•Steam accumulator storage provides primary and secondary power control.•Accumulated steam replaces turbine extractions for two condensate heaters.•Steam discharge from 600 m3 accumulator provides ...27.3 MWe for 27 min.•Additional 11.5 MWe power increase is available for 13 min.•Control reserve by fresh steam throttling reduces by 4.2% of 650 MWe nominal power.
Thermal power plants are forced to operate under frequent load changes in energy systems with increased share of solar and wind power plants. These load changes increase thermal power plant aging and reduce their efficiency. Therefore, solutions for energy storage are sought in order to accumulate energy in periods of lower power demand and produce excess power in periods of increased demands. In the present paper the steam accumulator as the thermal energy storage device is applied in a 650 MWe coal-fired thermal power plant to increase its flexibility under the demand for the variable load changes. The steam accumulator is charged during periods of lower power demand, as during the night hours, and discharged during peak load demands. The steam accumulator with volume of 600 m3 is charged with superheated steam (pressure 4.5 MPa and temperature 335 °C) extracted from the exit of high pressure steam turbine. The steam discharged from the steam accumulator replaces the turbine steam extraction for two low pressure heaters: a 59.5 MWt and a 54.6 MWt heater operating at 0.56 MPa and 0.23 MPa steam pressure. This replacement results in an increase of the steam flow rate through the turbine and the turbine power gain of 27.3 MWe during 27 min and 11.5 MWe for additional 13 min. The proposed solution of the steam accumulator installation in the power plant is a new design. The upgrade of power plant flexibility by steam accumulator is demonstrated.
•Explosive boiling of thin water film on hot cooper substrate is investigated.•Investigations are based on results of Molecular Dynamics simulations.•Quantitative analysis of involved explosive ...boiling mechanisms is done.•The results show that explosive boiling comprises several phases.
In this paper the dynamics of explosive boiling of a 72 Å thick water film on the hot copper substrate with the plain surface is analysed. The analyses are based on the results of molecular dynamics (MD) simulations of the transient in which the thermostat temperature within the solid substrate is increased from 298 to 800K. The obtained results show that explosive boiling comprises several phases. The first phase involves rapid thermoacoustic pressure build-up in the near wall region due to intensive water heating and its volumetric expansion. The generated compression wave propagates through the liquid film reaching the peak value higher than critical water pressure. Due to reflection at the free film surface the original compression wave turns into an expansion wave. This event leads to a rapid pressure decrease and occurrence of tension stress. The second phase starts when the pressure enters the negative domain. This phase is accompanied with nucleation of vapour embryos and their subsequent growth in the superheated water layer in the near wall region. As the intensity of vapour phase generation is moderate, the liquid film in this phase is still exposed to tension. The stress recovery starts in the third phase when the water temperature in the near wall layer attains the spinodal value. This leads to a massive vaporization, nanobubble coalescence and thermal explosion, which causes spallation of the liquid film and lift-off of the liquid slug. The last phase is characterized by suppression of heat transfer by the expanding vapour layer. As a consequence, after reaching the maximum, the pressure decreases. This paper gives quantitative description and detailed insight into the mechanisms associated with each of the aforementioned phases.
•Three-dimensional model of refrigerant-oil two-phase flow is developed.•R134a and 1234yf two-phase flows with PAG oil in an evaporator are simulated.•Oil retention and refrigerant mass in the ...evaporator are numerically predicted.•Numerical results are validated against measured data.•Locations of oil retention inside evaporator are predicted.
This paper presents a three-dimensional model of the refrigerant-oil two phase flow developed with the aim of predicting the oil retention in evaporators of air-conditioning systems. The developed model is based on the two-fluid model approach. The governing mass, momentum and energy balance equations are written for each phase. The gas phase is the refrigerant vapor, while the liquid phase is the mixture of liquid refrigerant and oil. The balance equation for the oil mass fraction in the mixture with liquid refrigerant is included. Transfer processes at the vapor-liquid interfaces and on the flow channel walls are predicted with closure laws. The model is solved by the in-house computer code based on the SIMPLE type numerical procedure. The model is validated by comparing numerically predicted refrigerant mass and oil retention data in a brazed plate and fin evaporator, typically used in automotive applications, against measured values. Two sets of experiments performed with refrigerants R134a and R1234yf in the mixture with PAG oil are simulated. The numerical results provide a complete picture of the two-phase flow structure in the evaporator. Model predicts that oil is mainly retained in the bottom header and in smaller amounts in the top header and in parallel evaporating channels with upward refrigerant flow. The developed three-dimensional modeling and numerical approach has the advantage of being more reliable for the prediction of oil retention than existing one-dimensional models.
