Zinc-air batteries are a promising alternative to expensive lithium-ion batteries. Compared with lithium-ion technology, zinc-air batteries have a greater energy density, very low production cost, and superior safety. However, their fundamental inability to recharge has lowered their wide-scale adoption.
Zinc-air batteries use charged zinc particles to store large amounts of electricity at a time. When electricity is required, the charged zinc is combined with oxygen from the air (and water), releasing the stored electricity and producing zincate. This process is known as oxygen reduction reaction (ORR).
Theoretically, this zincate can again be broken down into oxygen and zinc ions by passing electricity through it. This process, in turn, is called oxygen evolution reaction (OER). Using these reactions, zinc-air batteries can be made rechargeable, competing with lithium-ion batteries.
The major challenge of the recharging process is the sluggish kinetics of the reactions which lead to poor cycle life. These batteries require a catalyst that could potentially enhance the ORR and the OER reactions, making their kinetics fast. Hence, the development of highly efficient catalysts is of paramount importance for rechargeable zinc-air batteries.
Previous studies have suggested transition-metal oxides as great bifunctional ORR / OER catalysts because of their ability to provide sites for the reversible adsorption of oxygen. But the methods involved in creating well-defined defects for reversible adsorption of oxygen in such oxides are challenging.
To investigate the use of cobalt(II) oxide nanosheets deposited on stainless steel or carbon cloth as a bifunctional catalyst, a group of researchers from different universities of China and Canada collaborated and conducted several experiments. Their research findings were published in the journal Nano Energy .
Preparation of catalyst
Different nano-structures were prepared using simple heat treatment and electrodeposition to test them as bifunctional electrocatalysts. The type of nano-structures prepared were:
Cobalt hydroxide nanosheets on steel and carbon cloth
Layered cobalt (II) oxide nanosheet on steel and carbon cloth
Cobalt (II) oxide on steel
Layered cobalt tetroxide nanosheet on steel
To understand the characteristics of the prepared samples, various analyticaland tests were carried out:
Study of surface chemistry using atomic force microscopy and other equipment. To draw correlations between the oxygen vacancy defects and intrinsic ORR / OER activity linear sweep voltammogram of the samples were investigated in a three-electrode electrochemical cell.
Charging and discharging tests
Later discharge and charge cycling tests of single cells were operated by the battery testing system.
The simple heat treatment strategy created oxygen vacancy sites. According to the authors, layered cobalt-oxide nano-sheets exhibited excellent bifunctional ORR / OER performance. Investigations suggested abundant oxygen vacancies and cobalt sites be the reason for enhanced ORR / OER performance. Later, the developed layered cobalt-oxide nanosheets on steel were used as an electrode in a rechargeable zinc-air flow battery and a record-breaking cycle life of over 1,000 hours with nearly unchanged voltage was observed. Galvanostatic discharging-charging cycles also demonstrated long life and high energy efficiency.
This research carried out provides a new method to design highly efficient bifunctional ORR / OER catalysts that could be used to enhance the cycle life of rechargeable zinc-air flow batteries. At Frontis Energy we are looking forward to industrial applications.
Reference: https://doi.org/10.1016/j.nanoen.2020.105409 Wu et al., Cobalt (II) oxide nanosheets with rich oxygen vacancies as highly efficient bifunctional catalysts for ultra-stable rechargeable Zn-air flow battery, 2021
The demand for energy is increasing and raw material for the fossil fuel economy is diminishing. Moreover, the emission of gases from fossil fuel usage significantly degrades air quality. The carbon by-products produced from these fossil fuels severely affect the climate.
Hence, there is a need to find a renewable energy resource, that can be produced, stored, and used easily as per requirement. Hydrogen can be a promising energy resource as it is an abundantly available, non-toxic resource, and can be readily used to store excess electrical energy.
Hydrogen when combined with oxygen in a fuel cell produces electricity and the by-products obtained are water and heat. Based on the method of production of hydrogen it is categorized as blue hydrogen and green hydrogen. Blue hydrogen is produced from fossil fuels such as methane, gasoline, coal while green hydrogen is produced from non-fossil fuels / water. The cleanest way to produce eco-friendly hydrogen is via electrolysis of water where water is electrolyzed to separate hydrogen and oxygen. Renewable energy can be used as a power electrolyzer to produce hydrogen from water. Solar driven photo electrochemical (PEC) water splitting is one of the common method used these days. In photo electrochemical (PEC) water splitting, hydrogen is produced from water using sunlight.
PEC cells comprise of a working photoelectrode and a counter electrode. The photoelectrode consists of semiconductor material with a band gap to absorb solar light and generate an electron-hole pair. The photo-generated charges are responsible for the oxidation of water and its reduction into hydrogen. The PEC suffer devices from low stability and efficiency.
