Green energy

Green Energy Santiago Silvestre; Jordi Salazar. Annotation This course describes technologies, applications and basic concepts related to most relevant sources of renewable energies. Objectives After this course in Green Energy, the student will achieve knowledge about main characteristics and applications of most relevant sources of renewable energies as hydroelectric, wind and geothermal energies, biomass, and photovoltaic solar energy. Keywords hydroelectric energy, wind energy, photovoltaics, hydrogen technologies, biomass, geothermal energy Date of Creation 06.12.2021 Duration 10 hours Language English License Creative Commons BY-SA 4.0 ISBN Literature [1] E. D. Coyle, B. Basu, J. Blackledge and W. Grimson. Harnessing Nature: Wind, Hydro, Wave, Tidal and Geothermal Energy. Purdue University Press,2014. https://www.jstor.org/stable/j.ctt6wq56p.9 [1] Kumar, A., T. Schei, A. Ahenkorah, R. Caceres Rodriguez, J.-M. Devernay, M. Freitas, D. Hall, Å. Killingtveit, Z. Liu,2011: Hydropower. In IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation [O. Edenhofer, R. Pichs-Madruga, Y. Sokona, K. Seyboth, P. Matschoss, S. Kadner, T. Zwickel, P. Eickemeier, G. Hansen, S. Schlömer, C. von Stechow (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. [2] Hydro Power Basics. Energypedia: Hydro Portal. https://energypedia.info/wiki/Portal:Hydro [3] A. M. Bagher, M. Vahid, M. Mohsen and D. Parvin. Hydroelectric Energy Advantages and Disadvantages, American journal of energy Science, pp. 17-20,2015 [4] BizVibe. Hydropower Generation Industry: Top 20 Hydropower Producing Countries in the World2020. https://blog.bizvibe.com/blog/uncategorized/top-hydropower-producing-countries [5] European Commission. An EU Strategy to harness the potential of offshore renewable energy for aclimate neutral future, Brussels2020. COM(2020)741 - EU Strategy to harness the potential of offshore renewable energy for aclimate neutral future [6] O. Planas, What Is aWind Turbine? Types and Characteristics,2019. https://solarenergy.technology/renewable-energy/wind-power/wind-turbines#horizontal-axis-wind-generator [7] D. Clayton, Types of Wind Turbines: HAWT, VAWT and More Explained,2021. https://energyfollower.com/types-of-wind-turbines/ [8] J. Unwin and M. Farmer, The top 10 countries with the largest wind energy capacity in2021. Power Technology,2021. https://www.power-technology.com/features/wind-energy-by-country/ [9] A. Einstein. On aHeuristic Point of View about the Creation and Conversion of Light. Annalen der Physik 17 (1905): 132-148. [10] P. Bouguer. Essai d’optique, sur la gradation de la lumiere. Paris, Gauthier-Villars et Cie,1921. Collection “Les Maîtres de la Pensée scientifique. [11] Luis Castañer and Santiago Silvestre. Modelling photovoltaic systems using pspice. Wiltshire, Wiley,2002. [12] Fraunhofer Institute for Solar Energy Systems: Photovoltaics Report. Freiburg,2021. Photovoltaics Report (fraunhofer.de) [13] International Energy Agency Report: IEA-PVPS T1-39:2021. Strategic PV Analysis and Outreach,2021. Snapshot of Global PV Markets- 2020 (iea-pvps.org) [14] Green, Martin, et al. Solar cell efficiency tables (version 57).Progress in photovoltaics: research and applications29.1 (2021): 3-15. [15] Juan Ramón Morante, Teresa Andreu, Gotzon García, Jordi Guilera, Albert Tarancón, Marc Torrell. Hydrogen The energy vector of adecarbonised economy. Barcelona, IREC & Fundación Naturgy.2020. [16] European Commission. Ahydrogen strategy for aclimate-neutral Europe. Brussels,2020. Communication COM/2020/301: Ahydrogen strategy for aclimate-neutral Europe| Knowledge for policy (europa.eu) [17] European Commission, On the promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC,2009. https://eurlex.europa.eu/legal-content/EN/TXT/?uri=celex%3A32009L0028 [18] R. Singh, A. Prakash, B. Balagurumurthy, T. Bhaskar, Chapter10. Hydrothermal Liquefaction of Biomass, Recent Advances in Thermo-Chemical Conversion of Biomass, pp. 269-291,2015. https://doi.org/10.1016/B978-0-444-63289-0.00010-7 [19] Green Square, Advantages and disadvantages of Biomass energy, October,2020. https://doi.org/10.1016/B978-0-444-63289-0.00010-7 [20] EPA website, Astudent’s guide to Global Climate Change, Geothermal Energy,2017. https://archive.epa.gov/climatechange/kids/solutions/technologies/geothermal.html [21] TWI Ltd., What is geothermal energy? How does it work?, https://www.twiglobal.com/technical-knowledge/faqs/geothermal-energy [22] ThinkGeoEnergy, “Top 10 Geothermal Countries 2020 – installed power generation capacity”, https://www.thinkgeoenergy.com/thinkgeoenergys-top-10-geothermal-countries-2020-installedpower-generation-capacity-mwe/ CHAPTER 1 Introduction Main energy source before the nineteenth century was wood. The use of Coal as energy source started with the First Industrial Revolution and was replaced by oil and gas in the Second. Nowadays, major fossil fuels coexisting alongside with nuclear are the main energy sources used worldwide. Nuclear energy presents clear problems associated with the storage and recycling of the waste it produces. In addition, recent accidents in this type of nuclear plants have made clear the potential