Brian Elmegaard

Comparison between a 1D and a 2D numerical model of an active magnetic regenerative refrigerator

The active magnetic regenerator (AMR) refrigeration system represents an environmentally attractive alternative to vapour-compression refrigeration. This paper compares the results of two numerical AMR models: (1) a 1D finite difference model and (2) a 2D finite element model. Both models simulate a reciprocating AMR and can determine the cyclical steady-state temperature profile of the system as well as performance parameters such as the refrigeration capacity, the work input and the coefficient of performance (COP). The models are used to analyse an AMR with a regenerator made of flat parallel plates of gadolinium operating in the presence of a 1 T magnetic field. The results are used to discuss under which circumstances a 1D model is insufficient and a 2D model is necessary. The results indicate that when the temperature gradients in the AMR perpendicular to the flow are small a 1D model obtains accurate results of overall results such as the refrigeration capacity but that a 2D model is required for a detailed analysis of the phenomena occurring inside the AMR.

Analysis of Indirectly Fired Gas Turbine for Wet Biomass Fuels Based on Commercial Micro Gas Turbine Data

The results of a study of a novel gas turbine configuration is being presented. In this power plant, an Indirectly Fired Gas Turbine (IFGT), is being fueled with very wet biomass. The exhaust gas is being used to dry the biomass, but instead of striving to recover as much as possible of the thermal energy, which has been the practice up to now, the low temperature exhaust gases after having served as drying agent, are lead out into the environment; a simple change of process integration that has a profound effect on the performance. Four different cycles have been studied. These are the Simple IFGT fueled by dry biomass assuming negligible pressure loss in the heat exchanger and the combustion chamber, the IFGT fueled with wet biomass (Wet IFGT) assuming no pressure losses, and finally both the Simple and the Wet IFGT incorporating typical data for pressure losses of commercially available micro turbines. The study shows that the novel configuration, in which an IFGT and a drying unit have been combined, has considerable merit, in that its performance exceeds that of the currently available methods converting wet biomass to electric power by a factor of five. The configuration also has clear advantages with respect to corrosion and to the environmental friendliness and the quantity of the waste products and their usefulness.Copyright

Formulation and validation of a two-dimensional steady-state model of desiccant wheels

Desiccant wheels are rotary desiccant dehumidifiers used in air-conditioning and drying applications. The modeling of simultaneous heat and mass transfer in these components is crucial for estimating their performances, as well as for simulating and optimizing their implementation in complete systems. A steady-state two-dimensional model is formulated and implemented, aiming to obtain good accuracy and short computational times with the purpose of inclusion in complete system models. The model includes mass and energy balances and correlations for heat and mass transfer based on empirical relations from the scientific literature. Convective heat and mass transfer coefficients are computed locally accounting for the entrance length effects. Mass diffusion inside the desiccant material is neglected. Comparison with experimental data from the literature shows that the model reproduces the physical behavior of desiccant wheels satisfactorily, as the deviation between the computed results and available data is always within 12%. The simulation time is as low as 3 s for a model with 200 control volumes. It is found that for the applied case, the model provides accurate results for the overall flow using an equiangular control volume discretization with 200 control volumes and no axial discretization. More computationally expensive configurations with axial discretization give more accurate results and information on local flow and desiccant conditions inside the wheel.

Continuous versus pulsating flow boiling. Experimental comparison, visualization, and statistical analysis

This experimental study investigates an active method for flow boiling heat transfer enhancement by means of fluid flow pulsation. The hypothesis is that pulsations increase the flow boiling heat transfer by means of better bulk fluid mixing, increased wall wetting, and flow-regime destabilization. The fluid pulsations are introduced by a flow modulating expansion device and are compared with continuous flow by a stepper-motor expansion valve in terms of time-averaged heat transfer coefficient. The cycle time ranges from 1 to 9 s for the pulsations. The time-averaged heat transfer coefficients are reduced from transient measurements immediately downstream of the expansion valves at low vapor qualities. The results show that the pulsations improve the time-averaged heat transfer coefficient by 3.2% on average at low cycle time (1 to 2 s), whereas the pulsations may reduce the time-averaged heat transfer coefficient by as much as 8% at high heat flux (q ⩾ 35 kW/m2) and cycle time (8 s). The latter reduction is attributed to a significant dry-out that occurs when the flow modulating expansion valve is closed. Additionally, the effect of fluid flow pulsations is found to be statistically significant, disregarding the lowest heat flux measurements.

