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Lundqvist, P. & Öhman, H. (2017). Global Efficiency of Heat Engines and Heat Pumps with Non-Linear Boundary Conditions. Entropy, 19(8), Article ID 394.
Open this publication in new window or tab >>Global Efficiency of Heat Engines and Heat Pumps with Non-Linear Boundary Conditions
2017 (English)In: Entropy, ISSN 1099-4300, E-ISSN 1099-4300, Vol. 19, no 8, article id 394Article in journal (Refereed) Published
Abstract [en]

Analysis of global energy efficiency of thermal systems is of practical importance for a number of reasons. Cycles and processes used in thermal systems exist in very different configurations, making comparison difficult if specific models are required to analyze specific thermal systems. Thermal systems with small temperature differences between a hot side and a cold side also suffer from difficulties due to heat transfer pinch point effects. Such pinch points are consequences of thermal systems design and must therefore be integrated in the global evaluation. In optimizing thermal systems, detailed entropy generation analysis is suitable to identify performance losses caused by cycle components. In plant analysis, a similar logic applies with the difference that the thermal system is then only a component, often industrially standardized. This article presents how a thermodynamic "black box" method for defining and comparing thermal efficiency of different size and types of heat engines can be extended to also compare heat pumps of different apparent magnitude and type. Impact of a non-linear boundary condition on reversible thermal efficiency is exemplified and a correlation of average real heat engine efficiencies is discussed in the light of linear and non-linear boundary conditions.

Place, publisher, year, edition, pages
MDPI, 2017
National Category
Environmental Engineering
Identifiers
urn:nbn:se:kth:diva-214514 (URN)10.3390/e19080394 (DOI)000408739700016 ()2-s2.0-85028512889 (Scopus ID)
Note

QC 20170929

Available from: 2017-09-29 Created: 2017-09-29 Last updated: 2017-11-29Bibliographically approved
Öhman, H. (2016). Low temperature difference power systems and implications of multi-phase screw expanders in Organic Rankine Cycles. (Doctoral dissertation). Stockholm: KTH Royal Institute of Technology
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. p. viii, 98
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
Öhman, H. & Lundqvist, P. (2015). On high level evaluation and comparison of ORC power generators. In: Proceedings of the 3rd International Seminar on ORC Power Systems: . Paper presented at 3rd International Seminar on ORC Power Systems, October 12-14, 2015, Brussels, Belgium.
Open this publication in new window or tab >>On high level evaluation and comparison of ORC power generators
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.

National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-188016 (URN)978-2-9600059-2-9 (ISBN)
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
Öhman, H. & Lundqvist, P. (2015). Screw expanders in ORC applications, review and a new perspective. In: Proceedings of the 3rd International Seminar on ORC Power Systems: . Paper presented at 3rd International Seminar on ORC Power Systems, October 12-14, 2015, Brussels, Belgium.
Open this publication in new window or tab >>Screw expanders in ORC applications, review and a new perspective
2015 (English)In: Proceedings of the 3rd International Seminar on ORC Power Systems, 2015Conference paper, Published paper (Refereed)
Abstract [en]

Performance of Organic Rankine Cycles is sensitive not only to the entry  temperature ratio betweenheat source and heat sink but also to the temperature  degradation of the heat source flow, caused bythe heat transfer to the process in pre-heater, evaporator and super heater. In order to adopt the cycleto the great variety of heat sources a multitude of fluids are required. Alternatively fluid mixtures,trans-critical or supercritical fluid conditions can be used to match the process temperatures with theheat source. Screw expanders offer an alternative, new approach to the matching problem of ORC’s asthey allow for flexible multi-phase expansion. Hereby the vapour fraction at the expander entry can beused to  partially match the temperatures of the process to a particular heat source. To provide aperspective on the use of such screw expanders in ORC-systems previous experimental andcommercial experience have been reviewed and discussed.Screw expanders are versatile machines used for the production of mechanical work in power rangesfrom 3kW to 1.5MW. As the functional characteristics differ significantly  from dynamic expandersthe explanatory models used to generalise results are different. Plenty of research has resulted in wellgeneralized explanatory models for dynamic expander analysis. For screw expanders similarexplanatory models exist mainly in commercially confidential environments. A few public sourcesdisclose test data. In the few cases data has been investigated the analyses tend to rely onthermodynamic models suitable for dry gas expansion. Typically that leads to reasonable replicationof test results but seldom to models suitable for detailed understanding of the process. In applicationswith 2-phase expansion the theories used to simulate functional characteristics is entirely insufficient.The main reason for  the scarcity of work in this field is probably the empirical difficulties in obtaininggood measuring data in multi-phase conditions.This paper describes a review of multi-phase screw expander experiences and explains why a uniquetheory is required to model its characteristics. In the absence of such a unique theory a correlationbased in empirical data is presented. This allow for estimations of screw expander efficiency in multi-phase conditions. Measured efficiency with dry expansion, or such efficiency simulated, can be usedto estimate adiabatic efficiency with expansion entry vapor fractions ranging from 0 to 1 by using thiscorrelation. Hence estimating expansion efficiency during multi-phase expansion is simplified,allowing for better optimisation of the ORC-systems. This way a new perspective of screw expanderpotential in ORC system integration can be presented.

