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A brief review on solar flat plate collector by incorporating the effect of nanofluid
Materials Today: Proceedings xxx (xxxx) xxx
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A brief review on solar flat plate collector by incorporating the effect of nanofluid Ritvik Dobriyal, Prateek Negi, Neeraj Sengar, Desh Bandhu Singh ⇑ Mechanical Engineering Department, Graphic Era Deemed to be University, Bell Road, Clement Town, Dehradun 248002, India
a r t i c l e
i n f o
Article history: Received 31 October 2019 Received in revised form 12 November 2019 Accepted 27 November 2019 Available online xxxx Keywords: Flat plate collector Phase change materials Solar power Nanofluids Collector efficiency
a b s t r a c t The design, analysis and installation of solar collectors are the need of time as its application in the existing systems will reduce the use of conventional source of energy which is limited. On other hand, renewable energy source is unlimited and it will continue to exist till the existence of life on the planet earth. The main objective of solar collectors is to absorb heat from solar energy for increasing the temperature of fluid flowing through the solar collector and this heated fluid can be used for different applications namely heating to room in colder region, the heat of fluid can be utilized in cement industry and many other similar applications. This article attempts to provide an overview of the various techniques and improvements which allows the flat plate collectors to absorb as much solar radiation as possible while minimizing losses to the surroundings. It has been observed that the use of nanofluid enhances the performance of solar collectors. At last, recommendations have been provided. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Mechanical and Energy Technologies.
1. Introduction
2. Solar flat plate collector
Since fossil fuels such as coal, petroleum and natural gas, which are the main source of our energy today are exhaustible and polluting, therefore there is a need to migrate towards non-conventional sources of energy. Sun is a perpetual source of energy and the amount of energy it can provide is far in excess of the current global demand. Solar radiation can be directly converted into electricity using photovoltaic (PV) which convert light into electric current using the photoelectric effect or solar radiation can also be indirectly converted into electricity by using concentrated solar power (CSP). CSP uses lens or mirrors to collect solar radiation over a large area and concentrate it in a narrow beam. This concentrated heat can then be used to drive a conventional power plant (such as a steam turbine). Solar energy can also be harnessed as thermal energy using solar thermal collectors and can be used for a variety of purposes such as solar drying, cooking, distillation, heating water etc. Amongst the solar thermal collectors the most common are flat plate collectors. The current article contains an overview of the research aimed at development and enhancement of flat plate collectors.
Solar flat plate collector is a type of solar collector which is used to intercept and absorb solar radiation for heating a fluid. The basic idea behind a solar flat plate collector is simple. Solar radiation is used to heat up a dark flat surface known as absorber sheet or plate which converts the solar radiation energy into heat energy. This heat is then transferred to the fluid flowing through the pipes attached to the absorber sheet. The absorber sheets are made up of high thermal conductivity metals such as copper, aluminium or steel and are painted with selective coatings to maximise the absorption of incident solar radiation. The tubes or ducts carrying the fluid are either integral or attached to the absorber sheets. Cover sheets or plates, also known as glazing allows the high frequency solar radiation to pass through and reach the absorber plate but prevents the low frequency radiation from the absorber plate from escaping. It also prohibits surrounding cool air from flowing over the absorber plate and thus minimises heat loss due to convection. Material commonly used for this purpose is glass. Some plastic materials can also be used since they are cheaper and have higher transmittance, however are not as durable. Insulating material (such as fibreglass) is placed at the back and sides of the absorber plate for sealing and reducing heat loss (Fig. 1).
⇑ Corresponding author. E-mail address: [email protected] (D.B. Singh).
https://doi.org/10.1016/j.matpr.2019.11.294 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Mechanical and Energy Technologies.
Please cite this article as: R. Dobriyal, P. Negi, N. Sengar et al., A brief review on solar flat plate collector by incorporating the effect of nanofluid, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.294
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Fig. 1. Schematic diagram of a flat plate collector showing its different components.
Advantage of flat plate collectors is that they have a simple design which favours ease of manufacture. Other than cheap manufacturing they require little maintenance. They can collect energy from both beam as well as diffused radiation and are usually permanently fixed requiring no sophisticated positioning or mounting system. A liquid (water or oil) or air can be used as a fluid for flat plate collectors. Usually water is a common choice due to its ease of availability, high heat capacity, incompressibility and high mass density. One disadvantage of water is that it can freeze during periods of low temperature and may cause damage to the collector plate. Under these conditions antifreeze mixtures can be used. Phase-change liquids can also be used. They have a lower boiling point and hence eliminate the problem of freezing. Flat plate collectors are mainly used in residential buildings to cope up with the demand for hot water and are an alternative to conventional geysers. They can also be used for space heating of buildings with limited supply of electricity.
3. Development of flat plate collector Development of flat plate collector has focused on its effectiveness of trapping of sun’s radiation and enhancing its thermal efficiency. Many researchers have worked and are still working with different design and operating parameters as well as material selection to achieve a superior performance. 3.1. Mini channel Mansour [1] worked on collector having mini channel square array embedded in the absorber plate, which was covered by a glass plate. The pressure drop in the working fluid as well as heat transfer characteristics was studied using a CFD analysis. Heat removal from the absorber plate was found to be higher in comparison with the convectional collectors. However, maintaining higher thermal performance at higher flow rates required higher power input to the circulating pump. Robles et al. [2] experimentally investigated an aluminum based mini channel collector. Its performance was compared against a conventional collector under same operating conditions. An increase of thermal efficiency of 13% was obtained. In a recent study by Hota et al. [3], a back plate was added to the collector having mini channel tubes to enhance performance. Since, as discusses, for mini channels higher pumping power may be required, the authors also optimized the width of back plate and the geometry of mini channel tubes to balance the performance with pumping power.