A substantial potential exists for the recovery of flue gas waste heat at aged coal-fired power plants. High and low pressure economizers (HPE, LPE) can be applied for this purpose. Two cases of the ...feedwater supply to the HPE are considered: a supply of the “cold” feedwater taken from the feedwater pump discharge line and a supply of the “hot” feedwater taken after high pressure heaters. The LPE is fed by condensate taken from the low pressure condensate line. The heat transfer area of economizers is equal in all cases. The heat transfer rate of the LPE is maximized by the lowest possible condensate temperature at the economizer inlet, which is limited by the acid dew point temperature. The power upgrade by the HPE fed with the “cold” feedwater is by 2.5 times higher than in case with the LPE. The plant efficiency upgrade by the LPE is by 20% higher than in case with the HPE fed with “cold” feedwater. The HPE fed with the hot feedwater is not competitive. The presented overall benefits achieved by the HPE fed with the cold feedwater justify its implementation at the aged 620 MWe power plant.
•Big potential for the flue gas waste heat recovery at aged coal-fired power plants.•Plant retrofits with high and low pressure economizers (HPE, LPE) are compared.•Ratio of power upgrades by HPE and LPE is about 2.5.•Plant efficiency upgrade by LPE is by 20% higher than by HPE.•HPE is installed at the aged power plant due to overall benefits.
Coal-fired thermal power plants (TPPs) had been generally designed to cover base loads of electricity consumption. Nowadays, they should be flexible to participate in the primary frequency control of ...electric system, especially because of increased number of wind and solar plants with intermittent electricity production.
In this paper a new method for the primary power control in TPPs is presented, based on the feedwater flow rate changes between the feedwater heaters (FWHs) and an additional high pressure economizer. The achieved primary power control reserve is demonstrated by measurements at the lignite-fired 650 MWe TPP after its retrofit by the installation of the high pressure economizer in parallel with FWHs. In addition, the TPP transient operation during the primary control are simulated and analyzed with the developed thermal-hydraulic model of the steam turbine heat regeneration system. The control reserve of approximately 10 MWe is shown both by measured data and results of numerical simulations. The control reserve is originally provided by the fresh steam throttling in front of the turbine, but this throttling can be reduced by the application of the new method. The rates of change of wall temperatures of FWHs tubes are within allowed limits during the transients.
•A high pressure economizer was installed at the coal-fired 650 MWe power plant.•The economizer is in parallel with feedwater heaters.•Feedwater flow change through the economizer enables the primary power control.•Measurements and numerical simulations show 10 MWe of primary control reserve.•The primary control by the economizer reduces a need for the fresh steam throttling.
A new CFD modeling approach is developed and applied to a numerical study of pool boiling. It is based on a coupled simultaneous solving of the two-fluid model of two-phase flow in the boiling pool ...and the transient heat conduction within the heated wall. The applied two-fluid model consist in mass, momentum and energy balance equations for each phase and the closure laws for interface transport due to friction and phase transition. The boiling two-fluid mixture is thermally coupled with the heated wall by the heat transfer at the bubble footprint at randomly determined discrete locations of the nucleation sites and by the conjugate heat transfer from the heated wall to the wetting liquid on the surface outside the nucleation sites. The model is validated against detailed experimental data from the literature. Sensitivity analysis is performed to evaluate the influence of pool boiling parameters and conditions on the boiling curve, such as the nucleation site density, the wetting contact angle, the wall thermal conductivity, the wall thickness, the wall volumetric or bottom surface heating and the liquid volume in the pool. Temperature transients at the bubble nucleation sites are evaluated. The two-phase mixture pattern in pool boiling and the related swell level position can be well predicted with the nonuniform heat flux determined by the discrete nucleation sites of bubble growth and conjugate heat transfer from the heated wall to wetting liquid. The developed computational method is efficient and robust, while the reliability of its prediction is sensitive to the accuracy of nucleation site density and contact angle input data, as parameters that describe the coupled thermal and physical characteristics at the fluid-wall interface.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Second generation sequencing has permitted detailed sequence characterisation at the whole genome level of a growing number of non-model organisms, but the data produced have short read-lengths and ...biased genome coverage leading to fragmented genome assemblies. The PacBio RS long-read sequencing platform offers the promise of increased read length and unbiased genome coverage and thus the potential to produce genome sequence data of a finished quality containing fewer gaps and longer contigs. However, these advantages come at a much greater cost per nucleotide and with a perceived increase in error-rate. In this investigation, we evaluated the performance of the PacBio RS sequencing platform through the sequencing and de novo assembly of the Potentilla micrantha chloroplast genome.
