maximum thermodynamic efficiency. The possibility offered by dual-fuel (DF) systems to combine the efficiency and performance of diesel engines with the environmental advantages of gaseous fuels has been the subject of extensive investigations. However, the simple replacement of diesel fuel with gaseous fuel does not allow for optimising the engine performance, owing to the high percentage of unburned gaseous fuel, which compromises the potential reduction of CO2; therefore, more complex combustion strategies should be realised. In this study, with the aim of improving the DF combustion process, an experimental investigation was performed to analyse low-temperature combustion (LTC), using NG and two enriched hydrogen-compressed NG blends as primary fuels. The LTC mode was activated by means of a very early advanced pilot injection and carried out in two close steps. The double pilot injection was used to control the energy release rate in the first combustion stage, thereby minimizing the increase of the rate of pressure and allowing the extension of the operation range under LTC. The experimental activity was also focused on analysing the particle emissions, as it is well known that these emissions, together with those of nitrogen oxide, constitute the main pollutants resulting from diesel fuel combustion. The results demonstrated the potential to reduce the unburned fuel, NOx, and particle emissions simultaneously, while maintaining equivalent CO2 emissions to a diesel-only engine. Both the timing and pressure of the pilot injection proved to be critical parameters for optimising the emissions and performance.

A Gibbs energy-driving force method for the optimal design of non-reactive and reactive distillation columns

A simple Gibbs energy-driving force method for the design of non-reactive and reactive distillation columns has been developed. Based on the binary driving force concept and the equilibrium Gibbs energy computation, a systematic procedure for the design of non-reactive distillation columns (NRDC) and reactive distillation columns (RDC) is proposed. The design method exploits the connection between the driving force values and the equilibrium Gibbs energy to determine the number of stages, the optimal feed location, and the heat required at the top and bottom of the column and the minimum reflux ratio. The final design guarantees an optimal operation since maximum thermodynamic efficiency is achieved. The maximum thermodynamic efficiency criterion is equivalent to the minimum entropy condition required for a stable operation of the distillation columns. The method is applied for the design of two non-reactive systems: a) Benzene-Toluene ideal system and b) Ethanol-Water non-ideal system. A reactive distillation column considering the isomerization of n-butane in the presence of an inert compound is designed. The optimal thermal feed condition obtained through the maximum separation efficiency guarantees that the final designs obtained correspond to the minimum energy requirements for the design target of separation.

A thermodynamic and exergoeconomic numerical study of two-stage annular thermoelectric generator

A three-dimensional simulation of a two-stage annular thermoelectric generator is conducted to investigate the thermodynamic and the exergoeconomic performance of the thermoelectric device. The simulations are carried out considering the influence of Thomson effect and temperature-dependent material properties of thermoelectric legs to improve the precision of the study. The effect of two geometrical parameters, the height ratio of thermoelectric stages and angle ratio of legs as well as the influence of heat source temperature on the energy, exergy and economic performance of two-stage ATEG is studied. Results indicate that for a range of heat source temperatures, the output power, conversion efficiency and exergy efficiency of two-stage ATEG are higher compared to single-stage ATEGs based on the low-temperature and high-temperature materials. Moreover, the exergoeconomic study reveals that the single-stage ATEG performs more economical than the two-stage ATEG in all of the studied heat source temperatures. The amount of heat source temperature has a direct influence in determining the proper height ratio to reach the maximum thermodynamic and exergoeconomic efficiencies. Also, it is found that the angle ratio of 1 leads to better energy and exergy performance and least unit cost of output power.

