How Much Energy Is Stored In 1 Gallon Of Powerballs?
A gallon container, filled with powerballs will only be about 65% by volume NaH. The remaining volume will be made up of coating material and space between the pellets (water can be stored in the space between individual powerballs, also).
Thus, 1 gallon of sodium hydride powerballs is 65% X 3.8 Liters = 2470 cubic centimeters of NaH, or 3448 grams of NaH (NaH density = 1.396 g/cm^3, Merck Index). 1 gram of H2 is produced for each 12 grams of NaH that react with water.
Thus, 1 gallon of NaH powerballs (65% by volume NaH) when reacted with
287.3 grams of hydrogen
11. 2 KWH (assuming 100% conversion efficiency for H2 +.5 02 -> H2O)
3195.8 Liters of Hydrogen (at sea level, and room temperature)
841 gallons of Hydrogen (at sea level, and room temperature)
1 gallon of liquid hydrogen:
778 gallons of Hydrogen (at sea level, and room temperature)
1 gallon of methanol:
342 grams H2 = 1001 gallons Hydrogen (at sea level, and room temperature)
Many assumptions have to be made when comparing various hydrogen storage methods. For instance, from a strictly 'paper study', methanol appears to store more hydrogen by volume than does a gallon of sodium hydride powerballs. However, in order to get the hydrogen out of the methanol, a high temperature reformer system is needed. This reforming process is endothermic, meaning that you have to pump energy into methanol to get the hydrogen out. If this energy comes from burning the methanol, then this portion of the methanol is no longer available for hydrogen production. These considerations reduce the effective hydrogen storage density of methanol. In addition, the reformer itself must be included in the energy density calculations because without it, you just have methanol, not hydrogen. This consideration lowers the hydrogen storage density of methanol even further.
By stark contrast to a methanol based hydrogen storage system, when sodium hydride powerballs are reacted with water, there is an exothermic reaction. Thus, no additional fuel is needed to drive the NaH + H2O -> NaOH + H2 reaction.
Consider the following 2 reactions taking place on a powerball/fuel cell car of the future:
NaH + H2O -> NaOH + H2 (powerball reaction)
H2 +.502 -> H2O (fuel cell reaction)
NaH +.502 -> NaOH (overall)
These reactions suggest that water does not really need to be considered in energy density calculations for powerballs. Except for a small amount of water needed to start the process (stored between individual powerballs), water is available for the reaction as the waste product from the fuel cell.
In addition, although NaOH needs to be stored on the vehicle as a product of the reaction, its density is 2.130 g/cm^3. Considering that 1 gallon of NaH is 3448 grams of NaH or 143.7 moles. Thus, 143.7 moles or 5746.7 grams of NaOH will be produced as a product of the reaction. Thus, only 2.7 liters are required for NaOH storage or .7 gallons. So, with proper engineering, the NaOH product can easily be stored in the original gallon of space originally occupied by the NaH powerballs.
So, the original gallon of powerballs truly produces nearly 841 gallons of hydrogen even when the overall system is included in the energy density calculations.
When compared to alternative routes such as methanol reforming, liquid hydrogen or compressed hydrogen the powerball system is quite attractive because of its relative simplicity. Complicated reformers, as in the case of methanol, are not required. Energy to drive the reaction, as is the case with methanol, is not necessary. Cryogenic tanks or bleed-off losses, as is the case with liquid hydrogen, do not need to be considered in hydrogen storage calculations for powerballs. Evaporative emissions and fuel losses, as is the case with methanol, also do not need to be considered for powerball hydrogen storage calculations. In addition, water from the fuel cell can be used in the reaction, and the waste NaOH can be stored in the same space originally used by the powerballs.
Additionally, powerballs can be produced from other hydrides which would produce exceptionally large amounts of hydrogen from a very small space but at a higher cost; Consider a gallon of LiA1H4 for instance. When reacted with water, 1 gallon of LiA1H4 powerballs (net LiA1H4) would produce 1600 grams of hydrogen or 4683.6 gallons of hydrogen (at STP). LiA1H4 + 2H2O -> LiA1O2 + 4 H2 (8 g H2 per 38 g LiA1H4 @ 2. 0 g/cm^3). By comparison, to achieve this storage density with compressed hydrogen would require the hydrogen to be compressed to 4683.6 atmospheres or 68,848 psi!