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Power to gas (also power-to-gas) (often abbreviated P2G) is a technology that converts electrical power to a gas fuel.[1] When using surplus power from wind generation, the concept is sometimes called windgas. There are currently three methods in use; all use electricity to split water into hydrogen and oxygen by means of electrolysis.

In the first method, the resulting hydrogen is injected into the natural gas grid or is used in transport or industry.[2] The second method is to combine the hydrogen with carbon dioxide and convert the two gases to methane (see natural gas) using a methanation reaction such as the Sabatier reaction, or biological methanation resulting in an extra energy conversion loss of 8%. The methane may then be fed into the natural gas grid. The third method uses the output gas of a wood gas generator or a biogas plant, after the biogas upgrader is mixed with the produced hydrogen from the electrolyzer, to upgrade the quality of the biogas.

Impurities, such as carbon dioxide, water, hydrogen sulfide, and particulates, must be removed from the biogas if the gas is used for pipeline storage to prevent damage.[3]

Storage function

Power-to-gas systems may be deployed as adjuncts to wind parks or solar-electric generation. The excess power or off-peak power generated by wind generators or solar arrays may then be used at a later time for load balancing in the energy grid. Before switching to natural gas, the German gas networks were operated using towngas, which for 50–60 % consisted of hydrogen. The storage capacity of the German natural gas network is more than 200,000 GWh which is enough for several months of energy requirement. By comparison, the capacity of all German pumped storage power plants amounts to only about 40 GWh. The storage requirement in Germany is estimated at 16GW in 2023, 80GW in 2033 and 130GW in 2050.[4] The transport of energy through a gas network is done with much less loss (<0.1%) than in a power network (8%). The storage costs per kilowatt hour are estimated at €0.10 for hydrogen and €0.15 for methane.[5] The use of the existing natural gas pipelines for hydrogen was studied by the EU NaturalHy project[6] and US DOE.[7] The blending technology is also used in HCNG.

Efficiency

In 2013 the round-trip efficiency of power-to-gas-storage was well below 50%, with the hydrogen path being able to reach an maximum efficiency of ~ 43% and methane of ~ 39% by using combined-cycle powerplants. If cogeneration plants are used that produce both electricity and heat, efficiency can be above 60%, but is still less than pumped hydro or battery storage.[8] However, there is potential to increase efficieny of power-to-gas storage. In 2015 a study published in Energy and Environmental Science found that by using reversible solid oxide electrochemical cells and recycling waste heat in the storage process a round-trip efficiency electricity to electricity of more than 70% can be reached at low cost.[9]

Overall energy conversion efficiency by pathway and fuel
using electrolysis of water, plus methanation to produce methane
Fuel Efficiency Conditions
Pathway: Electricity→Gas
Hydrogen 54–72 % 200 bar compression
Methane (SNG) 49–64 %
Hydrogen 57–73 % 80 bar compression (Natural gas pipeline)
Methane (SNG) 50–64 %
Hydrogen 64–77 % without compression
Methane (SNG) 51–65 %
Pathway: Electricity→Gas→Electricity
Hydrogen 34–44 % 80 bar compression up to 60% back to electricity
Methane (SNG) 30–38 %
Pathway: Electricity→Gas→Electricity & heat (cogeneration)
Hydrogen 48–62 % 80 bar compression and electricity/heat for 40/45 %
Methane (SNG) 43–54 %
Source: Fraunhofer IWES, p. 18, February 2011 (in German)[10]

Electrolysis technology

  • Advantages and disadvantages of the predominantly considered electrolysis technologies.[11]
Alkaline Electrolysis
Advantage Disadvantage
Commercial technology (high technology readiness level) Limited cost reduction and efficiency improvement potential
Low investment electrolyser High maintenance intensity
Large stack size Modest reactivity, ramp rates and flexibility (minimal load 20%)
Extremely low hydrogen impurity (0,001 %) Stacks < 250 kW require unusual AC/DC converters
  Corrosive electrolyte deteriorates when not operating nominally
Proton Exchange Membrane Electrolysis (PEME)
Advantage Disadvantage
Reliable technology (no kinetics) and simple, compact design High investment costs (noble metals, membrane)
Very fast response time Limited lifetime of membranes
Cost reduction potential (modular design) Requires high water purity
Solid Oxide Electrolysis Cell (SOEC)
Advantage Disadvantage
Highest electrolysis efficiency Very low technology readiness level (proof of concept)
Low capital costs Poor lifetime because of high temperature and affected material stability
Possibilities for integration with chemical methanation (heat recycling) Limited flexibility; constant load required

Power to hydrogen

In this method, electricity is used to split water into hydrogen and oxygen by means of electrolysis. The resulting hydrogen is injected into the natural gas grid or is used in transport or industry.[2]

