Abstract
Titanium dioxide nanofluid is used in the thermosyphon solar flat plate collector at volume fractions of 0.1%, 0.3%, 0.5%, 0.7%, and 1% to test the collector’s effectiveness. The heat transfer coefficient, Nusselt number, friction factor, and pressure drop calculations were used to analyze the impact of adding nanoparticles to the working fluid. The effectiveness of the solar flat plate collector and the heat transfer coefficient are both greatly affected by the addition of titanium dioxide to the working fluid, with a tolerable rise in pressure drop. With concomitant variances of 15% and 13%, correlations were constructed for the Nusselt number and friction factor.
1. Introduction
Renewable energy is one of the most important sustainable energy resource for the future generation. The incident solar energy striking on the Earth’s surface an hour is greater than all of the human consumption of energy in a year. Collecting and converting the solar thermal energy into something useful for the human needs is one of the most challenging tasks for researchers. Thermosyphon solar flat plate collector is one of the principle methods of collection of solar thermal energy, which vary drastically in the amount of solar flux captured as well as the method for capture. Heat transfer enhancement in a solar water heater is the process of improving the performance of a system. The most common type of solar flat plate collector utilizes a black surface as the absorber plate, which then transfers heat to a fluid running in the riser tubes embedded with the absorber plate surface. In this case, the efficiency is limited by how the absorber plate captures solar energy and also how effectively the heat is transferred to the working fluid. An approach that has been proposed to enhance the efficiency of solar collectors while improving the thermal performance of the system is using the nanofluid as a heat transfer medium. The conventional heat transfer fluids have a low thermal conductivity which limits their heat transfer rate. Suspension of nano-sized solid particles in the base fluid is an innovative way of enhancing the fluid’s thermal performance. Nanofluid is a promising fluid for heat transfer enhancement due to its high thermal conductivity. Among the various passive techniques, nanofluids are used as a heat transfer medium in many engineering applications like heat exchangers, air heaters, and condensers also in solar collectors. Choi and Eastman [1] proposed nanofluid for heat transfer applications instead of conventional heat transfer fluids. Results show that nanofluids exhibit high thermal conductivities compare with conventional fluids for enhancement of heat transfer. Sekhar et al. [2] experimentally investigated the thermal conductivities of water and Al2O3 nanofluid at low concentrations in the range of 0.05%–1%. Results indicate that enhancement of thermal conductivity increases with particle concentrations. Xuan and Li [3] investigated the convective heat transfer and flow features of nanofluids, effect of volume concentration, convective heat transfer coefficient, friction factor, and flow features. Albadr et al. [4] experimentally studied forced convective heat transfer and flow characteristics of nanofluid at different volume concentrations of Al2O3–water in heat exchanger, results indicate that increasing volume concentration increasing the viscosity and friction factor. Natarajan and Sathish [5] analyze the heat transfer properties of the nanofluids and compare the results with conventional fluids. The thermal conductivity of the carbon nanotubes/sodium dodecyl sulfate (CNT/SDS) dispersion has been investigated; thermal conductivity enhancement depends on the volume fraction of the suspended particle and thermal conductivity of the base fluids. And the experiment proves that the nanofluids more effective than conventional heat transfer fluids. Also, the possibility of using nanofluids in solar water heater for increasing the efficiency was clearly explained. Said et al. [6] used TiO2 nanofluid and polyethylene glycol dispersant to evaluate the performance enhancement of solar flat plate collector in forced circulation mode. Energy efficiency, pressure drop, and pumping power were also calculated. Xuan and Roetzel [7] investigated the heat transfer mechanism of the nanofluid and proposing different methods to develop correlation for heat transfer. Khullar and Tyagi [8] studied the environmental impact of concentrating type solar water heater with nanofluid and a quantitative assessment for potential environmental benefits of solar water heater if substituted for those using fossil fuels. The proposed system introduces the experimental results of the material Al2O3–water-based nanofluid. Shareef et al. [9] experimentally studied the effect of Al2O3 nanofluid addition on the flat plate solar collector. The volume fraction of nanoparticles has been selected as 0.5%. The collector outlet, inlet temperatures difference increased with nanofluid than pure water for 1 l/min flow rates. Otanicar et al. [10] observed