Refrigerant effect on system performance

written by Pavel Makhnatch (under supervision of Rahmatollah Khodabandeh and Björn Palm)

Published May 08, 2012

The air-condition and refrigeration systems are significant contributors of greenhouse gases (GHG) in atmosphere. It is estimated that they are responsible for approximately 10% of total worldwide CO2 equivalent emissions released. Recent legislative measures have led to development of new low global warming potential (GWP) refrigerants with comparable performance to those refrigerants which they replace. However, the choice of new refrigerant is likely to influence the COP of the system, hence the amount of indirect GHG emissions. It is estimated, that indirect emissions in some cases can represent more than 84% of total GHG emissions, thus improving efficiency is vital to mitigate system impact on climate change.

Choice of refrigerant greatly affects COP. Thus, for the same temperature difference theoretical COP could vary greatly: compare 2.56 (CO2) and 4.60 (R134a) for a fixed condenser (+30 oC) and evaporator (-15 oC) temperatures. However, this comparison is not fare enough as it is extremely difficult to establish correct efficiency comparison of different refrigerants in the system as due to varying properties the components adjustment is usually required.

Nowadays, HFOs are seen to become new widely used low GWP refrigerants. With first cars using HFO-1234yf already sold in Europe and the first supermarkets’ chillers using HFO-1234ze already in UK, as well as considering HFO-1234yf and HFO-1234ze(E) use further expansion, performance issues are very important. In these systems these refrigerants replace other, with greater GWP values; therefore the performance is usually indicated in comparison to those refrigerants.

HFO-1234yf is seen as a replacement to HFC-134a in, primarily, automotive mobile air conditioning (MAC). If tested in similar conditions, the HFO-1234yf performance showed to be lower than that of HFC-134a currently used in similar MAC systems. The experimental measurements showed 4-8% decrease of COP, when HFC-1234yf is used as a drop-in replacement with no system modifications. However, with some simple modifications (e.g. installed IHX, optimized superheat, improved condenser and evaporator), the COP of such a system show to be even higher than that of HFC-134a. Similarly, HFO-1234yf has been considered as a replacement to R-410a in room air conditioning, however showed significantly lower performance in this application.

R1234ze(Z) most likely used as a CFC-114 replacement refrigerant in high temperature heat pumping applications. In this case it shows good performance showing nearly 5% higher COP compared to CFC-114. The low GWP potential of HFOs is utilized in the refrigerant blends, while also mitigating their flammability. R-32, R-125 and R-134a are seen as the most viable components for binary and tertiary blends. By mixing the refrigerant in different composition one is able to achieve similar performance or, in some cases, outperform conventional refrigerants used in various applications.

Creating an energy and environmentally efficient refrigerating system is a very complex process. With ongoing development in the field one can be sure to have low GWP high COP refrigerants in coming future, which, in addition to environmental benefits, bring an attractive reduction in energy consumption.

Page responsible:bpalm@energy.kth.se
Belongs to: Department of Energy Technology
Last changed: May 08, 2012

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