Evaporation Pressure Research Articles

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.

Molecular mechanisms of stomatal closure in response to rising vapour pressure deficit.

Vapour pressure deficit (VPD), the difference between the saturation and actual air vapour pressures, indicates the level of atmospheric drought and evaporative pressure on plants. VPD increases during climate change due to changes in air temperature and relative humidity. Rising VPD induces stomatal closure to counteract the VPD-mediated evaporative water loss from plants. There are important gaps in our understanding of the molecular VPD-sensing and signalling mechanisms in stomatal guard cells. Here, we discuss recent advances, research directions and open questions with respect to the three components that participate in VPD-induced stomatal closure in Arabidopsis, including: (1) abscisic acid (ABA)-dependent and (2) ABA-independent regulation of the protein kinase OPEN STOMATA 1 (OST1), and (3) the passive hydraulic stomatal response. In the ABA-dependent component, two models are proposed: ABA may be rapidly synthesised or its basal levels may be involved in the stomatal VPD response. Further studies on stomatal VPD signalling should clarify: (1) whether OST1 activation above basal activity is needed for VPD responses, (2) which components are involved in ABA-independent regulation of OST1, (3) the role of other potential OST1 targets in VPD signalling, and (4) to which extent OST1 contributes to stomatal VPD sensitivity in other plant species.

A calibrated organic Rankine cycle dynamic model applying to subcritical system and transcritical system

The organic Rankine cycle (ORC) technology is an attractive candidate for waste heat recovery and clean energy utilization. The operating condition of ORC system would change constantly under fluctuant heat source and a dynamic model is able to predict the transient behavior. The ORC dynamic model generally contains some unknown parameters while their identification is often ignored. This paper proposes a detailed calibration mechanism for ORC dynamic model including subcritical and transcritical system. Component models including pump, expansion valve and evaporator are well calibrated and the integrated calibrated system model is validated based on experimental data. Results indicate that calibrated model can capture the system dynamic behavior well. For key system state parameters such as working fluid evaporator pressure and outlet temperature, model prediction can achieve mean absolute relative deviation of less than 2% for both subcritical and transcritical system. Meanwhile, the conventional expansion valve model for transcritical system is found not efficient enough and a modification form is provided. This work can contribute to the calibration mechanism of similar thermodynamic system and enhance the confidence in model application.

Performance Analysis and Optimization of a Series Heat Exchangers Organic Rankine Cycle Utilizing Multi-Heat Sources from a Marine Diesel Engine.

Organic Rankine Cycle (ORC) is an effective way to recycle waste heat sources of a marine diesel engine. The aim of the present paper is to analyze and optimize the thermoeconomic performance of a Series Heat Exchangers ORC (SHEORC) for recovering energy from jacket water, scavenge air, and exhaust gas. The three sources are combined into three groups of jacket water (JW)→exhaust gas (EG), scavenge air (SA)→exhaust gas, and jacket water→scavenge air→exhaust gas. The influence of fluid mass flow rate, evaporation pressure, and heat source recovery proportion on the thermal performance and economic performance of SHEORC was studied. A single-objective optimization with power output as the objective and multi-objective optimization with exergy efficiency and levelized cost of energy (LCOE) as the objectives are carried out. The analysis results show that in jacket water→exhaust gas and jacket water→scavenge air→exhaust gas source combination, there is an optimal heat recovery proportion through which the SHEORC could obtain the best performance. The optimization results showed that R245ca has the best performance in thermoeconomic performance in all three source combinations. With scavenge air→exhaust, the power output, exergy efficiency, and LCOE are 354.19 kW, 59.02%, and 0.1150 $/kWh, respectively. Integrating the jacket water into the SA→EG group would not increase the power output, but would decrease the LCOE.