A comparative study between cobalt and nickel oxide nanoparticles deposited onto TiO 2 nanotubes prepared through anodization was carried out. The TiO 2 nanotubes were decorated with CoO (cobalt oxide) and NiO (nickel oxide) nanoparticles using the reactive pulsed laser deposition method. The surface loadings of CoO or NiO nanoparticles were controlled by the number of laser ablation pulses. The efficiency of CoO and NiO nanoparticles as co-catalysts for photo-electrochemical water splitting was studied by cyclic voltammetry, under both simulated sunlight and visible light illuminations and by external quantum efficiency measurements
The entire research work was carried out in the following steps:
In this study Cobalt (CoO) and Nickel (NiO) oxides were considered as effective co-catalysts for splitting water molecules. Both co-catalysts improved photo-electrochemical conversion of ultra violet as well visible light photons.
However, CoO nanoparticles were found to be the best co-catalyst under visible light illumination, with a Photo Conversion Efficiency almost 10 times higher than for TiO 2 . The performance of CoO nanoparticles got enhanced in the visible spectral region (λ> 400 nm). The possible reason can be a consequence of their visible bandgap which enables them to harvest more photon in the 400-500 nm range and transferring effectively the photo-generated electrons to TiO 2 nanotubes.
At Frontis Energy we are exited about these new discovery to improve hydrogen production from sunlight and hope to see an industrial application soon.
Energy-converting devices such as fuel cells are among the most efficient and clean alternative energy-producing sources. They have the potential to replace fossil-fuel-based power generators. More specifically, proton exchange membrane fuel cells (PEMFCs) are promising energy conversion devices for residential, transportation and portable applications owing to their high power density and efficiency at low operating temperatures (ca. 60–80 °C). For the complete approach, with electrolytic hydrogen renewable sources, PEM fuel cells can become one of the cleanest energy carriers. This is because water is the final product of such energy conversion systems. Currently, Nafion™ membranes are regularly used as hydrogen barriers in these fuel cells.
Sufficient hydrogen gas supply is crucial for practical application of the PEMFC systems. Currently, expensive high-pressure tanks (70 MPa) are state-of-the-art for hydrogen storage. Besides cost, there are other drawbacks such as portability and safety. In order to address these issues, alternative hydrogen storage materials have been extensively investigated. For example, metal hydrides and organic hydride materials, can fix and release hydrogen via covalent bonding.
Now, Dr. Junpei Miyake and colleagues of the University of Yamanashi, Japan, have proposed an “all-polymer” rechargeable PEMFC system (RCFC). The work has been published in Nature Communications Chemistry. Their strategy was to apply a hydrogen-storage polymer (HSP) sheet (a solid-state organic hydride) as a hydrogen-storage medium inside the fuel cell. With this approach, the issues like toxicity, flammability and volatility as well as concerns related to other components such as the fuel reservoir, feed pump and vaporizer were solved. The HSP structure is based on fluorenol/fluorenone groups that take over hydrogen-storage functionality.
In order to test the performance of their HSP-based rechargeable fuel cell, the scientists attached the HSP sheet of the membrane electrode to the catalyst layer of the anode. At the same time, the cathode side was operated as in a regular PEMFC. The researchers reported that an iridium catalyst has been applied to the inside of the HSP sheet to improve the hydrogen-releasing and fixing processes.
Fuel cell operation, cycle performance and durability were tested using cycles of 6 periodic steps. At first, hydrogen was infused into HSP sheet for 2 h, followed by nitrogen gas flushing to remove hydrogen from the anode. Then, heating of the cell up to 80°C to initiated hydrogen release from the HSP sheet. Together with oxygen gas supplied to the cathode side the fuel cell produced constant electrical current.
The team demonstrated that their HSP sheet released 20%, 33%, 51%, or 96% of the total fixed hydrogen gas in 20, 30, 60, or 360 min, respectively. The temperature was 80°C in the presence of the iridium catalyst. Also, the iridium catalyst could absorb up to 58 mol% hydrogen, which was considerably lower than that stored in the HSP. The maximum operation time was approximately 10.2 s / mgHSP (ca. 509 s for 50 mg of HSP) at a constant current density of 1 mA / cm2. The RCFCs reached cycleability of least 50 cycles. In addition, the utilization of a gas impermeable sulfonated poly-phenylene membrane (SPP-QP, another type of PEM) turned out to be a good strategy to enhance the opration time of the RCFC.
The advantageous features of the reported RCFC system include better safety, easier handling and lower weight. These are perfect for example in mobile application such as fuel cell vehicles. However, for the improvement of the RCFC performance, hydrogen storage capacity and kinetics (H2-releasing/fixing reactions) as well as catalyst stability need further improvements.
Life cycle analyzes of vehicles with different drive concepts are the subject of many studies. When it comes to CO2 emissions, the energy source is crucial. Two main developments are discussed today: the electrification of the propulsion system (i.e. fully and partially electrified vehicles) and the electrification of fuels (i.e. hydrogen and synthetic fuels).
In the manufacture of synthetic fuels, water is broken down into oxygen and hydrogen by electrolysis with renewable electricity. Due to the temporary oversupply of renewable electricity, this energy is particularly cheap. The hydrogen can then be used in hydrogen vehicles propelled by fuel cells. Alternatively, CO2 can be converted into hydrocarbons with hydrogen and then used in conventional combustion engines in a climate-neutral manner. The advantage of fuel cell vehicles is their high efficiency and the low cost of electrolysis. The disadvantage is the lack of a hydrogen infrastructure. Converting from hydrocarbons to hydrogen would cost trillions. The cheaper alternative would be synthetic hydrocarbons. However, the development is still in its infancy and the production of synthetic fuels cannot yet be carried out on a large scale.
