IIR GL2020 conference: Range of CO2 applications

At the highly successful 14^th IIR-Gustav Lorentzen Conference on Natural Refrigerants held in Kyoto, Japan, December 7-9, 2020, a central place has been given to CO₂ (R744); of the 121 presentations at this conference, 55 focused specifically on CO₂, vs 29 on hydrocarbons and 5 on air or water as refrigerants. 

 

Presentations on CO₂ applications in commercial refrigeration were predominant but it should be noted that many other promising CO₂ applications were presented. Here are a few examples: 

 

Refrigerated transport 

 

. In their paper (1), researchers from the University of Illinois (USA) stress that R744 is an attractive refrigerant for medium-(MT) and low-temperature (LT) refrigeration but suffers from reduced capacity and energy efficiency at high ambient temperatures, which is of particular concern for applications such as refrigerated transport. They describe the design of a multi-temperature refrigerated container, with a subcritical scroll compressor used to provide the LT capacity. To improve unit performance at high ambient conditions, recent advances in R744 technology have been implemented, including multi-stage transcritical compression with intercooling, improved gas cooler design to minimize approach temperature, and ejector expansion work recovery.  Experimental results show that for ambient temperatures between 25 and 50°C, the prototype R744 container achieves a MT capacity in the range of 5.4 to 3.4 kW, a LT capacity in the range of 4.1 to 2.3 kW, and a unit COP in the range of 1.60 to 0.75. 

 

. Italian researchers (2) have presented a dynamic numerical simulation of a CO₂ refrigerating unit, which can switch between different configurations during operation, and of a refrigerated truck insulated body during a short-distance delivery mission. The simulations are conducted for a reference mean day for each month of the year, under hot European climate conditions. Results show the annual refrigerating system performance, in terms of the total cooling energy supplied by the main and auxiliary evaporators and in terms of average COP.  Results demonstrate that, even if the ejector transcritical cycle configuration is selected only for a limited time during the hottest months of the year, a considerable amount of cooling energy (10.5% of the total yearly cooling energy) is provided by operating with this layout, achieving at the same time an optimisation of the COP of the refrigerating system. Since the system configuration optimisation does not require any changes in the structure of the cooling unit, but just a different load management between main and auxiliary evaporators, it can be concluded that switching between configurations during operation can lead to an improvement of the average system performance. 

 

Heat pump water heaters 

 

Several papers have been devoted to this application of CO₂. For example, researchers from the Oak Ridge National Laboratory (USA) (3) have presented a performance model that was developed to evaluate the characteristics of a transcritical CO₂ Heat Pump Water Heater (HPWH) system. The model, calibrated with existing experimental data, and configured with appropriate compressor discharge pressures, was used to evaluate the impact of supply water temperature. The authors have concluded that a CO₂ system had comparable performance to an R134a system, but with noticeably smaller capacity for similar ambient conditions. However, the gas cooler was relatively large owing to the single-phase vs. two-phase heat transfer for the CO₂ and R134a-based systems, respectively. The impact of the water circulation rate on the water temperature stratification in the tank, an essential requirement for higher performance for CO₂ HPWH systems, was also investigated. 

 

Food freezing and processing / Thermal storage 

 

. CO₂ is gaining in attractveness  for  fishing  vessels  due  to  the compactness of the units that use it, its negligible GWP  and  its non-toxic behaviour. On-board chilling and freezing are energy-intensive processes but necessary to guarantee high-quality products. Researchers from SINTEF (Norway) (4) have analysed an integrated freezing and heating refrigeration system with low-temperature CO₂ thermal storage. The reference refrigeration system is a two-stage transcritical CO₂ booster system with a freezing capacity of 240 kW. The simulation results show an average COP increased by a factor of 2.8 for combined freezing and heating system, compared to freezing alone. The average heat recovery of the refrigeration system is 298 kWh, which perfectly matches the heating demand of 261 kWh for fish oil production by the thermal treatment process. CO₂ thermal storage exhibits promising results. A 0.3m³ (75liters) aluminium thermal storage including an internal heat transfer area of 67m² is an ideal option for the reference refrigeration system to cover peak loads. A 50kWh stored energy reduces the peak time from 10 to 8 minutes. Each day corresponds to almost 28 peaks. An additional production capacity equivalent to 56 minutes was achieved in one day. 

