Thermocline Zone Research Articles

Temporal and Environmental Dynamics of Fish Stocks in the Marine Protected Area of the Artificial Reef of Kitros, Pieria (Northern Greece, Mediterranean Sea)

The impact of temporal factors and physiochemical properties on the quantities and biomass of fish in the Thermaikos Gulf and adjacent habitats around the Pieria artificial reef complex in Northern Greece was evaluated. Data were collected from edge habitats of an artificial reef made of submerged concrete blocks located offshore of Kitros. Between 2015 and 2017, sampling excursions took place in the spring, summer, and fall of each year. The artificial reef complex is positioned 11.5 km offshore from Kitros, near the delta of the Aliakmonas River, and is influenced by freshwater from the Axios, Aliakmonas, Ludias, and Gallikos Rivers. To estimate the biomass and numbers of the primary benthopelagic commercial fish in the region, nine experimental samples were taken from 2015 to 2017, employing a bottom trawl with a mesh size of 10 mm. Each trawl sample lasted for 30 min to reduce the environmental impact of fishing in the areas under study. The sampling schedule was aligned with factors like the breeding times of the dominant species and the seasonal changes in the thermocline zone. Analysis showed seasonal differences in average abundance and biomass values across the 3 years of the study. Mullus barbatus, recognized as the most commercially significant species caught, demonstrated the greatest abundance and biomass during the second sampling season. Conversely, Pagellus erythrinus, the second most vital commercial species caught, exhibited its highest abundance in the first season and reached its peak biomass during the third sampling season. Hierarchical cluster analyses showed that the two major resulting clusters of species proliferated in numbers over the 3-year period, while undergoing a concurrent reduction in their individual sizes.

Methane Distribution in Lake Baikal Water

Data on the distribution of methane concentration in water in the open water area and the northern end of Lake Baikal and in the source of the Angara R. collected in September 2016 and 2019 are analyzed. To apply correlation analysis, in addition to methane, various hydrochemical characteristics were also determined, including the temperature, pH, the concentrations of О2, suspended matter, Corg, Norg, Porg, mineral compounds of nitrogen and phosphorus. Methane concentration in Baikal water in 2016 varied within 0.44–3.41 µL/dm3 (on the average, 0.80 µL/dm3); in 2019, within 0.20–5.19 µL/dm3 (on the average, 1.22 µL/dm3). The maximal methane concentration was recorded in the water mass of the deepest central depression of the lake, and the minimal, in its southern depression. Among shallow areas, minimal methane concentrations were recorded in the coastal zone of Listvenichnyi Bay, and its maximal concentrations, in the northern part of the lake, into which many rivers empty, as well as in Selenginskoe shallows. Most stations, either deep-water or shallow, showed a peak of subsurface maximum of methane concentration at depths of 25–50 m (thermocline zone), after which its concentrations commonly dropped, reaching their minimal values either in the intermediate water mass or in bottom layers. Shallow areas, in addition to higher methane concentrations compared to deep-water areas, also featured a higher contrast in its vertical distribution in the water mass. The analysis of correlation relationships between the examined hydrochemical characteristics revealed significant direct correlations of methane concentration with the concentrations of Corg and Norg and an inverse correlation with O2 concentrations.

Experimental analysis of parameters influencing thermal stratification in single media single tank storage system with flow distributor

The design and operating parameters of a single media single tank (SMT) energy storage system affect the thermal stratification of the fluid. The objective of the present work is to propose a design configuration of an SMT to reduce the thermocline zone. To that end, experiments were performed to investigate the influence of the flow distributor, the fluid inlet, and outlet configuration on the thermal stratification. Experiments were also carried out to understand the scalability of the system. Results obtained from experiments performed for a range of Péclet numbers are analyzed using three performance parameters: the non-dimensional thermocline span ‘S’, mixing number Mix, and stratification number Str. The study shows that these parameters are scale-invariant. The result also shows that the use of a central flow distributor reduces the thermal gradient zone by 50% to 60% as compared to that achieved in a tank without the flow distributor operated with Péclet numbers between 262 and 945.

