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3. Doktoritööd

Selle kollektsiooni püsiv URIhttp://hdl.handle.net/10492/7081

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  • Kirje
    Multiple stage storage strategy for the increasing of renewable fraction of the consumer
    (Estonian University of Life Sciences, 2024) Hovi, Mart; Annuk, Andres; Karapidakis, Emmanuel; Institute of Forestry and Engineering; Galinš, Ainars (opponent); Hamburg, Arvi (opponent); Gevorkov, Levon (opponent)
    In this work, various solutions for storing renewable assets were studied. By renewable resources, the author understands sources that are based on natural resources that are constantly renewable and can be used in unlimited quantities without harming the environment or depleting resources. In a narrower sense, they are treated as sources of energy. The work focused on multistage storing in cases where receipts and consumption are of the same order of magnitude. The aim was to raise the share of the property used on the spot in relation to what was given away and received from other sources. To summarize: • The combination of different renewable storage devices increases the likelihood that the share of on-site consumption of the collected energy or other assets can be significantly increased. • When storing renewables, it is important that the system preserver is always ready to receive the flow of tops of the production schedule. For example, its increase in the considered model up to 6 kW·h leads to an increase in the self-consumption factor by 5%. • Priority is given to the supply of non-displaced customers, which will be covered by the battery in the event of an energy deficit. The lack of a battery leads to a self-consumption factor below the value of 32%. • When storing stored property, it is necessary to maintain it. For example, in the case of rainwater, its regular relocation between storage tanks should be carried out, during which additional filtration will also take place. An analogy of energy storage devices to this would be conversion, but also inevitable self-discharge. The transport, storage and use of non-renewables of an incidental nature is more complex and costly than the use of non-renewable energy sources and other assets. However, this is compensated by local use, taking into account the values of the environment and the community. The use of a pre-storage device in series with a liquid storage allows different values of the session peak cutting factor to significantly increase the self-consumption of renewable energy.
  • Kirje
    Chemical-free pretreatment of lignocellulosic biomass for the conversion of structural polymers
    (Estonian University of Life Sciences, 2024) Sjulander, Nikki; Kikas, Timo; Institute of Forestry and Engineering; Jäger, Alexander (opponent); Mežule, Linda (opponent)
    Due to the depletion of fossil fuels and global warming, caused by greenhouse gas emissions, industries and governments are searching for “greener” energy solutions. Bioenergy has already been investigated by scientists as an alternative to fossil fuels. The main focus in this field is to use lignocellulosic biomass for the production of heat, electricity, and chemicals. However, the use of lignocellulosic biomass is challenging due to its complex structure. This structure needs to be opened up before biomass can be further processed. Different pretreatment technologies have been developed in order to break the lignocellulosic structure; however, most of them have technological and economical limitations and they raise environmental concerns due to formed waste. As a result, the research continues to find the best pretreatment technology.
  • Kirje
    Post-fire recovery dynamics of hemiboreal Scots pine forest ecosystems
    (Estonian University of Life Sciences, 2024) Orumaa, Argo; Metslaid, Marek; Institute of Forestry and Engineering; Brazaitis, Gediminas (opponent)
    Hemiboreal forests are influenced by several natural disturbances. Forest fires are one of the most severe disturbances that can affect the entire forest ecosystem: stand structure and composition, understorey vegetation, soil nutrient quantities and properties, and soil biota. In this thesis, the chronosequence method was used to study the impact of fire on above- and below-ground components of hemiboreal Scots pine forest stands. The fire chronosequence consisted of areas burned in the years 2008, 1997, 1982, 1951, 1940, and 1837. Soil respiration was affected by time since fire and it was lowest in the most recently burned area. CH4 measurements showed that post-fire soil was CH4 sink in all the areas. The highest uptake of CH4 was measured in the most recently burned area and the lowest uptake was recorded in the oldest area, but there were no statistically significant differences between the areas. For N2O fluxes, the soil acted as both a sink and a source in post-fire areas, but there were no statistically significant differences between the areas. There were considerable compositional differences in the vascular plant and bryophyte assemblages between recently burned (burned in 2008, 1997 and 1982) and older burned (burned in 1951, 1940 and 1837) stands. The cover of the vascular plants and bryophytes layer increased with time since fire, while the trends for species richness and diversity along the post-fire chronosequence were less clear. For soil fungi, hemiboreal forests are typically dominated by Basidiomycota, but in the post-fire stands studied in this thesis, Ascomycota was dominating. Along the fire chronosequence, soil fungal communities were primarily dominated by fungi for which a functional group could not be determined, followed by soil saprotrophs, EcM fungi, and root endophytes.
