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Theoretical studies of the intermidiate band solar cell
No full textThis thesis presents theoretical work on the intermediate band solar cell (IBCS). It is divided into two main parts: The first part is an introduction to basic solar cell physics and a presentation of central parts of the physics of the IBSC. The intention is not to give a complete review of these topics, but to provide readers unfamiliar to solar cell physics the basic knowledge needed to understand the scientific papers that constitutes part two of the thesis.
The intermediate band solar cell is a photovoltaic concept with a theoretical efficiency limit of 63:2%. This is over 50% higher than the 40:7% limit for conventional photovoltaic cells. The feature that distinguishes the IBSC from other photovoltaic concepts is that it allows electrons to be excited from low energy electron states to high energy states in a two-step process via an intermediate band (IB). This allows a more effective harvest of the energy of the photons impinging on the cell, than in ordinary solar cells where only transitions directly from the low energy states to the high energy states are allowed.
Theoretical efficiencies can be calculated for various combinations of band gaps and light concentrations. For conventional solar cells it has been shown that the theoretical efficiency of cells with band gaps lower than the optimal can be increased when spectral selectivity is applied. Spectral selectivity means that the cell absorptivity of certain photon energies above the band gap is set to zero. In this work it is shown that the theoretical efficiency increases for several band gap combinations and light concentrations for the IBSC, when spectral selectivity is applied. For some spectra, the efficiency of a spectrally selective IBSC can surpass the efficiency limit found without spectral selectivity. To give an example, the efficiency limit for an IBSC under the 1 sun 6000K black body spectrum increases from 46:8% to 48:5% when spectral selectivity is applied. No change is found for the fundamental 63:2% limit for fully concentrated black body radiation.
For the two-step generation process to work properly, the intermediate band has to be partially filled with electrons to allow transitions both to and from the IB. It has previously been assumed that one either has to use materials that have a partially filled IB in thermal equilibrium or use doping to assure that the IB is partially filled. This work investigates the possibility of partially filling the IB with electrons that are photogenerated from the valence band, so called photofilling. In a model based on detailed balance principles it is found that a practically usable photofilling can be obtained when the density of states in the IB has values typical for IBs formed by quantum dot superlattices, particularly when the light is concentrated. It is also shown how the filling varies with the voltage of the solar cell and the position of the IB in the main band gap.
A drift-diffusion model for the IBSC which allows for photofilling, that is, the IB-filling is treated as a variable, is also developed. By use of this model it is shown how the mutual sizes of the absorption cross-sections for transitions over the sub-band gaps can give rise to spatial variations in the filling. These spatial variations will result in electric fields that can drive the carriers in the conduction band and valence band in opposite directions. If this effect is present in real cells this observation has the practical consequence that the cell should be designed in a way which assures that the electric field push the electrons in the right direction.
The drift-diffusion model is also used to determine whether an unfilled or a half-filled IB in thermal equilibrium results in the highest efficiency. This is found to depend on the mutual sizes of the absorption cross sections for transitions via the IB. The optimal filling is examined for a particular example.
To investigate the optimal filling further, models based on detailed balance principles are applied to see how the optimal filling is affected by various parameters. The optimal filling is found to vary with the band gaps, the overlap between the absorption coefficients, the light concentration and the mutual sizes of the absorption cross-sections for transitions over the subband gaps. The negative effect of a non-optimal filling is shown to depend on the absorptivity of the cell, the overlap between the absorption coefficients as well as the density of states in the IB. Two main effects are identified as determinative for the optimal filling. The first is the pursuit for a maximized net generation rate via the IB. The second effect is the irreversible losses due to overlapping absorption coefficients. These should be as small as possible.
An ideality that is assumed in most theoretical work on the IBSC is that the IB has no energetical width. Previously a fundamental limit has been found by Levy and Honsberg for the minimal effect of this width on the cell efficiency. In this thesis an attempt is made to investigate how the thermalized nature of the IB-electrons affects the efficiency of IBSCs with a wide IB. To obtain numerical results, two sets of idealized absorption coefficients based on different assumptions are derived. For IBs with nonoverlapping absorption coe_cients, the efficiency deviates only slightly from the fundamental limit when the IB-width is below 0:15 eV. When the width increases, so does the difference between the fundamental limit and the efficiency calculated with the thermalized nature of the IB-electrons taken into account. One of the sets of absorption coefficients increases more steeply with the photon energy than the other. It is found that the reduction in effciency is smaller for the absorption coefficients with the steeper increase.