3.2. Micro channel Deng et al. [4] worked on flat plat collector having micro channel heat type array. This type of channel was constructed with thin aluminum sheets having micro groves to enhance heat transfer. These micro groves improved heat transfer, reliability, provided greater compressive strength, reduced cost and had lesser contact resistance than the conventional type. A new structure of flat plate collector was reported in which one large integrated wickless heat pipe for enhancement of leak avoidance and stability of solarheating side and water-cooling side was used [5]. 3.3. Absorber plate designs Design of absorber plate and the selective coating plays an active role in the performance of flat plat collector for absorbing solar radiation. Jyothi et al. [6] developed a nanostructured tandem absorber especially for high temperature solar thermal applications. The performance of this tandem absorber was estimated by cyclically heating it in air as well as in vacuum. It showed high thermal stability for both short (2 h) and long terms (100 h–400 h), thus proving itself to be usable for high temperature solar thermal power generation. Del Col et al. [7] developed a special type of glazed flat plate collector, which was made of roll bond aluminum absorber plate. The study was performed both experimentally and numerically. These collectors helped in customizing and optimizing the flow of working fluid in the absorber leading to a improved and uniform temperature distribution across it. However, such absorbers are difficult to manufacture and are neither cost effective. El-Sawi et al. [8] came up with a chevron pattern absorber based on a continuous folding technique developed by Elsayed and Basily [9]. Experimental comparison between the conventional flat plate and chevron plate collector showed that the latter performed better under the considered operating conditions. Theoretical analysis also pointed towards a superior thermal efficiency of chevron plate collector. 3.4. Heat loss reduction Performance of a solar collector can be reduced due to heat loss from its various regions. Kumar and Mullick [10] did experimental study on unglazed solar collector to determine the coefficient of heat transfer from the upper region of the collector in outdoor conditions. Vestlund et al. [11] performed a computational study with replacement of air with some inert gas (Argon, Krypton and Xenon) in between glass cover and absorber for better performance of the
Please cite this article as: R. Dobriyal, P. Negi, N. Sengar et al., A brief review on solar flat plate collector by incorporating the effect of nanofluid, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.294
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collector. The results indicate that with the use of inert gases, the collector can be made thinner with comparable performance or a better performance can be achieved. The main complication with this approach is that a sealed space is needed for inert gages and variation of pressure and volume of the gas needs to be controlled. Akhtar and Mullick carried [12] repeated out numerical investigations to study the convective heat transfer coefficient along with radiative heat transfer coefficient for single as well as double glazed flat plate collector. Factors such as incidence angle, thickness of glass cover, declination, location and orientation of collector were taken into consideration. Zhang et al. [13] developed experimental set up to study the effect of heat shield in an evacuated tube solar collector. It was found that by employing a heat shield the thermal efficiency could be increased by as much as 54.07%. Beikircher et al. [14] worked on improving the insulation of flat plate collector from front and the back side. For reducing the heat loss from the front side of the collector, they made use of transparent insulation materials. They developed and experimentally investigated four types of transparent insulation materials. Out of these four, promising performance was delivered by single ETFE (ethylene tetrafluorethylene) film and double FEP (florinated ethylene-propylene) film. For preventing heat losses from the back side of the collector they employed vacuum super insulation (VSI) [15]. The combination of VSI with double FEP film was found to have the best performance. However, it was estimated that the production cost for incorporating VSI would be higher. Henshall et al. [16] demonstrated that the efficiency of a flat plate collector can be improved if housed in an evacuated enclosure. They even discussed the design of an enclosure for this purpose. A major concern is the stress on the enclosure due surrounding atmospheric pressure and thermal expansion. However, they performed a finite element analysis and realized that with proper mechanical design and selection of materials durable enclosures can be realized. Bhowmik and Amin [17] experimentally studied a flat plate reflector fitted with additional reflector plates. The reflectors are helpful in concentrating both the direct and diffused radiation on the collector plate, thus improving its performance. An increase in efficiency of 10% was reported by the authors.
improved efficiency by using PCM when compared with conventional system. In an another experimental work done by Xue [22]to characterize the Ba(OH)28H2O PCM, the solar collector used was of evacuated type while the PCM has melting temperature of 81.8 °C. The results showed an improvement in the performance of collector system. Molten salt was considered to be the most cost effective and commercially used energy storage material due to its high melting temperature [23]. Hamed and Brahim [24] developed a simple theoretical model to study the effect of PCM on flat plate collector. They found that due to incorporation of PCM there was a decrease in outlet temperature of water during the charging period and an increase in outlet temperature during the discharging period. The increase was greatest for higher inlet temperature and greater PCM thickness. Hamed et al. [25] theoretically studied flat plate collector with PCM (paraffin) installed below the absorber plate. Two different types of paraffins were installed with varying thickness. It was found that the best outlet temperatures for heat transfer fluid (water in this case) were for low thickness of PCM. Kabeel et al. [26] experimentally compared a flat plate collector and a vcorrugated plate collector having paraffin as the PCM. An increase in efficiency up to 12% was observed for v-corrugated plate collector with PCM as compared to the one without PCM. Also, an increase in efficiency of 21.3% was observed as compared to the conventional flat plate collector. Paraffin seems to be the most commonly used PCM in research. However, other materials such as eutectic mixture of salt hydrates and sodium acetate have also been used [27,28]. A thorough discussion on the different types of PCM used in flat plate collectors can be found in Tyagi et al. [29]. In addition, recent research on using nanoparticles in PCM is also picking up pace, Khan et al. [30]. In general, it is observed that selecting the thickness of PCM has an important effect on the efficiency of collector. Thicker PCM may provide greater performance for hotter regions. Another important aspect is to maximize the contact area between the absorbing plate and the PCM. For this purpose fins/metal foams are found to be beneficial.
3.5. Phase change material
Martinopoulos et al. [31] employed a low cost and lightweight polymer solar collector. In their study they performed both experimental investigations and CFD analysis. They could achieve a 50% reduction in weight and a low material cost with its help. Missirlis et al. [32] also used CFD analysis to study the heat transfer characteristics of polymer solar collector for various geometries. The author explored various behaviors of heat transfer of flat plat collector for different manifold configurations. This CFD model was employed to enhance the thermal behavior and flow field development of the collector. Mintsa et al. [33] performed numerical simulations to study the effect of both design parameters as well as operating parameters on polymer solar collector. It was found that the efficiency was not much influenced by the length of the collector plate, however, the air gap and the inlet temperature had a strong effect on it. Dela and Aguilar [34] analyzed a polymer solar collector developed by Modulo Solar (a Mexican company). They found that the thermal behavior of polymeric solar collector was comparable to metallic collector for ordinary use of home applications. Also, it was found that due to greater flexibility of polymers, they can resist low environmental temperatures.