ITM Power won a tender in March 2013 for a Thüga Group project, to supply a 360 kW self-pressurising high pressure electrolysis rapid response PEM electrolyser Rapid Response Electrolysis Power-to-Gas energy storage plant. The unit produces 125 kg/day of hydrogen gas and incorporates AEG power electronics. It will be situated at a Mainova AG site in the Schielestraße, Frankfurt in the state of Hessen. The operational data will be shared by the whole Thüga group – the largest network of energy companies in Germany with around 100 municipal utility members. The project partners include: badenova AG & Co. kg, Erdgas Mittelsachsen GmbH, Energieversorgung Mittelrhein GmbH, erdgas schwaben GmbH, Gasversorgung Westerwald GmbH, Mainova Aktiengesellschaft, Stadtwerke Ansbach GmbH, Stadtwerke Bad Hersfeld GmbH, Thüga Energienetze GmbH, WEMAG AG, e-rp GmbH, ESWE Versorgungs AG with Thüga Aktiengesellschaft as project coordinator. Scientific partners will participate in the operational phase.[12] It can produce 60 cubic metres of hydrogen per hour and feed 3,000 cubic metres of natural gas enriched with hydrogen into the grid per hour. An expansion of the pilot plant is planned from 2016, facilitating the full conversion of the hydrogen produced into methane to be directly injected into the natural gas grid.[13]

Units like ITM Power's HGas generates hydrogen to be directly injected into the gas network as Power to Gas

In December 2013, ITM Power, Mainova, and NRM Netzdienste Rhein-Main GmbH began injecting hydrogen into the German gas distribution network using ITM Power HGas, which is a rapid response proton exchange membrane electrolyser plant. The power consumption of the electrolyser is 315 kilowatts. It produces about 60 cubic meters per hour of hydrogen and thus in one hour can feed 3,000 cubic meters of hydrogen-enriched natural gas into the network.[14]

On August 28, 2013, E.ON Hanse, Solvicore, and Swissgas inaugurated a commercial power-to-gas unit in Falkenhagen, Germany. The unit, which has a capacity of two megawatts, can produce 360 cubic meters of hydrogen per hour.[15] The plant uses wind power and Hydrogenics[16] electrolysis equipment to transform water into hydrogen, which is then injected into the existing regional natural gas transmission system. Swissgas, which represents over 100 local natural gas utilities, is a partner in the project with a 20 percent capital stake and an agreement to purchase a portion of the gas produced. A second 800 kW power-to-gas project has been started in Hamburg/Reitbrook district[17] and is expected to open in 2015.[18]

In August 2013, a 140 MW wind park in Grapzow, Mecklenburg-Vorpommern ,owned by E.ON received an electrolyser. The hydrogen produced can be used in an internal combustion engine or can be injected into the local gas grid. The hydrogen compression and storage system stores up to 27 MWh of energy and increases the overall efficiency of the wind park by tapping into wind energy that otherwise would be wasted.[19] The electrolyser produces 210 Nm3/h of hydrogen and is operated by RH2-WKA.[20]

In January 2011, Secure Supplies Hydrogen, an internationally certified engineering and warrantied power to gas vendor of quality Australian and USA made Hydrogen Fueling equipment, Hydrogen Fueled Engines and Hydrogen Home /Business Solar fuel cell Kits entered the market. Project's deploying (2015-2020) are focused on renewable power investments. Value adding Sites to produce a Hydrogen fuel and transfer storage gas. Secure Supplies value adds all solar wind, geo thermal and hydropower projects. Enabling owners to achieve a higher ROI, to produce hydrogen gas grids to fuel industry and a variety of applications emission free. End users include Port Operators, Farms, Green Community residential development or and Renewable power operators. Gas is sold to gas grid or transported and used to fuel fixed engines or fuel cell that run 24r to pump water or make power for business and communities projects. Hydrogen Equipment along with Engineering and Service provision makes Secure Supplies is well positioned to supply key markets globally.[21]

The INGRID project started in 2013 in Apulia, Italy. It is a four-year project with 39 MWh storage and a 1.2 MW electrolyser for smart grid monitoring and control.[22] The hydrogen is used for grid balancing, transport, industry, and injection into the gas network.[23]

The surplus energy from the 12 MW Prenzlau Windpark in Brandenburg, Germany[24] will be injected into the gas grid from 2014 on.

The 6 MW Energiepark Mainz[25] from Stadtwerke Mainz, RheinMain University of Applied Sciences, Linde and Siemens in Mainz (Germany) will open in 2015.

