Browsing by Author "Lund, Peter D., Prof., Aalto University, Department of Applied Physics, Finland"
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- Analysis of single-layer and three-layer nanocomposite fuel cells
School of Science | Doctoral dissertation (article-based)(2020) Jouttijärvi, SamiFuel cells (FCs) convert the chemical energy of fuel directly to electricity. FCs are potential canditates for clean electricity sources in the future, provided that the main challenges halting their commercialization can be solved. Several different FC subtypes exist. This Thesis is focused on ceramic nanocomposite FCs (CNFCs) and single-layer FCs (SLFCs). Both of these FCs operate at intermediate temperatures, at around 500-600 °C. CNFC utilizes the traditional three-layer structure: anode, electrolyte, and cathode. The key component is the electrolyte, that consists of a composite of a solid oxide and a salt, here doped ceria and alkali carbonate mixture respectively. This composite electrolyte allows an efficient multi-ion conduction, reducing the ohmic losses in the cell. Excellent power densities, exceeding 1 W per square centimeter, were achieved with two different CNFCs in this Thesis. SLFC is a ground-breaking innovation where all FC functions are compressed into one single layer, consisting of a mixture of a semiconductor (here lithium nicke zinc oxide or copper iron oxide) and an ionic conductor (here doped ceria or doped ceria – alkali carbonate mixture). The SLFC desing allows to elimintate the challenges originating from the three-layer structure and to simplify the manufacturing procedure. In this Thesis, the working principle and performance-affecting factors of SLFCs were studied. The key findings include that the proton is dominating over the oxygen-ion in ionic conduction with the studied SLFC configuration and that applying the composite ionic conductor of CNFC to SLFC improves vastly the cell performance. Since both CNFCs and SLFCs are complex nanoscale structures, studying the microstructure of these devices with electron microscopy and X-ray spectroscopy are identified as crucial procedures to understand the macroscopic output. Systematic studies combined with modern microscopic methods are suggested as a pathway to push both SLFCs and CNFCs towards commercialization. - Anodic TiO2 nanotube arrays for photoconversion based hydrogen production
School of Science | Doctoral dissertation (article-based)(2022) Hou, XuelanSolar energy conversion and storage are potential technologies to improve energy security and achieve carbon neutrality. Among these technologies, the solar-to-chemicals conversion using photoelectrochemical cells to produce hydrogen fuel is a viable pathway. This work focuses on using highly active, stable, and flexible anodic TiO2 nanotube (TNT) electrodes in photoconversion cells for hydrogen production, such as the application of anode electrodes and cathode electrodes in water splitting (WS) cells. To improve solar energy capture, the two-anode reduction method and the stepwise cathodic reduction method were designed and verified as effective ways to modify pure anodic TNT. The performance of the reduced anodic TNT as a cathode in WS cell was -221.1 mA, which was 17,000-fold higher than that of TNT, and 5-fold that of commercial Ti. A stability test at CP@-100 mA for 24 h showed a decay of 1.3%. In the photoconversion cell, the onset potential of the reduced anodic TNT (TNT-C) has an anodic shift from -0.79 to 0.19 VRHE, corresponding to a performance increase from -110.06 to -210.66 mA at -1.0 VRHE. Self-improving performance was also found during the long-term tests, which were deduced to the self-formed composite structures, such as an n-p-n junction. When used as the anode, the reduced anodic TNT (TAR-2) gave a 3-fold enhancement over the pure one at 1.23 VRHE, 2.05 mA/cm2. The multi-size effects of the anodic TNT were investigated in a range of six orders of magnitude length scales ranging from 10-8 to 10-2 m. Increasing the nm-scale and cm-scale length size, the current density at 1.23 VRHE decreased. However, increasing the μm-scale length size, the performance at 1.23 VRHE increased. The light absorption intensity for reduced anodic TNT was enhanced, the light absorption range was broadened and the absorption edge had a redshift. The TNT-C showed six absorption peaks in the incident photon-to-electron conversion efficiency in the range of 365-1020 nm, while TNT showed an absorption edge at 430 nm. The electrical conductivity of the reduced samples has two arcs representing two electroactive interfaces with different kinetics in the Nyquist plot. The semiconductive arc was ten times smaller than that of pure anodic TNT, and the metallic arc was newly introduced indicating high electrical conductivity. The growth order of anatase TiO2 peaks was firstly reported and the growth rate of the anodic film was confirmed. The thesis shows that cathodic reduction methods, such as stepwise cathodic reduction, are promising for reducing the manufacturing costs of TNT-based electrodes. - Energy system resilience to extreme disruptions: reexamining impacts and their assessment