Maximum thermodynamic electrical efficiency of fuel cell system and results for hydrogen, methane, and propane fuels

The maximum electrical efficiency of fuel cell system, ηemax, is important for the understanding and development of the fuel cell technology. Attempt is made to build a theory for ηemax by considering the energy requirement of heating the fuel and air streams to the fuel cell operating temperature T. A general thermodynamic analysis is performed and the energy balances for the overall operating processes of a fuel cell system are established. Explicit expressions for the determination of ηemax are deduced. Unlike the Carnot efficiency, ηemax is found to be fuel specific. Except for hydrogen fuel, chemical equilibrium calculations are necessary to compute ηemax. Analytical solutions for the chemical equilibrium of alkane fuels are presented. The theoretical model is used to analyze the effects of T and the steam contents of CH4, C3H8, and H2 on ηemax for systems with various degrees of waste heat recovery. Contrary to the common perception concerning methane and propane fuels, ηemax decreases substantially with the increase of T. Moreover, ηemax of hydrogen fuel can be higher than that of methane and propane fuels for a system with a medium level of waste heat recovery and operated at 700 ℃≤T≤900 ℃.

Thermodynamic analysis of organic Rankine cycle with Hydrofluoroethers as working fluids

This paper presents the analysis of organic Rankine cycle (ORC) using hydrofluoroethers (HFEs) such as HFE7000, HFE7100 and HFE7500 as working fluids under external conditions. HFE's has been chosen over chlorofluorocarbons (CFC's) and hydro chlorofluorocarbons (HCFC's) as it is environmentally friendly. Both the CFC's and the HCFC's possess ozone depletion potential (ODP), while the hydrofluorocarbons (HFC's) have relatively significant global warming potential (GWP). The HFE's that have excellent thermophysical properties and low toxicity can be recommended as a long term solution to the environmental issues. The HFE's have zero ODP and very low GWP compared to the CFC's, HCFC's and HFC's. A thermodynamic model has been developed using Engineering Equation Solver (EES) software to simulate the system under steady state conditions. Parametric analysis is conducted to examine the effects of some thermodynamic parameters on the system performance using different working fluids. When turbine inlet temperature was varied from 70°C to 110 °C keeping condensation temperature fixed at 28 °C, HFE7000 produces the maximum thermodynamic efficiency and performs better in view of the net work output under the given working conditions. However, when evaporation pressure was kept constant at 1.2 bar and condensation temperature was varied from 20°C to 30 °C, HFE7500 produced the maximum efficiency of 12.3% in comparison with 7.6% for HFE7100 and 4.1% for HFE7000. The work demonstrates the use of hydrofluoroethers as working fluid in ORC.

Thermodynamic model for a reversible desalination cycle using weak polyelectrolyte hydrogels

The recently proposed use of hydrogels for water desalination is based on the decrease of salt concentration in the gel upon compression. In the first experiments, desalination cycles using hydrogels involved an irreversible mixing step, which inevitably reduced the thermodynamic efficiency. This approach could become competitive with membrane-based desalination methods if it could work close to maximum thermodynamic efficiency. In this work, we develop a thermodynamic model for compression of weak polyelectrolyte hydrogels in open and closed systems. We use this model to design a fully reversible desalination cycle which can, in principle, achieve maximum thermodynamic efficiency.We also show that compressing weak polyelectrolyte hydrogels at low salinity decreases their ionization, thereby leading to a non-monotonic dependence of salt concentration on the gel compression. Therefore, our model shows how to redesign the desalination cycle when using weak polyelectrolytes at low salinities.

Economic optimization of Organic Rankine cycle with pure fluids and mixtures for waste heat and solar applications using particle swarm optimization method

The optimization criterion for designing the thermodynamic layout of an organic Rankine cycle is often based on either achieving maximum thermodynamic efficiency or incurring minimum initial specific investment costs. Such designs, however, need not lead to the maximum utilization of waste heat potential or an optimal investment. For full potential utilization of a waste heat source, its temperature should be brought down to near ambient temperatures via transfer of enthalpy to the organic Rankine cycle working fluid. In the limit, however, pursuit of complete source utilization may lead to capital intensive organic Rankine cycle layouts that demand infinitesimal temperature gradients in heat exchangers leading to massive heat transfer areas. This paper defines a new objective function that reveals the tradeoffs between specific investment cost and the extent to which waste heat is utilized. A particle swarm optimization algorithm is used to optimize 7 and 8 dimensional search space for pure and mixture based working fluids, respectively, for case studies involving power capacities of 5, 50 and 500 kWe, waste heat source temperatures ranging from 75 to 275 °C and a number of working fluids. As a practical aid to designers, a methodology for generating high isentropic efficiency scroll geometries corresponding to optimized cycles is presented, and the optimization analysis is further extended to solar thermal applications.

