Download B4SKA-WP4-D.4.1 Diagram of the Power Conversion Unit

Energy storage Feasibility Report for the BIOSTIRLING4SKA COMMISSION OF THE
EUROPEAN COMMUNITIES
A cost effective and efficient
approach for a new generation
of solar dish-Stirling plants
based on storage and
hybridization
Grant Agreement number: 309028
BIOSTIRLING-4SKA
D.4.1. Diagram of the
Power Conversion Unit
WP4: Power Conversion Unit
Responsible partner: University of Seville
Dissemination level:
Version date: 30th May 2014 1 Energy storage Feasibility Report for the BIOSTIRLING4SKA TABLE OF CONTENTS TABLE OF CONTENTS ....................................................................................................... 2 DOCUMENT HISTORY ...................................................................................................... 2 1. EXECUTIVE SUMMARY ................................................................................................................ 3 2. ENERGY STORAGE ALTERNATIVES .................................................................................................. 4 2.1. Pumped hydro storage................................................................................................................. 4 2.2. Compressed Air Energy Storage ................................................................................................... 4 2.3. Batteries energy storage system. ................................................................................................ 5 2.3.1. Lead‐acid batteries ................................................................................................................... 6 2.3.2. Lithium‐ion batteries ................................................................................................................. 7 2.3.3. Nickel‐cadmium batteries ......................................................................................................... 7 2.3.4. Sodium‐sulfur batteries ............................................................................................................ 9 2.3.5. Flow batteries ......................................................................................................................... 10 2.4. Hydrogen energy storage system. ............................................................................................. 13 2.4.1. Electrolyzer ............................................................................................................................. 13 2.4.2. Fuel Cell ................................................................................................................................... 17 2.5. Flywheel energy storage system. ............................................................................................... 20 2.6. Supercapacitor energy storage system. ..................................................................................... 21 3. TECHNICAL AND ECONOMICAL FEASIBILITY ANALYSIS ..................................................................... 21 3.1. Hydrogen storage system with small batteries set .................................................................... 22 3.2. Batteries storage system. ...................................................................................................... 23 4. SUMMARY AND CONCLUSIONS ................................................................................................... 25 REFERENCES ............................................................................................................................... 26 Document History Version Status Date 00 Draft 27/03/2014 01 First Revision 13/05/2014 02 Second revision 30/05/2014 03 04 Approval Name Date Prepared US 13/05/2013 Reviewed Authorised 2 Energy storage Feasibility Report for the BIOSTIRLING4SKA 1. Executive Summary The objective of the document is to determinate the most suitable energy storage system to BIOSTILING‐4SKA demo plant in Moura (Portugal). The electrical requirements of SKA sensors infrastructure are summarized below (these requirements are being revised): Average Power [kWe]
Montly average energy demand [kWeh] Antenna 20 14880 Auxiliaries / Computing
20 14880 Air Conditioning 10 7440 Load Table 1. SKA sensors infrastructure electrical requeriments of Moura demo plant. The main objective of energy storage system is to assure the supply of electric energy demand of SKA. In normal operation, Stirling dishes supply the energy demand of SKA, if energy produced is higher than energy demanded, the surplus energy is sent to energy storage system. In the opposite way, if the energy produced by Stirling engines is lower than SKA electric demand, the defect is extracted from the energy storage system. Other objectives of the energy storage system:  The system has to serve as a grid stabilizer, due to an operation of grid as an ‘island’. The voltage of grid is 400 V (AC). The electric energy produced and demanded by grid are variable in time, so fluctuations of frequency and voltage of grid occur. These fluctuations have to be limited in order to not produce the collapse of grid.  If there is a problem in Stirling dishes that makes a full defect of energy in grid, the energy storage system has to assure the energy demanded during an enough period of time to permit the connection of grid to the general Portuguese net. The objective of document is to determine the most suitable Diagram of the Power Conversion Unit, so a technical and economical feasibility study of different storage technologies has to be performed. The document is divided in four parts:  First of all, different energy storage systems are presented.  In section 3, a technical and economical analysis with different scenarios is options are performed.  The document finishes with a summary and conclusions. 3 Energy storage Feasibility Report for the BIOSTIRLING4SKA 2. Energy storage alternatives Electrical energy can be converted to many different forms for storage:  As gravitational potential energy with water reservoirs.  As compressed air.  As electrochemical energy in batteries and flow batteries.  