School of Science | Doctoral dissertation (article-based)(2024) Jasiūnas, JustinasThe functioning of modern societies relies on an undisrupted energy supply, which is subject to a large number and variety of physical and nonphysical threats. The most severe disruptions, despite their rarity, are responsible for a major share of disrupted supply. Furthermore, cost-driving factors depend on disruption severity, which calls for knowledge about the magnitude of unprecedented but possible future disruptions. This work starts with a broad mapping of the landscape of threats to energy systems, later narrowed down to extreme weather threats and vulnerabilities of the Finnish electricity system. As the largest cause of electricity supply interruptions in Finland, windstorms are chosen for impact modeling in the rest of this thesis. To capture the magnitude of unprecedented windstorm impacts, a new fragility-based spatio-temporal impact model was developed with a unique combination of national scale and medium voltage grid detail. The development of this model necessitated rethinking relevant aspects and suitable approaches and the development of new methods across multiple impact chain steps. The most significant new modeling contribution is the synthetic grid generation method utilizing distribution grid operator (DSO) specific data in a country with many relatively small DSOs. This generation method combines spatially mapped grid component data with assumed standard feeder topology. The second most distinct methodological contribution is severity-dependent fixing time distribution derivation using a two-level fitting procedure. The first level of this procedure includes fitting fixing time distributions of faults in a storm and calm periods considered meteorologically independent events. The model is applied to Finland's three most impactful historical and historically unprecedented but meteorologically plausible windstorm cases. The model recreates lost load profiles for historical windstorms with errors of around 20%, despite omitting many windstorm impact driving environmental factors. The historically unprecedented windstorm's wind gust field is obtained by scaling the field of the historically most impactful windstorm upwards by 24%, a value obtained with the extreme-value-theory-based method. The lost load from 24% higher wind gust speeds increases tenfold. Impacts are limited by the significant cabling of powerlines done since 2011, which, despite high costs, would largely pay off during the unprecedented windstorm. That said, the cost of such an event requires a reevaluation of cost rates considering time dependency, critical services, and impacts on smaller economy and population segments. - Fabrication and electrochemical performance analysis of nanocomposite for low-temperature SOFC
School of Science | Doctoral dissertation (article-based)(2017) Jing, YifuLow-temperature solid oxide fuel cell (LTSOFC) offers a promising new energy conversion technology, which converts chemical energy into electrical energy. The benefits of LTSOFC technology include a low operating temperature, relatively high energy conversion efficiency, and potentially low costs. One of the key challenges with LTSOFC, however, is the power density and the ionic conductivity of the electrolyte, which still needs improvement. In this work, several different synthetic and fabrication processes, such as co-precipitation synthesis, freeze-drying synthesis, and spark plasma sintering (SPS) techniques were employed to enhance the performance of the composite electrolyte for the LTSOFC fuel cell. As the base electrolyte material, samarium-doped ceria (SDC) was employed, which was also modified by adding a carbonate element (CSDC). A LiNiCuZn electrode composite was utilized, which was synthesized using the slurry method. The ionic conductivity of the electrolyte could be improved via the freeze-drying and SPS methods as opposed to the co-precipitation method. The cold-pressing and hot-pressing methods were separately applied to prepare laboratory unit cells of the LTSOFC with the following results. The highest power density obtained was 1 W/cm2 at 470 oC. The best ionic conductivities were obtained by freeze-drying and the SPS, which exceeded 0.4 S/cm. In a carbonate-SDC electrolyte, adding CO2 to the air oxidant clearly improved the power density and the open circuit voltage of the fuel cell. The power density was improved by 30–100% and the OCV by 0.1–0.2 V compared to using pure air as an oxidant. - Hybrid heterojunction solar cells using single-walled carbon nanotubes and amorphous silicon thin films
School of Science | Doctoral dissertation (article-based)(2020) Pramod, Mulbagal RajannaSingle-walled carbon nanotubes possess extraordinary optical, electrical, chemical, and mechanical properties. Thin films of randomly oriented SWCNTs have a great potential in many opto-electro-mechanical applications. Moreover, recent developments in photovoltaics have been largely contributed by SWCNTs as a p-type transparent conductor that fulfill the requirements for continuous, fast, and cheap film manufacturing process compatible with the roll-to-roll technology. The scope of this thesis is the development of a conductive p-type SWCNT transparent conductor and its application in hybrid heterostructure solar cell based on amorphous silicon. For successful implementation of SWCNTs film in solar cells, it is very critical for the SWCNTs to have good