**5. Results and discussion**

In order to analyze the optical performance of proposed multicell concentrating assembly (MCA), the ray tracing simulation was conducted using TracePro software. The concentrating assembly was analyzed in terms of division of rays among four outlet apertures, uniformity of rays at the outlet aperture and investigation of deflected path of incident rays. The simulation model for proposed MCA, according to the discussed design, is shown in **Figure 8**. To conduct ray tracing simulation, a square grid of parallel rays was selected as the primary reflector is also of square shape. As the received solar radiation is not exactly parallel in nature, that is why the simulated MCA model was not only investigated for parallel ray grid but also for the grid with angle same as the solar subtended angle. The ray tracing simulation results of proposed MCA design with parallel ray grid are shown in **Figure 9**. It can be seen that a perfect division of rays among four outlet apertures of homogenizer is experienced. In addition, the rays are also uniformly distributed over the whole surface area of outlet aperture. Moreover, a concentrated collimated beam is also achieved after reflection of parallel incident rays from secondary reflector. As per discussed design, this concentrated collimated beam is being divided among four sections of homogenizer and also hitting the lower tapered portion of the homogenizer.

In order to simulate the optical performance of proposed MCA in real field environment, the simulation results for grid of rays with solar subtended angle are shown in **Figure 10**. These rays are not exactly parallel to the axis of primary reflector but have a small deviation which is same as the actual solar radiations, received during real field operation. It can be seen that there is still a perfect division of received radiations among four sections of the homogenizer and uniform distribution of these radiations over the outlet apertures of homogenizer. However, one of the differences which can be seen here is that the rays are also hitting the upper tapered portion of the homogenizer, without any induced ray deviation. The reason

**Figure 8.** TracePro model of multicell concentrating assembly (MCA) for ray tracing simulation.

from solar tracking sensor. Such real-time optical feedback avoids any chance of tracking error which may arise due to passive tracking method and possible backlash in the driving

**Figure 6.** Developed prototype of multicell concentrating assembly (MCA)-based CPV module.

**Figure 7.** Experimental prototype of MCA-based novel CPV unit.

assembly.

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After verifying the design of developed multicell concentrating assembly (MCA) through ray tracing simulation, it is also important to verify its performance under real conditions during field testing. The developed prototype of CPV system, with MCA-based design, was tested under field condition, and the output from each of the MJC, at four outlet apertures of homogenizer, was logged in real time using data logging unit. The power and voltage output of each of four MJCs is shown in **Figures 11** and **12**. From both figures, it is clear that there is equal distribution of solar radiations among four MJCs, during whole-day operation as their power and voltage plot look similar. It must be noted that the solar tracking unit was operated with tracking accuracy of 0.3°, instead of 0.1°. The equal output of all four MJCs, with such tracking accuracy, verifies the proposed design of multicell concentrating assembly (MCA)

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Another important test to investigate the response of concentrating assembly against different angular deviations of incident rays is expressed in the form of normalized power curve. So far, the performance of MCA, either simulated or experimental, is tested for parallel rays or rays with smaller deviations. In order to test the maximum response limit of concentrating assembly over different deviation angles, the normalized power output of the system is plotted against the angular deviation. Such normalized power curve also helps to predict the acceptance angle of concentrating assembly for efficient operation. The normalized power curve for developed multicell concentrating assembly (MCA)-based CPV is shown in **Figure 13**. The red line indicates the normalized power curve obtained from ray tracing simulation against different deviation angles of parallel rays. The simulated normalized power is obtained by taking the ratio of the total number of rays at the outlet apertures to the total incident rays. However, the blue line indicates the true normalized power output curve obtained through

and its compatibility in fulfilling the objective.

**Figure 11.** Power output for individual cell of MCA-based CPV system.

**Figure 9.** Ray tracing simulation of multicell concentrating assembly for parallel rays.

**Figure 10.** Ray tracing simulation of multicell concentrating assembly for solar subtended angle.

is that the incoming rays are already not parallel to the axis of primary reflector, and that is why they are reflected by the upper tapered portion which is designed to accommodate the tracking error or rays which are not parallel to the primary reflector axis.

