A portable renewable solar energy-powered cooling system based on wireless power transfer for a vehicle cabin

Applied Energy 195 (2017) 334–343

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

A portable renewable solar energy-powered cooling system based on wireless power transfer for a vehicle cabin Hongye Pan a, Lingfei Qi a, Xingtian Zhang a, Zutao Zhang a,⇑, Waleed Salman a, Yanping Yuan a, Chunbai Wang b a b

School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, PR China Department of Industrial & Manufacturing Systems Engineering, Iowa State University, Ames, IA 50011, USA

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


A novel portable solar collector

mechanism is optimally designed.
Wireless power transfer is first

applied to cooling systems.
A supercapacitor stores electricity

and outputs a regulated supply.
The proposed cooling system shows

high efficiency in a limited space.

a r t i c l e

i n f o

Article history: Received 3 January 2017 Received in revised form 12 March 2017 Accepted 14 March 2017 Available online 21 March 2017 Keywords: Photovoltaic Wireless power transfer Supercapacitor Portable cooling system

⇑ Corresponding author. E-mail address: [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.apenergy.2017.03.069 0306-2619/Ó 2017 Elsevier Ltd. All rights reserved.

a b s t r a c t As the greenhouse effect becomes increasingly serious, cooling a vehicle cabin parked under the blazing sun without running the engine or using an electric vehicle’s power has received considerable attention. In this paper, we develop a novel portable, renewable, solar energy-powered cooling system with wireless power transfer (WPT) and supercapacitors to cool the vehicle cabin. The proposed system consists of a solar collector mechanism, an energy conduit, and a temperature control and cooling module. First, consisting of folding solar photovoltaic (PV) panels, the solar collector mechanism making the proposed system portable. Once collected, the solar energy is converted into electricity and stored in the supercapacitors through wireless power transfer without breaching the vehicle body. Automatic temperature regulation is achieved with the cooling device via the temperature control and cooling module. The experimental results indicate that a maximum output power of 2.181 W and a maximum WPT efficiency of 60.3% are achieved when the prototype loaded with 3 X and 5 X respectively. Meanwhile, the simulation shows the temperature inside the cabin is reduced by as much as 4.2 °C in average, demonstrating that the proposed solar energy-powered cooling system is effective and feasible in cooling a hot vehicle cabin. Ó 2017 Elsevier Ltd. All rights reserved.

