An Insight into Organic Rankine Cycle Design

Nowadays, organic Rankine cycles (ORCs) are a widely studied technology. Currently, several research and academic institutions are focused on the design, optimization, and dynamic simulation of this kind of system. Regarding the numerical analysis of an ORC, several steps are required to select the optimal working fluid and the best cycle configuration, taking into account not only nominal performance indexes, but also economic aspects, off-design efficiency, the dynamic behaviour of the plant, and the plant volume or weight.

To begin, a detailed description of the heat source and heat sink, evaluation of all the technical constraints (component selection or plant layout), and both environmental and safety issues is needed. The most significant stage of the design is definitely the correct choice with both working fluid and cycle configuration. Making the wrong choice at this stage will result in poor cycle performance. A huge number of possible working fluids can be selected for ORC systems, which is one of the major advantages of these systems since they can be suitable for almost every heat source but, on the other hand, it makes the resolution of the optimization problem inevitably more complicated.

Organic fluids can be selected from various chemical classes like hydrocarbons, refrigerant fluids and siloxanes. Limiting the choice to well-known fluids, the number is around 100, with fluids differing remarkably from one another. Only a small number of pure fluids are commonly used in commercial systems (mainly toluene, pentane, butane, R134a, R245fa, and light siloxanes) because ORC manufacturers tend to keep the choice among a limited number of well-known fluids already used in previous installations. In addition, organic fluids differ appreciably in molecular weight and molecular complexity and they cover a large range of critical temperatures and critical pressures that can affect the configuration of the cycle and the design of the main components to a great extent.

Organic Rankine Cycle

After the selection of working fluid and plant layout, the system should be optimized according to a certain objective function. This is a crucial step to compare different solutions under varying assumptions. Such optimizations focus on the thermodynamic efficiency of the plant as well as on its economic analysis. From a thermodynamic point of view, this analysis allows one to obtain accurate results even without sophisticated efficiency correlations and it does not require cost functions. However, it is important to emphasize that the thermodynamic optimization may lead to unfeasible results if technological constraints and curves of components efficiency are not used during the plant design. On the other hand, techno-economic optimizations are generally more complex than thermodynamic optimizations because they require the use of reliable cost correlations, while the objective function depends on the application investigated. Different objective functions can be defined depending on the type of heat source and on the assumptions at the basis of the financial evaluation of the investment. For such analyses, information about the equipment cost, the fixed investment cost related to the exploitation of a certain heat source and much more should be known with good accuracy to obtain reliable final results during optimization. In addition to thermodynamic and economic optimization of the ORC, it is important to accurately analyze the off-design performance of the plant since the nominal operating conditions are often quite different from the average ones. Finally, other parameters that could be of an interest include the weight of the ORC and its environmental impact. To deal with all the different aspects of a good ORC design robust tools are needed and SoftInWay has developed AxCYCLE (thermodynamic cycle design) and AxSTREAM (component design) for that purpose. From a general point of view, the optimization of an ORC system may aim towards the minimization of different quantities that usually push the solution in different directions. Using both our tools, the user has the ability to explore different design scenarios from scratch to a complete solution within a very limited time frame.

For those interested in having a look at our tools you have now the opportunity to meet SoftInWay Inc. at the 4th International Seminar on ORC Power Systems in Milano, Italy on the 13th of September, where a paper on the applicability of radial-outflow turbines for a 3 MW WHR Organic Rankine Cycle will be presented! The paper presentation will take place at 5:30PM on Wednesday, the 13th of September in Building 28 on the first floor. Email us to learn more!

Source:

Astolfi, M., Martelli, E. & Pierobon, L. 2016, “Thermodynamic and technoeconomic optimization of Organic Rankine Cycle systems” in Organic Rankine Cycle (ORC) Power Systems: Technologies and Applications, pp. 173-249.

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