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On high level evaluation and comparison of ORC power generators
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.ORCID iD: 0000-0001-7732-6971
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
2015 (English)In: Proceedings of the 3rd International Seminar on ORC Power Systems, 2015Conference paper, Published paper (Refereed)
Abstract [en]

A review of the thermodynamic performance of ORCs from public, as well as non-public sources hasrevealed a correlation suitable to be used as a rule of thumb for  high-level performance estimation ofORC power generators. Using the correlation, the limited amount of available test data can begeneralised leading to a high level evaluation of the commercial benefits of any potential applicationfor ORCs.Power generators using ORC-technology exist in relatively low numbers. Furthermore, fieldinstallations seldom imply comparable boundary conditions. As ORCs generally  operate at lowtemperature differences between source and sink it has been shown that their relative sensitivity tovariations in temperatures i.e. the finiteness of source- and sink, is larger than the sensitivity of powergenerators operating with large temperature differences. Therefore the establishing of practical rule ofthumb performance estimation, similar to the figure of merit, Coefficient of Performance, COP, asused in refrigeration and air conditioning industry, has previously not been successful.In order to arrange field data in a manner suitable for comparison a refinement of suitable figures ofmerit was required. The suggested, refined terms are presented and explained as well as criticallyevaluated against the most common  efficiency terms traditionally used.The current lack of a performance rule of thumb leaves room for less serious vendors and laymen tomake performance claims unrealistic to practical achievements. Scrutinizing such questionablestatements requires detail process simulations and a multitude of technical assumptions. Henceargumentation becomes ineffective. If a suitable rule of thumb can be established argumentationagainst dubious claims would become significantly more forceful.This paper suggests a new term to be used as rule of thumb and explains a  method on how to use it.

Place, publisher, year, edition, pages
2015.
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-188016ISBN: 978-2-9600059-2-9 (print)OAI: oai:DiVA.org:kth-188016DiVA: diva2:933112
Conference
3rd International Seminar on ORC Power Systems, October 12-14, 2015, Brussels, Belgium
Note

QC 20160607

Available from: 2016-06-03 Created: 2016-06-03 Last updated: 2017-06-08Bibliographically approved
In thesis
1. Low temperature difference power systems and implications of multi-phase screw expanders in Organic Rankine Cycles
Open this publication in new window or tab >>Low temperature difference power systems and implications of multi-phase screw expanders in Organic Rankine Cycles
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

New and old data on screw expanders operating with 2-phase mixtures in the admission line has been combined to enable the first public correlation of adiabatic expansion efficiency as a function of entry vapour fraction. Although not yet perfected, these findings have enabled an entirely new approach to the design and optimisation of Organic Rankine Cycles, ORCs. By allowing a continuous variation of vapour fraction at expander entry optima for thermal efficiency, second law efficiency and cost efficiency can be found. Consequently one can also find maxima for power output in the same dimension.

This research describes a means of adapting cycle characteristics to various heat sources by varying expander inlet conditions from pure liquid expansion, through mixed fluid and saturated gas expansion, to superheated gas. Thermodynamic analysis and comparison of the above optimisations were a challenge. As most terms of merit for power cycles have been developed for high temperature applications they are often simplified by assuming infinite heat sinks. In many cases they also require specific assumptions on e.g. pinch temperatures, saturation conditions, critical temperatures etc, making accurate systematic comparison between cycles difficult. As low temperature power cycles are more sensitive to the ‘finiteness’ of source and sink than those operating with high temperatures, a substantial need arises for an investigation on which term of merit to use.

Along with an investigation on terms of merit, the definition of high level reversible reference also needed revision. Second law efficiency, in the form of exergy efficiency, turned out to be impractical and of little use. A numerical approach, based on a combination of first and second law, was developed. A theory and method for the above is described. Eventually low temperature power cycle test data was compiled systematically. Despite differences in fluid, cycle, temperature levels and power levels the data correlated well enough to allow for a generalised, rough correlation on which thermal efficiency to expect as a function of utilization of source and sink availability. The correlation on thermal efficiency was used to create a graphical method to pre-estimate key economic factors for low temperature site potential in a very simple manner. A major consequence from the findings of this thesis is the reduced dependency on unique choices of process fluid to match heat source characteristics. This development significantly simplifies industrial standardisation, and thereby potentially improves cost efficiency of commercial ORC power generators.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. viii, 98 p.
Series
TRITA-REFR, ISSN 1102-0245 ; 15/02
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-188015 (URN)978-91-7595-872-9 (ISBN)
Public defence
2016-09-02, Hörsal M3, Brinellvägen 64, KTH Campus, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2016-06-09 Created: 2016-06-03 Last updated: 2017-04-25Bibliographically approved

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Öhman, Henrik

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