Thermal and electrical behavior of built-middle photovoltaic integrated Trombe wall: Experimental and numerical study

Journal Pre-proof Thermal and electrical behavior of built-middle photovoltaic integrated Trombe wall: experimental and numerical study

Yuan Lin, Jie Ji, Xiangyou Lu, Kun Luo, Fan Zhou, Yang Ma PII:

S0360-5442(19)31868-7

DOI:

https://doi.org/10.1016/j.energy.2019.116173

Reference:

EGY 116173

To appear in:

Energy

Received Date:

09 April 2019

Accepted Date:

19 September 2019

Please cite this article as: Yuan Lin, Jie Ji, Xiangyou Lu, Kun Luo, Fan Zhou, Yang Ma, Thermal and electrical behavior of built-middle photovoltaic integrated Trombe wall: experimental and numerical study, Energy (2019), https://doi.org/10.1016/j.energy.2019.116173

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Thermal and electrical behavior of built-middle photovoltaic integrated Trombe wall: experimental and numerical study Yuan Lin b,a, Jie Ji a,*, Xiangyou Lub, Kun Luo a, Fan Zhou a, Yang Mac a

Department of Thermal Science and Energy Engineering, University of Science and Technology

of China, Hefei 230026, China b

Department of Architectural Environment and Energy Engineering, Anhui Jianzhu University,

Hefei 230009, China c

Sungrow, Hefei 230088, China

* Corresponding author. Tel.: 0086-551-63607346. E-mail address: [email protected]

ABSTRACT This study proposed a new system with the photovoltaic (PV) panel installed in the middle of channel of the Trombe wall (TW) system, called as built-middle photovoltaic integrated Trombe wall (PVMTW), which can realize multiple functions of electricity generation, space heating and heat preservation. The experiment rig was built to study the temperature field of the PVMTW system in heating seasons, in Hefei. A mathematical model of the PVMTW system was developed and validated against experimental data. Using the validated model, the thermal performance of the PVMTW system was investigated by comparison with that of the classic Trombe wall. The results showed that in the daytime, the average thermal efficiency of the PVMTW system was 65.2% higher than that of the classic TW system. In terms of room air temperature and interior surface temperatures on the walls, the indoor thermal comfortable of the PVMTW system was almost the same as that of the classic TW system. The average

Journal Pre-proof predicted mean vote (PMV) for two rooms (room with the PVMTW system and room with the classic TW system) were 0.05 and -0.36, respectively. Additionally, the average electrical efficiency and average total efficiency achieved 0.120 and 0.585, respectively. Keywords: PV Trombe wall; Thermal comfort; PMV index; Electrical performance; Total efficiency Nomenclature sim/S simulated data mea/M measured data G solar radiation intensity, W/m2 T Temperature, K c specific heat capacity, J/(kg·K) h heat transfer coefficient, W/(m2·K) u speed, m/s R,I thermal resistance, (m2·K)/W H height, m L length, m W width; external work, m, W D depth, m A area, m2 E output power, W Q energy, W U voltage of PV module, V I current of PV module, A P Hydraulic diameter, pressure m, pa C resistance coefficient,g gravity acceleration, m/s2 p person t time, s Nu Nusslet number,Gr Grashof number,Pr Prandtl number,Ra Rayleight number,K geometric factor n air ventilation rate M the body's metabolic rate, W Greek letters

ζ η ν β

PV cells coverage ratio, efficiency,dynamic viscosity of air, m2/s thermal expansion coefficient, K-1

Subscripts th thermal energy g glass amb ambient PV PV cells in inlet out outlet a air r room; radiation c convection, ceiling eq equivalent ch channel b base panel (absorber plate) w wall tw massive wall re reference power power generation f friction en envelop in interior surface i different walls; different data points cl clothes total total Abbreviations

Journal Pre-proof ρ

density, kg/m3

PMV

predicted mean vote

τ α σ e δ λ φ

transmissivity,absorptivity, -;thermal diffusivity, m2/s Stefen-Boltzmann constant, W/(m2·K4) emissivity,thickness, m thermal conductivity, W/(m·K) the relative humidity

RMSE TPT EVA PVOTW

root-mean –square error tedlar-polyester-tellar ethylene-vinyl acetate built-out photovoltaic integrated Trombe wall built-middle photovoltaic integrated Trombe wall built-in photovoltaic integrated Trombe wall photovoltaic blind-integrated Trombe wall

PVMTW PVITW PVBTW

1. Introduction

Due to energy crisis and environmental problems, reducing the total energy demand and the consumption of fossil fuels are crucial challenges. Building sector is one of the major components of the global energy consumption, accounting for approximately 40% of the global energy consumption [1] to maintain a comfortable indoor environment. Therefore, the construction of energy-saving buildings and the application of renewable energy such as solar energy in buildings are effective method. One of the maturest and cheapest approaches of using solar energy is Trombe wall (TW) system, realizing space heating and gaining indoor thermal comfort without any mechanical assistance in heating season. Trombe wall system has been popularized due to its unique features, such as simple implementation geometry, zero running cost and operating reliability[2, 3]. However, the traditional Trombe wall has the inevitable

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drawbacks of single function, low thermal resistance and some aesthetic problems caused by black appearance. Many researchers proposed various optimization schemes for the conventional Trombe wall, aiming to make the performance of the improved Trombe wall superior to that of the conventional Trombe wall. Luo et al.[4] conducted an experimental analysis on the energy performance of a modified Trombe wall using phase change materials. The results showed that effective prevention from summer overheating problem and good heating effect in winter can be achieved by the modified Trombe wall. Yu et al. [5]proposed an improved solar wall (i.e. a heating system combining solar air collector with hollow ventilated interior wall) for residential buildings on Tibetan Plateau, established the mathematics by EnergyPlus and analyzed the influence of various parameters on the thermal performance of the improved system. It was showed by simulations that the improved solar wall was applicable to the heating of residential buildings on Tibetan Plateau. Dong et al. [6] investigated experimentally the heating performance of a novel designed Trombe wall and the results showed the daily thermal efficiency of the improved Trombe wall was higher than 50% during the daytime. A mechanically ventilated Trombe wall with additional windows on the storage wall was proposed by Ma et al. [7]. The results showed that the heating load

