Thermal Losses Research Articles

Energy and exergy analysis of a 20-MW grid-connected PV plant operating under harsh climatic conditions

Abstract Large grid-connected photovoltaic (PV) plants are increasingly being installed around the world, including in harsh desert climates. Evaluating their performance can help improve the design and operation of PV systems. This study performed an energy and exergy analysis of a 20-MW grid-connected PV plant under desert climatic conditions in southern Algeria over a period of 1 year. The PV plant was divided into two 10-MW subsystems. Energy analysis was performed using actual irradiation, power output, wind speed and ambient temperature data. The annual average energy efficiency of the plant and subsystems was 10.82%, 10.95% and 10.69%, respectively. Solar radiation had the most significant impact (80% determination coefficient) on thermal exergy loss. The exergy efficiency of the plant was lower than the literature values, likely due to the harsh desert conditions. The comprehensive energy and exergy analysis provides insights into the performance of large-scale PV plants in desert climates. The results can help guide the system design and operation improvements for such conditions. Regular cleaning and cooling could improve performance.

Performance comparison of a standing-wave thermoacoustic engine with different resonator shapes with air working gas

The resonator is one of the key components of thermoacoustic machines and the choice of its geometry is of course of crucial importance, as it determines the way the machine operates. The resonator must be designed so that the quality factor of the resonance is as high as possible and must satisfy the technological production constraints related to the high pressurization of the working fluid and the presence of temperature non-uniformities. The shape of the resonator can be modified to reduce certain loss phenomena, particularly on the walls, and to limit the formation of harmonics in the high-level acoustic field. In this work, an acoustic power losses analysis and a numerical study of a standing-wave TAE with air working gas, considering changes in resonator shape, are performed to investigate the effect of resonator geometry on the thermoacoustic energy conversion. The acoustic power dissipation in the resonator is analyzed based on the simplified thermoacoustic theory and the numerical study is conducted based on a 2D numerical model based on a CFD analysis. Results show that the shape of the resonator significantly affects the frequency, the temperature along the stack, and the acoustic pressure produced on the TAE. In addition, it is shown that the resonator composed of two different diameters showed its ability to produce a high acoustic pressure amplitude (20.5% than the iso-diameter resonator) and minimize the viscous and thermal losses.

Mathematical model to optimize solar spectrum allocation for a Stirling engine-based concentrated photovoltaic thermal system

Incorporating a Stirling engine into a concentrated photovoltaic thermal system (CPVT) with spectral beam splitting converts wasted photovoltaic panel energy into electricity, boosting efficiency. However, complex Stirling engine development slows system performance estimation, which explains the scarce studies on the Stirling engine-based CPVT spectrum allocation range. Furthermore, the literature shows inconsistent remarks on low or high-energy photon allocation to a thermal system and uncertainty of solar irradiance variation effect on spectrum allocation. A generic methodology is presented for determining optimal spectrum allocation for a Stirling engine-based CPVT system with a dichroic filter. The analysis in this work shows that considering non-absorption and thermalization losses improves performance by 13.2 % compared to solely accounting for non-absorption loss in beam splitting. Additionally, a shift of up to 40 nm in the optimal spectrum allocation range is observed with a daily spectral irradiance source compared to the AM1.5D source. The PV mathematical model is validated with an average error of 8.9 %, and the Stirling engine model with an error of 5.2 %. Through this study, future researchers can determine an optimal spectrum allocation range for Stirling engine-based CPVT that maximizes PV performance based on a specified location with the proposed generalized CPVT model.

