Phase Change Of Water Research Articles

Sensitivity Analysis of the Noah‐MP Land Surface Model for Soil Hydrothermal Simulations Over the Tibetan Plateau

AbstractThe Tibetan Plateau (TP) features unique and highly heterogeneous soils, terrains, vegetation, and climate. Accurately modeling complex freeze‐thaw processes and their hydrothermal impacts remains a great challenge. This study focused on deciphering the spatiotemporal variability of diverse parameterization schemes in the soil hydrothermal simulations using the Noah‐MP land surface model. We first discussed the spin‐up time required by the model to reach the equilibrium state, and then performed a sensitivity analysis of these schemes. The Moderate Resolution Imaging Spectroradiometer land surface temperature and Soil Moisture Active Passive remote sensing products were used as benchmarks to evaluate the schemes' performance. Results show that longer spin‐up times are required in permafrost regions owing to water phase changes. Ground temperature and soil temperature are mainly sensitive to energy‐related schemes. Vegetation‐related schemes play an important role after the growing season begins on the southeastern TP. Soil water content shows strong sensitivity to schemes related to both water and energy transport. However, the sensitivity of these energy‐related schemes is weakened when simulating total soil moisture, including the total amount of water and ice, indicating that these schemes have marked impacts on soil freeze‐thaw processes. These results reveal the different spatial (both regional and depth‐related) and temporal effects of parameterization schemes; we also provided a preliminary selection of these schemes at a regional scale that could facilitate the further improvement of the soil hydrothermal simulations on the TP.

New approaches to modeling the critical refreezing length of a Philberth melt probe for ice sheet exploration

In recent years, interest in using Philberth melt probes in the exploration of subglacial lakes or extraterrestrial ice-covered planets has significantly increased. Consequently, several Philberth melt probes have been developed or are currently being developed. The critical refreezing length is a key parameter in determining the distribution of the lateral heater of the Philberth melt probe. In this study, two new approaches for calculating the critical refreezing length were proposed. Approach I was established based on the refreezing of a virtual borehole, whereas approach II was developed in the COMSOL Multiphysics 5.6a software by modeling the phase change of water and ice in the non-heated section of the Philberth melt probe. The two novel approaches were validated with the field test results of RECoverable Autonomous Sonde (RECAS) in Antarctica. The difference between the two approaches and the existing calculation methods was less than 32% and approach I presented a smaller critical refreezing length. The thermal head efficiency and ice temperature had significant influence on the critical refreezing length. Usually, the critical refreezing length increased with an increase in the thermal-head diameter, penetration rate, and ice temperature, whereas it decreased with an increase in the thermal head efficiency.

Explosion Differentiation Using Light Emissions—Nuclear Reactor, Steam, Water Hammer, Hydrogen, Piper Alpha, and Hydro-Volcanic Explosions

Abstract Light emissions during chemical reactions provide insights into various scenarios to better understand explosions and water hammers. For example, hydrogen burning and explosions emit blue light, organic carbon combustion and explosions emit combinations of blue and yellow light, water phase changes emit infrared and less visible white light, and white light explosions ignite when multiple chemical reactions are involved. Since experimental tests to observe infrared light during water hammers have not yet been performed, test data from water boiling tests and volcanoes are compared to larger water hammer and steam explosion incidents. Considering these facts and examining a series of photos and videos from the literature and Internet, determinations are proven with respect to water hammers, steam explosions, Piper Alpha water hammer explosions, and chemical explosions. Such evidence proves that steam explosions are important for water hammer accidents, but chemical explosions explain other explosions that have long been considered to be steam explosions. These other explosions include nuclear power plant explosions, hydro-volcanic explosions, and hydrogen explosions, where some of these explosions are, in fact, related to water hammers. This article is primarily a photographic essay to explain the differences between different types of explosions and water hammers, although combustion and explosion principles are expanded and explained to support this essay.

