In last two decades, metallic particles of nano-sizes (~10 m) are tested profoundly in volumetric absorption solar collectors (VASC) due to their excellent optical properties and broadband absorption in entire solar spectrum. However, availability, ease to synthesise, toxicity, and non-biodegradable nature of these nanofluids are some of the challenges that still needs attention of scientific community for future commercialisation of this technology. Further, based on literature review, very limited studies are available for understanding the performance of integrated energy storage VASC system using nanofluids. Considering all these issues, a hybrid nanofluid of gold nanoparticles in Azadirachta Indica leaves extract has been synthesised by chemical route. The prepared hybrid nanofluid has shown good absorption in 400-700nm wavelength range and hence high photo-thermal conversion efficiency for VASC. Further, commercial available paraffin wax is used as phase change material (PCM) in thermal energy storage (TES) and further integrated with VASC to analyse thermal performance of system even after sunshine hours. The real-time experiments were conducted using different working fluids and at three mass flow rates i.e. 0.5 lpm , 1 lpm and 2 lpm , respectively during mild winter days under tropical climate of India. The study revealed a photo-thermal efficiency enhancement of about 17.1% when hybrid heat transfer fluid was used in VASC system with TES as compared to base fluid water without TES. Further, a maximum zero loss efficiency of 83.8% was estimated at optimal mass flow rate of 2 lpm for TES integrated VASC system.

In the last decade, direct absorption solar collector has shown tremendous potential in improving the efficiency of solar water heaters by incorporating nanoparticles. Moreover, plasmonic nanoparticles laden nanofluids have shown high absorption in the visible region due to localized surface plasmon resonance (LSPR). However, these plasmonic nanoparticles are associated with high cost, complex preparation processes, and environmental degradation over prolonged use. To overcome such issues, a hybrid nanofluid based on gold nanoparticles in Azadirachta Indica leaves extract was prepared and tested in real outdoor conditions for the very first time. The absorbance curve of the prepared hybrid nanofluid showed broadband absorption from visible to near infrared spectrum. The maximum temperature gain of about 20 degrees C was observed with just 2 h of sun exposure and maximum photo-thermal efficiency reached nearly 62%, which is about 12% higher than base fluid i.e. water. Further, the prepared samples were found to be highly stable without any sign of sedimentation or agglomeration under repeated sun exposure for several hours. The study results show a new class of nanofluids that can be used for attaining high thermal conversion efficiency, which are more stable and eco-friendly for such novel solar thermal technologies.

Plasmonic nanofluids have emerged as strong candidates for future working fluids in direct absorption solar collectors (DASC). However, in novel DASC systems, a significant temperature gradient exists over working fluid depth increasing emissive losses and hence, results in low thermal conversion efficiency. Considering these issues, a laboratory scale prototype of reverse-irradiated DASC system (RR-DASC) has been tested under controlled laboratory conditions and compared with a direct irradiated DASC system (DR-DASC) using silver nanofluids at three different mass concentrations i.e., 0.1 mg/L, 0.2 mg/L, and 0.4 mg/L. Ag nanoparticles were having monodisperse spherical shape with surface plasmon resonance (SPR) peak occurring around 395-400 nm. The experimental results showed that RR-DASC maintains uniform temperature over the fluid depth, while about 3-4 degrees C temperature gradient was noticed in direct irradiated DASC system under similar operating conditions. A maximum photo-thermal conversion efficiency of about 80% was observed using 0.4 mg/L Ag nanofluids in RR-DASC system. The presented work can pave the ways for future development of highly efficient DASC systems for low to high temperature solar applications.