As chemical energy in hydrogen.  As kinetic energy in flywheels.  As electric field in capacitors. In this section, a review of several available technologies of energy storage that can be used for microgrids is explained. 2.1. Pumped hydro storage Pumped hydro storage is a large‐scale energy storage system. Its operating principle is based on managing the gravitational potential energy of water, by pumping it from a lower reservoir to an upper reservoir during periods of low power demand or an electric production. When the power demand is high, water flows from the upper reservoir to the lower reservoir, activating the turbines to generate electricity. The energy stored is proportional to the water volume in the upper reservoir and the height of the waterfall. This technology is most used for high‐power applications and needs two water reservoirs at different heights [1‐2]. This technology needs other elements to assure an adequate voltage stability of grid. 2.2. Compressed Air Energy Storage Compressed Air Energy Storage systems are based on conventional gas turbine technology. In this type of system, the energy is stored in form of compressed air in an underground storage cavern. When energy is required to be injected into the grid, the compressed air is drawn from the storage cavern, heated and then expanded in a set of high and low pressure turbines which convert most of the energy of the compressed air into rotational kinetic energy. The air is additionally mixed with natural gas and combusted. While the turbines are connected to electrical generators in order to obtain electrical energy, the turbine exhaust is used to heat the cavern air. The topology of the whole system is shown in Figure 1. 4 Energy storage Feasibility Report for the BIOSTIRLING4SKA Figure 1. Lay‐out of a Compressed Air Energy Storage. [1]
As the Pumped Hydro System, this alternative is also used for high‐power applications. 2.3. Batteries energy storage system. Batteries are one of the most used energy storage technologies available on the market. The energy is stored in the form of electrochemical energy, in a set of multiple cells, connected in series or in parallel or both, in order to obtain the desired voltage and capacity. Each cell consists of two conductor electrodes and an electrolyte, placed together in a special, sealed container and connected to an external source or load. The electrolyte enables the exchange of ions between the two electrodes; while the electrons flow through the external circuit. BESS is a solution based on low‐voltage power battery modules, connected in series / parallel in order to achieve the desired electrical characteristics. Batteries storage system comprises batteries, the Control and Power Conditioning System (C‐PCS, in BIOSITLING‐4SKA Project is called EMS) and the rest of the plant, which is in charge of providing good protection for the entire system.
Different technologies of batteries are suitable for isolated small grids:  Lead‐Acid batteries.  Nickel‐Cadmium batteries.  Sodium‐sulphur batteries.  Lithium‐ion batteries.  Flow batteries (Vanadium redox, Zinc‐bromine, Polysulphide‐bromide). Next, each type of battery are discussed: 5 Energy storage Feasibility Report for the BIOSTIRLING4SKA 2.3.1. Lead‐acid batteries Lead–acid batteries, invented in 1859, are the oldest type of rechargeable battery and they use a liquid electrolyte, as illustrated in Figure 2. The technology of lead–acid batteries is uncomplicated and manufacturing costs are low; however, such batteries are slow to charge, cannot be fully discharged and have a limited number of charge/discharge cycles, due to their low energy‐to‐weight ratio and their low energy‐to‐volume ratio [3]. The lead and sulfuric acid used are also highly toxic and create environmental hazards, which can be particularly ironic when used to accompany clean sources of power such as PV systems [4]. Figure 2. Structure of lead‐acid battery [5] The lead–acid battery chemistry can be modified for grid storage applications beyond stabilization applications by modification of the electrode structures. Lead–carbon electrodes are designed to combine high energy density of a well designed battery with the high specific power obtained via charging and discharging of the electrochemical double layer. Lead–carbon electrode research has been focused on the extension of cycle life durability and specific power. Carbon is added to the negative electrodes, and while the carbon does not change the nature of the charge transfer reactions, it increases specific power and reduces the incidence of sulfation during charging cycles, which is one of the principal failure modes of traditional lead– acid batteries [19]. In these applications, it is required to have relatively deep discharges with good cycle life. With new carbon enhanced negative electrodes in valve regulated lead–
acid (VRLA) batteries, the cycle life is improved up to a factor of 10 at significant rates. In Renewable Energy Source (RES) applications multiple deep‐cycle lead–acid (DCLA) batteries, which provide a steady current over a long time period, are connected together to form a battery bank. Indeed, banks of up to 1 MW of lead–acid batteries are already being used to stabilize wind farm power generation. For instance, DCLA are designed for backup and peak shifting in off‐grid and grid‐tied PV systems [6]. 6 Energy storage Feasibility Report for the BIOSTIRLING4SKA 2.3.2. Lithium‐ion batteries Lithium‐ion batteries, illustrated in Figure 3, which have achieved significant penetration into the portable consumer electronics markets and are making the transition into hybrid and electric vehicle applications, have opportunities in grid storage as wel. If the industry’s growth in the vehicles and consumer electronics markets can yield improvements and manufacturing economies of scale, they will likely find their way into grid storage applications too. Developers are seeking to lower maintenance and operating costs, deliver high efficiency, and ensure that large banks of batteries can be controlled [7]. Continued cost reduction, lifetime and state‐of‐
