Progress and latest developments of evacuated tube solar collector

Progress and latest developments of evacuated tube solar collectors
M.A. Sabiha a
, R. Saidur b,n
, Saad Mekhilef c
, Omid Mahian d
a
Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
b
Center of Research Excellence in Renewable Energy (CoRE-RE), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia
c
Power Electronics and Renewable Energy Research Laboratory (PEARL), Department of Electrical Engineering, University of Malaya,
50603 Kuala Lumpur, Malaysia
d
Department of Mechanical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
a r t i c l e i n f o
Article history:
Received 12 May 2014
Received in revised form
12 May 2015
Accepted 7 July 2015
Keywords:
Solar energy
Evacuated tube solar collector
Efficiency
Working fluids
Challenges
a b s t r a c t
Solar energy is the most available, environmental friendly energy source and renewable to sustain the
growing energy demand. Solar energy is captured by solar collectors and an evacuated solar collector is
the most efficient and convenient collector among various kinds of solar collectors. In this paper, a
comprehensive literature on why evacuated collector is preferable, types of evacuated collectors, their
structure, applications and challenges have been reviewed. Latest up to date literature based on journal
articles, web materials, reports, conference proceedings and thesis have been compiled and reported.
Applications of evacuated solar collectors in water heating, heat engines, air conditioning, swimming
pool heating, solar cooker, steam generation and solar drying for residential and industrial sectors have
been summarized and presented. Collector efficiency of different types of evacuated collectors and their
performance based on different working fluids have been reported as well. Based on the available
literature, it has been found that an evacuated tube collector has higher efficiency than the other
collector. An evacuated tube collector is also very efficient to be used at higher operating temperature.
There are few challenges that have been identified and need to be addressed carefully before installing
an evacuated tube solar collector. However, after critically analyzing the available literature, authors have
presented some future recommendations to overcome the barriers and for enhanced performance of an
evacuated tube solar collector.
& 2015 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1039
2. Evacuated tube solar collector (ETSC). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1039
2.1. Why an evacuated tube solar collector (ETSC) is preferable? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1039
2.2. Types of ETSC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1040
2.2.1. Single walled glass evacuated tube. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1040
2.2.2. Dewar tube. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1041
2.3. Mathematical modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1041
2.4. Applications of ETSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1043
2.4.1. Domestic applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1043
2.4.2. Industrial applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1046
3. Challenges of using evacuated tube solar collectors (ETSCs). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1050
3.1. Cost and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1050
3.2. Fragility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1050
3.3. Snow removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1050
3.4. Overheating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1050
4. Economic consideration on the usage of ETSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1050
5. Performance based on working fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1051
6. Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1051
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/rser
Renewable and Sustainable Energy Reviews
http://dx.doi.org/10.1016/j.rser.2015.07.016
1364-0321/& 2015 Elsevier Ltd. All rights reserved.
n
Corresponding author. Tel.: þ966 13 860 4628; fax: þ966 13 860 7312.
E-mail addresses: saidur@kfupm.edu.sa, saidur912@yahoo.com (R. Saidur).
Renewable and Sustainable Energy Reviews 51 (2015) 1038–1054

7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1051
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1052
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1052
1. Introduction
The most available source of renewable energy on earth is solar
energy as the earth receives millions of watts of energy everyday
coming from solar radiation. However, only a fraction of it in the
form of day lighting and photosynthesis is used by the natural
world, one third is reflected back into space and the rest is
absorbed by land, oceans and clouds. Thus, it is very reasonable
to collect solar energy and utilize it efficiently to generate electric
power, heat and also for cooling purposes in a viable way. The
effect of using solar energy on the environment for a variety of
applications is minimal as it produces no harmful pollutants.
Besides environmental consciousness, dwindling of traditional
energy sources marks solar energy as the appropriate energy
source to meet the increasing demand of energy worldwide.
Researchers have investigated and developed technologies on
how to harvest solar energy to serve human beings and are still
considering new technologies to maximize the collection and
utilization of solar energy [1].
There are particular challenges in the effective collection and
storage of solar energy though it is free for taking. As solar
radiation is only available during daytime, the energy must be
collected in an efficient manner to make use of most of the
daylight hours and then must be stored. Solar thermal collectors
are the existing components to capture solar radiation which is
then turned to thermal energy and transferred to a working fluid
subsequently. Therefore, solar collectors are the main and most
critical components of any solar system [2].
There are basically two types of collectors, stationary and
tracking [3] (Fig. 1). Different collector configurations can help to
obtain a large range of temperature for example, 20–80 1C is the
operating temperature range of a flat plate collector (FPC) [4] and
50–200 1C is for an evacuated tube solar collector (ETSC) [5,6]. The
most productive and mostly used solar collectors are FPCs but
these collectors have comparatively low efficiency and outlet
temperatures. FPC is popular due to its low maintenance cost
and simple design.
However, FPC has two major drawbacks:
i. convection heat loss through glass cover from collector
plate and
ii. absence of sun tracking.
ETSCs have considerably lower cost and heat loss than the
standard FPCs [7,8]. On the other hand, an ETSC overcomes both
these drawbacks due to the presence of vacuum in annular space
between two concentric glass tubes, which eliminates sun tracking
by its tubular design. Conventional FPCs are mainly designed for
sunny and warm climates. Their performance reduces during cold,
windy and cloudy days and they are greatly influenced by the
weather as moisture and condensation cause early erosion of
internal materials which might cause system failure. In contrast,
ETSCs have outstanding thermal performance, easy transportabil-
ity and expedient installation. In addition, ETSCs are suitable for
unfavorable climates [9,10].
This paper presents a review of previous studies on ETSC, their
applications, and suitability in solar thermal engineering systems.
The former studies on ETSC mainly related to their suitability and
performance in various applications. Therefore, this review mainly
investigates the performance of ETSC for domestic and industrial
applications, factors that influence the collector efficiency, chal-
lenges of using this collector as well as economic consideration
regarding the usage of this collector. Some suggestions are also
made for future research in this field. There is no review on ETSC
till now and thus this is the first systematic review paper on recent
developments of ETSC and their applications according to the
authors' opinion. Finally, it is the authors' hope that this review
will be useful to find more about ETSC, their applications, and
challenges and the future recommendations will help in future
research work.
2. Evacuated tube solar collector (ETSC)
A variety of technologies exist to capture solar radiation, but of
particular interest of authors is evacuated tube technology.
Numerous authors [3,11,12] have noted that ETSCs have much
greater efficiencies than the common FPC, especially at low
temperature and isolation. For instance, Ayompe et al. [13] con-
ducted a field study to compare the performance of an FPC and a
heat pipe ETSC for domestic water heating system. With similar
environmental conditions, the collector efficiencies were found to
be 46.1% and 60.7% and the system efficiencies were found to be
37.9% and 50.3% for FPC and heat pipe ETSC, respectively.
An ETC is made of parallel evacuated glass pipes. Each evac-
uated pipe consists of two tubes, one is inner and the other is
outer tube (Figs. 2 and 3). The inner tube is coated with selective
coating while the outer tube is transparent. Light rays pass
through the transparent outer tube and are absorbed by the inner
tube. Both the inner and outer tubes have minimal reflection
properties. The inner tube gets heated while the sunlight passes
through the outer tube and to keep the heat inside the inner tube,
a vacuum is created which allows the solar radiation to go through
but does not allow the heat to transfer. In order to create the
vacuum, the two tubes are fused together on top and the existing
air is pumped out. Thus the heat stays inside the inner pipes and
collects solar radiation efficiently. Therefore, an ETSC is the most
efficient solar thermal collector [12].
An ETSC, unlike an FPC, can work under any weather conditions
while it provides acceptable heat efficiency.
2.1. Why an evacuated tube solar collector (ETSC) is preferable?
According to many researchers [3,11,12] ETSCs have much
more higher efficiencies than FPCs. ETSCs can collect both direct
and diffuse radiations. Besides excellent thermal performances,
ETSCs have convenient installation and easy transportabilty.
Applications like desalination of sea water, air conditioning,
building heating, refrigeration, and industrial heating require
higher temperature and the performance of an ETSC is better than
an FPC for high temperature operations [15]. ETSCs are also able to
operate other higher temperature applications such as instanta-
neous gas heater, boost element integrated single solar tank
system, and boost tank incorporated solar pre-heaters [16].
Mangal et al. [17] mentioned that the peak energy output is
provided by an FPC only at mid-day when the sun is perpendicular
to the surface of the collector whereas the evacuated solar tubes
are able to track sun passively throughout the day as for cylindrical
M.A. Sabiha et al. / Renewable and Sustainable Energy Reviews 51 (2015) 1038–1054 1039