Efficiencies are also calculated when the absorption coe_cients are overlapping in the energy range which is assumed to be affected by the IB-width. The difference in efficiency between the fundamental limit and the efficiencies found in this work can be reduced, as compared to in the non-overlapping case, when the IB-width approaches 1 eV. In some of the investigated cases, the decrease in efficiency due to an increase in the IB-width is still significantly higher than the efficiency limit for single band gap cells. In other cases, however, the efficiency shows a devastating drop when the width of the IB goes from 0 to 1 eV.
The models presented in the thesis are idealized and based on assumptions that might not be fulfilled in real devices. Before transferring the results to real devices one should first be assured that the assumptions that have been made in the modeling are valid. If not, analyzes based on more advanced models might be required before conclusions can be drawn.PhD i fysikkPhD in Physic
Håndtering og Integrering av Solenergiens Overskuddsproduksjon i Distribusjonsnettet
No full textDenne masteroppgaven undersøker integrasjonen av solenergi i det norske kraftsystemet, med fokus på spenningsstabilitet og belastning i distribusjonsnettet på mellomspenningsnivå. Oppgaven har flere hovedmål: å analysere hvordan økt solenergiproduksjon påvirker nettstabiliteten, evaluere ulike energilagringsteknologier, utforske smarte styringssystemer, gjennomføre simuleringer, og undersøke optimal fordeling av solenergiproduksjon i kraftnettet.
Studien presenterer flere caser som illustrerer ulike scenarier for solenergiproduksjon og lagring. Case A undersøker et enkelt solkraftverk på 10 MW uten energilagring eller samlokalisert forbruk. Case B vurderer fire solkraftverk på 2,5 MW hver, plassert på ulike steder i nettet, også uten energilagring eller samlokalisert forbruk. Case C introduserer et 10 MW solkraftverk med implementert batteri for peak-shaving. Case D ser på et 10 MW solkraftverk med samlokalisert forbruk, som et hydrogenanlegg, men uten energilagring. Case E fokuserer på distribuert solproduksjon hos forbrukerne, der alle husstandene har solceller på taket.\\
Funnene fra disse casene viser at integrasjon av solenergiproduksjon kan føre til overspenninger og overbelastning i nettet. Riktig plassering og dimensjonering av solkraftverkene er essensielt for å unngå slike problemer. Batterier og andre energilagringsløsninger, som pumpekraft og hydrogenproduksjon, kan effektivt balansere produksjon og forbruk, og dermed bidra til å opprettholde nettstabiliteten. Videre viser det seg at bruk av overskuddsenergi til industrielle prosesser, som hydrogenproduksjon, kan forhindre struping av solproduksjonen og optimalisere bruken av tilgjengelig energi.
Oppgaven konkluderer med at integrasjon av solenergi i det norske kraftsystemet krever nøye planlegging og riktig dimensjonering for å sikre nettstabilitet. Det anbefales videre forskning på utvikling av avanserte styringssystemer og regulatoriske rammer som kan støtte en økt integrasjon av fornybar energi. Gjennom detaljerte analyser og simuleringer gir oppgaven innsikt i hvordan solenergi kan integreres mer effektivt i kraftsystemet til fordel for både energisystemet og samfunnet som helhet
Day-Ahead PV Power Forecasting for New Installations: Comparing Physical, Machine Learning, and Transfer Learning Models
Get PDFAccurate day-ahead power forecasting for new photovoltaic (PV) installations is challenging due to limited historical data, which hinders the training of traditional machine learning (ML) models and can impede the effective integration of new PV assets. This thesis addresses this problem by developing and evaluating physical baseline, Target-Only ML, and transfer learning (TL) models for day-ahead PV power forecasting. The study investigates their performance under varying levels of target site data availability (0, 1, 3, and 6 months) for installations in Norway, focusing on how baselines compare to data-driven approaches, if TL improves performance with limited data, and the impact of data quantity on TL versus Target-Only models.
The methodology involved preprocessing PV production and numerical weather prediction (NWP) data, engineering a consistent feature set, and systematically evaluating Persistence, Physical, Ridge Regression, Random Forest (RF), Light Gradient Boosting Machine (LGBM), and Multi-Layer Perceptron (MLP) models. TL models, using two distinct source sites, were assessed for zero-shot performance and adaptation with limited target data.
Results demonstrate that data-driven ML models outperform physical and persistence baselines, often with only 1-3 months of target data. TL models showed clear benefits: zero-shot TL frequently surpassed the Physical baseline, and adapted TL generally provided the highest accuracy, especially with minimal (1 month) target data. While some Target-Only ML models (notably RF and LGBM) became competitive with 3-6 months of data, adapted TL often maintained an advantage. Performance gains for both approaches typically diminished beyond three months of data.
