Abstract
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.
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Abbreviations
- A sa :
-
Solar-weighted absorption coefficient
- C ratio :
-
Solar concentration ratio
- D :
-
Diameter of the nanoparticle [nm]
- dλ :
-
Spectral interval [µm]
- E x :
-
Magnitude of electric field vector [NC−1]
- G surf :
-
Solar irradiance incident on the surface [Wm−2]
- K :
-
Optical coefficient [m−1]
- S λ :
-
Spectral solar irradiance [Wm−2µm−1]
- T :
-
Temperature [K]
- y :
-
Thickness of the fluid layer [m]
- ε :
-
Dielectric function [Fm−1]
- ε eff :
-
Effective emissivity
- ε λ :
-
Spectral emissivity
- λ :
-
Wavelength [μm]
- CNTs:
-
Carbon nanotubes
- HCE:
-
Heat collector element
- MWCNTs:
-
Multi-walled carbon nanotubes
- SAS:
-
Surface absorption system
- VAS:
-
Volumetric absorption system
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Khullar, V., Singh, H., Tyagi, H. (2018). Direct Absorption Solar Thermal Technologies. In: Tyagi, H., Agarwal, A., Chakraborty, P., Powar, S. (eds) Applications of Solar Energy. Energy, Environment, and Sustainability. Springer, Singapore. https://doi.org/10.1007/978-981-10-7206-2_5
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