T solar wall system in typical cities of China

Journal Pre-proof Annual analysis of a multi-functional BIPV/T solar wall system in typical cities of China

Lijie Xu, Jie Ji, Kun Luo, Zhaomeng Li, Ruru Xu, Shengjuan Huang PII:

S0360-5442(20)30205-X

DOI:

https://doi.org/10.1016/j.energy.2020.117098

Reference:

EGY 117098

To appear in:

Energy

Received Date:

10 July 2019

Accepted Date:

04 February 2020

Please cite this article as: Lijie Xu, Jie Ji, Kun Luo, Zhaomeng Li, Ruru Xu, Shengjuan Huang, Annual analysis of a multi-functional BIPV/T solar wall system in typical cities of China, Energy (2020), https://doi.org/10.1016/j.energy.2020.117098

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Journal Pre-proof

Annual analysis of a multi-functional BIPV/T solar wall system in typical cities of China Lijie Xu1, Jie Ji1*, Kun Luo1, Zhaomeng Li1, Ruru Xu1, Shengjuan Huang1

1. Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, 230027, China *([email protected])

0. Abstract Firstly, this paper introduces a novel multi-functional BIPV/T wall system in order to satisfy building’s seasonal energy demand in China. This system can generate electricity during the whole year. In heating season, hot air is created to decrease heating load for the building. In non-heating season, hot water is generated to supply the household demand. Meanwhile temperature of PV panel is decreased by water cycle, therefore higher electrical performance is able to be achieved. Secondly, mathematical model is established and verified by the experimental results. Thirdly, annual performance of the system in three different typical cities are evaluated. The annual electrical generation in Beijing, Hefei and Xining are 247.7kWh, 152.6kWh and 268.4kWh respectively. The solar fraction in Beijing, Hefei and Xining are 49.9%, 38.7% and 41.3% respectively. The system is able to satisfy 79.1%, 66.8% and 60.4% hot water energy demand in these cities respectively. The overall annual energy saving in Beijing, Hefei and Xining are 2661.8 kWh, 1908.4 kWh and 2412.3 kWh respectively. Fourthly, the impact of external/internal PV structures, Si/CdTe solar cells and aspect ratio of the system are investigated respectively.

Key words: Building integrated PV/T; PV/air-heating; PV/water-heating; Annual performance evaluation; Parameters discussions

1. Introduction With the development of the economic and society, high energy consumption and carbon emission are the most challenging global issue in today’s world. Energy cost in buildings accounts for a great part of total energy utilization and greenhouse gas emission. According to the literature [1], 40% of energy cost is created because of the building. The 2008 Climate Change Act requires a 34% reduction in 1990 greenhouse gas emissions by 2020, and an 80% reduction by 2050 [2]. Utilizing and popularizing green energy in buildings are very important method to achieve the goal. 1

Journal Pre-proof Solar energy is the most steady and easy-use clean energy, meanwhile buildings have large surface area. Therefore the utilization of solar energy technology in buildings is an effective method in order to improve the current situation of energy problem. PV/T is a high efficient solar system, which can generate hot air/ water and electricity in order to meet the building’s demand effectively. Building integrated PV/T (BIPV/T) systems have been investigated in recent 10 years. For BIPV/Air system, Sujata Nayak and G.N. Tiwari [3] investigated energy and exergy performance of a PV/Air collector integrated with a greenhouse in Delhi, India. Based on Köppen climate classification, the region is classified as BSh— an arid, steppe and hot climate [32]. The analysis was based on quasi-steady state condition. Exergy analysis calculations of the PV/T integrated greenhouse system showed an exergy efficiency level of approximately 4%. Basant Agrawal and G.N. Tiwari [4] researched a BIPV/T system fitted as rooftop of a building to generate electrical energy and also to produce thermal energy for space heating in New Delhi. A thermodynamic model was developed to determine energy, exergy and life cycle cost of the BIPV/T system. The energy and exergy efficiencies of the system were 33.54% and 7.13% respectively. The cost of power generation was found to be US $ 0.1009 per kWh. Tingting Yang and Andreas K. Athienitis [5] researched an open loop air-based BIPV/T system with a single inlet in Quebec, Canada. The climate in this area is classified as DFb based on Köppen climate classification— Cold (Continental), Without dry season, Warm summer. A numerical control volume model was developed and validated based on the results from the experiments. Simulation results indicated that the application of two inlets on a BIPV/T collector increased thermal efficiency by about 5% and increased electrical efficiency marginally. An added vertical glazed solar air collector improved the thermal efficiency by about 8%. Annamaria Buonomano et al. [6] analysed the energy and economic performance of roof and/or façades BIPV/T collectors for residential applications in four different European weather zones selected among those representative of European climates. They refer to the climate of South-Germany (CFb—Temperate, Without dry season, Warm summer), North-Italy (CWa— Temperate, Dry winter, Hot summer), South-Italy (CSa—Temperate, Dry and Hot summer) and South-Spain (CWa—Temperate, Dry winter, Hot summer). Suitable energy parametric analysis was performed by varying the thermal resistances and capacitances of the building envelope. The adoption of BIPV/T panels produced a decrease of the primary energy demands from 67% to 89%. For the investigated case studies, the pay back periods appeared quite long, varying from 11 years for South European weather zones to 20 for North European ones. As for BIPV/Water system, Adnan Ibrahim et al. [7] designed a BIPV/Water system to produce both electricity and hot water in useful temperature for the applications in Malaysia (AF, Tropical Rainforest climate). Results showed that the hourly variation for BIPV/T system, the PV/T energy efficiency of 55–62% was higher than the PVT exergy efficiency of 12–14%, and BIPV/T system was produced primary-energy saving efficiency from about 73% to 81%. T.N. Anderson et al. [8] designed of a BIPV/T solar collector and was theoretically analyzed through the use of a modified Hottel–Whillier model in New Zealand (CFb—Temperate, Without dry season and Warm summer). It was shown that by integrating the BIPV/T into the building rather than onto the building could result in a lower cost system. T.T. Chow et al. [9] introduced an experimental study of a centralized photovoltaic and hot water collector wall system which could serve as a water pre-heating system in Hong Kong (AM—Tropical, Monsoon). Collectors were mounted at vertical facades. Natural water circulation was found more preferable than forced circulation in this system. The thermal efficiency was found 38.9% at zero reduced temperature, and the corresponding electricity 2

