Understanding photo-thermal and melting mechanisms in optical charging of nano and micro particles laden organic PCMs

Engineering efficient optical charging process necessitates efficient photo-thermal energy conversion, transfer, as well as storage of the incident solar radiant energy. The present work is a determining step in deciphering, quantifying, and understanding the aforementioned steps involved in the optical charging of PCMs. Herein, the effect of particle size, concentration and thermochromism have been critically investigated. In particular, detailed optical characterization and photo-thermal experimentation pertinent to non-thermochromic nanoparticles laden PCM (referred to as NP-PCM) and thermochromic micro capsules laden PCM (referred to as TMCP-PCM) has been undertaken. Detailed optical analysis points out that opposed to NP-PCM, TMCP-PCM significantly scatter the incident radiations – the diffuse transmittance (at room temperature) in the two cases being approximately 2 % and 39 % respectively. Furthermore, whereas, optical properties of non-thermochromic nanoparticles are nearly invariant to temperature changes; in the case of thermochromic microcapsules, the magnitude of scattering further increases and diffuse transmittance reaches as high as 51 % at elevated temperatures. Although, thermochromism helps in enhancing the penetration depth at elevated temperatures, but, due to significant fraction of scattering, the photo-thermal energy conversion is not that effective. In terms of temperature field, spatial-temporal temperature distribution curves reveal that temperature spread (in the liquid phase) in case of optical charging of non-thermochromic particles (carbon soot nanoparticles) laden PCMs is significantly high (as high as approximately 24 °C) relative to that observed in case of thermochromic particles (microcapsules) laden PCMs (approximately, 4 °C). The magnitude of the temperature spread (being representative of the deviation from thermostatic optical charging) clearly points out that opposed to non-thermochromic laden PCMs, nearly thermostatic optical charging can be achieved in case of thermochromic particles laden PCMs.

Furthermore, in case of optical charging without thermochromic assistance, the temperature spread, peak temperatures and the melting rates increase with increase in particles concentration. Whereas, in the latter case, although the temperature spread and peak temperatures are nearly independent; the melting rates do depend on the particles’ concentration. Overall, the present work is a significant step towards accelerated-thermostatic thermal energy storage.