Hydrogen and synthetic fuels are a necessary addition to electromobility, especially for long-distance and load transport. The widespread view that the low level of efficiency of internal combustion engines makes these fuels uninteresting ignores the possibility of using them to store and transport energy and to enable climate neutrality for air and shipping traffic. If you compare the CO2 emissions from electric motors and electrified fuels, it becomes clear that these mainly depend on the CO2 pollution of the electricity used.
The decentralized production of these fuels brings not only climate neutrality but also geopolitical gains. Since CO2 and renewable energy – in contrast to lithium – are generally accessible resources, users of this technology become independent of energy imports. At Frontis Energy we think these are strong arguments in favor of synthetic fuels.
An abandoned or unproductive oilfield can be reused for methane production from CO2 using renewable electrical power. Exhausted oilfields can be reactors for the conversion of renewable energy to natural gas using microbes. To achieve this, an oilfield can be made electrically conductive and catalytically active to produce natural gas from renewable power sources. The use of natural gas is superior to any battery because of the existing infrastructure, the use in combustion engines, the high energy density and because it can be recycled from CO2. Oilfields are superior to any on-ground production because of the enormous storage capacities. They are already well explored and these geological formations underwent environmental risk assessments. Lastly, the microbial power-to-gas technology is already available.
Whole process (end-to-end via methane)
50% electrical efficiency
Energy density of methane
180 kWh kg−1
Storage capacity per oilfield
3 GWh day−1
Investment (electrodes, for high densities)
Cost per kWh (>5,000 hours anode lifetime)
To address the problem of storing renewable energy, batteries have been proposed as a possible solution. Lithium ion batteries have a maximum energy storage capacity of 0.3 kWh kg−1. To date, this is considered the best trade-off between cost and efficiency but these batteries are still too inefficient to replace gasoline, which has a capacity of about 13 kWh kg−1. This makes battery driven cars heavier than conventional cars. Lithium air batteries are considered a possible alternative because they can reach theoretical capacities of 12 kWh kg−1 but technical difficulties have prevented them from being used for transportation.
In contrast, methane has an energy density of 52 MJ kg−1 corresponding to 180 kWh kg−1 which is second only to hydrogen with 500 kWh kg−1, not counting in nuclear energy. This high energy density of methane and other hydrocarbons along with their facile usage, is the reason why they are used in combustion and jet engines that drive nearly all transportation to date. While electrical cars seem to be a tempting green alternative, the fact that combustion engines and the fueling infrastructure are so wide-spread makes it difficult to switch.
In addition to the difficulty of changing habits, battery-driven electrical cars need other limited natural resources such as lithium. To equip all 94 million automobiles produced worldwide in 2017, 3 mega tons lithium carbonate would need to be mined annually. This is nearly 10% of the entire recoverable lithium resources of 35 mega tons worldwide. Although lithium and other metals can be recycled, it is clear that metal based batteries alone will not build the bridge between green energy and traditional ways of transportation due to the low energy densities of metals. And this does not even take into account other energy demands such as industrial nitrogen fixation, aviation or heating.
For Germany, with its high proportion of renewable energy, fuel for cars is not the only problem. As renewable energy is generated in the north, but many energy consumers are in the south, the grid capacity is frequently reached during peak production hours. A steadier energy output can only be accomplished by decentralizing the production or by energy storage. To decentralize production, homeowners were encouraged to equip their property with solar panels or windmills. As tax incentives phase out, homeowners face the problem of energy storage. The best product for this group of customers so far are again lithium ion batteries but investment costs of $0.10 kWh−1 are still unattractive especially because these products store the energy as electricity which can only be used for a short time and is less efficient than natural gas when used for heating.
Natural gas is widely used as energy source today and the global energy infrastructure is designed for natural gas and other fossil fuels. Increasing demand and limited resources for these fossil fuels were the main drivers of oil and gas prices during the last years, slowed by the recent economic crises and hydraulic fracturing (fracking). The high oil price attracted investors to recover oil using techniques that become increasingly expensive and are environmental risks such as deep-sea drilling or tar sand extraction. Ironically, the high oil price made costly renewable energies an economically feasible alternative and helped driving down their cost. Since habits are difficult to change and building an entirely new infrastructure only for renewable energies does not seem economically feasible today while CO2 drives global warming, a more realistic solution needs to be found.
