Wind Power Based Hydrogen Production - Kökar Island Case Study

Transcription

1 Wind Power Based Hydrogen Production - Kökar Island Case Study Helka Martinoff Master s Thesis University of Jyväskylä, Department of Chemistry Masters Degree Programme in Renewable Energy

2 ABSTRACT Wind power is currently one of the fastest growing energy sources in the world and Finland has good potential of wind power partly because of its long coastline. If wind power is combined with energy storage system, wind power will also be a realistic energy source in isolated energy systems. This paper focuses on wind hydrogen system, where hydrogen is the storage medium. The components required in wind hydrogen system, which uses electrolyser to form hydrogen from water and fuel cell to form electricity from hydrogen, are discussed. Also hydrogen storage with external hydrogen load and fuel cell vehicles are introduced. Self-sufficiency with wind hydrogen system of Kökar island on the west coast of Finland was examined in the case study, and the sofware used in modeling was HOMER (Hybrid Optimization Model for Electric Renewables). The results show that an island like this could be entirely self-sufficient in terms of power with relatively competitive cost. Kökar island could be self-sufficient with 3 MW of wind power combined with kw electrolyser, kg hydrogen storage and 400 kw fuel cell. Economical profitability of this system is directly proportional to the distance of existing grid. 2

3 1 INTRODUCTION WIND ENERGY PRODUCTION GENERAL WIND SPEED DISTRIBUTION OPERATION CHARACTERISTICS WIND TURBINES THE LOSS OF VALUE OF WIND ENERGY AT HIGH PENETRATION ENERGY CONVERSION INTO H GENERAL ELECTROLYZER Alkaline Electrolyzer H 2 STORAGE COMPRESSED HYDROGEN LIQUID HYDROGEN METAL HYBRIDES H 2 USE IN A FUEL CELL FUEL CELL Fuel cells in general Fuel cell types Bipolar plates PEM FUEL CELL SOFC SYSTEM INTEGRATION AND CONTROL WIND HYDROGEN SYSTEM CONCEPT SHORT-TERM STORAGE DC-DC CONVERTERS Buck converter Boost controller ELECTROLYZER

4 6.5 PEMFC DESIGN TOOLS FOR RE SYSTEMS FUEL CELL VEHICLES DIRECT-HYDROGEN FUEL CELL VEHICLE MODEL FUEL CELL VEHICLE CHALLENGES CASE STUDY: KÖKAR ISLAND STAND-ALONE SYSTEM DESCRIPTION COMPONENT SIZING RESULTS CONCLUSIONS ACKNOWLEDGEMENT REFERENCES APPENDIX I APPENDIX II APPENDIX III

5 1 INTRODUCTION Renewable means of producing and storing electricity are expected to be increasingly important in the future, because fossil fuel supplies are expected to be less available, more expensive and of increasing environmental concern. The main concern in using renewable sources is their unpredictable nature. The electricity production can not be adjusted with load demand and, thus, energy storage should be added to solve the problem. Fossil fuels currently supply most of the world s energy needs, and traditionally the ideal energy storing has been realized by storing fossil and biofuels. Often other energy sources have been used side by side renewable energy systems to smooth out the intermittent supply. For example, a diesel generator is supplying the load in case of insufficient output of a wind turbine. In cases like this, however, the excess power of the wind turbine is not exploited. By connecting wind turbine to hydrogen storage system the excess energy from the wind is converted to hydrogen, and when the wind power is insufficient, the hydrogen is utilized in power production to meet the load demand. The hydrogen can also be used in other applications, such as transportation. Finland has a large potential of wind power, especially in coast line where the average wind speed is about 7 m/s (NASA). Therefore this study focuses on combining wind and hydrogen systems. The the basic wind power parameters are introduced in Chapter 2. This study introduces applications needed in wind hydrogen system, such as fuel cell and electrolyser. Also connections and operation of these components is studied. 5

6 Fuel cell vehicles are also in consideration. The stand-alone system in this study produces hydrogen, and fuel cell vehicles form the external hydrogen load. Fuel cell vehicles are a clean and sustainable alternative for today s cars using fossil fuels. Their structure and working principles are introduced in Chapter 7. A case study is introduced in Chapter 8. It considers the island Kökar in Åland archipelago, which has a population of 320 and already an existing 500 kw wind turbine. The feasibility to attach this kind of hydrogen system to the existing wind turbine is studied using the Hybrid Optimization Model for Electric Renewables (HOMER) software. 6

7 2 WIND ENERGY PRODUCTION Wind energy is the kinetic energy of moving air. The amount of available energy depends mainly on wind speed, but is also affected slightly by the density of the air. For any wind turbine, the power and energy output increases as the wind speed increases. Wind speed increases with height above the ground, so wind turbines are usually mounted on tall towers. In the next chapters basic parameters and working principles of wind power are introduced. 2.1 General The power in the wind can be extracted by allowing it to blow past moving wings that exert torque on a rotor. The amount of power transferred is directly proportional to the density of the air, the swept area of the rotor, and the cube of the wind speed: P 1 AV 2 3 = ρ, (1) where ρ is the air density (kg/m 3 ), A is the rotor swept area (m 2 ) and V is the wind velocity (m/s). Rotor swept area can be calculated from A= π (D/2) 2, where D is the rotor diameter (m). The air density varies with pressure and temperature which can in some cases be described by the ideal-gas law: pv=rt, (2) where p is the air pressure (Pa), T is the absolute temperature (K) and R is the gas constant for air (287 J/kgK). 7

8 Theoretically, if all the kinetic energy (100%) in the wind is captured, the wind would stop and the turbine wouldn t capture any energy. The other opposite is that if the wind speed doesn t change at all, it would flow throught the blades and again no energy is captured. The maximum power in the wind that can theoretically be extracted is given by the Betz limit and it is 16/27 (59,3%) of the power available in the wind. The power that can be converted to electrical power from the wind is illustrated in Figure 1. [1] Figure 1. The relation between the total power in the wind, the usable power input and turbine power output in a typical wind turbine as a function of the wind speed. 2.2 Wind speed distribution The power output of a wind turbine varies with wind speed and every turbine has a characteristic wind speed-power curve (Figure 2). The power curve is used to determine how much energy can be produced by a particular turbine on a given site under given wind conditions. 8

9 Figure 2. Typical wind turbine wind speed-power curve [1] The energy that wind turbine will produce depends also on the wind speed distribution at the site (see Figure 3). This curve represents number of hours for which the wind blows at different wind speeds during a given period of time. Combining these two curves (wind speed-power curve and wind speed distribution curve) a wind energy distribution curve can be plotted. The total energy produced is then calculated by summing the energy produced at all wind speeds within the operation range of the turbine. Figure 3 shows both the wind speed and energy distribution curves for a given turbine at a site. [1] 9

10 Figure 3. A typical distribution of wind speed compared to wind energy production. [2] 2.3 Operation Characteristics Because there are many differents sizes of wind power plants, their production capabilities are difficult to compare. Production parameters for the plants are usually compared by two indicators: top speed ratio (kwh/m 2 ) and rotor swept area (kwh/m 2 ). If these parameters are poor then it is usually a result from bad wind conditions, big amount of failure hours or technical failures. These and other commonly used parameters are introduced below: Production against rotor swept area e (kwh/m 2 ): (3) Capacity coefficient CF: CF = Production (kwh) (4) Nominal output (kw) x hours (h) Top speed ratio t h (h): t h = Production (kwh) (5) Nominal output (kw) 10

