RE/H 2 Production Micro-System Based on Standard Alkaline Electrolytic Technology

Transcription

1 RE/H 2 Production Micro-System Based on Standard Alkaline Electrolytic Technology A. Moschetto a, M. Ferraro b, N. Briguglio b, G. M. Tina a, V. Antonucci b a UNICT DIEES University of Catania Electric, Electronic and Systemcs Department, Viale A. Doria, Catania, Italy b CNR ITAE, National Council of Research Advanced Energy Technology Institute, Via Salita S. Lucia, Messina, Italy. ABSTRACT: This paper presents the first task of a more comprehensive research project focused on the development of micro-scale (1-20kW) Renewable Hydrogen (RE/H 2 ) production systems oriented to carry on a wide campaign of educational and demonstration projects. The paper proposes to rely on low-cost and rugged standard alkaline electrolytic technology, well suited for decentralised hydrogen production, but requiring a certain R&D effort to get technical competitiveness. An electrolyser test facility has been designed and carried out. Then performance assessment of a commercial electrolyser and its sub-systems has been accomplished. First experimental results stated that the unit under test gets an average production efficiency of 51%, versus a stack (cell) efficiency of about 62%, while the aged AC/DC power converter, to be removed or replaced to adapt the unit to DC link with renewables, requires more than 16% of the incoming power. KEYWORDS : Renewable hydrogen, electrolyser, alkaline, micro-system. INTRODUCTION Producing hydrogen via electrolysis is much more than the long-term solution to store and carry greenenergy. For a number of issues, it could be a real chance to overcome some barriers that today limit renewable energy potential and fuel cells market opening, inside the distributed and dispersed power generation scenario [1]. As a matter of fact, large diffusion of intermittent renewables often is limited by the need of high capacity storage systems, alternative to the traditional ones, like batteries or utility grids. Photovoltaic and wind energy generation systems in stand-alone and/or micro-grid configuration are typical applications in which it may be useful to get a storage method overcoming their well-known limitations [2]. Besides, development of large wind generation systems could be limited at low-penetration levels consistent with existing grids, as well as, often it is lacking an high-capacity storage medium that could buffer and shift energy surplus production versus more valuable markets [3]. In practice the integration of renewable energy sources (RES) with a high-capacity storage system, like hydrogen storage, offers major flexibility in operation, an increased energy availability and could provide also a lower payback time in niche applications. By another side, even if Fuel Cell market opening seems to be imminent, many issues linked at hydrogen production, storage and distribution are far to be solved in a cost-effective way. Concerning this, electrolysis is proven to be suited for decentralized hydrogen production, solving the chicken-egg dilemma about fuel cell or hydrogen first market incoming [4]. Diffuse use of Renewable Hydrogen (RE/H 2 ) is still far to come, but hydrogen thanks to its appeal could get a decisive role in spreading the best practice for energy and environment conservation. Then in order to submit to people and public authorities Green Hydrogen technologies, stimulating interest for widespread use of renewables PROIDRI PROgetto IDrogeno RInnovabile was born. It is a project focused on the development of low cost RE/H 2 production/storage micro-systems oriented to carry on a wide campaign of educational and demonstration projects. 1/8

2 PROIDRI aims to experimentally fix that for micro-size applications, today standard alkaline electrolytic technology could be profitable used, as an optimum matching of the components and a proper control strategy could be sufficient to assure good features about system efficiency, reliability and cost. The first project task, here reported, aims at performance assessment of a commercial electrolyser and its sub-systems. This experimental activity is required to fit electrolyser to new process requirements typical of renewable energy applications: work on highly variable power supply conditions; be supplied with high value primary energy; provide gas suited for storage and fuel cell (FC) usage. CURRENT RE/H2 SYSTEM SCENARIO The layout of a generic RE/H2 production plant is showed In fig. 1. In general it is based on the following main components: photovoltaic (PV) or wind (WIND) or hybrid photovoltaic-wind (PV-WIND) renewable energy generating systems; an hydrogen electrolytic generator (electrolyser); a generic single gas (H 2 ) or dual (H 2 /O 2 ) storage system. Other components, not less important, like electronic power converters and gas treatment units (purifiers, compressors) complete the system. RENEWABLE ENERGY GENERATORS H2 ELECTROLYTIC GENERATOR H2 STORAGE SYSTEM TO END-USERS Fig. 1: Generic Renewable Hydrogen production plant layout Afterwards the stored hydrogen could be used as electrochemical fuel, for re-electrification purposes in stationary, automotive, or portable applications using FC, as well as, with much lower efficiency, common fuel to be burnt in adapted internal combustion engine (ICE), gensets or heating/lighting appliances. Technical feasibility of hydrogen production systems devoted to energy storage purposes has been assessed thanks to research activities started in 70, but only in the last decade the emergency of environment and energy supply concerns has been making them receive an increasing attention. A large number of projects has been carried out worldwide. An overview of the main technologies involved in this kind of projects has been drawn examining literature related to decade [5]. Results of this work, presented in Fig. 2, show that the most diffuse plant layout includes a photovoltaic solar generator, an alkaline electrolyser, and conventional compressed gas storage tanks. WIND 27% PEM 15% LH 2 15% PV-WIND 10% PV 63% Alkaline 85% MH2 28% GH2 57% RES Technology Electrolyser Technology H 2 Storage Technology Fig. 2: Technology Overview ( ) 2/8