Energy and exergy analysis of conjugate 3D unsteady heat conduction with solidification of water by turbulent buoyancy-driven freezing of solid food in air

In this work, the influence of the position of salmon in a cooling cavity on heat transfer, freezing rate and local exergy destruction was investigated using mathematical modeling and numerical simulation. The predicted temperature evolution in the food during freezing by turbulent heat convection in air inside a cooling chamber was validated with experimental results. The conjugate turbulent model in three dimensions included the natural heat convection and the unsteady heat diffusion with the phase change of water to ice in the food meat. Three positions of the block of salmon in the freezer were studied: bottom-left, bottom-center, and center of the freezer. The local exergy destruction was calculated to quantify the contribution of viscous dissipation and heat transfer during the salmon freezing process. The unsteady description of air velocity, temperature, and freezing rate were analyzed. The highest cooling rate calculated was for the food at the bottom left, and the lowest when located at the center of the cooling cavity. The exergy destruction analysis determined the useful energy lost during the freezing process.

Conjugate free convection from an array of discrete heat sources with water and nano-encapsulated phase change particle in a cold enclosure

The present works investigates the conjugate free convection in a cold enclosure containing a hot circular cylinder. An array of discrete heat sources are attached to the cylinder surface. The free space between the enclosure and the cylinder was filled with a heterogeneous mixture of water and nano-encapsulated phase change particle (NEPCP). This nanoparticles have an outer shell and a core inside. Specifically, the NEPCP core can absorb thermal energy on charging state or releases energy on discharging state. The governing parameters considered are the number of heated segment, 1≤n≤4, the heat source thickness, 0≤A≤0.1, the fusion temperature, 0.1≤Θf≤0.5, the heating intensity, 104≤Ra≤106, and the NEPCP concentration, 0≤ϕ≤0.05. The results shows that the Θf, n, A, and Ra modify the position and size of the area of phase change zone. The maximum Nusselt number was found at different angular positions depending on the heat source arrangement. The maximum value increases by increasing the NEPCP concentration, but its position almost unchanged by adjusting the concentration. There is critical A=Ac and max(ϕ) for the optimum arrangement of discrete heat source.

Order reduction, simplification and parameters identification for cold start model of PEM fuel cell

The cold-start of proton exchange membrane fuel cell (PEMFC) has been one of the leading technical challenges for fuel cell vehicle commercialization. It is essential to understand the mechanism and process of a cold start to improve the cold start ability of the PEM fuel cell. In this study, a one-dimensional cold start transient model of PEMFC was developed for transfer species, heat, charge and water phase change during the cold start process. Based on the 1-D spatially distributed model, a reduced nonlinear cold-start model of the PEMFC is presented. Then the parameter identification is conducted to validate the model accuracy based on the experiment data. The identification result shows that the reduced model captures the detailed model's multiplicity behaviours, and good agreement is observed during cold start simulation with experiment results. In contrast, the computation time is reduced significantly. Thus, the proposed order reduction model can be used to investigate real-time estimation and develop model-based control strategies for automotive PEMFC cold start.

Impact of grid spacing, convective parameterization and cloud microphysics in ICON simulations of a warm conveyor belt

Abstract. Warm conveyor belts are important features of extratropical cyclones and are characterized by active diabatic processes. Previous studies reported that simulations of extratropical cyclones can be strongly impacted by the horizontal grid spacing. Here, we study to what extent and in which manner simulations of warm conveyor belts are impacted by the grid spacing. To this end, we investigate the warm conveyor belt (WCB) of the North Atlantic cyclone Vladiana that occurred around 23 September 2016 and was observed as part of the North Atlantic Waveguide and Downstream Impact Experiment. We analyze a total of 18 limited-area simulations with the ICOsahedral Nonhydrostatic (ICON) model run over the North Atlantic that cover grid spacings from 80 to 2.5 km, including those of current coarse-resolution global climate models with parameterized convection, as well as those of future storm-resolving climate models with explicit convection. The simulations also test the sensitivity with respect to the representation of convection and cloud microphysics. As the grid spacing is decreased, the number of WCB trajectories increases systematically, WCB trajectories ascend faster and higher, and a new class of anticyclonic trajectories emerges that is absent at 80 km. We also diagnose the impact of grid spacing on the ascent velocity and vorticity of WCB air parcels and the diabatic heating that these parcels experience. Ascent velocity increases at all pressure levels by a factor of 3 between the 80 and 2.5 km simulations, and vorticity increases by a factor of 2 in the lower and middle troposphere. We find a corresponding increase in diabatic heating as the grid spacing is decreased, arising mainly from cloud-associated phase changes in water. The treatment of convection has a much stronger impact than the treatment of cloud microphysics. When convection is resolved for grid spacings of 10, 5 and 2.5 km, the above changes to the WCB are amplified but become largely independent of the grid spacing. We find no clear connection across the different grid spacings between the strength of diabatic heating within the WCB and the deepening of cyclone Vladiana measured by its central pressure. An analysis of the pressure tendency equation shows that this is because diabatic heating plays a minor role in the deepening of Vladiana, which is dominated by temperature advection.

