Storing Energy as MgH2

Storing Energy as MgH2

With wind and solar power generation, it is often desirable to store energy locally rather than to put it onto the grid.  For example,

  • Sometimes the rates paid by the electric company are not favorable, e.g., at non-peak times, or are even negative.
  • Usually the electric company wants a “base load” to be provided, i.e., a minimum amount of electricity that goes onto the grid 7×24.  Neither wind nor solar can do this without burning another fuel such as natural gas or hydrogen.
  • A capital cost reduction can be partially realized if the generation system is designed NOT to generate more than a certain maximum level of electricity, with the rest being stored to address the base load issue.  The reason for “partially” is that the capital cost of the storage system needs to be factored into the total cost.

One storage possibility is to store the energy as hydrogen gas using electrolysis to break down water into hydrogen and oxygen:

2H2O + energy –> 2H2 + O2

With an 80% efficient electrolysis unit, it takes about 50 kWh of electricity to create 1 kg of hydrogen. The hydrogen could then be used in a fuel cell or converted gas engine to generate electricity.  This is great except for the “detail” of storing the hydrogen.  (What to do with the resultant oxygen is another detail.  It can be bottled and sold, or simply released into the air.)  Most people suggest compressing the hydrogen possibly to a liquid form, but that takes considerable energy, and the result is not economical for the production of electricity.  Not compressing it takes too much storage and pumping to move it around.

Multiple methods for storing the generated hydrogen as hydrides, e.g., magnesium hydride, MgH2, have been developed over the years.  For MgH2, they involve grinding the magnesium in a hydrogen environment at pressure (15 Bars or so), at high temperature (in excess of 300°C), and in the presence of a catalyst. (Various catalysts have been proposed, including MgH2 itself.) This process gives an initial supply of MgH2 for use in an energy storage cycle.

The folks at Safe Hydrogen mix the resulting MgH2 into a “slurry” by adding light mineral oil and some dispersants. The dispersants aid in keeping the MgH2 particles from agglomerating and help to stabilize the slurry. The mineral oil helps to protect the MgH2 particles from inadvertent contact with moisture in the air and provides the liquid medium for the slurry.  The resulting slurry is then cooled to room temperature for stable storage.  This process is described in their patent US 2010/0252423 A1. The slurry looks a lot like thick paint.  Here’s a photo from one of the Safe Hydrogen papers, links to which can be found on their web site.70 Percent MgH2 Slurry

At ambient temperatures, the slurry is stable, not highly flammable, and can be pumped, transported, and stored in standard tanks and trucks.  It has stored energy by kg or by liter (L) as high as [0] 3.9 kWh/kg and 4.8 kWh/L.  Compare this to gasoline which has 1.8 KWh/kg of practical energy storage within 12.8 kWh/kg of total theoretical energy storage. [1]

The slurry can be converted back to hydrogen and recycled as follows:

 

The first technique is to add water to release the hydrogen:

MgH2 + 2H2O –> Mg(OH)2 + 2H2

The resulting byproduct, magnesium hydroxide, Mg(OH)2, is, in its pure form, a white power, and when mixed with water forms a suspension that is essentially the same as “Milk of Magnesia.”  Recycling the Mg(OH)involves several steps:

  1. The separation of the oils from the byproduct slurry for reuse.
  2. The calcination of Mg(OH)2 to MgO by heating to around 300°C,

Mg(OH)2 –> MgO + H2O,

  1. The electrolytic reduction of the resulting magnesium oxide using the “solid-oxide • oxygen-ion-conducting membrane (SOM) process developed atBostonUniversity[2].

2MgO –> 2Mg + O2

  1. The hydriding of the Mg obtained in the preceding step and H2 (obtained by additional electrolysis of water) to MgH2, is done by mixing magnesium powder with magnesium hydride powder and hydrogen at about 300°C and 10 Bar.
  2. The production of new slurry from the MgH2 and the recovered oils.

In this process, slurry is pumped into a stream of hot water to mix the slurry with the water. The mixed stream then flows into a reaction chamber where the bulk of the reaction takes place, releasing hydrogen. Several injections take place in quick succession to take advantage of the heat from the previous reaction. The byproducts of this reaction collect in the bottom of the reactor. Periodically, the reaction chamber is flushed by filling it with water to recover the oils from the slurry. The water level is reset and the injections resume. Also periodically, the byproducts are moved from the bottom of the reactor to the byproduct separation chamber. In the byproduct separation chamber, the water is filtered from the solid byproducts and returned to the water storage container. One can readily hydride magnesium by mixing magnesium powder with magnesium hydride powder and hydrogen at about 300°C and 10 Bar [This step is described in detail in the above cited patent.].  The heat produced by the electrolysis process was estimated to be enough to satisfy the heating requirements of the system in continuous operation.

Effort needs to be performed to condense magnesium into a powder from the vapor produced by the SOM process. To minimize the fire hazards associated with magnesium powder, the powder should be immediately hydrided, and the resulting magnesium hydride should proceed directly into a slurry production process.

According to Andrew McClaine, CTO at Safe Hydrogen, “We are currently scaling up from a laboratory scale of 2 kWth to a scale of 20-40 kWth.”  In other words, this technology needs a lot of development before it can scale to utility sized operations, which would require tens to hundreds of MWh of energy stored.  Note that 10MWh would take about 2,000 liters of MgH2 slurry.  Safe Hydrogen’s estimates on cost seem reasonable, but the process is complex with lots of variables.  It usually takes a decade for such technology to become cost effective and mass produced.

McClaine also points out that there is another technique to release hydrogen from the slurry.  It is to heat it to around 280°C.  This releases some of the hydrogen rather efficiently and allows the slurry to be replenished later by adding hydrogen from the electrolysis process.  Apparently this process can be repeated efficiently approximately 100 or so times, before the complete first process needs to be performed.  Safe Hydrogen is working on optimizing this combined technique and on increasing the number of cycles from 100 to potentially 1000 or so.

Refining all these techniques may well produce a commercially viable product, even at a scale of 20-40 kWth.  Safe Hydrogen seems to think so.  It appears that smoothing out the energy produced from wind farms may have an earlier commercial product than transporting the slurry and producing hydrogen elsewhere. [3]

-gayn

[0] Certain proposed lower cost processes will produce a lower energy density of the slurry.  Safe Hydrogen is experimenting with various processes.  Personal communication Andrew McClaine, 8/28/2011.

[1] Journal of Physical Chemistry and Letters

[2] Solid-Oxide Oxygen-Ion-Conducting Membrane (SOM) Technology For Production Of Magnesium Metal By Direct Reduction Of Magnesium Oxide, D. E. Woolley and U. Pal, Department of Manufacturing Engineering, Boston University, Boston, MA 02215, G. B. Kenney. ElMEx, LLC, Medfield. MA 02052

[3]  Storing Wind Energy as Hydrogen, David Anthony and Ken Brown, reprinted in Green Economy Post, August 15, 2011.

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One Response to “Storing Energy as MgH2”

  1. Magnesium oil Says:

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