A new approach to optimize the energy efficiency of CO2 transcritical refrigeration plants

Abstract

This paper proposes a model-free real-time optimization and control strategy for CO2 transcritical refrigeration plants that assures covering the cooling demand and continuous tracking of conditions for maximum efficiency. Our approach obtains the feedback with only three measurements, and controls the opening degree of a back-pressure valve and the speed of the compressor. The strategy minimizes the power consumption of the compressor instead of maximizing the coefficient of performance, which avoids several sensors, and we demonstrate mathematically that both approaches are equivalent. We implemented the strategy with an algorithm that includes two independent auto tuned controllers, one devoted to regulate the high-pressure and another to regulate the outlet temperature of the secondary fluid of the evaporator. It also incorporates a real time perturb and observe procedure to locate on-line the optimum high-pressure that minimizes the compressor power consumption. The paper presents the experimental evaluation of the control strategy, verifying the stable operation of the algorithm and the energy optimization of the plant. KEYWORDS

Energy improvements of CO 2 transcritical refrigeration cycles using dedicated mechanical subcooling

“…Other improvements are the use of expanders (Li et al, 2004;Yang et al, 2007) and ejectors (He et al, 2014;Lee et al, 2014) to reduce irreversibilities during the expansion process, which also offered COP increments. In parallel, control strategies to regulate the heat rejection pressure were developed (Aprea and Maiorino, 2009;Ge and Tassou, 2009;Peñarrocha et al, 2014). And now, the improvements of performance of CO2 transcritical plants are also considered by combining them with other thermal systems via recovering heat in the gas-cooler.…”

Experimental evaluation of a CO2 transcritical refrigeration plant with dedicated mechanical subcooling

“…Others analyzed the effect of extracting vapour from the intermediate vessel to be injected in different points of the cycle, measuring maximum increments of 7 % in single-stage plants (Cabello et al, 2012) and 16.5 % in double-stage cycles (Cho et al, 2009). The use of expanders (Li et al, 2004), ejectors (Lee et al, 2014) and regulation strategies (Peñarrocha et al, 2014) are also considered. And now the improvements of the CO2 transcritical plants are looked for its combination with other systems, such as desiccant wheels (Aprea et al, 2015) or absorption plants (Arora et al, 2011).…”

Efficient control of automotive R744 heat pump using Nelder-Mead simplex method

“…Two control loops are commonly used for R744 heat pump control, first one for capacity control and the second one for high pressure control [20], [22]. We complemented them with COP feedback for Nelder-Mead method, see Fig.…”

“…The second approach is based on real-time (online) COP optimum searching, what can be done by extremum-seeking control (ESC), in general described in [18] and simulated for heat pump in [19]. Other perturbation-based algorithm for R744 heat pump energy efficiency optimization is described in [20]. Another solution of this problem is based on artificial neural networks [21].…”