shape of evacuated tube. The incident angle of sunlight on the
cylindrical tubes is at 90 1C throughout the day; hence the peak
absorption is always for an ETSC. It was also noted that the ETSC is
less affected by low temperature and wind because of the vacuum
envelop between the inner and outer tubes of evacuated pipe. The
vacuum is formed to reduce convective and conductive heat loss
by evacuating the air inside the interior tube of the ETSC.
They also reported that the maintenance of an ETSC is easy and
inexpensive. If a tube is damaged or broken, the system does not
leak or stop working, the collector still operates at lower efficiency.
In case of evacuated collector, without shutting down the whole
system, it is possible to replace the damaged tube whereas for the
FPC if the collector is damaged, the entire system needed to be
shut down to replace the collector. Thus FPCs have much higher
repair and maintenance cost than ETSCs.
To achieve the same heating performance as FPC, ETSC can be
used as well. Authors mentioned that approximately 250–300 l
water storage tank is required for a standard household with 4–5
members and the hot water needed for the household during the
summer and other seasons (large amount is needed in other
seasons) can be provided by 30 evacuated tubes. The same output
can be produced using an FPC in summer but the hot water is
mainly needed in other seasons. ETSCs are able to heat water all
the year round, even during overcast conditions as the tubes have
excellent isolative properties and highly efficient absorption of
solar radiation. Therefore, the average output of ETSCs over an
entire year is 25–40% higher than FPCs per net m2
.
Shriram Green Tech is a division of Shriram Industries Limited,
a very well-known industry for marketing solar energy based
efficient equipment and most economical solution provider, giving
service for 115 years in India. In their website (accessed on 14
April, 2014) comparisons are presented between ETSC and FPC.
According to their website, ETSCs have much lower convection and
convecting losses than FPC and the emissivity is lower for an
evacuated collector whereas for an FPC, emissivity is higher. An
ETSC is able to generate heat quickly and the heat loss in the
tubes is insignificant during daytime whereas for an FPC, heat
generation is slow and the heat loss in the collector and tank is
high due to convection during daytime. Grouting of evacuated
collectors is not required but grouting is required for FPCs. There is
no limitation about the placement of the collector unlike older
technology such as FPCs. They also reported that the performance
of an ETC is satisfactory even in extreme cold condition such as
À18 1C whereas an FPC will be damaged at high altitude due to
freezing of water [18] (Fig. 4).
2.2. Types of ETSC
According to Gao et al. [20] available types of evacuated tube
solar collectors can be categorized into two groups; one is the
single-walled glass evacuated tube and the other is the Dewar
tube. There are many variations of the two basic types; for
instance, heat extraction can be through a U-pipe, heat pipe or
direct liquid contact.
2.2.1. Single walled glass evacuated tube
The single-walled glass evacuated tube is popular in Europe.
Badar et al. [21] studied the thermal performance of an individual
single walled evacuated tube with direct flow type coaxial piping
based on analytical steady state model.
Kim et al. [22] investigated the thermal performance of an ETSC
with four different shaped absorbers both experimentally and
numerically. Four different shapes are: finned tube (Model I), tube
welded inside a circular fin (Model II), U tube welded on a copper
plate (Model III) and U tube welded inside a rectangular duct
(Model IV) as illustrated in Fig. 5.
Firstly, by considering only the beam radiation, the perfor-
mance of a single collector tube was observed and it was found
that the incidence angle has great influence on the collector
efficiency. Model III had the highest efficiency with small inci-
dence angle but the efficiency of model II became higher than
model III with the increment of incidence angle. The incidence
angle has negligible impacts on collector performance while
Nomenclature
ETSC evacuated tube solar collector
FPC flat plate collector
CPC compound parabolic collector
SHC solar heating and cooling program
IEA international energy agency
WGETSC water in glass evacuated tube solar collector
UPETSC U pipe evacuated tube solar collector
PLC Programmable Logic Controller
SWH solar water heating system
DSWH direct solar water heating system
COP coefficient of performance
PCM phase change material
HPA heat pipe absorber
DFA direct flow absorber
SAHP solar assisted heat pump
DFR diffuse flat reflector
HP-ETC heat pipe evacuated tube collector
c0 constant
c1 constant (W mÀ2
kÀ1
)
c2 constant (W mÀ2
kÀ1
)
τ transmittance
α absorptance
Q heat rate (W)
QL thermal loss (W)
Qu net heat energy absorbed by working fluid (W)
S solar energy absorbed by selective absorbing
coating (W)
D outer diameter of absorber tube (m)
L the length of absorber tube (m)
Ac surface area of collector (m2
)
G solar irradiation (W/m2)
Cp specific heat at constant pressure (j/kg 1C)
_m mass flow rate (kg/s)
Tout fluid outlet temperature (1C)
Tin fluid inlet temperature (1C)
Tm mean temperature of heat transfer fluid (1C)
Ta ambient temperature (1C)
FR collector heat removal factor
Ul overall loss coefficient (W mÀ2
kÀ1
)
Ut the edge loss coefficient of the header tube
(W mÀ2
kÀ1
)
Ue the loss coefficient from absorber tube to the ambient
(W mÀ2
kÀ1
)
Kθ incident angle modifier
a incident angle modifier constant
φ nanoparticles volume fraction (%)
η collector efficiency
M.A. Sabiha et al. / Renewable and Sustainable Energy Reviews 51 (2015) 1038–10541040