Numerical Study of the Cold Metal Transfer (CMT) Welding of Thin Austenitic Steel Plates with an Equivalent Heat Source Approach

The CMT (cold metal transfer) arc welding process is a valuable joining method for assembling thin sheets, minimizing heat transfers, and reducing subsequent deformations. The study aims to simulate the CMT welding of thin stainless-steel sheets to predict temperature fields and deformations. Both instrumented tests and numerical simulations were conducted for butt-welding of sheets with a thickness of 1 to 1.2 mm. Weld seam samples were observed to identify equivalent heat sources for each configuration. The electric current and voltage were monitored. Temperature measurements were performed using K-type thermocouples, as well as displacement measurements via the DIC (digital image correlation) technique. Thermomechanical simulations, considering phase changes and the actual seam geometry induced by filler material, were conducted using an equivalent heat source approach. A unique heat exchange coefficient accounting for thermal losses was identified. By incorporating these losses into thermal calculations, a good agreement was found between measured and calculated temperatures. Mechanical calculations allowed for the recovery of the horse saddle form after actual welding, with a relative difference of less than 10% in angular distortion between calculated and measured values.

Strong Dimensional and Structural Dependencies of Hot Carrier Effects in InGaAs Nanowires: Implications for Photovoltaic Solar Cells.

III-V nanowire structures are among the promising material systems with applications in hot carrier solar cells. These nanostructures can meet the requirements for such photovoltaic devices, i.e., the suppression of thermalization loss, an efficient hot carrier transport, and enhanced photoabsorption thanks to their unique one-dimensional (1D) geometry and density-of-states. Here, we investigate the effects of spatial confinement of photogenerated hot carriers in InGaAs-InAlAs core-shell nanowires, which presents an ideal class of hot carrier solar cell materials due to its suitable electronic properties. Using steady-state photoluminescence spectroscopy, our study reveals that by increasing the degree of spatial confinement and Auger recombination, the effects of hot carriers increase, which is in good agreement with theoretical modeling. However, for thin nanowires, the temperature of hot carriers decreases as the effects of crystal disorder increase. This observation is confirmed by probing the extent of the disorder-induced Urbach tail and linked to the presence of a higher density of stacking defects in the limit of thin nanowires. These findings expand our knowledge of hot carrier thermalization in nanowires, which can be applied for designing efficient 1D hot carrier absorbers for advanced-concept photovoltaic solar cells.

Distributed Coordinated Operation of Active Distribution Networks with Electric Heating Loads Based on Dynamic Step Correction ADMM

In order to change the centralized operation framework of the active distribution network with electric heating loads (EHLs), a distributed optimization method is proposed for the coordinated operation of the active distribution network with EHLs. Firstly, considering the thermal delay effect and heat loss of the thermal system, a centralized optimization operation model for active distribution networks with EHLs is established. Then, based on the centralized optimization operation model, it is rephrased as a standard sharing problem, and a distributed optimization operation model for the EHL active distribution network is established based on the alternating direction multiplier method (ADMM) solution. In the process of solving ADMM, dynamic step correction was further considered. By updating the steps during the iteration process, the number of iterations was reduced, and the convergence and computational efficiency of ADMM were improved. Finally, the effectiveness of the distributed coordinated operation method proposed in this paper was simulated and verified by constructing an IEEE33 distribution system. The results showed that the proposed distributed coordinated operation method has strong robustness to the randomness of the number of distributed units and parameters, and EHLs participating in coordinated operation can expand the consumption space of wind power and photovoltaic power, and improve the economic efficiency of system operation.

Thermophotovoltaic emitter design with a hyper-heuristic custom optimizer enabled by deep learning surrogates

Micro/nanostructures hold promise for use as emitters to boost the efficiency of thermophotovoltaic (TPV) systems. For example, periodic gratings may alter the spectrum of irradiation on the photovoltaic cell to better match the spectral response of the cell. Photons with energies slightly higher than the bandgap of the semiconductor are the most desired as they generate electron-hole pairs with minimal thermalization losses. This prompts the use of gratings as selective emitters, and the choice of grating geometry has been an active research topic. Even for a one-dimensional (1D) grating, millions of possible geometries exist, and each grating requires computationally intensive full-wave simulation, e.g., the rigorous coupled-wave analysis (RCWA), to calculate the spectral, directional emissivity. Since optimization algorithm performance is problem-dependent, in this work, a hyper-heuristic search enabled by a fully connected neural network surrogate of the native RCWA is employed to select an algorithm for a specific grating design problem. A comparison with existing untuned algorithms for an ideal emitter problem demonstrates that the hyper-heuristically generated algorithm yields superior performance. This algorithm is then employed for the optimization of the emitter for a full TPV system comprising a heated 1D tungsten binary grating paired with a 300 K InGaSb cell. The system is optimized for maximum power or efficiency at 2000 K and 1500 K, respectively, and the grating properties for the optimized cases are analyzed.

