Performance Analysis On Compound Parabolic Concentrator And Its Applications

As a kind of green,non-polluting new energy source,solar energy has many advantages,but it also has problems such as low energy density.If we absorb the solar energy on the earth’s surface by using an absolute black body,the temperature at the thermal equilibrium is about 91°C,which is slightly lower than the boiling point of water.In many applications(such as solar pump lasers,concentrating solar thermal power generation,etc.),the thermodynamic efficiency of a heat source with a temperature of 91°C is very low.So if an effective temperature(for example,higher than 300°C)is needed for generating energy,it is necessary to concentrate the sunlight.According to the second law of thermodynamics,the optical device on the earth can concentrate the solar to a theoretical maximum which does not exceed the surface temperature of the sun.The traditional method of concentrating is to use an imaging system,however,the actual imaging system cannot effectively achieve the maximum concentration ratio due to the existence of aberrations.In an optical system,the need of imaging can be sacrificed in exchange for the need of concentrating.Therefore,only by abandoning the imaging requirements can obtain a higher concentration ratio,a nonimaging concentrator is then designed.If this kind of concentrator is used for imaging,it will have very large aberrations,while they are much more efficient than imaging systems if used for concentrating,and can even reach or approach theoretical limits by special design.In this paper,a typical example of non-imaging optical concentrator,the compound parabolic concentrator,is analyzed.And in view of its excellent concentrating performance,this paper expands the application research of the compound parabolic concentrator.Firstly,the structure and concentrating performance of the compound parabolic concentrator are introduced and analyzed.Then its application on high solar concentration is given: combining a compound parabolic concentrator with a traditional imaging concentrator can achieve a two-stage high solar concentration system.The technical route adopted is to place the compound parabolic concentrator at the focus of the parabolic mirror,while the entrance size of the compound parabolic concentrator is equal to the size of the spot at the focus of the parabolic mirror,and the maximum acceptance angle of the compound parabolic concentrator is equal to the half angle of te light beam at the focus..Such a two-stage concentration system is well suited for fields such as solar pump lasers.Then an application of compound parabolic concentrator in large-scale is proposed: an annual compound parabolic concentrator is designed and used in the concentrating part of the tower solar thermal power generation system to improve the power generation efficiency.The power generation efficiency of the tower solar thermal power generation system is the product of the heat collection efficiency and the heat engine efficiency.Increasing the concentration ratio of the system can improve the heat collection efficiency of the system,thereby improving the power generation efficiency.The two usual methods to increase the system’s concentration ratio are increasing the number of heliostats or improving the tracking accuracy of the heliostat,but the cost of these two methods are high and the latter is technically difficult.So an annual compound parabolic concentrator is then designed and installed on the heat receiver for secondary concentration.Through simulation and experimental analysis,the annular compound parabolic concentrator not only can improve the power generation efficiency of the tower solar thermal power generation system,but also can prolong the daily working time of the system.So the economic benefits of the annular compound parabolic concentrator are considerable.Next,two applications of compooun parabolic concentrator in miniaturization are proposed:(1)A fiber coupling system based on compound parabolic concentrator is designed.The laser beams of the LDs are coupled to the multimode fiber after passing through the fiber combiner,and the size of the output beam of the multimode fiber is large.While using a compound parabolic concentrator coupling system,the size is greatly reduced and can match the core size of the active doped fiber,and most of the energy is concentrated within the numerical aperture of the active doped fiber.This device can improve the coupling efficiency.(2)A micro compound parabolic concentrator array is designed to improve the performance of a focal plane detector.The focal plane detector’s pixel consists of photosurface and dead zone,photosurface is the area where the photoelectric effect occurs,while dead zone is useless to detection and reduces the light energy utilization of the detector,which will influence the responsivity of the detector.Generally,by coupling a microlens array to concentrate the incident light as much as possible onto the photosurface can solve this problem.However,due to the imaging characteristics of the lens,when the detected target moves,a part of the light concentrated by lens will deviate from the photosurface to the dead zone,and this results in a loss of energy.So we used a micro compound parabolic concentrator array to replace the traditional microlens array.The compound parabolic concentrator has a large acceptance angle and can concentrate light beam with incident angle smaller than the acceptance angle,even if the incident light is deviated by a certain angle,most of the light can effectively be concentrated onto the photosurface as long as the deviate angle is smaller than the acceptance angle,so it has higher light energy utilization than microlens array.According to the simulation results,the advantage of the micro compound parabolic concentrator array was obvious.This method may have important application prospects in the field of high speed and high precision imaging.Based on the good concentrating performance of the compound parabolic concentrator,an annular compound parabolic concentrator used for the tower solar thermal power generation system is designed in this paper,and this is a groundbreaking design at home and abroad,which has important research significance in promoting the solar thermal power generation technology in China.In addition,according to our investigations and survey,this paper is the first to propose the application of compound parabolic concentrator to the focal plane detector,and this provides an idea for further research on focal plane detection.