danger it poses. On the other hand, fossil fuels are main responsible of the global warming and climate change. The energy demand is still growing worldwide, and this trend will continue in the future. However, it is necessary to change the energy production ways, trying to avoid the massive use of fossil fuels in order to reduce the greenhouse effect and stop climate change. DEFINITION Green energy is any energy type that comes from renewable energy sources. It is generated from natural resources, such as sunlight, wind, or water. Nowadays just the 27.3% of the electricity generation comes from renewable sources of energy, as it can be seen in Table1, while the other 72.7% is achieved by means of non-renewable electricity generation sources. Renewable Energy % Wind Power 5.9 Solar Photovoltaic 2.8 Bio-power 2.2 Geothermal, CSP and ocean power 0.4 In this context, renewable or green energies must play amajor role in the energy generation of the future. The key with green energy resources is that they do not harm the environment through factors such as releasing greenhouse gases into the atmosphere. This course describes main sources and technologies used in green energy generation. Table1. Estimated Renewable Share of Global Electricity production (End-2019) CHAPTER 2 Hydroelectric Energy Hydroelectric energy is the main renewable source in the world since the great availability of water allows to obtain ahigh yield. Currently, it covers about 20% of the world's electricity demand. Hydroelectric energy has an important role to play in the future since it plays amajor role in reducing greenhouse gas emissions. DEFINITION Hydroelectric energy is obtained from the use of kinetic and potential energies of the water current, waterfalls or tides, either through mills or dams. Hydroelectric energy has been exploited for centuries. Farmers since the ancient Greeks have used water wheels to grind wheat into flour. Placed in ariver, awater wheel picks up flowing water in buckets located around the wheel. The kinetic energy of the flowing river turns the wheel and is converted into mechanical energy that runs the mill. [1] DEFINITION Hydroelectric energy is the electrical energy that is generated in ahydroelectric plant from water in motion, by using flowing water or awaterfall to drive awater turbine and generator. The generation process is highly efficient, being able to reach levels of efficiency between 90and 95%. Hydroelectric energy is characterized by being one of the most profitable energies. The initial construction investment of ahydroelectric plant is high. However, maintenance costs for this type of infrastructure are low. Types of hydroelectric plants 2.1 Hydroelectric plants are usually located far from the large consumption centers and the place of their settlement is conditioned by the characteristics of the terrain. DEFINITION Not all hydroelectric plants are the same. Some hydroelectric plants use dams, and some do not. Depending on their mode of operation, there are basically three predominant types: Storage, Runof-river, and Pumped storage [2]. Storage This is the most widespread hydroelectric plant. They are large plants. By building one or more dams, reservoirs are formed that are used to retain large amounts of water and, in this way, regulate the flow that passes through the turbines that generate electricity. This allows energy production to be stable throughout the year and satisfy the electricity needs of each moment. Run-of-river Most small hydroelectric plants are based on the run-of-river type. All of them are characterized by not having awater storage capability which makes this type of plant subject to seasonal river flows. To minimize this, run-of-river plants are usually installed on rivers with aconsistent and steady flow or with alarge reservoir at the head of the river. In run-of-river plants the flowing water of the river is diverted to achannel to spin aturbine that generates electricity. Afterwards, the diverted water is returned to the main river. The generation of electricity depends on the speed of the water. Pumped storage Its operation is very similar to that of alarge rechargeable battery. The plant has two reservoirs at different heights and connected to each other. In times of low electricity consumption, the excess energy is used to raise the water contained in the reservoir located at the lowest level to the upper reservoir by means of ahydraulic pump. In the hours of greatest energy demand, the pumping station works like aconventional storage hydroelectric plant. Hydroelectric plants can also be classified by size according to the power output although there is no international consensus for setting the size threshold between small and large hydroelectric plants. In some countries the threshold is set at 30MW while in others it is set at 10MW. The following table shows the usual classification: [3] Size of the hydroelectric plant Power output Large plant Above 10-30 MW Small plant 1 MW to 10-30 MW Mini plant 100 kW to 1MW Micro plant Below 100kW Fig.1. Types of hydroelectric plants [2] Table2. Hydroelectric plants classification according to power generation Fig.2. Hydroelectric