| 32621. |
- Larsen, Ulrik, 1972, et al.
(författare)
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Design and optimisation of organic Rankine cycles for waste heat recovery in marine applications using the principles of natural selection
- 2013
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Ingår i: Energy. - : Elsevier BV. - 0360-5442 .- 1873-6785. ; 55, s. 803-812
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Tidskriftsartikel (refereegranskat)abstract
- Power cycles using alternative working fluids are currently receiving significant attention. Selection of working fluid among many candidates is a key topic and guidelines have been presented. A general problem is that the selection is based on numerous criteria, such as thermodynamic performance, boundary conditions, hazard levels and environmental concerns. A generally applicable methodology, based on the principles of natural selection, is presented and used to determine the optimum working fluid, boiler pressure and Rankine cycle process layout for scenarios related to marine engine heat recovery. Included in the solution domain are 109 fluids in sub and supercritical processes, and the process is adapted to the properties of the individual fluid. The efficiency losses caused by imposing process constraints are investigated to help propose a suitable process layout. Hydrocarbon dry type fluids in recuperated processes produced the highest efficiencies, while wet and isentropic fluids were superior in non-recuperated processes. The results suggested that at design point, the requirements of process simplicity, low operating pressure and low hazard resulted in cumulative reductions in cycle efficiency. Furthermore, the results indicated that non-flammable fluids were able to produce near optimum efficiency in recuperated high pressure processes.
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| 32622. |
- Larsen, Ulrik, 1972, et al.
(författare)
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Development of a model for the prediction of the fuel consumption and nitrogen oxides emission trade-off for large ships
- 2015
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Ingår i: Energy. - : Elsevier BV. - 0360-5442 .- 1873-6785. ; 80, s. 545-555
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Tidskriftsartikel (refereegranskat)abstract
- The international regulations on fuel efficiency and NOx emissions of commercial ships motivate the investigation of new system layouts, which can comply with the regulations. In combustion engines, measures to reduce the fuel consumption often lead to increased NOx emissions and careful consideration of this trade-off mechanism is required in the design of marine propulsion systems. This study investigates five different configurations of two-stroke diesel-based machinery systems for large ships and their influence on the mentioned trade-off. Numerical models of a low-speed two-stroke diesel engine, turbochargers and an ORC (organic Rankine cycle), are used for the optimisation of the NOx andfuel consumption at design and part-load conditions, using a multi-objective genetic algorithm. Moreover, the effects of engine tuning and exhaust gas recirculation are investigated. The results suggest that increased system complexity can lead to lower fuel consumption and NOx. Fuel consumption reductions of up to 9% with a 6.5% NOx reduction were achieved using a hybrid turbocharger and organic Rankinecycle waste heat recovery system.
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| 32623. |
- Larsen, Ulrik, 1972, et al.
(författare)
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Development of a multi-level approach to model and optimise the Kalina Split Cycle
- 2012
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Ingår i: Proceedings of the 53rd SIMS conference on Simulation and Modelling.
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Konferensbidrag (refereegranskat)abstract
- In the marine sector there is a strong motivation for increasing the propulsion system energy efficiency, mainly because of increasing fuel prices and stricter upcoming emission regulations. The Kalina cycle, based on a mixture of ammonia and water as working fluid, exhibits higher conversion efficiencies than conventional power cycles and could be suitable for this purpose. The Split Cycle technique provides a method to further increase the thermal efficiency, by reducing the thermodynamic losses in the heat recovery system. This is achieved by having two separate streams of different ammonia concentrations entering and leaving a first evaporator stage before being mixed at the inlet of a second evaporator stage. It seems that modelling efforts showing the advantages of the Split Cycle have not been presented in the literature yet. Thus, a thermodynamic model of the Split Cycle is introduced in this work. Modelling and optimisation of the rather complex cycle requires approaching the problem at different system levels. This paper investigates tools and methods suitable for demonstrating the feasibility and advantages of the Split Cycle. The integrated model developed and presented in this paper combines three sub-models all using the NIST REFPROP equations of state: a separator and mixing subsystem model to handle the inherent constraints of the Split Cycle, a component-based model to optimise the heat exchanger operating conditions, and a process model to investigate the complete thermodynamic cycle. Results suggest a 9% net power output increase and 7% higher thermal efficiency compared to the baseline case.
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| 32624. |
- Larsen, Ulrik, 1972, et al.
(författare)
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Emissions from Diesel and Gasoline Vehicles Fuelled by Fischer-Tropsch Fuels and Similar Fuels
- 2007
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Ingår i: Society of Automotive Engineering.
