Solar photovoltaic (PV) system harness the energy from the sunlight and convert into clean electricity to power homes and businesses. During an operation the solar panels get hot, the electrons inside the solar cells pick up that extra heat energy which puts them in a more excited state and when they are already excited, they have less room to absorb the energy from sunlight. As a result, electrical performance of a PV system reduces with increase in solar cell temperature. Efficiency of PV panels can be retained by establishing a hybrid PV-Thermal (PV-T) system. In this study, container filled with phase change material (PCM) embedded in porous metal is attached to back surface of the PV cell. As well as, to extract the excess heat from the PV cell, water is used as a heat transfer fluid (HTF) with constant mass flow rate of 30 kg/hr. During the simulation melting rate of PCM, amount of latent heat energy stored, thermal, electrical and overall efficiencies of the PV panel is studied and compared with the conventional PV-T system. The results show the enhanced melting fraction of PCM by 6% and 8% for the PV-T/PCM/Cu and PV-T/PCM/Al system, respectively compared with PV-T/PCM system. Moreover, in comparison with the conventional PV-T system, the overall efficiency of the PV-T/PCM/Cu and PV-T/PCM/Al is increased by 10.62% and 8.80%, respectively.

Thermal barrier coatings are widely used in gas turbines to protect the gas turbine metal components against very high combustion product temperature. To improve energy efficiency, higher combustion temperatures are needed. A limiting factor at present is the stability under extreme and prolonged heating of thermal barrier coatings. The coatings are typically made by the air plasma sprayed process in which fine particles of yttria-stabilized zirconia (YSZ) are melted or partially melted and ejected from plasma jet at high speed onto the bond coated substrate metal. With increasing combustion temperature and pressure in the modern gas turbine engines radiative heat transfer is becoming an important portion of the overall heat transfer in the thermal barrier coating. This study has demonstrated that the commonly used Kubelka-Munk method in the radiative property reduction from the measured transmittance and reflectance spectra of YSZ coatings will incur inaccurate result when the coating optical thickness is not sufficiently large. An alternative method — the discrete ordinates method with the asymmetric spherical ring angular quadrature — is used instead. The absorption and scattering coefficients of air plasma sprayed YSZ films are determined over the wavelength range from 1 to 2.6 μm at room temperature. Over this near infrared wavelength range, the scattering coefficient decreases with the increasing wavelength and the absorption coefficient is very small overall. The pore size distributions before and after the 50-hr temperature gradient, thermal cycling are compared. The sintering effect as well as the crack growth will impact the coating radiative properties. These results point to a clear need for better understanding of the radiative heat transfer process, which includes the microstructure-property relationship, progressive changes of the radiative properties with the operation condition and time.

Recent studies have indicated that droplet evaporation heat transfer can be substantially enhanced by fabricating a thin nanoporous superhydrophilic layer on a metal substrate. Such surfaces have immense potential to improve spray cooling processes, however, little durability testing of the surfaces have been performed. In spray cooling applications, as water evaporates any impurities in the water will be deposited onto the surface. Primarily, this investigation serves to demonstrate how minerals in hard water deposit on the surface and interact with the ZnO nanopillars of the superhydrophilic surface. Quantifying the effects of mineral scale on droplet spreading and vaporization heat transfer on the surface is important in determining implementation requirements to advance the surface into industry applications. Micrographs of the surface demonstrate minerals deposit nonuniformly, and quickly fill the nanostructure. Despite a reduction in the extent of droplet spreading due to the mineral deposition, scaled surfaces still demonstrated improved thermal performance compared to the uncoated, smooth copper surface. Scale tended to build up on previously deposited scale leaving largely uncoated areas where droplets chose to preferentially spread resulting in a continued low contact angle. Maintaining these uncoated areas and reducing the contaminants present in the water will extend the life and performance of the nanostructured surface.