charge improvements, will be critical for this battery chemistry to expand into these grid applications [8]. There are three types of lithium‐ion batteries in commercial use, such as, cobalt, manganese and phosphate. When lithium‐ion batteries are used for utility‐scale applications, it is to perform regulation and power management services and will be used for minutes of runtime [9,10]. Figure 3. Structure of lithium‐ion battery [11]. 2.3.3. Nickel‐cadmium batteries A nickel–cadmium battery is made up of a positive electrode with nickel oxyhydroxide as the active material and a negative electrode composed of metallic cadmium. These are separated by a nylon divider. The electrolyte, which undergoes no significant changes during operation, is aqueous potassium hydroxide. During discharge, the nickel oxyhydroxide combines with water and produces nickel hydroxide and a hydroxide ion. Cadmium hydroxide is produced at the negative electrode. To charge the battery the process can be reversed. However, during charging, oxygen can be produced at the positive electrode and hydrogen can be produced at 7 Energy storage Feasibility Report for the BIOSTIRLING4SKA the negative electrode. As a result some venting and water addition is required, but much less than required for a lead–acid battery. There are two nickel–cadmium battery designs, the sealed, which is shown in Figure 4 and the vented, which is shown in Figure 5. Sealed nickel–cadmium batteries are the common, everyday rechargeable batteries used in a remote controls, lamps, etc. No gases are released from these batteries, unless a fault occurs. Vented nickel–cadmium batteries have the same operating principles as sealed ones, but gas is released if overcharging or rapid discharging occurs. The oxygen and hydrogen are released through a low‐pressure release valve making the battery safer, lighter, more economical, and more robust than sealed nickel–cadmium batteries. Figure 4. Structure of sealed nickel‐cadmium battery [12]. Sealed nickel–cadmium batteries are used commonly in commercial electronic products such as a remote control, where light weight, portability, and rechargeable power are important. Vented nickel–cadmium batteries are used in aircraft and diesel engine starters, where large energy per weight and volume are critical. Nickel–cadmium batteries are ideal for protecting power quality against voltage sags and providing standby power in harsh conditions [13]. Recently, nickel–cadmium batteries have become popular as storage for solar generation because they can withstand high temperatures. However, they do not perform well during peak shaving applications, and consequently are generally avoided for energy management systems [12]. 8 Energy storage Feasibility Report for the BIOSTIRLING4SKA Figure 5. Structure of vented nickel‐cadmium battery [13]. 2.3.4. Sodium‐sulfur batteries Sodium–sulfur batteries are rechargeable high temperature battery technologies that utilize metallic sodium and offer attractive solutions for many large scale electric utility energy storage applications. Applications include load leveling, power quality and peak shaving, as well as renewable energy management and integration. A sodium–sulfur battery is a type of molten metal battery constructed from sodium and sulfur, as illustrated in Figure 6. This type of battery has a high energy density, high efficiency of charge/discharge (75–86%), long cycle life, and is fabricated from inexpensive materials. However, because of the operating temperatures of 300–350 ºC and the highly corrosive nature of the sodium polysulfide discharge products, such cells are primarily suitable for large‐scale, non‐mobile applications such as grid energy storage. Sodium β’’‐Alumina (beta double‐prime alumina) is a fast ion conductor material and is used as a separator in several types of molten salt electrochemical cells. The primary disadvantage is the requirement for thermal management, which is necessary to maintain the ceramic separator and cell seal integrity. In the mid‐1980s, the development of the sodium/metal‐
chloride system was launched. This technology offered potentially easier solutions to some of the development issues that sodium/sulfur was experiencing at the time. Sodium/metal chloride cells, referred to as ZEBRA cells (ZEolite Battery Research Africa), also operate at relatively high temperatures, use a negative electrode composed of liquid sodium, and use a ceramic electrolyte to separate this electrode from the positive electrode. In these respects, they are similar to sodium/sulfur cells. However, sodium/metal chloride cells include a secondary electrolyte of molten sodium tetrachloroaluminate (NaAlCl4) in the positive electrode section, and an insoluble transition metal chloride (FeCl2 or NiCl2) or a mix of such 9 Energy storage Feasibility Report for the BIOSTIRLING4SKA chlorides, as the positive electrode. The advantages are that the cells have a higher voltage, wider operating temperature range, are less corrosive and have safer reaction products. Figure 6. Structure of sodium‐sulfur battery [14]. 2.3.5. Flow batteries A flow battery is a form of rechargeable battery in which electrolyte containing one or more dissolved electro‐active species flows through an electrochemical cell that converts chemical energy directly to electricity. Additional electrolyte is stored externally, generally in tanks, and is usually pumped through the cell (or cells) of the reactor, although gravity feed systems are also available. Flow batteries can be rapidly recharged by replacing the electrolyte liquid (in a similar way to refilling fuel tanks for internal combustion engines) while simultaneously recovering the spent material that would be recharged in a separate step. Various classes of flow batteries exist including the redox (reduction–oxidation) flow battery, a reversible fuel cell in which all electro‐active components are dissolved in the electrolyte. If one or more electro‐active components are deposited as a solid layer, the system is known as a hybrid flow battery, that is, the