Two Thermoeconomic Diagnosis Methods Applied to Representative Operating Data of a Commercial Transcritical Refrigeration Plant

In order to investigate options for improving the maintenance protocol of commercial refrigeration plants, two thermoeconomic diagnosis methods were evaluated on a state-of-the-art refrigeration plant. A common relative indicator was proposed for the two methods in order to directly compare the quality of malfunction identification. Both methods were applicable to locate and categorise the malfunctions when using steady state data without measurement uncertainties. By introduction of measurement uncertainty, the categorisation of malfunctions became increasingly difficult, though depending on the magnitude of the uncertainties. Two different uncertainty scenarios were evaluated, as the use of repeated measurements yields a lower magnitude of uncertainty. The two methods show similar performance in the presented study for both of the considered measurement uncertainty scenarios. However, only in the low measurement uncertainty scenario, both methods are applicable to locate the causes of the malfunctions. For both the scenarios an outlier limit was found, which determines if it was possible to reject a high relative indicator based on measurement uncertainty. For high uncertainties, the threshold value of the relative indicator was 35, whereas for low uncertainties one of the methods resulted in a threshold at 8. Additionally, the contribution of different measuring instruments to the relative indicator in two central components was analysed. It shows that the contribution was component dependent.

Thermodynamic Performance Indicators for Offshore Oil and Gas Processing: Application to Four North Sea Facilities