The findings confirm TL as a valuable strategy for enhancing day-ahead PV forecasting accuracy for new installations, accelerating learning compared to models trained solely on limited target data. However, the optimal model and strategy are context-dependent, influenced by data availability and source-target similarity. This research offers practical insights for deploying effective forecasting solutions for new PV systems
Håndtering og Integrering av Solenergiens Overskuddsproduksjon i Distribusjonsnettet
No full textDenne masteroppgaven undersøker integrasjonen av solenergi i det norske kraftsystemet, med fokus på spenningsstabilitet og belastning i distribusjonsnettet på mellomspenningsnivå. Oppgaven har flere hovedmål: å analysere hvordan økt solenergiproduksjon påvirker nettstabiliteten, evaluere ulike energilagringsteknologier, utforske smarte styringssystemer, gjennomføre simuleringer, og undersøke optimal fordeling av solenergiproduksjon i kraftnettet.
Studien presenterer flere caser som illustrerer ulike scenarier for solenergiproduksjon og lagring. Case A
undersøker et enkelt solkraftverk på 10 MW uten energilagring eller samlokalisert forbruk. Case B vurderer fire solkraftverk på 2,5 MW hver, plassert på ulike steder i nettet, også uten energilagring eller samlokalisert forbruk. Case C introduserer et 10 MW solkraftverk med implementert batteri for peak-shaving. Case D ser på et 10 MW solkraftverk med samlokalisert forbruk, som et hydrogenanlegg, men uten energilagring. Case E fokuserer på distribuert solproduksjon hos forbrukerne, der alle husstandene har solceller på taket.
Funnene fra disse casene viser at integrasjon av solenergiproduksjon kan føre til overspenninger og overbelastning i nettet. Riktig plassering og dimensjonering av solkraftverkene er essensielt for å unngå slike problemer. Batterier og andre energilagringsløsninger, som pumpekraft og hydrogenproduksjon, kan effektivt balansere produksjon og forbruk, og dermed bidra til å opprettholde nettstabiliteten. Videre viser det seg at bruk av overskuddsenergi til industrielle prosesser, som hydrogenproduksjon, kan forhindre struping av solproduksjonen og optimalisere bruken av tilgjengelig energi.
Oppgaven konkluderer med at integrasjon av solenergi i det norske kraftsystemet krever nøye planlegging og riktig dimensjonering for å sikre nettstabilitet. Det anbefales videre forskning på utvikling av avanserte styringssystemer og regulatoriske rammer som kan støtte en økt integrasjon av fornybar energi. Gjennom detaljerte analyser og simuleringer gir oppgaven innsikt i hvordan solenergi kan integreres mer effektivt i kraftsystemet til fordel for både energisystemet og samfunnet som helhet
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Thermoradiative cells: proof of concept
No full textA new method for converting heat to electrical power called thermoradiative energy conversion is experimentally studied with the aim of proving this concept. The existing theory about this topic suggests that a semiconductors PN-junction should be able to produce electrical current by harvesting heat from an external source and emitting it as radiative energy to colder surroundings. For testing this principle, a commercial infrared photodiode is heated while it is placed in a cavity at lower temperature, the resulting voltage across the photodiode pins is measured. It will first be defined the design and manufacturing of a set-up that enables the required testing conditions. The obtained results show voltages with polarity corresponding to a reverse-bias performance when following the convention of photovoltaic literature. The absolute voltage values measured are very little (in the order of microvolts) but they show to increase together with the difference of temperature between the photodiode and its surroundings. These results match what first stated by the theory, regardless of their magnitude. Therefore, the principle of thermoradiative cells is proven along this work. There will be no discussions about how to improve the efficiency and delivered power of thermoradiative cells or whether if this method has practical potential or not.Outgoin
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Thermoradiative cells: proof of concept
No full textA new method for converting heat to electrical power called thermoradiative energy conversion is experimentally studied with the aim of proving this concept. The existing theory about this topic suggests that a semiconductors PN-junction should be able to produce electrical current by harvesting heat from an external source and emitting it as radiative energy to colder surroundings. For testing this principle, a commercial infrared photodiode is heated while it is placed in a cavity at lower temperature, the resulting voltage across the photodiode pins is measured. It will first be defined the design and manufacturing of a set-up that enables the required testing conditions. The obtained results show voltages with polarity corresponding to a reverse-bias performance when following the convention of photovoltaic literature. The absolute voltage values measured are very little (in the order of microvolts) but they show to increase together with the difference of temperature between the photodiode and its surroundings. These results match what first stated by the theory, regardless of their magnitude. Therefore, the principle of thermoradiative cells is proven along this work. There will be no discussions about how to improve the efficiency and delivered power of thermoradiative cells or whether if this method has practical potential or not.Outgoin
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