After verifying the design of developed multicell concentrating assembly (MCA) through ray tracing simulation, it is also important to verify its performance under real conditions during field testing. The developed prototype of CPV system, with MCA-based design, was tested under field condition, and the output from each of the MJC, at four outlet apertures of homogenizer, was logged in real time using data logging unit. The power and voltage output of each of four MJCs is shown in **Figures 11** and **12**. From both figures, it is clear that there is equal distribution of solar radiations among four MJCs, during whole-day operation as their power and voltage plot look similar. It must be noted that the solar tracking unit was operated with tracking accuracy of 0.3°, instead of 0.1°. The equal output of all four MJCs, with such tracking accuracy, verifies the proposed design of multicell concentrating assembly (MCA) and its compatibility in fulfilling the objective.

Another important test to investigate the response of concentrating assembly against different angular deviations of incident rays is expressed in the form of normalized power curve. So far, the performance of MCA, either simulated or experimental, is tested for parallel rays or rays with smaller deviations. In order to test the maximum response limit of concentrating assembly over different deviation angles, the normalized power output of the system is plotted against the angular deviation. Such normalized power curve also helps to predict the acceptance angle of concentrating assembly for efficient operation. The normalized power curve for developed multicell concentrating assembly (MCA)-based CPV is shown in **Figure 13**. The red line indicates the normalized power curve obtained from ray tracing simulation against different deviation angles of parallel rays. The simulated normalized power is obtained by taking the ratio of the total number of rays at the outlet apertures to the total incident rays. However, the blue line indicates the true normalized power output curve obtained through

**Figure 11.** Power output for individual cell of MCA-based CPV system.

is that the incoming rays are already not parallel to the axis of primary reflector, and that is why they are reflected by the upper tapered portion which is designed to accommodate the

tracking error or rays which are not parallel to the primary reflector axis.

**Figure 10.** Ray tracing simulation of multicell concentrating assembly for solar subtended angle.

**Figure 9.** Ray tracing simulation of multicell concentrating assembly for parallel rays.

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It can be seen that there is a good agreement between simulated and experimental normalized power curves, until point 'A'. However, the experimental curve starts deviating from simulated curve after point 'A'. There is gradual decrease in the simulated power curve. However, a stepwise drop is observed for experimental curve. Such a different response of simulated

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**Figure 14.** Simulated irradiance map at outlet apertures of multi-leg homogenizer for different angular deviations of

parallel rays.

**Figure 12.** Voltage output for individual cell of MCA-based CPV system.

**Figure 13.** Normalized power curves for multicell concentrating assembly-based CPV system.

experimentation. In order to obtain such plot, the tracker movement was stopped, while the CPV module was exactly facing the sun, during noontime. The angular deviation was calculated by taking the difference in the initial and current position of sun. The power output of all four MJCs, connected in series, was logged under real time for constant output load. It is clear from **Figure 13** that the simulated normalized power output is almost 100% for angular deviations up to 1°. It is also true for experimental results, which verifies the proposed design of MCA. If there is further increase in the incident ray deviation, than 1°, the normalized power starts to drop.

It can be seen that there is a good agreement between simulated and experimental normalized power curves, until point 'A'. However, the experimental curve starts deviating from simulated curve after point 'A'. There is gradual decrease in the simulated power curve. However, a stepwise drop is observed for experimental curve. Such a different response of simulated

**Figure 14.** Simulated irradiance map at outlet apertures of multi-leg homogenizer for different angular deviations of parallel rays.

experimentation. In order to obtain such plot, the tracker movement was stopped, while the CPV module was exactly facing the sun, during noontime. The angular deviation was calculated by taking the difference in the initial and current position of sun. The power output of all four MJCs, connected in series, was logged under real time for constant output load. It is clear from **Figure 13** that the simulated normalized power output is almost 100% for angular deviations up to 1°. It is also true for experimental results, which verifies the proposed design of MCA. If there is further increase in the incident ray deviation, than 1°, the normalized

**Figure 13.** Normalized power curves for multicell concentrating assembly-based CPV system.

**Figure 12.** Voltage output for individual cell of MCA-based CPV system.

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power starts to drop.

