Review on nanomaterials for thermal energy storage technologies

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Review on nanomaterials for thermal energy storage technologies

  1. 1. Send Orders of Reprints at reprints@benthamscience.net Nanoscience & Nanotechnology-Asia, 2013, 3, 000-000 1 2210-6812/13 $58.00+.00 © 2013 Bentham Science Publishers Review on Nanomaterials for Thermal Energy Storage Technologies Hussain H. Al-Kayiema,*, Saw Chun Linb,* and Afolabi Lukmonc,* a,b,c Department of Mechanical Engineering, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysia Abstract: To optimize the utilization of thermal conversion systems, it is essential to integrate them with thermal energy storage. Among many types of base materials, the phase change materials are the most satisfactory mediums to store and release the thermal energy due to their high latent heat of fusion. In general, the phase change materials have low thermal conductivity. Nanoadditives have been investigated to further enhance the thermal properties of the phase change materials. This paper reviews the research development of the various types of phase change materials, nanoadditives, nanofluids, and nanocomposites as possible materials for efficient thermal energy storage. Some deficit in the literature has been noticed on the dispersion of various types of nanoparticles in the various types of base materials. It is also recommended that further studies are required to understand the stability of the nanofluids and nanocomposites due to a large number of thermal cycles. Keywords: Nanoadditives, Nanomaterials, Nanocomposites, Nanofluids, Nanoparticles, TES, PCM. 1. INTRODUCTION Energy storage plays important roles in conserving the over demand of energy for utilization during the demand. On the other hand, many types of energy sources are intermittent in nature like solar energy which is affected by cloudy and dusty weather, as well as the non availability during the night. In this sense, the thermal energy storages (TESs) play essential roles in heat recovery and contribute considerably in improving the performance of the thermal systems. TESs are simply contained mediums which are able to store the thermal energy (charging mode) and release the stored thermal energy (discharging mode) to compensate for the shortage in the main thermal source. Commonly, the storing medium is a fluid or phase change material (PCM). In general, the thermal conductivity of these TES-based materials is poor leading to a slow charging and discharging rate. The charging and discharging rate can be enhanced by applying the heat transfer enhancement methods. The literature shows numerous methods to enhance the thermal conductivity of the TES materials varying from extended surfaces and fins, bubble agitation, metal ring and metal matrix insertion, encapsulation, and many others; among them are the nanoadditives. Nanomaterials are used as additives to enhance the properties of base materials. When they are added to the fluid, the produced mixture is denoted by nanofluid; while by adding the nanomaterials to PCM, the production is denoted by nanocomposites. Subsequently, they will be denoted by nanofluids/composites. Some review articles were published reporting progress on the enhancement in *Address correspondence to these authors at the Department of Mechanical Engineering, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysia; Tel: ??????????????; Fax: ??????????????; E-mails: hussain_kayiem@petronas.com.my, clsaw78@gmail.com, afoolabs@yahoo.com thermophysical properties investigated with different nanomaterials for different applications [1 to 4]. Many other reviews have emphasized the technological applications, synthesis and preparation methods in the production of nanomaterials [5 to 8]. The investigations on the characteristic mechanism enhancement (conductive and convective heat transfer) by these advanced materials have been carried out experimentally, numerically and theoretically. They have been aimed to determine the major phenomenon (Inter-layer surface and Brownian motion, particle aggregate etc.) which is responsible for the enhancement. The different synthesis methods of preparation and the characterization of nanofluids, suspension, and the convection and conduction heat transfer in nanofluids have been reported by [9 to 11]. Within the realm of nanomaterials for thermal energy applications, many researchers have focused on the thermal conductivity effectiveness of nanofluids/composites which can improve their thermophysical properties [12 to 17]. This paper focuses on the enhancement of TES by using nanomaterials as additives to the base storage materials. TESs as a whole and heat exchanger development in particular are mostly concerned with heating and cooling applications. The utilization of nanofluids/composites in heating and cooling processes is viewed in relation to different applications as grouped in Fig. (1). The paper critically reviews the existing studies dealing with the use of nanofluids and nanocomposites in TES applications. The types and classifications of the nanomaterials have been presented and discussed. The synthesis methods used in the preparation of the nanofluids and nanocomposites are also presented and discussed. 2. CLASSIFICATION OF WORKING MATERIALS This section presents and discusses the classification of the materials included in the process of TES enhancement.