 

. In another paper (5), SINTEF experts assess the existing energy system of a poultry processing plant, with focus on energy consumption, peak power requirement and heat recovery. A new concept for integrating cold thermal energy storages (CTES) with an R717/R744 cascade refrigeration system is presented.  A comparison between the systems is carried out by developing and running simulation models. The main findings from the comparison show that the CTES concept reduces peak power consumption with 52% and increases hot water production by 27%, with a 10% increase in total energy consumption. 

 

Desalination of water 

 

Membrane distillation (MD) is a desalination technique that consumes a lot of energy compared to reverse osmosis or multiple-effect distillation. Nevertheless, MD requires relatively low-temperature heat that can be recovered at the condenser of a cooling system. Experiments have shown that distilled water production increases with the feed water temperature. The transcritical CO₂ cycle and its ability to produce hot water at a higher temperature at low expense is an asset for MD. A simulation study (6) presents the expected performance of a CO₂ heat pump for simultaneous cooling and desalination. Under certain operating conditions, the production of fresh water is more than doubled compared to heat pumps with traditional refrigerants. The specific energy consumption per cubic meter of distilled water is still higher than with reverse osmosis. However, energy and exergy analyses considering the production of fresh water and cooling show promising research avenues. 

 

Mobile air conditioning 

 

In addition to its promising application for air conditioning in electric vehicles (see the other news on this topic in the Newsletter), CO₂ was found by University of Illinois researchers (7) to be potentially very efficient for air conditioning of buses and passenger rail cars. They designed and constructed a prototype R744 air-conditioning unit for passenger rail cars, with a target capacity of 44 kW, which it was able to achieve, and which can be further increased. A target COP was set at 5% above the COP of the baseline unit, and the R744 unit was able to exceed this target by offering a 16% improvement in COP at the target capacity. Additionally, despite the higher refrigerant pressure and more complex system design, the R744 unit proved to be approximately 5% lighter than the baseline R407C unit.   

 

Sources:

(1) Lawrence N. et al. Advanced R744 technology applied to a multi-temperature refrigerated container: https://iifiir.org/en/fridoc/advanced-r744-technology-applied-to-a-multi-temperature-refrigerated-142907 

(2) Fabris F. et al. Numerical investigation of a CO₂ cooling unit for refrigerating transport operating switching between different configurations: https://iifiir.org/en/fridoc/numerical-investigation-of-a-co-lt-sub-gt-2-lt-sub-gt-cooling-unit-for-142966 

(3) Nawaz K. et al. Performance evaluation of CO₂ HPWH system :  https://iifiir.org/en/fridoc/performance-evaluation-of-co-lt-sub-gt-2-lt-sub-gt-hpwh-system-142961 

(4) Saeed M. Z. et al. Integrated thermal storage and heat recovery of the CO₂ refrigeration system for fishing vessels: https://iifiir.org/en/fridoc/integrated-thermal-storage-and-heat-recovery-of-the-co-lt-sub-gt-2-lt-sub-gt-142990 

(5) Svendsen E. et al. Energy flow analysis of a poultry processing plant: https://iifiir.org/en/fridoc/energy-flow-analysis-of-a-poultry-processing-plant-142994 

(6) Byrne P. et al. Study of a heat pump for simultaneous cooling and desalination using carbon dioxide : https://iifiir.org/en/fridoc/study-of-a-heat-pump-for-simultaneous-cooling-and-desalination-using-142939 

(7) Lawrence N. et al. Improvement of a railway car air conditioning unit by conversion to transcritical R744 technology: https://iifiir.org/en/fridoc/improvement-of-a-railway-car-air-conditioning-unit-by-conversion-to-142906 

All these papers are downloadable in FRIDOC (free of charge for IIR members).