and experimental normalized power curves can be explained with the fact that the simulated power output is just based upon the number of radiation received at the outlet apertures of homogenizer. However, for experimental curve, the power output is the actual electrical power obtained from CPV module but at a constant load. As the load across CPV module is constant, therefore, excessive ray loss causes its maximum power point to shift, resulting in a decrease in its output and performance. That is why it also deviates from the simulated curve. To carefully understand the varying trend of simulated and experimental normalized power curves, the simulated irradiance map at all four outlet apertures of homogenizer is shown in **Figure 14**. The irradiance maps are plotted against different points mentioned on experimental normalized curve. It can be seen from **Figure 14(A)**, which shows the irradiance map at point 'A', that the solar cells 3 and 4 are coming under shadow. For simulated curve, this shadow is just a decrease in the number of rays coming out of the homogenizer. However, in actual system, the electrical output of the CPV system is greatly disturbed as all MJCs are connected in series. The maximum power point for cells under shadow changes, and they also pulls down the performance of other MJCs due to their series connection. Therefore, overall electrical output of complete CPV module decreases. At points 'B', 'C' and 'D', the normalized electrical power output of CPV system is almost same as 50%. This is because of the fact that at this point, only two cells are operating, while the other two cells are under complete shadow, as shown in **Figure 14(B)**–**(D)**. However, for simulated power curve, there is still a gradual decrease for these points which implies that the net flux output is not reduced to 50% for all these three points, i.e., 'B', 'C' and 'D'. The main reason for half of electrical output is because of the shift in the maximum power point for the entire CPV system, which is effected by the two cells under shadow.

concentrator can handle four MJCs, named as multicell concentrating assembly (MCA). Such proposed design will not only reduce the overall cost of the system, but it will also reduce the

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In this chapter, a detailed design of such multicell concentrating assembly is discussed and explained. For prototype purpose, the detailed calculation of concentrating assembly size is presented for concentration ratio of ×165. Such proposed design is transformed into working prototype of CPV module for actual field testing. The design was targeted to handle angular

A detailed performance investigation strategy was adopted to verify the proposed design with its prototype and its limitation during field operation. An optical simulation was conduction in TracePro to verify the optical performance of MCA. An equal division and uniform distribution of rays were observed at the outlet apertures of homogenizer. From the field testing, equal and uniform power output of each MJC verified the proposed design of multicell concentrating. However, to analyze the response of MCA against angular deviation of incident ray, normalized power output curve was presented against angular deviation, for both experimental and simulated performances. The system showed 100% power output for angular deviations up to 1°, as designed. However, the system showed maximum capability of handling 6.5° angular deviations. A great agreement was observed among simulated and

dimension of square primary reflector of multi-leg homogenizer concentrating

<sup>1</sup> depth of primary reflector of multi-leg homogenizer concentrating assembly (mm)

<sup>1</sup> focal length of primary reflector of multi-leg homogenizer concentrating assembly (mm)

)

corresponding diameter of primary reflector of multi-leg homogenizer concentrat-

dimension of square secondary reflector of multi-leg homogenizer concentrating

<sup>2</sup> depth of secondary reflector of multi-leg homogenizer concentrating assembly (mm)

focal length of secondary reflector of multi-leg homogenizer concentrating assem-

)

assembly and alignment efforts during CPV module fabrication.

deviations of 1° without any loss in performance.

experimental results.

**Nomenclature**

assembly (mm)

AM area of multi-junction solar cell (mm<sup>2</sup>

ACA area of concentrating assembly (mm<sup>2</sup>

ing assembly (mm)

CR<sup>g</sup> geometric concentration ratio

θ<sup>e</sup> tracking error (degree)

assembly (mm)

bly (mm)

d1

t

f

D1

d2

t

f2

After increasing deviation angle beyond point 'D', the experimental curve again starts to drop till point 'E'. This is because of the fact that now the other two cells are also coming under shadow, as can be seen in **Figure 14(D)**. At point 'E', the remaining two cells are under only partial concentration. That is why the power output reduced to a low value. The power output remains stable for a while after point 'E' which is just because of partial concentration that moves around the cell area due to leftover and scattered radiations. With further increase in the deviation angle, the power output slowly dies to zero. The normalized power output curve shows that the develop CPV system, based upon the proposed MCA, has acceptance angle of 1° as designed. However, it has a capability of operation for deviation angle as high as 6.5°. However, the power output drops significantly, but the system can still respond to the received radiations.