Recent studies have indicated that ZnO superhydrophilic surfaces on copper and aluminum substrates enhance surface wettability and evaporation processes in droplet evaporation and pool boiling experiments. At slightly superheated surface temperatures, droplet evaporation on a nanoporous superhydrophilic surface exhibits onset of nucleation and nucleate boiling effects found in pool boiling processes. This study explores water droplet evaporation and quenching experiments conducted on nanostructured surfaces of a 45° downward facing pyramid copper, aluminum, and 304 stainless steel substrate. The nanostructured surfaces were used to conduct adiabatic droplet deposition, droplet evaporation, and quenching experiments. Through the three experiments and through surface characterization, by means of scanning electron microscopy, the underlying heat transfer performance and mechanism of droplet evaporation and pool boiling on various host metal substrates can be understood. The six surfaces tested were the following: bare copper, aluminum, and 304 stainless steel surfaces; and ZnO nanostructured surfaces on copper, aluminum, and 304 stainless steel. The experimental variables in this study were temperature and time. Through temperature and time measurements, the mean heat flux at varying superheats could be determined. Experimental results indicate the ZnO nanostructure enhances wettability and water evaporation on a variety of metal substrates. The presence of the ZnO nanostructure increases the mean heat flux and critical heat flux (CHF) in comparison to the bare metal surfaces. These results provide insight into the potential of enhancing spray cooling systems by growing a ZnO nanostructure on a heat exchanger fabricated of various metals.

For continuous operation of Concentrated Solar Power (CSP) Plants it is necessary to integrate thermal energy storage module. High-density energy storage system at a high temperature is required for the new generation of large scale CSP plants. The Thermochemical Energy Storage (TCES) systems use the enthalpy of formation of a reversible chemical reaction for energy storage and release. Gas/solid reduction-oxidation (redox) reactions of solid metal oxides using air as heat transfer fluid (HTF) can be directly integrated with air operated CSP plants, and there is no need for HTF storage and any intermediate heat exchanger. A new generation of large scale CSP plants uses high-temperature solar collectors to increase power cycle efficiency. Such operating conditions require the development of suitable high-temperature TCES systems. The selection of suitable metal oxide reactant is very critical in the design of such high-temperature storage systems and requires a detailed study of the physics of reaction within the reactor. Cobalt oxide (Co 3 O 4 /CoO) has been verified to have a high reaction temperature, high enthalpy of reaction together with reasonable cyclic and thermal stability. Unique features of cobalt oxide require more fundamental study of the physics behind the redox reaction and its cyclic performance. Study of the physics of materials during the storage/release cycle is necessary for the design and improvement of the reactor and can be used as a benchmark for comparison of any implemented changes. A high precision, true differential TGA/DSC instrument is used for simultaneous measurement of weightchange (TGA) and true differential heat flow (DSC) for pure cobalt oxide (Co 3 O 4 ) powder. Storage cycle (charge/discharge) was conducted for five cycles. Complete re-oxidation was achieved within reasonable times by performing the two reactions at close temperatures and controlling heating/cooling rates. Basic performance parameters were derived as a benchmark for future references. Single-cycle controlling parameters such as heating/cooling rate, dwelling time, and purge gas rate were investigated. System response for few initial cycles was studied. It was shown that pure cobalt oxide could regain weight and complete re-oxidation with reasonable stability. A transition for heat flow was detected after a few initial cycles which reduced discharge heat and decreased overall performance.