  • Kirje
    Development of control and optimization methods for wood drying
    (Estonian University of Life Sciences, 2023) Tamme, Hannes; Muiste, Peeter; Tamme, Valdek; Institute of Forestry and Engineering; Spulle, Uldis (opponent)
    The purpose of wood drying is to obtain material with the desired final moisture content. During the process, it is necessary to find a balance between the quality of the wood and the energy and time spent. In order to find an optimal solution, it is important to know the condition of the material to be dried at a specific time. Often, the standard equipment of an industrial kiln is not enough when implementing a new drying schedule. In addition, there is a need for software that simulates wood drying, providing the initial drying regime, which can then be further optimized using the methodologies and sensors presented in this study. This work checks the occurrence of theoretically simulated moistening impulse response in experiments based on synchronous surface displacement, temperature and surface moisture sensors. For this purpose, it was necessary to develop specific sensors used for wood monitoring in the wood dryer climate. A physically justified measurement result is guaranteed when using sensors based on different working principles. When investigating the reliability of the sensors, typical issues and practically usable measurement ranges below and above the saturation point of the wood fiber were identified. The transition of the surface layer of wood from the first phase of drying to the second phase of drying was investigated. It was found that the transition of the surface layer to the second drying phase slows down the diffusion rate in the inner layers of the wood, which otherwise could still be in the first phase of drying due to their moisture content. In order to characterize the influence of the diffusion constant of the surface layer on the inner layers, the concept of critical diffusion constant was defined. In the process of wood drying, individually calibrated carbon fiber needle-electrodes were used to record the moisture content of wood at different depths from the surface of the board being dried. When creating an optimized drying plan taking into account the critical diffusion constant, it was possible to significantly shorten the drying time of a typical industrial drying plan at the same final moisture content. The optimized drying schedule was characterized by a higher drying strain, which can level down by a timed moistening pulse. In this thesis, the relative humidity value of the critical drying air corresponding to the critical diffusion constant for pine wood was found. It is easy for a timber company to use this information to modify its drying schedules on site. An electrical indicator of maximum wood drying stresses was also developed. Since the electrical resistance method is mostly used for wood drying monitoring in practice, the hardware and software of the patented polarization-type moisture meter prototype were improved. Galvanostatic charging mode and depolarization voltage monitoring capability was added.
  • Kirje
    Liquid fuels air-fuel mixture processing system with spark ignition engine for use in generators in a crisis situation
    (Estonian University of Life Sciences, 2023) Raide, Veljo; Olt, Jüri; Ilves, Risto; Institute of Forestry and Engineering; Horák, Vladimír (opponent); Gailis, Maris (opponent)
    In times of crisis and war, there is a high probability that the operation of electrical networks will be disrupted, and therefore some distribution networks may remain without electricity. As a result, important information- and communication technology nodes of the core networks of the country's critical infrastructure are also without electricity. In a crisis situation, the missing electrical energy can be produced with widespread spark ignition engine-generators, but in their case, flexibility in the use of fuels is important. In a crisis situation, all possible engine fuels, both light and heavy, must be used. The problem with the use of engine-generators for the production of electricity is their single-fuel nature. By heating the fuel mixture, better evaporation is achieved and thus it is possible to bypass the problem and use all liquid fuels intended for internal combustion engines to produce electricity in a spark-ignition engine-generator. The dissertation is based on six articles. The research paper described the effects of EU and NATO legislative acts on the liquid fuels used in Estonia, analyzed solutions for solving problems related to energy security in Estonia, analyzed the bottlenecks in the use of diesel and biodiesel fuels in compression ignition engines, developed a technical solution for a air-fuel mixture processing system made from different fuels, and improved the solution so that it is suitable for use with spark ignition engine and engine experiments were performed. The aim of the work was to develop a air-fuel mixture processing system made of different liquid fuels for a spark-ignition engine, and its suitability in practice was assessed. The aim of the system is to adapt the air-fuel mixture for use in a spark-ignition engine in order to ensure the production of distributed electricity in times of crisis and war to support the country's critical infrastructure. The conducted research and the developed solution contribute to the production of distributed electricity with spark ignition engine-generator in a crisis or war situation, using currently available liquid fuel to supply the country's information systems with electricity until NATO Article 5 is triggered, if necessary.
