air solar wall system

Journal Pre-proofs Seasonal experimental study of a hybrid photovoltaic-water/air solar wall system Kun Luo, Jie Ji, Lijie Xu, Zhaomeng Li PII: DOI: Reference:

S1359-4311(19)35373-6 https://doi.org/10.1016/j.applthermaleng.2019.114853 ATE 114853

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Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

1 August 2019 10 December 2019 26 December 2019

Please cite this article as: K. Luo, J. Ji, L. Xu, Z. Li, Seasonal experimental study of a hybrid photovoltaicwater/air solar wall system, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng. 2019.114853

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Seasonal experimental study of a hybrid photovoltaicwater/air solar wall system Kun Luo1, Jie Ji1*, Lijie Xu1, Zhaomeng Li1 1. Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, 230027, China *([email protected])

0. Abstract Traditional BIPVT systems are faced with the seasonal use issues that they run efficiently only during certain parts of the year, and perform poorly or even out of service for the rest of the time. To overcome the problems, this paper performs several tasks as follows. (1) A hybrid photovoltaic-water/air solar wall (HPSW) system is presented, which combines the air cooling channel and water cooling channel together in one single BIPVT system. (2) The HPSW system can run three modes according to different seasonal needs of the building: ① PV/Air mode in winter, ② PV/Water mode in summer, ③ PV-water/air mixing mode in transition season, which means that the new system is able to satisfy annual demands of the buildings. (3) The HPSW system were experimentally tested in different seasons under different modes during a whole year. (4) The results showed that the all-day average electrical efficiency can reach 15.3% by PV/Air mode in winter, 7.8% in summer and 11.6% in transition season by PV/Water mode. The average daily power generation of the system in summer, transition season and winter was 973.0kJ, 3226.4kJ and 4460.5kJ, respectively. As for thermal performance, the PV/Water mode performed well both summer and transition seasons, with the average thermal efficiency of 55.1% and 51.5% respectively. The temperature rise of experimental room reached 8.1℃ compared to the reference room in winter. The results proves that the HPSW system can operate efficiently as expected throughout the year and has great value in different seasons and different regions. Key words: Hybrid BIPVT system; PV/Air mode; PV/Water mode; mixing mode; Whole-year experiment; Performance evaluation

1. Introduction Building-Integrated Photovoltaic (BIPV) is an efficient energy conversion system that incorporates solar PV panels as part of the roof, windows, facades and shading devices. When active heat recovery is combined with BIPV systems either in closed loop with liquid or in an open loop with forced air they are known as building-integrated photovoltaic-thermal (BIPVT systems) [1]. As the BIPVT systems has significant benefits and potential for wide use in buildings, there are plenty of researches have been made on BIPVT systems in the past decades. As mentioned above, there are two types of BIPVT systems: BIPV/ Fluid (Water) system which

is combined BIPV systems with a closed liquid loop, and BIPV/Air system which is combined BIPV systems with an open air loop. For BIPV/Fluid system, some research had focused on the innovative structure of the system. A comparison among innovative building-plant system configurations was carried out by Buonomano et al. [2] and the results showed that the economic profitability was slightly better for roof BIPVT panels than for roof and facade applications. Chen et al. [3] designed a new BIPVT multifunctional roofing panel which was an aluminum/high-density polyethylene functionally graded material panel embedded with aluminum water tubes, and the performance indicated a very promising prospective of the new BIPVT multifunctional roofing panel. Gautam et al. [4] evaluated the potential of unglazed BIPVT system for multi-family apartment buildings and results showed that this system in warmer climates fared quite well against both solar thermal and BIPV technologies. Other studies had focused on numerical simulation and performance analysis of the system. Corbin et al. [5] established an experimentally validated CFD model of a novel BIPV/T collector on the roof to determine the effect of active heat recovery on cell efficiency, and found that cell efficiency could be raised by 5.3%. A mathematical model of the BIPV/T system was established by Li et al. [6], to analyze the effects of inlet and outlet velocity on thermal efficiency. The optimal inlet velocity of the BIPV/T– ASHP integrated system was determined to be 4 m/s, and the COP reached 4.6. Piratheepan et al. [7] developed a combined optical and thermal model to describe the performance of a BIPVT system and found that the key parameters such as tube spacing, and thermal conductivity between the solar cell and the absorber had a significant effect on the overall efficiency. Ibrahim et al. [8] conducted a performance analysis of BIPVT system and calculated the energetic improvement potential based on the metrological condition of Malaysia. For BIPV/Air system, a great deal of research had focused on the structure parameters that influence the performance of system. Ahmed-Dahmane et al. [9] designed a BIPVT system which was composed of multiple PV/T air collectors integrated into the building facade. The case of using exhaust air as coolant showed an important decrease in PV cells temperature. Tripathy et al. [10] used a HDKR model based insolation corresponding to optimum tilt angle of the panel to develop mathematical model of BIPVT system, which indicated that the semitransparent BIPVT systems were more efficient than opaque BIPVT systems for all values of tilt angle of PV panel. And Yadav et al. [11] introduced corresponding shadow effects to evaluate optimum tilt angle, insolation and performance of BIPVT system. The electrical and thermal energy output of the BIPVT system decreased with increased in storey heights as well as widths of surrounded buildings. Other than that, a prototype open loop air-based BIPV/T system with a single inlet was studied in a full scale solar simulator by Yang et al. [12], and simulation results indicated that the application of two inlets on a BIPVT collector increased thermal efficiency by around 5%. They also investigated the performance of air-based BIPVT System with Multiple Inlets in a Cold Climate [13]. Rounis et al. [14] introduced a numerical flow distribution model for the optimal design of multiple inlet BIPVT systems, which had been found to provide adequate estimation of the inflows. Energy performance of BIPVT system had also been investigated. Vats et al. [15] evaluated the energy and exergy performance of a BISPVT system integrated to the roof of a room and concluded that HIT PV module was suitable for producing electrical power while a-Si was suitable for space heating. The energy and exergy analysis of the natural ventilation BIPVT system was tested theoretically and

experimentally by Agathokleous et al. [16], which proved that the energy efficiency varies from 26.5% to 33.5%, and the exergy efficiency varies from 13% to 16%. However, in the above BIPVT research, most researchers only focused on the hot water or air heat collection of the system. It is difficult to solve the seasonal use limitation of the BIPVT system, which makes it difficult to improve the overall performance of the system throughout the year: The BIPV/Water system may be damaged without anti-freezing measures when the temperature drops below 0℃ in winter, especially at middle and high latitudes. The BIPV/Air system may be left unused and even cause overheating problems in the building in non-heating seasons, especially in summer. What’s worse, the electrical efficiency of BIPV/Air system would be limited due to the high operating temperature of PV cells. Consequently, neither the BIPV/Water system nor the BIPV/Air system is able to work effectively all over the year. The same drawbacks also exists in the flat-plate PV/T collectors, and the idea of combination water/air PV/T system was proposed by Ji et al. [17] and Guo et al. [18] in our lab. This trifunctional PV/T system was designed to provide electricity and hot air in winter, and to provide electricity and hot water in the rest of year. Therefore, considering the seasonal issues of the BIPVT system and the previous researches of our group [18] [19], a hybrid photovoltaic-water/air solar wall (HPSW) system is proposed in this paper. The HPSW system consists of two identical and independent modules and can run three modes to fulfill the requirements of buildings in different seasons, regions and applications. (1) PV/Air mode of both modules is used in winter and heated air is created to flow into the room to meet the indoor heating requirement. PV/Air mode is used in winter, which can both generate electricity and warm the indoor environment. (2) PV/Water mode of both modules is adopted in summer and the most time in spring or fall, and it can reduce cell temperature while producing hot water, to reach better electrical performance. (3) When there are both air heating and hot water demands in transition season (spring and fall), the mixing mode is applied, which means that one module runs PV/Air mode and the other runs PV/Water mode. The electricity energy and thermal energy is produced simultaneously in all modes. What’s more, different from the former active independent tri-functional PV/T system which used an air pump or water pump to achieve the heat transfer function, this HPSW system can achieve passive heating function and improve the indoor thermal environment. In this paper, experiments have been conducted to investigate the thermal and electrical performance of HPSW system in 3 working modes during a whole year. And the influence of the system on the indoor thermal environment of the building is discussed. (1) In summer, the electrical power output, the temperature of water tank, glass cover, experimental room, absorber plate temperature and insulation layer were measured to value the performance of PV/Water mode. (2) In winter, the electrical power output, temperature at different heights of the air channel, absorber plate, experimental room, and reference room were measured to value the performance of PV/air mode. (3) In transition season, the mixing mode was performed and valued by all the indexes. (4) Then the efficiencies in different modes are calculated and compared with each other.

