Abstract
A sensitivity study was conducted for three typical expander-based transcritical CO2 cycles with the developed simulation model, and the sensitivities of the maximum coefficient of performance (COP) to the key operating parameters, including the inlet pressure of gas cooler, the temperatures at evaporator inlet and gas cooler outlet, the inter-stage pressure and the isentropic efficiency of expander, were obtained. The results showed that the sensitivity to the gas cooler inlet pressure differs greatly before and after the optimal gas cooler inlet pressure. The sensitivity to the intercooler outlet temperature in the two-stage cycles increases sharply to near zero and then keeps almost constant at intercooler outlet temperature of higher than 45 °C. However, the sensitivity stabilizes near zero when the evaporator inlet temperature is very low of −26.1 °C. In two-stage compression with an intercooler and an expander assisting in driving the first-stage compressor (TEADFC) cycle, an abrupt change in the sensitivity of maximum COP to the inter-stage pressure was observed, but disappeared after intercooler outlet temperature exceeds 50 °C. The sensitivity of maximum COP to the expander isentropic efficiency increases almost linearly with the expander isentropic efficiency.
Highlights
With the ratification of the Montreal Protocol in 1987 and the Kyoto Agreement in 1997, the search for sustainable substances for refrigeration applications has become urgent
The throttling valve in all the existing transcritical CO2 refrigeration systems can be replaced by an expander, but only three typical expander-based transcritical CO2 cycles that are widely used and studied in the published literatures are investigated in the current study
CO2 refrigerant from leaking to external environment and reduce transmission losses of expansion work, the expander and the second-stage compressor were usually combined into one unit by a work, the expander and the second-stage compressor were usually combined into one unit by a common shaft, like as the prototypes developed by Nickl et al [8] and Zhang et al [9]
Summary
With the ratification of the Montreal Protocol in 1987 and the Kyoto Agreement in 1997, the search for sustainable substances for refrigeration applications has become urgent. In the experiment of evaluating a model-free real-time optimization and control strategy proposed by Peñarrocha et al, two electronic expansion valves (controlled by a PI controller) carried on the precise real-time control to the high-side pressure according to the feedback signal of compression power [18] The quantification of working characteristics for transcritical CO2 in different parameters is needed to find the appropriate control accuracy or method Many researchers, such as Robinson [4], Yang [5,6], Cho [7], Cecchinato [19], have studied the effects of parameters on COP for different transcritical CO2 cycles including single- and two-stage cycles with or without an expander.
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