  • Kirje
    Changes in agricultural land use in Estonia: opportunities for stable land use
    (Estonian University of Life Sciences, 2023) Rasva, Marii; Jürgenson, Evelin; Liibusk, Aive; Institute of Forestry and Engineering; Lisec, Anka (opponent); Kurowska, Krystyna (opponent)
    The issue of land concentration in the EU and many parts of the world remains basic and is one of the most serious land issues in the district today. Agricultural land is becoming increasingly concentrated into the hands of large businesses, a situation in which small farmers are losing control over the agricultural land. This phenomenon is affecting Estonian agriculture. After the regaining of Estonian independence in 1991, restitution of farmlands based on the pre-Second World War ownership and privatisation of collective farms took place. The land reform law favoured agriculture based on small farms. In the first ten years of regaining independence, the number of farms in Estonia increased to 55,748 in 2001. Many small agricultural producers arose but in the following years this number decreased and in 2020 there were 11,369 farms left. While the average land use per agricultural household in Estonia has increased, the agricultural land in Estonia has become increasingly concentrated into the hands of corporate bodies. Not only land use is concentrating but landownership is concentrating as well. This is a dangerous sign. Control over the land is concentrating in the hands of small number of large corporations and there is a need to take action against this development in Estonia. Estonia needs policy direction and regulations for the agricultural land market that help to mitigate the impact of land concentration. Relationship between large agricultural producers and small-scale farms must be enabled so that both farming types can stay in fair market competition. The direction of the policy and extent of the area of land use or ownership in Estonia is a matter for further research and even debate, to determine appropriate regulations, possible limitations to land areas, the usage of pre-emptive right and the creation of land bank in Estonia. Definitely, there is a need for transparent structure of enterprises in Estonia.
  • Kirje
    Genetic diversity, origin, and new hosts of the invasive pathogen Lecanosticta acicola in northern Europe
    (Estonian University of Life Sciences, 2022) Vester, Marili; Drenkhan, Rein; Institute of Forestry and Engineering; Müller, Michael M. (opponent)
    Lecanosticta acicola is an ascomycete causing Brown Spot Needle Blight (BSNB) – a foliar disease affecting pine species (Pinus spp.). The disease causes premature needle shedding which leads to reduced growth of the trees and to severe defoliation which may cause death of the infected trees. The native range of L. acicola is in North America but during the last century anthropogenic activity introduced the pathogen also into Europe and Asia. In Europe, L. acicola was first detected in southern and central regions, but during the current century the disease agent has significantly distributed northward and is now present also in the Baltic states and southern Sweden. So far, in northern Europe, the disease has mostly affected non-native pine species. Nevertheless, by today, L. acicola has already succeeded to infect native Scots pine (Pinus sylvestris), although so far damage on the P. sylvestris is insignificant and only the trees growing in the immediate proximity to non-native P. mugo have been found to be infected. The distribution range of L. acicola appears to be expanding also to the western Asia. In this thesis the pathogen was identified for the first time in Turkey and Georgia. In Turkey, the infection of BSNB was found also on Cedrus libani, proving that L. acicola is not only a Pinus-specific pathogen. Identical genetic strains of the pathogen were found on the distances that the fungus could not spread naturally. Identical strains were found to be shared even between continents, being present in Canada and Germany, but similarly strains were also shared between the countries in Europe. Lecanosticta acicola has reached northern Europe predominantly from pre-existing populations in central Europe and not directly from its native range in America. Surprisingly high diversity found from the populations in northern Europe is probably the result of repeated introductions from genetically different source populations. Although, by today, both mating types of the fungus are present in northern Europe and probably in some limited areas the sexual reproduction takes place, the clonal reproduction mode dominates in this region. Based on the so far distribution of this pathogen and that the ongoing climate changes support its dissemination, it can be assumed that northwards spread of L. acicola will continue in Europe. It is important to avoid any further increase in the diversity of pathogens’ populations, that could easily happen if new fungal strains are imported with insufficiently controlled planting material. An increase in diversity could lead to dangerous development of strains with higher virulence or higher suitability to local climate conditions which could result in higher damage to the forest stands.