Stratification Analysis and Behaviour of a Real Industrial Thermocline Thermal Energy Storage Tank for Cogeneration Purposes

Thermal energy storage systems help to couple thermal energy generation and process demand in cogeneration facilities. One single deposit with two design temperatures and one main temperature step in sensible thermal energy storage define the thermocline systems. Performance of one high size real thermocline thermal energy storage system is analysed. Starting from temperature and mass flow rate data registered by the plant control system, one advanced thermodynamic analysis is performed. The quality of heat storage is analysed in terms of evaluation of the stratification in the thermocline zone. The temperature data registered at 21 positions is extended by displacement analysis generating detailed profiles. Fraction of recoverable heat, thermocline width, stratification indices based on energy and exergy analysis, and mean temperature gradients in the thermocline region are calculated. These parameters are monitored under real operation conditions of the plant. The calculated parameters are studied to check their distribution and correlation. First and Second Law indices show parallel behaviour and two values are found that delimit situations of high and low values of mean temperature gradients. It was observed that buoyancy generates uniform forced movement with the right water temperature entering the diffusers, but good control strategies are essential to avoid mixing. The system demonstrated great stability in this use.

Dimensionless model based on dual phase approach for predicting thermal performance of thermocline energy storage system: Towards a new approach for thermocline thermal optimization

The focus of this study is the development of a dimensionless model that allows a better understanding of the thermocline storage system behavior. The proposed model is based on a dual phase approach. We had identified four remarkable dimensionless groups of parameters which control the thermocline system performances. The influence of each dimensionless group of parameters has been analyzed separately to understand its influence on the overall thermocline storage system performance. During this investigation, three concepts are deeply analyzed: the thermocline zone thickness, dimensionless discharge time and discharge efficiency. Using this model, we have demonstrated that, for each combination HTF/TESM, a universal optimum can be highlighted, which makes it possible to find, according to a given specification, the optimal configuration of the storage tank. Based on the obtained results, we had proposed a new reduced model based on dual-phase approach for rapid calculation of optimal sizing of a thermocline storage tank. The proposed method requires no discretization method and provides accuracy similar to those of the dual phase model but with a drastically reduced computation effort.

Multilevel comparison between magnetite and quartzite as thermocline energy storage materials

This paper presents a multilevel comparison between two thermal energy storage materials: quartzite as the most known thermocline energy storage material and magnetite as a new potential candidate. This comparison involves: thermal and thermophysical properties, cycling effect and thermocline thermal energy storage performances. In first, an experimental characterization of magnetite under cycling has been performed, and then magnetite was compared to quartzite from this viewpoint. It has been demonstrated that for the medium temperature range (i.e. from 100 °C to 500 °C), thermal cycling has a positive impact on magnetite characteristics and performances. Thereafter, a numerical model for thermocline storage has been presented and validated. Then, the thermocline behavior and the thermal energy storage performances of both materials during charging and discharging processes have been investigated and tested for various heat transfer fluidscommonly used, including natural oil, synthetic oils and molten salts. This study shows that, for the different HTF tested, no significant difference between the thermocline zone thicknesses can be noted between the two TESM. It has been concluded that for the same storage tank size and the same discharge time, magnetite can store and restore more energy and requires less storage volume although quartzite presents higher efficiencies. For magnetite, this can represent an advantage from an economic and technical standpoint. While for the same storage tank size and the same HTF speed, the magnetite charges and discharges more slowly. On the other hand, the different combinations HTF/TESM tested show that thermocline performances are driven by not only the filler material but also the nature of the heat transfer fluid. In that sense, the molten salt fluids arelargely more efficient for both TESMs than other fluids.

Biochemical Characteristics of Suspended Particulate Matter on a Latitudinal Transect in the North Atlantic

Dispersed sedimentary matter was studied along the latitudinal transect 59°30′ N from Great Britain to Greenland on cruise 49 of the R/V Akademik Ioffe in June 2015. The concentrations of suspended particulate matter (SPM) along the transect were low, increasing from 0.24 to 1.07 mg/L in the surface layer (0–5 m), from 0.24 to 0.99 mg/L in the pycnocline (thermocline) zone, and appreciably decreasing toward the bottom layer, fluctuating from 0.10 to 0.56 mg/L. The isotopic composition of organic carbon in SPM was determined in 18 samples. The δ13C-Corg values vary from –17 to –24.6‰. Judging from the chlorophyll a content, the isotopic composition of water masses, and their remoteness from the coast, the carbon in SPM is mainly represented by autochthonous organic matter of phytoplanktonic origin. The presence of SPM with a heavy isotopic composition in the surface layer of the water column can serve as evidence for the current processes of phytoplankton bloom. In this case, differences in the intensity of these processes can be observed, creating a mosaic bloom pattern even within a distance of 150 NM.

