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Energy Conversion and Management 76 (2013) 162168Contents lists available at ScienceDirectEnergy Conversion and Managementj o u r n a l h o m e p a g e : w w w . e l s e v ie r . c o m / l o c a t e / e n c o n m a nEnergy, economic and environmental analysis of metal oxides nanofluid for flat-plate solar collectorM. Faizal a,b, , R. Saidur b, S. Mekhilef c, M.A. Alim ba Engineering Division, ADP, Taylors University Lakeside Campus, 47500 Selangor, Malaysiab Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiac Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiaa r t i c l ei n f oArticle history:Received 19 April 2013Accepted 20 July 2013Keywords:NanofluidFlat-plate solar collector Energy saving EconomicEnvironmenta b s t r a c tFor a solar thermal system, increasing the heat transfer area can increase the output temperature of the system. However, this approach leads to a bigger and bulkier collector. It will then increase the cost and energy needed to manufacture the solar collector. This study is carried out to estimate the potential to design a smaller solar collector that can produce the same desired output temperature. This is possible by using nanofluid as working fluid. By using numerical methods and data from literatures, efficiency, size reduction, cost and embodied energy savings are calculated for various nanofluids. From the study, it was estimated that 10,239 kg, 8625 kg, 8857 kg and 8618 kg total weight for 1000 units of solar collec-tors can be saved for CuO, SiO2, TiO2 and Al2O3 nanofluid respectively. The average value of 220 MJ embodied energy can be saved for each collector, 2.4 years payback period can be achieved and around 170 kg less CO2 emissions in average can be offset for the nanofluid based solar collector compared to a conventional solar collector. Finally, the environmental damage cost can also be reduced with the nano-fluid based solar collector.2013 Elsevier Ltd. All rights reserved.1. IntroductionThe population and world energy demand is increasing and accelerating while fossil fuel sources are declining fast. The envi-ronment is polluted by fuel burning and climate change has becoming huge global problem. Therefore, a lot of studies related to energy efficiency and renewable energy have been conducted to address this issue 142. Commonly, most houses in Malaysia are using Electric Water Heater for shower. This is mainly because the price of Electric Heater is cheap and relatively easy to install. However, the world is facing a huge problem now because of declining source of energy and using the precious electrical energy for heating does not really a good idea since heat can be harnessed directly from the sun. Potentially, Malaysia is located on the equa-torial, with hot and humid climate throughout the year and monthly solar radiation approximately around 400600 MJ/m2 26. Solar energy source is sustainable, free, clean and infinite. However, current solar heater is still expensive, low in efficiency and big in size. One of the effective methods to increase the effi-ciency is to replace the working fluid with nanofluids. Researches on thermal efficiency by applying nanofluids on flat-plate solar col-Corresponding author at: Engineering Division, ADP, Taylors University Lake-side Campus, 47500 Selangor, Malaysia.E-mail addresses: .my, (M. Faizal).0196-8904/$ - see front matter 2013 Elsevier Ltd. All rights reserved. /10.1016/j.enconman.2013.07.038lector have been made in the past few years by numerous researchers 4352. Experimental investigation conducted by Yousefi et al. 45 on the effect of Al2O3 based nanofluid shown the increase of 28.3% efficiency of flat-plate solar collectors. Lenert and Wang 53 presented a model and experimental study of con-centrated solar power application using carbon-coated cobalt (C Co) nanoparticles and Therminol VP-1 base fluid and concluded that the efficiency is more than 35% with nanofluid and the efficiency will increase with increasing nanofluid height. Lu et al. 54 shown that the application of Copper Oxide (CuO) nanoparti-cles in evacuated tubular solar collector will significantly enhance the thermal performance of evaporator and evaporating heat transfer coefficient increased by 30% compared to water as work-ing fluid. Five percentage improvement in efficiency was found out by Otanicar et al. 55 by using diversity of nanoparticles with water as base fluid for micro-solar-thermal collector. Shin and Banerjee 56 applied novel nanomaterials in molten salts base fluid for concentrated solar power coupled with thermal storage and experienced an enhancement in operational efficiencies. They also concluded that the cost of electricity will be reduced. Taylor et al. 51 used graphite based nanofluid in high flux solar collec-tors that resulting in 10% increase in efficiency.Because of higher thermal conductivity and efficiency of nano-fluids, smaller and compact design of solar thermal collector has become possible without affecting the output desired. Smaller size collector can reduced the material usage, cost and energy requiredM. Faizal et al. / Energy Conversion and Management 76 (2013) 162168163in manufacturing 30. Some studies were made on the potential of size reduction of various engineering applications by using nanofl-uids. Work had been done by Saidur and Lai 1 in vehicles weight reduction, Kulkarni et al. 57 in building heat exchangers heat transfer area, Leong et al. 33 on the reduction of air frontal area of a car radiator and Leong et al. 30 on the size reduction of shell and tube