京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18969号 / 工博第4011号 / 新制||工||1618(附属図書館) / 31920 / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 小池 克明, 教授 松岡 俊文, 教授 田村 正行 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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The world’s demand for energy has steadily increased in recent years, driven by economic and population growth, coupled with the need to reduce greenhouse gas emissions. However, using fossil fuels to meet the energy demand has led to various environmental and social problems, including air pollution, climate change, and geopolitical conflicts. As a result, there has been a growing interest in shifting from fossil energy to green energy sources, which offer a cleaner, and often more cost-effective alternative. This essay will explore the opportunities and challenges of transitioning from fossil energy to renewable energy, with a focus on the economic, environmental, and social implications of this shift, throughout the current state of renewable energy technologies and their potential for widespread adoption, as well as the political and regulatory frameworks that facilitate or hinder this transition. The essay also provides an empirical literature overview of the costs and benefits of transitioning from fossil fuels to renewable energy, including the impact on the environment and employment. I argue that while the transition to renewable energy will not be easy, it is necessary to ensure a sustainable future for our planet and future generations

In this paper it is investigated the role that geothermal energy could play in theenergy mix, to meet new system requirements. As any other source, geothermal energy harnessing implies a number of risksmainly related to induced seismicity and landslides, together with the releaseof as greenhouse gases and metal salts. Moreover, important barriers to itsimplementation still exist, mainly concerning financial aspects and drillingoperations. As well, administrative status is uncertain and related investmentin R&D negligible. However, geothermal energy presents important advantages in relation toother energy sources, as its reliability and large capacity factor, comparable tonuclear and natural gas plants. It could help to reduce both the globalwarming, whose potential is up to 5 times lower than in the case of fossilfuels, and the landuse, the lowest of any power plant. Additionally, in spite ofthe high and risky initial investment, energy produced by geothermal means is amongst the cheapest. The geothermal potential is large enough to substantially contribute to theenergy mix, through locally available resources. Economic potential in Europeby 2050 is estimated in 100 – 4 000 TWheand 880–1 050 TWhth. Nevertheless, currently available technology strongly limits the access togeothermal resources. In addition, predictions about geothermal utilizationare modest and have hardly been achieved to date. The key for the future isthe development of the Engineered Geothermal Systems.

In winter it is common with frost formation in plate heat exchangers. During defrosting, the heat recovery efficiency decreases and the heat rate requirement for post-heating increases. One way to avoid frost formation is to preheat the ventilation air with geothermal energy, a so-called GeoMVHR system. This study examines the profitability of eliminating the requirement for post-heating using geothermal energy to preheat the incoming ventilation air. The study evaluates the profitability for two locations in Sweden, Stockholm and Gothenburg. The method used is the internal rate of return. The heat rate requirement for post-heating in an MVHR system without geothermal outdoor preheating and the power for operating a GeoMVHR system that eliminates the requirement for post-heating is calculated. The internal rate of return was 5.7% for Gothenburg and 5.3% for Stockholm. The conclusion was that the GeoMVHR systems could be seen as a profitable investment.

