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.


Energy Sources and Propulsion Choices

by Web FishAug 11, 2012 @ 08:18am

Warning: scientific content. If you are easily bored, please scroll down for final conclusions.

Now that we've established the rules, first order of business would be to figure out our energy source and the associated propulsion system. Not being able to rely on stored energy for propulsion (at first read) automatically discard several obvious sources:

  • Any exothermic chemical reaction based process - internal combustion engine, most forms of jet propulsion (fuel-powered jet engine), most forms of steam engine (where fuel is used for heat generation);
  • Any direct chemical reaction energy conversion for which at least one of the reagents is stored on board - chemical battery bank, classic fuel cell, pseudocapacitor;
  • Any direct electric energy storage system - pre-charged super capacitor;
  • Any mechanical energy storage - flywheels, etc.;
  • Any nuclear power source (not that our rules are the main limiting factor there :) 

What is left then? Actually, plenty (not necessarily in order of practicality):

  • Harvesting electromagnetic wave energy (to simplify things we'll refer to it as sunlight, even though any EM waves could do):
    • Direct conversion of  into mechanical energy. Theoretically (and somewhat practically - this, this or this) possible, nothing available yet that will work with diffused sunlight and produce the levels we need;
    • Conversion into electric current and from there on into mechanical energy for propulsion and (optionally) into other types of energy for storage. Plenty of variations of propulsion and storage methods are available. Solar panels driving electric motor and/or charging chemical battery bank can be achieved with mainstream components;
    • Conversion of sunlight into chemical energy and from there on into electric current and so forth. Various forms of photocatalysis with 100% external reagents (chemicals from ocean water?), photosynthesis. Not much available within our budget and size;
    • Conversion of sunlight into thermal energy and from there on into mechanical, electrical and/or chemical energy. Heating water by (concentrated) solar energy and powering steam engine, turbine, jet propulsion, etc.;
  • Harvesting mechanical energy from the surrounding ocean water:
    • Direct use of mechanical energy of ocean currents for propulsion. All forms of ocean gliders; 
    • Direct use of the mechanical energy of ocean waves for propulsion. Wave gliders;
    • Conversion of mechanical wave energy into electric current and so forth. Wave power generation - certain possibility but energy density is minimal within 1 cubic meter under the surface;
  • Harvesting mechanical energy from the surrounding air (wind):
    • Direct use of wind energy for propulsion. All types of sails
    • Conversion of mechanical wind energy into other forms of mechanical energy (i.e. driving a propeller by a wind generator). Complete decoupling of wind direction from vessel direction;
    • Conversion of mechanical wind energy into electric current and so forth. All types of Wind power generation;
    • Conversion of mechanical wind energy into other forms of energy - thermal, chemical, etc. Mentioned for completeness;
  • Harvesting chemical energy from the surrounding ocean water:
    • Harvesting and converting energy of phototrophic organisms;
    • Exothermic chemical reaction triggered by catalyst carried on board but with reagents derived 100% from the surrounding environment. Way out there... but still a cool idea :)

Not too bad. We will revisit some of the options above in the (hopefully not too distant) future, but for the time being, based on the other two factors in the rule book (volume constraints and budget considerations) our finalists are:

  • Harvesting sunlight energy via photovoltaics;
  • Harvesting sunlight energy via heat engine;
  • All forms of wind energy harvesting;

Time to see if anyone of these can actually get us across the ocean.