3 Focusing on system components and their integration, some general research needs, requiring still to be solved, emerge from the projects currently on going. They are centred mainly on increasing plant production efficiency, improving electrolyser reliability to dynamic power input and maintaining a minimum purity grade for the produced gases. However, that it is considered the major problem related to renewable electrolytic hydrogen production is its high investment costs [1, 2, 4]. This is related to a number of reasons such as the high density of expensive technology, the inherent low efficiency of renewable generation systems, the still not assessed hydrogen technology and the market still closed. PRELIMINARY PLAN OF THE RENEWABLE HYDROGEN GENERATION SYSTEM Since the system has to be configured for near term wide diffusion, its relative low investment cost has been fixed as the main design requirement. This is to get assuring obviously the highest safety and reliability levels. A useful design strategy to reduce sensibly system investment costs, gaining a large potential of application, could be to lower the content of high technology components forming the system, even if this means to pay a reduction in production efficiency. It is a fact that in such kind of systems the relative weight of H2 components on the system efficiency is much more lower than that of the power generation sources, therefore this project proposes to focus on market nearer technologies, aiming at getting a good system efficiency, by means of an optimum system integration and control strategy [6], rather than to stress performance of the single component. On the basis of the previous approach, some basic choices have been made to start system development. It has been chosen to develop micro-size systems with a maximum production capacity of the hydrogen generator of 3 Nm 3 /h (20 kw). Even if the general trend of increasing relative prices at the lowering of the plant size here is more steep because of the high density of expensive technologies, that size appears the best suited for demonstrative purposes in a number of market agreeable on-site hydrogen production application, tightly linked to the distributed and dispersed energy generation scenario: energy back-up, personal mini-vehicle fuelling, portable appliances recharging, new energy net-metering opportunities. Regarding primary energy sources, it has been chosen to investigate systems comprising always wind energy, alone or hybridized with solar. This opportunity is to be promoted thanks to many profitable issues, both technical and economical, such as: Smoother input power availability, on daily or seasonal basis; Relative lower investment costs; Strongly lower environmental impact over life cycle [7]; Opportunity to use large energy potential represented by dispersed micro-wind resources. This is true, even if, on account of its wide intermittency, wind energy requires a more accurate design and control. Finally, among the electrolysis technologies, it has been selected the standard alkaline technology, used in a variety of industrial applications and here considered the only one usable for near term diffuse RE/H 2 applications. Really, the promising pressurized technologies, like advanced alkaline and acidic polymer (PEM), even if more effective, are still affected by too high investment costs and not as a whole convincing reliability and operational life. Past research activities, centred on stack technology improvement, involved very high prices, hard to lower by future mass-production. As today micro-size standard alkaline electrolysers market is very limited, this technology has wide margins for technical improvement and further cost reduction. After having discarded liquid hydrogen (LH 2 ) as well as metallic hydrides (MH 2 ) storage in this project, for issues related to plant complexity, higher costs and limited lifetime, our attention has been focused on compressed gas storage applications. Comparing standard low pressure (1-5 bar) and advanced high pressure (25-30 bar) electrolysis technologies it could be noted that using low cost electrolysers even with the additional costs supported to compress gases until pressures useful for storage purposes (minimum 10 bar), the overall investment cost is half than that of pressurized electrolysers [8]. On the other hand, with a proper storage strategy the energy consumption for compression could be limited at 10% of the incoming energy. 3/8