physical contact with the material on which it is deposited. At first, quantitative measurements of the adhesion of SWCNT films with substrate materials in air and inert Ar atmosphere using atomic force microscopy was performed. It was found that adhesion of SWCNT films depends on the atmospheric conditions under which it is stored and deposited on a substrate material. The SWCNT film was measured to have higher adhesion in an inert atmosphere. With this understanding, a simple fabrication method of hybrid heterostructure solar cells was proposed in which the SWCNT-PEDOT:PSS composite p-type film forms a coupled continuous hybrid heterojunction with a-Si:H absorber. The optical and electrical properties of this composite was extensively characterized and further optimized by introducing multifunctional components like ultrathin MoO3 and SWCNT fibers. A rationally designed p-type transparent conductor with a combination of SWCNTs-MoO3-PEDOT:PSS-SWCNT fibers composite resulted in a state-of-the-art sheet resistance of 17 Ω/sq at 90% transmittance. Moreover, SWCNT fibers by itself can be used as replacement for traditional metal contacts as demonstrated here. This opens a new avenue in widespread energy technologies, where high hole conductivity and transparency of the material are prerequisites for their successful implementation. Integrating the developed p-type transparent conductor as a window layer and top electrode on a-Si:H in a nip configuration resulted in a dramatic increase in its power conversion efficiency reaching up to 8.8%. The energy level alignment of these solar cells is carefully engineered at a-Si:H and SWCNTs interface by introducing a ultrathin MoO3 layer that shows the carrier transport by means of band-to-band or trap-assisted tunneling. - Influence of Materials and Aging Test Design on Dye Solar Cell Stability
School of Science | Doctoral dissertation (article-based)(2021) Poskela, AapoDye solar cells are an emerging third-generation photovoltaic technology. Its main advantages are the low cost of materials and manufacturing, and the possibility to adjust the colour of the cell by selecting different dyes. Currently one of the main obstacles for commercialisation of dye solar cells is their relatively short lifetime. Silicon solar panels are often given guarantees of over two decades, while dye solar cells can reliably function only for a few years in outdoor conditions. This work focuses on dye solar cell stability from multiple perspectives: the impact of different materials and structures, effect of operating conditions to the cell lifetime, and analysis of the methods used for studying dye solar cell stability. A practical challenge in the production of robust and large dye solar cells is reliably sealing its liquid electrolyte within the cell. It was discovered that soaking a nanocellulose or nanochitin membrane in an electrolyte solution and then sealing it inside the dye solar cell not only simplifies the assembly process, but also increases the initial energy conversion efficiency of the solar cells. Research for improved dye solar cell lifetime can be supported studying how operating conditions affect it, i.e. what kind of factors and mechanisms are most detrimental to stability and should always be taken into consideration in cell design. Since a dye solar cell can function for some years in standard operation, stability studies generally rely on accelerated tests. One focus of this thesis was to study how well approximations made in accelerated tests correspond to realistic conditions. A common mistake in dye solar cell stability testing is to underestimate the degrading effects of UV light. It was found that even a UV filter is inadequate to protect dye solar cells fully, suggesting that UV degradation of dye solar cells is still an issue requiring further research. Another topic that is discussed is how the circuit state of the cells impacts their degradation rate. When solar cells are operating in practice, they are connected to an electric load that brings them close to their maximum power point. Often this is overlooked in research and dye solar cells are aged in open circuit conditions, which may artificially lengthen the lifetime of dye solar cells. This thesis demonstrates that the difference between the two states is small. The work also includes pioneering aging tests in harsh northern conditions. The stability research practices in the field of emerging photovoltaics was reviewed, providing suggestions on how to improve the corresponding research to obtain more accurate results. - Inkjet Printing for Low-Temperature Solid Oxide Fuel Cells: Comparative Fabrication Techniques and Microstructural Investigations