H. Pan et al. / Applied Energy 195 (2017) 334–343

1. Introduction Vehicles play an important role in transportation within modern society. The consumption of fossil fuels has also brought smog, hazardous emissions, and global warming, which has aroused increasing interest in new energy sources. Much research has been focused on electric vehicles (EVs) to achieve emissions reductions [1]. As another solution, many researchers have been fascinated by harvesting energy (especially vibration energy) from the environment [2–4]. Existing energy harvesting systems can be placed into different categories, including mechanical, piezoelectric, thermoelectric, wind energy, and solar [2–8]. It is vitally important to drivers that there be an air conditioner in their vehicles in the summer. One pervasive problem in the summer is that the temperature of the car cabin will rise rapidly, especially in vehicles parked outdoors. High temperatures speed the ageing of plastics, release harmful gas, consume more fuel to power the air conditioner, and compound energy problems. Solar energy, a renewable and clean energy, has been widely used to provide heat and electricity [9]. The use of renewable solar energy to power an air conditioner while reducing greenhouse gas production and energy consumption has attracted global attention [11–24]. Numerous researchers have proposed solar energy powered air-conditioning systems, which consist of two types: (1) photovoltaic (PV) conversion, and (2) photothermal conversion [10]. Photovoltaic collectors have increasingly been used to supply solar air-conditioning cooling systems, as the cost of a solar PV system is currently economical. Anis [11] proposed a PV-powered airconditioning system based on microprocessor control. Compared to a conventional PV powered air conditioning system, this system makes the high starting current of the motor more feasible. Daut et al. [12] presented a solar-powered air-conditioning system using PV panels consisting of a PV module, charge controller, batteries and an air conditioner. Porumb et al. [13] compared the performances of a solar-powered absorption air-conditioning cooling system and a solar-powered photovoltaic air-conditioning cooling system. They found that the photovoltaic cooling fraction was 12.1% greater than the thermal cooling fraction. Huang et al. [14] designed and built an air conditioner for a low-energy house driven by solar PV panels. The system is supported by a small buffer battery; no grid power is needed. A residential district-level cooling system combining photovoltaic and natural gas power is developed in [15]. At various times of a day, the PV generation and the gas turbine may work together or singly to provide electricity to meet the heating/cooling demand. Sanaye and Sarrafi [16] presented a combined cooling, heating and power (CCHP) solar generation system to supply cooling, heating, electricity and hot water for a building. Li et al. [17] presented a solar photovoltaic air conditioner that can meet temperature control needs in any weather with an inverter efficiency of 70–80%. The system has been deployed in a room, where the photovoltaic array is installed on the roof above the room. The solar air-conditioning system developed by [18] combines photovoltaic and thermal collectors and is able to produce both electricity and hot water. Many researchers have used solar energy as the power supply for an air-conditioning system via photothermal conversion. In [19], the authors analyse the technology and economy of the proposed solar-powered cooling systems for industrial applications to evaluate their advantages and limitations. Rosiek et al. [20] applied occupancy sensors and chilled water storage tanks in a solarassisted air-conditioning system. The system reduces the consumption of electrical energy by approximately 42% and the production of CO2 by 1.3 tons during an entire summer of operation under a partial load. To select the optimal solar collectors for a solar-driven ejector air-conditioning system, Zhang et al. [21] developed a simulation program. Based on the results of computer

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simulation and lab tests, the selected collectors can meet the full demands of powering a 5 kW solar-driven ejector airconditioning system for 10 h. A solar-assisted air-conditioning system combining a liquid desiccant ventilation system with an airhandling unit is proposed by Qi et al. [22]. The proposed system has been simulated in five cities representing four main climate regions, with savings of up to 45% in energy consumption. Zhai et al. [23] designed a solar-powered adsorption air-conditioning system and installed it in a green building. The solar-powered air-conditioning system outputs an average power of 15.3 kW and a maximum power of 20 kW. A review of a solar-powered ejector air conditioner is presented in [24]. The paper also analyses different cycles, one with compression enhancements and another without. In general, the main application of a solar-powered photothermal air-conditioning system is the cooling of an entire building. Although many researchers have studied solar-powered airconditioning systems, limited research has been conducted on applying one to a vehicle cabin. Abraham et al. [25] described a photovoltaic thermoelectric refrigerator for car heat dissipation. The group manufactured a small experimental prototype, and the results show that the temperature inside the cabin can be cooled to the ambient temperature. Another study [26] developed a solar powered cooling device for an electric car. The device in this study was installed in an electric car cabin, and the simulation results demonstrate that the proposed device can effectively displace the hot air with cooler ambient air. Zhang et al. [27] designed a solar-powered air-conditioning system for a vehicle including a foldable solar energy collector mechanism and an airconditioning system powered by solar photovoltaic panels. Zhang et al. [28] proposed a phase-change material cooling device for vehicles. The proposed device changes phase to absorb heat from the air inside a car. All this research makes it clear that solar-powered cooling systems are currently a hot topic. To meet the demands of thermal comfort and reduce fuel consumption and emissions, this paper presents a novel portable solar-powered cooling system for a vehicle cabin. For sake of portability and easy installation, wireless power transfer technology is applied to avoid a tangle of wires that will need to perforate the cabin. In recent decades, wireless power transfer has attracted more and more researches, such as charging electric vehicles, powering a biomedical capsule endoscope, and discussing the losses analysis [29–31]. The structure of the rest of this paper is as follows. In Section 2, a general overview of the proposed solar-powered cooling system’s design is given, including a solar collector mechanism, an energy conduit, a temperature control and a cooling model. Then Section 3 models and analyses the proposed system. Section 4 provides the experimental details and results for both the field experiments and the simulation experiments. Section 5 presents a discussion of the implications of the results. Finally, the study’s conclusions are presented in Section 6.

