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Abstract

Energy from the Sun brings as Solar energy which is abundantly available and utilized for various applications. Owing to the increase in energy demand, having a limit of non-renewable energy sources, more interest is given to Solar energy. One of the most fundamental applications of using Solar energy is a Solar collector. The efficiency of the Solar collector depends upon the fluids used in the Solar collector and thermal performance also can be enhanced by using heat pipes in the Solar collector. Compared to flat plate solar collectors, the major disadvantage of heat loss can be rectified in heat pipe solar collectors. Still, to improve the performance of heat pipe solar collectors, nanofluids can be used instead of base fluids to improve thermal performance. In this review paper, the application of nanofluid in Solar collectors results in the percentage of improvement in heat transfer by nanofluid at optimum condition is discussed. This paper reviewed widely the recent development and upcoming research that can be done to enhance the working of heat pipe Solar collectors using nanofluids.

Keywords

Solar collector thermal conductivity nanofluids heat pipe solar collector heat transfer

1 Introduction

Renewable Energy resources and their applications for energy systems exist over worldwide areas. Fast development and energy efficiency in Solar energy results in various economic benefits. Due to these economic benefits, now researchers are focussing on how to use Solar energy effectively. Only half of the Solar radiation makes it to the earth's surface and the remaining are either absorbed in the atmosphere or reflected directly back by the atmosphere. During day time, Solar energy is easily accessible, so the solar collector is considered the most important component among all systems. The working of the Solar collector is it consumes the radiation from the sun, converts it into thermal energy, and transfer the thermal energy to conventional fluid usually air, water, oil, etc. Thus, collected Solar energy is directly passed to a storage tank from which can be used for various applications. Solar collector exists in three types: Flat plate Solar collector (FPSC), Evacuated tube Solar collector (ETSC), Heat pipe Solar collector (HPSC). FPSC is considered as most fundamental technology used for the Solar hot water setup. FPSC is relatively inexpensive, collect energy as much as possible and has relatively easy installation due to their simple design. Although FPSC is technically and economically feasible, it is more efficient on sunny days than on cloudy or rainy days. But absorbing constant energy is not possible throughout the year. High energy loss occurs on sunny, cold days due to the large receiver surface. By applying anti-reflective glass, good absorbing surfaces, additional insulation can be given to avoid these issues. ETSC was introduced to overcome the challenges given by FPSC. In ETSC, there will be less heat loss, because of the vacuum between the tubes, especially on cloudy days. ETSC is strong and has low maintenance costs. As the panels are 90% efficient, they can produce more heat absorption compared to flat plate collectors. The most important of ETSC is they generate temperatures very high and get more efficient than other solar collectors. Problems arise in ETSC if the temperature exceeds more than the boiling point of water also exceeds. While experimenting with ETSC, it should handle with care as they are made from annealed glasses and these glasses are more fragile.¹ The limitation of ETSCs and FPSCs was overcome by using a heat pipe Solar collector (HPSC). The main advantage of HPSCs is it removes heat from the absorbing surface and transfer it to the fluid used in HPSCs with low heat resistance.² The efficiency of HPSC is more compared with FPSCs and EPSCs because it performs phase transition from liquid to vapour to transfer the heat. The temperature inside the HPSC can be controlled because the evaporation or condensation process will not occur above the liquid to vapour phase change temperature.³ In the heat pipe, heat is absorbed in the evaporator section, and heat is liberated in the condenser part. The heat loss can be minimized so that it acts as a thermal diode. Corrosion can be minimized by filling the working fluids in heat pipes and sealed.² In HPSC, the elimination of corrosion and freezing plays a major role, so the HPSC life period can be increased. In HPSC, conventional working fluids such as methanol, ethanol, water, etc. can be used based on the operating limit of the temperature. These working fluids have low thermal conductivity because it contains mm or µm size particle and they can clog with the devices. Current development in nanotechnology has been introduced nanofluids, which is a colloidal suspension of nanometre-sized particles instead of common base fluids. Compared to conventional base fluid, nanofluid has a high extinction coefficient. So, these nanofluids can consume more energy from Solar radiation.

In recent years, the advantages of HPSC over nanofluid gave important attention to Solar applications. This paper reviews the performance of nanofluids and how the efficiency is increased in heat pipe solar collectors by using nanofluid.