recovery exchanger. Some studies had been made to eval-uate the economic and environmental impact of solar hot water system 5861, one particular study focus on the environmental and economic analysis of direct absorption microsolar thermal col-lector utilizing graphite nanofluid 48, and also studies have been made on the potential of size reduction of flat-plate solar collectors when applying Al2O3 nanofluid 62 and MWCNT nanofluid 63. However, none of the studies focus on the size reduction of flat-plate solar thermal collector and its associated energy and cost sav-ing when applying various oxide nanofluids. Therefore, this study will focus on the potential of size reduction and its associated en-ergy saving of flat-plate solar collectors when applying Al2O3, SiO2, TiO2 and CuO nanofluid.2. Properties of nanofluidsVarious researchers have published the properties of nanoparti-cles and thermal properties of nanofluids as the basis of research on nanofluids applications. Table 1 shows the published specific heat, thermal conductivity and density of different nanoparticles.Improvement in thermal properties of nanofluids such as ther-mal conductivity and convective heat transfer that have been de-scribed in Section 1 above had a few mechanism contributing to it as listed by Keblinski et al. 66 such as Brownian motion, parti-cle and liquid interface nanolayer and heat transfer in nanoparti-cles. However, all this special characteristics cannot be achieved unless the nanoparticles are properly dispersed and stable. Surfac-tants can play a major role in achieving better dispersion and sta-bility of nanofluids 67,68.3. Collector size calculationThe important intention for this analysis is to investigate the potential of size reduction of flat-plate solar collector. The specifi-cation of flat-plate solar collector is taken from Yousefi et al. 43 45. The most important element in solar thermal collector is the working fluid. The fluid will pass through pipes attached to the ab-sorber plate that absorbed heat from sunlight. The fluid will absorb heat from the plate and as it flows through the pipes the temper-ature will increase. The useful heat gain and collectors efficiency can be calculated and compared between the conventional work-ing fluid and proposed nanofluids. The important useful heat gain by the working fluid can be expressed as:Q u mCpToutTinkJ1where Tout is the fluid outlet temperature (Kelvin), Cp is the heat capacity at constant pressure (kJ/kg K), m is the mass flow rate of the working fluid (kg/s).Table 1Properties of different nanomaterial and base fluid 64,65.The heat capacity and density of nanofluid can be calculated by 69:Cp;nf Cp;npu Cp;bf 1 u kJ=kg K2qnf 1 uqbf uqnp kg=m33where Cp,nf is the heat capacity of nanofluid (kJ/kg K), Cp,np the heat capacity of nanoparticles (kJ/kg K), Cp,bf the heat capacity of base fluid (kJ/kg K), u is the volume fraction of nanoparticles in nanofluid(%).Thermal efficiency of a flat-plate solar collector can be calcu-lated from:g Q u=IT Ac mCpToutTin=IT%4where IT is the the solar radiation incident on the tilted collector. After the thermal efficiency of solar collector been determined,the potential of reduction of the size of collectors area can be esti-mated by:Ac mCpToutTin=IT gm25And mass flow rate of the working fluid can be calculated by:_6m qV kg=sSize reduction calculation is carried out based from variations of efficiency of collectors using different nanofluids. Efficiency is the function of density, specific heat and mass flow rate of different nanofluids calculated with regard to volume fraction of nanoparticles.4. Nanofluid solar collector embodied energy analysisIn this section, the study is on the embodied energy of a solar collector. Only energy used to manufacture the solar collector is considered where else the distribution, maintenance and disposal phase of the collectors are neglected because according to Ardante et al. 70, more than 70% of the embodied energy of the system comes from the manufacturing of the collector. The analysis was done with the reduction of collector area as the functional unit that influences the overall weight and embodied energy of the collector. Two major materials that are being used in solar collector are glass and copper with the weight ratio of 30 kg glass and 10 kg copper for a 40 kg collector. The embodied energy index is 15.9 MJ/kg and 70.6 MJ/kg for glass and copper respectively 55. By using the result of size reduction, the weight and the embodied energy for solar collector can be calculated accordingly.5. Economic analysisThe results of the thermal performance of nanofluid solar col-lector and size reduction can also be used to estimate the cost sav-ing. By using nanofluid as working fluid in solar collector, large portion of copper and glass used in the system can be eliminated based on the scaling of the overall percentage weight of the collec-tor. The capital cost of the collector will then be offset by the cost of the nanoparticles. The energy usage per day in conjunction withMaterialSpecific heat, Cp (J/kg K)Thermal conductivity, k (W/m K)Density, q (kg/cu m)Alumina (Al2O3)773403960Copper Oxide (CuO)551336000Titanium Oxide (TiO2)6928.44230Silicon di Oxide (SiO2)765363970Water (H2O), base fluid41820.601000164M. Faizal et al. / Energy Conversion and Management 76 (2013) 162168the local electricity rates based in RM 0.40 per kWh is used to determine the amount saved by using solar thermal system. As can be seen in Fig. 1, 11.03% of electricity used for water heating in Malaysia can be saved by using solar hot water system.6. Environmental analysisBurning of fossil fuels to generate the energy to heat water will result in harmful gas emissions. Switching to solar hot water sys-tem can reduce that problem. The distribution of electricity from various fuel types and the key pollutants generated in Malaysia is shown in Table 2.With the embodied energy index of solar collector from Section 3, the emissions from the manufacturing of the collectors can be determined. The offset damage costs can be calculated for the three main pollutants of CO2, NOx and SOx based on the damage cost fac-tors 73. These offset damage cost are not costs directly applicable to the collector owner.7. Results and discussion7.1. Effect of varying volume fraction to the density of working fluidsFig. 