With on-going developments in the field of trans-critical carbon dioxide (R-744) vapour compression cycles, a need to effectively describe the heat transfer of supercritical carbon dioxide for application in larger diameter tube-in-tube heat exchangers was identified. This study focuses on the in-tube cooling of supercritical carbon dioxide for application in the gas cooler of a trans-critical heat pump. A literature study has revealed Nusselt number correlations specifically developed for the cooling of supercritical carbon dioxide. These correlations were proven to be accurate only for certain operating conditions and tube geometries. A shortcoming identified in the reviewed literature was a generic heat transfer correlation that can be applied over a wide range of fluid conditions for supercritical carbon dioxide cooling. The objective of this study was to compare experimental data obtained from a trans-critical heat pump with different Nusselt number correlations available in literature. The experimental tube diameter used for this study (16mm), was considerably larger than the validated tube diameters used by the researchers who developed Nusselt number correlations specifically for the supercritical cooling of carbon dioxide. The experimental Reynolds number (Re) ranges (350’000 - 680’000) were very high compared to the studies found in the literature (< 300’000), due to the test section from this study forming part of a complete heat pump cycle. Experimental results showed that correlations specifically developed for supercritical carbon dioxide cooling generally over-predicts experimental Nusselt numbers (Nuexp) with an average relative error of 62% to 458% and subsequently also over-predicts the convection heat transfer coefficient. Furthermore, generic heat transfer correlations were compared to the experimental results which over-predicted the Nuexp with an average relative error between 20% and 45% over the entire Re number range. More specifically, the correlation by Dittus & Boelter (1985) correlated with an average relative error of 9% for 350’000 < Re < 550’000. From the results of this study it was concluded that cooling heat transfer of supercritical carbon dioxide in larger tube diameters at higher Re numbers is more accurately predicted by the generic Dittus & Boelter (1985) and Gnielinski (1975) correlations mainly due to the absence of thermo-physical property ratios as seen in the CO2-specific correlations. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2014

With on-going developments in the field of trans-critical carbon dioxide (R-744) vapour compression cycles, a need to effectively describe the heat transfer of supercritical carbon dioxide for application in larger diameter tube-in-tube heat exchangers was identified. This study focuses on the in-tube cooling of supercritical carbon dioxide for application in the gas cooler of a trans-critical heat pump. A literature study has revealed Nusselt number correlations specifically developed for the cooling of supercritical carbon dioxide. These correlations were proven to be accurate only for certain operating conditions and tube geometries. A shortcoming identified in the reviewed literature was a generic heat transfer correlation that can be applied over a wide range of fluid conditions for supercritical carbon dioxide cooling. The objective of this study was to compare experimental data obtained from a trans-critical heat pump with different Nusselt number correlations available in literature. The experimental tube diameter used for this study (16mm), was considerably larger than the validated tube diameters used by the researchers who developed Nusselt number correlations specifically for the supercritical cooling of carbon dioxide. The experimental Reynolds number (Re) ranges (350’000 - 680’000) were very high compared to the studies found in the literature (< 300’000), due to the test section from this study forming part of a complete heat pump cycle. Experimental results showed that correlations specifically developed for supercritical carbon dioxide cooling generally over-predicts experimental Nusselt numbers (Nuexp) with an average relative error of 62% to 458% and subsequently also over-predicts the convection heat transfer coefficient. Furthermore, generic heat transfer correlations were compared to the experimental results which over-predicted the Nuexp with an average relative error between 20% and 45% over the entire Re number range. More specifically, the correlation by Dittus & Boelter (1985) correlated with an average relative error of 9% for 350’000 < Re < 550’000. From the results of this study it was concluded that cooling heat transfer of supercritical carbon dioxide in larger tube diameters at higher Re numbers is more accurately predicted by the generic Dittus & Boelter (1985) and Gnielinski (1975) correlations mainly due to the absence of thermo-physical property ratios as seen in the CO2-specific correlations. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2014