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was reduced by sending the air from the Trombe wall to the central air-conditioned room and installing a large heat capacity material on the floor in the Trombe wall. Rabani et al. [8] studied the thermal performance of a modified Trombe wall, which can receive solar radiation from the directions of south, west, and east. The results showed that the temperature range of the room with the improved Trombe wall was kept within 15 °C-30 °C on the coldest winter days and weeks in Yazd, implying that the improved Trombe wall can provide a comfortable indoor temperature. Saaadatian et al. [9] invented a zigzag Trombe wall, which can reduce glare and control unnecessary heat increase. Souayfane et al. [10] conducted a numerical simulations and experimental studies on the energy performance and economic analysis of the TIMPCM wall under different climates. The results showed that the energy-saving performance of building with the TIM-PCM wall can be significantly improved by using the TIM-PCM wall instead of a conventional insulated double-glazed window. Furthermore, they also concluded that the total loads decreased with the increase of the area of the TIM-PCM wall, except in the Mediterranean climate. Duan et al. [11] conducted a study on the thermal performance of two different types of Trombe wall. Type1 is that the absorber plate is pasted on the massive wall, and Type2 is that the absorber plate is placed between the glass cover and the massive wall. Results showed

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that the energy and exergy efficiencies of Type2 are greater than those of Type1. Furthermore, there are various schemes to help improve the efficiencies of the Trombe wall such as thermal insulation [12], DC fans [12-14] , glazing effect [15, 16], shading devices[17], the massive wall’s materials [18, 19], and size and thickness [20]. Although some improvements have been made to the traditional Trombe wall, its application has been restricted due to its single function, low energy efficiency and poor aesthetic appearance. The conjunctive use of Trombe wall with photovoltaic (PV) cells (PVTW) can solve the problems of the traditional Trombe wall and has attracted relatively more attention. The Trombe wall integrated with photovoltaic cells (PVTW) can simultaneously realize the dual functions of space heating and power generation, the overall efficiency of this hybrid system' solar energy harvesting will be enlarged dramatically compared to the respective independent systems. Furthermore, the color of the blue PV cells can improve the aesthetic appearance of the traditional Trombe wall to a certain extent. Moreover, the flowing air in the channel extracts the heat away from the PV cells, which reduces the temperature of the PV cells and improves the electrical performance of the hybrid system. Irshad et al. [21]used TRNSYW software to investigate the thermal and electrical performance of the three different types PVTW system. Type 1 is Trombe wall with PV Single Glazing, Type 2 is Trombe wall with

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PV Double Glazing, and Type 3 is Trombe wall with PV Double Glazing filled with gas of Argon. The results showed that in terms of saving in cooling energy cost and CO2 Emission, the order was Type 3> Type 2> Type 1. In addition, they also analyzed the effect of air mass flow on energy performance. Results showed that the air mass flow had a significant effect on the performance of three types of PVTW systems up to certain values after which its effect stagnates and the three types of PVTW systems had no further improvement. Ji et al. [22] proposed the concept by laminating PV cells made of crystalline silicon (c-Si) on the exterior glazing cover of the classical Trombe wall, which was named as built-out photovoltaic integrated Trombe wall (PVOTW) .They carried out a series of theoretical simulation and experimental studies on the thermal and electrical performance of the PVOTW system. They investigated the effects of the DC fan [13], PV coverage ratio [23] and south facade design [24] on performance of the PVOTW. The results showed that the assisted DC fan can not only improve the indoor temperature but also cool the PV cells. Results from simulations showed that the thermal efficiency of the PVOTW system with a southern facing window was reduced 27% compared to that without a southern facing window. With the increase of the PV coverage ratio, the electrical efficiency decreases slightly, while the thermal efficiency drops dramatically. The simulation data showed that compared with the traditional

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Trombe wall system, the PV coverage ratio of 33.4% and 100% reduced the thermal efficiency of the PVOTW system by 7% and 17%, respectively. This reason can be explained that the opaque c-Si PV cells hinder part of the sun rays into Trombe wall. In this case, Koyunbaba et al. [25, 26] presented a approach of replacing the opaque PV cells with semi-transparent PV cells. Results indicated that after adopting this approach, the thermal efficiency of PVTW system reached to 27 %, while the electrical efficiency was only 4.52 % due to the inefficient material of semi-transparent PV cells. Taffesse et al. [16] carried out a related study on the building integrated semi-transparent PV cells and the they drew the conclusion that the Trombe wall with semi-transparent PV cells was more suitable for space heating. Ahmed et al. [19] performed a numerical and experimental analysis on the performance of two different types of the hybrid PVTW systems using porous medium and DC fan. The difference between Type 1 and type 2 is that there is no glass cover in the hybrid system of type 1. The results showed that compared with the traditional the PVETW system, the thermal and electrical efficiencies of type 1 were increased by 13% and 4%, respectively, while those of type 2 were increased by 20% and 0.5%, respectively. In addition to the above-mentioned PVOTW system, Su et al. [27, 28] proposed a different structural design that the PV cells was installed on the interior massive wall surface of Trombe wall other than on