the improvement of 5% collector efficiency with nanofluid in the absorption medium with an increased efficiency with respect to increased volume fraction. Tyagi et al. [11] predicted efficiency of a direct absorption collector with nanofluid and found 10% higher efficiency compared to a flat-plate collector. Otanicar and Golden [12] reported the effective and economic utilization of nanofluids in the enhancement of solar collector efficiency. Yousefi et al. [13] investigated the effect of 0.2%, 0.4% with 15 nm Al2O3 nanofluid with and without surfactant and revealed the efficiency improvement of collector by 28.3% compared with water and surfactant. Said et al. [14] experimented natural and forced circulation on flat plate solar water heater with copper oxide nanofluid and observed addition of 0.05% volume fraction of nanopowder improved the thermal performance by 6.3%. Gangadevi et al. [15] studied the thermal efficiency of flat plate solar collector using Al2O3 nanofluid and observed an increase in volume fraction leads to an increase in the collector efficiency. In this work, the thermal performance of a flat plate solar collector employing MWCNT + Fe3O4/water hybrid nanofluids under thermosyphon conditions was examined. Various nanoparticle concentrations and Reynold’s numbers were tested in the field. Theil’s U2 was used to assess the prognostic paradigm’s uncertainties which were found to vary from 0.0099 to 0.1544 for BOBRT [16–19].
From the literature, it is observed that most of the experimental studies were mainly focused on the use of nanofluids in forced circulation [20–22]. Hence, an experimental investigation to improve the thermal performance of solar collector was attempted using 0.1%, 0.3%, 0.5%, 0.7%, and 1% volume fractions of titanium dioxide nanopowder. Further, the Reynolds number, Nusselt number, heat transfer rate, friction factor, and thermal efficiency for the solar flat plate collector were calculated from the experimental facts and compared with water.
2. Materials and Methods
2.1. Experimental Setup
An experimental setup was fabricated to investigate the thermal performance and pressure drop behavior of TiO2/water nanofluid in thermosyphon solar flat plate collector. The layout and photograph view of the experimental setup is shown in Figure 1 [23]. A riser tube was covered by black-coated absorbed plate. The cold fluid entered bottom side of riser tube and heated by black absorber plate and the hot fluid stored in top portion of tank. The unit has 2.2 m length and 0.22 m width and connected with insulated storage tank of 10 l capacity. This system working under natural buoyancy force is called thermosyphon effect. A transparent thin glass placed on the wooden cover and transmitted the solar energy to copper absorber plate and riser tube, and this setup called as leaf unit. The specification of the solar collector is given in Table 1. T-type thermocouples were brazed in absorber plate and riser tube, inlet and outlet temperatures of fluid were recorded by data logger. A magnetic flow meter was connected in the outlet of fluid passage which was used to measure the flow rate of the fluid.
2.2. Nanofluid Characterization
Nanofluid at different volume concentrations of 0.1%, 0.3%, 0.5%, 0.7%, and 1% is used to conduct the experiments. The TiO2/water nanofluids are prepared from 99.7% pure nanodurable metal oxide powders supplied by Alfa Easer Ltd., USA, surface area of nanometal powder is 40 cm2/g, and the density of the TiO2 nanometal powder is 4,170 kg/m3. Nanoparticle size plays a substantial role in enhancing the heat transfer of any thermal applications, increase in diameter of nanoparticle increases the viscosity of nanofluid and there is a great chance of increase in pressure in the riser tube. Hence, the TiO2 nanopowder with the average particle size of 20 nm is used for nanofluid. Figure 2 shows the SEM (JSM-6390, JEOL Ltd., Peabody, Massachusetts) image of TiO2 nanopowders; it reveals that the size of nanoparticle is in the range of 14.17–20.23 nm. Figure 3 shows the energy dispersive spectrum of TiO2 nanoparticles, which reveals the 34.65 wt% of titanium and 64.35 wt% oxide. Through the one step method, nanofluid was prepared, with the required quantity of TiO2 nanometal powder. With the help of magnetic stirrer, the fluid is enthused well and the sonication is done by using ultrasonic vibrating bath up to 1 hr. Stability of the nanofluid is measured for 0.1%, 0.3%, 0.5%, 0.7%, and 1% volume fractions. It is clear that the sonicated nanofluid has better stability and dispersion compared to unsonicated nanofluid. It is observed that the stability of TiO2 nanoparticles for 0.1%, 0.3%, and 0.5% concentration is good (ζ/mV = −28) and almost it is stable for up to 14 days, whereas the stability of 0.7% and 1% volume concentrations is found to be moderate. The thermal conductivity of the nanofluid was measured with KD2 Pro thermal property analyzer, and values of the thermal conductivity are 19.1%, 24.3%, 28.7%, and 32.8% for 0.3%, 0.5%, 0.7%, and 1% volume concentrations, respectively.
