Objective  The linear Fresnel solar concentrator is one of the technologies of concentrator solar thermal power generation (CSP). Because the primary mirror are discrete flat mirrors and installed near the ground, the system has the advantages of strong wind resistance, which is especially suitable for large-scale construction in the northwest region of China with excellent solar energy resources but high wind speed. The existing research results show that the solar irradiation on the surface of the collector tube is very uneven in linear Fresnel concentrator system due to the different aiming strategies and the reflection profile of the secondary mirror. In addition, the medium in the tube is difficult to quickly divert the heat from the surface of the collector tube, the collector tube is easy to bend and deform, resulting in the breaking of the outer glass tube of the vacuum tube and the increasing heat loss of the system. The uniformity of heat flux density distribution on the absorber tube surface has a significant impact on the optical and thermal performance, as well as safe operation of linear Fresnel concentrating solar systems. Therefore, it is necessary to study the method for improving the uniform distribution of energy flux density to improve system efficiency and system security.

  Methods  A linear Fresnel concentrator model was built using Tonatiuh optical simulation software, a cylindrical reflector was selected as the research object, and the relationship between the focal length (radius of the reflector) and the position of the aiming point on the aiming plane and the width of the spot was studied. Based on the fact that the width of the spot is not greater than the width of the secondary mirror opening, the focal length of the mirror and the distance from the center of the aiming point are determined. According to the different distribution of aiming points, five different aiming strategies are proposed, and the specific data are shown (Tab.2). Based on this, the optical efficiency and uniformity of heat flux distribution of the linear Fresnel concentrator under different aiming strategies were explored.

  Results and Discussions   When the height difference between the edge and the bottom of the mirror is 2.85 mm, the spot width is the smallest （Fig.3）, and it is far less than the aperture width of the secondary mirror opening. The aiming point is more than 0.04 mm off center, the spot width exceeds the aperture width of the secondary mirror (Fig.5). Using the aiming strategy of uniform distribution of aiming points, the concentrator will maintain high optical efficiency while improving the uniformity of energy flow distribution on the surface of the collector tube (Fig.7). With the optimized aiming strategy, the optical efficiency of the linear Fresnel concentrator can reach 87.4% (Fig.8), the standard deviation of energy flux density on the surface of the collector tube is reduced from 45.3% to 30.7% (Fig.8), and the energy flux density at the top of the collector tube is increased by 10.2% (Fig.9).

  Conclusions  The uniformity of energy flux distribution on the surface of the collector tube of linear Fresnel concentrators can be improved by adopting different aiming strategies. By rationally distributing aiming points evenly on both sides of the center, the best spot uniformity on the surface of the collector tube and the best optical efficiency of the system can be achieved. When the aiming strategy is applied to the preheating of the empty pipe of the linear Fresnel concentrators, the bending deformation of the collector tube is much smaller than the existing aiming strategy under the variable duty ratio tracking mode. The research results can provide theoretical support for the optimal design of linear Fresnel concentrator heat collection system.