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the exterior glass cover surface, which was called as built-in photovoltaic integrated Trombe wall (PVITW). They used CFD software to study the ventilation performance of the PVITW. Results showed the channel size and solar radiation influenced the ventilation performance remarkably. Obviously, this structural design can improve the thermal performance of the PVITW system, but it is not conductive to cooling of PV cells attached on the massive wall. Another different design structure of PV blindintegrated Trombe wall (PVBTW) system was proposed by Hu et al.[29, 30]. For this novel PVBTW system, the PV blind was used for shading devices as well as electricity generator. They investigated the performance of the PVBTW system through a comparative study with the PVETW system and the PVITW system. Results showed that annual electricity output of PVBTW system was almost the same as that of the PVETW system, which was 20% higher than that of the PVITW system under Hefei climate condition. Moreover, they also conducted research to optimize the inlet air flow rate and the PV blind angle which were linked to the calculation of electricity generation and heat gains. The results suggested that the inlet air flow rate of 0.45 m/s and the PV blind angle of 50° achieved the maximum electricity generation and the maximum heat gains. Though the PVBTW system presented its energy saving potential in the utilization of solar energy and the reduction of building cooling load, the control

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of the overhigh temperature of the PV blind cells in hot summer seems unattainable [31]. In general, PVTW can be grouped into three types in terms of the PV cells installation location in the Trombe wall system: PVOTW (Fig.1a), PVITW (Fig.1b) and PVBTW (Fig.1c). Fig.1 shows Schematic diagrams of three types of PVTW systems. This study presents a new structure of the PVTW system. This new design of Trombe wall combined PV cells structure is to install PV panel attached on the absorber plate into the middle of the air channel of the Trombe wall system, which is called as built-middle photovoltaic integrated Trombe wall (PVMTW). The structure of this new PVMTW system differs from that of the existing PVTW systems in that there is a sealed air layer in the PVMTW system, the structure diagram of the PVMTW system is shown in Fig.2c. This new PVMTW system has multiple functions: passive heating, thermal insulation and electricity generation. In order to learn a further insight into the performance of the PVMTW system, a test prototype of the PVMTW system was constructed and a series of tests were carried out on this test rig. Additionally, we established a dynamic mathematical model and validated it with experimental data. Then, using the validated model of the PVMTW system, the thermal performance is investigated by comparing with the classic TW system in terms of the heat gains, the air temperature distribution in the channel and room, the temperature distribution on the interior surface of walls and the indoor thermal comfort. In the end, the electrical and overall performance of the MPVTW system were analyzed by experimental data.

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Fig.1 Schematic diagram of the three existing types of PV- integrated Trombe wall systems (a) PVOTW, (b) PVITW, (c) PVBTW.

2. System description and experimental setup

2.1 System description

The proposed PVMTW system in this study is schematically shown in Fig.2, mainly consisting of a 5-mm-thick glass cover, a 20-mm-deep air layer, a PV panel integrated PV cells and absorber plate, a 100-mm-deep air channel, a 400-mm -thick massive wall, and 4 vents with the same size of 800 mm (length) ×80 mm (width). The dimensions of PV-Trombe wall system are height of 2000 mm and width of 994 mm. The PV panel as the power unit consists of 50 pieces mono-crystalline silicon cell of the same size 156 mm (length) ×156 mm (width) in parallel by a wire, with 0.60 PV cells coverage ratio (Fig.3b). For the MPVTW system, the PV panel integrated PV cells and absorber plate is installed in the air channel, which divides the air channel into two parts. A part of the space between glass cover and the PV panel is formed into a sealed

Journal Pre-proof air layer, which improves the heat preservation and heat insulation functions of buildings. Another part of the space between the PV panel and the massive wall is formed into the conventional air channel. Figs.2a and 2b depict the two operation models of the PVMTW system at different times in heating seasons. It should be noted that Vent 1 and Vent 4 are always closed during the whole heating season. In the daytime, Vent 2 and Vent 3 can be turned on/off to gain a moderate indoor temperature. When the solar radiation incidents on the PV cells, the PV cells convert a part of the captured solar energy into electricity to realize function of electricity generation. At the same time, the air in the channel is heated by PV cells, then the upward airflow is formed in the channel. The hot air flows into the room space through the outlet vent3 and mixed with the room air. The indoor cold air sinks and flow into the channel from inlet vent2, thus the heat transfer of natural convection is formed. In this procedure, the indoor cold air is continuously induced into the channel, taking away the heat generated by the PV cells while cooling the PV cells. So the function of passive heating is realized. In nighttime, Vent 2 and Vent 3 are closed due to the needing heat preservation of building, during which the Trombe wall releases stored heat for space heating. Moreover, the air layer with good thermal insulation can block heat loss to the outdoor environment.

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Fig.2. Schematic diagrams of winter operations and structure diagram of PVMTW system (a) operation mode in the nighttime, (b) operation mode in the daytime, (c) structure diagram of the PVMTW system

Fig.3. Photo of south wall installed with PVMTW system and front view of PV Cells Panel

2.2 experimental setup

The experimental test of the PVMTW system was conducted in the heating season. Two sets of the PVMTW systems are mounted on a southward existing facade of a hot-

Journal Pre-proof box building in Hefei (117° E, 32° N), China. The photo of the front view of the test building system is shown in Fig.3a. The dimension of test hot-box is 3900 mm (depth) × 3800 mm (width) × 2600 mm (height). Except for the south wall, the other walls of the hot-box have the same three-layer structure. The order from outside to inside is 40mm-thick steel panel, 900-mm-thick air layer and 50-mm-thick insulation panel. The experiment investigations was conducted from December 20th, 2017 to December 22nd, 2017. On the first day of the test, Vent2 and Vent3 were opened at 9:45 am and closed at 16:00 pm. On the second and third day of the experiment, Vent2 and Vent3 were opened at 9:00 am and closed at 16:00 pm. The global pyranometer with an error of  2% is used to measure the intensity of vertical solar radiation. Temperatures of all measuring point are measured by copper-constantan thermocouples within an error of  0.5°. The purpose of measuring temperature is to investigate the temperature distribution in the PVMTW system. The position of main temperature measuring points is shown in Fig.2. The power generated by PV cells is charged to the battery by the charging controller. The generated voltage and output current are recorded during the experiment. All test data will be recorded and saved by a portable data acquisition instrument (Agilent34980A). The time step for the data acquisition instrument is set to be 30s.