considering the diffuse radiation and the shadow effects and
model III is found to have the best performance for all ranges of
the incidence angle.
A prototype of solar water heating system with looped heat
pipe single walled evacuated tube was designed and both experi-
mental and theoretical research have been carried out by Zhao
et al. [23] (Fig. 6).
Nkwetta et al. [24] demonstrated a solar collector which
combines single walled evacuated tubes, heat pipe and an internal
or external concentrator for improving output temperatures.
2.2.2. Dewar tube
Dewar tube consists of inner and outer tubes which are made
of borosilicate glass and selective absorbance is used to coat the
outside wall of the inner tube to collect solar energy. The heat loss
is reduced in by evacuating the layer between the inner and outer
tubes. Tang et al. [25] investigated on dewar tubes and mentioned
that the cheap price of dewar water in glass evacuated tube solar
collector (WGETSC) makes it popular than dewar tube with U pipe
evacuated tube (UPETSC) with heat pipe. Tian [26] investigated the
thermal performance of dewar ETSC with an inserted U pipe. Yan
et al. [27] studied about the unsteady state efficiency of the dewar
tube solar collector having heat pipe inserted. Xu et al. [28] tested
the thermal performance of dewar tube solar collector under
various dynamic conditions and they used air as the heat transfer
fluid. Kim et al. [29] investigated the performance of dewar tube
where the inner tube was filled with coaxial fluid and the outer
tube was filled with an antifreeze solution and a one dimensional
mathematical model was established.
2.3. Mathematical modeling
There are two different procedures to measure the efficiency of
solar thermal collectors: steady state test method and quasi
dynamic test method [30]. The boundary conditions for solar
irradiation, ambient temperature and the inlet temperature of
the collectors are maintained constant during steady state test
method and for quasi dynamic test, the boundary conditions are
free to vary. In both the techniques, solar energy is the source of
Fig. 4. IEA SHC worldwide report 2012 [19].
Fig. 3. Evacuated tube solar collector.
Fig. 2. Evacuated tube [14].
Fig. 1. Types of solar collectors [3].

heat in the solar collectors; therefore, irradiation is the input
power which is received and absorbed by the collector and then
transferred to a working fluid. Thermal losses occur in the heat
transfer process involved in an ETSC. Heat transfer can occur
through conduction, convection and radiation. To perform heat
balance, heat transfer processes need to be included.
Thermal loss, QL can be expressed as
QL ¼ SÀQu ð1Þ
The useful heat which is delivered by a solar collector is the
difference between the energy absorbed by the working fluid and
the heat losses from the surface to the surroundings.
Qu ¼ SÀQL ð2Þ
The thermal performance of solar collector under steady state
conditions can be calculated as follows:
Qu=Ac ¼ FR ταð ÞGÀFRUL Tm ÀTað Þ ð3Þ
From Eq. (3), it is observed that the thermal performance of solar
collector depends on the intensity of the sunlight striking the
collector surface, the temperature of the surrounding environment
and the absorber plate and its optical and thermal performance
represented by the values of ταð Þ and UL respectively. The trans-
mittance (τ) of the glass cover and absorptance (α) of the absorber
plate depend on the incidence angle of the collector and according
to literature the product of the transmittance and absorptance (τα)
is approximately 0.836 [31–35].
Useful energy can also be expressed using Eq. (4) as
Qu ¼ _mCp Tout ÀTinð Þ ð4Þ
where Cp is the specific heat of water; in case of nanofluids specific
heat can be calculated using Eq. (5) [36]
Cp;nf ¼ ϕCp;np þð1ÀϕÞCp;bf ð5Þ
where the subscripts nf, np and bf are for nanofluid, nanoparticle
and base fluid, respectively.
The thermal efficiency of an ETSC can be measured by both Eqs.
(7) and (8) [30,37].
Efficiency; η ¼ Qu=AcG ð6Þ
Therefore; η ¼ _mCp Tout ÀTinð Þ=AcG ð7Þ
Eq. (7) gives the efficiency of evacuated collector with the known
value of fluid mass flow rate and the measured value of fluid inlet
and outlet temperature.
Another way of getting efficiency is by calculating the net
output power by considering the heat losses shown in Eq. (8).
η ¼ FR ταð ÞÀFRUL Tm ÀTað Þ=G ð8Þ
The heat loss coefficient UL is a function of the ambient
temperature and the temperature of the absorber plate but in
Fig. 5. Cross-section of (a) Model I, (b) Model II, (c) Model III and (d) Model IV [22].

reality it is not a constant. The collector heat removal factor is a
function of flow rate which is considered due to the fact that the
average fluid temperature and the average absorber temperature are
not same and the difference between these two temperatures is 0 to 1.
The efficiency of collector depends on the heat loss coefficient (UL) and
the design of the absorber plate of the collector. Therefore, the
approach to obtain (FRUL) is
FRUL ¼ c1 þc2 Tm ÀTað Þ ð9Þ
By combining Eqs. (8) and (9),
η ¼ FR ταð ÞÀc1
Tm ÀTað Þ
G
Àc2
Tm ÀTað Þ
G
2
ð10Þ
However, Eq. (10) is only applicable to calculate the efficiency when
sun strikes the collector perpendicularly but in reality sun is not
always perpendicular to the collector. Only at mid-day sun is perpen-
dicular to the collector but at morning and afternoon sun strikes the
collector with a different angle. An incidence angle modifier (IAM) is
the solution to get the performance for different incident angles which
can be described by the following equation [14]:
Kθ ¼ 1À tan θ=2
À Áa
; θ ¼ π=3 ð11Þ
An incidence angle modifier is actually the incidence angle modifier for
beam radiation (Kθb) and incidence angle for diffuse radiation (Kθd).
Incidence angle modifier; Kθ ¼ Kθb þKθd ð12Þ
The collector efficiency can be modified with the incidence angle
modifier which is expressed in Eq. (13) [3].
η ¼ FR ταð ÞKθ Àc1
Tm ÀTað Þ
G
Àc2
Tm ÀTað Þ2
G
ð13Þ
By combining Eqs. (12) and (13)
η ¼ FR ταð ÞKθb þFR ταð ÞKθd Àc1
Tm ÀTað Þ
G
Àc2
Tm ÀTað Þ
G
2
ð14Þ
2.4. Applications of ETSC
ETSCs are getting popular day by day for their uniqueness as
they are able to gather energy from the sun all day long at low
angles due to their tubular shape. Many researchers have done
researches on ETSCs which can be used for heating or cooling
purposes in industries like drug and pharmaceutical, textile, paper,
and leather and also for swimming pool, residential houses, boiler
house, hospitals, hotels and nursing home. The use of ETSC can be
discussed in two sections which are domestic and industrial
applications. Table 4 includes the summary of previous work on
applications of ETSC (Fig. 7).
2.4.1. Domestic applications
An ETSC is a mature technology for domestic applications as it
can operate over a wide range of temperatures from medium to
high according to the requirement. Fig. 8 demonstrates the
applications of an ETSC for domestic purposes.
2.4.1.1. Solar hot water. Since the last decade, the world market is
rapidly growing for solar water heaters which results in large
scale developments of improved quality products by various new
technologies. A Solar water heater is a device for heating water by
using solar energy to produce steam for domestic and industrial
purposes. Solar energy comes from the sun in infinite amount as
the form of solar radiation which falls on absorbing surface and
Fig. 6. Heat pipe evacuated tube collector [23].

