Study On The Multi-reflection Heliostat Field In Solar Power Tower Systems

With the rapid growth of global energy consumption,the pressure of energy supply and the ecological environment keeps increasing.As an important renewable energy source,solar energy is drawing more and more attention.Solar power tower(SPT)technology collects and focuses the sunlight through the heliostat field,and generates thermal power in the follow-up solar-thermal conversion.Due to the continuous and stable power output,as well as the advantage of scale economies,SPT technology has a wide application prospect.The solar collecting subsystem--heliostat field--is the key component of the SPT plant.Its cost has accounted for 40% of the total invest cost.Its performance can directly affect the power generation cost.However,the current heliostat field has high optical losses and low economic performance,which has obstructed the efficiency improvement and cost reduction in power generation.Aiming at those problems,this paper will study the design method for the high performance and low cost heliostat field in SPT plant.The principle of single-reflection heliostat field as well as the utilization theory of solar concentration are described in this paper.The calculation methods of the optical efficiency and levelized energy cost are proposed.Performance evaluation indexes for heliostat field design are defined.Taking the PS10 field for example,this paper calculates the optical performance of the traditional single-reflection heliostat,and find that the cosine loss is the largest.We propose a multi-reflection scheme which can eliminate the cosine effect.The heliostat concentrating unit to realize this scheme is designed,and several different sub-mirror shapes are modeled.The concentrating characteristics of those units with different sub-mirror shapes are simulated in the software Trace Pro.The variation of interception efficiency under the effect of the shading rate and radial distance in the field is specially discussed.The shape and parameters of the sub-mirror are optimized.On its basis,the general heliostat structure with mechanical group-control is proposed.The structural scheme reduces the number as well as manufacturing difficulty of the drive mechanism.The modular design of the heliostat can be achieved.Furthermore,the construction cost of the heliostat field can be reduced.The Monte Carlo ray-tracing model for optical performance calculation is built based on the random sampling idea.,The model is verified through the comparative analysis with Trace Pro.Then based on this model,the annual efficiency distribution and efficiency average of the multi-reflection heliostats are specially analyzed.The effects of the structure parameters and errors on the optical performance are also analyzed.The result shows that the multi-reflection heliostat has the efficiency advantage at the central zone,which lay a theoretical foundation for the heliostat field layout.For heliostat field layout,a methodology by adding the heliostats to the field in sequence is proposed.In this methodology,two layout modes,respectively based on annually equivalent efficiency distribution(AEED)and efficiency/cost rate distribution(ECRD),are developed for different optimization targets: field optical efficiency and electricity power cost.It’s used for the multi-reflection heliostat field layout.The results show that the optical efficiencies of the AEED-based and ECRD-based field are respectively 71.46% and 73.6%,both larger than the single-reflection heliostat field.Furthermore,the two fields’ land areas are both less than that of PS10.Therefore the multi-reflection heliostat has a better field performance.The multi-reflection heliostat field is arranged again in consideration of the shading rate.The results show that the LEC of the AEED-based and ECRD-based field are respectively 0.2548€/k Wh and 0.2475€/k Wh,3.0% and 5.8% lower than that of PS10 field.So the plant model with multi-reflection heliostat field has a better economic performance.The experiment on the multi-reflection heliostat is carried out.The heliostat prototype is first designed and manufactured according to the proposed experimental scheme.Then the tracking accuracy of the prototype is tested.The results show that the tracking error in one day is kept within 0.1° which has achieved the accuracy requirement of SPT plant.Then the flux density of the focusing spot is measured with the help of CCD system.Compared with the simulation result,the concentrating performance of the prototype is verified.