The effect of relative humidity on vapor dispersion of liquefied natural gas: A CFD simulation using three phase change models

LNG is stored in cryogenic conditions, where the temperature is under 111K. In case of an accidental leakage into a normal atmosphere, LNG will vaporize and form cold methane gas cloud, namely LNG vapor cloud. In the existing researches, few have studied the effect of the inevitable water vapor phase change on the dilution of LNG vapor cloud under different relative humidity conditions. In this paper, a two-phase multi-species model is used in combination with three common water phase change treatments to simulate the dispersion of LNG vapor cloud and analyze the effect of relative humidity on it. The three approaches to deal with water phase transfer are: the one neglecting water phase change, and the other two considering water phase change by using the Lee model and the Sun model respectively. It is found that the results obtained by the Lee model and the Sun model show a similar response to humidity. In addition, although the density of LNG vapor clouds is usually higher than that of the surrounding air, there is a chance for the ambient air to overweigh the LNG vapor cloud in a hot and rather wet setting (RH>80%) if water phase transfer is considered.

Improvement of cooling of a high heat flux CPU by employing a cooper foam and NEPCM/water suspension

In the current simulation, a copper porous media was inserted on a hot surface of a CPU releasing high heat flux and then embedded within a mini-channel. The working fluid was a mixture of water and Nano-encapsulated phase change material (NEPCM) with fusion temperature of 303.47 K. The proposed model was simulated by computational fluid dynamic (CFD) based on finite volume method (FVM) to evaluate the impacts of Reynolds number (Re = 80, 90, 100, 110, and 120), porous medium with different porosity number (ε = 0.9, 0.925, 0.95, 0.975, and 1). The important parameters in CPU cooling was selected including pressure drop and maximum temperature of CPU's surface. The evidence said that inserting a copper foam with 0.95 porosity can reduce the maximum temperature of CPU's surface and increase the pressure drop by 40 K and 100 times compared to without porous media, respectively. Also, rising Reynolds number from 80 to 120 enhanced the pressure drop by 57 % and decreased the maximum temperature of CPU's surface by only 5.4 K.

Analytical model to predict unfrozen water content based on the probability of ice formation in soils

AbstractThe variation in unfrozen water content with temperature substantially affects coupled heat and water transport in frozen soil, causing frost heave and thaw settlement owing to the ice and water phase change and influencing soil stability in cold regions. Thus, analyzing the mechanism of water freezing and building a predictive model for the unfrozen water content of soils is paramount. In this study, an analytical model based on equivalent contact angle was developed to predict the unfrozen water content. The relationship between the equivalent contact angle and temperature was obtained based on the assumption that the heterogeneous nucleation rate nonlinearly decreased with temperature. The proposed analytical model was validated using existing unfrozen water content data at various temperatures for a silty clay soil material from the Qinghai–Tibet Plateau, and compared to several existing numerical models which predict unfrozen water content in soil materials. The results revealed a close relationship between the unfrozen water content and equivalent contact angle, and the equivalent contact angle increased as the temperature decreased. Meanwhile, the pore water in the soil first froze when the contact angle was smaller. Moreover, the values predicted by the analytical model for the unfrozen water content agreed well with the experimental results, especially under low‐temperature conditions and during the early stage of water freezing.

Experimental study of the effect of metal foams on subcooled flow boiling heat transfer of water and developing a correlation for predicting heat transfer

This paper has investigated the effect of different porosities (0.80–0.90) of the metal foam pipes on heat transfer coefficient and pressure drop of subcooled flow boiling of water experimentally. The metal foam pipe with 0.80 porosity has increased the Nusselt number and pressure drop by 41 % and 21 % compared to the 0.90 metal foam pipe, respectively. The heat transfer of water phase change in a porous medium has received less attention due to the complexities of the problem. One of the challenges in this field is the absence of a correlation for predicting heat transfer in metal foams. In this paper, a systematic method based on dimensionless numbers considering the essential parameters, like mass flux, subcooling, and heat flux, was used to provide a correlation for calculating the Nu-number based on 106 experimental points. The mean absolute deviation (MAD) of the results predicted by the new correlation is 8.9 %, and it predicts 95 % of the database with ±20 %.