2. System design The proposed portable solar-powered cooling system as shown in Fig. 1, consists of three parts: a solar collector mechanism, energy conduit, and a temperature control and cooling module. When parked outdoors, place the solar collector mechanism on the roof of the vehicle. The solar collector mechanism, on which the solar PV panels are installed, is designed to use folding solar PV panels to increase portability. When the system is operating, the solar collector mechanism unfolds the solar PV panels to the proper angle to collect as much solar energy as possible. The energy conduit transfers energy from the solar PV panels to

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Fig. 1. Flowchart of the proposed portable solar-powered cooling system.

the wireless power transfer unit and the supercapacitor. The solar energy is then transformed to electrical energy, which is stored in the supercapacitor. The temperature control and cooling module can control the operation of the proposed system including obtaining the cabin temperature with the temperature sensor, starting the DC motor to unfold the solar PV panels, and enabling the cooling device. To decrease the temperature inside the vehicle, the prototype presented in this paper uses a small fan in the cooling module. Before driving, the solar collector mechanism must be folded and put away inside the cabin or trunk. Fig. 2 is a theoretical diagram of the proposed portable solar-powered cooling system that illustrates the system setup.

2.1. Solar collector mechanism The solar collector mechanism is designed as shown in Fig. 3. This mechanism contains ‘‘M-shape” linkages, ‘‘N-shape” link strut rods, a small DC driving motor, a ball screw and a transparent box, which are labelled in Fig. 3b. The overall size of this design is145  175  455mm. The weight of the solar collector mechanism is 3.5 kg. As presented in Fig. 3c and d, the portable solar collector mechanism is placed on the roof of the cabin for parking time. In Fig. 3c, the mechanism is unfolded, preparing for solar energy harvesting. Fig. 3d shows the mechanism folded when not in operation, and need to be put away inside cabin or trunk. The solar collector mechanism is folded and unfolded by the DC motor and ball screw. The solar PV panels are fixed on the symmetrical ‘‘M-shape” linkages. Each set of ‘‘M-shape” linkages is made of a

Fig. 2. Theoretical diagram of the proposed portable solar-powered cooling system.

hinge connected head to tail. The ‘‘M-shape” linkage base consists of two ‘‘N-shape” link strut rods. The small DC motor is connected through the coupling by the screw. The screw-nuts can move forward to close the ‘‘N-shape” link strut rods or backward to open them, depending on the motor rotation direction. Therefore, the clockwise or anticlockwise rotation of the motor can be converted into the linear opening and closing motions of the solar panels, respectively. Two switches are placed in the extreme movement

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Fig. 3. View of the solar collector mechanism.

positions of the solar panels to restrict their movement. The transparent portable box can be connected to the car roof. 2.2. Energy conduit Fig. 4 shows the energy flow of the proposed system. The solar PV panels collect solar energy and output electricity. Next, the wireless power transfer unit delivers electricity through the vehicle roof without perforating it. The DC/DC converter regulates the output to the set point voltage, and the controller controls the charging and discharging of the supercapacitor. Finally, the cooling device consumes the electricity stored in the supercapacitor. As shown in Fig. 5, the energy conduit consists of three main parts: solar PV panels, a wireless power transfer unit, and a supercapacitor. The solar PV panels convert solar energy to the desired electrical energy. Each monocrystalline silicon solar module provides 6 V and 150 mA electricity for the system with the size of 110 mm  60 mm. Through series and parallel connections, all of the solar PV panels can provide electricity totalling 12 V and 9 W. To avoid a jumble of wires the need of perforate the cabin, the proposed system uses wireless power transfer to transmit electricity from the roof to the cabin. The input voltage and most loading current of the WPT are 12 V and 1.3 A respectively, while the output voltage and output current are 5 V and 1.5 A respectively. The outside diameter of the coil is 43 mm. The efficiency of the WPT based on magnetic coupling resonance is about 60–70%, and the maximum transmission distance is 20 mm.