800 700 600 500 400 300 200 100 0

G

21

Te

18 15 12 9 6

Temperature(℃ )

Solar radiation(W/m2)

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3 0

10

20 30 Time (hour)

40

50

60

Fig.4. Environmental temperature and vertical solar irradiation

3.

Numerical research and behavior evaluation

3.1 Assumptions To gain a further insight into the thermal behavior of the PVMTW system, the temperature field of all components is built and the following assumptions have been made. 1) All physical parameters are constants and the air is regarded as ideal gas, the density and specific heat of the air is constant. 2) The materials of components of system, such as glass cover, PV cells, absorber plate, building envelop, etc., are considered to be isotropic. 3) Due to the small heat capacity of the glass material, it can be considered that the glass temperature is evenly distributed in space and only changes with time. 4) When all the vents are closed, the air in the channel is approximately regarded as a stationary state. 5) The heat flow direction in the building envelop is only considered to be onedimensional along its thickness.

Journal Pre-proof 6) Indoor air temperature gradient is simplified to vertical direction. 3.2 Mathematical model of system For glass cover:

 g cg  g

dTg dt

 hc ,r,amb (Teq  Tg )  hg , PV (TPV  Tg )  G  ag

(2)

where Teq is equivalent temperature and its value sets to be ambient temperature [32]; and hc,r.amb is a hybrid heat transfer coefficient combined by convective heat transfer (between the glass cover and ambient air ) coefficient and radiative heat transfer (between the glass cover and sky) coefficient, and its value equals to 29 W/(m2K) [32]; the heat transfer coefficient hg,PV between the glass cover and PV cells is defined as:

hg , pv   Tg 2  TPV 2   Tg  TPV  

1/  PV

Nu g , PV  a 1   1/  g  1 a

(3)

where Nug,PV is Nusslet number and defined as [33]: Nu g , PV  Max{1, 0.228  ( Ra / K )0.25 , 0.039 Ra 0.33 }

(4)

where K is the geometric factor, calculated by the formula K =Hch/Da; Rayleigh criterion number (Ra) is defined as:

Ra 

g   g , PV  (TPV  Tg )  H ch 3

 

,

 g , PV 

1 (TPV / 2  Tg / 2  273)

(5)

where βg,PV is the expansion coefficient of air in the sealed air layer. For PV cells:

 PV  cPV   PV 

TPV  2TPV  PV  hg ,PV Tg  TPV   G      PV t y 2

T  T   b PV    E RPV ,b

(6)

PV

where (τα)PV is the effective absorptivity of PV cells; and RPV,b is the thermal contact

Journal Pre-proof resistance between PV cells and absorber plate, and is expressed as [34]: Rb , PV  RPV ,b 

 TPT  EVA ; the electricity generation( EPV )is calculated by[35]:  TPT  EVA

EPV  G  ( ) PV re  [1  0.0045  ((TPV  273.15)  298.15)]

(7)

where ηre is the maximum power point cell efficiency at the reference temperature of 298.15 K; and (τα)PV is the PV cells effective absorptance and is given as:

  PV



 g 1  1     g

(8)

For absorber plate (base plane):

Tb  2Tb b  cb   b   b 2  hrb ,tw Ttw  Tb   hca ,b Ta  Tb   G  (1   )  g   b t y

T  T   PV b

(9)

RPV ,b

where hrb ,tw is the heat transfer coefficient between absorber plate and massive wall, which is expressed as Eq.(9); and hca,b is the surface convective heat transfer coefficient on the absorber plate and it is given by Eq.(10). hrb ,tw   Ttw 2  Tb 2  Ttw  Tb  hca ,b 

1 1/  tw  1/  b  1

(10)

Nuch  a Dch / 2

(11)

In the case of the natural convection heat transfer in vertical channel with openings of upper and lower, the Nuch is calculated by [36]: D 1 Nuch  Rach  ch 24  H ch

Rach 

g   ch  (Dch )3  (

Tb  Ttw  Ta ,ch ) 2 ,

 

3/4

     35  1  exp       Rach ( Dch / H ch )  

  ch

1 Ta ,ch  273.15

(12)

(13)

Journal Pre-proof For air in the channel: (1) Case in which vent2 and vent3 are open In the daytime, the air in the channel obtains thermal energy released from PV cells. The temperature of air varies along the channel height, so the model of air in the channel is expressed as:

 a  c  Dch

Ta T   a  c p ,a  ua  Dch a  hca ,b Tb  Ta   hca ,tw Ttw  Ta  t y

(14)

where hca,tw can be expressed by: hca,tw = hca,b = Nuch·λa/Dch; the air velocity (ua )is expressed as[24]:

ua 

0.5  g   a ,ch Ta ,out  Ta ,in   H ch 2

2

A   A  H  Cin  ch   Cout  ch   C f  ch   Ain   Aout   Pch 

 a ,ch 

2 Ta ,in  Ta ,out

(15)

(16)

where Pch is the hydraulic radius of the air flow channel, it can be expressed as Pch=2(Lch+Dch); and C is the friction coefficient of upper vent , lower vent and channel along the way, Cin=0.25,Cout=0.3 and C f  0.3 1.368  Gra ,ch 0.084 [24]. (2) Case in which vent2 and vent3 are closed

Ta  (Ttw  Tb ) / 2

(17)

For envelop of building:

Tw,i t



w,i

 2Tw,i

 w,i  cw,i  2 x

(18)

Eq. (17) is for walls, floor and ceiling, and the method of control volume is used to solve the equation.

