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The loss of heating load in the second gas cooler was not considered in the calculations for energy consumption. To further improve the SMER of the heat pump-assisted drying, the heat losses to the environment through the second gas cooler could be cut by applying an on/off regulation in case the drying temperature is getting to high due to the surplus heat from the compression work. Generally, the heat transfer properties between CO2 and moist air are a limitation towards the overall system efficiency. The application of intermediate water or glycol sub-cycles between the heat pump and the drying loop could enhance the overall heat transfer and provide more control in adjusting the dryer inlet temperature, with the utilization of water tanks to store heat and cold, respectively. 3 CONCLUSION A dynamic model for the drying chamber was developed in Modelica to demonstrate the potential energy saving for drying processes at common food drying temperatures. The model allows better understanding of the drying characteristics of the heat pump assisted drying system and enables more sophisticated dimensioning and design of future drying systems. The simulation results were successfully validated towards measurement data. The simulations show that the desired drying conditions can be reached with the suggested R744 heat pump system. It has been shown that the utilization of a heat pump to provide heating and cooling for a drying process can increase the energetic efficiency by up to 80 % but results in an increased drying time by 13 – 43 %. Furthermore, a variation of the heat pump system design holds further improvements in efficiency. A bypass for direct recirculation of moist air was investigated to decrease the loads in gas cooler and evaporator, providing energy savings of 28 % but demanding a drying time increase by 24 %. A second evaporator was introduced to control the low pressure and thus the evaporator load and water drain. Further studies are especially required in the overall control of the heat pump system. As indicated, a more sophisticated dynamic control for the bypass mass flow offers great potential for further optimizations during the different phases of the drying process, especially regarding the energy consumption. ACKNOWLEDGEMENTS The work was supported by the Research Council of Norway, grant 286127 – Core Organic Cofund: SusOrgPlus project as part of the ERA-NET action CORE Organic Plus. The authors acknowledge the financial support for this project provided by transnational funding bodies, being partners of the H2020 ERA-net project, CORE Organic Cofund, and the cofund from the European Commission. REFERENCES Atuonwu, J. C., Jin, X., van Straten, G., Deventer Antonius, H. C. v., & van Boxtel, J. B. (2011). "Reducing energy consumption in food drying: Opportunities in desiccant adsorption and other dehumidification strategies", Procedia Food Science, 1, 1799-1805. doi:https://doi.org/10.1016/j.profoo.2011.09.264 Baehr, H.D., K. Stephan (2003). "Wärme- und Stoffübertragung", 7. Auflage, Springer Verlag Chou, S.K., C. Kian Kiang, and Jon, 47 Heat Pump Drying Systems. 2006. Deans, J. (2000). "The modelling of a domestic tumbler dryer" Applied Thermal Engineering 21 (2001), 977-990 Gräber, M., K. Kosowski, C. C. Richter and W. Tegethoff (2009). "Modelling of heat pumps with an object-oriented model library for thermodynamic systems " Proceedings Mathmod 2009, Vienna, Austria: 1396-1404. Perera, C. O., & Rahman, M. S. (1997). Heat pump dehumidifier drying of food. Trends in Food Science & Technology, 8(3), 75-79. doi:https://doi.org/10.1016/S0924-2244(97)01013-3 Strumillo, C., P.L. Jones, and R. Żyłła, Energy Aspects in Drying. 2014. 1075-1101.PDF Image | Design of a CO2 heat pump drier with dynamic modelling tools
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