Storage of solar energy is a challenge. The storage tanks used for storing solar energy are currently bulky and expensive. Phase change materials (PCM) if integrated in solar collectors may reduce or eliminate the need of separate energy storage. Chen et al. [18] performed computational analysis of a flat plate collector having integrated aluminum foam pours structure filled with paraffin as the phase change material. When the sunlight is available, paraffin can absorb solar energy while during night the stored energy can be transferred to liquid contained in the capillary tubes embedded in the paraffin. Phase change material has the advantage of taking less space with higher energy storage capacity when compared with sensible heat storage material. PCM based solar collector was also investigated by Khalifa et al. [19]in Iraq by using paraffin wax having melting temperature in the range of 46–47 °C. It was used to absorb the waste heat of the collector system. The results showed efficiency within the range of 45% to 54% in the month of March due to high energy storage and provided a promising and alternate solution for the improvement of collector systems. However, with this system there is dependency on the climatic conditions. Many other investigation were done using paraffin wax [20,21] to study the performance of the solar collector with different inclination angle viz. 45°, 10°, 20°, & 30° respectively. Both the results showed an
3.6. Polymer collector
3.7. Nano fluids In order to improve the efficiency of solar collector, the energy absorbed by the collector from the sun should be maximised and
Please cite this article as: R. Dobriyal, P. Negi, N. Sengar et al., A brief review on solar flat plate collector by incorporating the effect of nanofluid, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.294
SDBS
1.0, 2.0 & 3.0 vol% 0.05 vol% Water
Colangelo et al. [45]
Michael & Iniyan [46]
CuO
45, 60, & 30, resp. 0.3 & 0.21 nm
Water & Ethyl Glycol (EG)/ Water 60:40 Water
Al2O3, ZnO& Fe2O3
13
20.8 1.05 m
Al2O3/Water nanofluid has better stability than others. At 3.0% Al2O3, thermal conductivity increased 607%. Higher enhancement in performance was found in natural circulation than forced circulation. A maximum increase of 6.3% in collector’s efficiency. N/A
N/A
0.34 m2
Polysorbate 80 No surfactant With 0.4, 0.5 & 0.6 wt% 0.05–0.1 vol% 1.0
Single walled carbon nanotube Al2O3 Water
0, 0.1, 0.2 & 0.3 wt% Water
20 TiO2
Water
Polvongsri and Kiatsiriroat [41] Chaji et al. [42]
Vijayakumaar et al. [43] Said et al. [44]
0.5 m 0.2 m
The increase in collector performance was less than that of MWCNT. Reduced efficiency than MWCNT. Surfactant were not used due to high foaming. Only 4 h of stability without surfactant. There was increase in collector performance at 0.5 wt% which was close to MWCNT. Water/Al2O3 has better stability than EG-water/ Al2O3 1.0 m 0.15 m
No surfactant No surfactant 0.1 & 1 wt%
0.2 & 0.4 wt% 10–30
Multiple layered carbon nanotube Ag (silver) Water Faizal et al. [40]
20
2m 0.2 & 0.4 wt% 15
Water Yousefi et al. [39]
Multiple layered carbon nanotube Al2O3 Water Yousefi et al. [38]
10–30
0.2 & 0.4 wt%
Triton X100 Triton X100 N/A
2 m2
Unstable nanofluid samples without surfactant. At 0.4 wt%, good increase in performance found Nanofluid was found unstable without surfactant. Increased performance was less when compared withMWCNT There was about 37% in reduction of collector’s area. 2
Remarks Solar collector used Surfactant Concentration Size (nm) Nanoparticles
Type
Base fluid type Author
the losses to the surroundings should be minimised as much as possible. Additionally, the heat transfer rate from the absorber plate to the heat transfer fluid and from that heat transfer fluid to the end client must be enhanced [35]. In this regard, improvement of energy transport properties of the working medium can be achieved by utilizing nanofluids, hence maximising amount of energy is transferred to the end user [36,37]. Yousefi et al. [38] experimentally worked on the efficiency of flat plate collector by using multiple layers of carbon nanotube at a concentration of 0.2% and 0.4%, while the Triton X-100 were used as surfactant (used to reduce the surface tension in the fluid) and water nanofluids as the heat transfer fluid, whereas copper was used as the absorber plate in a frame of Aluminium with some coating elements. Headers and riser tubes were made of copper along with a 4 mm of glass cover which was used to build the solar collector. Also, an internal heat exchanger is embedded within a tank to assimilate the heat received by the solar collector system. The result revealed that the use of surfactant helped the nanofluids to be stable up to 10 days along with the increase in efficiency; hence the performance of the solar collector increases while the nanofluids dispersion that were formed without the use of surfactant were found to be unstable and separated quickly. These multiple layers of carbon nano-tubes resulted in great impact on the efficiency of solar collector with 0.4% concentration. In another work by Yousefi et al. [39], aluminium oxide nanoparticles with and without surfactant with the same concentration were used and the result obtained showed the increased performance but less than when compared with multiple layered carbon nanotube. A theoretical work on the reduction of the size of solar collector was investigated by Faizal et al. [40] on the same mass flow rate as used by Yousefi et al. [39]. He concluded that for higher mass flow rate, the efficiency of the collector increases even without surfactant but with the loss of output temperature. He added that the size of the collector can also be reduced up to 37% for the same output temperature of the fluid. Polvongsri and Kiatsiriroat [41] investigated the performance of the flat plate solar collector by using silver and water nanofluids. The particles of the silver concentrated with water at 1000 ppm and 10,000 ppm and the result showed an increase of thermal conductivity at 10,000 ppm along with an increased heat transfer rate when compared with the results for 1000 ppm Chaji et al. [42] worked on TiO2 and water nanofluids by utilizing Triton X-100 and Cetyl Trimethyl-lammonium Bromide as a surfactant. They observed that by using the surfactant there is a formation of foam within the nanofluids which ultimately reduced the efficiency of the collector, hence from that point the surfactant were not used and the prepared nanofluids was found to be stable for up to 4 h due to which the TiO2 nanofluid has lesser efficiency as compared to multiple layered carbon nanotube. Vijayakumaar et al. [43] experimented the same carbon based nanofluids & water nanofluids but with a single wall of 1 nm while nanofluids having different weight fractions and mass flow rates. They observed the results and conclude that 39% efficiency has been achieved with a single walled carbon nanotube & water nanofluids which was found to be in a good efficiency range. The thermo-physical properties of ethylene glycol & water and water alumina nanofluids was obtained by Said et al. [44]. The characterization was done for different volume concentrations of nanofluids for both the solutions and obtained results showed that the stability of alumina/water nanofluids has better efficiency than ethylene glycol/water nanofluids. Colangelo et al. [45] worked on the stability of nanofluids by diffusing various nanoparticles like Aluminium Oxide, Zinc Oxide and Iron Oxide. Out of these three, Aluminium Oxide was selected due to higher stability of its nanofluid. Two different flat plate solar collectors were built with transparent tubes with header and riser diameter of 22 mm and 10 mm respectively
2 m2
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Table 1 Previous work done on the use of nanofluids in flat plate solar collectors.
