The prototype construction and performance evaluation of dish-type concentrator photovoltaic system

Abstract

In this paper, a dish-type concentrator photovoltaic system is taken as an object, whose tracking control strategy and energy conversion characteristics are studied in-depth. Specifically, the dish-type concentrator photovoltaic system is described in detail from the following three parts: dish-type concentrating system, tracking control system and cooling system. Then, an independently closed-loop tracking control system is designed based on programmable logic controller with the tracking accuracy under 1%, which can be demonstrated by the outer wall temperature of the secondary reflector and the temperature distribution of the focus spot. Finally, the maximum power output is estimated to evaluate the overall performance of the dish-type concentrator photovoltaic system.

1 INTRODUCTION

With the increasing development of the world economy, the fossil energy resource is depleting rapidly, which making the environmental pollution more and more serious. As a renewable energy resource, solar energy has become one of the alternative energy resources urgently needed for human development due to its unlimited storage, universality and economy [1–5]. However, the energy density of solar energy is relatively low, which making the direct utilization technology inefficient. It increases the device scale and initial investment, finally obstructs its commercial applications. In order to solve the above issues, the concentrating technology is presented, which is generally divided into four different categories: tower type, trough type, fresnel type and dish type. Among these four technologies, the dish type concentrating technology is one of the most feasible approaches because of its high tracking accuracy, concentration efficiency, and thermal power generation efficiency. Research shows that the solar thermal power conversion efficiency of the dish solar thermal power system is up to 48%, and the photoelectric conversion efficiency is as high as 31.25% [6–10].

At present, a lot of researches have been conducted to further enhance the energy efficiency of the dish-type concentrator photovoltaic system. For example, Jaramillo et al. [11] analyzed experimentally and theoretically the thermal energy and temperature distributions obtained in the focal plane. Peter et al. [12] took some main parameters into account, such as the geometry of the reflecting and receiving surfaces and the uncertainty of the reflecting surface. The energy distribution on the cylindrical central axis receiver was compared and analyzed. The results indicated that the coda wave comparison using a flat-plate calorimeter can be a useful technique to evaluate the thermal performance of point focus solar concentrating systems. Shuai Yong et al. [13] analyzed to the performance of the dish-type concentrating system by the Monte-Carlo method. The effect of the introduction of solar shape and reflection surface error on the performance of concentrating radiation was studied in depth. Du Shenghua et al. [14] also used the Monte-Carlo method to analyze the effect of non-parallelism of solar light, tracking accuracy, and reflection aperture ratio on the energy distribution of focal plane. Liu Ying et al. [15] proposed a new method for calculating the distribution of energyflow in the focal plane using the finite element method.

Researches mentioned above reveal the effect of the focus spot on the power generation efficiency and provide valuable ideas for uniform the focus spot in some extent. However, there are many factors affect the focus of light spot, such as the dish structure, the cell cooling system, and the tracking accuracy of the dish system. In order to analysis the energy distribution characteristics of the focus spot, a dish-type concentrator photovoltaic system is set up in this paper, which including concentrating power generation system, tracking control system, and cooling system. Then, an infrared camera is used to observe the focus spot in real time and the energy distribution of the cell surface is studied comprehensively. Besides, four thermal resistances were installed on the outer wall of the secondary reflector, which is used to judge the system tracking accuracy with the data of Infrared camera. Moreover, a water cooling circulation system was designed to guarantee the cell operation safety.

2 CONSTRUCTION OF DISH-TYPE CONCENTRATOR PHOTOVOLTAIC SYSTEM

As shown in Figure 1, the dish-type concentrator photovoltaic system is divided into three parts: dish-type concentrating system, tracking control system and cooling system. The dish-type concentrating system focuses a large area of sunlight into a small beam, which is uniformly distributed on the concentrating solar cell surface through the secondary reflector. The tracking control system keeps the sunlight perpendicular to the entrance plane of the condenser. Finally, a cooling system is designed to guarantee the operation safety and increase the overall energy utilization efficiency.

Figure 1.

Dish-type concentrator photovoltaic system schematic.

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2.1 Tracking control system

The tracking accuracy is the premise for efficient operation of the dish-type concentrator photovoltaic system. In this study, the tracking control system is mainly composed of photoelectric sensor, rod motor and deceleration motor, as shown in Figure 1. There are four photosensitive resistors in the sunlight sensor, whose resistance values are changeable with the sun’s position.

When the four resistances are different, the disk tracking system starts to operate. On one hand, the deceleration motor drives the corresponding mechanical structure to make the concentrator rotating from east to west to track the azimuth of the sun. On the other hand, the rod motor drives the concentrator to rotate around the push rod to track the sun’s height angle. Until the resistance error of the four photosensitive resistors is less than 1%, the tracking control system will stop operation and the sunlight is believed incident perpendicularly. Moreover, brushless motor and gearbox are adopt by the rod and decelerating motor to improve the motion accuracy.