then gets converted into heat which is used for water heating.
When evacuated tube collectors are used to heat water, it is called
evacuated tube solar water heater. There are various types of solar
water heaters such as flat plate solar water heater, concent-
rated solar water heater and evacuated tube solar water heater.
A concentrated solar water heater is used for very high
temperature water or steam and the flat plate solar water
heaters are getting replaced by evacuated water heaters due to
their numerous advantages [17]. ETSCs have been the core
attraction of modern development in the solar water heater
market as the manufacturing cost is comparatively lower and
ETSCs have better performance than FPCs particularly for
high temperature operations. Morrison et al. reported signifi-
cant developments of solar water heaters using ETSCs which
eventually include 65% of 6.5 million m2
/year in China [38].
Tang [25] studied the impact of different tilt angles on the
performance of solar water heaters with water in glass ETSC. For
the experimental purpose, two sets of water in glass evacuated
tube solar water heater were constructed which were identical but
had two different tilt angles, one inclined at 221 and the other at
461 from the horizon. It was reported that the heat removal to the
water storage tank from solar tubes is not influenced by collector
tilt angle but the daily solar heat gain of the system and daily
radiation are significantly influenced by collector’s tilt angle. The
thermal efficiency of a solar water heater does not depend on the
climatic conditions as the evacuated tube has lower heat loss to
the ambient air from solar tubes. Therefore, the collectors should
be inclined at such an angle which gives the maximum annual
solar radiation in order to maximize the heat gain of solar water
heaters annually.
Budihardjo et al. [15] studied about the long term performance
of water in glass evacuated tube solar water heater both experi-
mentally and numerically. They investigated the natural circula-
tion flow rate through the evacuated tubes, tank heat loss
coefficient and the collector efficiency of the solar water heater.
They used 21 evacuated tubes in the collector which has fluid in
Fig. 8. Application of evacuated tube solar collector for domestic purposes [42].
Fig. 7. Graph of efficiency (η) and temperature (T) ranges of various types of collectors [18].

direct contact with the glass tubes. The inlet and outlet tempera-
tures were measured under steady state conditions to determine
the useful energy from the collector and a sun tracking frame was
mounted under constant radiation to determine the collector
efficiency. The optical efficiency was found to be 0.58 using a
linear regression. They also reported that the collector design
which includes tube aspect ratio, reflector curvature, collector
inclination, operating temperature of the collector and the radia-
tion intensity has effects on the rate of natural circulation. By
considering each component, finally the results revealed that pre-
heater system with evacuated collector is capable of 45% annual
saving in Sydney.
Gao et al. [20] experimentally investigated the effects of
thermal flow and mass rate on forced circulation solar hot water
system. For the system two types of ETSCs namely water in glass
and U pipe evacuated collectors were used. A comparison was
made in terms of energy performance between WGETSC and
UPETSC. From the comparison, UPETSC has 25–35% higher energy
storage than WGETSC. The energy storage and also pump opera-
tions are influenced by the flow rate and fluid thermal mass. To
operate the pump in a stable condition and to take energy from
the collector adequately and timely, an appropriate mass flow rate
is important. It may be noted that the performance of energy
collection will be reduced for higher flow rate.
Morrison et al. [38] investigated the features of solar heaters
with WGETSC based on the circulation rate through single ended
tubes experimentally. To develop a numerical model of the heat
transfer and the fluid flow inside a single evacuated tube, it was
assumed that there is no contact between the neighboring tubes in
the collector array. It was investigated that the circulation flow
rate through the tubes has been significantly influenced by the
tank temperature and also the radiation intensity falling onto the
absorber whereas the circulation flow rate is influenced by the
inlet conditions of the tubes.
In another study, Budihardjo and Morrison [39] considered
optical and heat loss characteristics to investigate the performance
of water in glass evacuated tube solar heaters. The domestic water
heating system was compared with FPCs and the performance of
2 panel flat plate arrays was found to be higher than 30 evacuated
tube arrays.
Ayompe and Duffy [40] considered a heat pipe evacuated tube
collector (HP-ETC) to study the thermal performance of solar
water heating system. Experiment data was obtained over 1 year
period from a forced circulation solar water heating system with
3 m2
HP-ETC installation on a rooftop in the Focas Institute in
Dublin, Ireland. To mimic the domestic hot water system, an
automated hot water draw off system was developed which
comprises of electrical fittings, contactors, Programmable Logic
Controller (PLC), solenoid valve, thermostat, relays and impulse
flow meters. Water was used as the working fluid in the system
and the maximum outlet temperature of water was recorded as
70.31 while 59.51 was noted at the bottom of the hot water tank.
From the experimental investigation, it was revealed that the heat
pipe ETCs are more efficient than FPCs of a solar water heating
system.
Arefin et al. [41] investigated the characteristics and the
performance of different types of ETSCs for solar water heating
systems throughout the year. Besides determining the maximum
operating temperature for the solar water heater, they also
determined its feasibility by calculating the payback time. They
also reported that all glass evacuated tubes are the cheapest and
simplest and the heat loss is less than heat pipe collectors as the
glass tube collectors are directly connected with the tank. Rela-
tively small area is required for the system as the tank is mounted
over the collectors and less time is required for water to become
hot due to thermosiphon process. They found the operating
temperature of the system to be 50 1C which is good enough for
domestic purposes and their cost analysis shows that the solar
water heater using an ETSC is more cost effective than the electric
water heater. Table 1 summarizes the previous studies on the ETSC
used for solar water heating system.
2.4.1.2. Air conditioning. Nowadays researchers are investigating
environmental friendly technologies for air conditioning as
producing electrical energy causes some pollution. Mehta and
Rane [43] investigated the liquid desiccant based air conditioning
system which is adaptable to solar energy, a pollution free
renewable energy source. The solar radiation is highly available
in summer when the demand of air conditioning is also higher
which makes it logical to use solar energy source for air condi-
tioning. They developed a novel approach of using an ETSC with
heat pipes as regenerator for a liquid desiccant based solar
collector. They tested the collector at 100 1C to generate satu-
rated steam which offers 51–60% efficiency for average 9 h. The
average thermal COP of 0.82 was achieved as there is no heat loss
to air and the power consumption was less than 40 W because of
low pressure drop and flow rate of liquid desiccant collector. To
increase COP by regenerating further, a liquid desiccant in low
temperature stage which is possible by the latent heat produced in
the ETSC was introduced. The collector efficiency increased up to
44.7% and the power output of distilled water up to 5.14 kg/h
while regenerating liquid desiccant at 117 1C with 719 W/m2
global radiation.
Another experiment was done by Morthy [44] on the perfor-
mance of solar air conditioning system using HP-ETC. From his
experiment, it was concluded that to power the air conditioning
system, the solar system is capable of producing adequate energy.
The efficiency of heat pipe evacuated tube varies from 26% to 51%
and the overall system has efficiency from 27% to 48%. Using solar
air conditioning system with evacuated tube is very economical as
zero energy cost is provided by the solar powered chilled water
system. Besides, solar air conditioning system is a possible solution
to overcome environmental pollution.
2.4.1.3. Swimming pool. Sakhrieh et al. [45] conducted an
experiment on five types of solar collectors which are copper
thermosyphon with black coating (Type I), copper collectors with
blue coating (Type II), Copper solar collector (Type III), Aluminum
solar collectors (Type IV) and ETCs (Type V) for heating a
swimming pool based on overall performance, competence and
dependability. The aim of the experimental study was to replace
the heating system of the swimming pool at Hashemite University
of Jordan by a more efficient and cost effective solar heating
system. From the experimental investigation, it was found that an
ETSC has the highest efficiency. However, the lowest payback
period of 1.5 years was found to be for aluminum collectors and for
evacuated collectors, the payback period is found to be 1.9 years as
shown in Table 2. Even though aluminum collectors have the
lowest payback period, it is not suggested to use aluminum
collectors for large applications like heating a swimming pool.
Considering the efficiency, payback period and other perfor-
mances, they recommended that evacuated solar collector is the
best collector compared to other collectors to be used in the
heating system of the swimming pool.
2.4.1.4. Solar cooker. Sharma et al. [46] investigated the thermal
performance of a solar cooker based on ETSC with phase change
material (PCM) storage unit. The prototype in Fig. 9 was designed
in two separate parts, one for energy collection and the other one
for cooking.

