Power to Gas as an alternative energy storage solution to integrate a large amount of renewable energy: economic and technical analysis

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1 - Distribution Systems and Dispersed Generation - CIGRE SC C6 COLLOQUIUM Yokohama 2013 Power to Gas as an alternative energy storage solution to integrate a large amount of renewable energy: economic and technical analysis P. Lombardi, Fraunhofer Institute for Factory Operation and Automation IFF, Magdeburg, Germany T. Sokolnikova, K. Suslov, Irkutsk State Technical University, Irkutsk, Russia P. Komarnicki, Fraunhofer Institute for Factory Operation and Automation IFF, Magdeburg, Germany Z. Styczynski, Otto-von-Guericke University, Magdeburg, Germany Abstract: The European energy strategy for the coming decades aims to increase to usage of Renewable Energy Sources (RES) for the generation of electric and thermal power. Integrating the power generated by RES such as wind and solar into the electric network will be a big challenge. Many studies have outlined how Energy Storage System (ESS) might make this challenge easier if they are integrated into the power network. This study aims firstly to give an overview on ESS and secondly to analyze from an technical and economical point of view the energy storage concept Power to Gas. Keywords : Energy Storage Systems, Hydrogen,, Levelized Stored Energy Costs, Methane, Power to Gas, Renewable Energy Sources 1. INTRODUCTION The European energy strategy for the coming decades aims to increase to usage of Renewable Energy Sources (RES) for the generation of electric and thermal power. Among the RES, wind and solar energy will be the most used sources. By 2020 it is forecasted that 220 GW of wind power plants and 390 of photovoltaic plants will be installed in Europe [1],[2]. However, these sources produce power not when it is demanded, but instead when particular meteorological conditions are met. Integrating the power generated by wind and solar into the electric network will be, therefore, a big challenge. Many studies have outlined how Energy Storage System (ESS) might make this challenge easier if they are integrated into the power network [3],[4]. In the first part of this study an overview on ESS is given. The second part is dedicated to the energy storage concept Power to Gas, and the technical and economic aspects will be explained and evaluated. 2. ENERGY STORAGE SYSTEMS Energy storage systems are generally classified according to a bi-criterion method (Fig. 1): the discharge time and the rated power capacity. Depending on these two parameters, three ESS application fields can be pinpointed: ESS for power quality, ESS for bridging power and ESS for energy management [3]. Related to energy management application, the pumped hydro plants are currently the most used type in Europe. This technology is able to store a high amount of power (> 1 GW) with a discharge duration time up to 12 hours [1]. Besides the pumped hydro storage, another technology which is also able to store a large amount of energy is the Compressed Air Energy Storage System (CAES). In Europe a diabatic CAES system with a storage power of 290 MW and storage capacity of 2 hours has been in operation since 1978 [3]. An adiabatic CAES is in the planning phase in Staßfurt (Germany), where it will have a storage power of 90 MW and a storage capacity of 4 hours [5]. However, the usage of pumped hydro systems as well as of CAES systems mainly depends on geographic and geological factors such as rivers and mountains for the pumped hydro plants, and caverns for the CAES systems. These factors strongly limit new installations of such energy storage systems. With regard to the battery systems, they have registered a significant popularity in the last decades mostly thanks to the Sodium-Sulfur high temperature battery systems which can be used for power quality as well as for energy management applications [6]. Beyond pumped hydro CAES systems and batteries, the electricity can be stored in a large quantity for a very long time by converting it to hydrogen or methane. The produced gas can be partially (if it is hydrogen) or totally (if it is methane) fed into the natural gas network and stored, together with natural gas, inside cavers. Such a process, also called Power to Gas (P2G) is a candidate solution to store electric power in high quantities (> TW) and for a long time (> months). Discharge Time Months Days Hours Minutes Battery CAES Hydro Power Hydrogen-Methane Contact Address: Pio Lombardi, Fraunhofer Institute for Factory Operation and Automation IFF Sandtortstr. 22, Magdeburg, Germany pio.lombardi@iff.fraunhofer.de Seconds kw MW GW TW PW Rated Storage Power Fig. 1 Energy storage system classification for energy management application, based on [7] 1

electricity is firstly converted into hydrogen and then the hydrogen is stored as methane through the methanation process.

