The total amount of solar radiation that reaches the earth surface is substantial with relatively low energy flux density, which is often insufficient to meet the energy utilization demand. As a typical non-imaging concentrator, since a compound parabolic concentrator (CPC) is a non-tracking device featuring a stable operation state and easy integration construction, it has many applications in solar thermal conversion and photovoltaic power generation systems. For traditional CPC, the concentrator is in contact with the absorber. In practical applications, the concentrator will be deformed or even destroyed due to the concentrated thermal stress, which can reduce the heat collection efficiency of the CPC and affect its working stability. To address this problem, we propose the separation of the concentrator surface and absorber CPC (SCSA-CPC). Nevertheless, SCSA-CPC inherits some disadvantages from traditional CPC such as uneven energy flux density and poor economic feasibility due to its parabolic structure. Therefore, we optimally construct a compound plane concentrator with a congruent surface separated from the absorber (CSSA-CPC) and investigate various factors influencing its optical efficiency.

Firstly, the points requiring further improvement for SCSA-CPC are identified, followed by an investigation and study of corresponding solutions. Based on the principle of non-imaging optics and program calculation methods, CSSA-CPCs with varying numbers of congruent concentrated surface segments are constructed. Secondly, to verify the concentrating performance of CSSA-CPC, we print the CSSA-CPC model with the melting sediment molding additive manufacturing technology. Additionally, an experimental platform of laser verification is established to simulate solar ray incidence by the CSSA-CPC optical aperture and record the position at which the laser reaches the plane absorber. Simultaneously, optical simulation software is employed to conduct ray tracing simulations and determine theoretical values for positions on the absorber. Additionally, the comparison and analysis between experimental values and theoretical values are conducted to verify the reliability of the CSSA-CPC model. Finally, various factors affecting optical efficiency in different conditions are analyzed and studied by adopting simulation methods and knowledge about the motion of the earth.

During the laser experimental verification of CSSA-CPC, there are discrepancies between the experimental and theoretical values primarily due to the divergence angle of the emitted laser beam, surface form errors during the 3D printing and a flexible reflective film with a certain thickness covering the reflector. The errors remain within an acceptable range with these factors taken into consideration. The maximum absolute error is 1.64 mm, while the average absolute error is 0.66 mm (Fig. 6). The optical efficiency of SCSA-CPC sharply drops to 0 after acceptance half-angle, but CSSA-CPC exhibits a relatively stable change trend. Meanwhile, as the number of congruent concentrated surface segments increases, the geometric structure and optical efficiency of CSSA-CPC gradually approach that of SCSA-CPC (Fig. 7). Additionally, the average optical efficiency of CSSA-CPC is more advantageous than that of SCSA-CPC, and the improvement ratio reaches the maximum when the number of congruent surface segments is 4 (Fig. 8). For horizontally placed CSSA-CPC systems with four different lengths in axial north-south direction, optical efficiency gradually increases up to 92.90% with length. However, for CSSA-CPC, the cosine loss ratio decreasing with the increasing length results in a reduced growth rate in its optical efficiency (Fig. 10). In a tilted east-west CSSA-CPC concentrator array system, there is an initial turning point where optical efficiency declines when the spacing between concentrator units is 400 mm, and meanwhile the overall optical efficiency gradually declines until it converges toward similar change trends as the spacing increases (Fig. 11). The optimized cell spacing for CSSA-CPC arrays is determined to be 542 mm.

Based on the non-imaging optics principle, we optimize the CSSA-CPC and investigate the concentrator characteristics and optical performance of CSSA-CPC. The results of laser experiments demonstrate that the experimental values align with the theoretical ones, thus confirming the reliability of the CSSA-CPC model. The comparative optical efficiency analysis reveals that CSSA-CPC widens its acceptance angle compared to SCSA-CPC, while also exhibiting an advantage in average optical efficiency. This indicates that CSSA-CPC can continuously and stably collect solar radiation. The influence of condenser length on the optical efficiency of CSSA-CPC is limited due to cosine loss. As condenser length increases, optical efficiency gradually improves but at a decreasing rate. Therefore, selecting an appropriate condenser length according to practical application scenarios is crucial. Similarly, for array arrangements within the CSSA-CPC concentrator system, smaller or larger spacing between front and back concentrators is not suitable. Smaller spacing leads to an early decline in optical efficiency while larger spacing causes excessive emission of solar rays from gaps resulting in losses. In reality, optimal spacing for array placement should be calculated based on local conditions. Finally, we provide valuable insights for practical engineering applications involving CSSA-CPC.