During sunshine hours, PCM stores solar energy which is used
for cooking purpose at evening or night time. Different loads and
loading times were used to conduct cooking at noon and evening
and the evening cooking was not affected by noon cooking rather
it was found to be faster as the heat in PCM storage was used. The
solar cooker with an ETSC is expensive but it is able to provide
high temperatures up to 130 1C and allow users to cook in a
conventional kitchen with shade at evening and at non-sun
shining hours. The designed cooker has good prospective not only
in Japan but also in other regions with good sun shine.
Kumar et al. [47] designed a solar pressure cooker based on ETSC
having two separate parts for solar energy collection and both the
parts are attached with a heat exchanger. Besides experimental
investigation, they also developed a simulation model to determine
the performance of the cooker under various climatic circumstances.
The designed solar pressure cooker was able to achieve temperatures
up to 120 1C which is much higher than the pressure cooker based on
an FPC. Therefore, the solar pressure cooker with ETSC is reported to
be of high potential for community applications in Delhi.
2.4.2. Industrial applications
For industrial use, a higher temperature is required compared
to domestic applications. An ETSC is capable of generating
Table 2
Efficiency and payback period of different types of solar collectors.
Parameters Copper
thermosyphon
with black
coating (Type I)
Copper
collectors
with blue
coating (Type
II)
Copper
solar
collector
(Type III)
Aluminum
solar
collectors
(Type IV)
ETCs
(Type
V)
Efficiency
(at
2.00 pm)
80.1 77 71.3 78.7 93.5
Payback
period
(years)
2.2 2.0 1.9 1.5 1.9
Table 3
Performance of stationary collectors for solar drying.
Stationary collectors Temperature range (1C) Efficiency
500 W/m2
1000 W/m2
FPCs 30–80 0.71–0.75 0.72–0.75
ETCs 50–200 0.44–0.82 0.62–0.82
CPCs 60–240 0.45–0.73 0.58–0.72
Table 1
Summary of previous researches on evacuated tube solar collectors used in water heaters.
Author Type of
investigation
Working fluid Types of ETSC Findings
Hazami et al. [64] Experimental Water ETSC – ETSC DSWH (84%) has higher solar fraction (i.e. in average) than FPC
DSWH (68%) annually.
– ETSC can generate approximately 9% more energy than FPC.
Geo et al. [20] Experimental Antifreeze fluid
(40% glycol by
volume)
WGETSC and UPETSC – In average, the thermal efficiency of WGETSC is less than UPETSC.
– Comparing UPETSC and WGETSC with the same efficiency curves,
WGETSC achieves energy storage of 25–35% less than UPETSC.
Ayompe and Duffy [40] Analytical Water HP-ETSC – To operate solar water heating system, HP-ETCs are more efficient
than their FPCs.
Ma et al.[65] Analytical Water Glass evacuated tube
(with U shaped absorber
tube)
– Outlet temperature 38 1C, efficiency 59%, collector efficiency
increases within synthetic conductance.
– Heat losses reduced.
– Heat extraction is high.
– The challenge is to preserve vacuum environment.
Yamaguchi et al. [66] Experimental R744 CO2 UPETSC – Collector efficiency 66%
– Heat recovery¼65%
Rittidech et al. [67] Experimental R-134a Circular tube collector – Collector efficiency 76%.
– Non-corrosive.
– No freezing issues.
– The challenge is to maintain vacuum environment.
– Another problem is the release of non-condensable gases.
Morrison et al. [38] Experimental and
numerical
simulation
Water WGETSC – Storage tank temperature and the intensity of the radiation which
fall onto the absorber surface influence the circulation flow rate
through the tubes.
– Circulation rate is strongly influenced by the inlet conditions of
the tube.
Shah and Furbo[14] Water Double glass tube with
tubular absorber
– Outlet temperature is 32 1C, efficiency 60–70%.
– Suitable for high latitudes.
– Heat transfer coefficient is higher.
– The performance decreases because of shadow effects.
Morrison et al., 2004 [11] Numerical Water Water in glass