2 Heat Pipe

The Heat pipe utilizes the evaporator and condensation part, so it can absorb radiation from the sun in the evaporator part and transfer the heat in the form of vapour to the condenser part.⁴ High thermal conductivity, high thermal flux is the main advantage of using a heat pipe. The main advantage of this heat transfer device is the phase change of liquid to vapour of working fluids in the evaporation and condensation stages.⁵ Figure 1(a) represents the main module of the heat pipe are the evaporator, condenser, and the fluid transport of the working fluid. The evaporator part is referred to as a hot interface, in which the liquid phase changes into the vapour phase by absorbing heat. The vapour moves along with the heat pipe to the cold interface region (i.e.) condenser part, in which it releases heat and condenses back into the liquid. Due to capillary action, the liquid again reaches to condenser part and the cycle repeats as shown in Figure 1(b).

Figure 1.

Schematic represents of a) heat pipe and b) working principle of heat pipe for conventional rotation⁵.

If the transformation of heat in the evaporator section is large, a significant quantity of heat can move from one section to another. The selection of working fluid is mainly based on properties of the fluid, high latent heat, heat transfer, good thermal stability, high thermal conductivity, and high surface tension. Table 1 illustrates the study of heat pipes in various fields.

Table 1.

Illustration of heat pipes in various fields.

Source	Illustration	Remarks
Xue et al.⁵⁶	Heat transfer performance of a horizontal micro-Grooved heat pipe using CuO nanofluid	At operating pressure 7.45 KPa heat transfer improved by 46% Heat flux enhanced by 30% by CuO
Tun et al.⁵⁷	Thermal efficiency of heat pipe with Alumina Nanofluid	Thermal efficiency is 16.8% higher than distilled water when nanoparticles are at 0.1 Wt%
Bhawna et al.⁵⁸	Experimental studies on Thermal performance of a Pulsating heat pipe with Methanol/DI water	Experiment results show that PHP charged with methanol worked efficiently
Gabriela et al.⁵⁹	Thermal performance of Thermosyphon heat pipe using iron oxide nanoparticles	Performance was improved by using iron oxide nanoparticles in water compared with DI-water

3 Evacuated Heat pipe solar collector

The most popular design to heat water is the evacuated tube Solar collector. In this type of Solar collector evacuated tubes are fabricated using glass are arranged in parallel. In ETSCs each tube consists of a couple of glass tubes, in which the external tube is glassy and transparent, while the tube present inside is polished with the material having high absorption so that it can absorb the Solar energy with minimum reflection loss. The two glass tubes joined together, and the vacuum should be maintained between these two glass tubes. If a vacuum is present it acts as an insulator, so that performance of evacuated tubes can be improved. Figure 2 shows the cut-sectional view of the evacuated glass tube.

Figure 2.

A snapshot represents the cross-sectional view of the evacuated glass tube⁵⁵.

When compared to FPSC, ETSC due to its cylindrical absorbing surface it does not require sun trackers.⁶ One of the most advanced designs in the ETSC is combining Evacuated tubes with heat pipe; they form EHPSCs. The heat pipe is placed with an absorber plate, which is kept inside the evacuated glass tube. Inside the heat pipe working fluid is kept as shown in Figure 3. When the sunlight falls on the absorber plate, the working fluid inside the heat pipe absorbs the heat at the evaporator section, converts it into vapour, transfers it to the condenser part releases the heat, and flows back down to the tube.

Figure 3.

A snapshot represent the block diagram for evacuated heat pipe solar collector⁶.

Investigation on a Solar water heater with ETSC and FPSC was tested. Results show that the efficiency performance of the collector was 46% and 60.7% for FPSC and ETSC respectively.⁷ Investigation on thermosyphon heat pipe ETSC by using six conventional working fluids (Chloroform, Hexane, acetone, Petroleum ether, Hexane, Petroleum ether, methanol, and ethanol) with different air velocities. The Experiment was made to improve the performance of ETHPC using oil and foamed metals. Results revealed that the rate of heat transfer enhances by about 25%.⁸ Experimental work was carried out in evacuated tube Solar collectors for thermal efficiency using with and without heat pipe systems. It was observed that phase change materials with Heat pipe evacuated tube Solar collectors were found more efficient.⁹ Overall, the main advantage of HPSC i) Passive heat exchange ii) Space-efficient iii) Moisture removal capacity iv) Requires no mechanical or electrical input and v) Virtually maintenance free. Table 2 illustrates the various remarks of ETSC.