2. Description of the HPSW system. 2.1 Introduction of the HPSW system. The HPSW system is a new BIPVT system which can satisfy the annual demands of buildings and operate efficiently under different modes in all seasons. Compared to traditional air solar walls, we have added photovoltaic arrays on the back side of the glass cover to generate electricity, with an air interlayer between them. And we have a liquid-filled ribbed tube on the back of the photovoltaic heat absorbing plate to achieve multiple functions of water heat collection.

2.1.1 Working principle of the HPSW system. The HPSW system has three working patterns: PV/water mode, PV/air mode and the mixing mode. The operating mode diagrams of the system are shown in Fig. 1 (a) and (b). In the non-heating season (spring, summer, autumn and other high temperature seasons), when air temperature is higher than 0℃ or building needs heat insulation protection, PV/water mode is adapted, with the upper and lower air outlets of room closed , and the water circulation flow channel opened. The upper and lower air outlets of the experimental room should be closed and the water circulation flow path is opening. Cold water passes through the collector and take away a lot of heat generated on the photovoltaic heat absorbing plate. So the cell temperature is kept at a low level, and the electrical power and efficiency increase. Finally, the hot water flows out from the upper end of the device into the insulated water tank for storage. Water can be used as domestic water for civilian use during the non-heating season. During the heating season, when temperature is lower than 0℃ or there is indoor heating demand, PV/air mode is used. The water channel valves are close and the pipe is drained to prevent freezing cracks. The upper and lower air vent baffles are open, and the heat generated by the collector can heat the air in the flow channel. The cold air enters the collector through the lower vent, and under the action of thermosiphon, air flows upward from the lower part in the flow channel. The hot air returns to the room through the upper vent and forms air circulation with indoor air to raise the indoor air temperature and provide fresh air. In the cold weather of spring and autumn, when the ambient temperature is relatively low but higher than 0 °C, the mixing mode is applied. The building has a small amount of indoor heating demand in this time and the domestic hot water still has a huge demand. So we can make one system to run PV/air mode to meet the indoor heating needs, and another system run PV/water mode to provide domestic hot water, which can maximize meet the building's energy needs.

Fig. 1.

Two operating mode diagrams.

2.1.2 Structure of the HPSW system. Experimental device consists of glass cover, air interlayer, photovoltaic cells, heat absorbing plate, copper tube (sheet-and-tube), air flow channel, insulation layer, two vents, two air baffles, circulating water path and water tank. Fig. 2 shows the basic structure of the system.

Fig. 2. The basic structure of the HPSW system The core component of this device is a heat absorbing plate laminated with photovoltaic cells. The size of the glass cover on the outside of the unit is 1994 mm (height) * 994 mm

(width), and behind the cover there is a photovoltaic heat absorbing plate on which 50 monocrystalline silicon solar cells of size 156 mm*156 mm are laminated. The effective solar cell area on the photovoltaic heat absorbing plate is 1.2m2 and the coverage rate is 60%. The sealed air interlayer with a thickness of 30mm between the glass cover plate and the photovoltaic heat absorbing plate has no air flow, which can effectively prevent the efficiency and life of photovoltaic cells from being affected by surface ash. The other side of the photovoltaic heat absorbing plate is a heat insulating layer with a thickness of 50 mm and a material of glass fiber. The upper and lower parts of the heat insulating layer are respectively provided with two air vents of 800 mm (width) * 120 mm (height) and so the air can enter the experimental room through the vents. There is a 120 mm thick air flow channel between the absorber plate and the insulation layer, which has two baffles on the upper and lower sides and two indoor vents on the side of the wall. At the same time, the insulation material is evenly distributed around the device, which can effectively reduce the loss of heat to the environment. The photovoltaic cells layout on the photovoltaic heat sink, the structure of the air inlet of the insulation layer and the collector profile are shown in Fig. 3 (a), (b) and (c).

Fig. 3. Structure of the system. The PV cell is packaged by two layers of TPT (Tedlar-Polyester-Tellar). The photovoltaic heat absorbing plate is made by lamination process, which is vacuum laminated in a laminating machine. The glass cover used in the experiment is a low-iron glass with a transmittance of 0.9 and a thickness of 0.3 mm. The surface of the absorber plate is selected from a selective coating with high absorption rate and low emissivity. On the back side of the heat absorbing plate, eight copper branch pipes with an inner diameter of 8 mm are laser welded, and the upper and lower ends of the copper branch pipes are connected to the external water path through a header having an inner diameter of 20 mm. A water pump is installed as the power to maintain the circulation. The intermediate circulation waterway is connected by an aluminum-plastic pipe, and the valve is used to control the switch and the water circulation mode. Outside of the water pipe wrapped the insulation cotton, waterproof and tin foil paper to prevent heat dissipation, rain seepage and sunlight absorption. The waterway structure diagram is shown in Fig. 4.

Fig. 4. Waterway structure diagram The HPSW system is made in Guangdong Five Star Solar Energy CO.LTD. Table 1 shows the key structure parameters of the HPSW system. Table 2 shows the photovoltaic output characteristic parameters of two photovoltaic modules under standard conditions (solar radiation intensity is 1000W/m2, cells temperature is 29.9°C). The photoelectric conversion efficiency under standard conditions is 16.95%. Table 1. Structure parameters of the TFTW system Parameters Data Thickness of the glass cover 0.3 mm Width of the air interlayer 30 mm Thickness of the absorber plate 5 mm Width of the air channel 120 mm Thickness of the insulation layer 50 mm Inner diameter of the copper pipe 8 mm Inner diameter of the heat pipe 20 mm Thermal conductivity of the insulation board 0.0036 W/(m· K) Absorptivity of the panel 0.9 Emissivity of the panel 0.15 Depth of the south wall 0.4 m Power of the water pump 150 W Volume of the water tank 100 L Table 2. Photovoltaic output characteristic parameters of two photovoltaic modules Open circuit Short circuit Peak Operating Working Photoelectric voltage current power Voltage current conversion efficiency 31.83V 8.12A 206.28W 25.99V 7.94A 16.95% 31.95V 8.07A 206.25W 25.99V 7.94A 16.95%

2.2 Construction of experimental platform. The experimental test system platform is a hot box building with a peripheral enclosure in Hefei, China, 117.27°E and 31.86°N, a classic hot summer and cold winter area. The test system includes two hot box rooms, HPSW system, water pipes, water tank and the experimental parameter measurement system. The area of the HPSW system is 1.98𝑚2 and the internal text room area is 14.82𝑚2.The dimensions of the two internal hot box rooms are 3.8 m (length) * 3.9 m (width) * 2.6 m (height) and the south wall is a red brick wall with a thickness of 400 mm, while the other walls, the roof and floor are made of insulation materials. One hot box room is set as the experiment room with two HPSW systems and the other is set as the reference room. The hot box has double-layer insulation wall. The thickness of the inner insulation wall is 50 mm, and the thickness of the external insulation wall is 100 mm. The south wall of the experimental room has two vents with a size of 120mm (height) * 800 mm (width). Fig. 5is the hot box structure diagram. Table 3 shows the structural parameters of the hot box.

Layer Structure 1 Structure 2 Structure 3 South wall

Fig. 5. Hot box structure diagram Table 3. Structural parameters of the hot box Thermal Thickness Density Material Conductivity (mm) (kg/m3) (W/(m· K)) Steel plate 4 60.5 7854 Polystyrene 100/50 0.04 15 Steel plate 4 60.5 7854 Red brick 400 0.814 1760

Heat capacity (J/(kg· K)) 434 1210 434 840

The experimental appearance of the hybrid photovoltaic-water/air solar wall (HPSW) is shown in Fig. 6.