electrochemical cell contains one battery electrode and one fuel cell electrode. The main difference between these two types of flow batteries is that the energy of the redox flow battery, as with other fuel cells, is fully decoupled from the power, because the energy is related to the electrolyte volume, i.e., to the tank size, and the power to the electrode area, i.e., to the reactor size. The hybrid flow battery, similar to typical batteries, is limited in energy by the size of the battery electrode, i.e. to the reactor size. Energy producing electrochemical cells are generally divided into two categories. Cells that can be discharged only, with irreversible electrochemical reactions, are termed primary cells, while 10 Energy storage Feasibility Report for the BIOSTIRLING4SKA rechargeable cells with reversible reactions are termed secondary cells. Using this historical convention, a redox flow battery is better described as a secondary fuel cell or regenerative fuel cell, with the fundamental difference between batteries and fuel cells being whether energy is stored in a solid state electrode material (batteries) or in the electrolyte (fuel cells). This difference leads to the decoupling of energy and power in a fuel cell described above. Example of redox flow batteries is the vanadium redox flow battery, whereas for hybrid flow battery is the zinc–bromine battery. Redox flow batteries, and to a lesser extent hybrid flow batteries, have the advantages of:  Flexible layout, due to separation of the power and energy components,  Long cycle life, because there are no solid–solid phase changes,  Quick response times, no need for equalization charging since the over‐ charging of a battery to ensure all cells have an equal charge,  no harmful emissions. Some types also offer easy state‐of‐ charge determination, through voltage dependence on charge, low maintenance and tolerance to overcharge and/or overdischarge. On the negative side, flow batteries are rather complicated in comparison with standard batteries as they may require pumps, sensors, control units and secondary containment vessels. The energy densities vary considerably but are, in general, rather low compared to portable batteries, such as the lithium‐ion. Also, they have high initial self‐discharge rate. As a summary, most used batteries nowadays are Lead‐Acid batteries and Lithium‐ion batteries. Both present a good behaviour in terms of grid voltage stability. Lithium‐ion batteries have some advantages compared with Lead‐Acid batteries that make then more interesting to be installed in a micro‐grid:  The electric energy that can be obtained from batteries has a low dependence with the electric current demanded to them. In Lead‐Acid batteries the current demanded has a high influence in electric energy storage.  The number of cycles of charge‐discharge, which gives an idea of the durability of equipment, has no dependence with the discharge depth. In Lead‐Acid batteries exist this dependency. A deeper discharge induces a less number of cycles (less durability). In contrast, Lead‐Acid batteries are cheaper than Lithium‐ion batteries. In …, a summary with the principal batteries manufacturers are shown: 11 Energy storage Feasibility Report for the BIOSTIRLING4SKA Technology
Manufacturer Bosch Cell‐Solar DBE Enersol Exide Lead‐acid Fiam Hawker Kolibri Tudor U‐Power Basf Dong Energy General Electronics
Li‐ion IK4‐Cidetec Panasonic Saft Victron Energy Table 2. Battery manufacturers. Principal manufacturers of AC/DC converters and inverters for batteries storage system are: Manufacturer ABB Alstom Elia Gaelectric Green Power Kolibri Power Systems AG Siemens TDK Table 3. Electric power system manufacturers. These manufacturers are also valid for Hydrogen storage systems. 12 Energy storage Feasibility Report for the BIOSTIRLING4SKA 2.4. Hydrogen energy storage system. Hydrogen can be obtained from various ways: water electrolysis, thermochemical processes, fuel reforming, etc. Hydrogen is an “electric” storage system in the way that can be produced via water electrolysis and consumed in a Fuel Cell to produce electricity. An operation scheme is shown in Figure 7. Figure 7. Topology scheme of a Hydrogen energy storage system [2].
Hydrogen storage system as some advantages compared to batteries:  Electric power that can be absorbed (to produce hydrogen) is independent of electric power produced (via Fuel Cell). In Batteries are related with their size.  Electric current produced and level of storage are decoupled in hydrogen storage system. In batteries exist a relation between them. In contrast, batteries have a higher round‐trip efficiency (> 90%) compared with hydrogen storage system (<50%). Principal electrical components of hydrogen technology storage system are: electrolizer and fuel cell. Both technologies are described below. 2.4.1. Electrolyzer Water splitting in its simplest form uses an electrical current passing through two electrodes to break water into hydrogen and oxygen. Commercial low temperature electrolyzers have system efficiencies of 56–73% (70.1–53.4 kWh/kg H2 at 1 atm and 25 ºC). It is essentially the 13 Energy storage Feasibility Report for the BIOSTIRLING4SKA conversion of electrical energy to chemical energy in the form of hydrogen, with oxygen as a useful by‐product. The most common electrolysis technology is alkaline based, but more proton exchange membrane (PEM) electrolysis and solid oxide electrolysis cells (SOEC) units are developing. SOEC electrolyzers are the most electrically efficient, but are the least developed of the technologies. SOEC technology has challenges with corrosion, seals, thermal cycling, and chrome migration. PEM electrolyzers are more efficient than alkaline, do not have the corrosion and seals issues that SOEC, but cost more than alkaline systems. Alkaline systems are the most developed and lowest in capital cost. They have the lowest efficiency so they have the highest electrical energy costs. Electrolyzers are not only capable of producing high purity hydrogen, but recently, high‐
