A novel method proposed to choose the optimal working fluid—solely from the point of view of expansion route—for a given heat source and heat sink (characterized by a maximum and minimum ...temperature). The basis of this method is the novel classification of working fluids using the sequences of their characteristic points on temperature-entropy space. The most suitable existing working fluid can be selected, where an ideal adiabatic (isentropic) expansion step between a given upper and lower temperature is possible in a way, that the initial and final states are both saturated vapour states and the ideal (isentropic) expansion line runs in the superheated (dry) vapour region all along the expansion. Problems related to the presence of droplets or superheated dry steam in the final expansion state can be avoided or minimized by using the working fluid chosen with this method. Results obtained with real materials are compared with those gained with model (van der Waals) fluids; based on the results obtained with model fluids, erroneous experimental data-sets can be pinpointed. Since most of the known working fluids have optimal expansion routes at low temperatures, presently the method is most suitable to choose working fluids for cryogenic cycles, applied for example for heat recovery during LNG-regasification. Some of the materials, however, can be applied in ranges located at relatively higher temperatures, therefore the method can also be applied in some limited manner for the utilization of other low temperature heat sources (like geothermal or waste heat) as well.
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.
The shape of the working fluid’s temperature-entropy saturation boundary has a strong influence, not only on the process parameters and efficiency of the Organic Rankine Cycle, but also on the design ...(the layout) of the equipment. In this paper, working fluids are modelled by the Redlich-Kwong equation of state. It is demonstrated that a limiting isochoric heat capacity might exist between dry and wet fluids. With the Redlich-Kwong equation of state, this limit can be predicted with good accuracy for several fluids, including alkanes.
Maximizing the utilization of an available source serves as the ideal approach, provided that only technical factors are considered. For sources with low heat flux, however, cost‐effective solutions ...are more suitable due to the minimal net power generated, regardless of the effectiveness of the energy conversion. In such cases, utilizing low‐threshold technology may be the most fitting solution, the layout of these cycles should be simple and inexpensive. In the case of organic Rankine cycles (ORCs)‐based power cycles, this means the omission of superheaters or recuperative heat exchangers and the use of simple expanders and small heat exchangers. Simplifying the design, however, requires additional considerations about the elementary steps of the cycle. This work presents a procedure to select favorable working fluids for ORC while considering the expander's internal efficiency. The criteria for favourability is to have a nonideal expansion process starting and ending in (or very near) saturated vapor states to avoid problems related to wetness/dryness between the given maximal and minimal expansion temperatures. It is demonstrated that the design can be simplified under the simultaneous working fluid and expander selection method presented in this study, regardless of the type and isentropic efficiency of the expander. The resulting methodology applies the novel classification of working fluids using the sequences of their characteristic points on temperature‐entropy space. The proposed approach is illustrated with a case study finding optimal working fluid for an ORC system fitted to industrial waste heat, a low‐temperature geothermal, and a cryogenic heat source.
The importance of the saturated vapor line in the temperature‐entropy field is examined. A working fluid and expander selection method is introduced to simplify the layout. Software is developed, and its working principle is discussed. The applicability of the new selection method is demonstrated.
The shape of the temperature vs. specific entropy diagram of a working fluid is very important to understanding the behavior of fluid during the expansion phase of the organic Rankine cycle or ...similar processes. Traditional wet-dry-isentropic classifications of these materials are not sufficient; several materials remain unclassified or misclassified, while materials listed in the same class might show crucial differences. A novel classification, based on the characteristic points of the T–s diagrams was introduced recently, listing eight different classes. In this paper, we present a map of these classes for a model material, namely, the van der Waals fluid in reduced temperature (i.e., reduced molecular degree of freedom) space; the latter quantity is related to the molar isochoric specific heat. Although van der Waals fluid cannot be used to predict material properties quantitatively, the model gives a very good and proper qualitative description. Using this map, some peculiarities related to T–s diagrams of working fluids can be understood.
