Air Reversing R744 AC Systems a global solution, tests show

By team, Sep 24, 2008, 00:00 3 minute reading

An air reversing, turn-table residential air conditioning unit, using the natural refrigerant CO2, is a viable option for many global areas in need of both heating and cooling. This is the result of first studies jointly presented by SINTEF, Modine, and the US Army at this year’s Gustav Lorentzen event.

Even at extreme ambient temperatures of up to 52°C R744 residential air conditioning (RAC) units with a “one-way” refrigerant circuit are working reliably and efficient. Moreover, the simplicity of the system arrangement, its flexibility in the degree of automatisation and the lack of sophisticated switching devices make the air reversing RAC unit a cost-efficient and compact solution for developed and developing countries. The newly developed turn-table design, also suitable for portable applications, is thus a highly promising way to raise living standards in emerging economies in need of sustainable ways to provide heating and cooling.

These are main findings from tests performed with an Environmental Control Unit (ECU) prototype investigated by leading Scandinavian research institute SINTEF, global systems supplier Modine, and the U.S. Army. Results showed that this novel way of reversing the operating mode can further expand the application of CO2 air conditioning on a global scale. Initial promising results were presented by all three partners at this year’s Gustav Lorentzen Conference on Natural Working Fluids in Copenhagen.

“The investigated system is an interesting, viable low cost R744-HVAC solution for regions where both heating and cooling is required to achieve comfort in your office or at home,” confirmed Dr. Armin Hafner, Senior Researcher at SINTEF conducting the tests.

Rationale & Test Setup

An increasing interest for AC systems that can be reversed for heat pump operation has lead to numerous investigations on how to use CO2 (R744) in living areas where buildings require heating during the cold season and cooling during the hot season. Redirecting the refrigerant flow, however, requires several additional valves and fittings, thereby adding extra costs, possible leakage and pressure drop to the system. Therefore, SINTEF and its partners decided to investigate a small capacity ECU with a reversed air flow – a second, and more viable, option to supply cooling and heating.

The principle system layout, a turning table ECU with a tangential placement of the heat exchangers and stationary fans located in every corner of the ECU, was tested at different gas cooler pressures and air flow rates providing the desired capacity at maximum COP. The heat exchangers’ tangential arrangement guaranteed a uniform air side distribution by drawing air through the heat exchangers. The system comprised a four row heat exchanger evaporator, a gascooler designed as a four row cross counter flow heat exchanger, a combination of two expansion devices (thermal and manual back pressure valve), an accumulator originally designed for automotive R744 air conditioning, and a prototype hermetic piston compressor.

Test Results & Challenges

Tests proved that the small capacity RAC unit performed successfully at ambient temperatures of -20°C to 52°C. A life cycle climate performance (LCCP) analysis also confirmed that the air reversible CO2 unit is a viable alternative to traditional HFC-based residential split air conditioning units to be phased out under the Kyoto Protocol in the near future. Moreover, the system design keeping the refrigerant side relatively simple without any sophisticated switching devices, led to reduced heat conduction and pressure losses. The turning mechanism’s degree of automatisation from a simple manual handle to an electrical motor and the unchanged refrigerant circuit allowed for an application of standard components, including the heat exchanger and gascooler. Refrigerant charge problems could be handled since no “dead” lines are present.

Based on these positive results, further work is now needed to enhance the system efficiency by an improved refrigerant distribution inside the evaporator, high efficient compressor, an optimal length of the internal heat exchanger, an adjustment of the thermal expansion device, and the potential use of an ejector.


By team (@r744)

Sep 24, 2008, 00:00

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