In my last post I demonstrated that our raw supply of solar energy is relatively weak, perhaps 200 W/m2. Nonetheless, aside from the nuclear fuels we dig out of the earth, the sun is our only source of power. If we are to keep our technological world moving, we have no choice but figure out how to best use it.
Since direct solar energy is not adequate for many essential processes (I cited the automotive sector as an example), we have to store it up over time and space so that we can use it at more intense rates.
Mother nature has done this for us in the form of fossil fuels. And, though these are still the predominant choice of energy, they have hit a political speed bump. The push is on for alternate means of storing concentrated energy. I listed some of the common ones in my first posting. Here they are again:
|Energy Source||Energy Density||Relative To Gasoline|
|Burning Compressed Hydrogen||1.23x108 J/kg||2.67|
|Lithium Ion Battery||1.80x106 J/kg||0.0391|
|Alkaline Battery||6.70x105 J/kg||0.0146|
|Compressed Air||4.42x105 J/kg||0.00961|
|Nickel Metal Hydride Battery||2.90x105 J/kg||0.00630|
|Super Capacitor||1.80x104 J/kg||0.00039|
|Electrolytic Capacitor||1.00x103 J/kg||0.00002|
|Hydroelectric Power||1.00x103 J/kg||0.00002|
As an engineer, two things jump out at me here.
First, from an efficiency standpoint I would want to operate as high up the list as possible. Yes, I could build a 2,000 liter water tank on a 100 meter tower and use sunlight to pump water up into it. This potential energy could then be recovered with a hydroelectric turbine as it drains back out.
Or, I could build an enormous flywheel, with all its attendant mechanical issues, and use solar power to spin it up. An electric generator would then allow the stored energy to be tapped.
Or, I could simply store the energy in a briefcase-sized lithium ion battery. Relatively inexpensive. No moving parts. Little heat. Highly portable.
Finally, compressed hydrogen is very intriguing as a means of storing power. It can be made from sea water, and its only combustion product is water vapor. And, it contains almost three times the energy density as gasoline.
I will speak more on this compelling fuel in a future post.