CO₂ refrigerant pump from HERMETIC part of CTES proof of concept 

Proof of concept provided: Use of CO₂ for loading and offloading a cold thermal energy storage unit with a phase change material (PCM)

The research project included development of an innovative CTES unit with special type heat exchangers (pillow heat exchangers) and an experimental pilot plant was built, for which CO₂ in pump circulation was used as the refrigerant. The technology is based on utilization of the refrigerant CO₂ for loading and offloading the cold thermal energy storage unit with different phase change materials (PCM). During the loading process of the CTES system, heat is transferred from the PCM to the evaporating CO₂. The PCM solidifies in this process, in other words the relatively high latent thermal energy is withdrawn from the storage unit. When the CTES unit supports the refrigeration system, heat transfer occurs from the CO₂ (liquefied) to the PCM (melted). The experiments performed in the pilot plant have provided the proof of concept of the technology. The project demonstrated that effective and compact cold thermal energy storage with PCM is possible in conjunction with condensation and evaporation of CO₂ refrigerant. Multiple PCMs were tested in order to meet the temperature requirements of different refrigeration applications. For air conditioning systems, water/ice is an attractive, natural and inexpensive candidate, which was tested in the pilot plant. Finally, a commercial low-temperature PCM (–10 °C) that is suitable for food processing plants, was also demonstrated.

Refrigerant in pump circulation: Rigorous requirements imposed on pumped CO₂

A pumped refrigerant is suitable for this application, since the liquid refrigerant must be distributed uniformly in the special heat exchanger plates of the newly developed cold thermal energy storage unit. CO₂’s critical point is 31 °C and for heat dissipation, this critical point requires pressures that are much higher than the temperatures frequently used for industrial cooling. The components used must be able to meet these requirements. In the pilot plant, the refrigerant CO₂ circulates at a temperature range from 5 °C to –50 °C with a maximum nominal pressure of 52 bar. In addition to effectively dealing with the pressure, the low viscosity of CO₂ was also a challenge for the load capacity of the plain bearings. In addition, absolute leakproofness and a low MTBF value, in other words, the highest reliability, is required of the pump that is used.

Reliable refrigerant circulation under demanding test conditions

HERMETIC provided a CAMh2 series CO₂ pump. As a high-pressure design, the canned motor pump is optimally adapted to existing plant conditions. This series has been specially developed for CO₂ applications in order to meet the requirements of 52 bar operation and standstill pressure.

The research team gave the pump donated by HERMETIC outstanding marks, particularly in light of the demanding conditions in an experimental pilot plant. For example, compared to industrial applications, the pump was confronted with more frequent start-stop cycles, changing operating pressures and changing flow rates. Throughout the entire test, the pump ran with no problems, no leaks, and it ensured reliable distribution of the refrigerant in the pilot plant.

HERMETIC CAMh pump offers design safety and high MTBF values

Canned motor pumps offer design safety due to their absolute leak-free construction and high MTBF values, achieved by eliminating wear-prone shaft seals and couplings. This pump technology uses plain bearings that are lubricated by the refrigerant. The issue is the low viscosity of liquefied gases such as CO₂ that continues to decrease with increasing temperature and has a negative effect on the load capacity of the plain bearings. The wrong bearing materials or a surface that is too small cause mixed friction in operation. The results are wear and pump failure. Generously dimensioned HERMETIC plain bearings, made of state-of-the-art sintered materials, ensure non-contact operation and thus pump operation that is virtually wear-free. The construction and choice of materials for the CAMh pump series reliably meet the requirements imposed by the nominal pressure of PN 52. For example, pressure containing parts are made of cast steel. A pressure test at 78 bar is carried out on each pump in accordance with applicable standards such as ISO 15783 for canned motor pumps. In addition, the secondary containment was tested for leaks with nitrogen (N₂). With secondary containment, the HERMETIC pump ensures a reliable and controlled flow of refrigerant in the pilot plant.