Journal Pre-proof For air in the room: Four-point method is applied to build the model of room air [37],which is expressed as:

 a ca

n Tr  a (Ta ,out  Ta ,in )   hcwi ,a (Twi  Tr )Aw,i  hc ,c (Ta ,out  Tc )  Wr  Lr (19)  Vr  mc t i 1

where Tr is the average temperature of room air, which is calculated as: Tr = Ta,c - (

Ta,out - Ta,in H r ) Hr 2

(20)

where Ta,c is the air temperature near the ceiling, and is calculated as: 2  a ca ua  Ach  hc ,c (Ta ,out  Tc ) Wr  Lr

(21)

where Tc denotes the temperature of ceiling; the convective coefficient on the ceiling hc,c is calculated by the air ventilation rate n, which is given as[38]: hc ,c  0.166  0.48  n 0.8

(22)

3.3 Evaluation index of thermal comfort Predicted Mean Vote (PMV) [39] is an index that express the quality of the thermal environment as a mean value of the votes of a group of occupants on the seven-point thermal sensation scale (+3 hot,+2 warm,+1slightly warm,0 neutral,-1slightly cool,-2 cool,-3 cold). In this paper, PMV index is introduced to evaluate the thermal performance of the building with the PVMTW system. The mathematical model of PMV is given as: PMV =  0.303exp (-0.036 M ) + 0.028M -W - 3.05 10-3 5733 - 6.99 M - Pa  -0.42( M - 58.15) - 1.7 10-5 M (5867 - Pa ) - 0.0014 M (34 - Tr ) - 3.96 10-8  f cl  (Tw + 273.15) 4 - (Tr + 273.15) 4  - f cl  hp,a  (Tw - Tr )

(23)

Journal Pre-proof where Tw  35.7  0.028M  0.155 3.96 108 f cl  [(Tw  273.15) 4  (Tr  273.15) 4 ]  f cl hp (Tw  35.7  0.028 M)

hp  Max[2.38 Tw  Tr 

0.25

,12.1 v ]

1.00  1.29 I cl for I cl  0.078clo f cl   1.05  0.645 I cl for I cl  0.078clo Pa  0.1333exp(18.6686 

4030.183 )  Tr  235

(24) (25) (26)

(27)

where Icl is the thermal resistance of clothes; and M is the body's metabolic rate; the parameter W is the external work (equal to zero for most activity) and Pa is the partial pressure of vapor in the room air and φ is the relative humidity of room air. It can be found in literature[40], Icl=0.155 m2∙K/W, M=70W and φ=50%. 3.4 Performance evaluation The instantaneous electrical efficiency of MPVTW system is given by:

 PV 

EPV G  Ag  

(28)

The heat gains and instantaneous thermal efficiency of system can be defined as following [41, 42]: Qth  ua  Ach   a  (Ta ,out  Ta ,in )

th 

Qth G  Ag

(29) (30)

In addition, this paper introduces the total efficiency to assess comprehensively the utilization of solar energy by MPVTW system. the total efficiency is calculated as [23]:

Journal Pre-proof total  th    pv /  power

(31)

where ηpower is the power generation efficiency of coal–fired thermal power plants, and its value is 0.38. 4. Results and discussion 4.1 Verification The model of the PVMTW system is solved by MATLAB software and validated by test data. The index of RMSE ( root-mean-square error) is now introduced to evaluate the reliability of the proposed model, which is given as[43]: RMSD 



i k i 1

(M i  Si ) 2 / k M

(32)

where Mi and Si are the measured data and simulate values, respectively; k indicates the number of data points; M represents the mean of measured data. For the thermal performance, the accurate temperature prediction is helpful to accurate calculation of heat gain. The temperature of glass, PV cells and room air are related to the heat gains and thermal comfort,which are also indexes for evaluating the thermal performance of the PVMTW system. Figs.5-7 present the comparisons of simulated and mesured temperature of glass cover, PV cells and room air. From these figures, it can be seen that the simulated temperature are in well agreement with the measured temperature. The RMSD values of glass cover, PV cells and room air are 7.9%， 9.3% and 4.3%, respectively. Since the RMSD values of the temperature parameters are within the regulated values, the proposed model are capable of simulating the thermal performance of the PVMTW system. Fig.8 shows a comparison

Journal Pre-proof of measured and simulated results of 3-day electricity generation. It is clear from Fig.8 that results obtained in both studies are in good agreement, and the RMSD value of electricity generation is 3.7%. Thus, the model proposed in this paper is validated. 40

Tg(mea) Tg(sim)

Temperature(°C)

35 30 25 20 15 10 5 0

0

9

18

27

36

45

54

63

72

Time (h)

Fig.5. Comparison of measured and simulation temperature on glass cover

(mea-measured data;

sim-simulation result)

27

Tr(mea) Tr(sim)

Temperature (°C)

24 21 18 15 12 9 0

9

18

27

36

45

54

63

72

Time (hour) Fig.6. Comparison of measured and simulation temperature in room (mea-measured data; simsimulation result)

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PV cells temperature (°C)

75

TPV(sim) TPV(mea)

60 45 30 15 0

0

9

18

27

36

45

54

63

72

Time (hour) Fig.7. Comparison of measured and simulation temperature of PV cells (mea-measured data; simsimulation result) 120

100

100

Epv(sim) Epv(mea)

80 60 40 20

Electricity generation (W)

Electricity generation (W)