Multi-objective optimization of an innovative power-cooling integrated system based on gas turbine cycle with compressor inlet air precooling, Kalina cycle and ejector refrigeration cycle

In this paper, an innovative power-cooling integrated system based on gas turbine, Kalina cycle system, ejector refrigeration cycle (GT-KCS-ERC) is proposed. The ERC driven by GT flue gas and KCS low concentration liquid waste heat is utilized to precool GT inlet air for producing extra power from GT and provide some cooling capacity for users, simultaneously. The comprehensive thermodynamic and thermo-economic analyses are conducted to demonstrate the feasibility of novel GT-KCS-ERC hybrid system by comparing with standalone GT-KCS system. Furthermore, the effects of seven key operation parameters on the system performances are investigated. The multi-objective optimization of GT-KCS-ERC hybrid system and standalone GT-KCS system is carried out through Non-dominated Sorting Genetic Algorithm-II, in which the objectives are maximum total energy efficiency and minimum levelized cost of energy (LCOE). The results show that the total energy efficiency of GT-KCS-ERC increases with increasing pinch point temperature difference of boiler ΔTKCS,boi and decreasing ammonia concentration of working solution in KCS, while that of standalone GT-KCS shows opposite trends. The LCOE of GT-KCS-ERC is lower than that of standalone GT-KCS as ΔTKCS,boi is larger than 21℃ or turbine inlet pressure of KCS is higher than 6.6 MPa. The optimal saturated evaporator temperature and pressure ratio of vapor generator to condenser in ERC with optimal refrigerant of R290 are 0℃ and 4, respectively. Under the optimal condition, the GT-KCS-ERC with an ERC secondary flow split ratio of 0.162 for precooling GT inlet air presents 219.4 kW more net power and 764.2 kW more cooling capacity than standalone GT-KCS system. The LCOE decreases by 0.802% and total energy efficiency increases by 5.347% in novel GT-KCS-ERC system comparing with standalone GT-KCS system.

Optimal hydrocarbon based working fluid selection for a simple supercritical Organic Rankine Cycle

In this paper, several hydrocarbons' performance as a working fluid for a supercritical Organic Rankine Cycle are studied. Energy, exergy, and exergoeconomic analysis are conducted for the cycle operating with a low-grade waste heat source. Initially, fifteen different working fluids, primarily hydrocarbons, were considered based on their environmental characteristics, global warming potential, ozone depletion potential, flammability, and toxicity. The cycle efficiency for each working fluid was analyzed for different cycle operating pressures and different inlet heat source temperatures from 400 K to 500 K for a constant turbine work output. Three different trends of cycle performance characteristics were observed for various working fluids with the cycle operating at different evaporator pressures and source inlet temperatures. The working fluids that exhibited similar characteristic trends were then categorized into families for further analysis. Three hydrocarbons, i.e., Dimethyl Ether, R1234yf, and Diethyl Ether, were each selected as the representative of their respective family, along with Isobutane as the reference working fluid for comparison. After detailed energy and exergy analyses, Dimethyl Ether and Isobutane were found to be promising working fluids for the range of pressure and source inlet temperatures considered. Finally, the exergoeconomic analysis showed Dimethyl Ether to be the optimal working fluid based on the unit cost of net work produced and the total cost of exergy destruction, which were calculated to be (59.18 ± 0.49) $/MWh and (8.92 ± 0.48) $/h, respectively, when operating at 450 K.

Investigation of water column separation induced by pressure pulsation based on critical cavitation rate model

Significant pressure fluctuation may exists at the inlet of the draft pipe during load shedding of water pump and turbine sets. When the tail-water level is low, the minimum water hammer pressure at the inlet of the draft pipe set is higher than evaporation pressure; however, the overall pressure after superposition with pulsating pressure may reach the evaporation pressure in an instant. In these cases, it is unknown if a cavity is formed or water column separation is induced. It is also unknown how frequency and amplitude of the pulsating pressure affect the formation of cavity or water column separation. However, in terms of the physical formation of water column separation, liquid pressure reaching evaporation pressure is only the necessary condition of water column separation rather than a sufficient condition because the growth and aggregation of cavitations take time. In addition, water column separation could be induced only when the cavitation rate in the water reaches a specific value and gas-liquid relative motion occurs. In this study, based on the uniform cavitation distribution model, the critical of flow velocity gradients are calculated both in front and at the back of the section and are the sufficient condition of water column separation. This study uses the criterion of when the ratio of the vaporous cavitation volume to the volume of the pipe segment with a length of Δx exceeds a critical cavitation rate for classifying water column separation segments and non-water column separation segments. In water column separation segments, a concentrated vaporous cavitation model is used for calculation; however, dynamic meshes should be applied for tracking the change of vaporous cavitations. In the non-water column separation segments, the vaporous cavitation volume can be calculated according to the continuity equation but is converted into a cavitation rate in the pipe segment and substituted into gas-liquid two-phase equation to calculate wave velocity. Next, a case study was performed on a pipe-valve system. By taking into account the pulsating pressures with different frequencies and amplitudes on the downstream side in the valve closing process, water column separation and merging processes were analyzed and the change in flows, cavitation volumes and pressures on various sections and their laws during the transient process were concluded.

