Concentrator Solar Cell for Solar Thermal Boiler Dong Il Leea,* and Seung Wook Baekb a,b

School of Mechanical, Aerospace and System Engineering, Division of Aerospace Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-Dong, Yuseong-Gu, Daejeon 305-701, Korea a

[email protected], [email protected]

Abstract In this study, the generating efficiency of the limited area of a concentrator solar cell was increased using a solar concentrator and a tracking device. Heat generated from the solar cell was collected with a thermal absorber to supply hot water or heating. Thus, the concentrator solar cell system provided electricity and heat simultaneously. A heat transfer simulation model was developed to compare the actual temperature profile of the concentrator solar cell and thermal absorber, with good agreement between the simulations and experiments. Keyword: Concentrator solar cell, Solar tracking device, Thermal absorber 1. Introduction A concentrator solar cell is one of the most popular renewable energy products due to its high efficiency and preference for operation under highly concentrated solar light conditions. Additionally, a concentrator solar cell offers lower generation cost compared to silicon solar cells; this accounts for a high proportion of the total cost of the system [1]. However, the electrical efficiency of the solar cell decreases as its operating temperature increases due to the concentration of light. Therefore, cooling is needed to decrease the temperature of the solar cell to boost electrical efficiency [2,3]. In the past, natural convective cooling fins or heat pipes have been utilized to control the temperature of concentrator solar cells. However, these methods are inefficient; thermal energy is released from the concentrator solar cell into the atmosphere [4,5]. The purpose of this research was to increase the generating efficiency of concentrator solar cells within a restricted area using light concentration and solar tracking. At the same time, we demonstrated that the proposed system had the ability to extract thermal energy from a concentrator solar cell using thermal absorbers, which could then be used for a heating system or hot-water supply.

2. Experimental set-up Figure 1 shows a schematic diagram for a heating device using a concentrator solar cell. The system is composed of three parts: the positioning part, the concentrator solar cell, and the thermal absorber. The positioning part tracks the sun. The concentrator solar cell generates electricity and is operated through light concentration while tracking the sun. The thermal absorber extracts thermal energy from the concentrator solar cell using cooling water. A triple-junction-based concentrator solar cell (Emcore Co., USA), 10 mm in length and width, was used for this research. The raw material of the solar cell consisted of InGaP, InGaAs, and Ge. A Fresnel lens with a 290-mm focal length (Fresnel Factory, South Korea), 28 mm in length and width, was used. Solar power was measured using a TM-207 meter (Tenmars Electronics Co., TAIWAN), which had an average error of ±1W m−2.

Figure 1. Schematic diagram for the heating device using concentrator solar cells. The multi-junction solar cell used several wavelengths; thus, a Fresnel lens was required for light concentration. Simultaneously, a solar-tracking device was necessary to ensure that the orientation of the solar-cell plane was perpendicular to the incoming light. If the angle of the sun with respect to the solar cell was not positioned properly, then the light was out of focus, thereby significantly degrading electricity generation. Figure 2 shows the configuration of the solar-tracking device, which consisted of a controller, two stepping motors, and four CdS sensors. The controller used Simulink to calculate the sun’s position and operate the stepping motors. Stepping motors are commonly used in precisely positioning control applications. The stepping motor itself is brushless and load-independent, and has open-loop positioning capability, good holding torque, and excellent response characteristics. The stepping motor used for the prototype solar-tracking device operated with the following specifications: 24V, 0.072° per step, with five phases. The voltage of the CdS sensor changed depending on the brightness of the light. The cylindrical

pillar had differing shadows that depended on the sun’s position. On the cylinder’s exterior, north-, south-, east-, and west-facing CdS sensors detected the brightness of the ambient light. The voltage values, which varied with the brightness, were processed by an analog-to-digital converter. Of the four CdS sensors (A, B, C, and D), A and B were used to detect the azimuthal angle, while C and D measured the elevation angle. The cylindrical housing height was calculated as follows. If h is the height of the cylindrical housing and d is the diameter of the CdS sensor, then from trigonometry, tan  

d . If the h

angle of the shadow formed by the cylindrical pillar is within 1°, then h can be calculated

d 0.003   0.176m . o tan1 0.017 The output voltage for the sensors was calculated as follows. If a CdS sensor (R1) and a resistor (R2) are connected to a circuit voltage, then the output voltage is determined by the from h 

following expression: Vout 

R2 Vin . When using this equation, the output voltage R 2  R1

increases (decreases) in bright (dark) places because R1 corresponding to the CdS sensor decreases (increases). The error of the CdS sensor was