temperature up to 200 1C [6]. Therefore, ETSC can be used for
industrial applications.
2.4.2.1. Heat engines. Madduri et al. [48] studied a commercial
evacuated tube solar hot water system which was used in a
thermodynamic engine as a thermal power source. According to
them, it is important to use concentrators to achieve high
efficiency solar thermal conversion to a heat engine from a
commercial evacuated tube system which supplies input thermal
powers at temperatures of 180–220 1C. It was concluded that at
higher temperatures, the concentrated evacuated tube is very
efficient to convert incident solar radiation to thermal power.
The mechanical output power per unit of installed collector area is
also increased by this system from a heat engine.
2.4.2.2. Solar drying. The most common technique to preserve
agricultural products is sun drying but this procedure has many
drawbacks and weaknesses since products can be spoiled by dust,
wind, rain and moisture or loss of products due to animals and
birds. Decomposition can cause deterioration in the harvested
crops, fungi and insect also can attack the products, etc. Besides,
the process of sun drying requires large area to spread the
harvested crops which is time consuming and labor demanding.
Therefore, to process the agricultural products such as vegetables
and fruits with zero energy costs in hygienic, clean and standard
conditions, solar drying technology is an alternative solution. For
small scale food processing industries or for agricultural purposes,
solar dryer technology is convenient, environment friendly, and
reliable to produce hygienic and good quality food products as this
technology requires less area, saves time, energy and labor costs
and also improves the product quality [49]. Sharma et al. also
mentioned that the solar drying system is faster, healthier,
cheaper, more hygienic and efficient than traditional drying
systems.
Lamnatou et al. [50] investigated the thermodynamic perfor-
mance analysis of a solar dryer which was used to dry apples,
carrots and apricots using an evacuated tube air collector. As there
are many disadvantages of traditional sun drying, many research-
ers have done research and applied solar dryers to dry agricultural
products but most of them using an FPC. They mentioned that
dryers with ETSCs are special of type which have significant
advantages over FPCs, higher efficiency is one of them. Their
experimental results showed that the warm outlet air of the ETSC
is able to attain the suitable temperature level for drying agricul-
tural products. They also reported that the solar dryer with ETSC
Flow Meter
Data logger
PC
Solar irradiance at 20
degree inclined and
horizontal surface
Evacuated tube
solar collector
Relief Valve
PCM storage unit
Cooking vessel
PCM temperatures
Flow rate
valve
Collectorinletandoutlet
temperature
Water source
Fig. 9. Prototype of solar cooker based on ETSC [46].

Table 4
Summary of previous work on applications of evacuated tube solar collectors.
Author Type of
investigation
Working fluid Types of ETSC Objective Results
Nkwetta et al. [24] Experimental Water – HP-ETC
– Pipe
absorber
(EICPC-HPA)
or direct
flow
absorber
(EICPC-DFA)
Experimental characterization of the
‘concentrator augmented solar collector
array’, a component of solar collector.
– Concentrator augmented solar collector
arrays proved to be more economical
for providing heating and cooling
demands due to the reduced number of
HP-ETCs.
– Reduction in reflector size and related
reflector losses.
Kumar et al. [68] Experimental Air One ended
evacuated tubes
Production of hot air without using any
intermediate fluid to produce hot air at
different air flow rates and efficiency.
– Performance of ETC significantly
increases with the use of reflector.
– -Efficiency, outlet temperature and
temperature difference also increase
with the use of reflector.
Xu et al.[28] Experimental Air All glass ETC Testing the thermal performance of all glass
ETC under dynamic outdoor conditions.
– The energy balance analysis revealed
that the proposed dynamic method
(steady state) is effective to solve the
operational limitations of steady state
test caused by uncontrollable weather
conditions.
Madduri et al. [48] Experimental ETC To achieve high efficiency at higher
temperature (180–220 1C) using
concentrated evacuated tube system.
– Thermal efficiency is 35%
– Mechanical efficiency is 12%.
– At higher temperatures incident solar
radiation is efficiently converted to
thermal power and the mechanical
output power increases.
Caglar and Yamal [69] Theoretical
and
experimental
Water ETSC Designing a solar assisted heat pump (SAHP)
with ETC for gaining higher efficiency.
– Efficiency of ETSC varies from 0.728
to 0.807.
– From cost analysis, electricity
consumption has reduced by 19–45% for
space heating application by using the
designed SAHP with ETC.
Nkwetta et al. [34] Experimental Water Concentrated
and non-
concentrated HP-
ETC
Investigating the performance of HP-ETC
compared to a concentrated evacuated tube
single-sided coated HP absorber for medium
temperature applications.
– Comparing to the non-concentrated HP-
ETC, the concentrated HP-ETC showed
an improvement of 30% in overall
average outlet and inlet fluid
temperature differential and 25.42%
total daily energy collection.
Lamnatou et al. [50] Experimental Air Evacuated tube
air collector
Thermodynamic performance investigation
of a solar dryer using an evacuated tube air
collector
– Suitable for solar drying applications
without preheating the outlet air.
– Proposed model showed very good
correlation coefficients for all the
products (apple, carrots and apricots)
used for testing purpose.
– Can be used for drying products of
larger quantities in both industrial and
agricultural sectors.
Kumar et al. [47] Experimental Water ETSC Investigation of thermal performance of a
community type solar pressure cooker
based on ETSC.
– ETSC based system supply heat at
higher temperature at about 120 1C.
– On a clear sunny day, the system is able
to provide cooking facilities on several
batches.
– Potential for community based
applications.
Tang et al. [25] Experimental Water Glass ETSC Investigating the impacts of the collector tilt
angle on daily thermal conversion and water
flow characteristics inside solar tubes.
– Tilt angle has noteworthy influence on
daily collectible radiation and heat gain.
– Tilt angle has minor influence on
thermal conversion efficiency and heat
removal from solar tubes to water

Table 4 (continued )
Author Type of
investigation
Working fluid Types of ETSC Objective Results
storage tank.
– Evacuated tube has lower heat loss and
thus thermal efficiency of SWHs was
independent of the climatic conditions.
Sharma and Diaz [4] Numerical Material ETSC based on
minichannels
To study about the thermal performance of
solar collector based on minichannels.
– Under identical operating conditions,
ETC based on minichannels has higher
efficiency than a similar ETC.
Liang et al. [70] Theoretical
and
experimental
Water Two layered glass
U-tube evacuated
tube
Investigating thermal performance of a
filled type and the copper fin evacuated tube
with a U tube.
– Satisfactory thermal performance is
achieved by finned type evacuated tube
with U tube.
– Thermal performance is 12% higher for
filled type evacuated tube considering
the component heat transmission
is 100.
Hayek et al. [30] Experimental Water WGETSC and HP-
ETC
The eastern coast of the Mediterranean was
considered for overall performance of solar
collectors under local weather conditions.
– HP-ETC has 15-20% higher efficiency
than WGETSC.
– HP-ETC has better design than WGETSC.
Zambolin and Del Col
[71]
Experimental Mixture of water
and propylene
glycol (to prevent
freezing during
winter)
FPC and ETC with
CPC
1. Comparing FPC and ETSC in term of steady
state and quasi dynamic test methods.2.
Comparing the daily energy performance of
FPC and ETSC.
– For a larger range of operating
conditions, ETSC found to have higher
efficiency than FPC based on daily tests.
Gao and Ge [72] Experimental Water All glass ETSC Using all glass ETSC for heating purpose. – Average collector efficiency ranged from
50% to 54% on daily basis.
– Collector efficiency varies from 51% to
55% in winter.
Budihardjo and
Morrison [39]
Experimental
simulation
Water WGETSC Comparing the performance of WGETSC and
FPCs in a range of places.
– The performance of a 2 panel FPC is
higher than an ETC with 30 tube
evacuated tube for domestic water
heating in Sydney.
– The performance of ETC is not sensitive
to the tank size.
Hayek [73] Numerical Water WGETSC Investigating the features and design and
overall performance of WGETSC.
– Better performance of standard water-
in-glass tube under high heat inputs.
– ETCs can perform well under cloudy
conditions.
Tang et al.[74] Numerical Single tube of all-
glass ETSC
Developing a mathematical model to
calculate irradiation daily on single tube of
all-glass ETSCs based on solar geometry,
knowledge of two dimensional radiation
transfers.
– Collector type, central distance between
tubes, size of solar tubes, tilt and
azimuth angles, use of diffuse flat
reflector (DFR) effect the annual
radiation collection.
– Use of DFR can significantly improve the
collectors’ energy collection.
– To maximize annual energy collection,
all glass ETSC should be fixed with a tilt
angle which is smaller than the site
latitude.
Kim et al. [75] Numerical
and
experimental
Water Evacuated CPC
solar collector
Investigation and improvement of thermal
performance of evacuated CPC collector
with a cylindrical absorber
– Comparing to stationary CPC solar
collector, tracking CPC solar collector
has about 14.9% higher efficiency.
– Solar radiation and incident angle both
effect the efficiency of the collector.
Sharma et al. [46] Experimental Water ETSC Investigating the thermal performance of a
prototype solar cooker based on an ETSC
with phase change material (PCM) storage
unit.
– During summer months in Japan, the
solar cooker is capable of cooking twice
(noon and evening) in a single day.
– Evening cooking was not disturbed by
noon cooking rather evening cooking
was faster because of PCM heat storage.