Journal Pre-proof conversion efficiency was 8.56% in Hong Kong. With the PVT wall, the space thermal loads were much reduced both in summer and winter. Khem Raj Gautam et al. [10] researched the potential of unglazed BIPVT for electricity production and domestic hot water heating application for multifamily apartment buildings in Denmark (CFb—Temperate, Without dry season and Warm summer). The author compared the energy output and the cost savings of BIPV, solar thermal and BIPV/T systems of different sizes but with the same initial investment cost. The results indicated that the BIPVT system in cold climates could be competitive with traditional technologies only under certain conditions, such as favorable electricity to heat price ratio or a particular range of collector areas. A similar system in warmer climates, however, fared quite well against both solar thermal and BIPV technologies. But up to now, the BIPV/T systems are restricted to single function structures: PV water mode or PV air mode solely. Both of them are not able to satisfy building’s seasonal demand during the whole year in China, where thermal requirements of the building is multiple and changing with seasons. Passive heating function is required in heating season, and in non-heating season, hot water demand for residential building should be satisfied. Hot water or hot air supplement individually is not enough during the whole year. Not only that, system with single function may increase the energy load of the building instead of decreasing it. For example, BIPV-Water system can’t work without expensive anti-freeze measures in cold winter, and space heating demand of the building is not able to be satisfied. As for BIPV-Air system, hot air created by the system is unnecessary in hot summer seasons. More than that, high temperature of PV panel leads to low electrical efficiency in summer, overheat problem and the increase of building’s cooling load should also be faced [28]. In conclusion, a BIPV system which can satisfy the requirements of space heating in winter and hot water in other seasons is needed eagerly. Besides that, electricity efficiency of the PV cells should be ensured during the whole year. In this paper, a multi-functional BIPV/T wall system is introduced. There are several advantages of this multi-functional BIPV/T system compared with the single functional BIPV/T system. Firstly, the system has two different working modes, BIPV Water mode for non-heating season and BIPV Air mode for heating season, in order to satisfy the annual energy demand of the building. Secondly, in non-heating season, residential hot water demand can be satisfied. Meanwhile, because hot water is able to decrease temperature of PV panel, electrical efficiency is increased. Thirdly, in heating season, passive space heating function is provided based on thermos syphon theory, and electricity is generated by the PV cells simultaneously. Building’s heating load is able to be decreased significantly. Fourthly, compared with the single functional system in which PV panel and solar thermal collector is installed with side-by-side arrangement, this hybrid system is more compact than the single functional system, which leads to lower installation, operation and maintenance cost. Thermal and electrical performance of the system are studied in three typical cities of China: Beijing (North China; DWa: Cold (continental), Dry winter, Hot summer), Hefei (Middle China; CFa: Temperate, Without dry season, Hot summer) and Xining (North-west China; DWb: Cold (continental), Dry winter, Warm summer) respectively. Based on the literatures quoted previously, the climates and regions are not researched at present to authors’ knowledge. This article aims to: (1) propose a novel multi-functional BIPV/T solar wall system; (2) establish the mathematical model of the system, and verify accuracy of the model by comparing the simulated and experimental results; (3) investigate the annual thermal and electrical performance of 3

Journal Pre-proof the system in three typical cities of China; (4) Performance of the system and energy consumption of the building under different parameter conditions are presented.

Nomenclature A Area, m2 C Loss coefficient, D External diameter, m d Distance, m E Electrical output, W/m2 g Gravitational acceleration, m/s2 h Convective heat transfer coefficient, I Irradiation intensity, W/m2 k Conductive heat transfer coefficient, L length, m m Mass, kg 𝑚 Mass flow rate, kg/s Q Auxiliary power, W/m2 R Thermal resistance, K/W T Temperature, K t Time, s V Wind speed (m/s) l Distance, m H Height, m q Heat flow, W/m2

W/m2 K W/m2 K

Greek Symbols α Absorptivity, ν Dynamic viscosity, ε Emissivity, σ Declination angle, τ Transmittance, ρ density, kg/m3 θ Beam radiation φ Latitude β Inclination angle γ Solar azimuth ω Hour angle ψ Acceptance angle ζ Reflectivity Subscripts g Glass cover s Sky pv PV panel c,i Different room wall w Water pipe t Water tank p Insulation layer

2. System construction and operation description 2.1 System presentation This BIPV/T system is installed on building’s south wall and constituted by: glass cover, PV cells, sheet-and-tube, air channel, two vents and insulation layer. Fig. 1 (a) and (b) are the illustrative diagrams of the system. The size of the absorber plate is 1.95 m·0.95 m (H·W), and the PV cells are laminated on it with the coverage ratio of 60%. PV cells are constituted by 50 single PV units which are connected in series, and the size of single PV cell is 156 mm·156 mm. There is airtight layer between glass and the PV panel to prevent heat loss. Eight copper tubes are welded on the back of the panel to generate hot water. Fig. 2 is the phantom drawing of the system. In this study, two BIPV systems are installed on the building with a window in the middle, which is a classic building structure in the vast countryside of China.

4

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Fig. 1 Illustrative diagrams of the system showing: (a) PV/Air mode and PV/Water mode

Figure 1 (b) Vertical view of the BIPV/T wall system

5

Journal Pre-proof Fig. 2 Phantom drawing of the system

2.2 Operation description The multi-functional BIPV/T wall system has two working modes: PV-Air mode and PV/Water mode for heating season and non-heating season respectively. PV cells are able to generate electricity during the whole year, which is able to decrease the electricity costs of the building. And by connecting an inverter, PV grid-connected system is able to be established if necessary. Under different modes, the system is able to satisfy the seasonal demand of the building during different times of the year. The followings are the introduction of the two working conditions.

2.2.1 PV-Air mode: In heating season, PV-Air mode is activated. In the daytime (Beijing and Hefei from 8:00 to 17:00, Xining from 9:00 to 18:00) electricity is generated by PV cells. Besides of that, vents 1 and 2 are opened, hot air from the air channel flows into the room to meet the space heating requirements of the building. Auxiliary power is adopted to keep the room temperature over 16 ℃. Therefore the auxiliary power is activated if the room temperature is lower than 16 ℃ [25, 26]. Solar fraction is introduced to evaluate the system’s daily thermal effect on the building in heating season, and is defined as the proportion of solar energy in total thermal energy consumption of the building.

