Materials Today: Proceedings 45 (2021) 4931–4935

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Materials Today: Proceedings

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Thermal augmentation in evacuated tube solar collectors using

reflectors, nano fluids, phase change materials and tilt angle: A review
Sorabh Aggarwal a, Sushil Kumar b,⇑, Raj Kumar a,⇑, Robin Thakur a
a
Faculty of Engineering and Technology, Shoolini University, Solan, H.P, 173229, India
b
Department of Physics, Hansraj College, University of Delhi, India

a r t i c l e i n f o a b s t r a c t

Article history: This review presents influence of reflectors, tilt angle, phase change materials and nanofluids in solar
Available online 19 February 2021 evacuated tube solar collectors (ETSCs). In order to increase efficiency, various designs of solar ETSCs have
deduced that ETSCs integrated with reflectors are one of the most efficient and powerful designs. In order
Keywords: to increase the solar irradiation directed to the basin liner or the water as well as the efficiency of the
Solar water heater ETSCs, the reflectors are a nice and cheap upgrade. Reverse flow rate (RWR) measured using water tem-
Heat transfer enhancement perature measurements in solar tubes is dependent on collector tilt-angle. The RWR increases with
Phase change materials
increase in the collector’s tilt-angle. Phase change Materials (PCMs) has emerged as an option by serving
Tilt angle
Nano fluids
as thermal storage batteries to increase the efficiency of the ETSCs. The presence of nanofluids with
enhanced thermal, optical, and magnetic properties has become very common to efficiently use solar
energy by converting it into thermal.
Ó 2021 Elsevier Ltd. All rights reserved.
Second International Conference on Aspects of Materials Science and Engineering (ICAMSE 2021).

1. Introduction power assets, accordingly we need to search for the sustainable

power assets in light of the fact that the inexhaustible assets are
ETSCs comprise layers of translucent, parallel glass tubes sup- sans contamination, yet they are accessible in plenty and they
ported on a frame. Diameters of tube vary from 2.5 cm to 7.5 cm won’t wiped out, so it is exceptionally basic for us to catch the
and length varies from 1.5 m to 2.4 m based on the maker. Each inexhaustible source of energy as much as could reasonably be
tube has outer glass tube of high thickness and inner tube of low expected. Sustainable sources can be utilized in different manners
thickness glass. The inner tube is coated with a covering for like wind energy, solar energy, geothermal energy and so forth. It is
absorbing the solar energy and preventing the heat loss. The mate- prominent that the sun is the fundamental source of energy in this
rial of tubes is borosilicate or soda lime glass having properties of universe and any remaining types of energy like wind energy and
high temperature resistance and solar irradiation transmittance geothermal energy are likewise made by the sun. Thus it is astute
[1]. Fig. 1 depicts the schematic diagram of the ETSC. Creation of to utilize solar energy straightforwardly to satisfy our energy
vacuum the lead towards its nomenclature evacuated tubes. The needs. This article instructs us about utilizing solar energy and
vacuum created acts as an insulator leading to very small heat loss we have evaluated the different sorts of procedures utilized for
to the surroundings. water warming utilizing solar energy. Solar water heaters have
been utilized from endless years and since the time we began uti-
lizing solar energy to warm water, we experienced different issues
1.1. Comparative study of different types of evacuated tube collectors like non accessibility of sun during night. Subsequently it becomes
significant that we can store the energy got from the sun so it can
With the expanding populace, the interest for energy is likewise likewise be used during nights. Presently energy from the sun can
expanding step by step and it is preposterous to expect to satisfy basically be gotten in two different ways either as electrical energy
all these energy necessities with the non-environmentally friendly through PV cells or as Thermal energy through sun based thermal
frameworks. Electrical energy obtained from the sun (converting
⇑ Corresponding authors. solar to electrical) can be put away in the batteries, yet the thermal
E-mail addresses: sushil8207@gmail.com (S. Kumar), errajap@gmail.com energy got from the sun needs a protecting holder with the goal
(R. Kumar).

https://doi.org/10.1016/j.matpr.2021.01.371
2214-7853/Ó 2021 Elsevier Ltd. All rights reserved.
Second International Conference on Aspects of Materials Science and Engineering (ICAMSE 2021).
S. Aggarwal, S. Kumar, R. Kumar et al. Materials Today: Proceedings 45 (2021) 4931–4935

Fig. 3. U-pipe ETC.

effective and results in high thermal performance. Fig. 3 depicts

the ETC U-pipe geometry [3].