Table 2.

Various illustration of working of evacuated tube solar collector.

Source	Illustration	Remarks
Tahmineh et al.⁶⁰	Thermo economic and environmental analysis of Solar Flat Plate and Evacuated tube collector in cold climate condition	Performance of ETC is 41% better than FPC system, an Energy gain of ETC is 30% more than FPC
Feliński et al.⁶¹	Experimental study of evacuated tube collector/storage A system containing paraffin as a PCM	Results show that ETC with Paraffin has increased the total amount of heat by 45–79%.
Ruobing et al.⁶²	Thermal performance of filled type Evacuated tube with U-tube	Results show that Evacuated tube with U tube filled type have enhanced thermal efficiency compared with Evacuated tube with copper fin
Liangdong et al.⁶³	Experimental study of Thermal performance analysis of the glass evacuated tube Solar collector with U-tube	Investigation shows that the diameter, thickness, length of the copper tube increases thermal performance and outlet mean temperature.

4 Flat Heat pipe solar collector

A schematic of FPSC as represented in Figure 4 having an absorber plate usually made of Aluminium treated with a black coating surface to enhance the absorption of incident radiation. Convection losses can be minimized by using glazing. To transfer the heat to the working fluid Aluminium sheet is joined to the copper pipe.¹⁰

Figure 4.

Schematic diagram of flat plate liquid type solar collector.

Flat plate HPSC is another type of HPSC in which the absorption/transmission process of Solar radiation is the same, in which heat pipes are attached to flat plate absorbers. Advantages of FPHSC are heat flows in uni-direction, preventing overheating, and minimizing heat loss. The Experiment was made in an FPSC using a WHP having three various cross-section geometries. Three various distilled water substantial ratios of 10%, 20%, and 35% were charged. Results show that among the different three cross-section geometries, the elliptical cross-section WHP shows better performance.¹¹

Enhancement of thermal performance of the Heat pipe FPSC using a cross-flow heat exchanger was analyzed experimentally. This method is applied to view the performance of Heat pipe FPSCs.¹²

5 Working Fluids

Based on the characteristics of working fluid only, the working performance of the Solar collector can be improved. There are two types of working fluids: (i) Conventional fluids and (ii) Nanofluid. For Heat transfer, a working fluid can be gas or liquid that transfers heat into or out of the region. Conventional working fluids like Ethanol, distilled water, Methanol etc., are so far used in heat pipes for heat transfer. If a working fluid is chosen, that fluid should work based on the operating temperature range of heat pipe. The condensation process will not occur if the working operating temperatures of the fluid are high and if the temperatures are low the fluid will not do evaporation process. The main aspect is how to enhance the working of the Solar collector. Based on these criteria, the important base in the functioning of solar collectors is heat transfer for better thermal conductivity. To enhance the thermal performance of the base fluid, researchers have done more experiments by suspending nanometer-size solid particles in the base fluids, because in solid the thermal conductivity is more than in liquid as shown in Table 3.

Table 3.
Thermal conductivity of various materials.

Material Thermal conductivity (W/m-K)

Carbon nano tubes 3000

Copper 401

Silver 406

Aluminium 235

Gold 314

Non-metallic solids Silicon 148

Alumina (Al₂O₃) 40

Metallic liquids Sodium (644 K) 72.3

Non-metallic liquids Water 0.613

Ethylene glycol (EG) 0.253

Engine oil (EO) 0.145

Material	Thermal conductivity (W/m-K)
Carbon nano tubes	3000
Copper	401
Silver	406
Aluminium	235
Gold	314
Non-metallic solids Silicon	148
Alumina (Al₂O₃)	40
Metallic liquids Sodium (644 K)	72.3
Non-metallic liquids Water	0.613
Ethylene glycol (EG)	0.253
Engine oil (EO)	0.145

