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Wu, Jinshun, Zhang, Xingxing, Shen, Jingchun, Wu, Yupeng, Connelly, Karen, Yang, Tong
ORCID logoORCID: https://orcid.org/0000-0002-1254-5628, Tang, Llewellyn, Xiao, Manxuan,
Wei, Yixuan, Jiang, Ke, Chen, Chao, Xu, Peng and Wang, Hong (2017) A review of thermal
absorbers and their integration methods for the combined solar photovoltaic/thermal (PV/T)
modules. Renewable and Sustainable Energy Reviews, 75 . pp. 839-854. ISSN 1364-0321
[Article] (doi:10.1016/j.rser.2016.11.063)
Final accepted version (with author’s formatting)
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1
A review of thermal absorbers and their integration methods for the combined
solar photovoltaic/thermal (PV/T) modules
JinshunWu
a
,XingxingZhang
b
,
c
,*,JingchunShen
b
,YupengWu
d
,KarenConnelly
d
,TongYang
b
,LlewellynTang
b
,ManxuanXiao
b
,
YixuanWei
b
,KeJiang
b
,ChaoChen
a
,PengXu
e
,HongWang
d
a
CollegeofArchitectureandCivilEngineering,BeijingUniversityofTechnology,Beijing,China
b
FluidsandThermalEngineering(FTE)Group,UniversityofNottingham,Ningbo,China
c
SolarEnergyResearchCenter(SERC),DalarnaUniversity,Borlänge,Sweden
d
DepartmentofArchitectureandBuiltEnvironment,UniversityofNottingham,UK
e
SchoolofEnvironment&EnergyEngineering,BeijingUniversityofCivilEngineeringandArchitecture,China
⁎
Corresponding author at: Solar Energy Research Center (SERC), Dalarna University, Borlänge, Sweden
E-mail addresses: Xingxing.zhang@nottingham.edu.cn, Xing0520@gmail.com (X. Zhang).
Abstract
Thermalabsorbersandtheirintegrationmethodsarecriticaltosolarphotovoltaic/thermal(PV/T)modules.Thesetwoelements
directlyinfluencethecoolingeffortofPVlayersandasaresult,therelatedelectrical/thermal/overallefficiency.Thispaper
conductsacriticalreviewontheessentialthermalabsorbersandtheirintegrationmethodsforthecurrently‐availablePVmodules
forthepurposeofproducingthecombinedPV/Tmodules.AbriefoverviewofdifferentPV/Ttechnologiesisinitiallysummarized,
includingaspectsoftheirstructure,efficiencies,thermalgoverningexpressionsandtheirapplications.Sevendifferenttypesof
thermalabsorbersandfourcorrespondingintegrationmethodsaresubsequentlydiscussedandsummarizedintermsoftheir
advantages/disadvantagesandtheassociatedapplicationforvariousPV/Tmodules.Comparedtotraditionalthermalabsorbers,
suchassheet‐and‐tubestructure,rectangulartunnelwithorwithoutfins/groovesandflat‐platetube,thesefourtypes,i.e.micro‐
channelheatpipearray/heatmat,extrudedheatexchanger,roll‐bondheatexchangerandcottonwickstructure,arepromising
duetothesignificantenhancementintermsofefficiency,structure,weight,andcostetc.Theappropriateorsuitableintegration
methodvariesindifferentcases,i.e.theethylene‐vinylacetate(EVA)basedlaminationmethodseemsthebestoptionfor
integrationofPVlayerwiththermalabsorberwhencomparedwithotherconventionalmethods,suchasdirectcontact,thermal
adhesiveandmechanicalfixing.Finally,suggestionsforfurtherresearchtopicsareproposedfromfiveaspects.Theoverall
researchresultswouldprovideusefulinformationfortheassistanceoffurtherdevelopmentofsolarPV/Tmoduleswithhigh
feasibilityforwidespreadapplicationinenergysupplyevenatdistrictorcity‐levelinthenearfuture.
Keywords:
Solar,PV/T,Thermalabsorber,Integrationmethod
1.
Introduction
Global current energy demand is continuously growing, therefore
new solutions for energy conservation, energy supply and simultaneous
environmental protection is highly desirable. The utilization of renew-
able energy is, without a doubt, one of the most encouraging ecological
solutions towards sustainable and resilient development. Solar energy,
as an inexhaustible, renewable and eco-friendly energy source is
currently promising to o
ff
er potential solutions for sustainable
development [1]. At present, the most widely available solar
technologies are
solar photovoltaic (PV) and solar thermal heat,
which combined contribute towards a large share of global energy
supply as illustrated
in Fig. 1 [2].
