The upper limit of total efficiency of solar power tower system

Determining the upper limit of concentrated solar power (CSP) systems’ total efficiency is fundamental to guiding system design and optimization. The energy transfer and conversion processes in such systems primarily include solar concentration, optical–thermal conversion, and thermal–power conversion. While the mutual constraint between the optical–thermal conversion efficiency (ηrec) and the thermal–power conversion efficiency (ηt) has been well recognized, the relationship between the optical efficiency of the concentration process (ηopt) and the other two efficiencies remains unclear. Consequently, the upper limit of systems’ total efficiency (ηtotal) and its key influencing parameters have yet to be fully elucidated. In assessments of systems’ total efficiency, the optical efficiency is often treated as a constant—either idealized to 1 without specifying the type of concentration system, or assigned an empirical value. These simplifications fail to account for its actual variation with the geometric configuration of the concentration system. The aforementioned problems make it difficult to further guide the design of the system. To address this issue, this study focuses on solar power tower (SPT) systems and develops a theoretical full-chain efficiency limit model for an ideal SPT system, followed by an investigation of the upper limit of systems’ total efficiency. The results reveal that, in addition to the receiver operating temperature (T), which is widely acknowledged, the concentration ratio (C) is also a key parameter affecting the upper limit of systems’ total efficiency. The relationship between ηtotal and key parameters can be expressed as: ηtotal(C,T)=ηopt(C)·ηrec(C,T)·ηt(T). Quantitative analysis indicates that as C increases, the heliostat field optical efficiency limit ηopt(C) gradually decreases, whereas the optical–thermal–power conversion efficiency limit ηrec(C,T)·ηt(T) increases, revealing a clear trade-off. It can be found that an optimal combination of C and T exists that maximizes ηtotal(C,T). Based on this insight, a multi-stage optimization strategy—“temperature by thermal performance, ratio by optical efficiency”— is proposed for the practical design of SPT plants. This strategy operates on two distinct principles. First, it identifies the optimal operating temperature for each concentration ratio by maximizing the system’s optical-thermal-power conversion efficiency—a principle termed “temperature by thermal performance.” Second, it determines the best concentration ratio by maximizing the total system efficiency, which incorporates coupled optical performance, referred to as “ratio by optical efficiency”. Taking the Hami 50 MW SPT plant as an example, the highest total efficiency under the constraints of the real heliostat field and cycle configuration is presented, along with the corresponding optimal concentration ratio and the optimal operating temperature of the receiver. This highest total efficiency (27.01%) represents a 12% increase compared to the current total efficiency of the Hami 50 MW SPT plant (approximately 15%). The proposed analysis of the upper limit of systems’ total efficiency and multi-stage optimization approach can provide valuable guidance for the design and performance enhancement of next-generation solar power tower plants.