Power to gas and other energy storage schemes to store and utilize renewable energy are part of Germany's Energiewende (energy transition program).[26]

Grid injection without compression

The core of the system is a proton exchange membrane (PEM) electrolyser. The electrolyser converts electrical energy into chemical energy, which in turn facilitates the storage of electricity. A gas mixing plant ensures that the proportion of hydrogen in the natural gas stream does not exceed two per cent by volume, the technically permissible maximum value when a natural gas filling station is situated in the local distribution network. The electrolyser supplies the hydrogen-methane mixture at the same pressure as the gas distribution network, namely 3.5 bar. [27]

Power to methane

Methanation of CO2 by electrolytically obtained hydrogen

The Power to Gas Methane method is to combine hydrogen from an electrolyzer with carbon dioxide and convert the two gases to methane[28] (see natural gas) using a methanation reaction such as the Sabatier reaction or biological methanation resulting in an extra energy conversion loss of 8%, the methane may then be fed into the natural gas grid if the purity requirement is reached.

ZSW (Center for Solar Energy and Hydrogen Research) and SolarFuel GmbH (now ETOGAS GmbH) realized a demonstration project with 250 kW electrical input power in Stuttgart, Germany. The plant was put into operation on October 30, 2012.[29]

The first industry-scale Power-to-Methane plant was realized by ETOGAS for Audi AG in Werlte, Germany. The plant with 6 MW electrical input power is using CO2 from a waste-biogas plant and intermittent renewable power to produce synthetic natural gas (SNG) which is directly fed into the local gas grid (which is operated by EWE).[30] The plant is part of the Audi e-fuels program. The produced synthetic natural gas, named Audi e-gas, enables CO2-neutral mobility with standard CNG vehicles. Currently it is available to customers of Audi's first CNG car, the Audi A3 g-tron.[31]

In April 2014 the European Union’s co-financed and from the KIT coordinated[32] HELMETH[33] (Integrated High-Temperature ELectrolysis and METHanation for Effective Power to Gas Conversion) research project started.[34] The objective of the project is the proof of concept of a highly efficient Power-to-Gas technology by thermally integrating high temperature electrolysis (SOEC technology) with CO2-methanation. Through the thermal integration of exothermal methanation and steam generation for the high temperature steam electrolysis a conversion efficiency > 85% is expected (higher heating value of produced methane per used electrical energy). The process consists of a pressurized high-temperature steam electrolysis and a pressurized CO2-methanation module which are planned to be coupled in 2016. A methane output of approximately 30 kW (higher heating value) is targeted.

Microbial methanation

The biological methanation combines both processes, the electrolysis of water to form hydrogen and the subsequent CO2 reduction to methane using this hydrogen. During this process, methane forming microorganisms (methanogenic archaea or methanogens) release enzymes that reduce the overpotential of a non-catalytic electrode (the cathode) so that it can produce hydrogen.[35][36] This microbial power-to-gas reaction occurs at ambient conditions, i.e. room temperature and pH 7, at efficiencies that routinely reach 80-100%.[37][38] However, methane is formed more slowly than in the Sabatier reaction due to the lower temperatures. A direct conversion of CO2 to methane has also been postulated, circumventing the need for hydrogen production.[39] Microorganisms involved in the microbial power-to-gas reaction are typically members of the order Methanobacteriales. Genera that were shown to catalyze this reaction are Methanobacterium,[40][41] Methanobrevibacter,[42] and Methanothermobacter (thermophile).[43]

Biogas-upgrading to biomethane

In the third method the carbon dioxide in the output of a wood gas generator or a biogas plant after the biogas upgrader is mixed with the produced hydrogen from the electrolyzer to produce methane. The free heat coming from the electrolyzer is used to cut heating costs in the biogas plant. The impurities carbon dioxide, water, hydrogen sulfide, and particulates must be removed from the biogas if the gas is used for pipeline storage to prevent damage.

2014-Avedøre wastewater Services in Avedøre, Kopenhagen (Denmark) is adding a 1 MW electrolyzer plant to upgrade the anaerobic digestion biogas from sewage sludge.[44] The produced hydrogen is used with the carbon dioxide from the biogas in a Sabatier reaction to produce methane. Electrochaea[45] is testing another project outside P2G BioCat with biocatalytic methanation. The company uses an adapted strain of the thermophilic methanogen Methanothermobacter thermautotrophicus and has demonstrated its technology at laboratory-scale in an industrial environment.[46] A pre-commercial demonstration project with a 10,000-liter reactor vessel was executed between January and November 2013 in Foulum, Denmark.[47]

In 2016 Torrgas, Siemens, Stedin, Gasunie, A.Hak, Hanzehogeschool/EnTranCe and Energy Valley intend to open a 12 MW Power to Gas facility in Delfzijl (The Netherlands) where biogas from Torrgas (biocoal) will be upgraded with hydrogen from electrolysis and delivered to nearby industrial consumers.[48]

Power to syngas

Power-to-syngas process
WaterCO2
Electrolysis of Water
OxygenHydrogen
Conversion Reactor
WaterHydrogenCO
  • 1st step: Electrolysis of Water (SOEC) −water is split into hydrogen and oxygen.
  • 2nd step: Conversion Reactor (RWGSR) −hydrogen and carbon dioxide are inputs to the Conversion Reactor that outputs hydrogen, carbon monoxide, and water.

