Application of volumetric expanders in small vapour power plants used in distributed energy generation – Selected design and thermodynamic issues

Energy Science & Engineering

2022

Efficiency is a crucial factor for power cycles. Generally, in the same temperature range, the cycle efficiency for a basic organic Rankine cycle (ORC) is higher than a similar trilateral flash cycle (TFC) for almost all working fluids and heat source/heat sink temperature pairs. According to some recent results, by using super dry working fluids, the thermal efficiency of TFC can outperform its ORC counterpart at high temperatures. This study performed the thermodynamics analysis using three dry working fluids with subcritical basic ORC and TFC with zero to partial recuperated heat ratio. The results showed that using recuperative heat exchange effectively contributes to increasing the efficiency of ORC and TFC. However, the rate of efficiency increase in ORC is higher than the TFC, which can be clearly seen, especially at high maximal cycle temperatures. Using super dry working fluids, the recuperator‐free TFC has favorable cycle efficiency at high heat source temperatures. In contrast, by adding heat recovery for both systems, the ORC can outperform the TFC at 0.4, 0.2, and 0.1 recuperated heat effectiveness for Dodecane, Decane, and Nonane, respectively.

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Investigation of thermal efficiency for subcritical ORC and TFC using super dry working fluids

Ahmed

Energy Science & Engineering

2022

“…Preferences may change the key factors slightly, yet fundamental properties influencing the outcomes are easily identifiable, regardless of the circumstances. Even though the optimum behavior of an ORC is not directly connected to the isentropic behavior of the fluids, 15 the shape of the saturated vapor line of the selected fluid in the T–s diagram plays an important role 16 when the purpose is to maximize the power output or when cycle efficiency is to be improved 17,18 . Namely directly affecting the layout and indirectly affecting the investment and operational costs.…”

Section: Introductionmentioning

confidence: 99%

Simultaneous working fluid and expander selection method for reaching low‐threshold technology organic Rankine cycle (ORC) design

Groniewsky

Kustán

Energy Science & Engineering

2023

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.

“…-efficiency is higher (see later); -due to the evaporation (between points 3* to 3), the heat input is higher and therefore, power output is also higher (even with equal efficiency); -for the realization of a TFC cycle, a very special expander is needed because of the very high fraction of droplets (especially in the early stage of the expansion); this can be, for example, a volumetric expander [7,8]. Concerning the efficiency, one can realize very easily, that by using wet (ACZ-type) working fluid, the efficiency of ORC is higher than the efficiency of TFC.…”

Section: Introductionmentioning

confidence: 99%

The effect of recuperator on the efficiency of ORC and TFC with very dry working fluid

Ahmed

2021

MATEC Web Conf.

Organic Rankine Cycles (ORC) and Trilateral Flash Cycles (TFC) are very similar power cycles; ideally, they have a reversible adiabatic (isentropic) compression, an isobaric heating, an isentropic expansion and an isobaric cooling. The main difference is that for ORC, the heating includes the full evaporation of the working fluid (prior expansion); therefore, the expansion starts in a saturated or dry vapour state, while for TFC, the heating terminates upon reaching the saturated liquid states. Therefore, for TFT, expansion liquid/vapour state (in bubbly liquid or in vapour dispersed with droplets), requiring a special two-phase expander. Being ORC a more “complete” cycle, one would expect that its thermodynamic efficiency is always higher than for a TFC, between the same temperatures and using the same working fluids. Surprisingly, it was shown that for very dry working fluids, the efficiency of TFC can exceed the efficiency of basic (i.e. recuperator- and superheater-free) ORC, choosing sufficiently high (but still subcritical) maximal cycle temperature. Therefore in these cases, TFC (having a simpler heat exchange unit for heating) can be a better choice than ORC. The presence of a recuperator can influence the situation; by recovering the proper percentage of the remaining heat (after the expansion), the efficiency of ORC can reach and even pass the efficiency of TFC.