ESS TABLE 1 ENERGY STORAGE EFFICIENCY FOR DIFFERENT ESS NaS CAES Hydro P2G (with Battery* System**,*** Power** methanation process) Efficiency 80% 56%-70% 70%-80% 42%-58% * [10],** [3],***[5], (1) In

2 3. POWER TO GAS: TECHNICAL ANALYSIS 3.1 POWER TO GAS EFFICIENCY The term Power to Gas generally refers to two different ways to store electricity: in the first one the electricity is stored as hydrogen, while in the second approach the electricity is firstly converted into hydrogen and then the hydrogen is stored as methane through the methanation process. Both processes need an electrolyzer system which converts the de-ionized water into hydrogen and oxygen by using electricity (see equation (1)). ESS TABLE 1 ENERGY STORAGE EFFICIENCY FOR DIFFERENT ESS NaS CAES Hydro P2G (with Battery* System**,*** Power** methanation process) Efficiency 80% 56%-70% 70%-80% 42%-58% * [10],** [3],***[5], (1) In the electrolyzer a part of the electricity used to split the water into hydrogen and oxygen is lost as heat, while the additional energy is lost in the rectifier and in the auxiliaries (i.e. compressor). All these effects decrease the efficiency of the electrolyzer system, which is generally referred to the Heat High Value (HHV) of the gas (hydrogen s HHV is kwh/kg). Nowadays, two different industrial electrolyzers are mostly used which can be classified in two types: alkaline electrolyzers and Proton Exchange Membrane (PEM) electrolyzers. The overall efficiency of these systems ranges from 56% to 73% [9] depending on the electrolyzer type (the the PEM electrolyzer has the lowest efficiency value). The methanation process consists in combining four moles of hydrogen to one mole of carbon dioxide to obtain one mole of methane and two moles of steam (see equation (2)). Such a process, also known as the Sabatier reaction, is an exothermic process ( kwh/mol) and requires high temperature and high pressure to produce a high amount of methane. The efficiency of such a reaction ranges from 75% to 80% [19]. Therefore, the entire efficiency of the P2G process (with methanation) ranges from 42% to 58%. Such a value is definitely the lowest one if it is compared to the efficiency values of the other energy storage systems used for the energy management application (see Table I). However, the P2G efficiency may increase if the heat generated during the splitting of the water is recovered. A possible application is to use it for the fermentation process in biogas plants. In fact, for the fermentation process the biogas plants need heat and emit free CO2 to the environment which can be re-used to methanize the hydrogen. Biogas can therefore be viewed as complementary to the P2G systems for making it a CO2 neutral emission. In Fig. 2 a possible scheme of the P2G concept is depicted. The electrolyzers convert the electricity surplus generated by RES into hydrogen. The methanizators combine the hydrogen with the carbon dioxide emitted by biogas plants. The generated methane is then fed into the natural gas network. The gas is successively used either to generate electric power (i.e. through gas turbine plants) or to generate thermal power for heating or industrial purposes. (2) Fig. 2 Power to Gas scheme 3.2 POWER TO GAS: TECHNICAL ADVANTAGES AND DISADVANTAGES Although the P2G concept has a lower energy efficiency in comparison with the other energy storage systems, the main advantages of the P2G concept are of an environmental and economic nature. The combustion of hydrogen or methane, obtained through the P2G approach, is a carbon free process if the gas is produced by using RES. Related to the economic advantages of the P2G, it is mainly based on the usage of the existing natural gas infrastructure (pipelines, compressors, caverns, etc.) and therefore, through it, huge investments may be avoided. Nowadays in Europe more than 200,000 km of natural gas transmission pipelines and more than 200 caverns are in operation (see Fig. 3), which transmitted about 5,058,000 GWh of energy in 2011 [11], [12]. In comparison with the European natural gas system, the US natural gas system is considerably larger. It uses about 490,000 km of high pressure pipelines, 1400 compressors and 400 caverns [13]. Fig. 3 On the left side: European natural gas pipeline, 2011 [11]. On the right side: the US natural gas pipeline 2009 [13] However, the P2G also has some limitations. Considering the P2G concept in which the electricity is stored as hydrogen, some complications may occur if the hydrogen is fed into the natural gas 2

