Quasi-steady State Modeling Of Solar Air-Brayton Cycle With Thermal Storage And Experimental Study Of Key Components

Distributed solar air-Brayton cycle system has the advantages of high operation reliability,no water consumption,flexible system structure and high efficiency of power generation and comprehensive utilization of energy,which is very promising.In addition,integration with low-cost thermal energy storage(TES)can further improve the power generation stability and the utilization efficiency of the system.It is suitable for some remote areas with abundant solar energy resources but scarce water resources and electricity supplies.This paper focuses on a 10 k W solar high-temperature air-Brayton cycle power system coupled with thermal storage.Research on key components including thermal energy storage,recupeartor and turbomachinery are carried out.Quasi-steady state model of the system is then studied for analyzing the feasible system operation strategies.The design,tests and mathematical modeling of a sensible TES tank with stacked mullite honeycomb ceramics as storage medium are conducted.Charging tests of the TES are carried out by using a high-temperature solar air receiver.Under the inlet conditions of maximum air temperature at 810?,maximum air pressure at125 k Pa,average mass flow rate at 0.077 kg/s,the thermal efficinency of TES reaches about 79% for 6-hour charging period and pressure loss is less than 4.4%.The one-dimensional dynamic mathematical model of TES can predict the module temperature,outlet air temperature and outlet air pressure with relative roor mean square errors below 8.03%,5.96% and 0.33%,respectively.Compared with the tested results,the relative error of the calculated thermal energy amount in charging or discharging process is less than 10%.The heat transfer performance tests of the primary surface counter-flow recuperator are carried out,which show that the recuperator effectiveness can reach0.81.Pressure loss coefficients of the cold and hot sides are less than 2% and 0.55%,respectively.The established one-dimensional dynamic recuperator model is validated by test results.The relative root mean square errors of outlet temperature and pressure are the less than 3.5% and 0.5%,respectively.The relative error of the recuperator effectiveness is below 5.5%.The structural design and numerical simulation of both the radial turbine(nozzle and rotor)and the centrifugal compressor(impeller and diffuser)matching with the system design parameters are carried out,by which their performance maps are obtained.Under the design condtion,simulated adiabatic total-to-static expansion efficiency of the turbine and simulated total-to-total compression efficiency of the compressor can reach 83.9% and 74.3%,respectively.Their other performance parameters,such as shaft power,expansion or pressure ratio and surge margin,also achieve the design values.Both the steady-state and quasi-steady state models of solar air-Brayton cycle power system coupled with thermal storage are established for system performance analysis under three proposed operation strategies,which are constant rotation speed(N=120 krpm),constant turbine exhaust temperature(TET=620?)and constant output power(Pe=12 k W).When operating in constant N and constant TET strategies,the system has a maximum output power of 14 k W and a maximum power generation efficiency around 12.5%.Compared with constant TET strategy,constant N strategy stands out because it's more reliable and easy to handle for the system studied in this paper.With integration of the TES into the system,turbine inlet temperature(TIT)and rotation speed can be simultaneously kept constant by two controlling variables,the mass flow rate through the TES and the load,so that stable power output of the system can be realized.The operation results show that the system power generation duration can be extended for 3 hours with an average output electric power above 5 k W under no solar radiation condition.What's more,TES can reduce the fluctuation of turbine inlet temperature and shorten the preheating time when the system starts from the cold state for earlier net power generation by utilizing the initial heat stored in the TES.The research work is expected to offer references for the design,modeling and operation of distributed solar thermal air-Brayton cycle system coupled with thermal energy storage.