  2. 2. 2 Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 Al-Kayiem et al. 2.1. Phase Change Materials, (PCM) The trends of using PCMs as base materials for TES have been increasing tremendously since the energy crisis during the period of 1973 – 1974. There have been wide research applications on PCMs such as space heating, space cooling, water heating, food storage, electricity generation, air conditioning etc. revealed [18]. The PCMs consist of organic, inorganic and eutectic materials as shown in Fig. (2). Paraffins are mainly hydrocarbons. Non-paraffins are fatty acids and form the largest group in the phase change materials. Inorganic PCMs are divided into salt hydrate and metallic. Hydrate salt undergoes hydration and dehydration of the salt during the phase change but not all of the the hydrate salts melt congruently. Metallics are low melting metals and eutectics. These metallics are very light and have a high thermal conductivity like metal but are low in terms of the latent heat of fusion. Another alternative in PCMs is eutectics. Eutectics are compositions of two or more components such as a combination of organic and organic, inorganic and inorganic, and inorganic and organic. Eutectics have shown a positive sign to solve the incongruent melting of hydrate salts. Mixing two types of hydrate salts, (inorganic and inorganic) as an example, to form a eutectic, helps to prevent incongruent melting and atthe same time reduces the melting point and improves the thermal conductivity. The advantages of organics compared to inorganics are stated in Table 1. Although organic PCMs possess lower latent heat compared to inorganic PCMs, the thermal cycles are stable and have a low super cooling effect. It is important to have stable thermal cycles. In such, the storage and release of the latent heat will be stable with no weight loss happening [20]. However, the thermal conductivity of PCMs is low. The introduction of nanomaterials mixed with PCMs will enhance the thermal conductivity. 2.2. Nanomaterials Nanomaterials can be classified into organics and inorganics as shown in Fig. (3), which has been concluded from [21 and 22]. Organic nanomaterials consist of fullerenes, carbon nanotubes (CNT), single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), graphite and nanofibers. Most of the organic nanomaterials are carbon-based nanomaterials [23]. While, metal and metal oxide-based nanomaterials such as aluminium, zinc, copper, iron, aluminium oxide, iron oxide, titanium oxide are categorized asinorganic nanomaterials [24]. Quantum dots, such as CdSe, ZnS, ZnOetc are metalloid nanomaterials and are also categorized as inorganic nanomaterials [25 and 26]. Hybrid nanomaterials are a new class included in the chart in Fig. (3). Hybrid nanomaterials are the combination of organic – organic nanomaterials, organic - inorganic nanomaterials and inorganic – inorganic nanomaterials through synthesis such as chemical vapor deposition (CVD), Electrospinning, atom transfer radical polymerization (ARTP) etc. [27 and 28]. Fig. (1). Nanomaterials for TES Applications.
  3. 3. Review on Nanomaterials for Thermal Energy Storage Technologies Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 3 Although, nanomaterials possess many advantages, there are risks and they having a toxic effect on humans and the environment. Reference [29] studied 935 nanomaterials including their particle size and surface area measurement. The investigation also revealed 483 toxicity effects of nanomaterials including CNT. CNT nanomaterials have stable characteristics, are not reactive with air and have a high thermal conductivity as compared to metal nanomaterials. Carbon-based nanomaterials have been prioritized and widely studied by many researchers as summarized by, e.g., [29 and 30]. 2.3. Classification of Nanocomposites and Nanofluids Nanocomposites and nanofluids are the mixtures resulting from the incorporation of nanoadditives with the base materials of TES as classified in Fig. (4). Nanocomposites can be organics and inorganics. Most of the nanofluids reported for TES are water based mixtures. Some other base fluids may be ethylene, glycol, and oil. Many reported nanocomposites are mainly the organic- based such as paraffin blended nanocomposites, fatty acid blended nanocomposites, HDPE blended nanocomposites etc. [31 and 32]. The organic-based nanocomposites are also called polymer nanocomposites [33]. Nanofluids, as thermal energy storage, have been investigated by many researchers but most of the studies have been focused on indoor experiments [34 and 35]. The term nanofluid was first introduced by the Argonne National Laboratory (ANL) through the seminal work of Dr. Choi and his team. They developed the noble concept of nanofluids in 1995 [36]. Nanofluids are nanoscale colloidal suspensions containing nanomaterial such as nanofibers, nanorods, nanosheets, nanoparticles, nanodroplets, nanowires or nanotubes in a conventional base fluid like ethylene or tri-ethylene-glycols, water, bio-fluids, polymer solutions, oil and other common coolants. These nanofluids possess a higher specific surface area and dispersion stability, adjustable properties including thermal conductivity and surface wettability, reduced particle clogging and pumping as compared to conventional solid-liquid suspension for heat transfer intensification [37]. Advanced fabrication technologies provide great opportunities to actively process materials at the micro and nanometer scales. Nanostructured or nanophase materials are made of nanometer-sized substances engineered on the atomic or molecular scale to produce either new or enhanced physical properties not exhibited by conventional bulk solids. Much emphasis has been given to this new material technologicalinnovation in recent times by several researchers particularly over the past ten years [38]. Yet further investigations are needed to be carried out to determine the major factors influencing the performance of nanofluids. The major driving force for nanofluid researches lay in its verserange of applications and inert potentials. 