plant of Mequinenza (Spain) Advantages and disadvantages of hydroelectric energy 2.2 The following are some advantages of the use of hydroelectric energy: [4] Flexibility. It is possible to adapt the flow of water that passes through the turbines to the electricity needs of each moment. Clean energy. The generation process of electrical energy is clean, since it does not produce waste, as occurs with energy derived from fossil fuels or nuclear energy. Safety. The risks of water leaks are quite low, due to the safety measures that are currently taken in the hydroelectric plants today. Inexhaustible resource. The source of energy, water, is free and inexhaustible since it is renewed with the rains and thaws. Stable source of energy. Hydroelectric energy is very stable unlike, for example, solar energy which directly depends on the sky situation. In other words, hydroelectric energy does not depend on rain every day to produce electricity because there are very important water reserves. There are also several negative points about this type of energy, such as: Environmental effects. The construction of adam has important environmental consequences, since it influences the course of ariver and floods an area of land, which produces effects on flora and fauna. On the other hand, when the dams open and close there are effects on the fish and on the river ecosystem. High cost of ahydroelectric plant. In the long run, hydroelectric energy is very cheap, and maintenance is simple, but the construction of ahydroelectric plant involves ahigh cost. Depends on environmental conditions. It is true that hydroelectric energy does not depend on rain on adaily basis. However, the lack of rains will affect energy production. So, seasons with many droughts reduce the amount of water stored and the amount of electricity that can be produced. A dam cannot be built anywhere. The characteristics of the terrain and the height that the dam may have been essential for ahydroelectric plant to be installed. CHAPTER 4 Photovoltaic Solar Energy The solar energy that affects the entire earth's surface annually represents about five thousand times the energy demand of the world population, for that reason it is the main source of renewable energy within our reach. In addition, solar energy offers aseries of advantages over other renewable energies: It is not polluting since it does not emit any type of waste into the atmosphere, nor does it produce noise pollution, since its generation is silent, and it is available all over the planet. Although not all points on the earth receive the same amount of energy from the sun. Two types of technologies are used to take advantage of solar energy: photovoltaic solar technology and solar thermal technology. The term photovoltaic (PV) comes from the Greek word Photo (light) and the name of the Italian physicist Alessandro Volta, inventor of the chemical battery. DEFINITION Photovoltaic solar technology allows direct transformation of sunlight or solar radiation into electricity. It is responsible for harnessing the energy of photons into electrical energy, generating electrons, with photovoltaic solar modules, for this purpose, it uses devices called solar cells. Edmund Bequerel discovered the photovoltaic effect in1839. Albert Einstein proposed amathematical description of the photoelectric effect in which the emission of electrons was produced by the absorption of quanta of light that would later be called photons. [10] For this explanation of the photoelectric effect, Einstein received the Nobel Prize in Physics in1921. Solar thermal technology takes advantage of the sun's energy to heat water that can be used for domestic consumption: Heating, sanitary hot water, or to produce mechanical energy, which we can then transform into electrical energy. Solar thermal energy is also used to power absorption cooling machines, which instead of using electricity to produce cold, use heat. The basic elements of solar thermal energy are solar collectors, responsible for capturing solar radiation and converting it into thermal energy: Heat, which is used to heat water directly or aspecial liquid that in alater stage would heat water through an exchanger. SUMMARY In this chapter, we will focus on analyzing photovoltaic solar energy in detail. Video 2 Solar Radiation 4.1 DEFINITION Solar radiation is the energy emitted by the Sun, which propagates in all directions through space using electromagnetic waves. The radiation that comes from the sun is equivalent to that radiated by ablack body at atemperature of 6000K. Part of the energy that reaches us from the sun, about 30% of this radiation, is reflected into space. Gas molecules and suspended particles that are present in the different layers of the atmosphere absorb another part and the rest reaches the earth's surface. In the equatorial zones the solar radiation affects directly, in the poles, the radiation that arrives is much less. Because the atmosphere filters sunlight, when we are at ahigher altitude and, therefore, the atmospheric layer is thinner, the energy we receive from the sun is greater. DEFINITION The term solar radiation refers to the values