Analysis of cold thermal energy storage concepts in CO2 refrigeration systems.

The environmentally friendly refrigerant CO2 becomes more and more popular and is widely used for different applications, such as heat pumps or refrigeration systems, especially in commercial refrigeration. Options to increase efficiency of CO2 refrigeration systems, also under unfavourable boundary conditions as high ambient temperatures, are available. However, these options, as for instance ejectors or parallel compressors, can only increase efficiency in certain operation points. Depending on the boundary conditions, the overall system performance can be affected by the operation in part load. Cold Thermal Energy Storages (CTES) can shift the energy consumption in time and thus help to avoid inefficient part load operation of the refrigeration system. This can potentially lead to a higher overall efficiency of the system. Being able to shift the energy consumption to periods of lower electricity prices can further reduce energy costs. Additionally, they can be used to reduce electrical peak consumption and thus enable the integration of refrigeration systems in smart grids. Furthermore, the downsizing of certain equipment, as for instance compressors, can be an advantage of CTES. There are many kinds of CTES, which can be categorized regarding their size, their working principle (sensible/latent) and integration (central/decentral) into the refrigeration system. This paper describes and analyses various CTES concepts for CO2 refrigeration systems. The performance of the concepts will be assessed via thermodynamic analysis for different boundary conditions. Furthermore, the advantages and shortcomings from a practical point of view will be discussed.