  • Kirje
    Biosensor array for BOD measurements in different types of wastewater
    (Estonian University of Life Sciences, 2022) Pitman, Kätlin; Raud, Merlin; Nerut, Jaak; Kikas, Timo; Institute of Forestry and Engineering; Kokina, Kristīna (opponent)
    The basis of all life on Earth is the existence of clean water. The state of water must be constantly monitored to ensure that the water is clean. One critical parameter in accessing water purity is the content of organic pollutants. A universal parameter - biochemical oxygen demand (BOD) – is widely used to assess this. The BOD test determines the amount of biodegradable organic compounds in wastewater. The dissolved oxygen concentration in the test sample is measured at the beginning and end of the incubation period, and the BOD value is calculated from the difference. The incubation period during which the decomposed organic matter content is measured is 5 days according to the American standard (BOD5) and 7 days according to the Swedish standard (BOD7). Such a long incubation time is a severe disadvantage of this method, as, during this time, elevated BOD can have irreversible consequences for the water body. An alternative method for measuring BOD is to use biosensors that can estimate the BOD value of the sample in minutes. A biosensor is an analytical device consisting of a biological recognition element and physicochemical transducer. BOD biosensors use microorganisms as the biological recognition element and the dissolved oxygen sensor as the transducer. The accuracy of the biosensor depends mainly on the microorganisms used. Microorganisms with a broad substrate spectrum that use different types of organic compounds for their activities provide a better match to the BOD7 value. There are several other solutions for the selection of microorganisms. Mixed bacterial cultures have a broader substrate spectrum however, their composition changes over time and causes signal instability. One possibility, when it comes to approaching these issues, is to use different bacterial cultures in a sensor array in which different cultures are spatially separated from each other. The use of spatially separated pure microbial cultures makes it possible to prevent contamination between different sensors in the array while also ensuring a longer lifetime for the sensor. Different possibilities for constructing a BOD sensor array were investigated in this work. Microchips based on various technologies were first studied, and their suitability for electrochemical measurements was tested. The experiments showed that the screen-printed electrode array was suitable for the biosensor. The biosensor array was then constructed using the bacterial culture Pseudomonas putida Pc15, and its behaviour in different solutions was investigated. It was shown that the BOD of water samples could be estimated with the obtained biosensor array. In the future, it may be feasible to measure BOD values with the sensor array consisting of several membranes that have been modified with different pure bacterial cultures.
  • Kirje
    Occupational noise exposure and hearing problems among active military service personnel in Estonia
    (Estonian University of Life Sciences, 2022) Luha, Assar; Merisalu, Eda; Orru, Hans; Institute of Forestry and Engineering; Pääkkönen, Rauno (opponent); Kic, Pavel (opponent)
    Hearing loss is one of the most common health problems among military personnel. Active service members are mostly exposed to high-level noise from gunfire, blasts, and military vehicles commonly used by the Land, Naval and Air Forces. Prolonged exposure to noise increases the risk of noise-induced hearing loss (NIHL) and tinnitus. However, noise exposure can be reduced by using personal protective equipment or technologies with reduced noise levels. Noise levels were measured in vehicles and from weapons or weapon systems commonly used in the Estonian Defence Forces. The subjects of the study consisted of two groups: one group of subjects had to complete a questionnaire about their health and work-related risk factors. The second group had to first answer questions about their exposure to noise during their routine medical examinations. Subsequently, they underwent an audiometric examination in which testing was carried out at two frequencies. Based on the collected data, the associations between noise exposure, risk factors and NIHL were studied using various statistical methods. The data were also interpreted using mathematical equations and laws relevant to noise propagation and attenuation. Almost half of the active service members suffered from noise-induced hearing loss during their service period. The most important factors that could affect and facilitate prediction of the occurrence of hearing health problems in the military are the age of the active service member, service duration, shooting blanks, and driving vehicles such as PASI and Bandvagn. The most important finding was that military personnel who do not use hearing protection have more hearing problems than others who do. NIHL is difficult to prevent in the military during the active service period. Therefore, working out less noisy military technologies, vehicles, and weapon systems and using appropriate hearing protectors are prerequisites for reducing the incidence of hearing problems in the Defence Forces.
  • Kirje
    Development and optimization of a reagent-free pretreatment method for the production of liquid biofuels from lignocellulosic waste
    (Estonian University of Life Sciences, 2022) Rooni, Vahur; Kikas, Timo; Institute of Forestry and Engineering; Inayat, Abrar (opponent)
    Global energy consumption and demand are increasing every year. In 2018, the distribution of global energy consumption was as follows: 51% heat production, 17% electricity production and 32% transportation sector. There are a number of sustainable “green” alternatives to fossil resources for heat and electricity production however, alternative solutions for the transportation sector are lacking. Presently, the biofuels used to reduce the share of fossil fuels are mostly produced from sugar, starch or oil-rich food crops. In order to avoid competition between food and fuels, the greatest potential is seen in the second-generation biofuels, such as bioethanol, which are produced using different plant (lignocellulosic) residues as a feedstock. It is possible to produce bioethanol, biodiesel and, for example, synthesis gas, but compared to the first generation, the production is much more complex due to characteristics of raw material therefore requiring a higher energy input. The second-generation bioethanol production chain includes several steps: pretreatment, hydrolysis, fermentation, and distillation. The most important process in the entire production chain is pretreatment. The more efficient the pretreatment, the higher the yield of sugars and ethanol that can be achieved. In order to increase the competitiveness of the second-generation bioethanol as an alternative motor fuel, various pretreatment methods were studied in the framework of this doctoral thesis to ensure maximum cellular structure disruption with minimal energy and no chemical input. Freezing, as a potential zero-energy pre-treatment method for the Nordic climate, and explosive decompression method were investigated in order to determine the optimal pretreatment process parameters.