of solar irradiation, that is, the amount of energy received per unit area in each time. The values of solar radiation express the energy that comes from direct radiation from the sun and diffuse radiation that, scattered through the atmosphere, comes from the rest of the sky. The radiant power of the sun at the outer limit of the Earth's atmosphere, at about 150 million kilometers, corresponds to about 1,360 W/m2. This value is called the Solar Constant: Gsc. Fig. 4 shows the irradiance spectral density that we receive from the sun on the earth's surface as afunction of its wavelength, outside the atmosphere and on the earth's surface, as well as the absorption in the atmosphere associated with different elements present in it. As can be seen in the figure, most of the energy is centered in the visible spectrum, between 0.4and 0.8 µm wavelength. DEFINITION The energy of the photons depends on the wavelength as shown by the following equation: where h is Planck's constant, cthe speed of light and λ the wavelength associated with the photon. For aphoton to be absorbed and generate apair of carriers, electron /hole, in amaterial it is necessary that the energy of the photon is higher than the energy of the gap, E.g., of the material. In addition, each semiconductor material, the materials most used in the manufacture of solar cells, has acertain absorption coefficient, α. When light passes through amaterial, it is attenuated, and its absorption is proportional to its intensity at each wavelength. The number of photons that penetrate asemiconductor material decreases exponentially as afunction of αand the distance they travel following Lambert's law. [11] DEFINITION The reference solar spectrum outside the Earth's atmosphere is known as AM0, (AM: Air mass, amount of atmosphere that solar radiation passes through). At the earth's surface, the reference spectrum is AM1.5. Fig.4. Incident solar radiation on the earth's surface Photovoltaic Generator 4.2 DEFINITION The electronic device in charge of transforming the sun's energy, incident photons, into electrical energy, aflow of free electrons that give rise to electric current, from the photoelectric effect is the solar cell. An important characteristic of the solar cell is its efficiency. DEFINITION The efficiency of asolar cell, , is defined as the relationship between the electrical power that it can deliver at its output with respect to the power of light incident on its surface: η where Vm, Im are respectively the maximum voltage and current at the output of the solar cell under standard conditions (STC): Irradiance, G =1000W/m2 and temperature of 25°C and A is the area of the solar cell. W.G. Adams and R.E. Day manufactured the first solar cell on selenium (Se) in1877, but it was not until 1954 that the first silicon (Si) solar cell was manufactured at Bell Laboratories, with efficiencies of 6%. This gives us an idea of how recent solar photovoltaic technology is. Despite this, today it is avery mature technology. Most solar cells are based on semiconductor materials where ap-n junction has been created, aregion with adefect of electrons (p) in contact with another one with excess electrons (n), which generates an electric field that allows the electrons to move in acertain direction and thus obtain an electrical current at the output of the device. Solar cells are connected in series to obtain ahigher output voltage and thus form PV modules. [12] Some PV modules have several branches of cells in series connected in parallel, which allows obtaining greater output current from the PV module. On the market, we can find PV modules with very different output powers depending on the number of solar cells they incorporate and how they are connected to each other. The efficiencies of PV modules depend on the efficiency of their solar cells and therefore on the material and technologies used in their manufacture. PV is amodular and scalable technology. Thus, aPV generator is also constituted by the interconnection of parallel branches of PV modules connected in series, as shown in Fig.5. Video 3 Fig.5. From solar cells to PV arrays PV modules manufacturing technologies 4.3 The 95% of the solar cells manufactured in 2020 were developed on silicon (Si) wafers, most of them, 84%, used mono-crystalline silicon (c-Si) and the rest multi-crystalline silicon (mc-Si). [13] The efficiency record for c-Si cells stands at 26.7%, while for mc-Si the maximum efficiency achieved for the solar cell is 24.4%. Some thin film technologies are used in the manufacture of solar cells that are based on cheaper materials, but with lower efficiencies such as: CIGS or CdTe. Recently, research and production of solar cells has increased in materials such as Perovskites or Kesterites, also cheaper than Si, reaching efficiencies of 25.5% in the case of solar cells based on Perovskites. The highest efficiency values in solar cells, up to 47% [14], are obtained in tandems of solar cells that take better advantage of the solar spectrum, based on compounds of groups III-V. These tandems are made up of several solar cells stacked in series and are expensive to manufacture, which is why their use is