2 shows that the density of nanofluids is proportional to volume fraction of nanoparticles. It can be explained by Eq. (3) and data from Table 1 where density of nanofluids will increase by increasing the volume fraction of nanoparticles. Fig. 2 also shows that CuO nanofluids have the highest possible density com-pared to other fluids based on the higher density of CuO nanopar-ticles. These results are similar to studies by Pandey and Nema 74.7.2. Effect of varying volume fraction to the specific heat of working fluidsFig. 3 shows that specific heats of nanofluids are inversely pro-portional to volume fraction of nanoparticles. Similar results had also been shown by other researchers 74. Substitution of lower value of specific heats of nanoparticles from Table 1 will decrease the overall specific heats of nanofluids as shown in Eq. (2).Specific heat can be explained as the energy required to raise the temperature of a unit mass of a substance by one degree. It means that a different amount of heat energy is needed to raise the tem-perature of similar masses of different substances by one degree. Smaller number of specific heats for nanofluids will leads to smal-ler amount of energy needed to raise the temperature of it.Fig. 1. Average electricity consumption breakdown (%) in Malaysia 71.7.3. Effect of varying volume flow rate to the efficiency of solar collectorFig. 4 shows that the thermal efficiency of solar collector with nanofluids is proportional to the volume flow rate of the working fluid. In Eq. (1), the important useful heat gain by the working fluid is calculated and the value is then substituted in Eq. (4) to deter-mine its efficiency. Efficiency was calculated by the function of the working fluid density, specific heat and mass flow rates. To cal-culate the efficiency, mass flow rates had been determined first by using Eq. (5) and density data from Fig. 2 for volume flow rate be-tween 1 to 3.8 L/min. After that, the value of mass flow rates and specific heats of working fluid at 3% volume fraction of nanoparti-cle for nanofluids had been substituted in Eq. (4) for varying vol-ume flow rate and by keeping the inlet temperature, outlet temperature and solar radiation value constant. As shown in Fig. 3, the efficiency of solar collector increased by 38.5% by using CuO nanofluid and 28.8% for Al2O3, SiO2 and TiO2 nanofluids com-pared to water as working fluid. These results are in good agree-ment with experimental results by other researchers like Yousefi et al. 45 and Tyagi et al. 75.There are some reasons for the higher efficiency of nanofluids solar collector compared to water. One of it is the higher output temperature associated with nanofluids solar collector 4345. Output temperature of solar collector can be influenced by the spe-cific heat of working fluids. As seen in Table 1 and Fig. 3, nanopar-ticles and nanofluids have lower specific heat than water and Copper have the lowest value of all others. Because of that, less heat is required to raise the temperature of nanofluids and thus making the output temperature and efficiency becomes higher 10,35.7.4. Size reductionThe potential of size reduction of solar collector by using vari-ous nanofluids is shown in Fig. 5. Compared to water, solar collec-tors area can be reduced up to 25.6%, 21.6%, 22.1% and 21.5% for CuO, SiO2, TiO2 and Al2O3 respectively. Potential of collectors area reduction is calculated by substituting efficiency data in Fig. 4 into Eq. (5). Because of higher efficiency of nanofluids solar collector, the surface area of the solar collector that acts as the input energy of the system can be adjusted to give the same amount of output temperature with water as conventional working fluid for solar thermal collector.The weight reduction of solar collector can be estimated by using the area reduction data in Fig. 5. As shown in Fig. 6, the total weight for 1000 units of solar collectors can be reduced up to 10,239 kg for CuO nanofluid solar collector and around 8624.6 kg, 8856.5 kg and 8617.8 kg for SiO2, TiO2 and Al2O3 respectively.7.5. Energy savingsTable 3 presents the embodied energy for each collectors as well as the percentage of energy savings when applying nanofluidsTable 2Electricity generation by fuel type and primary emissions mix for Malaysia 71.Fuel% ofCarbonSulfur Oxides,NitrogenelectricityDioxide, CO2SOx (kg/MJ)Oxides, NOxgenerated(kg/MJ)(kg/MJ)Coal36.50.2740.000310.0005Oil0.20.22000Natural55.90.11300.00003gasHydro5.6000Others1.8000M. Faizal et al. / Energy Conversion and Management 76 (2013) 162168165Fig. 2. Effect of varying volume fraction to the density of working fluids.Fig. 3. Effect of varying volume fraction to the specific heat of working fluids.Fig. 4. Effect of varying volume flow rate to the efficiency of solar collector.Fig. 5. Percentage of size reduction for solar coll