11 Failure time (h): The time, when wind power plant has operation break due to maintenance, failure, transient disturbance or other stop. The normal operation of wind power plant includes times, when wind speed is below cut-in speed (3-5 m/s) or over storm-limit (20-25 m/s), or when temperature is below plant operation temperature ( C). These figures are not counted on the failure time. Technical usability (%)Hours - (Disturbance time electric network disturbances) (6) Hours Production index (%): Production against average long-term observations based on weather station information about wind speeds. Height Z (m): Height from earth surface to the centre of the rotor 2.4 Wind turbines Wind energy has been used for thousands of years first in sailing boats and later in windmills. Therefore, today there is a variety of machines that use wind as an energy source. Modern wind turbines are electricity generating devices and can be devided into horizontal axis and vertical axis wind turbines. Vertical axis turbines have an axis of rotation that is vertical, and they can exploit winds from any direction without the need to change the position of the rotor. Horizontal axis wind turbines usually have two, three or more blades. The three bladed wind turbines are the most common turbines manufactured today and also this study refers to these turbines. In Figure 4 is a cross-section of a typical horizontal axis wind turbine. [1], [3] 11

12 Figure 4. A cross-section of a typical horizontal axis wind turbine [3] From the outside, horizontal axis wind turbines consist of three parts: The tower, the blades, and a box behind the blades, called the nacelle. Inside the nacelle is where most of the action takes place, where motion is turned into electricity. Blades rotate around a horizontal hub that is attached to an axle that runs into a gearbox. The gearbox increases the rotation speed for the generator, which converts the rotational energy to electrical energy. Brake is needed to stop rotation in case of over-speed. [1], [3] 2.5 The loss of value of wind energy at high penetration At high penetration levels of the wind energy production the electricity produced is greater than the load demand. Uncontrollability of wind poses operational problems on the electricity supply system at high penetration levels, lessening the value of wind-generated electricity extensively. Ensuring power reliability and quality and maintaining the necessary reserve capacity requires changes in system management, and usually demands costly grid reinforcements. These factors reduce remarkably the value of wind energy in high penetration levels. [4] 12

13 Efficiency of the wind power has been improved by new technologies and strategies, such as forecasting, geographical dispersion, interconnections and new materials. In spite of that, a large scale wind energy production will ultimately require the uptake of energy storage, because of the fluctuating nature of the wind. [4] 13

14 3 ENERGY CONVERSION INTO H2 In the future, wind-powered water electrolysis is envisaged as an important source of zero-emissions hydrogen. Hydrogen systems can help to overcome problems arising in the electric systems with high wind energy penetration and offset the gradual reduction in value of this energy. The hydrogen produced and stored can either be supplied to stationery fuel cells, in order to generate power again when needed, or used for transportation. [4] 3.1 General Hydrogen production using wind power via electrolyzer has many attractive features. It can be stored as energy, which can either be converted back into electricity by fuel cell, or used as a non-polluting fuel for other applications, such as transport. All over the world, transportation is very dependent on fossil fuels, so hydrogen can find its market as a clean, sustainable fuel. [4] Using hydrogen as an energy storage medium provides a manner of storing energy that has many advantages compared to conventional batteries that are not appropriate for long-term energy storage because of their low energy density, self-discharge, and leakage. The energy that is stored as hydrogen can be retained for long periods of time and is insensitive to cycle life, temperature, or self-discharge. In off-grid applications, where batteries are coupled to diesel generator, batteries supply power until their stored energy is depleted after which the generator provides additional power while recharging the batteries. For these off-grid applications, the fuel cell could replace most of the batteries and greatly reduce or eliminate the need for a back-up generator. Compared to battery storage, fuel cells used as a back-up or standby power systems can provide a 14

15 higher degree of utility providing longer periods of back-up power with less installation impact at lower overall cost. [5], [6] 3.2 Electrolyzer The decomposition of water into hydrogen and oxygen can be achieved by passing an electric current (DC) between two electrodes separated by an aqueous electrolyte with good ionic conductivity. The anodic and cathodic reactions taking place there are: Anode: 2 OH - (aq) ½O 2 (g) + H 2 O(l) + 2 e - (7) Cathode: 2 H 2 O(l) + 2 e - H 2 (g) + 2 OH - (aq) (8) Thus, the total reaction of splitting water is H 2 O(l) + electrical energy H 2 (g) + ½O 2 (g) (9) For this reaction to occur a minimum electric voltage must be applied to the two electrodes. This minimum voltage can be determined by Gibbs energy for water splitting, which is dependent on temperature. For example, to split 10 kg of water to hydrogen and oxygen at 25 C requires 36,6 kwh energy (And fuel cell used in case study produces 39,5 kwh energy from that same amount of hydrogen). [7] In alkaline solution the electrodes must be resistant to corrosion and must have good electric conductivity and catalytic properties, as well as good structural integrity, while the diaphragm should have low electrical resistance. This can be achieved by using anodes based on nickel, cobalt and iron (Ni, Co, Fe), cathodes based on nickel with a platinum activated carbon catalyst (Ni, C-Pt), and nickel oxide (NiO) diaphragms. [8] 15

16 Electrolyser is the key to the functionality of a regenerative fuel cell as this must both generate and pressurize the hydrogen to allow it to be easily stored. Water is introduced in the anode where it is electrolytically decomposed to oxygen, protons, and electrons. The oxygen evolves as gaseous O 2 at the surface of the electrode while the protons are driven through the membrane. The electrons move through the external circuit. The protons combine with the electrons to evolve into gaseous hydrogen at the cathode. Since electrolyzer is a crucial component in storing energy as hydrogen, the technical challenge is to make it to operate smoothly with intermittent power from renewable energy source. [5], [8] Alkaline Electrolyzer In conventional alkaline water electrolyzers the electrolyte has traditionally been aqueous potassium hydroxide (KOH), mostly with solutions of 20-30% because of the optimal conductivity and remarkable corrosion resistance of stainless steel in this concentration range. The typical operation temperatures and pressures of these electrolyzers are C and 1-30 bar, respectively. [8] The most common alkaline electrolyzers manufactured today have a bipolar design, where the individual cells are linked electrically and geometrically in series. In monopolar design the electrodes are either negative or positive with parallel electrical connection of the individual cells. Bipolar electrolyzer stacks are more compact than monopolar systems, and the advantage of this is that it gives shorter current paths in the electrical wires and electrodes. This reduces the losses due to internal ohmic resistance of electrolyte and, therefore, increases the electrolyser efficiency. Bipolar design has also some disadvantages. One example is the parasitic currents that can cause corrosion problems. Furthermore, the compactness and high pressures of the bipolar 16

17 electrolysers require relatively sophisticated and complex system designs, which consequently increase the manufacturing costs. [8] In the new advanced alkaline electrolyzers the operational cell voltage has been reduced and the current density increased compared to conventional electrolyzers. Reducing the cell voltage reduces the unit cost of electrical power and thereby operation costs, while increasing current density reduces the investment costs. However, there is a conflict of interest because the ohmic resistance in the electrolyte increases with increasing current due to increasing gas bubbling. [8] 17

18 4 H2 STORAGE The hydrogen is produced through electrolyser. The electrolyser input power is controlled, with respect to the energy available at the DC bus, the power line in which wind turbine produces electricity. The H 2 is temporarily stored in a water-sealed tank of the electrolyser system. When this tank is full, the electrolyser compressor starts automatically and sends the H 2 at high pressure through the purification and drying processes. The stored electrolytic hydrogen can then be utilized later to produce electrical energy as per load requirement through fuel cell. Practical implementation of a stand-alone system needs an effective hydrogen storage that achieves both technical and commercial success. Effective storage should optimize cost, lifetime, installation and other factors to a degree that is acceptable for a given application. Available methods of storing hydrogen are as liquid hydrogen, compressed hydrogen and as metal hybrids. 4.1 Compressed Hydrogen Compressed hydrogen needs a high-pressure tank. The energy required to compress the hydrogen amounts to 4-15 % of the energy that the stored hydrogen contains. High-pressure tanks are fitted to bar, which reduces the volume of the tank. High pressure though causes safety issues and requires enhanced security measures. 4.2 Liquid Hydrogen Liquid hydrogen is usually stored at 20 K (-263 C). Compressing and cooling hydrogen into its liquid state requires considerable energy costs. This consumes 18

