# Highly Concentrated Solar Flux of Large Fresnel Lens Using CCD Camera-Based Method

^{1}

^{2}

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## Abstract

**:**

^{2}. This method confirms the simulation results of previous studies that using the rays tracing method, that is, the flux level of the Fresnel lenses can reach 5000 suns. The experimental results demonstrated the CCD camera-based method combined with a heat flow meter is competent in measuring the intensity of flux with a level of 5000 suns.

## 1. Introduction

^{2}. Concentrated solar systems can produce high flux levels which use parabolic dish solar concentrators [1,2], convex lenses, and Fresnel lenses. Parabolic dish solar concentrators [3,4,5] are often combined with Stirling engines for power generation [6]. The concentrated solar radiation at the receiver is absorbed by the working fluid, liquid, or gas, raising its temperature to 120 to 1500 °C. The working fluid temperature inside a commercial solar power tower absorber can be as high as 565 °C, while experimentally proven values are in the range of 1000 °C [2,7]. Convex lenses and Fresnel lenses are commonly used in concentrating photovoltaic systems and daylighting systems [8]. With the Fresnel lenses reaching multiple thousand suns [9,10,11] and a high concentrator with 5800 suns, geometrical concentration ratio based on multiple primary Fresnel lenses was proposed in 2018 [12]. Benefiting from the advantages of lightweight and small space occupation [13,14], the diameter of Fresnel lenses can reach up to 1 m, which can produce a concentrated flux with the level of 5000 suns, and can reach over 1500 °C when the solar irradiation is a high level. They are also widely used in applications where high flux are required [15,16].

## 2. System Configuration

#### 2.1. Instruments

#### 2.2. Method

^{2}, the peak flux value of the concentrated spot can reach 4.61 MW/m

^{2}, as shown in Figure 5d, exceeding the range of heat flow meter. The quartz-glass rod here as a homogenizer, can reduce the peak flux to 3.15 MW/m

^{2}, within the measurement range of the heat flow meter, as shown in Figure 5e.

## 3. Experiment and Discussions

#### 3.1. Verification

^{6}dB/km. The relationship between attenuation rate and transmittance is shown below.

^{2}). In the case of an incident area ratio accounting for 1/4 and 1/2, which is shown in Figure 10b,c, the total measured energy of the spot obtained by the heat flow meter is 114 W and 229 W, respectively. The total measured energy of the concentrated spot obtained by the CCD camera is 112 W and 233 W, respectively. The relative error is −1.8% and 1.7%, respectively. The relevant data are shown in Table 3.The measurement results are consistent, indicating that the measurement method used in this study is reliable.

#### 3.2. Experiment

^{2}, the flux density distributions of the concentrated spot obtained by rays tracing simulation are shown in Figure 11a–c. When the incident area ratio is 1, 1/2 and 1/4, the peak flux density of the spot reaches 4.60 MW/m

^{2}, 2.26 MW/m

^{2}, and 1.13 MW/m

^{2}, respectively. The actual flux density distribution of the CCD images measured by the CCD camera-based method is shown in Figure 11d–f, and the peak values are 4.06 MW/m

^{2}, 2.12 MW/m

^{2}, and 1.12 MW/m

^{2}, respectively.

#### 3.3. Discussion

^{2}level in this study. The spot is matched with the size of the heat flow meter through the homogenizer so that the verification of CCD image was carried out. The combination of a homogenizer, heat flow meter, and CCD camera provides a virtually unlimited flux measurement range. The homogenizer made of quartz glass can withstand millions of times of suns, and the CCD camera can be combined with multiple layers of neutral density filters to prevent overexposure when shooting a concentrated spot. The upper limit of the measurement method is determined by the intensity and gradient of flux that the incident end of the homogenizer can withstand. The damage threshold of the quartz material used in the homogenizer exceeds 10

^{6}suns, which means that the method can measure the flux density in any solar concentrated system. The theoretical upper limit of the geometric spot ratio of solar concentrators is 46,000 suns, which is below the damage threshold of the homogenizer. Therefore, this method can be used for the measurement of focused energy flows in solar concentrated systems with high flux levels.