Thermodynamic analysis of a hydrogen fuel cell waste heat recovery system based on a zeotropic organic Rankine cycle

In this study, a thermodynamic model of a hybrid system consisting of a proton-exchange membrane fuel cell (PEMFC) system and an organic Rankine cycle (ORC) system is established using Aspen Plus software. The optimal working fluid is selected by comparing the performance of pure working fluids and zeotropic mixtures of working fluids in an ORC system. The zeotropic working fluid consisting of R245fa/R123 with a mixing ratio of 0.6/0.4 shows the best performance. The effects of some key parameters, including the current density and operating temperature, on the performance of the PEMFC–ORC system with zeotropic mixing fluids are discussed. The results indicate that the power and efficiency of the PEMFC–ORC system with the zeotropic mixing fluid increase with increasing operating temperature. When the R245fa/R123 zeotropic working fluid with a mixing ratio of 0.6/0.4 is used, the net power and efficiency of the hybrid system reach optimum values under the studied boundaries, with improvements of 12.87% and 4.84%, respectively. The influence of the maximum allowable evaporation pressure and stack operating temperature on the selection of optimum working fluid for the hybrid system is discussed in addition.

Improving Comfort and Air Conditioner Performance by Optimizing Controllers under Actual Usage Conditions

This study is to increase the efficiency of the air conditioning system under actual conditions of use. In order to improve the increase in energy consumption due to control without considering indoor cooling load fluctuations, we review it compared to conventional control methods in two respects. First, we examined the control method to reduce energy consumption by varying the evaporative pressure of the system according to the cooling load, and secondly, further reducing energy consumption by controlling the revolutions per minute (RPM) of the fan of the indoor unit at the same time as the pressure fluctuation under the cooling load. We found that changing the target pressure depending on the difference between the target temperature and room temperature can control temperature more efficiently and save energy than using fixed target pressure, and that the effect increases when applying the control of the air volume of the indoor air. Cleaning up the resulting values, the fan-load-control condition of the cooling-load-estimation control method showed an energy savings effect of 16.1–48.7% compared with the fixed-pressure control method and 1.2–37.7% compared with the cooling-load-estimation control method.

Proposal and analysis of a novel cooling-power cogeneration system driven by the exhaust gas heat of HCCI engine fuelled by wet-ethanol

A novel combined power and cooling cycle consisting of the ejector refrigeration cycle (ERC) and organic Rankine cycle (ORC) is proposed to enhance the overall efficiency of wet-ethanol fuelled homogeneous charge compression ignition (HCCI) engine. The proposed combined system of cooling-power cogeneration was simulated by Engineering Equation Solver (EES) software. Combined system responses to altering the operative conditions on the energy and exergy performances are ascertained to obtain guidance for system design. The results are computed for R134a, R290, and R600a working fluids. Increase in turbocharger pressure ratio from 2.5 to 3.5 raises the thermal efficiency of cooling-power cogeneration from 47.87% to 50.09% when R134a is used as working fluid. Cooling capacity and exergy of refrigeration are decreased by greater than 2.0% in case of R134a operated system when the vapor generator pressure is elevated from 1800 kPa to 2200 kPa. Increase in evaporator pressure of ERC from 327.4 kPa to 348.7 kPa is greatly beneficial to thermodynamic performance of cogeneration and its cooling capacity is improved by 11.34% when R134a is utilized as the working. When PEvap rises from 175.7 kPa to 186.9 kPa and R600a is employed as the working fluid, the cooling capacity is increased by 12.58%.