6 Roles of nanofluids in heat transfer process

Recently many researchers have been analyzed, using nanofluids in the concept of transfer of heat in heat pipe Solar collector. Nanofluids are the homogeneous mixture of nanoparticles with the size range of the order of 1–100 nm in base fluids like methanol, oil, water, ethylene glycol etc. When nanoparticles are suspended into base fluids each nanoparticle absorbs and releases an electron and gets electrically charged. An optimistic outlook for nanofluid is how the performance of heat transfer improved, by increasing the heat transfer coefficient. Millimeters and Micrometers range fine particles failed to give a better thermal performance, but this can be overcome by using nanoparticles. In the past, many analyses were done to improve the function of the thermal conductivity of base fluids. But recent studies show that nanofluids suits for better thermal conductivity. In 1993, Masuda et al.¹³ analyzed the improvement of thermal conductivity by suspending micro-sized solid particles in the base fluid. But the depositions of solid particles in the base fluid lower its thermal performance. In 1995, Choi investigated the preparation of nanofluids by dispersing the nanosized solid particles in the base fluid. This analysis gave good experimental results without creating any stability problems. Suresh Kumar et al.¹⁴ reviewed the heat transfer and fluid flow properties of nanofluids in heat pipes. Heat transfer rate, adequate strength, stable dispersion, negligible agglomerations are the important characteristics of nanofluids. In Nanofluids, Brownian motion is improved because Vander Waals repulsive forces are larger. For better performance of thermal conductivity, Brownian motion plays an important role. The High surface area in nanoparticles makes to have a great heat transfer.

7 Nanofluids Preparation

To carry out the experimental studies preparation of Nanofluids plays the main role. To use nanofluid in various applications, it needs special requirements such as even dispersion, low agglomeration, and without any chemical change. Preparation of nanofluids can be done by two processes namely Single-step and Two-step methods.¹⁵

(i) Single-Step Method:

In this methodology, the nanoparticles were prepared and dispersion carried out in the base fluid like water etc. Coagulation of nanoparticles can be minimized because storage, the drying process is not required in this method. The physical vapour deposition (PVD) technique and Liquid chemical method are the most commonly used in the preparation of nanoparticles. The main concept of the one-step method is cluster formation in a nanoparticle can be reduced so that nanoparticles can be easily dispersed in the base fluid. Using only vapour pressure fluids is the main drawback of this method. Copper nanofluids were prepared by a one-step method under microwave irradiation by reducing CuSO₄·5 H₂O with NaH₂PO₂·H₂O. Results show that the rate of reaction and Cu nanofluids properties get disturbed due to the presence of microwave irradiation and NaH₂PO₂·H₂O.¹⁶ Silver nanofluid can be obtained by decomposition of silver lactate in the presence of Korantin SH and Mineral oil using the One-step method.¹⁷ Copper nanofluid was obtained by reducing CuSO₄ with the reducing agent EG using the one-step method. Copper nanofluids prepared from this method have high thermal conductivity and stability. In this method, various terms like microwave irradiation, Ultrasonic vibration, and dispersant are studied.¹⁸ In a one-step process, Laser ablation is also another method to produce nanofluids without using surfactants. In the one-step method, controlling particle size in the nanofluid is a big issue because the particle size is not uniform in nanofluids produced by this method. Another disadvantage is some part of the reaction is present in the nanosuspension due to incomplete reaction and it will disturb the purity and properties of nanofluids.

(ii) Two-Step Method

Most commonly used for nanofluid preparation is the two-step method.¹⁹ In this method, various forms of nanofibers, nanoparticles, and nanotubes are produced. Using base fluid these nanomaterials are directly dispersed using an ultrasonic bath, magnetic stirrer, homogenizer, or ultrasonic processor, etc., as shown in Figure 5. Agglomeration of particles can be reduced by dispersing the particles by ultrasonic equipment.

Figure 5.

The process of the two-step method for direct mixing of nanoparticles followed by addition of dispersant and ultrasonication.