1.4. Heat pipe (HPS)

Fig. 1. Evacuated tube solar thermal collector.

that it tends to be put away and utilized at whatever point required It is a twofold stage framework with high conductivity and is
[2,3]. utilized with the end goal of heat transfer. A vaporizing liquid
and capillary wick structure are utilized. This process is based on
the evaporation–condensation cycle, which includes the evapora-
1.1.1. Types of ETC tion phase and condensation phase. The working fluid moves due
The general categories of ETC are: to natural circulation among the phases in order to transport the
necessary heat. Heat pipes in ETSCs must result in efficient trans-
 Thermosyphon portation of heat flow and high heat conduction. Recently various
 U-pipe studies have utilized HPS in solar collectors. Fig. 4 displays the
 Heat pipe ETSC ETC heat pipe [2].
The aim of current study is to review the studies on heat trans-
fer enhancement in ETSCs using Reflectors, Nano fluids, Phase
1.2. Thermosyphon
Change Materials, Tilt angle and to provide the significant informa-
tion about crucial for optimum heat enhancements in ESTCs. The
The Thermosyphon collector contains single-ended immersed
above study will be useful for researchers to choose optimum
tubes, which are directly attached to a horizontal tank. The ETC
geometry for their future work.
incorporates two tubes, which are insulated due to vacuum
between them and coating on the inner tube’s outer surface. Nor-
mal water convection through the single-ended tube is the mech- 2. Technical developments in ETC SWH
anism of heat transfer here. Solar radiation also allows the water
flow to be heated, then increases it to the storage along the top Several problems have been experienced over the years when
of the pipe. Then the water is finally replaced with cooler water. using the SWHs. Solar radiation is available for a very short time,
In the last decade, the efficiency of SWHs and the performance of so maximum solar energy needs to be collected when available,
numerical works have been experimentally tested by several and this captured solar energy needs to be stored for use at night
researchers. Fig. 2 displays water in ETC glass [2]. when there is no light. In this respect, many technological develop-
ments have been made to allow optimal use of SWHs, as shown
below:
1.3. U-pipe evacuated tubular solar collectors (UPETSCs)

The UPETSCs is being progressively applied to the solar heating

system, called the evaporator draft. The HTF temperature range is
commonly indicated to be below the ambient air range [3]. The U-
pipe, a common type of heat exchanger in the ETCs, is cost-

Fig. 2. Shows the water in glass ETC. Fig. 4. Shows the heat pipe ETC.

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S. Aggarwal, S. Kumar, R. Kumar et al. Materials Today: Proceedings 45 (2021) 4931–4935

