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NOVEL TRANSCRITICAL CO2 HIGH- TEMPERATURE HEAT PUMP ( novel-transcritical-co2-high--temperature-heat-pump )

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Paper ID: 134, Page 2 2015). Additionally, the price of CO2 refrigerant per unit mass is around three times cheaper than HFC blends (R404A and R407A) (Bruno, Belusko & Halawa 2019). According to data released by the Australian government in 2012, the energy consumed for heating water accounts for around 21% of total energy, which produces around 23% of green gas emissions (Chris Riedy 2013). Compared to the electric water heater and gas water heater, a CO2 heat pump water heater can contribute to a significant reduction in energy consumption and is able to provide hot water with a relatively high COP (Hepbasli & Kalinci 2009). In regards to the current CO2 water heaters, it has been investigated by researchers for various purposes of potable water heating (Saikawa & Koyama 2016), sanitary water heating (Tammaro et al. 2017), general water heating (Qi et al. 2013), and high- temperature water heating (White et al. 2002). Also, CO2 heat pump systems with the combinations of water heating and space heating have been developed and investigated (Stene 2005). The reciprocating compressors are normally selected for current CO2 heat pump systems because of their higher compression ratio and better efficiency, compared with turbo and scroll compressors (Kus & Nekså 2013), (Zheng et al. 2020). However, due to the upper temperature and pressure limits of current CO2 reciprocating compressors (140°C and 140 bar for long-term operation), there is limited research focusing on a CO2 high-temperature heat pump with a heat delivery temperature above 100°C. Therefore, the objective of this study is to investigate a method for achieving a high-enough CO2 discharge temperature and generating sufficient heat capacity with the consideration of real component constraints. The present study aims to address the limitations by operating an electrically driven auxiliary heater and employing an IHX to deliver a low-cost steam/water production solution. 2 METHODOLOGY 2.1 Thermodynamics Cycles Until now, four possible thermodynamic cycles (P-h cycles) have been considered to achieve a higher discharge temperature of a transcritical CO2 heat pump system, as shown in Figs. 1 to 4. Although all these cycles are theoretically achievable, they still need to be investigated considering industrial applicability. Theoretically, the simplest way to achieve a targeted discharge temperature (e.g. 180 °C) is to compress CO2 vapour to a very high discharge pressure (e.g. 200 bar), as shown in Fig. 1. Then, the water heating process can be operated during the gas cooling process from state 2 to state 3. However, the high-pressurized CO2 cycle is impractical in industry, due to the upper-pressure limit of compressors, e.g.140 bar is the maximum pressure of CO2 compressors in Bitzer (which is a world- leading manufacturer of refrigeration compressors) (Bitzer 2019). The second proposed thermodynamics cycle is to operate an auxiliary heating device, where the compressor reaches its maximum pressure, i.e. at around 140 bar. Then, the discharge temperature can be further increased from T2 to T2’, e.g. from 110°C to 180°C, as shown in Fig. 2. Although this scenario is practicable in industry, it relies more on auxiliary heater other than CO2 heating, that is, considerable energy is required from the auxiliary input, leading to relatively low heating efficiency. The third proposed cycle is to employ an IHX for transferring a part of rejection heat to superheat the suction CO2 vapour (e.g. to around 45°C at state 1’), as shown in Fig.3. The targeted discharge temperature can be achieved at a lower discharge pressure after the compression process, compared to that of the second thermodynamics cycle. The advantages of this cycle include that: no more extra energy is required for the auxiliary heater and the efficiency can be improved by using an IHX. However, this scenario is still impractical due to the upper-temperature limit of available compressors. After several correspondences with experts at Bitzer in Germany, it has been told that the highest discharge temperature of compressors for the long-term is 140°C, although with only short events the system is able to operate above 140°C but not exceed 160°C. This is for limiting the low frequency of the compressor, as motor cooling is required with overly high superheat. Additionally, it was suggested that it is not sensible to go higher temperature through a compressor since even electric heating or gas heating would be more cost-efficient. Therefore, the final thermodynamic cycle has been proposed by both using an IHX and operating an auxiliary heater, as shown in Fig.4. The system with an IHX is able to reach 140°C of the discharge temperature at relatively low pressure (e.g. 100 bar), followed by auxiliary heating of CO2 to the targeted temperature. Based on this cycle, the CO2 cycle operates within the temperature and pressure range of 6th International Seminar on ORC Power Systems, October 11 - 13, 2021, Munich, Germany



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