Nanofluids in solar collectors

Background

Dispersing trace amounts of nanoparticles in a common base fluid has a significant impact on the optical ^{[12] and} thermo physical properties of base fluid . This characteristic can be used to effectively capture and transport solar radiation . Enhancement of the solar irradiance absorption capacity leads to a Higher heat transfer resulting and in more efficient heat transfer as shown in Figure 2. The efficiency of a solar thermal system is connecting On Several energy conversion steps, qui are in turn-governed by the effectiveness of the heat transfer processes. While higher conversion efficiencyof solar to thermal energy is possible, the key components that need to be improved are the solar collector . An ideal solar collector will absorb the concentrated solar radiation, convert that incident solar radiation into heat and transfer heat to the heat transfer fluid. Higher the heat transfer to the fluid, the higher temperature and the higher temperature lead to the improved conversion efficiency in the power cycle . Nanoparticles have several orders of magnitude higher heat transfer coefficient when transferring heat immediately to the surrounding fluid. This is simply due to the small size of nanoparticle .

Thermal conductivity of nanofluids

We know that thermal conductivity of solids is greater than liquids . Commonly used fluids in heat transfer applications such as water , ethylene glycol and engine oil have low thermal conductivity when compared to thermal conductivity of solids, especially metals . So, addition of solid particles in a fluid can increase the conductivity of liquids.

• Mixtures are unstable and hence, sedimentation occurs.
• Presence of large solid particles also require large pumping power and increased cost.
• Solid particles can also erode the channel walls.

Due to these drawbacks, the use of solid particles has not become practically feasible. Recent improvements in nanotechnology made it possible to introduce small particles with diametersmaller than 10 nm. Liquids, thus obtained higher thermal conductivity and are known as Nanofluids . As carbon nanotubes have highest thermal conductivity as compared to other materials.

Maxwel model ^[13]

{\ displaystyle k_ {nf} = k_ {bf} {\ Biggl (} {\ frac {k_ {p} + 2k_ {bf} +2 \ varnothing (k_ {p} -k_ {bf})} {\ k_ { p} + 2k_ {bf} – \ varnothing (k_ {p} -k_ {bf})}} {\ Biggr)}}

Pak and Choi model ^[14]

{\ displaystyle k_ {nf} = k_ {bf} (1 + 7.47 \ varnothing)}

Koo and Kleinstreuer model ^[15]

{\ displaystyle k_ {nf} = k_ {bf} {\ Biggl (} {\ frac {k_ {p} + 2k_ {bf} +2 \ varnothing (k_ {p} -k_ {bf})} {\ k_ { p} + 2k_ {bf} – \ varnothing (k_ {p} -k_ {bf})}} {\ Biggr)} + ​​5000 \ theta \ rho _ {bf} C_ {pbf} {\ sqrt {\ frac {K_ {B} T} {\ rho _ {p} d_ {p}}}}}

Udawattha and Narayana model ^[16]

{\ displaystyle k_ {nf} = k_ {bf} {\ Biggl (} 1 + {\ frac {3 \ varnothing _ {e} (k_ {p} -k_ {bf})} {\ k_ {p} + 2k_ {bf} – \ varnothing _ {e} (k_ {p} -k_ {bf})}} + {\ frac {\ rho _ {bf} (\ varnothing ^ {0.0009T + 0.25}) C_ {pbf} k_ {p} d_ {p} V_ {B}} {200 \ mu _ {bf}}} {\ sqrt {\ frac {\ pi} {18}}} {\ Biggr}}}

{\ displaystyle V_ {B} = {\ sqrt {\ frac {18K_ {B} T} {\ pi \ rho _ {p} {d_ {p}} ^ {3}}}}}

{\ displaystyle \ varnothing _ {e} = \ varnothing {{\ Biggl (} 1 + {\ frac {h} {r}} {\ Biggr)}} ^ {3}}

Where

{\ displaystyle k}is the thermal conductivity of the sample, in [ W · m ^-1 · K ^-1 ]

{\ displaystyle nf} is nanofluid

{\ displaystyle bf} is basefluid

{\ displaystyle p} is particle

{\ displaystyle \ varnothing} is volume fraction

{\ displaystyle \ rho}is density of the sample, in [ kg · m ^-3 ]

{\ displaystyle C_ {p}}is specific heat capacity of the sample, in [J · kg ^-1 · K ^-1 ]

{\ displaystyle K_ {B}} is the constant Boltzmann

{\ displaystyle T} is Temperature of the sample, in [K]

{\ displaystyle d_ {p}} is diameter of a particle

{\ displaystyle h} is nanolayer thickness (1 nm)

{\ displaystyle r} is radius of a particle

Mechanism for enhanced thermal conductivity of nanofluids

Keblinski et al. ^[17] had named four possible mechanisms for the anomalous increase in nanofluids heat transfer which are:

Brownian motion of nanoparticles

Due to Brownian motion particles randomly move through liquid. And hence better transportation of heat. Brownian motion increased mode of heat transfer.

Liquid layering at liquid / particle interface

Liquid molecules can form a layer on the solid particles and there by the atomic structure of the atomic structure at the interface region.hence, the atomic structure of such liquid layer is more than that of the bulk liquid.

Effect of nano-particles clustering

The actual volume of a cluster is regarded much larger than the volume of the particles due to the lower packing fraction of the cluster. Since, heat can be transferred rapidly within these clusters, the volume fraction of the highly conductive phase is larger than the volume of solid, thus increasing its thermal conductivity

Comparison

In the last years, many experiments have been conducted and analyzed to validate the importance of nanofluids.

From the table 1 ^[14] it is clear that nanofluid-based collector have a higher efficiency than a conventional collector. So, it is clear that we can only collector by adding trace amounts of nano-particles. It has beens aussi Observed through numerical simulation That mean outlet temperature Increase By Increasing volume fraction of nanoparticles, length of tube and decreases by decreasing velocity. ^[14]

Benefits of using nanofluids in solar collectors

Nanofluids poses the following advantages as compared to conventional fluids which makes them suitable for solar collectors:

• Absorption of solar energy will be maximized with change of size, shape, material and volume fraction of the nanoparticles.
• The suspended nanoparticles increase the surface area but decrease the heat capacity of the particle size.
• The suspended nanoparticles enhance the thermal conductivity which results in the efficiency of heat transfer systems.
• Properties of fluid can be changed by varying concentration of nanoparticles.
• Extremely small size of nanoparticles ideally enables them to pass through pumps.
• Nanofluid can be optically selective (high absorption in the solar range and low emittance in the infrared .)

The fundamental difference between the conventional and nanofluid-based collectors in the mode of heating of the working fluid. In the former case, the sunlight is absorbed by a surface, which is directly absorbed by the working fluid (through radiative transfer ). On reaching the receiver the solar radiation transfer energy to the nanofluid via scattering and absorption .

See also

• Nanofluid
• Solar collector
• Solar Energy
• Absorption
• scattering
• Fluid
• Radiation

References