  • Kirje
    Quantitative assessment of stand structural traits and health condition in hemiboreal forest ecosystems
    (Estonian University of Life Sciences, 2022) Põldveer, Eneli; Korjus, Henn; Laarmann, Diana; Institute of Forestry and Engineering; Marozas, Vitas (opponent)
    In recent years, the focus of Estonian forestry has been on the better integration of economical, ecological and social functions, and consideration of ecosystem services. Although forest conservation is an important step for maintaining biodiversity, it may insufficiently provide expected ecological value. Efforts should promote stand structural diversity also in commercial forests, as the variety of stand structural elements that form structural patterns will promote resources for different species and ecosystem services. Loss of structural diversity negatively affects forest resistance to different abiotic and biotic factors. The current thesis synthesizes several methods that could be used in forest management and planning to focus on ecosystem values. The main aims of the thesis are to quantify the structural heterogeneity and spatial patterns of stands with different management history, to quantify the proportion of visually healthy trees with internal decay indicating decreased vitality in intensively managed conifer forest stands and to distinguish the spatial location of trees with internal decay. The thesis is based on the data collected from the Estonian Network of Forest Research Plots and different structural indices were used to reach the set aims. The results showed that the variety of structural elements and patterns (multiple tree species and sizes, such as large old trees or regrowth, deadwood abundance, different spatial distribution of dead and alive trees) in forest stands with different management history showed significant differences but depended on the main tree species. Heterogeneous stand structures were generally significantly more abundant in unmanaged (natural, recovering or conservation) forests compared to other forest types. For example, unmanaged Norway spruce stands were significantly more heterogeneous in mingling of species and differences in tree dimensions when compared to managed stands, and the positioning of trees was more irregular. Living trees were significantly more often surrounded with dead trees in Scots pine stands belonging to natural forests than in recovering or managed forests, and dead trees more frequently clumped in natural forests. Despite the lack of visual symptoms of internal damage, conifers in intensively managed stands had well-developed decay at the root collar, especially in Norway spruce stands. Additionally, visually healthy trees with internal decay were relatively often clumped, especially in Norway spruce stands. Structural indices are effective tools that provide quantitative information about spatial forest structure, e.g., how trees of different species, dimensions or vitality status are mixed in space and how heterogeneous the structure of a forest stand is. The importance of such indices will increase in forest management as high-resolution remote sensing data providing tree-level information will be used on a large scale.
  • Kirje
    Cultivation technology for lowbush blueberry cultivation in milled peat field plantations
    (Eesti Maaülikool, 2021) Arak, Margus; Olt, Jüri; Szabó, István (opponent); Šarauskis, Egidijus (opponent)
    Blueberry cultivation on milled peat fields is not particularly common in Estonia, while also not being very profitable. The basis for the development of blueberry cultivation is the mechanization and automation of production. This consists, on the one hand, in the development of machines and technical equipment with suitable productivity and, on the other hand, in reducing the operating costs of the machines. This doctoral thesis is largely based on six original publications and two intellectual properties. The aims of the thesis were to describe the technological peculiarities of a blueberry orchard planted on milled peat fields, to collect basic data for the development of a machinery which allows to reduce the importance of manual labour and to replace it with machinery to reduce the unit cost involved in technological operations and, thereby, to reduce manufacturing costs. In order to compile the initial task of designing a mechanized blueberry harvesting technology, the relationship between the various elements which are involved in blueberry cultivation (berry-plant-field-machine), all of which have been described, methodology has been developed to determine the physical properties of the blueberry plant. A methodology has been developed for determining the design and kinematic parameters of a motoblock-type blueberry harvester and for selecting the material of the harvester, the duration of the vernalisation period in Estonia was also determined. The patents that have been issued in the development of blueberry cultivation technology show that novel solutions have been elaborated. The studies that have been carried out and the solutions which have been developed could help in and become a prerequisite for the development of new equipment which will serve to foster the establishment of new blueberry plantations, first and foremost on milled peat fields, but also in terms of increasing profitability levels and reducing the ecological footprint in already established blueberry plantations.