Fig. 5. Installation of the main parts of the energy conduit.

ning, the solar PV panels are unfolded, and the conversion of the solar energy to electrical energy begins. The electricity is stored in the supercapacitor via wireless power transfer. The temperature control and cooling module detects and regulates the cabin temperature in real time using the sensor. After sufficient sun exposure, the temperature inside the cabin will exceed a pre-set threshold which is 30 °C. Then, the temperature control switches on, and the cooling device starts. The supercapacitor supplies the electricity to the cooling device, until the cabin temperature is reduced to its threshold value. The system can also be controlled via Bluetooth. An application has been developed for Android mobile phones. Users can operate the proposed system from their mobile phones in advance, so that the cooling process will be complete before the users step into the car. The cooling device presented in this paper is shown in Fig. 6(a). The nominal voltage of the small fan is 5 V. Fig. 6(b) illustrates the cooling process. 3. Modelling and analysis 3.1. Solar collector mechanism A model of the solar collector mechanism is shown in Fig. 7, where v1 is the velocity of the rectilinear motion of the ball screw, rm is the revolving speed of the DC motor. P represents the screw lead. The relationship between them can be expressed as

v 1 ¼ P  rm

ð1Þ

2.3. Temperature control and cooling module When a vehicle is parked outdoors, the solar collector mechanism is placed on the car roof, and the main switch of the portable solar-powered cooling system is turned on. The motor starts run-

Solar PV Panels

Wireless Power Transfer Unit

DC/DC

Cooling Device

Supercapacitor

Controller

Fig. 4. Energy flow.

(a) Cooling device

(b) Cooling effect

Fig. 6. View of the cooling device.

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B ¼ ðn  1Þ

360 365

ð7Þ

where n is the day of the year, with a range from 1 to 365.

T ¼tþ

Lloc  Lst E þ 60 15

ð8Þ

x ¼ 15ðT  12Þ

ð9Þ

where T represents the local solar time (h); h is the solar hour angle (°); t is the standard time (h - in this paper, t is Beijing time); Lst is the standard meridian for the local time zone (in this paper, Lst is 120° East); and Lloc is the longitude of the location in question (the longitude of Chengdu is 104.06° East).

sin as ¼ sin u sin d þ cos u cos d cos x

ð10Þ

sin cs ¼ cos d sin x= cos as

ð11Þ

Fig. 7. Analysis of the solar collector mechanism.

c ¼ c 0 þ 2v 1 t

ð2Þ

As it is a right triangle, the displacement (L) between a and c can be obtained by Pythagorean Theorem, which is expressed as

L¼

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a  c2 2 b  2

Taking the derivative of L,

ð3Þ

v2 means the velocity of L.

v 2 ¼ dL=dt

ð4Þ

Fig. 8 shows the result of L and v2. At the first 2 s, the displacement (L) increased very fast, which is also proved by the curve of velocity (v2). Then the velocity decreased slowly till it stopped.

where as represents the solar altitude angle (°); cs stands for the solar azimuth angle (°); and u is the latitude of the location (the latitude of Chengdu is 30.67° North). 3.3. Solar radiation 3.3.1. Beam radiation The beam radiation of the plane normal to the solar rays can be expressed as:

Ibr ¼ Gsc Pm

ð12Þ 2

where Gsc represents a solar constant, which is 1367 W/m ; and m is the air mass, which can be written as:

1 sin as

3.2. Sun position

m¼

Duffie and Beckman [32] put forward the following expressions to estimate the declination of the sun (d) and the time correction factor (E).

