Energy Density and Power Budget

by Web FishAug 12, 2012 @ 01:51pm


From an energy density perspective, here is a (approximate) comparison of our energy source finalists and their currently available harvesting methods:

  • Photovoltaics: 100 Watts/sq.m. (~10% efficiency);
  • Solar thermal: 100 Watts/sq.m. (~10% efficiency);
  • Wind turbine to electric and/or mechanical (from extrapolated data from here and here): 60 Watts/sq.m. (assuming overall 20% efficiency and the energy levels at 10 m altitude. In reality - probably lower);
  • Wind sail soft/rigid (assumed - cannot find proper data): 100-250 Watts/sq.m. (same wind assumptions as above);

Those are the theoretical maximums we could derive from each energy source type if we were to use 100% of the available area of our virtual cube. In reality, the harvested energy will be lower (due to partial surface utilization, variable angle towards the energy source, environment factors, etc.). This doesn't really leave us with a lot to work with. So we need to be frugal :)

We have a few "fixed needs" that will have to be addressed before we can even start expending energy for propulsion:

  • Navionics: computer guidance system, sensors, servo control (minimized). Allocated budget: 5 Watts; 
  • Comm: we will need to phone home. Only required in short burst mode. Average allocated budget: 1 Watt;
  • Navigation lights: Optional. Average allocated budget: 1 Watt;
  • Image acquisition: camera(s) / storage. Optional. Only required in short burst mode. Average allocated budget: 1 Watt;

So yes, we DEFINITELY need to be frugal.

Assuming a displacement hull (and at these energy levels that is the only type that we can figure out how to build and get moving) our 1 meter maximum length will give us a maximum displacement speed of 2.4 to 2.7 knots (4.5 to 5.0 km/h).

From past experience two 6V Decaperm motors will propel a 4 foot "fat" hull (tug-boat scale model - ~12 kg. displacement) to approx. 3 knots drawing roughly 5 Amps each. 5 Amps * 6 Volt * 2 = 60 Watt total. This is under ideal conditions and in calm water. But we are in the ballpark! Even with the fixed needs, each of the energy sources above are still in the game. And for now this is without combining two or more of them.

The engineering considerations will be the deciding factor.


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