The Two-step method is best for oxide nanoparticles and this method is not suited for metallic nanoparticles. Copper oxide powder is prepared by the sol-gel method and these copper oxide nanoparticles are dispersed in water by using an ultrasonic vibrator, nanofluid produced. pH value is measured for the nanofluid around 4.83.²⁰ TiO₂ – H₂O nanofluid was prepared by a two-step process. Proper ultrasonication improves the stability and the stability properties of TiO₂ – H₂O were tested by varying ultrasonication duration.²¹ In this method the stability of nanofluids can be obtained by adding surfactants. A copper-water-based nanofluid was prepared and showed that absorbency and zeta potential is very important. Distribution of Particle size leads to a better dispersion process.²² TiO₂ – water-based nanofluid obtained by a two-step method in which the surfactants used are cetyltrimethylammonium bromide and oleic acid to get proper stability and a better dispersion process.²³ Copper nanoparticles were suspended in water and transformer oil by sonication process in the presence of suitable stabilizers.²⁴

8 Parameters That enhance the thermal conductivity in nanofluids

In nanofluids, the thermal conductivity was depending on temperature, the volume fraction of nanoparticles, size of the particle, particle shape, etc. There are many studies related to the outcome of particle volume fraction in nanofluids. It is amain parameter investigated in many experimental studies and results also have good agreement qualitatively. The report showed that if particle volume fraction increases increase the performance of thermal conductivity properties in nanofluids. Investigation of Thermal conductivity for grapheme nanoplatelets dispersed in two different base fluids (water and EG) was investigated based on volume concentration. The experiment revealed that if concentration increases it results in enhancement in thermal conductivity. Thermal conductivity was improved for 0.5% volumetric concentration.²⁵ An effect of size of the nanoparticle, volume concentration, and temperature was investigated for α – Al₂O₃/Glycerol nanofluid. The volume concentration varied between 0.5% and 4%. The range of temperature taken is between 20°C and 45°C. The particle size of the nanoparticle was 31 nm, 55 nm, and 135 nm. Results revealed that if particle size increases it results in a decrease in nanofluid thermal conductivity.²⁶ The shape of the nanoparticle also plays a major role in the improvement of thermal conductivity. The performance of thermal conductivity of TiO₂/water nanofluid was investigated based on the shape of nanomaterials and volume concentration. Variation of Volume concentration was done from 0.5% and 2.5%, the temperature varied 303K and 353K. The nanoparticles are taken in the form of shapes like spherical, rod, and cubic was investigated. It was observed that cubic-shaped particles result in high thermal conductivity. In addition to that, it also is shown that rod-shaped nanoparticles show better performance compared to spherical-shaped particles.²⁷ Al₂O₃ nanoparticles were taken with two different sizes (36 nm and 47 nm) and it is meant for the investigation of the thermal conductivity of Al₂O₃/water nanofluids. It is revealed that at room temperature both nanoparticle size of Al₂O₃ shows the same enhancement in thermal conductivity. But if temperatures increased, smaller particles of Al₂O₃/water nanofluid show improvement.²⁸ Thermal conductivity of CuO/water and Al₂O₃/water nanofluid was investigated with the impact of temperature and particle size 29 nm and 36 nm respectively. For both nanofluids, there is an increase in thermal conductivity due to a temperature rise.²⁹ Using hybrid nanofluids is also considered an important feature for the improvement of thermal conductivity.^30–32 The Enhancement of thermal conductivity was also improved by using hybrid nanofluids. The thermal conductivity of SiO₂/EG and SiO₂/water was compared with hybrid nanofluids SiO₂-Cu/EG and SiO₂-Cu/water. It shows that the deposition of Cu nanoparticles on SiO₂ results in the enhancement of thermal conductivity.³³

9 Application Of nanofluids in solar collector

The effect of CuO-water nanofluid was investigated experimentally in FPSC. The volume concentration of nanoparticles is considered as 0.4% and the average particle size as 40 nm. The mass flow rate of working fluid is changed between 1 to 3 Kg/min. The collector efficiency increases by about 21.8% for the value of 1 kg/min mass flow rate. Results show that nanoparticle with base fluid shows improved thermal characteristics. It also shows that there is an optimum mass flow rate for the various working fluids which maximizes collector efficiency.³⁴ The exergy and energy efficiency of FPSC was examined by using Al₂O₃ – water nanofluid. The size of the nanoparticle was 13 nm and the volume fraction used was 0.1% and 0.3%. Al₂O₃ – water nanofluid was prepared by controlling pH for 30 days period. The nanofluid mass flow rate was varied from 0.5 to 1.5 kg/min. Results showed that nanofluid increase the energy efficiency by 83.5% for 0.3% v/v and 1.5 kg/min and exergy efficiency was increased by 20.3% for 1.1% v/v and 1 kg/min.³⁵ The influence on the thermal efficiency of nanofluid and water in FPSC was performed by using Al₂O₃, CuO, and TiO₂ nanoparticles at 0.2,0.4, and 0.8 volume concentration in distilled water. The results showed that nanofluid gives more collector efficiency compared with water.³⁶