A genset and mini-photobioreactor association for CO2 capturing, enhanced microalgae growth and multigeneration

Abstract A multigeneration system is proposed to recover waste heat from a genset driven by a diesel engine, and capture CO2 in its emissions for microalgae growth. Scenedesmus sp. and an algae mixture were separately cultivated in 20-L jugs by supplying two CO2 sources: air and diesel engine emissions. Microalgae growth rates were determined from absorbances and analyzed to construe whether emissions in lieu of air enhanced the microalgae growth. Also, a mini-photobioreactor (mPBR) was employed to grow local algae mixture with air and emissions as CO2 sources. The experimental results demonstrated that diesel engine emissions increased the growth of both Scenedesmus sp. and local algae mixture. The multigeneration system thermal efficiency was defined as the sum of the total electrical power produced for cultivation in PBR, biomass harvesting, flocculation, separation, drying and oil extraction, followed by biodiesel production via transesterification reaction, plus the waste heat recovery and the heat generation rate potential of the produced biodiesel, divided by the heat input rate due to the combustion of consumed diesel fuel. In the absence of waste heat recovery, maximum thermodynamic efficiency of the system was 26%, while it increased to 36.2% with waste heat recovery and microalgae biodiesel, i.e., by 39.2%.

Rigorous thermodynamic evaluation of the extractive distillation process

The extractive distillation process has received special attention from researchers due to its high energy consumption and low thermodynamic efficiency. However, the thermodynamic performance has been approached in the literature in a superficial way and it is common to affirm that intensifying processes increases the thermodynamic efficiency without presenting quantitative results. The main objective of this work is to determine the operational conditions that result in maximum thermodynamic efficiency of the extractive distillation process (global optimum). Thus, the process was divided into three blocks in order to identify the irreversibility sources. The results show that the use of a solvent to enable the separation is the main cause of the low thermodynamic efficiency of the extractive distillation process, and depending on the chemical system and the operating condition, negative values ​​for the extractive column are observed. In order to make the thermodynamic analysis more didactic, a methodology to calculate the individual and overall lost work is proposed, which allows for identifying and quantifying, as well as where to act in order to increase the thermodynamic efficiency of the separation. The azeotropic binaries used as case studies consisted of ethanol (ETOH), water (H2O), acetone (ACET) and methanol (METOH), while the solvents used to promote separation were ethylene glycol (EG) and dimethyl sulfoxide (DMSO), respectively.

Energy, economic and environmental (3E) aspects of internal heat exchanger for ORC geothermal power plants

Installing Internal Heat Exchanger (IHE) after the turbine, can increase the effectiveness of heat exchange process and consequently plant efficiency in the Organic Rankin Cycles (ORCs). The main objective of this study is to investigate and also compare the energy, economic and environmental issues for two different configurations of geothermal binary cycles including conventional ORC and ORC with Internal Heat Exchanger (IHE-ORC). Energy-Economic-Environment (3E) models for both cycles were developed and a parametric study was conducted with respect to the maximum thermodynamic efficiencies and minimum cost rate. Evaporator pressure, superheating degree of the steam, the minimum pinch difference temperature in the evaporator and also condenser temperature were analyzed to obtain the optimal performance of the system. The aim of this study was to investigate the effect of IHE in binary cycles on the exergy destruction rates for all components and finding the most sensitive equipment to IHE. The results indicated that under optimized economic conditions, IHE-ORC had higher net power output (5245 vs. 5063 kW) with lower production cost. Furthermore, IHE-ORC, had superior energy and exergy efficiencies (16.82%, 59.71% vs. 14.13%, 52.2%). Additionally, IHE system was more environmental friendly and in comparison with conventional ORC, it can save the fuel consumption by 3.6% higher value and consequently about 3.6% reduces the CO2 emission more than conventional ORC.