Effect of metalimnetic gradient on phytoplankton and zooplankton (Rotifera, Crustacea) communities in different trophic conditions

Theory predicts and recent study revealed that depth of the thermocline can strongly influence the nutrient availability and composition of plankton communities. We are focused on the effect of metalimnetic gradients on water chemistry and plankton communities in three stratified lakes with different trophic conditions. Vertical changes in water chemistry revealed significant increase of macroelement concentrations in the metalimnion of all studied lakes. However, there was no significant increase of nutrient concentrations in the thermocline of lakes with smoother metalimnetic gradient, whereas sharp and deep thermocline zone caused higher concentration of orthophosphates and dissolved inorganic nitrogen in the metalimnion. The maximum concentrations of phytoplankton were observed just below the thermocline and were caused mostly by the abundance of diatoms and cryptophytes. Vertical distribution of the crustacean zooplankton was similar to the distribution of phytoplankton. Especially, Daphnia cucullata was strongly related with the phytoplankton distribution and reached maximum densities in deep layers with high chlorophyll concentrations, and, conversely, smaller crustacean species and rotifers were not affected by the vertical distribution of phytoplankton.

Single-tank TES System – Transient Evaluation of Thermal Stratification According to the Second-law of Thermodynamics

Abstract Single-tank thermal energy storage (TES) systems represent a valuable alternative, to the most common two-tank systems with molten slat, to effectively store thermal energy in concentrating solar power (CSP) applications.From an economic standpoint, the gap between the two TES solutions is relevant. A remarkable cost reduction can be achieved if a single-tank TES system, with a low-cost filler material, is exploited.In this kind of TES system,the buoyancydriven effects of the heat transfer fluid are exploited to establish and maintain a thermocline zone which separates the hot region on top and the cold region at the bottom of the tank.The thinner the thermocline thickness, the higher the thermodynamic quality of the stored energy. As soon as the TES is charged for the first time, i.e. startup of the system, the extent of thermal stratification may vary sharply during the first cycles before achieving a stable condition. For this reason, this study aims at evaluating, by means of accurate time-dependent 3D CFD simulations, the transient evolution of thermal stratification of a single-tank TES system exploited to fulfill the round-the-clock energy requirement of a reference 80 MWeCSP plant which uses air as heat transfer fluid.A total of 30 consecutive cycles, composed by charge/discharge phases, were simulated.Since the thermal energy stored is exploited to produce electrical energy, the performances of the TES system, operating under cyclic conditions, were qualitatively characterized by means of a stratification efficiency indexbased upon the second-law of thermodynamics.

Study of Thermocline Tank Performance in Dynamic Processes and Stand-by Periods with an Analytical Function

The behavior of an Andasol-like thermocline tank with and without solid filler has been predicted with an analytical Logistic Cumulative Distribution Function not only during dynamic processes of charge/discharge but also during stand-by periods of no operation. The evolution of thermocline thickness with dwelling time in stand-by periods has been studied under the condition of energy conservation when thermocline zone is total or partially inside the tank. Thermocline degradation is faster for the tank with molten salt and solid filler than for the tank with only molten salt because thermal diffusivity of the molten salt is lower than thermal diffusivity of the solid material. The strong decrease in tank efficiency with consecutive charge/discharge cycles when thermocline zone is left inside the tank can be relieved if a certain percentage of thermocline zone is extracted. Actually, if 50% thermocline is extracted, efficiency after a second process can be as high as 96% comparing with the 77% obtained if thermocline is not extracted. However, extracting thermocline zone implies a decrease of outlet temperature in discharge or an increase of outlet temperature if the process is a charge. Therefore, temperature limits and operation strategies should be defined for the solar thermal power plant that includes a thermocline storage tank in order to establish which amount of thermocline zone can be extracted.