Continuous power supply in Concentrated Solar Power (CSP) plants can be achieved via integration of efficient, cost-effective and reliable Thermal Energy Storage (TES) system. The new generation of CSPs operates at higher temperatures and requires thermal storage systems with higher energy density at high storage temperature. Thermochemical Energy Storage (TCES) is the available solution which can meet performance requirements of energy density, temperature, and stability. TCES systems apply reversible endothermic/exothermic chemical reaction through which energy is stored as the enthalpy of reaction and released during the reverse mode. Among several available potential reversible chemical reactions, metal oxides, with high reaction temperature and enthalpy of reaction, have remarkable advantages compared to others. They use air both as Heat Transfer Fluid (HTF) and oxidation reactant, which eliminates the need for storage and intermediate heat exchanger integration between HTF and collector working fluid. Using air as HTF has made them perfectly fitted for the new generation of air operated solar collectors. Among several screened available potential metal oxides, cobalt and manganese oxides were selected as best candidates for high-temperature storage. Pure manganese oxide does not meet the cyclic operation requirement, but the iron-doped solid solution has proven reasonable cyclic storage performance. In this study, iron-doped manganese oxide (Fe-Mn 1:3 molar ratio) has been selected as a redox agent for TCES reactor. The cylindrical packed bed configuration is considered as a reactor bed configuration. A two-dimensional axisymmetric numerical model is developed using the finite element method. Performance analysis for both charge and discharge is provided separately. The effect of inflow rate and bed porosity variations on reactor performance in complete storage cycle were studied.

Performance enhancements in heat exchanger design and manufacturing have been achieved over the past several decades through a combination of improved thermal-hydraulic modeling and experimentation tools, enhancements in material formulations and associated property characterizations, and new manufacturing methods. Most recently, Additive Manufacturing (AM) methods have matured sufficiently that they are now being considered as realizable heat exchanger fabrication methods. More complex, compact, and efficient designs can be achieved with AM methods that could not be easily obtained through more traditional manufacturing techniques. This study expands upon a previous work [1] in which an optimized twisted tube shell and tube stainless steel heat exchanger was designed, analyzed, and fabricated with a Direct Metal Laser Sintering (DLMS) AM method. In that study, the twisted tube heat exchanger performance was a considerable improvement over that of a traditional straight tube shell and tube heat exchanger. In the present study, the AM twisted tube heat exchanger was subjected to thermal-hydraulic tests to measure its performance and to identify any necessary refinements to the previous CFD model. For the conditions used in this study, the experimental data will show how the previous CFD model over-predicted the twisted tube heat exchanger’s heat transfer rate of 2,297 W and under predicted its overall heat transfer coefficient of 1,008 W/m 2 /K. Interrogation of the CFD model found that this discrepancy was due to the utilization of a k-ε turbulence model. Once this turbulence characterization was replaced with a more suitable shear transport model, the numerical predictions and experimental measurements of total heat transfer rate and overall heat transfer coefficient were in very close (∼10%) agreement. When combined with the previous study, this current work reveals how a complex, twisted tube shell and tube heat exchanger can be designed with existing CFD modeling tools and efficiently manufactured with current AM technologies to significantly improve its performance over a more traditionally manufactured straight tube version of the heat exchanger.

Recent studies have indicated that droplet evaporation heat transfer can be substantially enhanced by fabricating a thin nanoporous superhydrophilic layer on a metal substrate. Earlier investigations have focused on how these surfaces affect low Weber number deposition of droplets and their subsequent evaporation on a horizontal, upward-facing heated surface. This investigation explores the effects of changing impact parameters — specifically how deposition, spreading, and vaporization on nanoporous superhydrophilic surfaces are affected by changing impact velocity and incident angle of the droplet motion relative to the surface. The results of droplet deposition and evaporation experiments are reported here for multiple droplet sizes (2–6 μL), and multiple incident angles (0–45°), and of 8 μL droplets from different drop heights (1.2, 40, and 80 mm). The results indicate that the strong capillary forces that enhance spreading on these surfaces remain dominant in the spreading and vaporization processes even when droplets strike the surface with significant velocity, and when the incident angle is oblique. The results indicate that increasing the Weber number has little effect on droplet evaporation and decreasing the incident deposition angle further enhances spreading and evaporation heat transfer. This paper also explores the implications of these results for spray cooling applications.