Microbial Power-to-Gas could be a bridging technology that integrates renewable energy into the existing fossil fuel infrastructure. It reaches break even in less than 2 years if certain preconditions are met. This is accomplished by integrating methane produced from renewable energy into the current oil and gas producing infrastructure. The principal idea is to use carbon instead of metals as energy carrier because of its high energy density when bound to hydrogen. The benefits are:
High energy density of 180 kWh kg−1 methane
Low investments due to existing infrastructure (natural gas, oilfield equipment)
Carbon is not a limited resource
Low CO2 footprint due to CO2 recycling
Methane is a transportation fuel
Methane is the energy carrier for the Haber-Bosch process
Inexpensive catalysts further reduce initial investments
Low temperatures due to bio-catalysis
No toxic compounds used
No additional environmental burden because existing oilfields are reused
Methane can be synthesized by microbes or chemically. Naturally, methane is produced by anaerobic (oxygen-free) microbial biomass decomposition. The energy for biomass synthesis is provided by sunlight or chemical energy like hydrogen. In the case of methanogens (methane producing microbes), energy is harvested after CO2 and hydrogen were released from biomass decomposition following a 1-to-4 stoichiometry:
CO2 + 4 H2 → CH4 + 2 H2O
Without microbes, methane is produced by the Nobel-prize winning Sabatier reaction and several attempts are currently underway to use it on industrial scale. It is necessary to split water into hydrogen and use this to reduce CO2 in the gas phase. A major drawback of the Sabatier reaction is the need for high temperatures around 385ºC, and a nickel catalyst that becomes quickly spent. Methanogens use iron-nickel enzymes called hydrogenases to harvest energy from hydrogen, but do so at ambient temperatures.
The future challenge will be to accelerate methane production rates as has been reported for a high temperature oilfield cultures. Besides increasing the temperature, the most obvious solution is to use a higher reactive surface and bringing both electrodes closer together. Using carbon brushes that are poor hydrogen catalysts but provide a higher surface for microbial attachment is one possibility. Methane production correlates with microbial cell numbers in the reactors.
To overcome the problem of expensive carbon (and also steel) brushes for large scale applications,exhausted gas and oilfields could be used. They provide a high surface area and are usually economic liabilities and not assets. Methanogens inhabit oilfields where they carry out the final step in anaerobic petroleum degradation. Hence, oilfields can be seen as bioreactors at geological scale. Geological formations provide ideal conditions for producing, storing and extracting methane.
But how fast can microbes produce methane in an hypothetical oilfield? Under optimal conditions, methanogens that grow on electrodes (typically the genus Methanobacterium or Methanobrevibacter) can produce methane at a rate of 100-200 nmol ml−1 day−1 (equals 2.24-4.48 ml l−1 day−1) depending on catalyst and potential. Using a production rate of 15 J ml−1 day−1 of methane (190 nmol ml−1 day−1), the entire microbially accessible oilfield (2%) has a capacity of 3.6 million MBtu per year. Microbes would theoretically consume 1 TWh per year for 3.6 million MBtu methane production if there were no losses and electrical power is translated into methane 1-to-1. A power generator of 121 MW would be sufficient to supply the entire oilfield at these rates. However, all German off-shore windfarms produce 7,000 MW meaning that only 3% off-peak power can be captured by our example oilfield. Therefore, the catalytic surface and activity must be increased to accelerate methane conversion rates.
Since methanogens produce methane from hydrogen, not only the 2% porespace big enough for cells can be used resulting in an increased catalytic surface to nearly 60%. A hydrogen catalyst needs to be found that does not out-pace methanogen growth to keep the reservoir pH within the limits of 6-8 required for methanogen growth. This hydrogen catalyst must be cheap and render an oilfield electrically conductive. A chemical formulation that mimics microbial hydrogen catalysis could be used. It has the potential to turn a non-conductive and non-catalytic oilfield into a conductive hydrogen catalyst sufficient to sustain methane production needed to store all of Germany’s electricity produced by off-shore windfarms. This catalyst is soluble in water when inactive. To become active, it coats mineral surfaces by precipitation that can be triggered by indigenous microbes or by electrical polarization. The investment would be $2.3 million per MW storage capacity ($16 billion for the entire 7,000 MW). Due to microbial growth, the catalytic activity of the system improves during operation and there is no need for the second component if an immediate production is not crucial. The investments made on the cathode side would then be as low as $600 per MW ($4.2 million for 7,000 MW).
As the cathodic side of the reaction can be excluded as limiting factor, the anode needs to be designed. Several commercially available anodes such as mixed metal oxides (up to 750 A m−2) with platinum on carbon black or niobium anodes (Pt/C, 5-10 kA m−2) could be used. Anodes based on platinum are the most cost-efficient material available on the market. Investments made for Pt/C (10%, 6 mg cm−2) anodes will amount to $50,000 per MW ($350 million for 7,000 MW). However, the exact amount of Pt needed for the reaction still needs to be evaluated in an experiment because the corrosion rate at 2 V cell voltage is unknown. An often cited value for the lifetime of fuel cells is 5,000 hours and is used here to determine the costs per kWh. For 5,000 hours lifetime, the costs per kWh will be at the targeted limit of $0.01 but may be well below that because Pt/C anodes can be recycled and the Pt load may be reduced to 3 mg cm−2 (5%). Alternatively, steel anodes (SS316, 2.5 kA m−2, $54,000 per MW) can be used but it is unclear when steel anodes fail to electrolyze. In conclusion, the anodic side is the cost-driving factor. Hopefully, better water splitting anodes will lower these costs in future.