has the capacity to dry agricultural products in larger quantities
and thus it can be used in industrial sector.
Pirasteh et al. [51] investigated the performance of stationary
collectors (FPCs, CPCs and ETSCs) for agricultural and industrial
products drying, presented in Table 3. Industries like textile,
cement, clay brick production, wood and timber, waste water
treatment and dairy etc. can use solar drying in order to decrease
fossil fuel consumption and to become more economical and
environmental friendly.
Fadhel et al. [52] designed, fabricated and tested a solar assisted
chemical heat pump dryer and investigated the performance of
the system. An ETSC is used as the main component for the system
whereas the other components are storage tank, dryer chamber
and solid gas chemical heat pump unit. The climatological condi-
tions of Malaysia were considered for the performance investiga-
tion of solar chemical heat pump dryer. The experimental result
shows an efficiency of 74% of ETSC whereas the simulation gives
the efficiency of 80%. Chemical heat pump dryer with ETSCs is
suitable for industrial drying purposes for instance drying textile
products.
Fudholi et al. [53] also mentioned about chemical heat pump
dryer with evacuated tube for drying agricultural and marine
products.
2.4.2.3. Steam generation. An ETSC can be used for applica-
tions requiring high temperature such as steam cooking, boilers,
laundry etc. as this is known as the best alternative thermal
technology for generating high temperature up to 200 1C [5].
Vendan et al. [54] studied on the design of an ETSC for high
temperature steam generation for the applications of steam
cooking, boilers, laundry, etc.
3. Challenges of using evacuated tube solar collectors (ETSCs)
3.1. Cost and maintenance
Morisson et al. [38] mentioned that the world market of solar
water heater with ETSCs is significantly expanding due to the low
cost manufacturing process of tubular solar collectors. According
to China industry in 2001, about 65% of 6.5 million m2
/year solar
water heaters are double glass evacuated tubular solar water
heaters.
According to Mangal [17], evacuated tubes are strong and long
lasting. In case if any tube is broken, it just requires replacing the
broken tube which is cheap whereas for an FPC it is expensive as
the whole collector is needed to be replaced.
Arefin et al. [41] did the cost analysis for a solar water heater
and compared with the cost of an electric heater. They reported
that the lifetime of a solar water heater is 30 years whereas the
lifetime for an electric water heater is only 5 years. Therefore,
almost every 5 years, the electric heater is needed to be replaced
which is expensive. On the other hand after installation, solar
water heater does not require any maintenance cost and thus it is
more beneficial and cost effective to use solar water heater instead
of electric water heater.
Tang et al. [25] also mentioned that the manufacturing costs of
evacuated tubes are decreasing recently. Budihardjo and Morrison
[39] mentioned that the water in glass, an ETSC, is the mostly used
collector among other evacuated collectors in solar water heater
due to its simple construction and low manufacturing costs.
Shukla et al. [55] mentioned in the review of recent advances in
the solar water heating system that the performance of an ETSC is
better than mostly used FPC due to its ability to produce high
temperature but ETSCs are not widely used because of its high
initial cost.
As ETSCs have natural frost protection, without any damage
ETSCs can be used in sub-zero temperatures whereas antifreeze
systems need to be installed for flat plate panels under same
temperatures which is complicated and expensive. Regular repla-
cement is required for the glycol used in flat plate systems as it can
cause damage by freezing. The glycol needs to be replaced every
few years (3 years). Therefore, it is an ongoing cost. In addition,
leaking might happen while replacing glycol or due to damage of
flat panel by storms which is an added risk.
3.2. Fragility
Evacuated tubes are made of two layers of annealed borosili-
cate glass and the glasses which are made of annealed glass are
much more fragile than tempered glass. Because of fragility, glass
tubes can be shattered easily due to small hail, jostling or poor
handling. Therefore, extra care must be taken while transporting
or handling ETSCs.
3.3. Snow removal
ETCs do not shed snow as the collector surface is not always
warm, the tubes are insulator in nature and the collector surface is
irregular which lets the snow stick on tubes for a long time. As the
glass tubes are fragile, it is not possible to scrape off the
accumulated snow which might make the system ineffective.
3.4. Overheating
One of the characteristics of ETSCs is that they produce high
temperature and get much hotter than other collectors. Therefore,
ETSCs are not recommended for domestic solar water heating or
for solar space heating system as the high temperature can cause
significant problem when it exceeds boiling point of water; rather,
it is recommended for commercial applications. For domestic use,
it is essential to keep the temperature below 100 1C which
continuously requires ample load on the system; otherwise
weaknesses will be exposed in the material of evacuated tube
due to overheating and eventually the vacuum will be lost.
4. Economic consideration on the usage of ETSCs
ETSCs have not provided any real competition to FPCs in the
past years though ETSCs have been commercially available for
more than 20 years. Recently, there has been a major expansion of
the evacuated tube solar water heater market in China, Europe,
and Japan as a result of globally growing industries of evacuated
tube collectors. This review has attempted to identify the eco-
nomic advantages of ETSC over FPC by comparing the initial and
maintenance cost as well as the payback period.
From Table 5, considering the initial cost of installation, main-
tenance and operational cost, and the payback period of the
system, it can be observed that the SWH system using an ETSC
is more cost effective than an FPC. However, this scenario is for
China. A recent study by Pappis et al. [56] was conducted in Brazil
regarding the economic and environmental comparison between
FPC and ETSC. They concluded that the ETSCs are the best choice
from the environmental perspective due to least impact generated
during the manufacturing process. However, an FPC is preferable
from the economic point of view as greater investment is required
for an ETSC system.
Installing an ETSC is expensive in some countries but it is an
advanced technology at competitive price which requires very less
maintenance afterwards. Mostly used FPC is an old technology
with higher price and it requires high maintenance as well.