Performance of 2 m² FPSC using graphene oxide-based nanofluid was analyzed. By ultrasonication, method nanofluid was prepared with a mass concentration of 0.005, 0.01, and 0.02. Thermal efficiency was improved by about 7.3% for graphene oxide nanofluid for the mass fraction of 0.02%.³⁷ Enhancement of 0.375 m² FPSC for the working fluids SiO₂/H₂O, TiO₂/ H₂O, Al₂O₃/ H₂O, CuO/ H₂O, graphene/ H₂O, and MWCNT/ H₂O was performed. Triton X-100 surfactant was used. The volume fraction 0.25%,0.5%,0.75%,1.0%,1.5% while the mass flow rates of 0.01 to 0.05 Kgs⁻¹ were used. The Thermal efficiency of FPSC was enhanced by 23.47%, 16.97%, 12.64%, 8.28%, 5.09%, and 4.08% for MWCNT/water, graphene/water, CuO/water, Al₂O₃/water, TiO₂/water, and SiO₂/water were observed.³⁸ So far as discussed above, FPSC shows good improved thermal efficiency by using nanofluids. But we know that FPSC is more efficient only during sunny days not on rainy or cloudy days. Even though if we use nanofluids in FPSC, during cloudy days the thermal efficiency will not be up to the mark. These difficulties can be overcome by using nanofluids in ETSC. The efficiency of ETSC using water-based SWCNTs nanofluids was investigated. The experiment was performed by using SWCNTs nanofluids having a mass flow rate of 0.08,0.017 and 0.025 kg/s and volume concentration of 0.05, 0.1 and 0.2 vol %. The result shows that increase in efficiency was found to be 93.43% for 0.2 vol % at a mass flow rate of 0.025 kg/s. The efficiency of the collector was improved by using SWCNTs nanofluids.³⁹ Dispersing nanoparticle Cu₂O in distilled water and using the nanofluid in the volume fraction range from 0.01 to 0.08 in ETSC was experimentally verified. Thermal efficiency was enhanced for the 0.08% volume fraction of nanofluid in ETSC.⁴⁰ Experimental investigation was performed on EPSC with a spherical coil kept inside a horizontal tank by using Al₂O₃ / water nanofluid. Triton X-100 is used as a surfactant and the volume fraction of nanoparticles was 0.03% and 0.06%used in the experimental study. The result shows that efficiency was increased by 57.63% for 0.06 vol % by using Al₂O₃ nanofluids.⁴¹ Performance of graphene nanoplatelets (GNP)/ water nanofluid on ETSC was analyzed. Viscosity, stability, thermal conductivity, and specific heat capacity were also analyzed. Experiment was performed for mass concentration of GNP at 0.025, 0.5, 0.075 and 0.1 wt % and volumetric flow rate of 0.5, 0.1 and 1.5 L/min. Results show that the thermal efficiency of ETSC was improved by 90.7% at a flow rate of 1.5 L/min. Increasing the mass concentration of nanoparticles, gain in thermal energy also increases.⁴² When compared to FPSC, ETSC can absorb more Solar radiation with minimum reflection loss, in addition to that using nanofluids in ETSC makes these Solar collectors more efficient compared to nanofluids used in FPSC.