High Temperature Rock-bed TES System Suitable for Industrial-scale CSP Plant – CFD Analysis Under Charge/Discharge Cyclic Conditions

The present study aims at dimensioning and modeling, by means of accurate time-dependent 3D computational fluid dynamics simulations, the behavior of a high temperature rock-bed TES system. The latter is exploited to fulfill the round-the-clock energy requirements of a reference 80 MWe industrial-scale CSP plant, based upon the Airlight Energy technology, which uses air as heat transfer fluid. The TES system behavior was analyzed through 15 consecutive charge/discharge cycles to evaluate the thickness evolution of the thermocline zone, and hence the overall thermal efficiency of the system, under cyclic conditions.The numerical model was satisfactorily validated with experimental data, gathered from a 6.5 MWhth TES system prototype, located in Biasca, designed and built by the Swiss company Airlight Energy SA. The good agreement between CFD simulations results and experimental data allowed the authors to assess the relevance of radiative heat transfer, even at relatively low temperature (300 ÷ 350°C), on the thermodynamics behavior of the TES system. Moreover, a porosity variation, with the packed bed depth, was also observed numerically and experimentally mainly due to the own weight of the packings (25m3 of natural river pebbles with 3cm average diameter).The CFD simulations were performed with Fluent code from ANSYS.

Analytical function describing the behaviour of a thermocline storage tank: A requirement for annual simulations of solar thermal power plants

Thermocline tank behaviour during dynamic processes of charge/discharge and stand-by periods has been described in dimensionless form by means of the Logistic Cumulative Distribution Function (CDF). The Logistic-CDF parameter, S, has been correlated with both thermocline tank features and operation conditions (i.e. dimensionless velocity, v∗, and time, t∗) with the help of a single-phase one-dimensional numerical model previously developed and validated. For the dynamic processes, it has been found that S becomes independent on v∗ when v∗>2500, but it always increases proportionally with v∗t∗ due to the progress of thermocline zone inside the tank. For adiabatic stand-by periods, in which v∗=0, S parameter proportionally increases with t∗ whereas zc∗ varies in order to keep constant the stored energy inside the tank. In terms of thermocline thickness it has been demonstrated that it is proportional to S parameter but its exact value strongly depends on the temperature accuracy that determines thermocline zone limits. This parameterised Logistic-CDF provides a general analytical equation for the outlet temperature of the storage liquid, which allows implementing thermocline storage systems in the annual performance simulations of solar thermal power plants.

Controls of plankton production by pelagic fish predation and resource availability in the Alboran and Balearic Seas

A one-dimensional coupled physical and intermediate-complexity biochemical model comprising large and small phytoplankton and zooplankton groups, particulate organic nitrogen, ammonium and nitrate was developed to study the physical–biogeochemical interactions and parameters that control plankton production in the Alboran and Balearic Sea ecosystems. The model findings suggest that pelagic fish predation and resource availability through lateral and vertical nutrient inputs jointly characterize the plankton community structures. In agreement with previous observations, a typical annual plankton structure of the mesotrophic systems involves a vertically homogeneous biomass of large groups of phytoplankton and zooplankton within the upper 50-to-100m layer from mid-November to April and a subsurface biomass accumulation distributed roughly within 25–75m depths in the following months. Their light and temperature limitations constrain the smaller groups into the thermocline zone (25–50m) during late spring and summer. These obtained results were dependent on the zooplankton actively switching between preys (i.e., the food preference coefficients dependent on prey biomass). In the case of no switching, spurious dynamic equilibrium solutions may arise in the case of a constant and weak fish predation rate and using the quadratic predation formulation. The choice of a Holling Type II (i.e., hyperbolic) predation function may, however avoid ambiguous representation of the annual plankton structure in the case of a constant food preference choice under relatively weak predation pressures.

Finite Element Modeling for Cathodic Protection of Pipelines under Simulating Thermocline Environment in Deep Water Using a Dynamic Boundary Condition

A finite element model (FEM) was established to calculate the cathodic protection (CP) potential distribution on pipelines, under the environment of the thermocline structure in deep water. Supplemental boundary conditions were applied on the interfaces of two different mediums in the thermocline model. To solve the problem of time-dependent polarization curves due to calcareous deposits formed on the steel surface during CP in seawater, a dynamic boundary condition based on Ohm's law was proposed, instead of polarization curves or the Butler-Volmer equation. A good agreement was obtained between the calculated and experimental potential distributions, which proved that the model was practical and feasible. The anode placement studies revealed that the anode could supply a more efficient protection when it was placed at the thermocline zone rather than the surface zone or the deep water zone.

