Vikrant Khullar

Solar Thermal Energy: Use of Volumetric Absorption in Domestic Applications

Solar thermal systems are one of the renewable energy systems used in the residential buildings for the heating purpose, and with these systems, the usage of non-renewable energy resources decreases. To improve the performance of solar collectors, engineers and scientists are regularly working on it. Direct absorption-based solar thermal collectors (DASTC) are kind of solar collectors in which the fluid can be heated directly (without any absorption surface). The present study deals with numerical model of direct absorption-based solar collector which can be used for residential purposes. The absorbed energy fraction, effect of the height, length of collector, and mass flow rate on the collector efficiency have been determined. The analysis shows that collector efficiency increases with the increase of mass flow rate when the height of the fluid in the collector is same and the efficiency of the collector deceases with the increase of channel length. Further, it has been observed that it is beneficial to use an optimum volume fraction of the nanoparticles in DASTC because at an optimum volume fraction, the collector achieved maximum efficiency.

Direct Absorption Solar Thermal Technologies

Solar collectors that can directly absorb radiation represent an emerging realm of solar thermal systems wherein the collection as well its subsequent conversion to the useful thermal energy happens within the working fluid. Nanofluids (stable dispersions of nanoparticles in the basefluid) have been found to be promising working fluids for realizing such direct solar to thermal energy conversion owing to their enhanced (and the ease of tuning) thermo-physical and optical properties. Seeding trace amounts of carefully chosen nanoparticles into the basefluid has been shown to significantly enhance the solar weighted absorptivity of the basefluid—hence rendering them is suitable for solar thermal applications. Firstly, a brief description relevant to the incumbent surface absorption-based solar thermal technologies has been presented. A critical analysis of the fundamental limits of performance that can be achieved in the incumbent solar thermal systems reveals that solar selectivity could only be beneficial up to a certain temperature and solar concentration ratios, beyond which we cannot further improve the efficiency. Subsequently, the candidature of direct absorption solar thermal systems has been assessed to ascertain if these could be deployed under conditions which are not so amenable for the conventional surface absorption-based solar thermal technologies. Finally, a representative experimental study is presented that points out that even for low solar concentration ratios (conditions which are more suitable for the conventional surface-based absorbers), the two classes of solar thermal technologies can have comparable thermal efficiencies. It is envisaged that the benefits of the direct absorption-based solar thermal systems over the conventional ones shall be more pronounced for high-flux conditions, i.e. high solar concentration ratios.

Solar Selective Volumetric Receivers for Harnessing Solar Thermal Energy

Given the largely untapped solar energy resource, there has been an ongoing international effort to engineer improved solar-harvesting technologies. Towards this, the possibility of engineering a solar selective volumetric receiver (SSVR) has been explored in the present study. Common heat transfer liquids (HTLs) typically have high transmissivity in the visible-near infrared (NIR) region and high emission in the mid-infrared region, due to the presence of intra-molecular vibration bands. This precludes them from being solar absorbers. In fact, they have nearly the opposite properties from selective surfaces such as cermet, TiNOx, and black chrome. However, liquid receivers which approach the radiative properties of selective surfaces, can be realized through a combination of anisotropic geometries of metal nanoparticles and transparent heat mirrors. Solar selective volumetric receivers represent a paradigm shift in the manner in which solar thermal energy is harnessed and promise higher thermal efficiencies (and lower material requirements) than their surface-absorption based counterparts. In this paper, the ‘effective’ solar absorption to infrared emission ratio has been evaluated for a representative SSVR employing copper nanospheroids and Sn-In2O3 based heat mirrors. It has been found that a solar selectivity comparable to (or even higher than) cermet-based Schott receiver is achievable through control of the cut-off solar selective wavelength. Theoretical calculations show that the thermal efficiency of Sn-In2O3 based SSVR is 6 to 7% higher than the cermet-based Schott receiver. Furthermore, stagnation temperature experiments have been conducted on a lab-scale SSVR to validate the theoretical results. It has been found that higher stagnation temperatures (and hence higher thermal efficiencies) compared to conventional surface absorption-based collectors are achievable through proper control of nanoparticle concentration.Copyright

Parametric study of the energy efficiency of the HDH desalination unit integrated with nanofluid-based solar collector

Humidification–dehumidification desalination (HDH) systems have been found to be ideal for treating seawater at a smaller scale. It requires thermal energy to drive the processes inside the HDH system which can be supplied by a renewable source of energy (such as solar energy). For this purpose, a nanofluid-based direct absorption solar collector (DASC) can be used which has a relatively higher thermal efficiency, as compared to the conventional surface absorption-based solar thermal collector. In this study, these two sub-systems—HDH and DASC, are coupled through a heat exchanger. In this paper, a numerical model has been prepared for DASC-based HDH system which aims to evaluate the energy efficiency of this combined system by calculating gained output ratio as a function of various parameters, related to the DASC, such as particle volume fraction (fv), height (H) and length (L) of the collector, mass flow rate of nanofluid inside the collector

Potential Heat Transfer Fluids (Nanofluids) for Direct Volumetric Absorption-Based Solar Thermal Systems

(dot{m}_{text{nf}} )

Performance evaluation of a brine-recirculation multistage flash desalination system coupled with nanofluid-based direct absorption solar collector

(m˙nf) and amount of solar energy incident (q) on the collector. The performance of the combined or integrated system has also been verified against the various parameters related to HDH system such as ratio of mass flow rate of the saline water to the dry air

Community-level solar thermal systems

(dot{m}_{text{w}} /dot{m}_{text{da}} )