The beam radiation of the horizontal plane and slant plane are expressed as:

0

0:006918  0:399912 cos B þ 0:070257 sin B

B d ¼ ð180=pÞ@ 0:006758 cos 2B þ 0:000907 sin 2B

1

IbH ¼ Ibr sin as

ð14Þ

C A

Ibb ¼ Ibr ðsin as cos b þ cos as cos cs sin bÞ

ð15Þ

0:002697 cos 3B þ 0:001480 sin 3B ð5Þ  E ¼ 229:2

0:000075 þ 0:001868 cos B  0:032077 sin B 0:014615 cos 2B  0:04089 sin 2B

ð13Þ

 ð6Þ

where b is the slope angle of the plane (°). 3.3.2. Diffuse radiation The diffuse radiation of the horizontal plane is given as:

IdH ¼ 0:5Gsc

1  Pm sin as 1  1:4 ln P

ð16Þ

And the diffuse radiation of the slant surface is written as:

Idb ¼ cos2

b IdH 2

ð17Þ

where IdH is the diffuse radiation of the horizontal plane (W/m2), and Idb is the diffuse radiation of the plane with an angle b to the horizontal plane (W/m2). For a slant surface, the total solar radiation can be expressed as:

IR ¼ Ibb þ Idb

ð18Þ

Fig. 9 shows the different solar radiation on a horizontal plane in different days during one year, in Chengdu. The maximal solar radiation can reach 1356 W/m2 in 20th Jun. While even in 21st Dec. the solar radiation can achieve 801 W/m2. 3.4. Power generation of PV cell

Fig. 8. Simulation of the solar collector mechanism.

For the sake of obtaining output power of the PV cell, the equivalent circuit model of PV cell with single diode was designed, as shown in Fig. 10. According to the equivalent circuit diagram, the

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Fig. 9. Simulation for solar radiation of one year in Chengdu.

Fig. 11. Wireless power transmission model.

Fig. 10. The equivalent circuit model of PV cell with single diode.

correlativity between the output voltage (V) and output current (I) of PV cell can be written as

    qðV þ IRS Þ V þ IRS 1  I ¼ Iph  Isat exp AkT Rsh

ð19Þ Fig. 12. Experimental environment of solar energy powered cooling system.

where Iph and Isat are the photo-generated current and reverse saturation current when there is no light on the PV cell, respectively. The Rs and Rsh are the equivalent series resistance and equivalent parallel resistance, respectively. A, K and T are the ideal performance coefficient of diode, Boltzmann constant and absolute temperature of PV cell. The maximum power output of the PV cell is expressed as

Pm ¼ Isc V oc F F

ð20Þ

where Voc and Isc are the open-circuit voltage and short-circuit current, respectively, and FF is Fill Factor (0.75  08). Therefore, the photoelectric conversion efficiency of PV cell (g1) can be given by

g1 ¼

F F Isc V oc  100% Pin

ð21Þ

where Pin was the input power of the PV cell. 3.5. Wireless power transfer The wireless power transfer system model and equivalent circuit model of this system were developed, as shown in Fig. 11 (a) and (b), respectively. Where the Uin and Uo are the highfrequency power supply and load voltage, respectively. R1 and R2 are the sum of the eternal resistance of power and internal resis-

Table 1 Different parameters in six conditions. Serial number

Load resistance (X)

Distance between induction coils (mm)

No. No. No. No. No. No.

3 3 3 5 5 5

0 5 10 0 5 10

1 2 3 4 5 6

tance of transmitting coil and internal resistance of receiving coil, respectively. The inductance of transmitting coil and receiving coil are L1 and L2, respectively. The capacitance of transmitting coil and receiving coil are given by C1 and C2, respectively. The M is the coefficient of mutual induction between the transmitting coil and the receiving coil. RL stands for load resistance. The output power (Po) and energy transfer efficiency (g2) of the wireless power transfer system are given by

Po ¼

2

x2 M þ

U 2in RL R21 ðR2 þRL Þ2 x2 M2

þ 2R1 ðR2 þ RL Þ

ð22Þ

340

g2 ¼

H. Pan et al. / Applied Energy 195 (2017) 334–343

pffiffiffiffiffiffiffiffiffiffiffiffi

x2 RL x2 ðR2 þ RL Þ þ

ð23Þ

R1 ðR2 þRL Þ2 M2

Moreover, the mutual inductance is affected by the distance between the transmitting coil and receiving coil which the relationship can be expressed as the following

(a) 3 , 0mm.