19 about 30 % of the energy that the stored hydrogen contains. Liquid hydrogen is expensive form of storing hydrogen compared to other forms. 4.3 Metal Hybrides Metal hybrids can incorporate hydrogen into their surface, emitting heat in the process. When the metal hybrid vessel is heated, the hydrogen is released. By weight, the absorbed hydrogen is only 1-2 % of the total weight of the storage, and this is due to the high weight of metal alloys and low weight of hydrogen. In terms of volumetric storage capacity, metal hybrid tanks store approximately 60 kg H 2 /m 3. Some metal hybrids can absorb hydrogen 6-7 % of their weight, but these require the unloading temperature to be at least 250 C. Hybrids offer a safe alternative to store hydrogen as they can deliver hydrogen at constant pressure (30-60 bar) over a broad range of discharging levels. The disadvantage of metal hybrid vessels is their mass and the lifetime of these vessels that is directly related to the purity of hydrogen. [9], [10] 19

20 5 H2 USE IN A FUEL CELL 5.1 Fuel cell Fuel cells are combined with a fuel generation device, commonly an electrolyzer, when used as an energy storage device to create a regenerative fuel cell system. Regenerative fuel cell system can convert electrical energy to a storable fuel and then use this fuel in a fuel cell to provide electricity when needed. Most common fuel cell types use hydrogen, which is generated via electrolysis of water, as the energy storage medium. This kind of a system provides full back-up power for extended time periods. By contrast, storage of equivalent amounts of energy via traditional lead-acid batteries requires an environmentally controlled room, which leads to significant quantities of lead and acid being present in the facility and also typical loss of batteries is 1-5 % of their energy content per hour. [5], [11] Fuel cells in general Fuel cells are electrochemical devices that convert a fuel's chemical energy directly to electrical energy with high efficiency. Electricity is produced from hydrogen and oxygen through an electrochemical reaction. Chemical reactions can be the same as in batteries, but the main difference is that fuel cells produce electricity as long as there is fuel, made by reactive chemicals, and the electrodes are functional. As the reagents are hydrogen and oxygen, the emissions are only water and heat. [12] Fuel cell operates like a conventional galvanic cell with the exception that the reactants are supplied from outside rather than forming an integral part of it. A typical fuel cell is based on the reaction of hydrogen and oxygen to form water. Hydrogen gas is diffused through the anode, a porous electrode with a catalyst. 20

21 Oxygen is diffused through the cathode that is a porous electrode impregnated with a catalyst. The two electrodes are separated by an electrolyte. The anode half-reaction is the oxidation: H 2 (g) + 2OH - (aq) 2H 2 O(l) + 2e - E= -0,83 V (10) And the cathode half-rection is the reduction: O 2 (g) + 2H 2 O(l) + 4e - 4OH - (aq) E= +0,40 V (11) Since the overall reaction 2H 2 (g) + O 2 (g) 2H 2 O(l) E= +1,23 V (12) is exothermic as well as spontaneous, it is less favourable thermodynamically at 200 C than at 25 C, so the cell potential is lower at the higher temperature. The flow of electrons from anode to cathode represents the direct generation of electric power from flameless oxidation of the hydrogen fuel. [13], [14], [15] An advantage of fuel cells is that they are expected to be highly reliable because of the absence of moving parts. The other important advantage of fuel cells is zero or close to zero pollution emissions: water is the only waste stream. [15] The expected life span of the fuel cells range from years with minimal maintenance. Hydrogen safety is an issue, although hydrogen quickly disperses into the environment, making it less of a fire hazard than gasoline. Disadvantages include the high cost of fuel cells, although this is expected to decline as fuel cells are mass-produced. [15] 21

22 5.1.2 Fuel cell types Several types of fuel cells have been developed or are under development. Fuel cell types are generally characterized by the electrolyte material: proton exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC). Despite differences in materials and operating conditions, all of these fuel cells are based on the electrochemical reaction of hydrogen and oxygen and the only by-products are water and heat. PEMFC and SOFC are the most studied fuel cell types today. Bipolar plates are used between fuel cell stacks to reduce the stack size. The properties of different types of fuel cells are described in Table 1 and in Figure 5 and more precise information of the PEMFC and SOFC in Chapters 5.2 and 5.3. [15] 22

23 Table 1. Properties of fuel cells [12] Property PEMFC AFC PAFC MCFC SOFC Electrolyte Polymer KOH Phosphoric acid Molten carbonate Ceramic material Operation temperature , 80 C C , 190 C , 650 C , 1000 C Fuel H 2 H 2 H 2 H 2, CO H 2, CO, CH 4 Oxidant O 2 O 2 O 2 O 2 + CO 2 O 2 Charge carrier H + OH - H + CO 2-3 O 2- Catalyst Platinum Platinum Platinum Nickel Nickel Power (e.g.) 100W- 10MW 300W-5 kw -200 kw (11 MW) -2 MW (100 MW) -220 kw Efficiency % 89 % % % % Ideal application Advantages Disadvantages Distribution of electricity, portables, transportation, CHP production Low temperature, fast starting, solid electrolyte Low temperature demands expensive electrolyte, clean fuel Space technology, war technology Fast reaction CO 2 in the air ages electrolyte Distribution of electricity, transportation Efficiency can be as high as 85 % in combined heat and electricity production, can use also impure H 2 Big size, platinum catalyst, small power and current Distribution of electricity, CHP production High temperature: can use diversity of fuels and cheap catalyst High temperature is demaning to components Distribution of electricity, CHP production High temperature: can use diversity of fuels and cheap catalyst High temperature is demaning to components 23

24 Figure 5. The five principal types of fuel cells and their electrochemical reactions [16] Bipolar plates Bipolar plates are flat, gas impermeable, electrically conductive separators between individual fuel cells in a stack, containing "flow fields" on each side. These flow fields are usually channels machined or molded into the composite, ceramic, or metal plate that carry fuel (usually hydrogen) on one side and oxidant on the other side from entry and exit points in the fuel cell. [17] Bipolar plates (one plate doing the ion-conducting job on both sides) help reduce the size and weight of a total fuel cell stack, remove water produced in the electrochemical reaction, assist in heat removal as part of a stack s thermal management, and preserve power density. For PEMFCs, initial pure graphite plates with machined flow fields have proved expensive; lower cost processes can be yielded by the use of composite materials. [17] 24

25 5.2 PEM fuel cell Proton Exchange Membrane (PEM) fuel cell is also known as the solid polymer or polymer electrolyte fuel cell. It usually operates at lower temperatures around 80 C. A PEM fuel cell contains an electrolyte that is a layer of solid polymer. The polymer is chemically inert, mechanically stable, a good proton conductor, a good absorbator for water and its ph is below 7 (usually a sulfonic acid polymer). Best electrolyte for both anode and cathode is platinum and in most PEM fuel cells the anodes and cathodes are similar. The high cost of platinum has been considered a constraint in fuel cell production, but at the present price of platinum, about 10 /kwh it is not a restricting factor anymore. [5], [15], [16] The operation of the PEM fuel cell is illustrated in Figure 6. PEM fuel cells require hydrogen and oxygen as inputs, though the oxidant may also be ambient air, and these gases must be humidified. Water is formed in cathode, and both anode and cathode needs little humidity to work ideally. However, water should not block the gas diffusion layer in neither electrode. Water is moving in the cell both directions. Water may back-diffuse from the cathode to the anode, if the cathode holds more water. Water will also move from anode to cathode by protons moving through the electrolyte. Water will be removed from the cathode by evaporation into the air circulating over the cathode. Externally humidifying the hydrogen supply in the anode and externally humidifying the air supply in the cathode may supply water. The air that flows through cathode dries the cell more quickly than the reaction produces water. Therefore the gasous hydrogen fuel is humidified with water. [16] 25