Cost estimation summary
Already in place
Natural gas capturing equipment
Already in place
Wastewater from oil rig
Total (>5,000 hours anode lifespan)
Energy and conversion efficiencies
The whole cell voltage for microbial power-to-gas reactions varies from 0.6 to 2.0 V, depending on cathodic rates, anodic corrosion and the presence of a membrane. Higher voltages will accelerate anode corrosion, again, making anodes the limiting factor. As the voltage decreases, methane production rates become slower but also more efficient. The voltage also depends on the pH of the oilfield. An oilfield that underwent CO2 injection as enhanced recovery method will have a low pH, providing better conditions for hydrogen production but not for microbial growth and must be neutralized using seawater. As stated above, the oilfield, being the cathode, is not limiting the the system. The use of Pt/C anodes eliminates the overpotential problem on the anode side. Hence, we can assume an ideal system that splits water at 1.23 V. However, the voltage is often 2 V due to anode and cathode overpotentials. Optimized cultures and cathodes produce about 190 nmol ml−1 day−1 methane which equals 0.15 J ml−1 day−1 using the energy of combustion of 0.8 MJ mol−1. The same electrolysis cell consumes 0.2 mW at a cell voltage of 2 V which equals 0.17 J ml−1 day−1 and the resulting energy efficiency is 91%. The anodes can be simple carbon brushes and the two chambers of the cell are separated by a Nafion™ membrane. The system can still be optimized by using Pt/C anodes and by avoiding membranes.
The overall electricity-methane-electricity efficiency also depends on the consumption side efficiency where methane is used in combustion engines and gas fired power plants. Such power plants frequently operate at efficiencies of 40- 60%. Assuming a reasonable power efficiency of 80% (see above), the overall electrical power recovery using gas fired power plants will be up to 50%. Besides the high efficiency of gas fired power plants, they are also easy to build and therefore contribute the a better power grid efficiency. Coal fired power plants can be upgraded to gas fired power plants.
The conversion efficiencies of charge (Coulombs) transported across the circuit are usually between 70-100% in these systems depending on the electrode material. Another efficiency limitation could arise from mass transport inhibition. Mass transport can be improved by pumping electrolyte adding more costs for pumping which still have to be determined. However, since most oilfields undergo seawater injection for enhanced oil recovery the additional cost may be negligible. The total efficiency has yet to be determined in scale-up experiments and will depend on the factors mentioned above.
Controlling the pH is crucial. Alkaline pHs significantly impede hydrogen production and therefore methanogenesis. This can be addressed by a software that monitors the pH and adjusts the potential accordingly. Addition of acids is not desired as this drives the costs. The software can also act as potentiostat that then fully controls the methane production process. To test the process under more realistic conditions, a drill core from an oilfield must be obtained.
Return of investment of the microbial power-to-gas process
The the microbial power-to-gas process in unproductive oilfields is economically superior to all other storage strategies because of the low start-up and operating costs. This is achieved because the major investments are the installation of oil- and gas production equipment and renewable power plants which are already in place as a precondition. These investments break even in a short amount of time.
But how can the microbial Power-to-Gas process accelerate the return of investment in renewable energy? Only 8 out of 28 active off-shore windfarms reported their investment costs. These 8 produce roughly half the overall power of 1,600 MW corresponding to $7 billion. While the maximum production of an oilfield with unlimited supply of electricity would yield hypothetical 3.6 million MBtu natural gas per year resulting a return of $13 million per year the real production is limited by off-peak power generated by renewable energy production. Assuming that the maximum annual methane production corresponds to 10% excess electrical power, $15 million per year can by generated by selling 4.3 million MBtu methane per year on the market. These are $15 million that are not lost during off-peak shutdowns. Clearly, this conservative estimate can help to compensate the investment in renewable energy earlier. It also decreases the investment risk because the investment calculations for new wind farms can be made on a more reliable basis.
In the example using all German windfarms (7,000 MW) this compensation roughly doubles. Using the $60 million generated by methane sales per year, the investment of $4 million for the cathodic catalyst and the $36 million for the Pt/C anodes are compensated for within less than a year. No other investments are required because the target oilfield already produced oil and gas and all necessary installation are in working condition. The target oilfield is swept using seawater as secondary extraction method. Electrical installations are in place for cathodic protection of production equipment in order to prevent microbial corrosion, which, however, may need to be upgraded to pass the now higher power densities. Moreover, CO2 is used from CO2 injection as tertiary enhanced oil recovery method. Only the pH may then need to be adjusted to sustain life by sweeping with seawater.
And this is not the end of oilfield storage capacity. In theory, an oilfield can store the entire amount of renewable energy produced in one year globally, allowing more than enough head room for future development and CO2 sequestration.
In addition to well-established Nafion™ membranes which are currently the best trade-off between high-performance and cost in proton exchange fuel cells (PEM), methanol fuel cells, electrolysis cells etc. As our energy resources are diversifying, there is a growing demand for efficient and selective ion-transport membranes for energy storage devices such as flow batteries.
Redox flow batteries – the energy storage breakthrough
The high demand for a reliable and cost-effective energy storage systems is reflected in the increased diversity of technologies for energy storage. Among different electrochemical storage systems, one of the most promising candidates are redox-flow batteries (RFBs). They could meet large-scale energy storage requirements scoring in high efficiency, low scale-up cost, long charge/discharge cycle life, and independent energy storage and power generation capacity.