2.3. Location of the Experimentation and Data Collection
The experimental work was carried out in Star Lion College of Engineering and Technology, Thanjavur, which is situated at 250 miles to the north of equator and its geographical location is 10°ʹN of latitude, 80°11ʹE of longitude and 88 m above mean sea level. The experimental work was performed during the period of March–June and the local weather conditions were measured. The day times are clear and bright sunny days, and 55% relative humidity present on those days with an average wind speed of 2.5 m/s and the average atmospheric temperature is 36°C. Six leaf units such as water, TiO2/water nanofluid of volume concentration of 0.1%, 0.3%, 0.5%, 0.7%, and 1% are kept in open atmosphere with tilt angle of 10°. During the experimentation, data were recorded by data logger from 9.00 am to 4.00 pm at equal interval of every 15 min for 4 days and the readings are averaged and counted for calculations. Characteristics and heat transfer of thermosyphon solar flat plate collector have been divided into two phases, such as phase 1 (09:00–13:00 hours) and phase 2 (13:00–16:00 hours).
2.4. Data Processing
The correlations provided by Pak and Cho [24] and Equation (1) were used to calculate the density of the nanofluid:
The specific heat of the nanofluid is calculated by using Equation (2):
Thermal conductivity of the nanofluid is calculated by using Equation (3) and it was introduced by Yu and Choi [25], as given below:
The viscosity of the nanofluid is calculated by using Equation (4):
The kinematic viscosity can be calculated by using Equation (5):
Calculation of Reynolds and Prandtl numbers is as follows:
Thermal diffusivity is given by the following equation:
The Nusselt number, thermal performance, and friction factor are calculated separately for phases 1 and 2.
The solar flat plate collector heat transfer rate is calculated based on the following equation:
The factors such as specific heat, mass flow rate, and inlet and outlet temperatures difference of fluid govern the heat transfer rate. By combining Equations (10) and (11), the internal convective heat transfer coefficient (hi) is calculated and the values of the Nusselt number were obtained:
Pressure drop measurement is done by using pressure transducers. The pressure drop is found to be more at low temperature and less at high temperature due to the decreased fluid viscosity with respect to the temperature rise. Based on the following equation, friction factor is calculated:
With Hottel and Whillier equation, solar collector thermal performance is obtained:
Heat loss coefficient, heat removal factor, and transmittance–absorbance product are calculated by using Duffie and Beckman equation:
Based on Equation (17), heat removal factor is obtained:
Data reduction process is performed to find the experimental uncertainties with Coleman–Steele method and ASME. The maximum uncertainties are attained as ±1.9%, ±2.7, and ±3.3% for the Reynolds number, friction factor, and Nusselt number, respectively, with the thermal performance of ±1.17%.
3. Results and Discussion
3.1. Nusselt Number Data Verification
The theoretical values of Nusselt number for the plain water and the nanofluid at phases 1 and 2 are calculated and compared with experimental values with maximum deviation of ±7%. The values of the Reynolds number and Nusselt number gradually increase at phase 1 due to the high intensity of solar radiation which directly influences the mass flow rate and velocity of the working fluid, whereas the values of the Reynolds number and Nusselt number gradually decrease at phase 2 due to the reduction in the intensity of solar radiation.