Experimental study on pore water phase transition in saline soils

Sharp variations of thermophysical properties during the freezing process are caused by phase change of water and salt, which necessitates examining the water and salt transfer as well as deformation of saline soils. Meanwhile, the study on phase change in saline soils can provide the insight conditions for numerical heat-moisture coupling simulation. In order to investigate the principle of phase transformation and crystallization in saline soil with Na2SO4 and NaCl under freezing and freezing-thawing cycles, heat curves of saline soils with different salt concentrations were measured by differential scanning calorimetry (DSC). The temperature of phase transition and supercooling were obtained from the curves. According to the conservation of heat and mass, the proportions of salt crystal, ice crystal and unfrozen water produced during the phase transition were calculated, respectively. The precipitation sequence of salt crystal and ice crystal in saline soils was investigated, and the effect of NaCl on the phase transition of Na2SO4 was analyzed based on the phase change diagram of the salt solution. Finally, the effect of salt type and their concentration on soil structure was analyzed under freezing-thawing cycles, combining with soils' scanning electron microscope (SEM). The results indicated that the rules of supercooling for the soil with NaCl or mixture of Na2SO4 and NaCl were the same, whereby the supercooling diminished as the concentration or soil volume increased. Further, the addition of NaCl can significantly reduce the temperature of phase transition and ice content in sulfate saline soil, while Na2SO4 had little effect on the temperature of soil phase transition. Ice firstly generated in a small portion in soil and salt crystallized with the phase transition occurred for the most of water, and the curve of unfrozen water content in soils shifted toward low-temperature direction as the salt content increased. Phase transition sequence of water and salt will be changed for high salty saline soils under freezing-thawing cycles which has little effect on the precipitation amount of salt crystal in saline soils.

Experimental evaluation of performance of a hybrid solar photovoltaic (PV/T) panel integrated with effective cooling solutions with water base nanofluids and phase change materials

ABSTRACT The overall performance of photovoltaic (PV) panels is prejudiced by the operating temperature of the solar cell owing to the absorbed solar radiation. In this experimental study, the in-house hybrid PV/thermal (PV/T) system is designed, fabricated, and parametrically studied for effective and practical applications of the solar panels. The water and water-based nanofluids are used for the active cooling of the PV/T system. The effect of phase change material (PCM) along with the active cooling is also studied, and results are comprehensively discussed. The in-house stable water-based Al2O3 and TiO2 nanofluids are engineered by the two-step method with two different nanoparticle concentrations of 0.05 and 0.1 vol.%. The thermal conductivity and stability of the nanofluids are experimentally measured and discussed to understand their effects on heat transfer from the PV panel. The thermal conductivity of nanofluids is increased with the concentration of nanoparticles in the base fluid. The maximum enhancement of around 30% is observed with Al2O3-water nanofluid at a concentration of 0.1 vol.%. The PV/T system is experimentally simulated in the indoor experimental setup with the solar simulator to provide the variable intensity of solar irradiance from 450 to 1050 W/m2. The maximum decrement in the panel’s temperature is found to be around 25% using both active and passive cooling simultaneously and the electrical efficiency is improved by approximately 10.3% at the 1050 W/m2. The electrical efficiency has found 13% more with Al2O3-water nanofluid of 0.1 vol% as compared to water cooling at 450 W/m2. The use of nanofluids also improved the system’s thermal efficiency compared to the water, and the maximum improvement of around 10% is achieved with the Al2O3-water nanofluid at 0.1 vol.% concentration. These results are significant for the design of the PV/T system from the application viewpoint.

