Scholarly article on topic 'A Review of Nano Fluid Role to Improve the Performance of the Heat Pipe Solar Collectors'

A Review of Nano Fluid Role to Improve the Performance of the Heat Pipe Solar Collectors Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Ahmed Kadhim Hussein, Dong Li, Lioua Kolsi, Sanatana Kata, Brundaban Sahoo

Abstract This paper gives a comprehensive review about the recent advances related with the application of the nano fluid in the heat pipe solar collectors. Papers reviewed including theoretical, numerical and experimental up to date works related with the nanotechnology applications in this type of the solar collectors. A lot of literature are reviewed and summarized carefully in a useful table (Table 1) to give a wide overview about the role of the nano fluid in improving the heat pipe solar collectors. It was found that the use of the nano fluid in the heat pipe solar collectors can play a significant role in increasing the efficiency of these devises.

Academic research paper on topic "A Review of Nano Fluid Role to Improve the Performance of the Heat Pipe Solar Collectors"

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Energy Procedia 109 (2017) 417 - 424

International Conference on Recent Advancement in Air Conditioning and Refrigeration, RAAR 2016, 10-12 November 2016, Bhubaneswar, India

A Review of Nano Fluid Role to Improve the Performance of the

Heat pipe Solar Collectors

Ahmed Kadhim Hussein*a, Dong Lib, Lioua Kolsic, Sanatana Katad, Brundaban Sahooe

a Mechanical Engineering Department, College of Engineering, Babylon University,Hilla, Babylon, Iraq b School of Architecture and Civil Engineering, Northeast Petroleum University, Fazhan Lu Street, Daqing 163318, China c College of Engineering, Mechanical Engineering Department, Hail University, Hail City, Saudi Arabia dDirectorate of Technical Education & Training, Odisha e Bhubaneswar College of Engineering, Bhubaneswar.

Abstract

This paper gives a comprehensive review about the recent advances related with the application of the nano fluid in the heat pipe solar collectors. Papers reviewed including theoretical, numerical and experimental up to date works related with the nanotechnology applications in this type of the solar collectors. A lot of literature are reviewed and summarized carefully in a useful table (Table 1) to give a wide overview about the role of the nano fluid in improving the heat pipe solar collectors. It was found that the use of the nano fluid in the heat pipe solar collectors can play a significant role in increasing the efficiency of these devises.

© 2017 The Authors.Publishedby ElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of RAAR 2016.

Keywords: Nanofluid, Heat pipe solar collector, Solar energy, Thermosyphon , Renewable energies.

1. INTRODUCTION

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. The solar energy is widely believed to be the most sustainable form of energy among another renewable energy sources. By mid of the 21st

* Corresponding author. Ahmed Kadhim Hussein, Phone: 009647813769317.

E-mail address: ahmedkadhim7474@gmail.com

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of RAAR 2016. doi:10.1016/j.egypro.2017.03.044

century, renewable sources of energy could account for 60% of the world's electricity market and 40% of this market are come for fossil fuels [1-2]. The solar energy can be defined as the energy which comes from the sun and can be converted into electricity and heat. Solar energy is a natural result of electromagnetic radiation released from the Sun by the thermonuclear reactions occurring inside its core. It has produced energy for billions of years, so the utilization of solar energy has received significant attention especially in the last ten years [3-4]. For example, some studies have indicated that about 1000 times from the global energy requirements can be achieved by using solar energy ; however, only 0.02% of this energy is currently utilized [5]. The main reasons of this huge attention in the solar energy applications are due to the increased demand of energy , limited availability of fossil fuels and environmental serious problems related with them especially the carbon dioxide emissions. From the other side , the huge increase in the human population can be considered as an extra serious problem [6]. In fact , the sun radiates every day , enormous amount of energy and the hourly solar flux incident on the earth's surface is greater than all of human consumption of energy in a year [7]. In spite of this huge amount of available solar energy, approximately 80% of energy used worldwide still significantly comes from fossil fuels [8]. Recently , one of the future projections is to reduce global carbon dioxide (CO2) emissions by 2050 to 75% of its 1985 level if we can improve and use the solar energy equipments such as the solar collectors.