The system adopts Siemens PLC-1200 as the main controller which installed in the control box. There are two kinds of tracking modes for the system, time tracking and sunlight tracking as shown in Figure 2. When the weather is clear, the radiation intensity is higher than the threshold set by the photoelectric sensor. The control system automatically enters into the sun tracking mode. If the radiation intensity is lower than the threshold of the sensor, the system automatically enters into the time tracking mode and calculates the sun azimuth and height angle at this time, and then drives the motor. The motor feedback data are collected to detect the angle of motor operation.

Figure 2.

Tracking control flow chart.

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2.2 Cooling system

Through the tracking control system and the concentrating system, ~80% of the solar energy is absorbed by the solar cell. Specifically, one part is converted to electricity by photoelectric effect and the remainder is dissipated into heat, which leads to the energy wastage and the cell efficiency reduction. In order to guarantee the operation safety and the energy conversion efficiency, some cooling structure is designed as shown in Figure 3, such as the water cooling structure of the outer wall in the secondary reflector and heat dissipation structure in the cell.

Figure 3.

Cooling structures in the dish-type concentrator photovoltaic system.

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As described above, most of the energy is redistributed by the secondary reflection unit, which means higher energy density and temperature on the reflective film under the normal working condition. Therefore, a water cooling structure is design on the outer wall of the secondary reflector to prevent from the reflective film damage as shown in Figure 3a. In addition, as the solar cell can only absorb the solar light of a specific band, the radiation not used will be converted into thermal energy, which increasing the cell temperature and affects the cell efficiency correspondingly. In order to maintain the efficient operation of the cell in the dish-type concentrator photovoltaic system, the bottom of the cell board is equipped with cooling devices as shown in Figure 3b. The design structure of the U shape increases the contact area between the water and the inner wall, making the cooling system more effective.

The cooling water of the entire system is pumped from water tank, and then enters the inlet pipe. A part of the cooling water passes through the outer wall of the secondary reflector. The other part flows into the cell heat dissipation structure and then passes through the outlet pipe. The waste heat taking away by the cooling water is stored for later use, as shown in Figure 1.

2.3 Dish-type concentrating system

Aiming to increase the energy density of the solar radiation on the cell surface, the vertical incident solar light is firstly converged to the entrance of the secondary reflector through the concentrator’s reflection and focusing. Then, the gathering light of the entrance is reflected by the secondary reflector, which makes the gathering light distributed uniformly on the cell surface of the, so that the solar cell can absorb the solar energy and then convert into electric energy as much as possible.

The solar disk concentrator used in the dish-type concentrator photovoltaic system has a regular octagonal cross section and a diameter of 384 cm. In order to increase its mechanical strength and reduce the production difficulty, each paraboloid consists of three sub-mirrors. Under the joint action of the eight sub-mirrors, a dish-shaped converging unit with a converging ratio of 570:1 is formed. The specific parameters of each sub-mirror are shown in Figure 4.

Figure 4.

Concentrator sub-mirror.

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3 EXPERIMENTAL METHOD AND SYSTEM VERIFICATION

3.1 Experimental method

In order to analyze the thermal efficiency of the dish-type concentrator photovoltaic system, the pump is first opened to ensure the operation of the water cooling system. Meanwhile, the software on the computer records the irradiated data in real time. The control system automatically enters the automatic control state and tracks the sun location, and compares the resistance of the four thermistors (K3–K6). When the temperature difference is within 0.1, the infrared camera starts to collect the temperature images on the entrance of the secondary reflector.

In order to verify the feasibility and actual performance of the disk tracking control system, four thermal resistors are placed on the center of the outer wall of the secondary reflector in Figure 3a. When the central point of the focus spot is not consistent with the bottom central point of the secondary reflector, the temperature error of the outer wall of the secondary reflector will exceed the specified value. Therefore, it is necessary to wait for the tracking system to recalibrate automatically.

In order to obtain the temperature distribution image of the cell surface, the infrared camera is used to record the information of the focus spot in real time. Meanwhile, the image can also be used as an auxiliary method to calibrate the control system accurately, so as to obtain better tracking precision. The actual equipment platform of the dish-type solar concentrator is shown in Figure 5. The direct normal insolation (DNI) radiation sensor is installed at the edge of the dish surface to measure the sunlight intensity in real time. Moreover, the inlet and outlet of the test bed also measure the inlet and outlet water temperature by the thermal resistance, and the thermal efficiency of the experimental table can be reflected by the temperature of the inlet and outlet.

Figure 5.

Dish-type concentrator photovoltaic system.