4 At the outlet of the electrolyser gas purity is about 99.8 vol%. It is enough to feed purity sensitive FC such as PEM. But for a safe compressed storage it is required the gases must be dried achieving a gas purity of 99.95%. This could be easily assured without any energy consumption. After these preliminary design choices, trade-off associated with hydrogen production rate, operational efficiency and capital cost need to be optimized on a case by case basis, especially taking into account partload and intermittent operation conditions [9]. TARGETS AND KEY ACTIONS OF THE PROJECT The present project requires to adapt a conventional hydrogen generator, to new process requirements typical of Renewable Hydrogen production applications: Work on fluctuating power supply conditions; Be supplied with high value primary energy; Store safely H 2 and O 2 as compressed gases; It has been defined a set of technical and economical features that should provide a viable near term storage solution: Specific Power Consumption 4.8 kwh/nm 3 (η 62% H2 LHV ) Average Storage Efficiency = 40% (from incoming DC power to H 2 stored gas) Average Gas Purity = 99.95% (on daily basis) No increase in stack aging (current lifetime is near h [11]) Finally electrolyser investment cost has to be much lower than that of pressurized units (at present higher than 6.0 k /kw). Just with small-scale production, it could be maintained at 2.5 k /kw, gas tank excepted. Taking into account these targets it follows that electrolyser development have to be tightly linked to the entire renewable energy system design and development. On the contrary, most of demonstration projects on renewable hydrogen has focused on the integration of commercially available electrolysers designed for continuous and grid-supplied operation, therefore complete with their own dedicated controller and power electronics (to convert AC power from the grid to DC power required by the cell stack). These power converters, representing 25-30% of the total cost of the electrolyser, contribute to keep high the system cost and are redundant to the other power converters generally required for the renewable energy sources (MPPT, AC/DC, DC/DC converters) [10]. This high power electronics content in small-size RE/H 2 systems is to be avoided to lower investment costs and increase reliability. Therefore designing single power electronics packages and optimizing the sizing and integration of components are needed to improve efficiency, cost and robustness of these systems. The following points represent the set of key actions that has been selected to achieve the targets of the project: 1. Electrolyser and renewables have to be matched for their optimum direct current coupling, avoiding to use power converters, as much as possible. 2. A small battery bank is to be integrated to assure power quality at the stack and to support electrolyser in stand-by operations, i.e. when input current density falls under the electrolyser lower operative limit, for predictable and limited periods of time. 3. A dual pressure storage strategy is to be studied. Energy required for compression, from a buffer (at production pressure) to medium pressure (10-30 bar) storage, has to be provided by means of a strategy focused on reducing wasted energy, due to electrolyser upper operative limit. 4. And at last, as an accurate control strategy is of primary concern in this kind of plant, a novel control system, integrating also weather forecast capabilities, has to be designed. EXPERIMENTAL SET-UP AND ELECTROLYSER CHARACTERIZATION Commercial electrolysers technical figures provided by manufactures must be handled with care. For example, only electrolyser power consumption and purity at rated power are provided, while none information is mentioned about auxiliaries power consumption and gas purity versus inlet power. In general it is showed a lack of data indispensable to assess equipment true potential as novel storage device. Therefore submitting an alkaline electrolyser to a wide characterization activity, it has been chosen to assess technology weak points and to determine any process enhancements, as primary concern in the development of the novel hydrogen production/storage system. 4/8

5 Particularly, tests focused on studying the effects of variable input power on electrolyser performances and its aging. A set of measurements has been defined, in order to fix and carry out, reproducible test procedures on a selected electrolyser. In the first step the electrolyser has been supplied as usual by utility grid under constant AC power supply. Then it wi ll be submitted to fluctuating DC power supply under simulated conditions to be choice as representative of typical time and space dependent available power levels. As main experimental task it was designed and carried out a semi-automatic electrolyser test facility, integrating data acquisition, process control and safety features, comprising simulated power supply. Fig. 3 reports mechanical layout of the electrolyser test facility, with its three conceptual sections: gas handling and measurements, storage and venting. It allows to measure the following process variables: stack voltage and current; electrolyte temperature; flow, pressure and purity of the gases; voltages and currents of the auxiliaries. In Fig. 4 the test facility is showed as built at IDRILAB, a laboratory set up to be totally dedicated to Renewable Hydrogen systems development. LEGEND Hydrogen Pipeline (D ½ ) Oxygen Pipeline (D ½ ) VS Ball Valve VR Check Valve EV Powered Valve N.C. PS Pressure Sensor GS Gas Sampling MFC Mass Flow Controller (0-15 slpm) FLM Flow Meter FC Fuel Cell VSO1 VSH1 PS P VSH4 MFC VSH 5 VSH2 VRH1 VSH 3 PS FLM VSH6 FC GS P EVH 1 VSO3 VSO4 VSO2 VRO1 GS O2 H 2 O2 H2 H2 Low Pressure Storage (PMAX 7 bar) DEIONIZED WATER TANK ELECTROLYSER (Unit Under Test) PROCESS GAS OUTLETS VENTED GAS OUTLETS SECTION 1 - STORAGE SECTION 2 GAS HANDLING & MEASUREMENT SECTION 3 - VENTING Fig. 3: Electrolyser Test Facility Mechanical Layout Fig. 4: Electrolyser Test Facility at IDRILAB 5/8