pressure units (pressures > 1000 psig) are being developed. The advantage of high‐pressure operation is the elimination of expensive hydrogen compressors. Currently, electrolysis is more expensive than using large‐scale fuel processing techniques to produce hydrogen. And, if non‐ renewable power generation is used to make the electricity for electrolysis, it actually results in higher emissions compared to natural gas reforming. However, it should be noted, that if the hydrogen must be shipped in cylinders or tankers, then on site production via electrolysis may be less expensive. Several different approaches have been proposed to address these short comings. These include using renewable sources of energy such as solar, wind, and hydro, to produce the electricity, or excess power from existing generators to produce hydrogen during off‐peak times, and high temperature electrolysis. There have been several studies on the cost of using renewable energy for electrolysis, all reaching the conclusion that as the cost of natural gas increases renewable energy will become economically competitive at central production facilities as well as at distributed generation points especially if carbon dioxide and other pollutants are included in the analysis [15‐17]. Alkaline electrolyzers are typically composed of electrodes, a microporous separator and an aqueous alkaline electrolyte of approximately 30wt% KOH or NaOH. In alkaline electrolyzers nickel with a catalytic coating, such as platinum, is the most common cathode material. For the anode, nickel or copper metals coated with metal oxides, such as manganese, tungsten or ruthenium, are used. The liquid electrolyte is not consumed in the reaction, but must be replenished over time because of other system losses primarily during hydrogen recovery. In an alkaline cell the water is introduced in the cathode where it is decomposed into hydrogen and OH‐. The OH‐ travels through the electrolytic material to the anode where O2 is formed. The hydrogen is left in the alkaline solution. The hydrogen is then separated from the water in a gas liquid separations unit outside of the electrolyser. The typical current density is 100–300 mA cm2 and alkaline electrolyzers typically achieve efficiencies 50–60% based on the lower heating value of hydrogen. The overall reactions at the anode and cathode are: Anode: 4
Cathode: 2
Overall: →
2
→
2
→2
Δ
288 /
14 Energy storage Feasibility Report for the BIOSTIRLING4SKA Proton Exchange membrane (PEM) electroyzers build upon the recent advances in PEM fuel cell technology. PEM‐based electrolyzers typically use Pt black, iridium, ruthenium, and rhodium for electrode catalysts and a Nafion membrane which not only separates the electrodes, but acts as a gas separator. In PEM electrolyzers water is introduced at the anode where it is split into protons and oxygen. The protons travel through the membrane to the cathode, where they are recombined into hydrogen. The O2 gas remains behind with the unreacted water. There is no need for a separations unit. Depending on the purity requirements a drier may be used to remove residual water after a gas/liquid separations unit. PEM electrolyzers have low ionic resistances and therefore high currents of >1600 mA cm2 can be achieved while maintaining high efficiencies of 55–70%. The reactions at the anode and cathode are: →
Anode: Cathode: 2
2
2
→
2
Overall is the same as for alkaline electrolyzers. Solid oxide electrolysis cells (SOEC) are essentially solid oxide fuel cells operating in reverse. These systems replace part of the electrical energy required to split water with thermal energy, as can be seen in Figure 8. The higher temperatures increase the electrolyzer efficiency by decreasing the anode and cathode overpotentials which cause power loss in electrolysis. For example, an increase in temperature from 375 to 1050 K reduces the combined thermal and electrical energy requirements by close to 35%. A SOEC operates similar to the alkaline system in that an oxygen ion travels through the electrolyte leaving the hydrogen in unreacted steam stream. Other advantages for high temperature electrolysis with a solid oxide based electrolyzer include: the use of a solid electrolyte which, unlike KOH for alkaline systems, is non‐corrosive and it does not experience any liquid and flow distribution problems. Of course the high temperature operation requires the use of costly materials and fabrication methods in addition to a heat source. The materials are similar to those being developed for solid oxide fuel cells (SOFC), yttria stabilized zirconia (YSZ) electrolyte, nickel containing YSZ anode, and metal doped lanthanum metal oxides, and have the same problems with seals which are being investigated. High temperature electrolysis efficiency is dependent on the temperature and the thermal source. The efficiency as a function of electrical input alone can be very high with efficiencies 85–90% being reported. However, when the thermal source is included the efficiencies can drop significantly. For example, SOEC operating from advanced high temperature nuclear reactors may be able to achieve up to 60% efficiency. In addition to using conventional combustion or nuclear energy to produce the high temperature source, solar energy is under development and may result in higher efficiencies. 15 Energy storage Feasibility Report for the BIOSTIRLING4SKA Figure 8. Energy demand for wáter and steam electrolysis [18]. Combining SOEC with a SOFC for co‐generation of hydrogen and electricity has been proposed. In this hybrid system a SOFC and SOEC are manifolded into the same stack and fed the same fuel, such as natural gas. Hydrogen is then produced by the SOEC and electricity is produced by the SOFC. Proof‐of‐concept short stacks have been demonstrated with efficiencies of up to 69%. However, the fuel utilization is still relatively low at approximately 40% and coking is a serious issue in addition to the other challenges faced by SOEC. Principal electrolyzer manufacturers are presented in Table 4. 16 Energy storage Feasibility Report for the BIOSTIRLING4SKA Technology