2.2.2 PV-Water mode: During other time of the year, PV-Water mode is operated. Vents are closed all day long to provide thermal insulation protection in hot summer season. In daytime water valve and water pump are opened to control the flow rate of water cycle. Water circle behind the panel is able to decrease the temperature of the PV panel, leads to better electrical performance, meanwhile 200 L of hot water is generated in each day. To meet the minimum temperature requirements of hot water for household application, auxiliary power is installed inside the water tank. If the final water temperature is less than 45 ℃, the auxiliary power is activated. Water tank is filled with fresh cold water each day at 0:00, and initial water temperature equals to the temperature of the local underground water. In summer, an auxiliary power keeps the room temperature lower than 26 ℃, thus the auxiliary power works in the occasion when the room air temperature is higher than 26 ℃.

3. Mathematical model and validation Based on the energy balance, dynamic mathematical model is established including: glass cover, PV panel, water circle, air channel, wall system and indoor air. Transmittance of glass cover is determined by solar angle changing in the whole year.

6

Journal Pre-proof 3.1 Transmittance of glass cover: θ is incidence angle of solar beam radiation [22]:

cos  sin  (sin  cos   cos  sin  cos  )  cos  cos  (cos  cos   sin  sin  cos  )  cos  sin  sin  sin 

(1)

Where σ is the declination angle, φ is the latitude, β is the inclination angle, 90 degree. γ is the solar azimuth, ω is the hour angle. θ1 is effective incidence angle of solar diffuse radiation [12]:

1 =44.86  0.0716  0.00512 2  0.00002798 3

(2)

ψ is the acceptance angle, in this case equals to 90 degree.

n1 sin n2

(3)

n1 sin 1 n2

(4)

sin2 = sin  '2 =

θ2 and θ’2 are refraction angle of beam radiation and diffuse radiation respectively, n1 and n2 are refraction coefficient of air and glass.

 sin2    tan2 2    r  0.5  2 2  2  sin    tan 2       2 

(5)

 sin2 2 1  tan2 2 1   r1  0.5  2  2  sin    tan 2  1     2 1 

(6)

a1 =1 e(Kd /cos2 ) '

a2 =1  e(  Kd /cos 2 )

(8)

a(1 r) 1 r(1 a)

(9)

a1 (1  r1 ) 1  r1 (1  a1 )

(10)

1 = 2 =

(7)

 (1  a1 ) 2 1  r 2    r 1   1  r 1  a1   

(11)

 (1  a2 ) 2 1  r1 2  1  r1 1   1  r1 1  a2   

(12)

ζ and ζ1 are reflectivity of beam radiation and diffuse radiation respectively. 7

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1=11 1

(13)

2 =12 2

(14)

τ1 and τ2 are transmittance of the beam radiation and diffuse radiation respectively. The total transmittance is calculated by:

=

1Ib  2 Id

(15)

I b  Id

Fig. 3 is the daily average glass transmittance changing with time in Beijing, Hefei and Xining, respectively. The maximum transmittance shows up in winter and the minimum transmittance appears in summer. 1.0 0.9

Transmittance of glass cover

0.8

Beijing

0.7 0.6 1.0 0

60

120

180

240

300

360

0.9 0.8

Hefei

0.7 0.6 1.0 0

60

120

180

240

300

360

0.9 0.8

Xining

0.7 0.6 0

60

120

180

240

300

360

Day Fig. 3 Glass transmittance of three cities during the year

3.2 Solar radiation Hourly data of the annual solar radiation are obtained from the history records. But only the horizontal solar radiation are generally available which includes the beam radiation Ib, the diffuse radiation Id and the ground reflected radiation Ir. For any titled angle:

I =Ib Rb  0.5Gd (1 cos  )  0.5g (Gb  Gd )(1 cos  ) Rb =

8

cos cosz

(16)

(17)

Journal Pre-proof Where θ is incidence angle, θz is the zenith angle [13].

cosz  cos coscos  sin sin

(18)

3.3 Glass cover:

mgcg

dTg t

 ha Ag (Ta  Tg )  hs Ag (Ts  Tg )  hg-pv Ag (Tpv  Tg )  Ag I

(19)

Where the mg, cg and Tg are the mass, specific heat and temperature of the glass cover, respectively. Ta and Ts are the temperature of the ambient air and the sky respectively. ha and hs are the convection and radiation heat transfer coefficient between the glass cover and surroundings, and defined as [14]:

ha  5.63.8V hs   g (Ts 2  Tg 2 )(Ts  Tg )

(20) (21)

hg-pv is the heat transfer coefficient between the glass cover and the absorber plate which includes two parts: heat convection and radiation. The hg-pv can be defined as:

  Nuk 1 hg-pv   Tpv2  Tg 2  Ts  Tg     1  1  1  l pv g  

(22)

The Nu is given by [15]:

Nu  0.197(Gr Pr )1 / 4 ( / H )1 / 9

(23)

And Gr is Grashof number, calculated by:

Gr 

g Tl 3

2

(24)

Where g is acceleration of gravity, β is air expansion coefficient, l is the distance between glass cover and the PV panel, ∆T is the temperature difference between the glass cover and the absorber plate, and ν is dynamic viscosity of air.

3.4 PV panel

 cd pv

dTpv dt

=hgpv (Tg  Tpv )  hppv (Tp  Tpv )  Qc  I     Epv

(25)

Where ρ, c, d and Tpv are the density, heat capacity, thickness and temperature of the PV panel, respectively. I, τ and α are the solar radiation, transmittance of the glass and the absorptivity of the PV panel, respectively. hg-pv is heat transfer coefficient between the PV panel and the glass cover, defined by equation (4). Tp is temperature of the insulation layer, and hp-pv 9

Journal Pre-proof is heat transfer coefficient between the PV panel and the insulation layer, which is calculated by [16]:

  Nuk 1 hp-pv   Tpv2  Tp2  Tpv  Tp     1   1  1  l pv p  

(26)

The Nusselt number is calculated by [17]:

Nu = 0.12  GrPr 

1

3

(27)

Gr is the Grashof number

Gr 

g  Tpv  Tp  d 3

2

(28)

Qc is the energy transmitted from the PV panel to the water tube, and is defined as [20]:

Qc = Tpv  Tc   RA 

(29)

Where Tc is temperature of the copper water tube, R is thermal resistance between water pipe and PV panel, and A is area of the PV panel grid. Epv is the electric output power per unit area, and it is shown as [18]:

E pv  I  g ref 1  B r Tpv  Tref  

(30)

Where ηref is the PV efficiency under standard condition, 14%. Br is the temperature coefficient, set as 0.0045 K-1, and Tref is 25℃ (298 K).