Fig. 6. Experimental appearance of the TFTW system During the experiment, the temperature, irradiation, and the maximum power output of the photovoltaic cell were measured. Copper-constantan thermocouples with ice-point compensation (±0.2 ℃) was used in the temperature measurement, and the measurement accuracy was 0.5°C. Three temperature measurement points were arranged in the indoor room, which were located at the height of 0.3m, 1.3m, and 2.3m on the 2.6m high room central axis. There are three temperature measurement points on the glass cover plate, heat absorption plate and copper tube, respectively, which were located at the height of 0.2m, 1m, and 1.8m on the central axis of the collector. In the 1m high water tank, we arranged four temperature measurement points with the heights of 0.2m, 0.4m, 0.6m, 0.8m. The three temperature measurement points in the air flow channel were located at the upper vent, lower vent and the center of the flow channel, respectively. The location of the temperature measurement points can be seen in the places marked by the red dots in Fig. 2 and Fig. 4. The solar radiation was measured by the TBQ-2 irradiation meter which was placed in the south and vertical direction, parallel to the lighting plane of the wall. The maximum electric power was tracked by MPPT and the electric current & voltage were collected by 34980A data logger. The energy generated by the photovoltaic cells was stored in a sealed lead-acid battery (12 V, 65 AH) and the water speed was controlled by a 150W water pump. All the above measured parameters, especially the real-time temperature of the measurement points, were collected by the Agilent 34980A data collector in good weather.

3. Performance evaluation of the HPSW system. The comprehensive performance of the TFTW system has two parts: photovoltaic performance and thermal performance.

The instantaneous photoelectric efficiency of a photovoltaic cell is defined as the ratio of the output of the photovoltaic cell to the total solar radiation received by the photovoltaic cell: 𝑃𝑚𝑎𝑥

𝑈𝐼

𝜂𝑝𝑣 = 𝐺𝐴𝑝𝑣 = 𝐺𝐴𝑝𝑣

(1)

Where 𝑃𝑚𝑎𝑥 is the maximum instantaneous power of the PV cells. G is the total solar radiation intensity and 𝐴𝑝𝑣 is the PV cell module area. U and I are the voltage and current when the power reaches the maximum value. This formula is applicable to both PV/water mode and PV/air mode. The thermal performance evaluation is divided into two cases according to the different operating modes of the system. When the system operates the PV/water mode, the instantaneous thermal efficiency is defined as the ratio of the heat gain of the water tank per unit time to the solar radiation on the glass cover. 𝜂𝑡ℎ =

𝑀𝐶𝑝(𝑇2 ― 𝑇1) 𝐺𝐴𝑐∆𝑡

(2)

The average thermal efficiency is defined as the ratio of the total heat gain of the tank to the total solar radiation of the glass cover throughout the day. 𝑄

𝜂𝑡ℎ = 𝐻𝐴𝑐 =

𝑀𝐶𝑝(𝑇𝑓𝑖𝑛𝑎𝑙 ― 𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙) 𝐻𝐴𝑐

(3)

In the formula, M is the mass of water in the water storage tank, kg. 𝐶𝑝 is the specific heat capacity of water, taking 4.2kJ/(kg·K). 𝑇1 and 𝑇2 are the initial and final water temperature in the unit time, °C. 𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙 and 𝑇𝑓𝑖𝑛𝑎𝑙 are the initial water temperature and final water temperature respectively during the experiment, °C. H is the total amount of solar radiation on the collector, kJ/𝑚2. 𝐴𝑐 is the total area of the collector that can receive the radiation. 𝑚2. ∆t is the unit time length taken for the calculation, s. In this experiment, four thermocouple temperature measuring points are evenly arranged in the water storage tank along the height direction, and the average water temperature is: 𝑇=

𝑇𝑡𝑎𝑛𝑘1 + 𝑇𝑡𝑎𝑛𝑘2 +𝑇𝑡𝑎𝑛𝑘3 +𝑇𝑡𝑎𝑛𝑘4 4

(4)

Where 𝑇𝑡𝑎𝑛𝑘𝑛(n=1, 2, 3, 4) is the temperature of each measuring point from top to bottom in the water storage tank, °C. When the collector operates the PV/air mode, the instantaneous thermal efficiency is defined as the ratio of the heat gain of the air in the air flow channel to the solar radiation on the glass cover of the collector. 𝜂𝑡ℎ =

𝑚𝑐𝑝(𝑇𝑜𝑢𝑡 ― 𝑇𝑖𝑛) 𝐺𝐴𝑐

(5)

Where 𝑚 is the air mass flow rate in the flow channel, kg/s; 𝑐𝑝 is the air constant pressure specific heat, J / (kg · K). 𝑇𝑖𝑛 and 𝑇𝑜𝑢𝑡 are the inlet and outlet temperatures of the air flow channel, °C. The formula for calculating the flow rate 𝑚 is as follows: 𝑚 = 𝜌𝑉𝐴𝑖𝑛 (6) Where ρ is the air density, kg/m3. V is the air flow rate in the flow channel, m/s. 𝐴𝑖𝑛 is the flow channel cross-sectional area， 𝑚2. There are still many difficulties in measuring the air flow rate of the solar wall runner. Firstly, due to the structural reasons of the solar wall, the anemometer cannot be placed inside the air flow channel to measure the flow rate. Secondly, the air flow rate in the flow channel or the vent, is unevenly distributed. So this paper uses the

Bernoulli equation to derive the flow equation [19]: 𝑔𝛽(𝑇𝑜𝑢𝑡 ― 𝑇𝑖𝑛)𝐿

𝑉=

𝐴𝑐 2

𝐿

𝐴𝑐

𝐶𝑖𝑛(𝐴 ) + 𝑓𝐷 + 𝐶𝑜𝑢𝑡(𝐴 𝑖𝑛

ℎ

2

)

(7)

𝑜𝑢𝑡

Where L is the height of the collector, m; 𝐷ℎ is the power dimension of the channel, m. 𝐷ℎ = 2 (w + d), where w, d are the width and thickness of the channel, m. 𝑇𝑖𝑛 and 𝑇𝑜𝑢𝑡 are the air temperatures for the inlet and outlet of the solar wall, °C; 𝐴𝑖𝑛 and 𝐴𝑜𝑢𝑡 are the lower air outlet area and upper air outlet area of the solar wall,𝑚2. 𝑓、𝐶𝑖𝑛、𝐶𝑜𝑢𝑡 are the resistance coefficient along the air flow channel, the loss coefficient at the lower air outlet, and the loss coefficient at the upper air outlet. The loss coefficient of the upper and lower vent is set to 𝐶𝑖𝑛=1.0、𝐶𝑜𝑢𝑡=1.5, respectively. As for the value of the resistance coefficient along the channel, if it is turbulent, 𝑓 = 0.3164𝑅𝑒 ―0.25 (8) And if it is laminar [20], 96

𝑓 = 𝑅𝑒(1 ― 1.20244𝑥 + 0.88119𝑥2 + 0.88819𝑥3 ― 1.69812𝑥4 + 0.72366𝑥5)

(9)

Where x is the aspect ratio of the solar wall. Based on the first law of thermodynamics, the total solar energy utilization efficiency of the TFTW system is: 𝜂𝑡𝑜𝑡𝑎𝑙 = 𝜂𝑡ℎ +𝜀𝜂𝑝𝑣 (10) 𝐴𝑝𝑣

Where 𝜀= 𝐴𝑐 is the coverage factor of the TFTW system, and in this experiment, 𝜀 is defined as the ratio of the area of the photovoltaic cell to the area of the collector, which is set to 0.6. Considering that compared with thermal energy, electric energy is a higher-grade energy source, in order to fairly evaluate the solar energy utilization performance of the TFTW system, we use the thermal energy consumed by these electric energy under normal conditions as the energy benefit of the system. Huang et al. [21] introduced the concept of photovoltaic photo-thermal integrated efficiency: 𝜂𝑝𝑣

𝜂𝑓 = 𝜂𝑡ℎ +𝜀𝜂𝑝𝑜𝑤𝑒𝑟

(11)

In the formula, 𝜂𝑝𝑜𝑤𝑒𝑟 is the power generation efficiency of a common thermal power plant, and is generally 38%.