3H2 + CO2 → (2H2 + CO)syngas + H2O

Power-to-syngas feedstock is the same as feedstock derived from other sources.

Initiatives

Other initiatives to create syngas from carbon dioxide and water may use different water splitting methods.

The US Naval Research Laboratory (NRL) is designing a power-to-liquids system using the Fischer-Tropsch Process to create fuel on board a ship at sea,[85] with the base products carbon dioxide (CO2) and water (H2O) being derived from sea water via "An Electrochemical Module Configuration For The Continuous Acidification Of Alkaline Water Sources And Recovery Of CO2 With Continuous Hydrogen Gas Production".[86][87]

See also

Notes

  1. ^ DLR-Power to gas in transport-Status quo and perspectives for development
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  6. ^ NaturalHY Project. "Using the Existing Natural Gas System for Hydrogen". EXERGIA. Retrieved 2014-06-21.
  7. ^ NREL - Blending hydrogen into natural gas pipeline networks A review of key issues
  8. ^ Volker Quaschning, Regenerative Energiesysteme. Technologie - Berechnung - Simulation, Hanser 2013, p 373.
  9. ^ Jensen et al, Large-scale electricity storage utilizing reversible solid oxide cells combined with underground storage of CO2 and CH4. In: Energy and Environmental Science 8, (2015), 2471-2479, doi:10.1039/c5ee01485a.
  10. ^ (German) Fraunhofer -Energiewirtschaftliche und ökologische Bewertung eines Windgas-Angebotes, p. 18
  11. ^ Power-to-gas: Climbing the technology readiness ladder
  12. ^ http://www.itm-power.com/news-item/first-sale-of-power-to-gas-plant-in-germany/
  13. ^ Ground broken at ITM Power power-to-gas pilot plant in Frankfurt
  14. ^ http://www.itm-power.com/news-item/injection-of-hydrogen-into-the-german-gas-distribution-grid/
  15. ^ http://www.eon.com/en/media/news/press-releases/2013/8/28/eon-inaugurates-power-to-gas-unit-in-falkenhagen-in-eastern-germany.html
  16. ^ Hydrogenics and Enbridge to develop utility-scale energy storage
  17. ^ E.on Hanse starts construction of power-to-gas facility in Hamburg
  18. ^ E.ON power-to-gas pilot unit in Falkenhagen first year of operation
  19. ^ German wind park with 1 MW Hydrogenics electrolyser for power-to-gas energy storage
  20. ^ RH2-WKA
  21. ^ [1]
  22. ^ INGRID Project to Launch 1.2 MW Electrolyser with 1 Ton of Storage for Smart Grid Balancing in Italy
  23. ^ Grid balancing, Power to Gas (PtG)
  24. ^ Prenzlau Windpark (Germany)
  25. ^ Energiepark Mainz
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  34. ^ "Karlsruhe Institute of Technology - Press Release 044/2014". Retrieved 2014-10-31.
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  45. ^ Electrochaea
  46. ^ http://www.hindawi.com/journals/archaea/2013/157529/
  47. ^ http://www.electrochaea.com/technology.html
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  87. ^ The total carbon content of the world's oceans is roughly 38,000 GtC. Over 95% of this carbon is in the form of dissolved bicarbonate ion (HCO3 −). (Cline 1992, The Economics of Global Warming; Institute for International Economics: Washington D.C.). The dissolved bicarbonate and carbonate of the ocean is essentially bound CO2 and the sum of these species along with gaseous CO2, shown in the following equation, represents the total carbon dioxide concentration [CO2]T, of the world's oceans. Σ[CO2]T=[CO2(g)]l+[HCO3 −]+[CO3 2−]

Further reading

  • Manuel Götz, Jonathan Lefebvre, Friedemann Mörs, Amy McDaniel Koch, Frank Graf, Siegfried Bajohr, Rainer Reimert, Thomas Kolb. Renewable Power-to-Gas: A technological and economic review. In: Renewable Energy 85, (2016), 1371–1390, doi:10.1016/j.renene.2015.07.066.
  • Méziane Boudellal. "Le Power-to-Gas, Stockage de l'électricité d'origine renouvelable". 192 pages. In French only. Editor: Dunod, June 2016.
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