Figures 4(a) and 4(b) show the Nusselt number vs. Reynolds number for phases 1 and 2. It is observed that the Nusselt number is found to be increased with respect to the increase in Reynolds number for all the conditions. When comparing with water, the Nusselt number is found to be higher for the TiO2/water nanofluid with respect to the Reynolds number. The average values of the Nusselt number are obtained as 4.69, 16.29, 19.56, 19.97, 20.44, and 20.61 for plain water, 0.1% φ, 0.3% φ, 0.5% φ, 0.7% φ and 1% φ, respectively. It is perceived that the Reynolds number for the nanofluid of all conditions is found to be increased linearly with respect to the particle concentration and the Nusselt number for the nanofluid condition is found to be increased four times of the plain water. With the help of Gauss elimination method, a mathematical relationship was developed for phases 1 and 2 conditions with 95% of confidence level and their values were compared with the experimental values.
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(b)
3.2. Enhancement of Heat Transfer in Nanofluids Collectors
The values of heat transfer coefficient with respect to Reynolds number of a thermosyphon solar flat plate collector with nanofluid are presented in Figures 5(a) and 5(b) that show the convective heat transfer coefficient with respect to the Reynolds number for water, titanium dioxide–water nanofluid at varying volume concentrations of 0.1%, 0.3%, 0.5%, 0.7%, and 1%. The heat transfer for the TiO2 nanofluid is found to be higher when compared to plain water. Thermal conductivity of the fluid is enhanced by the addition of titanium oxide nanoparticles in the base fluid, which results in the increase in heat transfer by compared to the plain water collector. The average value of hi for both phases 1 and 2 were obtained as 754.89, 787.44, 829.42, 877.05, and 904.17 W/m2K for 0.1%, 0.3%, 0.5%, 0.7%, and 1% volume concentrations, respectively, which indicates the intensification of increase in heat transfer coefficient directly proportional to the particle volume concentration. Though the viscosity of the nanofluid increases with an increase in particle volume concentration which leads to decrease in heat transfer rate and Nusselt number, whereas the minimum percentage reduction of Nusselt number is observed at lower particle concentrations.
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(b)
3.3. Friction Factor and Pressure Drop
The suspended TiO2 nanoparticle increases the density of the base fluids and it leads to increase in viscosity and friction factor. Figures 6(a) and 6(b) show the variation in the friction factor with respect to the Reynolds number. The friction factor is found to be increased by 9%, 13%, 21%, 31%, and 37% for 0.1% φ, 0.3% φ, 0.5% φ, 0.7% φ, and 1% φ, respectively, which indicates that the friction factor increases linearly with the volume concentration of nanoparticle in base fluid. Friction factor is a function of Reynolds number and particle volume fraction. The predicted values of friction factor were compared with the experimental values. Figures 7(a) and 7(b) show the variation in the pressure drop with respect to the Reynolds number. It is observed that pressure drop increases with the increase in friction factor due to nanoparticle concentration in the base fluid. The pressure drop is found to be higher for the higher concentration of nanoparticles, i.e., 1% volume concentrations whereas the pressure drop is found to be much lower for the water. From the pressure drop measurements, the average percentage increase for the 0.1% φ, 0.3% φ, 0.5% φ, 0.7% φ, and 1% φ TiO2 nanofluid is in the order of 90.51%, 92.81%, 93.78%, 94.60%, and 95.21% of the pressure drop due to the occurrence of nanoparticles in the working fluid when compared with water.