26 Figure 6. Operation of PEM fuel cell [16] 5.3 SOFC Solid oxide fuel cell is one of the two high-temperature fuel cell types (along with molten carbonate fuel cells), typically operating at 800 to 1.000ºC and designed in either planar (flat plate) or tubular configuration. As with the MCFC, the SOFC produces heat as high-quality by-product that can be used particularly effectively in cogeneration and other applications. The fundamental electrode reaction in the SOFC is different from that of other kinds of fuel cells. Ionic conduction is accomplished by oxygen ions (O 2- ). Typically the anode of an SOFC is cobalt or nickel zirconia (Co-ZrO 2 or Ni-ZrO 2 ) and the cathode is strontium-doped lanthanum manganite (Sr-doped LaMnO 3 ). At the anode, hydrogen gas reacts with O 2- ions to produce water and electrical energy is released in the form of the electrons: H 2 + O 2- H 2 O + 2e - (13) 26

27 At the cathode, oxygen reacts with the electrons taken from the electrode, and oxygen ion O 2- is formed: ½O 2 + 2e - O 2- (14) Solid Oxide Fuel Cells (SOFCs) are currently being demonstrated in sizes from 1 kw up to 250 kw plants, with plans to reach the multi-mw range. Hardware materials tend to be ceramic or metallic or a mixture of both. SOFCs utilize a non-porous metal oxide (usually yttria-stabilized zirconia, Y 2 O 3 -stabilized ZrO 2 ) electrolyte material. SOFCs have traditionally been operated at ~1000 C because the cell support was a thick doped-zirconia electrolyte layer. Recently, the operating temperature has been lowered to C by supporting the cell on a thick ceramicmetal (cermet) anode layer and decreasing the thickness of the electrolyte layer to <20 µm, thus decreasing its resistance. [16], [17], [18], [19] SOFCs offer the stability and reliability of all-solid-state ceramic construction. High-temperature operation, up to 1000 C, allows more flexibility in the choice of fuels and can produce very good performance in combined-cycle applications. SOFCs approach 60 percent electrical efficiency in the simple cycle system, and 85 percent total thermal efficiency in co-generation applications. The flat plate and monolithic designs are at a much earlier stage of development typified by sub-scale, single cell and short stack development (kw scale). At this juncture, tubular SOFC designs are closer to commercialization. [16] However, there are still barriers to the use of SOFCs for these applications, including (1) susceptibility to cracking due to vibration, impact, and thermal shock; (2) contact resistance between the cell components; and (3) high materials and manufacturing costs. The bulk of the material costs of the anode- 27

28 supported SOFC lie in the large amount of zirconia in the thick anode support and the cost of expensive alloys in the bipolar plate. SOFCs are more fuel flexible than PEMFCs, but sealing technology of individual planar ceramic cells requires additional research and enhancement. [18] 28

29 6 SYSTEM INTEGRATION AND CONTROL The electrolyzer and the fuel cell system are major components of a selfsufficient wind hydrogen system. The excess energy produced, with respect to load requirement, is being sent to the electrolyzer, which splits water into hydrogen and oxygen. The oxygen is released into the atmosphere and the hydrogen is stored in a tank under pressure. When the input power is insufficient to feed the system load, stored hydrogen is reconverted through a fuel cell to the required electricity. The energy available for storage depends on the load profile and meteorological parameters. [6], [15], [20] A typical self-sufficient renewable energy system must include both short-term and long-term energy storage. Typical renewable energy system designs rely on battery for short-term energy storage, and hydrogen is used for long-term energy storage. In stand-alone systems the electrolyser, the fuel cell, the batteries, the buck and boost converters and the storage system are integrated together. These components are described in later chapters. More detailed equations required in the modeling of the system are also introduced. Modeling softwares (also HOMER used in the case study chapter) use these equations. The renewable energy system components have substantially different voltagecurrent characteristics and they are integrated through power conditioning devices on a DC bus for autonomous operation by using a developed control system. Schematic of this type of a system is shown in Figure 7. [6], [15], [20] 29

30 Figure 7. Wind hydrogen system [15] 6.1 Wind Hydrogen System Concept All system components are integrated through power conditioning devices on the DC bus. The control system with power conditioning devices should manage the energy flow throughout a renewable energy system to assure continuous supply of energy at the load. Control systems vary from simple switches, fuses and battery charge regulators to computerized systems for control of yaw systems and brakes. [6], [21], [22] Systems with hydrogen storage are generally designed for a nominal DC bus voltage that is about 48 V. The real voltage on the DC bus depends on the operating conditions of the system. When energy production exceeds demand and the battery is being charged, the input power tends to impose the output voltage on the DC bus. Therefore peaks in input power can increase the bus voltage notably. Similarily, when input energy production is below what is needed and the load draws on the battery, it is the battery that will impose its voltage on the DC bus. This variability of the bus voltage is a major control problem. Due to this effect, the DC bus voltage alone cannot be considered an 30

31 appropriate variable through which to control the operation of renewable energy plant. It is mainly the battery energy that will be used as a systemcontrolling variable. Examples of grid-connected wind hydrogen and standalone wind hydrogen systems are shown in Figures 8 and 9, respectively. [6], [20], [22] grid Peak Shaving ICE/Fuel Cell O 2 Gas H 2 Gas Hydrogen Storage Power Conditioner -Grid Interconnector -MaxPower Tracker -AC/DC converter -PowerSupply Switch -etc. Control Systems Water Supply + V - Electrolyzer - Water purification - Regulators - Gas dryer - Shutdown Switch - etc. Local H 2 Use H 2 H 2 Figure 8. Grid-connected Wind-Hydrogen system concept [23] 31

32 Consumer Load Desalination Fuel Cell/ H 2 ICE/µTurbine ~ = Electrolyzer Figure 9. Stand-alone Wind-Hydrogen System [23] 6.2 Short-term storage A battery bank is commonly used for short-term energy storage in renewable energy systems because of its ability for fast charging/discharging. The battery is the main component on the dc bus, and plays the role of an energy buffer to handle current spikes. The combination of a battery bank with long-term energy storage in the form of H 2 can significantly improve the performance of stand-alone renewable energy system for energy storage as H 2 and its reutilization. Their performance characteristics depend mainly on their voltage, current and temperature. [6], [20] The battery state-of-charge (SOC) is one of the energy management logics in stand-alone renewable energy systems. The batteries are highly efficient as a buffer to deliver energy quickly during rapid load increase. The electrolyzer will start its operation depending on the battery SOC, excess energy available from the renewable energy system, and the load demand, and stops as per defined SOC in a control algorithm. Similarly the fuel cell generator starts or stops as per battery SOC, energy available from the renewable energy system, availability of stored hydrogen, and the load demand, as defined in the control 32