4. Experimental results and discussions. The arrangements for the different working mode experiments are as follows: (a) In the summer, PV/water mode is used for two HPSW modules. (b) In the winter, PV/air mode is adopted for two HPSW modules. (c) In the transitional seasons, the mixing mode is applied, which means that one HPSW module run the PV/water mode and the other run the PV/air mode.

4.1 Summer experiment results. A series of full-day summer experiments were arranged from July 13th to July 19th, 2018. On July 14th, the experiment started at 8:30am, and the system was run stable at 9:00am. The pump was turned off at 17:00pm, which meant that the experiment was over. Fig. 7 shows the changing irradiation and ambient temperature throughout the day, where 𝐼r is the total irradiation of the south wall facade, and 𝑇e is the environment temperature. It can be seen from the figure that the whole day irradiation was good and the fluctuation was small. The maximum irradiation occurred at 12:30, which was 270.6W m2, and the total irradiation amount was 5.0 MJ m2. The ambient temperature fluctuation was more obvious, and the maximum ambient temperature was 38.3°C, the average temperature was 35.1°C. Te Ir 42

300

40

200

36 34

150

Irradiation(W/m2)

Temperature（℃ ）

250 38

32 100 30 28

8

9

10

11

12

13

14

15

16

17

50

Time

Fig. 7. Radiation and ambient temperature change diagram.

4.1.1 The temperature of room, absorber plate and insulation layer. The temperatures of the two rooms are shown in Fig. 8(a). 𝑇exp and 𝑇ref are average temperatures of the experimental room and reference room respectively. The difference between the temperature of experimental room and reference room was shown in the figure and the temperature in the experimental room was a little lower than the temperature in the reference room, with an average difference temperature of 0.6℃. This explained that the HPSW system can protect the building from overheating in the summer. The temperature distribution of the thermal insulation layer and the heat absorption plate are shown in Fig. 8(b). 𝑇insulation1, 𝑇insulation2 and 𝑇insulation3 are the temperature measurement points at the upper, middle and lower heights of the insulation layer respectively. 𝑇absorber1 , 𝑇absorber2 and 𝑇absorber3 are temperature measuring points at different heights of the absorber plate respectively. There was a significant stratification phenomenon on the insulation layer with a high temperature on the upper side and a low temperature on the lower side. The maximum temperature of the insulation layer was 39.8 °C. The temperature

distribution on the absorber plate was relatively uniform, as the temperature difference at different heights was not large. The maximum temperature of the absorber plate was 42.9 °C, which was obviously higher than the temperature of the insulation layer. The average temperature difference between the absorber plate and insulation layer was 2.8℃. Texp Tref

33

42

32

40 31

Temperature(℃ )

Indoor temperature(℃ )

Tinsulation1 Tinsulation2 Tinsulation3 Tabsorber1 Tabsorber2 Tabsorber3

44

30

29

38 36 34

28

27

32

9

10

11

12

13

14

15

16

30

8

9

10

11

12

Time

13

14

15

16

17

18

Time

(a) The temperatures of two rooms.

(b) The temperature of insulation layer and absorber plate. Fig. 8.Temperature of rooms, insulation layer and absorber plate.

4.1.2 Thermal performance of the PV/water mode. The temperature of the water tank is shown in Fig. 10. The temperature at different heights of the water tank was basically the same, which means that there was no stratification phenomenon in the tank. The reason of this phenomenon is that a water pump in the water path was used to power the water cycle, and the water in the tank and circulation path flowed at a high speed. The heat transfer rate was fast, and the water inside the tank was evenly mixed. As the initial water temperature was low, which made a fast heat transfer, the temperature rose obviously from 9:00 to 14:00. After 14:00, the water temperature increased slowly, as the irradiation began to drop significantly. And the water temperature not rose any more after 16:00, even slightly decreased. Because the temperature of the water tank was already high, the rate of heat dissipation from the water to the system and the environment increased. As the figure shows, the initial water temperature was 28.4 °C, the water temperature was terminated at 40.8 °C, and the overall temperature rise was 12.4 °C. Instantaneous thermal efficiency was shown in Fig.9. When the initial water temperature was low, the heat collection efficiency was relatively high. With the increase of water temperature, the thermal efficiency gradually decreased, even dropped to zero. And the maximum instantaneous thermal efficiency was 75.9%. The overall thermal efficiency of the whole day was 52.3%, indicating that the thermal effect of the system was very obvious.

42

70

Ttank1 Ttank2 Ttank3 Ttank4 ηth

Temperature(℃ )

38 36 34

60 50 40 30

32 20 30 28

10

9

10

11

13

12

14

15

16

Instantaneous thermal efficiency（%）

80 40

0

Time

Fig. 9. The temperature of the water tank and instantaneous thermal efficiency.

4.1.3 Electrical performance of the PV/water mode. The electric power and electric efficiency of the whole day is shown in Fig. 10. The trend of power generation is basically consistent with the change of irradiation, and the maximum electric power of the whole day is 48W, reaching at around 12:30. The total power generation in the whole day is 838.4KJ. What can be seen from the all-day irradiance curve is that the total amount of solar radiation of the south wall is very small, so that the total power generation in the whole day is very little. The electricity efficiency is slightly higher in the morning and evening, which can reach more than 9%. And the electricity efficiency is basically stable during the period from 10:00 to 15:00, with less fluctuation, keeping at around 7%. The maximum electrical efficiency all the day is 10.3%, the average electrical efficiency is 7.2%. The efficiency of all-day electricity is lower than it in other seasons below. The main reason is that in the summer of Hefei, the solar elevation angle on the facade of the south wall of the building is very large, so that a large part of sunlight is reflected through the glass cover and the irradiation of the plate onto the photovoltaic panel is small. The comprehensive efficiency of the whole day is 63.4%.

ηpv P

50

14 13

Electrical efficiency(%)

11

40

10 35

9 8

30

7 25

6 5

20

4 3 2

Electric power（W）

45

12

15 16

15

14

13

12

11

10

9

Time

Fig. 10. The electric power and electric efficiency.

4.2 Winter experiment results. The winter experiments were conducted from January 16th to January 18th, 2019. The upper and lower air vents were opened to form an air circulation with the indoor air. The air outlets were opened at 9:00 and closed at 17:00 every day. Select experimental data for two consecutive typical meteorological days from 6:00 on the 17th to 21:00 on the 18th. Fig. 11 showed the solar radiation and ambient temperature curves.

28

800

24

700

Temperature（℃ ）

500

16

400 12 300 8

200

4

Irradiation（W/m2）

600

20

100

0 -4

Te Ir

0 6

9

12

15

18

21

24

27

30

33

36

39

42

45

-100

Time

Fig. 11. The irradiation and ambient temperature of two consecutive days. Irradiation was good on 17th and 18th, and the maximum irradiation for the two days of the experiment were 677.3W m2 and 760.3W m2, respectively, which appeared between 12:00 and 12:30. The total irradiation during the opening period of the vents were 14.3MJ m2 and 16.3MJ m2, respectively. The ambient temperatures could be reduced to 0 °C at night and up

to 17 °C during the day. The maximum ambient temperatures for the two days of the experiment were 17.0 °C and 16.4 °C, respectively. The average ambient temperatures for the two days of the experiment was 13.2 °C and 11.6 °C, respectively.