Plate fin heat exchanger is widely used for air side cooling. To enhance its thermal performance, a novel self-agitator for air-side heat transfer enhancement was developed and validated both numerically and experimentally. Self-Agitator made of 3D printing material connected to well-designed metal beam was placed between the plate fins. It could lose stability and start to oscillate in the channel due to fluid structure interaction. The oscillation enhanced the mixing so that self-agitator can improve the heat transfer in plate fin. Wind tunnel experiment was carried out and self-agitator can save pumping power up to 40% with the same rejected heat. Numerical model was also developed and verified for this fluid-structure interaction process.

Frequency-Selective Surfaces (FSS) type metasurfaces consist of periodic arrays of antenna elements. They can be scaled from microwave frequencies to the infrared wavelengths where they allow the scattering response to be engineered. This includes the spectral absorptance/emittance. At IR wavelengths, the features sizes of the metasurface are sub-micron which poses manufacturing issues for the meter squared scales required for most heat transfer applications. In this paper, we investigate the use of Microsphere Photolithography for creating spectrally selective metasurfaces. This approach uses a self-assembled array of microspheres as a lens array to focus a lattice of photonic jets into photoresist. These can be used with lift-off to create metal-insulator-metal (MIM) or even five-layer of resonant structures. We study the design constraints and synthesize a broadband emitter in the mid-infrared. The spectral absorptance is measured experimentally using FTIR. The structures are then tested at moderate temperatures to demonstrate the ability to affect surface temperature/heat flux in practical applications.

Additive manufacturing, the layer-by-layer creation of parts, was initially used for rapid prototyping of new designs. Recently, due to the decrease in the cost and increase in the resolution and strength of additively manufactured parts, additive manufacturing is increasingly being used for production of parts for end-use applications. Fused Deposition Modeling (FDM), a type of 3d printing, is a process of additive manufacturing in which a molten thermoplastic material is extruded to create the desired geometry. Many potential heat transfer applications of 3d printed parts, including the development of additively manufactured heat exchangers, exist. In addition, the availability of metal/polymer composite filaments, first used for applications such as tooling for injection molding applications and to improve wear resistance, could lead to increased performance 3d printed heat exchangers because of the higher thermal conductivity of the material. However, the exploitation of 3d printing for heat transfer applications is hindered by a lack of reliable thermal conductivity data for as-printed materials, which typically include significant void fractions. In this experimental study, an apparatus to measure the effective thermal conductivity of 3d printed composite materials was designed and fabricated. Its ability to accurately measure the thermal conductivity of polymers was validated using a sample of acrylic, whose conductivity is well understood. Finally, the thermal conductivities of various 3d printed polymer, metal/polymer composite, and carbon/polymer composite filaments were measured and are reported in this paper. The materials used are acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), stainless steel/PLA, Brass/PLA, and Bronze/PLA.

Nucleation of hydrates requires very long induction (wait) times, often ranging from hours to days. Electronucleation, i.e. nucleation stimulated by the presence of an electric field in the precursor solution can reduce the induction time significantly. This work reveals that porous aluminum foams enable near-instantaneous electronucleation at very low voltages. Experiments with tetrahydrofuran hydrate nucleation reveal that open-cell aluminum foam electrodes can trigger nucleation in only tens of seconds. Foam-based electrodes reduce the induction time by as much as 150X, when compared to non-foam electrodes. This work also discusses two mechanisms underlying electronucleation. These include bubble generation (due to electrolysis), and the formation of metal-ion coordination compounds. These mechanisms depend on electrode material and polarity, and affect the induction time to different extents. This work also shows that foams result in more deterministic nucleation (compared to stochastic) when compared with non-foam electrodes. Overall, electronucleation can lead to a new class of technologies for active control of formation of hydrates.