Two-dimensional metamaterials as meta-foams for optimized surface-enhanced solar steam generation

We report on a new class of metamaterials, which consists of meta-foams, optimized for high performance in enhanced solar steam generation. The design of these meta-foams involves precise dimensional control of a periodic two-dimensional micro-pore array. Proof-of-concept of meta-foams efficiency is demonstrated using two fabrication technologies. The first one involves microscale plasma etching onto a nanostructured black silicon substrate. As an alternative low-cost technique, we also use 3D printing of a graphene-containing polymer material. The experimental validation shows that the best evaporation rate reaches 1.34 kg/(h·m2) under 1 sun illumination, in open-air conditions under normal temperature and relative humidity conditions of 20 °C and 58%, an unmatched value so far under similar conditions, while the theoretical limit is estimated at 1.5 kg/(h·m2). This corresponding to a 89% conversion efficiency. Experimental data concur well with the predicted performance obtained by numerical simulations. The proposed model simultaneously accounts for both heat and mass transfer, including photothermal conversion and water phase change, to guide the optimization towards the maximization of the evaporation rate, an important figure of merit in many applications, including solar heat harvesting and solar water purification.

A reconfigurable and magnetically responsive assembly for dynamic solar steam generation

Interfacial solar vapor generation is a promising technique to efficiently get fresh water from seawater or effluent. However, for the traditional static evaporation models, further performance improvement has encountered bottlenecks due to the lack of dynamic management and self-regulation on the evolving water movement and phase change in the evaporation process. Here, a reconfigurable and magnetically responsive evaporator with conic arrays is developed through the controllable and reversible assembly of graphene wrapped Fe3O4 nanoparticles. Different from the traditional structure-rigid evaporation architecture, the deformable and dynamic assemblies could reconfigure themselves both at macroscopic and microscopic scales in response to the variable magnetic field. Thus, the internal water transportation and external vapor diffusion are greatly promoted simultaneously, leading to a 23% higher evaporation rate than that of static counterparts. Further, well-designed hierarchical assembly and dynamic evaporation system can boost the evaporation rate to a record high level of 5.9 kg m−2 h−1. This proof-of-concept work demonstrates a new direction for development of high performance water evaporation system with the ability of dynamic reconfiguration and reassembly.

Modeling of path-dependent phase change in sorption and freezing of pore water for cementitious materials

Many physical and mechanical behaviors of cement-based materials can be affected by the phase change and phase equilibrium of pore water, such as shrinkage, creep, ion transport, frost damage etc. In addition, the thermodynamic behavior and sorption of pore water are closely related to the pore size distribution (PSD) and pore shape. Practical prediction of liquid/gas/ice content during adsorption-desorption and freezing-thawing process is thus needed for an engineering purpose. This paper presents a path-dependent phase change model based on the empirical PSD model, in consideration of the humidity history, temperature history and the hysteresis during drying-wetting and freezing-thawing. The adsorption-desorption curves and ice formation during freezing-thawing are calculated for different mix proportion and different environmental conditions. A good agreement can be found with the selected experimental data. The developed empirical formulations can be beneficial for further theoretical models and numerical simulations.

Unmanned aerial vehicle transport of frozen blood samples using phase change materials

Appropriate cryopreservation of blood samples during transit from the point of collection to testing sites is an important element of quality sample management in clinical and veterinary diagnostics and research. Here, a heat-insulated design that employs 23.3% w/w salt water as the phase change material (PCM) for encapsulating a 1.5-mL vial of frozen blood sample was studied to assess its efficacy in maintaining sub-zero temperatures during transport. A two-dimensional finite element numerical simulation model with 5457 elements was found to exhibit mesh independence with 10% convergence error. For the PCM pre-cooled in dry ice and vial sample kept initially at 20 °C before flight, this model is predicted to maintain a stable transport temperature of −20 °C for at least 70 min. It also predicted to have uniform temperature distribution across the PCM holder, PCM, sample vial and sample. Experimental data obtained using a thermocouple confirmed the accuracy of the simulation model. Sensor data collected from a bespoke unmanned aerial vehicle (UAV) allowed mapping of the acceleration forces during flight and facilitated the development of a mechanical setup to experimentally show that frozen samples are protected from shear forces generated under mimicked transport conditions. The capability of real-time sample temperature monitoring with feedback control on UAV flight path demonstrates the viability of the drone delivery system for quality transport of thermal-sensitive samples. • UAV transport of frozen blood samples done here with salt water phase change material. • 2D finite element model achieved mesh independence with 10% convergence error. • 70 min stable transfer at −20 °C attained after 30 min of dry ice pre-cooling. • Sensor data from UAV mimicked the flight conditions in a sloshing test setup. • Preliminary tests with frozen sheep's blood show lower coagulation from sloshing.