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3.2 Theoretical analysis

According to the above experimental measurements, the total energy of the dish-type concentrator photovoltaic system at the entrance of the secondary reflector can be calculated by Equation (1). Then, the concentrated solar energy is reflected by the secondary reflector and evenly distributed on the surface of the cell, which is actually defined as Equation (2).

Q_{i n} = G_{D} \cdot γ \cdot ρ_{1 m} \cdot F_{s h a p e} \cdot F_{d i r t}

(1)

{Q_{i n}}^{'} = ρ_{2 m} \cdot Q_{i n}

(2)

Where, $ρ_{1 m}$ is the specular reflectivity, $F_{s h a p e}$ is the mirror shape factor, $F_{d i r t}$ is the dust influence factor, $γ$ is the theoretical converging ratios and $G_{D}$ is the direction solar radiation, $ρ_{2 m}$ is the specular reflectivity.

The energy reaching the cell surface can be partly converted into electric power as Equation (3) shown. And the overall energy balance can be obtained by Equation (4):

Q_{e l e c} = \frac{I_{p \max} * V_{p \max}}{A_{c e l l}}

(3)

{Q_{i n}}^{'} = Q_{w} + Δ Q + Q_{e l e c}

(4)

Where,

Q_{t h}

is the total energy taken away by the cooling water;

Δ Q

is the heat dissipation of the system;

Q_{e l e c}

is the energy consumed by the solar cell, which can be calculated by Equation (4);

I_{p \max}

and

V_{p \max}

are the currents and voltages of solar cells at maximum power, which can be measured by electronic load;

A_{c e l l}

is the actual area of solar cells.

For the conventional power generation unit, the net efficiency is commonly considered to evaluate the thermal performance of the energy utilization system, which is the ratio of the power output to the total energy input and can be calculated as follows:

η = \frac{Q_{e l e c}}{{Q_{i n}}^{'}} * 100 %

(5)

The main parameters needed for the performance evaluation are shown in Table 1.

Table 1

Main parameters for performance evaluation

Theoretical converging ratios

γ

570

Specular reflectivity

ρ_{1 m}

0.9

Mirror shape factor

F_{s h a p e}

0.98

Dust influence factor

F_{d i r t}

0.9

Specular reflectivity

ρ_{2 m}

0.9

Collect frequency

p

2 Hz

3.3 Experimental verification

(1) Tracking control system verification

During the operation of the control system, PLC controller collect the four temperature on the outer wall of the secondary reflectors and the DNI data at the same time from 9:00 am to 2:00 pm.

At the beginning of operation, the radiation intensity is 0, and the outer wall temperature of the secondary reflector is equal. When the system automatically enters the tracking mode, the thermal resistance temperature of the south face increases sharply and reaches the peak at 9:33 in 45 seconds, indicating that the focus spot entering the secondary reflector from the south. As the control system continuously tracking the sun location and the sunlight is eventually incident on the concentrator vertically, the temperature of the outer wall of the secondary reflector enters an equilibrium state (i.e. the difference of the four temperature within 1%), which means the representative control system completes an automatic tracking cycle.

Besides, when the focus spot just enters the secondary reflector, DNI increased rapidly first as shown in the phase 1 of Figure 6. When the focus spot enters from the edge to the center, the control system will calibrate many times to improve the tracking accuracy. Correspondingly, DNI presents a slow growth state as shown in phase two in Figure 6. Then, DNI presents a regular variation and the period is 71 seconds, which is exactly consistent with the control system calibration period (70 seconds). During this period, the tracking accuracy (i.e. the ratio of the actual power output to the theoretical power output) is above 90% as shown in Figure 7, which means the control error is within 0.1. Conclusively, the variation of the radiation intensity and the outer wall temperature of the secondary reflector are consistent with working state of the control system, which demonstrating the tracking accuracy of the system.

Figure 6.

The temperature of the outer wall of the secondary reflector and irradiation intensity data.

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Figure 7.

The power output and control error.

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(2) Cooling system verification

This cooling performance of the dish-type concentrator photovoltaic system can be mainly verified by the variation of the K1-K2 temperature, whose location is shown in Figure 3. The temperature at the back of the cell fluctuates between 39 and 41°C. Moreover, the temperature of the thermistor K1 is higher than K2. This is because the thermistor K1 cling to back of the cell. Within this temperature range, the concentrating solar cell can work normally, which means that the cooling system of the dish-type concentrator photovoltaic system has played a vital role. It provides the cooling protection for the later installation of concentrating solar cells for testing (Figure 8).

Figure 8.

The back temperature of the solar cell.

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(3) Dish-type concentrating system verification

In the experiment, the infrared camera collects the temperature distribution image at the entrance of secondary reflector from 10:00 am to 2:00 pm. As time goes on, the temperature in the concentrator is getting higher with the increasing of DNI and the central temperature is always higher than the surrounding temperature. From this point of view, the design of the dish-type concentrator photovoltaic system is reasonable and can converge the vertical incident sunlight into the concentrator. In addition, it is worth to note that there is a high temperature anomaly point (dark blue) in the lower left corner in each image, which is attributed that the reflective film on the inner wall of the secondary reflector is partially melted under the high temperature condition. The damage of the reflective film leads to the nonuniform energy distribution on the solar cell, as shown in Figure 9.