In the following study, two of the most commonly used and analyzed organic Rankine cycles (ORC), one with a basic set and another one with a regenerative heat exchanger, are investigated at different ...temperature levels below 450 K. The work involved implementing the continuous-molecular targeting approach to computer-aided molecular design in an optimization procedure using thermodynamic modeling software for generating the different ORC configurations. After finding the optimal operating parameters and ideal working fluid for each temperature range with a genetic algorithm, the performances of the cycles are compared. The results show that the most frequently investigated setup when the cycle is extended with a regenerative heat exchanger is not necessarily superior over the basic ORC. Under certain conditions, the simpler and more affordable design could lead to higher performance. Therefore, a thorough examination requires the investigation of both topologies. Also, groups of substances that may become fourth-generation refrigerants in the future due to zero ODP and low GWP were also examined.
Group contribution methods (GCMs) are widely employed across various disciplines to estimate compound properties when experimental data is lacking in the literature. While several methods exist, none ...are comprehensive, as they exhibit gaps either in functional groups or the predicted properties or simply do not offer the required accuracy. As a result, different methods may be necessary to evaluate distinct properties of the same compound. Typically, switching between these models is performed manually due to variations in the sets of functional groups employed. However, with the advancements in computational power and numerical optimum search, there is a growing need to automate the conversion between different group contribution methods. This study presents a procedure to extend the property estimation of the Constantinou and Gani method with vapor pressure by supplementing it with the Tochigi method using an automated group conversion algorithm. The difficulties of automatic conversion procedures, resulting from the differences in the group sets and the shortcomings of the GCMs, are also highlighted. It is also demonstrated that the accuracy of the acentric factor estimation can only be refined to a limited extent by incorporating the Tochigi method, which is, however, indispensable for the several groups where the Constantinou and Gani group contribution values are missing.
Display omitted
Unlike with gases, for liquids and solids the pressure of a system can be not only positive, but also negative, or even zero. Upon isobaric heat exchange (heating or cooling) at p = 0, the volume ...work (p-V) should be zero, assuming the general validity of traditional δW = dWp = −pdV equality. This means that at zero pressure, a special process can be realized; a macroscopic change of volume achieved by isobaric heating/cooling without any work done by the system on its surroundings or by the surroundings on the system. A neologism is proposed for these dWp = 0 (and in general, also for non-trivial δW = 0 and W = 0) processes: “aergiatic” (from Greek: Ἀεργία, “inactivity”). In this way, two phenomenologically similar processes—adiabatic without any heat exchange, and aergiatic without any work—would have matching, but well-distinguishable terms.
Power generation from low-temperature heat sources (80–300 °C) like thermal solar, geothermal, biomass or waste heat has been becoming more and more significant in the last few decades. Organic ...Rankine Cycle (ORC) uses organic working fluids, obtaining higher thermal efficiency than with water used in traditional Rankine Cycles, because of the physical (thermodynamic) properties of these fluids. The traditional classification of pure (one-component) working fluids is based on the quality of the expanded vapour after an isentropic (adiabatic and reversible) expansion from saturated vapour state, and distinguishes merely three categories: wet, dry and isentropic working fluids. The purpose of this paper is to show the deficiencies of this traditional classification and to introduce novel categorisation mostly to help in finding the thermodynamically optimal working fluid for a given heat source.
•The need for a refined working fluid classification (beyond the classical categories wet/isentropic/dry) is demonstrated.•A novel classification based on characteristic points is introduced.•Potential technical applications for the new classification are presented.•Categories and characteristic points for 57 pure working fluids are provided.
•Conditions for working fluids to show wet, dry or isentropic character were described for van der Waals fluid.•Wet-to-dry transition was found by increasing the degree of freedom of the molecules.•A ...limiting isochoric heat capacity (around 45J/molK) was given for isentropic fluids.
Conventional steam power cycles have their limitations on recovering low grade waste heat, therefore other alternatives are required in these cases. Organic Rankine Cycle (ORC) is suitable for power generation based on various heat sources including solar, geothermal, biomass or waste heat. ORC working fluids can be characterized as wet, dry or isentropic. The aim of this paper is to give a method to find novel dry or isentropic working fluids based on simple physical properties, like degree of freedom and isochoric heat capacity.
PDF