3.5 Water pipe and water tank The energy balance of the water pipe is defined as [19]: dT (31)   Dc  d c   c cc d c c    Dc  2 d c  hw Tw  Tc    Dc hc  b Tb  Tc   Qc t Tc, Dc and dc are the temperature, external diameter and thickness of the water pipe separately. hw is the heat convection coefficient between water and copper pipe. hc-b is the heat coefficient between the water pipe and the insulation layer. Qc is the energy transmitted from the PV panel to the water tube. The energy balance equation of water in the pipe is:

Tw Tw 2Tw w Accw =  Ac wuwcw  Ackw 2  Ph c wc (Tt  Tw ) t x x

(32)

Where ρw, cw, uw and Tw are the density, heat capacity, flow rate and temperature of the water, Ac and Pc are the sectional area and the circumference of the water pipe. For water tank, the temperature is calculated by:  T (33)  w V t c w t  m w c w Tw-in  Tw  out   ht At Ta  Tt  t Where Vt, 𝑚, ht and Tt are the volume, mass flow rate, heat loss coefficient and temperature of the water tank respectively. 10

Journal Pre-proof 3.6 Air channel In heating period, vents 1 and 2 are opened, temperature of the air channel is defined as:  dT m air cair air  m cair (Tair-in  Tair-out )  hchannel A (Tpv  Tair )  hchannel A (Tp  Tair ) (34) dT Where mair, cair and Tair are the mass, heat capacity and temperature of the air channel, respectively. hchannel is the convection coefficient between air and two panel, and 𝑚 is mass flow rate of the air, Tair-in and Tair-out are inlet and outlet air temperature. The mass flow rate and convection coefficient are calculated by [17]:

g  (Tout  Tin ) H Cin ( A / Ain )  C out ( A / Aout )  C d ( H / D )

V 

(35)



m = AV

(36)

D is the duct hydraulic diameter; H is the height of the duct; A is cross section area of the duct; Ain and Aout are the areas of the inlet and outlet vents; Cin, Cout and Cd are the loss coefficient of the channel, represent the inlet loss coefficient of the vent, outlet loss coefficient of the vent and friction factor along the duct. hchannel is the convection coefficient, and is defined as [18]:

Gr 

g  (Tpv  Tp ) L3



2

1

Nu  0.12(Gr  Pr）3 (38)

(37)

hchannel 

Nu  k L

(39)

L is length of the air duct, k is thermal conduction coefficient of air.

3.7 Wall In this model heat transfer only through the thickness direction is considered.

T  2T c  2 , t x

(40)

Boundary condition of the outside surface is:



T x

x 0

  I solar  hc , o (Ta  T ) ,

(41)

Boundary condition of the inside surface is:



T x

N

xL

 hc , i (Tindoor  TL )   q j , i , j i

11

(42)

Journal Pre-proof Where αIsolar is the solar radiation absorbed by the outer surface, Tindoor is the room temperature， hc,o and hc,I are convection coefficients of the outer and inner surface, N is number of the wall；qj,i is the radiation between surface i and j, which is defined by:.

q j ,i  hrj ,i Ai (T j  Ti ) ,

(43)

hrj,i is the radiation coefficient between surface i and j, Ai is area of the surface i, Ti and Tj are temperature of the surface i and j respectively. hrj,i is calculated by：

hrj ,i 

 i j f i  j (T j2  Ti 2 )(T j  Ti ) 2 [1  (1   i )(1   j ) f i  j Ai / A j ] N

(1   k ) f i  k f j  k

k 1

Ak [1  (1   i )(1   j )(1   k ) f i  k f k  j f j i ]

 i j Aj (T j2  Ti 2 )(T j  Ti )

(44)

Where ε represents the emissivity, and f is the view factor between two surfaces.

3.8 Room air In PV-Water mode and night of the PV-Air mode, vents are closed, room air temperature is calculated by:

aVroomCa

dTindoor   hc , i Ai (TL  Tindoor )  Q dt

(45)

While in the daytime of the PV-Air mode, vents are opened, and the room air temperature is defined as:

aVroomCa

dTindoor   hc,i Ai (TL  Tindoor )   aVd Ag (Tv  Tindoor )  Q dt

(46)

Where Vroom is volume of the room，Ai represents the area of surface i, (m2)，Tv is the inlet air temperature of the channel, Q is auxiliary power. Based on the section 2.2, auxiliary power is adopted in order to keep the room temperature over 16 ℃ under BIPV/Air mode and lower than 26 ℃ under BIPV/Water mode. In BIPV/Air mode, if the room air temperature is lower than 16 ℃, Troom is assumed as 16 ℃ and heating load Q of that typical time step can be calculated. If the room air temperature is higher than 16 ℃, the heating load Q is 0. In BIPV/Water mode, if the room air temperature is higher than 26℃, temperature room air 12

Journal Pre-proof is assumed as 26 ℃ and cooling load Q of that time step is able to be calculated. When the room air temperature is lower than 26 ℃, the cooling load Q is 0.

3.9 Simulation process The simulation program is established on MATLAB platform. The hourly weather data in typical meteorological year are configured as a data input file in the program. Fig.4 is the annual ambient air temperature and solar radiation on vertical surface in Beijing, Hefei and Xining. Located on North China Plain, Beijing (39.26 N, 116.23 E) has sub-humid continental monsoon climate, characterized by hot rainy summer and cold dry winter. Hefei (31.9 N, 117.3 E) is in east China and has subtropical monsoon climate, characterized by warm winter and hot summer. Xining (36.6 N, 101.8 E) is in Northwest China, characterized by plateau continental climate. Xining has cold winter, cool summer and high quality solar energy resources. Time step in the simulation is one second, therefore time step of weather data is managed as one second as well using interpolation method. The following list is the particular data which is used in the simulation. Fig. 5 is the flow chart of the simulation program. (a) House: 6 m·3 m· 3.3 m (L,W,H); (b) Window: 1m·1m (L,W); (c) Brick wall: d=15 cm, k=0.72 W/m2, ρ=1800 kg/m3, α=0.75; (d) Solar wall system: 2 m·1 m(H,W); (e) Glass cover: ρ=2500 kg/m3; d=3 mm; (f) PV plate: 1.2 m2, ρ=2702 kg/m3,c =903 J/kg K (g) Mass flow rate of water: 0.1 L/s; (h) Initial temperature in the water tank: 10-15 ℃ in Hefei and Beijing, 5-10 ℃ in Xining depending on season. (i) Room air temperature: higher than 16 ℃ in PV-Air mode and lower than 26 ℃ in PV-Water mode.