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Please cite this article as: R. Dobriyal, P. Negi, N. Sengar et al., A brief review on solar flat plate collector by incorporating the effect of nanofluid, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.294
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and installed at an angle of 30°. The result were obtained at a concentration of 3% nanoparticles of aluminium oxide, there was sharp increase of thermal conductivity by 6.7%, also, a 25% increase in the convective heat transfer coefficient was observed. Michael & Iniyan [46] led investigations to study the impact of utilizing copper oxide/water nanofluid as the working medium on the performance of flat plate solar collector. Both natural & forced circulation was used during the operation and a volumetric concentration of 0.05% of CuO was utilized. The maximum possible efficiency of 6.3% was observed when natural circulation was utilized as compared to the forced circulation. From this research, it tends to be seen that the productivity of the solar collector was lower than it was when carbon nano-structured based nanofluids were utilized. Previous works on the utilization of nanofluids in flat plate solar collectors are summarized in Table 1.
4. Conclusion Demand for non-convectional energy is on rise and solar energy is perhaps the most suitable option. It is also expected that by 2030 solar energy will cover the demand of newly built houses. Therefore cheap and efficient technology for capturing solar energy will be on rise. Flat plate solar collectors have been in use for this purpose and also are a promising alternative for future since they are cheap and simple in construction. This review article has discussed the technology behind these collectors and traced their development including design and operating aspects such as absorber plate designs, use of novel materials for absorber plate, use of nanofluids for enhancing the heat transfer, considering the ways to reduce heat loss to the surroundings. The use of nanofluids in flat plate collectors enhances the performance of the collector. Flat plate collectors have successfully captured the interest of researchers for decades and as a result have gone through significant improvement in more than one way. With the increase in demand for solar energy in the future it can be expected that flat plate collectors will play an important role in satisfying the global energy needs.
5. Recommendations To improve the flat plate solar collector, a lot of experimental and computational research has been conducted. Effect of changes in various operating and design parameters on the performance of flat plate collector has been evaluated, yet as for any other field of research ample scope remains for future progress. New materials can be explored for developing absorbing plate that offer better absorptance and have higher thermal conductivity. Flow turbulence in the tubes can be increased for better heat transfer. One way of doing this is by carefully designing the inner sides of the tubes or by optimizing the tube geometry. With advancement in nanotechnology, use of nanofluids in flat plate collector is rapidly gaining popularity and may likely become a norm in future due to the overall increase in the thermal conductivity of the working fluid it provides. Another future challenge is to integrate solar collectors into the buildings. They could be made a part of roof, wall or window, in this way they can absorb solar energy and also provide shade at the same time.
CRediT authorship contribution statement Ritvik Dobriyal: Writing - original draft, Conceptualization. Prateek Negi: Methodology, Resources. Neeraj Sengar: Data curation, Formal analysis. Desh Bandhu Singh: Writing - review & editing.
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[28] W.S. Lee, B.R. Chen, S.L. Chen, Latent heat storage in a two-phase thermosyphon solar water heater, J. Sol. Energy Eng. 128 (1) (2006) 69– 76. [29] V.V. Tyagi, N.L. Panwar, N.A. Rahim, R. Kothari, Review on solar air heating system with and without thermal energy storage system, Renew. Sustain. Energy Rev. 16 (4) (2012) 2289–2303. [30] M.M. Khan, N.I. Ibrahim, I.M. Mahbubul, H.M. Ali, R. Saidur, F.A. Al-Sulaiman, Evaluation of solar collector designs with integrated latent heat thermal energy storage: a review, Sol. Energy 166 (2018) 334–350. [31] G. Martinopoulos, D. Missirlis, G. Tsilingiridis, K. Yakinthos, N. Kyriakis, CFD modelling of a polymer solar collector, Renewable Energy 35 (2010) 1499– 1508. [32] D. Missirlis, G. Martinopoulos, G. Tsilingiridis, K. Yakinthos, N. Kyriakis, Investigation of the heat transfer behaviour of a polymer solarcollector for different manifold configurations, Renewable Energy 68 (2014) 715– 723. [33] A.C.D. Mintsa, M. Medale, C. Abid, Optimization of the design of a polymer flat plate solar collector, Sol. Energy 87 (2013) 64–75. [34] J.L.P. Dela, R. Aguilar, Polymer solar collectors: A better alternative to heat water in Mexican homes, Energy Procedia 57 (2014) 2205–2210. [35] F.S. Javadi, R. Saidur, M. Kamalisarvestani, Investigating performance improvement of solar collectors by using nanofluids, Renew. Sustain. Energy Rev. 28 (2013) 232–245. [36] X.Q. Wang, A.S. Mujumdar, Heat Transfer Characteristics of nanofluids: a review, Int. J. Therm. Sci. 46 (1) (2007) 1–19. [37] L. Godson, B. Raja, D.M. Lal, S. Wongwises, Enhancement of heat transfer using nanofluids – an overview, Renew. Sustain. Energy Rev. 14 (2) (2010) 629–641.