 Reflectors
 Tilt angle
 Phase change materials
 Nano Fluids

2.1. Reflectors

We need to increase the area of the collector to absorb the full

solar radiation, for which reflectors can be of great benefit because
the solar rays that do not occur on the collector can be made to fall
on it using reflectors, thereby increasing the area of heat capture
without substantial expenditure. Elfadl et al. have studied varia-
tion in the thermal energy output by the integrating reflectors with
an ETSC-heat pipe (ETSC-HP) device. Three separate cases are being
investigated; a reflector mounted on the collector’s bottom surface,
on the collector’s top surface, and two reflectors on the collector’s
top and bottom surfaces. The results show that the use of reflectors Fig. 6. (a) ETC without CPC (b) ETC with CPC.
with ETSC-HP increases performance of the system by approxi-
mately 16 percent in comparison to collector without reflectors.
plate collectors. Heat loss because of reverse flow was very small,
Fig. 5 represents ETC with upper and lower reflectors. P. Felinski
approximately 8–10 percent of the gross heat loss. Tang et al. [7]
and R. Sekret [5] tested the output of an ETSC-HP fitted with a com-
have investigated variation of thermal efficiency of solar systems
pound parabolic concentrator (CPC) with phase change material
with collector tilt angle. They have also studied characteristics of
(PCM). The findings reveal rapid increase in the temperature of
water flow inside solar tubes.
paraffin especially during and after paraffin melting. In addition,
Results show that the collector tilt-angle of SWHs had a major
the utilization of CPC in optimized ETSCs with PCM increased
impact on a system’s consistent collection of radiation. The collec-
charging efficiency by 5% and overall charging efficiency by 9%.
tor tilt-angle influences solar heat gain but has negligible impact
Fig. 6(a & b) ETC without CPC and ETC without CPC, respectively.
on the heat removal and the efficiency of the day-to-day solar ther-
Milani and Abbas [6] have attached the diffuse flat reflector
mal shift. Tang et al.[8] have investigated thermal augmentation of
(DFR) with an ETC array for enhancing the heat capture rate for col-
water-in-glass evacuated tube SWH. They found that the rate of
lectors of evacuated tubes. They created an STC model for estima-
the reverse flow increases with the collector’s tilt-angle. Compared
tion of energy savings annually.
to that of a thermosyphonic domestic SWH, the reverse stream in
Four metro cities of Australia representing 4 major solar zones
the SWH was much greater. The inverse stream in SWHs results in
have implemented the created model. For these four regions, the
small heat loss from collectors and is around one tenth of the entire
tilt and azimuth angles are optimized. Together with the inclusion
heat losses.
of DFR, this optimum environment increases STC privileges by
The efficiency of water-in-glass evacuated tube SWHs was
14.6, 20.2, 25.9 and 27.9 percent for zone 1, 2, 3 and 4 respectively.
experimentally tested by Zhang et al. [9]. They noticed that the
Table 1 shows ETC’s increase in performance by integrating
thermal efficiency of SWHs was affected by the solar collector’s
reflectors.
power and heat loss from the storage tank. Tang et al. [10] mea-
sured the regular accumulation of radiation from all-glass evacu-
2.2. Tilt angle ated solar tube collectors on a single tube based on solar
geometry by developing a mathematical method. The results show
Reverse flow rate (RWR) measured using water temperature that the twelve-monthly collectible radiation is influenced by var-
measurements in solar tubes is dependent on collector tilt-angle. ious parameters such as collector type and spacing between adja-
Previous studies have revealed higher RWR in the SWH in compar- cent tubes, solar tube’s size, the angle of tilt and azimuth, site’s
ison to thermosyphonic domestic solar water heaters with flat- latitude and various environmental conditions.
The utilization of DFR will greatly enhance collectors’ energy
collection. The optimum tilt angles for the thermosyphonic SWH
were investigated by annual solar fraction [11]. The expedient
energy composed by the device is substantially greater than the
load energy during the summer, specifically in the case of a collec-
tor with an area of 30000 cm2 or larger. Table 2 demonstrates
enhancement in ETC performance with tilt angle.

2.3. Phase change materials

Phase change materials (PCM) release/absorb adequately due

at phase transition .The phase transition is from solid to liquid
and vice versa to provide useful heat. PCM can help in storing
the excessive thermal energy during peak hours and release this
stored energy when this energy is required. CaCl2.6H2O (calcium
chloride hexahydrate), sodium acetate trihydrate mixtures, paraf-
fin wax etc. are different PCM that can be used. The performance
of a domestic SWH with a ETSC using PCM (DSWHSCPHES) is
investigated by H. Sheng Xue [12]. The efficiency of the exposed
Fig. 5. ETC with upper and lower reflectors. DSWHSCPHES is worse than conventional solar water-in-glass
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S. Aggarwal, S. Kumar, R. Kumar et al. Materials Today: Proceedings 45 (2021) 4931–4935

Table 1
Efficiency enhancement of ETC by integrating reflectors.

Author Collector Type Integration type Studied parameter Results Study type
Elfadl et al. [4] ETC with heat pipe Upper and lower reflectors Thermal efficiency Increased by Experimental
15.68%
Felinski and ETC with heat pipe PCM in heat pipe evacuated tube collector equipped with a maximum charging Increased by Experimental
Sekret [5] compound parabolic concentrator efficiency 9%
Milani and Etc with heat pipe, U pipe diffuse flat reflector (DFR) at the back of ETC array Extra Energy 16.7%energy Experimental
Abbas [6] and water in tube saving is saved

Table 2
Effect of tilt angle in efficiency enhancement of ETC.