Following error-correction, a total of 28,638 PacBio RS reads were recovered with a mean read length of 1,902 bp totalling 54,492,250 nucleotides and representing an average depth of coverage of 320× the chloroplast genome. The dataset covered the entire 154,959 bp of the chloroplast genome in a single contig (100% coverage) compared to seven contigs (90.59% coverage) recovered from an Illumina data, and revealed no bias in coverage of GC rich regions. Post-assembly the data were largely concordant with the Illumina data generated and allowed 187 ambiguities in the Illumina data to be resolved. The additional read length also permitted small differences in the two inverted repeat regions to be assigned unambiguously.
This is the first report to our knowledge of a chloroplast genome assembled de novo using PacBio sequence data. The PacBio RS data generated here were assembled into a single large contig spanning the P. micrantha chloroplast genome, with a higher degree of accuracy than an Illumina dataset generated at a much greater depth of coverage, due to longer read lengths and lower GC bias in the data. The results we present suggest PacBio data will be of immense utility for the development of genome sequence assemblies containing fewer unresolved gaps and ambiguities and a significantly smaller number of contigs than could be produced using short-read sequence data alone.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
An additional high pressure economizer was installed at Unit B1 of the 620 MWe lignite-fired Power Plant “Nikola Tesla B” after 30 years of its operation. An innovative connection of the new ...additional economizer was applied. It is in parallel connection to the first section of the originally built economizer and it is directly fed with the feedwater from the outlet of the feedwater pump. The analysis of Unit B1 operation with such an economizer arrangement is performed and it is supported by measured data. It is shown that more than 30 MWth of the flue gas waste heat is recovered. The Unit gross efficiency is increased by 0.53 percentage points, which leads to 9.4 MWe of the electric power production. The parallel connection of the additional economizer also leads to the partial feedwater bypass of the high pressure heaters, which enables an increase of the plant electric power by up to 24.5 MWe. The accompanying effects are the reduction of the pressure drop in the feedwater line and the economizers, which leads to the decrease of the energy consumption for the main feedwater pump operation. The applied solution is presented together with measured and calculated energy and economic benefits.
•An additional economizer was installed at the steam boiler of an aged power plant.•An innovative parallel connection of new and old economizer sections was applied.•Energy analysis of the retrofitted plant is performed for observed operation.•Energy upgrades depend on the feedwater distribution between economizers' sections.•The unit gross efficiency increases by up to 0.53 percentage points.
Severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) emerged in late 2019 and has since caused a global pandemic. Experimental studies and sporadic reports have confirmed susceptibility of ...dogs and cats to SARS‐CoV‐2 infection. However, the importance of pet animals in the epidemiology of this infection is unclear. This study reports on a first large‐scale serosurvey of SARS‐CoV‐2 infections in dogs and cats in Europe. From 26 February 2020, just one day after the first confirmed human case of SARS‐CoV‐2 infection in Croatia, to 15 June 2020, dog and cat serum samples were collected from animals admitted to three veterinary facilities in Croatia. Additionally, on 25 May 2020, a total of 122 serum samples from employees of the Faculty of Veterinary Medicine University of Zagreb were collected. Total of 656 dogs and 131 cat serum samples were tested using an in‐house microneutralisation test (MNT). Human serum samples, as well as 172 randomly selected, dog sera were tested using enzyme‐linked immunosorbent assay (ELISA). ELISA‐positive human sera were subsequently tested using MNT. Neutralising antibodies were confirmed in 0.76% cats and 0.31% dogs. ELISA reactivity was recorded in 7.56% tested dog sera. On the other hand, 5.19% of administrative, basic and pre‐clinical sciences department personnel and 5.13% of animal health service providers and laboratory personnel tested ELISA positive. Neutralising antibodies were not confirmed in any of the human samples. In conclusion, seropositivity among pet animals in Croatia is low, especially when compared to results from China. A small number of seropositive animals with a low titre of neutralising antibodies suggest infections are rare and are following infections in the human population. Additionally, contact with animals does not seem to be an occupational risk for veterinary practitioners.
A prediction of thermal transients in district heating systems is important in order to adjust in an energy efficient manner the loads of heat power plants and pump stations with dynamic consumption ...needs during changes of the outside air temperature, the wind intensity, the solar radiation, or system start-ups and shut-downs. A model and corresponding computer code are developed with the aim to simulate the thermal transients in district heating systems. They are based on the high-order accurate numerical solution of the transient energy equation, and the hydraulic prediction of pressure and fluid flow rates within the complex pipe network. Thermal transients caused by an increase and decrease of the heat power plant load are simulated for real operating conditions of the district heating system. Predicted temperature front propagations show a good agreement with data measured at three consumer substations located in different parts of the district heating network and at different distances from the heat plant. The developed computational tool provides reliable information about time periods of temperature fronts propagation and heat distribution from the heat source to consumers within the whole district heating network.
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