First results on bathymetry and limnology of high-altitude lakes in the Gokyo Valley, Sagarmatha (Everest) National Park, Nepal

The aim of this study was to investigate for the first time the morphology, physical and chemical characteristics of three high-altitude lakes in the Gokyo Valley, Everest National Park, Nepal. The moraine-dammed glacial lakes were studied for three seasons to create baseline data. The second, third, and fourth lakes in the Gokyo Valley are deeper than previously assumed. The vertical profiles of temperature and dissolved oxygen indicated that the thermocline zone varied between 10 and 20 m below the surface during the post-monsoon season. Although most of the analyzed metal concentrations were below the level recommended by the WHO for safe drinking water, lead (Pb) and cadmium (Cd) levels exceeded the limits in some of the samples. This has created great concern for human health. The probable route of these two pollutants is monsoon precipitation that carries industrial pollutants along its route. The sedimentation rates of Gokyo Lakes range from 0.069 to 0.089 cm per annum, which is within the limits of other high-altitude lakes. The present findings created a baseline database for some of the remote high-altitude lakes in Nepal that can be used for lake management and for assessing future changes in lake characteristics in the Himalayan region.

The seasonal variation of ph and dissolved oxygen (dO2) concentration in Asa lake Ilorin, Nigeria

The vertical and surface water variations of pH and dissolve oxygen (DO2) concentration were studied for 12 months (March 2005 to February 2006) in Asa lake Ilorin, Nigeria. The mean surface pH values ranged between 7.30 and 8.10, with lower values (7.30 - 7.70) obtained October to December while higher values (8.0/8.10) were obtained in January to July. Vertical distribution of mean pH also ranged from 8.1 (Surface) to 5.20 (Bottom). Similarly the monthly variations of mean DO2 concentration at the surface waters were between 7.60 - 8.20 mg/l, with lower values in October/November and higher values in March/April. Vertical distribution of DO2 concentration also ranged from 8.20 (surface) to 2.50 (bottom) mg/l. A fairly defined pH and DO2 stratification were noted from March to September and also in January/ February, while destratification was noted for these parameters in November and December. However a sharp drop in DO2concentration usually occurred at the thermocline zone of 5 m depth. This sharp drop also became shifted to 7 m depth in October and November due to thermal mixing condition of the lake around this season, and DO2 concentration were almost uniform from the surface to 5 m depth (mean = 7.60 mg/l). The effects of this on some of the fish fauna of this water body were discussed. The pH and dissolved DO2 concentration were positively correlated (0.54). There were no marked variations of the pH and dissolved oxygen concentration along the 3 sampling sites across the lake hence fish species composition along the water body is likely to be less varied. Key words: Dissolve oxygen, pH, fishes, Asa lake, stratification.

Zooplankton ecology in the eastern tropical Pacific oxygen minimum zone above a seamount: 2. Vertical distribution of copepods

The abundance and vertical distribution (0-1230 m) of copepods were studied in the eastern tropical Pacific near the seamount Volcano 7 to examine the influence of the extreme oxygen minimum zone (OMZ). Maximum zooplankton biomass and copepod abundance were in the thermocline zone. A secondary peak in biomass and copepod abundance was evident between 600 and 1000 m, which included the depth of the lower interface of the OMZ. This prominent secondary peak in zooplankton is a feature unique to OMZ regions. There were four general trends of vertical distribution of copepod abundance. These trends appeared to be related to the oxygen concentration and gradients. The most common vertical distribution was a pattern of maximum abundance in the mixed layer and thermocline zones, with a secondary maximum in the zone of the lower OMZ interface (600-1000 m). Clausocalanus spp., Oncaea, spp., Euchaeta spp., Oithona spp. and Corycaeus spp. showed this trend. Low oxygen concentration did not appear to restrict these groups, since they were present throughout the OMZ. The second vertical distributional pattern was vertical migration between the thermocline and the OMZ. Pleuromamma robusta showed this pattern, with maximum abundance at night in the thermocline zone and during the day in the core of the OMZ. In addition, there was a secondary maximum of abundance at the lower OMZ interface zone. The third type of distribution was shown by copepods that were abundant in the upper OMZ and at the lower OMZ interface zones. Eucalanus inermis, Haloptilusparalongicirrus and Heterostylites longicornis were dominant copepod species that exhibited this pattern. They were either absent from the mixed layer or at similar abundances in the mixed layer and upper OMZ. The fourth pattern was shown by copepod species that live primarily above the OMZ day and night. The majority of the species appeared to be tolerant of the extreme low oxygen concentrations. Rhincalanus spp. was the dominant copepod that was excluded by low oxygen concentrations. In general, species also found in other OMZ regions showed similar distributions in this study, indicating that low oxygen is a major controlling factor. Some vertical niche separation among congeneric species was indicated for Eucalanidae, Metridiidae and Augaptilidae.