3. SYNTHESIS TECHNIQUES One of the major difficulties in nano technologies lies in the synthesis of the nanofluids/composites and their control over the size and dimension. Since the early nineties, Fig. (2). Basic categoriesof phase change materials [19]. Table 1. Comparison of Organic and Inorganic Materials for Thermal Storage [20] Organics Inorganics 1. Advantages Non corrosives Greater phase change enthalpy Low or non-undercooling - Chemical and thermal stability - 2. Disadvantages Lower phase change enthalpy Undercooling Lower thermal conductivity Corrosive Non-flammable Phase separation Phase segregation, lack of thermal stability
  4. 4. 4 Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 Al-Kayiem et al. innumerable efforts have been given by the researchers in the field of nanofluid/compositesynthesis. Upon that effort, several techniques have been evolved and considered as standard nanofluid/composite synthesis procedures. The synthesis of nanofluids/composites with respect to heating and cooling applications in thermal energy storage is the primary focus in this review paper. The following procedures have beenadopted for the preparation of nanofluids/ composites. 3.1. Synthesis and Preparation of Nanofluids The adequate preparation of nanofluids is an essential step towards experimental studies because nanofluids are not just the dispersion of solid particles in a fluid. Special features are expected such as a stable and uniform suspension, durable suspension, and the low agglomeration of particles and stable chemical change of the fluid. Nanofluids are produced by dispersing nanometer-scale solid particles into base liquids such as water, ethylene glycol, oil etc. In the synthesis of nanofluids, agglomeration is a major problem. There are mainly two methods to produce nanofluids: 3.1.1. One-step Direct Evaporation Method This method involves one single process of simultaneously producingand dispersing nanoparticles directly into the base fluid; this is best known for metallic nanofluids. The Vacuum Evaporation Running Oil Substrate (VEROS) technique was the first known single-step direct evaporation method used. Although, it was deficient due to Fig. (3). Classification of nanomaterials. Fig. (4). Classification of enhanced TES nanofluids/composites.
  5. 5. Review on Nanomaterials for Thermal Energy Storage Technologies Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 5 its subsequent difficulty to separate the particles from the fluids to produce dry nanoparticles, the VEROS method was modified by employing high pressure magnetron sputtering for the preparation of the suspensions with different metal particles such as silver (Ag) and iron (Fe) [39]. Cu vapor wasdirectly condensed into nanoparticles by contact with a flowing low-vapor pressure liquid (EG) using the modified VEROS technique. Cu-ethylene glycol nanofluids were developed by the one-step physical vapor condensation method [40]. Stable ethanol-based nanofluids containing silver nanoparticles can be produced by the one-step microwave-assisted reduction of silver nitrate (AgNO3) with polyvinylpyrrolidone (PVP) as the stabilizing agent [4]. Various methods have been tried to produce different kinds of nanoparticles and nanosuspensions. The initial materials tried for nanofluids were oxide particles primarily because they were easy to produce and chemically stable in solutions. Various investigators have produced aluminum oxide (Al2O3)and copper oxide (CuO) nanopowders by an inert gas condensation process. They were found to be 2–200 nm- sized particles. The major problem with this method is its unsuitability to produce pure metallic nanoparticles. The problem of agglomeration is reduced to a good extent by using a direct evaporation condensation method. 3.1.2. Two-step Method This method is the most widely used method for preparing nanofluids. The process involves: i Firstly, producing the nanomaterial (nanofibers, nanorods, nanosheets, nanoparticles, nanodroplets, nanowires or nanotubes) in dry powder using various methods like (chemical: vapor condensation, micro-emulsions, spray pyrolysis, thermal spraying; or physical: grinding, inert- gas condensation). ii Then, dispersingthe nano-sized powders into the base fluid and mixing with the aid of mechanical agitation: high-shear mixing, homogenizing, ball milling or ultrasonication). The two-step method has a high tendency of agglomeration problems due to the high surface area contaction of the nanoparticles. The agglomeration can be reduced with the addition of surfactants; however, the effectiveness of the surfactant under high temperature applications is yet to be established fully. The method is the most economic method to produce nanofluids on alarge scale, because nanopowder synthesis techniques have been scaled up to industrial production levels. Several other methods (chemical vapor condensation, chemical vapor deposition, LASER deposition, inert gas condensation etc.) have been employed towards the production of nanoparticles and suspensions. Some good overviews of the synthesis methods have beenhighlighted by [5]. Zinc oxide (ZnO) nanoparticles have been dispersed in deionized water to enhance the critical heat flux (CHF); the enhancement obtained was in the concentration range of 0.001 – 0.1% [41]. 