According to the International Energy Agency (IEA)
’
s
projections [3] by 2050 there will be 3000 GW of installed PV
capacity worldwide,
generating 4500 TW h per year and
contributing 11% of expected
global electricity supply (Fig. 2).
China has now overtaken Germany
and the US to become the
world's top generator of solar PV power.
During the period of
the Twelfth Five-Year Plan, China's PV capacity
increased 168
times, far beyond the speed of all previously observed
renewable
energy development. [4]. With 15 GW added in 2015, China
has
reached 43 GW of solar PV capacity at a mean 40% rise annually
[5]. Fig. 3 shows the cumulative installed PV capacity in China
from 2000 to 2015.
Currently, solar thermal only provides around 0.5% of the
estimated global water and space heating demand in the
buildings sector
within the European area [6]. In 2005 Europe had a solar thermal
system capacity of around 10 GW
th
. This is expected to grow to
200 GW
th
by 2030, of which up to 50% will be used for the delivery
of low and medium temperature water [7]. In the UK, around
131 GW
th
of domestic hot water has already been provided by solar
systems, partly replacing conventional gas and electrical heating
systems in 2011 [8]. In 2013, it was indicated that 148.2 million
tonnes of oil equivalent was consumed, with 66% used for space
heating and another 17% for water heating, with a total estimated cost
of around £33 billion to the UK economy [9]. Meanwhile, solar driven
water heating systems have been identified as having the potential to
offset around 70–90% of the total energy required for water heating,
thus enabling significant savings in household fossil fuel energy use
[10]. Supporting the UK Government's Renewable Heat Premium
Payment scheme, solar thermal is expected to offer great potential
for heat source diversity and for the development of towns and cities in
sustainable and affordable ways.
As for China, the installed operating capacity of solar thermal in
2013 was 262.3 GW
th
[10], far beyond the installed capacity in any
other country (Fig. 4). At the beginning of 2015 the Chinese authorities
released its “Renewable Energy Development Roadmap 2050” as a
long-and-medium-term plan for the development of solar technologies.
This plan includes the huge expansion of low-median temperature
solar thermal applications to support a stronger growing Chinese
economy and a low carbon future.
PV/thermal (PV/T) technologies enable dual function of solar
collection within one module with an output of both electricity and
heat. Such synergetic integration of PV and thermal collection results
not only in improved PV efficiency [3], but also generates more energy
per unit area than a stand-alone PV or solar thermal module. The
market potential of PV/T technology is therefore significantly higher
than for individual PV and solar thermal systems. This strategic
concept will boost solar energy application in line with future devel-
opment trends of both PV and solar thermal technologies as addressed
above.
Nomenclature
a width of duct, m
A
c
collector aperture area, m
2
b length of duct, m
C
b
thermal conductance of the bond between fin and tube, J/
kg k
C
p
heat capacity of flowing medium, J/kg k
D
h
hydraulic diameter, m
D
i
inside diameter of flow tubes, m
D
o
outside diameter of flow tubes, m
F finefficiency
F’ module efficiency factor
F
R
heat-removal factor
h heat transfer coefficient, W/m
2
k
I incident solar radiation, W/m
2
P power, W
Q energy rate, W
T temperature, °C
U overall heat transfer coefficient, W/m
2
k
W distance between tubes, m
Greek
α absorptivity
δ thickness, m
η efficiency, %
τ transmittance of the material
Subscripts
a air
e electricity
fi fluid
in inlet
L loss
o overall
p,m mean value of plate
PV-roll bond PV layer to roll-bond plate
th thermal
Fig. 1. Global capacity in operation and annual energy yields in 2014 [2].
Fig. 2. Global PV power generation and relative share of total electricity generation [3].
Fig. 3. Installed PV power capacity in China from 2000 to 2015 [5].
