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Organic Rankine cycle integrated hybrid arrangement for power generation
Published in Anoop Kumar Shukla, Onkar Singh, Meeta Sharma, Rakesh Kumar Phanden, J. Paulo Davim, Hybrid Power Cycle Arrangements for Lower Emissions, 2022
Mohammad Bahrami, Fathollah Pourfayaz, Ali Gheibi
As can be seen, ORC are divided into main systems, single pressure level cycle and multi-pressure level cycles, subcritical and supercritical/transcritical cycles are in the group of single pressure level and multi-pressure levels cycles. In addition, there are triangular and complete flash cycles that can be used for solar power applications (Macchi 2017). The thermodynamic properties consist of enthalpy, vapor density, and temperature of the working fluid, which have a great effect on the cycle performance. ORC cycles can be classified into subcritical and transcritical/supercritical cycles based on the operating pressures. The main differences are: (1) both heat addition and heat rejection at subcritical pressures for subcritical cycles, (2) both heat addition and heat rejection at supercritical pressures for supercritical cycles, and (3) heat addition at supercritical pressure and heat rejection at subcritical pressure for transcritical cycles. A simple ORC system and corresponding T-S diagram are shown in Figure 12.2. In a subcritical cycle, the working fluid at the evaporator outlet is in the saturated state and the superheated working fluid at the turbine condensed to a saturated liquid. In the transcritical cycle, the working fluid is compressed and heated to the supercritical state. So there is no change in the fluid state. The evaporator in the transcritical cycle is called a vapor generator (Lecompte et al. 2015).
Improving Process Efficiency by Waste Heat Recuperation
Published in Sheila Devasahayam, Kim Dowling, Manoj K. Mahapatra, Sustainability in the Mineral and Energy Sectors, 2016
Ibrahim A. Sultan, Truong H. Phung, Ali Alhelal
If the highest temperature of a Rankine cycle falls below the critical temperature of the fluid, that cycle is described as ‘subcritical’. As the highest temperature of a cycle approaches the critical temperature, the cycle efficiency increases. This motivated the further increase in highest temperature to exceed the critical value thus producing the transcritical cycle. When both the highest and lowest temperatures of a cycle exceed the critical temperature (Tchanche et al., 2014), the cycle is referred to as ‘supercritical’ as shown in Figure 25.5. The efficiency of the transcritical cycle is known to be up to 40%; however, shifting from subcritical cycle to transcritical cycle results in only about 8% efficiency increase. Such upturn is not significant compared with the requirements of specific materials to manufacture the transcritical cycle components and high operational safety precautions owing to very high pressures. Nevertheless, the supercritical cycle has not been fully investigated with any working fluid (Shengjun et al., 2011; Kim et al., 2012; Tchanche et al., 2014).
The optimum discharge pressure of CO2-based refrigeration cycles operating under subcritical and transcritical conditions
Published in International Journal of Ambient Energy, 2023
Devendra Kumar Sharma, Mihir Mouchum Hazarika, Maddali Ramgopal
The use of CO2 as a refrigerant for refrigeration and air-conditioning applications is increasing as CO2 is non-toxic, non-flammable, environment-friendly and has excellent thermo-physical properties. However, due to its relatively low critical temperature (31.1°C), condensation of CO2 is not possible if the heat sink temperature is higher than 31°C. To improve the performance of the system for operation under high heat sink temperatures, Lorentzen and Pettersen proposed a CO2-based transcritical cycle (Lorentzen and Pettersen 1993). In a transcritical cycle, the condensation process of a conventional subcritical cycle is replaced with a gas cooling process that takes place above the critical point. It was shown by Lorentzen and Pettersen that with suitable cycle modifications and component designs, it is possible to develop CO2-based refrigeration systems that can compete well with systems based on synthetic refrigerants. This has had huge implications because of the ozone depletion and global warming potential of most of the conventionally used synthetic refrigerants. Hence the pioneering work of Lorentzen and Pettersen led to many other studies over the last two decades. A bulk of research has been done in this field, particularly for commercial refrigeration, heat pump water heating systems, mobile air conditioning systems and supermarket refrigeration (Austin and Sumathy 2011; Groll and Kim 2007; Gullo, Hafner, and Banasiak 2018; Kim, Pettersen, and Bullard 2004; Ma, Liu, and Tian 2013; Sarkar 2012).