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3.4. Thermal Performance
Several factors influence the collector’s instantaneous efficiencies such as absorber plate design, type of coating, coating material, glazing property, and the condition of operation. Figure 8 shows the efficiency curve for the flat plate solar collector at different volume concentrations of TiO2 nanofluid. It is found that there is an increase in the instantaneous efficiency with an increase in solar radiation insolation. During phase 1, the instantaneous efficiency of the system is increased due to the increase in heat transfer whereas the instantaneous efficiency of the system is decreased in phase 2. The efficiency of TiO2 nanofluid collector is found maximum when compared with the water collector because of the fact that the surface area of the fluid increases with increase in nanosized metal particle suspended in working fluid which influences the heat removal rate from the absorber plate to working fluid, whereas in case of the water, the surface area is less compared with nanofluid that results in the reduced heat removal rate. It is found that collector with nanofluid of 0.1% φ, 0.3% φ, and 0.5% φ is rectilinear under the specified operating conditions. In collector with plain water minimum amount of heat is removed from the absorber plate and the rate of heat removal is 0.61. In case of TiO2 nanofluid, the heat removal factor gradually increases with increase in particle volume concentrations. The average heat removal rate for phases 1 and 2 for 0.1% φ, 0.3% φ, 0.5% φ, 0.7% φ, and 1% φ are found as 0.88, 0.91, 0.93, 0.96, and 0.98, respectively. This indicates that the fluid thermal conductivity has a relatively strong influence on the mean plate temperature and heat removal rate solar flat plate collector. Thermal efficiency of the collectors with 0.3% φ and 0.5% φ was increased by almost 31% and 42%, respectively, when compared with the water tube collector, which shows the TiO2 nanofluid gives a significant increment in efficiency with insignificant drop in the pressure. The temperature of the absorber plate is the quantification of heat transfer between the riser tube and working fluid and it is found to be increased with an intensity rise of the solar radiation and atmospheric temperature during phase 1. The heat removal rate is found to be higher for the nanofluid collector from the absorber plate when compared with the water collector.
4. Conclusion
The effectiveness of the titanium dioxide nanofluid at concentrations of 0.1%, 0.3%, 0.5%, 0.7%, and 1% in plain water and the thermosyphon solar flat plate collector was tested. It was discovered that when the volume concentration of TiO2 grew, the heat transfer coefficient increased and the pressure drop dropped; the resulting values were 77.21% and 16.08 N/m2, respectively. In comparison to water, the TiO2 nanofluid’s immediate thermal efficiency is found to rise with increasing volume concentration; a greater instantaneous thermal efficiency of 91% is recorded at 1% TiO2 nanofluid.
Nomenclature
| Ac: | Collector aperture area, m2 |
| Ai: | Inside surface area of the riser tube, m2 |
| Ao: | Outside surface area of the riser tube, m2 |
| Cp: | Specific heat, kJ/kg°C |
| Do: | Outside riser tube diameter, m |
| f: | Friction factor for water tube collector, (dimensionless) |
| fnf: | Friction factor for nanofluids collector, (dimensionless) |
| Ht: | Total intensity of solar radiation, W/m2°C |
| k: | Thermal conductivity of water, W/m°C |
| kw: | Thermal conductivity of the riser tube wall, W/m°C |
| L: | Length of the riser tube, m |
| m: | Mass flow rate, kg/s |
| Nu: | Nusselt number for water riser tube, dimensionless |
| Nunf: | Nusselt number for nanofluid |
| Pr: | Prandtl number, dimensionless |
| Q: | Heat transfer rate, W |
| Re: | Reynolds number based on the internal diameter of the riser tube, dimensionless |
| dp: | Diameter of the nanoparticle, nm |
| S: | Surface area of the particle |
| Cpnf: | Heat capacity of the nanofluid |
| Cpf: | Heat capacity of the base fluid |
| Cpp: | Heat capacity of the nanoparticles |
| Tm: | Bulk mean temperature of fluid in the riser tube, °C |
| Tin: | Average outlet temperature of water, °C |
| Tout: | Average outlet temperature of water, °C |
| Two: | Average wall surface temperature outside |
| Ui: | Overall inside heat transfer coefficient, W/m2K |
| Uo: | Overall outside heat transfer coefficient, W/m2K |
| Ul: | Overall heat loss coefficient, W/m2K |
| η: | Collector efficiency |
| FR: | Collector heat removal factor |
| : | Transmittance–absorptance produce |
| Ul: | Overall heat loss coefficient |
| Ta: | Ambient temperature |
| Tp: | Absorber plate temperature |
| Ht: | Total solar radiation |
Greeks
| ρ: | Density of water, kg/m3 |
| φ: | Volume fraction of nanoparticle, % |
| µ: | Dynamic viscosities of water at bulk mean temperature, Ns/m2 |
| µw: | Dynamic viscosity at wall temperature, Ns/m2 |
| ΔP: | Pressure drop of water, N/m2. |
Data Availability
Data available on request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.