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S. Mohan et al.
1 Introduction
Renewable energy being a clean and economical energy resource has high demand
in the society. Different types of renewable energy sources currently in use include
solar, wind, tidal, and geothermal energy. Among these, solar energy is the most
common on account of its abundant availability in the earth. Tapping of solar energy
by means of photo-thermal conversion can meet most of the energy needs of the
society [1]. A solar thermal power system employs both concentrating and nonconcentrating type of energy tapping out of which the former is more effective [2].
In concentrating type, the light energy is focused onto a receiver which converts
it into heat energy. Solar thermal conversion using direct absorption was known to
be one of the modes through which thermal energy conversion could be achieved
directly by the working fluid. This energy is transferred for various applications
like water heating, drying, etc. Dispersing nanoparticles into the base fluid enhances
its thermo-physical and optical properties and aids in better capture and transport
of solar radiation [3, 4]. The investigations on nanofluids for energy conversion
started in the early 1980s wherein prime focus was given to enhance the optical and
thermal properties of the heat transfer fluid for thermal transport. However, most
of the studies were conducted with nanofluid containing single nanoparticles which
include but not limited to Al, Cu, Ag, Au, Ti, and its oxides [2]. In course of time,
researchers developed binary (hybrid) nanofluid which exhibited superior properties
than their counterpart containing single nanoparticles.
Nambiar et al. [5] suspended copper oxide nanoparticles in ethylene glycol in a
weight ratio of 60:40 and noted an increase in thermal conductivity by as much as
40%. Allen Zennifer et al. [6] noted about 13% reduction in the viscosity and 11%
enhancement in thermal conductivity of ethylene glycol when 26% (by volume)
of CuO nanoparticles was added to it. Muraleedaran et al. [7] evaluated the thermooptical properties of Al 2 O 3 -Therminol 55 nanoheat transfer fluid using a line concentrating Fresnel lens. The authors carried out the solar conduction tests in real-time
environment and found that the addition of nanoparticles dramatically increased the
thermal conductivity of the nanofluid at lower concentrations. Accordingly, there
was an increase in the refractive index of the nanofluid. The authors further observed
that higher the concentration of nanoparticles, higher the absorbance of the heat
transfer fluid. Amiri et al. [8] found out the favorable effect of addition of SiO 2 –Cu
nanoparticles on the thermal conductivity of two base fluids, water, and ethylene
glycol. Murshed et al. [9] prepared a nanofluid by dispersing spherical-shaped and
rod-shaped TiO 2 nanoparticles in deionized water. The thermal conductivity of the
nanofluid was found to increase with an increase in concentration of TiO 2 nanoparticles. Arulprakasajothi et al. [10] used TiO 2 -water based nanofluids to enhance the
performance of a heat exchanger. The thermal conductivity was increased with the
concentration of nanoparticles. Zeiny et al. [11] concluded that the direct absorption nanofluid is an effective alternative in increasing the efficiency of solar-thermal
energy conversion. The study experimented different categories of nanofluid under
well-controlled conditions.
S. Mohan et al.
1 Introduction
Renewable energy being a clean and economical energy resource has high demand
in the society. Different types of renewable energy sources currently in use include
solar, wind, tidal, and geothermal energy. Among these, solar energy is the most
common on account of its abundant availability in the earth. Tapping of solar energy
by means of photo-thermal conversion can meet most of the energy needs of the
society [1]. A solar thermal power system employs both concentrating and nonconcentrating type of energy tapping out of which the former is more effective [2].
In concentrating type, the light energy is focused onto a receiver which converts
it into heat energy. Solar thermal conversion using direct absorption was known to
be one of the modes through which thermal energy conversion could be achieved
directly by the working fluid. This energy is transferred for various applications
like water heating, drying, etc. Dispersing nanoparticles into the base fluid enhances
its thermo-physical and optical properties and aids in better capture and transport
of solar radiation [3, 4]. The investigations on nanofluids for energy conversion
started in the early 1980s wherein prime focus was given to enhance the optical and
thermal properties of the heat transfer fluid for thermal transport. However, most
of the studies were conducted with nanofluid containing single nanoparticles which
include but not limited to Al, Cu, Ag, Au, Ti, and its oxides [2]. In course of time,
researchers developed binary (hybrid) nanofluid which exhibited superior properties
than their counterpart containing single nanoparticles.
Nambiar et al. [5] suspended copper oxide nanoparticles in ethylene glycol in a
weight ratio of 60:40 and noted an increase in thermal conductivity by as much as
40%. Allen Zennifer et al. [6] noted about 13% reduction in the viscosity and 11%
enhancement in thermal conductivity of ethylene glycol when 26% (by volume)
of CuO nanoparticles was added to it. Muraleedaran et al. [7] evaluated the thermooptical properties of Al 2 O 3 -Therminol 55 nanoheat transfer fluid using a line concentrating Fresnel lens. The authors carried out the solar conduction tests in real-time
environment and found that the addition of nanoparticles dramatically increased the
thermal conductivity of the nanofluid at lower concentrations. Accordingly, there
was an increase in the refractive index of the nanofluid. The authors further observed
that higher the concentration of nanoparticles, higher the absorbance of the heat
transfer fluid. Amiri et al. [8] found out the favorable effect of addition of SiO 2 –Cu
nanoparticles on the thermal conductivity of two base fluids, water, and ethylene
glycol. Murshed et al. [9] prepared a nanofluid by dispersing spherical-shaped and
rod-shaped TiO 2 nanoparticles in deionized water. The thermal conductivity of the
nanofluid was found to increase with an increase in concentration of TiO 2 nanoparticles. Arulprakasajothi et al. [10] used TiO 2 -water based nanofluids to enhance the
performance of a heat exchanger. The thermal conductivity was increased with the
concentration of nanoparticles. Zeiny et al. [11] concluded that the direct absorption nanofluid is an effective alternative in increasing the efficiency of solar-thermal
energy conversion. The study experimented different categories of nanofluid under
well-controlled conditions.