Oil and gas extraction have been responsible for 25—28% of the total greenhouse gas emissions in Norway the last 10 years. The part from offshore oil and gas processing, including power production, flaring, and cold ventilation on production platforms, accounted for 20—22%. Exergy analysis is a method for systematic assessment of potential to perform work. It gives the possibility to identify where in a process inefficiencies occur: both losses to the surroundings and internal irreversibilities, and can be used as a tool for pinpointing improvement potential and for evaluation of industrial processes. When used in the petroleum sector, this can motivate more efficient oil and gas extraction, leading to a better utilisation of the resources and less greenhouse gas emissions.The objectives of this thesis were to: (i) establish exergy analyses of the oil and gas processing plants on different types of North Sea platforms; (ii) identify and discuss improvement potentials for each case, compare them and draw general conclusions if possible; and (iii) define meaningful thermodynamic performance parameters for evaluation of the platforms.Four real platforms (Platforms A—D) and one generic platform of the North Sea type were simulated with the process simulators Aspen HYSYS and Aspen Plus. The real platforms were simulated using process data provided by the oil companies. The generic platform was simulated based on literature data, with six different feed compositions (Cases 1—6). These five platforms presented different process conditions; they differed for instance by their exported products, gas-to-oil ratios, reservoir characteristics and recovery strategies.Exergy analyses were carried out, and it was shown that for the cases studied in this work, the power consumption was in the range of 5.5—30 MW, or 20—660 MJ/Sm3 o.e. exported. The heat demand was very small and covered by electric heating for two of the platforms, and higher, but low enough to be covered by waste heat recovery from the power turbines and by heat integration between process streams, for the other three platforms. The main part of the power was consumed by compressors in the gas treatment section for all cases, except Platform B and Case 4 of the generic model. Platform B had lower pressures in the products than in the feeds, resulting in a low compression demand. Case 4 of the generic model had a high content of heavy hydrocarbons in the feed, resulting in large power demand in the oil export pumping section. The recompression and oil pumping sections appeared to be the other major power consumers, together with the seawater injection system, if installed.The total exergy destruction was in the range of 12—32 MW, or 43—517 MJ/Sm3 o.e. exported. Most exergy destruction was related to pressure increase or decrease. Exergy destruction in the gas treatment section made up 8—57% of the total amount, destruction in the recompression section accounted for 11—29%, while 10—28% took place in the production manifolds. Exergy losses due to flaring varied in the range of 0—13 MW.Platforms with high gas-to-oil ratios and high pressures required in the gas product presented the highest power consumption and exergy destruction.Several measures were proposed for reduction of exergy destruction and losses. Two alternatives included use of mature technologies with potential to increase efficiency significantly: (i) limit flaring by installation of gas recovery systems, and (ii) improve gas compression performance by updating/exchanging the compressors.Several thermodynamic performance indicators were discussed, with Platforms A—D as case studies. None of the indicators could at the same time evaluate (i) utilisation of technical achievable potential, (ii) utilisation of theoretical achievable potential and (iii) total use of energy resources. It was concluded that a set of indicators had to be used to evaluate the thermodynamic performance. The following indicators were suggested: BAT efficiency on exergy basis, exergy efficiency, and specific exergy destruction.The formulation of exergy efficiency for offshore processing plants is difficult because of (i) the high throughput of chemical exergy, (ii) the large variety of chemical components in the process streams and (iii) the differences in operating conditions. Approaches found in the literature for similar processes were applied to Platforms A—D. These approaches had several drawbacks when applied to offshore processing plants; they showed low sensitivity to performance improvements, gave inconsistent results, or favoured platforms operating under certain conditions. A new exergy efficiency, called the component-by-component efficiency, was proposed. This efficiency could successfully evaluate the theoretical improvement potential.Eksergianalyse av offshore olje- og gassprosesseringOlje- og gassutvinning har vaert kilde til 25—28% av de totale klimagassutslippene i Norge de siste 10 arene. Den delen som stammer fra offshore olje- og gassprosessering (kraftproduksjon, fakling og kaldventilering pa produksjonsplattformer) stod for 20—22%. Eksergianalyse er en metode for systematisk bestemmelse av potensiale til a utfore arbeid. Det gir mulighet til a identifisere hvor i en prosess ineffektiviteter oppstar: bade i form av tap til omgivelsene og i form av interne irreversibiliteter. Det kan brukes som et verktoy for a finne forbedringsmuligheter og for evaluering av industrielle prosesser. Ved bruk innen petroleumssektoren kan dette motivere for mer effektiv olje- og gassutvinning, noe som gir bedre utnyttelse av ressursene og mindre utslipp av klimagasser.Formalet med denne avhandlingen er a: (i) etablere eksergianalyser av olje- og gassprosessering pa ulike typer Nordsjo-plattformer; (ii) identifisere og diskutere forbedringspotensialer for hvert tilfelle, sammenligne dem og trekke generelle konklusjoner om mulig; og (iii) definere meningsfulle termodynamiske ytelsesindikatorer for evaluering av plattformene.Fire virkelige plattformer (Plattform A—D) og en generisk Nordsjo-type plattform er simulert med prosessimulatorene Aspen HYSYS og Aspen Plus. De virkelige plattformene er simulert ved a bruke prosessdata stilt til radighet av operatorene av plattformene. Den generiske plattformen er simulert basert pa litteraturdata, med seks ulike fodesammensetninger (Case 1—6). Disse fem plattformene har ulike prosessbetingelser; de har for eksempel ulike eksporterte produkter, gass/olje-forhold, reservoaregenskaper og utvinningsstrategier.Eksergianalyser viser at for tilfellene studert i dette arbeidet er kraftforbruket i storrelsesorden 5,5—30 MW, eller 20—660 MJ/Sm3 o.e. eksportert. Varmebehovet er svaert lite og blir dekket med elektrisitet for to av plattformene, og noe hoyere men lavt nok til a bli dekket med varmegjenvinning fra kraftturbinene og ved varmeveksling mellom prosesstrommer for de tre andre plattformene. Hoveddelen av kraften blir konsumert av kompressorene i gassbehandlingsseksjonen for alle tilfellene bortsett fra Plattform B og Case 4 i den generiske modellen. Plattform B har lavere trykk i produktstrommene enn i fodestrommene, noe som resulterer i lavt behov for kompresjon. Case 4 i den generiske modellen har et hoyt innhold av tunge hydrokarboner i foden, noe som resulterer i hoyt kraftbehov i seksjonen for eksportpumping. Seksjonene for rekompresjon og eksportpumping viser seg a vaere de andre viktigste kraftforbrukerene, sammen med systemet for sjovannsinjeksjon hvis dette er installert.Den totale ekserginedbrytingen er 12—32 MW, eller 43—517 MJ/Sm3 o.e. eksportert. Mest ekserginedbryting er relatert til trykkoking eller trykkreduksjon. Ekserginedbryting i gassbehandlingsdelen utgjor 8—57% av den totale mengden, nedbryting i rekompresjonsseksjonen utgjor 11-29%, mens nedbryting i produksjonsmanifoldene utgjor 10—28%. Eksergitap pa grunn av fakling varierer mellom 0—13 MW.Plattformene med hoye gass/olje-forhold og behov for hoyt trykk i gassproduktene har hoyest kraftforbruk og ekserginedbryting.Ulike tiltak for reduksjon av ekserginedbryting og eksergitap er foreslatt. To alternativer inkluderer bruk av modne teknologier og har potensiale til a oke effektiviteten betydelig: (i) begrensning av fakling av gass ved installasjon av gassgjenvinningssystemer, og (ii) forbedring av gasskompresjonen ved a oppdatere/bytte ut kompressorer.Flere termodynamiske ytelsesindikatorer er diskutert med utgangspunkt i Plattform A—D. Ingen av indikatorene kan pa samme tid evaluere (i) utnyttelse av teknisk oppnaelig potensiale, (ii) utnyttelse av teoretisk potensiale og (iii) total bruk av energiressurser. Det konkluderes med at et sett med indikatorer ma brukes for a evaluere termodynamisk ytelse. De folgende indikatorene foreslas: BAT (best tilgjengelig teknologi) effektivitet pa eksergibasis, eksergieffektivitet og spesifikk ekserginedbryting.Formuleringen av eksergieffektivitet for offshore olje- og gassprosessering er utfordrende pa grunn av (i) den hoye gjennomgangen av kjemisk eksergi, (ii) den store variasjonen av kjemiske komponenter i prosesstrommene og (iii) de store forskjellene i driftsbetingelser. En ny type eksergieffektivitet foreslas. Denne effektiviteten kan evaluere utnyttelsen av det teoretiske potensialet pa tross av punktene nevnt ovenfor.

