Use In Thermal Applications Research Articles

Experimental and Simulation Studies of Temperature Effect on Thermophysical Properties of Graphene-Based Polylactic Acid.

Overheating effect is a crucial issue in different fields. Thermally conductive polymer-based heat sinks, with lightweight and moldability features as well as high-performance and reliability, are promising candidates in solving such inconvenience. The present work deals with the experimental evaluation of the temperature effect on the thermophysical properties of nanocomposites made with polylactic acid (PLA) reinforced with two different weight percentages (3 and 6 wt%) of graphene nanoplatelets (GNPs). Thermal conductivity and diffusivity, as well as specific heat capacity, are measured in the temperature range between 298.15 and 373.15 K. At the lowest temperature (298.15 K), an improvement of 171% is observed for the thermal conductivity compared to the unfilled matrix due to the addition of 6 wt% of GNPs, whereas at the highest temperature (372.15 K) such enhancement is about of 155%. Some of the most important mechanical properties, mainly hardness and Young’s modulus, maximum flexural stress, and tangent modulus of elasticity, are also evaluated as a function of the GNPs content. Moreover, thermal simulations based on the finite element method (FEM) have been carried out to predict the thermal performance of the investigated nanocomposites in view of their practical use in thermal applications. Results seem quite suitable in this regard.

Enhanced in‐plane and through‐plane thermal conductivity and mechanical properties of polyamide 4.6 composites loaded with hybrid carbon fiber, synthetic graphite and graphene

AbstractPolyamide 4.6 has excellent properties, such as high temperature resistance, crystallinity, fatigue resistance, melting temperature, excellent creep resistance, toughness, and good wear properties, but low thermal conductivity values. For this reason, its use in thermal applications has been limited. In this study, its use in thermal applications can be increased by increasing its thermal conductivity with carbon‐based fillers and reinforcements added to the PA46 structure. The aim of this study is to examine the carbon fiber (CF), synthetic graphite (SG) and graphene nanoplatelet (G) loading on the mechanical, thermal, physical, and electrical properties of polyamide 4.6, which are produced using co‐ rotating twin‐screw extrusion. Tensile and flexural strength of polyamide 4.6 increased with the addition of CF at all weight fractions. The highest electrical conductivity value was measured as 4.01 S/cm in PA46‐20CF‐20SG‐3G composite material. The highest in‐plane thermal conductivity achieved in this study at 20 wt% CF, 20 wt% synthetic graphite, and 5 wt% graphene loading was 20.43 W/mK. However, the highest through‐plane thermal conductivity value was obtained to be 4.19 W/mK at 20 wt% CF. Graphene and synthetic graphite are more efficient to increase the in‐plane thermal conductivity, while CF is more efficient to increase the through‐plane thermal conductivity.

Parametric investigation of a nanofluid-NEPCM based spectral splitting photovoltaic/thermal system

Photovoltaic cells can convert incoming sunlight within their responsive wavelength range into electricity. Unfortunately, the majority of the incident radiation is reflected, absorbed directly as heat, or indirectly converted to heat due to internal losses (i.e., ≥ 52.9%, even world-record efficiency cells). In concentrated photovoltaic systems, this added heat can further degrade the electrical conversion efficiency of the cells if it is not removed, resulting from an elevated operating temperature. Spectral splitters and absorptive filters can remove the unused portion of the solar spectrum before it is converted to heat in the photovoltaic cells and harvest this energy for active use in thermal applications. In this paper, an innovative concentrated photovoltaic/thermal system that utilizes a layered design containing an optical fluid and a phase change material was investigated as a new alternative absorptive filter. Five possible configurations of this filter were compared from energy, exergy, and energy storage perspectives. These layers consist of a phase change material (dotriacontane, a paraffin) layer and fluid channel layers (a glycerol fluid) with suspended Ag and/or Au nanoparticles. The optical properties of the nanofluid and the nano-enhanced phase change material (NEPCM) were modeled using Mie and Rayleigh scattering theories. The comparison showed that the configuration containing the nanofluid channel at the top of the nano-enhanced phase change material performs well in all aspects of energy, exergy, and energy storage. A parametric study was conducted to determine the effects of nanoparticles volume fraction, concentration ratio, mass flow rate, nanofluid thickness and phase change material thickness on PV/T system performance with this filter configuration. It was found that volume fractions lower than 0.3 × 10−5 and 0.2 × 10−5 for Ag and Au, respectively, provided the best system performance for the assumed geometric and operational conditions. At an optimal mass flow of 0.005 kg s−1 for the nanofluid, this design achieved a maximum exergetic efficiency of 24% for a concentration ratio of 30. The results also revealed that the nanofluid-NEPCM filter is not suitable for concentration ratios of less than 10. Overall, this study finds that nanofluid-NEPCM-based spectral splitter, in addition to successfully filtering radiation, can store energy efficiently.