  • Kirje
    Методика оптимизации структуры посевных площадей и размещения севооборотов в условиях Эстонской ССР : автореферат диссертации на соискание учёной степени кандидата экономических наук
    (Тартуский государственный университет, 1974) Elmet, Henn
    Среди мер, направленных на повышение объема и эффективности производства сельскохозяйственной продукции, большое значение имеет усовершенствование методики планирования производства. В директивах XXIV съезда КПСС указывается на необходимость дальнейшего усовершенствования существующей системы планирования, на связь размещения сельскохозяйственного производства с местными природно-экономическими условиями и на широкое применение экономико-математических методов и ЭВМ при планировании сельского хозяйства. Проблемы настоящей работы обусловлены требованиями практики дать экономическое обоснование структуры сельскохозяйственных культур и размещения севооборотов с детальнoстью свойственной точным наукам.
  • Kirje
    Distribution of and damages by Dutch elm disease agents in Northeastern Europe
    (Eesti Maaülikool, 2021) Jürisoo, Liina; Drenkhan, Rein; La Porta, Nicola (opponent)
    As elsewhere in the world, elms in Northeastern Europe are threatened by Dutch elm disease (DED), which is one of the most devastating diseases of the elms. The aim of the doctoral thesis was to investigate the spread of Dutch elm disease, its agents and their impact on the health status of different elms, vectors and the health of the elms in different habitats in Estonia and North-West Russia. The results of the work demonstrated that U. glabra is more susceptible to DED than U. laevis and elm hybrids. Therefore, we recommend to use U. laevis rather than U. glabra in landscaping. Elm hybrids may be promising in green areas, but they still need to be tested before they can be widely used in the conditions of Northeastern Europe. DED is known to have caused damage to Estonia's elms since 1930s. First, the pathogen was Ophiostoma ulmi which has now been replaced by a new species, Ophiostoma novo-ulmi. In Europe the European subspecies (subsp. novo-ulmi) and to a lesser extent the North American subspecies (subsp. americana) are both the causal agents of DED in Estonia; however, the latter was detected only in Northwestern Estonia, incl. Hiiumaa island. As the American subspecies of DED is more aggressive than the European one, it causes the mortality of elms at a faster rate as was the case in North Estonia. In the current work the presence of an aggressive hybrid of this pathogen was also proved for the first time in Estonia in 2020. At the same time, our investigations demonstrated that the pathogen´s hybrids between subspecies are even more aggressive as revealed by their significantly faster growth in pure cultures. The reason may be the poor health status of elms in Russia where these hybrids were particularly often detected. Additionally, in this work it was found that Xyleborinus saxesenii and Xyleborus dispar are new potential vectors for DED. This is clear evidence of wider spread potential of DED and greater risk to elms.
  • Kirje
    Novel application principles for energy storage technologies in nearly zero energy buildings
    (Eesti Maaülikool, 2021) Lill, Heiki; Allik, Alo; Annuk, Andres; Colak, Ilhami (opponent); Hamburg, Arvi (opponent)
    In this thesis the renewable energy storage options in residential buildings are under investigation. This is to store cheap electricity due to the temporary overproduction of large wind farms and also on-site solar and wind farms. In an electric system, there should be a balance at all times between energy production and consumption: as much as is produced should also be consumed. Deviating significantly from this balance can damage electrical equipment or cause serious network failures and even blackouts. Unfortunately, both solar and wind energy generation possibilities are associated with (rapid) changes in production. The simplest examples are wind gusts for wind turbines and intermittent cloud cover for solar panels where the electric output power changes in seconds. In order to smooth out the rapid changes in electricity production, the work proposes the possibility to add ultracapacitors to the battery bank for temporary energy storage, which would act as a buffer and are able to temporarily store the produced electricity. So far, the sale of green energy to the electricity grid has been supported at the state level. However, this paper examines the next step in how to support the storage capacity of the produced energy in order to increase self-consumption. To this, a state subsidy measure for battery banks is proposed. Due to short-term overproduction of electricity, there are more and more situations where electricity is sold at zero or even negative prices on the power exchange. The reason is simple - it is more practical for producers to temporarily pay to consumers for electricity consumption than to stop production for a while. This work also proposes a method for storing energy in heat carriers under favorable conditions for the consumer in order to ensure a balance between the production and consumption of the electricity network.