2. NANOFLUID : A GROWING FIELD.

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, A^Os , TiO2, ZnO, FesO^O, Carbides (SiC, TiC), Nitrides (AlN, SiN) and different types of carbon (diamond, graphite, single/multi wall carbon nanotubes). Classical liquids, such as water, ethylene glycol and engine oil are some examples of base fluids. Choi [9] from Argonne National Laboratory in Unites States was the first person which was invented this fluid in 1995. He observed experimentally that the addition of high thermal conductivities metallic/non-metallic nano- particles into the base fluid was increased the thermal conductivity of these fluids dramatically and thus enhancing their overall heat transfer capability . For example, the thermal conductivity of copper at room temperature is about 700 times greater than that of water and about 3000 times greater than that of engine oil [10]. Nanofluid was used for various industrial applications. Some of these applications including nuclear reactors, transportation industry, electrical energy , solar absorption and biomedical fields [11-12]. Nanofluid have a good properties of radiation absorption and it has a high thermal conductivity. For example, the thermal conductivity at the room temperature of individual multi-walled carbon nanotubes (MWCNTs) were found to have values greater than 3000 W/ m.K [13]. Moreover , Assael et al. [14] indicated that about 1% volumetric fraction of MWCNT was enhanced the thermal conductivity of water by about 40%. In order to prepare nanofluids by dispersing nanoparticles in a base fluid, a proper mixing and stabilization of the particles is required. The size of nanoparticles is very small and in the range of 1-100 nm. It is highly recommended not to add large solid particles in the base fluids ( more than 100 nm ) due to the following main problems [15] :-

1-The classical millimeter or micrometer-sized particles sediment quickly in the fluid and forming a layer on the surface. This fouling layer reduces the heat transfer effectiveness of the fluid.

2- Existence of large solid particles require a large pumping power and this increases the cost.

3-The pressure drop in the fluid increases considerably due to the high increase in the viscosity.

4- The large size of the particles in the classical suspensions does not work with the emerging "miniaturized" devices because they can clog the tiny channels of these devices.

Therefore , nanofluid can be used efficiently to solve these drawbacks , since it has many features such as :-

1- High effective thermal conductivity.

2- It has a very small size , so it fluidizes easily inside the base fluids and can be moves faster inside solid blocks such as the porous media.

3- High specific surface area.

4- Small concentration of the particles helps the fluid to stay in its Newtonian behavior.

5-Viscosity, specific heat , thermal conductivity and density can be varied easily by changing particle concentrations to be suitable with different engineering applications [16].

6- Low pumping power [17].

7-The heat transfer increases as a result of increase in the heat transfer surface area between the particles and fluids.

8-Nanofluid has a high extinction coefficient ( a function of the particle diameter and wave length of the light) compared with the conventional base fluid. Therefore, it has more capability to absorb the energy from incident light in a solar energy systems such solar collectors.

9- High dispersion stability.

10- Prevent the erosion or clogging phenomena due to the small size of the particles.

11-Nanfluid has a high thermal capacity , since the small volume of nanoparticles make them easily to store a large quantity of heat. This of course will decrease the energy losses and increasing the efficiency of the system [18].

12- Nanoparticles increase significantly the optical properties of the base fluid [19].

However, the science which deals with the nanofluids is called the nanotechnology and it provides a new area of research to deal with these new types of fluids [20]. This technology has the potential to dramatically re-define the methods used for developing lighter, stronger and high-performance structures and processes with clear and non-traditional properties. For comprehensive details about the applications and challenges of nanofluids , the reader can be go back to the review by Saidur et al. [21].

