A Simple Method of Finding New Dry and Isentropic Working Fluids for Organic Rankine Cycle

Abstract

One of the most crucial challenges of sustainable development is the use of low-temperature heat sources (60–200 °C), such as thermal solar, geothermal, biomass, or waste heat, for electricity production. Since conventional water-based thermodynamic cycles are not suitable in this temperature range or at least operate with very low efficiency, other working fluids need to be applied. Organic Rankine Cycle (ORC) uses organic working fluids, which results in higher thermal efficiency for low-temperature heat sources. Traditionally, new working fluids are found using a trial-and-error procedure through experience among chemically similar materials. This approach, however, carries a high risk of excluding the ideal working fluid. Therefore, a new method and a simple rule of thumb—based on a correlation related to molar isochoric specific heat capacity of saturated vapor states—were developed. With the application of this thumb rule, novel isentropic and dry working fluids can be found applicable for given low-temperature heat sources. Additionally, the importance of molar quantities—usually ignored by energy engineers—was demonstrated.

Thermodynamic Selection of the Optimal Working Fluid for Organic Rankine Cycles

“…Some of these real isentropic fluids (characterized by five-letter sequences) were considered formerly as dry ones, being the point N well below ambient temperatures. Using this classification, one can easily correlate simple material properties like molar volume [12] or molar isochoric heat capacity [14] with the classification, i.e., separate wet and dry working fluids. Also, it can be used easily to study the behaviour of working fluids during the expansion process of ORC and other similar cycles [17].…”

“…Using our method, for a given heat source-heat sink pair one can select a real, one-component working fluid from a database [14], with an ideal adiabatic (isentropic) expansion process starting from a saturated vapour state and terminating also in a saturated vapour state (or at least in the vicinity of this state), utilizing the "belly" of the reverse S-shaped saturated vapour branches of various materials. In this way, one can use the simplest ORC layout, consisting of only a pump, two heat exchangers (evaporator, condenser) and an expander, avoiding the use of superheater or droplet separator and a recuperative heat exchanger (recuperator).…”

Approximating the Temperature–Entropy Saturation Curve of ORC Working Fluids From the Ideal Gas Isobaric Heat Capacity

“…which are expected to yield good results for the entropies of the saturated vapor and liquid in the range 0.6 < T r <0.99. Equations (8) and (9) require the parameter b as an input. Differentiating Equation (8) with respect to T r one has…”

“…Finally, fluids like the refrigerants RE143a, R11, or R116 present a wide range of temperatures for which the saturated vapor curve is almost vertical. These fluids are usually termed as isentropic, although isentropic behavior can also be obtained with a particular class of dry fluids [8][9][10]. To summarize, the slope of the vapor branch of the T-s saturation curve gives rise to a basic classification of working fluids into three categories: wet, dry, and isentropic.…”

Performance and economic investigation of a combined phosphoric acid fuel cell/organic Rankine cycle/electrolyzer system for sulfuric acid production; Energy‐based organic fluid selection

“…Three categories of the WFs (isentropic, wet, and dry) are used in the ORC systems . The organic fluids enable the ORCs to use the recovered heat from lower temperature sources (biomass combustion, low‐temperature FCs, or industrial WHs . The following literature shows the previous studies that have been performed on the application of ORC, FC, and electrolyzers for producing electricity, heating energy, cooling energy, hydrogen, and freshwater.…”