Concern with global climate change has led to the interest in the use of natural refrigerants, such as carbon dioxide, as replacements in heat pump systems. When operating in a transcritical cycle, carbon dioxide heat pumps are well suited for use in high temperature water heating heat pumps. In this work, four systems are analyzed: with and without the use of a suction line heat exchanger, and two water heating schemes. These two schemes involve the heating of water to its desired temperature at a low water flow rate in a single pass through the heat pump, and the heating of water at a high water flow rate, but requiring multiple passes. The performance and resulting heat exchanger size of these four systems is analyzed through the development of an overall system model. This system model uses component-level models that were developed based upon heat exchanger geometry and subsequently validated through experimental testing on a test facility developed for this purpose.

With more restrictions regarding the use of refrigerants with high GWP-values, refrigeration systems that uses R744 (Carbon dioxide) as a refrigerant increases. Supermarkets are energy intense facilities where the refrigeration system utilize a significant part of the total utilized energy. Heat recovery have been proven beneficial and reduces the total energy utilization for the building.  This master thesis evaluates an existing R744 refrigeration system with heat recovery, for the purpose to find an optimal system configuration and control strategy that minimizes the operational costs. Today the refrigeration system uses a water-cooled condenser/gas cooler that during the period from February to November 2021 consumed 8 194 m3 of cooling water, that resulted in high water costs for the supermarket. With the development of a model that takes historical data as input can different system configurations and control strategies be tested and compered to the historical analysis of the existing refrigeration system. Four system configurations with different heat recovery solutions were evaluated.  The result of this study shows that the operational cost can be reduced by 31% (55 400 SEK) with a control strategy that adapts the discharge pressure to the facility’s heating demand. With maximized heat recovery the condenser/gas cooler load decreases and therefore the cooling water flow decreases too. With an additional heat exchanger in the supermarkets ventilation exhaust duct can the water consumption further be reduced, compared with similar system without an additional heat exchanger. The additional heat exchanger reduced the cooling water consumption by 1 400 m3 for the investigated period and reduced the total operational cost by 9 800 SEK. However, it is recommended to first apply the control strategy that adapts the discharge pressure to the heating demand, and from there evaluate the benefits of a heat exchanger in the exhaust duct. / Med ett allt strängare regelverk angående användandet av köldmedier med höga GWP-värden, ökar antalet kylanläggningar som använder R744 (Koldioxid) som köldmedium. Livsmedelsbutiker är energikrävande där kylsystemen är en stor del av den totala energimängd som nyttjas. Studier visar att värmeåtervinning för dessa system kan med fördel användas för att minska energianvändningen i fastigheten.  Detta masterexamensarbete utvärderar en befintlig R744-kylanläggning med värmeåtervinning, i syfte att finna en optimal systemkonfiguration och styr-strategi som minimerar driftkostnaderna. Idag används en vattenkyld kondensor/gaskylare som under perioden februari till november 2021 förbrukade 8 194 m3 kylvatten, vilket resulterade i höga vattenkostnader för butiken. Genom att utveckla en modell som använder historisk mätdata som indata kan olika systemlösningar och styr-strategier testas och jämföras med analysen av det befintliga kylsystemet. Fyra systemkonfigurationer med olika grad av värmeåtervinning utvärderades. Studiens resultat visar att driftkostnaderna kan reduceras genom att anpassa kylsystemens högtryck till fastighetens värmebehov. Genom att maximera värmeåtervinningen minskar nyttjandet av kondensorn/gaskylaren och därmed flödet av kylvatten. Med en extra värmeväxlare i butikens frånluftskanal kan vattenförbrukningen reduceras ytterligare jämfört med liknande system utan denna värmeväxlare. Dock rekommenderas att initialt nyttja en styrstrategi som anpassar högtrycket efter värmebehov, för att sedan utvärdera fördelarna med en värmeväxlare i ventilationens frånluftskanal.