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(a)T-S chart of carbon dioxide transcritical refrigeration cycle (b) logP-H chart of carbon dioxide transcritical refrigeration cycle (from EES)
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Carbon dioxide transcritical cycles have become more and more investigated during the last decade. For all systems operating with such a cycle, there will be at least one heat exchanger to either heat or cool the supercritical carbon dioxide. Unlike in the sub-critical region, the supercritical carbon dioxide's thermophysical properties will have s...
Context in source publication
Context 1
... the heat rejection pressure, Liao et al. (2000) proposed a correlation to predict the optimum heat rejection pressure in terms of evaporation temperature and the GC's outlet temperature, which is expressed as equation (1) Based on equation (1), the optimum heat rejection pressure for the proposed working condition will be 8.7 Mpa Moreover, a 5 ºC superheat after the evaporator is assumed as a fixed value to ensure that there is no moisture at the compressor inlet. The cycle operating conditions are given in table 1 and the corresponding cycle T-S chart and logP-H chart are also plotted in figure 4. The model implanted in EES shows that under the pre-described working condition, the air temperature after passing the condenser will be 46.3 °C and the supercritical carbon dioxide's temperature at the IHX's inlet will be 36.21 ...
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Citations
Concentrated solar power (CSP) plants represent a viable technology already operational in several locations worldwide. Among the numerous challenges associated with this technology, the proper design of the receiver is arguably one of the most critical. In that sense, the present study proposes using supercritical carbon dioxide (s-CO2) as the working fluid for a cylindrical solar tower receiver. The receiver is modeled at steady state and considers a hybrid formulation that combines a 1-D model for the fluid flow and a 2-D CFD-based model for the conduction heat transfer process within the receiver walls. The analysis parametrically considers the number of plates composing the receiver and the number of channels in each plate, the s-CO2 mass flow rate, and the receiver aspect ratio as independent variables while focusing on the receiver's efficiency as the figure of merit. The analysis also considers two radiation boundary conditions over the receiver surface: (i) an idealized uniformly distributed heat flux and (ii) a real TMY-based spatially distributed radiation heat flux. As expected, the results indicate that the number of plates and the mass flow rate of the cooling fluid highly influence the receiver's efficiency. More interesting, however, is that by assuming a fixed external area for the receiver and parametrically varying the number of plates composing that receiver, it is possible to identify the design that maximizes the receiver's efficiency. The optimal number of plates changes with the receiver height, while the maximized efficiency is lightly sensitive to it. Furthermore, the analysis reveals that the appearance of a maximal value for the receiver's efficiency is associated with a competition between the radiation heat losses and the pumping expenditure needed to move the s-CO2 through the receiver.
The applications of carbon dioxide in heat pump and refrigeration systems are based on a good understanding of its thermophysical properties. Equations of state (EoSs) are largely used to describe the thermophysical behaviors of fluids and can usually give satisfactory results at conditions far from the critical region. However, in the vicinity of the critical point, the long-wavelength of density fluctuations induce an asymptotic singular behavior of fluids, which cannot be described properly by the classical mean-field based EoS. The use of the renormalization group (RG) method with EoSs can correct this nonanalytical behavior near the critical point and not affect the classical behavior far away from the critical region. In this work, an RG method is applied to the Cubic-Plus-Association (CPA) EoS to build a crossover equation. A comprehensive study on thermophysical properties of carbon dioxide, containing vapor-liquid phase equilibrium (VLE) of pure carbon dioxide and its binary mixtures with n-decane and water, compressed density, derivative properties, and transport properties, is performed using this crossover CPA (c-CPA) and the original CPA EoSs to analyze the effects of the RG method on these properties. Our results show that the RG approach can improve the EoS's performances on VLE, compressed density, and some derivative properties such as isobaric heat capacity, while there is no remarkable influence observed on the calculation of transport properties.