33 algorithm. The SOC levels to start and stop the electrolyzer and fuel cell system are defined as: SOC start, electrolyzer > SOC stop, electrolyzer and SOC start, fuel cell < SOC stop, fuel cell with SOC stop, fuel cell < SOC start, electrolyzer. As the specified energy levels of the batteries are reached, the control algorithm sends a conditioned signal to the DC/DC (Buck/Boost) converter for effective operation of the electrolyser and fuel cell generator sub-systems. The battery SOC thresholds in the control algorithm have been selected in such way that the fuel cell should not operate simultaneously with the electrolyser. [24], [25] The main parameters, which determine battery s performance, are its internal resistance, the polarization effect, and the long-term self-discharge rate. The battery voltage U B (t), which takes these three parameters into account is given by: U B ( B, 0 i + i R t t) = (1 + α t) U + R ( t) I( t) K Q ( ), (15) where α is the self-discharge rate (s -1 ); U B,0 is the open circuit voltage (V) at t=0; R i (t) is the internal resistance (Ω), K i is the polarization coefficient (Ωh -1 ); and Q R (t) is the rate of accumulated ampere hours. If I(t)>0 then the battery is charging; if I(t)<0 then the battery is discharging. The battery energy is then: t W ( t) = W + P d 0 in ( t ) t, (16) 0 where P in (t )= U B (t)i(t) is the input power to the battery and W 0 is the battery s initial energy. Battery s state of charge (SOC) is defined by:, (17) where W max is the maximum battery energy without overcharge. [20] 33

34 6.3 DC-DC Converters The renewable energy system components have substantially different voltagecurrent characteristics, and they are integrated on a 48V DC bus through proper power conditioning devices for effective power management. The excess energy provided to the electrolyzer for energy storage as H 2 is controlled through a buck converter connected between the DC bus and the electrolyzer. Similarly, a boost converter has been used to control the fuel cell system output to provide the required energy at the DC bus. The boost converter has been connected between the PEM - fuel cell system and the DC bus. [22] These secondary micro-controllers manage the power flow with respect to the energy availability at DC bus through the digitally controlled DC-DC converters. These DC-DC converters use a multiphase technique to generate pulse width modulation signals to control the power flow. The DC-DC converters are important components in the system for effective operation and power flow control of the electrolyzer and fuel cell system. [22] The limits of the energy levels in the control algorithm at which the electrolyzer and fuel cell system kick in or out in response to variations in the systems (source power, load demand, etc.) are implemented through a double hysteresis strategy, meaning that the energy level at which either device is turned on is not the same as the level at which is turned off. The developed control algorithm needs to take into account that the PEM fuel cell system and the electrolyzer cannot operate at the same time. The proper choice of pre-defined energy levels at the DC bus should produce effective operation of the electrolyzer and the PEM fuel cell systems. This choice depends on the environmental conditions, the load profile, etc. In Figure 10 (page 40) are system components with the control units. [22], [24] 34

35 6.3.1 Buck converter Electrolyzer starts its hydrogen production, when the electrical energy exceeds load demand. To control this hydrogen production, a buck converter controls the input current to the electrolyzer cells. This buck converter is a dc voltage reducer designed to maximize the power transfer from the dc bus to the electrolyzer cells. The following equation gives the relation between the buck output voltage and the dc bus voltage: ( B + B z ) D ( n) U Bu, (18) 1 Bu,0 Bu,1 Bu, Out ( n) = U B ( n) 1 ABu,0 + ABu,1z where A Bu,0, A Bu,1, B Bu,0, and B Bu,1 are parameters which have to be determined. U B is the dc bus voltage; D Bu is the duty cycle and U Bu,Out is the buck converter output voltage (and applied to the electrolyzer cells). The buck converter input voltage (U Bu,In ) is equal to the dc bus voltage because of its direct connection to the dc bus. Taking into account the buck power efficiency (η Bu ), the input current (I Bu,In ) to the buck converter is: I Bu, In U Bu, Out ( n) I Bu, Out ( n) ( n) =, (19) η U ( n) Bu Bu, In where I Bu,Out is the input current to the electrolyzer cells. [20] Boost controller When power from the renewable source is insufficient, the fuel cell starts its operation to convert hydrogen to electrical energy. Boost converter controls the PEM fuel cell system output to provide the required energy at the DC bus. The boost converter has been connected between the PEM fuel cell system and the 35

36 DC bus. The relation between duty cycle D Bo and the input current I Bo,In (t) of the boost converter is given by: ( B + B z ) D ( n) I Bo, (20) 1 Bo,0 Bo,1 Bo, In ( n) = I FC, Max 1 ABo,0 + ABo,1z where I FC,Max is the maximum output current of the fuel cell; A Bo,0, A Bo,1, B Bo,0 and B Bo,1 are parameters to be determined. The output current (I Bo,Out ) of the boost converter is obtained from th boost power efficiency (η Bo ): I Bo, Out U FC ( n) I Bo, In ( n) ( n) = ηbo, (21) U ( n) B where U FC is the fuel cell output voltage, and η Bo is determined by direct measurement. [20] 6.4 Electrolyzer The excess energy is stored in the form of electrolytic H 2 produced through the electrolyzer unit, which consists of a control unit, a compressor, and purification and drying unit. The electrolyzer input power is controlled, with respect to the energy available at the DC bus, through a duty ratio of the DC- DC converter. The electrolyzer characteristics depend mainly on voltage, current and cell temperature. The electrolyzer voltage is given by:, (22) where U el,0 (V), C l (V C -1 ), C 2 (V C -1 ), I el,0 (A) and R el (Ω C -1 ) are parameters of the electrolyzer and can be determined experimentally and they depend on the 36

37 type of electrolyzer and the stack structure. The first two terms of equation (22) represent the theoretical potential of an ideal cell, the third term gives the activation potential of the electrodes, and the last term represents ohmic losses. The total electrolyzer cell input power goes into four applications: the main H 2 production (P el,h2 ) and three losses to heat production (P el,heat ), process control (P el,ctrl ) and the gas handling equipment power (P gh ) (i.e., the compressor): ( P + P P ) P + el = Pel, H + 2 el, heat el, ctrl gh, (23) and P el, H = N cellv 0 ηcell I 2 el, (24) where V 0 is the reversible voltage of the electrolysis reaction (which at room temperature is 1,48 V), N cell is number of cells in series and η cell is the electrolyzer current efficiency (i.e., the utilization factor) and depends on the cell temperature. The hydrogen production rate V el (t) is given by: V el ηi, el I el ( t) = N Cell, (25) C H 2 where C H2 is the conversion coefficient, i.e., 2,39 Ah/l H 2. The overall electrolyzer performance depends on the power consumption of the buck converter, compressor and control unit, and on hydrogen leakage. The energy efficiency of the electrolyzer can be given by:, (26) 37

38 where P el is the electrolyzer input power which is avalable for storage as H 2, and T is the system operating time. [20], [22], [25] 6.5 PEMFC The stored electrolytic H 2 is converted back into electricity via the fuel cell system as per the energy demand and pre-defined energy levels in the control algorithm. The polarization characteristics of the fuel cell system depend on the thermodynamic potential, ohmic losses, stack temperature, and oxygen concentration. The PEM fuel cell voltage is given by: V cell fc ( CI fc ) R fc I fc, = A B ln, (27) where A, B and C are the parameters of the PEM fuel cell system which can be determined experimentally, and which depend on the type of fuel cell and its performance. R fc is the PEM fuel cell stack resistance, and I fc is the PEM fuel cell stack current. During operation the PEM fuel cell system experiences hydrogen leakage, losses due to purging, and heat losses. The net consumption rate of hydrogen in the PEM fuel cell system is given by: Q fch N sfc I fc =, (28) 2 c η c cfc where η cfc is the utilization factor, i.e. current efficiency, of the PEM fuel cell system. N sfc is the number of cells connected in series, and I fc is the PEM fuel cell system output current. The PEM fuel cell system power will be:, (29) 38