3. SOLAR COLLECTOR.

Solar energy collectors are a special kind of heat exchangers that transform solar radiation energy to an internal energy of the transport medium. The major component of any solar system is the solar collector. It is a device which absorbs the incoming solar radiation, converts it into heat, and transfers this heat to a fluid (usually air, water, or oil) flowing through the collector. The collected solar energy is carried from the circulating fluid either directly to the hot water or space conditioning equipment, or to a thermal energy storage tank from which can be drawn for use at night and/or cloudy days. Therefore , higher the heat transfer to fluid, means higher outlet temperature and higher the collector efficiency in the power cycle [22]. So , the major challenge is how can we improve this device to increase its efficiency to convert the solar energy into a thermal or electrical energy . Solar collectors can be used for a variety of residential and small commercial applications such as water heating systems in homes, solar space heating , solar desalination , solar drying devices , electricity production and small solar power plants. For solar thermal applications, the solar irradiation is absorbed by a solar collector as a heat and then transferred to its working fluid (air, water or oil). The heat carried by the working fluid can be used to either provide domestic hot water/heating, or to charge a thermal energy storage tank where the heat can be used later at night or cloudy days. For photovoltaic applications, a PV module not only converts solar irradiation directly into electric energy, but it also produces plenty of waste heat, which can be recovered for thermal use by attaching PV board with recuperating tubes filled with carrier fluids [23]. The performance of the solar collector depends upon the properties of the working fluid which are used to maximize the solar energy absorption in the solar collector. Examples of solar thermal collectors are solar water heaters , solar cookers and solar ponds. Many researchers are presented a literature review papers about the solar collectors such as Kalogirou [24] , Jaisankar et al. [25] and recently by Wang et al. [26].

4. HEAT PIPE SOLAR COLLECTOR (HPC).

The heat pipe or some times called the evacuated tube solar collector (ETC) (Fig.1) consists of a heat pipe maintained inside a glass enclosure . This type of collector are invented in order to solve the problems which are appeared in the conventional flat-plate solar collectors. Since , the latter collectors efficiency are connected significantly with the weather conditions. In other words , their performance decreases dramatically in the cold, cloudy and windy days [27]. In addition to the severe weather conditions, both the condensation and moisture cause early failure of internal pipe materials and reduces the collector performance. Also, the limited quantity of heat transferred by the classical base fluid Moreover, the forced circulation system due to the pump and its extracted power, extra space required for the natural circulation system due to the position limitations required , the night cooling due to the reverse flow of cooled water, freezing of the water on cold nights, pipe corrosion due to the use of water can be considered as an additional problems. All these mentioned drawbacks can be solved by using the heat pipe solar collector which consists from a heat pipe (a highly efficient thermal conductor) inside a vacuum-sealed

tube, as shown in Fig.2.The heat pipe solar collector has significant advantages over the flat-plate solar collector in terms of the heat loss. Available types of heat pipe solar collector can be classified into two groups; one is the single-walled glass heat pipe and the other is the Dewar tube. There are many variations of the two basic types; for example, the heat extraction can be through a U-pipe, heat pipe or direct liquid contact [28].

Collector plate

Evacuated tube

Fig. 1 Photos of the heat pipe solar collector [ Du et al. [29] ].

Heat pipe Manifold condenser

Fluid flow

Heat pipe evaporator

Huid out

Fluid in

Heat pipe Etwualcd glass tube

Fig 2. Schematic diagram of heat pipe solar collector [ Kalogirou [ 24]]

Fig 3. Diagram of working operation of heat pipe solar collector [ suman et al. [3] ].

The pipe, which is a sealed copper pipe, is attached to a black copper fin that fills the tube (absorber plate). Protruding from the top of each tube is a metal tip attached to the sealed pipe (condenser).