Figure 9.

Focal spot images.

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4 MAXIMUM POWER ESTIMATING

When the control system locates the sun accurately, the maximum power output of the solar cell is measured by the electronic load, and the irradiation intensity is recorded by the irradiator as well. As shown in Figure 10, the direct irradiance and solar cell power increase firstly and then decrease, which reaches a peak between 14:00 and 14:30.

Figure 10.

Irradiance and power.

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Moreover, it can be seen that the efficiency of the dish-type concentrator photovoltaic system is between 25% and 30% as in Figure 11, which is lower than the common value [16]. This is because the energy intensity received by the cell is much lower than the value calculated theoretically. Specifically, the concentrating ratio is calculated theoretically by 570. But in the actual process, the test-bed has experienced severe weather such as gale, rain and snow, and the plane mirror of the concentrator may tilt, which affecting the incident angle of the sunlight, the reflected light deviating from the pre-set angle correspondingly and leading to the decrease of the receiving energy at the entrance of the secondary reflector. This part of the optical loss can’t be neglected, so the actual concentrating ratio of the solar cell is much lower than 570.

Figure 11.

Power generation efficiency.

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As shown in Figure 11, the efficiency at noon is lower than that in the morning and afternoon. It is attributed that the concentrating solar cell has a working temperature range. The dish-type test platform is installed on the roof of the building, and the temperature of the roof environment reaches the highest from 12:00 to 14:00, which affects the work of solar cells and leads to the low efficiency.

5 CONCLUSION

In this paper, a dish-type concentrator experiment platform has been constructed. In order to demonstrate the feasibility of the platform and the accuracy of the control system, some verification experiments are systematically designed, including the measurements of the outer wall temperature of the secondary reflector and the temperature distribution of the focus spot. By analyzing the real-time temperature distribution image of focal spot, it is concluded that the concentrator and the secondary reflector can focus the sunlight evenly on the surface of the solar cell. By analyzing the outer wall temperature of the secondary reflector, the tracking control system can make the sunlight incident vertically in the concentrator. In addition, the designed cooling system could take away the waste heat efficiently, so that the temperature of the concentrating solar cells can be kept within the normal working range. Finally, with the changing of the DNI, the power generation efficiency varies between 25% and 30%, which can be further improved by the reutilization of the waste heat in later research.

FUNDING

This study was supported by the Open Foundation of Hubei Collaborative Innovation Center for High-efficiency Utilization of Solar Energy, Hubei University of Technology (No. HBSKFTD2016001), the International Science & Technology Cooperation Project of Wuhan Science and Technology Bureau under (No. 2017030209020254), the International Science & Technology Cooperation Program of China (No. 2016YFE0124300) and the PhD Research Startup Foundation of Hubei University of Technology (No. BSQD2017069).

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Article Contents

The prototype construction and performance evaluation of dish-type concentrator photovoltaic system

Abstract

1 INTRODUCTION

2 CONSTRUCTION OF DISH-TYPE CONCENTRATOR PHOTOVOLTAIC SYSTEM

2.1 Tracking control system

2.2 Cooling system

2.3 Dish-type concentrating system

3 EXPERIMENTAL METHOD AND SYSTEM VERIFICATION

3.1 Experimental method

3.2 Theoretical analysis

3.3 Experimental verification

4 MAXIMUM POWER ESTIMATING

5 CONCLUSION

FUNDING

REFERENCES

Citations

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Theoretical converging ratios	$γ$	570
Specular reflectivity	$ρ_{1 m}$	0.9
Mirror shape factor	$F_{s h a p e}$	0.98
Dust influence factor	$F_{d i r t}$	0.9
Specular reflectivity	$ρ_{2 m}$	0.9
Collect frequency	$p$	2 Hz

Article Contents

The prototype construction and performance evaluation of dish-type concentrator photovoltaic system

Abstract

1 INTRODUCTION

2 CONSTRUCTION OF DISH-TYPE CONCENTRATOR PHOTOVOLTAIC SYSTEM

2.1 Tracking control system

2.2 Cooling system

2.3 Dish-type concentrating system

3 EXPERIMENTAL METHOD AND SYSTEM VERIFICATION

3.1 Experimental method

3.2 Theoretical analysis

3.3 Experimental verification

4 MAXIMUM POWER ESTIMATING

5 CONCLUSION

FUNDING

REFERENCES

Citations

Views

Altmetric

Email alerts

Citing articles via

Latest

Most Read

Most Cited

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