  • Kirje
    Short-term effect of felling on carbon fluxes and storages in different Estonian forest ecosystems
    (Eesti Maaülikool, 2021) Aun, Kristiina; Uri, Veiko; Pumpanen, Jukka (opponent)
    Boreal forests are large C sinks that play an essential role in the fight with climate changes. However, forest management and especially fellings are one of the important factors affecting stand C accumulation capacity. In the present thesis, the short-term effect of thinning and clear-cutting was studied on the ecosystem C cycling in silver birch and Scots pine stands. On the basis of a compiled C budget, a 6-year-old pine stand acted as a moderate C source (NEP -1.37 t C ha-1yr-1), which was in accordance with the results of eddy-covariance measurements at the same study site (NEE -1.19 t C ha-1yr-1). Since the magnitude of C loss via soil heterotrophic respiration was similar with that in middle-aged and matured stands, then the C source status of the site was attributed mostly to the small biomass of trees and thus to the low annual production of the stand. However, since the annual production of the trees increased in the following years and, assuming a stable annual Rh flux, the ecosystem will become a C sink already during a short period after clear-cut. Therefore, the recovery of the C budget after clear-cut depends strongly on timely and vigorous regeneration. Moderate thinning from below was used in the current study. Thinning did not significantly increase soil heterotrophic respiration in silver birch or Scots pine stands of different ages in the second post-thinning year. As expected, thinning decreased the aboveground C storage of the stand, as well as the production of the tree layer, but all thinned stands still remained their C sink status (NEP 1.5-4.4 t C ha-1 yr-1). Therefore, the use of moderate thinning from below can be recommended the retain a stand as a C neutral ecosystem in the immediate post-thinning period both for silver birch and Scots pine. The recovery of C accumulation rate to the pre-thinning level may occur roughly during the following five years.
  • Kirje
    The effects of land use and climate change on the hydropower potential in Estonian rivers
    (Eesti Maaülikool, 2020) Tamm, Ottar; Tamm, Toomas; Koivusalo, Harri (opponent)
    Water-related changes are currently receiving the most focus in the context of climate change around the world. The changing climate conditions have already redistributed water resources, including the hydropower potential in Estonian rivers. This phenomenon is expected to continue in the future. However, the effects on land use and climate change on the hydropower potential in Estonia are unclear. The technical hydropower potential was assessed for the territory of Estonia (excluding the Narva River). According to the developed method, the total technically feasible hydropower potential in Estonia was calculated to be approximately 80 MW, which was considerably higher than that previously reported. The SWAT model was employed to assess the potential impact of future climate and land-use change on river flow in various Estonian river watersheds, where hydropower is or has been harvested. The SWAT model demonstrated satisfactory performance in describing the hydrological processes in Estonian rivers by using series of mathematical equations. An evident linear trend between the effects of deforestation and afforestation on yearly river flow was observed. The following general rule can be applied to Estonia; a 5% forest change induces a 1% change in annual average flow. Hydrological modeling indicated a positive change in river flow according to both climate scenarios. Increases in the mean annual flow of 10% and 33% were predicted by the climate projections KNMI and DMI, respectively. Hydropower potential is expected to increase in Estonia by the end of the century, compared with the baseline period. The installation of additional turbines, along with upgrading existing turbines, could increase the installed capacity. Furthermore, with the projected overall increase of hydropower potential in Estonia, the construction of new stations becomes more economically feasible and profitable.
  • Kirje
    Solar and wind energy seasonal heat storage systems for residential buildings in Nordic climate
    (Eesti Maaülikool, 2020) Kalder, Janar; Annuk, Andres; Kokin, Eugen; Galinš, Ainars (opponent)
    In this doctoral thesis are handled possibilities, how to increase renewable energy on-site usage, which is produced from wind and sun. In the Nordic climate, the main share of energy consumption in buildings is the need for heating, being ~ 80% in Estonia. To increase the share of renewable energy for heating buildings, the energy storage systems are needed, because this renewable energy is fluctuating. The doctoral thesis is based on a four scientific publications, the first of which examines the behaviour of a system without storing devices, changing the ratio of wind and solar energy production, if the heat pump is used for heating. In this case, if ratio of wind and solar energy is 70% and 30%, the annual coverage of electricity supply can be up to 37.5%. The second publication presents the results of using the hot water boilers as energy storage in different wind conditions. In a good wind conditions, the share of renewable energy can be 68%. In the third and fourth publications, the results of using the vacuum insulated tank in the house heating system are presented. In this tank, the heat energy can be stored for a long time. Producing the heat energy with the solar collectors in summertime and storing it in the vacuum insulated tank, allows cover 41% of the heat energy demand from the renewable sources. Integrating the vacuum insulated tank in to the renewable energy system, where the electricity is produced from sun and wind, the annual share of renewable energy is 79%. In this case, the additional heating devices are not needed, because amount of the electricity from the grid is comparable small.