Ethyl alcohol (ethanol) as secondary fluids is very popular as heat transfer fluid for indirect refrigeration system with ground source heat pump systems (GSHP) in several countries such as Sweden, Norway, Switzerland, Finland and other European countries. There have been several researches about the future ofthe refrigeration sector, refrigerants and refrigeration systems. Moreover, strict regulations such as F-gasregulation and Kigali Amendment forcing a phase down of many current widely used high global warming potential (GWP) refrigerants, i.e. R134a or R410A. Therefore, secondary refrigeration systems and their working fluids are expected to play a key role in order to minimize the refrigerant charge in the systems, reduce the indirect refrigerant leakages as well as increase the safety during operation. The aim of this thesis is to investigate the effect different additives to increase the flame point together with ethanol-based secondary fluids and validate their thermophysical properties by comparing them with reference values for pure ethanol water solutions. The study aims to design a new commercial ethyl alcohol-based product for GSHP system that could replace existing ones in the Swedish market and could workwith natural or flammable low GWP refrigerants. Different high flash point additives were tested such as 1-propyl alcohol, n-butyl alcohol, glycerol andpropylene carbonate. Thermophysical properties were investigated and a GSHP model in Excel was created in order to assess the energy performance of the resulted blends. After screening different blends and assessing the energy performance, glycerol as additive in low concentration seems to be the future for the ethyl alcohol-based secondary fluids because of its high flashpoint (160ºC) that will reduce the flammability risk associated to ethyl alcohol blends, the low viscosity (by 12% lower compared to pure ethyl alcohol blends) that help reduce pumping power by 4.5% compared topure ethyl alcohol blends. Moreover, ethyl alcohol and glycerol blend showed the lost in heat transfer coefficient by 4% lower compared to pure ethyl alcohol blends due to lower thermal conductivity compared to pure ethyl alcohol blends. Finally, it is a rather cheap and natural product which has no problem related to corrosion since ethyl alcohol and glycerol are less corrosive than water. Although, flash point test was not conducted so there is no data regarding the flash point, it is expected the flash point is increased due to the high flash point of glycerol compared to ethyl alcohol or other possible additives. Therefore, it is expected that the flammability risk associated to ethyl alcohol-based secondary fluids is reduced. / Etylalkohol (etanol) som köldbärare är mycket populärt som värmeöverföringsvätska för indirekt kylsystemmed bergvärmepumpsystem (BVP) i Sverige, Norge, Schweiz, Finland och andra europeiska länder. Fleraundersökningar har gjorts om kylsektorns framtid, köldmedier och kylsystem. Dessutom strängaförordningar som F-gas förordning och Kigali- förordning tvingar en utfasning av många nuvarande allmäntanvända köldmedier med den höga globala uppvärmningspotentialen (GWP), dvs. R134a eller R410A. Därför förväntas det att kylsystem och deras köldbärare spela en nyckelroll för att minimera köldmediumsmängd i systemen, minska de indirekta köldmedieläckage och öka säkerheten under drift. Syftet med detta examensarbete är att undersöka effekten av olika tillsatser för att öka flammanpunkten tillsammans med etanolbaserade köldbärare och validera deras termofysikaliska egenskaper genom att jämföra dem med referensvärden för rena etanolvattenlösningar. Studien syftar till att utforma en nykommersiell etylalkoholbaserad produkt för BVP-system som skulle kunna ersätta befintliga produkter på den svenska marknaden och kan arbeta med naturliga eller brandfarliga köldmedier med låg GWP. Olika tillsatser med hög flampunkt testades såsom 1-propylalkohol, n-butylalkohol, glycerol och propylenkarbonat. Termofysikaliska egenskaper undersöktes och en BVP-modell i Excel skapades för att bedöma energiprestanda för olika blandningarna. De erhållna resultaten för olika blandningar visar att glycerol i en låg koncentration som tillsats kan vara framtidens additiv för de etylalkoholbaserade köldbärare på grund av dess höga flampunkt (160 ºC) som förmodligen kan minska brandrisken för etylalkoholblandningar. Dessutom hade glycerol och etanolblandningar den lägsta viskositeten (c.a.12% lägre jämfört med ren etylalkoholblandningar) som bidrar tillen minskning av pumpeffekten med c.a. 4,5% jämfört med rena etylalkoholblandningar. Däremot visade etylalkohol och glycerol blandningen c.a. 4% lägre värmeöverövergångstal jämfört med de rena etylalkoholblandningar på grund av lägre värmeledningsförmåga jämfört med ren etylalkoholblandningar. Slutligen är glycerol en ganska billig och naturlig produkt som inte har några korrosionsproblem eftersom etylalkohol och glycerol är mindre frätande än vatten. Även om flampunkttest inte genomfördes i projektet, förväntas det att flampunkten ökas lite på grund av den höga flampunkten av glycerol jämfört med etylalkohol och andra tillsatser. Därför förväntas det att brännbarhetsrisken förknippad med etylalkoholbaserade köldbärare reduceras.