Fabrication of micro and nanoscale electronic components has become increasingly demanding due to device and interconnect scaling combined with advanced packaging and assembly for electronic, aerospace and medical applications. Recent advances in additive manufacturing have made it possible to fabricate microscale, 3D interconnect structures but heat transfer during the fabrication process is one of the most important phenomena influencing the reliable manufacturing of these interconnect structures. In this study, optical absorption and scattering by three-dimensional (3D) nanoparticle packings are investigated to gain insight into micro/nano heat transport within the nanoparticles. Because drying of colloidal solutions creates different configurations of nanoparticles, the plasmonic coupling in three different copper nanoparticle packing configurations were investigated: simple cubic (SC), face-centered cubic (FCC) and hexagonal close packing (HCP). Single-scatter albedo (ω) was analyzed as a function of nanoparticle size, packing density, and configuration to assess effect for thermo-optical properties and plasmonic coupling of the Cu nanoparticles within the nanoparticle packings. This analysis provides insight into plasmonically enhanced absorption in copper nanoparticle particles and its consequences for laser heating of nanoparticle assemblies.

Bulk density measurements of radioactive materials can give significant information about radiation-induced compositional and structural changes. Traditionally, density measurements of solids are performed using hydrostatic immersion techniques based on Archimedes‘ principal, the precision of which is highly dependent on knowing the density profile of the immersion fluid. However, radioactive materials are inherently self-heating, producing thermal gradients within the immersion fluid, which induce convective flow and density gradients around the sample. In this study, COMSOL Multiphysics models were generated to study how the immersion fluid thermo-physical properties, sample size, shape and orientation, and heating power output affect the buoyancy force on the sample and ultimately the precision of the density measurements. A dimensionless equation was developed and will be fit to simulation and experimental data to build a correlation between the thermo-physical properties of the immersion fluid and the variance in the resulting buoyancy force on the samples with varying heat outputs. Thermal models will be validated experimentally by immersing self-heating metal mimics — tungsten samples with embedded surface mount resistors connected to a controllable power source — into various immersion fluid baths. Schlieren and laser-induced fluorescence flow visualization techniques around the self-heating mimics can be used to validate the density profiles predicted by the numerical models. Only the beginning setup of Schlieren experiments will be discussed as they are ongoing. This correlation, along with concurrent chemical compatibility studies not discussed in this publication, will help derive fluid selection parameters to increase the precision of density measurements. The validated numerical models can also be used to improve density measurement setups and understand where data probes, e.g., thermocouples, should be located to realize the density profile in the immersion fluid during sample measurement.

The design and construction of solar concentrators heavily affects their cost, heat utilization and optical efficiency. Current trough concentrators support the reflector with an equivalent uniform beam configured from a metal grid sub-structure. Under gravity and wind loads, the support-structure stress distribution varies as a function of position of the structure and the tracking angle. In the conventional design, there is ample surplus stiffness and strength designed into some beams of the structure, which increases the overall weight and cost of the structure. This paper describes an approach towards structural optimization of trough concentrators (with the Eurotrough design taken as an example, that means that the safety factors and structure is similar with Eurotrough design) using a variable cross section beam. The main improvement of this approach comes from keeping the beams rigid and strong near the two ends (at the torque box structure) while allowing the middle of the structure to be relatively weak. Reducing the cross-sectional area of the central beams not only reduces amount of material needed for the structure but also reduces the deflection of the reflector. The simulated results show that the concentrator’s structural weight (including the torque box, endplates and cantilever arms) and the maximum displacement of the reflector are reduced about 15.3% (about 151.2kg per 12-metre long element) and 15.5%, respectively. This represents a meaningful capital and installation cost savings while at the same time improving the optical efficiency.

