An innovative ecological hybrid refrigeration cycle for high power refrigeration facility

What is it about?

Searching for new refrigerants is one of the most significant scientific problems in refrigeration. There are ecological refrigerants commonly known: H2O and CO2. H2O and CO2 known as natural refrigerants, but they have problems:a high freezing point of H2O and a low triple point of CO2. These problems can be solved by the application of a hybrid sorption-compression refrigeration cycle. The cycle combines the application possibility of H2O in the high temperature sorption stage and the low temperature application of CO2 in the compression stage. This solution gives significant energy savings in comparison with the two-stage compressor cycle and with the one-stage transcritical CO2 cycle. Besides, the sorption cycle may be powered by low temperature waste heat or renewable heat. This is an original idea of the authors. In the paper an analysis of the possible extension of this solution for high capacity industrial refrigeration is presented. The estimated energy savings as well as TEWI (Total Equivalent Warming Impact) index for ecological gains are calculated.

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Why is it important?

Refrigeration nowadays is searching for new refrigerants, since the Montreal Protocol (1987) regulations strongly affected the refrigerant market as well as scientific research trends. The new findings, for example, R1234yf meet the Montreal Protocol requirements in terms of air pollution but have many disadvantages, which makes it very sensible to look for other opportunities. The application of carbon dioxide as a refrigerant is becoming increasingly popular. However due to the temperature limit and resulting efficiency limitation CO2 is used mostly for the LT (Low Temperature) stage compression cycle. In the HT (High Temperature) cycle another refrigerant, such as for example R410A, is used. A number of papers on CO2 applications in different refrigerating cycles has been published in recent years (Suamir et al., 2012; Suamir and Tassou, 2013; Yamaguchi and Zhang, 2009). There is also interest in modelling and simulation of adsorption heat pumps, which allows the analysis of the winter cycle of the combined systems (Evola et al., 2013; Starace et al., 2014). The range of produced absorption or adsorption units is from 8kW up to 21MW cooling capacity. The choice of adsorption or absorption technology depends on load, price, and most of all temperature of the heat source for desorber heating (Ullah et al., 2013). The COP of the absorption cycle with high temperature generation (above 120°C) may be higher than that for adsorption using two stage, double effect cycles (Misra et al., 2003; Misra et al., 2005). For lower temperature sources such as solar cooling or central heating network, the application of absorption may not be possible at all. Different heat sources for cooling or refrigerating have been also analysed in * Corresponding author, e-mail: pcyklis@mech.pk.edu.pl cpe.czasopisma.pan.pl; degruyter.com/view/j/cpe P. Cyklis, K. Janisz, Chem. Process Eng., 2015, 36 (3), 321-330 322 a number of published papers (Joudi and Dhaidan, 2001; Saha et al., 2003; Syed et al., 2005; Wang et al., 2000). The adsorption technology can work using 65°C as the driving temperature for a desorber (Habib et al., 2011; Saha et al., 2003; Saha, et al., 2006. This is the main difference between the possibilities of contemporary industrial adsorption and absorption technologies. There is also scientific interest in hybrid sorption-compression cycles, their application however, has other aims and refrigerants than presented here (Banker et al., 2008; Garimella et al., 2011; Han et al., 2013; Zheng and Meng, 2012). The problem with H2O application as a refrigerant is the triple point 0.01 °C and the CO2 cycle has reduced COP terminology explanation above the critical point 31 °C. A possible solution of this problem is a cascade system combining two cycles: low temperature compression stage (LT) with CO2 and high temperature sorption stage (HT) with H2O. There are ecological solutions for HT cycle: adsorption or absorption cycles. Both can be driven with waste heat or solar heat. This reduces the electric energy consumption. The solar driven sorption only cycles have been reported earlier in previous publications (Cyklis and Brak, 2008; Florides, 2003; Grzebielec, 2009; Hassan et al., 2012; Kim and Infante Ferreira, 2003; Kim and Infante Ferreira, 2008; Wang et al., 2000; Zhang et al., 2011).

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