M¼

pl N1 N2 ðr1 r2 Þ2

ð24Þ

3

2d

where l is permeability of the medium. The N1 and N2 were the circle number of the transmitting coil and receiving coil, respectively. The radius of the transmitting coil and receiving coil were r1 and r2,

(d) 5

, 0mm.

(b) 3 , 5mm.

(e) 5

(c) 3 , 10mm.

(f) 5 , 10mm.

Fig. 13. Power curves in field test for WPT at different solar radiation.

, 5mm.

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respectively. The distance the between the transmitting coil and receiving coil was given by d. By comparing Eqs. (22) and (24), the following expression can be obtained.

Po ¼

x2 A2 6 d

þ

U 2in RL h i2 R1 ðR2 þRL Þ þ xA

2R1 ðR2 þ RL Þ

ð25Þ

x2 RL 2

6 LÞ x2 ðR2 þ RL Þ þ R1 ðR2AþR d 2

ð27Þ

Therefore, the output power of the whole system was calculated by

Pout ¼ Pin g

Load resistance

Before WPT

After WPT

(a)

3X

2.025 4.173 3.046

1.001 2.181 0.32

5X

2.016 3.307 3.163

1.145 1.995 1.62

(d) (e) (f)

Peak power (W)

ð26Þ

Under the condition of other factors which cause energy loss are neglected, the total energy transfer efficiency, g, was evaluated by

g ¼ g1 g2

Fig. 13

(b) (c)

Similarly, by comparing Eqs. (23) and (24), the following expression can be obtained.

g2 ¼

Table 2 Experimental results from Fig. 13.

ð28Þ

4. Experimental details As shown in Fig. 12, the experiment comprises the proposed system, the solar simulator, a pyranometer, a data recorder, and a multimeter. The prototype of solar energy powered cooling system is manufactured for the experiment to evaluate the performance of the proposed system. The solar simulator is installed high that can simulate the different solar irradiance in the range of 0–1100 W/m2. As shown in Fig. 9, the solar irradiance in daytime (8:00 am–18:00 pm) during summer days are higher than 500 W/m2, so we simulate the solar irradiance in the range of 500–1100 W/m2. A pyranometer placed under the solar simulator is periodically gauging the solar irradiance intensity every 5 s during the experiment. The multimeter is used to measure the voltage and current signal at different places in the circuit, for example, the voltage and current signals before WPT, and the voltage and current signals after WPT. For sake of convenient measurement, the cooling device is replaced by the load resistances, and the resistance values are 3X and 5X. In order to test the performance of wireless power transfer, we set 3 distance values, as (1) no distance; (2) 1 piece of glass; and (3) 2 pieces of glasses between the transmitter coil and receiving coil. The thickness of every piece of glass is 5 mm. Six experimental conditions according to Nos. 1–6, are set up with different load resistances and distances between the induction coils, which are listed in Table 1.

The maximum output power appeared as 2.181 W shown in Fig. 13(b), while the maximum efficiency of WPT is 60.3% as shown in Fig. 13(e). As the distance between the induction coils increased, both the power before WPT and after WPT rises first and fall later. This appearance is caused mainly by the distance transfer characteristics of the WPT. For the magnetic resonant WPT system, there is a best distance for the power transfer. When the solar irradiance is same, the power and efficiency of the system circuit increase first and then decrease as the distance increased. As shown in Fig. 13(c) and (f), when the distance is 10 mm, the powers reached a maximum value and did not increase again as the solar irradiance increased. One thing distinguished solar PV panels from other kind of power source is that the output voltage and current change with different load, results in the change of output power. Therefore, there must be an optimal load resistance which lead to a maximum output power. Regarding the efficiency curves, when the distances are same, the efficiencies of the WPT that load a 5 X resistance are always higher than that load 3 X resistance. The highest solar irradiance is 1100 W/m2 because of the output power limit of this solar simulator. When the transfer distance is optimal, the system operate well and the output power is smooth when the solar irradiance or the area of solar PV panels increased. Limited by the size of the solar simulator, it’s difficult to do the field test on the real vehicle cabin to study the thermal performance of the prototype. To simulate the cabin temperature, we used SOLIDWORKS to build a cabin model with the size of about 3177 mm  1408.5 mm  1246.5 mm, as shown in Fig. 14. The model was

