Abstract: different types of solar collectors, photovoltaic systems

Abstract: Solar
energy is considered nowadays as one of the most important sources of clean ,
free and renewable energy with minimum environmental effects. The negative
impact of human activities on the environment receives tremendous attention,
especially on the increased global temperature. Using nano fluids as absorber
fluid is an effective method to enhance thermal properties of solar devices . Nanofluids
are suspension of nanoparticles in base fluids, a new challenge for thermal
sciences provided by nanotechnology. Nanofluids have unique features different
from conventional solid-liquid mixtures in which mm or µm sized particles of
metals and non-metals are dispersed. Due to their excellent characteristics
such as unique mechanical, optical, electrical, magnetic, and thermal
properties, nanofluids find wide applications in enhancing heat transfer. More recently
researchers have become interested in the use of nanofluids in Solar collectors,
solar water heating systems, solar cooling systems, solar cells, solar stills,
solar cooling systems.

The aim of this review paper is the
investigation of the nanofluids’ applications in solar applications. Modern
nanotechnology can produce metallic or nonmetallic particles of nanometer
dimensions which have. Studies in this field indicate that exploiting nanofluid
in solar systems, offers unique advantages over conventional fluids. In this
paper, the applications of nanofluids on different types of solar collectors,
photovoltaic systems and solar thermoelectrics are reviewed. Beside the wide
range of energy conversion, the efforts done on the energy storage system (ESS)
have been reviewed. In the field of economics, nanotech reduces manufacturing
costs as a result of using a low temperature process. The aim of this appraisal
manuscript is the study of the nanofluids in solar Energy applications. In order
to overcome these drawbacks, direct solar absorption collector has been used
for solar thermal exploitation.

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Introduction:
Solar energy technology is considered as one of the ideal candidates, which
directly converts solar energy into electricity and heat without any greenhouse
gas emissions Solar energy is considered nowadays as one of the most important
sources of clean , free and renewable energy with minimum environmental
effects. After industrial revolution (1970s) energy consumption increased
sharply ,so threat of energy shortages led scientists to find new sources of
energy. Renewable sources of energy from sun are fairly non-polluting and
considered clean. Solar energy as the green and environmental friendly energy
has produced energy for billions of years. Considering the environmental protection
and great uncertainty over future energy supplies, solar energy is a better
alternative energy form in spite of its  slightly higher operation costs.

Nano
Particles and Nano fluid Application

Nanofluids can be defined as a
solid-liquid composite materials consisting of nanometer sized solid particles,
fibers, rods or tubes suspended in different base fluids. Some examples of
nanoparticles are pure metals (Au, Ag, Cu, Fe), metal oxides (CuO, SiO2,
Al2O3 , TiO2, ZnO, Fe3O4),
Carbides (SiC, TiC), Nitrides (AlN, SiN) and different types of carbon
(diamond, graphite, single/multi wall carbon nanotubes).How to select suitable
nanofluids in solar applications is a key issue. The effectiveness of
nanofluids as absorber fluids in a solar device strongly depends on the type of
nanoparticles and base fluid, volume fraction of nanoparticles, radiative properties
of nanofluids, temperature of the liquid, size and shape of the nanoparticles,
pH values, and stability of the nanofluids. Nanofluid have a good properties of
radiation absorption and it has a high thermal conductivity. A nano fluid is
prepared by dispersing nanoparticles in a base fluid by proper mixing and
allowing stabilization of mixture. The size of nanoparticles is very small and
in the range of 1–100 nm. The Nano particles are classified as   Ultrafine particles sized between 1 and
100 nm in size, fine particles are sized between 100 and 2,500 nm and
coarse particles cover a range between 2,500 and 10,000 nm. Due
to their large surface area, less particle momentum, and high mobility,
nanoparticles emerged as suitable candidates for suspending in fluids.5

Figure 1 TEM (a, b, and c)
images of prepared mesoporous silica nanoparticles with mean outer diameter:
(a) 20nm, (b) 45nm, and (c) 80nm. SEM (d) image corresponding to (b).                       ( Source:
https://en.wikipedia.org/wiki/Nanoparticle)

 

 Application of Nano Fluids in Solar Energy

Solar Water heating: Solar
collectors transform solar radiation into heat and transfer that heat to a
medium (water, solar fluid, or air). The solar heat can be used for heating
water, to heating or cooling systems, or for heating swimming pools. Solar
cooling technologies demand high temperatures and not all the type of solar
collectors are capable of producing them. Lu, L., 6 developed an open
thermosyphon device used in high-temperature evacuated tubular solar collectors
 to investigate the thermal performance
of the open thermosyphon using respectively the deionized water and water-based
CuO nanofluids as the working liquid. Effects of filling rate, kind of the base
fluid, nanoparticle mass concentration and the operating temperature on the
evaporating heat transfer characteristics in the open thermosyphon were
investigated and discussed. Experimental 
results showed the optimal filling ratio to the evaporator is 60% and
the thermal performance of the open thermosyphon increase generally with the
increase of the operating temperature.

Sujit  Kumar Verma7 explored the
variable performance of solar system with and without the use of nanofluid. Parametric
studies of variables such as  thermal
conductivity, viscosity, specific heat and the heat transfer coefficient are
studied. A study of the works earlier done seems to be suggesting that the
nanofluids have great potential to enhance the functioning of various thermal systems.
It can be safely assumed further that it might enhance the overall performance
of the solar energy conversion process. Khullar 9  carried out parametric studies  to  harvest  solar radiant energy through usage of
nanofluid-based concentrating parabolic solar collectors (NCPSC) by developing
mathematical model, and the governing equations were numerically solved using finite
difference technique. The results of the model were compared with the
experimental results of conventional concentrating parabolic solar collectors
under similar conditions. It was observed that while maintaining the same
external conditions (such as ambient/inlet temperatures, wind speed, solar
insolation, flow rate, concentration ratio, etc.) the NCPSC has about 5–10%
higher efficiency as compared to the conventional parabolic solar collector.
Furthermore, parametric studies were carried out to discover the influence of
various parameters on performance and efficiency. The following parameters were
studied in the present study: solar insolation, incident angle, and the
convective heat transfer coefficient. The theoretical results clearly indicate
that the NCPSC has the potential to harness solar radiant energy more
efficiently than a conventional parabolic trough.

Solar
Absorption: An
air-conditioning system utilizing solar energy would generally be more
efficient, cost wise, if it was used to provide both heating and cooling
requirements in the building it serves. Research on solar absorption air
conditioning system to make it in order to more economical and technically
viable is ongoing. Due to its renewable and nonpolluting nature, solar energy
is often used in applications such as electricity generation, thermal heating,
and chemical processing. The most cost-effective solar heaters are of the
“flat-plate” type, but these suffer from relatively low efficiency and outlet
temperatures. Tyagi10 carried theoretical study to investigate performance   of
using a nonconcentrating direct absorption solar collector (DAC) and compare
its performance with that of a typical flat-plate collector using a
nanofluid  —a mixture of water and
aluminum nanoparticles—as the absorbing medium. It was observed that the presence
of nanoparticles increased the absorption of incident radiation by more than
nine times over that of pure water.