Since this technology is still young, the development of commercially and economically viable systems demands:
improvement of the core components e.g. membranes with special properties,
improvement of energy efficiency
reduction in overall cost system.
Meeting demands for redox flow batteries
Two research teams in the United Kingdom, one from Imperial College and the other from the University of Cambridge, pursued a new approach to design the next generation of microporous membrane materials for the redox-flow batteries. They recently published their data in the well renown journal Nature Materials. Well-defined narrow microporous channels together with hydrophilic functionality of the membranes enable fast transport of salt ions and high selectivity towards small organic molecules. The new membrane architecture is particularly valuable for aqueous organic flow batteries enabling high energy efficiency and high capacity retention. Importantly, the membranes have been prepared using roll-to-roll technology and mesoporous polyacrylonitrile low-cost support. Hence, these innovative membranes could be cost effective.
As the authors reported, the challenge for the new generation RFBs is development of low-cost hydrocarbon-based polymer membranes that features precise selectivity between ions and organic redox-active molecules. In addition, ion transport in these membranes depends on a formation of the interconnected water channels via microphase separation, which is considered a complex and difficult-to-control process on molecular level.
The new synthesis concept of ion-selective membranes is based on hydrophilic polymers of intrinsic microporosity (PIMs) that enable fast ion transport and high molecular selectivity. The structural diversity of PIMs can be controlled by monomer choice, polymerization reaction and post-synthetic modification, which further optimize these membranes for RFBs.
Two types of hydrophilic PIM have been developed and tested: PIMs derived from Tröger’s base and dibenzodioxin-based PIMs with hydrophilic and ionizable amidoxime groups.
The authors consider their approach innovative because of
The application of PIMs to obtain rigid and contorted polymer chains resulting in sub-nanometre-sized cavities in microporous membranes;
The introduction of hydrophilic functional groups forming interconnected water channels to optimize hydrophilicity and ion conductivity;
The processing of the solution to produce a membrane of submicrometre thickness. This further reduces ion transport resistance and membrane production costs.
Ionic conductivity has been evaluated by the real-time experimental observations of water and ion uptake. The results suggest that water adsorption in the confined three-dimensional interconnected micropores leads to the formation of water-facilitated ionic channels, enabling fast transport of water and ions.
The selective ionic and molecular transport in PIM membranes was analyzed using concentration-driven dialysis diffusion tests. It was confirmed that new design of membranes effectively block large redox active molecules while enabling fast ion transport, which is crucial for the operation of organic RFBs.
In addition, long-term chemical stability, good electrochemical, thermal stability and good mechanical strength of the hydrophilic PIM membranes have been demonstrated.
Finally, it has been reported that the performance and stability tests of RFBs based on the new membranes, as well as of ion permeation rate and selectivity, are comparable to the performances based on a Nafion™ membranes as benchmark.
There has been criticism directed towards the Netherlands for the progress made. According to projections in their 2009 National Renewable Energy Action Plan, the Netherlands should have reached nearly 20% renewable electricity in 2014. This lackluster progress prompted a statement from the EU Commission in its 2017 Second Report on the State of the Energy Union, where the EU Commission stated the Netherlands were the only member state to not exhibit average renewable energy shares which were equal or higher than their corresponding action plan trajectories in 2013/2014.
The EU Commission also stated that the Netherlands was one of the three countries (others: France, Luxembourg) with the biggest efforts required to fill 2020 targets.
Existing Energy Storage Facilities
To date, the Netherlands has almost 20 MW of energy storage capacity either operating (14 MW), contracted (1 MW), or under construction (4 MW).
All energy storage facilities in the Netherlands are electro-chemical, with the exception of the contracted 1 MW Hydrostar underwater compressed air energy storage project in Aruba (Caribbean). Hydrostar is a Canadian company specializing in underwater compressed air energy storage technologies.
The vast majority of the 20 MW of installed energy storage capacity in the Netherlands is spread over just three facilities: the Netherlands Advancion Energy Storage Array (10 MW Li-ion), the Amsterdam ArenA (4 MW Li-ion), and the Bonaire Wind-Diesel Hybrid project (3 MW Ni-Cad battery).
The Netherlands Advancion Energy Storage Array was commissioned in late 2015 and provides 10 MWh of storage to Dutch transmission system operator TenneT. The project, which represents 50% of all Dutch energy storage capacity, provides frequency regulation by using power stored in its batteries to respond to grid imbalances.
The 4 MW Amsterdam ArenA lithium-ion project was commissioned 2017 for PV integration and back up power purposes. The 3 MW Bonaire Wind-Diesel Hybrid project is a battery array located on the Dutch Caribbean island of Bonaire and used as a buffer between intermittent wind energy and the diesel-generation stations on the island.
The remaining 3 MW of Dutch energy storage projects are spread over 21 sub-100 kW facilities, mainly geared towards electric vehicle (EV) charging. Mistergreen, a leading developer of EV charging stations in the Netherlands has constructed 750 kW of LI-ion energy storage arrays at its various electric vehicle charging stations.