2
Thermal absorbers for PV/T modules are complementary to solar
cells as another way of harvesting solar energy. The overall conversion
efficiency of a PV/T module increases with the efficiency of its thermal
absorber according to the laws of thermodynamics. Different methods
for thermal absorber design, namely sheet-and-tube structure, rectan-
gular tunnel with or without fins/grooves, flat-plate tube, micro-
channel heat pipe array/heat mat, extruded heat exchanger, roll-bond
heat exchanger and cotton wick structure, are being comprehensively
developed. Generally, a PV/T module is constructed by attaching a
commercially available PV layer to a thermal absorber using integra-
tion methods such as mechanical or chemical adhesive bonding. This
combination provides gap filler that transfers heat between the PV
layer and the thermal absorber and must have a good elongation
property to compensate for the different expansion of various compo-
nents of the PV layer and thermal absorber. Poor thermal contact
between the PV layer and the thermal absorber underneath leads to a
temperature difference of about 15 °C for an unglazed PV/T module.
This is due to reduced solar energy absorption, and increased heat
transfer resistance in the cell to the absorber interface, resulting in
poor module heat removal factor [11,12]. Moreover, the thermal
resistance between the PV layer and thermal absorber may become
extremely large if a small air gap or air bubbles exist within the
integration layer. Therefore, both the thermal absorber and the
integration method used is critical to the solar PV/T modules as they
directly affect cooling of PV layers and therefore also the related
electrical/thermal/overall efficiency.
This paper thus conducts a critical review on recent research and
development of thermal absorbers and the integration methods
required for their use within combined PV/T modules; categorised
into flat-plate, flexible and concentrated thermal absorbers. The overall
research provides useful information for the assistance of further
development of PV/T modules with high feasibility for widespread
application in energy supply even at district or city-level approaching
the near future.
2. Photovoltaic/thermal (PV/T) technologies
2.1. Basic concept and theory of PV/T operation
PV modules come in a variety of forms including conventional
framed flat-plate, flexible and concentrated types. However, owing to
the low energy output of solar PV modules combined with the low
exergy of solar thermal collectors, a solar PV/T module, combining
both electrical and thermal components in a single unit area, could
potentially provide a solution to the low overall (sum of electrical and
thermal) efficiency. Present PV technology has a major inherent
drawback in its inability to absorb solar radiation from the complete
solar spectrum. In addition, PV cells suffer from a drop in efficiency
with a rise in temperature. Increasing the temperature of PV cells by
1 °C causes a reduction in electrical efficiency by around 0.4–0.5% for
crystalline silicon PV cells and around 0.25% for amorphous silicon (a-
Si) PV cells [13]. This results in PV cells delivering relatively low
electrical efficiencies since a major part of the incident solar energy is
either lost due to convection and radiation or converted as heat. Solar
PV/T can harvest these thermal energy and therefore increase overall
thermal and electrical efficiency [14,15].
Fig.
5
shows the basic operation principle of a PV/T module. From
the point of view of the first law of thermodynamics, the overall
efficiency of a PV/T module is the sum of the module's thermal
efficiency ηth and the module's electrical efficiency η
e
, which are
defined as the ratios of useful heat gain and electricity gain to the
incident solar irradiation striking on the module's collecting surface,
given as below:
η
Q
IA
U
I
TTη==1−(−)−
th
th
c
L
pm a
e
,
(1)
where Qth can alternatively be expressed by the difference in absorbed
solar radiation, heat loss and the generated electricity.
η
Q
IA
P
IA
==
e
e
c
e
c
(2)
where Q
e
is equal to the measured electrical power (P
e
).
η
ηη
QQ
IA
Q
IA
U
I
TT=+=
+
=1 − =1 − ( −
)
othe
th e
c
L
c
L
pm a,
(3)
Apart from categorization by working fluid (i.e. air, water, refrig-
erant, phase change material, nano-fluid etc.) [15,16] the hybrid PV/T
technologies can further be divided into flat-plate, flexible and con-
centrated, depending on the type of PV module, as indicated in Fig. 6.
The following section will give an overview of the different PV/T
technologies including aspects of their structure, efficiencies, applica-
tions etc.
2.2. Flat-plate PV/T modules
The flat-plate PV/T modules usually combine a flat-plate PV
module in the front, which converts sunlight into electricity, with a
solar thermal absorber at the back, which captures the remaining
energy and removes excessive heat from the PV module. Such modules
can be engineered to carry heat away from the PV cells thereby cooling
the cells and therefore improving their efficiency by lowering resis-
tance. The capture of both electricity and heat allow these devices to
have higher exergy [17] and thus have greater overall energy efficient
than solar PV or solar thermal alone [18]. A significant amount of
research has gone into developing the flat-plate PV/T technology since
Fig. 5. : Basic operation principle of PV/T modules.
