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Organic Rankine Cycles with variable vapour fraction expansion entry: Reduced sensitivity to choice of working fluid in modified Organic Rankine Cycles by using wet vapour expansion entry conditions
KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.ORCID-id: 0000-0001-7732-6971
KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
2014 (Engelska)Rapport (Övrigt vetenskapligt)
Abstract [en]

The task of reducing global carbon dioxide emissions leads to a need to reduce the average CO2-emission in power generation. A more energy efficient mix of power generation on national, or regional level, will require the re-use of waste heat and use of primary, low temperature heat for power generation purposes. Low Temperature Power Cycles, such as Organic Rankine Cycles, Trilateral Flash Cycles, Kalina Cycles offer a large degree of freedom in finding technical solutions for such power generation.

Theoretical understanding of LTPC’s advance rapidly though practical achievements in the field show very humble improvements at a first glance. Cost of applying the new knowledge in real applications seems to be an important reason for the discrepancy. One central reason for the high cost level is the diversity of process fluids required and consequently the lack of standardization and industrialization of equipment. Uses of supercritical power cycle technology tend to cause the same dilemma. Furthermore upcoming regulations prohibiting the use of several process fluids tend to lead to remedies increasing plant cost.

By using 2-phase, variable vapour fraction, expansion inlet conditions the need to use many different process fluids is reduced, allowing simpler and more cost efficient LTPC’s by easier matching with heat source temperature characteristics. This article explores some of the associated effects on cycle output and cost efficiency. A waste heat recovery application is investigated simulating cost efficiency, thermodynamic efficiencies and power generation while using fundamentally different working fluids, lumped component efficiencies, variable utilization of the waste heat and optimisation on expansion inlet vapour fraction.

The conclusion made is that the sensitivity to choice of working fluid is lower than intuitively anticipated, in contrast to common consensus in science. Furthermore it is shown that exceptional component efficiencies are not required in order to achieve a performance comparable to current practise and that a good business case is possible under the assumed economic conditions.

Ort, förlag, år, upplaga, sidor
KTH Royal Institute of Technology, 2014. , s. 26
Serie
TRITA-REFR REPORT 14:2
Nationell ämneskategori
Energiteknik
Forskningsämne
Energiteknik
Identifikatorer
URN: urn:nbn:se:kth:diva-188002ISBN: 978-91-7595-224-6 (tryckt)OAI: oai:DiVA.org:kth-188002DiVA, id: diva2:932994
Anmärkning

QC 20160603

Tillgänglig från: 2016-06-02 Skapad: 2016-06-02 Senast uppdaterad: 2016-11-30Bibliografiskt granskad
Ingår i avhandling
1. Low temperature difference power systems and implications of multi-phase screw expanders in Organic Rankine Cycles
Öppna denna publikation i ny flik eller fönster >>Low temperature difference power systems and implications of multi-phase screw expanders in Organic Rankine Cycles
2016 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: KTH Royal Institute of Technology, 2016. s. viii, 98
Serie
TRITA-REFR, ISSN 1102-0245 ; 15/02
Nationell ämneskategori
Energiteknik
Forskningsämne
Energiteknik
Identifikatorer
urn:nbn:se:kth:diva-188015 (URN)978-91-7595-872-9 (ISBN)
Disputation
2016-09-02, Hörsal M3, Brinellvägen 64, KTH Campus, Stockholm, 10:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2016-06-09 Skapad: 2016-06-03 Senast uppdaterad: 2017-04-25Bibliografiskt granskad

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