3 network. These complications mainly depend on the typical components which make up the natural gas network structure such as pipelines, compressors, caverns, gas turbines and measurement devices. Related to the pipeline component, the highest limitation is due to a reaction between the materials composing the pipeline and the hydrogen. In the literature there are different opinions with regard to the amount of hydrogen that the natural gas pipelines are able to transport. Such amounts range between 10% to 50% of hydrogen volume [14],[16]. Related to the compressor component, in general centrifugal compressors with a compression ratio of about 15 bars are used. They are usually driven by a gas turbine which burns natural gas taken from the natural gas pipeline. The gas turbines are generally open cycle and require a power of circa MW to drive the compressor. The limitation on the compressor depends on the materials composing the compressor blades, on the amount of the energy required to compress the hydrogen-methane mixture and on boiler of the gas turbine. With reference to the materials, the compressors actually used in the natural gas network should be able to compress up to 5% of the hydrogen volume without any problems [14]. Related to the energy to compress the gas, it can be evaluated as in equation (3), where γ is the ratio of the specific heat, p is the pressure, V is the volumetric density and the subscript 0 and 1 indicate the initial and final compression state. By considering the same compression ratio and the same mass flow rate, the energy to compress the hydrogen-methane mixture increases by increasing the volumetric percentage of hydrogen (see Fig. 4). It is important to note that the energy necessary to compress only hydrogen is circa 9 times higher than that to compress only methane. With a volumetric concentration of hydrogen of 10% - 15%, the power required to compress the mixture increases from 10% to 17% (see Fig. 5). Since the compressors are generally designed for a power higher than the nominal one (circa 10%-15% higher) then is possible to assert that a mixture of hydrogen-methane up to 14% can be fed into the actual natural gas network. (3) Fig. 5 Energy to compress the hydrogen-methane mixture compared to the energy to compress only methane (H2 volume up to 20%) The limitation in the gas turbine boiler is different. Here, the main problems for burning a mixture of hydrogen-methane arise from the different burning velocities of the mixture (see Table II) which may destabilize the burning process and damage the boiler. The commercial gas turbines available today are able to burn a mixture of hydrogen-methane containing from 0% up to 8.5% hydrogen [14]. However, some tests have shown that by modification it is possible to burn a mixture containing up to 60% hydrogen [15]. Table 2 Burning velocity for hydrogen and methane [18] Hydrogen Methane Burning velocity [cm/s] Lesser limitations come from the usage of the caverns. The experience of the past years with the so called town gas has shown that the methane-hydrogen mixture with up to 55% of hydrogen by volume could be stored inside caverns [16]. With regard to the measurement devices, different studies show that the devices being used nowadays in the natural gas network are able to work without any particular problem with a hydrogen volume of between 10%-30% [14], [16]. Table III summarizes the maximal tolerable limits of hydrogen in the natural gas network. Table 3 Maximal tolerable limits of hydrogen within the natural gas network Components of natural gas network Transport pipeline Up to 50% Compressors Up to 14% Caverns Up to 55% Measurement devices Up to 30% Gas Turbine Up to 1-3% Maximal tolerable limits [Volume % of H2] 4 POWER TO GAS: ECONOMIC ANALYSIS 4.2 LEVELIZED STORED ENERGY COSTS Fig. 4 Energy to compress the hydrogen-methane mixture as the percentage of H2 increases from 0 to 100% Despite the low storage efficiency, the main advantages of the P2G concept lie in its economics. For a particular energy system, 3

Instead, the analysis takes into consideration the investment costs to upgrade the natural gas infrastructure and the operative costs (maintenance and costs of electricity).

4 the advantages may be higher if this system already makes use of a robust natural gas infrastructure, since their related costs may be avoided. In this analysis the costs related to the construction of the gas pipelines, compressor stations and to the mining of the caverns, where the gas is stored, will not be considered. Instead, the analysis takes into consideration the investment costs to upgrade the natural gas infrastructure and the operative costs (maintenance and costs of electricity). To evaluate the costs to store the electricity in methane, the Levelized Stored Energy Costs method was used (see equation 4). Where LEC are the Levelized Stored Energy Costs [ /kwh] I are the investment costs [ /kw] r is the discount factor [%] y is the life time of the plants [years] CF is the Capacity Factor of the plant [hours/year] (M&O) var are the variable maintenance and operation costs [ /kwh] p el is the price of the electricity [ /kwh] E in the electricity taken from the grid for storing in methane [kwh] Related to the investment costs, the main components to be added to the natural gas infrastructure are the electrolyzer-methanizer. The investment costs for these plants depend on its size; the larger the plant the lower the investment costs. For an electrolyzer-methanizer plant with an installed capacity ranging from 5 MW to 50 MW the specific investment costs may be estimated as 2000 per kw [20]. The costs for the electricity may depend on the energy system (power generation technology) in which the P2G is used. Table IV depicts the main assumption used for evaluating the LEC (4) Fig. 6 Partition of the total costs to produce methane, with a CF of 2000 hours Fig. 7 Portion of the total costs to produce methane, with a CF of 500 hours Table 4 Assumption for the costs analysis Values Investment costs [ /kw] 2000 Discount factor [%] 5 Life time [years] 20 (M&O) var [ /kwh] Fig. 6 shows the partition of the storage costs to produce methane through the P2G. Besides the assumption listed in Table IV, the following assumption were taken: Efficiency of the electrolyzer-methanizer: 55% Cost of electricity: 8 c/kwh Capacity Factor: 2000 hours The costs to store 1 kwh of electricity in methane were calculate as 20 c/kwh. These costs mainly depend on the electricity costs (71,4%) and on the investment costs (26%). However when the CF of the plants is decreased, the dependency of the total costs on the investment increase (see Fig. 7 and Fig. 8) as do the storage costs. In the case in which the CF is 500 hours the storage costs are 36 c/kwh. Fig. 8 Relation between the storage costs and the CF In Fig. 9 the dependency of the storage costs on the CF and on the electricity price is depicted. Three different electricity price were assumed: 20 c/kwh, 8 c/kwh und zero /kwh. If the P2G if fed with the surplus of electricity generated by renewable energy sources which could not be fed in the electric grid, then the storage costs range from 22 c/kwh to 2 c/kwh, depending on the plant s CF. 4