3.2. Synthesis and Preparation of Nanocomposites Nanocomposites, because of their controlled and advanced properties, are used in a wide range of applications in various fields, such as medicine, textiles, cosmetics, agriculture, optics, food packaging, optoelectronic devices, semiconductor devices, aerospace, construction and catalysis. Nanoparticles are dispersed into polymeric materials to form polymer nanocomposites. There are three major groups of nanocomposite matrices depending on the temperature applications: Polymer, Metal and Ceramic nanocomposites. Polymer matrices are the most commonly based materials used especially for low temperature applications of less than 250o C. Polymer nanocomposites are mostly polymer blended with inorganic and organic nanoparticles. These polymer nanocomposites represent a new class of materials that exhibit improved performance compared to the base materials. Incorporation of inorganic nanoparticles into a polymer matrix can significantly affect the properties of the matrix. The resulting nanocomposites might exhibit improved thermal, mechanical, rheological, electrical, catalytic, fire retardant and optical properties. The properties of polymer nanocomposites depend on the type of nanoparticles that are dispersed, their size and shape, their concentration and their interactions with the polymer matrix. Particle agglomeration is a major problem in polymer nanocomposites, too. It is extremely hard to produce evenly dispersed nanoparticles in a polymer matrix due to the nanoparticle agglomeration as a result of their specific surface area and volume effects. Modification of the surface of the inorganic particles improves the interfacial interactions between the inorganic particles and the polymer matrix. The two methods bywhich the surface modifications are accomplished are: first, through surface absorption or reaction with small molecules, such as silane coupling agents, and the second method is based on grafting polymeric molecules through covalent bonding to the hydroxyl groups existing on the particles [42]. The advantage of the second procedure over the first lies in the fact that the polymer grafted particles can be designed with the desired properties through a proper selection of the species of the grafting monomers and the choice of grafting conditions. The investigations on the use of nanomaterials for storing thermal energy have gained prominence with many researchers exploring different techniques and the applications of the utilization of this new technological advancement in the field of nanotechnology. Micron scale additives have experienced hindrances in the development of thermal enhancing technologies, especially with the problem of particle clogging and sediments. On the other hand, the nanoscale additives have drastically improved the thermal properties of the heat transfer of fluids. Also, they are able to produce an ultrafine performance for cooling in electronics, enhanced heat transfer surface techniques and improving the specific heats. 4. APPLICATIONS OF NANOFLUIDS/COMPOSITES IN TES Nanoparticles have one dimension that measures in 100 nanometers or less and the properties of the bulk material changes with changes in the nanoscale particles [43]. The verse changes in the physio-chemical properties are partly due to the kinetic movement of the nanoparticles at the surface layer. They exhibit greater surface area per weight
  6. 6. 6 Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 Al-Kayiem et al. than larger particles which causes them to be more reactive and have a continuous random motion. The dispersion of the nanoparticles in polymers, metals, ceramics and fluids has further opened pathways for more engineering of flexible composites that exhibit advantageous thermophysical properties [44]. These new hybrid nanofluids/composites are applied in many technological applications ranging from biomedical and transportation to energy etc. The application of nanoparticles dispersed into base fluids for TES is still a new research area. The amount of nanoparticles needing to be dispersed into base fluids to suit certain applications for specific TES is still under study and investigation. To date, carbon nanotubes are the most widely used additives as their thermophysical properties have been established and the advantages have been proven compared to other nanoparticles. TES is a combination of different technologies that store thermal energy in a reservoir for later use. They can be used to balance energy demand at peak and off peak periods. Among the several diverse technologies in TES are the heating and cooling applications especially with respect to buildings. As an example to the statistical data, the total area of China’s residential buildings is about 40 billion m2 . The total national building energy consumption (TNBEC) is 16 billion tons of standard coal which accounts for 20.7% of the total end energy consumption [45]. Likewise, in the United States, buildings account for 40% of all energy used in the United States. This sector consumes more energy than either industrial or transportation, surpassing industry as the number one consuming sector in 1998 [46]. The transportation sector (Automobile), micro-electronics and electrical smart devices, medical, spacecraft and agriculture (livestock) requires heating and cooling systems either for enhancing efficiency or for thermal comfort. Many works have been carried outon efficient trends in the heating and cooling of systems for general applications but with fewer investigations being conducted on the nanomixture application for heating and cooling using TES. 4.1. Heating and cooling of Buildings Space cooling in industrial areas is an enormous scientific challenge, which also applies to many other diverse production areas, including transportation, manufacturing, microelectronics, sporting arenas etc. Traditionally, the heating and cooling of a building or space has been in practice for many decades. The construction of buildings amidst trees to aid for cooling in hot weather, or the use of bricks in the building of walls to aids cooling in hot weather. The need for the