Energy Storage Market Outlook
Gearing up for significant market growth for electric vehicles in the Netherlands, there has been a considerable amount of effort to expand the country’s network of quick charging stations. This trend will have to continue in order meet the demand for the 1-million electric vehicles expected in the Netherlands by 2025, so one could expect that there will be large growth in the sub-100 kW Li-ion stations that have already started popping up around the country.
There is little information available regarding the need for large-scale energy storage but the overall need is likely low due to the low penetration of renewables in the electricity sector. However, there is significant focus on energy efficient/independent/self-sufficient housing.
Like Italians, the Dutch are very accustomed to using natural gas in their homes. This, coupled with the push for energy self-sufficient housing could present a unique market for residential power-to-gas systems in the Netherlands.
While fuel oil is still used for electricity in Spain, it should be noted that this is exclusive to the non-peninsular areas of Spain (i.e. Canary Islands, Balearic Islands, Cueta, Melilla, and several other small islands).
The policy changes and self-consumption taxes allude to the Royal Decree 900/2015 on self-consumption, a law enacted by the Spanish government in October 2015, which aims to financially penalize the self-consumption of electricity. Under the new law solar PV producers (residential PV owners, for example) are required to not only pay a tax on the energy they self-consume, but also must pay the same transmission & distribution fees they would have paid on an equivalent amount of electricity purchased from the grid. In addition to these charges and taxes, owners of systems 100 kW and smaller – most residential system owners – are prohibited from selling excess electricity from the grid. Instead, they must give it to the grid for free. Furthermore, this law is retroactive; meaning existing PV systems must comply or face a penalty. Penalties under the self-consumption law range from as low as EUR 6-million up to a maximum of EUR 60-million – about twice the fine for leaking radioactive waste. The Spanish government see’s self-consumption as a risk to tax revenues at the current high electricity prices.
Spain is still the world leader in concentrated solar power capacity (2.5 MW). However, no new plants were constructed since and there are currently no new plants under construction or in planning.
Energy Storage Market Outlook – Spain
Although initial drafts of the “self-consumption” law had strict provisions against battery storage systems, the final version does permit energy storage systems – although under conditions that make them impractical. While owners of solar-plus-storage systems are subject to additional charges, they also cannot reduce the amount of power that they have under contract from their utility company.
At this point in time, it appears as if the self-consumption law has effectively halted any investment in renewable energy and/or energy storage projects in Spain.
Proximity to both Scandinavia and mainland Europe makes exporting and importing power rather easy for the Danish system operator, Energinet.dk. This provides Denmark with the flexibility needed to achieve significant penetration of intermittent energy sources like wind while maintaining grid stability.
While the results to-date have been promising, getting to 100 percent renewable energy will still require a significant leap and the official policies that Denmark will use to guide this transition have yet to be delivered. However, there has been some indication at what the ultimate policies may look like. In their report Energy Scenarios for 2020, 2035 and 2050, the Danish Energy Agency outlined four different scenarios for becoming fossil-free by 2050 while meeting the 100 percent renewable electricity target of 2035. The scenarios, which are primarily built around deployment of wind energy or biomass, are:
Wind Scenario – wind as the primary energy source, along with solar PV, and combined heat and power. Massive electrification of the heat and transportation sectors.
Biomass Scenario – less wind deployment that in the wind scenario, with combined heat and power providing electricity and district heating. Transportation based on biofuels.
Bio+ Scenario – existing coal and gas generation replaced with bioenergy, 50% of electricity from wind. Heat from biomass and electricity (heat pumps).
Hydrogen Scenario – electricity from wind used to produce hydrogen through electrolysis. Hydrogen used as renewable energy storage medium, as well as transportation fuel. Hydrogen scenario would require massive electrification of heat and transport sectors, while requiring wind deployment at faster rate than the wind scenario.
Agora Energiewende and DTU Management Engineering, have postulated that this scenario report does in fact show that transitioning the Danish energy sector to 100 percent renewables by 2050 is technically feasible under multiple pathways. However, Danish policy makers must decide before 2020 whether the energy system will evolve into a fuel-based biomass system, or electricity-based wind energy system (they must decided which of the four scenarios to pursue).
Energy Storage Facilities – Denmark
Regardless of which energy policy scenario Denmark decides to pursue, energy storage will be a central aspect of a successful energy transition. There are currently three EES facilities operating in Denmark, all of which are electro-chemical (batteries). A fourth EES facility – the HyBalance project – is currently under construction and will convert electricity produced by wind turbines to hydrogen through PEM electrolysis (proton exchange membrane).
The HyBalance project is the pilot plant undertaking of Power2Hydrogen, a working group comprised of major industry players and academic research institutions aimed at demonstrating the large-scale potential for hydrogen from wind energy. The plant will produce up to 500 kg/day of hydrogen, used for transportation and grid balancing.
Worth noting is the decommissioned BioCat Power-to-Gas project, a pilot plant project which operated from 2014 to 2016 in Hvidovre, Denmark. The project, a joint collaboration between Electrochaea and several industry partners (funded by Energienet.dk), was a 1 MWe Power-to-Gas (methane) facility built to demonstrate the commercial capabilities of methane power-to-gas. The BioCat project was part of Electrochaea’s goal of reaching commercialization in late 2016, however, as of early 2017 no further updates have been given.