10 Application of nanofluid in HPSC

To increase the efficiency of solar collectors more when compared to using nanofluids in the Solar collector, we can use nanofluids in heat pipes which can be installed in different Solar collectors to improve their thermal efficiency. The advantages of using nanofluids in heat pipe Solar collectors are (i) Using a greater number of heat pipes results in higher efficiency in Solar collectors. (ii) Thermal diode (iii) Redundancy in the Solar collector and (iv) Co-axial heat pipe Solar collector improves the efficiency in heat pipe evacuated Solar collector. The absorption properties in the base fluid limit the performance of the Solar collector. It is overcome by the nanofluids, which is used in Heat pipe Solar collector to improve efficiency. Silver-water nanofluid is used in thermosyphon heat pipe ETSC for commercial applications. The result shows that the performance of Solar collectors increases between 20.7% and 40% by using Silver nanofluid in contrast to water.⁴³ Using a two-step method MgO – water nanofluid was prepared. Results show that the efficiency of the collector increases by using MgO nanofluid in contrast to pure water.³ The Thermal performance of HPSC is investigated by using water and CuO – water nanofluid at different flow rates and with different volume fractions was studied. It was observed that the function of Solar collectors gets enhanced with flow rate and volume fraction of nanofluid. Two flat plate Solar collectors were used which are identical, thermal efficiency was studied by using different working fluids: pure water and CNT – water nanofluid. Results show that using CNT – water nanofluid collector was better when compared to pure water.⁴⁴ The performance of a heat pipe Solar collector (wickless) was designed and fabricated by using CNT nanofluids for various tilt angles and concentrations. Experiment shows that the efficiency was enhanced at 73% for 63% CNT nanofluids.⁴⁵ The performance of copper oxide nanofluid in the Thermosyphon Heat pipe Solar collector at the different operating systems was investigated. Results show that by using nanofluid thermal enhancement of heat pipe was increased. Enhancement of the Solar collector was high with an increase in the concentration of nanofluid.⁴⁶

11 Inferences From the survey

Future work can be done by choosing toxin-free and relatively inexpensive nanoparticles compare to high-expensive nanoparticles.

An increase in exergy and energy efficiency in experimental studies encourages future research in the field of nanofluids.

Investigation on hybrid nanofluid for enhancement in heat transfer has been confirmed in recent studies.^47–52 Future work can be done by using these types of hybrids nanofluid in Solar collectors to ensure maximum performance.⁵³

Further research can be done to avoid the corrosion and erosion of Solar collectors by using nanoparticles in nanofluid for a longer time.

It is recommended by designing the parameter of heat pipe like diameter, materials, and input parameters like mass flow rate, the inclination angle also can be done as future research work to enhance the thermal efficiency of heat pipe Solar collector.⁵⁴

Comparisons of heat pipe Solar collectors using nanofluids in FPSC and ETSC under using different absorber plates, different climatic conditions are also needed. This would result in a more accurate selection of Solar collectors to improve their efficiency.

12 Conclusions

This Paper reviews the various research carried out in characteristics of transferring heat in nanofluids which are used as working fluid in FPSC and ETSC. In addition to that this paper gave a compact review to enhance the thermal efficiency more in FPSC and ETSC by using nanofluids in the heat pipe. It summarized that using nanofluids, heat pipe Solar collector has the great enhancement of heat transfer

Heat removal and transfer of heat are improved if nanoparticles are dispersed into a base fluid.

When compared with conventional working fluids used in FPSC and ETSC, the thermal efficiency of FPSC and ETSC are improved when the volume concentration of nanoparticles increases compared to conventional working fluids.

The dispersion of nanoparticles in different base fluids should be a better dispersion process so that the performance of the Solar collector can be improved. To get uniform dispersion surfactants were used. Thermophysical properties of base fluids and nanofluids will also be affected by using surfactants.

The major challenge in the nanofluid is the increase in viscosity and pumping power of nanofluid.

Mostly metal-based nanofluids are used for investigation purposes. Using CNT and GNP nanofluids in the Solar collector is very less.

Instability, pumping power, pressure drop, high cost, and agglomeration are the major drawbacks of using nanofluids in Heat pipe Solar collectors.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship and/or publication of this article.

ORCID iD

N. Jayanthi

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A review on performance of nanofluids in various heat pipe solar collector

Abstract

Keywords

1 Introduction

2 Heat Pipe

7 Nanofluids Preparation

(i) Single-Step Method:

(ii) Two-Step Method

9 Application Of nanofluids in solar collector

10 Application of nanofluid in HPSC

11 Inferences From the survey

12 Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

ORCID iD

Appendix

References