Synchrotron X-ray Analysis of the Influence of the Magnesium Content on the Absorptance during Full-Penetration Laser Welding of Aluminum

Full-penetration laser beam welding is characterized by a weld seam whose depth equals the material thickness. It is associated with a stable capillary and is therefore widely used for welding of sheet metal components. The realization of lightweight concepts in car body production requires the application of high-strength aluminum alloys that contain magnesium as an alloying element, which significantly influences the evaporation temperature and pressure. This change of the evaporation processes influences the geometry of the capillary and therefore its absorptance. In order to quantify the influence of magnesium on the capillary, their geometries were captured by means of high-speed synchrotron X-ray imaging during the welding process of the aluminum alloys AA1050A (Al99.5), AA5754 (AlMg3) and AA6016 (AlSi1.2Mg0.4). The 3D-geometries of the capillaries were reconstructed from the intensity distribution in the recorded X-ray images and their absorptance of the incident laser beam was determined by the analysis of the reconstructed 3D-geometry with a raytracing algorithm. The results presented in this paper capture for the first time the influence of the magnesium content in high-strength aluminum alloys on the aspect ratio of the capillary, which explains the reduced absorptance in case of full-penetration laser beam welding of aluminum alloys with a high content of volatile elements. In order to improve the absorptance in full-penetration welding, these findings provide the information required for the deduction of new optimization approaches.

Identifying the key system parameters of the organic Rankine cycle using the principal component analysis based on an experimental database

The organic Rankine cycle (ORC) is a promising technology for medium-and-low temperature heat utilization. However, the mechanism of how system parameters affect output have been investigated very little in the experimental aspect. Experimental investigation on the impact of each system parameter on system performance requires decoupling these system parameters. In this work, a series of experiments are conducted on a 10 kW scale ORC experiment setup. Statistical analysis is performed to identify a key parameter subset based on an experimental database. 6 system parameters, including temperature (Te) and pressure (pe) at the evaporator outlet, temperature (Tc) and pressure (pc) at the condenser inlet, expander shaft efficiency (ηSSE), and working fluid pump efficiency (ηP) are obtained. Combined with the ORC net power output and thermal efficiency, an experimental database of system operation conditions is constructed. Subsequently, the principal component analysis (PCA) of ORC is conducted based on the experimental database. Prediction models are developed using multi-linear regression (MLR), back propagation artificial neural network (BP-ANN), and support vector regression (SVR). Finally, accounting for the prediction performance of models and system parameter inter-correlation behavior, the key parameter subset is determined with the exhaustive feature selection method. The results imply that the key parameter subset is (pe, ηP, pc, ηSSE). Further removing or including more system parameters would reduce the accuracy of prediction models. In addition, the MLR models are slightly less accurate than the more sophisticated BP-ANN and SVR models.

Multi-criteria assessment and optimization of waste heat recovery for large marine diesel engines

The energy crisis combined with increasingly severe environment pollution have highlighted the significance of the energy conservation and emission reduction in shipping industry. To improve the energy utilization rate and reduce pollution, a dual-pressure organic Rankine cycle system is proposed to reuse the heat of exhaust gas in marine engines. Many researches focused on the thermodynamics and thermos-economics of the waste heat recovery system, and optimized it from these two goals to recover more energy and decrease cost. However, the impact of the system itself on the environment, which was significant to assess the greenness, was often neglected in these studies. The system with high energy recovery rate may also cause serious environment pollution in the manufacturing or operation process, hence the environmental factor should be taken into account in the optimization. To solve the problems, an evaluation method combined thermodynamics, economics and environment is proposed. The parametric study of the dual-pressure organic Rankine cycle system with six common used working fluids is assessed by this method. Finally, taking the indicators of evaluation method as objective function, the multi-objective optimization method is conducted to figure out the optimum operation conditions for the system. Results show cyclopentane is the optimal working fluid considering thermodynamic, economic and environmental properties. Under the conditions of the evaporation pressure in high-pressure circuit 2.79 MPa, the evaporation pressure in low-pressure circuit 1.97 MPa, the condensation temperature 35 °C, the superheating temperature 1.2 °C, the system obtains a good performance with exergy efficiency 60.24%, electricity production cost 0.167 $/kWh and the environmental impact of net power output 11831.9 mPts/GJ.