4.2.1 The temperature of the air channel and absorber plate. The temperature variation curve of the air flow channel is shown in Fig. 12(a). 𝑇channel ― 1 , 𝑇channel ― 2 and 𝑇channel ― 3 are the temperatures at different heights of the air flow channel and △T is the temperature difference at the upper and lower vents of the flow channel. It can be seen from the figure that after the experiment started, the air temperature at different heights of the flow channel all rose rapidly. The air temperature of the flow channel exhibited an obvious stratification phenomenon. The maximum temperatures at the upper, middle and lower measuring points of the flow channel were 26.0 °C, 19.9 °C, 14.3 °C in the first day and they were 32.0°C, 24.9°C, 17.1°C in the next day. The maximum temperatures of three points appeared at around 13:30, 14:10, and 14:40 respectively. The maximum temperature of the upper side appeared earliest, and the lower side appeared the latest. As the upper air temperature was greatly affected by the irradiation, after 13:30, the irradiation decreased rapidly and the heat effect of the system was weakened, so the maximum temperature appeared around 13:30. While the middle and lower air of the flow channel were further mixed with the indoor air, heat was continuously transmitted from the upper to the middle and lower inside in the room. The heat exchange caused the temperature of the lower side and the middle side to further increase, so the maximum temperature of the air on the lower side and the middle side of the flow channel appeared later. The temperature difference between the upper and lower vents of the channel rose and then fell, and the maximum temperature difference was 12.5 °C and 13.4 °C in two days, which occurred at around 12:30 at noon, the same time when the maximum irradiation time of the whole day occurred. The average temperature difference was 9.4 °C. After the indoor air flowed through the channel, it obtained a great amount of heat and brought the heat to the room, so that the indoor temperature rose and the system realized the function of indoor building heating. Fig. 12(b) shows the temperature variation of the absorber plate and insulation layer. It can be seen that both absorber plate and insulation layer appeared significant temperature stratification. And the temperature of the absorber plate was higher than the temperature of insulation layer. The temperature of the absorber plate varied greatly with the lowest temperature 15.2 °C in the morning, and the highest temperature 58.6 °C at 13:00 noon in the daytime. According to previous research, the temperature of the absorber plate has a great influence on the power generation efficiency of the photovoltaic cell. Increasing the temperature of the PV module by 1 °C may result in efficiency decreases of 0.5% and 0.25% respectively for crystalline and amorphous silicon PV cells. [23-24] As the temperature of the absorber plate increases, the photovoltaic power generation efficiency will decrease. The temperature of the insulation layer was slightly higher than the ambient temperature, and the maximum temperature was 27.0 °C. When the air vents were closed at 17:00 pm, the temperature of the absorber plate, and the temperature of the insulation layer were all lowered to the ambient temperature.

Tchannel-1 Tchannel-2 Tchannel-3 △T

30

50 40

20

Temperature(℃ )

Temperature(℃ )

25

Tabsorber-1 Tabsorber-2 Tabsorber-3 Tinsulation-1 Tinsulation-2 Tinsulation-3 Te

60

15

10

30 20 10

5

0

0

6

9

12

15

18

21

24

27

30

33

36

39

42

45

-10

6

9

12

15

Time

18

21

24

27

30

33

36

39

42

45

Time

(a) Air flow channel temperature.

(b) Absorber plate and insulation layer temperature. Fig. 12. The temperature of the air flow channel, absorber plate and insulation layer.

4.2.2 Thermal performance of the PV/air mode. Fig. 13(a) shows the temperature distribution in the experimental and reference room. 𝑇exp1,𝑇exp2 and 𝑇exp3 are the air temperatures at three different heights in the experimental room. △T is the difference of the average temperatures between two rooms. During the two days of the experiment, when the two vents opened at 9:00, 𝑇exp1,𝑇exp2 and 𝑇exp3 were identical which signified that the indoor temperature was consistent. After the vents were opened, as the air in the flow channel and the indoor air formed a circle, the indoor air temperature began to rise and the air temperature distribution showed obvious stratification. The difference between 𝑇exp1,𝑇exp2 and 𝑇exp3 was also getting bigger. The maximum values of the air temperature difference at the upper and lower positions of the room were 5.4 °C, 5.6 °C in two days, respectively. The maximum values appeared at around 12:30 noon, which was basically the same as the maximum irradiation appeared time. The reason is that when the solar irradiation is maximum, the heating effect of the air in the channel is the most obvious. So at this time, the indoor air temperature difference is the largest, and the indoor temperature stratification phenomenon is the most obvious. During the two days of the experiment, the maximum temperature of the upper part of the room were 18.8 °C and 21.6 °C, respectively. The maximum values appeared at between 13:30 and 14:00 and there is a delay time difference compared to the time at which the maximum irradiation appeared. The reason is that the irradiation has started to decline at noon, but still maintains a high level, and the ambient temperature continues to rise. The heating effect on the air flow channel is still obvious, making a large amount of heat into the room, so the temperature in the upper part of the room continues to rise until 14:00 pm. During the two days of the experiment, when the vents were opened, the average temperature in the experimental room increased rapidly, and the rising speed was much larger than the average temperature of the reference room. At the same time, the difference between the indoor temperatures of the two rooms also rapidly increased. The heating effect of the indoor air in the experimental room is obvious, and the heating function of the system in the winter can be realized. The maximum value of the average temperature of the experimental room from 9:00 to

17:00 was 16.4 °C and 19.0 °C, which were appeared at around 14:00 pm. And the maximum temperature of the reference room was 9.0 °C and 11.3 °C, which were appeared at around 15:30pm. This indicated that the temperature of the experimental room reached the maximum 1.5 hour earlier than the temperature of the reference room. The difference of temperature between two rooms risen first and then fallen, and the maximum value of the temperature difference is 8.4 °C and 8.3°C, which were appeared at around 14:00 pm, too. This time is also similar to the maximum temperature of the indoor upper temperature. The reason of this phenomenon is same as above. In order to further analyze the influence of the system on the indoor thermal environment in the experimental room at night, we select the indoor temperature difference data from 17:00 on the first day to 8:00 on the next day, when the two vents were closed. And it was shown in Fig. 13(b). It can be seen from the figure that after the vents being closed at 17:00, the difference in room temperature gradually decreased for the reason that the temperature in the experiment room was higher than the reference room, so the external heat dissipation was larger. Until 20:00 at night, the downward trend in temperature difference was slowing down, and the temperature difference maintained about 1 °C to 1.5 °C. By 24:00 at night, the difference in room temperature tended to be stable, which was maintained at about 1 degree from 24:00 to 8:00 on the next day. At about 7:00 am the next day, the temperature difference between the two rooms showed a minimum value which was 0.6 °C. Throughout the night, the indoor temperature of the experimental room had always been higher than that of the reference room, with an average temperature difference of up to 1.3 °C, which indicated that the HPSW system is conducive to improve the insulation performance of the building at night. There are three reasons why the system can keep the building warm at night. First, when the vents are closed, the temperature of the air inside the experimental room is higher than that of the reference room in the initial period, which means that the heat content of the air in the experimental room is higher. Second, the south brick wall of the experimental room is heated by the hot air from the flow channel and it accumulated a lot of heat in the daytime. When the indoor air temperature is decreased at night, the heat in the wall is radiated to the indoor air, slowing down the temperature drop in the room. And the last reason is that the outdoor air temperature is very low in winter. Two solar wall systems are added on the experiments south wall which can have a certain thermal insulation effect.

20 18

Temperature(℃ )

16 14 12 10 8 6

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

4

0.5

2 0

△T 5.0

Temperature（℃ ）

Texp1 Texp2 Texp3 Texp Tref △T

22

6

9

12

15

18

21

24

27

Time

30

33

36

39

42

45

0.0

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Time

(a) Temperature of the two rooms. (b) Temperature difference on two rooms. Fig. 13. The temperature at different heights of the experiment room and the temperature difference on two rooms at night.

4.2.3 Electrical performance of the PV/air mode. The electrical power and electrical efficiency during the period from 9:00 am to 16:00 pm in the two days are shown in Fig. 14(a) and (b). The all-day electric power gradually increased with the increase of irradiation, and the maximum value was 103.2W at 12:33 on the first day, the other was 115.8W at 12:15 on the second day. The average electric power of the whole day was 82.9W and 94.8W and the total power generation was 4098.8kJ and 4553.3kJ. After the system started running in the morning, the electric efficiency first risen rapidly first and then lightly dropped. At 13:00, the electric efficiency had a lower point. After that, the electric efficiency risen again, and after 15:00 in the afternoon, it started to drop until the end of the experiment. In this process, the electrical efficiency had three lower points, which appeared at the morning, noon and dusk respectively. The reason for the low electrical efficiency in the morning and evening is that the solar incident angle is large at this time, and the solar radiation entering the photovoltaic cell through the outer glass is small. While the temperature of the photovoltaic absorber plate at noon is high, the electrical efficiency is affected by the cell temperature. With the temperature of the absorber plate increasing, the photovoltaic efficiency will decrease. The maximum electrical efficiency of the whole day was 15.8% and 15.6%, and the average electrical efficiency was 15.3% and 15.1%. Pday1 Pday2

120

ηpv-day1 ηpv-day2

16.0

110 15.5

Electric efficiency (%)

Electric power(W)

100 90 80 70 60 50

15.0

14.5

14.0

40 30

9

10

11

12

13

14

15

16

13.5

Time

9

10

11

12

13

14

15

16

Time

(a) Electrical power (b) Electrical efficiency Fig. 14. The electrical power and electrical efficiency of the system in the two days.