Improvement of thermal energy storage density of parabolic dish solar absorber with organic phase change materials

The enhancement of the thermal storage capacity of the solar absorber using phase change material was investigated in this work. A 16 m2 paraboloidal dish concentrator was employed to concentrate the solar rays on the absorber. Erythritol and D-Mannitol are selected based on the temperature region of the absorber during the outdoor testing at the test site (Chennai; 13°N, 80°E). The mass flow rate of heat transfer fluid was in the range of 25-150 kg/h. The solar absorber with phase change material will ensure a uniform thermal output during a sudden discontinuity of solar radiation, for a few to several minutes. The absorber is ensured with the uniform temperature. PCM solar absorbers will act as a thermal battery for later use in thermal applications with proper insulation of the absorber.

In situ preparation and characterization of encapsulated high-chain fatty acid ester-based phase change material (PCM) in poly(urethane-urea) by using amino alcohol

A fatty acid ester-based phase change material (PCM) has been encapsulated with a proper poly-functional amino alcohol and toluene diisocyanate under in situ polymerization conditions for further use in thermal applications. N-(2-hydroxyethyl)ethylenediamine has been utilized for the first time for encapsulation of PCMs to form poly(urethane-urea) shell material. The research reports the influence of utilized anionic and non-ionic surfactants on properties of the encapsulated end products at different surfactant/PCM stoichiometric ratios. The thermal properties are reported in terms of phase change temperature, enthalpy and PCM encapsulation efficiency with necessary statistical calculations by differential scanning calorimeter (DSC) analyses after the conducted effective scrubbing procedure for excess PCM in the boundary layer and on walls of the shell material. Particle size distribution analysis and high-resolution transmission electron microscope (HRTEM) imaging have been utilized for further discussions on relations between surfactant types, stoichiometric ratios and structural properties. The prepared PCM composites with micro- and nano-structures are suitable candidates for use in desirable exterior building materials and solar units as outdoor thermal energy storage agents for construction of energy efficient buildings.

Catalytic self-cleaning coatings for thermal oxidation of organic deposits on glass

Self-cleaning catalytic coatings were developed for use in thermal applications, such as domestic oven windows. Based on a modified citric acid complexation method, oxide coatings were synthesized on commercial glass substrates. Their performance was evaluated in the oxidation of a layer of pre-oxidized lipid at 250 °C for 1 h, as followed by FTIR or visual/UV-Vis spectroscopic inspection. Based on both evaluation methods, manganese and mixed manganese–ceria oxide coatings with a thickness of around 35 nm were selected as the most promising ones: they combine the excellent activity for oxidative removal of the contaminant with the desired optical characteristics. XPS analysis revealed that during reaction with the contaminant the valence of the manganese oxide decreased from its initial value between +3 and +4. A less pronounced reduction in the presence of ceria suggests the transfer of lattice oxygen from ceria to Mn oxide; nevertheless, the reoxidation of the oxide by gas phase oxygen at 250 °C seems to be the rate limiting step. Gas phase analysis indicates that ca. 3 times as much CO2 is produced on the catalyst-coated glass than on a blank glass slide in an oxygen atmosphere. The liquid phase products resemble those of a catalyzed liquid phase autoxidation, with too high catalyst concentrations inhibiting the conversion of the lipid. Therefore, the oxidation mechanism on the catalytic coating shows characteristics of a Mars–van Krevelen mechanism (consumption of lattice oxygen) combined with a metal ion catalyzed liquid phase mechanism.

An assessment of linear Fresnel lens concentrators for thermal applications

This paper describes an assessment of linear Fresnel lens concentrator technology for use in thermal applications. System performance and cost data for a Fresnel lens concentrator were developed and compared with data for a parabolic trough system. Two thermal applications of these technologies were chosen for the comparison. The first is a water preheat application where energy is transferred from an antifreeze, collector field working fluid to preheat 60 °C water through a heat exchanger. The other application is the generation of saturated steam at 177 °C. Annual performance predictions were generated using a simulation code that includes the effects of both thermal capacitance and heat losses from all piping and system components. Performance was simulated at three different geographic locations. Costs for the collectors were based on documented estimates. Overall costs included all major equipment, site development, and installation. The performance and cost data were combined to arrive at a levelized cost of energy. Based on the performance predictions developed here and on the current documented costs, results indicate that the Fresnel lens systems have a levelized energy cost about one-third less than the parabolic trough systems. This is primarily due to less expensive concentrators.