[38] T. Yousefi, E. Shojaeizadeh, F. Veysi, S. Zinadini, An experimental investigation on the effect of ph variation of MWCNT-H2O nanofluids on the efficiency of a flat plate solar collector, Sol. Energy 86 (2) (2012) 771–779. [39] T. Yousefi, F. Veisy, E. Shojaeizadeh, S. Zinadini, An experimental investigation on the effect of MWCNT-H2O nanofluid on the efficiency of flat-plate solar collectors, Exp. Therm. Fluid Sci. 39 (2012) 207–212. [40] M. Faizal, R. Saidur, S. Mekhilef, Potential of size reduction of flat-plate solar collectors when applying MWCNT nanofluid, IOP Conf. Series: Earth Environ. Sci. 16 (2013) 012004. [41] S. Polvongsri, T. Kiatsiriroat, Enhancement of flat-plate solar collector thermal performance with silver nano-fluid, Paper presented at the Second TSME International Conference on Mechanical Engineering, 2011. [42] H. Chaji, Y. Ajabshirchi, E. Esmaeilzadeh, S.Z. Heris, M. Hedayatizadeh, M. Kahani, Experimental study on thermal efficiency of flat plate solar collector using TiO2-water nanofluids, Modern Appl. Sci. 7 (10) (2013). [43] S.C. Vijayakumaar, R.L. Shankar, K. Babu, Effect of CNT-H2O nanofluids on the performance of solar collector – an experimental investigation, Paper Presented at the International Conference on Advanced Nanomaterials and Emerging Engineering Technologies, New Delhi, India, 2013. [44] Z. Said, M.H. Sajid, M.A. Alim, R. Saidur, N.A. Rahim, Experimental investigation of the thermophysical properties of Al2O3-nanofluid and its effect on a flat plate collector, Int. Commun. Heat Mass Transfer 48 (2013) 99–107. [45] G. Colangelo, E. Favale, A. Risi, D. Laforgia, A new solution for reduced sedimentation flat panel solar collector using nanofluids, Appl. Energy 111 (2013) 80–93. [46] J.J. Michael, S. Iniyan, Performance of copper oxide/water nanofluids in a flat plate solar water heater under natural and forced circulations, Energy Convers. Manage. 95 (2015) 160–169.
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A brief review on solar flat plate collector by incorporating the effect of nanofluid Ritvik Dobriyal, Prateek Negi, Neeraj Sengar, Desh Bandhu Singh ⇑ Mechanical Engineering Department, Graphic Era Deemed to be University, Bell Road, Clement Town, Dehradun 248002, India
a r t i c l e
i n f o
Article history: Received 31 October 2019 Received in revised form 12 November 2019 Accepted 27 November 2019 Available online xxxx Keywords: Flat plate collector Phase change materials Solar power Nanofluids Collector efficiency
a b s t r a c t The design, analysis and installation of solar collectors are the need of time as its application in the existing systems will reduce the use of conventional source of energy which is limited. On other hand, renewable energy source is unlimited and it will continue to exist till the existence of life on the planet earth. The main objective of solar collectors is to absorb heat from solar energy for increasing the temperature of fluid flowing through the solar collector and this heated fluid can be used for different applications namely heating to room in colder region, the heat of fluid can be utilized in cement industry and many other similar applications. This article attempts to provide an overview of the various techniques and improvements which allows the flat plate collectors to absorb as much solar radiation as possible while minimizing losses to the surroundings. It has been observed that the use of nanofluid enhances the performance of solar collectors. At last, recommendations have been provided. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Mechanical and Energy Technologies.
1. Introduction
2. Solar flat plate collector
Since fossil fuels such as coal, petroleum and natural gas, which are the main source of our energy today are exhaustible and polluting, therefore there is a need to migrate towards non-conventional sources of energy. Sun is a perpetual source of energy and the amount of energy it can provide is far in excess of the current global demand. Solar radiation can be directly converted into electricity using photovoltaic (PV) which convert light into electric current using the photoelectric effect or solar radiation can also be indirectly converted into electricity by using concentrated solar power (CSP). CSP uses lens or mirrors to collect solar radiation over a large area and concentrate it in a narrow beam. This concentrated heat can then be used to drive a conventional power plant (such as a steam turbine). Solar energy can also be harnessed as thermal energy using solar thermal collectors and can be used for a variety of purposes such as solar drying, cooking, distillation, heating water etc. Amongst the solar thermal collectors the most common are flat plate collectors. The current article contains an overview of the research aimed at development and enhancement of flat plate collectors.
Solar flat plate collector is a type of solar collector which is used to intercept and absorb solar radiation for heating a fluid. The basic idea behind a solar flat plate collector is simple. Solar radiation is used to heat up a dark flat surface known as absorber sheet or plate which converts the solar radiation energy into heat energy. This heat is then transferred to the fluid flowing through the pipes attached to the absorber sheet. The absorber sheets are made up of high thermal conductivity metals such as copper, aluminium or steel and are painted with selective coatings to maximise the absorption of incident solar radiation. The tubes or ducts carrying the fluid are either integral or attached to the absorber sheets. Cover sheets or plates, also known as glazing allows the high frequency solar radiation to pass through and reach the absorber plate but prevents the low frequency radiation from the absorber plate from escaping. It also prohibits surrounding cool air from flowing over the absorber plate and thus minimises heat loss due to convection. Material commonly used for this purpose is glass. Some plastic materials can also be used since they are cheaper and have higher transmittance, however are not as durable. Insulating material (such as fibreglass) is placed at the back and sides of the absorber plate for sealing and reducing heat loss (Fig. 1).
⇑ Corresponding author. E-mail address: [email protected] (D.B. Singh).
https://doi.org/10.1016/j.matpr.2019.11.294 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Mechanical and Energy Technologies.
Please cite this article as: R. Dobriyal, P. Negi, N. Sengar et al., A brief review on solar flat plate collector by incorporating the effect of nanofluid, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.294
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Fig. 1. Schematic diagram of a flat plate collector showing its different components.