5 Fig. 9 Relation between the storage costs and the electricity price for different CF 5 CONCLUSIONS Electricity Price 20 c/kwh Electricity Price 8 c/kwh Electricity Price 0 c/kwh In this study the P2G concept has been analyzed as a storage technology. The technical advantages as well as disadvantages of both P2G versions have been pointed out. The P2G version in which only hydrogen is generated has a higher storage efficiency in comparison with the version in which methane is generated. However, the natural gas system in use today is not able to tolerate a high amount of hydrogen (up to 3% max.) since some components, such as the Gas Turbine power plants, may begin to work in unstable conditions. The P2G version with a methanizer has lower storage efficiency, but it can fully use the natural gas infrastructure. In addition, the costs to store electricity as methane have been estimated. These costs mainly depend on the electricity price, on the investment cost of the electrolyzer-methanizer and on the capacity factor of the plant. [9] J. Ivy, Summary of electrolytic hydrogen production National Renewable Energy Laboratory (NREL), September [10] EPRI-DOE, Handbook of energy storage for transmission and distribution applications, Washington DC, December [11] European Network of Transmission System Operators for gas (ENTSOG), Gas Infrastructure Europe, System development map Available : 1.pdf [12] Gas Infrastructure Europe, Storage Map 2013, available on line: [13] U.S. Energy Information Administration, Natural Gas, available on line: ngpipeline/index.html [14] C. Guardamagna, F. Polidoro, M. Scagliotti, G. Torsello, M. Verga, Logistica di trasporto e distribuzione dell idrogeno CESI ricerca, February 2008 [15] G.Benelli, S.Calvetti, P.Carrai, M.Faleni, G.Tanzini "Verifiche sperimentali sull'utilizzo di miscele Gas Naturale-Idrogeno in combustori per turbogas", Rapporto Ricerca di Sistema, 2003 [16] M. Henel, Ergebnisse aus der Innovationsoffensive p2g Projekt (Konzepte, Fallstudien, Einspeisung un das Gasnetz, etc), Power To Gas Meeting, October 8th 2012, Regensburg, Germany. [17] V. Barbarossa, A. Capriccioli, B. Sardella, S. Tosti, Carbon dioxide utilisation for methane production by renewable energy sources, Sustainable Fossil Fuel For Future Energy ROMA, 8th - 11th July [18] H-J Tomczak, G Benelli, L Carrai, D Cecchini, Investigation of a gas turbine combustion system fired with mixture of natural gas and hydrogen IFRF Combustion Journal Article Number , December, 2002 ISSN X [19] M. Sterner, Bioenergy and renewable power methane in integrated 100% renewable energy systems, Kassel (Germany) September 2009 [20] Gas und Umwelttechnick GmbH, Power to Gas: Konzepten, Kosten und Potenziale, DBI Fachforum: Energiespeicherkonzepte und Wasserstoff Berlin, 13./14. September 2011author : title, journal title, Vol.00, No.000, pp (pub. year) References [1] S. Inage, Prospects for Large-Scale Energy Storage in Decarbonised Power Grids, International Energy Agency, 2009 [2] EWEA, Wind Energy and EU climate policy, achieving 30% lower emission by 2020, European Wind Energy Association (EWEA), Brussels, BE, October Available: _ClimateReport.pdf. [3] Cigre WG C6.15 Technical Report, Electric energy storage systems, April 2011, ISBN: [4] Z. A. Styczynski, P. Lombardi, R. Seethapathy, M. Piekutowski, C. Ohler, B. Roberts, S.C. Verma, Electric Energy Storage and its Tasks in the Integration of Wide-Scale Renewable Resources, CIGRE- PES Symposium, July 2009, Calgary, Canada. [5] RWE, ADELE Adiabatic Compressed Air Energy Storage (CAES) for electricity supply, unpublished. [Online]. Available web site: [6] EPIA, Connecting the sun, solar photovoltaic on the road to large scale grid integration, European Photovoltaic Industry Association (EPIA), Brussels, BE, September Available: ng_the_sun_full_report_converted.pdf. [7] Norris B. L., Newmiller J., Peek G.: NAS Battery Demonstration at American Electric Power A Study for the DOE Energy Storage Program, Sandia National Laboratories, March [8] International Energy Agency Energy Technology Perspective how to secure a clean energy future ISBN

OPPORTUNITIES FOR HYDROGEN-BASED ENERGY STORAGE FOR ELECTRIC UTILITIES 1

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