Evaporation/Condensation Heat Transfer and Pressure Drop of R1336mzz (E) and Its Mixture Working Fluid for High-temperature inside Horizontal Microfin Tubes

Research is currently in progress on developing and improving efficiency of high-temperature heat pumps and organic Rankine cycle power systems, both of which generate high-temperature steam by effectively utilizing low-quality heat sources from ship engines and factories for industrial purposes. From an environmental point of view, development efforts are also underway to replace low-pressure working fluids used in these systems with new alternative working fluids with low global warming potentials (GWPs). The purpose of this study is to experimentally clarify the evaporation and condensation flow characteristics of alternative low-GWP working fluids inside two types of horizontal microfin tubes with outer diameters of 9.5 mm. The Hydrofluoroolefin (HFO) working fluids R1336mzz(E) and R1336mzz(Z), both of which are expected to replace the working fluid R245fa that is used widely at present, two pure components and their mixture were used as test fluids in this study. In this experiment, the authors measured the evaporation and condensation heat transfer coefficients and pressure drop under the conditions of 50-300 kg/(m2s) mass velocity, a saturated evaporation temperature of 40 °C and a saturated condensation temperature of 60 °C. The results of the experiment clarified the effects of mass velocity and fin geometries on the evaporation and condensation heat transfer and pressure drop characteristics of R1336mzz(E), R1336mzz(Z) and their mixture working fluid. In addition, comparisons were made between the evaporation and condensation heat transfer coefficients and pressure drop of these working fluids and those of R245fa to highlight differences in their performance. Furthermore, the measured values were correlated with previous correlations for evaporation and condensation heat transfer for microfin tubes to verify the applicability of these predicted correlations.

Technical research on vacuum distillation to purify magnesium to 99.99% purity

In this study, the distribution and evaporation principles of impurities in distilled magnesium metal were investigated using a low vacuum (8 × 104 Pa) distillation purification experiment and theoretical analysis. Provided all other factors remain unchanged (distillation time, pressure, and total area of evaporation), the optimum temperature for preparing high-purity Mg (99.99%) is 750 °C. A detailed analysis of purified Mg was obtained by inductively coupled plasma mass spectrometer (ICP-MS) for 10 major impurity elements. After distillation at 750 °C, low vapor pressure impurities in condensed magnesium, including Si, Mn, Al, Fe, Cu, Ni, and Sn were significantly reduced as other impurities were slightly reduced. Our analysis confirmed a decrease in the following impurities: Fe, Si, Mn, Cu, and Al were reduced from 21.8, 78.6, 68.4, 4.4, and 39.4 ppm to 1.2, 9.3, 6.0, 1.0, and 5.4 ppm, respectively; satisfying the 99.99% Mg standard. The evaporation rate and separation coefficient were calculated under experimental conditions. To better describe the distillation process of metallic magnesium under low vacuum conditions, the mean free path is also calculated in this study under actual conditions.

Capillary, Film, and Vapor Flow in Transient Bare Soil Evaporation (2): Experimental Identification of Hydraulic Conductivity in the Medium to Dry Moisture Range

AbstractBare‐soil evaporation involves coupled flow of liquid water, water vapor, and heat. As evaporation results in non‐isothermal conditions in the soil, the temperature dependence of transport properties and thermal fluxes of water and vapor must be accounted for. In a companion paper, we showed that the Richards equation, that is, a single‐phase flow model assuming isothermal conditions, is applicable to accurately determine soil hydraulic properties including the medium to dry range from evaporation experiments by inverse modeling. This is warranted if pressure head data across a wide moisture range, that is, from almost saturated to almost air‐dry, are used in the objective function and a suitable parameterization of the hydraulic conductivity function including vapor and non‐capillary flow is used. In this article, we confirm the theoretical results by examining real evaporation experiments, in which we measured the temporal dynamics of evaporation rate, soil temperature, and pressure head in laboratory soil columns. Pressure head was measured with mini‐tensiometers and relative humidity sensors. The measurements were evaluated by inverse modeling with the Richards equation assuming isothermal conditions and ambient temperature in the soil. Our results for a sandy and a loamy soil show that the observed transient water and vapor dynamics in the drying soil could be accurately matched, provided the hydraulic conductivity curve considered isothermal vapor diffusion and film flow. These components dominate hydraulic conductivity in the medium to dry soil moisture range and were uniquely identified in agreement with the theoretical analysis in the companion article.