5. Results and discussion The power and efficiency curves of the proposed system in different load resistance and difference distance between the induction coils are shown in Fig. 13, and all the waveforms of the conditions occur to be similar with a tendency of increase by the enhancement of the solar irradiance. In Fig. 13(a)–(f) are the power and efficiency curves of the six conditions as shown in Table 1, where the serial numbers are consistent with Nos. 1, 2, 3, 4, 5, and 6. Table 2 shows the experimental results from Fig. 13. When the load resistance is 3 X, the peak powers before WPT, are 2.025 W, 4.173 W and 3.046 W, and the peak powers after WPT are 1.001 W, 2.181 W and 0.320 W. As shown in Fig. 13(d)–(f), when the load resistance is 5 X, the peak powers before WPT, are 2.016 W, 3.307 W and 3.163 W, and the peak powers after WPT are 1.145 W, 1.995 W and 1.620 W.

Fig. 14. The temperature distribution inside the cabin.

Table 3 Specification parameters. Surfaces

Area (m2)

Slope (°)

Radiation (W/m2)

Windshield Side windows Rear Window Roof Side gates Tailboard Floor

1.178 1.023  2 1.020 3.643 2.834  2 1.524 5.828

30 90 60 0 90 90 0

1021.4 176.6 108.3 1053.4 176.6 54.1 0

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imported to GAMBIT for pre-processing such as meshing and setting the boundary and continuum types, and the mesh files were exported to FLUENT 14.5 to solve the equations. The specification parameters of the numerical simulation are shown in Table 3. For this model, two situations are evaluated: sitting under the blazing sun; and running the proposed system while sitting under the blazing sun. When the proposed system is used, the temperature and wind speed are 28 °C and 5 m/s. To simplify the calculations, we make some assumptions: (1) (2) (3) (4)

The air inside the cabin is ideal and incompressible. The cabin is well-sealed. The air flow inside the cabin is a steady turbulent flow; and The temperature of every wall is fixed and is the same as the integrated temperature of the corresponding faces.

From the software we obtain the results shown in Figs. 15 and 16. Fig. 15 shows the simulated results on the two planes of the cabin shown in Fig. 14, to compare the temperatures inside the cabin. To illustrate fully the effect of the proposed system, we take a line in each plane and its associated data and then draw the graphs of Fig. 16(a) and (b). In Fig. 16 there is a gap in each curve. The gap occurs because the line goes through the seat, and the seat area is removed during modelling time. The benefits of the proposed system are very obvious, with average temperature reductions of 3.0 °C and 3.5 °C, and a maximum temperature reduction of about 8 °C.

Fig. 16. Temperature along lines inside the cabin.

6. Conclusions A portable solar-powered cooling system with wireless power transfer technology is developed for a vehicle cabin. The experimental results show that the output power is in phase with solar irradiance. Moreover, a peak output power of 2.181 W and a peak efficiency of WPT of 60.3% are generated from the prototype when the load resistances are 3X and 5X respectively. The achieved output power and high efficiency indicate that the proposed solar energy-powered cooling system is effective and feasible in cooling a hot cabin. If the proposed system in this paper matches with PV panels in larger areas, the output power may be higher to power the higher power fan, which will make a better cooling effect. Acknowledgements

Fig. 15. Temperature distribution inside the cabin.

This work was supported by the National Natural Science Foundation of China under Grant No. 51675451, by the Science and Technology Projects of Sichuan and Chengdu under Grant Nos. 2016GZ0026, 2017RZ0056 and 2015-HM01-00338-SF, and by the Fundamental Research Funds for the Central Universities under Grant No. 2682016ZDPY03. The first four authors contributed equally to this work. The asterisk indicates the author to whom all correspondence should be directed.

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