limited to space applications. Currently, most commercial PV modules based on c-Si or mc-Si present efficiencies around 20%. The highest efficiency values achieved in PV solar modules of different technologies are summarized in the following Table. Technology Efficiency (%) Manufacturer c-Si 24.4 Kaneka mc-Si 24.4 Hanwha Qcells GaAs (thin film) 25.1 Alta devices CdTe (thin film) 19 First Solar Perovskite 17.9 Panasonic Multi Junction (III-V) 31.2 Sharp The cost of photovoltaic modules has dropped 26% in the last 40 years. Today, photovoltaic solar energy is amature technology and allows producing electricity at competitive costs, between 14and 20 USD /MWh [13], with the rest of traditional energies based on fossil fuels. As an example, in 2021the cost of electricity in Spain stands at 200 Euros /MWh and as in the rest of the countries of the European community, the price of electricity produced from non-renewable sources is expected to continue rising in the future. DEFINITION The total photovoltaic power installed worldwide is today 760GW and allows avoiding the emission of 875 million tons of CO2 per year into the atmosphere. However, it still represents asmall percentage of global electrical energy production. Table3. PV module top efficiencies2020. [14] Undoubtedly, the growth of photovoltaic solar energy will continue to be exponential worldwide, as in recent decades, and it is called to be one of the most important solutions to global warming and to play akey role in the energy transition. Photovoltaic Systems 4.4 Photovoltaic systems can be divided into two types: Stand-alone PV systems and Grid connected PV systems. DEFINITION Stand-alone PV systems are used when it is not possible to connect to an electrical distribution network. Stand-alone PV systems were the first systems used in this field and their applications are multiple: Water pumping, telecommunication stations, radio and television repeaters, space applications in satellites or space vehicles, applications in households etc. Element Description [1] PV modules Depending on the application, the size of the generator can vary from very low power systems with asingle module to generators with several kW of peak power. Fig.6. Stand-alone PV system Table4. Elements included in astand-alone PV system [2] Charge regulator This element is responsible for protecting the batteries and keeping them in asuitable state of charge, depending on the energy demand of the system and the power available at the output of the photovoltaic generator. They can incorporate maximum power point trackers: MPPT, to maximize the use of the energy produced. This allows the photovoltaic generator to work at the point of maximum output power while the rest of the system works at the battery voltage and the current required by the loads at all times. [3] Batteries They are the energy accumulators that allow supplying electrical current to the loads when the PV generator does not produce, for example, at night. When the generator produces power, it recharges the batteries. [4] System loads Electrical loads that the PV system must supply in DC. If there are loads present that need to be powered in AC, it is possible to incorporate aDC /AC converter called an inverter. DEFINITION The second type of PV systems are systems connected to the electrical distribution network or gridconnected PV systems. Nowadays, more than the 90% of existing PV installations are connected to the grid. In this group of systems, we can find facilities designed as large electricity generators, up to hundreds of MW of power, which are used to inject energy into the distribution network. In addition, medium power systems, up to 500kWp, designed for industrial or commercial warehouses that pour energy into the grid, but also used to feed the internal consumption of these facilities. Every day more PV systems connected to the grid are installed in homes for self-consumption applications, where the energy generated is used for domestic consumption and if there is asurplus, it can be poured into the grid if the country's regulations allow it. In these cases, there are two types of strategies to compensate for this energy injected into the grid. DEFINITION Net metering, where the electricity companies to which the system is connected discount the energy injected into consumption costs, and the second case in which electricity companies pay directly for the energy injected into the grid by the photovoltaic system to the owner of the same, feed-in tariffs. In grid-connected PV systems, especially in applications designed for self-consumption, batteries are also included to store the energy produced by the PV generator, thus reducing direct consumption from the grid when the generator is not able to satisfy the energy demand of the loads. Its main elements are the following: Element Description Photovoltaic generator From afew kW to hundreds of MW depending on the application. Inverter In charge of converting the DC output of the photovoltaic generator into AC. Depending on the size of the system, the number of inverters and their nominal power may vary in each application. These inverters incorporate MMPT systems