Development of nanofluids for thermal energy storage based on phase change materials

Resumo

In recent decades, the growing international concern for environmental problems and other issues directly or indirectly related to energy consumption has motivated a great interest in improving the efficiency of thermal installations. In this sense, energy storage is expected to play a key role in the coming years. The two main objectives of this Doctoral Thesis were to develop different nano-enhanced phase change materials, NePCMs, and characterize them on the basis of those thermophysical properties that are considered most representative for thermal energy storage. Novel NePCMs were designed by dispersing different nanomaterials, viz. commercial Baytubes® multi-walled carbon nanotubes (c-MWCNT) and MWCNTs synthesized within the framework of this PhD Thesis (s-MWCNT), Iolitec functionalized graphene (fGnP) nanoplatelets, and PVP-coated Ag nanoparticles (PVP-Ag). NePCM preparition has been carried out using a two-step method for samples designed using carbon-based nanoadditives, and in one-step for those designed with silver metal nanoparticles. Special attention has been given to study the morphology and/or purity of base fluids and nanoparticles as well as the temporal and thermal stability of NePCMs by using different physico-chemical techniques. Thus, five different nanofluids sets were designed at nanoparticle mass concentrations ranging from 0.025 to 1.1% in different types of poly(ethylene glycol)s, i.e. c-MWCNT/PEG200, c-MWCNT/PEG300, s-MWCNT/PEG400, fGnP/PEG400 and PVP-Ag/PEG400, which makes a total of twenty-four dispersions. Regarding the thermal and physical characterization, solid-liquid phase change transitions were determined for fGnP/PEG400 and PVP-Ag/PEG400 systems using a DSC Q2000 differential scanning calorimeter. The same device, working with a quasi-isothermal temperature-modulated differential scanning calorimetry (TMDSC) method, was used to obtain the isobaric heat capacity for several PEG400-based systems. The study of the influence of nanoparticles on the thermal conductivity was carried out by means of a KD2 Pro Thermal Properties Analyzer, working with the transient hot wire technique, for the fGnP and s-MWCNT dispersions based on PEG400, while studies on PVP-Ag/PEG400 samples were conducted using a Hot Disk Thermal Constants, which relies on transient plane source technique. Results show that thermal conductivity improves with the nanoparticle concentration for all investigated NePCM systems. Experimental data were also compared with the correlated or predicted values by using different theoretical models such as those of Maxwell, Hamilton-Crosser, Nan, Murshed or Xue. Density measurements were performed for fGnP/PEG400, s-MWCNT/PEG400 and PVP-Ag/PEG400 systems using a DMA501 densimeter based on the well-known vibrating U-tube technique. Dynamic viscosity was studied as a function of temperature and nanoparticle concentration for all designed NePCMs. Experiments were carried out by means of a SVM 3000 rotational Stabinger viscometer-densimeter in the case of fGnP/PEG400 samples, using a Physica MCR 101 rotational rheometer for c-MWCNT/PEG200, c-MWCNT/PEG300 and s-MWCNT/PEG400 systems and utilizing an AR-G2 Magnetic Rotational Bearing Rheometer for PVP-Ag/PEG400 samples. Experimental data on dynamic viscosity were also compared with the values provided by means of Einstein, Batchelor, Brinkman, Krieger-Dougherty, Maron-Pierce or Brenner-Condiff models. Oscillatory rheology experiments were also conducted for c-MWCNT/PEG200 and c-MWCNT/PEG300 sets. In addition, surface tension at the air-sample surface was experimental investigated for pure PEG400 and PVP-Ag/PEG400 NePCMs using a DSA30 droplet shape analyzer.