13

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Ambient Temperature(Beijing)

Vertical Radiation(Beijing)

800 600 400

Radiation (W/m2)

200 0 1000 0 800

1000

2000

3000

4000

5000

6000

7000

8000

Ambient Temperature(Hefei)

Vertical Radiation(Hefei)

40 30 20 10 0 -10 -20 40 30

600

20

400

10

200

0

0

-10

1000 0 800

1000

2000

3000

4000

5000

Vertical Radiation(Xining)

6000

7000

8000

Ambient Temperature(Xining)

600 400 200 0 0

1000

2000

3000

4000

5000

6000

7000

8000

Time (h) Fig.4 Annual ambient temperature and vertical solar radiation in three cities

Fig. 5 Flow chart of the simulation program

14

40 30 20 10 0 -10 -20

Temperature (℃ )

1000

Journal Pre-proof 3.10 Model validation Experimental test of the system under BIPV/Water and BIPV/Air mode were conducted in 24th June 2018 and December 18th 2018, respectively. The system is installed on a hot box platform in Hefei, China. Fig. 6 is the exterior view of the experiment system. The black dots in Fig. 1 (a) and (b) are location of the main temperature measure points. Temperature of measure points are obtained by copper-constantan thermocouples with ice-point compensation. The maximum instantaneous electric power is tracked by Maximum Power Point Tracking (MPPT), a solar charge controller which enables the system to charge the battery with maximum power output [29]. The electric current & voltage are collected by 34980A data logger. The water cycle velocity is controlled by a water pump. Solar radiation intensity on vertical surface is obtained by TBQ-2 Pyranometer. Accuracy of the experimental test equipment are listed in Table 1. Table 1 Accuracy of the experimental test equipment Apparatus Accuracy Thermocouple ± 0.5℃ TBQ-2 Pyranometer ± 2% Water meter ± 2% MPPT ± 0.5% Current transducer ± 0.1%

Fig. 6 Exterior view of the experiment system RMSD (root mean square deviation) is introduced to evaluate the accuracy of the resulting 15

Journal Pre-proof model. The RMSD is calculated by the equation (26):

RMSD 

 [(T

2

sim

 Texp ) / Texp ] n

 100%

(26)

Tsim and Texp represent the results of experimental and simulated data respectively, and Centigrade temperature scale is adopted in this situation.

3.10.1 BIPV/ Water mode This section compares the simulated and experimental results in BIPV/Water mode. Fig. 7 (a) compares the experimental and simulated temperature of water tank. Texp and Tsim are the experimental and simulated average water tank temperature respectively. Fig. 7 (b) is the comparison between the simulated and experimental results of the room air, Rexp and Rsim are the experimental and simulated room air temperature. Fig. 7 (c) is the contrast between the simulated and experimental results of the electric output. Eexp and Esim are the experimental and simulated results of the electrical generation. Fig. 7 (d) is the comparison between simulated and experimental temperature of the absorber plate. Aexp and Asim represent the experimental and simulated temperature respectively. RMSD in Fig. 7 (a), (b), (c) and (d) are 0.7 %, 1.9 %, 10% and 2%respectively. The deviation of the simulated electric power is a little high, the reason is experimental data was measured by MPPT, and the data fluctuated acutely. But difference of the electrical generation throughout the day is very small. 24

39 38 37 36

Texp Tsim

35 34

8

10

12

Temperature (℃ )

32

14

16

18

31 30 29 28

26

Rexp Rsim

(b) 8

10

12

16 Eexp Esim

12 8 4

8

10

12

44

Time

27

20

(c)

(a)

14

16

Time

Temperature (℃ )

Temperature (℃ )

40

Electrical power (W)

41

18

42 40 38 36

32

Aexp Asim

(d) 8

10

12

14

Time

16

16

Time

34

18

14

16

18

Journal Pre-proof Fig. 7 Experimental and simulated results in BIPV/Water mode

3.10.2 PV/Air mode

24

110

22

100 90 80 70 60

Eexp Esim

50 40 30

8

9

10

11

12

13

14

15

16

Rtop-sim Rmid-sim Rlow-sim Rtop-exp Rmid-exp Rlow-exp

16 14 12

(c) 8

9

10

11

60

12

13

14

15

16

17

Time

55

28 26 24 22

Ttop-sim Tmid-sim Ttop-exp Tmid-exp

20 18 16 14 12

18

8

Time

30

20

10

(a)

32

Temperature (℃ )

Temperature (℃ )

120

(b) 8

9

10

11

12

13

14

15

16

Temperature (℃ )

Electical power (W)

In this section, comparisons between the simulated and experimental results in PV/Air mode are conducted. Fig. 8 (a) is the comparison between simulated and experimental results of the electrical generation. Eexp and Esim are the experimental and simulated electrical generation respectively. The RMSD of electric power output is 3.3%. Fig. 7 (b) represents the simulated and experimental room air temperature, Rtop-sim, Rmid-sim and Rlow-sim are the top, middle and lower point of the simulated room air temperature separately, Rtop-exp, Rmid-exp and Rlow-exp are the top, middle and lower point of the experimental room air temperature. The RMSD value of each point are 1.7 %, 1.5 % and 3.3 % respectively. Fig. 7 (c) shows the contrast between simulated and experimental temperature of the air tunnel. The Ttop-sim and Tmid-sim are the top and middle simulated temperature of the air tunnel respectively. And Ttop-exp and Tmid-exp are the top and middle experimental temperature of the air tunnel respectively. The RMSD value are 2.4 % and 2.0 % respectively. Fig. 7 (d) is the comparison between experimental and simulated results of the absorber plate. Aexp and Asim are the experimental and simulated temperature of the absorber plate. The RMSD of the absorber plate is 1.3%.

50 45 40 35 30

17

25

(d) 8

9

10

11

12

Time Time Fig. 8 Experimental and simulated results in BIPV/Air mode

17

Aexp Asim 13

14

15

16

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4. Results and discussion 4.1Results of system’s annual performance In this section, annual performance of the system and energy saving condition of the building are investigated based on the simulated results.

