Improvement of the Methodology for Selecting the Working Fluid and Flash Tank for Organic Rankine Cycle Units

Although the production of units operating on the basis of the organic Rankine cycle (ORC) began in the 1980s, the constant development of refrigeration technology and thermodynamics has led to the use of different working fluids and types of flash tanks. A specific feature of designing the Rankine cycle on organic coolants is the type of their phase curves and the value of the critical and triple points. The correct choice of the working fluid is important for efficient operation of the ORC unit. It should ensure that the maximum efficiency of system operation is achieved and should be stable under various temperature conditions. Optimization of the technique consists in parametric modeling of cycle efficiency and power dependences versus the saturation temperature and the working fluid. As a constraint, to avoid problems associated with the supercritical Rankine cycle caused by the use of liquids with low critical temperatures, it is necessary to make sure that the working fluid critical temperature does not fall below 50°C. Turbines in the ORC system can be radial or axial, depending on the size of the unit, mass flow rate and pressure ratio. Axial turbines are preferred for high mass flow rates and low pressure differences, while radial turbines are suitable for low mass flow rates and high pressure differences. An algorithm for selecting a working fluid for an arbitrary heat source and a procedure for determining the flash tank efficiency are proposed. The influence of the choice of the optimal working fluid on the heat source parameters is shown, and the criterion of complete heat recovery (absorption) is formulated.