Thermodynamic Selection of the Optimal Working Fluid

orc2019 Tracking Number 156

Using a recently proposed novel working fluid classification scheme based on the entropy-sequences of various characteristic point on the temperature-entropy curves [1], we are proposing a method to select the thermodynamically most suitable working fluid for a given heat source. The maximal and minimal temperatures of the potentially usable ORC are determined by the temperature of the heat source (high-temperature reservoir), the temperature of the environment (low-temperature reservoir) and the pinch point temperature differences. Using these data, we are able to select a working fluid (from a database [2]), with an ideal adiabatic (isentropic) expansion step starting from a saturated vapour state and terminating also in a saturated vapour state (or at least in the vicinity of this state). In this way, one can use the most simple ORC layout, using only a pump, a fluid heater, an evaporator, an expander, and a condenser, avoiding the use of superheater (or droplet separator) and recuperator. Since the fluid is always in dry condition during expansion, droplet erosion can also be avoided. Presently we have a database of 30 pure fluids with T-s data taken from the NIST Chemistry Webbook. Most of these fluids were termed formerly as “dry”, while in the novel classification they are in various isentropic sub-classes, namely in ANCMZ, ACNMZ. ANZCM and ANCZM. From the present working fluid set, one can choose the thermodynamically most suitable cryogenic cycles, but after proper expansion, the database will be available for other temperature ranges (like geothermal or waste heat applications). [1] G. Györke, U. K. Deiters, A. Groniewsky, I. Lassu, A. R. Imre: Novel Classification of Pure Working Fluids for Organic Rankine Cycle, Energy, 145 (2018) 288-300. [2] G. Györke, A. Groniewsky, A. R. Imre: A simple method to find new dry and isentropic working fluids for Organic Rankine Cycle, Energies, 12 (2019) 480.