Synthesis of preliminary system designs for offshore oil and gas production

Abstract The present work deals with the design of oil and gas platforms, with a particular focus on the development of integrated and intensified petroleum processing plants. It builds on a superstructure-based approach that includes all the process steps, transformations and interconnections of relevance, to generate and compare a large number of alternatives. The superstructure is formulated based on engineering knowledge and is coupled to process models developed in Aspen and Matlab, together with multi-objective optimisation routines and uncertainty assessments. It takes actual measurements from North Sea fields and three petroleum compositions as starting points. The significance of the uncertainties associated with the feed properties, and the capital costs, taxes and lifetime, is assessed. The results indicate that (i) the system performance strongly depends on the level of mass integration within the platform, (ii) the oil and gas recoveries are markedly impacted by the number of separation stages and heat exchangers, and (iii) disregarding the interactions between the several plant sections lead to sub-optimum solutions. The application of this framework proves to be useful for eliminating inadequate configurations and screening potentially novel solutions at early stage designs, with respect to technical, energetic and economic criteria.

Sustainable Production of Asphalt Using Biomass as Primary Process Fuel

The production of construction materials is very energy intensive and requires large quantities of fossil fuels. Asphalt is the major road paving material in Europe and is being produced primarily in stationary batch mix asphalt factories. The production process requiring the most energy is the heating and drying of aggregate, where natural gas, fuel oil or LPG is burned in a direct-fired rotary dryer. Replacing this energy source with a more sustainable one presents several technical and economic challenges, as high temperatures, short startup times and seasonal production variations are required. This paper analyses different pathways for the use of biomass feedstock as a primary process fuel. The analysed cases consider the gasification of straw and wood chips and the direct combustion of wood pellets. The additional use of syngas from the gasifier for the production of heat or combined heat and power is further evaluated during hours without asphalt production. The challenges of having varying seasonal production can be solved by this integration of the production unit to the utility system. The results show the economic and technical feasibility of using biomass for process heating in the asphalt factory. The dryer demand of 6.4 MW can be covered with a biomass input between 7.1 and 8.6 MW. District heat can be produced at competitive prices below 40 € per MWh.

Regenerative Gas Turbines With Divided Expansion

Recuperated gas turbines are currently drawing an increased attention due to the recent commercialization of micro gas turbines with recuperation. This system may reach a high efficiency even for the small units of less than 100kW. In order to improve the economics of the plants, ways to improve their efficiency are always of interest. Recently, two independent studies have proposed recuperated gas turbines to be configured with the turbine expansion divided, in order to obtain higher efficiency. The idea is to operate the system with a gas generator and a power turbine, and use the gas from the gas generator part for recuperation ahead of the expansion in the power turbine. The present study is more complete than the predecessors in that the ranges of the parameters have been extended and the mathematical model is more realistic using an extensive simulation program. It is confirmed that the proposed divided expansion can be advantageous under certain circumstances. But, in order for todays micro gas turbines to be competitive, the thermodynamic efficiencies will have to be rather high. This requires that all component efficiencies including the recuperator effectiveness will have to be high. The advantages of the divided expansion manifest themselves over a rather limited range of the operating parameters, that lies outside the range required to make modern micro turbines economically competitive.Copyright

Performance of a reversible heat pump/organic Rankine cycle unit coupled with a passive house to get a positive energy building

This paper presents an innovative technology that can be used to deliver more renewable electricity production than the total electrical consumption of a building while covering the heat demand on a yearly basis. The technology concept uses a heat pump (HP), slightly modified to revert its cycle and generate electricity, coupled to a solar thermal collector roof. This reversible HP/organic Rankine cycle unit presents three operating modes: direct heating, HP and organic Rankine cycle. This work focuses on describing the dynamic model of the multi-component system followed by a techno-economic analysis of the system under different operational conditions. Sensitivity studies include: building envelope, climate, appliances, lighting and heat demand profiles. It is concluded that the HP/ORC unit can turn a single-family house into a PEB under certain weather conditions (electrical production of 3012 kWh/year and total electrical consumption of 2318 kWh/year) with a 138.8 m2 solar roof in Denmark.