Optimization with genetic algorythms of PVT system global efficiency

Photovoltaic (PV) solar panels generally produce electricity in the 6% to 12% efficiency range, the rest being dissipated in thermal losses. To recover this amount, hybrid photovoltaic thermal systems (PV/T) have been devised. These are devices that simultaneously convert solar energy into electricity and heat. A significant amount of research on PV/T collectors has been carried out over the last decade and water PV/T glazed flat plate collector systems turned out to be the most promising to develop (Zondag[1]). It is thus interesting to study the PV/T system as part of a closed loop single phase water CDU (coolant distribution unit) in laminar forced convection. In particular, the analysis was conducted on the optimal cooling performance of the thermal part, testing polynomial channel profiles of varying order (from zero to fourth) for channels of a real industrial module heat sink, under the following conditions: ideal flux of 1000 W/m² on one side, insulation on the opposite side, periodic conditions on the remaining sides, fully developed thermal and velocity profile in laminar flow of water. Through the use of a genetic algorithm, we have optimized the shape of the channel’s sidewalls in terms of heat transfer maximization. In terms of Nusselt number, results show that fourth order profiles are the most efficient. When limits to allowable pressure loss and module weight are introduced, these bring generally to a lower efficiency of the system than the unconstrained case.

Numerical and experimental study of a multiphase flow inside a self-contained drum motor drive system

The multiphase Taylor-Couette flow of air and oil enclosed within the annular space between two concentric cylinders has been studied experimentally and numerically. The outer cylinder rotates at a constant speed and exchanges heat by convection and radiation with ambient air. The inner cylinder is stationary and subjected to a constant uniform heat flux. Such flow can be found in a self-contained drum motor drive system (SCDMDS). The heat flux imposed on the inner cylinder mimics the amount of heat generated due to several thermal losses within the SCDMDS. A layer of lagging material is sometimes added to the outer cylinder to enhance belt traction.The effect of the inner-to-outer cylinder radius ratio (RR), Reynolds number (Re), oil volume (OV), outer cylinder surface emissivity (ε), and lagging material thickness (δ) on the flow and heat transfer between the two cylinders and the ambient has been investigated considering a quasi-steady state analysis. The current investigation considered the following ranges found in a typical SCDMDS: RR = 0.35–0.85, Re = 200–3000, OV = 50–100 %, ε = 0–0.95, and δ = 0–100 mm. The numerical part of the present study has been carried out using ANSYS-CFX software package. The numerical results have been validated using experimental data. Resultsindicated that increasing RR, Re and ε enhanced the heat transfer within the SCDMDS. The rate of heat transfer within the annular space reached its highest value at OV = 65 % above which the rate of heat transfer is deteriorated. Because the lagging material is acting as an insulation over the outer cylinder, a critical thickness of δ = 35 mm resulted in the minimum overall resistance above which the rate of heat transfer is deteriorated.The parametric study revealed that the best overall thermal performance was attained at RR = 0.85, Reynolds number = 3000, OV = 65 %, ε = 0.95 and δ = 35 mm. In this case, the heat transfer coefficient between the inner cylinder and the oil reached its highest value of 410 W/m2 °C, giving the lowest possible inner cylinder temperature. A correlation of Nusselt number in terms of the geometrical and operating parameters of a SCDMDS considered in the current study has been developed and validated. The developed correlation gave a maximum deviation of ±8 %.