4.3 Transition season experiment results. The transitional season experiments were arranged in spring and autumn. The autumn experiments were conducted from October 25th to October 27th, 2018, and the spring experiments were conducted during the period of March 15th to April 20th, 2019. In the transitional season, the ambient temperature span in Hefei is large. The minimum temperature may be below 10 °C, and the maximum temperature may be nearly 30 °C. The building has the demand both for indoor heating and domestic hot water. Therefore, in order to better meet the energy demand of the building, we used the PV/water and PV/air mixing mode. The experimental data of the mixed mode on April 6, 2019 was selected for analysis. The PV/water mode was operated on the left side HPSW system and the PV/air mode was operated on the right side HPSW system. The changes of the environmental parameters are shown in the

Fig. 15. Ir Te

40

500 450

35

30

350 300

25 250 20

200

Temperature(℃ )

Irradiation(W/m2)

400

150 15 100 50

7

8

9

10

11

12

13

14

15

16

17

18

10

Time

Fig. 15. The irradiation and ambient temperature.

4.3.1 Thermal performance of the mixing mode. The thermal efficiency, the temperature of the water tank and absorber plate of the right HPSW system were shown in Fig. 16. During the period from 8:00 to 14:00, the temperature of the absorber plate and the water tank showed a significant upward trend due to the continuous increase of the irradiation intensity. The rising trend of the two was almost the same, and the temperature of the absorber plate was kept larger than the water tank. But after 14:00, the temperature rise of the absorber plate slowed down and began to decline. The temperature difference between the two was gradually reduced, which made the water temperature in the tank not rise. Even after 15:00, the temperature of the absorber plate was lower than that of the water tank, and so the water which flowed quickly through the copper tube, will lost the heat soon. It caused the water temperature dropping. It can be observed from the instantaneous thermal efficiency curve that thermal efficiency was declining all the daytime and even less than zero after 16:00. The reason of this matter is the reverse heat conduction of water to the absorber plate as mentioned above. It can be seen that if the water pump was turned off at around 15:00 pm and the water flow path was stopped, water heat transfer rate will reduce and the temperature of the water tank can be maintained at a higher temperature. The initial water temperature was 18.2°C, the maximum water temperature was 42.3°C, and the temperature rise was 23.3°C. The maximum instantaneous thermal efficiency was 82.0% and the total thermal efficiency throughout the day was 53.5%. Fig. 17 shows the indoor temperature of the experimental room and the reference room. The temperature of the experimental room was higher than the reference room, and at 13:00, the temperature difference between the two rooms reached a maximum of 2.4 °C. The average temperature of the experimental room in the daytime was 23.6°C，while the reference room was 22.0°C. This shows that only one HPSW system can also effectively improve the indoor thermal environment and realize the function of indoor heating at transition seasons.

44

90

27

42

80

26

40

70

34

50

32

40

30

30

28

20

Ttank Tabsorber ηth

26 24 22

10 0

25

Temperature(℃ )

60

36

Thermal efficiency(%)

Temperature(℃ ）

38

24 23 22 21 20

20

-10

19

18

-20

18

8

9

10

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12

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14

15

16

17

Texp Tref

7

8

9

10

11

Time

12

13

14

15

16

17

18

Time

Fig. 16. The thermal efficiency, the temperature of the water tank and absorber plate of the right HPSW system. Fig. 17. The temperature of the experimental room and the reference room.

4.3.2 Electrical performance of the mixing mode. Fig. 18 shows the electrical power and electrical efficiency of the water module and the air module. The trend of the electric power of the two photovoltaic cells was consistent with the irradiation situation, and the maximum values were obtained at around 12:40, which were 66.2W and 63.1W. The water module generated 1195.5kJ of electricity and the air module generated 1144.5kJ of electricity throughout the day. It can be seen from the figure that the electric power and electric efficiency of the water module were slightly larger than the air module from 8:00 to 13:00. The reason is that the water temperature was lower at the beginning of the experiment and the water module had better cooling effect on the photovoltaic cell, so that the temperature of the absorber plate was lower. The lower PV cell temperature, the better electrical performance. After 13:00, the absorber plate temperature of the air module dropped faster, making its electrical performance slightly better than the water module. The maximum electrical efficiency of the water module was 12.2%, and the average electrical efficiency was 10.7%. The maximum efficiency of the air module was 11.6%, and the average efficiency of the entire day was 10.2%. The trend of two module’s electricity efficiency was basically consistent with the trend of irradiation. The temperature of the photovoltaic cells was gradually increased and then gradually decreased. According to theory of PV cell, the higher the temperature of the PV cell was, the smaller the electrical efficiency was, but the experimental result was that the electrical efficiency was highest at noon. Considering the change of the transmittance of the glass cover with the incident angle change, the incident angle at noon is small, and so the transmittance is large. The solar radiation reaching the surface of the PV cell is large, so the electrical efficiency is higher at noon. It can be seen that the influence of the incident angle on the electrical efficiency of the system is greater than the influence of the PV cell temperature.

15

65

Pwater Pair ηpv-water ηpv-air

Electrical power(W)

55 50

14 13 12

45

11

40 10 35 9

30

8

25 20

7

15

6

10

7

8

9

10

11

12

13

14

15

16

17

18

Electrical efficiency(%）

60

5

Time

Fig. 18. The electrical power and electrical efficiency of two modules.

4.4 Comprehensive analysis of different mode experiments. More than 30 days of experiments under three modes were carried out in different seasons and the test results were shown in Table 4. In the table, 𝑇exp and 𝑇ref are the average temperatures of the experiment room and the reference room throughout the day. 𝑇final and △ 𝑇tank are the final temperature of the tank and the temperature rise of the whole day. Q is the total power generation of the system throughout the day. Table 4. The test results of experiments in different seasons. 𝑇final(℃) △𝑇tank(℃) Q(kJ) 𝜂pv(%) 𝜂th(%) Season Pattern 𝑇exp(℃) 𝑇ref(℃) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Water active Summ er Water passive

Spring and Autum n

Water active

/ / / / / / / / / / / / / / / / /

/ / / / / / / / / / / / / / / / /

40.9 40.7 42.4 44.0 38.1 37.6 37.7 40.2 51.6 31.0 31.5 37.9 38.2 44.3 44.7 36.6 37.0

13.5 13.3 15.0 14.7 10.0 10.1 10.0 9.7 32.8 25.6 25.9 26.7 26.4 28.5 28.7 22.4 22.4

848.0 829.0 1114.8 873.4 1115.8 1095.4 1094.8 892.2 5091.4 3910.8 3982.6 3334.4 3384.4 3615.6 3674.4 2770.6 2789.0

7.3 7.2 8.8 6.9 8.2 8.1 8.7 7.1 12.3 12.5 12.7 12.4 12.5 11.9 12.1 11.1 11.2

52.3 51.1 58.3 58.8 36.2 36.4 39.0 38.7 43.8 46.0 46.5 58.3 57.7 47.8 48.0 46.0 46.0

𝜂f(%) 63.8 62.5 72.2 69.7 49.1 49.2 52.7 49.9 63.2 65.7 66.6 77.9 77.4 66.6 67.1 63.5 63.7

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Water passive

Winter

Air

/ / / / / / / 18.6 17.5 10.1 10.8 13.2 13.3 15.7 15.8

/ / / / / / / 12.0 11.9 4.5 4.7 6.4 6.8 9.1 9.3

38.5 41.5 47.2 45.7 35.1 47.9 30.6 / / / / / / / /

22.9 23.3 26.8 27.4 20.9 29.9 16.3 / / / / / / / /

2441.0 2391.0 2448.6 2460.8 2209.6 4864.8 2253.0 4525.2 4680.8 4568.4 4605.0 4053.2 4144.4 4494.6 4612.0