39 where P fc,h2 is the net power needed for hydrogen conversion, P fc,heat is the heat loss in the PEM fuel cell system, and P fc,ctl is the power required for the PEM fuel cell system control process. The losses in the PEM fuel cell system are given by: P loss fc V ccq, 0 fch P 2 fc, out = (30) The instantaneous efficiency of the PEM fuel cell is obtained from: Pfc, out η fc = (31) P + P fc, out loss, fc The energy conversion efficiency of the PEM fuel cell system over the operation time T is given by: T fc, out, net 0 H conv = 2, T c V c P 0 0 Q dt η (32) fch 2 dt The hydrogen energy storage efficiency depends on the combined efficiency of electrolyzer, PEM fuel cell system, storage system, and power conditioning device effincies. Figure 10 shows the system components. [6], [22], [25] 39

40 Wind Turbine Load Battery Bank ~ - ~ - DC Bus Buck Converter Boost Converter Electrolyser Press. H 2 Storage PEM Fuel Cell Figure 10. Simplified stand-alone wind hydrogen system with control units. [24] 6.6 Design tools for RE systems When designing off-grid and grid-connected systems, there are many different size possibilities and hardware configurations to be determined. For example, the size of wind turbine, electrolyzer, hydrogen storage tank and fuel cell must all be determined to ensure that system is able to meet the required load at the least possible cost. [26], [27] In this study the focus is in combining wind turbine and fuel cell. Often used software for this effort is Hybrid Optimization Model for Electric Renewables (HOMER), which is freely availale from National Renewable Energy Laboratory (NREL). HOMER determines the operation of a system by making energy balance calculations for each hour in a year. It finds the least cost combination of components that meet electrical and thermal loads and simulates thousands of system configurations, optimized for lifecycle cost, and generates results of sensitivity analyses on most inputs. Inputs to HOMER include load data, renewable resource data, system component specifications and costs, and various optimization parameters. [26], [27] 40

41 7 FUEL CELL VEHICLES The high dependency of transportation on fossil fuels, the raising awareness to many environmental problems and the fact that oil is actually diminishing and the price of it is increasing, will demand a clean, sustainable alternative fuel in the future. Governments, car manufactures and energy enterprises are realising the increasing need of an alternative fuel. It is in these mobile applications that hydrogen will find its first and largest market. [4] Internal combustion engines for automotive industry can be replaced with environmentally friendly fuel cells using electrolytic hydrogen. Fuel cell vehicles fueled with pure hydrogen emit no pollutants, only water and heat. The benefits of using hydrogen will be fully realized when produced from renewable energies. [28] Fuel cell vehicles are propelled with electric motors, like battery-electric vehicles. The main difference in these is that a fuel cell vehicle creates its own electricity, while battery electric vehicles use electricity from an external source and store it in a battery. Fuel cells in the vehicle can be fueled with pure hydrogen gas, which is stored in a high-pressure tank in the vehicle. These vehicles can also use hydrogen-rich fuels, such as methanol or natural gas, but these fuels must first be converted into hydrogen gas. This conversion needs a special device in the vehicle, which turns the hydrogen-rich fuels into hydrogen gas. The vehicles using hydrogen-rich fuels emit in addition to water and heat, also small amounts of pollutants (mainly carbon dioxide). [29] The shift from a transport system based on fossil fuels to hydrogen requires huge investments and involves major infrastructural changes both in the fuel distribution and supply and in the vehicles. Iceland has already undertaken the 41

42 ambitious goal to become the first hydrogen economy in the world, producing hydrogen by electrolysis using excess renewable energy.[4] 7.1 Direct-hydrogen fuel cell vehicle model Direct-hydrogen version of the FCVSim tool (DH-FCVSim) has four major subsystems, which are fuel cell stack, air supply system, water and thermal management system and hydrogen supply. DH-FCVSim system diagram is represented in Figure 11. Figure 11. DH fuel cell system diagram.[30] The fuel cell stack uses hydrogen gas and air to produce electricity to power the electric motor. The fuel cell works as a fuel cell described earlier. Oxygen is supplied from the ambient air by a compressor or a blower. The conditions on anode and cathode (pressure, stoichiometry and humidity) have a strong effect on the stack performance. Supply conditions on anode and cathode, electric power demand and the shape of the fuel cell polarity plot have a critical impact on fuel cell vehicle performance. This impact occurs through the voltage 42

43 feedback effect and is due to the depedence of the available motor torque on the supply voltage at the terminals of the power electronics. The fuel cell stack typically consists of over 400 individual fuel cells. [29], [30] The air supply system illustrated in Figure 11 includes compressor with a variable speed electric motor. The compressor drives pressure and mass flow to the cathode, where oxygen is depleted for power generation. The air compressor controls the rate at which air is supplied to the stack according to the need for power. The exhaust path from cathode includes a condenser to recover liquid water to H 2 O tank, but no humidifier is included. It is necessary to recover liquid water for recirculation into the anode fuel loop. The air supply system also interacts with water and thermal managemant system components. [29], [30] Water and thermal management system includes the radiator and the condenser, for heat transfer and water recovery, respectively. Water and thermal management system consideres all other component in the system, but only in the context of heat and water balance. The heat load in the system is given by the combination of the stack heat rejection due to inefficiency and due to water condensation in the stack. Relatively little heat is carried away in the exhaust of the fuel cell (<10%). The required flow rates of coolant through the radiator can be substantial, because the fuel cell stack has a small temperature range for optimal operation. The hydrogen is generally stored as a gas in high-pressure tanks, so that enough fuel can be stored to give the vehicle a suitable driving range. In most current fuel cell vehicles, the fuel tank is cabable of storing hydrogen at about 35 bar, and even higher pressure tanks are under development. This current 35 bar tank can store enough hydrogen to allow vehicles go more than 300 km before refuelling. The hydrogen supply system also includes a recirculation of 43

44 hydrogen from the anode back to the flow from the hydrogen tank. The compressed hydrogen does not effect on the dynamic response of the fuel cell system, nor on the energy conversion efficiency. [29], [30] Some fuel cell vehicles contain a battery to store electricity produced from regerative braking or from the fuel cell stack. Battery can be used to help power the electric motor or other electrical devices. Fuel cell vehicles are being controlled by a power controller unit. This unit contains the electronic control mechanisms that manage the production and storage of electricity. In Figure 12 is shown a fuel cell vehicle. [30] Figure 12. A fuel cell vehicle. 7.2 Fuel cell vehicle challenges Fuel cell vehicles have big challenges to overcome before becoming a competitive alternative for conventional vehicles. The hydrogen storage in the vehicle needs development to allow fuel cell vehicles travel the same distance as conventional vehicles with a full tank of fuel. Although fuel cells are more energy-efficient than internal combustion engines in terms of the amount energy used per weight of fuel and the amount of fuel used versus the amount vasted, hydrogen is very diffuse and hence, in terms of weight, very small 44

45 amount can be stored in reasonable size tank. This can be overcome by increasing the pressure under which the hydrogen is stored or through the development of chemical or metal hybride storage options. Also new facilities and systems will be required to get hydrogen to consumers, because the existing gasoline delivery and filling stations cannot be used for transporting or storing hydrogen. The operation of fuel cell at cold weather is also a great challenge. Fuel cell system always contains water, both as a byproduct and for humidifying the fuel cell, and water freezes at cold temperatures. Also the efficiency of the fuel cell is best at certain temperature. The handling of hydrogen has safety risks, which are different from conventional gasoline. Therefore, safe storing and transportation systems of hydrogen must be optimized. Fuel cell vehicles are more expensive than conventional vehicles, mainly because of the expensive catalyst (platinum) and electrolyte membrane. To survive in the competitive market, fuel cell vehicles will have to offer consumers a viable alternative, in terms of performance, durability and cost. [30] 45