Molecular dynamics simulation and experimental study of heat transfer and phase change of water with slit effect

The condensation water-bridge phenomenon of the heat exchanger surface fins is a common undesirable phenomenon in refrigeration systems and heat pump systems, which would significantly affect the heat exchanger's thermal efficiency due to insulation and obstruction of airflow. Based on the lack of experimental studies on louvered-fin heat exchangers and surface energy for fin-surface condensation, this paper experimentally investigates the effect of wettability (surface energy) on the condensation mechanism and heat exchanger performance. Firstly, hydrophilic and hydrophobic coatings are formed by reagent deposition, which regulates the surface energy of the fins. Experiments are designed to investigate the effect of surface energy on heat exchanger performance at different temperatures, humidity, and wind speed. Finally, the key issues such as condensation morphology are discussed in detail by molecular dynamics simulation, thus filling the gap and providing a reference for heat exchanger surface construction. The experimental results show that the wet condition has a faster condensation rate, with an average rate reduction of 33.2%. Both hydrophilic and hydrophobic fins could improve the performance of the heat exchanger compared to bare fins due to their inherent uniqueness. The hydrophilic fins have a smaller pressure drop, maintain a more stable heat transfer rate, and improve the heat transfer performance by 13.4% in a typical environment (60%) or with lower water content. While the hydrophobic fins show advantages in a wet environment (80%), improving the heat transfer performance by 17.8% due to the severe deterioration of the bare heat exchanger performance. Further, molecular-scale condensation patterns were analyzed, and condensation rates and adsorption forces were quantified. The experimental and simulation results guide the construction and selection of surface energy to optimize the heat transfer surface.

Receiving heat from a PCM tank by using natural convection of water and NEPCM: A simulation for LHTES application

Latent heat thermal energy storage (LHTES) systems are widely used due to their ability to store a lot of heat energy as latent, and few studies have directly or indirectly investigated on the heat exchange methods with LHTES systems. In the present study, the free convection of a mixture of water and Nano-encapsulated phase change material (NEPCM) with a porous medium was used to exchange heat energy between a cold-water stream and a hot tank filled phase change material (PCM). NEPCM are nanostructures consisted of a solid shell and a PCM core, and the addition of these nanostructures to water has remarkable effects on the heat transfer parameters. In this research, a number of cases were simulated using computational fluid dynamic (CFD) to observe the impacts of injecting NEPCM to water, the porosity of the porous media and the location of the phase change zone on the average heat flux of the PCM tank. The results showed that increment of porosity number from 0.85 to 0.95 decreased the average heat flux of the PCM tank by 13.8%. Moreover, adding 3% NEPCM to water enhanced the average heat flux of the PCM tank by 22.2% and 18.4% when porosity numbers were 0.85 and 0.95, respectively.

설비공학 분야별 최근 연구 동향: 2021년 설비공학논문집 발표논문에 대한 종합적 고찰

This article reviews the papers published in the Korean Journal of Air-Conditioning and Refrigeration Engineering in 2021. The purpose of this article is to understand the status of current research issues in the areas of heating, cooling, ventilation, sanitation, and indoor environments of buildings and plant facilities. Summary of this paper is follows.<BR> (1) The topics of the paper in the field of building mechanical system were the energy consumption, indoor environment analysis, building energy saving, ventilation system, renewable energy system and law of mechanical facilities.<BR> (2) The topics of the research in the field of indoor environment were the IAQ, ventilation, air cleaning, cooling/heating load, renewable energy, zero-energy building, building energy performance etc.<BR> (3) The research topics in the field of refrigeration in 2021 include robust temperature control of variable speed refrigeration system, R32 liquid-injection showcase vapor compression system, hybrid dessicant dehumidifying system, compressor torque calculation by differential pressure measurement, FAPO zeolite adsorbent, compression-adsorption hybrid refrigeration system and so on.<BR> (4) The topics of the papers published in the field of heat were heat transfer, a heat exchanger and a battery, etc. <BR> (5) The topics of the papers published in the field of heat transfer were the air flow rate inside drum of clothes dryer, the dynamic behavior and thermal performance of a solar water heater, a higher energy density storage system using sensible heat or phase change of water, factors affecting performance characteristics of falling film evaporation using R-1233zd(E) refrigerant and the underground seawater intake system in a power plant.