Simple Design Scheme for a 300 kW Modular NPC Inverter Using Thermal RC Network Analysis

This paper proposes a simple design scheme for a modular NPC inverter using thermal RC network analysis. The proposed design process is an efficient and straightforward approach to designing the heatsink for a 300 kW modular neutral-point-clamped inverter. The heatsink design plays a crucial role in achieving high power density of a power converter because the weight and size of the heatsink are primarily influenced by its type. The structure and dimensions of the heatsink are mainly determined based on the generated heat by losses of the power semiconductor switches. In this paper, a thermal RC network model was established using parameters from the power switch module and was applied to the simulation of the power converter. The thermal losses of the power semiconductor switches were calculated via this process, and the heatsink was designed according to the calculated thermal losses. The proposed design scheme was analyzed and compared with the thermal fluid dynamic model. To validate the feasibility of the proposed design process, The simulation results were compared with experimental results.

Heat transfer characteristics of liquid cooling system for lithium-ion battery pack

To improve the thermal uniformity of power battery packs for electric vehicles, three different cooling water cavities of battery packs are researched in this study: the series one-way flow corrugated flat tube cooling structure (Model 1), the series two-way flow corrugated flat tube cooling structure (Model 2), and the parallel sandwich cooling structure (Model 3). Based on the fluid-solid coupling method, this study analyzes the cooling performance of the three models, including thermal uniformity, heat dissipation, and pressure loss. At a high discharge rate, compared with the series cooling system, the parallel sandwich cooling system makes the average temperature and maximum temperature of the battery pack decrease by 26.2% and 26.9% respectively, and the battery pack temperature difference decreases by 62%, and the coolant pressure loss decreases by 95.8%. The results show that the Model 3 overcomes the temperature accumulation caused by the series flow of coolant and achieves a better level of thermal uniformity while improving the heat dissipation and pressure loss performance. The research provides scholars and industries with a reference for upgrading thermal management and improving the stability of the power battery pack for electric vehicles, which has both theoretical and practical significance.

Scale-Up Considerations of the Sundial Rotating Fresnel Solar Collector

In the frame of the ASTEP project two prototypes of the Sundial collector, together with a Phase Change Material Thermal Energy Storage, will be tested integrated with the industrial processes of two sites. However, the size of the pilot systems will not be optimized for commercialization. The present work considers the scale-up of the concept for one of the pilot industrial sites: a dairy industry located in Greece, and how will the thermal losses of the system must be considered with the increased aperture, thermal storage size and energy output. For the scaling up of the system, the capacities of the Sundial, demand and TES were doubled and multiplied by 10, and the thermal losses of both nominal conditions and annual performance were considered. The results showed that the ratio between the piping heat losses and the total available energy decreased when scaling-up the system both in the nominal and in the annual results. Hence, the heat losses lost importance when scaling up, when more energy was available. These facts should be considered when evaluating the performance of the pilots and feasibility of the concept.

Research progress on classification, source, application of phytosterol esters, and their thermal oxidation stability

Phytosterol esters can effectively decrease serum cholesterol concentration in the human body and prevent cardio-cerebrovascular diseases. It was found that phytosterol esters exhibited better solubility and bioavailability than free phytosterols. In recent years, phytosterol esters have attracted increasing attention. However, during food processing, phytosterol esters are susceptible to degradation at high temperatures, resulting in certain losses and formation of potentially harmful substances for humans. This paper reviews the relevant literatures and updates on the thermal oxidation stability of phytosterol esters in recent years from the following aspects: (i) Sources, physiological activities, and applications of phytosterol esters; (ii) Oxidation mechanism of phytosterol esters; (iii) Effects of phytosterols species, the volume of addition, food matrix, heating temperature and time, and antioxidants on the thermal loss and oxidation stability of phytosterol esters. The research progress on the safety of phytosterol esters is also discussed in detail. Additionally, the prospects for future research are highlighted.