Therefore for the long term, an ETSC is more economical and cost
effective than an FTC. Recent huge production and large scale
implementation of evacuated tubes have proven this technology as
matured with 3 years payback period without any subsidies.
5. Performance based on working fluids
To improve the efficiency of solar collectors, researchers have mainly
focused on several structural changes such as changing the structure of
solar collectors or changing the coating to improve absorptivity but
from the literature, only few studies focused on changing the working
fluid in order to improve the collectors' efficiency [57]. From recent
studies, it is found that the working fluid can influence the performance
of solar collector significantly. Water, oil, and air are the most common
working fluids used in solar energy system but the thermal conductivity
of these fluids is relatively low [58]. Recently, researchers are investigat-
ing on other working fluids such as nanofluids rather than water and air
to improve the collector's efficiency. Nanofluids consist of base liquid
and nanomaterials that have enhanced thermophysical properties such
as higher thermal conductivity, thermal diffusivity and convective heat
transfer coefficients [59,60]. Besides improving the effectiveness of heat
transfer, nanofluids also improve optical properties, transmittance as
well as extinction coefficient of solar collectors. By using nanomaterials,
the efficiency of an FPC has increased up to 10% and the incident
radiation is found to be 9 times higher than a conventional FPC. For a
direct absorption solar collector, the efficiency increased up to 10% using
nanofluids [61]. Table 6 represents the summary of the previous studies
regarding the performance of ETSCs based on different working fluids.
6. Future work
i. One of the drawbacks of ETSC is that the collector tubes are
very fragile and easy to be damaged. To overcome this
drawback, research can be carried out on improving the
structure of evacuated collector to make their body harder.
For example, nanotechnology can be used to build a harder
and powerful evacuated collector.
ii. Evacuated tubes are made of annealed glass which is much more
fragile than tempered glass and the material mostly used is
borosilicate glass. Experiments can be done on materials of
glasses used in evacuated collector to have better efficiency.
iii. Grooved tubes which have spirally running grooves in inner
surface can be used instead of usual tubes inside the collector
to improve the efficiency. The heat transfer coefficient of
grooved tube is said to be 2–3 times higher than plan tube
with same specification.
iv. The effectiveness of heat transfer is directly related to the
working fluids of the collector to absorb the heat energy from
the absorber plate. From the literature, ETSCs have been
commercially available for more than 20 years and water is
being used as the working fluid which has several hundred
times low thermal conductivity than working fluids with
metal or metal oxide [55]. Based on comprehensive studies, it
has been also realized that very few studies were conducted
on ETSCs using nanofluids. As the evacuated collectors have
better performance in producing high temperature due to
minimal convection and radiation losses, using nanofluids in
ETSC is expected to raise the efficiency significantly.
v. Solar collectors are basically of two types namely stationary
and tracking, ETSCs are of stationary type. For stationary type
solar collectors sun tracker can be used to track the maximum
sunlight throughout the day. Though the cylindrical shape of
the ETSCs helps to track the sun passively throughout the
whole day but it is not able to absorb the maximum sunlight
as the solar panel is positioned with a fixed angle. Solar
tracker is able to orient the collector along the direction of the
sunlight and ensures the absorption of maximum sunlight
throughout the day by adjusting its orientation according to
the sun [62]. It is not essential to use a sun tracker but in
different geographical conditions it can boost the collector
energy from 10 to 100% [63]. It is expected that the use of
solar tracker in ETSC panels will maximize the performance
efficiency especially for industrial or large scale uses.
vi. It is found from the previous studies that the use of nano-
fluids in solar collectors reduces CO2 emissions and also
annual electricity cost [58]. As it is expected that the effi-
ciency of ETSCs will increase by using nanofluids, an eco-
nomic analysis can be done to find the payback period of an
ETSC with different types of nanofluids and the also annual
electricity savings.
vii. The working fluids inside evacuated pipe move at a slow
speed as the fluid boundary layer is close to the pipe wall.
Therefore, the heat transfer coefficient in heat exchangers is
limited. The thickness of the boundary layer can be reduced
by creating a turbulent flow which requires turbulators to be
placed inside the pipes and thereby the heat transfer coeffi-
cient can be increased.
viii. For industrial applications, a hybrid system can be developed
to minimize the evacuated collector area and to improve the
overall efficiency of the system by combining ETSCs with
concentrating collector. To achieve high temperature, concen-
trating collectors use mirrors and lenses by concentrating
sunlight of a large area onto a small area.
7. Conclusion
This paper presents an overview of recent studies on ETSCs and
revealed that this collector has great potential in industrial, residential
and agricultural sectors. ETSCs have been used for water heaters, solar
cooker, air conditioning, heating swimming pool, drying agricultural
products and for many other purposes. An ETSC is highly recom-
mended for higher temperature applications as they can gain higher
temperatures easily and are able to preserve heat even when the
outside weather is cold. For the countries with good sunshine, an ETSC
Table 5
Comparisons of economic performance between ETSC and FPC.
Author Location of
study
Type of
collector
Tank volume
(L)
Initial cost
($)
Maintenance and operational
cost
Payback period (Compared to electric
heaters) (years)
Saxena and
Srivastava [76]
China ETSC 100 377 Negligible 4.41
Allen et al. [77] China FPC 110 4823-5627 Negligible 17
Han et al. [78] China ETSC 200 330 $3.2/year 0.7-3.0
Gastli [79] China FPC 200 2000 $20/year 19.7
Koroneos and Nanaki
[80]
China FPC 200 $1911 approximately 1–4% of the
investment cost
5

shows outstanding efficiency and the countries with cold weather
ETSCs are very cost effective with excellent efficiency because of their
freezing protection characteristics. The performance of an ETSC using
various working fluids is also presented in this paper and it was found
after analyzing the available literatures that the ETSC performs much
better with nanofluids as working fluid rather than conventional
working fluids such as water and air. Some recommendations are
made on future research. It is expected that it will be very useful for
energy producing industries as well as for research organizations.
Acknowledgments
The authors would like to acknowledge the Ministry of Higher
Education Malaysia (MOHE) for financial support. This work
was supported by UM-MOHE High Impact Research Grant
Scheme (HIRG) (Project no: UM.C/HIR/MOHE/ENG/40).
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Zhang and
Yamagu-
chi [57]
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more higher efficiency.
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