An electrolyzer is an electrochemical device with two electrodes which acts somewhat like the energy storage phase of a rechargeable battery; When an electric potential is applied between the two electrodes chemical changes occur which produce two chemical species which are capable of reaction with each other to produce energy. That is to say electrical energy is converted in to chemical potential energy. Electrolysis often takes the form of decomposing a compound chemical substance, the most well known of such reactions being the decomposition of water in to molecular hydrogen and oxygen, although many other substance can also be electrolytically decomposed. For example this paper claims that a solid oxide electrochemical cell can be designed to electrolytically decompose carbon dioxide into oxygen and carbon monoxide just as efficiently as water can be decomposed. Another example of great practical importance is the electrolytic production pure aluminum from its oxide Al2O3
If water is electrolyzed into hydrogen and oxygen, hydrogen and oxygen can later be combined in a fuel cell to produce electricity. This combination of an electrolyzer and a fuel cell is effectively a storage battery. Such a 'battery' has one advantage and a couple of disadvantages compared to more conventional electrochemical storage devices:
Advantage: The substance destined to store energy (water) is cheap and widely available.
Water is certainly cheap compared to substance like lithium or nickel, but it is nevertheless a commodity in high demand. Therefore an interesting question is: How much water would be required to run a large scale energy storage system? For H2 the HHV=142MJ/kg=39kWh/kg. Electricity usage (industrial and domestic combined) in the United States averages 12,000kWh/person. Twelve hours worth of electricity at this usage rate is 16.4kWh. If the round trip efficiency of energy storage by this method was 42% then 1kg of water per person would be required to store 12 hours of electricity, which does not seem like excessive drain on water resources.
1. The round trip efficiency is low.
The much bally-hooed Bloom Box solid oxide fuel cell claims to have an energy efficiency of greater than 50%. (Although this claim has been disputed. The efficiency of water electrolysis is about 73%. If the Bloom Box efficiency is 55% then the round trip efficiency of an electrolyzer/Bloom Box combination would be: 100*0.73*0.55=40%. This efficiency implies an electricity cost multiplier of 2.5 even if the energy storage capital cost are next to nothing (However, see disadvantage 2). If your solar panel produces electricity at a cost of 0.1$/kWh then cost of 1 kWh coming out of the storage system will be 0.25$/kWh + Storage costs.
Disadvantage 2: Using a separate electrolyzer and fuel cell for energy storage/energy delivery means that the whole storage system requires four catalytic electrodes rather than just two.
It is true that the electrodes are catalytic (i.e. they promote the desired chemical reaction but are not themselves chemically transformed) and therefore do not have the bulk of the energy storage electrodes in traditional batteries, but nevertheless the cost of such electrodes is important. As I have pointed out before an energy storage system is also a power delivery system, and the cost per unit of power is important. A tank full of water and an empty tank to hold hydrogen gas might be relatively cheap per kWh of storage (although see disadvantage 3), but if the cost of moving energy in and out of the storage system at a reasonable rate is too high then the system will not be cost competitive with traditional generation. Four electrodes in the energy transfer path is a greater cost challenge than just two
Disadvantage 3: Hydrogen storage is not problem free
Hydrogen has a high gravimetric energy density but a low volumetric energy density. Storing hydrogen gas a high pressure solves the volumetric density problem, but energy is lost during the compression process further lowering an already low round trip efficiency. Furthermore hydrogen is a corrosive substance which will embrittle steel tanks. Fancy carbon fiber tanks can solve this problem, but at a substantial cost.
Of the three disadvantages of water electrolysis energy storage systems the low round trip efficiency is probably the worst. Furthermore, the thermodynamics properties of water/hydrogen/oxygen limits this efficiency even with ideally performing electrodes. My knowledge of chemical thermodynamics is weak, but I doubt that round trip efficiencies of greater than 50% are possible for a water based system. This low efficiency implies a doubling of generation costs on top of the capital costs for electrolyzers, fuel cells, and hydrogen storage tanks.
Recently a group of researchers and Northwestern University and the Colorado School of Mines have proposed a new electrolyzer/fuel cell energy storage concept which addresses all three disadvantages of this energy storage method. They call this energy storage system a Solid Oxide Flow Battery (SOFB). Solid Oxide Fuel Cells (SOFC) are electrochemical devices for converting the chemical potential energy of certain kinds of fuel gases into electricity. Like PEM fuel cells they can use hydrogen (H2 as a fuel gas, but they can also use carbon monoxide (CO) or methane (CH4). In this research project the investigators propose using a reversible solid oxide electrochemical device to electrolyze a mixture of steam (H2O) and carbon dioxide (CO2) into mixture of methane (CH4), hydrogen (H2), and carbon monoxide (CO). When the device is reversed and operated in fuel cell mode then the fuel gas mixture of CH4, H2, and CO is oxidized back to H2O and CO2, producing and electrical current in the process. Although the proposed energy storage system is different that flow batteries with external tanks of liquid electrolyte such as Vanandium Redox Flow Batteries, the fact that the fuel gas and its oxidised products are stored in tanks and pumped by the mebrane electrode assembly when chemical transformation is desired make the designation 'Flow Battery' appropriate. This type of solid oxide device can potentially address all three disadvantages of more traditional designs.
1. Low round trip efficiency.
The researchers claim that this fuel gas mixture can, under proper conditions of temperature and pressure can achieve round trip efficiencies of greater than 80% (i.e. 80% of the electrical energy used to electrolyze H2O and CO2 during the energy storage phase of operation is recovered during the fuel cell phase of operation.). Admittedly the operating point indicated by the model which give this high efficiency occurs at a current density which is too low for an economically competitive device (Remember that an energy storage system is also a power delivery system), but nevertheless this result gives hope than materials set can be found which will be able to delivery high efficiency and high power.
2. The expense of four catalytic electrodes.
A reversible electrolyzer/fuel cell requires only two electrodes. It is not a certainty of course that two electrode will have half the cost of four, but the potential for a large cost reduction exists.
3. Difficulties of Hydrogen storage.
The fuel gas of the proposed system will be methane rich with a relatively small component of H2. This gas will not require a large amount of compression, or specialized containers. The researchers suggest that this gas can be stored in the same kinds of pipes and underground storage facilities that are currently used for natural gas.
My view of this idea is that it is an interesting and worthwhile research project, but holding one's breath waiting for a low cost, high power, high efficiency, long cycle life product to start shipping in volume is probably not a good plan.
Jun 28, 2013Energy Storage News
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