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INTRODUCTION Based on the choice of heat source, heat pumps are often divided into three main categories: ground source, water source and air source heat pumps. The choice of heat source is strongly influenced by regional conditions such as the availability and temperature of the heat source (for instance the air temperatures for air source heat pumps and ground conditions for ground source heat pumps, etc.) Another way to categorize heat pumps could be based on the choice of refrigerants. Heat pumps could again be divided into three categories: heat pumps with pure synthetic working fluids, heat pumps with different types of mixtures and heat pump working entirely with natural working fluids such as CO2, Ammonia or Water. The refrigerant Carbon Dioxide, CO2 was widely used in marine and air conditioning applications during the first decades of the 20th century but were gradually passed-out when the CFC fluids were introduced in the 1930s. Since then CO2 has been “gone” for a long time until the late 1980s, when the search for alternatives started due to the findings concerning ozone depletion and global warming caused by CFC and HFC refrigerants. Ever since Professor Gustav Lorenz proposed the CO2 transcritcal cycle in early 1990s, carbon dioxide (CO2) has been more and more “popular” all over the world [1]. Due to its low critical temperature (31.3 °C), a carbon dioxide refrigeration cycle will, in many applications, be a so-called transcritical cycle. This means that the refrigerant will encounter no sharp phase change on the high temperature side – the condenser in a conventional system becomes a gas cooler of a supercritical fluid. The high critical pressure of CO2 also gives a challenge in designing the heat exchangers and above all the compressors. Due to all these reasons, the introduction of suitable components for CO2 refrigeration systems has to be introduced on the market and mass-produced in the same way as the components for conventional refrigeration systems. This has so far made CO2 heat pumps more expensive and thus not yet so widely spread. Compared to heat pumps using conventional refrigerants, a carbon dioxide heat pump has an advantage when heating cold water with a large temperature glide producing high temperature hot water directly. This is due to the fact that the temperature glide of supercritical carbon dioxide matches the hot water heating process better than conventional refrigerants do, which will lead to less irreversibilities and can give higher COP than conventional heat pumps have (Figure 1) provided the system is designed accordingly. However, this also makes the demand profile of the users a critical factor that strongly influences the performance of a CO2 heat pump. 9PDF Image | CO2 heat pumps for the Swedish market
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CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
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