On the stabilization mechanism of high-speed deflagrations in narrow channels with heat loss

Statistically steady supersonic deflagrations are numerically investigated in narrow channels with strong thermal expansion and heat loss. Four modes of flame propagation are observed, namely, extinction, low-speed deflagration, high-speed deflagration, and DDT. It is determined that larger thermal expansion facilitates initiation of high-speed deflagrations while the heat loss can suppress the transition to detonation. The high-speed deflagration mode is shown to be the result of the dynamic balance between thermal expansion and wall heat loss. The limits of high-speed deflagration in terms of the thermal expansion and heat loss coefficients are determined. The statistically steady oscillatory high-speed deflagrations propagate at average velocities close to half of the CJ detonation velocity. The dynamics of the flame front and shock waves are visualized using numerical schlieren. Periodic acceleration and deceleration of the leading shock are identified, and the mechanism of DDT suppression is elucidated.

A data analysis system for thermal energy loss in electrical automation management

In order to achieve data analysis of thermal energy loss for electrical automation management, the author proposes a Broyden Fletcher Goldfarb Shanno (BFGS) trust region algorithm based thermal energy loss calculation method for the integrated energy system of electricity and gas, the BFGS trust region algorithm is utilized to obtain the thermal energy loss distribution of natural gas system, which is not highly dependent on the initial value, this solves the problem of non-convergence in power flow calculation when using the Newton method for calculation. The experimental results show that the relative errors of natural gas system node pressure, pipe-line flow and power system node voltage and phase angle calculated by Newton?s method and BFGS trust region algorithm are within 1 ? 10-4, within the allowable range, which verifies the correctness of the calculation results of the method described by the author. It has been proven that the BFGS trust region algorithm can achieve data analysis of thermal energy loss for electrical automation management.

Design and Performance Comparison of District Heating Systems in Milan and Riga

Abstract The work proposes a comparison between three heating configurations covering the demand of a new settlement: 1) centralized district heating system (DHC); 2) 5th generation district heating & cooling system (5GDHC) and 3) individual home heating and cooling (HHC) systems. Thermal and electrical loads are evaluated by transient simulations of a residential area with 80 buildings. The energy plants are based on different technologies: combined heat and power plants, gas-fired boilers, and domestic heat pumps. A transient numerical model has been developed for each solution. Every component is modelled according to performance maps provided by the manufacturers, allowing an accurate simulation in both design and off-design operating conditions. On an annual basis, the Latvian residential complex requires almost twice as much energy as the Milan one. The thermal losses in the district systems are 4.21 % in the Milan solution and 5.65 % in Riga. The district heating system coupled with a heat pump represents the best layout in terms of primary energy consumption in both locations, with energy savings of 50 % compared to other solutions. The use of 5GDHC is a good compromise that could increase the use of renewable energy. The adoption of a cogeneration plant is a good choice in the case of a centralized district system that allows the installation of high-efficiency genset. On the contrary, for small applications such as residential, the cogeneration results are expensive, and the conversion efficiency does not justify the installation.

Interface chemical coupling enables Janus elastomer-hydrogel composites for a roof-free evaporator with efficient hydrocooling condensation

A solar-driven roof-free evaporator (SRFE) has been designed based on an asymmetric elastomer/photothermal hydrogel composite. This device offers the advantages of high solar absorption and low thermal convection loss, making it suitable for use as a portable purified water collection device.

Design and thermal test of high-vacuum insulator for heat delivery pipes

Thermal piping insulation of implants is crucial for heat delivery, production, collection, or storage at high temperature values. It is currently obtained by enveloping low thermal conductivity materials such as rockwool, fiberglass, polyurethane, polystyrene, and aerogel. However, better performances can be reached by adopting vacuum technology. In this case, conductive losses are annihilated, and the radiative heat transfer mechanism represents the only loss mechanism. Here, we compare a high vacuum-based novel solution and the traditional insulation for heat delivery applications. We propose a high vacuum- based solution consisting of an evacuated gap that surrounds the hot pipe coated by a thin aluminium foil. Experimental results using this novel solution show a fivefold reduction of the thermal radiation losses compared to the traditional solutions when in the temperature range between 100 °C and 250 °C.

