IIR 2023 conference in Ohrid: focus on CO2 heat pumps

CO₂ (R744) was the dominant topic at the last IIR conference in Ohrid in April 2023, featuring in 27 of the 39 papers presented. 

 

According to the latest ATMO report (1), the penetration rate of transcritical CO₂ installations in supermarkets, grocery and convenience stores continues to progress, reaching 18.4% in Europe (57,000 installations), 8.6% (6960) in Japan and 0.7% (1150) in the United States as of December 2022. 

 

Due to its thermodynamic and environmental properties, it has promising prospects for even larger use, particularly in the field of heat pumps, as evidenced by two papers presented at this conference. 

 

Experts from DTU and from Danfoss in Denmark (2) presented a review of CO₂ heat pump technologies at component and system level and suggested areas for research to optimise their energy performance. 

 

The use of heat pumps has grown considerably in the wake of recent energy challenges. According to the International Energy Agency, the global stock of heat pumps was around 190 million in 2021, and it is expected to at least double by 2030. In particular, heat pumps that are in the range of 20 kW-200 kW heating capacity, are good candidates for replacing fossil fuel boilers. Among other heat pump solutions, CO₂-based solutions are now more popular due to their energy-saving and environmentally-friendly operating conditions. 

 

The experts point out that both the evaporator and gas cooler (condenser) should be compact and lightweight to improve heat pump performance while heat flux and mass flux should be well defined through optimisation or other decision-making processes. Both heat exchange processes are in the two-phase region; thus, flow arrangements and operating conditions also affect heat absorption and rejection processes. Furthermore, compressors and expansion devices operate with a large pressure difference between supercritical and subcritical conditions; in this regard, several alternatives are possible to overcome energy losses, such as multi-stage compression and/or compressor optimisation on the compression side and ejector integration into the system on the expansion side. 

 

The main challenges for air-source and water-source heat pumps are frosting and water freezing, respectively. Water and heating demand are the two main criteria that affect the operation limits and, inherently, the overall performance of ,water-source heat pumps. In addition, they are quieter. On this basis, improving the performance of CO2 heat pumps in the future will involve optimising control, ejectors and multi-stage compression, and better heat exchange mechanisms for defrosting. 

 

Researchers from NTNU in Trondheim, Norway (3) presented an overview of the current state of the art of CO₂ heat pump solutions for thermal management of electric vehicles, i.e. temperature control of the vehicle’s cabin and battery, as well as cooling the motor and ,power electronics. 

 

They conclude that CO₂ refrigerant is suitable for use in electric vehicles. In heating mode, CO₂ has a decisive advantage over R1234yf or R134a. Especially at temperatures below 0 °C, the CO₂ system is more efficient due to the higher density of the suction vapour. In addition, no auxiliary heater is necessary, which further increases efficiency and saves the system from having to add a very expensive component. Satisfying the cooling demand takes on new importance in electric vehicles as it impacts battery performance and safety. Research is needed to evaluate the system under conditions of further increased charging and discharging power of the battery (for example, during fast charging). The use of both heat exchangers in the HVAC system (gas cooler and evaporator) seems promising to increase the cooling/heating capacity.  

 

Due to the different properties of CO₂, traditional thermal management solutions suitable for other refrigerants or vehicle types are often not suitable and lead to inefficient and dangerous operating conditions. Further research is needed to determine the design, operation, and appropriate control strategies for CO2 heat pump systems for electric vehicles. 

 

Proceedings of the 2023 Ohrid conference can be downloaded here. 

Papers can be downloaded individually here. 

 

(1)ATMO Report, Natural Refrigerants: State of the Industry, 2022 Edition. Link.

(2) Kanbur B. et al. Mini review on a technological map of R744 heat pumps. Link. 

(3) Piesch N. et al. R744 heat pump solutions for electric vehicles. Link.