Effect of ambient temperature organic Rankine cycle geothermal power plant with Aspen plus

This study concerns a thermodynamic analysis of the Organic Rankine Cycle for energy conversion from geothermal resources. Some thermodynamic variable was simulated by using Aspen plus. The equation of state is expressed by the Peng-Robinson formulation. Analysis carried out considering the variance of ambient temperature (27°C, 30°C, 33°C, 35°C, and 40 °C) on air-cooled in the condenser. Result is obtained for several set operating parameters, such as the evaporation and condensation pressure for the variance of ambient temperature, the mass flow rate of the geothermal fluid. From results, the ambient temperature has affected for power generated of ORC. The lower ambient temperature then will increase the power generate of the system.

Optimization and Analyses of an Organic Rankine Cycle System Utilizing Cold Energy of Liquefied Argon

In order to study the feasibility of liquefied argon cold energy recovery by organic rankine cycle (ORC), an ORC system cooling by liquefied argon was proposed and the mathematical model was established in this paper. The thermophysical properties of several organic working substances were compared, and R218 demonstrates the best circulating performance under working condition. The operating characteristics of the ORC system under various parameters were analyzed and optimized. When the condensing pressure is 13.6kPa, the system output power reaches the peak value of 1.003kW. With the raising of the evaporation pressure, the net output power and the thermal efficiency of the ORC both increase. The raising of ambient temperature can effectively promote the circulating working substance in the evaporator to absorb heat from the air. The working substance at the exit of condenser under saturated state is propitious to the net output power of the system.

A Novel Oil-free Dual Piston Compressor Driven by a Moving Coil Linear Motor with Capacity Regulation Using R134a

Improving compressor efficiency is very important to save energy and reduce greenhouse gas emissions. A novel oil-free dual piston compressor prototype driven by a moving coil linear motor was developed, and its working principle was described in detail. The prototype was integrated with a test rig to measure the operation characteristics, the compressor efficiencies and the coefficient of performance (COP). The results show that the dual piston structure results in extraordinary sinusoidal gas force and electromagnetic force and significantly reduces piston offset, which is completely different from the traditional single piston structure. Compared with the variable frequency method, the variable stroke method has lower energy consumption and a higher COP, which is more suitable to cooling capacity regulation for the prototype. The maximum COP, motor efficiency and volumetric efficiency are 5.34, 87.9% and 79.1%, respectively, under the design condition (the evaporation pressure is 0.35 MPa, and the pressure ratio is 2.54). The COP of the linear compressor is 38%, 24% and 12% higher than the commercial crank-driven reciprocating compressor at the pressure ratios of 2.54, 2.80 and 3.90, respectively, which reflects the efficiency advantage of the dual piston linear compressor in household refrigeration.

Effects of superheat degree and wettability on droplet evaporation time near Leidenfrost point through Lattice Boltzmann simulation

The evaporation of droplet on a superheated surface is a very important and common phenomenon in industrial production, such as spray cooling and metallurgical treatment. In this paper, a two-dimensional numerical simulation based on Lattice Boltzmann Method has been adopted to analyze the evaporation behavior of liquid droplet on an over-heated surface. The effects of superheat degree (Ja number) and surface wettability on droplet evaporation are analyzed respectively. During the evaporation process, the evaporation pressure at the bottom of the droplet exhibits periodic oscillation corresponding to bouncing movement. The oscillation gradually decays in the case of low Ja numbers, but for high Ja numbers, the oscillation doesn't decay. Leidenfrost vapor layer is more likely to be generated on a hydrophobic surface. Meanwhile, the comprehensive influence of wettability and Ja number on droplet evaporation time was studied systematically. On hydrophilic surfaces, there is a critical Ja value, around which the evaporation time exhibits a U-type tendency. However, the evaporation time continues to decrease as Ja number increases on hydrophobic surfaces.