to ensure that the PV generator always works at the maximum power point. Protections Fuses, bypass diodes, blocking diodes, manual and automatic switches or breakers, earth connections, power protection systems, etc. (depending on the regulations of each country). Meters They measure the energy injected into the grid. In the case of self-consumption applications, bidirectional meters can be used that measure both the injection into the network and its consumption. Loads AC loads present in the system. Interaktivní prvek Interaktivní prvek Interaktivní prvek Fig.7. Block diagram of agrid-connected PV system Table5. Main elements of grid-connected PV systems Finally, other applications are combustion in boilers for domestic heating or applications for heat generation in industry. Interaktivní prvek Interaktivní prvek Interaktivní prvek Interaktivní prvek Interaktivní prvek Interaktivní prvek Fig.9. H2-powered car CHAPTER 6 Biomass The use of biomass energy is, in fact, one of the oldest known, already used by the earliest cavemen for keeping warm or cooking food. However, with the arrival of the industrial revolution and the need to generate agreater amount of energy in an increasingly reduced space, it promoted the use of fossil fuels and slowed down the use of biomass energy. Nowadays, however, the use of biomass energy is gaining in popularity since is aclean, sustainable, and renewable source of energy. DEFINITION Biomass refers to all organic matter existing in the biosphere, whether of plant or animal origin, as well as those materials obtained through their natural or artificial transformation. According to the 2009/28/EC Directive related to the promotion of the use of energy from renewable sources, ‘biomass’ means the biodegradable fraction of products, waste, and residues from biological origin from agriculture (including vegetal and animal substances), forestry and related industries including fisheries and aquaculture, as well as the biodegradable fraction of industrial and municipal waste. [17] DEFINITION In other words, biomass is avery broad concept that includes everything from waste from forestry, agricultural and livestock activities to the organic fraction of domestic and industrial waste, including by-products from the agri-food and wood processing industries. Biomass is asource of renewable energy. Biomass energy comes in the last instance from the Sun. Plants absorb the Sun’s energy through the process of photosynthesis and convert carbon dioxide and water into nutrients. Also, animals that eat plants transforms these nutrients into biomass. Biomass can be transformed into usable energy through direct and indirect conversion. Biomass can be burned to create heat or converted into electricity (direct) or processed into biofuel (indirect). Fig.10. Biomass creation Types of biomass 6.1 According to its origin, biomass can be classified into three main groups: Natural biomass. It occurs spontaneously in nature, without human intervention. For example, the natural pruning of forests. Residual biomass. Waste products that come from agricultural or forestry activities (e.g. sawdust), from agri-food industries or from biodegradable waste formed by urban, industrial and livestock wastewater (e.g. guano). Produced biomass. These are energy crops. Aspecific crop is grown in farms with the sole purpose of being used as energy source. Conversion technologies 6.2 Conversion technologies use biomass in an efficient and sustainable way to generate heat, electricity, biofuels, chemicals, and biomaterials. DEFINITION There are two main conversion technologies to convert biomass into energy: thermo-chemical and biochemical. Thermo-chemical conversion consists of directly burning biomass at very high temperatures (between 600and 1,300 °C) and in the presence of large amounts of air and with yields of up to 95%. Different thermochemical conversion processes include combustion, gasification, and pyrolysis, being combustion the most practiced process. Bio-chemical conversion involves use of bacteria, microorganisms, and enzymes to breakdown biomass into gaseous or liquid fuels, such as biogas or bioethanol. The most popular biochemical processes are anaerobic digestion and fermentation. Anaerobic digestion occurs in the absence of oxygen. In the process, the degradation of organic matter is achieved thanks to the activity of the microorganisms, which transform it into agas with ahigh energy content (biogas) and other products that can be used to produce secondary products. Thermochemical Biochemical Effectively applied to almost any biomass feedstock Involves the use of microbes, enzymes, and/or chemicals No pretreatment Pretreatment is essential Relatively higher productivity due to completely chemical nature of reaction Productivity is limited due to biological conversion Multiple high-value products possible using fractional separation of products Normally, limited to one or few products and would require additional microbes, enzymes for more products Independent of climatic conditions Mostly susceptible to ambient temperature, anaerobic digester Complete utilization of waste/biomass Production of secondary