effective utilization of TES has led to the use of PCM in different building technologies to aid cooling and heating depending on the desired comfort. PCMs have a high latent heat storage capacity and as such havebeen considered for thermal storage in building applications [47]. With the advent of PCM impregnated in trombe walls, wallboards, shutters, under-floor heating systems and ceiling boards can be used as a part of the building for heating and cooling applications [48]. The relatively low thermal conductivity of most PCMs brings a furtherset back in adequate utilization, hence the need to integrate the PCM with nanomaterial to enhance the thermophysical properties has been a recent development. Many literatures have shown the possibilities of the development and use of PCM nanocomposites to enhance the TES capacity in building applications. The application of PCM enhanced nanocomposites in buildings or spaces mainly focuses on using natural heat that is solar radiation for heating purposesincold weather and for cooling during hot weather conditions using man-made heat or cold sources. In any case, storage of thermal energy is important to match availability and demand with respect to time and also with respect to power. Table 2 summarizes the reported studies dealing with the use of nanomaterials in TES for the heating and cooling of buildings. 4.2. Heating and Cooling of Electrical and Electronic Components The technological advancement in the development of smart materials and components to further reduce the weight and size of electromechanical devices has made greater the need for an efficient heat dissipation mechanism especially on the microelectronic devices which operate at high speeds, on higher power engines, and brighter optical devices. This has driven increased thermal loads that require advances in Table 2. Nanomaterial usage in TES for Building Heating and Cooling Authors / Year Nanomaterials & Composites Mechanism/Applications Remarks [49] Granqvist et al., 2007 Electro- chromic material with nanostructure photocatalyst Smart material for a benign indoor environment The solar photocatalysts are used in air purification and the nanostructured control properties of smart windows to reduce heat absorptance thereby reducing the demand for ventilation. [50] Colella et al., 2012 Paraffin Graphite composite Piping network for district heating The thermal conductivity of the paraffin-based (HTF) was enhanced with a 15% graphite mixing fraction. The charging behavior was improved and the discharging extendibility was observed without physical damage to the (LHTES) units’ medium. [51] Sciacovelli et al., 2012 Graphite matrix composite Cylindrical shell thermal storage unit for space heating The PCM was enhanced by a 10% increase in the thermal conductance and the efficiency of the system was determined as a28% increment.
  7. 7. Review on Nanomaterials for Thermal Energy Storage Technologies Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 7 cooling. Conventionally, heat dissipation is a factor of increasing the area available for exchanging heat and the use of a better conductive fluid [52]. However, this approach undesirably increases the size of the thermal network system; hence, the need arises for novel coolants with improved performance. The use of oscillatory heating pipes, heat sinks and micro channels to reduce the heat dissipated in a compact microelectronic device integrated with micro- electromechanical systems (MEMs) all have limitations. The extremely high rates of heat transfer obtained by employing micro-channels make them an attractive alternative to conventional methods of heat dissipation. This is especially so in applications related to the cooling of micro-electronics. The researcher [53] carried out a compilation and analysis of the results from investigations on fluid flow and heat transfer in micro- and mini-channels and micro-tubes. The advent of nanofluids has opened new possibilities of integrating the MEMs with nanocomposites/fluids for the purpose of heat removal and cooling in micro-electronic systems. Fig. (5) describesthe CNT’s assembled structure for use in micro- channel cooling. Thermal management issues are limiting barriers to the high density electronic packaging and miniaturization. In the electronic industry, improvement of the thermal performance of cooling systems together with the reduction of their required surface area has always been a great technical challenge. The continually increasing power of micro- processors and other electronic components requires a search for a more efficient heat dissipating system. Different approaches have been investigated in times past and more recently in the use of nanoadditives incorporated with the conventional heat transfer fluids. The reported works on the use of nanofluids/composites in electronics and computers for heating and cooling applications of micro-electronic devices integrated with micro-electromechanical systems (MEMS) for heat dissipated and heat transfer system are summarized in Table 3. Nanoparticles of metals and metal oxides are mostly used as nanoadditives to the base materials [55, 56, 57, 58 and 59]. 4.3. Automobile Applications Radiant energy that falls on the vehicle through the windows of the car is another storage source for automobiles during daytime especially at times of high solar intensity. Various methods to store this kind of energy and use it when needed have been tried and used so far. Effective TES of this Fig. (5). (a) Positioning and (b) soldering of the flip chip on the Cu landing pads of the substrate (this structure also served as a reference). (c) Solder paste dispensing, CNT array positioning, and (d) soldering on the Cu coated backside of the chip. (e) Field emission scanning electron microscopy image of an assembled structure (scale bar: 500 μm) [54].