National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-188018 (URN)978-2-9600059-2-9 (ISBN)
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
Öhman, H. & Per, L. (2014). 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 Royal Institute of Technology
Open this publication in new window or tab >>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
2014 (English)Report (Other academic)
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.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2014. p. 26
Series
TRITA-REFR REPORT 14:2
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-188002 (URN)978-91-7595-224-6 (ISBN)
Note

QC 20160603

Available from: 2016-06-02 Created: 2016-06-02 Last updated: 2016-11-30Bibliographically approved
Öhman, H. & Per, L. (2014). Thermodynamic pre-determination of power generation potential in geothermal low-temperature applications. Geothermal Energy, 2(1), 1-10, Article ID 4.
Open this publication in new window or tab >>Thermodynamic pre-determination of power generation potential in geothermal low-temperature applications
2014 (English)In: Geothermal Energy, ISSN 2195-9706, Vol. 2, no 1, p. 1-10, article id 4Article in journal (Refereed) Published
Abstract [en]

Background: Small capacity, low temperature, geothermal heat sources providesignificant opportunities for distributed, small scale power generation. Projectdefinitions and pre-dimensioning however require advanced thermodynamicengineering at a cost independent of project size. In order to reduce this cost, anew method has been developed to allow basic level engineers to performpre-optimisation of thermodynamic potential as well as expected performance usingavailable power plant technology at such pre-optimized conditions.Results: By reducing the complexity of second-law computations, a simpletwo-dimensional diagram is shown representing the dimensioning criteria requiredfor maximum power generation using the particular heat source and sink whileconsidering expected power plant performance, using systems and componentson the market.Methods: By sensitivity analysis the combination of thermodynamic analysis and realworld data correlations was simplified and arranged for pre-dimensioning ofbusiness cases.Conclusions: Optimal pre-dimensioning of power generation system for anygeothermal heat source, with a defined heat sink, can be determined withoutadvanced thermodynamic expertise. This reduces the cost for business case proposals,pre-dimensioning and tender specifications of small-scale power generation systemsfor low temperature heat sources. The intended implication of this work is to increasethe use of low-temperature geothermal wells for distributed power generation.

Place, publisher, year, edition, pages
Springer, 2014
Keywords
ORC, Kalina, Waste heat, Power cycle, Geothermal, Performance
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-188003 (URN)10.1186/s40517-014-0004-2 (DOI)2-s2.0-84942116253 (Scopus ID)
Note

QC 20160603

Available from: 2016-06-02 Created: 2016-06-02 Last updated: 2017-01-11Bibliographically approved
Öhman, H. (2011). Implementation and evaluation of low temperature waste heat recovery power cycle using NH3 in an organic rankine cycle. In: Zvonimir Guzovic (Ed.), : . Paper presented at Sustainable development if energy, water and environment systems / 2011 / Croatia, SDEWS-11, 25-29 September. Zagreb: Faculty of Mechanical Engineering and Naval Architecture
Open this publication in new window or tab >>Implementation and evaluation of low temperature waste heat recovery power cycle using NH3 in an organic rankine cycle
2011 (English)In: / [ed] Zvonimir Guzovic, Zagreb: Faculty of Mechanical Engineering and Naval Architecture , 2011Conference paper, Published paper (Refereed)
Abstract [en]

With increasing cost for power generation the opportunities for small scale powergeneration from waste heat have increased. The awareness of untapped resources such aslocal waste heat streams as well as the available range of technology and products toharvest such streams is increasing steadily. For ORC power plant applications the numberof open parameters is large though the reported field data is limited, particularly for lowtemperature waste heat recovery.This paper presents field data and a performance analysis of an ORC powergeneration plant operating with NH3 as media. The ORC unit operates on waste heat froma Swedish pulp mill at an available temperature level of 75 to 85degC. Performance at lowwaste heat temperatures and during capacity variation is reported as well as an analysisof the particular investment case.The field data was generated by remote logging of control system information during a 15day period.The results show a thermal efficiency of 8 to 9% during a capacity range of 50 to 100%power generation. The results indicate a flat thermal efficiency curve from 20 to 100%power generation. The investment case is a supplier own-and-operate type of arrangement supplying thepulp mill with electric power at a predefined cost during a long period of time.

Place, publisher, year, edition, pages
Zagreb: Faculty of Mechanical Engineering and Naval Architecture, 2011
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-188001 (URN)
Conference
Sustainable development if energy, water and environment systems / 2011 / Croatia, SDEWS-11, 25-29 September
Note

QC 20160603

Available from: 2016-06-02 Created: 2016-06-02 Last updated: 2016-06-09Bibliographically approved
Öhman, H. (2004). Test Results of A Screw Type Expander/. Compressor and the Implication of Phase. Separators on the Refrigeration Process. In: 2004 International Refrigeration and Air Conditioning Conference: . Paper presented at International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004. West Lafayette, Indiana, USA: Purdue University Press
Open this publication in new window or tab >>Test Results of A Screw Type Expander/. Compressor and the Implication of Phase. Separators on the Refrigeration Process
2004 (English)In: 2004 International Refrigeration and Air Conditioning Conference, West Lafayette, Indiana, USA: Purdue University Press, 2004Conference paper, Published paper (Refereed)
Abstract [en]

Running a refrigeration cycle at saturated liquid conditions at the evaporator inlet has a few interesting advantages in comparison to the conventional 2-phase situation. Opportunities in using novelty heat exchangers, increased cycle efficiency and significantly reduced main compressor sizes occur. Using the two rotor integrated

compressor/expander idea developed by Olofsson (1993) in a refrigeration cycle theoretically offer the possibility of pure liquid evaporator inlet conditions, the device is called Phase Separator. This paper reports results from hardware tests as well as some theoretical results. To evaluate the efficiency of a Phase Separator three terms of efficiency are suggested. The test results prove that pure liquid conditions to the evaporator are obtainable with a practical Phase Separator.

Place, publisher, year, edition, pages
West Lafayette, Indiana, USA: Purdue University Press, 2004
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-188000 (URN)
Conference
International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004
Note

QC 20160607

Available from: 2016-06-02 Created: 2016-06-02 Last updated: 2016-06-07Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-7732-6971

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