120

20/12/2017

80

Epv(sim) Epv(mea)

60 40 20

21/12/2017

Electricity generation (W)

09:00 10:30 12:00 13:30 15:00 16:30 09:00 10:30 12:00 13:30 15:00 16:30 Time of day (hour) Time of day (hour) 120 100 80

Epv(sim) Epv(mea)

60 40 20

22/12/2017

09:00 10:30 12:00 13:30 15:00 16:30 Time of day (hour)

Fig.8. Comparison of measured and simulation electricity generation (mea-measured data; simsimulation result)

Journal Pre-proof 4.2 Thermal behavior analysis Under identical ambient temperature and radiation intensity, the classic Trombe wall system is selected as the comparative object to evaluate the thermal performance of the PVMTW system. Fig.4 shows ambient temperature and radiation intensity from December 20th, 2017(9:45) to December 23rd, 2017(7:45). During the test procedure, weather was fine, the ambient temperature took ranges from 4.7 °C to 18.6 °C and the average irradiation intensity was 629 W/m2.

Heat gains (W)

550

Q(PVMTW℃ Q℃ TW℃

440 330 220 110 0 0

9

18

27 36 Time (hour)

45

54

63

72

Fig.9. The comparisons of heat gains between the PVMTW system and classic TW system (Q (PVMTW) - heat gains from the PVMTW system; Q (TW) - heat gains from the classic TW system)

Fig.9 shows the comparison of the instantaneous heat gains obtained by natural ventilation between the PVMTW system and the classic TW system. It can be seen clearly that in the daytime (i.e. during the opening of the vent2 and vent3), the instantaneous heat gain from the PVMTW system is significantly larger than that from the classic Trombe wall system. The values of the daily total heat gains from the PVMTW system are 11.7 MJ, 11.6 MJ and 12.4 MJ, respectively, while those from the classic TW system are 8.9 MJ, 8.3 MJ, and 9.3 MJ on 20th December, 21st December

Journal Pre-proof and 22nd December. The comparison of instantaneous thermal efficiencies of the PVMTW system with that of the classic TW system are depicted in Fig.10. The results show that the average daily thermal efficiencies of the former are 39.6%, 38.0% and 38.5% on 20th to 22nd December, while those of the latter are 23.8%, 23.3% and 23.8%. Through calculation, the average thermal efficiency of the PVMTW system is 65.2% higher than that of the classic TW system. The three subfigures in Fig.10 also clearly show the thermal efficiency variation scope of the PVMTW system are smaller than those of the classic TW system. For example, on 21st December the instantaneous thermal efficiency changed in the scope of 0.10-0.36 for the classic TW system, 0.350.44 for the PVMTW system. This is due to the fact that the heat gains of the PVMTW system comes from the energy released from the PV cells. According to the law of conservation of energy ( 1   PV  th  loss ), it can be inferred that the instantaneous thermal efficiency changes within a narrow range due to the stability of PV cells electrical efficiency (Fig.15) and the good thermal insulation of the PVMTW system. However, the thermal efficiency of the classic TW system is affected by various uncertain factors, such as meteorological parameters, temperature on the glass cover and the massive wall, indoor air temperature and air flow patterns in the channel, etc. Thus the thermal efficiency of the classic TW system varies widely. It is clear that the PVMTW system provided the more desirable thermal behavior considering the heat gains and thermal efficiency.

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Thermal efficiency

0.48 0.40

ηth(TW) ηth(PVMTW)

0.32 0.24 0.16 0.08 09:00

20/12/2017 10:00

11:00

Thermal efficiency

0.7 0.6

14:00

15:00

16:00

12:00

14:00

15:00

16:00

14:00

15:00

16:00

ηth(TW) ηth(PVMTW)

0.5 0.4 0.3 0.2 0.1 09:00 0.6

Thermal efficiency

12:00 13:00 Time (hour)

0.5

21/12/2017 10:00

11:00

13:00

Time (hour) ηth(TW) ηth(PVMTW)

0.4 0.3 0.2 0.1 09:00

22/12/2017 10:00

11:00

12:00

13:00

Time (hour)

Fig.10. The comparisons of instantaneous thermal efficiency between the PVMTW system and classic TW system (ηth (PVMTW) – instantaneous thermal efficiency of the PVMTW system; ηth (TW) - instantaneous thermal efficiency of the classic TW system)

Fig.11 shows the relationship between the average air temperature in the channel and the average air temperature in the room. It can be seen clearly that in the nighttime, the air temperature in the channel is lower than that in the room because the heat loss rate in the channel is much greater than that in the room. To ensure that the heat gains is positive, the optimum time to open/close vents is a key factor. The results in Fig.11 show that average air temperature in the channel is higher than that in the room during

Journal Pre-proof the period from 7:53 am to 16:53 pm for the PVMTW system. This means that the optimum time to open/close vents can be set at around 8:00 am/17:00 pm. Fig.11 also shows that in the daytime, the air temperature curve in the room with PVMTW system is very close to that in the room with the classic TW system. The value of the average air temperature in the room with PVMTW system is 21.8 °C and that of the average air temperature in the room with classic TW system is 21.6 °C. It is concluded that the multi-functional PVMTW system proposed can realize the same space heating function as the classic TW system. Fig.11 also shows that during the opening of the vents, the air temperature in the room with PVMTW system maintains above 18°C for 20.8 hours,which can reduce auxiliary heat energy consumption of building. However, in the nighttime, the average air temperature of 15.6 °C in the room with the PVMTW system is lower than that of 17.8°C in the room with the classic TW system. The reason for this phenomenon is that the massive wall of the classic TW system absorbs more solar energy in the daytime and, correspondingly, releases more energy to the room in the nighttime. Based on the above analysis, it can be concluded that the room with the PVMTW system is suitable for office buildings in daytime, while the room with the classic TW system is suitable for residential buildings in all-day use.