4.1.1 Annual electrical performance in three cities Fig. 9 is the comparison of annual electrical output and efficiency in Beijing Hefei and Xining. The annual average efficiency of the three cities are 12.1%, 11.2% and 10.6% respectively. The total electrical generation in these cities are 268.4 kWh, 247.7 kWh and 152.6 kWh respectively. Xining has the highest electrical efficiency and electric energy production. From the Fig. 9 it can be concluded that the electrical power and efficiency are lower in summer and higher in winter. The reason is solar altitude is higher in summer than in winter, resulting in the variation of glass transmittance. The variation of annual glass transmittance in three cities are shown in Fig. 3, section 3.1. Electrical Efficiency(Beijing) Electrical Efficiency(Hefei) Electrical Efficiency(Xining)

13.5

35

13.0

30

12.5

25

12.0

20

11.5

15

11.0

10

10.5

5

10.0

0

9.5

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month

Fig. 9 Annual electrical performance in three cities

18

Efficiency (%)

Electrical power (kWh)

40

Electrical Power(Beijing) Electrical Power(Hefei) Electrical Power(Xining)

Journal Pre-proof 4.1.2 Annual performance in three cities during heating season Because of the climate difference, heating season is different in three cities. In Beijing, the heating season is from January to March and from October to December. In Hefei, the heating season is from January to March and from November to December. And in Xining, the heating season is from January to April and from September to December. Fig. 10 is the heating load of the building without the system and solar fraction in three cities from January to March and from November to December. The solar fraction is the share of solar energy in the building’s heating load. In Beijing, the annual heating load is 2112.6 kWh, and the solar fraction during heating season in Beijing is 49.9%. In Hefei, the annual heating load is 1482.2 kWh, and the solar fraction is 38.7%. In Xining, the annual heating load is 3656.1 kWh, the solar fraction is 41.3%. The monthly detailed heating load and solar fraction are showed in Table 2. In Table 2, the solar fraction in January and December are lower than other month. Because in January and December, ambient temperature is lower than other month, hence the daily heating load of the building is increased. Therefore solar fraction in winter is lower than other season. Solar fraction is the proportion of solar energy in daily thermal energy consumption of the building and defined as:

F=

Esolar

Eheating

(47)

Where F is the solar fraction, Esolar is the daily energy provided by the system, and Eheating is the daily thermal consumption of the system. The system is only able to work in the daytime and in winter’s nighttime without solar radiation, effect of space heating is unsatisfactory. Considering the solar fraction is a full-day index, therefore the solar fraction in winter is not very high. But from the experimental results in Fig.8 (b), the maximum room temperature is over 30℃ in winter’s daytime, which proves the system is able to provide passive heating during the daytime effectively. And during the nighttime, auxiliary power is required to keep the room air temperature. Because the building is made by bricks with heat storage capacity, heating load of the building during the night is able to be decreased because of the energy obtained by the brick walls in the daytime.

19

Journal Pre-proof Heating load (Beijing) Heating load (Hefei) Heating load (Xining)

800

Solar fraction (Beijing) Solar fraction (Hefei) Solar fraction (Xining)

80

60

400

40

Solar fraction (%)

Enenrgy (kWh)

600

200

0

20 Jan

Feb

Mar

Nov

Dec

Month Fig. 10 Energy consumption and solar fraction in three cities during heating season Table 2 Monthly heating load and solar fraction in three cities Month

1

2

3

4

9

10

11

12

Total

Heating load(Beijing)

742.5 kWh

419.3 kWh

173.5 kWh

0 kWh

0 kWh

0kWh

220 kWh

482 kWh

2112.6 kWh

Solar fraction(Beijing)

27%

52.9%

122.8%

0

0

0%

93.2%

36.9%

49.9%

Heating load (Hefei)

502.2 kWh

311.8 kWh

202.3 kWh

0 kWh

0 kWh

0 kWh

138.6 kWh

327.3 kWh

1482.2 kWh

Solar fraction(Hefei)

20%

32%

53%

110%

35%

38.7%

Heating load(Xining)

923.3 kWh

620 kWh

400.3 kWh

149.5 kWh

90.3 kWh

221.2 kWh

434.7 kWh

816.8 kWh

3656.1 kWh

Solar fraction(Xining)

21%

34%

44%

107%

161%

92%

52%

24%

41.3%

0

0

0

4.1.3 Hot water supplement in non-heating season PV-Water mode is active in different moths in three cities because of the weather difference. In Beijing, it’s from April to September. In Hefei, it’s from April to October. And in Xining, it’s from May to August. Fig. 11 is the monthly heat gain of hot water and energy provided by auxiliary power in order to keep final temperature of the water tank over 45 ℃. From the Fig. 11 it can be concluded that there is more energy demand from auxiliary power in summer than in winter. The reason is that the illumination intensity on vertical surface in summer is lower than winter, and the initial water temperature is low because underground water is used. Table 3 shows the monthly thermal efficiency and the ratio of solar energy in the total hot water energy demand. The average thermal efficiency in Beijing, Hefei and Xining are 46.6%, 50.9% and 44.8% respectively. The average ratio in three cities are 79.1%, 66.8% and 60.4% respectively. The result indicates that this wall system has the highest thermal efficiency in Hefei, and can meet more demand of hot water in Beijing.

20

Journal Pre-proof Heat source (Beijing) Solar energy (Beijing)

300

250

Energy (kWh)

Heat source (Xining) Solar energy (Xining)

Heat source(Hefei) Solar energy(Hefei)

200

150

100

50

0 Apr

May

Jun

Jul

Aug

Sep

Oct

Month Fig. 11 Energy provided by solar energy and auxiliary power for hot water supplement Table 3 Hot water supplement in three cities Month

4

5

6

7

8

9

10

Total

Efficiency(Beijing) Efficiency(Hefei) Efficiency(Xining) Ratio(Beijing) Ratio(Hefei) Ratio(Xining)

35.7% 41.3% 0.0% 84.5% 59.4% 0.0%

42.6%

51.2%

54.4%

49.1%

46.8%

0.0%

50.7% 38.9% 70.1% 57.% 58.5%

62.0% 45.2% 71.9% 57.4% 53.7%

60.2% 48.7% 73.4% 65.6% 61.5%

55.8% 46.5% 86.6% 74.4% 67.8%

48.0% 0.0% 88.1% 76.6% 0.0%

38.5% 0.0% 0% 77.0% 0.0%

46.6% 50.9% 44.8% 79.1% 66.8% 60.4%

4.1.4 Heat gain of south wall with and without the system in summer In summer this system can prevent most of the energy from transmitting into the building, therefore the cooling load is able to be decreased. In this section heat gain of south wall with & without the system in summer are compared in three cities. Fig. 12 is the monthly heat gain though south wall with & without the system per meter square in summer. It can be proved that heat gain though south wall is decreased significantly because of this wall system. Table 4 is the detail of the heat gain though south wall per meter square. The system can decrease 18.4, 20.2 and 9.2 kWh/m2 heat gain though south wall during summer season in Beijing, Hefei and Xining respectively.