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Heating and cooling needs have been highly demanded as the extreme weathers become increasingly frequent and global warming becomes well-founded. Because ground temperature keeps relatively constant at 20-30 feet below the surface, using the earth as a thermal mass for temperature conditioning and thermal management creates an energy-efficient and environmentally beneficial approach to surface heating and cooling, which has been used in self-heated pavement, greenhouse, and building integrated photovoltaic thermal systems. Inspired by the human body wherein a blood circulation system keeps skin nearly at a constant temperature under environmental changes, a thermal fluid circulation system is introduced to the geothermal well system. Through bi-directional heat exchange between surface space with the ground, heat harvested at high temperatures can be stored underground for utilization at low temperatures, so that the surface temperature variations can be significantly reduced for daily and yearly cycles minimizing the heating/cooling needs. Understanding the heat transfer under the ground and thermal stress of the heat exchange systems induced by the temperature changes is critical for system design, performance prediction and optimization, and system control and operation. This dissertation studies heat transfer and thermomechanical problems for different geothermal systems. The temperature field of the earth can be calculated given the heat source and ambient temperature. Due to nonuniform thermal expansion caused by temperature differences or material mismatches, thermal stress will be induced. Its interaction with surface mechanical load and displacement constraint will be investigated for the design and failure analysis of the fluid circulation and heat exchange system. In the theoretical study, the earth is approximated as a semi-infinite domain. Green's function technique has been used in the analysis of heat conduction, elastic, and thermoelastic problems respectively. The semi-infinite domain with a surface boundary condition can be considered a special case of two semi-infinite domains with a perfectly bonded interface, which forms an infinite bi-material domain. For a Dirichlet boundary value problem with a constant temperature or displacement, the top semi-infinite domain can be considered with infinitely large conductivity or stiffness, respectively; for a Neumann boundary value problem with zero flux or traction, the top semi-infinite domain can be considered with a zero conductivity or stiffness, respectively. The general Green's functions of an infinite bi-material domain can recover the classic solutions for Boussinesq's problem, Mindlin's problem, Kelvin's problem, etc. The three-dimensional (3D) problems can be used to recover the corresponding two-dimensional (2D) problems by an integral of Green's function in one dimension through the Hadamard regularization. Firstly, the heat transfer problem in an infinite bi-material is introduced and the Green's function is formulated for the temperature change caused by a point heat source in the material. It is used to simulate heat transfer for a spherical heat exchanger embedded underground in geothermal energy applications. The temperature field of the spherical inhomogeneity embedded in an infinite bi-material subjected to a uniform far-field steady-state or sinusoidal heat flux is determined by solving the boundary value problem. Eshelby’s equivalent inclusion method (EIM) is used to consider the mismatch of the thermal conductivities of the particle from the matrix, which is simulated by a prescribed temperature gradient. When the material of one semi-infinite domain exhibits zero or infinite thermal conductivity, the above solution can be used for a semi-infinite domain containing a heat source with heat insulation or constant temperature on the boundary, respectively. The analytical solution has been verified with the finite element method. The formulation is used to simulate a spherical heat source embedded in a semi-infinite domain. The method can be immediately applied to the design of geothermal energy systems for heat storage and harvesting. When the heat exchanger is a long horizontal pipe, a similar procedure can be conducted for the corresponding 2D problem. If the temperature exhibits a cyclic change, such as daily variation, the formulation is extended to the harmonic transient heat conduction problems. Secondly, a similar formulation has been introduced for the elastic problem of an infinite bi-material. The Green's function is formulated for the displacement caused by a point force in the bi-material. It is used to simulate the stress transfer for a spherical heat exchanger embedded underground in geothermal energy applications. The formulation of the heat transfer problem is extended to the corresponding elastic problem. How a surface mechanical load is transferred to the underground heat exchanger is illustrated. The interactions between a heat exchanger and the surface load are investigated. Finally, the thermoelastic problem of an infinite bi-material is introduced and the Green's function is formulated for the displacement field caused by a point heat source in the material. It can be straightforwardly used to derive the thermoelastic stress caused by a distributed heat source by volume integrals. However, when the thermal conductivity and elasticity of the heat exchanger are different from the earth in actual geothermal energy applications, the Green's function cannot be directly used. By analogy to Eshelby's equivalent inclusion method, a dual equivalent inclusion method (DEIM) is introduced to address the dual material mismatch in thermal and elastic properties. The fundamental solutions of a bi-material for thermal, elastic, and thermoelastic problems are versatile and recover the ones of the single material domain for both 2D and 3D problems. The equivalent inclusion method is successfully extended to the thermoelastic problems to simulate the material mismatch. The formulation can be used in designing a geothermal heat exchanger for heat storage and supply for energy-efficient buildings as well as other geothermal applications. Future work will extend the fundamental solutions to time-dependent thermomechanical load and investigate the daily and seasonal heat exchange with the ground using different configurations of the pipelines. The algorithms will be integrated into the inclusion-based boundary element method (iBEM) for geothermal system design and analysis.

Individual and societal desires for fossiluel independence are an increasingly popular goal. This research investigates residential geothermal space heating and cooling as a viable technical and financial alternative. The road to net-zero energy is then assessed, weighing the benefits and detriments to the consumer. First, the template for location-specific geothermal space heating and cooling is developed through a pilot analysis of a home in Memphis, Tennessee. A methodical process of soil investigation, prototype home characteristics, and financial incentives is designed. Expanding upon existing studies, accurate soil data is extracted from beneath the foundation of a specific address, rather than region-wide soil averages. This high level of precision allows the owner of a specific address to preview realistic results and develop truthful expectations. Payback period and system lifetimes savings are calculated using two methods. Second, the framework developed through the Memphis, Tennessee pilot home is used to investigate 11 additional cities across the continental United States. The increase in breadth uses a representative city from its respective climate zone. While each city within a single climate zone will vary from the representative city, a general climate performance can be determined. With each location’s soil properties and heating and cooling demands, the borefield design and heat pump system capacity is customized and applied for analysis. Using human interest surveys from previous energy projects, a climate is ultimately classified as viable or nonviable for geothermal heating and cooling. Finally, the increasingly popular net-zero energy building concept is explored through a complementary solar photovoltaic (PV) array to the geothermal system. An array capacity is sized and priced to offset the total facility energy use in each climate’s representative city. Once determined, the payback and lifetime savings values are calculated and the GHP + PV system results are compared to a baseline + PV system. From this, a system type is identified as the more viable option for each of the 12 climate zones. The final touch on this research is the introduction of the human perceptions toward environmentally friendly renewable energy in general and how it affects a consumer’s ultimate decision.