46 8 CASE STUDY: KÖKAR ISLAND Remote areas can be considered as those that lie outside of grid systems. Those areas are typically geografically remote and characterised by sparse populations. In such areas the energy supplies are susceptible to interruption and improved utilization of locally available renewable resources would offer significant benefit. This study focuses on a small island in the Åland archipelago in Finland, which already has a 500 kw wind turbine. The aim of this study is to evaluate in which configuration the island could be self-sufficient in terms of power with a system including wind turbines, electrolyser, hydrogen tank and a fuel cell. Software used for this effort is Hybrid Optimization Model for Electric Renewables (HOMER). Data from the existing turbine is shown on Table 2. The energy demand of the island is estimated based on an assumption that average energy consumption is 5 MWh/person/year. The population in the island is 320, so the load demand is approximately 1600 MWh/year. It is seen from the statistics that the island can not be completely self-sufficient with only the existing turbine. 46

47 Table 2. Statistics on 500 kw wind turbine in Kökar, height (Z) 44 m and rotor diameter (D) 40 m. [31], [32] Year 2005 Year 2006 Estimate (MWh) Production (MWh) t h (h) e (kwh/m 2 ) CF 0,32 0,31 (h) Usability (%) 98 % 100 % 8.1 Stand-alone system description The energy output is provided by wind turbines. Six 500 kw wind turbines are considered, because the power of existing wind turbine in insufficient. The electrolyzer in this study is assumed to be of alkaline type. The electrolyzer unit consists of H 2 purification, filtration, and compression systems for long-term hydrogen storage. The hydrogen produced in electrolyzer is stored in a pressurized hydrogen tank from where it can either be delivered to a fuel cell to produce electricity or used as fuel for hydrogen vehicles. Electricity production from hydrogen is done through a proton exhange membrane fuel cell (PEMFC) system. A battery storage system is included for balancing the wind power with different loads. The schematic of the system is shown in Figure 13 (page 50). 47

48 8.2 Component sizing On that precise area there is no wind speed statistics. Therefore, wind speed data for that location was obtained from NASA website according to latitude and lognitude (N E ). The existing data is used for the wind turbines. The software uses values in dollars for economic calculations and therefore most values are in dollars. Currency convertion was made with the exchange rate 1 $ = 0,6717 ( , Reuters website). The initial cost of 500 kw wind turbine is assumed to be $, and the operation and maintenance cost 4 % ( $/year). It has been taken into consideration that Kökar island already has one 500 kw wind turbine. [33] Electrolyzer is assumed to be of unit size kw and the cost per unit $. Sizes of 1.000, and kw are considered. Fuel cell is assumed to be of unit size 400 kw and the cost $, according the medium cost case in research of T.E. Lipman et al. [34]. Hydrogen consumption is set to 40 kg/h when fuel cell output power is 400 kw, and 32 kg/h when output is 300 kw, which gives fuel curve parameters: intercept coefficient 0,02 kg/h/kw rated and slope 0,08 kg/h/kw output. The unit size for hydrogen tank is assumed to be kg, which stands for about kwh. The cost for hydrogen tank is 5 $/kw. Sizes 4.000, and kg are considered. The hydrogen tank would also serve as fuelling station for vehicles. The average hydrogen load is assumed to be 0,13 kg/hr, which is calculated from assumptions that there is 100 vehicles in the island, each of them drives km/y, the tank size is 50 l, the pressure in the tank is 5000psi(~345 bar) and that you can drive 300 km with one tank of hydrogen. 48

49 Converter is placed between AC and DC bus and the unit size is 20 kw. The price for converter is 50 $/kw and sizes 20 and 40 kw are considered. Batteries selected to the system are the biggest in HOMER, Ah. The price for batteries is assumed to be approximately the same as in car batteries, 1 $/Ah. One battery unit would cost then $ and the system is considered with 10, 20 and 30 battery units. Case study parametres are shown in Table 3 and the overall HOMER input summary is in Appendix I. Table 3. Case study parameters. Unit Size Wind turbine Sizes in consideration Cost $/unit Operation & Maintenance cost / year 500 kw 2,4,6,8 units % Electrolyzer kw 1.000, 3.000, kw % Fuel cell 400 kw 400 kw ,0 $/h (6 %) Battery Ah (6 kwh) 10, 20, 30 units % Hydrogen storage kg 4.000, 8.000, kg % Converter 20 kw 20, 40 kw % 49

50 8.3 Results The different component sizes were fed to HOMER, which then calculated all possible combinations with the given figures. There were 216 combinations with these different component sizes, starting from 2 x 500 kw wind power where the capacity shortage was 38 %, to 8 x 500 kw wind power where 37 % of the total electrical energy production was excess electricity. The criterion to the chosen combination of component sizes was that there should be no capacity shortage with the most affordable cost. The results suggest the stand-alone system to be most cost-effective and without capacity shortage, with 3 MW wind power, kw electrolyser, kg hydrogen tank, 400 kw fuel cell, 20 kw converter and 10 units of Ah batteries. System components in and sizes are shown in Figure 13. Figure 13. Case study system components and sizes. 50

51 Total net present cost with these equipments would be about $ ( ), initial capital cost $ ( ) and the price of electricity would be 0,289 $/kw (0,194 /kwh). More precise cost breakdown is listed in Table 4. Table 4. Cost breakdown in Kökar island case study Initial Annualized Annualized Component Capital Capital Replacement Annual O&M Annual Fuel Total Annualized ($) ($/yr) ($/yr) ($/yr) ($/yr) ($/yr) Wind Turbine 500kW Generator Battery Converter Electrolyzer Hydrogen Tank Totals

52 Kökar Island already has a grid connection, but if compared to a place without grid connection, it would be economically more beneficial to build this kind of system if existing grid would be further than 287 km. This result is based on the assumptions that electricity price in grid is 0,10 (0,148 $), capital cost for building a grid $/km and operation and maintenance cost 160 $/km/year. Results show, that 95 % of the islands energy is produced by wind turbines and 5 % by fuel cells. From the Figure 14 can be seen that in winter the production of wind turbines is highest, and the need for fuel cells is very small. Respectively, the need for fuel cells is larger in the summer, when the wind turbine production is lower. Figure 14. Annual electric energy production in Kökar island case study. Average electrical output of the wind turbines was 837 kw and fuel cell 46,3 kw. Respectively, maximum electrical ouputs for those components were kw and 356 kw. The fuel cell consumed hydrogen as fuel about kg/year, 0,253 kg/kwh, and external hydrogen load consumed hydrogen 0,13 kg/h. Electrolyser produced hydrogen kg/year. Hydrogen tank level varied with the fuel cell production, as in the summer the fuel cell produced more electricity and consumed more fuel. In Figure 15 is shown the hydrogen tank level throughout one year. 52

53 Figure 15. Hydrogen tank level in Kökar island case study. Battery usage was very small, mostly they were fully charged. The entire system report is in Appendix II and in Appendix III are diagarams showing hourly data between different components. 53

