CO2热泵热水器气冷器的热力性能研究
Study on Thermal Performance of Gas Cooler for CO2 Heat Pump Water Heater

本文以CO2定压比热容最大的准临界温度为分界点，将气冷器的换热过程分为：准临界前区间、准临界区间和准临界后区间3个温度区间，CO2在100～0 ℃的放热过程，计算发现准临界区间温度的放热量占比达49.8%，基于(火积)理论的模拟结果也表明放热量主要集中在第二阶段的准临界区间内。以准临界区间的换热特性为基础，提出一种在名义工况下，最佳气冷器进气压力和温度的简单算法：发现制取65 ℃热水时，气冷器最佳进气压力为11.34 MPa，进气温度为79.96 ℃。在不同进气压力下进行实验验证，并分析气冷器中水的温度分布。结果表明：CO2和水流量均为15.0 g/s时，11.3 MPa进气压力下，在准临界温度区间内，水温从17 ℃迅速升至41 ℃，加热量占比为50.1%，出水温度为64.1 ℃，系统COP最高为3.23。
The heat transfer process in a gas cooler can be divided into three parts, namely pre-, quasi-, and post-critical temperature intervals, which are based on the carbon dioxide (CO2) critical temperature at a maximum constant pressure specific heat capacity. Particularly for the quasi-critical interval, the proportion of heat release was reportedly 49.8% under exothermic process with CO2 critical temperature being lowered from 100 ℃ to 0 ℃. This finding was supported by a simulation based on the theory of entransy, which has been proven in subsequent experiments. On this basis, a simple algorithm for the optimal inlet pressure and temperature in the tested gas cooler was proposed according to the heat transfer characteristics of the quasi-critical section. The optimal inlet pressure was 11.34 MPa and temperature was 79.96 ℃ when the hot water (65 ℃) was obtained under the standard test condition through the algorithm. A test involving different inlet pressures with different compressor speeds was designed to validate the previous algorithm by calculating the system coefficient of performance (COP) and analyzing the heat transfer characteristics in a gas cooler byvarying water temperature with discharge tube distance. Consequently, when both CO2 and water flow rate was 15.0 g/s, the water temperature increased rapidly from 17 ℃ to 41 ℃ in the quasi-critical temperature intervals, which possessed huge entransy under 11.3 MPa inlet pressure with critical temperature of CO2 as 51.04 ℃. Finally, when the water outlet temperature was 64.1 ℃, heating amount was approximately 50.1%, and system COP was 3.32, which provided the best test condition among the three different sets