wastes such as biomass sludge Less reaction time High reaction time Table7. AGeneral Comparison of Biochemical and Thermochemical Processes [18] Advantages and disadvantages of biomass 6.3 Some advantages of the use of biomass as asource of energy are: [19] ADVANTAGE It is arenewable source of energy. It helps to reduce volumes of waste, with the added benefit of putting it to use. It is found in large quantities. It does not mean agreater impact than the greenhouse effect, since when it is used as afuel it causes less emission of gases that are harmful to the environment. It is quite inexpensive. It can economically benefit rural sectors. There are also some drawbacks, including: DISADVANTAGE Its scope is still limited. Its performance is lower than that of other types of energy sources such as fossil fuels. Biomass plants require alarge land available for its production and alot of space for subsequent storage. Can lead to deforestation. Its distribution channels are not sufficiently developed. It can make the price of some foods consumed by people and animals more expensive because certain crops are used to produce this energy source. Biomass energy is not as efficient as fossil fuels. The use of biomass in the world 6.4 The use of biomass in the world differs depending on each country. It continues to be the main source of energy in less developed countries. In some areas of Asia, Africa, and Latin America two thirds of the energy generated comes from biomass. On the other hand, countries such as Finland or the United Kingdom are at the forefront in the use of biomass as an energy source and use more complex transformation processes. For instance, Finland covers 50% of its heat needs and 20% of its primary energy consumption with biomass. Interaktivní prvek Interaktivní prvek CHAPTER 7 Geothermal energy The first geothermal plant was built in 1904in Larderello, Italy. Although geothermal energy has been exploited for more than 100 years, it is less well-known than other alternative energy sources such as photovoltaic solar and wind energies. DEFINITION Geothermal energy is the thermal energy generated and stored beneath the Earth’s crust. It is arenewable energy resource that does not require the combustion of any material, thus avoiding emissions of carbon dioxide. Geothermal energy is obtained by harnessing the heat stored in rocks, soils, and groundwater, whatever their temperature and depth. To exploit this type of energy, it is necessary to drill the earth's surface 1.6 miles or deeper to reach areservoir of steam or hot water. Generally, the deeper you drill, the hotter you get. Geothermal applications depend on the temperature of the geothermal resource. So, high temperature geothermal reservoirs, above 100°C, can be used to generate electricity. Heat is used to heat water to produce steam and drive aturbine connected to agenerator to produce electricity. When the reservoir temperature is below 100°C, the heat is used directly to provide heating and cooling in homes and businesses by using heat pumps. Finally, low temperature reservoirs, below 25-30 °C, are used in airconditioning applications and for obtaining hot water. How it works 7.1 Geothermal energy is generated in ageothermal plant. Wells are drilled to reach areservoir of steam or hot water. Geothermal plants are located close to tectonically active regions where the potential for geothermal energy is high. Next figure illustrates how ageothermal plant works: [20] 1. Hot water is pumped from deep underground through awell under high pressure 2. When the water reaches the surface, the pressure is dropped, which causes the water to turn into steam. 3. The steam spins aturbine, which is connected to agenerator that produces electricity. 4. The steam cools off in acooling tower and condenses back to water. 5. The cooled water is pumped back into the Earth to begin the process again. Fig.11. Geothermal power plant diagram [20] Advantages and disadvantages of geothermal energy 7.2 Geothermal energy has some advantages over fossil fuels but also advantages over other renewable energies such as solar and wind energy. Some of these advantages include: [21] ADVANTAGE Compared with fossil fuels, geothermal energy is cleaner, often has no emissions, and cheaper. It is constantly available. Unlike other renewable energies, it does not matter if it is day or night or what are the current weather conditions. It can be produced domestically and with less land surface than wind and solar energy. It works very well for heating and cooling Despite the great number of advantages, the geothermal energy has, there are also some disadvantages that must be considered: DISADVANTAGE Production is limited to sites close to tectonic plate boundaries. High initial costs. Although it is cheaper than fossil fuels once aplant has been built, the drilling and exploration of these sites is expensive. Risk of accidentally releasing harmful greenhouse gases in the process Can trigger surface instability which can lead to earthquakes. Power electricity can only be generated on an industrial scale. In the case of homes, geothermal energy can only be used for heating and cooling.