5. PRINCIPLE OF OPERATION OF (HPC).

The heat pipe contains a small amount of fluid (e.g. methanol , water or ethanol) that transfers the heat inside an evaporating-condensing cycle and this fluid used to capture the heat from the solar radiation. In this cycle, the solar radiation evaporates the liquid, while the vapour travels to the heat sink region where it condenses and releases its latent heat. The condensed fluid return back to the solar collector and the process is repeated. When these tubes are mounted, the metal tips up, into a heat exchanger (manifold) as shown in Fig.2. Water or glycol, flows through the manifold and picks up the heat from the tubes. The heated liquid circulates through another heat exchanger and gives off its heat to a process or to water that is stored in a solar storage tank.

6. MERITS OF (HPC).

1- The vacuum envelope of HPC reduces both the convection and conduction losses, so it can be operate effectively at the high temperatures.

2-They collect both direct and indirect radiation and their efficiency is high at low incidence angles.

3- The efficiency of HPC is high in comparison with the others collectors. Since , it uses the liquid-vapour phase change materials to transfer the heat inside the collector.

4-The HPC has an inherent protection from the freezing and overheating phenomena . Since , there is no evaporation or condensation occurs above the phase-change temperature. Therefore , the temperature inside the HPC can be controlled automatically.

5- These collectors depend on the heat pipes technology which that working under gravity with the condenser above the evaporator so they don't require external power or capillary action to return again the working fluid from condenser to the evaporator.

6-The size of the solar collector can be reduced by using the heat pipe technology and as a result of this reduction , the overall cost was reduced also.

7- The HPC is very suitable for very cold regions.

7. APPLICATIONS OF NANOFLUID IN HEAT PIPE SOLAR COLLECTOR.

Lu et al. [30] designed a special open thermosyphon device applied in an evacuated tubular solar collectors to obtain high fluid temperature. The open thermosyphon consisted of three fundamental sections including a tubular evaporator, a condenser box and a condensing coil (Fig.4) .They used both the deionized water and water-based CuO nanofluids as a working fluids in the modified open thermosyphon. They concluded that the thermal performance of the evaporator and evaporating heat transfer coefficients were increased by about 30% at optimum mass concentration of 1.2 wt % compared with those of the deionized water. Effects of filling rate, kind of the base fluid, nanoparticle mass concentration and operating temperature on the evaporating heat transfer coefficient in the open thermosyphon were also investigated and discussed.

Fig 4 photo of the modified open thermosyphon [ lu et al. [30] ] Fig 5. Photo of the evacuated tubular solar air collector [ liu et

al. [ 34 ] ]

Senthil Kumar et al. [31] fabricated a two identical flat plate solar collectors. In each collector , a three identical wickless copper heat pipes of length 620 mm and outer diameter of 18 mm were used. The working fluid in the first collector was the pure water , while in the another one was CNT-water nanofluid. The diameter of CNT nano particles was 10-12 nm while its length was 0.1-10 |i.They concluded that the performance of the nanofluid collector was better than the pure water collector in the all test conditions. Chougule et al. [32] investigated experimentally the performance of a two symmetrical collectors , each of them was connected with a three wickless heat pipes. The

first collector was used the pure water as a working fluid , while the second one was used carbon nanotubes (CNT) - water nanofluid. It was found that the second collector gave a better performance in all the experiment conditions. Moorthy et al. [33] made an experimental evaluation of evacuated tube solar collector performance by using TiO2 -water nanofluid. It was found that the collector efficiency was increased from 58 % when using the pure water to about 73 % when the nanofluid was used. Liu et al. [34] designed and analyzed experimentally the thermal performance of a novel evacuated tubular solar air collector (Fig.5) integrated with a simplified CPC (compound parabolic concentrator) and open thermosyphon using the water based CuO nanofluid to provide air with high and moderate temperatures. Experimental results showed that the air outlet temperature and the collector efficiency using nanofluid as the open thermosyphon's working fluid were both higher than that using water only. They observed that the maximum air outlet temperature exceeded 170 oC at the air volume rate of 7.6 m3/h in winter. Also , they concluded that the solar collector using nanofluid as the open thermosyphon's working fluid had much better thermal performance than that using water only.