Fig. 4. Share of the total installed solar thermal capacity in operation (glazed and
unglazed water and air collectors) by economic region at the end of 2013.
Solar PV/T modules
Flat-plate
PV/T modules
Flexible
PV/T modules
Concentrated
PV/T modules
Fig. 6. : Category of solar PV/T modules according to PV types.
3
the 1970s [19].
The main components of the flat-plate PV/T modules, given in
Fig. 7, are the glazing cover (optional), flat-plate PV module, adhesive,
thermal absorber and insulation. The adhesive often consists of
ethylene-vinyl acetate (EVA) and a layer of tedlar-polyester-tellar
(TPT). Glass cover is optional for flat-plate PV/T module and can
either be single or double glass. PV/T devices with more than three
glass covers are not recommended because their electrical efficiency is
very low, due to the low transmittance of the aperture and the
enhanced thermal resistance of the triple glazing cover [20]. The
purpose of the thermal absorber, also called an “extracting heat device”
is to reduce the temperature underneath the PV module. The fluid
flowing inside the channels transport the collected thermal energy in
low-temperature applications. The insulation layer prevents heat from
escaping into the surrounding area. Fig. 8 shows the typical classifica-
tion of flat-plate PV/T according to different working fluids [21], which
have electrical, thermal and combined efficiencies in the range of 6.7–
15%, 22–79%, and 40–87% respectively [22].
Flat-plate PV/T modules are produced in regular flat shapes that
could be applied in both urban and rural areas, i.e. ground mounted,
wall/roof mounted, etc., and for industry or building energy supply.
Each module is fitted with a tubular inlet and outlet at the back or the
side that allow for connection between module to module in either a
serial pattern to allow the working fluid to pass through from one to
another, or a parallel pattern. It is feasible to make further use of the
absorbed heat through a heat pump for one or more of the following
purposes, i.e. hot water supply [23–25], space heating [26,27], solar
cooling [28,29], thermal storage [30], desalination [31], drying [32,33]
and pool heating [34,35] etc. Electricity generation from the PV cells,
either exported to the national grid or stored in batteries, will meet the
electrical load or drive the system component, i.e. water pump, heat
pump compressor. The combination of these concepts could create a
low (zero) carbon industry process and building driven by solar energy.
2.3. Flexible PV/T modules
Flexible PV/T modules have an almost identical structure as
compared to flat-plat modules aside from the PV layers are often made
of amorphous silicon (a-Si). The
flexible
PV/T modules typically
include thermal pipes or air space beneath the metal sheet supporting
the thin film, which may be installed above the current roof structure in
the case of building retrofits. Amorphous silicon is the most popular
thin film technology used at low-and-medium temperature with cell
efficiencies of 5–7%, and double- and triple-junction designs raising it
to 8–10% [36]. The additional thermal efficiencies of flexible PV/T
modules could be in the same range as observed for flat-plate modules
1: Glaazing cover; 2
5: TPT bac
2: EVA-encap
ck sheet; 6: Th
psulate; 3: Sola
hermal absorb
ar PV cells; 4
ber; 7: Therma
: EVA-encaps
al insulation
sulate;
Fig. 7. Schematic of a typical flat-plat PV/T module. 1: Glazing cover; 2: EVA-
encapsulate; 3: Solar PV cells; 4: EVA-encapsulate; 5: TPT back sheet; 6: Thermal
absorber; 7: Thermal insulation.
Fig. 8. Classification of flat-plate PV/T module [21].
1: TThin film PV; 2: Metal rooff; 3: Fin sheet; 4: Thermal ppipes; 5: Insulaation
Fig. 9. Schematic of a typical flexible PV/T module [37]. 1: Thin film PV; 2: Metal roof;
3: Fin sheet; 4: Thermal pipes; 5: Insulation.
Fig. 10. Schematic of a typical concentrated PV/T module [42].
4
![Fig. 21. Roll-bond heat exchanger for PV/T modules - two aluminium sheets [77].](/figures/fig-21-roll-bond-heat-exchanger-for-pv-t-modules-two-13kbmqeg.webp)
![Fig. 22. Roll-bond heat exchanger for PV/T modules – single aluminium sheet [78].](/figures/fig-22-roll-bond-heat-exchanger-for-pv-t-modules-single-1ldk4e4e.webp)