Advantage of flat plate collectors is that they have a simple design which favours ease of manufacture. Other than cheap manufacturing they require little maintenance. They can collect energy from both beam as well as diffused radiation and are usually permanently fixed requiring no sophisticated positioning or mounting system. A liquid (water or oil) or air can be used as a fluid for flat plate collectors. Usually water is a common choice due to its ease of availability, high heat capacity, incompressibility and high mass density. One disadvantage of water is that it can freeze during periods of low temperature and may cause damage to the collector plate. Under these conditions antifreeze mixtures can be used. Phase-change liquids can also be used. They have a lower boiling point and hence eliminate the problem of freezing. Flat plate collectors are mainly used in residential buildings to cope up with the demand for hot water and are an alternative to conventional geysers. They can also be used for space heating of buildings with limited supply of electricity.
3. Development of flat plate collector Development of flat plate collector has focused on its effectiveness of trapping of sun’s radiation and enhancing its thermal efficiency. Many researchers have worked and are still working with different design and operating parameters as well as material selection to achieve a superior performance. 3.1. Mini channel Mansour [1] worked on collector having mini channel square array embedded in the absorber plate, which was covered by a glass plate. The pressure drop in the working fluid as well as heat transfer characteristics was studied using a CFD analysis. Heat removal from the absorber plate was found to be higher in comparison with the convectional collectors. However, maintaining higher thermal performance at higher flow rates required higher power input to the circulating pump. Robles et al. [2] experimentally investigated an aluminum based mini channel collector. Its performance was compared against a conventional collector under same operating conditions. An increase of thermal efficiency of 13% was obtained. In a recent study by Hota et al. [3], a back plate was added to the collector having mini channel tubes to enhance performance. Since, as discusses, for mini channels higher pumping power may be required, the authors also optimized the width of back plate and the geometry of mini channel tubes to balance the performance with pumping power.
3.2. Micro channel Deng et al. [4] worked on flat plat collector having micro channel heat type array. This type of channel was constructed with thin aluminum sheets having micro groves to enhance heat transfer. These micro groves improved heat transfer, reliability, provided greater compressive strength, reduced cost and had lesser contact resistance than the conventional type. A new structure of flat plate collector was reported in which one large integrated wickless heat pipe for enhancement of leak avoidance and stability of solarheating side and water-cooling side was used [5]. 3.3. Absorber plate designs Design of absorber plate and the selective coating plays an active role in the performance of flat plat collector for absorbing solar radiation. Jyothi et al. [6] developed a nanostructured tandem absorber especially for high temperature solar thermal applications. The performance of this tandem absorber was estimated by cyclically heating it in air as well as in vacuum. It showed high thermal stability for both short (2 h) and long terms (100 h–400 h), thus proving itself to be usable for high temperature solar thermal power generation. Del Col et al. [7] developed a special type of glazed flat plate collector, which was made of roll bond aluminum absorber plate. The study was performed both experimentally and numerically. These collectors helped in customizing and optimizing the flow of working fluid in the absorber leading to a improved and uniform temperature distribution across it. However, such absorbers are difficult to manufacture and are neither cost effective. El-Sawi et al. [8] came up with a chevron pattern absorber based on a continuous folding technique developed by Elsayed and Basily [9]. Experimental comparison between the conventional flat plate and chevron plate collector showed that the latter performed better under the considered operating conditions. Theoretical analysis also pointed towards a superior thermal efficiency of chevron plate collector. 3.4. Heat loss reduction Performance of a solar collector can be reduced due to heat loss from its various regions. Kumar and Mullick [10] did experimental study on unglazed solar collector to determine the coefficient of heat transfer from the upper region of the collector in outdoor conditions. Vestlund et al. [11] performed a computational study with replacement of air with some inert gas (Argon, Krypton and Xenon) in between glass cover and absorber for better performance of the
Please cite this article as: R. Dobriyal, P. Negi, N. Sengar et al., A brief review on solar flat plate collector by incorporating the effect of nanofluid, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.294
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collector. The results indicate that with the use of inert gases, the collector can be made thinner with comparable performance or a better performance can be achieved. The main complication with this approach is that a sealed space is needed for inert gages and variation of pressure and volume of the gas needs to be controlled. Akhtar and Mullick carried [12] repeated out numerical investigations to study the convective heat transfer coefficient along with radiative heat transfer coefficient for single as well as double glazed flat plate collector. Factors such as incidence angle, thickness of glass cover, declination, location and orientation of collector were taken into consideration. Zhang et al. [13] developed experimental set up to study the effect of heat shield in an evacuated tube solar collector. It was found that by employing a heat shield the thermal efficiency could be increased by as much as 54.07%. Beikircher et al. [14] worked on improving the insulation of flat plate collector from front and the back side. For reducing the heat loss from the front side of the collector, they made use of transparent insulation materials. They developed and experimentally investigated four types of transparent insulation materials. Out of these four, promising performance was delivered by single ETFE (ethylene tetrafluorethylene) film and double FEP (florinated ethylene-propylene) film. For preventing heat losses from the back side of the collector they employed vacuum super insulation (VSI) [15]. The combination of VSI with double FEP film was found to have the best performance. However, it was estimated that the production cost for incorporating VSI would be higher. Henshall et al. [16] demonstrated that the efficiency of a flat plate collector can be improved if housed in an evacuated enclosure. They even discussed the design of an enclosure for this purpose. A major concern is the stress on the enclosure due surrounding atmospheric pressure and thermal expansion. However, they performed a finite element analysis and realized that with proper mechanical design and selection of materials durable enclosures can be realized. Bhowmik and Amin [17] experimentally studied a flat plate reflector fitted with additional reflector plates. The reflectors are helpful in concentrating both the direct and diffused radiation on the collector plate, thus improving its performance. An increase in efficiency of 10% was reported by the authors.