Manufacturer AccaGen Erre Due Hydrogenics Idroenergy Alkaline IHT McPhy Norsk Hydro Piel Hogen PEM Hydrogenics Teledyne Table 4. Electrolyzer manufacturers. 2.4.2. Fuel Cell A fuel cell is an electrochemical device that converts the chemical energy of a fuel directly into electrical energy. The one‐step (from chemical to electrical energy) nature of this process, in comparison to the multi‐step (e.g. from chemical to thermal to mechanical to electrical energy) processes involved in combustion‐based heat engines, offers several unique advantages. For instance, the current combustion‐based energy generation technologies are very harmful to the environment and are predominantly contributing to many global concerns, such as climate change, ozone layer depletion, acidic rains, and thus, the consistent reduction in the vegetation cover. Furthermore, these technologies depend on the finite and dwindling world supplies of fossil fuels. Fuel cells, on the other hand, provide an efficient and clean mechanism for energy conversion. Additionally, fuel cells are compatible with renewable sources and modern energy carriers (i.e., hydrogen) for sustainable development and energy security. As a result, they are regarded as the energy conversion devices of the future. The static nature of fuel cells also means quiet operation without noise or vibration, while their inherent modularity allows for simple construction and a diverse range of applications in portable, stationary, and transportation power generation. In short, fuel cells provide a cleaner, more efficient, and possibly the most flexible chemical‐to‐electrical energy conversion. Polymer electrolyte membrane, also proton exchange membrane, fuel cells (PEMFC) in particular are one of the most promising types already in the early commercialization stage. Nonetheless, further development and research are required in order to reduce their costs, enhance their durability, and further optimize and improve their performance. Most of the research currently being conducted on PEMFCs is on the individual cell‐level and the general system‐level. Stack‐level research, on the other hand, is an area that requires further research and development. 17 Energy storage Feasibility Report for the BIOSTIRLING4SKA A fuel cell is composed of three active components: a fuel electrode (anode), an oxidant electrode (cathode), and an electrolyte sandwiched between them. The electrodes consist of a porous material that is covered with a layer of catalyst (often platinum in PEMFCs). Figure 9 illustrates the basic operational processes within a typical PEMFC. Molecular hydrogen (H2) is delivered from a gas‐flow stream to the anode where it reacts electrochemically. The hydrogen is oxidized to produce hydrogen ions and electrons, as shown in Figure 9, per the following equation: →2
2
The hydrogen ions migrate through the acidic electrolyte while the electrons are forced through an external circuit all the way to the cathode. At the cathode, the electrons and the hydrogen ions react with the oxygen supplied from an external gas‐flow stream to form water, as shown in Figure 9, per the following equation: 1
2
2
2
→
The overall reaction in the fuel cell produces water, heat, and electrical work as follows: 1
2
→
The heat and water by‐products must be continuously removed in order to maintain continuous isothermal operation for ideal electric power generation. Hence, water and thermal management are key areas in the efficient design and operation of fuel cells. Figure 9. Typical fuel cell operation [19]. 18 Energy storage Feasibility Report for the BIOSTIRLING4SKA Principal PEM Fuel Cell manufacturers are indicated in next table: Manufacturer AFC Energy Altergy Systems Ballard Power Systems Cellkraft AB ClearEdge Power Convion Ltd. EnerFuel FutureE Fuel Cell Solutions GmbH
Hydrogenics Infinity Fuel Cell and Hydrogen Nuvera Oorja Protonics PowerCell Toho Gas US Hybrid VP Energy Table 5. PEM Fuel Cell manufacturers. Hydrogen storage system is used as a long‐term storage system, and is not well suited to give good voltage stability in an isolated grid. So it is necessary to combine it with other storage technologies: batteries or supercapacitors. Considering the specifications of BIOSTIRLING‐4SKA project, where a stream of hot gases, from a combustion process, could feed the Stirling engines when the solar irradiation is not enough (or during night). Two possible fuels are considered in project: syngas produced by gasification of biomass or bio‐methane. Both can be blended with hydrogen. The advantage of the mix are summarized below: 

Hydrogen is produced in electrolyzer using the electric energy produced by Stirling engines when they are fed with solar irradiation. So hydrogen will come from a renewable energy source. The electric energy used will be the surplus of energy produced compared with the energy demand from SKA sensors. Hydrogen contributes to reduce the primary fuel (syngas or bio‐methane) and increases the heat value of blended fuel. The percentage of hydrogen introduced in fuel depends on: the type of fuel, the temperature of combustion desired and the type of burner. Experiences and analysis are related in bibliography. Melaina et al. [20], realized a review of possibilities of hydrogen to enrich natural gas in pipelines, the conclusions show low 19 Energy storage Feasibility Report for the BIOSTIRLING4SKA modifications on transportation system and natural gas burners. Zhen et al. [21], perform an analysis of burning behaviour of different bio‐methane enriched fuel (with hydrogen). They concluded that a percentage of hydrogen between 10‐15% in fuel permit better flame and temperature distribution in burner. 2.5. Flywheel energy storage system. A Flywheel energy storage system is an electromechanical system that stores energy in form of kinetic energy. A mass rotates on two magnetic bearings in order to decrease friction at high speed, coupled with an electric machine. The entire structure is placed in a vacuum to reduce wind shear. The scheme of the system is presented in Figure 10. Figure 10. Topology of Flywheel Energy Storage System.