Chougule et al. [35] examined the thermal performance of two phase thermosyphon on flat-plate solar collectors using both the pure and nanofluids.The experiments were performed at the outdoor test conditions. They used carbon nanotubes (CNT) particles with the pure water as a nanofluid for various concentrations (0.15%, 0.45%, 0.60% and 1% by volume).It was found that the thermal performance of the nanofluid heat pipe solar collector was much better than the corresponding collector that used the pure water only. Aruna et al. [36] investigated experimentally the effect of the working fluid type on the performance of the flat absorber plate solar water heater with thermosyphon (Fig.6). In their test , a Propanol and ( TiO2 + DI water ) nanofluid was used. The experimental rig consisted from a thermosyphon heat pipe, solar collector with a grooved flat absorber plate and a water storage tank. Glass wool insulation was provided below the absorber plate in order to reduce the conduction losses , while the sides of the absorber plate were covered by a thermo-cool insulator to minimize the convection losses. It was found that the nanofluid exhibited a better thermal behaviour such as improved thermal conductivity and convection coefficient in comparison to the pure fluid.

Fig 6 Photo of the thermosyphon solar collector [ aruna et al. [ 36] ]

Fig 7. effect of working fluid on efficiency of heat pipe solar collector [ saravanan and karunakaran [ 37]]

Saravanan and Karunakaran [37] investigated experimentally the effect of methanol , ethanol , DI water and ( TiO2 +DI water ) nanofluid on the performance of a cylindrical heat pipe solar collector with a V-type absorber plate . The designed V -type absorber plate solar collector with heat pipe was found to increase the solar intensity significantly. It was found a significant rise in the thermal efficiency of the collector by using the nanofluid in the cylindrical heat pipe as shown in Fig.7.

8. CHALLENGES AND DRAWBACKS.

The utilizing of the nanofluid in the heat pipe solar collectors suffers from many defects which can be summarized in the following points :-

1- Long term stability of nanoparticles dispersion. 2-The nanofluid has a low specific heat in comparison with the base fluid. 3- The toxicity of the nanofluid is high , so it needs to be careful during the preparation of it.4- The cost

of preparation and testing of the nanofluid is high. 5- Increased pressure drop and pumping power.

6- The presence of nanoparticles in the nanofluid may leads to a corrosion and erosion of solar collector for a long

time. 7- The nanofluid has a high viscosity. The reason of this is due to the adding solid particles to the base fluid.

9. CONCLUSIONS

The present paper gives a comprehensive survey about the recent advances related with the application of the nanofluid in the heat pipe solar collector. Some important conclusions are summarized below : -

1-The future attention must be directed towards the effect of the optical properties of nanofluid on the performance of HPC together with the other fluid properties except the thermal conductivity.

2-The future works must be directed towards inventing a non-toxic and low cost nanoparticles to reduce further the cost of nanofluid based solar collector and to meet quickly with the market needs.

3- More efforts are needed to study the reliability of using nanofluids in HPC from both environmental and economical point of view.

4- Nanoparticles must be dispersed uniformly in the base fluid to enhance the solar-weighted absorption and increase the efficiency of the solar collector.

5-Volume fraction of nanoparticles must be chosen accurately to enhance the performance of nanofluid collector.

6- It is recommended to use carbon nanohorns (CNHs) as a nanoparticles to improve the optical properties of the HPC. This is due to their large surface area and large number of cavities.

7-Further efforts must be directed towards various significant challenges in the field of nanotechnology and its application in the solar collector such as : Brownian motion of particles , particle migration , changing thermophysical properties with temperature , tendency of nanoparticles to agglomeration , changing nanofluid properties by using additives and the stability of nanofluids.

8-The results of the reviewed papers indicated that the overall performance of HPC is a function of nanofluid properties and the other properties of system.

9- Further researches must be directed towards using of a mixture of more than one nanoparticles to increase the

efficiency of HPC.

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