improved efficiency by using PCM when compared with conventional system. In an another experimental work done by Xue [22]to characterize the Ba(OH)28H2O PCM, the solar collector used was of evacuated type while the PCM has melting temperature of 81.8 °C. The results showed an improvement in the performance of collector system. Molten salt was considered to be the most cost effective and commercially used energy storage material due to its high melting temperature [23]. Hamed and Brahim [24] developed a simple theoretical model to study the effect of PCM on flat plate collector. They found that due to incorporation of PCM there was a decrease in outlet temperature of water during the charging period and an increase in outlet temperature during the discharging period. The increase was greatest for higher inlet temperature and greater PCM thickness. Hamed et al. [25] theoretically studied flat plate collector with PCM (paraffin) installed below the absorber plate. Two different types of paraffins were installed with varying thickness. It was found that the best outlet temperatures for heat transfer fluid (water in this case) were for low thickness of PCM. Kabeel et al. [26] experimentally compared a flat plate collector and a vcorrugated plate collector having paraffin as the PCM. An increase in efficiency up to 12% was observed for v-corrugated plate collector with PCM as compared to the one without PCM. Also, an increase in efficiency of 21.3% was observed as compared to the conventional flat plate collector. Paraffin seems to be the most commonly used PCM in research. However, other materials such as eutectic mixture of salt hydrates and sodium acetate have also been used [27,28]. A thorough discussion on the different types of PCM used in flat plate collectors can be found in Tyagi et al. [29]. In addition, recent research on using nanoparticles in PCM is also picking up pace, Khan et al. [30]. In general, it is observed that selecting the thickness of PCM has an important effect on the efficiency of collector. Thicker PCM may provide greater performance for hotter regions. Another important aspect is to maximize the contact area between the absorbing plate and the PCM. For this purpose fins/metal foams are found to be beneficial.
3.5. Phase change material
Martinopoulos et al. [31] employed a low cost and lightweight polymer solar collector. In their study they performed both experimental investigations and CFD analysis. They could achieve a 50% reduction in weight and a low material cost with its help. Missirlis et al. [32] also used CFD analysis to study the heat transfer characteristics of polymer solar collector for various geometries. The author explored various behaviors of heat transfer of flat plat collector for different manifold configurations. This CFD model was employed to enhance the thermal behavior and flow field development of the collector. Mintsa et al. [33] performed numerical simulations to study the effect of both design parameters as well as operating parameters on polymer solar collector. It was found that the efficiency was not much influenced by the length of the collector plate, however, the air gap and the inlet temperature had a strong effect on it. Dela and Aguilar [34] analyzed a polymer solar collector developed by Modulo Solar (a Mexican company). They found that the thermal behavior of polymeric solar collector was comparable to metallic collector for ordinary use of home applications. Also, it was found that due to greater flexibility of polymers, they can resist low environmental temperatures.
Storage of solar energy is a challenge. The storage tanks used for storing solar energy are currently bulky and expensive. Phase change materials (PCM) if integrated in solar collectors may reduce or eliminate the need of separate energy storage. Chen et al. [18] performed computational analysis of a flat plate collector having integrated aluminum foam pours structure filled with paraffin as the phase change material. When the sunlight is available, paraffin can absorb solar energy while during night the stored energy can be transferred to liquid contained in the capillary tubes embedded in the paraffin. Phase change material has the advantage of taking less space with higher energy storage capacity when compared with sensible heat storage material. PCM based solar collector was also investigated by Khalifa et al. [19]in Iraq by using paraffin wax having melting temperature in the range of 46–47 °C. It was used to absorb the waste heat of the collector system. The results showed efficiency within the range of 45% to 54% in the month of March due to high energy storage and provided a promising and alternate solution for the improvement of collector systems. However, with this system there is dependency on the climatic conditions. Many other investigation were done using paraffin wax [20,21] to study the performance of the solar collector with different inclination angle viz. 45°, 10°, 20°, & 30° respectively. Both the results showed an
3.6. Polymer collector
3.7. Nano fluids In order to improve the efficiency of solar collector, the energy absorbed by the collector from the sun should be maximised and
Please cite this article as: R. Dobriyal, P. Negi, N. Sengar et al., A brief review on solar flat plate collector by incorporating the effect of nanofluid, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.294
SDBS
1.0, 2.0 & 3.0 vol% 0.05 vol% Water
Colangelo et al. [45]
Michael & Iniyan [46]
CuO
45, 60, & 30, resp. 0.3 & 0.21 nm
Water & Ethyl Glycol (EG)/ Water 60:40 Water
Al2O3, ZnO& Fe2O3
13
20.8 1.05 m
Al2O3/Water nanofluid has better stability than others. At 3.0% Al2O3, thermal conductivity increased 607%. Higher enhancement in performance was found in natural circulation than forced circulation. A maximum increase of 6.3% in collector’s efficiency. N/A
N/A
0.34 m2
Polysorbate 80 No surfactant With 0.4, 0.5 & 0.6 wt% 0.05–0.1 vol% 1.0
Single walled carbon nanotube Al2O3 Water
0, 0.1, 0.2 & 0.3 wt% Water
20 TiO2
Water
Polvongsri and Kiatsiriroat [41] Chaji et al. [42]
Vijayakumaar et al. [43] Said et al. [44]
0.5 m 0.2 m
The increase in collector performance was less than that of MWCNT. Reduced efficiency than MWCNT. Surfactant were not used due to high foaming. Only 4 h of stability without surfactant. There was increase in collector performance at 0.5 wt% which was close to MWCNT. Water/Al2O3 has better stability than EG-water/ Al2O3 1.0 m 0.15 m
No surfactant No surfactant 0.1 & 1 wt%
0.2 & 0.4 wt% 10–30
Multiple layered carbon nanotube Ag (silver) Water Faizal et al. [40]
20
2m 0.2 & 0.4 wt% 15
Water Yousefi et al. [39]
Multiple layered carbon nanotube Al2O3 Water Yousefi et al. [38]
10–30
0.2 & 0.4 wt%
Triton X100 Triton X100 N/A
2 m2
Unstable nanofluid samples without surfactant. At 0.4 wt%, good increase in performance found Nanofluid was found unstable without surfactant. Increased performance was less when compared withMWCNT There was about 37% in reduction of collector’s area. 2
Remarks Solar collector used Surfactant Concentration Size (nm) Nanoparticles
Type
Base fluid type Author