Author Year Major findings

Tang et al. [7] 2011 Performance of two identical evacuated tube collector solar water heater was compared with tilt angles as 22° and 46° with the horizon
Tang et al. [8] 2014 Evaluated the thermal efficiency of ETC with water in glass type, where the higher angle of tilt causes enhancement in reverse flow rate
Zhang et al. [9] 2014 Found that in china there is no effect of tilt angle on the performance of SWH
Tang et al. [10] 2009 Mathematical model was formed for ETC collectors to determine optimum tilt angles for optimum utilization of sun radiation in china and
the study revealed that with latitudes above 30°, the best tilt angle should be about 10° lower than latitude of the sites.
Shariah et a. [11] 2002 Best tilt angle was evaluated for Solar water heaters with thermosyphon effect

ing heat pipes (OHP) to enhance thermal efficiency. In daytime,

PCM can effectively minimize CE fluctuations in high and low
intense solar radiations. They found 30% fewer CE fluctuations
using PCM. The EWT with PCM has sustainability above 50° C.
Felinski et al. [16] have developed evacuated tube collector/storage
device that uses paraffin as the PCM. They found that the gross
heat received from the paraffin combined ETC/S device increased
by 45–79 percent compared to the ETC. With a constant stream
volume, DSWHSCPHES also performs more effectively than under
the condition of exposure. Fig. 7 displays a photographic view of
ETCSWH’s experimental setup. Table 3 demonstrates the increase
of ETC with PCM in thermal efficiency and thermal energy.

Fig. 7. Photographic view of experimental set up of ETCSWH.

tube heaters (TWGETSWH) with an equivalent collector area.

Yongtai et al [13] have designed a heat storage ETSC intubated with
heat storage tubing was developed by They found that mean con-
version efficiency reached 56.9 percent and 48.46 percent in paral-
lel and series–parallel combinations respectively. The parallel and
series–parallel prototype have 10 percent and 8 percent higher
conversion efficiency respectively in comparison to non-heat stor-
age prototypes. These prototypes have better heating time. Ther-
mal efficiency of a compact solar collector based on a standard U
pipe ETC combined with PCM as thermal energy storage was stud-
ied by Abokersh et al. [14]. The system is tested in two forms,
namely un-finned and finned U-pipe ETC. Under the same operat-
ing and weather conditions, the performance of the two forms is
compared side by side with FSWHS. Wu et al. [15] produced a Fig. 8. Photographic view of PCM/NCPCM containers and their integration with the
SWHS using PCM for thermal energy storage and inserted oscillat- water storage tank.

Table 3
Enhancement in thermal efficiency and thermal energy of ETC with PCM.

Author Collector Type Integration Studied parameter Results Study type

H. Sheng Xue [12] ETC with U Tube inside U tube ETSC with PCM Collector thermal efficiency 7% increase Experimental
Yongtai et al. [13] ETC with U Tube inside 11% increase Experimental
Abokersh et al. [14] ETC with U Tube inside 13% increase Experimental
Wu et al. [15] ETC with Heat pipe Heat pipe ETSC with PCM Collector output thermal energy 24.6% increase Experimental
Felinski and Sekret [16] ETC with Heat pipe 45–79% increase Experimental

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S. Aggarwal, S. Kumar, R. Kumar et al. Materials Today: Proceedings 45 (2021) 4931–4935

Table 4
Enhancement in the efficiency of SWH using different nano materials.

Author Type of Nanofluid Results

ETSC
Mahbubul et al. [17] Heat pipe Water/SWCNT 10% Improvement in efficiency by operating ETSC with 0.2 vol% nanofluid
Kaya and Arslan [18] U-pipe Water-Ethylene glycol /MgO, 26% increase in collector efficiency by using water-ethylene glycol/Ag nanofluid as
Ag & ZnO compare to pure working fluid
Ozsoy and Corumlu [19] Heat pipe Water/silver 20% enhancement in the collector efficiency of ETC SWH as compared with the pure water
Dehaj and Mohiabadi [20] Heat pipe water /MgO Enhancement in the collector performance using MgO nanofluid

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Declaration of Competing Interest Amarnath, Experimental analysis on solar water heater integrated with Nano
351 composite phase change material (SCi and CuO), Materials Today:
roceedings. DOI: https://doi.org/10.1016/j.matpr.2020.05.093.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.

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