  • Kirje
    Evaluation of forest management in the context of carbon fluxes: Eddy-Covariance method
    (Eesti Maaülikool, 2020) Rebane, Sille; Jõgiste, Kalev; Metslaid, Marek; Nilsson, Urban (opponent)
    Boreal forest, being a widely distributed vegetation type as well being a substantial part of the global carbon (C) cycle, must be addressed to understand the consequences of climate change. Forest ecosystems are part of the biosphere as a whole and have a role in maintaining global equilibrium. Also, forest ecosystems have the ability to regulate Earth´s climate and energy fluxes. The forests, including boreal and hemiboreal forests, can absorb CO2 from the atmosphere and store carbon in biomass. This kind of ecosystem study is conducted by the widely used eddy covariance (EC) method. It is the most adequate way to measure net ecosystem exchange (NEE) between ground and atmosphere. EC is a direct micrometeorological measurement method for identifying C-fluxes in forest ecosystems. Clear-cutting changes the forest ecosystem C-balance. It is assumed that immediately after a clearcut, a huge amount of C is released, which make a forest stand act as a C-source. The time necessary for forest ecosystem recovery after clear-cutting also depends on the choice of regeneration method. Natural regeneration usually takes more time than artificial regeneration by planting or sowing and the proper regeneration method may shorten the recovery period. In Estonia, clear-cutting is the common forest management practice followed by planting to establish the new forest generation. Planting may speed up revegetation that increases C sequestration. Thus, proper forest management practices provide important strategies to mitigate global climate change. After clear-cutting, a 5-year-old mixed stand acted as C-neutral forest ecosystem during the measurement period (May to August). In the 6- and 8-year-old study stands, the ecosystem showed already C-sink status during the measurement period (June to September). Literature review showed that after clear-cutting, recovery to C-sink status could take 10 years and in some cases even 20 years. Forest recovery after wildfire may take up to 50 years and most likely even more. Measurements after insect outbreaks and windstorm disturbance detected recovery after 3 to 6 years; however there were few such studies and more research is needed for further comparisons.
  • Kirje
    Second-generation bioethanol production: strategies for sidestreams valorisation in a sustainable circular economy
    (Eesti Maaülikool, 2020) Da Rocha Meneses, Lisandra Marina; Kikas, Timo; Orupõld, Kaja; Gilroyed, Brandon Harvey (opponent)
    Due to the increase of the energy production mainly from non–renewable energy sources, there is a need for alternative sources of energy. Second–generation biofuel production, using lignocellulosic biomass as a feedstock, is emerging as an important liquid biofuel for the transportation sector. However, its production is still very costly, and inefficient mainly due to the large quantity of sidestreams that are generated. Sidestreams from bioethanol production process bring environmental, economic and energetic constraints. a. Environment From the environmental point of view, sidestreams from bioethanol production are highly pollutant. There are several handling options for these residues and it includes: discharge; marine outfall; agricultural fields; sewage treatment; lagoon treatment; anaerobic digestion; incineration; and drying. From all these solutions, anaerobic digestion can be used to reduce the pollution potential of the production waste, and to reduce CO2 and other greenhouse gas (GHG) emissions. b. Economics From the economic point of view, the high costs of bioethanol production require additional valorisation of its stillage to create new revenue streams. Pretreatment and enzymatic hydrolysis are the two most costly parts of the production process. Anaerobic digestion of bioethanol stillage is a prospective solution to offset the costs of bioethanol production and add value to the production chain. c. Energetics Finally, stillage has a high energetic value and can be used to produce energy in the form of methane. Its utilization can give higher net energy outputs and improve the efficiency of the production chain. In addition, ethanol from biomass has an energy return on energy invested (ERoEI) lower than for fossil fuels. Thus, stillage and bioethanol sidestreams can be used for processing energy, through anaerobic digestion to add value to the production chain and improve the energy balance. To sum up, there is a continuous search for strategies that make use of the high energy content of bioethanol sidestreams. Anaerobic digestion to produce energy in the form of methane has been reported as an effective strategy to increase the energy output from the biomass, help to balance the costs of bioethanol production and as an environmental solution to this highly pollutant residues.