Abstract
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.
Highlights
The utilization of low-temperature heat sources for energy conversion required novel technologies; one of the novelty is the use of alternative working fluids for Rankine cycles [1]
In point 3*, the working fluid is in a high-temperature saturated liquid state, while in point 3*, it is in a high-temperature saturated vapour state
Disadvantages using recuperated Organic Rankine Cycles (ORC) over simple Trilateral Flash Cycles (TFC) with very dry working fluid - Need of complex initial heating unit; i.e. higher installation and maintenance costs. - Need of recuperative heat exchanger to recover part of the leftover heat after expansion; i.e. higher installation and maintenance costs
Summary
The utilization of low-temperature heat sources for energy conversion required novel technologies; one of the novelty is the use of alternative working fluids (like organic materials or carbon dioxide) for Rankine cycles [1]. In this case, the maximum for ORC efficiency disappears, while the efficiency curve for TFC can show a small, insignificant maximum [9]. It is important to remark, that in both cases, the shape of the curves would be the same by choosing higher minimal temperature, only the loops would be smaller and the effect would be less remarkable
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
