Under the annual German Refrigeration Award, the German government rewards promising concepts promoting energy efficient and climate friendly refrigeration. This year, the German Federal Environment Ministry considered more than 60 ideas of both scientists and companies in three categories.

The project “expander compressor unit (ECU) for carbon dioxide refrigeration systems with hot gas defrosting”, which received the 2^nd place in the category “climate-friendly special applications in refrigeration”, was a co-operative initiative between Saxony-based kke GmbH Kesselsdorf and the BITZER Chair of Refrigeration, Cryo and Compressor Technology of the Technische Universität Dresden. Other companies involved in the project were TEKO, Wurm Elektronische Systeme, thermofin, Danfoss, Viessmann and Küba.

About the project

kke GmbH built a transcritical CO₂ booster refrigeration system connected to a demonstration store that simulated a sales area of approximately 850-1000m².

The main components of the system are:

• two frequency controlled subcritical compressors for TK (t0 = -30 °C) from Bitzer
• two frequency controlled transcritical compressors for NK (t0= -10 °C) from Bitzer
• liquid /vapor separator
• air cooled gas cooler and intermediate cooler from thermofin GmbH
• internal heat exchanger
• expander compressor unit (ECU)
• coriolis mass flow meters
• hot gas defrosting

An expander compressor unit (ECU) was integrated in this system (see fig. 1). Within the scope of the project a control strategy for the ECU was developed and tested.

The ECU unit expands the CO₂ from high pressure to middle pressure (li./v. separator) and in a second expansion stage the flash gas. The expansion stages are direct coupled with a compression stage. This additional compression stage allowed to also integrate an intermediate cooler (see fig. 2). With such a unit the energy consumption and the discharge temperature of the main compressors can be reduced.

Another means of reducing energy consumption is through hot gas defrosting. While in conventional refrigeration systems, additional electrical heating is used to defrost the evaporators, the use of hot gas defrosting can save this energy. Moreover, the excellent heat transfer characteristics of CO₂ allow a much quicker defrosting. First tests have confirmed feasibility and verified the predicted energy savings.

In addition, the system is used by kke GmbH to train its staff and for customer demonstrations.