Energy is transferred to the flywheel when the machine operates as a motor (the flywheel accelerates), charging the energy storage device. The Flywheel is discharged when the electric machine regenerates through the drive (slowing the flywheel). In fact, the energy stored by the flywheel is dependent on the square of the rotating speed and its inertia. Commercially, the two major types of machines used for flywheels systems are the axial‐flux and the radial‐flux permanent magnet machines. Apart from the permanent magnet machine used in almost all flywheels, there is also the possibility of using synchronous reluctance or induction machines. A Flywheel energy storage system presents good features regarding high efficiency (around 90% at rated power), long cycling life, wide operating temperature range, freedom from 20 Energy storage Feasibility Report for the BIOSTIRLING4SKA depth‐of‐discharge effects, higher power and higher energy density. On the other hand, flywheels present relatively high standing losses. Self‐ discharge rates for complete flywheel systems are about 20% of the stored capacity per hour. This is the reason why flywheels are not adequate devices for long‐term energy storage. 2.6. Supercapacitor energy storage system. Supercapacitors are also known as ultracapacitors or double‐ layer capacitors. Like batteries, supercapacitors are based on electrochemical cells, which contain two conductor electrodes, an electrolyte and a porous membrane whereby ion transit between the two electrodes is permitted. However, no redox reactions occur in the cells, because the operating voltage is lower, in order to electrostatically store charge on the interface between the surfaces of the electrolyte and the two conductor electrodes. In fact, this structure creates two capacitors (due to both interfaces, electrolyte – negative electrode and electrolyte – positive electrode), and for this reason, they are called double‐layer capacitors. The energy stored in the capacitors is directly proportional to their capacity and the square of the voltage between the terminals of the electrochemical cell, while the capacity is proportional to the electrode‐surface area and inversely proportional to the distance between the electrodes. Therefore, the main difference between capacitors and supercapacitors is the use of porous electrodes with high surface‐areas by the latter ones, providing higher energy densities to the system. Due to their low‐cell voltage (about 3V), the desired voltage and capacity of the supercapacitor are achieved by the series and parallel connection of a set of cells. Electrolyte and electrode materials have a fundamental influence on the energy and power capacity of the supercapacitor, as well as its dynamic behaviour. Actually, the product of the equivalent resistance of the electrolyte and the capacity of the supercapacitor determine its charge and discharge time constants. This equivalent resistance is very small, therefore short time constants can be achieved. In addition, power densities 10 times higher than batteries can be achieved. These features, combined with the high self‐discharge of the system (which can be 20% of its rated capacity in 12 h, due to the non negligible equivalent resistance of the contact between the electrolyte and the electrodes), define the system as a candidate for short time scale applications with short time responses. 3. Technical and economical Feasibility Analysis Taking into account the different energy storage systems presented in before section, three alternatives are selected:  Hydrogen storage system with small batteries set (for transient switch).  Batteries storage system. 21 Energy storage Feasibility Report for the BIOSTIRLING4SKA Other storage alternatives are not considered. Pumped hydro storage needs two water reservoir at different heights (do not exit near demo plant) and the cost for small‐scale project are prohibitive. Also Compressed Air storage is not economically feasible. Flywheel and supercapacitor energy storage are not considered because they are not a mature technology. The technologies selected to the batteries storage system are: Lead‐acid and Lithium‐ion. These are the most used in RES systems and the number of manufacturers is higher compared with other battery technologies. 3.1. Hydrogen storage system with small batteries set A scheme of this system is shown in Figure 11. The components of the system are: 
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Hydrogen technology: electrolizer, hydrogen storage and fuel cell. Batteries. Power management: AC/DC converter, DC/AC inverter. Auxiliaries: Water purifier, water storage and water pump. Control system. In this alternative, the massive storage system is the hydrogen technology and batteries are used to assure the stability of AC Bus (frequency and voltage). The operating principle is:  If electric energy produced by Stirling‐engines is higher than electric demanded from SKA‐