  8. 8. 8 Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 Al-Kayiem et al. waste heat can be used in the preheating of the engine, preheating the catalytic converter, heating and cooling the passenger compartment and defrosting the car windows. A huge amount of energy is lost due to incomplete combustion in automobile engines, as such considerable amounts of gasoline are consumed thereby adding to environmental pollution. Also, a significant amount of thermal energy generated by automobile engines is wasted to the environment especially in the case of diesel engines. Hence, various heat recovery and reuse systems are under investigation for probable use of nanomaterials as thermal energy storage. Engine cylinders (liners) are being envisaged to be coated with nanocrystalline ceramics, such as zirconia and alumina, for purposes of heat retention and a more efficient combustion system. The conventional heat transfer fluid like ethylene glycol and a water mixture in the automotive coolant have a relatively poor heat transfer due to low thermal conductivity [60]. The addition of nanoparticles to this engine coolant has the potential to improve the cooling rate of automotive and heavy-duty engines. Such improvements will reduce the size of the radiator which will result in smaller and lighter coolant systems and reduce heat waste. The studies on the performance enhancement by integration with nano-TES are summarized in Table 4. 4.4. Solar Thermal Heating Water heating is a major source of energy consumption in domestic and commercial buildings where low temperatures are mostly required. The primary energy sources used to generate hot water are non-renewable. Regrettably, this condition remains despite efforts to develop and promote the use of renewable, solar thermal water heaters for over a century [66]. Solar thermal collector systems are by far the most reversed means of harvesting solar radiation to be used for the heating of water. There are many types of solar Table 3. Nanomaterials as TES in Electrical, Electronic and Computer Components Authors / Year Nanomaterials & Composites Technology /Applications Remarks [55] Nguyen et al., 2007 Silver oxide nanoparticle– water mixture Liquid Cooling /Fluidic circuit of micro- electronic devices The inclusion of nanoparticles into distilled water has produced a considerable enhancement of the cooling block convective heat transfer coefficient. For a particular nanofluid with a 6.8% particle volume concentration, heat transfer coefficient has been found to increase as much as 40% compared to the base fluid. Experimental results have also shown that the nanofluids with a 36 nm particle size provided higher convective heat transfer coefficients than the ones given by nanofluids with 47 nm particles. [56] Putra et al., 2011 Alumina–water and Titania–water nanofluids Thermo-electric / Heat pipe liquid block The critical heat flux wasreduced due to a reduced temperature difference between the heated wall and the coolant. The CPU temperatures as well as the thermal resistance werereduced by 4-6o C below the ambient temperature. The Al2O3 showed the best result with the reduced thermal temperature of 23.9o C, while TiO2 and distilled water showed 24.2o C and 26.5o C, respectively, at a 1% mixture concentration. [57] Selvakumar et al., 2012 CuO / water nanofluids Heat sink / Compact Electronic cooling applications The interface temperature of the water block wasmeasured and a maximum reduction of 1.15°C was observed when nanofluids of 0.2% volume fraction were used as the working fluid compared to deionized water. The convective heat transfer coefficient of the water block was found to increase with the volume flow rate and the nanoparticle volume fraction and the maximum rise in the convective heat transfer coefficient wasobserved as 29.63% for the 0.2% volume fraction compared to deionized water. The average increase in the pumping power was 15.11% for the nanofluid volume fraction of 0.2% compared to deionized water. [58] Rafati et al., 2012 Silica, Alumina and Titania nanoparticles dispersed in base fluid Heat pipe / Thermal performance in computer cooling and micro-electromechanical systems (MEMS) The use of nanofluids resulted in the considerable reduction of the processor operating temperature as compared to the pure base fluid. The surface heat transfer wasalso reduced. The largest decrease observed was for alumina nanofluids, which decreased the processor temperature from 49.4o C to 43.9o C for 1.0% of the volumetric concentration. The flow rate was 1.0 L per minute when compared with the pure base fluid with the same flow rate. [59] Jeng et al., 2013 Al2O3/ Water nanofluid Hybrid cooling system / vapor compression refrigeration system (VCRS) for micro- electronic devices and components The hybrid cooling system for CPUs combinedthe advantages of a liquid cooling system with an Al2O3/water nanofluid and a VCRS with a hydrocarbon refrigerant. The experimental results demonstrated that the maximum cooling capacities of the liquid cooling system, VCRS, and hybrid cooling system were 450 W, 270W and 540 W, respectively. The Al2O3/water nanofluid increasedthe heat dissipation performance and the power consumption of the water pump, yet decreasedthe surface temperature of the heater.