11.1 10.7 10.9 10.9 10.5 11.7 10.7 12.1 12.5 14.6 14.7 14.9 15.3 14.9 15.3

53.4 53.5 61.3 62.7 50.6 40.0 39.5 25.2 26.5 23.5 25.2 20.4 24.8 20.0 22.8

4.4.1 Thermal performance analysis In PV/water mode, the average temperature rise of the water tank in the summer multi-day experiment was 12.0 °C, and the average final temperature of the water tank was 40.2 °C. In spring and autumn, due to the large difference in ambient temperature, the difference of the initial water temperature of the water tank was obvious during the experiments of different time nodes. However, the temperatures rise of the water tank in different periods were basically maintained above 20 °C. The average water tank temperature rise was 25.2 °C from fourteenday experiment data, and the final temperature of the water tank was up to 51.6 °C. In the comparison of thermal efficiency, the difference between transition seasons and summer was not big, but the active cycle mode showed better thermal performance. In summer and transition seasons, the average thermal efficiency of active cycle mode was 55.1%, 51.5%, respectively, and the average thermal efficiency of the passive mode was 37.6% and 39.8%, respectively. Active cycles were significantly more efficient than passive cycles in both summer and the transition seasons. The thermal efficiency of the active cycle mode can up to 62.7%. According to the results of the multi-day water circulation mode experiment, we have fitted a linear relationship between thermal efficiency and (𝑇initial-𝑇e)/H to evaluate the effect of initial water tank temperature on the heat efficiency, which is shown in Fig. 19. The 𝑇initial is water tank’s initial temperature, 𝑇e is daily average ambient air temperature, H is daily total irradiation intensity, MJ/m2. The initial temperature is controlled from 5.4 ℃ to 20.8 ℃. The thermal efficiency decreases approximately linearly as the initial water temperature rises, and the equation between the two variate can be concluded as: 𝑇initial ― 𝑇e

𝜂th=0.3485-0.4500

𝐻

It can be seen from the Fig. 19 that thermal efficiency and (𝑇initial-𝑇e)/H show a good linear

(12)

70.9 70.4 78.5 79.9 67.2 58.5 56.4 44.3 46.2 46.6 48.4 43.9 49.0 43.5 47.0

correlation and the correlation coefficient is -0.9739. 65

ηth Linear Fit of ηth

Thermal efficiency(%)

60

55

50 y = a + b*x 34.8508 ± 1.45971 Intercept -45.00308 ± 3.4954 Slope 22.9892 Sum of squared residual -0.97391 Pearson's r 0.94278 Squared R Equation

45

40 -0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

(Tinitial-Te)/H (℃ /MJ)

Fig. 19. Plot of daily thermal efficiencies of the PV/Water mode. In PV/air mode, the heating effect on the experimental room was very obvious. From the 8day experimentation data at different time periods in winter, the temperature of the experimental room was obviously improved compared with the reference room. The average temperature difference between the experimental and reference rooms was 6.1°C, and the average value of the maximum temperature difference was 8.1 °C. The indoor temperature of the experimental room reached an average of 14.4 ° C, which can meet certain indoor heating needs in winter.

4.4.2 Electrical performance analysis In terms of electrical performance comparison, the average power generation in the summer experiments was 973.0kJ, and the electrical efficiency was also a little low, with an average of 7.8%. The power generation in the transition seasons experimental days increased significantly, and the average power generation was 3226.4kJ. The average electrical efficiency was 11.6%. The electrical performance in winter was the best, with the average power generation 4460.5kJ and the average electrical efficiency 14.3%. The maximum solar energy utilization rate of summer experiment and transition seasons experiment can reach 72.2% and 79.9%, respectively. The utilization rate is very high, indicating that the PV/water mode of the experimental system can effectively utilize solar energy and achieve the original design effect. From the perspective of system power generation in different seasons, it can be found that the winter experimental system had the highest power generation, while the summer experimental system had the smallest power generation. The reason is that the total irradiation received on the facade of the south wall is different in different seasons. The area where the experiment conducted is located at 31°N, north of the Tropic of Cancer. Therefore, the direct irradiation that can be received on the south wall of the building changes with seasonal changes. The vertical surface irradiation in the summer is the weakest and in the winter is the strongest.

The spring and autumn seasons are between the two. Therefore, the system achieved the maximum power generation in the winter. [24] From the perspective of system electrical efficiency in different seasons, it can be found that the winter experimental system had the highest electrical efficiency, while the summer experiment is the lowest. The winter experimental electrical efficiency was almost twice the summer experimental. The first reason is that the solar elevation angle is different in different seasons. In summer, the solar elevation angle on the south wall of the system is very large, which can reach 80°. When the solar shines on the glass cover of the system, a large portion of the irradiation is reflected off, and the irradiation that is projected into the interior of the system and illuminates the cell array is weak. The second reason is that during the summer experiment, the shadow effect of the border is obvious. Because the angle between the sunlight and the system facade is large, the frame will block part of the sunlight, resulting in a part of the edge of the PV cells without direct sunlight. Since all of our PV cells are connected in series, the poor electrical performance of part of the cells will affect the total performance of the system [22]. In the winter experiments, due to the small solar elevation angle, a large part of the direct irradiation of the south wall can enter the interior of the system, and the shadow effect is weak, so the electrical efficiency is higher. Considering the problem of solar elevation angle, it can be concluded that the HPSW system will be used to a greater extent in areas with high latitude. In these areas, because of the high latitude, the solar elevation angle of the south wall facade is small and the direct solar radiation is very large, which can make the system has more excellent electrical performance. At the same time, the ambient temperature of the high latitude areas is generally low, causing a better thermal performance. In areas with high altitude and inconvenient power transmission, the system is more applicable. In these areas, the solar elevation angle is also small and the irradiation conditions are good, which can get higher power generation and electrical efficiency. If the lighting area of the system is reasonably designed, it can basically meet the daily power consumption of home users. At the same time, there is a greater demand for domestic hot water and building heating in these areas due to the economic underdevelopment.

5. Comparison with traditional systems. 5.1 Electrical and thermal performance The comparison of the electrical and thermal performance of the HPSW system with traditional single BIPVT-water systems and BIPVT-air systems is putting forward. As shown in Table 5, it can be found that the energy efficiency of the HPSW system under water mode is roughly the same as that of single BIPVT-water system. And thermal efficiency of the HSPW system under air mode is much higher than the BIPVT-Air system. Because the hybrid HPSW system is more compact and has more function than the BIPVT-Air and BIPVT-Water system, which leads to lower installation, operation and maintenance cost.

Table 5. Comparison of the performance of three systems. 𝜂pv 𝜂th(water) 𝜂th(air) Collector type BIPVT-Water [25] 0.43 0.10 — BIPVT-Air [26] 0.21 0.116 — HSPW(water mode) 0.52 0.12 — HSPW(air mode) 0.24 0.143 —

𝜂f 0.70 0.31 0.71 0.47

5.2 Economical analysis Based on the previous research by our group [26-27], the cost of the HSWP system, BIPVTWater system and BIPVT-Air system are shown in Table 6 and Table 7. The cost of the HPSW system, BIPVT-Water system and BIPVT-Air system are 381.8 $, 372.7 $ and 147.5 $, respectively. The cost of HPSW system and BIPVT-Water system are nearly the same. Considering the HPSW system is able to achieve the function of both BIPVT-Water and Air system, and sum of the two system’s cost is 520.2 $, which is much higher than the HPSW system (380.8 $). If the installation, operation and maintenance cost are considered, economical efficiency of the HPSW system is much higher than the single functional system. Table 6. Cost list of the HSWP system. Material Unit Unit price($) Quantity Amount($) 7 Copper pipe (TP2) branch 2.2 15.4 2 Header (TP2) branch 6.4 12.8 30 Water pipe m 1.1 33.0 30 Waterway Parts pcs 1.4 42.0 1 Water tank (100 l) pcs 130.8 130.8 2 6 Anti-PID front membrane EVA 1.8 10.8 𝑚 2 3 TPT (transparent) 2.3 6.9 𝑚 1 Ultra-white cloth pattern tempered glass piece 16.8 16.8 2 3 Galvanized sheet 5.0 15.0 𝑚 50 Polycrystalline silicon cell sheet 0.8 40.0 1 Black chrome plated aluminum plate piece 12.7 12.7 1 Aluminum frame article 17.2 17.2 Other packaging adhesive materials — — — 28.4 Total — — — 381.8