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12

With(Xining) Without(Xining)

With(Hefei) Without(Hefei)

With(Beijing) Without(Beijing)

Enerngy (kWh)

10

8

6

4

2

0 May

Jun

Jul

Month

Aug

Fig. 12 Monthly heat gain though south wall per meter square in summer. Table 4 heat gain though south wall Month

May

Jun

Jul

Aug

Total(kWh)

Beijing(With) Hefei(With) Xining(with) Beijing(Without) Hefei(Without) Xining(without)

1.5 0.7 2.1 4.6 3.4 4.2

1.5 0.8 1.2 6 4.3 3.2

1.7 1.7 1.1 6.8 7.4 3.6

2.4 0.1 1.8 8.1 8.4 4.4

7.1 3.3 6.2 25.5 23.5 15.4

4.1.5 Annual energy performance of the system in three cities Table 5 is the annual energy performance of the three cities. The total energy saving in Beijing, Hefei and Xining are 2661.8, 1908.4 and 2412.3 kWh respectively. From the Table 5 it can be concluded that in Beijing, the system can achieve the highest total energy saving and solar fraction in winter. The system blocks most heating load and has the highest thermal efficiency in Hefei. And in Xining, system has the highest electrical power output and efficiency. Table 5 Annual energy performance of the system in different regions Region

Winter Heating

Solar fraction

Hot water load

Solar supply

Electricity

Blocked heating

Total

load (kWh)

(%)

(kWh)

rate (%)

generation (kWh)

load(kWh)

(kWh)

Beijing

2112.6

49.9

1470.8

79.1

247.7

36.8

2661.8

Hefei

1482.2

38.7

1705.9

66.8

152.6

40.2

1908.4

Xining

3656.1

41.3

998.0

60.4

268.4

18.4

2412.3

22

Journal Pre-proof 4.2 Discussions In this section, annual thermal and electrical performance under difference situations are investigated, the impact on the building’s energy saving is also putting forward. In this case, weather data of typical meteorological year in Beijing is configured as a data input for the simulation.

4.2.1

Internal and external PV cells

In the previous section, the structure of the system is internal, thus the PV cells are laminated on the absorber plate. In this section, external PV cells is discussed, which means the PV cells are attached on the glass cover. Surface area of the PV cells in both structure are the same. The reason why internal and external PV structures are discussed is based on the present technology and material, if PV cells are laminated on the absorber, the PV cells deform easily after the sheet-and-tube is welded on the back of the panel. Moreover, thermal stress of the PV panel under internal structure is large, therefore the lifetime and security of the system is not able to be ensured. When PV cells are laminated on the glass, using life of the PV is guaranteed. But the following question is because PV cells are on the glass, part of the solar radiation is blocked, therefore thermal performance of the system is affected. Fig. 13 is the annual electrical power output and efficiency of the internal and external structure respectively. The power output and efficiency of the external structure are both significantly higher than the internal structure. The reason is firstly, the temperature of glass cover is lower than the temperature of absorber plate. Secondly, the external PV cells can absorb more solar radiation than the internal PV system. The annual electrical output and efficiency of the internal system are 247.7 kWh and 11.2% respectively. As for the external system, the annual electrical output and efficiency of the internal system are 291.5 kWh and 12.3%, up by 17.7% and 9.8% respectively.

Electrical power (kWh)

30

14

Electrical Efficiency (External) Electrical Efficiency (Internal) 13

25 20

12 15 10

11

5 0

10 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

23

Efficiency

35

Electrical Power (Internal) Electrical Power (External)

Journal Pre-proof Fig. 13 Annual electrical power output and efficiency of the internal and external structure Fig. 14 is the heating load of the building under external and internal system during the heating season respectively. Because the PV cells block a lot of solar radiation under the external system, energy from the solar radiation is lower than the internal structure. Annual energy provided by solar are 1214.5 kWh and 710.7 kWh for internal and external system. Annual energy provided by auxiliary power are 1414 and 1483 kWh for internal and external system respectively. The above prove the external system will not raise the energy consumption of the auxiliary power, which means the external system is able to provide passive heating in the winter’s daytime as well as the internal system.

Auxiliary power(Internal) Solar energy(Internal)

800

Auxiliary power(External) Solar energy (External)

700

Energy (kWh)

600 500 400 300 200 100 0 Jan

Feb

Mar

Oct

Nov

Dec

Month Fig. 14 The heating load of the external and internal system In non-heating season, the external system is not able to provide 200 L hot water as well as the internal system because the PV cells block half of the radiation and in summer, the solar radiation on vertical surface is already small. Thus, 100 L tank is used in the external system. Considering the house is 6m·3m·3m, family formed by two adults and one child is appropriate to live. Based on the Technical standard for solar water heating system of civil buildings in China (GB 503642018)[31]，the daily average hot water consumption per person for residential building is 20-60 L. Therefore the system is able to meet the requirements of an ordinary family in this case. Fig. 15 is the thermal efficiency and energy provided by solar & auxiliary power. Energy provided by solar and auxiliary power are 123.1 and 37.2 kWh respectively, which means the solar energy can meet 76.8% hot water energy demand. Table 6 is the monthly temperature of water tank. For 132 days in non-heating season, the water temperature can reach 40 ℃ or higher. For 85 days in non-heating season, the water temperature can reach 45 ℃ or higher. The average water tank temperature in this period is 42.5 ℃. Table 6 Monthly temperature of water tank Month

No. days (>40℃)

No. days (>45℃)

Average temperature(℃)

4

23

20

43.6

24

Journal Pre-proof 5

21

9

38.9

6

20

2

40.1

7

21

13

40.8

8

27

20

45.8

9

20

21

45.7

Total

132

85

42.5

Supplementary heat source Solar energy

180

Thermal efficiency 64

Energy (kWh)

56 120

90

48

60 40

Thermal efficiency (%)