Energy Storage Market Outlook − Denmark
The energy storage market in Denmark will be most primed for growth should policy follow the Hydrogen Scenario, where massive amounts of hydrogen production will be needed to eliminate the use of fossil fuels across all sectors.
Renewable energy produced gases (hydrogen, methane) have the potential to balance the electricity grid in two primary ways: balancing supply and demand (“smart grid”), and balancing through physical storage. The smart grid, an intelligent electricity grid where production and consumption are administered centrally, presents significant opportunity for electrolysis technologies as short-term “buffer” storage (seconds to minutes). Bulk physical storage of renewable energy produced gases can act as a longer-term storage solution (hours, days, weeks, months) to help maintain flexibility in a fossil-free energy grid (The Danish Partnership for Hydrogen and Fuel Cells).
Without the hydrogen scenario, the potential for hydrogen-based energy storage in Denmark will be limited. In their 2016 report “potential of hydrogen in energy systems”, the Power2Hydrogen working group concluded that:
hydrogen electrolysers would not provide any significant upgrade on flexibility for renewables integration over today’s sufficiently flexible system, and;
by 2035, with the increased wind production, it was concluded that hydrogen electrolysers would in fact improve system flexibility, allowing for even more extensive penetration of wind energy in the system.
The potential for renewable energy produced gases in Demark is extremely high. There is a very distinct possibility that power-to-gas type of systems will be the linchpin of Denmark’s energy transition. While there appears to be little opportunity in the short-term, there will be extensive opportunity in the medium-to-long-term should the official energy transition policy focus on the hydrogen scenario, or a similar renewable gas based policy.
In order to make better use of wind currents, the air mass dynamics and its interactions with land and turbines must be understood. Our knowledge of wind currents in complex terrain and under different atmospheric conditions is very limited. We have to model these conditions more precisely so that the operation of large wind turbines becomes more productive and cheaper.
To gain more energy, wind turbines have grown in size. For example, when wind turbines share larger size areas with other wind turbines, the flow changes increasingly.
As the height of wind turbines increases, we need to understand the dynamics of the wind at these heights. The use of simplified physical models has allowed wind turbines to be installed and their performance to be predicted across a variety of terrain types. The next challenge is to model these different conditions so that wind turbines are optimized in order to be inexpensive and controllable, and installed in the right place.
The second essential direction is better understanding and research of the wind turbine structure and system dynamics . Today, wind turbines are the largest flexible, rotating machines in the world. The bucket lengths routinely exceed 80 meters. Their towers protrude well over 100 meters. To illustrate this, three Airbus A380s can fit in the area of one wind turbine. In order to work under increasing structural loads, these systems are getting bigger and heavier which requires new materials and manufacturing processes. This is necessary due to the fact that scalability, transport, structural integrity and recycling of the used materials reach their limits.
In addition, the interface between turbine and atmospheric dynamics raises several important research questions. Many simplified assumptions on which previous wind turbines are based, no longer apply. The challenge is not only to understand the atmosphere, but also to find out which factors are decisive for the efficiency of power generation as well as for the structural security.
Our current power grid as third essential direction is not designed for the operation of large additional wind resources. Therefore, the gird will need has to be fundamentally different then as today. A high increase in variable wind and solar power is expected. In order to maintain functional, efficient and reliable network, these power generators must be predictable and controllable. Renewable electricity generators must also be able to provide not only electricity but also stabilizing grid services. The path to the future requires integrated systems research at the interfaces between atmospheric physics, wind turbine dynamics, plant control and network operation. This also includes new energy storage solutions such as power-to-gas.
Wind turbines and their electricity storage can provide important network services such as frequency control, ramp control and voltage regulation. Innovative control could use the properties of wind turbines to optimize the energy production of the system and at the same time provide these essential services. For example, modern data processing technologies can deliver large amounts of data for sensors, which can be then applied to the entire system. This can improve energy recording, which in return can significantly reduce operating costs. The path to realize these demands requires extensive research at the interfaces of atmospheric flow modeling, individual turbine dynamics and wind turbine control with the operation of larger electrical systems.
Advances in science are essential to drive innovation, cut costs and achieve smooth integration into the power grid. In addition, environmental factors must also be taken into account when expanding wind energy. In order to be successful, the expansion of wind energy use must be done responsibly in order to minimize the destruction of the landscape. Investments in science and interdisciplinary research in these areas will certainly help to find acceptable solutions for everyone involved.
Such projects include studies that characterize and understand the effects of the wind on wildlife. Scientific research, which enables innovations and the development of inexpensive technologies to investigate the effects of wild animals on wind turbines on the land and off the coast, is currently being intensively pursued. To do this, it must be understood how wind energy can be placed in such a way that the local effects are minimized and at the same time there is an economic benefit for the affected communities.
These major challenges in wind research complement each other. The characterization of the operating zone of wind turbines in the atmosphere will be of crucial importance for the development of the next generation of even larger, more economical wind turbines. Understanding both, the dynamic control of the plants and the prediction of the type of atmospheric inflow enable better control.
As an innovative company, Frontis Energy supports the transition to CO2-neutral energy generation.