Impact of Thermal non-equilibrium phenomena during two phase expansion
Xander van Heule; Michel De Paepe; Steven Lecompte
- Publisher
- Zenodo
- Language
- en
Abstract
Today, climate change is one of the biggest issues facing humanity. One method to reduce its effects is to utilise a larger amount of the total used primary energy. One well-known method is the utilisation of waste heat by an organic Rankine cycle (ORC). However, the efficiency of this method is reduced when lower-temperature heat sources are used. A Promising variant of the ORC for the application in low-temperature heat recovery is the trilateral flash cycle (TFC) and the partially evaporating organic Rankine cycle (PEORC). Both these cycles require an expansion machine that is capable of expanding a two-phase liquid-vapour mixture. For this, volumetric machines lend themselves naturally compared to turbomachines. However, the behaviour of these volumetric two-phase expansion machines has not been broadly studied yet. It has been shown that thermodynamic non-equilibrium phenomena occur within these expansion machines. It was also shown that these effects introduce additional losses which have been unaccounted for in studies of the TLC and PEORC. In previous work, the authors modelled these non-equilibrium expansion effects within a free linear reciprocating expander. The model predicts a reduction of the indicated work of an expansion stroke by around 20% when this stroke occurs over a duration of 0.1 seconds compared to a duration of 1 second. This work also tested these same expansion profiles, but experimentally. The same expansion profiles and durations show a reduction of around 25% experimentally compared to the 20% of the model. This shows that the model is capable of predicting the two-phase expansion behaviour. However, the empirical parameters within the model will have to be slightly adapted.
Full text
A lot of wasted heat potential Recuperating waste heat •Heat recovery through the organic Rankine cycle (ORC) is possible •But the ORC loses performance for low temperature heat sources, which is the largest share of waste heat •Variants of the ORC for the low temperature heat sources are needed •Trilateral cycle (TLC) and partially evaporating ORC (PEORC) are possible •These obtain higher second law efficiencies than the basic ORC •They both require a two-phase expansion machine Non-equilibrium phenomena Experimental setup Indicated work •Lower pressure during expansion will reduce the indicated work •The modelling results showed a 20% reduction in work for the same movement profile when comparing an expansion time of 0,1 seconds to an expansion time of 1 second •The experiments showed a 25% reduction in work for the same conditions •The model is capable of describing the phenomenon, but it still slightly underpredicts the non-equilibrium losses IMPACT OF THERMAL NON-EQUILIBRIUM PHENOMENA DURING TWO PHASE EXPANSION SUSTAINABLE THERMO-FLUID ENERGY SYSTEMS (STFES) Xander van Heule, Michel De Paepe, Steven Lecompte Contact [email protected]e https://www.linkedin.com/in/xander-van-heule Funded by Foundation-Flanders through FWO-Flanders grants 1SD9723N Conclusions •Waste heat recovery is a necessary measure to limit GHG emissions •The PEORC is a promising technology to utilise low temperature waste heat • Non-equilibrium effects occur within two-phase mixtures •The thermal non-equilibrium effects have a significant impact •The experiments show a reduction on the work of around 25% compared to a prediction of around 20% by the model. •Metastable conditions occur if the process is fast enough •Superheated liquid (point B) during two-phase expansion •Corresponds to delayed evaporation to return to thermodynamic stability •The delay in evaporation results in lower pressure during the expansion •This phenomenon is typically not investigated in literature but was added in a model via the HRM Experimental: 25% reduction Model: 20% reduction