System BIPV-Water BIPV-Air HPSW

Table 7. Cost comparison of the three systems. Quantity Area 2 1 2𝑚 (1m*2m) 1 2𝑚2 (1m*2m) 1 2𝑚2 (1m*2m)

cost($) 372.7 147.5 381.8

6. Conclusions A new design of the hybrid photovoltaic-water/air solar wall (HPSW) system is proposed and constructed in this paper. And a corresponding experimental platform with two hot box rooms is implemented. A large number of experiments have been conducted to investigate the thermal and electrical performance of HPSW system in different seasons under three working modes. Users can apply the system to achieve three effects of electric energy, hot water and hot air. At the same time, users can flexibly choose different operating modes depending on the season and their needs. Through experiments, we reached several results as follows: 1. In summer, PV/water mode was adopted. The average electrical efficiency of the system was 7.8%, with the total power generation was 973.0 kJ. The water tank fullday temperature rise was 12.0 °C, and we got 200L hot water with a final temperature of 40.2 °C in a day, enough to meet the needs of domestic. The total thermal efficiency was 52.3% and the comprehensive utilization rate of photovoltaic and thermal throughout the day reached 63.4%. 2. In winter, PV/air mode was applied. The average electrical efficiency reached 15.3%, and the whole day power generation was 4460.5kJ. The system increased the indoor temperature of the experimental room with the space volume of 38.5m3 by 8.4 °C compared with the reference room, and the thermal efficiency was 23.9%. The results showed that the HPSW system can effectively improve the indoor thermal environment. 3. In the transitional season, the mixing mode was used. The system reached the average electric efficiency of 11.6% and total power generation was 3226.4KJ. The highest temperature difference between experimental room and reference room was 2.4 °C. And the average temperature rise of the water tank was 25.2°C, with the maximum final temperature of 51.6 °C. The HPSW system effectively meet the complex energy needs of buildings during the transitional season, including indoor heating and hot water demands. 4. From summer to winter, the power generation of system was gradually increased due to the solar elevation angle and the electrical efficiency was increased for the reason of transmittance. 5. The comprehensive efficiency of solar energy was highest in the transitional season with the maximum of 79.9%. 6. These works will be carried out in the following research: (1) Set up the numerical model of HPSW system and perform parametric analysis. (2) Energy saving of the system throughout the year is able to be researched. (3) Optimize the structure of HPSW system to have better performance, and conduct experimental and theoretical research.

7. 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 Dong Guan Innovative Research Team Program (No. 2014607101008).

References [1] Debbarma M, Sudhakar K, Baredar P. Comparison of BIPV and BIPVT: a review[J]. Resource-Efficient Technologies, 2017, 3(3): 263-271. [2] Buonomano A, Calise F, Palombo A, et al. BIPVT systems for residential applications: An energy and economic analysis for European climates[J]. Applied energy, 2016, 184: 1411-1431. [3] Chen F, Yin H. Fabrication and laboratory-based performance testing of a buildingintegrated photovoltaic-thermal roofing panel[J]. Applied energy, 2016, 177: 271-284. [4] Gautam K R, Andresen G B. Performance comparison of building-integrated combined photovoltaic thermal solar collectors (BiPVT) with other building-integrated solar technologies[J]. Solar Energy, 2017, 155: 93-102. [5] Corbin C D, Zhai Z J. Experimental and numerical investigation on thermal and electrical performance of a building integrated photovoltaic–thermal collector system[J]. Energy and Buildings, 2010, 42(1): 76-82. [6] Li H, Cao C, Feng G, et al. A BIPV/T System Design Based on Simulation and its Application in Integrated Heating System[J]. Procedia Engineering, 2015, 121: 1590-1596. [7] Piratheepan M, Anderson T N. Performance of a building integrated photovoltaic/thermal concentrator for facade applications[J]. Solar Energy, 2017, 153: 562-573. [8] Ibrahim A, Fudholi A, Sopian K, et al. Efficiencies and improvement potential of building integrated photovoltaic thermal (BIPVT) system[J]. Energy conversion and management, 2014, 77: 527-534. [9] Ahmed-Dahmane M, Malek A, Zitoun T. Design and analysis of a BIPV/T system with two applications controlled by an air handling unit[J]. Energy Conversion and Management, 2018, 175: 49-66. [10] Tripathy M, Yadav S, Panda S K, et al. Performance of building integrated photovoltaic thermal systems for the panels installed at optimum tilt angle[J]. Renewable Energy, 2017, 113: 1056-1069. [11] Yadav S, Panda S K, Tripathy M. Performance of building integrated photovoltaic thermal system with PV module installed at optimum tilt angle and influenced by shadow[J]. Renewable energy, 2018, 127: 11-23. [12] Yang T, Athienitis A K. A study of design options for a building integrated photovoltaic/thermal (BIPV/T) system with glazed air collector and multiple inlets[J]. Solar Energy, 2014, 104: 82-92. [13] Yang T, Athienitis A K. Performance evaluation of air-based building integrated photovolta-ic/thermal (BIPV/T) system with multiple inlets in a cold climate[J]. Procedia Engineering, 2015, 121: 2060-2067. [14] Rounis E D, Bigaila E, Luk P, et al. Multiple-inlet BIPV/T modeling: wind effects and fan

induced suction[J]. Energy Procedia, 2015, 78: 1950-1955. [15] Vats K, Tiwari G N. Energy and exergy analysis of a building integrated semitransparent photovoltaic thermal (BISPVT) system[J]. Applied energy, 2012, 96: 409-416. [16] Agathokleous R A, Kalogirou S A, Karellas S. Exergy analysis of a naturally ventilated Building Integrated Photovoltaic/Thermal (BIPV/T) system[J]. Renewable Energy, 2018, 128: 541-552. [17] Ji J, Guo C, Sun W, et al. Experimental investigation of tri-functional photovoltaic/thermal solar collector[J]. Energy Conversion and Management, 2014, 88: 650-656. [18] Guo C, Ji J, Sun W, et al. Numerical simulation and experimental validation of trifunctional photovoltaic/thermal solar collector[J]. Energy, 2015, 87: 470-480.67. [19] Irshad K, Habib K, Thirumalaiswamy N. Performance evaluation of PV-Trombe wall for sustainable building development[J]. Procedia Cirp, 2015, 26: 624-629. [20] Spiga M, Morini GL. A symmetric solution for velocity profile in laminar flow through rectangular ducts. Int Commun Heat Mass Transfer 1994;21(4):469–75. [21] Huang B J, Lin T H, Hung W C, et al. Performance evaluation of solar photovoltaic/thermal systems[J]. Solar energy, 2001, 70(5): 443-448. [22] Luque A, Hegedus S. Handbook of photovoltaic science and engineering. 2003[J]. England: John Wiley & Sons Ltd, 1996. [23] Skoplaki E, Palyvos J A. On the temperature dependence of photovoltaic module electrical performance: A review of efficiency/power correlations[J]. Solar energy, 2009, 83(5): 614-624. [24] Wang Y, Pei G, Zhang L. Effects of frame shadow on the PV character of a photovoltaic/thermal system[J]. Applied energy, 2014, 130: 326-332. [25] Ji J, Chow T.T, He W. Dynamic performance of hybrid photovoltaic/thermal collector wall in Hong Kong[J]. Building and Environment, 2003, 38(11):1327-1334. [26] Sun W , Ji J , Luo C , et al. Performance of PV-Trombe wall in winter correlated with south fade design[J]. Applied Energy, 2011, 88(1):224-231.

1. A hybrid PV-water/air solar wall (HPSW) system was investigated by annual experiments. 2. The HPSW runs 3 different modes according to different seasonal needs of building in 4 seasons. 3. The testing thermal, electrical and overall efficiencies in summer are 52.3%, 7.8% and 63.4%. 4. The HPSW can effectively improve the indoor thermal environment in winter. 5. The HPSW effectively meet the complex energy needs of buildings during the transitional season. 6. Economic efficiency of the HPSW system is much higher than the single functional system.

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Kun Luo, Jie Ji, Lijie Xu