Friday, August 19, 2011

Agricultural Robots: Power Source

The choice for a robot harvester/tender's power source will depend on its power requirements.  But what exactly these power requirements are going to be is a big unknown. 

When, as a engineer, I am faced with a new design challenge, and I don't have any idea were to start, one of the first things I try is to make some order of magnitude estimates (OME's) of the problem I'm dealing with. 

Coming of age as an engineer, before the introduction of pocket calculators, I did all of my calculations using a slide rule.  A slide rule only works with the significant digits of a calculation; you have to keep track of the exponents separately; so using a slide rule forces you, over time, to acquire the skill of doing quick order-of-magnitude calculations in your head. 

This is a skill that I don't believe engineering students are being exposed to anymore.  This is unfortunate, since being able to do a quick OME, while not giving you an answer to your design question, does at least give you a good estimate of its "size".  Which, often by itself, will be enough information to let you know which design solutions won't work and help you narrow down the range of design solutions you will need to look at. 

Another useful aspect of doing OME's is that you can often get a feel for the size of a design challenge, one that you have little prior knowledge of, by comparison with problems you do have past experience with.  I guess one of the observations I'm trying to get at here is that doing OME's requires an engineer to trust his/her intuition on a problem.  But the use of calculators, with their apparent exactness of answers, has taken yet one more opportunity away for students to develop their engineering intuition. 

So let's try this OME approach to the power source question.  First, I know from my running experience that a runner will burn about 110-140 calories per mile, about 0.15 kWh/mile.  I also recall from memory that a moped got about 100-mpg, about 0.33 kWh/mile.  This suggests to me that a machine will probably consume, maybe, 5-times as much energy [1] as a human to accomplish the same manual labor task.  The other thing I recall from my running experience is that an in-shape human is capable of 100-150 watts of sustained power output over the course of an 8-10 hour workday. 

Putting these two OME's together indicates that our robot harvester/tender will require a power source on the order of a 1-2 kW's; with a total energy use for an 8-hour workday of about 10-15 kWh's. 

The next step is to look into possible power sources that fall into this range. 
  • One gallon of gas, burned efficiently in an internal combustion engine, has an energy equivalent of 30-35 kWh; weight, 7 lbs, and volume, 0.15 cubic-ft.
  • The Chevy Volt's Li battery has a usable charge of about 12 kWh, weight, 435 lbs, and volume, approx 15.0 cubic-ft.
  • For comparison, a standard 12V lead-acid car battery holds around 500 Wh.
  • A 1 sq-meter solar panel has a sunny day capacity on the order of 1 kWh per day. But it must be remembered that a harvester/tender robot will be required to work rain or shine, day or night.

One thing that immediately stands out is the weight and size of a battery capable of powering our robot harvester/tender.  At 400+ lbs, the Li battery will be almost twice as heavy as our robot's frame alone! 

Future Posts: The only realistic option appears to be a liquid-fuel power source of some kind; this could be either an internal combustion engine or a fuel cell.  Of the ICE’s there have been some interesting developments in the area of micro-turbine engines and, on the fuel cell front, the reformed methanol fuel cell.

[1] Remember, actual numbers are not critical here, only the general magnitude of the value.  Also, I added some extra design fudge-factor to get to the number 5.

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