Abstract
In the last decade, particular attention has been paid to the organic Rankine cycle (ORC) power plant, a technology that implements a classical steam Rankine cycle using low-boiling fluid of organic origin. Depending on the specific application and the choice of the designer, the ORC can be optimized using one or several criteria. The selected objectives reflect various system performance aspects, such as: thermodynamic, economic, environmental or other. In this study, a novel criterion called exergy utilization index (XUI) is defined and used to maximize the utilization of an energy source in the ORC system. The maximization of the proposed indicator is equivalent to bring the heat carrier outlet temperature to the ambient temperature as close as possible. In the studied case, the XUI is applied along with the total heat transfer area of the system, and the multi-objective optimization is performed in order to determine the optimal operating conditions of the ORC. Moreover, to reveal a relationship between the XUI and important ORC performance indicators, a parametric study is conducted. Based on the results, it has been found that high values of the XUI (~80%) correspond to optimal values of exergy-based indicators such as: exergy efficiency, waste exergy ratio, environmental effect factor or exergetic sustainability index. Furthermore, the values of the XUI = 60%–80% are associated with beneficial economic characteristics reflected in a low payback period (<11.3 years). When considering the ecological aspect, the maximization of XUI has resulted in minimization of exergy waste to the environment. In general, the simple formulation and straightforward meaning make the XUI a particularly useful indicator for the preliminary evaluation and design of the ORC. Furthermore, the comparative analysis with respect to other well-known performance indicators has shown that it has a potential to be successfully applied as the objective function in the optimization of ORC power plants.
Highlights
The global increase in the demand for electricity necessitates the search for alternative energy sources and the development of technologies that can efficiently convert this type of energy into electricity
The maximization of the energy utilization in the organic Rankine cycle (ORC) is inseparably connected with minimizing the outlet temperature ths2 of the heat carrier
The exergy utilization index XUI was analysed by comparing its relationships with selected ORC parameters and performance indicators
Summary
The global increase in the demand for electricity necessitates the search for alternative energy sources and the development of technologies that can efficiently convert this type of energy into electricity. One way to characterize an energy source for the ORC is to classify it as one of the two types: closed (sealed) or open [1]. The first type is characterized by a constant temperature of the heat carrier exiting the vapor generator of the ORC. This occurs in applications in which the heat carrier is circulating in a closed loop and it must have a predefined temperature at the outlet of the heat exchanger.
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