## 4. Conclusions

^{2}under DNI 800 W/m

^{2}, the equivalent of 5000 suns. Experiments demonstrated the simulation result of the literature, where there is a serious gradient distribution inside the spot of the lens and the central flux reaches three times the average flux density. The measured peak value of the concentrated spot is about 10% lower than the theoretical value, and the reason for this phenomenon is that there is a gravity deformation of the large-aperture plastic Fresnel lens, which means the flux density distribution of a concentrated system with a high flux level can be obtained by using this method.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Measurement experiment system of concentrated flux of large Fresnel lens. (

**a**) Side view. (

**b**) Prototype. (

**c**) Front view of 3D structure. (

**d**) Shelters.

**Figure 2.**High-temperature resistance test of quartz-fiber board. (

**a**) Gray scale image of the flux. (

**b**) Infrared thermal images (°C).

**Figure 3.**The heat flow meter experiment system. (

**a**) Sketch. (

**b**) CCD camera with neutral density filter. (

**c**) Water-cooling system. (

**d**) Homogenizer. (

**e**) Heat flux meter.

**Figure 5.**Function of homogenizer. (

**a**) Focus length of the Fresnel lens. (

**b**) Size of focused spot. (

**c**) Internal optical path of homogenizer. (

**d**) Flux distribution without homogenizer. (

**e**) Flux distribution with homogenizer.

**Figure 6.**Images of CCD camera at different exposure times under ISO 6400: (

**a**) 1/4000 s; (

**b**) 1/2000 s; (

**c**) 1/1000 s; (

**d**) 1/500 s; (

**e**) 1/250 s; (

**f**) 1/125 s. (

**g**) Response function extracted by the camera with a light sensitivity of ISO 6400.

**Figure 10.**Gray images of focus flux. (

**a**–

**c**) Incident area ratio of 1, 1/2, and 1/4. (

**d**–

**f**) Lambertian target CCD images with incident area ratios of 1, 1/2, and 1/4.

**Figure 11.**Comparison of flux distribution between results of CCD image and rays tracing simulation. (

**a**–

**c**) Results of rays tracing simulation with area ratios 1, 1/2, and 1/4. (

**d**–

**f**) Results of CCD image with area ratios 1, 1/2, and 1/4.

Parameters | Units | Values |
---|---|---|

Lens aperture | mm | 968 |

Focal length | mm | 1300 |

Lens ring distance | mm | 0.5 |

Lens thickness | mm | 5 |

Lens transmittance | % | 89 |

Size of homogenizer | mm | Φ20 × 200 |

Transmittance of homogenizer | % | 90 |

Diameter of heat flow meter | mm | 20 |

Range of heat flow meter | MW/m^{2} | 0–3.14 |

Maximum operating temperature of heat flow meter | °C | 1600 |

Size of Lambertian target | mm | 100 × 100 × 3 |

Parameters | Unit | Value |
---|---|---|

Model | Z-10 | |

manufacturer | Ever fine | |

Test range | lx | 0.01–300,000 |

Spectral range | nm | 380–760 |

**Table 3.**Comparison of heat flow meter measured value and CCD camera measured value under DNI = 800 W/m

^{2}.

Area Ratio | Energy of the Focus Flux (W) | ||
---|---|---|---|

Theoretical | Heat Flow Meter | CCD Image | |

1/4 | 117 | 114 | 112 |

1/2 | 233 | 229 | 233 |

Reference [12] | This Manuscript | |
---|---|---|

Concentration ratio | 5831 | 4164.5 |

Half acceptance angle (deg) | 0.4 | 0.33 |

f−number | 0.74 | |

Maximum concentrated level | 14k suns | 6k suns |

Flux distribution map |

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**MDPI and ACS Style**

Zhang, K.; Su, Y.; Wang, H.; Wang, Q.; Wang, K.; Niu, Y.; Song, J.
Highly Concentrated Solar Flux of Large Fresnel Lens Using CCD Camera-Based Method. *Sustainability* **2022**, *14*, 11062.
https://doi.org/10.3390/su141711062

**AMA Style**

Zhang K, Su Y, Wang H, Wang Q, Wang K, Niu Y, Song J.
Highly Concentrated Solar Flux of Large Fresnel Lens Using CCD Camera-Based Method. *Sustainability*. 2022; 14(17):11062.
https://doi.org/10.3390/su141711062

**Chicago/Turabian Style**

Zhang, Kexin, Ying Su, Haiyu Wang, Qian Wang, Kai Wang, Yisen Niu, and Jifeng Song.
2022. "Highly Concentrated Solar Flux of Large Fresnel Lens Using CCD Camera-Based Method" *Sustainability* 14, no. 17: 11062.
https://doi.org/10.3390/su141711062