A novel design of a mini heat exchanger utilizing forced convection heat transfer enhancement with electrohydro-dynamic (EHD) technique has been numerically investigated. When a high voltage is applied to a metal wire, air in its vicinity will be ionized and the injected ions will travel towards electrically grounded heat exchanger surfaces, leading to the corona wind. As a result, the corona wind disturbs the heat exchanger boundary layer and thus enhances heat transfer between the heat exchanger surface and its ambient air. A three dimensional numerical model has been developed to evaluate the heat transfer coefficient (HTC) and air side pressure drop of this EHD enhanced mini heat exchanger. Influences of position and size of the wire are evaluated in order to achieve the highest enhancement. In addition, the swirling flow pattern induced by EHD has been studied due to its important role in heat transfer enhancement. The results show a three times increase of HTC enhanced by EHD effect in present design comparing to the one without EHD effect. The most promising result shows an overall heat transfer coefficient equal to 318 W/(m 2 ·K) for a bare tube in cross flow configuration with airside pressure drop of 2.8 Pa.

Constructal principles are used to investigate the optimization of material utilization in a metal matrix heat sink that maximizes the total heat transfer rate through the base of heat sink. This approach utilizes a two-dimensional geometry to examine spatial heat flow and optimal material distribution in a metal matrix in the plane perpendicular to the coolant flow direction. The matrix is composed of multiple layers of conductive tees built up from the smallest constituent, the unit T-cell. The unit cell consists of a conductive tee-shaped geometry with the two rectangular void regions each making up half of a cooling channel. The horizontal boundaries must match the temperature and heat flux at the boundaries of the neighboring unit cells as this is a conjugate conduction/convection problem. The geometry of the unit cell is characterized by aspect ratios of channel width to height, overall cell width to height, and channel height to cell height. The matrix structure is assembled by stacking unit cells into multiple layers where the number of cells in each layer is an integer multiple of the cells contained in the lower layer. The solution is obtained for optimal T-cell geometric parameters under a set of predetermined constraints including overall volume, solid fill fraction, and number of layers. When a large number of stacked unit cells are considered, the results describe the ideal spatial distribution of porosity and pore sizes for two dimensional functionally graded metal-matrix heat sink. These results will lead to a better understanding of the role played by the porosity distribution in a metal-matrix heat sink and may be applied to the analysis, optimization, and design of more effective heat sinks.

Metal nanoparticle has been a promising option for fillers in thermal interface materials due to its low cost and ease of fabrication. However, nanoparticle aggregation effect is not well understood because of its complexity. Theoretical models, like effective medium approximation model, barely cover aggregation effect. In this work, we have fabricated nickel-epoxy nanocomposites and observed higher thermal conductivity than effective medium theory predicts. Smaller particles are also found to show higher thermal conductivity, contrary to classical models indicate. A two-level EMA model is developed to account for aggregation effect and to explain the size-dependent enhancement of thermal conductivity by introducing local concentration in aggregation structures.

In this work, we have observed 60% reduction in total interfacial resistance by adding an intermediate metal layer nickel between gold and aluminum oxide. Two temperature model is applied to explain the change of interfacial resistance, including both lattice mismatch with diffuse mismatch model and electron-phonon coupling effect. Simulation result agrees reasonably well with experimental data. Even though interfacial resistance due to electron-phonon coupling effect for Au-aluminum oxide is much larger than that of Ni-aluminum oxide interface, lattice mismatch is still the dominant factor for interfacial resistance.

As an advanced welding method, the diffusion welding is widely applied in aerospace, instrumentation industry, engine manufacturing, etc. Not only the diffusion welding can realize the connection of different metals, but also it solves the difficult welding problem between metals and ceramics. Aiming to research the diffusion process during welding, this study investigates the diffusion behavior and mechanisms of Cu and Al under different pressures and surface roughnesses in the aspect of atomic level by using the molecular dynamics (MD) method. From the results, we can find that the pressure influences obviously the second thickness growth stage, which is one of the three stages in the whole diffusion process. The grooves generated by the surface roughness, is filled mainly by the deformation of Al, which fits the initial surface profile. The diffusion is promoted as the contacting area increases. Whether the surface is smooth or rough, the pressure has little impact on the final thickness. This study succeeds in explaining the diffusion mechanism, which is significant for the experiments of diffusion welding in practice.