Zooplankton ecology in the eastern tropical Pacific oxygen minimum zone above a seamount: 1. General trends

The distribution of zooplankton in the oxygen minimum zone (OMZ) of the eastern tropical Pacific and near a seamount was investigated. The oxygen minimum layer appeared to be an important factor influencing the vertical distribution of zooplankton taxa below the thermocline. The maximum zooplankton in terms of biomass and numbers was in the thermocline zone, with a secondary maximum in the depth zone that included the lower OMZ interface (600-1000 m). Most taxonomic groups showed this secondary peak in abundance in the lower OMZ interface depth zone and minimum abundances above it. The secondary maximum in zooplankton at the lower OMZ interface seems to be unique to OMZ regions. Only larvaceans and mollusks did not appear to be strongly influenced by the minimum concentrations of oxygen. Four vertical distribution patterns characteristic of different taxonomic groups may be indicative of their different tolerances to minimum oxygen concentrations. The physical intrusion of the seamount (summit at 730 m depth) did not cause major changes in the distribution of pelagic zooplankton. Significant biological and physical differences between locations above and away from the seamount were found only within the upper OMZ (100-300 m).

Influence of thermal stratification on the behaviour of a deep wastewater stabilization pond

Deep lagooning is an alternative to conventional lagooning and it implies smaller land requirements as an additional advantage. Treatment in deep wastewater stabilization ponds is influenced by thermal stratification. The object of this paper is the study, over a 1-yr period (October 1986–December 1987), of the influence of temperature on the behaviour of a treatment unit (an 8 m deep pond) which receives domestic wastewater. For this study, a spatiotemporal follow-up of the natural treatment process has been conducted. The removal efficiencies achieved for the following parameters varied as follows: COD, 71–92%; BOD 5, 83–97%; total Kjeldahl nitrogen, 51–98.8%; ammonia nitrogen, 38.8–99.9%; total phosphorus, 42.2–92.9% and orthophosphate, 40–99%. Log reduction of microorganisms varied between 2.0 and 3.0 for total coliforms, 1.1–3.0 for faecal coliforms and 1.7–3.3 for faecal streptococcus. Other physicochemical parameters observed were: temperature, dissolved oxygen, pH, nitrates and nitrites. The continuous flow of wastewater did not hinder the appearance of a marked stratification in spring-summer: an aerobic epilimnetic zone and an anaerobic hypolimnetic one remained clearly differentiated. The evolution of all the parameters observed was similar. In the absence of thermal stratification, the system was found to function in approximately complete mixing conditions, and the value of each parameter was almost constant throughout the water column; nevertheless, the presence of stratification caused a vertical distribution with marked gradients in the thermocline zone.

210Pb in GEOSECS water profiles from the North Pacific

Ten GEOSECS profiles from the North Pacific have been analyzed for 210Pb. GEOSECS 226Ra data on the same profiles are used to calculate 210Pb excess or deficiency relative to secular equilibrium. The resultant profiles are divisible into a thermocline zone (<2000m) showing an expected decrease with depth, a mid-water zone of about 2000 m showing small constant deficiencies with a zone of increasing deficiency to a bottom zone of about 1000 m having the highest deficiency virtually invariant with depth. The exponentially decreasing portion in the thermocline yields a “diffusion” coefficient of 3 cm 2/s. The mid-water deficiencies yield ☐ model residence times of 400 years northeast of Hawaii decreasing to 100 years at the most marginal stations.