  9. 9. Review on Nanomaterials for Thermal Energy Storage Technologies Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 9 collectors such as the flat plate collector, evacuated tube collector, concentrated collector and integrated collector [67]. The ideal thermal collector should efficiently absorb solar radiation and convert it to a thermal energy, and minimize heat losses in the system. The major setbacks of the thermal collectors are the intermittent nature of solar energy thereby requiring a storing medium. PCMs have been used as storage mediums to store and release latent heat for integrated solar collectors; while some integrated collectors use water as the storage medium. However, PCMs are proven to have higher heat storage capacity because of the latent heat. During the daytime, PCMs will absorb heat from solar radiation to store thermal energy while in the night the PCMs will release the thermal energy to heat the water [68 and 69]. The low thermal conductivity of the PCM requires enhancement, mainly on the thermal conductivity. Nano- materials have been incorporated with PCMs to enhance the heat transfer thermal conductivity and heat transfer rate. Advanced heat transfer fluid and nanocomposites cascaded with well transitioning temperatures in various designs has improved the thermal storage capacity of solar thermal collector systems. Experimental results of [68] demonstrate thermal conductivity enhancement by 12.2% by dispersion of 1% of the nano Cu powder in paraffin wax as a base material. By increasing the dispersed nano Cu to 2%, by weight, the thermal conductivity of the nanocomposite is enhanced by 24%. There have been many attempts to enhancethe performance of solar thermal systems by integration with nano-TES. Table 5 displays the achievements gained by nanoadditives to the TES base materials. Researchers [70, 71, 72, and 75] mainly conducted the experimental characterization in the laboratory environment to improve the PCMs and heat transfer fluids by dispersion of nanoadditives. Only researchers [73 and 74] tested the blended nanofluid for the solar water heating application on the real operational condition. The solar collector performance was enhanced by 25% and 30% by using the nanofluids. Researcher [76] experimentally investigated the direct absorption of solar radiation on nanofluids but no solar collector model was revealed However, the researcher proposed a 1% nanofluid concentration and a lower than 20 nm aluminium nanoparticle diameter for the maximum absorption of the daylight scattering of solar radiation. There is a lack of solar thermal heating and cooling experiments incorporated with nanomaterials being applied at in-situ projects. This will provide wider opportunitiesto test nanocomposites and nanofluids withsolar collector underreal operational conditions. Table 4. Enhancing the IC Engine Performance by Nanomaterials as TES Authors / Year Nanomaterials & Composites Technology/Application Comments [61] Peyghambarzadeh et al., 2011 Al2O3-nanoparticle and conventional heat transfer fluid (water) Heat transfer /Automobile radiator coolant The Al2O3 – water nanofluid enhanced the heat transfer rate of the automobile radiator by 45% compared with pure water. Increasing the fluid flow rate increased the heat transfer performance and gave a slight variation between the inlet water temperature to the radiator and the ambient. The cooling effectiveness yielded a better performance and reduced the radiator space and weight. [62] Gumus et al., 2009 Salt Hydrate (Na2SO4. 10H2O) as the PCM Thermal energy storage system for pre-heating of internal combustion engine The engine temperature was increased to 17.4o C by pre-heating and the pre- heating took500s. The maximum thermal efficiency recorded was57.5% and 2277kJ of heat absorbed during charging. Emissionswithpre-heating of the enginewas less than normal engines where CO and HC emissions decreased by about 64% and 15%, respectively, with the effect of pre-heating the engine. [63] Bewilogua et al., 2009 TiAIN base Nanocomposites Coating and self- cleansing surface Higher surface hardness wasattained compared to the conventional coating and enormous improvement in heat resistance. Addition of nanomaterial to improve the coating material property gave improved properties and enhanced the system performance. [64] Pandiyarajan et al., 2011 PCM Heat exchanger in diesel engine exhaust The effectiveness of the heat recovery exchanger (HRHE) reached 99% efficiency. 10-13% wasted heat wasrecovered using the (HRHE). The cascading of the exhaust increased the temperature to 14.8o C. The disadvantage of the (HRHE) is the external space and added weight. The integration of nanomaterial to the PCM couldreduce the space and use of an external storage heat medium, and increase the temperature as well as efficiency. [65] Bokde et al., 2013 PCM Catalytic converter for pre-heating An average of 23% and 21% reduction in %CO and HC emission occurred during the cold start emission. The temperature of the engine increased by 12% and the charging period was extended thereby allowing for an increased temperature and reducing the cold start effect. Further modification of the catalytic converter integrated with nanomaterial to improve the thermal property of the PCM further increased the efficiency of the system.