sensors, the surplus energy is sent to storage system. The first system to charge is the batteries set, if it is not in its maximum state of charge (SOC), and the second the hydrogen storage system (via the electrolyzer).  If electric demanded from SKA is higher than energy produced by Stirling‐engines, the deficit of electric energy is supplied by the fuel cell, which consumes hydrogen and air (from ambient). The batteries set is destined to stabilize the AC bus during the transient time of switching‐on the fuel cell. The control system is the responsible for carrying out this scheme of principle and follow the orders issued by the EMS. The electrolyzer, the batteries set and the fuel cell work on DC current, so an AC/DC converter and a DC/AC inverter are needed. In this proposal scheme, all these components have to work with the same DC voltage, so a careful designing and sizing of components has to be done. 22 Energy storage Feasibility Report for the BIOSTIRLING4SKA Figure 11. H2 storage system with batteries set scheme. An approximate unit cost of system 1 components is shown in next table: Item Unit Cost Fuel Cell 4 k€/kWe Hydrogen Storage system (pressurized @ 30 bar)
1 k€/Nm3 H2 Electrolyzer (alkaline technology) 2 k€/kWe DC/AC inverter 600 €/kWe AC/DC converter 600 €/KWe Batteries set (Li‐ion technology) 5 k€/KWhe Engineering and control system 100 k€ Auxiliaries 10 k€ Table 6. Unit cost for system 1 components. 3.2. Batteries storage system. The second storage system proposed is only composed by a batteries set. The batteries have to assure the grid stability and take the roll of massive storage system. To reach this two objectives the size of the batteries set have to be higher compared with previous system. The principle operation of system is the same than in system 1, but only intervenes the batteries storage system. In Figure 12, a scheme of system is shown. In a batteries storage system, a bidirectional AC/DC converter is needed. This kind of power converters is more expensive compared with traditional converters, but only one is needed (in previous system two converters are used). 23 Energy storage Feasibility Report for the BIOSTIRLING4SKA Figure 12. Batteries storage system. The unit cost of components in this is shown in next table: Item Unit Cost Batteries set (Li‐ion technology) 5 k€/KWhe DC/AC converter (bidirectional) 1 k€/kWe Engineering and control system 60 k€ Table 7. Unit cost for system 2 components. This option is technically and economically available. The benefits of this option have been explained in section 2.3. 24 Energy storage Feasibility Report for the BIOSTIRLING4SKA 4. Summary and conclusions According to the analysis made along this report, we can provide this summary. Technical feasibility (Y/N) Hydrogen + batteries set Y Batteries Y Both systems proposed are technically feasible. Taking into consideration the number of components and the unit cost, the preferable scheme for the storage system is the batteries storage system. Thus, the Diagram of the Power Conversion Unit has to be similar to scheme of Figure 12. 25 Energy storage Feasibility Report for the BIOSTIRLING4SKA References [1] Díaz‐González F, Sumper A, Gomis‐Bellmunt O, Villafafila‐Robles R, “A review of energy storage technologies for wind applicantions”, Renewable and Sustainable Energy Reviews 2012; 16:2154‐2171. [2] Tan X, Li Q, Wang W, “Avances and trends of energy storage technology in Microgrid”, Electrical Power and Energy Systems 2013; 44:179‐191. [3] Parker CD, “Lead–acid battery energy‐storage systems for electricity supply networks”. Journal of Power Sources 2001; 100:18–28. [4] Roselund C., “Energy storage and solar power”, www.solarserver.com. [5] Lead–acid batteries, micro.magnet.fsu.edu. [6] Bayar T., “Batteries for energy storage: new developments promise grid flexibility and stability”, Renewable Energy World magazine 2011, www.renewableenergyworld.com. [7] Adachi K, Tajima H, Hashimoto T., “Development of 16 kWh power storage system applying Li‐ion batteries”, Journal of Power Sources 2003; 11:119–21. [8] Daniel HD, Paul CB, Abbas AA, Nancy HC, John DB, “Batteries for large‐scale stationary electrical energy storage”, The Electrochemical Society Interface 2010: 49‐53, www.electrochem.org. [9] Fergus JW, “Recent developments in cathode materials for lithium ion batteries”, Journal of Power Sources 2010; 195:939–54. [10] Majima M, Ujiie S, Yagasaki E, Koyama K, Inazawa S. “Development of long life lithium ion battery for power storage”, Journal of Power Sources 2001; 101:53–59. [11] Lithium‐ion battery separators, gm‐volt.com. [12] Connolly D, Lund H, Mathiesen BV, Leahy M., “The first step towards a 100% renewable energy‐system for Ireland”. Applied Energy 2011; 88:502–507. [13] Goncalves‐Lacerda V, Barbosa‐Mageste A, Boggione‐Santos IJ, Henrique‐ Mendes L., “Separation of Cd and Ni from NiCd batteries by an environmentally safe methodology employing aqueous two‐phase systems”, Journal of Power Sources 2009; 193:908–913. [14] Sodium–sulfur battery , www.en.wikipedia.org. [15] Aroutiounian VM, Arakelyan VM, Shahnazaryan GE, Solar Energy 2005; 78:581–592. [16] Scott PB, Fuel Cell Review 2005; 2:21–25. [17] Erickson PA, Goswami PY, “Proceedings of the Intersociety Energy Conversion Engineering Conference, Hydrogen from solar energy: an overview of theory and current technological status”, Institute of Electrical and Electronics Engineers Inc., Savannah, GA, United States, 2001, 573–580. 26 Energy storage Feasibility Report for the BIOSTIRLING4SKA [18] Holladay JD, Hu J, King DL, Wang Y, “An overview of hydrogen production technologies”, Catalysis today 2009; 139:244‐260. [19] Polymer electrolyte membrane fuel cells. Helsinki University of Technology. http://tfy.tkk.fi/aes/AES/projects/renew/fuelcell/pem_index.html. [20] Melaina MW, Antonia O, Penev M, “Blending hydrogen into natural gas pipeline networks: a review of key issues”, NREL, 2013. [21] Zhen HS, Leung CW, Cheung CS, “Effects of hydrogen addition on the characteristics of a biogas diffusion flame”, International Journal of Hydrogen Energy 2013; 38:6874‐6881. 27