6 Fig. 5 shows an inner view of the unit currently under test, a 0.6 Nm 3 /h hydrogen production capacity electrolyser manufactured by Idroenergy. Table 1 reports unit main technical features as derived from its data sheets. Efficiency tests have been based on requiring a certain gas flow rate variable by means of a mass flow controller (MFC) and measuring the corresponding values of voltages and currents at various measurement points of the electrolyser power line, Fig. 6, as well as auxiliaries (water pump and fun), and process controller hardware. Hydrogen Nominal Flow Rate: 0.6 Nm 3 /h; Oxygen Nominal Flow Rate: 0.3 Nm 3 /h; Pressure: 1.6 bar max; Electrical Power Consumption: 4.5 kw; Electrical Power Supply: 220 V 50 Hz; Standard Gas Purity: 99.5%; Water Supply Demineralised: MW*cm; Dimensions (l x p x h): 85x70x110 cm; Weight: 280 kg; Fig. 5: The Unit Under Test Table 1: Unit Under Test Main Data Fig. 6 Electrolyser Electrical Power Line RESULTS AND DISCUSSION Fig. 7 and Fig 8 report the measured data points and the corresponding best curve fitting obtained from the first measurement session. Fig 7 shows the stack characteristic curve V-I (polarization curve) following its typical trend, while Fig. 8 shows the curve, quite linear, representing stack required power versus the amount of produced hydrogen. Fig. 9 shows the stack electrical efficiency calculated as function of the current drained by the stack. Fig. 7 Polarization Curve (V-I) Fig. 8 Characteristic Power- H 2 Flow 6/8

7 Fig. 9 Stack Efficiency Curve (η - I) Again from the measurements carried out, the efficiencies of each electrolyser component have been calculated. Fig. 10 represents the electrolyser power flow and the efficiencies of its main components at the rated production capacity Nm3/h (i.e slpm) of the unit under test. First experimental results stated that the unit under test gets an average production efficiency of 51%, versus a stack (cell) efficiency of about 62%, while the power conditioning stage requires more than 16% of the available net power. Therefore just removing or replacing it to adapt the unit to DC link with renewables, it could be saved a significant amount of energy. Such preliminary results will be useful to manage next experimental activities of the present project. Fig. 10 System and component efficiencies at rated production capacity Nm3/h References: [1] The European Hydrogen and Fuel Cell Technology Platform Steering Panel Strategic Research Agenda 2005 [2] Market potential Analysis for Introduction of Hydrogen Energy Technology in Stand-Alone Power Systems HSAPS Final Report Contract. N /Z/01-101/2001 ALTENER Programme [3] G. M. Tina, C. Brunetto, A. Moschetto, M. Ferraro, V. Antonucci, Analysis of Hydrogen Storage Opportunities for Wind Energy Generation in the Italian Electricity Market, European Wind Energy Conference & Exhibition, Athens, Greece, 27 February - 2 March 2006 [4] Ivy J., Summary of Electrolytic Hydrogen Production. Milestone Completion Report. NREL. NREL/MP , 2004 [5] A. Moschetto, M. Ferraro, V. Antonucci, G. M. Tina, Development and testing of a low cost PV-WIND/H2 system for small-size applications, 2nd European Hydrogen Energy Conference, November 2005, Zaragoza, Spain. 7/8

8 [6] O. Ulleberg, The Importance of Control Strategies in PV-Hydrogen Systems, Solar Energy, 76, ,Elsevier, 2004 [7] C. Koroneos et al., Life Cycle Assessment of Hydrogen Fuel Production Processes, International Journal of Hydrogen Energy, 29, , Elsevier, 2004 [8] E. Varkaraki et al., Hydrogen based emergency back-up system for telecommunication applications, Journal of Power Sources, 118, 14-22, Elsevier, [9] A.G. Dutton et al., Experience in the design, sizing, economics and implementation of autonomous windpowered hydrogen production systems, International Journal of Hydrogen Energy, 25, , 2000 [10] C. Elam, B. Kroposki et al., Renewable Electrolysis Integrated System Development and Testing, FY 2004 Progress Report, , DOE Hydrogen Program 8/8