Journal Pre-proof Ta(PVMTW) Tr(PVMTW) Ta(TW) Tr(TW)

40

Temperature(°C)

32 24 16 8 0

Fig.11.

0

9

18

27 36 Time (hour)

45

54

63

72

The comparisons of air temperature in channel and room between the PVMTW system

and classic TW system (Ta- air temperature in channel,Tr- room air temperature) Temperature of westwall (°C)

25 20 15 10 5

Temperature of northwall (°C)

30

Tew(TW) Tew(PVMTW)

a 0

30 25

15

30 45 Time (hour) Tnw(TW) Tnw(PVMTW)

60

20 15 10 5

c 0

15

30 Time (hour)

Fig.12.

45

75 Temperature of southwall (°C)

Temperature of eastwall (°C)

30

60

75

Tww(TW) Tww(PVMTW)

25 20 15 10 5 40

b 0

15

30 45 Time (hour)

60

75

60

75

Tsw(TW) Tsw(PVMTW)

35 30 25 20 15 10 5

d 0

15

30

45

Time (hour)

The comparisons of temperatures on interior surface walls between the PVMTW system

and the classic TW system (Tew,Tww,Tsw,Tnw- the interior surface temperature on east wall,west wall, north wall and south wall; TW-Trombe wall system; PVMTW- the PVMTW system)

The interior surface temperatures on walls have an important effect on the thermal

Journal Pre-proof comfortableness of the human body. In the winter, the thermal comfortableness of the human body will decrease with the decrease of the interior surface temperatures on walls. The variations of the interior surface temperature on each wall with time are presented in Fig.12. The results show that the average interior surface temperature on walls (excluding south wall) of building with the PVMTW system is 21.6 °C in the daytime, and that on walls of building with the TW system is 21.0 °C. Nevertheless, the average interior surface temperature of 15.2 °C on the walls of building with the PVMTW system is lower than that of 16.9°C on the walls of building with TW system in the nighttime, and the temperature of the former is 1.7 °C lower than that of the latter. It is should be noted that the scope of the surface temperature range on the massive wall is larger than that on the ordinary walls. Fig.12d shows that during the three-day study, the average interior surface temperature on massive wall of the PVMTW system and the classic TW system is 17.4 °C and 18.7 °C, respectively. The maximum temperature values on the massive wall of the PVMTW system and the classic TW system reaches 34.4 °C and 38.4 °C on the third day, respectively. In a word, from the view of the interior surface temperature on the walls, the PVMTW system is more beneficial for the thermal comfort in the daytime, while the classic TW system is more conductive to the thermal comfort in the nighttime. The average predicted mean vote (PMV) has been extensively used throughout the world and it is mainly used to assess the thermal comfort of the indoor environments. Fig.13 depicts the comparison of the PMV value between the room with the PVMTW system and that with the classic TW system. The results show that the average value of

Journal Pre-proof PMV of room with the PVMTW system is 0.05 in the daytime, while that of room with the classic TW system is -0.36. According to the thermal and cold sensory index, the indoor thermal comfort of the PVMTW system is close to thermal neutrality, which most occupant feel comfortable, while the room with the classic TW system is a little cold. Also note that when the air temperature difference between the person’s head position and ankle position exceeds 3 °C, the thermal comfort of human body will decrease even if the person is in a neutral environment. Fig.14 presents the trends of air temperatures difference in two rooms (room with the PVMTW system and room with the classic TW system) with time. The results shows that For the PVMTW system, during the 3-day study, the durations of temperature difference staying above 3 °C are summed up to 8.5 h, and the maximum value of 3.5 °C occurs at 12:51 on the third day. Based on the local thermal discomfort theory [44], the room with PVMTW system can cause a percentage of dissatisfied of about 8% at the maximum temperature difference of 3.5 °C. In order to improve this unsatisfactory phenomenon, it is a good measure to adjust the opening of valves of Vent2 and Vent3 at noon. 1.0

PMV(TW) PMV(PVMTW)

0.5

PMV

0.0 -0.5 -1.0 -1.5 -2.0 -2.5 0

10

20

30

40

50

60

70

Time (hour) Fig.13. Values of PMV index between the room with the PVMTW system and the room with the

Journal Pre-proof traditional Trombe wall system

Tempetature difference (°C)

4

∆T(TW) ∆T(PVMTW)

3 2 1 0 0

10

20

30

40

50

60

70

Time (h) Fig.14.

Air temperature difference between head position and ankle position (∆T(TW) - air

temperature difference of the classic TW system; ∆T(PVMTW) - air temperature difference of the PVMTW system).

4.3 Electrical behavior and total efficiency analysis The PVMTW system proposed in this paper provides space heating, as well as the high-quality electricity. The electricity generation of the PVMTW system are shown in Fig.8. The results show that daily electricity generation is 0.57 kWh, 0.60 kWh and 0.62 kWh on 20th, 21st and 22nd December, respectively. Fig.15 depicts the tendencies of the instantaneous electrical efficiency curves for three days. It can be seen that the instantaneous electrical efficiencies first drop and then increases. The main reason for this phenomenon is attributed to the temperature variation of PV cells. Fig.7 shows the temperature variation of the PV cells. It can be seen clearly that when the vents are opened, the PV cells temperature firstly increases till it reaches a maximum temperature point (about 12:30 on the day) at which no further temperature increase can be achieved

Journal Pre-proof thus, it starts decreasing. Fig.7 and Fig.15 also show that the electrical efficiencies decrease with the increase of temperature of PV cells. For example, when the PV cells reached the maximum temperature of 69.2 °C at 12:30 on the third day, and the electrical efficiency dropped to a minimum value of 0.113 at the corresponding time. In addition, Fig.15 shows the range of change in electrical efficiency from 0.096 to 0.131. During the three-day study of the PVMTW system, the daily average electrical efficiencies were 0.118, 0.122 and 0.119, respectively. Average electrical efficiency value of the PVMTW system of 0.120 is obtained. Total efficiency is a popular indicator to evaluate the overall system performance of the PV-Trombe wall system, which is calculated using Eq. (30). The values of the average daily total efficiencies were calculated to be 0.590, 0.583 and 0.581, respectively. The maximum and average values of total efficiency value are 0.590 and 0.585, respectively, during the studying period. Whether compared to current PV cells or the classic Trombe wall system, the PVMTW system increases solar energy harvesting under zero energy consumption.