150

30

0

32 Apr

May

Jun

Jul

Aug

Sep

Month

Fig. 15 Hot water supplement in non-heating season

4.2.2

CdTe and Si solar cells

In this section, a novel thin film CdTe solar cells is compared with traditional Si solar cells which are adopted in the system. The advantages of CdTe material are its suitable band gap, and high optical absorption coefficient nearly about 100% due to the fact of thickness being approximately 2 μm [23]. Because of the simplification of its preparation technology, the production cost is very low. Besides that, it can keep relatively higher electrical efficiency in low light situation like cloudy or rainy days [24]. Temperature coefficient is -0.21% [27], nearly 50% lower than traditional Si solar cells, which is 0.4%. Many research team is trying to raise the efficiency of the CdTe solar cell, up to now, the highest efficiency of CdTe is over 22%. In spite of the relatively lower efficiency than Si solar cell, the CdTe solar cells have a good application prospects especially in BIPV/T field. In China, the CdTe solar panels available in the market reach the highest efficiency of 10% in engineering application. As for the life circle of the CdTe, 90% power output is guaranteed in 10 years and 80% power output is guaranteed in 25 years. Fig. 16 compares the electrical power output and efficiency of CdTe and Si solar cells. The annual average efficiency of CdTe and Si solar cells are 8.3% and 11.2% respectively. The annual electrical output are 216.5 and 247.7 kWh respectively. The electrical output difference between the CdTe and Si is rarely small under the circumstances 25

Journal Pre-proof that the difference in electrical efficiency is significant. Because the CdTe solar cells are thinner, more beautiful, higher temperature resistance, cheaper and have better low light performance and longer service life, once the efficiency of CdTe for engineering application is promoted, the power output can exceed the traditional Si solar cells. Especially in BIPV/T field, excellent low light performance is able to generate electricity steadily, which is absolutely a great news for PV Gridconnected system. Low temperature coefficient and high temperature resistance is friendly for thermal application in the building.

Electrical Power (Si) Electrical Power (CdTe)

Electrical Efficiency(Si) Electrical Efficiency (CdTe)

13

25

12

20

11

15

10

10

9

5

8

0

Efficiency (%)

Power (kWh)

30

7 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Fig. 16 Comparison of CdTe and Si solar cells

4.2.3

Aspect ratio of the BIPV/T system

To evaluate the impact of the aspect ratio on the indoor thermal environment, the ratio is changed from 1:1 to 4:1 under the same surface area, which is shown in Fig. 17. From Fig. 18, with the growth of the aspect ratio, more energy are transferred into the room. The reason is larger aspect ratio resulting in higher wind speed in the air channel, therefore more hot air is able to flow into the room. Considering floor height and structural strength, the ratio is recommended as 3:1 when the coverage area of the system is 2m2.

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Fig. 17 Schematics of different aspect ratio 1:1 2:1 3:1 4:1

280 240

Energy (kWh)

200 160 120 80 40 0 Jan

Feb

Mar

Oct

Nov

Dec

Month

Fig. 18 Energy provide by the system under different aspect ratio

5.Conclusion This paper introduced a novel multi-functional BIPV/T solar wall system. The system has two different working modes, BIPV Water mode for non-heating season and BIPV Air mode for heating season, in order to satisfy the seasonal energy demand of the building. Based on the heat transfer analyses, dynamic mathematical model of the system is established and verified by the experimental results. Annual analysis of the system in three typical cities: Beijing (North China), Hefei (Middle China) and Xining (North-west China) is putting forward. Monthly thermal and electricity 27

Journal Pre-proof performance of the wall system under different working modes are discussed. The impact of external/internal system, Si/CdTe solar cells and aspect ratio of the system are investigated respectively. The results lead to the following conclusions: 1. The annual average efficiency are 11.2%, 10.6% and 12.1% in Beijing, Hefei and Xining respectively. The overall electrical generation in these cities are 247.7 kWh, 152.6 kWh and 268.4 kWh respectively. The electricity efficiency changes significantly with time, the reason is the change of the sun position and illumination intensity of the glass. 2. The annual heating load in Beijing, Hefei and Xining are is 2112.6 kWh, 1482.2 kWh and 3656.1 kWh respectively. The solar fraction in these three cities are 49.9%, 38.7% and 41.3%. 3. The system can provide 79.1%, 66.8% and 60.4% of the hot water energy demand in these cities respectively. 4. Heat gain though south wall is decreased significantly because of this wall system in summer. The system can decrease 18.4, 20.2 and 9.2 kWh/m2 energy though south wall during summer season in Beijing, Hefei and Xining respectively. 5. The annual energy saving in Beijing, Hefei and Xining are 2661.8, 1908.4 and 2412.3 kWh respectively. 6. External system can generate 291.5 kWh of electricity, which is 17.7% higher than the internal system. The external system is able to heat the room in the winter’s daytime as well as the internal system. But in summer, only 100L of hot water can be created by the external system. 7. The annual electrical output of CdTe and Si solar cells are 216.5 and 247.7 kWh respectively, while the difference is small. Considering the low light performance, low temperature coefficient and high temperature resistance of CdTe, this new thin film solar cell has great potential in the BIPV/T field. 8. With the growth of the aspect ratio, more energy are brought into the room. And considering floor height and structural strength, for a 2m2 system, the ratio is recommended as 3:1. There are still several restrictions of the present study. Firstly, the mathematical model is one dimensional, heat transfer in other directions are not considered. Secondly, thermal stress on the PV panel is not deeply discussed. Thirdly, hot water generated by the system is only for residential demand, more application methods are able to be discussed. Therefore, in the future study mathematical model will be improved firstly. Secondly, thermal stress will be investigated in the system. Thirdly, more applications of hot water generated by the system will be researched, heat pump as an example.

6.Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51878636), Key Research and Development Project of Anhui Province (201904a07020014), National Key Research and Development Project, Ministry of Science and Technology of China (No. 2016YFE0124800), Bureau of International Cooperation, Chinese Academy of Sciences (No. 211134KYSB20160005) and the DongGuan Innovative Research Team Program (No. 2014607101008).

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Journal Pre-proof Conflict of interests statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in the manuscript entitled “Annual analysis of a multi-functional BIPV/T solar wall system in typical cities of China”

Journal Pre-proof Highlights: 1. This paper introduces a novel multi-functional BIPV/T wall system for building in cold winter and hot summer area. 2. Mathematical models are established and verified by experimental results. 3. Annual performance of the system in three different typical cities are evaluated. 4. System’s performance under different parameters are conducted.