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Konferensbidrag (refereegranskat)abstract
- The described investigation was carried out under the umbrella of IEA Advanced Motor Fuels Agreement. The purpose was to evaluate the emissions of carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx), particulate matter (PM) and polycyclic aromatic hydrocarbons (PAH) from vehicles fuelled by Fischer Tropsch (FT) based diesel and gasoline fuel, compared to the emissions from ordinary diesel and gasoline. The comparison for diesel fuels was based on a literature review, whereas the gasoline comparison had to be based on our own experiments, since almost no references were found in this field. In this context measurement according to the Federal Test Procedure (FTP) and the New European Driving Cycle (NEDC) were carried out on a chassis dynamometer with a directly injected gasoline vehicle. Experiments were carried out with a reference fuel, a fuel based 70% on FT and an alkylate fuel (Aspen), which was taken to be the ultimate formula of FT gasoline. FT based diesel generally showed good emission performance, whereas the FT based gasoline not necessary lead to lower emissions. On the other hand, the Aspen fuel did show many advantages for the emissions from the gasoline vehicle.
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| 32625. |
- Larsen, Ulrik, 1972, et al.
(författare)
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Expansion of organic Rankine cycle working fluid in a cylinder of a low-speed two-stroke ship engine
- 2017
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Ingår i: Energy. - : Elsevier BV. - 0360-5442 .- 1873-6785. ; 119, s. 1212-1220
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Tidskriftsartikel (refereegranskat)abstract
- Electricity and power produced from waste heat is particularly relevant in shipping because fuel expenses constitute the majority of the cost of operating the ships; however, the cost-benefit aspect limits the widespread implementation of waste heat recovery power units on ships. This paper presents the thermodynamic analysis of a concept that aims at reducing the cost of an organic Rankine cycle unit by using one of the cylinders in a large diesel engine as expansion device. Numerical models were used to optimise the process parameters and thereby determine the power potential for this concept. The evaluation of 104 working fluids points to cyclopropane, R245fa and R1234ze(z) as the most promising. The results suggest that the power produced by the organic Rankine cycle cylinder is at least equivalent to that of the cylinders operating with the diesel process. This enables potential fuel savings and emissions reductions of about 8.3% in the studied scenario.
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| 32626. |
- Larsen, Ulrik, 1972, et al.
(författare)
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Multiple regression models for the prediction of the maximum obtainable thermal efficiency of organic Rankine cycles
- 2014
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Ingår i: Energy. - 0360-5442 .- 1873-6785. ; 65
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Tidskriftsartikel (refereegranskat)abstract
- Much attention is focused on increasing the energy efficiency to decrease fuel costs and CO2 emissions throughout industrial sectors. The ORC (organic Rankine cycle) is a relatively simple but efficient process that can be used for this purpose by converting low and medium temperature waste heat to power. In this study we propose four linear regression models to predict the maximum obtainable thermal efficiency for simple and recuperated ORCs. A previously derived methodology is able to determine the maximum thermal efficiency among many combinations of fluids and processes, given the boundary conditions of the process. Hundreds of optimised cases with varied design parameters are used as observations in four multiple regression analyses. We analyse the model assumptions, prediction abilities and extrapolations, and compare the results with recent studies in the literature. The models are in agreement with the literature, and they present an opportunity for accurate prediction of the potential of an ORC to convert heat sources with temperatures from 80 to 360 C, without detailed knowledge or need for simulation of the process
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| 32627. |
- Larsen, Ulrik, 1972, et al.
(författare)
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System analysis and optimisation of a Kalina split-cycle for waste heat recovery on large marine diesel engines
- 2014
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Ingår i: Energy. - 0360-5442 .- 1873-6785. ; 64
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Tidskriftsartikel (refereegranskat)abstract
- Waste heat recovery systems can produce power from heat without using fuel or emitting CO2, therefore their implementation is becoming increasingly relevant. The Kalina cycle is proposed as an efficient process for this purpose. The main reason for its high efficiency is the non-isothermal phase change characteristics of the ammonia-water working fluid. The present study investigates a unique type of Kalina process called the Split-cycle, applied to the exhaust heat recovery from large marine engines. In the Split-cycle, the working fluid concentration can be changed during the evaporation process in order to improve the match between the heat source and working fluid temperatures. We present a system analysis to identify the governing mechanisms of the process, including a comparison of the efficiency of the Split-cycle and a conventional Kalina cycle and an investigation of the effects of using reheat in both cases. Results of a multi-variable optimisation effort using a genetic algorithm suggest that the Split-cycle process can obtain a thermal efficiency of 23.2% when using reheat compared to 20.8% for a conventional reference Kalina cycle. Reheat can increase the thermal efficiency by 3.4-5.9%. A simplified cost analysissuggests higher purchase costs as result of increased process complexity.
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| 32628. |
- Larses, Patrik, et al.