Mechanism analysis and optimal design of sound-absorbing metastructure constructed by slit-embedded Helmholtz resonators

<sec>Low-frequency noise has always been a thorny problem in the field of noise control. In recent years, the development of sound-absorbing metastructures has provided new ideas for controlling low-frequency noise. In this work, we propose a low-frequency sound-absorbing metastructure constructed by Helmholtz resonators with embedded slit. Analytical and numerical models are established to analyze the sound absorption performance and mechanism of the proposed sound-absorbing metastructure, and optimization design is conducted to achieve low-frequency wideband absorption performance. The analytical modeling method and the performance of the proposed sound-absorbing metastructure are also experimentally verified. The main conclusions are summarized as follows.</sec><sec>1) By using transfer matrix method and finite element method, analytical and numerical models for calculating sound absorption coefficient are established. It is shown that analytical predictions are in good agreement with numerical calculations. It is demonstrated that a typical design of a 30-mm-thick single-cell metastructure can achieve a sound absorption coefficient of 0.88 at 404 Hz. Typical designs of two-cell parallel structure and the four-cell parallel structure (both with a thickness of 50 mm) can achieve two and four nearly perfect low-frequency sound absorption peaks in a frequency band of 200–400 Hz, respectively.</sec><sec>2) The low-frequency sound absorption mechanisms of the proposed metastructures are explained from four aspects: simplified equivalent model parameters, normalized acoustic impedance, complex-plane zero/pole distribution, and sound pressure cloud image and particle velocity field distribution. It is demonstrated that the main sound absorption mechanism is related to the thermal viscous loss of sound waves, caused by the inner wall of embedded slit.</sec><sec>3) The design for broadband low-frequency absorption performance is optimized by using differential evolution optimization algorithm. An optimized parallel-multi-cell coupled metastructure with multiple perfect sound absorption peaks below 500 Hz is realized. For a thickness of 90 mm, the sound absorption coefficient curve of an optimized metastructure exhibits 8 almost perfect sound absorption peaks and an average sound absorption coefficient of 0.86 in a frequency range of 170－380 Hz.</sec><sec>4) Experimental samples are fabricated to test sound absorption. Experimental results are basically consistent with the analytical predictions. The results from analytical model, numerical calculations and experimental measurements are mutually verified.</sec><sec>In summary, the sound-absorbing metastructures with a thickness of sub-wavelength, proposed in this work, exhibit outstanding sound absorption performance at low frequencies. We demonstrate that they are suitable for low frequency broadband sound absorption below 500 Hz. Owing to their thin thickness and relatively simple construction, they have broad application prospects in practical noise control engineering.</sec>

Parâmetros térmicos do UHPC em situação de incêndio: uma abordagem à norma brasileira ABNT NBR 15200

Abstract Ultra-high-performance concrete (UHPC) has exceptional mechanical properties at room temperature. However, there are no standardized thermal procedures proposed by ABNT NBR 15200 to characterize UHPC at high temperatures, such as in the case of fire. This is due to the lack of research on the subject, leaving gaps for experimental and numerical investigations. Based on experimental analyzes on standardized, small-scale specimens, this research collects a set of parametric data for UHPC at high temperatures. Thermal diffusivity, thermal conductivity, thermal expansion and specific heat as thermal parameters were defined for different temperature ranges. The results were compared with other structural concretes proposed in the literature (NSC, HSC and UHSC). The UHPC exhibited a particular fire behavior. Compared to NSC, HSC and UHSC, the thermal expansion and mechanical parameters of UHPC are less affected at high temperatures, but its thermal conductivity and mass loss are higher. UHPC also has the highest specific heat compared to other concretes. The thermal field of UHPC tends to be higher compared to the other concretes.