School of Science | Doctoral dissertation (article-based)(2024) Zarabi Golkhatmi, SanazSolid oxide fuel cells (SOFCs) are emerging as a promising technology for clean energy generation, yet their market penetration is hampered by du-rability and stability challenges. This thesis addresses these challenges by focusing on new fabrication methods for SOFC components and their microstructure. It employs advanced basic materials in the manufacturing process, enabling much lower operating temperatures than traditional materials, which could potentially improve the longevity of the cells. The thesis introduces a unique inkjet printing technique, a mask-free, accurate, and contactless method for fabricating high-performance materials with customized microstructures. This is particularly beneficial for cathodes, where oxygen reduction reactions contribute to activation loss in SOFCs. The research involved developing and optimizing three distinct ink formulations: La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), CuFe2O4, and CuFe2O4 – Gd:CeO2 (GDC) nanocomposite. These formulations were compared with other low-viscosity inks used in drop casting and spin coating. The created inks have undergone extensive evaluation, which includes particle size analysis, rheological characteristics, and thermal analysis, as well as microstructural investigations and electrochemical performance measurement. All inks demonstrated excellent jetting performance, with Z parameters indicating their suitability for the inkjet printing process. For instance, fresh LSCF ink and CuFe2O4 – GDC nanocomposite ink showed Z parameters of 2.77 and 5.5, respectively, at their printing temperature. Electrochemical performance analysis revealed improvements compared to drop casting and spin coating techniques with the same ink. Inkjet print-ing reduced the ohmic resistance of the LSCF symmetric cell from 1.05 Ω cm² to 0.37 Ω cm² at 550°C in an air atmosphere and decreased the mass diffusion resistance by 4.25 times compared to a drop-casted cell. Further comparisons using Electrochemical Impedance Spectroscopy (EIS) showed that inkjet printing could lower the area-specific resistance (ASR) of a 100-layer cell significantly from 19.59 Ω cm² to 5.99 Ω cm² under similar conditions. For the CuFe2O4 – GDC nanocomposite ink – Samba cartridge case at 650°C under H2 and air atmospheres, the best inkjet-printed complete fuel cell gave a Rohm and ASR of 0.96 and 1.12 Ω cm², respectively, using just 2.16 mg of deposited ink (1.63 mg cm-2). In contrast, a spin-coated cell with the same ink amount exhibited higher Rohm (3.2 Ω cm²) and ASR (37.82 Ω cm²). The drop-cast cell with 6.2 times more deposited ink showed even higher values (Rohm = 8.84 Ω cm² and ASR = 15.96 Ω cm²). These findings highlight the potential of inkjet printing for morphological control, improving gas transport, lowering ion transport losses, and speed-ing up charge transfer reactions. This resulted in improved electrochemical performance, emphasizing the potential of inkjet printing in tailoring cathode morphology for the development of high-performance materials in electrochemical energy conversion. - Photovoltaic Output Modeling: Monitoring, Forecasting, and Applications
School of Science | Doctoral dissertation (article-based)(2021) Böök, HermanPhotovoltaics (PV) has emerged from a niche market towards becoming a potential mainstream electricity source. Despite the rapidly increasing share of PV systems in electricity production, the wide-spread adoption of PV still pose a handful of challenges related to the intermittent and weather-dependent nature of its output. Even though certain best practices in PV output modeling have gradually taken shape, no comprehensive solutions have yet been established for handling the entity, as a whole. In this doctoral thesis, modeling methods for monitoring and forecasting the output of specific PV systems were developed and validated, also demonstrating the utilization of these methods in real-world applications for assessing the general viability of PV technology in a Nordic context. In-situ measurements at two PV sites were used for optimizing and evaluating the performance of the PV output conversion model. The model was shown to perform well in snow-free conditions, demonstrating its value by estimating distinct system losses and its PV system monitoring capability. As a part of this process, the proposed quality control method for treating calculated direct normal irradiance (DNI) values was shown to provide a feasible approach for processing calculated DNI values. In order to attain an accurate and truthful depiction of a set of unique PV systems, the occurring characteristics of the investigated systems are to be taken into account. For sites where no external measurements are available, a novel approach for adjusting the baseline model for forecasting the output of specific PV systems was validated at 23 separate PV sites. The method was shown to capture time-dependent losses, proving it as a feasible approach for providing adjusted site-specific PV output forecasts. The studied PV output modeling tools were demonstrated in three different applications, covering domestic hot water heating cost optimization with a PV output forecast -based control method, a residential PV profitability study in Finland, coupled with energy storage and optimization, and a virtual power plant concept, required to regulate and aggregate active consumer behaviour in the future markets. In each case, a clear added value of PV output modeling through distinct cost reduction potential and imbalance mitigation, could be demonstrated. The contributions of this thesis demonstrate the performance of the selected modeling approaches, and provide tangible information about their application and value creation potential within the selected subject areas. - Reliability and sustainability analyses of frugal solar photovoltaic micro-grid systems in emerging markets