Volumetric absorption solar collectors (VASC) have recently shown better photo-thermal performance than conventional solar collectors. The present study describes a novel idea for thermal performance enhancement of Volumetric absorption solar collector system using natural extract of Azadirachta Indica leaves in ethanol as heat transfer fluid (HTF) for the very first time. The optical characterization of Azadirachta Indica leaves extract was carried out using a UV-Vis spectrophotometer, which shows the first absorbance peak around 353 nm and second peak at 671 nm showing full use of visible spectrum. The real outdoor experiments showed a maximum tem-perature rise of 9 ? and a thermal efficiency enhancement of 30.4% using Azadirachta Indica based heat transfer fluids compared to the base fluid water. Furthermore, the tested heat transfer fluids were found to be very stable with no sign of agglomeration/sedimentation for 30 days. Economic analysis of natural extract based volumetric absorption solar collector system was found to be highly economical compared to flat plate solar collector with the same dimensions.

The use of nanofluids improves the overall performance of solar thermal collectors and has been studied explicitly in the last decade. More recently, to overcome the limitations of surface based solar collectors, novel nanofluid seeded direct absorption solar collectors (DASCs) have been proposed for effective solar energy conversion. Plasmonic nanofluids, the colloids of plasmonic nanoparticles such as Au, Ag, Cu, Al etc. in base fluids, have emerged as promising thermal media for novel DASCs. Due to the inherent localized surface plasmon resonance effect (LSPR), these novel media show high thermal gain and photo thermal conversion potentials compared to other types of nanofluids. Therefore, this review focuses on recent progress and challenges in the use of plasmonic nanofluids in DASC-based solar thermal applications. The state-of-the-art includes reporting the recent experimental and numerical results in low, medium and high-temperature DASCs as well as hybrid photovoltaic/thermal (PV/T) technology, which utilises nanofluids as beam splitter. We have also tried to provide a review of optical characteristics of plasmonic nanoparticles along with the latest developments in the synthesis of complex nanoparticle morphologies and blends for broadband absorption of solar spectrum. Finally, authors have tried to highlight the challenges, grey areas and possible future research directions for the potential applications of plasmonic nanoparticles in futuristic solar harvesting applications.

The prime objective of the hostel buildings is to provide an adequate thermal environment to the students for their good health and learning performance. However, a few studies have been reported so far in such buildings around the world. This paper reports the findings of a questionnaire based thermal comfort study in six naturally ventilated hostel buildings located in Jalandhar city, lies in composite climate of India, during the monsoon season (August-September 2018). The study was carried out in two newly constructed (aged less than 5 years) and four traditional (aged more than 25 years) multi-storey naturally ventilated (NV) hostel buildings. During the study, a total of 945 completely filled questionnaires from 470 occupants were collected. About 80% and 75% of subjects voted within central three categories of ASHRAE thermal sensation scale in traditional and modern NV hostel buildings, respectively. Results from the probit analysis revealed that 80% of subjects voted within +/- 1 TSV when indoor operative temperature ranged between 27.2 and 31 degrees C. The mean indoor comfort temperature as calculated by Griffiths method is 29.9 degrees C (sd(+/-sigma) = 2.16). To restore comfort primary adaptive action of occupants was found to be switching on the fans followed by the opening of external doors and windows.

Hostel buildings prime objective is to provide better thermal environments to the students for their good health and learning performance. In India, a very few studies are done on the thermal environments of multi-storied naturally ventilated hostel buildings. We carried out a thermal comfort study in two mid-rise (~G+5 floors) naturally ventilated (NV) hostel buildings during monsoon season (August-September, 2018). The field study conducted for three consecutive weeks collecting 642 valid subjective responses with objective information regarding thermal parameters of 253 rooms. Statistical analysis of student’s responses and measured thermal environment variables was performed for assessing inter buildings effects, different weather conditions (rainy or cloudy) and daytime duration (morning, afternoon and evening), respectively. The study finds the mean thermal neutrality at 29.9°C for the studied group using Griffiths’ method. The results suggested that more than 80% of subjects were voting within central three categories when indoor operative temperature ranged between 28-32.1°C. The primary adaptive action of occupants includes switching on the fans (100%) followed by the opening of external doors (80%) and opening or closing of windows (55%) to restore thermal comfort in built environments.