Evacuated Tube Solar Collector Integrated with Multifunctional Absorption-Storage Materials




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Solar water heaters (SWHs) are a well-established renewable energy technology that have been widely adopted around the world. This work presents a novel method of integrating Phase Change Materials (PCMs) within the evacuated solar tube collectors for solar water heaters (SWHs). In this method, the heat pipe is immersed inside the phase change material, where heat is effectively accumulated and stored for an extended period of time due to thermal insulation of evacuated tubes. The benefit of this method includes improved functionality by delayed release of heat, thus providing hot water during the hours of high demand or when solar intensity is insufficient. The proposed solar collector utilizes two distinct phase change materials (dual-PCM), namely Tritriacontane and Erythritol, with melting temperatures of 72◦C and 118◦C respectively. The operation of solar water heater with the proposed solar collector is investigated during both normal and on-demand stagnation) operation. Beyond the improved functionality for solar water heater systems, the results from this study show efficiency improvement of 26% for the normal operation and 66% for the stagnation mode, compared with standard solar water heaters that lack phase change materials.

The obtained results from the experimental work is validated by computational fluid dynamic model of one single evacuated tube solar collector (large scale size) with ANSYS Fluent. Also the concept of enhancing the convective heat transfer and uniform melting process by silicone oil as a heat transfer medium in direct contact with PCMs is investigated.

In addition, the Evacuated Tube solar Collectors (ETCs) are significantly improved by utilizing the “dry-drawable” Carbon Nanotube (CNT) sheet coatings to increase the solar energy absorption and Phase Change Materials (PCMs) to increase the heat accumulation for application in solar water heaters. The proposed solar collector utilizes a phase change material namely Octadecane paraffin, with melting temperatures of 28 ◦C which is categorized as nontoxic with long-term chemical stability PCM. As PCMs particularly in powder form may not be effective by itself due to the poor heat transfer rate, low thermal diffusivity and thermal conductivity, by combining CNT layers with the high thermal diffusivity and thermal conductivity compare to phase change materials, we are able to overcome the shortcomings of PCMs and design an innovative and efficient solar water heater. With the current technology, we can provide a near ideal “black body” surface, absorbing a maximum of 98%, between 600-1100 nm, of solar light striking the surface, and providing additional spectral absorption which improves the performance of the solar heater. Applying CNT sheets in conjunction with PCM enables heat storage directly on the collector for a more constant output, even on a cloudy day and prolonged output of heat at night.



Solar collectors, Solar water heaters, Carbon nanotubes, Computational fluid dynamics



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