School of Science | Doctoral dissertation (article-based)(2019) Numminen, SiniMassive over-consumption of natural resources has resulted in severe environmental and climate problems, which in turn threaten life supporting ecosystems. Moreover, despite some nations driving this over-consumption, millions of people in other parts of the world, still suffer from energy poverty: 2.7 billion people live without access to clean cooking facilities, and one billion without access to electricity mainly in the sub-Saharan Africa and the developing regions of Asia. The main motivation of this dissertation is to better understand the relevance of frugal energy innovations in providing affordable energy services to impoverished people in emerging economies in a sustainable manner. The dissertation contains a novel five-criteria method for the identification of frugal energy innovations, which underlines local appropriateness, affordability, sustainability and technical durability. A frugal technology, a solar micro-grid system with locally manufactured pre-paid energy meters was investigated in rural Northern India through a set of field measurement trials. The micro-grids included an advanced pricing method that could potentially decrease system cost, which is of the utmost importance in low-income communities. However, the dynamic pricing function was not found beneficial in the field trial, because the impoverished customers minimized their power consumption, so demand response benefits could not be demonstrated. Therefore it is recommended to design technology and business models with an essential user-centered approach without overlooking the potential necessity of educating end-users in areas where people may have not had access to power grid services before. Moreover, the frugal electronic component quality requires attention. A particular focus in this dissertation is on the reliability of solar micro-grid systems. In the field trial, power was measured to be available for the households 87% of the time. The number of days without blackouts was 200 out of 356 days measured. The reasons for a lack of reliability were related to a lack of solar energy in the winter season, component failures, and unexpected user activities such as power theft. This study also presents a new reliability assessment method for renewable off-grid power systems, based on an interview study with solar micro-grid operators in India. The method is descriptive, which has an advantage of better highlighting localized problems, such as long maintenance outages in remote regions or a lack of sufficient protective measures of some critical system units. The use of the framework may encourage reliability thinking and assist in designing more reliable power systems. - Strategies for decarbonising the energy system in Finland
School of Science | Doctoral dissertation (article-based)(2020) Pilpola, SannamariFinland has ambitious climate mitigation goals and intends to become carbon-neutral already by 2035. However, the pathways to these targets remain unclear. Previous studies on the Finnish energy system have often focused on assuming certain future system configurations, seldom considering possible future pathways for the whole energy system through energy system optimization, and particularly not with an hourly level resolution. The focus of this thesis is to analyse and compare future scenarios for decarbonising the Finnish energy system. A particular focus is given to the three main pillars of the Finnish energy and climate policies: forestry biomass, nuclear power, and wind power, while also considering the uncertainty of future demand, policies, costs and renewable resource availability. As wind power is often an integral part of a decarbonised future energy system, the thesis also covers wind power integration with different system flexibility measures. The main research question of the thesis is what kind of an energy system would meet the climate targets most cost-effectively and be the most resilient to future uncertainties within given system limitations. The main methodology of the thesis is studying possible energy system pathways by constructing and analysing cost-optimized energy system scenarios with techno-economic energy system modelling. For this purpose, a comprehensive simulation model of the Finnish energy system was developed, covering the electricity, heat and fuel sectors. The thesis concludes that there are many different optimal decarbonised energy system pathways for Finland. Common denominators for all scenarios seem to be a high level of decarbonised electricity production (nuclear and wind power), electrification of heat production, active international power exchange, and fossil fuel replacement with biomass. Effective wind power integration would also require system-level thinking and flexibility measures, the most cost-effective options being power-to-heat and curtailment. The thesis also suggests that the energy system's resilience against political disruptions could be improved by increasing and diversifying the renewable resource base, utilising sector-coupling and other system flexibility measures, and improving energy efficiency. However, no single system configuration stands out as the most resilient to future uncertainties.