the losses to the surroundings should be minimised as much as possible. Additionally, the heat transfer rate from the absorber plate to the heat transfer fluid and from that heat transfer fluid to the end client must be enhanced [35]. In this regard, improvement of energy transport properties of the working medium can be achieved by utilizing nanofluids, hence maximising amount of energy is transferred to the end user [36,37]. Yousefi et al. [38] experimentally worked on the efficiency of flat plate collector by using multiple layers of carbon nanotube at a concentration of 0.2% and 0.4%, while the Triton X-100 were used as surfactant (used to reduce the surface tension in the fluid) and water nanofluids as the heat transfer fluid, whereas copper was used as the absorber plate in a frame of Aluminium with some coating elements. Headers and riser tubes were made of copper along with a 4 mm of glass cover which was used to build the solar collector. Also, an internal heat exchanger is embedded within a tank to assimilate the heat received by the solar collector system. The result revealed that the use of surfactant helped the nanofluids to be stable up to 10 days along with the increase in efficiency; hence the performance of the solar collector increases while the nanofluids dispersion that were formed without the use of surfactant were found to be unstable and separated quickly. These multiple layers of carbon nano-tubes resulted in great impact on the efficiency of solar collector with 0.4% concentration. In another work by Yousefi et al. [39], aluminium oxide nanoparticles with and without surfactant with the same concentration were used and the result obtained showed the increased performance but less than when compared with multiple layered carbon nanotube. A theoretical work on the reduction of the size of solar collector was investigated by Faizal et al. [40] on the same mass flow rate as used by Yousefi et al. [39]. He concluded that for higher mass flow rate, the efficiency of the collector increases even without surfactant but with the loss of output temperature. He added that the size of the collector can also be reduced up to 37% for the same output temperature of the fluid. Polvongsri and Kiatsiriroat [41] investigated the performance of the flat plate solar collector by using silver and water nanofluids. The particles of the silver concentrated with water at 1000 ppm and 10,000 ppm and the result showed an increase of thermal conductivity at 10,000 ppm along with an increased heat transfer rate when compared with the results for 1000 ppm Chaji et al. [42] worked on TiO2 and water nanofluids by utilizing Triton X-100 and Cetyl Trimethyl-lammonium Bromide as a surfactant. They observed that by using the surfactant there is a formation of foam within the nanofluids which ultimately reduced the efficiency of the collector, hence from that point the surfactant were not used and the prepared nanofluids was found to be stable for up to 4 h due to which the TiO2 nanofluid has lesser efficiency as compared to multiple layered carbon nanotube. Vijayakumaar et al. [43] experimented the same carbon based nanofluids & water nanofluids but with a single wall of 1 nm while nanofluids having different weight fractions and mass flow rates. They observed the results and conclude that 39% efficiency has been achieved with a single walled carbon nanotube & water nanofluids which was found to be in a good efficiency range. The thermo-physical properties of ethylene glycol & water and water alumina nanofluids was obtained by Said et al. [44]. The characterization was done for different volume concentrations of nanofluids for both the solutions and obtained results showed that the stability of alumina/water nanofluids has better efficiency than ethylene glycol/water nanofluids. Colangelo et al. [45] worked on the stability of nanofluids by diffusing various nanoparticles like Aluminium Oxide, Zinc Oxide and Iron Oxide. Out of these three, Aluminium Oxide was selected due to higher stability of its nanofluid. Two different flat plate solar collectors were built with transparent tubes with header and riser diameter of 22 mm and 10 mm respectively
2 m2
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Table 1 Previous work done on the use of nanofluids in flat plate solar collectors.
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Please cite this article as: R. Dobriyal, P. Negi, N. Sengar et al., A brief review on solar flat plate collector by incorporating the effect of nanofluid, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.294
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and installed at an angle of 30°. The result were obtained at a concentration of 3% nanoparticles of aluminium oxide, there was sharp increase of thermal conductivity by 6.7%, also, a 25% increase in the convective heat transfer coefficient was observed. Michael & Iniyan [46] led investigations to study the impact of utilizing copper oxide/water nanofluid as the working medium on the performance of flat plate solar collector. Both natural & forced circulation was used during the operation and a volumetric concentration of 0.05% of CuO was utilized. The maximum possible efficiency of 6.3% was observed when natural circulation was utilized as compared to the forced circulation. From this research, it tends to be seen that the productivity of the solar collector was lower than it was when carbon nano-structured based nanofluids were utilized. Previous works on the utilization of nanofluids in flat plate solar collectors are summarized in Table 1.
4. Conclusion Demand for non-convectional energy is on rise and solar energy is perhaps the most suitable option. It is also expected that by 2030 solar energy will cover the demand of newly built houses. Therefore cheap and efficient technology for capturing solar energy will be on rise. Flat plate solar collectors have been in use for this purpose and also are a promising alternative for future since they are cheap and simple in construction. This review article has discussed the technology behind these collectors and traced their development including design and operating aspects such as absorber plate designs, use of novel materials for absorber plate, use of nanofluids for enhancing the heat transfer, considering the ways to reduce heat loss to the surroundings. The use of nanofluids in flat plate collectors enhances the performance of the collector. Flat plate collectors have successfully captured the interest of researchers for decades and as a result have gone through significant improvement in more than one way. With the increase in demand for solar energy in the future it can be expected that flat plate collectors will play an important role in satisfying the global energy needs.
5. Recommendations To improve the flat plate solar collector, a lot of experimental and computational research has been conducted. Effect of changes in various operating and design parameters on the performance of flat plate collector has been evaluated, yet as for any other field of research ample scope remains for future progress. New materials can be explored for developing absorbing plate that offer better absorptance and have higher thermal conductivity. Flow turbulence in the tubes can be increased for better heat transfer. One way of doing this is by carefully designing the inner sides of the tubes or by optimizing the tube geometry. With advancement in nanotechnology, use of nanofluids in flat plate collector is rapidly gaining popularity and may likely become a norm in future due to the overall increase in the thermal conductivity of the working fluid it provides. Another future challenge is to integrate solar collectors into the buildings. They could be made a part of roof, wall or window, in this way they can absorb solar energy and also provide shade at the same time.
CRediT authorship contribution statement Ritvik Dobriyal: Writing - original draft, Conceptualization. Prateek Negi: Methodology, Resources. Neeraj Sengar: Data curation, Formal analysis. Desh Bandhu Singh: Writing - review & editing.
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