  10. 10. 10 Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 Al-Kayiem et al. 5. CONCLUSION The literature survey has demonstrated that the TESs are effective when integrated with thermal conversion systems like heat exchangers. However, the performance of the TESs could be considerably improved by improving the thermal conductivity of the TES base materials via nanoadditives. Among the major base materials that have been used and investigated are the PCMs. The paper has reviewed the available PCMs, nanocomposites and nanofluids for TES applications. Almost all of the researches agreed on the enhancement of the thermal properties of the base materials but with some inconsistency in the results. As the case is, further investigations are required to reach standardized status on the effect of the nanoadditives to the base materials. In spite of the numerous investigations on the nanoadditives and theirenhancement of the thermophysical properties of the TES base materials, more investigations are needed to understand the dispersion and settlement of the nanoparticles in the base materials. Also, it is recommended that more work is required to study the thermal cycle’s effect on the stability of the nanofluids/composites. CONFLICT OF INTEREST • The main author would like to acknowledge Universiti Teknologi PETRONAS for sponsoring the work under the Universiti Internal Research Fund (URIF no. 19/2012). • The second author would like to acknowledge the Ministry of Higher Education, Malaysia, (MOHE) for sponsoring his PhD scholarship. • The third author would like to acknowledge Universiti Teknologi PETRONAS for granting his PhD under the GA scheme. ACKNOWLEDGEMENTS The main author would like to acknowledge Universiti Teknologi PETRONAS for sponsoring the work under the University Internal Research Fund (URIF no. 19/2012). The second author would like to acknowledge the Ministry of Higher Education, Malaysia, (MOHE) for sponsoring his PhD scholarship. The third author would like to acknowledge Table 5. Summary of Nano-TES Integration to Enhance the Solar Collectors Authors / Year Nanomaterials Composites Nanofluids Technology/ Application Remarks [70] Sani et al., 2010 Single wall carbon nanohorns (SWCNHs) in an aqueous suspension Solar Collector Heat Exchanger Thermal conductivity was increased to 10% with the investigated concentration. The spectral transmission showed that single wall carbon nanohorns (SWCNHs) improved the platonic properties of the fluid, leading to a significant increase of the light extinction level even at a very low concentration [71] Jung et al., 2011 Mica nanoparticle and pure nitrate salt eutectic Concentrated Solar Power (CSP) There was a 13-15% enhancement on the specific heat at the solid phase and 13- 19% at the liquid phase compared to a pure nitrate salt eutectic in 60:40 molar ratio. Enhancement in the specific heat value in the liquid phase increasedwith an increase in the mass concentration [72] Shin et al., 2011 Silica nanoparticle dispersed to carbonate salt eutectic Concentrated Solar Power (CSP) A 24% enhancement at the solid phase and 75% at the liquid phasewerereported with an error analysis of 3.3% and 1.5% for the solid and liquid phases. The enhancement increased with an increase in the volume ratio of the mixture. [73] Lu et al., 2011 CuO nanofluid Evacuated Solar collector The CuO nanofluid concentration range of 0.8 % - 1.5% was tested. 1.2% was found to provide the highest thermal conductivity. The performance of the evaporator improved by 30% as compared to ionized water. [74] Yousefi et al., 2012 Multi-walled carbon nanotube (MWCNT) nanofluid Flat Plate Solar Collector 0.2% of the MWCNTs were investigated with water as the base fluid. Different pH rangeswere selected. The nanofluid withthe pH of 6.5 achieved the highest heat absorption coefficient, FRUL (38.84), while the nanofluid with the pH of 9.5 (FRUL=30.2). The enhancement increasedthe collector efficiency by 25%. [75] Poinern et al., 2012 Carbon nanowalls (CNWs) in an aqueous solution Solar Thermal Collector The largest CNS mass content (0.04 g) nanofluid had the largest temperature enhancement of 8.1°C, which clearly demonstrates the efficient absorption capabilities of the CNS nanofluids towards solar irradiation The results indicate that functionalized CNS nanofluids have the potential to effectively improve the solar absorption capabilities of direct-absorption solar collectors [76] Saidur et al., 2012 Aluminium nanofluid Direct Solar Absorption Researchers have investigated experimentally the effect of the absorption coefficient and the scattering coefficient range of the 0.1 – 2.5% concentration of aluminium nanofluid. The nanoparticle diameters chosen werefrom 1 – 20 nm. The absorption and scattering coefficient was themaximum at the solar wavelength of 0.3 μm before starting to decrease for all the parameters. The scattering coefficient showed a good value only at the solar wave length of 0.2μm.
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