Research On The Influence Of Additives On Expansion Process And System Performance In CO₂Trans-critical Heat Pump

Based on CO2transcritical water to water heat pump system with expander, aconcept adding additives into CO2heat pump system is put forward in thisdissertation, in order to optimize the operation of the CO2expander and improvesystem performance. The influence of additives, such as nitrogen, argon, dimethylether (DME) and R32, on the performance of the system, the expander and gas coolerin CO2transcritical heat pump system is investigated.The thermodynamics analyses of CO2transcritical cycle with additives areconducted. The recovery power of expander increases using nitrogen and argon asadditives, but the heating capacity and the input power of compressor decrease.However, COP of the system increases first and then decrease with increasing theconcentration of nitrogen or argon, and an optimal COP exists. The heating capacityand the input power of compressor increase at first and then decrease with increasingthe concentration of DME, but for R32, the two parameters monotonically increases.The phase change of CO2with additive in the expander is analyzed with theory.According to the quasicrystal structures theory, the nucleation rate of CO2withadditives increases compared with that of pure CO2. The dilute solution’sthermodynamics performance and the superheat limits of CO2with or withoutadditives are also conducted. The phase change is accelerated with the presence ofnitrogen or argon, but the calculation results show the opposite behavior when DMEand R32are considered. The heat transfer characteristic of CO2with additives in thegas cooler is performed. The heat transfer performance of CO2gas cooler decreaseswith the addition of nitrogen or argon, but that increases with DME or R32.To observe the process of CO2depressurization, a high-pressure CO2test block isdesigned and produced. The rapid depressurization of pure CO2and CO2withnitrogen and DME are tested. The addition of nitrogen accelerate the phase change ofCO2, to the contrary, the phase change of CO2with DME is postponed.At last, the heat pump tests of CO2with and without additives are carried out. Theresults show that the recovery power in the expander of CO2increases with thepresence of nitrogen or argon. The heating capacity increases firstly and then dropwith increasing the concentration of nitrogen and argon. The system gets the maximum COP when the concentration of nitrogen or argon is1%. When R32orDME exists in CO2heat pump system, the recovery power increases at first, and thendescends with increasing the concentration of R32or DME. COP of the systemincreases with the existence of R32or DME.