(författare)
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Hydrogen evolution at mixed α-Fe1 − xCrxOOH
- 2018
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Ingår i: Journal of Electroanalytical Chemistry. - : Elsevier BV. - 1572-6657 .- 0022-0728. ; 819, s. 114-122
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Tidskriftsartikel (refereegranskat)abstract
- The activity of mixed α-Fe 1 − x Cr x OOH oxides towards hydrogen evolution in alkaline solution is discussed based on Density Functional Theory (DFT) calculations, cyclic voltammetry and steady state measurements. Thermogravimetric and XRD measurements indicate an isomorphic substitution of Fe by Cr. Electrochemical characterization shows increasingly sluggish hydrogen evolution reaction (HER) kinetics with increased Cr loadings. This decrease in activity is accompanied by the inhibition of the reduction of iron in α-Fe 1 − x Cr x OOH. To investigate the origin of this decrease in activity, DFT calculations were performed for mixtures of Fe and Cr placed at Fe(OH) 2 . Based on phase diagrams, the most stable structures under HER conditions are identified and used to estimate the theoretical overpotential. In contrast to experiment, no decrease in HER activity is observed. Instead, mixed FeCr sites display an overpotential comparable to that of Pt. Taking into account the inhibition of the bulk reduction in the presence of Cr, the activity decreases in agreement with experiment. These results are very important in the search for cathodes in the chlorate process that are active also in the absence of chromate in solution and may offer a new route for Pt free HER catalysts in alkaline solution.
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| 32629. |
- Larsman, Pernilla, 1976, et al.
(författare)
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Perceived work demands, felt stress, and musculoskeletal neck/shoulder symptoms among elderly female computer users. The NEW study.
- 2006
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Ingår i: European Journal of Applied Physiology. - : Springer Science and Business Media LLC. - 1439-6319 .- 1439-6327. ; 96, s. 127-135
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Tidskriftsartikel (refereegranskat)abstract
- The aim of the present study was to test a structural model of the relationship between the perceived quantitative (time pressure and unevenly distributed workload) and emotional work demands and self-reported musculoskeletal symptoms from the neck and shoulder region with felt stress (rested, relaxed, calm, tense, stressed, and pressured at the end of a normal workday) as a mediating variable. As part of the NEW (Neuromuscular assessment in the Elderly Worker) study, a European case-control study, the present cross-sectional study was based on a questionnaire survey among Danish, Dutch, Swedish and Swiss female computer users aged 45 or older (n = 148). The hypothesized structural model was tested using structural equation modelling. The results indicate that perceived work demands influence neck/shoulder musculoskeletal symptoms through their effect on felt stress. The results further indicate complete mediation, which means that all of the effect of the perceived work demands on symptoms could be attributed to the stress mechanism. As regards the percentage of explained variance in the endogenous variables, 36% of the variation in felt stress was explained by the perceived work demands, and about 20% of the variation in musculoskeletal neck/shoulder symptoms was explained by the combination of the perceived work demands and the felt stress
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| 32630. |
- Larsman, Pernilla, 1976, et al.
(författare)
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Prognostic factors for intervention effect on neck/shoulder symptom intensity and disability among female computer users
- 2009
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Ingår i: Journal of Occupational Rehabilitation. - : Springer Science and Business Media LLC. - 1053-0487 .- 1573-3688. ; 19:3, s. 300-311
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Tidskriftsartikel (refereegranskat)abstract
- Introduction: It has been suggested that treatments may be more effective when they are matched to patient characteristics. This study aimed at investigating potential prognostic factors for clinically relevant improvement in symptom intensity and symptom-related disability among employees with symptoms in the neck/shoulder area, receiving either ergonomics counseling or such counseling in combination with myofeedback training. Methods: A randomized controlled study was performed among female computer users aged 45 or older (n = 36). A clinical examination and a questionnaire survey were performed before inclusion in the study. Symptom intensity and disability was assessed using questionnaires before the start of the interventions (baseline) and at follow-ups directly after the end of the interventions (T0) and after 3 (T3) and 6 (T6) months. Logistis regression analyses were performed in order to assess prognostic factors for clinically relevant improvement in symptom intensity and disability. Results: Improvement in symptom intensity was consistently predicted by symptom intensity at baseline. Diagnosis and stress-induced lack of muscular rest were prognostic factors for improvement in symptom intensity at short-term follow-up. Baseline disability and passive coping consistently served as prognostic factors for outcome in disability. Few substantial differences were found between the interventions in terms of prognostic factors. Conclusions: Myofeedback training in combination with ergonomics counseling seem to be an especially beneficial tool for secondary prevention among employees with moderate levels of symptom intensity and symptom-related disability, who respond to work-related stress by increased/sustained muscle activation, and who tend to employ passive coping to deal with their neck/shoulder symptoms.
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