54 9 CONCLUSIONS The imminent climate change forces us to cut up large scale use of fossil fuels and replace them with alternative energy sources. Renewable means of producing energy are ancient, but the modern study of them has begun no more than few decades ago. Today technology is quite ready, although it still needs large investment from governments. Finland has a great capacity to produce wind power, especially in coastlines. The Kökar island case study in this research gives an example of a fully renewable system in remote areas. The results show that the island can not be self-sufficient in terms of power with the existing 500 kw wind turbine. If a fuel cell system were used with the existing turbine, the island would need % of the total electricity consumption from the grid. By adding wind power to six units (3 MW) and building fuel cell system with it, the island could be entirely self-sufficient with power. Results suggests the price of electricity to be 0,191 /kwh, which is slightly more than electricity price from the grid. Dominant figure in cost calculations is the price of wind turbines, followed by fuel cell costs. Costs of these renewable systems are about to decrease within the next years by technology developments, standardisation, mass-production, and greater competitivness. The results of the case study show that this type of wind hydrogen system is ideal for remote areas, which are far away from the existing grid. Particularly when the place in question is an island and the grid would need to be built under sea or lake. This type of system would also be good in smaller scale, for example in summer/winter cottages, where the other resource of power could be solar cell. 54

55 ACKNOWLEDGEMENT I would like to thank everyone who has helped me along the way. Especially Jussi Maunuksela for guidance and support. I also express my gratitude for Professor Korppi-Tommola from giving me the idea of this topic and kindly red this draft and adviced me. 55

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59 [24] M. Kolhe, K. Agbossou, J. Hamelin, T.K. Bose, Analytical model for predicting the performance of photovoltaic array coupled with a wind turbine in a stand alone renewable energy system based on hydrogen, Renewable Energy 28 (2003), [25] K. Agbossou, M. Kolhe, T.K. Bose, Autonomous operation and control of stand-alone renewable energy system with hydrogen storage, 22 nd IASTED International Conference Modelling, Identification, and Control (MIC2003), Feb 2003 [26] J. Cotrell, W. Pratt, Modeling the feasibility of using fuel cells, and hydrogen internal combustion engines in remote renewable energy systems (Preprint), National Renewable Energy Laboratory (NREL) Conference paper, May 2003 [27] National Renewable Energy Laboratory (NREL), HOMER Brochure, Internet: sited [28] E. Bernier, J. Hamelin, K. Agbossou, T.K. Bose, Electric round-trip efficiency of hydrogen and oxygen based energy storage, International Journal of Hydrogen Energy, Volume 30, Issue 2, February 2005, pages [29] United States Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Vehicles, sited

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61 APPENDIX I HOMER Input Summary AC Load: Primary Load 1 Data source: Synthetic Daily noise: 15% Hourly noise: 20% Scaled annual average: 4,377 kwh/d Scaled peak load: 403 kw Load factor: AC Wind Turbine 500kW Quantity Capital ($) Replacement ($) O&M ($/yr) 1 333, ,000 16,000 Quantities to consider: 6 Lifetime: Hub height: 15 yr 25 m 61

62 Wind Resource Wind Speed Month (m/s) Jan 8.60 Feb 7.80 Mar 7.35 Apr 6.96 May 6.47 Jun 6.16 Jul 5.98 Aug 6.13 Sep 7.26 Oct 7.91 Nov 7.97 Dec

63 Weibull k: 2.00 Autocorrelation factor: Diurnal pattern strength: Hour of peak wind speed: 15 Scaled annual average: Anemometer height: Altitude: Wind shear profile: 7.23 m/s 40 m 0 m Logarithmic Surface roughness length: 0.01 m Fuel Cell Size (kw) Capital ($) Replacement ($) O&M ($/hr) , , Sizes to consider: Lifetime: 400 kw 15,000 hrs Min. load ratio: 0% Heat recovery ratio: 0% Fuel used: Stored hydrogen Fuel curve intercept: 0.02 L/hr/kW Fuel curve slope: 0.08 L/hr/kW 63

64 Battery: Hoppecke 24 OPzS 3000 Ah Quantity Capital ($) Replacement ($) O&M ($/yr) 10 30,000 5,000 1, Quantities to consider: 10, 20, 30 Voltage: Nominal capacity: Lifetime throughput: 2 V 3,000 Ah 10,196 kwh Converter Size (kw) Capital ($) Replacement ($) O&M ($/yr) , Sizes to consider: Lifetime: 20, 40 kw 15 yr Inverter efficiency: 90% Inverter can parallel with AC generator: Yes Rectifier relative capacity: 100% Rectifier efficiency: 85% Grid Extension Capital cost: $ 8,000/km O&M cost: $ 160/yr/km Power price: $ 0.148/kWh AC Electrolyzer Size (kw) Capital ($) Replacement ($) O&M ($/yr) 1, ,000 20,000 2,000 Sizes to consider: 1,000, 3,000, 5,000 kw Lifetime: 15 yr Efficiency: 85% Min. load ratio: 0% 64

65 Hydrogen Tank Size (kg) Capital ($) Replacement ($) O&M ($/yr) 1, ,000 1, Sizes to consider: Lifetime: 4,000, 8,000, 10,000 kg 25 yr Initial tank level: 10% Constrain year-end tank level: No Economics Annual real interest rate: 6% Project lifetime: 25 yr Capacity shortage penalty: $ 0/kWh System fixed capital cost: $ 0 System fixed O&M cost: $ 0/yr Generator control Check load following: Check cycle charging: No Yes Setpoint state of charge: 80% Allow systems with multiple generators: Allow multiple generators to operate simultaneously: Yes Yes Allow systems with generator capacity less than peak load: Yes 65

66 APPENDIX II System Report System architecture Wind turbine: Generator 1: 6 Copy of WES 30/ Enercon 500kW 400 kw Battery: 10 Hoppecke 24 OPzS 3000 Inverter: Rectifier: Electrolyzer: Hydrogen Tank: 20 kw 20 kw 3,000 kw 10,000 kg Dispatch strategy: Cycle Charging Cost summary Total net present cost: 5,903,496 $ Levelized cost of energy: $/kwh Cost breakdown Initial Annualized Annualized Annual Annual Total Component Capital Capital Replacement O&M Fuel Annualized ($) ($/yr) ($/yr) ($/yr) ($/yr) ($/yr) Copy of WES 30/ Enercon 500kW 1,999, ,453 63,758 96, ,211 Generator 1 280,000 21,903 77,279 17, ,649 Battery 30,000 2, , ,600 Converter 1, Electrolyzer 150,000 11,734 1,594 6, ,328 Hydrogen Tank 50,000 3, , ,911 Totals 2,510, , , , ,811 66

67 Annual electric energy production Component Production Fraction (kwh/yr) Wind turbines 7,327,758 95% Generator 1 403,931 5% Total 7,731, % Annual electric energy consumption Load Consumption Fraction (kwh/yr) AC primary load 1,595,512 24% Electrolyzer load 5,193,606 76% Total 6,789, % Variable Value Units Renewable fraction: Excess electricity: Unmet load: Capacity shortage: 942,579 kwh/yr 2,091 kwh/yr 2,433 kwh/yr 67

68 AC Wind Turbine: Copy of WES 30/ Enercon 500kW Variable Total capacity: Average output: Minimum output: Maximum output: Value Units 3,139 kw 837 kw kw 3,139 kw Wind penetration: 459 % Capacity factor: 26.6 % Hours of operation: 7,575 hr/yr Generator Variable Value Units Hours of operation: Number of starts: Operational life: Average electrical output: Minimum electrical output: Maximum electrical output: Annual fuel usage: Specific fuel usage: 8,733 hr/yr 4 starts/yr yr 46.3 kw kw 356 kw 102,178 L/yr L/kWh Average electrical efficiency: 11.9 % 68

69 Battery Variable Battery throughput Value Units 144 kwh/yr Battery life 20.0 yr Battery autonomy hours 69

70 HydrogenTank Variable Hydrogen production: Hydrogen consumption: Hydrogen tank autonomy: Value Units 111,919 kg/yr 102,178 kg/yr 1,828 hours 70