Thermodynamic Analysis And Performance Comparison Of Supercritical Carbon Dioxide Brayton Cycle

The supercritical carbon dioxide(S-CO2)power cycle has the characteristics of high cycle efficiency,compact system and small footprint.In recent years,it has attracted wide attention in the fields of solar energy,nuclear energy,waste heat utilization and ship power.In S-CO2 Brayton cycle,the simple regenerative cycle is the most basic circulation system.Because of the simple structure of the system,the effective solution to the pinch point problem in the heat exchanger,and the high efficiency of the system,the recompression cycle is considered to be the most classic circulation system in the Brayton cycle with the most application prospects.S-CO2 Brayton cycles with different topological schemes are suitable for capacity utilization systems in different application fields and temperature ranges.There are five S-CO2 Breton cycle systems analyzed in this paper,such as simple heating,recompression,reheating and recompression,exergy cooling and precompression.Thermodynamic calculation and exergy analysis models for each system are established.Exergy analysis shows the impact of key parameters on the performance of the system,while exergy analysis shows the link where exergy damage is greatest and corresponding improvement suggestions are put forward.Finally,according to Sandia Laboratory’s high and low working conditions,the five Breton circulation systems were compared and analyzed,and the following conclusions were drawn:There is a pinch point in the regenerator of Breton circulation system.Simple regenerator cycle is inefficient because of the pinch point problem.The pinch point of the high temperature regenerator with the shunt recompression cycle is at the outlet of the high temperature side.The pinch point of the low temperature regenerator in the recompression and reheat recompression cycles moves from the inlet to the outlet of the high temperature side with the increase of the shunt ratio.The value range of the shunt ratio of the pinch point in the low temperature regenerator increases with the increase of the outlet pressure of the precompressor in the intermediate cooling and precompression cycles.The greater the temperature difference between the pinch points,the lower the return heat of the regenerator,and the thermal efficiency and exergic efficiency of the system decreased accordingly.For the other four S-CO2 cycles except for simple regenerative cycle,as the shergy ratio increased,the thermal efficiency of the system and exergy efficiency went up first and then went down,while the overall exergy loss went down first and then went up,there are optimal and minimum shergy ratios.Pressure ratio is a key parameter affecting S-CO2 Brayton cycle.With the increase of pressure ratio,the optimal efficiency of the system rises first and then decreases,and the increase amplitude of the optimal efficiency curve of the cycle gradually slows down.Therefore,the cyclic pressure ratio should not be too high.Under the given conditions,the optimal pressure ratio of simple regenerative cycle is 6.65,that of shunt recompression cycle is 4,that of reheat recompression cycle is 5,and that of intermediate cooling cycle and precompression cycle is 4.4.The higher the maximum temperature of the cycle,the better the system performance.Influenced by the inlet temperature of the compressor,the performance of the simple regenerative cycle system is different from that of the recompression cycle system with shunt.The optimal inlet temperature of the simple regenerative cycle compressor is 37℃,and that of the main compressor with the Breton cycle with shunt recompression is 32℃.In addition to the simple reheating cycle,there are four other S-CO2 cycles.For the reheating and recompression cycle,the optimal reheating pressure formula is the square root of the product of the highest pressure and the lowest pressure,which is deduced theoretically and verified by simulation calculation.The performance of the intermediate cooling and precompression cycles varies with the outlet pressure of the precompressor as follows: When the turbine inlet pressure is low or the pressure ratio is small,the circulation efficiency decreases monotonically with the increase of the precompressor outlet pressure.When the turbine inlet pressure is large(25MPa or above)or the pressure ratio is large,the optimal compressor outlet pressure exists.When the turbine inlet pressure is low,the precompressor should not be used,and when the turbine inlet pressure is high(the corresponding pressure ratio is large),Intermediate cooling and precompression schemes are beneficial to improve the performance of S-CO2 Brayton cycle.For Breton cycle,exergy loss of heater,regenerator and cooler is the greatest,that is,exergy loss of exergy is the greatest during heat exchange.Strengthening heat exchange and reducing heat exchange end difference is the key to improve system performance.S-CO2 Brayton cycle is suitable for operation under high operating parameters.Under high operating parameters,the system layout of reheating,intermediate cooling and precompression can effectively improve the system performance,and reheating is the most effective way to optimize the circulation layout.