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News – What`s happening in energy research

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The expander compressor unit that has been installed in the booster system
© kke GmbH
Renaissance for natural refrigerants
14.11.2011

Cold store and retail display equipment containing dummy items for a test supermarket
© TU Dresden

Supermarket refrigeration using carbon dioxide

Researchers from the Technical University of Dresden have succeeded in significantly improving the efficiency of chillers that use carbon dioxide as a refrigerant. One possible application of their system is in refrigeration for supermarkets, where cooling is required for equipment ranging from refrigerated display cases right through to large cold storage rooms. The researchers expect average annual energy savings of 10% compared to conventional refrigeration systems, with savings of up to 25% on warm days. The researchers are cooperating with an industrial partner to obtain initial operating experience with these systems.

Carbon dioxide (CO2) is a natural refrigerant that was already being employed at the start of the last century. However, the use of carbon dioxide as a refrigerant has been confined to specialist applications since the development of chlorofluorocarbons in the 1930s. The intensification of efforts to reduce greenhouse gas emissions has led to increased attention being focussed on CO2 as a refrigerant again. The effectively unlimited natural availability of CO2 and its minimal greenhouse gas impact make it the ideal environmentally friendly refrigerant. For example, the Global Warming Potential (GWP) of CO2 is 1, compared to a value of 3,780 for the refrigerant R404A. 

So where is the problem?

The physical properties of carbon dioxide differ significantly from those of synthetic refrigerants. Its high volumetric cooling capacity is beneficial and, in combination with its excellent heat transfer behaviour, allows for the design of very compact equipment. Carbon dioxide is a non-toxic refrigerant, meaning that leaks do not pose environmental problems. However, other physical properties of carbon dioxide are less favourable. These properties make high pressures of up to 100 bar and also large pressure differences between the cold and warm sides necessary in equipment. Furthermore, energy consumption is significantly increased if recooling temperatures are above 31 °C (transcritical operation).

The solution: harnessing the pressure difference

In the refrigerant cycles of chillers, the refrigerant is allowed to expand across a throttle valve between the warm and cold sides before being compressed again. The energy of the pressure difference is not harnessed. This loss is not substantial when conventional refrigerants are used, as the pressure difference is very low. However, the opposite is true in the case of the high pressure differences that occur in systems based on carbon dioxide. The researchers established that it is worth harnessing this energy for the use in the refrigerant cycle. They developed an expander compressor unit (ECU) for this purpose. This unit uses the energy released by the expansion step in order to help drive the compression process. This increases the energy efficiency, particularly at high ambient temperatures.

In early 2011, this research work by the TU Dresden and the project partner  kke GmbH from Kesselsdorf near Dresden was awarded second place in the “Environmentally friendly special applications of refrigeration technology” category at the 3rd German Refrigeration Awards. As part of this project, the ECU was integrated into one of the project partner’s systems and tested. This system is a so-called booster system that combines both normal and freezer refrigeration. This test and demonstration system simulates the conditions actually encountered in a supermarket with retail area of around 1,000 square metres.

Another system that does not have a freezer unit is currently undergoing field testing in a supermarket in Switzerland.

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