Electrical efficiency

0.13

ηpv(20th) ηpv(21st) ηpv(22nd)

0.12

0.11

0.10 09:00

10:00

11:00

12:00

13:00

14:00

15:00

16:00

Time (hour) Fig.15. Variation of instantaneous electrical efficiency with time (ηpv(20th), ηpv(21st), ηpv(22nd)Instantaneous electrical efficiency on 20th , 21st ,22nd

December).

Journal Pre-proof 5. Conclusion

This study proposed a PVMTW system which can realize passive heating, thermal insulation and electricity generation. A dynamic model of the PVMTW system was established and validated. Then the validated model was applied to evaluate the thermal performance by comparison with the classic Trombe wall system on various aspects. Furthermore, the electrical performance of the PVMTW system was investigated in terms of electricity generation and electrical efficiency. The main results are as follows: The dynamic model can be used to evaluate the thermal and electrical performance of the PVMTW system. (1) By the comparisons between the PVMTW system and the classic TW system, the results showed that: the average thermal efficiency of the PVMTW system was 65.2% higher than that of the classic TW system, and the thermal efficiency of the former was more stable than that of the latter. Additionally, in the daytime, the average air temperature of 21.8 °C in the room with MPVTW system was close to that of 21.6 °C in the room with classic TW system, while the former of 15.6 °C was lower than that the latter of 17.8°C in the nighttime, which demonstrated that the room with MPVTW system was more suitable for office buildings in daytime. Moreover, the interior surface temperature on the walls of the building with the MPVTW system was slightly higher than that on the walls of the building with the classic TW system in the daytime, but in the nighttime, the temperature of the former is 1.7 °C lower than that of the latter. (2) In terms of value of PMV index in the daytime, the results showed that the room with the PVMTW system achieved a PMV of 0.05, which made the indoor thermal

Journal Pre-proof comfort approach neutral. While the PMV of the classic TW system was -0.36, and the room was a little cold. (3) The average values of electrical efficiency and total efficiency of the PVMTW system were 0.120 and 0.585, respectively.

Acknowledgements

This research is supported by the National Natural Science Foundation of China (No. 51878636), Ministry of Science and Technology of China (No.2016YFE0124800), the Dongguan Innovative Research Team Program (No.2014607101008), and the Chinese Academy of Sciences (No.211134KYSB2016005). References [1] Vanaga R, Blumberga A, Freimanis R, Mols T, Blumberga D. Solar facade module for nearly zero energy building. Energy. 2018;157:1025-34. [2] Omrany H, Ghaffarianhoseini A, Ghaffarianhoseini A, Raahemifar K, Tookey J. Application of passive wall systems for improving the energy efficiency in buildings: A comprehensive review. Renewable and sustainable energy reviews. 2016;62:1252-69. [3] Zhang T, Tan Y, Yang H, Zhang X. The application of air layers in building envelopes: a review. Applied energy. 2016;165:707-34. [4] Luo C, Xu L, Ji J, Liao M, Sun D. Experimental study of a modified solar phase change material storage wall system. Energy. 2017;128:224-31. [5] Yu T, Liu B, Lei B, Yuan Y, Bi H, Zhang Z. Thermal performance of a heating system combining solar air collector with hollow ventilated interior wall in residential buildings on Tibetan Plateau. Energy. 2019. [6] Dong J, Chen Z, Zhang L, Cheng Y, Sun S, Jie J. Experimental investigation on the heating performance of a novel designed trombe wall. Energy. 2019;168:728-36. [7] Ma Q, Fukuda H, Lee M, Kobatake T, Kuma Y, Ozaki A. Study on the utilization of heat in the mechanically ventilated Trombe wall in a house with a central air conditioning and air circulation system. Applied energy. 2018;222:861-71. [8] Rabani M, Kalantar V, Dehghan AA, Faghih AK. Experimental study of the heating performance of a Trombe wall with a new design. Solar Energy. 2015;118:359-74. [9] Saadatian O, Sopian K, Lim CH, Asim N, Sulaiman MY. Trombe walls: A review of opportunities and challenges in research and development. Renewable and Sustainable Energy

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Journal Pre-proof

Thermal and electrical behavior of built-middle photovoltaic integrated Trombe wall: experimental and numerical study Yuan Lin b,a, Jie Ji a,*, Xiangyou Lub, Kun Luo a, Fan Zhou a, Yang Mac a

Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei 230026, China b Department of Architectural Environment and Energy Engineering, Anhui Jianzhu University, Hefei 230009, China c Sungrow, Hefei 230088, China * Corresponding author. Tel.: 0086-551-63607346. E-mail address: [email protected]

Conflict of interest We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.

Journal Pre-proof A PVMTW system was proposed and studied. Thermal and electrical performance were validated by experiments. Simulation method were used for comparative study. Indoor thermal comfort of PVMTW system reached thermal neutrality. The average electrical efficiency and total efficiency achieved 0.120 and 0.585.